Biomedical Engineering Reference
In-Depth Information
A.1 Fabrication: Electrochemical Deposition
During deposition it is important to keep the applied potential within a
certain window, dictated by the specific conducting polymer in use. The
minimum value is the potential at which the monomer unit is oxidised
and can begin to form oligomers and deposit onto the working elec-
trode. The maximum value is the potential at which the electrochemical
stability of the conducting polymer is exceeded, resulting in an ir-
reversible inactivation of the electrochemical properties of the polymer.
PEDOT is much more electrochemically stable than PPy (over-oxidation
at 1.6 V and 0.6 V, respectively).
Electrochemical deposition can be carried out in either galvanostatic or
potentiostatic modes. The potentiostatic mode allows for control of
oxidation/over-oxidation whereas the galvanostatic mode allows for
control over the kinetics of deposition (growth rate and film thickness).
It is the authors' opinion that galvanostatic deposition produces more
reliable samples.
Depositing films will initially grow in a 2D manner, but will begin to
shift to a 3D nodular growth as film thickness increases. Smooth 2D
growth can be encouraged through the use of low current densities (i.e.,
slower deposition). The use of poly(styrene sulfonate) as a dopant will
also result in smoother films compared to films doped with p-
toluenesulfonate.
Smaller working electrodes with counter-electrodes of equivalent size
and lower current densities will help to produce more uniform coatings.
The type of electrode substrate chosen for deposition will considerably
impact upon the quality of the electrode coating. Platinum and gold
electrodes have been found to produce mechanically and electrically
superior coatings compared to other substrate types.
The authors typically use a precursor solution containing 0.1 M EDOT
monomer and 0.05 M dopant in a 1 : 1 water-acetonitrile solution
and deposit galvanostatically at current densities in the range of
0.5-3 mA cm
2
. Deionised water should always be used to minimise
contamination with ionic species. PPy has better aqueous solubility
than PEDOT and can be deposited from entirely aqueous solutions;
however, it is less stable in aqueous environments and typically requires
distillation and purging.
d
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A.2 Characterisation
A.2.1 Cyclic Voltammetry
A three-electrode set-up is utilised for CV. The coated electrode (sample) is
the working electrode placed at the base of the cell. A length of platinum
wire is used as the counter electrode and is inserted into the top of the
cell (to maximise the distance between counter and working electrodes).
A Ag/AgCl reference electrode is inserted into the cell and placed down near
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