SITI NUR ZAWANI BINTI MOHAMAD ZAIN

UNIVERSITI PUTRA MALAYSIA

MODIFICATION OF SCREEN PRINTED CARBON ELECTRODE WITH SILICON NANOWIRES FOR ELECTROCHEMICAL DETECTION OF Hg(II)

AND Cd(II) IN WATER

SITI NUR ZAWANI BINTI MOHAMAD ZAIN

FS 2015 61

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MODIFICATION OF SCREEN PRINTED CARBON ELECTRODE WITH

SILICON NANOWIRES FOR ELECTROCHEMICAL DETECTION OF Hg(II)

AND Cd(II) IN WATER

By

SITI NUR ZAWANI BINTI MOHAMAD ZAIN

Thesis Submitted to the School of Graduate Studies, Universiti

Putra Malaysia, in Fulfilment of the Requirements for the Master of

Science

May 2015

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All material contained within the thesis, including without limitation text, logos, icons,

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may only be made with the express, prior, written permission of Universiti Putra

Malaysia.

Copyright © Universiti Putra Malaysia

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Abstract of thesis presented to the Senate of Universiti Putra Malaysia in fulfilment of

the requirement for the degree of Master of Science

MODIFICATION OF SCREEN PRINTED CARBON ELECTRODE WITH

SILICON NANOWIRES FOR ELECTROCHEMICAL DETECTION OF Hg(II)

AND Cd(II) IN WATER

By

SITI NUR ZAWANI MOHAMAD ZAIN

May 2015

Chairman: Ruzniza Mohd Zawawi, PhD

Faculty: Science

Water pollution resulting from heavy metal ions such as mercury and cadmium tends to

have lethal effects on the environment and living organisms. This indicates the need for

further research to develop on heavy metal sensors that are fast, potable and cost

effective. In this research, Silicon Nanowires (SiNW’s), a 1-dimensional nanowire was

used as a modifier for disposable screen printed carbon electrodes (SPCE) for detection

of metal ions. The SiNW’s was characterized by different spectroscopic techniques and

the application of SiNW’s on the surface of the electrode was found to increase the

sensitivity of the electrode.

The screen printed carbon electrode was modified by casting SiNW’s with 3-

aminopropyl-triethoxysilane (APTES) onto the working electrode surface. The

modified electrode (SiNW’s/APTES/SPCE) was then applied for Hg2+ and Cd2+ ion

detection. Electrochemical studies using linear sweep stripping voltammetry performed

with SiNW’s/APTES/SPCE were found to give a better response through the

optimization of some analytical parameters.

Concentration study of mercury with SiNW’s/SPCE gave linear calibrations and a

detection limit value of 42.59 μg L−1 was achieved by applying deposition potential of

-1.2V and deposition time of 160s. The electrode showed very good recovery

indicating the accuracy of the method, while concentration study of cadmium with

SiNW’s/SPCE gave a linear calibration with R2 = 0.994. A detection limit of 251.7

μg L−1 was also achieved by applying deposition potential of -1.2V and deposition time

of 160s. Validation of the method with inductively coupled plasma-mass spectroscopy

(ICP-MS) and atomic absorption spectroscopy showed very good correlation. This

modified screen printed carbon electrodes with Silicon Nanowires

(SiNW’s/APTES/SPCE) also found for simultaneously detection of Hg2+ and Cd2+ ions

in the solution.

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Abstrak tesis yang dikemukakan kepada Senat Universiti Putra Malaysia sebagai

memenuhi keperluan untk ijazah Sarjana Sains

PENGUBAHSUAIAN ELEKTROD KARBON SKRIN TERCETAK DENGAN

SILICON NANOWIRES(SiNW’S) UNTUK PENGESANAN ELEKTROKIMIA

BAGI Hg(II) DAN Cd(II) DI DALAM AIR

Oleh

SITI NUR ZAWANI MOHAMAD ZAIN

Mei 2015

Pengerusi: Ruzniza Mohd Zawawi, PhD

Fakulti: Sains

Pencemaran air dengan ion logam berat seperti merkuri dan kadmium memberi kesan

kepada persekitaran dan organisma hidup. Ini menunjukkan penyelidikan mengenai

penderia logam berat yang memberi keputusan yang pantas, ringkas dan kos efektif

adalah diperlukan. Dalam kajian ini, Silicon Nanowires(SiNW’s) yang merupakan 1-

dimensi dawai nano telah digunakan sebagai bahan pengubah suai untuk elektrod

karbon skrin tercetak terpakai buang untuk mengesan logam berat ion. SiNW’s telah di

cirikan dengan menggunakan teknik spektroskopi, dan SiNW’s diaplikasikan ke atas

permukaan elektrod untuk meningkatkan kepekaan electrod tersebut.

Elektrod karbon skrin tercetak telah diubah suai dengan melekatkan SiNW’s dengan

APTES diatas permukaan elektrod kerja. Elektrod yang diubah suai

(SiNW’s/APTES/SPCE) telah diaplikasikan untuk mengesan Hg2+ dan Cd2+ ion. Kajian

elektrokimia menggunakan voltametri sapuan linear pelucutan dilakukan menggunakan

SiNW’s/APTES/SPCE memberi kesan yang lebih baik dengan mengoptimumkan

sebahagian parameter analisis.

Kajian kepekatan merkuri dengan SiNW’s/APTES/SPCE telah memberikan

penentukuran linear dan had pengesanan 42.59 μg L−1 dengan menggunakkan potensi

pemendapan -1.2V dan masa pemendapan selama 160s. Elektrod ini menunjukkan

pengambilan semula yang baik yang menunjukkan ketepatan kaedah ini. Sementara

kajian kepekatan kadmium dengan SiNW’s/APTES/SPCE telah memberikan

penentukuran linear dengan R2 = 0.994. Had pengesanan 251.7 μg L−1 telah dicapai

dengan menggunakan potensi pemendapan -1.2V dan masa pemendapan selama 160s.

Pengesahan kaedah ini dengan Spektrometriberat–induktif pasangan plasma (ICP-MS)

dan spektroskopi serapan atom menunjukan korelasi yang sangat baik. Dengan elektrod

karbon skrin tercetak dengan SiNW’s (SiNW’s/APTES/SPCE) ini juga didapati

mampu untuk mengesan Hg2+ dan Cd2+ secara serentak.

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ACKNOWLEDGEMENTS

Firstly, I would like to thanks to my supervisor and co-supervisor Dr. Ruzniza Mohd

Zawawi and Prof Nor Azah Yusof for their encouragement and help. Without their

help, guidance encouragement and knowledge I can’t reach this state.

To my beloved parents, mama and ayah thank you so much support my journey to

continue my study. Without them I can’t achieved my dream. To my siblings thank you

for financial support and advice. To my husband thank you so much for your fully

support to me to finish this study.

To my friends and colleagues, izzah, k.emy, iffah, wani thank you for support and

make me enjoying life at UPM. The enjoying time in UPM will be in my memory

forever. Their encouragement to me to finish my study, love all of you. Special thanks

farhana, zida, zira, salama always advise me and make me motivated to finish my

journey in this study.

Best Regards

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I certify that a Thesis Examination Committee has met on 11th May 2015 to conduct

the final examination of Siti Nur Zawani Mohamad Zain on her thesis entitled

Modification Of Screen Printed Carbon Electrode With Silicon Nanowires (SiNW’s)

For Electrochemical Detection Of Hg(II) And Cd(II) In Water in accordance with the

Universities and University Colleges Act 1971 and the Constitution of the Universiti

Putra Malaysia [P.U.(A) 106] 15 March 1998. The Committee recommends that the

student be awarded the Master of Science.

Members of the Thesis Examination Committee were as follows:

Haslina Binti Ahmad, PhD

Faculty of Science

Universiti Putra Malaysia

(Chairman)

Zulkarnain B Zainal, PhD

Faculty of Science

Universiti Putra Malaysia

(Internal Examiner)

Yatimah Alias, PhD

Department of Chemistry

Faculty of Science

Universiti Malaya

(External Examiner)

____________________________________

ZULKARNIAN ZAINAL, PhD

Deputy Dean

School of Graduate Studies

Universiti Putra Malaysia

Date: 12 August 2015

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This thesis was submitted to the Senate of Universiti Putra Malaysia and has been

accepted as fulfilment of the requirement for the degree of Master of Science. The

members of the Supervisory Committee were as follows:

Ruzniza Mohd Zawawi, PhD

Senior Lecturer

Faculty of Science

Universiti Putra Malaysia

(Chairman)

Nor Azah Yusof, PhD

Professor

Faculty of Science

Universiti Putra Malaysia

(Member)

_________________________________

BUJANG BIN KIM HUAT, PhD

Professor and Dean

School of Graduate Studies

Universiti Putra Malaysia

Date:

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Declaration by graduate student

I hereby confirm that:

this thesis is my original work;

quotations, illustrations and citations have been duly referenced;

this thesis has not been submitted previously or concurrently for any other degree

at any other institutions;

intellectual property from the thesis and copyright of thesis are fully-owned by

Universiti Putra Malaysia, as according to the Universiti Putra Malaysia

(Research) Rules 2012;

written permission must be obtained from supervisor and the office of Deputy

Vice-Chancellor (Research and Innovation) before thesis is published (in the form

of written, printed or in electronic form) including books, journals, modules,

proceedings, popular writings, seminar papers, manuscripts, posters, reports,

lecture notes, learning modules or any other materials as stated in theUniversiti

Putra Malaysia (Research) Rules 2012;

there is no plagiarism or data falsification/fabrication in the thesis, and scholarly

integrity is upheld as according to the Universiti Putra Malaysia (Graduate

Studies) Rules 2003 (Revision 2012-2013) and the Universiti Putra Malaysia

(Research) Rules 2012. The thesis has undergone plagiarism detection software.

Signature: ________________________ Date: __________________

Name and Matric No.: _________________________________________

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Declaration by Members of Supervisory Committee

This is to confirm that:

the research conducted and the writing of this thesis was under our supervision;

supervision responsibilities as stated in the Universiti Putra Malaysia (Graduate

Studies) Rules 2003 (Revision 2012-2013) are adhered to.

Signature: _______________________

Name of

Chairman of

Supervisory

Committee: _______________________

Signature: _______________________

Name of

Member of

Supervisory

Committee: _______________________

Signature: _______________________

Name of

Member of

Supervisory

Committee: _______________________

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TABLE OF CONTENTS

Page

ABSTRACT i

ABSTRAK ii

ACKNOWLEDGEMENT iii

APPROVAL iv

DECLARATION vi

LIST OF FIGURES x

LIST OF TABLES xiii

LIST OF ABBREVIATIONS xiv

CHAPTER

1 INTRODUCTION 1.1 Heavy Metals 1 1.2 Screen Printed Electrode (SPEs) 3

1.3 Problem Statement 3

1.4 Objective of the studies 4

1.4.1 General Objective 4

1.4.2 Specific Objective 4

2 LITERATURE REVIEWS 5 2.1 Cadmium toxicity 5

2.2 Mercury toxicity 5

2.3 Voltammetry 5

2.4 Linear sweep voltammetry (LSV) 7

2.5 Anodic stripping voltammetry (ASV) 8

2.6 Preparation of sensors 10

2.6.1 Modification of Screen printed electrode by nanomaterial 10

2.6.2 Silicon Nanowires 11

2.6.3 Ionophore 12

2.6.4 Application of Ionophore in Development of Heavy Metal

Detection 13

3 METHODOLOGY 15 3.1 Material and reagents 15

3.1.1 Transmission Electron Microscopy (TEM) 14

3.1.2 Inductively Coupled Plasma (ICP) 16

3.1.3 Atomic Absorption Spectroscopy (AAS) 16

3.2 Preparation of modified electrodes (Screen printed carbon electrode) 17

3.3 Preparation of working electrode 17

3.4 General procedures for electrochemical measurements 17

3.5 General procedures for sample measurements 18

3.5.1 Transmission Electron Microscopy (TEM) 18

3.5.2 Inductively Coupled Plasma (ICP) 18

3.5.3 Atomic Absorption Spectroscopy (AAS) 18

3.6 Optimization of the Screen Printed Carbon Electrode 19

3.6.1 Effect of supporting Electrolyte 19

3.6.2 Effect of Deposition Potential 19

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3.6.3 Effect of Deposition Time 19

3.6.4 Effect of Varying Scan Rate 19

3.6.5 Effect of Varying Hg2+ and Cd2+ Concentration 19

3.6.6 Effect of foreign Substances 20

3.6.7 Analysis of Cd2+ and Hg2+ in Waste Water 20

3.6.8 Reproducibility 20

4 RESULT AND DISCUSSION 21 4.1 Morphology of electrode 21

4.2 Effect of supporting Electrolyte for detection of Hg2+ and Cd2+ ions 23

4.3 Effect of Deposition Time for Hg2+ and Cd2+ ions detection 27

4.4 Effect of Deposition Time for Hg2+ and Cd2+ ions detection 28

4.5 Effect of Varying Scan Rate for Hg2+ and Cd2+ ions detection 29

4.6 Effect of Varying Hg2+ and Cd2+ Concentration 33

4.7 Electrochemical Characterization of the Modified Screen

Printed Electrode for detection of Hg2+ and Cd2+ 36

4.8 Effect of Foreign Substances for Hg2+ and Cd2+ ions detection 38

4.9 Analysis of Hg2+ and Cd2+ in SeaWater 40

4.10 Reproducibility and Stability for Hg2+ and Cd2+ ions 40

4.11 Modification SiNW’S/APTES/Mercury ionophore /SPCE 41

4.11.1 Effect of ionophore/SiNW’s in stripping current of Hg2+ 41

4.11.2 Comparison study 42

4.11.3 Effect of various Hg2+ concentrations 43

5 CONCLUSION AND RECOMMENDATIONS FOR FUTURE

RESEARCH 45

REFERENCES 47

APPENDICES 53

BIODATA OF STUDENT 55

LIST OF PUBLICATION 56

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LIST OF FIGURES

Figure Page

1.1 Source of Heavy Metal Pollution to Aqueous System

(http://www.sagasiki-kankyo.co.jp)

1

1.2 Schematic diagram of bio/chemical sensor 2

1.3 Screen printed carbon electrode consists of working electrode;

counter electrode and reference electrod

3

2.1 Schematic diagram of Linear Sweep Voltammetry (LSV) (A)

voltage scanned from a lower limit to an upper limit by

changing the time & (B) current response plotted as a function

of voltage

7

2.2 Series of linear sweep voltammograms recorded at different

scan rates

8

2.3 Schematic diagram of Anodic Stripping Voltammetry (ASV)

process with Cd2+ analytes at the SPE

9

2.4 Schematic diagram of the ASV process with 4 steps (A)

Cleaning step, (B) Electroplating step, (C) Equilibration step

and (D) Stripping step

9

3.1 Schematic diagram of modern analytical electron microscope 15

3.2 Schematic diagram of experimental setup used in this study 18

4.1 FESEM images of unmodified SPCE and SiNW’s modified

SPCE: (a) unmodified SPCE; (b) SiNW’s/APTES modified

SPCE.

21

4.2 The fabrication process for modification of SiNW’s

/APTES/SPCE.

22

4.3 FESEM-EDS analysis result for a) unmodified SPCE and b)

modified SPCE with SiNWs/APTES

22

4.4 TEM image of the functionalize of SiNW’s with APTES 23

4.5 LSASV voltammograms for SiNW’s/APTES/SPCE in 100

ppm Hg2+ with (a) 0.1 M Na2SO4, (b) 0.1 M Acetic Buffer (c)

0.1 M KCl and (d) 0.1 M HCl electrolyte. Potential scanning

in positive direction from -1.0 V to -0.3 V vs Ag/AgCl at scan

rate of 100 mVs-1 , deposition potential;-1.2, deposition time:

120 secs.

24

4.6 LSASV voltammograms for SiNW’s/APTES/SPCE in 100

ppm Cd2+ with (a) 0.1 M Na2SO4, (b) 0.1 M Acetic Buffer (c)

0.1 M KCl and (d) 0.1 M HCl electrolyte. Potential scanning

in positive direction from -1.0 V to -0.3 V vs Ag/AgCl at scan

rate of 100 mVs-1, deposition potential: -1.2V, deposition

time; 120 secs.

25

4.7 Effect of the HCl concentration on the peak current of Hg2+

and Cd2+ in the solution of 100 ppm Cd2+ and 100 ppm Hg2+,

Deposition time: 120 secs; Deposition potential -1.2 V.

26

4.8 Effect of deposition potential on the peak current of Hg2+ and

Cd2+ in the solution of 100 ppm Cd2+ and 100 ppm Hg2+.

27

4.9 Effect of deposition time on the peak current of Hg2+ and Cd2+

in the solution of 10 ppm Cd2+ and 5 ppm Hg2+: Deposition

potential -1.2.

28

4.10 Effect of varying the scan rate on the LSV of 29

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SiNW’s/APTES/SPCE in 100 ppm Hg2+ and 0.05 M HCl as

supporting electrolyte. Potential scanning in positive direction

from -0.2 V to 0.5 V vs Ag/AgCl at a scan rate 0.01 Vs-1 to

0.1 Vs-1, deposition potential: -1.2, deposition time: 160 secs.

4.11 (A) Dependence of log cathodic current on log (V) (B) current

vs V and (C) current vs V1/2 for SiNW’s/APTES/SPCE in 10

ppm Hg2+ and 0.05 M HCl as supporting electrolyte. Potential

scanning in positive direction from -0.2 V to 0.5 V vs

Ag/AgCl at scan rate 0.01 Vs-1 to 0.1 Vs-1, deposition

potential:-1.2, deposition time: 160 secs.

31

4.12 Effect of varying the scan rate on the LSV of

SiNW’s/APTES/SPCE in 10 ppm Cd2+ and 0.05 M HCl as

supporting electrolyte. Potential scanning in positive direction

from -1.1 V to -0.3 V vs Ag/AgCl at scan rate of 0.01 Vs-1 to

0.1 Vs-1, deposition potential: -1.2, deposition time: 160 secs.

31

4.13 (A) Dependence of log cathodic current on log scan rate (B)

current vs V and current vs V1/2 for SiNW’s/APTES/SPCE in

10 ppm Cd2+ and 0.05 M HCl as supporting electrolyte.

Potential scanning in positive direction from -0.2 V to 0.5 V

vs Ag/AgCl at scan rate of 0.01 Vs-1 to 0.1 Vs-1, deposition

potential: -1.2, deposition time: 160 secs.

33

4.14 Votammograms of SiNW’s/APTES/SPCE in 0.05M HCl

supporting electrolyte with different concentrations of Hg2+.

Potential scanning in positive direction from -0.5 V to 0.5 V

vs Ag/AgCl at scan rate of 100 mVs-1.

34

4.15 Current intensity versus concentration of Hg2+ in range of 4 -

18 ppm using SiNW’s/APTES/SPCE in 0.05 M HCl as

supporting electrolyte. Potential scanning in positive direction

from -0.5 V to 0.5 V vs Ag/AgCl at scan rate of 100 mVs-1.

34

4.16 Votammograms of SiNW’s/APTES/SPCE in 0.05M HCl

supporting electrolyte with different concentrations of Cd2+.

Potential scanning in positive direction from -1.0 V to -0.6 V

vs Ag/AgCl at scan rate of 100 mVs-1.

35

4.17 Current intensity versus concentration of Hg2+ in range of 4 –

18 ppm using SiNW’s/APTES/SPCE in 0.05 M HCl as

supporting electrolyte. Potential scanning in positive direction

from -1.0 V to -0.6 V vs Ag/AgCl at scan rate of 100 mVs-1.

35

4.18 Voltammogram of simultaneous detection at constant

concentration of 10 ppm of Cd2+ and various concentrations of

Hg2+ ions.

36

4.19 Comparison of LSASV voltammograms for (a) unmodified

SPCE and (b) modified SiNW's/APTES/SPCE in 10 mg/L

Hg2+ and 0.05M HCl, deposition potential: -1.2, deposition

time:160 secs, potential scanning in positive direction from -

0.4 V to 0.4 V vs Ag/AgCl at scan rate of 100 mVs-1

37

4.20 Comparison of LSASV voltammograms for (a) unmodified

SPCE and (b) modified SiNW's/APTES/SPCE in 10 mg/L

Cd2+ and 0.05M HCl, deposition potential:-1.2, deposition

time;160 secs, potential scanning in positive direction from -

1.1 V to -0.3 V vs Ag/AgCl at scan rate of 100 mVs-1

37

4.21 Effect of foreign substances in Hg2+ using 39

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SiNW’s/APTES/SPCE in 0.05M HCl as supporting

electrolyte.

4.22 Effect of foreign substances in Cd2+ using

SiNW’s/APTES/SPCE in 0.05M HCl as supporting

electrolyte.

39

4.23 Stability Study for 10 ppm Hg2+ with SiNW’s/APTES/SPCE 41

4.24 Effect of Mercury ionophore concentration versus current 42

4.25 Voltammogram for mixture of 10ppm of Cd2+Pb2+and Hg2+in

0.05M HCl using SiNW’s/APTES/SPCE and

SiNW's/APTES/Hg ionophore/SPCE

42

4.26 Hg ionophore and the interaction of Hg ionphore with Hg2+

ions

43

4.27 Votammograms of SiNW’s/APTES/Hg ionophore screen

printed carbon electrode in 0.05M HCl supporting electrolyte

with different concentration of Hg2+Potential scanning in

positive direction from -0.5 V to 0.5 V vs Ag/AgCl at a scan

rate 100mVs-1.

43

4.28 Current intensity versus concentration of Hg2+ in range of

2ppm to 10ppm using SiNW’s /APTES/Hg ionophore

modified screen printed carbon electrode in 0.05 M HCl as

supporting electrolyte. Potential scanning in positive direction

from -0.5 V to 0.5 V vs Ag/AgCl at a scan rate 100mVs-1.

44

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LIST OF TABLES

Table Page

2.1 Electrochemical Sensors applied in Hg2+ detection 6

2.2 Electrochemical Sensors applied in Cd2+ detection 7

2.3 Various examples of ionophores for determination of heavy

metal ions

14

3.1 Hg2+ and Cd2+ concentration study on modified electrode 20

4.1 Summary of detection limit for Hg2+ and Cd2+ modified

SiNW’s/APTES/SPCE under optimum condition with LSASV 33

4.2 Determination of Hg2+ and Cd2+ ions in sea water using

SiNW’s/APTES/SPCE in 0.05M HCl as supporting electrolyte.

40

4.3 Reproducibility study for 100 ppm Cd2+ and 10 ppm Hg2+ ions

with SiNW’s/APTES/SPCE

41

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LIST OF ABBREVIATIONS

A Electrode surface area

APTES 3-aminopropyl-triethoxysilane

ASV Anodic Stripping Voltammetry

AuNP Gold nanoparticles

C Concentration of analyte or reactant in the bulk solution

CNT Carbon nanotubes

CSV cathodic stripping voltammetry

CV Cyclic Voltammetry

DPASV Differintial Pulse Anodic Stripping Voltammetry

GCE Glassy carbon electrode

I Current

LSASV Linear Sweep Anodic Stripping Voltammetry

MWCNT Multi wall carbon nanotubes

N Numbers of electrons

ppb part per billion

ppm part per million

SAM Self-assembled monolayer

SiNW’s Silicon Nanowire’s

SPCE screen printed carbon electrode

SPE screen printed electrode

SWASV Square Wave Anodic Stripping Voltammetry

v Scan rate

WHO World Health Organization

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CHAPTER 1

INTRODUCTION

1.1 Heavy Metals

Heavy metals are mostly defined by their chemical properties such as malleability,

ductility with metallic luster and the ability to conduct heat and electricity as well as

possessing the ability to lose its outer most electrons to form cations with basic oxides

(John et al., 2002). They are group of metals with a high density which is greater than

5 gcm-3. Heavy metals are known to form stable complexes or chelates with a variety of

ligands and their average stability decreases with electronegativity of the metal in

which they are attached to in the following order Pd > Cu > Ni > Co > Zn > Cd > Mn

(Mellor et al., 1947, Irving et al., 1948). And several factors such as pH, are found to

influence this order.

Heavy metal ions are regarded as the most toxic pollutants due to their acute toxicities,

carcinogenicities and non-biodegradability. The main sources of heavy metal

contamination are mostly contributed from metal related industries such as metal

plating facilities, mining operations, fertilizer and electronic device manufactures,

agricultural activities, vehicle emission, and domestic activities (Juang et al., 2002).

The oxidation of the different metal species causes the solubility of the metal ions thus

released into the surrounding environment through water drainages.

Wastes discharged from industrial and agricultural activities containing heavy metal

can accumulate into sludge and drain down to cultivated soil; hence, this could easily

be transferred into the food chain (Zhao et al., 2014). In many developing countries

around the world, an increasing concentration of heavy metals in the environment is a

serious problem to both human and animal health as well as protection and production

of food stuffs.

Figure 1.1: Source of Heavy Metal Pollution to Aqueous System

(http://www.sagasiki-kankyo.co.jp)

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Previous studies have proved that the use of spectroscopic techniques, such as

inductively coupled plasma mass spectrometry (Hassan et al., 2004), surface-enhanced

raman scattering (Enzhong et al., 2012) and cold vapour atomic fluorescence

spectroscopy (Yu et al., 2005) in detecting the presence of this heavy metals can yield

an accurate result. However, these techniques are highly costly because it requires

expensive instruments, routine maintenance and specialized personnel in order to run

the samples. These techniques also do not allow onsite analysis due to non-portability

of the equipment.

For these reasons electrochemical sensors were developed and extensively being used

in studying environmental pollution with diverse industrial applications. There is high

demand of sensors in the world market that are easy to handle, compatible and can give

a fast result. These sensors also offer advantages of low detection limits, a wide linear

response range, good stability and reproducibility. Electrochemical sensor can be

portable, simple to use,in-situ and miniature in size.These features are ideal for real

time on field manaagement, thus the errors caused by the sample transportation and

storage can largely be reduced (Cammann et al., 1996). The biochemical sensor usually

consists of the analyte for example food sample, environmental sample or human

sample and receptor that is specifically binded to the analyte, transducer element for

example nanomaterial converted to an electronic signal by electrodes and amplified by

a detector circuit using an appropriate reference and sent for processing to the computer

software to be converted to a meaningful physical feature for example light, signal,

and data display (Belluzo et al., 2007).

Figure 1.2: Schematic diagram of a bio/chemical sensor

The electroanalytical techniques that are usually employed in chemical sensor analysis

includes; cyclic voltammetry(CV), differential pulse voltammetry (DPV), square wave

voltammetry (SWV) and stripping voltammetry (SV). The stripping voltammetry is a

sensitive electroanalytical technique used for the determination of trace amount of

metals in a given solution. The technique consists of three steps. In the first step, metal

ions are deposited onto an electrode which is held at a suitable potential. The solution

is stirred during this step in order to maximize the amount of metal deposited. For the

second step, stirring is stopped so that the solution will settles and finally in the third

step, the metal deposits are stripped from the electrode by scanning the potential. The

observed current during the stripping step can be used as an indirect measuremnt for

the amount of metals in the solution.

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The stripping step may consist of a positive or negative potential scan by creating either

an anodic or cathodic current respectively. Hence, they are named as anodic stripping

voltammetry (ASV) and cathodic stripping voltammetry (CSV). Anodic stripping

voltammetry (ASV) has been recognized as a powerful technique for electrochemical

measurements of trace metal ions in various samples of environmental, clinical, and

industrial origin (Injang et al., 2010 , Yi et al., 2012) in particular due to its capability

of preconcentrating analytes on the surface of the working electrode. It also allows

quantification of heavy metal ions down to part per million (ppm) or even to part per

billion (ppb) (Gu et al.,2013). Cyclic voltammetry (CV) is also widely used to study

the presence of heavy metal but is not sensitive enough for environmental analysis

never the less it is useful in optimization of analytical conditions (Buffle et al., 2005).

1.2 Screen Printed Electrode (SPEs)

The advent of disposable screen printed electrodes (SPEs) as an alternative to the

conventional electrodes in development of analytical methods that respond to perform

rapid ‘in situ’ analyses has become necesary due to many advantages it possess such as

low cost, ease of handling and ease of preparation and moreover can be treated as

disposable, thus avoiding the need for surface cleaning and the problem of memory

effects.

Figure 1.3: Screen printed carbon electrode consists of working electrode; counter

electrode and reference electrode

The production of the SPEs comes with different ink on planar ceramic or

plastic supports. SPEs can be modified by adding versatile substances of different

nature such as nanomaterial, enzymes, self assembled monolayer (SAM), polymers and

complexing agents (Honeychurch et al.,2012).

1.3 Problem Statement

Development of heavy metal ions detection and quantification in the environment is

crucial to our present society. There is an increasing need for analytical systems that

deliver fast and reliable data in the development of novel sensors. Sensitivity

enhancement and lowering of the detection limit of heavy metal ions are the effort

neede to be focused on.

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Nanoparticles is receiving particular attention in sensor and biosensor application, for

it’s excellent conducting ability and capability in increasing sensitivity. Nanoparticles;

have shown a great potential application in a wide range of existing and emerging

technologies. Nanoparticles are used in heavy metals detection in order to avoid the

previous used of mercury electrodes due to toxicity of mercury.

In this study SiNW’s was used as the nanomaterial because it have excellent

conducting capability and high surface-to-volume ratio to enhance the detection limit.

Selectivity is another important issue in heavy metals detection. Macrocyclic molecules

as ionophore can be the solution to this problem, because ionophore provides binding

sites for interaction with metal ions. In this research, Hg2+ and Cd2+ were detected by

electrochemical method using SiNW’s modified SPCE. The detection limit of Hg2+ also

much lower when modified SPCE with SiNW’s/ionophore.

1.4 Objective of the studies

1.4.1 General Objective

The obective of this research is to develop a sensor for selective and specific detection

and quantification of toxic metal ions such as Hg2+ and Cd2+ ions in environmental

samples by utilizing silicon nanowires (SiNW’s).

1.4.2 Specific Objective

1. To modify the screen printed carbon electrode (SPCE) for selective and specific

detection of heavy metal ions Hg2+ and Cd2+ by utilizing silicon nanowires

(SiNW’s).

2. To optimize the deposition time, deposition potential, supporting electrolyte and

characterize with SEM the modified SPCE.

3. To test the fabricate sensor on sea water samples.

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