Transcript Class-4-

BIOMEMS
Class II. Electrochemistry Background (I)
Winter 2011
Dr. Marc Madou
Contents
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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
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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
Double layer-(in case of a metal 10-40 µF cm-2)
Inner Helmholtz plane (IHP)
Outer Helmholtz plane (OHP)
Gouy-Chapman layer (GCL)
Metals in Solution
Metals in Solution
Metals in solution
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In order for current to pass the
O + ne_  R 107-10 8 V cm-1
interface Me/solution an
_
R
ne
O
electochemical reaction must occur: an
Oxidant O (say Fe 3+) gets reduced in
Metal (working electrode, sensing
a cathodic reaction (on the cathode, electrode, detector electrode)
also the working electrode in this case
Electrolyte
(WE)) to become a reductant R (say
Fe 2+)
For a complete circuit a counter
electrode must also be present in the
Anode also CE (in this case)
cell for the reverse or anodic reaction
on the anode (Counter electrode (CE))
Redox couple
e.g. Fe3+ + 1 e_  Fe 2+
Without applied bias the potential
drop across the Helmholtz layer on the Cathode also WE (in this case)
WE (e.g. a Pt electrode) is determined

by the redox species with the largest
exchange current density i0,e
Metals in solution
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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)
Redox couple
e.g. Fe3+ + 1 e_  Fe 2+

Electroactive redox couple means electrons
are exchanged wit h t he sensing electrode at a
reasonable rat e
e.g. Fe3+ + 1 e_  Fe3+
Metals in solution
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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
Metals in Solution
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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
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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
Semiconductor
e.g. TiO2
Electrolyte
Semiconductors in solution
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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.
Solution
V FB
Solid electrolytes in solution
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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
Solution
F-
Solid electrolyte
e.g. LaF3
Electrolyte
Insulators in solution
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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 !!
Insulator
Electrolyte
Mixed conductors in solution
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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 700
have made this mixed conductor (e, H+)
into a very good pH sensor with small 600
redox interference
500
EMF / mV
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pH 1.68
pH 2.00
pH 4.01
pH 5.00
400
pH 7.00
300
pH 8.00
200
pH 10.00
100
1
2
3
Test
4
Homework
1.
2.
3.
4.
Suggest an array of sensors that could be used for an
electronic tongue (five tastes)
Make a list of biosensors that have been used in-vivo. How
long is the longest that a biosensor has been used in-vivo?
Explain why the more selective biosensors are the least
reversible (compare in this context an enzyme sensor with an
immuno sensor)
Draw the equivalent electrical circuit of a metal/electrolyte
interface with the electrode at a potential so that a redox
reaction occurs