Evaluation of Diffusion Coefficient of Thiocholine in Enzyme-Loaded Polypyrrole Composite Film through Different Methods and Electrode Polarization

In this work, the diffusion
of thiocholine ion into an enzyme-loaded polypyrrole film was evaluated
by different methods, and the results were compared to identify the
most suitable method. The enzyme-loaded polypyrrole film was coated
with a thin layer of gelatin and gluteraldehyde so as to prevent enzyme
leaching. Diffusion coefficients under normal and prepolarized conditions
were calculated by five different methods, namely, the Cottrell method,
the method of Peerce and Bard, the theoretical impedance model, the
electrochemically stimulated conformational relaxation (ESCR) method,
and the direct impedance measurement method. The theoretical model
of Vortynstev was used to calculate the parameters from the impedance
spectra using simplex technique in MATLAB. The results indicate that
under normal unpolarized condition the ESCR method gives a diffusion
coefficient close to that given by Vortynstev method, but under polarized
conditions the Cottrell method can provide a better value of diffusion
coefficient than ESCR. The diffusion coefficient of thiocholine in
PPy composite film from an electrolytic background of phosphate buffer
of pH 7.4 was found to be 1.00 × 10–8 cm2 s–1 based on Vortynstev method. The mechanism
of thiocholine diffusion into the positively charged/polarized matrix
is attributed to be through the formation of a dinegative ion between
thiocholine and phosphate anion via electrostatic attraction.