Dr. Marc Madou Class II. Electrochemistry Background (I) Winter 2009 BIOMEMS.

Dr. Marc Madou

Class II. Electrochemistry Background (I)

Winter 2009

BIOMEMS

Contents

Sensors and interfaces Electrodes (materials) in

solutions– Metals in solution– Semiconductors in solution– Solid electrolytes in

solution– Insulators in solution– Mixed conductors in

solution

Sensors and interfaces Every chemical sensor is about an

interface with the environment, the better one can control the sensor surface the better one can control that interface and thus say something about the environment

We will look into some detail at solid/liquid before we discuss any type of electrochemical sensor in detail

Interfaces are very complex often involving fractals (beach, trees, snow flakes, etc.) rather than smooth transitions, this implies that perfect selectivity will be hard to achieve (too many different binding sites)

Electrodes (materials) in solution

Charge carriers in electrode materials:

– Metals (e.g. Pt) : electrons

– Semiconductors (e.g. n-Si) : electrons and holes

– Solid electrolytes (e.g. LaF3 ) : ions

– Insulators (e.g. SiO2):no charge carriers

– Mixed conductors (e.g. IrOx) : ions and electrons

– Solution (e.g. 1 M NaCl in H2O): solvated ions

Inner Helmholtz plane (IHP)Outer Helmholtz plane (OHP)Gouy-Chapman layer (GCL)

Double layer-(in case of a metal 10-40 µF cm-2)

Metals in Solution

Metals in Solution

Metals in solution In order for current to pass the

interface Me/solution an electochemical reaction must occur: an Oxidant O (say Fe 3+) gets reduced in a cathodic reaction (on the cathode, also the working electrode in this case (WE)) to become a reductant R (say Fe 2+)

For a complete circuit a counter electrode must also be present in the cell for the reverse or anodic reaction on the anode (Counter electrode (CE))

Without applied bias the potential drop across the Helmholtz layer on the WE (e.g. a Pt electrode) is determined by the redox species with the largest exchange current density i0,e

Electrolyte

Metal (working electrode, sensing electrode, detector electrode)

O + ne_ → R

R - ne_ → O

A

Cathode also WE (in this case)

Anode also CE (in this case)

€

Redox couple

e.g. Fe3+ + 1 e_ ↔ Fe2+

107-10 8 V cm-1

Metals in solution The fastest electron-exchange reaction

(the rate of electrons going back and forth between redox species and electrode in equilibrium i.e. at zero current) determines the potential of the electrode ---zero external current and no net reaction

Often there are different redox species involved in establishing the equilibrium potential in which case we speak about a mixed potential---zero external current but no net reaction (e.g. corrosion)

A working electrode (e.g. Pt) that changes potential with the redox couple present is called an electrode of the first kind and it is our first sensor we encounter in this course

An electrode that does not change it’s potential with solution composition is an electrode of the second kind i.e. a reference electrode (see below)

V

Reference electrode (second kind)

Working electrode or sensor (first kind)

Electroactive redox couple means electrons

are exchanged with the sensing electrode at a

reasonable rate

e.g. Fe3+ + 1 e _ ↔ Fe3+

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Redox couple

e.g. Fe3+ + 1 e_ ↔ Fe2+

Metals in solution

The inert species in solution (e.g. NaCl ) also called indifferent electrolyte do not (at zero or low bias) exchange electrons with the Pt electrode but they provide solution conductivity

The inert electrolyte ensures that the electroactive species reaches the electrode by diffusion and not by migration

All redox couples have a known redox potential as measured against a standard reference electrode (e.g. Standard Hydrogen Electrode or SHE)

A first type of sensor measures redox potential of a solution and it consists of a voltmeter, a Pt electrode and a reference electrode

10 -4 M Fe2+/Fe3+

1 M NaCl

10 -4 M Fe2+/Fe3+

Migration Diffusion

V

Metals in Solution The two E° values shown refer

to "standard" conditions of unit H+ activity (pH=0) and gas pressures of 1 atm. At combinations of pH and E that lie outside the shaded area, the partial pressures of O2 or H2 exceed 1 atm, signifying the decomposition of water. The unity partial pressures are of course arbitrary criteria; in a system open to the atmosphere, water can decompose even at much lower H2 partial pressures, and at oxygen pressures below 0.2 atm. Fortunately, these processes are in most cases quite slow.

Semiconductors in solution In this case most of the potential drop is in the

semiconductor instead of in the solution Transport of charges to and from solution is

limited to those redox systems that have states that overlap with the semiconductor bands

ElectrolyteSemiconductore.g. TiO2

Semiconductors in solution When the semiconductor is in contact with the

solution a band bending results just as in the case of a conductive solution contacting a metal

The flat-band potential (V FB) is that potential one needs to apply to make the bands flat in the semiconductor all the way to the surface (it can be deduced from a capacitance measurement of the interface)

For a semiconductor covered with an oxide (e.g. Si with SiO2 , TiO2) the flat band potential is a function of pH (ionization of the surface OH groups changes with pH) and is often independent of redox systems (depending on their overlap with the semiconductor bands)

This is the second sensor we have encountered in this case the sensor is mainly a pH sensor.

EF

EC

EV

hν

-n Si

OH -

-OH

-OH

-OH-O--O-

-O-

-O-

Solution

V FB

Solid electrolytes in solution No electrons exchange at the

surface just ions exchange with the solid often with very high selectively

The fastest ion-exchange reaction determines the potential i.e. i0,i in the case of LaF3 that is F- (also glass for H+)

This is a third type of sensor we encounter here i.e. an ion selective sensor

ElectrolyteSolid electrolytee.g. LaF3

Solution

F-

Insulator Electrolyte No electron exchange and no ion exchange If it is an oxide insulator it will exhibit pH

sensitivity like an oxide semiconductor But how do you measure such a high

impedance, the voltmeter will just show an overload ? See later under ISFET !!

Insulators in solution

Mixed conductors in solution Both ions and electrons may exchange

at the surface Depending on the relative magnitude of

i o,e vs. i o,i the electrode will be a redox sensor or an ion sensor, for most mixed conductors i o,e >>> i o,i

IrOx may be one of the exceptions we have made this mixed conductor (e, H+) into a very good pH sensor with small redox interference

pH 1.68

pH 2.00

pH 4.01

pH 5.00

pH 7.00

pH 8.00

pH 10.00

100

200

300

400

500

600

700

Test

EMF / mV

4321

Homework

1. Suggest an array of sensors that could be used for an electronic tongue (five tastes)

2. Make a list of biosensors that have been used in-vivo. How long is the longest that a biosensor has been used in-vivo?

3. Explain why the more selective biosensors are the least reversible (compare in this context an enzyme sensor with an immuno sensor)

4. Draw the equivalent electrical circuit of a metal/electrolyte interface with the electrode at a potential so that a redox reaction occurs