Catalysis of slow charge transfer reactions at polypyrrole-coated glassy carbon electrodes

Oxidation of ascorbic acid, dihydroxyphenylacetic acid and dopamine are compared at polypyrrole-coated glassy carbon and naked glassy carbon electrodes. These currents are mass-transport limited and not limited by permeation into or through the polypyrrole film. Ascorbic acid oxidation occurs at potentials 300 mV more negative at polypyrrole-coated electrodes and the rising slope of rotated disk voltammograms changes by over 100 mV. A similar enhancement in electrochemical reversibility is observed for dihydroxyphenylacetic acid, whereas dopamine is oxidized at slightly more positive potentials at polypyrrole-coated electrodes. Comparing the electrochemistry of dopamine and dihydroxyphenyl-acetic acid, it appears that the electrochemical reversibility differences for these substances are to some degree result of electrostatic interactions between the anionic solutes, or anionic reaction intermediates, and anionic functional groups on carbon or cationic fixed sites in oxidized polypyrrole.