Chemical Sensors: Fundamentals of Sensing Materials, Vol. 3: Polymers and Other Materials

Momentum Press is proud to bring you Chemical Sensors Volume 3, the newest addition to The Sensors Tech-nology Series, edited by Joe Watson. In this third Volume, Polymers and Other Materials, new applications for chemical sensing, using materials developments in polymers, calixarenes, biological and biomimetic systems, novel semiconductors, and ionic conductors, are fully explored. This book will make it clear that chemical sensors based on these materials comprise a large part of the chemical sensors market. Inside, you will find background and guidance on:• Anoverviewofpolymersusedinchemicalsensingmaterialsincludingwhattheyare,howtheyaremade,

and how their functionality can be designed and enhanced for sensing applications• Molecular-Imprinting—anewapproachfordesigningpolymer-basedsensors• New developments in calixarene-based materials for chemical sensors including their synthesis,

characterization and properties and use for such things as probes for sensing and extraction, sensing ions and molecules, and use as ditopic molecular receptors

• Advancesinbiologicalandbiomimeticsystems,includingpolymer-biomoleculeassemblies,membranes,and nanobiosensors

• Newsemiconductingsensingmaterialsusingdiamond,galliumnitrideandIII-nitrides,andnano-scalesemiconducting structures

• Auniqueanalysison the limitsof sensingmaterials—andhowtobestchoose therightmaterial foraparticular sensing objective

Chemical sensors are integral to the automation of a myriad industrial processes, as well as everyday moni-toring of such activities as public safety, testing and monitoring, medical therapeutics, and many more. This massive reference work will cover all major categories of both the materials used for chemical sensors and their applications. This is THE reference work on sensors used for chemical detection and analysis.TheChemicalSensorsreferencesbookswillspan6volumesandcover in-depthdetailsonbothmateri-

als used for chemical sensors and their applications, with volumes 1 through 3 exploring the materials used forchemicalsensors—theirproperties,theirbehavior,theircomposition,andeventheirmanufacturingandfabrication.Volumes4 through6willexplore thegreatvarietyofapplications forchemical sensors—frommanufacturing and industry to biomedical uses.

AbouT The ediTorGhenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and Devices in 1976 and his Habilitate Degree (Dr. Sci.) in Physics and Mathematics of Semiconductors and Dielectrics in 1990. He was for many years the leader in the Gas Sensor Group at the Technical University of Moldova. He is currently a research professor at Gwangju Institute of Science and Technology, in Gwangju, RepublicofKorea.Dr.Korotcenkov is theauthoroffivepreviousbooksandhasauthoredover180peer-reviewedpapers.Hisresearchhasreceivednumerousawardsandhonors,includingtheAwardoftheSupremeCouncilofScienceandAdvancedTechnologyoftheRepublicofMoldova.

ISBN: 978-1-60650-230-3

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CHEMICAL SENSORS VoLuME 3: PoLyMErs And otHEr MAtErIALsEdited by Ghenadii Korotcenkov, Ph.d., dr. sci.

AvolumeintheSensors Technology SeriesEdited by Joe WatsonPublished by Momentum Press®

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CHEMICAL SENSORSFUNDAMENTALS OF SENSING MATERIALSVOLUME 3: POLYMERS AND OTHER MATERIALS

CHEMICAL SENSORSFUNDAMENTALS OF SENSING MATERIALSVOLUME 3: POLYMERS AND OTHER MATERIALS

EDITED BYGHENADII KOROTCENKOV

GWANGJU INSTITUTE OF SCIENCE AND TECHNOLOGYGWANGJU, REPUBLIC OF KOREA

MOMENTUM PRESS, LLC, NEW YORK

Chemical Sensors: Fundamentals of Sensing Materials. Volume 3: Polymers and Other MaterialsCopyright © Momentum Press®, LLC, 2010

All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means—electronic, mechanical, photocopy, recording or any other—except for brief quotations, not to exceed 400 words, without the prior permission of the publisher.

First published in 2010 byMomentum Press®, LLC222 East 46th Street, New York, NY 10017www.momentumpress.net

ISBN-13: 978-1-60650-230-3 (hard back, case bound)ISBN-10: 1-60650-2309-1 (hard back, case bound)ISBN-13: 978-1-60650-232-7 (e-book)ISBN-10: 1-60650-232-8 (e-book)DOI forthcoming

Cover design by Jonathan PennellInterior design by Derryfi eld Publishing, LLC

First Edition: December 2010

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Printed in the United States of America

v

CONTENTS

PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS xi

PREFACE TO VOLUME 3: POLYMERS AND OTHER MATERIALS xiii

ABOUT THE EDITOR xv

CONTRIBUTORS xvii

1 POLYMERS IN CHEMICAL SENSORS 1B. Adhikari P. Kar

1 Introduction 12 What Are Polymers? 33 Parameters of Polymers Promising for Chemical Sensor Application 44 Synthesis of Polymers 75 Deposition of Polymers 96 Functionalization of Polymers 13

6.1 Structure Modifi cation 136.2 Surface Modifi cation 146.3 Composition Modifi cation 15

7 Polymers in Chemical Sensors 167.1 Optical and Fiber Optic Polymer-Based Sensors 207.2 Conductometric Gas Sensors 277.3 SAW and QCM Polymer-Based Sensors 407.4 Electrochemical Polymer-Based Sensors 457.5 Chemically Sensitive FET-Based Sensors 57

8 Outlook 619 Acknowledgments 61References 62

vi CONTENTS

2 MOLECULAR IMPRINTING (TEMPLATING)—A PROMISING APPROACH FOR DESIGN OF POLYMER-BASED CHEMICAL SENSORS 77

G. Korotcenkov B. K. Cho

1 Introduction 772 General Principles of Molecular Imprinting (Templating) 793 Methods of Imprinting (Templating) 80

3.1 In-Block Imprinted Polymers 803.2 In Situ Imprinted Polymers 823.3 Polymer-Imprinted Beads 82

4 Components of Imprinting Technology 834.1 Target Molecules 834.2 Th e Imprinting Matrix 854.3 Cross-Linkers 864.4 Solvents (Porogens) 884.5 Initiators 89

5 MIP Preparation Methods 896 Combination of MIPs and Monomolecular Host Molecules 917 Control of the Imprinting Eff ect 928 Application of Imprinting Polymers in Chemical Sensors 92

8.1 Advantages of MIP-Based Chemical Sensors 928.2 Detection Principles Used in MIP Chemical Sensors 938.3 Interfacing the MIP with the Transducer 998.4 Factors Controlling the Sensing Characteristics of MIPs-Based Chemical Sensors 1018.5 Micro- and Nanofabricated MIPs 103

9 Outlook 10610 Acknowledgments 108References 109

3 CALIXARENE-BASED MATERIALS FOR CHEMICAL SENSORS 117H. M. ChawlaN. PantS. KumarD. StC. BlackN. Kumar

1 Introduction 117

CONTENTS vii

2 Molecular Receptors and Generation of Signal for Sensing Target Species 1193 Calixarenes and Th iacalixarenes 1204 Synthesis of Calix[n]arenes 123

4.1 Base-Catalyzed Condensation Reactions 1234.2 Acid-Catalyzed Condensation Reactions 124

5 Synthesis of Th iacalix[n]arenes (Sulfur-Bridged Calixarenes) 1246 Physical Properties of Calixarenes and Tetrathiacalixarenes 127

6.1 Melting points 1276.2 Solubilities and pKa Values 127

7 Spectral Properties and Characterization of Calixarenes 1287.1 Infrared Spectra 1287.2 Ultraviolet Spectra 1287.3 NMR Spectra 129

8 Conformational Structures of Calixarenes and Th iacalixarenes 1299 Conformational Characterization of Calix[n]arenes 13210 Calixarenes as Materials for Chemical Sensors 13211 Calixarene-Based Materials for Recognition of Alkali and Alkaline Earth

Metal Ions 13312 Calixarene-Based Materials for Recognition of Transition and Heavy-

Metal Ions 13713 Calixarene-Based Materials as Dual Probes for Sensing and Extraction 13914 Calixarene-Based Materials for Sensing Lanthanides and Actinides 13915 Sensor Materials Based on Polymeric Calixarenes 14316 Naked-Eye Sensing: Calixarene-Based Chromogenic Materials for Sensing

Ions and Molecules 14317 Calixarene-Based Electroactive Sensing Materials 15618 Calixarene-Based Materials for Sensing Anions 161

18.1 Calixarene-Based Electron-Defi cient or Positively Charged Anion Receptors 16118.2 Calixarene-Based Neutral Anion Receptors 16518.3 Calixarene-Based Ditopic Molecular Receptors 175

19 Calixarene-Based Sensor Materials for Neutral Molecules and Biological Amines 176

20 Calixarene-Based Materials for Gas Sensors 17921 Outlook 181

viii CONTENTS

22 Acknowledgments 181References 182

4 BIOLOGICAL AND BIOMIMETIC SYSTEMS IN CHEMICAL SENSORS 201R. JelinekS. Kolusheva

1 Introduction 2012 Polymers and Polymer/Biomolecule Assemblies 202

2.1 Conductive Polymers 2022.2 Luminescent Conjugated Polymers 205

3 Membranes in Chemical Sensors 2103.1 Chemical Membranes 2103.2 Biological Membranes 213

4 Biomimetic Systems for Molecular and Ionic Recognition 2194.1 Biological Receptors and Channels 2194.2 Synthetic Receptors 2214.3 Biomimetic Enzyme-Based Sensors 2234.4 Nanobiosensors 2254.5 Other Biomimetic Sensors 232

5 Monolayers and Films 2425.1 Self-Assembled Monolayers 2425.2 Langmuir-Blodgett Films 244

6 Challenges and Limitations of Biosensors 2477 Conclusions and Outlook 248References 248

5 NOVEL SEMICONDUCTOR MATERIALS FOR THE DEVELOPMENT OF CHEMICAL SENSORS 263N. Chaniotakis N. Sofi kiti V. Vamvakaki

1 Introduction 2632 Th e Silicon Era—Classical Semiconductors in Chemical Sensing 2653 Fundamentals of Sensor Development 2694 Surface Chemistry of Semiconductors in Chemical Sensing 2695 Band Gap Th eory and Its Relationship to Sensor Design 2716 Pinning of the Surface Fermi Level 272

CONTENTS ix

7 New Semiconductor Substrates 2737.1 Diamond 2747.2 Silicon Carbide 2777.3 Gallium Nitride and III-Nitrides 279

8 Nanosemiconductor Structures in Chemical Sensors 2819 Forecasting the Future 283References 284

6 ION CONDUCTORS AND THEIR APPLICATIONS IN CHEMICAL SENSORS 291R. V. KumarC. Schwandt

1 Introduction 2911.1 Solid Electrolytes 2921.2 Chemical Sensors 293

2 Ionic Conduction in Solids 2943 Oxygen Ion–Conducting Solid Electrolytes 296

3.1 Zirconia-Based Solid Electrolytes 2973.2 Defect Chemistry of Stabilized Zirconia 3043.3 Preparation of Stabilized Zirconia 3053.4 Oxygen Sensors Based on Stabilized Zirconia 308

4 Proton-Conducting Solid Electrolytes 3194.1 High-Temperature Proton-Conducting Solid Electrolytes 3194.2 Defect Chemistry of Substituted Perovskites 3204.3 Preparation of Substituted Perovskites 3234.4 Hydrogen Sensors Based on Substituted Perovskites 3244.5 Low-Temperature Proton-Conducting Solid Electrolytes 330

5 Metal Ion–Conducting Solid Electrolytes 3325.1 Defect Chemistry and Preparation of -Aluminas 3325.2 Sensors Based on -Aluminas 337

6 Outlook and Future Trends 343References 344

7 SENSOR MATERIALS: SELECTION GUIDE 351G. Korotcenkov

1 Acceptable Materials for Chemical Sensors 3512 Which Metal Oxides Are Better for Gas Sensors? 358

x CONTENTS

3 Choosing a Polymer for a Chemical Sensor Application 3624 Technological Limitations in Sensing Material Applications 3625 Future Trends 3636 Toward a Th eory of Chemical Sensors 3687 Summary 3708 Acknowledgments 370References 370

INDEX 375

xi

PREFACE TO CHEMICAL SENSORS:

FUNDAMENTALS OF SENSING MATERIALS

Sensing materials play a key role in the successful implementation of chemical and biological sen-sors. Th e multidimensional nature of the interactions between function and composition, preparation method, and end-use conditions of sensing materials often makes their rational design for real-world applications very challenging.

Th e world of sensing materials is very broad. Practically all well-known materials could be used for the elaboration of chemical sensors. Th erefore, in this series we have tried to include the widest pos-sible number of materials for these purposes and to evaluate their real advantages and shortcomings. Our main idea was to create a really useful “encyclopedia” or handbook of chemical sensing materials, which could combine in compact editions the basic principles of chemical sensing, the main properties of sensing materials, the particulars of their synthesis and deposition, and their present or potential ap-plications in chemical sensors. Th us, most of the materials used in chemical sensors are considered in the various chapters of these volumes.

It is necessary to note that, notwithstanding the wide interest and use of chemical sensors, at the time the idea to develop these volumes was conceived, there was no recent comprehensive review or any general summing up of the fundamentals of sensing materials Th e majority of books published in the fi eld of chemical sensors were dedicated mainly to analysis of particular types of devices. Th is three-volume review series is therefore timely.

Th is series, Chemical Sensors: Fundamentals of Sensing Materials, off ers the most recent advances in all key aspects of development and applications of various materials for design of chemical sensors. Regarding the division of this series into three parts, our choice was to devote the fi rst volume to the fundamentals of chemical sensing materials and processes and to devote the second and third volumes to properties and applications of individual types of sensing materials. Th is explains why, in Volume 1: General Approaches, we provide a brief description of chemical sensors, and then detailed discussion of desired properties for sensing materials, followed by chapters devoted to methods of synthesis, deposi-tion, and modifi cation of sensing materials. Th e fi rst volume also provides general background informa-tion about processes that participate in chemical sensing. Th us the aim of this volume, although not ex-haustive, is to provide basic knowledge about sensing materials, technologies used for their preparation, and then a general overview of their application in the development of chemical sensors.

xii PREFACE TO CHEMICAL SENSORS: FUNDAMENTALS OF SENSING MATERIALS

Considering the importance of nanostructured materials for further development of chemical sen-sors, we have selected and collected information about those materials in Volume 2: Nanostructured Materials. In this volume, materials such as one-dimension metal oxide nanostructures, carbon nano-tubes, fullerenes, metal nanoparticles, and nanoclusters are considered. Nanocomposites, porous semi-conductors, ordered mesoporous materials, and zeolites also are among materials of this type.

Volume 3: Polymers and Other Materials, is a compilation of review chapters detailing applications of chemical sensor materials such as polymers, calixarenes, biological and biomimetric systems, novel semiconductor materials, and ionic conductors. Chemical sensors based on these materials comprise a large part of the chemical sensors market.

Of course, not all materials are covered equally. In many cases, the level of detailed elaboration was determined by their signifi cance and interest shown in that class of materials for chemical sensor design.

While the title of this series suggests that the work is aimed mainly at materials scientists, this is not so. Many of those who should fi nd this book useful will be “chemists,” “physicists,” or “engineers” who are dealing with chemical sensors, analytical chemistry, metal oxides, polymers, and other materials and devices. In fact, some readers may have only a superfi cial background in chemistry and physics. Th ese volumes are addressed to the rapidly growing number of active practitioners and those who are interested in starting research in the fi eld of materials for chemical sensors and biosensors, directors of industrial and government research centers, laboratory supervisors and managers, students and lecturers.

We believe that this series will be of interest to readers because of its several innovative aspects. First, it provides a detailed description and analysis of strategies for setting up successful processes for screen-ing sensing materials for chemical sensors. Second, it summarizes the advances and the remaining chal-lenges, and then goes on to suggest opportunities for research on chemical sensors based on polymeric, inorganic, and biological sensing materials. Th ird, it provides insight into how to improve the effi ciency of chemical sensing through optimization of sensing material parameters, including composition, struc-ture, electrophysical, chemical, electronic, and catalytic properties.

We express our gratitude to the contributing authors for their eff orts in preparing their chapters. We also express our gratitude to Momentum Press for giving us the opportunity to publish this series. We especially thank Joel Stein at Momentum Press for his patience during the development of this project and for encouraging us during the various stages of preparation.

Ghenadii Korotcenkov

xiii

Th is volume covers a variety of topics in the rapidly developing fi eld of chemical sensors. Th e purpose of this volume is to explain and illustrate the use of multifunctional materials such as polymers, calixarenes, ion conductors, biological systems, and novel semiconductors in chemical sensors. Th ese materials diff er fundamentally from standard metal oxides and metals, so their application provides opportunities to de-sign sensors based on entirely diff erent mechanisms of sensing. As a result, new trends in the elaboration of sensors with diff erent functional attributes and for building instruments with previously unavailable capabilities demanded by new applications have opened up. Th erefore, this book is intended to be a pri-mary source for both fundamental and practical information related to these multifunctional materials, which will be necessary for future development.

Th is volume comprises seven chapters written by active researchers who are well-known experts in their fi elds and who have made signifi cant contributions to the fi eld over the past several years. Th us, this book presents the most recent advances in all the key aspects of development and application of polymers and other multifunctional materials for chemical and biological analysis. Th e chapters in this book have been written to give the reader the “big picture,” from the design phase to implementation of chemical sensors of various types. Every chapter addresses the particulars of multifunctional materials synthesis and characterization. In every chapter you will also fi nd descriptions of a very wide range of devices that may be designed using such multifunctional materials.. Th ese chapters thus highlight the materials, the physics, the devices, and even key fabrication issues. We hope that the information pre-sented in this volume will help the reader understand the details of sensing material design for specifi c applications and establish quantitative structure–function relationships.

Th e intended audience is scientists, researchers, and engineers in industries and research laborato-ries. With its many references to the vast resources of recently published literature on the subject, this book serves as a signifi cant and insightful source of valuable information pertaining to the ongoing scientifi c debates, the current state of understanding, and future directions. Students will also fi nd the book to be very useful in their research and understanding of chemical sensors and multifunctional materials. Th e structure of this book off ers a basis for a high-throughput instrumentation course at the

PREFACE TO VOLUME 3: POLYMERS AND

OTHER MATERIALS

xiv PREFACE TO VOLUME 3: POLYMERS & OTHER MATERIALS

advanced undergraduate or graduate level. As such, it should be very useful to university post-docs and professors as well.

Ghenadii Korotcenkov

xv

ABOUT THE EDITOR

Ghenadii Korotcenkov received his Ph.D. in Physics and Technology of Semiconductor Materials and Devices in 1976, and his Habilitate Degree (Dr.Sci.) in Physics and Mathematics of Semiconductors and Dielectrics in 1990. For a long time he was a leader of the scientifi c Gas Sensor Group and manager of various national and international scientifi c and engineering projects carried out in the Laboratory of Micro- and Optoelectronics, Technical University of Moldova. Currently, he is a research professor at Gwangju Institute of Science and Technology, Gwangju, Republic of Korea.

Specialists from the former Soviet Union know G. Korotcenkov’s research results in the study of Schottky barriers, MOS structures, native oxides, and photoreceivers based on Group III–V compounds very well. His current research interests include materials science and surface science, focused on metal oxides and solid-state gas sensor design. He is the author of fi ve books and special publications, nine invited review papers, several book chapters, and more than 180 peer-reviewed articles. He holds 16 patents. He has presented more than 200 reports at national and international conferences. His articles are cited more than 150 times per year. His research activities have been honored by the Award of the Supreme Council of Science and Advanced Technology of the Republic of Moldova (2004), Th e Prize of the Presidents of Academies of Sciences of Ukraine, Belarus and Moldova (2003), the Senior Research Excellence Award of Technical University of Moldova (2001, 2003, 2005), a Fellowship from the International Research Exchange Board (1998), and the National Youth Prize of the Republic of Moldova (1980), among others.

xvii

Basudam Adhikari (Chapter 1)Materials Science CentreIndian Institute of TechnologyKharagpur 721302, India

David St. Clair Black (Chapter 3)School of Chemistry University of New South Wales Sydney 2052, New South Wales, Australia

Nikos Chaniotakis (Chapter 5)Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of CreteVoutes 71003 Iraklion, Crete, Greece

Har Mohindra Chawla (Chapter 3)Department of ChemistryIndian Institute of Technology DelhiNew Delhi 110016, India

Beongki Cho (Chapter 2)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of Korea

Raz Jelinek (Chapter 4)Department of Chemistry and Ilse Katz Institute for Nanotechnology Ben Gurion UniversityBeer Sheva 84105, Israel

CONTRIBUTORS

xviii CONTRIBUTORS

Pradip Kar (Chapter 1)Polymer Engineering DepartmentBirla Institute of Technology, MesraRanchi 835215, India

Sofi ya Kolusheva (Chapter 4)Department of Chemistry and Ilse Katz Institute for NanotechnologyBen Gurion UniversityBeer Sheva 84105, Israel

Ghenadii Korotcenkov (Chapters 2 and 7)Department of Material Science and EngineeringGwangju Institute of Science and TechnologyGwangju 500-712, Republic of KoreaandTechnical University of MoldovaChisinau, Republic of Moldova

Naresh Kumar (Chapter 3)School of ChemistryUniversity of New South Wales Sydney 2052, New South Wales, Australia

Ramachandran Vasant Kumar (Chapter 6)Department of Materials Science and MetallurgyUniversity of CambridgeCambridge CB2 3QZ, United Kingdom

S. Kumar (Chapter 3)Department of ChemistryIndian Institute of Technology Delhi New Delhi 110016, India

Nalin Pant (Chapter 3)Department of ChemistryIndian Institute of Technology DelhiNew Delhi 110016, India

Carsten Schwandt (Chapter 6)Department of Materials Science and MetallurgyUniversity of Cambridge Cambridge CB2 3QZ, United Kingdom

CONTRIBUTORS xix

Nikoletta Sofi kiti (Chapter 5) Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of CreteVoutes 71003 Iraklion, Crete, Greece

Vicky Vamvakaki (Chapter 5) Laboratory of Analytical ChemistryDepartment of ChemistryUniversity of Crete,Voutes 71003 Iraklion, Crete, Greece

1

CHAPTER 1

POLYMERS IN CHEMICAL SENSORS

B. Adhikari P. Kar

1. INTRODUCTION

Plants and animals have built-in natural sensor devices targeted either for detection of external agencies in their surroundings or for performing some specifi c function. Plants have electromagnetic detectors for sensing external attacks. Animals have devices for sensing through fi ve diff erent sense organs—the tongue, skin, eye, ear, and nose. Th ey use these organs to perform normal activities as well as to re-main away from unfavorable situations. To help them remain away from deadly poisonous chemical substances, for instance, animals are alerted by smell and by irritation of the eyes and skin. Both the feeling of irritation and detection of abnormal odors occur via their built-in sensor system. Th ese sensor systems, formed through biosynthesis from organic molecules, are embedded in an aqueous environ-ment containing soluble electrolytic salts. Th us the sensing network in living systems is organic as well as polymeric in nature.

Gathering knowledge by studying biological sensing systems and their functioning can help in mim-icking such sensor systems using synthetic macromolecules. Such sensor networks consist of a chemical detection or recognition element, a transducing element, and a signal-processing element, which appear to be extremely complex even though their function and response appear to be very simple and spon-taneous (see Figure 1.1). Th e fi rst element is a chemical detection element of some sensing material, which interacts with the environment and generates a response. Th e second element is a transducer, which reads the response from the chemical detection element and converts it into an interpretable and quantifi able term for the third element, the signal processor. Th e chemical detection element is the

2 CHEMICAL SENSORS: FUNDAMENTALS. VOLUME 3: POLYMERS & OTHER MATERIALS

heart of the sensor system and can be considered the primary part of the sensor. Th e response, recovery, selectivity, and sensitivity of a chemical sensor depend on the chemical detection element used. Sensor technology depends on progress in materials science and technology for this chemical detection element layer. Th e choice of a particular interactive material is based on the sensitivity, selectivity, reliability, and the reversibility of the related sensing mechanisms. Various chemical detection materials are available, including metal and metal oxide semiconductors, solid electrolytes, insulators, catalytic materials, poly-mers, composites, and others.

Exposure to toxic and hazardous chemicals may cause serious problems to mammals, such as irrita-tion, vomiting, suff ocation, or illness; the chemical may even be deadly poisonous. Th us, as a measure of protection to living bodies, it is necessary to detect hazardous and toxic chemicals using artifi cial sen-sor devices. Toward that goal, sensing of toxic and hazardous chemicals is an emerging fi eld, in which modern research tries to develop more effi cient sensor devices than those in living bodies.

Th e chemical sensors fi eld is one of the fastest-growing areas in both research and commercial ap-plication. During the last 25 years, global research and development in the fi eld of sensors has expanded exponentially in terms of fi nancial investment, published literature, and number of active researchers. Most of the research work in this fi eld is concentrated toward reducing the size of sensors and enabling identifi cation and quantifi cation of multiple species. Since easy handling, quick response, good revers-ibility and reproducibility, sensitivity, and selectivity are qualities expected of an excellent sensor, there is a need for further research.

Exploring the present state of the art of materials used in chemical sensors is our primary goal in this chapter, with special reference to the role of organic polymers as chemical detecting elements in sensors. Since chemical sensors need to either detect or estimate chemical analytes, the sensing element should be selective as well as interactive with the analyte. In terms of general operating principles, there are three major categories of chemical sensors: electrochemical, optical, and mass sensors. Th erefore, the detecting element, which is the key element of a sensor device, must respond to electrochemical or optical stimu-lation or undergo structural changes due to a change in its mass as a result of absorption of a minute quantity of an analyte. From this point of view, polymers represent a class of highly tailorable materials, which have already been qualifi ed as detecting elements that respond at ambient temperature to chemical or electrochemical stimulation, to optical stimulation, and that express piezoelectric behavior after ab-sorbing a small mass. In addition, polymers are easy to synthesize and process to develop a suitable device for sensor application. Th e properties of polymers, such as their polyelectrolytic nature, intrinsic con-ductivity, electrochromism, etc., have made them today’s materials of choice in modern sensor devices, gradually replacing the metal oxides and inorganic semiconductors that earlier dominated the fi eld.

Analyte or substrate

Chemical detection element (polymer)

Transducer Signal processor

Figure 1.1. Schematic representation of simplifi ed sensor setup.

POLYMERS IN CHEMICAL SENSORS 3

2. WHAT ARE POLYMERS?

In contrast to discrete small molecular compounds, polymers are macromolecules. Except for a few, polymers are organic macromolecules made of carbon and hydrogen atoms in major percentage with some heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, halogens, etc., as minor constituents. A polymer molecule is formed by the repetitive union of a large number of reactive small molecules in a regular sequence (see Figure 1.2). Th e repeated unit in the backbone of the polymer molecule is known as the mer unit, and the reactive small molecule from which the polymer is formed is called the monomer. Th e simplest example of a polymer is polyethylene, in which the ethylene moiety is the mer unit.

Figure 1.2. Repeating unit structures of some common conducting polymers.

n

N

H

n

S

R

n

Nn

HPolyacetylene Polypyrrole Poly(3 alkylthiophene) Polyaniline

Sn O

n

n

Sn

Polythiophene Polyfuran Polyphenylene Polyphenylene sulfide

n

S

n

S

n

Polyphenylenevinylene Polythienylenevinylene Polyisothianapthene

n

NH

n

n

Polyazulene Polycarbazole Polyfluorene


As a class, polymers are unique over other materials with respect to their tailorability and broad range of properties as well as versatility. In terms of size and molecular weight, in general, polymers are more than a million times bigger than small molecular compounds. Properties of polymers, in general, depend on their chemical composition, molecular structure, molecular weight, molecular-weight distri-bution, and morphology. Morphologically, polymers are quasi-crystalline in nature, having small crys-tallites dispersed in an amorphous matrix in which a single molecule may extend from one amorphous region to a distant amorphous region while passing through several crystallite regions. Both the bulk and the surface of a polymer sample may or may not contain active functional groups which can respond to a stimulus in chemical sensing. Although primary covalent bonds predominate in polymers, secondary bonding infl uences both their processing and their functional performance. Due consideration should be given to the role of secondary bonding on the interaction of a polymer with the analyte during the sensing function. Extensive secondary bonding interaction can be a major cause of insolubility of a poly-mer in solvents, which may restrict its processability to produce a suitable device for chemical sensing.

Interpretation of sensing response and recovery is easier if it can be correlated with chemical bond-ing in the polymer. On the other hand, organic macromolecules are known which can exhibit ionic or electronic conduction properties. Th ese are polymers that have ionizable functional groups, viz., poly-electrolytes, and extended π-electron conjugation, viz., intrinsically conducting polymers. In general, polymers are electrically insulating in nature due to the nonavailability of free electrons, since the four valence electrons of carbon are fully saturated. However, some organic polymer molecules are semicon-ducting in nature, by virtue of their extended π-electron conjugation along the backbone chain of the macromolecule. Th e ability of conducting polymers to conduct electricity depends on the alternating double bond–single bond structure in the polymer backbone, coupled with the formation of some charged centers on the chain by partial oxidation. Th e introduction of such extra charges on the poly-mer by doping (in analogy to inorganic semiconductors), alters the conductivity of such polymers from almost insulators to something approaching a metallic conductor. Due to the more reactive nature of the ionic groups in polyelectrolytes and π-electron conjugation in conducting polymers, these are the important sensing sites for corresponding analyte compounds.

3. PARAMETERS OF POLYMERS PROMISING FOR CHEMICAL SENSOR APPLICATION

It has been pointed out that built-in sensing devices in biological systems are polymeric in nature, al-though they are complex. Th e actual structure and chemistry of the polymers in the biological sensing devices of various sense organs are not known. However, by virtue of their light weight, ease of synthesis, good processability, stability, nonhazardous nature, and low cost, polymers off er a lot of advantages as sensor materials over other materials in the majority of sensor technologies. Th e sensing ability of many synthetic polymers in various artifi cial sensor devices has been established by their excellent tailorability and easy processability to form very-thin-layer devices.

Polymers have high tailorability of their molecular structure and composition for better processabil-ity or to improve specifi c behavior in the bulk material or on its surface. As a result, a broad spectrum of properties can be easily obtained with polymers. In addition, the ease of creation of new functional groups


on the polymer backbone, their ability to respond to redox systems, their charge-carrying capability, and their ability to respond to optical stimulation are other advantages of using them in sensor devices. Due to the fl exible nature of the polymer chains, compact and miniaturized sensor arrays can be easily fabricated. Unlike metal and metal-oxide semiconductor materials, there is no need for special clean room, high temperature, or special high-cost process techniques to fabricate sensor devices using polymers.

Good selective sensing of a specifi c analyte in a mixture is also possible with a polymer because of its high structural tailorability. Polymers have operational/functional advantages at ambient temperature. Th e suitability of a polymer in a sensor device for use at a particular temperature may be suggested by the glass transition temperature (Tg) of the polymer. A polymer in a sensor device may provide proper function if the temperature of the sensing measurement is kept close to its Tg but well below its melting temperature (Tm).

As a basic principle, a polymer in a sensor device can function by absorption/adsorption of an analyte in the form of gas or liquid, reversibly or irreversibly, followed by interaction and change in properties of the polymer, which is transduced electronically, electrochemically, or optically. Processing of the transduced signal provides the sensor output. Large numbers of articles and reviews have been published on chemical sensors, in which many polymers have been utilized to fabricate sensor devices (Armstrong and Horvai 1990; Bidan 1992; Adhikari and Majumdar 2004; Persaud 2005). Although few such polymer-based sensor technologies have been commercialized as yet, many are in the explora-tion stage, and understanding the structure and properties of suitable polymers for use in sensor devices may lead to more advanced devices for chemical sensing.

Polymers have unique characteristics that have been proved to infl uence the operating parameters of sensor devices. Properties of polymers that infl uence the operating parameters of sensors can be physicochemical, electrical (conductivity, resistivity), chemical, optical (photo- and electrolumines-cence, optoelectronic), redox, hydrophobic/hydrophilic, piezoelectric/pyroelectric, etc. Being organic in nature, polymers provide an inherent affi nity to chemical species that need to be detected or estimated. Comparing the solubility parameters of both the polymer as sensing element and the analyte to be sensed can lead to correlation of this affi nity. Because of its long backbone chain and fl exible nature, a polymer can accommodate a large quantity of a foreign substance in the form of fi ne particle (as fi ller) or fi ber or even highly viscous liquid. Th us, a polymer in its solid state contains ample free volume in the bulk. Depending on its affi nity toward a foreign agent, the polymer can hold it for quite a long period. Th is can happen in two ways: by simple physical entrapment provided the foreign agent is compatible with the host polymer; or by chemical bonding between them. Th e latter is well known to provide co-valent immobilization of the foreign agent as the recognition element. A polymer to be used as either a solid carrier or directly as the sensing element can be best selected based on the solubility parameters of both the polymer and the analyte chemical compound.

Another important parameter to be considered in selecting a polymer for sensor application is diff u-sion. Better sensing response in terms of sensitivity and sensor recovery can be obtained from a polymer-based sensor if the polymer off ers a lower diff usion barrier to the analyte compound. Th is way, short response time as well as short recovery time with good sensor repeatability can be available from a par-ticular polymer–analyte system. Exploring the nature of interactions between polymer and analyte can also help in a major way to obtain a good sensing device. Such interactions can be judged by looking at the chemical structure of the polymer chain and its pendant groups vis-à-vis the chemical structure of the


incoming analyte compound. Adsorption/desorption, stability, morphological features (crystalline/amor-phous), surface area, and the population of the active sensing site also infl uence sensing characteristics.

Th e stability of the polymers in the sensor device is another issue to be considered before selecting a polymer (Durst et al. 1997). In polymers, some percentages of heteroatoms such as nitrogen, oxygen, sulfur, phosphorous, and halogens are present together with a long chain of carbon and hydrogen. Th e carbon–carbon or carbon–hydrogen bonds are comparatively more stable than the bonds between carbon and the heteroatoms. A polymer that has very good electrical conductivity may not have good stability in the ambient environment, or its stability may decrease when it is in contact with the analyte compound. Th is occurs because the unsaturated bonds in conducting polymers are often very reactive when exposed to environmental agents such as oxygen or moisture. Apart from the inherent stability of the polymers, the stability of some foreign chemical compounds, which are used as dopants in conduct-ing polymers, are also important. Th ese dopants may not be very stable within the polymer matrix and may also react with environmental agents. Th e polymer selected should maintain its intrinsic sensing characteristics across a wide temperature range. It must possess adequate mechanical strength to sustain handling and other stresses. Stability and degradation of the polymer is also important when it may be exposed to chemical environments during sensing. Th e polymer should be resistant to degradation or dissolution in organic solvents but should interact with the reactive sensing element. Th e permeability of the polymer fi lm used as the detecting layer is also important, because this property aff ects the transport properties of the other components. Th erefore, criteria for polymers to be used in chemical sensors need attention, and some information about the stability and degradation of such polymers is essential.

Experimental and theoretical work on polymeric and supramolecular compounds helps in designing highly selective chemical sensors. Diff erent transducer principles are used for sensing molecules in air and water by monitoring changes in mass, temperature, capacitance, and thickness. Such changes in physical and chemical properties are monitored using resonator, calorimetric, impedance, or fi ber optic sensors. Supramolecular chemistry aims at the preparation of molecules with specifi c binding sites inside their “cavities.” Self-organized layers with a well-defi ned architecture make it possible to design highly spe-cifi c chemical sensors utilizing very fast adsorption processes between recognition sites at the surface of monolayers and molecules to be detected (Schierbaum 1992). Permselective polymeric membranes, used to separate diff erent gaseous and liquid constituents, can also be used in analytical chemistry and the fi eld of sensors. Practical examples include membranes in gas sensors that have improved selectivity to certain gas constituents, membranes in ion-selective electrodes and ion-sensitive fi eld-eff ect transistors (ISFETs), and diff usion membranes in amperometric electrochemical cells. Polyethylene, polytetrafl uoro ethylene (PTFE), polyfl uoroethylene-propylene (PFEP), cellulose acetate, silicone rubber, plasticized polyvinyl chloride (PVC), polydimethylsiloxane (PDMS), and other polymers are used as membranes.

Development of miniaturized sensor arrays using nanostructured polymers and composites has come to the forefront of chemical sensor research in order to obtain specifi c selectivity of one particular chemical analyte in a mixture. By virtue of their inherently fl exible nature, the backbones of polymer chains can undergo conformational change to provide reversible adsorption/desorption in sensing re-sponse, or they can undergo oxidation/reduction when exposed to an analyte with an attendant change in electrical conductivity as a sensing signal. Th e major functions of polymers in sensor devices are to serve as a solid support for a chemical recognition element, as a selective agent for a specifi c analyte, or as the recognition element itself.


4. SYNTHESIS OF POLYMERS

Using some processing and fabrication techniques, pure polymers of adequate molecular weight and properties can be employed in sensor devices. In many cases, however, sensor device fabrication is dif-fi cult because many solubility and processing techniques are not compatible with all polymers. Th ese polymers either possess in-built sensing sites or are modifi ed to attach sensing functionalities. Th is indi-cates that some polymers have inherent properties that off er sensing function, whereas other polymers are used as carriers of sensing elements. So, depending on the need for a specifi c sensing function, a polymer should be selected before attempting its synthesis. Most carrier polymers are electrical insula-tors in which sensing elements are either physically immobilized or chemically attached. In these cases the polymer of choice should be procured from market or synthesized as per the requirement.

Polymers used in sensor devices are either formed in situ or processed from a fully grown polymer prepared by conventional techniques. Polymers are synthesized from their respective monomers either by condensation of reactive functional groups in the monomer or by addition chain reaction through olefi nic unsaturation in the monomer followed by isolation and purifi cation. Flory (1953) described briefl y these two types of polymerization, chain-growth polymerization and step-growth polymeriza-tion, by which most of the polymer molecule may be built up. Th e procedures generally followed are bulk, solution, suspension, emulsion, or interfacial polymerization (Flory 1953; Odian 2004).

In chain-growth polymerization, an initiator reacts with a monomer molecule to create a reactive site, and the reactive site then reacts with successive monomer molecules to yield the polymer. A ho-mopolymer that forms via chain-growth polymerization usually forms from one monomer; copolymers result from chain-growth polymerization of two or more monomers with the same type of reactive functional site (olefi nic unsaturation). Th is type of polymerization is popular for the monomers that have double bonds, which can act as the reactive functional site during polymerization. Th e formation of polyethylene from ethylene in the presence of an initiator is an example of chain-growth polymer-ization. Other examples are styrene, butadiene, propylene, vinylene monomer, and acryl monomer. Initiators that are commonly used for polymerization include peroxide (R–O–O–R), azo compounds (R–N=N–R), and redox compounds [FeSO4, FeCl3, K2S2O8, (NH4)2S2O8, etc.] by thermal or photo-chemical pathways (Odian 2004).

Step-growth polymerization begins when one monomer with two reactive functional groups reacts with another monomer containing functional groups of another type such that a small by-product molecule leaves the chain. Polymerization usually proceeds by reactions between two diff erent reactive functional groups, e.g., hydroxyl and carboxyl groups, isocyanate and hydroxyl groups, amine and acid groups, etc. So, according to the pair of functional groups in the monomers, a number of diff erent chemical reactions may be used to synthesize polymeric materials by step polymerization, e.g., esterifi ca-tion, amidation, the formation of urethanes, aromatic substitution, etc. (Odian 2004).

Th e synthesis of condensation polymers by ring-opening polymerization is also possible. For the general principle behind the synthesis of polymers, readers are referred to standard textbooks (Flory 1953; Odian 2004).

Intrinsically conducting polymers are widely used in various electrochemical sensing devices. Presently used conducting polymers for these applications are mostly polyheterocycles such as poly-pyrrole, polythiophene, polyfuran, polyisothionapthalene, polyindole, polyaniline, polycarbazole, etc.,


and polyaromatics such as polyazulene, poly-p-phenylene (PPP), poly p-phenylene vinylene (PPV), polypyrene, etc. (see Figure 1.2) (Gurunathan et al. 1999). Th ose conducting polymers are usually syn-thesized by one of two popular methods, chemical or electrochemical oxidation of the corresponding monomers. Th iophene, furan, carbazole, aniline, indole, azulene, and their derivatives are the major monomers used for synthesis of conducting polymers. Synthesis of conducting polymers by oxidative coupling polymerization is a very easy and simple method and, therefore, this method is suitable for producing bulk quantities of polymer in a conducting oxidized state with associated counterions from the polymerization medium. Th e procedure involves simple mixing of monomer and oxidant in aqueous or organic protonic acid solution. Commonly used oxidants are ammonium persulfate, ferric chloride, hydrogen peroxide, potassium dichromate, cerium sulfate, etc.

Th e majority of the redox polymers are synthesized by chemical polymerization. Th e oxidative polymerization of inexpensive, simple aromatic benzenoid or nonbenzenoid (mostly amines, e.g., ani-line, o-phenylenediamine), and heterocyclic compounds (e.g., pyrroles, thiophenes, indoles, azines, etc.) is of greatest interest (MacDiarmid and Epstein 1989; Syed and Dinesan 1991; Martin et al. 1993). Polythiophene and its derivatives are synthesized via oxidative coupling reactions (McCullough 1998). Quite a large number of published reports are available in the literature on the chemical synthesis by oxi-dative coupling reaction of aniline, pyrrole, thiophene, etc. A comprehensive picture may be obtained from some review reports (Toshima and Hara 1995; Feast et al. 1996; Smith 1998).

Diff erent electrochemical principles are followed to synthesize intrinsically conducting polymers, viz., galvanostatic, potentiostatic, cyclic voltammetry, and other potentiodynamic methods (Toshima and Hara 1995; Smith 1998; Feast et al. 1996). Th ese techniques utilize a three-electrode system: a working electrode, a counter electrode, and a reference electrode. During electrochemical synthesis, the conducting polymers are electrochemically deposited on the working electrode, which is made of ma-terials such as platinum, stainless steel, gold, indium tin oxide (ITO), or glass. Th e polymer-deposited electrodes are either used directly or the polymers deposited on the electrode surface are peeled off as self-standing fi lms for a particular application.

In electrochemical polymerization, the electrochemically active groups are either built into the poly-mer structure or are added as a pendant group. Th ese groups can also be incorporated into the polymer during polymerization, or attached to the polymer network in an additional step after the coating proce-dure (postcoating functionalization) to obtain polymer fi lm electrodes (Bruce 1995; Inzelt 2000). Sato et al. (1986) showed that the electrochemical polymerization of long-chain alkyl-substituted thiophene and pyrrole yields highly conducting fi lms, some of which are soluble in common organic solvents in their conducting state. Although chemical oxidation of pyrrole by Fe(ClO4)3 leads to conducting polypyrrole (MacDiarmid and Epstein 1989; Syed and Dinesan 1991; Martin et al. 1993), electrochemical polymer-ization is a preferred technique for polymer fi lm electrodes, thin-layer sensors, in microtechnology, etc., be-cause the control of potential is a prerequisite for polymer fi lm deposition on the anode during synthesis.

Th e oxidation state of the polymer can be varied electrochemically by cycling the potential between oxidized, conducting, and the neutral, insulating state, or by using suitable redox compounds. Varying the composition of the polymerization medium leads to a change in the conductivity of the polymer. For example, increasing the pH of the polymerization medium (MacDiarmid and Epstein 1989; Paul et al. 1985) or including an electron-donor molecule (e.g., NH3) in the gas phase decreases the conductivity of polyaniline (PANI) or polypyrrole (PPy) fi lms (Miasik et al. 1986; Pei and Inganas 1993).


Many conducting polymers and their derivatives are usually synthesized through well-known chem-ical routes rather than by oxidative polymerization. Th ese chemical routes include various well-known reactions, such as the Wittig reaction (Diaz et al. 1979), the Heck reaction (Kobayashi et al. 1985), and Gilch polymerization (Malhotra et al. 1986).

Plasma polymerization or electropolymerization of monomers are preferred when the polymers are diffi cult to process because of insolubility or infusibility. Plasma-polymerized thin fi lms from various monomer precursors have been prepared by employing radio-frequency plasma polymerization tech-niques (Saravanan et al. 2004). Th e apparatus consisted of a 50-cm glass tube of 8 cm diameter, with provision for charging monomer vapors by evacuation. A schematic of the experimental setup is shown in Figure 1.3. Chemically and ultrasonically cleaned glass substrates were placed inside the glass tube exactly under the space separated by the aluminum foil electrodes, which were capacitively coupled and wrapped around the glass tube, separated by a distance of 5 cm.

5. DEPOSITION OF POLYMERS

Th e deposition of polymer as a sensing element on an electrode surface is a very tricky process, since the sensor sensitivity depends on the thickness, chemical composition, crystallinity, conductivity of the coated polymer, etc. Th e present state of the art of polymer-coated electrode preparation is based on ex-tensive research and development, and numerous research articles are available in various sensor-related journals and books. In a chemical sensor, a properly functionalized polymer acts as a chemoreceptor, which is a selective receiving site for analyte recognition and reaction. In the case of a biologically de-rived receptor, the more specifi c term biochemical receptor or bioreceptor may be used. Th e bioreceptor might be an enzyme, tissue organelles, antigen/antibodies, etc. Th ese biorecognition elements might be immobilized in a polymer by covalent attachment, physical entrapment, or cross-linking.

Electrodes are usually fabricated by chemical modifi cation or by deposition of polymer on the elec-trode surface by one of the following techniques.

Figure 1.3. Schematic plasma polymerization experimental setup. A gaseous monomer or monomer vapor such as acetylene or aniline and a gaseous dopant such as iodine, chlorine, or HCl gas are used within the chamber. (Reproduced with permission from Saravanan et al. 2004. Copyright 2004 IOP Publishing Ltd and Deutsche Physikalische Gesellschaft.)


1. Th e chemisorption-adsorption technique utilizes valence forces of the same kind as those operat-ing in the formation of chemical compounds, where the chemical polymeric fi lm is strongly and, ideally, irreversibly adsorbed (chemisorbed) onto the electrode surface to provide a monolayer fi lm (Gold et al. 1987). Th is modifi cation creates substrate-coupled self-assembled monolayers (SAMs) in which uncorrelated molecules spontaneously chemisorb at specifi c sites on the surface of the electrode to form a superlattice (Allara 1995).

2. Covalent attachment of one to several monomolecular layers of the chemical modifi er to the elec-trode surface involves some combination of chemisorption and low solubility in the contacting solution, or physical anchoring on a porous electrode.

3. Formation of a polymer fi lm using a chemical modifi er is also done. Th e polymer fi lm can be organic, organometallic, or inorganic as long as it contains the desired chemical modifi er. Other forms of possible modifi cation are substrate-decoupled SAMs, in which adsorbate molecules are arranged on the electrode surface independent of any substrate structure (Allara 1995).

In order to get a polymer-coated ion-selective electrode, two conditions must be met. First, the cations must be freely mobile within the polymer matrix, which can be achieved by saturating the poly-mer with a solvent. Th e solvent helps to dissociate the cations associated with the pendant acidic groups and allows them to move within the polymer. Second, the membrane surface must be made conductive by depositing a thin metal electrode on both surfaces of the membrane. Typically, the ion-exchange property of the polymer is exploited to facilitate metal deposition. Th e polymer surface needs to be pretreated by sand blasting, hydrating, and cleaning with a strong acid, thus ensuring that the polymer is fully saturated with protons. Th e membrane is then placed in an aqueous solution containing ions of the metal to be plated. Th ese ions are allowed to exchange with the protons in the polymer for a predetermined amount of time and are then reduced to their neutral state at the surface of the polymer by a reducing agent (typically NaBH4 or LiBH4) in the outer solution. In this solvated and electroded form, a Nafi on membrane can be made to bend toward the anode side when a small voltage (1–5 V) is applied across its thickness, thus making it a soft, distributed actuator. Membranes in this form can also be used as distributed sensors. Several researchers have shown that the transient voltage generated across the membrane is correlated to the quasi-static displacement of the membrane (Sadeghipour et al. 1992; Shahinpoor et al. 1998; Keshavarzi et al. 1999).

Th e active polymer layer may be in free-standing fi lm form or on a suitable substrate and forms the heart of the sensor. Polymer fi lm or polymer coated on a substrate can be obtained using the fol-lowing methods.

1. Dip coating. Th is process consists of immersing the electrode material in a solution of the polymer in a suitable solvent for a suffi cient period to allow spontaneous fi lm formation onto the substrate by adsorption. Th e fi lm thickness may be controlled by adjusting the polymer solution viscosity and the speed of withdrawing the electrode from the solution, followed by solvent evaporation to form the polymer fi lm on the electrode.

2. Solvent evaporation. Applying a drop of polymer solution of the required consistency on to the electrode surface, followed by solvent evaporation, creates a fi lm. Th e quality and thickness of a polymer fi lm formed by this manual technique depends on the personal skill of the research


worker, but the method is advantageous because the thickness of the polymer-coated fi lm on the electrode can be known from the original concentration of polymer solution and droplet volume. Th is is the oldest and still popular method for free-standing polymer fi lm casting.

3. Spin coating. In spin coating or spin casting, a dilute polymer solution droplet is placed on the surface of a rotating electrode. Because of the rotation, excess solution is spun off the surface and a very thin polymer fi lm is formed. Following the same procedure, multiple layers can be formed until the desired thickness is obtained. Th is procedure provides pinhole-free thin fi lms.

4. Layer-by-layer (LBL) self-assembly. A composite of two polymeric electrolytes on some suitable substrate can be fabricated (Ram et al. 2005a; Nohria et al. 2006). In this method the sub-strate is alternatively immersed into a polymeric anion solution and a polymeric cation solution. Insoluble doped conducting polymers, e.g., polyaniline with positive charge on the backbone, can be deposited on a polymeric anion layer. Th e thickness of the LBL fi lm depends on the num-ber of times the process is repeated (Nohria et al. 2006).

5. Langmuir-Blodgett fi lm casting. In Langmuir-Blodgett (LB) fi lm casting, polymer molecules with hydrophilic heads and hydrophobic tails are spread on a water surface and then the molecules are compressed using a barrier to align the molecules (see Figure 1.4). Here, the polymer itself contains hydrophobic and hydrophilic groups within the backbone, or monomers having such groups are polymerized, which causes the polymer to orient itself during fi lm casting. A single-layer fi lm is cast on a substrate which is drawn up from beneath the water surface. Generally, LB fi lms form by balancing the interactions at the polymer–water, air–water, and polymer–air interfaces (Wegner and Remmers 1995). Th e resulting fi lm is very well ordered, single-layered, and in the range of molecular thickness.

6. Electrochemical polymerization. A monomer solution is oxidized or reduced to an activated form that leads to a polymer fi lm formed directly on the electrode surface. Th is procedure results in few pinholes, since polymerization would be accentuated at exposed (pinhole) sites at the electrode surface. Unless the polymer fi lm itself is redox-active, electrode passivation occurs and further fi lm growth is prevented.

Presently, polymers are being used in layer or fi lm form as an active part of inorganic solid-state sensing devices. Polymers can be easily deposited on various substrates by simple techniques. Easy

Water

Substrate (hydrophobic) Polymer

layer

Barrier

Polymer layer in substrate

Figure 1.4. LB fi lm deposition mechanism.


fi lm casting is also possible for fabrication of diff erent sensor systems. In the case of electrode preparation by direct electrochemical deposition of the conducting polymer, knowledge of the kinetics of the electrodeposition process is also of utmost importance in order to obtain proper sensing function of the electrode.

Along with the above-mentioned infl uencing parameters on polymer growth during electrode-position, the eff ect of the electrode material and its surface properties need attention. For exam-ple, in the autocatalytic oxidation of aniline over a platinum electrode, specifi c interactions and wetting may determine the nucleation and dimensionality of the growth process. Two or more stages of the polymerization process have been distinguished in the case of PANI: a compact layer (200 nm) formed on the electrode surface via a potential-independent nucleation and a two-dimensional (2-D, lateral) growth of PANI islands followed by 1-D growth of the polymer chain with continuous branching in the advanced stage leading to an open structure (Bade et al. 1992; Cruz and Ticianelli 1997). It was established that the formation of the aniline-based polymer involves two electrons in the formation of one monomer–monomer bond (Linford 1987; Lyons 1994, 1996). Th e growth rate is proportional, except for the early induction period, to the 0.5 power with respect to the fi lm volume, and it is fi rst-order with respect to aniline concentration (Stilwell and Park 1988). To avoid hydrolytic degradation of the oxidized PANI (pernigraniline form), the positive potential limit of cycling can be decreased after 2–10 cycles because of the autocatalytic nature of the electropolymerization (Horanyi and Inzelt 1989; Stilwell and Park 1989). In addition to the head-to-tail coupling, formation of p-aminodiphenylamine by tail-to-tail dimerization (benzidine) also occurs as a minor intermediate, as evidenced by the rate constant of dimerization for radical–cation coupling to produce the former product, which is about 2.5 times higher than that for the tail-to-tail dimer (Yang and Bard 1992). Generally, a mixed material is deposited on the surface, containing electrochemically active and conducting as well as inactive and insulating parts, if the polymerization conditions are not carefully opti-mized (Otero and Rodriguez 1994). Since polythiophene does not adhere on a Au or Ti surface, it prefers electrochemical formation and precipitation from the medium. Th erefore, a suitable approach is the deposition of a thin polypyrrole layer on Ti or Au that ensures the deposition of polythiophene on these substrates (e.g. Ti, Au) (Gratzl et al. 1990).

7. Deposition by radio-frequency polymerization of a suitable monomer. In this method, a monomer va-por is exposed to a radio-frequency (RF) plasma discharge to form a thin polymer fi lm on the elec-trode surface. Chemical damage of the polymer fi lm producing unknown functionalities and struc-tural modifi cations due to high energetics of the RF discharge is a disadvantage of this technique.

8. Polymer deposition followed by cross-linking. In this technique, chemical components of a polymer fi lm are bonded with the electrode to impart stability, decreased permeability, or altered electron-transport characteristics to the polymer fi lm. Cross-linked fi lms are often formed by polymeriza-tion of bifunctional and polyfunctional monomers or by chemical, electrochemical, photolytic, radiolytic, or thermal activation.

9. Pellet preparation. In this technique the well-dried polymer powder is made into a thin pellet of particular thickness using a steel die in a hydraulic press under a certain constant pressure. Th e pellet preparation technique is very useful for polymers which are not soluble in organic solvents, such as conducting polymers—especially, doped forms of conducting polymers.


6. FUNCTIONALIZATION OF POLYMERS

6.1. STRUCTURE MODIFICATION

Th e polymer fi lm may also be functionalized subsequent to fi lm application, because immobilizing the fi lm is easier than working with monolayers, such modifi ed fi lms are usually more stable, and stronger electrochemical responses can be available from multiple layers of redox sites. Some questions remain, however, as to how the electrochemical reactions of multimolecular layers of electroactive sites in a polymer matrix occur, for example, the mass-transport and electron-transfer processes by which the multilayers exchange electrons with the electrode and with reactive species such as molecules and ions in the contacting solution (Murray 1984; Murray et al. 1987; Inzelt 1994). Lack of suffi cient know-ledge about the structure and properties of polymer fi lms, as well as morphological changes arising out of various chemical, electrochemical, and physical processes during use, may lead to uncertainty in the ultimate sensing performance. Electrocatalysis on a modifi ed electrode is usually an electron-transfer reaction between the electrode and some solution substrate, which, when mediated by an immobilized redox couple, proceeds at a lower overpotential than would otherwise occur at the bare electrode (Durst et al. 1997).

After about four decades of research on conducting polymers and their utilization in sensor devices, there is suffi cient scope and many avenues for enhancing the conductivity as well as improving sensing ability. From the literature on polymer-based sensor research, it has become apparent that effi ciency in sensor output depends on parameters such as polymer structure, composition, morphology, and active functional sites. Th erefore, to achieve a specifi c sensor device, it is usually necessary to modify the struc-ture of the conducting polymer and perhaps substitute for one of the common conducting polymers.

Not only the sensor output but also the stability of the polymer in the device needs special atten-tion. Polymers having excellent sensing characteristics have been found to be environmentally unstable. Processing and fabrication of the synthesized conducting polymers into a suitable device is a challeng-ing problem today. Many researchers have tried to solve this problem by chemical substitution in the monomer stage or by changing the polymerization conditions.

Although both electrically insulating as well as conducting polymers have been successfully utilized in the fabrication of chemical sensor devices, conducting polymers have clear advantages in sensing performance over insulating polymers. By virtue of tailor-made characteristics, structural as well as functional modifi cation of conventional conducting polymers can add a new dimension to polymers as materials for chemical sensors.

Some of the common modifi cation strategies being practiced for conducting polymers intended for use in sensor devices are described in the following paragraphs, together with some research outcomes.

1. Chemical group substitution on monomers and polymers. Appropriate substitutions in the monomer can improve the stability in air of the electrochemically produced polymers. Although prepared under identical electrochemical conditions, the substitution of a methyl group, e.g., poly(3-methyl thiophene) has shown better air stability than that of polythiophene (Tourillon and Garnies 1982; Bryce et al. 1987). In addition to its increased stability in air, poly(3-methyl thio-phene) has also shown improved conductivity over that of the parent polythiophene (Waltman


et al. 1983). Unfortunately, such an increase in electrical conductivity has not been observed in a pyrrole system after alkyl-group substitution. Th e methyl-substituted polypyrroles had lower conductivities than the parent polypyrrole (Diaz et al. 1982). It appears that a balance between electronic and steric eff ects in substituted polymers of fi ve-membered heterocycles might account for either more or less conductivity than in the parent polymers (Waltman 1984; Waltman and Diaz 1985).

Several polyaniline derivatives have been developed by polymerizing various alkyl- and alkoxy-substituted polyanilines with the objective of improving conductivity as well as achieving bet-ter-processable conducting polyanilines. Common derivatives have included ortho- and meta- substituted anilines with –NH2 (Gouette and Leclerc 1995; Levin et al. 2005), –OH (Rivas et al. 2002; Salavagione et al. 2004; Levin et al. 2005; Kar et al. 2008) –OCH3 (Dao et al. 1989; Park et al. 1989, Cataldo and Maltese 2002), –CH3 (Leclerc et al. 1989; Falcou et al. 2005), –SO3H (Yue and Epstein 1990; Kuo et al. 1998), –Cl, –F, –I (Kang et al. 1990), –CH2CH3 (Dao et al. 1989; Leclerc et al. 1989; Yue and Epstein 1990), etc., giving rise to similar products. A few of these derivatives had improved processability, but very little or no increase in electrical conductiv-ity was achieved.

2. Copolymerization. Electrochemically synthesized conducting polymers suff er from the drawback of poor mechanical strength, which restricts their use in commercial products. Since copoly-merization is one of the most eff ective methods for improving the mechanical strength of brittle polymers, this approach may be followed for conducting polymers without compromising their conductivity. In this respect, copolymerization of polyheterocycles with poly(p-phenylene sul-fi de) (PPS) can lead to better mechanical strength, less pronounced O2 sensitivity, and increased conductivity of the copolymer by doping of PPS. However, copolymerization by the electro-chemical technique is a challenging task due to the diff erent electrochemical oxidation potentials of individual monomers. Graft copolymerization and blending are other tailoring approaches to improve atmospheric stability and mechanical strength of conducting polyheterocycles.

6.2. SURFACE MODIFICATION

A wide variety of modifi cations to the surface morphology of polymers is possible for suiting specifi c sensor architecture, e.g., lithographic and soft lithographic techniques, replication from masters, pattern formation using self-assembly and controlled deposition, nanomachining, emulsion templating, and wetting-assisted templating (Xia et al. 1999; Hamley 2003; Helt et al. 2004; Drain and Batteas 2004; Barbetta et al. 2005). Surface modifi cation is also possible using plasma polymerization techniques. For example, one existing plasma polymerization setup (see Figure 1.3) was slightly modifi ed for the surface modifi cation of inorganic materials by organic coating (Sunny et al. 2006). Th e controlled fl ow of monomer vapor in the radio-frequency plasma polymerization chamber creates a thin deposition on the existing ceramic or semiconductor surface. Th e monomer is plasma-polymerized inside the chamber due to the RF source and is deposited on substrate surfaces placed inside the bottom of the discharge tube. In this method, conducting polymers such as polyaniline, polypyrrole, and polythiophene can be deposited on the surface of conventional semiconducting materials.


Coating of polymers is also possible by thermal spraying of polymer powders onto a wide variety of materials. In this method, polymer powder is injected into a heat source (hot fl ame or plasma) and transported to a preheated substrate. Th is is an eff ective method of producing protective barrier coat-ings. Polymers that have been sprayed include polyethylene, polymethyl methacrylate, polyether ether ketone, polyphenylene sulfi de, nylon, phenolic, epoxy, Tefzel (modifi ed ethylene-tetrafl uoroethylene polymer), and postconsumer commingled polymers. Th e thickness of the coating depends on the num-ber of repeated passes of the spray gun across the substrate. However, polymers with large particle size or higher molecular weight may form more heterogeneous microstructures within the coating, creating voids, trapped gasses, unmelted particles, splats, and pyrolized material.

Direct surface modifi cation of polymer substrates such as the polyimide Nafi on, a sulfonated ionic fl uoropolymer, which is commonly used in polymer sensor-actuator devices, can be done by the plasma technique. Here the polymer surface is treated with a suitable gas plasma to obtain reactive functional groups on the surface of the treated polymer. For example, Kapton, a polyimide, was treated in argon plasma for 2 min at 35 W and 0.2 torr, and Nafi on was treated in oxygen plasma for 10 min at 50 W power and 0.2 torr (Supriya and Claus 2004). Th e plasma treatment activates the polymer surface to fa-cilitate the deposition of other polymers having active functionality, or the polymer can be used directly as the detecting layer for a chemical sensing device. Other important surface modifi cation methods include grafting, surface coupling reactions, electron beam, glow discharge, corona discharge, UV treat-ment, etc. (Uyama et al. 1998).

6.3. COMPOSITION MODIFICATION

It has been established that π-electron conjugation along the backbone of a polymer chain is one of the criteria for a polymer to exhibit good electrical (conducting and semiconducting) behavior. Additionally, it is also known that the presence of heteroatoms in polymers can lead to improved electrical (conduct-ing/semiconducting) performance. We have already mentioned that, for chemical sensor detection by the electrochemical principle, it is important to have inherent electrical properties which can alter dur-ing analyte interaction with the polymer detection element. As is known, the electrical properties de-pend on the mobility of free or loosely bounded electrons.

For polymers to be used as detection elements, the inherent electrical conduction may be obtained in two ways: by metal or metal compound dispersion within the polymer matrix; or by -electron con-jugation along the polymer chain. Th e free electrons or charges on the metal are responsible for the elec-trical conductivity of the metal or metal compound dispersed polymer detection system. In this system the polymer does not play a direct role in the alteration of electrical properties. However, the polymer can increase the performance of metal or metal compounds during sensing by chemical interaction of some particular groups. On the other hand, a particular group of polymers may aid interaction with the detection element. Since the discovery of intrinsically conducting polymers (ICPs), which show inher-ent electrical properties through the movement of a loosely bounded -electron cloud, these have been used successfully as chemical detection elements in electrochemical transducer systems.

Metal or metal compounds can also be dispersed within the ICP matrix to increase the sensing per-formance of the polymer. One of the easiest routes to composition modifi cation is composite formation.


Gurunathan and Trivedi (2000) incorporated TiO2 in polyaniline by chemical and electrochemical tech-niques to study the eff ect of photoconducting inorganic semiconductors on thermal stability and appli-cation of a new composite.

Th e structure and conductivity of the polymer can also be altered by further chemical reactions. Active sensing polymers are used in sheet or fi lm form as an integral part of inorganic solid-state de-vices. Conducting polymer composite sheets/fi lms contain conductive fi ller loaded in an electrically insulating polymer matrix. Th e change in resistivity as a function of fi ller concentration in such com-posites can be understood according to the percolation concept, which describes conduction through electrically conducting paths between two fi ller particles. Th e number of these paths dramatically decreases below a critical volume fraction of fi ller (Lundberg and Sundqvist 1986). Common fi ller ma-terials are metals (Cu, Pd, Au, Pt), carbon black, and semiconducting metal oxides (V2O3, TiO2, etc.). Th e insulating matrix polymers have included polyethylene, polyimides, polyesters, poly(vinyl acetate) (PVAc), PTFE, polyurethane, poly(vinyl alcohol) (PVA), epoxies, acrylics, poly(methyl methacrylate) (PMMA), and others. Th ese composites have been used successfully in positive temperature coeffi cient (PTC) thermistors and in piezoresistive pressure, tactile, humidity, and gas sensors (Lundberg and Sundqvist 1986).

Th e ionic conductivity of polyelectrolytes is modulated by several environmental parameters, which is the basis of their application in sensors. Th e conductivity of polyelectrolyte fi lms can be increased by increasing the number of ionic carriers through addition of ions from the environment and modify-ing the degree of dissociation of the polymer electrolyte (Sadaoka et al. 1986; Sakai et al. 1989). Ion-conducting polymers, viz., Nafi on, polyhydroxyethyl methacrylate (polyHEMA) and its copolymers, are widely used as solid electrolytes in electrochemical cells for the detection of various gases or ionic components. Alkali salt–polyether complexes, such as polypropylene oxide (PPO) and polyethylene ox-ide (PEO) with LiClO4, LiCl, LiCF3SO3, LiSCN (Watanabe et al. 1985; Sadaoka et al. 1986), polysty-rene sulphonate, and quaternized polyvinyl pyridine (PVPy), were used in impedance-type or semicon-ductor-based humidity sensors. Th e structures of π-electron conjugated conducting polymers, such as polyacetylene, polyaromatics, and polyheterocycles contain a one-dimensional organic backbone, which enables formation of a superorbital for electronic conduction. Conduction through these polymers takes place by charge hopping both along the polymer chains and also between the macromolecules that make up individual fi bers as well as between the fi bers themselves. However, in the neutral (undoped) state, these materials behave like semiconductors. Th e electronic conductivity in these materials is obtained by doping, i.e., by injecting electrons or holes into the superorbital (Bidan 1992).

7. POLYMERS IN CHEMICAL SENSORS

As we have indicated, many types of chemical sensors have been designed for diff erent analytical tasks, such as detection of hazardous gas concentration, control of groundwater pollution, medical analysis and diagnostics, industrial quality and process controls, environmental safety, and so on. Some toxic chemicals and their common sources are listed in Table 1.1. All these various sensor types work by dif-ferent principles and require sensing materials with diff erent properties. However, polymers, due to their unique physical and chemical properties, can be used in all types of chemical sensors (see Table 1.2).

CAT

EGO

RY

TO

XIC

CH

EMIC

ALS

C

OM

MO

N SO

UR

CES

Toxi

c in

dust

rial

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n Pe

troch

emic

al in

dustr

ies a

nd m

otor

fuel

sch

emic

als

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, h

eavy

met

als a

nd th

eir o

xide

s, et

c.

M

ethy

l eth

yl k

eton

e, a

ceto

ne, m

etha

nol,

etha

nol,

chlo

rofo

rm, c

arbo

n te

trac

hlor

ide,

So

lven

ts an

d cl

eani

ng a

gent

s use

d in

te

trah

ydro

fura

n, is

opro

pano

l, m

ethy

l chl

orid

e, tr

ichl

oroe

thyl

ene,

ben

zene

, tol

uene

, ch

emic

al in

dustr

ies

xy

lene

, am

mon

ia, i

norg

anic

aci

ds, h

ydro

gen

sulfi

de, o

xide

s of s

ulfu

r and

nitr

ogen

,

isocy

anid

es, h

alog

ens,

etc.

Fr

eon-

22, c

hlor

ofl u

oroc

arbo

ns, m

ethy

l chl

orid

e, m

etha

nol,

etc.

Re

frige

rant

s and

coo

ling

syste

ms

V

inyl

chl

orid

e, m

ethy

l eth

yl k

eton

e, a

ceto

ne, m

etha

nol,

etha

nol,

chlo

rofo

rm, c

arbo

n Po

lym

er, r

ubbe

r, an

d te

xtile

indu

strie

s

tetr

achl

orid

e, te

trah

ydro

fura

n, is

opro

pano

l, m

ethy

l chl

orid

e, tr

ichl

oroe

thyl

ene,

ben

zene

,

tolu

ene,

xyl

ene,

inor

gani

c ac

ids,

etc.

Toxi

c ho

useh

old

Met

hane

, ker

osen

e, p

heno

l, cr

esol

, ino

rgan

ic a

cids

, sod

ium

hyd

roxi

de, b

leac

h,

Hou

se c

lean

ing

chem

ical

s iso

prop

anol

, 2-b

utox

yeth

anol

, nap

htha

lene

, etc

.

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n K

itche

ns

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, e

tc.

M

orph

olin

e, m

etha

nol,

etha

nol,

carb

on d

ioxi

de, c

arbo

n m

onox

ide,

etc

. M

edic

inal

and

hum

an h

abits

Wat

er-p

ollu

ting

C

arbo

n m

onox

ide,

car

bon

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, e

tc.

Diss

olve

d ga

ses i

n w

ater

chem

ical

s

Amm

onia

, ino

rgan

ic a

cids

, hyd

roge

n su

lfi de

, oxi

des o

f sul

fur a

nd n

itrog

en, i

socy

anid

es,

Rai

nwat

er

halo

gens

, etc

.

Am

mon

ia, i

norg

anic

aci

ds, h

ydro

gen

sulfi

de, o

xide

s of s

ulfu

r and

nitr

ogen

, iso

cyan

ides

, G

roun

dwat

er

halo

gens

, ars

enid

es, t

oxic

met

als,

etc.

M

etha

ne, e

thyl

ene,

ben

zene

, tol

uene

, xyl

ene,

form

alde

hyde

, car

bon

mon

oxid

e, c

arbo

n M

arin

e po

llutio

n

diox

ide,

oxi

des o

f sul

fur a

nd n

itrog

en, p

etro

leum

pro

duct

s, he

avy

met

als a

nd th

eir

ox

ides

, etc

.

Che

mic

al

Chl

orin

e, c

hlor

opic

rin, h

ydro

gen

cyan

ide,

ars

ines

, psy

chot

omim

etic

age

nts,

toxi

ns,

Che

mic

al g

as w

eapo

nsw

eapo

ns

carb

on m

onox

ide,

car

bon

diox

ide,

pho

sgen

e, m

usta

rd g

as, t

ear g

as, l

ewisi

te, G

-ser

ies

ne

rve

agen

ts, V

-ser

ies n

erve

age

nts,

etc.

M

ercu

ry fu

lmin

ate,

lead

styp

hnae

, lea

d az

ide,

dyn

amite

, TN

T, R

DX

, PET

N, H

MX

, Ex

plos

ives

am

mon

ium

nitr

ate,

etc

.

Tabl

e 1.

1. S

ourc

es o

f tox

ic a

nd h

azar

dous

che

mic

als

in th

e en

viro

nmen

t

PO

LYM

ER

USE

S SP

ECIA

L FE

ATU

RES

R

EFER

ENC

ES

Cop

olym

ers o

f pol

y (E

DM

A-co

-MAA

) D

etec

tion

of te

rpen

e in

Pi

ezoe

lect

ric se

nsor

coa

ted

with

Pe

rciv

al e

t al.

2001

at

mos

pher

e m

olec

ular

impr

inte

d po

lym

er

Poly

ethy

lmet

hacr

ylat

e, c

hlor

inat

ed p

olyi

sopr

ene,

Id

entifi

cat

ion

of g

ases

and

Po

lym

er–c

arbo

n bl

ack

com

posit

e fi l

m

Zee

and

Judy

200

1po

lypr

opyl

ene

(isot

actic

, chl

orin

ated

), sty

rene

/ ga

s mix

ture

sbu

tadi

ene,

ABA

blo

ck c

opol

ymer

, sty

rene

/et

hyle

ne/b

utyl

ene

ABA

bloc

k co

poly

mer

, po

lyep

ichl

oroh

ydrin

Nafi

on

Det

ectio

n of

eth

anol

gas

Fu

el c

ell w

ith p

olym

er e

lect

roly

te

Kim

et a

l. 20

00

conc

entr

atio

n m

embr

ane

Poly

anili

ne (P

ANI)

, PAN

I–ac

etic

aci

d m

ixed

D

etec

tion

of N

O2

Laye

rs o

f pol

ymer

fi lm

s for

med

by

Xie

et a

l. 20

02fi l

m, P

ANI–

poly

styre

nesu

lfoni

c ac

id (P

SSA)

Lang

mui

r-Bl

odge

tt (L

B) a

ndco

mpo

site

fi lm

self-

asse

mbl

y te

chni

ques

Poly

[3-(

buty

lthio

)thio

phen

e]

Gas

sens

or

Film

s of p

olym

er p

repa

red

via

LB

Rella

et a

l. 20

00

de

posit

ion

and

casti

ng

Poly

viny

l chl

orid

e (P

VC

) D

etec

tion

of N

O2 i

n ai

r So

lid p

olym

er e

lect

rode

of 1

0% P

VC

H

rnci

rova

et a

l. 20

00

Poly

pyrr

ole

nano

com

posit

e D

etec

tion

of C

O2,

N2,

CH

4 N

anoc

ompo

site

of ir

on o

xide

Su

ri et

al.

2002

ga

ses a

t var

ying

pre

ssur

es

poly

pyrr

ole

prep

ared

by

simul

tane

ous

gela

tion

and

poly

mer

izatio

n

Prop

ylen

e–bu

tyl c

opol

ymer

D

etec

tion

of to

luen

e, x

ylen

e

Poly

mer

fi lm

–coa

ted

quar

tz re

sona

tor

Nan

to e

t al.

2000

bala

nce

Poly

dim

ethy

lsilo

xane

(PD

MS)

mem

bran

e O

2 and

NO

Am

pero

met

ric se

nsor

M

izuta

ni e

t al.

2001

Poly

pyrr

ole-

impr

egna

ted

fi lte

r pap

er

NH

3

Elec

tric

al p

rope

rty

mea

sure

men

t N

ylan

der e

t al.

1983

Poly

anili

ne (P

ANI)

N

H3

Elec

tric

al p

rope

rty

mea

sure

men

t H

irata

and

Sun

199

4

Nan

ocom

posit

e ul

trat

hin

fi lm

s of P

ANI a

nd

NH

3 and

NO

2 El

ectr

ical

pro

pert

y m

easu

rem

ent

Li e

t al.

2000

isopo

lym

olyb

dic

acid

(PM

A)

Acry

lic a

cid–

dope

d PA

NI

NH

3

Cha

buks

war

et a

l. 20

01

Poly

pyrr

ole

NH

3

Yado

ng e

t al.

2000

Tabl

e 1.

2. P

olym

ers

used

in g

as s

enso

rs

Poly

pyrr

ole–

poly

(vin

yl a

lcoh

ol) (

PVA)

com

posit

e N

H3

G

ango

padh

yay

and

De

2001

Ag-m

etal

ized

poro

us T

efl o

n m

embr

ane

H2S

C

atho

dic

strip

ping

vol

tam

met

ry

Ope

kar a

nd B

ruck

enste

in 1

984

Solid

pol

ymer

ele

ctro

lytic

(SPE

) ion

-exc

hang

e

H2S

El

ectro

chem

ical

det

ectio

n Sc

hiav

on e

t al.

1995

mem

bran

e

Poly

(4-v

inyl

pyr

idin

e) o

n pa

lladi

um a

nd ir

idiu

m

SO2

Elec

troch

emic

al se

nsor

Sh

i et a

l. 20

01ox

ide

(PV

P/Pd

/IrO

2)–co

ated

pla

tinum

m

icro

elec

trode

Nitr

ocel

lulo

se m

embr

ane

and

a sil

icon

e ru

bber

N

O

Ampe

rom

etric

sens

or

Ichi

mor

i et a

l. 19

94;

oute

r lay

er, p

heny

lene

dia

min

e-m

odifi

ed c

arbo

n

Frie

dem

ann

et a

l. 19

96fi b

er

Poly

styre

ne fi

lm

NO

2 Ir

reve

rsib

le in

crea

se o

f con

duct

ivity

C

hrist

ense

n et

al.

1993

due

to se

lf-io

niza

tion

of N

2O4 t

o

N

O+ N

O3−

Pt/N

afi o

n el

ectro

de

NO

2 Am

pero

met

ric se

nsor

H

o an

d H

ung

2001

Mul

tilay

er fi

lms o

f PAN

I and

PSS

A

NO

2 C

ondu

ctiv

ity in

crea

se

Xie

et a

l. 20

02

Poly

[3-(

buty

lthio

)thio

phen

e] d

epos

ited

on

NO

2, N

H3

Elec

tric

al c

ondu

ctiv

ity

Rella

et a

l. 20

00al

umin

a su

bstr

ates

Nafi

on

as so

lid p

olym

er e

lect

roly

te

CO

El

ectro

chem

ical

O

taga

wa

et a

l. 19

90

Poly

ethy

lene

gly

col–

alka

li ca

rbon

ate

com

plex

fi lm

C

O2

Elec

tric

al c

ondu

ctiv

ity

Saka

i et a

l. 19

95

Cu/

Pd-d

oped

PPy

fi lm

C

O a

nd H

2 El

ectr

ical

con

duct

ivity

To

rsi e

t al.

1998


7.1. OPTICAL AND FIBER OPTIC POLYMER-BASED SENSORS

Sensors based on optical and fi ber optic properties are a real alternative to electronic ones in elec-trically noisy environments where electronic sensors do not operate correctly. In fact, optic and fi -ber optic chemical sensor technology is a fi eld of growing importance, with diversifi ed applications in oceanography, chemical process, clinical diagnosis, and environmental monitoring. Optical sensors off er advantages such as light weight, a passive nature, low attenuation, and the possibility of multiplexing, among others. In a recent review, Elosua et al. (2006) classifi ed these devices according to the sensing mechanism, taking into account the sensing materials and the diff erent methods of fabrication.

7.1.1. Requirements for Sensing Materials

A variety of polymers and polymer composites [e.g., PMMA, poly(2-hydroxyethyl methacrylate) (PHEMA), PTFE, cellulose, Nafi on, PVC, polyacrylamide, polyvinyl pyrrolidone, etc.] together with indicator dyes are widely used in chemical sensors, including ion, gas, humidity, and enzyme sensors (Wolfbeis 1991; Gauglitz and Reichert 1992).

Polymer materials must fulfi ll various requirements to enable optical sensing. First of all, the indi-cator dye and all additives need to be dissolved well in the polymer (and must not be washed out). Th e polymer should be of such chemical nature that it can dissolve the analyte and allow it to diff use easily within the polymer. For practical applications, the polymer must be chemically and physically stable in order to achieve good operational lifetime and shelf life. Furthermore, no crystallization/migration/ reorientation of the indicator molecules must occur within the polymer. Th ese phenomena can occur even after weeks or months if the indicator lacks solubility as expected. Th e polymer must be thermally stable so that it can sustain steam sterilization without degradation. Th e polymer should be stable against sunlight, acids, bases, oxidants, etc., and nontoxic as well as biocompatible for clinical and biochemical applications. Th e polymer should not have any intrinsic colour/luminescence, and it should be optically transparent in the spectral range where measurements will be performed. Finally, the polymer should possess adequate stability to mechanical stresses (Mohr 2006).

If it has apparent optical properties (AOP), the polymer can be used as the analyte-sensitive layer for various analytes. Polymers that have good mechanical properties, homogeneity, preparation simplicity, and optical transparency, are very good materials for the AOP sensing layer. Th ey also allow reproduc-ibility of fi lm production and therefore are excellent for sensor fabrication. Polymers with very high molecular weights and capable of forming a cagelike structure may be helpful when trying to strengthen the required properties. When sensing analytes, in some polymers a mechanism of analyte dissolution is accompanied by diff usion, in others the analyte simply diff uses through the pores of the polymers. So, here the bulk free volume, which can be monitored from the fi lm thickness or by tailoring the polymer structure, is useful for the sensing of the particular analytes.

Hydrophobic polymers which can be penetrated by gases and hydrophilic polymers which can be penetrated by specifi c ionic species are the most important polymers for selective sensing of analytes. Films of these polymers can also be attached directly to an optical fi ber and can be deposited along the length of the fi ber before curing. Th e intrinsic fl uorescence of some polymers caused by their backbone


units [e.g., PPy, PPV, polyphenylene, polyfl uorene, and phenylthiohydantoin (PTH) derivatives] or fl uorescence of fl uorophores introduced into the polymer can be used.

7.1.2. Mechanisms of Polymer Sensing

Optical sensing is based on colorimetric, fl uorescence, or luminescence eff ects and on changes in light refraction/propagation. Reseach has shown that the change in optical properties of a polymer can pro-vide good sensing response (Wolfbeis 1991; Gauglitz and Reichert 1992). It has been established that an interaction with an analyte that oxidizes/reduces or protonates/deprotonates a conducting polymer infl uences not only electrical properties but also optical properties. For example, in a polyaniline system, as a result of oxidation/reduction, a wide range of reversible visible color changes can be observed (see Table 1.3). Using this phenomenon, a polyaniline-based optical pH sensor as well as ozone, NO2, and H2S gas sensors have been developed (Ge et al. 1993; Agbor et al. 1997; Hu et al. 2002; Jin et al. 2001; Ando et al. 2002).

In intrinsically conducting polymers, the color of the polymer depends on the band gap of the polymer. Changes in the redox or protonation state of an intrinsically conducting polymer lead to strong modifi cation of its electronic band structure. Th erefore, certain conjugated conducting polymers are extremely sensitive to structural perturbations as a result of the absorption of external analyte. Th ese structural perturbations in the electronic network of the conducting polymer causes a self-amplifying fl uorescence quenching response upon binding of analytes. Depending on the system, these polymers can have strong luminescence properties, which can be used as good fl uorescent sensor material. Figure 1.5 shows the mechanism, which is known as photo-induced electron-transfer fl uorescence quenching. For conjugated polymers it amplifi es the molecular recognition signal via migration of electrons along the polymer chain (Desmonts et al. 2007). Photon irradiation of the polymer causes promotion of an electron to the lowest unoccupied molecular orbital (LUMO) from the highest occupied molecu-lar orbital (HOMO). Now, due to analyte binding, trapping of the electron occurs to the quencher’s

Table 1.3. Oxidation states of polyaniline (PANI) with visible colors

Structure of PANI

Value of y 1 ≥0.5 0 0–1

Form Reduced Half-oxidized Oxidized Protonated salt

Name Polyleucoemeraldine or Polyemeraldine or Polypernigraniline Polyemeraldine polyprotoemeraldine base polynigraniline base base salt

Color Transparent Blue Purple Green

N

H

N

H y

N N

1-yn


LUMO (here, analyte) and eff ectively deactivates the electron transfer (see Figure 1.5). Th is destroys the polymer-based excited state, and the polymer can no longer fl uoresce. Interaction of an analyte which oxidizes/reduces or protonates/deprotonates an intrinsically conducting polymer can also be measured by UV-vis or photoluminescence spectroscopy. Th is principle is especially applicable for detection of gaseous analytes.

Using the above-discussed techniques, various metal ions have been detected by fl uorescent con-jugated polymers, e.g., 2,2,9-bipyridyl-phenylene-vinylene–based polymers (Wang and Wasielewski 1997), terpyridine-based poly(p-phenylene-ethynylene)-alt-(thienylene-ethylene) polymers (Zhang et al. 2002), and poly(p-phenylene-ethynylene)–based polymers (Chen et al. 2004; Kim and Bunz 2006). Successful detection of fl uoride ions as quencher in some fl uorescent conjugated polymers has also been reported (Kim and Swager 2003; Tong et al. 2003; Saxena et al. 2004; Zhou et al. 2005). Th e most successful use of these types of fl uorescent polymers has been in detection of vapors of nitroaromatic explosives as quencher, such as trinitrotoluene (TNT) and dinitrotoluene (DNT) (Mcquade et al. 2000; Cumming et al. 2001, 2004; Sohn et al. 2003) by photochemical quenching.

Another important mechanistic approach for the optical sensing of analyte by dye-solvated polymer matrices is the polarity change of the polymer. By incorporating solvochromic dye in polymer matrices of varying polarity, hydrophobicity, pore size, fl exibility, and swelling tendency, unique sensing regions are created with diff erent fl uorescence responses for diff erent organic vapors (Dickinson et al. 1996; Albert et al. 2000). Here the permeation of analyte for some polymers is due to dissolution by diff usion; for some other polymers it is due to diff usion only. In general, hydrophobic polymers have the abil-ity to permeate gases, and hydrophilic polymers have the ability to permeate ionic species (Brook and Narayanaswamy 1998). Th is mechanism is utilized with silicone, Nafi on, and PVC fi lms for the deter-mination of gases (chlorine, nitrogen dioxide, oxygen, etc.) and moisture (Brook and Narayanaswamy 1998). Nafi on used as an ion-exchange matrix for crystal violet for the determination of relative humid-ity is a suitable matrix in an optical fi ber–based moisture sensor (Sadaoka et al. 1992).

Figure 1.5. Electron-transfer fl uorescence quenching mechanism for a conjugated polymer upon interaction with the analyte. (Reproduced with permission from Desmonts et al. 2007. Copyright 2007 The Royal Society of Chemistry.)


Th e optical path length in a given medium is determined by the refractive index and the geometric path length. Changes in both parameters result in a change in the phase shift, which can be detected by interferometry. Based on the same eff ects, in optical waveguides the waveguide eff ective refractive index or transmission eff ectiveness variation can also be measured by the change in refractive index of the cladding. By this means, fi lm materials that formerly could not be used may now be used for fi ber optic or optical integrated chemical sensors. Th e refractive index of the polymeric fi lm and/or cladding varies with the permittivity when the vapor to be detected is absorbed in it.

Another frequently used method is to detect small changes in thickness of a transparent layer using optical refl ection mode interferrometry, which can also be used in fi ber optic sensors. Polydimethylsiloxane (PDMS) was examined in several studies and seems to be a good candidate for applications in opti-cal sensors. It shows both swelling and refractive index shift when exposed to organic solvent vapors (Gauglitz et al. 1993).

In solution, binding with analytes can also change the conformation of polymers, which leads to a corresponding modifi cation of optical adsorption. Th is principle was applied to detection of cations in the solution phase (Ewbank et al. 2004). More sensitive detection of optical changes in ICPs can be realized by application of surface plasmon resonance (SPR) or Raman spectroscopy. Other optics-based sensors such as optodes utilize colorimetric changes as well or fl uorescence quenching of indicator dyes using excited light for detection (Wolfbeis 1991).

It is also possible to fabricate chemical and biochemical sensors based on interferrometry at thin (multi-) layers (Gauglitz et al. 1993). In some cases, the addition of indicators such as dyes or fl uorescent materials to the fi lm is not necessary, because the physical-chemical changes take place with the direct participation of the sensing polymer material. Polysiloxane polymers seem to present these properties and can be used successfully in chemical sensor applications.

A chemical vapor sensor working with a monochromatic light source and based on optical fi ber coated with a thin siloxane polymer fi lm has been developed for in situ monitoring of volatile organic compounds, such as ethylbenzene, o-xylene, heptane, octane, chloroform, carbon tetrachloride, ethanol, and butanol in indoor atmospheres and confi ned areas of industrial environments. Th e sensor consists of a monomode optical fi ber with an end surface coated with a thin polymeric fi lm by dip coating. Th e light source utilized was a stabilized laser diode at 1550 nm, and the light power changes were measured with a photodiode. Th e sensor was tested for diff erent volatile organic compounds and for diff erent individual concentrations in terms of stability, sensitivity, repeatability, and reversibility of the analytical signal. Th e response and desorption times were found to be 30 s, and good reproducibility and accuracy were also obtained. Finally, the analytical performance of the sensor was also evaluated and found to be adequate for actual monitoring in indoor atmospheres (Silva et al. 2008).

7.1.3. Examples of Polymer-Based Optical Sensors

7.1.3.1. HCL SENSORS

Hydrochloric acid is a source of dioxin produced during the incineration of plants and in acid rain. It is also known as a workplace hazard with a short-term exposure limit of 5 ppm. Composites of alkoxy-


substituted tetraphenylporphyrin–polymer composite fi lms were developed by Nakagawa et al. (2001) for the detection of HCl at sub-ppm levels. Th e alkoxy group imparts basicity to the material and hence increases its sensitivity to HCl. Nakagawa et al. achieved high selectivity to sub-ppm levels of HCl gas using 5,10,15,20-tetra (4-butoxyphenyl)porphyrin-butylmethacrylate [TP (OC4H9)PH2-BuMA] composite fi lm. Supriyatno (2001) detected HCl gas optochemically using a mono-substituted tetra-phenylporphin–polymer composite fi lm. In this system a higher and preferred sensitivity to sub-ppm levels of HCl was achieved using a polyhexylmethacrylate matrix in the composite.

7.1.3.2. SENSORS OF GASES AND VOLATILE ORGANIC COMPOUNDS

Changes in the optical and electrical properties of π-conjugated polyaniline occurs as a result of the interaction of the emeraldine salt (ES) with NH3 gas (Nicho et al. 2001). Th e interaction of polyaniline with gas molecules decreases the polaron density in the band gap of the polymer. It was observed that PANI–PMMA composite coatings are sensitive to very low concentrations of NH3 gas (10 ppm).

Optical fi ber–based sensors for volatile organic compounds (VOCs) off er new and interesting prop-erties, which can overcome some of the limitations of traditional gas sensors. Horiuchi et al. (2006), for instance, developed a portable gas sensor using fi ber optic microfl uidic devices (concentration and detection cells) for detecting and identifying gaseous aromatic VOCs such as benzene, toluene, xylene, styrene, and ethylbenzene. Th ey used precise control of gas transfer from the concentration cell to the detection cell and optimized the spectrum measurement conditions in order to improve the sensitivity of the sensor. A detection level of 10 ppb for benzene gas (an aromatic VOC) in a sampling time of 50 min was reported. Good linearity was shown in the calibration curve in the range of 10–100 ppb of benzene. Th e same authors (Horiuchi et al. 2006) carried out gas monitoring experiments in a garage. Th ey also showed the applicability of the aromatic VOC sensor for fi eld monitoring.

Blum et al. (2001) designed an alcohol sensor in which two lipophilic derivatives of Reichardt’s phenolbetaine were dissolved in thin layers of plasticized poly(ethylene vinylacetate) copolymer coated with microporous white PTFE in order to facilitate refl ectance (transfl ectance) measurements. Th e sen-sor layers were found to respond to aqueous ethanol with a change of color from green to blue with increasing ethanol content. Th e highest signal changes were observed at a wavelength of 750 nm, with a linear calibration function up to 20% ethanol (v/v) and a detection limit of 0.1% (v/v). Th is sensor also responded to acetic acid, thus aff ecting measurements on beverages, but that limitation was overcome by adjusting the pH of the sample solution.

Patra and Mishra (2001) developed an optical sensor for detecting nitro aromatic compounds such as nitrobenzene, m-dinitrobenzene, o-nitrotoluene, m-nitrotoluene, p-nitrobromobenzene, o-nitroaniline, p-nitrophenol, etc., by fl uorescence quenching of benzo[k]fl uoranthene (BkF) in poly(vinyl alcohol) fi lm. Th e fl uorescence spectra of BkF-doped PVA fi lms in various solvents are shown in Figure 1.6.

Figure 1.6 demonstrates that, due to the enhanced swelling of the fi lm in methanol, the sensor fi lm showed good fl uorescence quantum yield in methanol compared to other solvents. However, less fl uorescence was observed with the same PVA fi lm due to the lower quantum yield of BkF in water than in methanol, although PVA swells more in water. Polypyrrole–nitrotoluene copolymer provides selective response to aromatic hydrocarbons (Josowicz et al. 1987).


Several polymers have been used to detect nitro aromatic explosives by a variety of transduction schemes. Detection relies on both electronic and structural interactions between the sensing material and the analyte. Quenching of luminescent polymers by electron-defi cient nitro aromatic explosives, such as trinitrotoluene, may be monitored to detect explosives. Luminescent polymetallocenes have recently been investigated for sensing explosives in aqueous-based solutions and for improved visual detection of trace particulates on surfaces (Toal and Trogler 2006). Photoluminescence quenching of the various metallocene polymers and their copolymers for picric acid, TNT, DNT, and nitrobenzene were reported by Sohn et al. (2003). Th e quenching of photoluminescence spectra by the nitro aromatics was better for the substituted silole–silane copolymer (see Figure 1.7) in the order picric acid > TNT > DNT > nitrobenzene at various concentrations.

Figure 1.6. Emission spectra of PVA fi lm doped with benzo[k]fl uoranthene in various solvent media (ex = 308 nm). (Reproduced with permission from Patra and Mishra 2001. Copyright 2001 Elsevier.)

Si

PhPh

Ph Ph

MeO SiOMe

H Phn

Figure 1.7. Structure of substituted silole–silane copolymer.


7.1.3.3. OXYGEN SENSORS

Luminescent sensors made of composites of transition-metal complexes dispersed in polymer matrices were developed for sensing oxygen in biomedical and barometric applications. Phosphorescent dyes were dispersed in a polymer matrix that had high gas permeability. Pang et al. (1996) controlled the sensitivity of the phosphorescent oxygen sensors over a wide range by using S–N–P polymers as a novel matrix material. Miura et al. (1984) developed a concentration cell–type O2 sensor by using Nafi on membrane as a proton conductor. Th e concept of using oxygen-quenchable luminescent dyes in chemi-cally homogenous polymer layers may lead to promising applications in oxygen sensing.

Th e oxygen-sensing behavior was studied by exploiting the luminescence quenching behavior of tris(4,7-diphenyl-1,10-phenanthroline) Ru(II) perchlorate dissolved in a polystyrene layer (Hartmann et al. 1995). Th e photophysical and photochemical properties of aluminum phthalocyanine– polystyrene fi lm were measured for use in a fl uorescence-quenching oxygen sensor (Amao et al. 2000). Next they developed an optical oxygen sensor based on the luminescence intensity changes of tris(2- phenylpyridine anion) iridium(III) complex ([Ir(ppy)3]) immobilized in a fl uoropolymer, poly(styrene-co-2,2,2- trifl uoroethyl methacrylate) [poly (styrene-co-TFEM)] fi lm (Amao et al. 2001). Th ey ob-served a decrease of luminescence intensity of [Ir(ppy)3] in poly(styrene-co-TFEM) fi lm with increasing oxygen concentration.

7.1.3.4. OPTICAL pH SENSORS

Munkholm et al. (1986) prepared a pH sensor device using photochemically polymerized copolymer of acrylamide-methylenebis(acrylamide) containing fl uoresceinamine covalently attached to an optical fi ber surface (core diameter 100 mm). Th e need for organic dyes has been eliminated by the use of con-ducting polymers in the preparation of optical pH sensors. Demarcos and Wolfbeis (1996) developed an optical pH sensor with polypyrrole by oxidative polymerization. Th is polymer fi lm, having suitable optical properties for optical pH sensing, has eliminated the immobilization step for an organic dye during preparation of the sensor layer. Th e fi rst commercial optical blood pH sensor was developed in 1995 by Leiner.

Likewise, several groups of researchers (Ge et al. 1993; Pringsheim et al. 1997; Grummt et al. 1997) have developed optical pH sensors from polyaniline for measurement of pH in the range 2–12 and reported that the polyaniline fi lms, synthesized within a time span of 30 min, are very stable in water. Jin et al. (2000) reported an optical pH sensor based on polyaniline fi lm prepared by chemical oxida-tion at room temperature. Th ey signifi cantly improved the stability of the polyaniline fi lm by increasing the reaction time up to 12 h. Th e fi lm showed a rapid, reversible color change as a result of a change of pH. Th e solution pH was determined by monitoring either the absorption at a fi xed wavelength or the maximum absorption wavelength of the fi lm. Th e change in the electronic spectrum of polyaniline was explained by the diff erent degrees of protonation of the imine nitrogen atoms in the polymer chain as a result of the change in pH (Chiang and MacDiarmid 1986). Th at these optical pH sensors were environmentally stable was verifi ed by keeping the sensors exposed in air for over 1 month without any deterioration in sensor performance.


Ferguson et al. (1997) developed a pH sensor by incorporating acryloyl fl uorescein as pH indicator in poly(hydroxyethyl methacrylate) hydrogel. Shakhsher et al. (1994) developed a fi ber optic pH sensor based on changes accompanying the swelling of a small drop of aminated polystyrene (quaternized) on the tip of the single optical fi ber. A low-cost optical pH sensor was developed by immobilizing a direct indicator dye, e.g., naphthyl red or alizarine yellow, to a porous and transparent acetylcellulose fi lm. Th e membranes were durable (>12 months) and had short response times (<5 s) (Ensafi and Kazemzadeh 1999).

Janowiak et al. (2001) developed a technology to support specifi c analytical reagents on polymeric membranes for fi ber optic–based pH sensors. A synthesis strategy resulting in a three-layered probe was designed. Th e technique involves the growth of a hydrophilic polymer matrix from a fi ber optic and incor-porates specifi c pH indicator dyes. Wolfbeis (2006) has reviewed developments in optical sensors for gases, vapors, and humidity. A review by Korostynska et al. (2007) describes current state-of-the-art methods of measuring pH levels that are based on polymer materials, viz., polymer-coated fi ber optic sensors, devices with electrodes modifi ed with pH-sensitive polymers, fl uorescence pH indicators, potentiometric pH sen-sors, as well as sensors that use a combinatory approach for ion concentration monitoring.

7.1.4. Future Trends in Optical Sensor Design

Optical sensing devices should ideally have high sensitivity, selectivity, and reliability, and they should be able to perform measurements in real time, in a site-specifi c fashion. However, although optical sen-sors are important because they are both very simple and cost-eff ective to manufacture and they enable rather sophisticated multisensor applications, they also suff er from some serious disadvantages. Existing optical sensing principles can be applied to a huge number of applications, but doing so requires inter-disciplinary understanding of the detection principles, sensitive layers, kinetics, and thermodynamics of the interaction processes and of the fl uidics. Th us, fundamental research must be performed on these problems to characterize the layers and the interaction processes. For better prediction of sensor performance, characterization of (plasticized) polymers with respect to lipophilicity, dielectric constant, polarity, water uptake, etc., under reproducible conditions is needed.

7.2. CONDUCTOMETRIC GAS SENSORS

7.2.1. Conducting Polymers in Gas Sensors

A change in conductivity upon exposure to an analyte is the main requirement for a sensing material intended for use in a conductometric gas sensor. Conducting polymers having acid–base or oxidizing/reducing characteristics fulfi ll this requirement and therefore they are widely used in various conducto-metric sensing devices. It has been demonstrated (Heeger 2001) that the molecular arrangement in a conducting polymer must contain alternating single and double bonds in order to allow the formation of delocalized electronic states. Th e driving force for the delocalization of these states is associated with the resonance-stabilized structure of the polymer. Electrically conducting polymers display electrical


conductivities that are dependent on the concentration of dopant ions incorporated in the material. As we have shown before, most of the conducting polymers are doped/undoped by redox reactions; there-fore, their doping level can be altered by transferring electrons from or to the analyte. Electron transfer causes the changes in resistance and work function of the sensing material.

Th e conductivity, usually measured using direct current (DC) techniques, may be modulated re-versibly and rapidly at ambient temperature by adsorption and desorption of volatile chemicals (Amrani et al. 1996; Bissell et al. 2002). As the conducting polymer undergoes a reversible redox reaction during interaction with various analytes, sensing by the conducting polymer is possible due to interaction with the polymer chain or the dopant, either chemically or electronically.

Conducting polymers that are currently being used in various gas sensors are listed in Table 1.2. Composites of conducting polymers with PVC, PMMA, etc., with active functional groups and solid polymer electrolytes (SPEs) are also used to detect such gases.

Nowadays, a very simple electrochemical sensor setup is used for conducting polymers (see Figure 1.8). Either two probes or four probes are connected to the polymer-detecting substrate to measure directly the resistivity (for two probes) or voltage at a particular applied current or vice versa (for four probes).

A special feature of semiconducting electronic devices is their effi cient performance at ambient tem-perature with low power consumption. From this point of view, conducting polymers are superior over inorganic semiconductors. Reasons for choosing conducting polymers as sensing elements in conducto-metric gas sensors are as follows (Persaud 2005): (1) Th e sensors show rapid adsorption and desorption

Figure 1.8. Conductometric gas sensor setup: (a) two-probe; (b) four-probe.

(a) (b)


kinetics at room temperature; (2) the sensor elements feature low power consumption (of the order of microwatts), because no heater element is required; (3) the polymer structure can be correlated to speci-fi city toward particular classes of chemical compounds; and (4) the sensors are resilient to poisoning by compounds that would normally inactivate some inorganic semiconductor sensors. In addition, one can expect good sensitivity from a higher surface area of deposited conducting polymer.

7.2.2. Mechanism of Conducting Polymer Sensing

7.2.2.1. CHEMICAL REACTIONS

Th e sensing mechanism of conducting polymers depends on the detecting analyte as well as the detect-ing polymer layer. Th e Lewis acid and base characteristics of these compounds have led to the concept called secondary doping: Electron donation or withdrawal by the analyte vapors leads to conductivity changes in the sensor fi lms, in addition to polymer doping by counter ions before gas exposure (pri-mary doping) (Persaud 2005). A similar idea has been applied to the more common analytes of volatile organics, leading to a model of partial charge transfer between the polymer and the analytes, although the Lewis acid and base features of these compounds are much less prominent. Partial electron transfer may increase or reduce the concentration of the charge carriers (polarons and bipolarons) in the polymer backbone and, hence, polymer conductivity; the direction is determined by the relative magnitude of the electronegativity of the vapor and the work function of the polymer (Janata and Josowicz 2003). In addition to this model based on the band theory of semiconductors, Charlesworth et al. (1993) have presented a model of a dielectric eff ect on electron hopping. Polymer conductivity is considered to be determined by electron hopping between polymer chains or over intrachain defects, and the hopping rate is aff ected by the dielectric property of the vapor. Others have reported “physical eff ects” on con-ductivity as a result of polymer swelling by the organic vapors. It has also been recognized that variation in the extent of sorption of diff erent vapors by the polymer may lead to signifi cant diff erences in sensor performance. So, conducting polymers interact with a gas or vapor analyte mainly in two ways: chemi-cally or physically. Such interactions of various gases with conducting polymers are represented in Table 1.4 (Lange et al. 2008).

As the conducting polymer interacts with gaseous species, it can act either as an electron donor or as an electron acceptor. If a p-type conducting polymer donates electrons to the gas, its hole conductivity increases. Conversely, when the same polymer acts as an electron acceptor, its conductivity decreases. In addition to the change in the number of carriers, there may also be a change in bulk mobility. Th is is usually due to conformational changes of the polymer backbone (Zheng et al. 1997).

It has been observed that nucleophilic gases (H2S, NH3, N2H4, methanol, ethanol, etc.) cause a decrease in conductivity in most widely used conducting polymers in gas-sensing applications, such as polythiophene, polypyrroles, polyaniline, and their derivatives or composites. For example, ammonia (Mohammad 1998; Sakurai et al. 2002) and H2S (Hanawa et al. 1989) gases were found to decrease the conductivity of conducting polythiophene due to the reduction of the conducting polymer. Reduction of polypyrrole (Torsi et al. 1998; Roy et al. 2003) and polyaniline (Roy et al. 2003) also resulting in a decrease in conductivity during hydrogen sensing.

TYP

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F IN

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WIT

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DU

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ING

PO

LYM

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EXA

MP

LES

SEN

SIN

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Phys

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D

iff us

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CH

Cl 3,

CH

2Cl 2,

D

ecre

ase

or in

crea

se in

At

haw

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and

Kul

karn

i 200

0; T

an a

ndin

tera

ctio

n

alco

hols,

ace

tone

, el

ectr

ical

con

duct

ivity

Bl

ackw

ood

2000

; Ver

celli

et a

l. 20

02;

acet

onitr

ile, a

lkan

es,

V

irji e

t al.

2004

; Ree

mts

et a

l. 20

04;

cycl

ohex

ane,

ben

zene

,

Tors

i et a

l. 20

04; R

uang

chua

y et

al.

tolu

ene

20

04; S

egal

et a

l. 20

05; L

i et a

l. 20

07

W

eak

hydr

ogen

bon

ding

Al

ipha

tic a

lcoh

ols,

In

crea

se in

ele

ctric

al

Ruan

gchu

ay e

t al.

2004

; Kar

et a

l.

ac

eton

e co

nduc

tivity

20

09

Che

mic

al

Oxi

datio

n N

O2,

SO2,

O3

Incr

ease

in e

lect

rical

Pr

issan

aroo

n et

al.

2000

; And

o et

al.

inte

ract

ion

cond

uctiv

ity

2002

, 200

5; X

ie e

t al.

2002

; Ram

et a

l.

20

05; Y

an e

t al.

2007

Re

duct

ion

H2,

N2H

4, N

H3,

H2S

D

ecre

ase

in e

lect

rical

R

atcl

iff e

1990

; Elli

s et a

l. 19

96;

co

nduc

tivity

M

oham

mad

et a

l. 19

98; T

orsi

et a

l.

19

98; S

akur

ai e

t al.

2002

; Virj

i et a

l.

20

04 ;

Yang

et a

l. 20

06; Th

om

as a

nd

Sw

ager

200

6

Pr

oton

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n/de

prot

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ion

HC

l, N

H3

Dec

reas

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ease

in

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et a

l. 20

03; R

eem

ts et

al.

2004

;

elec

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ctiv

ity

Virj

i et a

l. 20

04; H

ao e

t al.

2005

;

K

rond

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t al.

2006

; Sen

gupt

a et

al.

2006

, 200

9

Tabl

e 1.

4. In

tera

ctio

n of

gas

mol

ecul

es w

ith c

ondu

ctin

g po

lym

ers

and

thei

r effe

cts


Ammonia sensors based on protonic acid–doped polyaniline fi lms have been extensively studied (Koul et al. 2001; Debarnot and Epaillard 2003; Sengupta et al. 2009). Th e mechanism of ammonia sensing by HCl-doped conducting PANI is somewhat diff erent (Sengupta et al. 2006). Here, the analyte ammonia is attached to the dopant H+ and, due to the weak doping eff ect of the resulting NH4

+ ion, the conductivity of the fi lm is decreased (see Figure 1.9). Th is deprotonation of PANI by ammonia is a cause of increased resistance.

Polypyrrole shows the same eff ect as polyaniline at low ammonia concentrations, but at higher con-centrations, in the presence of humidity, the deprotonation process becomes reversible (Gustafsson et al. 1989). Th e resistance of the fi lm changes through the formation of a positively charged electric barrier of NH4

+ ions in the submicrometer fi lm. Th e electron lone pair of the NH3 gas acts as a donor to the p-type polypyrrole semiconductor, thereby reducing the number of holes in the polypyrrole and hence causing the resistivity to increase. Th e reduction of conducting polymers such as polypyrrole (Ratcliff e 1990), poly-3-hexylthiophene (P3HTH) (Ellis et al. 1996; Yang et al. 2006), and PANI (Virji et al. 2004) by hydrazine vapors has also been reported.

On the other hand, electrophilic gases with higher electron affi nity than the conducting polymer (NOx, PCl3, SO2, O2, etc.) show the opposite eff ect (Slater and Watt 1991), by increasing the number of charge carriers in the conducting polymer through oxidative doping. For this reason, NO2 gas was found to decrease the resistance in polyaniline (Xie et al. 2002) and P3HTH (Ram et al. 2005b); how-ever, an increase in resistance was observed for emeraldine salt nanofi ber, due to oxidation by NO2 to pernigraniline base (Yan et al. 2007). SO2 also decreases the resistance of PPY by a similar mechanism as proposed for the NO2–P3HTH system (Prissanaroon et al. 2000).

A decrease in resistance of a PANI fi lm was observed when it was exposed to CO gas (Misra et al. 2004; Dixit et al. 2005; Densakulprasert et al. 2005; Watcharaphalakorn et al. 2005). According to Densakulprasert et al. (2005), no diff erence was observed in UV-vis and x-ray diff raction patterns of the fi lm before and after CO exposure. According to their explanation, the stable resonance structure of CO with +C=O− withdraws electrons to form amine nitrogen (–NH), which results in a net increase of positive charge carriers on the polymer backbones and therefore an increase in conductivity. A well-defi ned scheme has been drawn by Bai and Shi (2007). For the case of chlorinated hydrocarbon sensing by PANI, similar phenomena were observed by Anitha and Subramanian (2005).

NH NHA

NH3NH3

N NHANH4

Figure 1.9. Sensing mechanism of ammonia by HCl-doped polyaniline fi lm.


Another explanation for the response of PANI to CO is also based on the redox reaction. Th e decrease in resistance was interpreted as reduction of the barrier height between grains (Misra et al. 2004; Dixit et al. 2005). Th e conductance of the sensing fi lm is governed by potential barriers between polymer grains. Th e oxidation that occurs at the grain surfaces in the presence of CO gas causes the sur-face coverage of adsorbed oxygen to decrease. Th us, the surface potential, barrier height, and depletion length are reduced, which leads to a decrease in resistance.

7.2.2.2. WEAK INTERACTIONS

Many important organic analytes are not reactive at room temperature and under mild conditions. Th erefore, it is diffi cult to detect them by their chemical reactions with conducting polymers. However, they may have weak physical interactions with the sensing polymers, involving absorbing or swelling of the polymer matrixes. Like other polymers, conducting polymers can swell in many organic solvents. Th is is controlled by the vapor molecular volume, the affi nity of the vapor to the sensing polymer, and the physical state of the polymer (Segal et al. 2002). It is necessary to note that the swelling is the main mechanism of the conductivity response of sensors based on nanocomposites, including isolated polymer and carbon black.

Weak physical interactions with conducting polymers are observed for nonreactive volatile organic compounds such as chloroform, acetone, aliphatic alcohols, benzene, toluene, and some other VOCs. Th ese interactions do not change the oxidation levels of the conducting polymers, but they can infl uence the properties of the sensing materials and thus make these gases detectable (Ruangchuay et al. 2003; Virji et al. 2004). For example, the diff usion of those gases to the intermolecular space or surface void space of the polymer can lead to some eff ect on the conductivity of the polymer. It has been found that for a pure conducting polymer, inserting analyte molecules into the polymer matrix generically increases interchain distance, which aff ects electron hopping between diff erent polymer chains. Experiments have shown that this type of diff usion of chloroform, acetone, ethanol, acetonitrile, toluene, and hexane re-duced the conductivity of PANI, PPY, PTH, and polythiophene derivatives (Torsi et al. 1998; Vercelli et al. 2002; Ruangchuay et al. 2004; Virji et al. 2004; Li et al. 2007).

Th e swelling process in a composite conducting polymer is complicated. One or more components can swell to diff erent extents, which results in various changes in overall conductivity. In some cases, the analyte dissolves conducting polymer better than the other component, and it will swell fi rst. Segal et al. (2005) synthesized polyaniline/polystyrene (PS) composite fi lms and tested their response to alcohols. Because PANI has a higher solubility in polar alcohols, it swelled much more than PS, which in fact increased the eff ective volume of the conducting PANI. Th is resulted in increasing the conductivity of the PANI. In some cases, other components than the conducting polymers in the composite swelled more. For example, when PPy/PMMA composite fi lm was exposed in acetone, PMMA swelled much more than PPy and separated conducting PPy. Th us, the conductivity of the composite fi lm was de-creased (Ruangchuay et al. 2003). Similar results were also obtained in a PANI/PVA composite sensor for humidity (McGovern et al. 2005) and in PPy/polyvinyl acetate (PVAc), PPy/PS, and PPy/polyvinyl chloride (PVC) for some toxic gases (Hosseini and Entezami, 2003).

In some cases, a catalyst incorporated in the conducting polymer fi lm can help in detecting some inert analytes. Athawale et al. (2006) prepared a nanocomposite of Pd/PANI and found that its electrical


resistance responded rapidly and reversibly in the presence of methanol. Athawale et al. assumed that the eff ective positive charges on the imine nitrogen atoms were reduced by the methanol molecules in the presence of Pd nanoparticles.

Experimental results also demonstrated that some analyte gases, especially alcohols (Svetlicic et al. 1998; Athawale et al. 2000) and ketones (Ruangchuay et al. 2004), could change the crystallinity of conducting polymers. For example, the increase in crystallinity is explained as the reason for the conductivity increase during aliphatic alcohol sensing by polyaniline and its derivatives (Athawale and Kulkarni 2000). Recently, Kar et al. (2009) reported the eff ect of functional groups on aliphatic alco-hol vapor sensing by poly(m-aminophenol). Th e –OH groups of methanol or alcohol molecules were hydrogen-bonded with the phenolic –OH groups present in the polymer molecule (see Figure 1.10), and ultimately a week electron fl ow throughout the polymer chain increased. Adsorption of ethanol and hexanol on dipentoxy-substituted polyterthiophene changes the potential barrier at the boundaries between polymer grains and is the reason for the change in conductivity (Torsi et al. 2004). Hydrogen bonding between acetone and polypyrrole makes electron hopping diffi cult, and so the conductivity decreases in acetone sensing by polypyrrole (Ruangchuay et al. 2004).

7.2.3. Examples of Polymer-Based Conductometric Gas Sensors

Conducting polymers such as polypyrrole, polythiophene, polyaniline, polyindole, and their derivatives and composites are the most widely used materials as transduction matrices sensitive to gases, vapors, ions, and biomolecular systems, resulting in a straightforward conductance, impedance, or redox poten-tial change via modulation of their doping level (Bidan 1992). Th ey are also very popularly used as sens-ing materials for gas and chemical odors in sensor arrays (Persaud and Pelosi 1985; Amrani et al. 1993).

N

SO

OO

OH

O

OH

HO R

H

_+

d -

W h ere , R = C H 3: M eth an o l = C H 2C H 3: E th an o l

Figure 1.10. Aliphatic alcohol sensing mechanism by sulfuric acid–doped conducting poly(m-amino-phenol). (Reproduced with permission from Kar et al. 2009.Copyright 2009 Elsevier.)


An “electronic nose,” which is an array-based system that attempts to mimic the mammalian olfactory system, can recognize complex, distinct, and diverse odors of cheeses, beers, olive oil, explosives, and pathogenic bacteria (Gardner and Bartlett 1999).

Polypyrrole, the fi rst conducting polymer used in gas sensors, showed low sensitivity, long response time, and incomplete reversibility of the sensor response. During the last decade, the properties of polypyrrole-based polymers were considerably improved. However, due to easy synthesizability, better processability, easy dopability, and better stability, polyaniline (PANI), its derivatives, and their compos-ites have become the important conducting polymer materials for gas sensors (Bai and Shi 2007).

7.2.3.1. AMMONIA GAS SENSORS

Nylander et al. (1983) reported ammonia gas sensing using polypyrrole-impregnated fi lter pa-per and obtained linear performance of the sensor at room temperature with higher concentrations (0.5–5%) with around 1.0 min response time. Composites of electrically conducting polyacrylonitrile (PAN)/ polypyrrole (Park and Han 1992), polythiophene/polystyrene, polythiophene/polycarbonate, polypyrrole/polystyrene, and polypyrrole/polycarbonate (Wang et al. 1990) have been prepared by electropolymeriza tion of the conducting polymers within the matrix of the insulating polymers PAN, polystyrene, and polycarbonate, respectively.

Ammonia has been detected by electroactive nanocomposite ultrathin fi lms of PANI and isopoly-molybdic acid (PMA). Th e device was prepared by alternate deposition of PANI and PMA following Langmuir-Blodgett and self-assembly techniques (Li et al. 2000). Th e process was based on a doping-induced deposition eff ect of the emeraldine base. Chabukswar et al. (2001) used acrylic acid as dopant in polyaniline for sensing of ammonia vapor over a broad range of concentrations, viz., 1–600 ppm. Th e change in DC electrical resistance of the polymer was found to increase linearly upon exposure to ammo-nia up to 58 ppm concentration and to saturate thereafter. Th ey explained the decrease in resistance on the basis of removal of a proton from the acrylic acid dopant by the ammonia molecules, thereby rendering free conduction sites in the polymer matrix. A plot of the variation of relative response of the ammonia gas sensor with increase in the concentration of ammonia gas is shown in Figure 1.11. For an acrylic acid–doped polyaniline sensor, a sharp increase in relative response was obtained for around 10 ppm ammonia, which subsequently remained constant beyond 500 ppm. On the other hand, the nanocomposite of poly-aniline and PMA showed a decrease of relative response with the increase in ammonia concentration.

NH3 sensitivity was also detected by the change in resistance of a submicrometer polypyrrole fi lm (Yadong et al. 2000). A polypyrrole–poly(vinyl alcohol) composite was prepared by electropolymer-izing pyrrole in a cross-linked matrix of PVA for the purpose of sensing NH3 gas (Gangopadhyay and De 2001).

Linsey and Street (1984) studied gas-sensing behavior of PPy-PVA fi lms prepared by electrochemi-cal polymerization on to a precoated PVA matrix. Th ese studies contrasted the advantages of mechanical properties of the host polymer with the electrical properties of PPy-PVA composite fi lms. To obtain good response and reproducibility in gas sensing, a thin fi lm is better than a pellet, because absorption and desorption of gas will be faster in a fi lm than in a pellet, due to molecular order and compactness in-side the fi lm. Th e drawback of previously reported results (Bidan 1992; Hirata and Sun 1994) is related


to insuffi cient gas sensitivity and irreversibility for ammonia sensing by HCl-doped polyaniline. Th is is due to the deprotonation interaction of ammonia with the doped polymer (see Figure 1.9). Th e same eff ect has been observed in ammonia sensing by polypyrrole. Although the response is somewhat revers-ible at low ammonia concentration, it becomes irreversible at higher ammonia concentration, especially in the presence of humidity (Gustafsson et al. 1989).

7.2.3.2. NITROGEN OXIDE GAS SENSORS

Although sensor development for acidic–basic gases (e.g., CO2, NH3) and oxygen have a long history, there is a challenge in the need for rapid, sensitive detection of nitric oxide (NO). Th ere is increasing interest in determination of NO, primarily because of its role in intra- and intercellular signal transduc-tion in tissues (Cunningham 1998). Christensen et al. (1993) developed a NO2 sensing device using a polystyrene fi lm. Upon exposing the fi lm to a 1:10 v/v mixture of NO2/N2, the conductivity of the fi lm increased irreversibly and rapidly, by several orders of magnitude. It was believed that the increase in conductivity of the fi lm was due to self-ionization of N2O4 to NO+NO3

−.

Figure 1.11. Variation of relative response of ammonia gas by acrylic acid–doped polyaniline sensor with increased concentration. (Reproduced with permission from Chabukswar et al. 2001. Copyright 2001 Elsevier.)


Xie et al. (2002) reported a polyaniline-based gas sensor made by ultrathin-fi lm technology. Th ey prepared multilayer fi lms of PANI, PANI and acetic acid, and PANI and polystyrenesulfonic acid (PSSA) composite by Langmuir-Blodgett (LB) and self-assembly (SA) techniques. Th e pure PANI fi lms prepared by the LB technique showed good sensitivity to NO2, while the SA fi lms exhibited faster recov-ery. PANI is oxidized in contact with NO2. Contact of NO2 with the π-electron network of polyaniline results in the transfer of an electron from the polymer to the gas, making the polymer positively charged, which gives rise to increased conductivity. Th e PANI–acetic acid fi lms showed lower sensitivity, due to the occupation of the chemically blocked sensitive sites responsive to NO2 by acetic acid molecules. Films of poly [(3-butylthio) thiophene] were prepared by LB deposition and casting techniques for ap-plications in gas sensor devices (Rella et al. 2000). Th e sensing layer was prepared by the two techniques separately: the LB deposition of the polymer in a mixture with arachidic acid and direct casting from a solution of the polymer in chloroform. In both cases the deposition was done on alumina substrates equipped with gold interdigitated electrodes. Th e deposited devices showed changes in electrical con-ductivity when exposed to a mixture of NO2-oxidizing and NH3-reducing agents at about 100°C (see Figure 1.12). According to Rella et al. (2000), the fi rst drop of current is due to the reducing interaction

Figure 1.12. Response curve for poly-3-(butylthio)thiophene Langmuir–Blodgett fi lms in NH3 and NO2 mixture with dry air at 100°C. (Reproduced with permission from Rella et al. 2000. Copyright 2000 Elsevier.)


of 1000 ppm ammonia, while the next rise of current is because of the p-type doping by 10-ppm NO2 (see Figure 1.12).

7.2.3.3. CO2 GAS SENSORS

A CO2 gas sensor, consisting of K2CO3–polyethylene glycol solution supported on porous alumina ceramics, was developed by Wu et al. (1989). Upon exposure to CO2 and under an applied voltage, the resistance of the device increased. A linear relationship between the sensitivity (the ratio of resistance in CO2 to that in air) and the CO2 concentration from 1% to 9% was reported. An improvement of this sensor was reported by solidifying the sensing layer using a solid polyethylene glycol of high molecular weight doped with a solution comprised of liquid polyethylene glycol and K2CO3 (Sakai et al. 1995). Th e change in resistance was attributed to the change in concentration of the charge carrier K+ ion.

7.2.3.4. H2 AND CO SENSORS

Doping of electrochemically synthesized conducting polymers, such as polypyrrole and poly-3-methyl-thiophene, with copper and palladium creates a good material for gas sensing (Torsi et al. 1998). Th ese metals were deposited potentiostatically, either on pristine conducting fi lms or on partially reduced samples. Th e resistance of the PPy and Cu-doped PPy fi lm sensors after exposure to H2 and CO reduc-ing gases was enhanced. On the other hand, there was a drastic drop in resistivity of the Pd–PPy sensor to H2 and CO, while a resistivity enhancement was shown upon exposure to ammonia. Moreover, the responses of the Pd–PPy sensor to CO and H2 were highly reversible and reproducible. Roy et al. (2003) reported the hydrogen gas–sensing characteristics of doped polyaniline and polypyrrole fi lms.

7.2.3.5. SENSORS OF VOCS

Environmental pollution arising out of volatile organic compounds from chemicals and fertilizers, pesti-cides, and waste streams is a global concern today. For detection as well as assessment of levels of pollu-tion from those volatile organic compounds, chemical sensors play a major role. Detection of VOCs is a topic of growing interest, with applications in diverse fi elds ranging from environmental uses to food and chemical industries. Recently, highly crystalline nanostructures of regioregular polythiophene-based conductive copolymers have been shown to hold considerable promise as an active layer in VOC chemiresistor sensors (Li et al. 2006). Th e regioregular polythiophene polymer chain provides a charge conduction path, and its chemical sensing selectivity and sensitivity are altered either by incorporating a second polymer to form a block copolymer or by making a random copolymer of polythiophene with diff erent alkyl side chains. Th ese copolymers were exposed to diff erent VOC vapors. Th e electrical con-ductivity of the copolymers either increased or decreased depending on the polymer composition and the specifi c analytes. Measuring at room temperature, the responses were found to be fast and appeared to be completely reversible. Various copolymers of polythiophene in a sensor array provided much better discrimination to various analytes compared to existing solid-state sensors.


Since conducting polymers have gained popularity as competent sensor materials for organic va-pors, a few reports are available describing the use of polyaniline as a sensor for alcohol vapors, such as methanol, ethanol, and propanol (Sukeerthi and Contractor 1994; Hatfi eld et al. 1994). Polyaniline doped with camphor sulfonic acid (CSA) also showed a good response to alcohol vapors (Xia et al. 1995; MacDiarmid and Epstein 1994, 1995; Svetlicic et al. 1998). Th ese reports discussed the sensing mechanism on the basis of the crystallinity of polyaniline. Polyaniline and its substituted derivatives, such as poly(o-toluidine), poly(o-anisidine), poly(N-methyl aniline), poly(N-ethyl aniline), poly(2,3-dimethyl aniline), poly(2,5-dimethyl aniline), and poly(diphenyl amine) were reported to be sensitive to various alcohols such as methanol, ethanol, propanol, butanol, and heptanol vapors by Athawale and Kulkarni (2000). All the polymers were found to respond to saturated alcohol vapors by undergoing a change in resistance. While the resistance decreased in the presence of short-chain alcohols, viz., metha-nol, ethanol, and propanol, an opposite trend in the change of resistance was observed with butanol and heptanol vapors. Th e authors attributed the change in resistance of the polymers upon exposure to diff erent alcohol vapors to their chemical structure, chain length, and dielectric nature. Th ese polymers showed a sensitivity of 60% for short-chain alcohols, at up to 3000 ppm concentration, but none of them were suitable for long-chain alcohols. As a reason, the authors indicated a vapor-induced change in the crystallinity of the polymer.

Recently we have reported sulfuric acid–doped poly(m-aminophenol) as a very good aliphatic al-cohol vapor sensor material (Kar et al. 2009). Th e polymer shows very good response for methanol and

Figure 1.13. Variation of response for sulfuric acid–doped poly(m-aminophenol) with te methanol and ethanol vapor concentration (the polynomial equations are given in the corner, and R2 is the cor-relation coeffi cient). (Reproduced with permission from Kar et al. 2009. Copyright 2009 Elsevier.)


ethanol but is a very poor sensor for other higher alcohols such as isopropanol. Th e variation of response with methanol and ethanol vapor concentration is shown in Figure 1.13 with the equations for the poly-nomial fi t. From the equation on the plot, one can calculate the lower limits for alcohol sensing. Th ese were obtained as 80 ppm for ethanol and 100 ppm for methanol.

Polypyrrole has also been studied as a sensing layer for alcohols. Jun et al. (2003) incorporated do-decyl benzene sulfonic acid (DBSA) and ammonium persulfate (APS) in polypyrrole in a sensor, which showed a linear change in resistance upon exposure to methanol vapor in the range 87–5000 ppm. Bartlett and Ling-Chung (1989) also detected methanol vapor by measuring the change in resistance of a polypyrrole fi lm and obtained a rapid and reversible response at room temperature. Th e eff ects of metha-nol concentration, operating temperature, and fi lm thickness on the response were investigated. A liquid-phase alcohol sensor, based on a refl ection hologram distributed within a poly(hydroxyethyl methacry-late) fi lm, was reported by Mayes et al. (1999) for measuring alcohol-induced thickness changes. With advantages of high selectivity and operation at ambient temperatures over inorganic gas sensor materials, polyaniline and polypyrrole have found use in multicomponent gas sensing (Harsányi et al. 1999).

7.2.4. Problems of Polymer-Based Conductometric Gas Sensors

Although conducting polymers as sensor materials have advantages over inorganic semiconductors, no commercial systems have yet been developed. Th e reasons can be attributed to the following problems.

1. Th e response times of conductometric sensors are long. Usually, response times exceed hundreds of seconds, and only for some ultrathin-fi lm sensors can this time be as short as about several seconds. For example, the response time for ammonia sensing by polyaniline is around 4 min, and the response of the polyaniline fi lm generally depends on the concentration of ammonia va-por (Kukla et al. 1996). Using polyacrylic acid–blended polyaniline, Athawale and Chabukswar (2001) reported good response for ammonia at very low concentration with moderate response time but long recovery time.

2. Th ese sensors possess long-time instability. Th e performance of this kind of sensors decreased dra-matically when they were stored in air for a relatively long time. Th is phenomenon can be ex-plained as de-doping of the conducting polymers. Many conducting polymers such as PPy and PTh are easily de-doped when they are exposed to air. In addition, oxygen may cause degenera-tion of some conducting polymers. However, it was reported that PPy doped with big anions can retain conductivity for 20 years (Ricks-Laskoski and Buckley 2006), and PPy doped with an amphiphilic anion can reduce the infl uence of water and oxygen (Bay et al. 2002).

Polymer sensors used for environmental control are also inclined to degradation due to their sensitivity to UV radiation and the presence of oxidizing gases. It has been reported that ozone and other oxidizing components of the polluted atmosphere of industrial centers may be initia-tors or accelerators of photochemical destruction of polymers (Razumovskii and Zaikov 1982; Heeg et al. 2001). As a result, gas sensors based on these materials have short life spans, especially in normal atmospheres containing water and active gases. It was found that among other poly-mers, undoped PPy, as a semiconducting polymer, is rather stable toward UV irradiation, which


can actually increase its conductivity (Fang et al. 2002). However, the stability of PPy against UV irradiation depends on the type of dopant present in the polymer and the power density of the UV irradiation (Rabek 1995).

3. High sensitivity to air humidity is other important disadvantage of polymer-based gas sensors (Sandier and Karo 1974; Kumar and Sharma 1998). Th is means that humidity must be consid-ered when detecting other gases in air.

4. Conducting polymers have low selectivity. Not only sensors based on conducting polymers, but also other sensors have to face this problem. A single sensor cannot distinguish diff erent analytes, and the response can be easily infl uenced by the presence of other gases. Nearly all the conducting polymers are sensitive to redox-active gases, such as NH3 and NO2, and to organic vapors (Bai and Shi 2007).

5. Irreversibility is another disadvantage of many polymer-based sensors. It was found that the re-sponse of sensors, especially ammonia sesnors, gradually fell during sensing cycles, or the signal did not return to the original value after exposure to analytes (Hao et al. 2005; Kim et al. 2005). Th e irreversibility of PPy in ammonia may be caused by nucleophilic attack on the carbon back-bone (Kemp et al. 2006), but the mechanism of irreversibility is still not clear.

It is necessary to note that all the disadvantages of conductometric gas sensors mentioned above are problems of other polymer-based sensors as well.

7.3. SAW AND QCM POLYMER-BASED SENSORS

Mass sensing is a popular method for chemical analysis, since the mass is a fundamental physical prop-erty of any matter. Mass-sensitive devices transform the change in mass after interaction with analyte into a change in a property of the sensor detection element. Quartz crystal microbalance (QCM) and surface acoustic wave (SAW) sensors are very stable devices, capable of measuring the change of an ex-tremely small mass (Forster 1998). Piezoelectric devices are based on the measurement of the frequency change of the quartz oscillator plate caused by adsorption of a mass of the analyte at the oscillator. In SAW sensors a change in velocity of surface acoustic waves, caused by a change in mass of the coating on the sensor due to absorption of an analyte species, alters the resonant frequency of the wave (Buff 1992). To achieve the necessary sensitivity, QCM and SAW devices contain a special coating layer with adsorptive properties. Th is coating determines the selectivity of QCM and SAW sensors. Oscillations are applied to the sensor through a set of metallic electrodes formed on the piezoelectric surface, over which an adsorptive coating is deposited (Forster 1998).

For example, Figure 1.14 shows that the formation of an acoustic wave by an AC voltage applied to a set of interdigited electrodes at one end of the device. SAW propagates toward the acoustic aperture of the crystal by distortion of the piezoelectric material under an electric fi eld. When the wave arrives at the other end, a duplicate set of interdigited electrodes generates an AC signal as the acoustic wave passes underneath them. Th is AC signal can be monitored in terms of amplitude, frequency, and phase shift. Most SAW sensors were developed for continuous operation in situ for detection of volatile organic compounds (Slater and Paynter 1994; Grate et al. 1997; Ho et al. 2003). Special packages that encase


the SAW device and integrated circuit board allow the sensor to be used in air, soil, and even water. Th ese SAW devices operate well at ultrahigh frequencies and sense as little as 1 pg of material.

7.3.1. Requirements to Sensing Materials

QCM-, SAW-, and cantilever-based sensors are adsorption-type sensors, in which the change in weight of the sensing element is the determining factor (Houser et al. 2001). Th erefore, the role of the sens-ing material in such devices is selectively and reversibly to sorb an analyte of interest from sampled air or liquid, and to concentrate it so that lower concentrations can be detected. Maximum and reversible sorption of specifi c analytes or classes of analytes, with rapid sorption kinetics and minimal sorption of interferents, are key aims in the development of a successful chemoselective coating for SAW-, QCM-, and cantilever-based sensors (Th ompson and Stone 1997). Th is means that the selective coating should be such that it interacts well, either physically or chemically, with the chemical species to be sensed.

Polymer fi lms are normally chosen to coat the surface of QCM and SAW sensors. Phthalocyanine (Zhou et al. 1993), polymer–ceramic composites (Dias et al. 1993), epoxy resin (Ema et al. 1989), cel-lulose (Nakamoto et al. 1990), and many other polymers have been tested. As we have shown before, polymers can have diff erent chemical properties, allowing sensing of a variety of organic chemical classes such as hydrocarbons, alcohols, ketones, and oxygenated, chlorinated, and nitrogenated compounds. Th erefore, by choosing the proper polymer fi lms with suitable active free functional groups, each chemi-cal vapor of interest can be determined. Just this property of polymers is used in arrays of QCM or SAW devices for use as an “electronic nose.” Every sensor in the array has a diff erent polymer fi lm. Each fi lm is chosen to have chemical absorption characteristics diff erent from the others.

Th e sensitivity and selectivity of an analyte for a polymer-based SAW or QCM sensor can be correlated from the relation of the solubility parameters of the polymer and the analyte (Gopel et al.

Figure 1.14. Layout of a single acoustic aperture surface acoustic wave (SAW) device. (Reproduced with permission from Bai and Shi 2007. Copyright 2007 MDPI.)


1998). Th e closer the solubility parameters of the polymer layer are to those of the analyte, the better the sensitivity and selectivity, because of the better affi nity. For example, in a QCM device, a polybu-tyl methacrylate layer showed better sensitivity and selectivity for carbon tetrachloride, benzene, and chloro benzene than for ethanol, and the response was maximal for carbon tetrachloride vapor (Koshets et al. 2003) (see Figure 1.15a). A copolymer of propylene–butyl fi lm coated on a quartz resonator mi-crobalance was used as a material for sensing harmful gases such as toluene, xylene, diethyl ether, chlo-roform, and acetone, the solubility parameters of which are close to those of the polymer (Nanto et al. 2000). Th e quartz resonator microbalance sensor coated with this copolymer exhibited high sensitivity and excellent selectivity for these harmful gases, especially for toluene and xylene gas, suggesting that

Figure 1.15. QCM sensitivity of sensors coated with (a) polybutyl methacrylate and (b) Polyvinyl formal ethylal toward various volatile organic vapors. (Reproduced with permission from Koshets et al. 2003. Copyright 2003 V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine.)


the solubility parameter is an eff ective parameter for use in functional design of the sensing membrane for quartz resonator gas sensors.

Th e dipole–dipole interaction between the polymer layer and the analyte can also be taken into ac-count as a sensing mechanism for some analytes. For instance, polyvinyl formal ethylal copolymers show a high sensitivity toward alcohol but low sensitivity toward chlorine organic vapors (see Figure 1.15b) (Koshets et al. 2003).

7.3.2. Examples of QCM and SAW Polymer-Based Sensors

A QCM-type SO2 gas sensor was fabricated by Matsuguchi et al. (2001) with amino-functional poly (styrene-co-chloromethylstyrene) derivative on a quartz surface. Th ey used three diff erent diamine com-pounds, N,N-dimethyl ethylene diamine (DMEDA), N,N-dimethyl propane diamine (DMPDA), and N,N-dimethyl-p-phenylene diamine (DPEDA), to attach an amine group onto the copolymer back-bone. Th e functioning of this sensor is based on the absorption of the basic amino group by SO2, which is a strong Lewis acid gas. Sensing characteristics are infl uenced by many factors, such as the mole fraction of chloromethyl styrene in the copolymers, the structure of the diamine compound attached, measurement temperature, and addition of organically modifi ed siloxane oligomer. Th e sensor contain-ing DPEDA functional copolymer showed the shortest response time (t100 = 11 min), with complete reversibility even at 50°C.

Figure 1.16 shows the response characteristics of a SO2 gas sensor using various amino-functional copolymers (DMEDA, DMPDA, DPEDA) measured for 50 ppm SO2 gas at 30°C (Matsuguchi et al.

Figure 1.16. Response characteristics of sensor using aminofunctional copolymers measured for 50 ppm of SO2 at 30°C; (●) DMEDA; (▲) DMPDA; (■) DPEDA. (Reproduced with permission from Matsuguchi et al. 2001. Copyright 2001 Elsevier.)


2001). A thin fi lm of 1,4-polybutadiene was used to construct a small and very sensitive (<10 ppb) ozone sensor (Fog and Rietz 1985).

As in QCM sensors, the nature of the coating on the SAW sensor also determines the selectivity of the device; for example, LiNbO3 (Wohltjen and Dessy 1979) and fl uoropolymers selectively sense pollutant organophosphorus gas (Dejous et al. 1995), and commercially available gas chromatographic phases as coatings sense toluene in dry air (Arn et al. 1991) with a response time of the order of 1 s. Micropatterned polymeric diff raction gratings were developed (Bailey and Hupp 2003) for sensing volatile organic chemicals, operating under nonresonant conditions. Th e sensor elements were found to respond in a rapid (response time 5–15 s) and reproducible fashion to each analyte investigated. Th e detection limits of micropatterned polyepichlorohydrin, polyisobutylene, and polybutadiene gratings were found, respectively, to be 8, 11, and 7 ppm for toluene; 25, 258, and 72 ppm for methyl ethyl ketone; 41, 102, and 34 ppm for chloroform; and 460, 60, and 59 ppm for hexane. While generally less than one order of magnitude higher than those observed for identical polymer/analyte combinations in SAW studies, the observed limits of detection were at or below governmental standards for each analyte evaluated with these polymeric gratings.

SAW sensors using three diff erent kinds of polymers to detect selectively three diff erent volatiles, di-methyl methylphosphonate, CH3CN, and CH2Cl2, have been reported by Joo et al. (2005). In an inter-digital transducer (IDT) SAW device the polymers used as the sensing material were polyisobutylene (PIB), polyepichlorohydrin (PECH), and polydimethylsiloxane (PDMS) on an aluminum substrate. Th e thin fi lms were coated on quartz substrate by spin coating.

Figure 1.17. Calibration curves for frequency shift using polystyrene-coated quartz crystal electrode of benzene (■), toluene (●), ethylbenzene (▲), and xylene (×). (Reproduced with permission from Mirmohseni and Rostamizadeh 2006. Copyright 2006 MDPI.)


Koshets et al. (2003) reported a quartz crystal microbalance sensor coated with polybutyl meth-acrylate (PBMA) and polyvinyl formal ethylal (PVFE). It was found that the PBMA fi lm–coated sensor showed good sensitivity as well as selectivity for chlorobenzene, although it also responded to chloro-form, chloromethylene, and toluene. However, the fi lm did not show any signifi cant response for alco-hols. On the other hand, the responses of the PVFE fi lm–coated device show high sensitivity toward alcohol but low sensitivity toward chlorine organic vapors. A QCM sensor was developed (Mirmohseni and Hassanzadeh 2001, 2006) for the detection of volatile BTEX vapors (benzene, toluene, ethylben-zene, and xylenes). Th e adsorption of BTEX organic vapors on a thin layer of polydimethylsiloxane (PDMS) (Mirmohseni and Hassanzadeh 2001) or polystyrene (Mirmohseni and Rostamizadeh 2006) coated on AT-cut quartz crystals with gold electrodes was studied. Calibration graphs were constructed for the frequency shifts due to the sorption of BTEX compounds at the ppm level. As an example, calibration curves for the frequency shift using a polystyrene-coated quartz crystal electrode of benzene, toluene, ethylbenzene, and xylene is shown in Figure 1.17.

7.4. ELECTROCHEMICAL POLYMER-BASED SENSORS

In sensor technology, the electrochemical transducer principle is the oldest and most widely used strategy. Generally, electrochemical methods are based on the transformation of chemical information (chemical reaction) into an analytically useful signal. As a result of electron transfer, the change of electrical proper-ties of the detection element can be measured with the help of a well-defi ned electrical circuit by various modes of measurements, e.g., voltammetric (measurement of voltage change), impediometric (mea-surement of impedance change), conductometric (measurement of conductivity or resistance change), amperometric (measurement of current change), or potentiometric (measurement of potential change) (Bakker 2004). Th e last two techniques are the most utilized ones. In potentiometric sensors, a local equilibrium is established at the sensor interface, and information about the composition of a sample is obtained from the potential diff erence between two electrodes. Amperometric sensors exploit the use of a potential applied between a reference and a working electrode, to cause the oxidation or reduction of an electroactive species; the resulting current is measured. Th e most widely used potentiometric device is the pH electrode, which has been used for several decades. More detailed description of these methods can be found in reviews (e.g., Bakker 2004) and in other chapters of this series.

Any sensor used in electroanalytical determinations contains two basic functional units: a receptor, which transforms the chemical information into a form of energy; and a transducer, which transforms the energy, bearing chemical information, into a useful signal. Given the multifunctionality and the large variety of possible structures, polymers in electrochemical sensors can act as elements with diff erent functions, i.e., as solid electrolytes, membranes, or solid contacts (Bobacka 2006).

7.4.1. Ion-Selective Electrodes

Potentiometric ion sensors (Uhlmann et al. 1998), a subgroup of electrochemical sensors (Bakker 2004), are attractive for practical applications because they are characterized by small size, portability, low


energy consumption, and low cost compared to many other analytical techniques. Typical constructions of ion-selective electrodes (ISEs) are shown in Figure 1.18.

Ion-selective electrodes based on polymeric membranes containing neutral or charged carriers (ionophores) are among the most successful electrochemical sensors in routine use today. In the classic confi guration, the electrodes are usually based on porous membranes, consisting of permselective mem-brane for the comparable size of the ions which separate the sample from the inside of the electrode. When such systems come in contact with analytes to be sensed, some ionic exchange/interaction occurs. Due to this interaction and the movement of the selected ions, a potential diff erence is generated, which is measured in the sensor device. So, the ISE is an indicator electrode capable of selectively measuring the activity of a particular ionic species. ISEs are suitable for determination of specifi c ions in a solu-

Figure 1.18. Construction principles for various ion-selective electrodes: (a) conventional ISE with an internal reference electrode and internal fi lling solution; (b) coated-wire ISE; (c) ISE with a hy-drogel contact; (d) ISE with a conducting polymer contact; (e) ISE with a conducting polymer dis-solved in the ion-selective membrane; (f) ISE with a (functionalized) conducting polymer as sensing membrane. 1, electronic conductor; 2, internal reference electrode; 3, inner fi lling solution; 4, ion-selective membrane; 5, hydrogel; 6, electronic conductor with a high work function; 7, conducting polymer; 8, ion-selective membrane containing a conducting polymer; 9, conducting polymer con-taining ion- recognition sites (ionophores). (Reprinted with permission from Bobacka 2006. Copyright 2006 Wiley-VCH Verlag GmbH&Co.)


tion in the presence of other ions. Th e quantitative analysis of ions in solutions by ISEs is a widely used analytical method. Ion sensors fi nd wide application in medical, environmental, and industrial analysis (Bakker and Meyerhoff 2000).

According to Bobacka (2006), conducting polymers are useful for application in solid-state ISEs for several reasons:

1. Conducting polymers are electronically conducting materials that can form an ohmic contact to materials with high work function, such as carbon, gold, and platinum.

2. Conducting polymers can be conveniently electrodeposited on the electronic conductor by electro chemical polymerization of a large variety of monomers.

3. Alternatively, several conducting polymers are soluble and can therefore be deposited from solution.

4. Conducting polymers are electroactive materials with mixed electronic and ionic conductivity, which means that they can transduce an ionic signal into an electronic one in the solid state. Th ese multifunctional properties of conducting polymers are excellent for ion-to-electron trans-duction in the solid state.

Ion selectivity can be introduced in two diff erent ways (Bobacka 2006). One possibility is to use the conducting polymer only as an ion-to-electron transducer (solid contact) in combination with a classical ion-selective membrane (Cadogan et al. 1992). In these solid-contact ISEs (see Figure 1.18d), the ion selectivity is determined mainly by the ion-selective membrane, and this methodology allows full utiliza-tion of a large number of existing ionophore-based polymeric membrane formulations (Uhlmann et al. 1998). Th e operating principle is shown schematically in Figure 1.19a.

By using a conducting polymer that is soluble in the same solvent as the components of the ion-selective membrane, it is possible to construct a so-called single-piece ISE, in which the conduct-ing polymer, acting as an ion-to-electron transducer, is integrated into the ion-selective membrane (Figure 1.18e).

Another possibility for inducing selectivity is to incorporate the ion-recognition sites directly into the conducting polymer matrix, e.g., by doping the conjugated polymer with counter ions containing ion-complexing groups or by covalent binding of ion-recognition sites to the conjugated polymer chain (Figure 1.18f ) (Garnier 1989). Th e operating principle is shown schematically in Figure 1.19b. Th is approach requires precise control of both the electronic and the ionic transport properties of the mem-brane in order to optimize the ion-recognition and transduction processes (Bobacka 2006).

Th e main challenge is to obtain a selective ionic response while minimizing redox interference. However, the possible benefi ts are huge, because covalent binding of ion-recognition sites to the con-ducting polymer backbone allows integration of the ion-recognition sites and ion-to-electron transducer even within the same macromolecule. Th is approach is still in an early stage of development, but one can expect that it will become of great importance for the construction of durable micro- and nano-sized ion sensors in the future (Bobacka 2006).

Polymers that are usually applied for design of ISEs are listed in Tables 1.5 and 1.6.As follows from previous discussion, membrane materials for ISEs must possess adequate porosity.

Initially, porous polymeric ISE membranes were obtained by soaking a viscous solution of a water-


immiscible, nonvolatile polymer containing the dissolved ionophore. Recently, homogeneous polymer membrane matrices having ionic functional groups have replaced these materials. For preparation of a fl exible sensing membrane, the polymer is mixed with a compatible plasticizer. In such membranes, the polymers must have some special physical properties, e.g., elasticity and mechanical stability.

7.4.2. Electrochemical Polymer-Based Gas Sensors

One approach to designing a room-temperature gas sensor is to use solid polymer electrolytes (SPEs). Solid polymer electrolytes became important during the mid-1970s because of the ineffi ciencies and maintenance requirements of liquid electrolytes then used in amperometric and potentiometric electro chemical sensors (Korotcenkov et al. 2009). Th e simplest setup for these technologies is shown in Figure 1.20.

Originally, the SPE was a solid plastic sheet of perfl uorinated sulfocationite polymer that, when saturated with water, became an excellent ionic conductor. Ionic polymers in contact with a conductive medium such as a metal allow electrochemical reactions at the interface. (It is necessary to note that SPEs are not electronic conductors.)

(a) (b) Figure 1.19. Operating principles of (a) solid-contact ISE based on an oxidized (p-doped) conduct-ing polymer as ion-to-electron transducer and (b) solid-state ISE based on an oxidized (p-doped) conducting polymer as sensing membrane. EC, electronic conductor; CP, conducting polymer; ISM, ion-selective membrane; S, solution; e, electron; +, “hole” (oxidized CP); L, ion-recognition site ( mobile/fi xed). (Reproduced with permission from Bobacka 2006. Copyright 2006 Wiley-VCH Verlag GmbH&Co.)


SOLID CONTACT ION-SELECTIVE

SUBSTRATE MATERIAL MEMBRANE PRIMARY ION

Glassy carbon PPy PVC Cl−; Ca2+; K+

Graphite PPy PVC Cl−Pt PPy PVC K+; pHPt PPy (+ Nafi on) Glass pHAg/AgCl + Pt PHDP SR CO3

2−

Glassy carbon PEDOT PVC N-MePy+; Bupivacaine; Ag+; K+

Au PEDOT PVC K+

EPD + graphite PEDOT PVC K+

PEDOT PEDOT PVC K+; Ca2+; Cu2+

Glassy carbon PMT PVC Ca2+

Ag/AgCl + Pt POT SR CO32−

Au or Pt POT PVC PUR Cl−Au POT MMA/DMA Ca2+; Pb2+

Au PANI PVC , PUR Cl−Pt PANI PVC pHGlassy carbon PANI PVC K+

Pt PNA PVC DimedrolPt PAP PVC Chlordiazepoxide

PPy = polypyrrole; PVC = ion-selective membrane based on plasticized poly(vinyl chloride); PHDP = poly(1-hexyl-3,4-dimethylpyrrole); SR = ion-selective membrane based on plasticized silicone rubber; PEDOT = poly(3,4-ethylenedioxythiophene); N-MePy+ = N-methyl pyridinium; bupivacaine = 1-butyl-N-[2,6-dimethylphenyl]-2-piperidinecarboxamide; EPD = poly(ethylene-co-propylene-co-5-methylene-2-norbornene); PMT = poly(3-methylthiophene); POT = poly(3-octylthiophene); PUR = ion-selective membrane based on plasticized polyurethane (Tecofl ex); MMA/DMA = ion-selective membrane based on poly(methyl methacrylate)/poly(decyl methacrylate) copolymer; PANI = polyaniline; PNA = poly(-naphthylamine); PAP = poly(o-aminophenol).

Source: Data from Bobacka 2008.

Table 1.5. Solid-state ion-selective electrodes using polymer-based materials as solid contact (ion-to-electron transducer)

Nafi on, a typical solid polymer electrolyte, is a hydrated copolymer of polytetrafl uoroethylene (PTFE) and polysulfonyl fl uoride vinyl ether containing pendant sulfonic acid groups. It is a cation exchanger that contains hydrophilic SO3

− radicals fi rmly bound to the hydrocarbon backbone, whose charge is com-pensated by counter ions (mostly H+). Th e counter ions are dissociated and solubilized by water present within the polymer structure and give rise to the ionic (proton) conductivity of the polymer. Th e water


required for proton solubility is bound in the hydration mantles of the ions present, so the polymer is a solid which contains no macroscopic liquid phase unless excess water is present (Opekar and Stulik 1999). Nafi on has good proton conductivity, high gas permeability, outstanding chemical stability, and good mechanical strength, and it has been widely used in fuel-cell and sensor applications.

CONDUCTING

POLYMER SUBSTRATE DOPANT PRIMARY ION

PPy Pt, glassy carbon Arsenazo-I Ca2+; Mg2+; Cu2+

Pt, glassy carbon Calcon; SSA; Tiron; Calcion; ATP Ca2+; Mg2+

Graphite Heparin pH Pt Co-bis(dicarb) pH Pt HCO3

− Zn2+

Graphite-epoxy TPB− DS−

Pencil lead DS− NO3−

Glassy carbon NO3− ClO4

−

Glassy carbon ClO4− K+

Glassy carbon Fe(CN)63−/4− Ag+

Sulfonated calixarenes

PEDOT Pt, glassy carbon Heparin Ca2+; Mg2+

Glassy carbon PSS− K+

Glassy carbon Cl−; Fe(CN)63−/4− Cl−

Glassy carbon Sulfonated calixarenes; Ag+

methylsulfonated resorcarenes

PANI Carbon fi ber Cl− pH Au SO4

2− pH Glassy carbon Cl− pH Pt DS− DS−

Pt DBS− DBS−

Pt ClO4− Aniline

PPy = polypyrrole; TPB = tetraphenylborate; Arsenazo-I = 2-(o-arsenophenylazo)-1,8-dihydroxynaphtalene-3,6-disulfonic sodium salt; Calcon =[1-(1-hydroxy-2-naphthylazo)-2-naphtol-4-sulfonic]sodium salt; SSA = 1-hydroxy-4-sulfobenzoic acid; Tiron = 1,2-dihydroxy-benzene-3,5-disulfonic disodium salt; Calcion (or calcichrome) = C3OH14O22N4S6Na5; heparin = highly sulfonated glycosaminoglycan constituted by disaccharic repeating units of D-glucosamine and L-iduronic acid; co-bis(dicarb)jcobaltbis(dicarbollide) [3,3-Co(1,2-C2B9H11)]; DS = dodecylsulfate; PEDOT = poly(3,4-ethylenedioxythiophene); DBS = dodecylbenzene sulfonate.

Source: Data from Bobacka 2008.

Table 1.6. Examples of solid-state ion-selective electrodes using conducting polymers as ion-selective membranes


However, the geometric dimensions of Nafi on and its electrical properties (primarily its ionic con-ductivity) are strongly dependent on the amount of water in the polymer. Th e maximum water content, corresponding to 22 water molecules per single sulfo group of the polymer, is attained by boiling Nafi on in water, and this number decreases to 14 for the polymer in contact with a gaseous phase saturated with water vapor; the water content fl uctuates with the relative humidity (RH) of the surrounding medium (Zawodzinski et al. 1993). In general, perfl uorinated polymer membranes show high ionic conduc-tivities at high water vapor pressure (Anantaraman and Gardner 1996). Nafi on electrolyte gas sensors, because Nafi on conductivity is a function of RH, typically produce a gas response that depends on the RH (Yan and Liu 1993; Samec et al. 1995). Th is RH response is not desired for an ambient-air sensor, because the RH can change over wide limits and so it is typically either eliminated or compensated for.

Nafi on and polymer electrolytes such as sulfonated polybenzimidazole (PBI), sulfonated polyether ether ketone (S-PEEK) (Bouchet et al. 2002; Sundmacher et al. 2005), and PVA/H3PO4 (Ramesh et al. 2003) can be used in H2 sensors. Some of these solid polymer electrolytes have excellent mechanical and thermal properties and good protonic conductivity even in dry atmospheres (Rosini and Siebert 2005). Th e remarkable properties of these polymers lie in the combination of the high hydrophobicity of the perfl uorinated polymer backbone and the high hydrophilicity of the sulfonic acid branch. Th e hydro-philic branches act as a plasticizer, and the backbone retains strong mechanical properties (Colomban 1999). More detailed information about polymers used in electrochemical gas sensors can be found in various reports in the literature (Colomban 1999; Maksymiuk 2006; Korotcenkov et al. 2009).

In some cases, hydrogels or an electrolyte inside a porous matrix is used instead of a free liquid electrolyte in order to raise the viscosity, lower evaporation rates, and resist leakage of the electrolyte from sensor devices. Th e polymers or hydrogels can prevent the evaporation of electrolyte during sensor fabrication, especially in microsensor devices in which very small amounts of electrolyte are used. Using polymer electrolytes provides opportunities for the design of planar sensors and the applications of standard microelectronic fabrication technologies (Shi andAnson 1996). Polymers also allow decreasing both the size and weight of electrochemical sensors.

In addition, polymer electrolytes allow a larger range of operating temperatures for the electro-chemical sensor. Polymer-based gas sensors can operate successfully in the range from room temperature to approximately 100°C (Sakthivel and Weppner 2006). Further, compared to liquid electrolytes, solid polymer electrolytes can be used as separators in electrochemical cells, do not dissolve impurities from the gas as easily, and permit the construction of miniaturized devices that are leakproof to help avoid premature sensor failure.

Polymer-coated sensing electrode

Reference electrode

Counter electrode

Electrolyte

Figure 1.20. Electrode setup for an electrochemical sensor.


7.4.3. Examples of Polymer-Based Electrochemical Sensors

7.4.3.1. ION-SELECTIVE ELECTRODES

7.4.3.1.1. Polyvinyl Chloride–Based Sensors

Polyvinyl chloride (PVC) has a long history as a polymeric matrix in various electrochemical sensors. Ion-selective electrodes based on plasticized PVC with valinomycin (an ionophore) as neutral carrier are now widely used (Janata and Bezegh 1988; Armstrong and Horvai 1990). Th e ionophore forms a complex with the primary ion being sensed rather than with any other similar ions of the membrane. Ionophores, in most cases small molecules, must be lipophilic to remain in the polymer phase without entering into aqueous phase in contact. However, research has shown that PVC sometimes shows devia-tion due to leaching of plasticizer and ionophores. Th erefore, considerable eff orts have been made to fabricate electrodes with chemically modifi ed PVC.

A PVC-based membrane electrode was prepared by Shamsipur et al. (2001) using 3,4-di[2-(2-tetra-hydro-2H-pyranoxy)]ethoxy styrene–styrene copolymer as ionophore to successfully determine beryl-lium in a mineral sample. Th e membrane was composed of oleic acid (OA) and sodium tetraphenylbo-rate (STB) as anionic additives, and dibutyl phthalate (DBP), dioctyl phthalate (DOP), acetophenone (AP), and nitrobenzene (NB) as plasticizing solvent mediators. A membrane made of PVC:NB:I:OA in the ratio 3%:55%:10%:5% gave the best performance.

Liu et al. (2000) reported the selective determination of silver ions in electroplating wastewater using PVC membrane electrodes with 5% bis(diethyldithiophosphate) as ionophore and 65% 2-nitro-phenyl octyl ether (o-NPOE) as plasticizer.

A 7-ethylthio-4-oxa-3-phenyl-2-thioxa-1,2-dihydropyrimido[4,5-d]pyrimidine (ETPTP) iono-phore incorporating a PVC matrix membrane (Saleh et al. 2001) showed good potentiometric re-sponse for Al3+ over a wide concentration range (10−5–10−1 M). Th e sensor provided a stable response with a slope of 19.5 mV per decade for at least 1 month, and was also selective for Al3+ in the pres-ence of alkali, alkaline earth, transition, and heavy metal ions with minimal interference from Hg2+ and Pb2+.

Zhang et al. (2000) prepared a PVC membrane electrode based on chloro[tetra(m-aminophenyl) porphinato]-manganese [T(m-NH2)PPMnCl] and 2-nitrophenyl octyl ether (o-NPOE) in the composi-tion 3:65:32 [T(m-NH2)PPMnCl:o-NPOE:PVC] to determine fl uoroborate in electroplating solution. A Hg(II) ion-selective PVC membrane sensor based on ethyl-2-benzoyl-2-phenylcarbamoyl acetate (EBPCA) was developed by Hassan et al. (2000), which showed selectivity for Hg(II) ion in comparison with alkali, alkaline earth, transition, and heavy metal ions. Th e sensor was applied for the determina-tion of Hg(II) content in some amalgam alloys.

High selectivity for Ag+ ions over Na+, K+, Ca2+, Sr2+, Pb2+, and Hg2+ was observed with a PVC membrane containing bis-pyridine tetramide macrocycle (Mahajan and Parkash 2000). Hassan et al. (2001) described a novel uranyl ion–responsive sensor based on a uranyl PVC matrix membrane con-taining tris(2-ethylhexyl) phosphate (TEHP) as both the electroactive material and plasticizer, and sodium tetraphenylborate (NaTPB) as an ion discriminator. Th e sensor displayed a rapid and linear response for UO2

2+ ions over a wide concentration range in a pH range of 2.8–3.6 with a life span


of 4 weeks. Th ey also reported another PVC-based uranyl sensor containing o-(1,2-dihydro-2-oxo-1-pyridyl)-N,N,N,N-bis(tetramethylene) uranium hexafl uorophosphate (TPTU) as a sensing material, sodium tetraphenyl borate as an ion discriminator, and dioctyl phenylphosphonate (DOPP) as a plas-ticizer. Linear and stable response was obtained with the sensor in a working pH range of 2.5–3.5 and over a life span of 6 weeks.

Gorton and Fiedler (1977) developed a zinc-sensitive polymeric membrane electrode to detect the end point during potentiometric titration of Zn2+ against ethylenediamminetetraacetic acid (EDTA). Th e membrane, having (by weight) 8% ligand [zinc salt of di-n-octylphenylphosphoric acid (HDOPP)], 62% solvent (di-octylphenylphosphonate (DOPP-n), and 30% polymer (PVC) composition, was found to have at least a 3-month lifetime.

A potentiometric method was described by Abbas et al. (2000) for the determination of cetylpyri-dinium (CP) cation using a PVC powder membrane sensor as an end-point indicator electrode in potentiometric titration of some anions, which was applied for the determination of anionic surfactants in some commercial detergents and waste water. Mousavi et al. (2000) constructed a PVC membrane nickel(II) ISE using 1,10-dibenzyl-1,10-diaza-18-crown-6 (DbzDA18C6) as a neutral carrier that ex-hibited relatively good selectivity for Ni(II) over a wide variety of other metal ions, and could be used in a pH range of 4.0–8.0.

Wroblewski et al. (2000) investigated the infl uence of the membrane components on phosphate selectivity by anion-selective PVC plasticized membranes containing uranyl salophene derivatives. Th e highest H2PO4 selectivity over other anions tested was obtained for lipophilic uranyl salophene III in PVC/o-nitrophenyl octylether membrane containing 20 mol% of tetradecylammonium bromide.

7.4.3.1.2. Other Polymers

Other polymers besides PVC can also be used to design eff ective ion-selective electrodes. For example, a calcium ion-sensitive electrochemical sensor device (Artigas et al. 2001) was fabricated with a photocur-able polymer membrane based on aliphatic diacrylated polyurethane instead of PVC to measure calcium activity in water samples extracted from agricultural soils. Th e authors claimed their results to be well correlated with those obtained by standard methods.

Torres et al. (2001) developed fi ve diff erent types of membranes by solubilizing poly(ethylene-co-vinyl-acetate) copolymer (EVA) and tri-caprylyl-trimethyl-ammonium chloride (Aliquat-336S) in chloroform without using any plasticizer, followed by fi lm casting, for the detection of iodide, periodate, perchlorate, salicylate, and nitrate ions. Th e membrane performance was in the concentration range 10−5–10−1 mol L−1 at steady state.

Gupta and Mujawamariya (2000) found a signifi cant dependence of sensitivity, working range, response time, and metal-ion interference on the concentration of ionophore, plasticizer, and molecular weight of cyanocopolymers for a Cd(II) ISE based on a cyanocopolymer matrix and 8-hydroxyquino-line as ionophore. Th e cyano groups of the copolymers contributed signifi cantly to enhance the Cd(II) selectivity of the electrode for Cd2+ ions in the presence of alkali and alkaline-earth metal ions in the pH range 2.5–6.5. Th e selectivity coeffi cients for cesium ion over alkali, alkaline-earth, and ammonium ions were also determined.


Polysiloxane membranes and membranes with polyHEMA hydrogel intermediate fi lm were de-signed by Sudhölter et al. (1989). A modifi ed silicone rubber–based membrane was used for detection of Na+ in body fl uids (Tsujimura et al. 1995). Teixeira et al. (2000) used a MnO2-based graphite–epoxy electrode for determination of lithium ions by the potentiometric method. In that study the best potentio metric response of 30-s response time and 6 months lifetime was obtained for an electrode composition of 35% -MnO2, 15% graphite, and 50% epoxy resin.

A lipophilic acrylate resin as a matrix for the sensing membrane with long-term stability was de-veloped by Numata et al. (2001) to determine water hardness by analyzing Ca2+ and Mg2+ with equal selectivity. Th e performance of the electrode was maintained during a test of the electrode conducted in tap water at a continuous fl ow rate of 4 ml min−1 with long-term stability of the electrode for 1 year due to the strong affi nity of 1-decylalcohol for the lipophilic acrylate resin.

A new Ca2+-selective polyaniline (PANI)-based membrane was also developed (Lindfors and Ivaska 2001) for all-solid-state sensor applications. Th e membrane contained electrically con-ducting PANI as the matrix polymer, bis[4-(1,1,3,3-tetramethylbutyl) phenyl] phosphoric acid (DTMBP-PO4H), dioctyl phenylphosphonate (DOPP), and cationic (tridodecylmethylammo-nium chloride, TDMACl) or anionic [potassium tetrakis(4-chlorophenyl) borate, KTpClPB] as lipophilic additives.

7.4.3.2. pH SENSORS

Since the solution pH has a signifi cant eff ect on chemical reactions, the measurement and control of pH is very important in chemistry, biochemistry, clinical chemistry, and environmental science. Th e pH, of course, indicates the amount of hydrogen ions in a solution. A pH-sensitive layer was obtained by reacting aminoethylcellulose fi bers with 1-hydroxy-pyrene-3,6,8-trisulfochloride, fol-lowed by attachment of the sensitive layer to the surface of a polyester foil, and embedding the composite in an ion-permeable polyurethane (PU)–based hydrogel material. Hydrogen ion-selective solid-contact electrodes based on N,N’-dialkylbenzylethylenediamine (alkyl = butyl, hexyl, octyl, decyl) were also prepared. Solid-contact electrodes and coated-wire electrodes were fabricated from polymer cocktail solutions based on N,N’-dialkylbenzylethylenediamine (alkyl = butyl, hexyl, octyl, decyl). Th e response ranges and slopes were infl uenced by the alkyl chain length. Th e solid-contact electrodes showed linear selectivity to hydrogen ion in the pH ranges 4.5–13.0, 4.2–13.1, 3.4–13.0, and 3.0–13.2. Stability was also improved in comparison to coated-wire electrodes (Han et al. 2001).

Pandey et al. (2000) developed a solid-state poly(3-cyclohexyl)thiophene-treated electrode as a pH sensor and, subsequently, a urea sensor. Later, Pandey and Singh (2001) reported the pH-sensing func-tion of a polymer-modifi ed electrode (a novel pH sensor) in both aqueous and nonaqueous media. Th e sensor was prepared with a polymer-modifi ed electrode by electrochemical deposition of PANI in dry acetonitrile containing 0.5 M tetraphenyl borate at 2.0 V versus Ag/AgCl.

Han et al. (2001) have shown that N,N’-didecylbenzylethylenediamine–based solid-contact electrodes can also be successfully used for design selectivity and a reproducible potentiometric pH sensor.


7.4.3.3. ELECTROCHEMICAL GAS SENSORS

7.4.3.3.1. Nitric Oxide Sensors

Ichimori et al. (1994) introduced a commercial amperometric NO-selective electrode. Th e Pt/Ir (0.2) electrode was modifi ed with a NO-selective nitrocellulose membrane and a silicone rubber outer layer. Th e electrode showed linear response at nanomolar concentration with a time constant of 1.5 s. A three-fold increase in sensitivity was achieved by raising the temperature from 26°C to the physiological value of 37°C. In an in vivo application, the electrode was used for the measurement of NO in rat aortic rings under acetylcholine stimulation.

Friedemann et al. (1996) used a carbon-fi ber electrode modifi ed with an electrodeposited o- phenylenediamine coating. A Nafi on underlayer with this modifi ed electrode provided good sensitivity to NO, and a three-layer overcoat of the Nafi on optimized selectivity against nitrite. Using a polystyrene fi lm, Christensen et al. (1993) developed a NO2-sensing device. Upon exposing the fi lm to a 1:10 v/v mixture of NO2/N2, the conductivity of the fi lm increased irreversibly and rapidly by several orders of magnitude. It was believed that the increase in conductivity of the fi lm was due to self-ionization of N2O4 to NO+NO3

−. Ho and Hung (2001) prepared a Pt/Nafi on electrode–based amperometric gas sensor for NO2 de-

tection with a concentration range from 0 to 485 ppm. Reticulated vitreous carbon (RVC) (Hrncirova et al. 2000) was used as the indicator electrode in solid-state gas sensors. A typical sensor contained a RVC indicator, a platinum auxiliary, and a Pt/air reference electrode, with a SPE of 10% PVC, 3% tetra-butylammonium hexafl uorophosphate (TBAHFP), and 87% 2-nitrophenyloctyl ether (NPOE). Th e gaseous nitrogen dioxide analyte in air was monitored by reduction at 500 mV versus a Pt/air electrode. Th e RVC was found to successfully replace noble metals in solid-state gas sensors. With hydrophobic SPEs, the sensitivity decreases with increasing humidity, while with hydrophilic ones (e.g., Nafi on), it increases. Due to the extraordinary chemical inertness, the signal stability of RVC is aff ected not only for detection in solution, but also in sensors in which RVC remains in contact with a SPE.

Mizutani et al. (2001) fabricated amperometric gas sensors for the measurement of dissolved oxy-gen and nitric oxide at lower concentrations using a permselective polydimethylsiloxane (PDMS) mem-brane at room temperature.

7.4.3.3.2. Other Gases

Ammonia and carbon dioxide gas-sensing systems in both static and continuous fl ow conditions were prepared using polymer-membrane pH electrodes as internal sensing elements (Opdycke et al. 1983). Th e pH-responsive polymer membranes were prepared with plasticized PVC by incorporating trido-decylamine as the neutral carrier. It was suggested that for miniature static gas sensors, the internal polymer pH electrode could be made with or without an internal reference solution. In the latter case, the polymeric membrane was coated directly onto a graphite substrate. Response times and reproduc-ibility of these new gas-sensing systems were evaluated using optimized internal electrolytes, fl ow rates, and gas-permeable membrane materials.


Opekar and Bruckenstein (1984) determined gaseous hydrogen sulfi de by cathodic stripping vol-tammetry after preconcentration on a silver metalized porous Tefl on membrane electrode. Th e repro-ducibility of the determination, expressed in terms of the relative standard deviation, was 3.2%. SPE ion-exchange membrane–supported porous silver electrodes were used in an electrochemical sensor for the detection of trace hydrogen sulfi de in gaseous samples (Schiavon et al. 1995). Th e high sensitivity and fast response of this sensor was due to the elimination of oxygen interference. One side of this sensor faced the sample and the other side of the membrane faced an internal electrolyte solution containing the counter and reference electrodes. Th e sensor functioned by electroanalysis of H2S by amperometric monitoring, cathodic stripping measurements, and fl ow injection analysis.

Shi et al. (2001) reported an electrochemical sensor for the detection of sulfur dioxide in both gas and solution. Th e sensor was constructed by chemical modifi cation of the electrode by polymerizing 4-vinyl pyridine (4-VP) onto a palladium and iridium oxide (PVP/Pd/IrO2)–coated platinum micro-electrode, which exhibited excellent catalytic activity toward sulfi te with an oxidation potential of +0.50 V. In this SO2 gas sensor, the PVP/Pd/IrO2-modifi ed electrode functioned as the detecting electrode, a Ag/AgCl electrode as reference electrode, Pt as counter electrode, and a porous fi lm in direct contact with the gas-containing atmosphere. Th e sensor was found to have high current sensitivity, a short re-sponse time, and good reproducibility for the detection of SO2, and it showed good potential for use in the fi eld of environmental monitoring and control.

Otagawa et al. (1990) fabricated a planar miniaturized electrochemical CO sensor that was com-prised of three Pt electrodes (sensing, counter, and reference) and a solution-cast Nafi on as solid polymer

Figure 1.21. Induced voltage signals at different ethanol concentrations using Nafi on membrane on Pt/C electrode. (Reproduced with permission from Kim et al. 2000. Copyright 2000 Elsevier.)


electrolyte. Th e sensor showed a linear response to the CO concentration in air with a sensitivity of 8 Pa/ppm and 70 s response time.

Methane gas was determined via pre-adsorption on a dispersed Pt electrode backed by a SPE mem-brane (Nafi on) in contact with 10 M sulfuric acid (Jacquinot et al. 2001). Th e adsorption process was strongly temperature-dependent, with an activation energy of 8.7 kcal mol−1. Th e measurement of other hydrocarbons, viz., ethane, propane, and butane, was also shown to be possible using this sensor, with a signifi cant reduction of cross-sensitivity to carbon monoxide and hydrogen by means of suitable chemi-cal adsorption fi lters.

Fuel cells using a polymer electrolyte membrane were fabricated and tested by Kim et al. (2000) for the measurement of ethanol gas concentration. Th e polymer electrolyte (Nafi on 115 membrane) and 10% Pt/C sheets with 0.5 mg/cm2 Pt loading were used as catalyst electrodes. In Figure 1.21, the voltage responses of the fuel cell for various ethanol concentrations are shown at a load resistance of 100 Ω. Kim et al. (2000) correlated the ethanol concentration with the peak height of the signals instead of the area because the shape of voltage signals is the same even at diff erent concentrations (see Figure 1.21).

Analytical performance of a potentiometric pCO2 (partial pressure of CO2) sensing catheter was reported by Opdycke and Meyerhoff (1986). Th e sensing catheter consisted of an inner tubular PVC pH electrode in conjunction with an outer gas-permeable silicone rubber tube. Th e results of conven-tional blood-gas instruments correlated well with the continuous pCO2 values obtained by the sensor during 6 h of in vitro blood pump studies. Based on the preliminary results of study with this sensor, the suitability of this catheter was demonstrated by intravascular implantation in a dog for continuous in vivo monitoring of pCO2.

7.5. CHEMICALLY SENSITIVE FET-BASED SENSORS

As we have shown, conductometric polymer-based sensors possess good sensitivity to various gases and organic vapors, but they can suff er from temperature and humidity dependence. Work using electro-chemically deposited polypyrrole as a fi eld-eff ect transistor (FET) gate material has demonstrated devices which can possess better operating characteristics. Such a combination of technologies could lead to a new generation of low-cost, low-power gas sensors for hand-held monitors (Persaud 2005). Polymers used in chemically sensitive FET (CHEMFET)–based sensors are listed in Table 1.7.

7.5.1. Insulated-Gate FETs

Chemically sensitive insulated-gate fi eld-eff ect transistors (IGFETs) are a class of sensors which contain a polymer gate as detecting element on a silicon base (Janata and Josowicz 2003). In IGFETs, the gate contact is on the surface of the device and is the sensing part that is exposed to the analyte. Th e proper-ties of the polymer determine the selectivity of such devices. Th e buried inorganic semiconductor, where the fi eld-eff ect transport takes place, normally is not exposed to the analyte. A simplifi ed IGFET setup is shown in Figure 1.22 (Reese et al. 2004).


Th e IGFET consists of a pair of electrodes forming contacts (top or bottom contact) to the con-ducting polymer, deposited on an insulating substrate; under a constant current, the resulting potential diff erence at the electrodes becomes the response signal. By interacting with the analyte, the detecting element changes the semiconductor work functions (M − S) of the gate system. Th e transduction principle is based on the linear relationship between the transistor threshold voltage (Vt

IGFET) and the gate-metal/semiconductor work-function off set (Bergveld 2003):

( )IGFETt M SV µ f -f

Th e sensitivity and selectivity of the device depends on the nature of the gate materials and the operat-ing temperature (usually between 50 and 200°C). Incorporation of small analyte receptor molecules into the polymeric semiconductor layer by chemical and physical methods causes a change in the gate of IGFETs.

Several intrinsically conducting polymers (ICPs) have been successfully employed as IGFET gate layers (Janata and Josowicz 1998). For example, PANI-based FET devices have been demonstrated by Potje-Kamloth et al. (2002). Th e morphology of the polymeric semiconductor layer is important to the sensing behavior, so it is necessary to fi nd an optimum semiconductor morphology that will interact well with the analyte.

POLYMER RECOGNITION MECHANISM CHEMICAL ANALYTE REFERENCES

Polythiophene and its Alkyl or alkoxy side Ammonia, water vapor, Assadi et al. 1990; derivatives chains chloroform, alcohols, Ohmori et al. 1991; ketones, thiols, nitriles, Crone et al. 2001; esters, ring compounds, Torsi et al. 2003a, 2003b alkanes

Poly(phenylene Enantioselective pendant Volatile chiral molecules Tanese et al. 2004ethynylene) groups

Poly(3-hexylthiophene) Proton-sensitive dielectric H+ Bartic et al. 2002, 2003; layer Loi et al. 2005

Polypyrrole Membrane H+ Nishizawa et al. 1992

Poly(3-methylthiophene) Platinum particles H+, IrCl6, O2, H2 Th ackeray et al. 1985; Th ackeray and Wrighton 1986

Polyaniline Moisture-sensitive solid-state H+, Ru(NH3)63+/2+, Paul et al. 1985;

electrolyte, crown ether Fe(CN)6, water vapor, Chao and Wrighton 1987; SO2, K+ Hoa et al. 1992 ; Dabke et al. 1997; Nilsson et al. 2002

Source: Data from Mabeck and Malliaras 2006.

Table 1.7. Polymers used in CHEMFETs


A wide variation in the work function of ICPs is possible due to the tailorability and porous mor-phology of ICPs, which helps easy permeation of analyte molecules and alters the work function of the polymer/gate dielectric system.

One of the major advantages of ICP-based IGFET sensors is the ability to detect changes in voltage (Vt) by passing a negligible current. It has also been established that ICP-based IGFETs are more stable than chemiresistors in terms of signal-to-noise ratio and very high detection limits (ppm level) (Janata and Josowicz 2003).

7.5.2. Polymeric FETs

Another variant of FET-based devices which is interesting for chemical sensor design is polymeric fi eld eff ect transistors (PolyFETs) (Covington et al. 2004; Dodabalapur 2006). In this type of CHEMFET device, all the elements are fabricated from polymeric materials. Th e interaction of an analyte with a polymer layer of the PolyFET directly aff ects the conductive channel, where sensing events occur at the gate or gate/insulator boundary and indirectly modulate the current by capacitive coupling.

Figure 1.22. Schematic presentation of a simplifi ed CHEMFET. (Reproduced with permission from Reese et al. 2004. Copyright 2004 Elsevier.)


Polymer-based FET devices are promising on account of their lower power dissipation and ease of circuit design. Th e fi rst polymer large-scale integrated circuits were implemented using this circuit approach. PolyFET backplanes are ideally suited for electronic paper applications and other display schemes due to their low cost and processing advantages. Polymer materials require no epitaxial templat-ing, and most PolyFET processes are simple, with low thermal energy requirements, and are compatible with a range of substrates.

In fact, among polymeric semiconductors with higher charge carrier mobilities, π-conjugated lad-der polymers have the best prospects for application in PolyFETs (Torsi et al. 2000; Babel and Jenekhe 2003). In particular, Torsi et al. (2003, 2004) have developed gas-sensitive organic thin-fi lm transistors in which alkyl- and alkoxy-substituted regioregular polythiophenes, such as dipentoxy-substituted poly-terthiophene, are used as active layers, and a set of volatile organic compounds carrying diff erent chemi-cal functionalities is employed as analytes. It was shown that rapid and reversible responses of PolyFETs with remarkable response repeatability are obtainable for a series of alcohol vapors (Crone et al. 2001; Torsi et al. 2000, 2003, 2004).

An introduction to PolyFET principles and history, as well as state-of-the-art organic semiconduc-tor structure and performance of PolyFETs, is available in reviews (Facchetti 2007; Kymissis 2009). Th e operation mechanism of the transistors is explained in terms of bulk conductivities and fi eld-eff ect mo-bilities along with temperature dependencies on the basis of variable-range hopping for heavily doped systems and polaronic thermally activated transport for lightly doped systems. Th is is a consequence of the density of states of conjugated systems, which changes dynamically upon introduction of charge either by a fi eld eff ect or through doping (Brown et al. 1997). Th erefore, the charge transport inside the detecting layer in terms of both holes and electrons in conjugated polymer semiconductors is important for PolyFET device applications.

When the detecting element, i.e., the conducting polymer, interacts with gaseous species, it can act as either an electron donor or an electron acceptor, depending on the oxidizing or reducing character of the gas on that particular conducting polymer (see Table 1.3). As the charge carriers in conjugated polymers are a measure of the mobility of holes and electrons, their injection by the analyte may cause the change in the overall voltage of the device. Because of this interaction, both the electron mobility and the carrier number varies, and their relative contribution to the overall change of conductivity is observed in the PolyFET device. Th is change in conductivity at the electrode/conducting polymer con-tacts can be attributed to modulation of the height of the Schottky barrier, which is determined by the diff erence in work function or potential of the organic semiconductor and the metal.

7.5.3. Future Trends in Design of Polymer-Based Chemically Sensitive FET Sensors

Th e fi rst PolyFET sensors were proposed in 1986–1987 (Tsumura et al. 1986; Laurs and Heiland 1987). However, despite the availability of a wide range of sensing materials and sensor technologies, com-mercial implementation of a sensitive, selective, reliable, and inexpensive portable system based on PolyFETs and IGFETs for detecting volatile analytes is still a major issue today. Although many diff erent types of gas sensors have been developed, only a few of them seem to satisfy the minimum requirements


for portable sensing instruments capable of performing fast and in situ detection of volatile analytes. In addition to sensitivity and selectivity robustness, low power consumption and compact size of sensor devices have yet to be achieved. Facchetti (2007) believes that the problems lie with the stability and processability of the conducting polymer. To achieve the required parameters in PolyFETs, Facchetti (2007) proposes the following activities.

1. Realization of high carrier mobility. Th e fi eld-eff ect mobilities of electrons or holes could be increased by doping, due to the underlying hopping transport of charge, which results in an increase in bulk conductivity.

2. Design of environmentally stable semiconductors. It is also well known that the surface conduc-tivity of the conducting polymer is aff ected by water vapor or moisture, so moisture is the most common interferant for polymer-based devices operating at room temperature;

3. Achievement of high FET performance from solution-deposited semiconducting fi lms. 4. Achievement of high performance using inexpensive processes for polymer circuit fabrication,

such as those employed in the graphic arts industry. 5. Addressing the fundamental FET operational stability parameters related to charge transport and

trapping.

8. OUTLOOK

Th is chapter has shown that polymers possess unique properties that make them interesting and impor-tant materials for chemical sensing. A large number of various types of chemical sensing devices have been explored through the tireless eff orts of scientists and researchers. Researchers are trying hard to make polymers as sensor materials to replace the commercially available inorganic semiconductors. So far, however, commercial polymer-based sensors are not yet available, because there are still some serious unresolved problems in using polymers as sensing materials in sensor devices. Recent results, however, indicate that many such sensors are now moving from the research stage to potentially commercial phases. It can be predicted that the impact of conducting polymer materials on chemical sensing will depend on the cost of the sensors themselves and on the ability to fabricate sensors that are repeatable, robust, and able to operate reliably in real environments.

Understanding sensor devices, sensor materials, sensing analytes, and mechanisms of detection can be helpful for variety of academic areas. Such knowledge might result from more collaboration among researchers in diff erent fi elds, e.g., physics, chemistry, engineering, biology and biochemistry, materials science, and others. Th e interdisciplinary nature of sensor technology makes progress diffi cult for any single discipline and requires interdisciplinary scientists and engineers to work together on complex goals to develop useful physical and chemical/biochemical sensors.

9. ACKNOWLEDGMENTS

We would like to express our gratitude to the Ghenadii Korotcenkov for his valuable scientifi c and tech-nical suggestions and his contribution in preparing this chapter.


REFERENCES

Abbas M.N., Mostafa G.A.E., and Homoda A.M.A. (2000) Cetylpyridiniumiodomercurate PVC membrane ion selective electrode for the determination of cetylpyridinium cation in Ezafl uor mouth wash and as a detector for some potentiometric titrations. Talanta 53, 425–432.

Adhikari B. and Majumdar S. (2004) Polymers in sensor applications. Prog. Polym. Sci. 29, 699–766.Agbor N.E., Creswell J.P., Petty M.C., and Monkman A.P. (1997) An optical gas sensor based on polyaniline

Langmuir-Blodgett fi lms. Sens. Actuators B 41, 137–145.Albert K. J., Lewis N.S., Schauer C.L., Sotzing G.A., Stitzel S.E., Vaid T.P., and Walt D.R. (2000) Cross-reactive

chemical sensor arrays. Chem. Rev. 100(7), 2595–2626.Allara D.L. (1995) Critical issues in applications of self-assembled monolayers. Biosens. Bioelectron. 10(9–10),

771–783.Amao Y., Asai K., and Okura I. (2000) Fluorescence quenching oxygen sensor using an aluminum phthalocyanine–

polystyrene fi lm. Anal. Chim. Acta 407, 41–44.Amao Y., Ishikawa Y., and Okura I. (2001) Green luminescent iridium(III) complex immobilized in fl uoropolymer

fi lm as optical oxygen-sensing material. Anal. Chim. Acta 445, 177–182.Amrani M.E.H., Payne P.A., and Persaud K.C. (1996) Multi-frequency measurements of organic conducting poly-

mers for sensing of gases vapours. Sens. Actuators B 33, 137–141.Amrani M.E.H., Ibrahim S., and Persaud K.C. (1993) Synthesis, chemical characterisation and multifrequency

measurements of poly N-(2-pyridyl) pyrrole for sensing volatile chemicals. Mater. Sci. Eng. C 1, 17–22.Anantaraman A.V. and Gardner C L. (1996) Studies on ion-exchange membranes. Part 1. Eff ect of humidity on the

conductivity of Nafi on®. J. Electroanal. Chem. 414, 115–120.Ando M., Swart C., Pringsheim E., Mirsky V.M., and Wolfbeis O.S. (2002) Optical ozone detection by use of

polyaniline fi lm. Solid State Ionics 152–153, 819–822.Ando M., Swart C., Pringsheim E., Mirsky V.M., and Wolfbeis O.S. (2005) Optical ozone-sensing properties of

poly(2-chloroaniline), poly(N-methylaniline) and polyaniline fi lms. Sens. Actuators B 108, 528–534.Anitha G. and Subramanian E. (2005) Recognition and exposition of intermolecular interaction between CH2Cl2

and CHCl3 by conducting polyaniline materials. Sens. Actuators B 107, 605–615.Armstrong R.D. and Horvai G. (1990) Review article: Properties of PVC based membranes used in ion-selective

electrodes. Electrochim. Acta 35(1), 1–7.Arn D., Blom N., Dubler-Studle K., Graber N., and Widmer H.M. (1991) Surface acoustic wave gas sensors:

Applications in the chemical industry. Sens. Actuators A 26, 395–397.Artigas J., Beltran A., Jimenez C, Bartroli J., and Alonso J. (2001) Development of a photopolymerisable membra-

ne for calcium ion sensors. Application to soil drainage waters. Anal. Chim. Acta 426, 3–10.Assadi A., Gustafsson G., Willander M., Svensson C., and Inganas O. (1990) Determination of fi eld-eff ect mobility

of poly(3-hexylthiophene) upon exposure to NH3 gas. Synth. Met. 37(1–3), 123–130.Athawale A.A. and Chabukswar V.V. (2001) Acrylic acid-doped polyaniline sensitive to ammonia vapors. J. Appl.

Polym. Sci. 79, 1994–1998.Athawale A.A. and Kulkarni M.V. (2000) Polyaniline and its substituted derivatives as sensors for aliphatic alcohols.

Sens. Actuators B 67, 173–177.Athawale A.A., Bhagwat S.V., and Katre P.P. (2006) Nanocomposite of Pd-polyaniline as a selective methanol sen-

sor. Sens. Actuators B 114, 263–267.Babel A. and Jenekhe S.A. (2003) High electron mobility in ladder polymer fi eld-eff ect transistors. J. Am. Chem.

Soc. 125, 13656–13657.Bade K., Tsakova V., and Schultze J.W. (1992) Nucleation, growth and branching of polyaniline from microelec-

trode experiments. Electrochim. Acta 37(12), 2255–2261.


Bai H. and Shi G. (2007) Gas sensors based on conducting polymers. Sensors 7, 267–307.Bailey R.C. and Hupp J.T. (2003) Micropatterned polymeric gratings as chemoresponsive volatile organic com-

pound sensors: Implications for analyte detection and identifi cation via diff raction-based sensor arrays. Anal. Chem. 75, 2392–2398.

Bakker E. and Meyerhoff M.E. (2000) Ionophore-based membrane electrodes: New analytical concepts and non-classical response mechanisms. Anal. Chim. Acta 416, 121–137.

Bakker E. (2004) Electrochemical sensors. Anal. Chem. 76, 3285–3298.Barbetta A., Carnachan R.J., Smith K.H., Zhao C.T., Cameron N.R., Kataky R., Hayman M., Przyborski S.A., and

Swan M. (2005) Porous polymers by emulsion templating. Macromol. Symp. 226(1), 203–212.Bartic C., Palan B., Campitelli A., and Borghs G. (2002) Monitoring pH with organic-based fi eld-eff ect transistors.

Sens. Actuators B 83, 115–122.Bartic C., Campitelli A., and Borghs S. (2003) Field-eff ect detection of chemical species with hybrid organic/inor-

ganic transistors. Appl. Phys. Lett. 82, 475–477.Bartlett P.N. and Ling-Chung S.K. (1989) Conducting polymer gas sensors part II: Response of polypyrrole to

methanol vapour. Sens. Actuators 19, 141–150.Bay L., Mogensen N., Skaarup S., Sommer-Larsen P., Jorgensen M., and West K. (2002) Polypyrrole doped with

alkyl benzenesulfonates. Macromolecules 35, 9345–9351.Bergveld P. (2003) Th irty years of ISFETOLOGY: What happened in the past 30 years and what may happen in

the next 30 years? Sens. Actuators B 88, 1–16. Bidan G. (1992) Electroconducting conjugated polymers: New sensitive matrices to build up chemical or electro-

chemical sensors: A review. Sens. Actuators B 6, 45–56.Bissell R.A., Persaud K.C., and Travers P. (2002) Th e infl uence of nonspecifi c molecular partitioning of ana-

lytes on the electrical responses of conducting organic polymer gas sensors. Phys. Chem. Chem. Phys. 4, 3482–3491.

Blum P., Mohr G.J., Matern K., Reichert J., and Spichiger-Keller U.E. (2001) Optical alcohol sensor using lipophi-lic Reichardt’s dyes in polymer membranes. Anal. Chim. Acta 432, 269–275.

Bobacka J. (2006) Conducting polymer-based solid-state ion-selective electrodes. Electroanalysis 18(1), 7–18.Bouchet R., Rosini S., Vitter G., and Siebert E.A. (2002) Solid-State potentiometric sensor based on polybenzimi-

dazole for hydrogen determination in air. J. Electrochem. Soc. 149, H119–H122.Brook T.E. and Narayanaswamy R. (1998) Polymeric fi lms in optical gas sensors. Sens. Actuators B 51, 77–83.Brown A.R., Jarrett C.P., de Leeuw D.M., and Matters M. (1997) Field-eff ect transistors made from solution-

processed organic semiconductors. Synth. Met. 88, 37–55.Bruce P.G. (1995) Solid State Electrochemistry. Cambridge University Press, Cambridge, UK. Bryce M.R., Chissel A., Kathirgamanathan P., Parker D., and Smith N.R.M. (1987) Soluble, conducting polymers

from 3-substituted thiophenes and pyrroles. J. Chem. Soc. Chem. Commun. 6, 466–467.Buff W. (1992) SAW sensors. Sens. Actuators A 30, 117–121.Burgi L., Sirringhaus H., and Friend R.H. (2002) Noncontact potentiometry of polymer fi eld-eff ect transistors.

Appl. Phys. Lett. 80, 2913–2915.Cadogan A., Gao Z., Lewenstam A., Ivaska A., and Diamond D. (1992) All-solid-state sodium-selective electrode

based on a calixarene ionophore in a poly(vinyl chloride) membrane with a polypyrrole solid contact. Anal. Chem. 64(21), 2496–2501.

Cataldo F. and Maltese P. (2002) Synthesis of alkyl and N-alkyl-substituted polyanilines: A study on their spectral properties and thermal stability. Eur. Polym. J. 38, 1791–1803.

Chabukswar V.V., Pethkar S., and Athawale A.A. (2001) Acrylic acid doped polyaniline as an ammonia sensor. Sens. Actuators B 77, 657–663.

Chao S. and Wrighton M.S. (1987) Characterization of a solid-state polyaniline-based transistor: Water vapor


dependent characteristics of a device employing a poly(vinyl alcohol)/phosphoric acid solid-state electrolyte. J. Am. Chem. Soc. 109(22), 6627–6631.

Charlesworth J. M., Partridge A.C., and Garrard N. (1993) Mechanistic studies on the interactions between poly(pyrrole) and organic vapors. J. Phys. Chem. 97(20), 5418–5423.

Chen Z., Xue C.H., Shi W., Luo F.T., Green S., Chen J., and Liu H.Y. (2004) Selective and sensitive fl uorescent sensors for metal ions based on manipulation of side-chain compositions of poly(p-phenyleneethynylene)s. Anal. Chem. 76(21), 6513–6518.

Chiang J.C. and MacDiarmid A.G. (1986) Polyaniline: Protonic acid doping of the emeraldine form to the metallic regime. Synth. Met. 13, 193–205.

Christensen W.H., Sinha D.N., and Agnew S.F. (1993) Conductivity of polystyrene fi lm upon exposure to nitrogen dioxide: A novel NO2 sensor. Sens. Actuators B 10, 149–153.

Colomban Ph. (1999) Latest developments in proton conductors. Ann. Chim. Sci. Mater. 24, 1–18.Covington J.A., Gardner J.W., Bartlett P.N., and Toh C.-S. (2004) Conductive polymer gate FET devices for va-

pour sensing. In: IEE Proc.—Circuits Devices Syst. 151(4), 326–334.Crone B., Dodabalapur A., Gelperin A., Torsi L., Katz H.E., Lovinger A.J., and Bao Z. (2001) Electronic sensing

of vapors with organic transistors. Appl. Phys. Lett. 78, 2229–2231.Cruz C.M.G.S. and Ticianelli E.A. (1997) Electrochemical and ellipsometric studies of polyaniline fi lms grown

under cycling conditions. J. Electroanal. Chem. 428(1–2), 185–192.Cumming C.J., Fisher M., and Sikes J. (2004) Amplifying fl uorescent polymer arrays for chemical detection of

explosives. In: Gardner W. and Yinon J. (eds.), Electronic Noses and Sensors for the Detection of Explosives. Kluwer Academic Publishers, Dordrecht, Th e Netherlands, 53–70.

Cumming C.J., Aker C., Fisher M., Fox M., La Grone M.J., Reust D., Rockley M.G., Swager T.M., Towers E., and Williams V. (2001) Using novel fl uorescent polymers as sensory materials for above-ground sensing of chemical signature compounds emanating from buried landmines. IEEE Trans. Geosci. Remote Sensing 39(6), 1119–1128.

Cunningham A.J. (1998) Introduction to Bioanalytical Sensors. John Wiley, New York, 95.Dabke R.B., Singh G.D., Dhanabalan A., Lal R., and Contractor A.Q. (1997) An ion-activated molecular electro-

nic device. Anal. Chem. 69(4), 724–727.Dao L.H., Leclerc M., Guay J., and Chevalier J.W. (1989) Synthesis and characterization of substituted poly(anilines).

Synth. Met. 29, 377–382.Debarnot D.N. and F.P. Epaillard (2003) Polyaniline as a new sensitive layer for gas sensors. Anal. Chim. Acta 475,

1–15.Dejous C., Rebiere D., Pistre J., Tiret C., and Planade R. (1995) A surface acoustic wave gas sensor: Detection of

organophosphorus compounds. Sens. Actuators B 24, 58–61.Demarcos S. and Wolfbeis O.S. (1996) Optical sensing of pH based on polypyrrole fi lms. Anal. Chim. Acta 334,

149–153.Densakulprasert N., Wannatong L., Chotpattananont D., Hiamtup P., Sirivat, A., and Schwank, J. (2005) Electrical

conductivity of polyaniline/zeolite composites and synergetic interaction with CO. Mater. Sci. Eng. B—Solid State Mater. Adv. Technol. 117, 276–282.

Desmonts L.B., Reinhoudt D.N., and Calama M.C. (2007) Design of fl uorescent materials for chemical sensing. Chem. Soc. Rev. 36, 993–1017.

Dias C., Das-Gupta D.K., Hinton Y., and Shuford R.J. (1993) Polymer/ceramic composites for piezoelectric sen-sors. Sens. Actuators A 37, 343–347.

Diaz A.F., Kanazawa K.K., and Gardini G.P. (1979) Electrochemical polymerization of pyrrole. J. Chem. Soc., Chem. Commun. 14, 635–636.

Diaz A.F., Castillo J., Kanazawa K.K., Logan J.A., Salmon M., and Fajardo O. (1982) Conducting poly-N-alkyl-pyrrole polymer fi lms. J. Electroanal. Chem. 133(2), 233–239.


Dickinson T.A., White J., Kauer J.S., and Walt D.R. (1996) A chemical-detecting system based on a cross-reactive optical sensor array. Nature, 382, 697–700.

Dixit V., Misra S.C.K., and Sharma B.S. (2005) Carbon monoxide sensitivity of vacuum deposited polyaniline semiconducting thin fi lms. Sens. Actuators B, 104, 90–93.

Dodabalapur A. (2006) Organic and polymer transistors for electronics. Mater. Today 9(4), 24–30.Drain C.M. and Batteas J.D. (2004) A benchtop method for the fabrication and patterning of nanoscale structures

on polymers. J. Am. Chem. Soc. 126(2), 628–634.Durst R.A., Baumner A.J., Murray R.W., Buck R.P., and Andrieux C.P. (1997) Chemically modifi ed electrodes:

Recommended terminology and defi nitions. Pure Appl. Chem. 69(6), 1317–1323.Ellis D.L., Zakin M.R., Bernstein L.S., and Rubner M.F. (1996) Conductive polymer fi lms as ultrasensitive chemi-

cal sensors for hydrazine and monomethylhydrazine vapor. Anal. Chem. 68(5), 817–822.Elosua C., Matias I.R., Bariain C., and Arregui F.J. (2006) Volatile organic compound optical fi ber sensors: A re-

view. Sensors 6, 1440–1465.Ema K., Yokoyama M., Nakamoto T., and Moriizumi T. (1989) Odour sensing system using a quartz-resonator

sensor array and neural-network pattern recognition. Sens. Actuators 18, 291–296.Ensafi A.A. and Kazemzadeh A. (1999) Optical pH sensor based on chemical modifi cation of polymer fi lm.

Microchemical J. 63, 381–388. Ewbank P.C., Loewe R.S., Zhai L., Reddinger J., Sauve G., and McCullough R.D. (2004) Regioregular

poly(thiophene-3-alkanoic acid)s: Water soluble conducting polymers suitable for chromatic chemosensing in solution and solid state. Tetrahedron 60, 11269–11275.

Facchetti A. (2007) Semiconductors for organic transistors. Mater. Today 10(3), 29–37.Falcou A., Dhcuene A., Hourquebie P., Marsaeq D., and Balland-Longeall A. (2005) A new chemical polymeriza-

tion process for substituted anilines: Application to the synthesis of poly(N-alkylanilines) and poly(o-alkyl-anilines) and comparison of their respective properties. Synth. Met. 149, 115–122.

Fang Q., Chetwynd D.G., and Gardner J.W. (2002) Conducting polymer fi lms by UV-photo processing. Sens. Actuators A 99, 74–77.

Feast W.J., Tsibouklis J., Pouwer K.L., Groenendaal L., and Meijer E.W. (1996) Synthesis, processing and material properties of conjugated polymers. Polymer 37(22), 5017–5047.

Ferguson J.A., Healey B.G., Bronk K.S., Barnard S.M., and Walt D.R. (1997) Simultaneous monitoring of pH, CO2 and O2 using an optical imaging fi ber. Anal. Chim. Acta 340, 123–131.

Flory P.J. (1953) Principles of Polymer Chemistry. Cornell University Press, Ithaca, NY, chap. 2.Fog H.M. and Rietz B. (1985) Piezoelectric crystal detector for the monitoring of ozone in working environments.

Anal. Chem. 57, 2634–2638.Forster R.J. (1998) Miniaturized chemical sensors. In: Diamond D. (ed.), Principles of Chemical and Biological

Sensors. John Wiley, New York, 239.Friedemann M.N., Robinson S.W., and Gerhardt G.A. (1996) o-Phenylene diamine-modifi ed carbon fi ber

electrodes for the detection of nitric oxide. Anal. Chem. 68, 2621–2628.Gangopadhyay R. and De A. (2001) Conducting polymer composites: noble materials for gas sensing. Sens.

Actuators B 77, 326–329.Gardner J.W. and Bartlett P.N. (1999) Electronic Noses: Principles and Application. Oxford Science, Oxford, UK.Garnier F. (1989) Functionalized conducting polymers—Towards intelligent materials. Angew. Chem. 101(4),

529–533.Gauglitz G. and Reichert M. (1992) Spectral investigation and optimization of pH and urea sensors. Sens. Actuators

B 6, 83–86. Gauglitz G., Brecht G., Kraus G., and Nahm W. (1993) Chemical and biochemical sensors based on interferometry

at thin (multi-) layers. Sens. Actuators B 11, 21–27.


Ge Z., Brown C.W., Sun L., and Yang S.C. (1993) Fiber-optic pH sensor based on evanescent wave absorption spectroscopy. Anal. Chem. 65, 2335–2338.

Gold V., Loening K.L., McNaught A.D., and Sehmi P. (1987) Compendium of Chemical Terminology, IUPAC Recommendations. Blackwell Scientifi c, Oxford, UK.

Göpel W., Ziegler C., Breer H., Schild D., Apfelbach R., Joerges J., and Malaka R. (1998) Bioelectronic noses: A status report part I. Biosens. Bioelec. 13(3–4), 479–493.

Gorton L. and Fiedler U. (1977) A zinc-sensitive polymeric membrane electrode. Anal. Chim. Acta 90, 233–236.Gouette M.A. and Leclerc M. (1995) Structure-property relationships in poly(o-phenylenediamine) derivatives.

J. Electroanal. Chem. 382(1–2), 17–23.Grate J.W., Abraham M.H., and McGill R.A. (1997) Sorbent polymer materials for chemical sensors and arrays. In:

Kress-Rogers E. (ed.), Handbook of Biosensors and Electronic Noses: Medicine, Food and the Environment. CRC Press, Boca Raton, FL.

Gratzl M., Hsu D.F., Riley A.M., and Janata J. (1990) Electrochemically deposited polythiophene. 1. Ohmic drop compensation and the polythiophene paradox. J. Phys. Chem. 94(15), 5973–5981.

Grummt U.W., Pron A., Zagorska M., and Lefrant S. (1997) Polyaniline based optical pH sensor. Anal. Chim. Acta 357, 253–259.

Gupta K.C. and Mujawamariya J.D. (2000) Eff ect of concentration of ion exchanger, plasticizer and molecular weight of cyanocopolymers on selectivity and sensitivity of Cd(II) ion selective electrode. Talanta 52, 1087–1103.

Gurunathan K. and Trivedi D.C. (2000) Studies on polyaniline and colloidal TiO2 composites. Mater. Lett. 45(5), 262–268.

Gurunathan K., Murugan A.V., Marimuthu R., Mulik U.P., and Amalnerkar D.P. (1999) Electrochemically syn-thesised conducting polymeric materials for applications towards technology in electronics, optoelectronics and energy storage devices. Mater. Chem. Phys. 61(3), 173–191.

Gustafsson G., Lundstrom I., Liedberg B., Wu C.R., Inganas O., and Wennerstrom O. (1989) Th e interaction between ammonia and poly(pyrrole). Synth. Met. 31(2), 163–179.

Hamley I.W. (2003) Nanotechnologie mit weichen Materialien. Angew. Chem. 115(15), 1730–1752.Han W.S., Park M.Y., Chung K.C., Cho D.H., and Hong T.K. (2001) Potentiometric sensor for hydrogen ion

based on N,N(-dialkylbenzylethylenediamine neutral carrier in a poly(vinyl chloride) membrane with polyani-line solid contact. Talanta 54, 153–159.

Hanawa T., Kuwabata S., Hashimoto H., and Yoneyama H. (1989) Gas sensitivities of electropolymerized polythio-phene fi lms. Synth. Met. 30(2), 173–181.

Hao Q., Kulikov V., and Mirsky V.M. (2003) Investigation of contact and bulk resistance of conducting polymers by simultaneous two- and four-point technique. Sens. Actuators B 94, 352–357.

Hao Q., Wang X., Lu L., Yang X., and Mirsky V.M. (2005) Electropolymerized multilayer conducting polymers with response to gaseous hydrogen chloride. Macromol. Rapid Commun. 26(13), 1099–1103.

Harsányi G., Réczey M., Dobay R., Lepsényi I., Illyefalvi-Vitéz Z., Steen V.J., Vervaet A., Reinert W., Urbancik J., Guljajev A., Visy C., Inzelt G., and Bársony, I. (1999) Combining inorganic and organic gas sensor elements: A new approach for multicomponent sensing. Sens. Rev. 19(2), 128–134.

Hartmann P., Leiner J.P., and Lippitsch M.E. (1995) Lumunescence quenching behavior of an oxygen sensor based on a Ru(II) complex dissolved in polystyrene. Anal. Chem. 67, 88–93.

Hassan S.S.M., Saleh M.B., Abdel Gaber A.A., Mekheimer R.A.H., and Abdel Kream N A. (2000) Novel mercury (II) ion-selective polymeric membrane sensor based on ethyl-2-benzoyl-2-phenylcarbamoyl acetate. Talanta 53, 285–293.

Hassan S.S.M., Ali M.M., and Attawiya A.M.Y. (2001) PVC membrane based potentiometric sensors for uranium determination. Talanta 54, 1153–1161.


Hatfi eld V., Neaves P., Hicks P.J., Persaud K., and Travers P. (1994) Towards an integrated electronic nose using conducting polymer sensors. Sens. Actuators B 18–19, 221–228.

Heeg J., Kramer C., Wolter M., Michaelis S., Plieth W., and Fisher W.J. (2001) Polythiophene-O3 surface reactions studied by XPS. Appl. Surf. Sci. 180, 36–41.

Heeger A.J. (2001) Semiconducting and metallic polymers: Th e fourth generation of polymeric materials (Nobel Lecture). Angew. Chem. Int. Ed. 40, 2591–2611.

Hirata M. and Sun L. (1994) Characteristics of an organic semiconductor polyaniline fi lm as a sensor for NH3 gas. Sens. Actuators A 40, 159–163.

Ho K.C. and Hung W.T. (2001) An amperometric NO2 gas sensor based on Pt/Nafi on electrode. Sens. Actuators B 79, 11–16.

Ho C.K., Lindgren E.R., Rawlinson K.S., McGrath L.K., and Wright J.L. (2003) Development of a surface acoustic wave sensor for in-situ monitoring of volatile organic compounds. Sensors 3, 236–247.

Hoa D.T., Kumar T.N.S., Punekar N.S., Srinivasa R.S., Lal R., and Contractor A.Q. (1992) A biosensor based on conducting polymers. Anal. Chem. 64(21), 2645–2646.

Horanyi G. and Inzelt G. (1989) In situ radiotracer study of the formation and behaviour of polyaniline fi lm elec-trodes using labelled aniline. J. Electroanal. Chem. 264(1–2), 259–272.

Horiuchi T., Ueno Y., Camou S., Haga T., and Tate A. (2006) Portable aromatic VOC gas sensor for onsite conti-nuous air monitoring with 10-ppb benzene detection capability. NTT Tech. Rev. 4(1), 30–37.

Hosseini S.H. and Entezami A.A. (2003) Conducting polymer blends of polypyrrole with polyvinyl acetate, poly-styrene, and polyvinyl chloride based toxic gas sensors. J. Appl. Polym. Sci. 90, 49–62.

Houser E.J., Mlsna T.E., Nguyen V.K., Chung R., Mowery E.L., and McGill R.A. (2001) Rational materials design of sorbent coatings for explosives: applications with chemical sensors. Talanta 54, 469–485.

Hrncirova P., Opekar F., and Stulik K. (2000) An amperometric solidstate NO2 sensor with a solid polymer electro-lyte and a reticulated vitreous carbon indicator electrode. Sens. Actuators B 69, 199–204.

Hu H., Trejo M., Nicho M.E., Saniger J.M., and Garcia-Valenzuela A. (2002) Adsorption kinetics of optochemical NH3 gas sensing with semiconductor polyaniline fi lms. Sens. Actuators B 82, 14–23.

Ichimori K., Ishida H., Fukahori M., Nakazawa H., and Murakami E. (1994) Practical nitric oxide measurement employing a nitric oxide selective electrode. Rev. Sci. Instrum. 65, 1–5.

Inzelt G. (1994) Mechanism of charge transport in polymer-modifi ed electrodes. In: Bard A.J. (ed.), Electroanalytical Chemistry, Vol. 18. Marcel Dekker, New York, 89.

Jacquinot P., Muller B., Wehrli B., and Hauser P.C. (2001) Determination of methane and other small hydrocar-bons with a platinum-Nafi on electrode by stripping voltammetry. Anal. Chim. Acta 432, 1–10.

Janata J. and Bezegh A. (1988) Chemical sensors. Anal. Chem. 60, 62R–74R.Janata J. and Josowicz M. (2003) Conducting polymers in electronic chemical sensors. Nat. Mater. 2, 19–30.Janata J. and Josowicz M. (1998) Chemical modulation of work function as a transduction mechanism for chemical

sensors. Acc. Chem. Res. 31, 241–248.Janowiak M., Huang H., Chang S., and Garcia-Rubio L.H. (2001) Development of a fi ber optic pH sensor for on-

line control. ACS Symp. Ser. 795, 195–210.Jin Z., Su Y., and Duan Y. (2000) An improved optical pH sensor based on polyaniline. Sens. Actuators B 71,

118–122.Jin Z., Su Y., and Duan Y. (2001) Development of a polyaniline-based optical ammonia sensor. Sens. Actuators B

72, 75–79.Joo B.S., Lee J.H., Lee E.W., Song K.D., and Lee D.D. (2005) Polymer fi lm SAW sensors for chemical agent detec-

tion. In: Proceedings of 1st International Conference on Sensing Technology, November 21–23, Palmerston North, New Zealand, 307–310.


Josowicz M., Janata J., Ashley K., and Pons S. (1987) Electrochemical and ultraviolet-visible spectroelectro-chemical investigation of selectivity of poltentiometric gas sensors based on polypyrrole. Anal Chem. 59, 253–258.

Jun H.K., Hoh Y.S., Lee B.S., Lee S.T., Lim J.O., Lee D.D., and Huh J.S. (2003) Electrical properties of polypyr-role gas sensors fabricated under various pretreatment conditions. Sens. Actuators B 96, 576–581.

Kang E.T., Neoh K.G., Tan K.L., and Tan B.T.G. (1990) Structural studies of halogen-substituted polyanilines by X-ray photoelectron spectroscopy. Synth. Met. 35, 345–355.

Kar P., Pradhan N.C., and Adhikari B. (2008) A novel route for the synthesis of processable conducting poly(m-aminophenol). Mater. Chem. Phys. 111(1), 59–64.

Kar P., Pradhan N.C., and Adhikari B. (2009) Application of sulfuric acid doped poly (m-aminophenol) as aliphatic alcohol vapor sensor material. Sens. Actuators B 140, 525–531.

Kemp N.T., Kaiser A.B., Trodahl H.J., Chapman B., Buckley R.G., Partridge A.C., and Foot P.J.S. (2006) Eff ect of ammonia on the temperature-dependent conductivity and thermopower of polypyrrole. J. Polym. Sci. B 44, 1331–1338.

Keshavarzi A., Shahinpoor M., Kim K.J., and Lantz J. (1999) Blood pressure, pulse rate, and rhythm measurement using ionic polymer–metal composites sensors. In: Bar-Cohen Y. (ed.), Proc. SPIE—Int. Soc.Opt. Eng. 3669, 369–376.

Kim K.C., Cho S.M., and Choi H.G. (2000) Detection of ethanol gas concentration by fuel cell sensors fabricated using a solid polymer electrolyte. Sens. Actuators B 67, 194–198.

Kim T.H. and Swager T.M. (2003) A fl uorescent self-amplifying wavelength-responsive sensory polymer for fl uoride ions. Angew. Chem. Int. Ed. 42(39), 4803–4806.

Kim B.K., Kim Y.H., Won K., Chang H.J., Chi Y.M., Kong K.J., Rhyu B.W., Kim J.J., and Lee J.O. (2005) Electrical properties of polyaniline nanofi bre synthesized with biocatalyst. Nanotechnology 16, 1177–1181.

Kim I.B. and Bunz U.H.F. (2006) Modulating the sensory response of a conjugated polymer by proteins: An agglu-tination assay for mercury ions in water. J. Am. Chem. Soc. 128, 2818–2819.

Kobayashi M., Colaneri N., Boysd M., Wudl F., and Heegh A.J. (1985) Th e electronic and electrochemical proper-ties of poly(isothianaphthene). J. Chem. Phys. 82(12), 5717–5723.

Korostynska O., Arshak K., Gill E., and Arshak A. (2007) Review on state-of-the-art in polymer based pH sensors. Sensors 7, 3027–3042.

Korotcenkov G., Han S.-D., and Stetter J.R. (2009) Review of electrochemical hydrogen sensors. Chem. Rev. 109(3), 1402–1433.

Koshets I.A., Kazantseva Z.I., and Shirshov Y.M. (2003) Polymer fi lms as sensitive coatings for quartz crystal micro-balance sensors array. Semicond. Phys. Quan. Electron. Optoelectron. 6(4), 505–507.

Koul S., Chandra R., and Dhawan S.K. (2001) Conducting polyaniline composite: A reusable sensor material for aqueous ammonia. Sens. Actuators B 75, 151–159.

Krondak M., Broncova G., Anikin S., Merz A., and Mirsky V. (2006) Chemosensitive properties of poly-4,4-dialkoxy-2,2-bipyrroles. J. Solid State Electrochem. 10(3), 185–191.

Kukla A.L., Shirshov Y.M., and Piletsky S.A. (1996) Ammonia sensors based on sensitive polyaniline fi lms. Sens. Actuators B 37, 135–140.

Kumar D. and Sharma R.C. (1998) Advances in conductive polymers. Eur. Polym. J. 34(8), 1053–1060.Kymissis I. (2009) Organic Field Eff ect Transistors: Th eory, Fabrication and Characterization. Series on Integrated

Circuits and Systems (Chandrakasan A., ed.). Springer-Verlag, New York. Lange U., Roznyatovskaya N.V., and Mirsky V.M. (2008) Conducting polymers in chemical sensors and arrays.

Anal. Chim. Acta 614, 1–26.Laurs H. and Heiland G. (1987) Electrical and optical properties of phthalocyanine fi lms. Th in Solid Films 149,

129–135.


Leclerc M. Guay J., and Dao L.H. (1989) Synthesis and characterization of poly(alkylanilines). Macromolecules 22, 649–653.

Leiner J.P. (1995) Optical sensors for in vitro blood gas analysis. Sens. Actuators B 29, 169–173.Levin O., Kondratiev V., and Malev V. (2005) Charge transfer processes at poly-o-phenylenediamine and poly-o-

aminophenol fi lms. Electrochim. Acta 50(7–8), 1573–1585.Li B., Sauve G., Iovu M.C., Jeff ries-EL M., Zhang R., Cooper J., Santhanam S., Schultz L., Revelli J.C., Kusne

A.G., Kowalewski T., Snyder J.L., Weiss L.E., Fedder G.K., McCullough R.D., and Lambeth D.N. (2006) Volatile organic compound detection using nanostructured copolymers. Nano Lett. 6(8), 1598–1602.

Li B., Santhanam S., Schultz L., Jeff ries-EL M., Iovu M.C., Sauve G., Cooper J., Zhang R., Revelli J.C., and Kusne A.G. (2007) Inkjet printed chemical sensor array based on polythiophene conductive polymers. Sens. Actuators B 123, 651–660.

Li D., Jiang Y., Wu Z., Chen X., and Li Y. (2000) Self-assembly of polyaniline ultrathin fi lms based on doping-induced deposition eff ect and applications for chemical sensors. Sens. Actuators B 66, 125–127.

Linford R.G. (ed.) (1987) Electrochemical Science and Technology of Polymers, Vol. 1. Elsevier, London.Lindfors T. and Ivaska A. (2001) Calcium-selective electrode based on polyaniline functionalized with bis[4-

(1,1,3,3-tetramethylbutyl) phenyl]phosphate. Anal. Chim. Acta 437, 171–183.Lindsey S.E. and Street G.B. (1984) Conductive composites from polyvinyl alcohol and polypyrrole. Synth. Met.

10, 67–69.Liu D., Liu J., Tian D., Hong W., Zhou X., and Yu J.C. (2000) Polymeric membrane silver-ion selective electrodes

based on bis(dialkyldithiophosphates). Anal. Chim. Acta 416, 139–144.Loi A., Manunza I., and Bonfi glio A. (2005) Flexible, organic, ion-sensitive fi eld-eff ect transistor. Appl. Phys. Lett.

86(10), 103512–103514.Lundberg B. and Sundqvist B. (1986) Resistivity of a composite conducting polymer as a function of temperature,

pressure, and environment: Applications as a pressure and gas concentration transducer. J. Appl. Phys. 60(3), 1074–1079.

Lyons M.E.G. (ed.) (1994) Electroactive Polymer Electrochemistry, Part 1. Plenum Press, New York.Lyons M.E.G. (ed.) (1996) Electroactive Polymer Electrochemistry, Part 2. Plenum Press, New York.Mabeck J.T. and Malliaras G.G. (2006) Chemical and biological sensors based on organic thin-fi lm transistors.

Anal. Bioanal. Chem. 384, 343–353.MacDiarmid A.G. and Epstein A.J. (1989) Polyanilines: A novel class of conducting polymers. Faraday Discuss.

Chem. Soc. 88, 317–332.MacDiarmid A.G. and Epstein A.J. (1994) Th e concept of secondary doping as applied to polyaniline. Synth. Met.

65, 103–116.MacDiarmid A.G. and Epstein A.J. (1995) Secondary doping in polyaniline. Synth. Met. 69, 85–92.Mahajan R.K. and Parkash O. (2000) Silver (I) ion selective PVC membrane based on bis-pyridine tetramide

macro cycle. Talanta 52, 691–693.Maksymiuk K. (2006) Chemical reactivity of polypyrrole and its relevance to polypyrrole based electrochemical

sensors. Electroanalysis 18, 1537–1551.Malhotra, B.D., Kumar N., and Chandra S. (1986) Recent studies of heterocyclic and aromatic conducting poly-

mers. Prog. Polym. Sci. 12(3), 179–218.Martin C.R., Parthasarathy R., and Menon V. (1993) Template synthesis of electronically conductive polymers—A

new route for achieving higher electronic conductivities. Synth. Met. 55(2–3), 1165–1170.Matsuguchi M., Tamai K., and Sakai Y. (2001) SO2 gas sensors using polymers with diff erent amino groups. Sens.

Actuators B 77, 363–367.Mayes A.G., Blyth J., Kyrolainen-Reay M., Millington R.B, and Lowe C.R. (1999) A holographic alcohol sensor.

Anal Chem. 71, 3390–3396.


McCullough R.D. (1998) Th e chemistry of conducting polythiophenes. Adv. Mater. 10, 93–116.McGovern S.T., Spinks G.M., and Wallace G.G. (2005) Micro-humidity sensors based on a processable polyaniline

blend. Sens. Actuators B 107, 657–665.Mcquade D.T., Pullen A.E., and Swager T.M. (2000) Conjugated polymer-based chemical sensors. Chem. Rev.

100(7), 2537–2574.Miasik J., Hooper A., and Tofi eld B. (1986) Conducting polymer gas sensors. J. Chem. Soc. Faraday Trans. 1 82(4),

1117–1127.Mirmohseni A. and Hassanzadeh V. (2001) Application of polymer-coated quart crystal microbalance (QCM) as a

sensor for BTEX compounds vapors. J. Appl. Polym. Sci. 79, 1062–1066.Mirmohseni A. and Rostamizadeh K. (2006) Quartz crystal nanobalance in conjunction with principal component

analysis for identifi cation of volatile organic compounds. Sensors 6, 324–334.Misra S.C.K., Mathur P., and Srivastava B.K. (2004) Vacuum-deposited nanocrystalline polyaniline thin fi lm sen-

sors for detection of carbon monoxide. Sens. Actuators A 114, 30–35.Miura N., Kato H., Yamazoe N., and Seiyama T. (1984) A proton conductor oxygen sensor operative at lower

temperature. Denki Kagaku 52, 376–377.Mizutani F., Yabuki S., Sawaguchi T., Hirata Y., Sato Y., and Iijima S. (2001) Use of a siloxane polymer for the pre-

paration of amperometric sensors: O2 and NO sensors and enzyme sensors. Sens. Actuators B 76, 489–493.Mohammad F. (1998) Compensation behaviour of electrically conductive polythiophene and polypyrrole. J. Phys.

D: Appl. Phys. 31(8), 951–960.Mohr G.J. (2006) Polymers for optical sensors. In: Baldini F., Chester A.N., Homola J., Martellucci S. (eds.),

Optical Chemical Sensors. Springer-Vrlag, Dordrecht, Th e Netherlands, 297–321.Mousavi M.F., Alizadeh N., Shamsipur M., and Zohari N. (2000) A new PVC-based 1,10-dibenzyl-1,10-diaza-18-

crown-6 selective electrode for detecting nickel II ion. Sens. Actuators B 66, 98–100.Munkholm C., Walt D.R., Milanovich F.P., and Klainer S.A. (1986) Polymer modifi cation of fi ber optic chemical

sensors as a method of enhancing fl uorescence signal for pH measurement. Anal. Chem. 58, 1427–1430.Murray R.W. (1984) Chemically modifi ed electrodes. In: Bard A.J (ed.), Electroanalytical Chemistry, Vol. 13. Marcel

Dekker, New York, 191.Murray R.W., Ewing A.G., and Durst R.A. (1987) Chemically modifi ed electrodes. Molecular design for electro-

analysis. Anal. Chem. 59(5), 379A–390A.Nakagawa K., Sadaoka Y., Supriyatno H., Kubo A., Tsutsumi C., and Tabuchi K. (2001) Optochemical HCl gas

detection using alkoxy substituted tetraphenyl porphyrin–polymer composite fi lms. Eff ects of alkoxy chain length on sensing characteristics. Sens. Actuators B 76, 42–46.

Nakamoto T., Fukunishi K., and Moriizumi T. (1990) Identifi cation capability of odor sensor using quartz- resonator array and neural-network pattern recognition. Sens. Actuators B 1, 473–476.

Nanto H., Dougami N., Mukai T., Habara M., Kusano E., Kinbara A., Ogawa T., and Oyabu T. (2000) A smart gas sensor using polymer-fi lm-coated quartz resonator microbalance. Sens. Actuators B 66, 16–18.

Nicho M.E., Trejo M., Garcıa-Valenzuela A., Saniger J.M., Palacios J., and Hu H. (2001) Polyaniline composite coatings interrogated by nulling optical-transmittance bridge for sensing low concentrations of ammonia gas. Sens. Actuators B 76, 18–24.

Nohria R., Khillan R.K., Su Y., Dikshit R., Lvov Y., and Varahramyan K. (2006) Humidity sensor based on ultra-thin polyaniline fi lm deposited using layer-by-layer nano-assembly. Sens. Actuators B 114, 218–222.

Nilsson D., Kugler T., Svensson P.O., and Berggren M. (2002) An all-organic sensor–transistor based on a novel electrochemical transducer concept printed electrochemical sensors on paper. Sens. Actuators B 86, 193–197.

Nishizawa M., Matsue T., and Uchida I. (1992) Penicillin sensor based on a microarray electrode coated with pH-responsive polypyrrole. Anal. Chem. 64(21), 2642–2644.


Numata M., Baba K., Hemmi A., Hachiya H., Ito S., Masadome T., Asano Y., Ohkubo S., Gomi T., Imato T., and Hobo T. (2001) Determination of hardness in tapwater and upland soil extracts using a long-term stable diva-lent cation selective electrode based on a lipophilic acrylate resin as a membrane matrix. Talanta 55, 449–457.

Nylander C., Armgarth M., and Lundstrom I. (1983) An ammonia detector based on a conducting polymer. Anal. Chem. Symp. Ser. 17, 203–207.

Odian G. (2004) Principles of Polymerization, 4th ed. John Wiley, Hoboken, NJ.Ohmori Y., Takahashi H., Muro K., Uchida M., Kawai T., and Yoshino K. (1991) Gas-sensitive Schottky gated fi eld

eff ect transistors utilizing poly(3-alkylthiophene) fi lms. Jpn. J. Appl. Phys. 30(7B), L1247–L1249.Opdycke W.N., Parks S.J., and Meyerhoff M.E. (1983) Polymer-membrane pH electrodes as internal elements for

potentiometric gas-sensing systems. Anal. Chim. Acta 155, 11–20.Opdycke W.N. and Meyerhoff M.E. (1986) Development and analytical performance of tubular polymer membra-

ne electrode based carbon dioxide catheters. Anal. Chem. 58, 950–956.Opekar F. and Bruckenstein S. (1984) Determination of gaseous hydrogen sulfi de by cathodic stripping voltamme-

try after preconcentration on a silver metalized porous membrane electrode. Anal. Chem. 56, 1206–1209.Opekar F. and Stulik K. (1999) Electrochemical sensors with solid polymer electrolytes. Anal. Chim. Acta 385,

151–162Otagawa T., Madou M., Wing S., Rich-Alexander J., Kusanagi S., Fujioka T., and Yasuda A. (1990) Planar micro-

electrochemical carbon monoxide sensors. Sens. Actuators B 1, 319–325.Otero T.F. and Rodriguez J. (1994) Parallel kinetic studies of the electrogeneration of conducting polymers: Mixed

materials, composition and properties control. Electrochim. Acta 39(2), 245–253. Pandey P.C., Upadhyay S., Singh G., Prakash R., Srivastava R.C., and Seth P.K. (2000) A new solid-state pH sensor

and its application in the construction of all solid-state urea biosensor. Electroanalysis12, 517–521.Pandey P.C. and Singh G. (2001) Tetraphenylborate doped polyaniline based novel pH sensor and solid-state urea

biosensor. Talanta 55, 773–182.Pang Z., Gu X., Yekta A., Masoumi Z., Coll J.B., Winnik M.A., and Manners I. (1996) Phosphorescent oxygen

sensors utilizing sulfur–nitrogen–phosphorus polymer matrixes. Adv. Mater. 8, 768–771.Park Y.H. and Han M.H. (1992) Preparation of conducting polyacrylonitrile/polypyrrole composite fi lms by

electro chemical synthesis and their electrical properties. J. Appl. Polym. Sci. 45, 1973–1982.Park Y.W., Moon J.S., Bak M.K., and Jin J.I. (1989) Electrical properties of polyaniline and substituted polyaniline

derivatives. Synth. Met. 29, 389–394.Patra D. and Mishra A.K. (2001) Fluorescence quenching of benzo[k] fl uoranthene in poly(vinyl alcohol) fi lm: A

possible optical sensor for nitro aromatic compounds. Sens. Actuators B 80, 278–282.Paul E.W., Ricco A.J., and Wrighton M.S. (1985) Resistance of polyaniline fi lms as a function of electrochemical

potential and the fabrication of polyaniline-based microelectronic devices. J. Phys. Chem. 89(8), 1441–1447.Pei Q. and Inganas O. (1993) Conjugated polymers as smart materials, gas sensors and actuators using bending

beams. Synth. Met. 57(1), 3730–3735.Percival C.J., Stanley S., Galle M., Braithwaite A., Newton M.I., McHale G., and Hayes W. (2001) Molecular-

imprinted, polymer-coated quartz crystal microbalances for the detection of terpenes. Anal. Chem. 73, 4225–4228.

Persaud K.C. and Pelosi P. (1985) An approach to an artifi cial nose. Trans. Am. Soc. Artif. Int. Organs 31, 297–300.

Persaud K.C. (2005) Polymers for chemical sensing. Mater. Today 8, 38–45.Potje-Kamloth K., Polk B.J., Josowicz M., and Janata J. (2002) Doping of polyaniline in solid-state with photo-

generated trifl ic acid. Chem. Mater. 14, 2782–2787.Prissanaroon W., Ruangchuay L., Sirivat A., and Schwank J. (2000) Electrical conductivity response of dodecylben-

zene sulfonic acid-doped polypyrrole fi lms to SO2–N2 mixtures. Synth. Met. 114(1), 65–72.


Pringsheim E., Terpetschnig E., and Wolfbeis O.S. (1997) Optical sensing of pH using thin fi lms of substituted polyanilines. Anal. Chim. Acta 357, 247–252.

Rabek J.F. (1995) Polymer Photodegradation: Mechanism and Experimental Methods. Chapman & Hall, London.Ram M.K., Yavuz O., Lahsangah V., and Aldissi M. (2005a) CO gas sensing from ultrathin nano-composite con-

ducting polymer fi lm. Sens. Actuators B 106, 750–757.Ram M.K., Yavuz O., and Aldissi M. (2005b) NO2 gas sensing based on ordered ultrathin fi lms of conducting

polymer and its nanocomposite. Synth. Met. 151(1), 77–84.Ramesh C., Velayutham G., Murugesan N., Ganesan V., Dhathathreyan K.S., and Periaswami G. (2003) An

improved polymer electrolyte-based amperometric hydrogen sensor. J. Solid State Electrochem. 8, 511–516.Ratcliff e N.M. (1990) Polypyrrole-based sensor for hydrazine and ammonia. Anal. Chim. Acta 239, 257–262.Razumovskii S.D. and Zaikov G.Y. (1982) Eff ect of ozone on saturated polymers. Polym. Sci. U.S.S.R. 24(10),

2805–2325.Reemts J., Parisi J., and Schlettwein D. (2004) Electrochemical growth of gas-sensitive polyaniline thin fi lms across

an insulating gap. Th in Solid Films 466(1–2), 320–325.Reese C., Roberts M., Ling M., and Bao Z. (2004) Organic thin fi lm transistors. Mater. Today 7(9), 20–27.Rella R., Siciliano P., Quaranta F., Primo T., Valli L., Schenetti L., Mucci A., and Iarossi D. (2000) Gas sensing

measurements and analysis of the optical properties of poly[3-(butylthio)thiophene] Langmuir–Blodgett fi lms. Sens. Actuators B 68, 203–209.

Ricks-Laskoski H.L. and Buckley L.J. (2006) Twenty-year aging study of electrically conductive polypyrrole fi lms. Synth. Met. 156, 417–419.

Rivas B.L., Sanchez C.O., Bernede J.C., and Mollinie P. (2002) Synthesis, characterization, and properties of poly(2- and 3-aminophenol) and poly(2- and 3-aminophenol)-Cu(II) materials. Polym. Bull. 49(4), 257–264.

Rosini S. and Siebert E. (2005) Electrochemical sensors for detection of hydrogen in air: model of the non- Nernstian potentiometric response of platinum gas diff usion electrodes. Electrochim. Acta 50, 2943–2953.

Roy S., Sana S., Adhikari B., and Basu S. (2003) Preparation of doped polyaniline and polypyrrole fi lms and appli-cations for hydrogen gas sensors. J. Polym. Mater. 20, 173–180.

Ruangchuay L., Sirivat A., and Schwank J. (2003) Polypyrrole/poly(methylmethacrylate) blend as selective sensor for acetone in lacquer. Talanta 60, 25–30.

Ruangchuay L., Sirivat A., and Schwank J. (2004) Electrical conductivity response of polypyrrole to acetone vapor: Eff ect of dopant anions and interaction mechanisms. Synth. Met. 140(1), 15–21.

Sadaoka Y., Matsuguchi M., Sakai Y., and Murata Y. (1992) Optical humidity sensing characteristics of Nafi on-dyes composite thin fi lms. Sens. Actuators B 7, 443–446.

Sadaoka Y., Sakai Y., and Akiyama H. (1986) A humidity sensor using alkali salt-poly(ethylene oxide) hybrid fi lms. J. Mater. Sci. 21(1), 235–240.

Sadeghipour K., Salomon R.. and Neogi S. (1992) Development of a novel electrochemically active membrane and “smart” material-based vibration sensor/damper. Smart Mater. Struct. 1, 172–179.

Sakai Y., SadaokaY., and Matsuguchi M. (1989) A humidity sensor using cross-linked quaternized polyvinylpyridine. J. Electrochem. Soc. 136(1), 171–174.

Sakai Y., Sadaoka Y., Matsuguchi M., Yokouchi H., and Tamai K. (1995) Eff ect of carbon dioxide on the electrical conductivity of polyethylene glycol-alkali carbonate complex fi lm. Mater. Chem. Phys. 42, 73–76.

Sakthivel M. and Weppner W. (2006) Development of a hydrogen sensor based on solid polymer electrolyte mem-branes. Sens. Actuators B 113, 998–1004.

Sakurai Y., Jung H.S., Shimanouchi T., Inoguchi T., Morita S., Kuboi R., and Natsukawa K. (2002) Novel array-type gas sensors using conducting polymers, and their performance for gas identifi cation. Sens. Actuators B 83, 270–275.


Salavagione H.J., Arias J., Garces P., Morallon E., Barbero C., and Vazquez J.L. (2004) Spectroelectrochemical study of the oxidation of aminophenols on platinum electrode in acid medium. J. Electroanal. Chem. 565(2), 375–383.

Saleh M.B., Hassan S.S.M., Abdel Gaber A.A., and Abdel Kream N.A. (2001) Novel potentiometric membrane sensor for selective determination of aluminum(III) ions. Anal. Chim. Acta 434, 247–253.

Samec Z., Opekar F., and Crijns G.J.E.F. (1995) Solid-state hydrogen sensor based on a solid-polymer electrolyte. Electroanalysis 7, 1054–1058.

Sandler S.R. and Karo W. (1974) Polymer Synthesis. Academic Press, New York.Saravanan S., Mathai C.J., Venkatachalam S., and Anantharaman M.R. (2004) Low K thin fi lms based on RF

plasma-polymerized aniline. New J. Phys. 6, 64–68.Sato, M., Tanaka S., and Kacriyama K. (1986) Electrochemical preparation of conducting poly(3- methylthiophene):

comparison with polythiophene and poly(3-ethylthiophene). Synth. Met. 14(4), 279–288.Saxena A., Fujiki M., Rai R., Kim S.Y., and Kwak G. (2004) Highly sensitive and selective fl uoride ion chemosen-

sing, fl uoroalkylated polysilane. Macromol. Rapid Commun. 25(20), 1771–1775.Schierbaum K.D., Gerlach A., Haug M., and Göpel W. (1992) Selective detection of organic molecules with poly-

mers and supramolecular compounds: Applications of capacitance, quartz microbalance, and calorimetric transducers. Sens. Actuators A 31, 130–137.

Segal E., Tchoudakov R., Narkis M., and Siegmann A. (2002) Th ermoplastic polyurethane-carbon black com-pounds: Structure, electrical conductivity and sensing of liquids. Polym. Eng. Sci. 42, 2430–2439.

Segal E., Tchoudakov R., Narkis M., Siegmann A., and Yen W. (2005) Polystyrene/polyaniline nanoblends for sensing of aliphatic alcohols. Sens. Actuators B 104, 140–150.

Schiavon G., Zotti G., Toniolo R., and Bontempelli G. (1995) Electrochemical detection of trace hydrogen sulfi de in gaseous samples by porous silver electrodes supported on ion exchange membranes (solid polymer electro-lytes). Anal. Chem. 67, 318–323.

Sengupta P.P., Kar P., and Adhikari B. (2009) Infl uence of dopant in the synthesis, characteristics and ammonia sensing behavior of processable polyaniline. Th in Solid Films 517, 3770–3775.

Sengupta P.P., Barik S., and Adhikari B. (2006) Polyaniline as a gas-sensor material. Mater. Manuf. Proc. 21, 263–270.

Shahinpoor M., Bar-Cohen Y., Simpson J., and Smith J. (1998) Ionic polymer-metal composites (IPMCs) as bio-mimetic sensors, actuators and artifi cial muscles: a review. Smart Mater. Struct. 7(6), R15–R30.

Shakhsher Z., Seitz W.R., and Legg K.D. (1994) Single fi ber-optic pH sensor based on changes accompanying polymer swelling. Anal. Chem. 66, 1731–1735.

Shamsipur M., Ganjali M.R., Rouhollahi A., and Moghimi A. (2001) Beryllium-selective membrane sensor based on 3,4-di[2-(2-tetrahydro-2H-pyranoxy)]ethoxy styrene-styrene copolymer. Anal Chim Acta 434, 23–27.

Shi G., Luo M., Xue J., Xian Y., Jin L., and Jin J.Y. (2001) Th e study of PVP/Pd/IrO2 modifi ed sensor for ampero-metric determination of sulfur dioxide. Talanta 55, 241–247.

Shi M. and Anson F.C. (1996) Eff ects of hydration on the resistances and electrochemical responses of nafi on-coatings on electrodes. J. Electroanal. Chem. 415, 41–46.

Silva L.I.B., Rocha-Santos T.A.P., and Duarte A.C. (2008) Sensing of volatile organic compounds in indoor atmos-phere and confi ned areas of industrial environments. Global NEST J. 10(2), 217–225.

Slater J.M. and Paynter J. (1994) Prediction of gas sensor response using basic molecular parameters. Analyst 119, 191–195.

Slater J.M. and Watt E.J. (1991) Examination of ammonia-poly(pyrrole).interactions by piezoelectric and conduc-tivity measurements. Analyst 116, 1125–1130.

Smith J.D.S. (1998) Intrinsically electrically conducting polymers. Synthesis, characterization, and their applica-tions. Prog. Polym. Sci. 23, 57–79.


Sohn H., Sailor M.J., Magde D., and Trogler W.C. (2003) Detection of nitroaromatic explosives based on photo-luminescent polymers containing metalloles. J. Am. Chem. Soc. 125(13), 3821–3830.

Stilwell D.E. and Park S.M. (1988) Electrochemistry of conductive polymers. J. Electrochem. Soc. 135(9), 2254–2262.

Sudhölter E.J.R., Van der Wal P.D., Skowronska-Ptasinska M., Van den Berg A., and Reinhoudt D.N. (1989) Ion-sensing using chemically-modifi ed ISFETs. Sens. Actuators 17, 189–194.

Supriyatno H., Nakagawa K., and Sadaoka Y. (2001) Optochemical HCl gas detection using mono-substituted tetraphenyl porphin–polymer composite fi lms. Sens. Actuators B 76, 36–41.

Sukeerthi S. and Contractor A.Q. (1994) Applications of conducting polymers as sensors. Indian J. Chem. Sect. A 33, 565–571.

Sundmacher K., Rihko-Struckmann L.K., and Galvita V. (2005) Solid electrolyte membrane reactors: Status and trends. Catal. Today 104, 185–199.

Sunny V., Narayanan T.N., Sajeev U.S., Joy P.A., Kumar D.S., Yoshida Y., and Anantharaman M.R. (2006) Evidence for intergranular tunnelling in polyaniline passivated -Fe nanoparticles. Nanotechnology 17, 4765–4772.

Suri K., Annapurni S., Sarkar A.K., and Tandon R.P. (2002) Gas and humidity sensors based on iron oxide– polypyrrole nanocomposites. Sens. Actuators B 81, 277–282.

Supriya L. and Claus R.O. (2004) Fabrication of electrodes for polymer actuators and sensors via self-assembly. In: Proceedings of IEEE Sensor Conference, October 24–27, Vienna, 2, 619–622.

Svetlicic V., Schmidt A.J., and Miller L.L. (1998) Conductometric sensors based on the hypersensitive response of plasticized polyaniline fi lms to organic vapors. Chem. Mater. 10, 3305–3307.

Syed A.A. and Dinesan M.K. (1991) Review: Polyaniline—A novel polymeric material. Talanta 38(8), 815–837. Tan C.K. and Blackwood D.J. (2000) Interactions between polyaniline and methanol vapour. Sens. Actuators B 71,

184–191.Tanese M.C., Torsi L., Cioffi N., Zotti L.A., Colangiuli D., Farinola G.M., Babudri F., Naso F., Giangregorio

M.M., Sabbatini L., and Zambonin P.G. (2004) Poly(phenyleneethynylene) polymers bearing glucose substitu-ents as promising active layers in enantioselective chemiresistors. Sens. Actuators B 100, 17–21.

Teixeira M.F.S, Fatibello-Filho O., Ferracin L.C., Rocha-Filho R.C., and Bocchi N. (2000) A l-MnO2-based graphite-epoxy electrode as lithium ion sensor. Sens. Actuators B 67, 96–100.

Th ackeray J.W., White H.S., and Wrighton M.S. (1985) Poly(3-methylthiophene)-coated electrodes: Optical and electrical properties as a function of redox potential and amplifi cation of electrical and chemical signals using poly(3-methylthiophene)-based microelectrochemical transistors. J. Phys. Chem. 89(23), 5133–5140.

Th ackeray J.W. and Wrighton M.S. (1986) Chemically responsive microelectrochemical devices based on platinized poly(3-methylthiophene): Variation in conductivity with variation in hydrogen, oxygen, or pH in aqueous solution. J. Phys. Chem. 90(25), 6674–6679.

Th omas S.W. III and Swager T.M. (2006) Trace hydrazine ddetection with fl uorescent conjugated polymers: A turn-on sensory mechanism. Adv. Mater. 18(8), 1047–1050.

Th ompson M. and Stone D.C. (1997) Surface-Launched Acoustic Wave Sensors: Chemical Sebsing and Th in-Film Characterization. John Wiley, New York.

Toal S.J. and Trogler W.C. (2006) Polymer sensors for nitroaromatic explosives detection. J. Mater. Chem. 16, 2871–2883.

Tong H., Wang L.X., Jing X.B., and Wang F.S. (2003) “Turn-on” conjugated polymer fl uorescent chemosensor for fl uoride. Macromolecules 36(8), 2584–2586.

Torres K.Y.C., Garcia C.A.B., Fernandes J.C.B., de Oliveira Neto G., and Kubota L.T. (2001) Use of self- plasticizing EVA membrane for potentiometric anion detection. Talanta 53, 807–814.

Torsi L., Pezzuto M., Siciliano P., Rella R., Sabbatini L., Valli L., and Zambonin P.G. (1998) Conducting polymers doped with metallic inclusions: New materials for gas sensors. Sens. Actuators B 48, 362–367.


Torsi L., Dodabalapur A., Sabbatini L., and Zambonin P.G. (2000) Multiparameter gas sensors based on organic thin-fi lm-transistors. Sens. Actuators B 67, 312–316.

Torsi L., Tanese M., Cioffi N., Gallazzi M., Sabbatini L., Zambonin P., Raos G., Meille S., and Giangregorio M. (2003a) Side-chain role in chemically sensing conducting polymer fi eld-eff ect transistors. J. Phys. Chem. B 107(31), 7589–7594.

Torsi L., Tafuri A., Cioffi N., Gallazzi M.C., Sassella A., Sabbatini L., and Zambonin P.G. (2003b) Regioregular polythiophene fi eld-eff ect transistors employed as chemical sensors. Sens. Actuators B 93, 257–262.

Torsi L., Tanese M.C., Cioffi N., Gallazzi M., Sabbatini L., and Zambonin P.G. (2004) Alkoxy-substituted poly-terthiophene thin-fi lm-transistors as alcohol sensors. Sens. Actuators B 98, 204–207.

Torsi L., Tanese M.C., Crone B., Wang L., and Dodabalapur A. (2007) Organic transistor chemical sensors. In: Bao Z. and Locklin J. (eds.), Organic Field-Eff ect Transistors. CRC Press, Boca Raton, FL.

Toshima N. and Hara S. (1995) Direct synthesis of conducting polymers from simple monomers. Prog. Polym. Sci. 20, 155–183.

Tourillon G. and Garnies F. (1982) New electrochemically generated organic conducting polymers. J. Electroanal. Chem. 135(1), 173–178.

Tsujimura Y., Yokoyama M., and Kimura K. (1995) Practical applicability of silicone rubber membrane sodium-selective electrode based on oligosiloxane-modifi ed calix[4]arene neutral carrier. Anal. Chem. 67, 2401–2404.

Tsumura A., Koezuka H., and Ando T. (1986) Macromolecular electronic device: Field eff ect transistor with a poly-thiophene thin fi lm. Appl. Phys. Lett. 49, 1210–1219.

Uhlmann P.B., Pretsch E., and Bakker E. (1998) Carrier-based ion-selective electrodes and bulk optodes. 2. Ionophores for potentiometric and optical sensors. Chem. Rev. 98, 1593–1688.

Uyama Y., Kato K., and Ikada Y. (1998) Surface modifi cation of polymers by grafting. Adv. Polym. Sci. 137, 1–39.Vercelli B., Zecchin S., Comisso N., Zotti G., Berlin A., Dalcanale E., and Groenendaal L.B. (2002) Solvoconductivity

of polyconjugated polymers: Th e roles of polymer oxidation degree and solvent electrical permittivity. Chem. Mater. 14(11), 4768–4774.

Virji S., Huang J., Kaner R.B., and Weiller B.H. (2004) Polyaniline nanofi ber gas sensors: Examination of response mechanisms. Nano Lett. 4(3), 491–496.

Waltman J., Bargon J., and Diaz A.F. (1983) Electrochemical studies of some conducting polythiophene fi lms. J. Phys. Chem. 87(8), 1459–1463.

Waltman J., Diaz A.F., and Bargon J. (1984) Substituent eff ects in the electropolymerization of aromatic hetero-cyclic compounds. J. Phys. Chem. 88(19), 4343–4346.

Waltman J. and Diaz A.F. (1985) Th e electrochemical oxidation and polymerization of polycyclic hydrocarbons. J. Electrochem. Soc. 132(3), 631–634.

Wang B. and Wasielewski M.R. (1997) Design and synthesis of metal ion-recognition-induced conjugated poly-mers: An approach to metal ion sensory materials. J. Am. Chem. Soc. 119(1), 12–21.

Wang H.L., Toppare L., and Fernandez J.E. (1990) Conducting polymer blends: Polythiophene and polypyrrole blends with polystyrene and poly(bisphenol A carbonate). Macromolecules 23, 1053–1059.

Watanabe M., Sanui K., Ogata N., Kobayashi T., and Ohtaki Z. (1985) Ionic conductivity and mobility in network polymers from poly(propylene oxide) containing lithium perchlorate. J. Appl. Phys. 57(1), 123–128.

Watcharaphalakorn S., Ruangchuay L., Chotpattahanont D., Sirivat A., and Schwank J. (2005) Polyaniline/ polyimide blends as gas sensors and electrical conductivity response to CO-N2 mixtures. Polym. Int. 54, 1126–1133.

Wegner G. and Remmers M. (1995) Th in fi lms. In: Ullmann’s Encyclopedia of Industrial Chemistry, A26 Th in Films. VCH, Weinheim, Germany, 103.

Wolfbeis O.S. (1991) Biomedical applications of fi bre optic chemical sensors. Int. J. Optoelectron. 6(5), 425–441.Wolfbeis O.S. (2006) Fiber-optic chemical sensors and biosensors. Anal. Chem. 78, 3859–3874.


Wohltjen H. and Dessy R.E. (1979) Surface acoustic wave probe for chemical analysis. I. Introduction and instru-ment description. Anal. Chem. 51, 1458–1464.

Wroblewski W., Wojciechowski K., Dybko A., Brzozka Z., Egberink R.J.M, Snellink-Ruel B.H.M., and Reinhoudt D.N. (2000) Uranyl salophenes as ionophores for phosphate-selective electrodes. Sens. Actuators B 68, 313–318.

Wu X.Q., Shimizu Y., and Egashira M. (1989) Carbon dioxide sensor consisting of potassium carbonate- polyethylene glycol solution supported on porous ceramics. II. Study of carbon dioxide-sensing mechanism by employing cyclic voltammetry. J. Electrochem. Soc. 136, 2892–2895.

Xie D., Jiang Y., Pan W., Li D., Wu Z., and Li Y. (2002) Fabrication and characterization of polyaniline-based gas sensor by ultra-thin fi lm technology. Sens. Actuators B 81, 158–164.

Xia Y., Rogers J.A., Paul K.E., and Whitesides G.M. (1999) Unconventional methods for fabricating and patterning nanostructures. Chem. Rev. 99(7), 1823–1848.

Xia Y., Wiesinger J.M., MacDiarmid A.G., and Epstein A.J. (1995) Camphorsulfonic acid fully doped polyaniline emeraldine salt: Conformations in diff erent solvents studied by an ultraviolet/visible/near-infrared spectroscopic method. Chem. Mater. 7, 443–445.

Xie D., Jiang Y., Pan W., Li D., Wu Z., and Li Y. (2002) Fabrication and characterization of polyaniline-based gas sensor by ultra-thin fi lm technology. Sens. Actuators B 81, 158–164.

Yadong J., Tao W., Zhiming W., Dan L., Xiangdong C., and Dan X. (2000) Study on the NH3-gas sensitive pro-perties and sensitive mechanism of polypyrrole. Sens. Actuators B 66, 280–282.

Yan H. and Liu C. (1993) Humidity eff ects on the stability of a solid polymer electrolyte oxygen sensor. Sens. Actuators B 10, 133–136.

Yan X.B., Han Z.J., Yang Y., and Tay B.K. (2007) NO2 gas sensing with polyaniline nanofi bers synthesized by a facile aqueous/organic interfacial polymerization. Sens. Actuators B 123, 107–113.

Yang H. and Bard A.J. (1992) Th e application of fast scan cyclic voltammetry. Mechanistic study of the initial stage of electropolymerization of aniline in aqueous solutions. J. Electroanal. Chem. 339(1–2), 423–449.

Yang H., Wan J., Shu H., Liu X., Lakshmanan R.S., Guntupalli R., Hu J., Howard W., and Chin B.A. (2006) Hydrazine leak detection using poly (3-hexylthiophene) thin-fi lm micro-sensor. Proc. SPIE—Int. Soc. Opt. Eng. 6222(1), 62220S-8.

Yue J. and Epstein A.J. (1990) Synthesis of self-doped conducting polyaniline. J. Am. Chem. Soc. 112, 2800–2801.

Zawodzinski T.A., Springer T.E., Uribe F., and Gottesfeld S. (1993) Characterization of polymer electrolytes for fuel cell applications. Solid State Ionics 60, 199–211.

Zee F. and Judy J.W. (2001) Micromachined polymer-based chemical gas sensor array. Sens. Actuators B 72, 120–128.

Zhang X.B., Guo C.C., Jian L.X., Shen G.L., and Yu R.Q. (2000) Fluoroborate ion sensitive PVC membrane electrode based on chloro[tetra(m-amino-phenyl)porphinato]-manganese as neutral carrier. Anal. Chim. Acta 419, 227–233.

Zhang Y., Murphy C.B., and Jones W.E. (2002) Poly[p-(phenyleneethynylene)-alt-(thienyleneethynylene)] polymers with oligopyridine pendant groups: Highly sensitive chemosensors for transition metal ions. Macromolecules 35(3), 630–636.

Zheng W., Min Y., MacDiarmid A.G., Angelopoulos M., Liao Y.H., and Epstein A.J. (1997) Eff ect of organic va-pors on the molecular conformation of nondoped polyaniline. Synth. Met. 84, 63–64.

Zhou R., Haug M., Geckeler K.E., and Gopel W. (1993) NOx sensitivity of monomeric and polymeric N-macrocyclic compounds. Sens. Actuators B 16, 312–316.

Zhou G., Cheng Y.X., Wang L.X., Jing X.B., and Wang F. (2005) Novel polyphenylenes containing phenol- substituted oxadiazole moieties as fl uorescent chemosensors for fl uoride ion. Macromolecules 38(6), 2148–2153.

Chemical Sensors: Fundamentals of Sensing Materials, Vol. 3: Polymers and Other Materials

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