Biomedical Engineering Reference
In-Depth Information
In this circuit, IC1, IC2A, and IC2B form a potentiostat, which tries to maintain the volt-
age applied between the working and reference electrodes and allows negligible current
through the reference electrode (IC2B is con
fi
gured as a unity-gain bu
ff
er, presenting virtu-
ally in
nite input impedance to the reference electrode). In voltammetric measurements, the
current is measured as a function of this applied potential. The counter electrode acts as a
source or sink of electrons to balance the redox reaction occurring at the working electrode.
IC2C, IC3, R4, and R6 act as a current-to-voltage converter. Therefore, the potential applied
between the counter and working electrodes must be su
fi
cient both to drive the appropriate
electron transfer reaction at the counter electrode and to compensate for the potential drop due
to the solution resistance between the counter and reference electrodes (this potential drop is
given by Ohm's law,
V
iR
solution
, where
i
is the current generated by the electrochemical reac-
tion and
R
solution
is the solution resistance between the electrodes). The compliance voltage is
then the maximum potential that can be supplied between the working and counter electrodes.
The voltage waveform is applied to a counter electrode (a large platinum electrode) in
the electrolyte. The iridium electrode to be activated is the working electrode and provides
a return current path. A saturated calomel electrode (SCE) provides a reference (Fisher
Scienti
c 13-620-52). The voltage on the counter electrode is cycled between anodic and
cathodic potentials while the iridium electrode is exposed to an electrolyte.
During the anodic sweep, an inner oxide (IrO
2
) is formed from pure iridium. As the
potential increases, the inner oxide changes to a hydrous outer layer [Ir(OH)
3
]. The fact
that this layer is hydrated (water molecules are attached) limits formation to a monolayer.
The cathodic sweep causes reduction of the inner oxide back to iridium but does not go
low enough to reduce the outer layer. The outer layer remains. Since the oxide is porous,
the metal maintains contact with the electrolyte. Therefore, on the next potential sweep the
process will repeat. In this way, a hydrous porous layer of iridium oxide is created. The
potential limits depend on the electrolyte but should not exceed the potential that results in
oxygen or hydrogen evolution.
The University of Michigan Center for Neural Communication Technology recom-
mends the following method for the formation of high-quality IROX
fi
fi
films:
1.
Initially, hold sites at potentials of
3.0 and 2.5 V for approximately 3 minutes each
to remove any oxide that has formed, essentially cleaning the metal.
2.
Cycle the activation potential between
0.85 and 0.75 V. These limits are usually
wide enough to grow an oxide but narrow enough to remain within the
water window
(the potential range that does not result in oxygen or hydrogen evolution). As a rule,
the voltage limits should be set approximately 100 mV inside the water window.
3.
Use a square wave (0.5 to 1 Hz) to activate. This allows the metal to remain at the
critical levels for hydrous oxide formation (0.75 V) and inner oxide reduction
(
0.85 V) for a longer time than a ramp wave would. Holding the potential at these
levels allows more complete oxide formation and reduction and also reduces the
number of potential cycles (500 to 1000 cycles) needed to grow the oxide.
4.
Activate to a limit of 30 mC/cm
2
. Although studies have shown 100 mC/cm
2
to be the
maximum usable storage capacity for activated iridium, 30 mC/cm
2
allows the oxide
to better maintain electrical characteristics and should su
ce for most neural stim-
3
You can still use this circuit as a classical cyclic voltammeter. Just change the output of the function generator
to a triangular wave and connect the voltage- and current-monitoring outputs to a two-channel oscilloscope capa-
ble of working in the
x
-
y
mode. For a detailed discussion of the cyclic voltammetry technique, we recommend
D. K. Grosser,
Cyclic Voltammetry Simulation and Analysis of Reaction Mechanisms
, Wiley-VCH, Weinheim,
Germany, 1993. In addition, the accompanying CD-ROM includes a cyclic voltammetry simulator (VirtualCV
v1.0 freeware for Windows 9x by Andre Laouenan) to help you understand how the technique works and how to
analyze results.
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