Electrochemical overoxidation of polyindole and its cation-permselective behavior

Polyindole films prepared by potentiostatic growth in dichloromethane solution were subjected to overoxidation studies in aqueous media. Overoxidation at potentials greater than 1.1 V (vs. SCE) in 0.1 M KNO3 or 0.1 M H2SO4 was possible. Overoxidation in 0.1 M NaOH resulted in mechanically unstable films which were not adherent to the electrode surface. The overoxidation process in 0.1 M KNO3 involved removal of one electron per four indole monomer moieties in the polymer film. Nucleophilic attack led to introduction of carboxylate functionality and to cation permselective behavior, as tested by cyclic voltammetry and hydrodynamic voltammetry of hexamminoruthenium(III) and hexacyanoferrate (III). Such films may be useful in various electrochemical sensor applications.