Coupled electron-proton transport in electropolymerized methylene blue and the influences of its protonation level on the rate of electron exchange with β-nicotinamide adenine dinucleotide

This paper describes electrochemical characteristics of poly(methylene blue) electrolytically deposited on glassy carbon and examines the electrocatalytic activity of the polymer toward oxidation of the coenzyme NADH. Redox-active properties of the cationic polyelectrolyte arose from both electron self-exchange between electroactive sites and a high ionic film-conductivity. The diffusion coefficient of charge carriers in the film increased with decreasing solution pH, indicating the pH dependence of the electron diffusion coefficient. The electrocatalytic oxidation of NADH at the polymer-modified electrode proceeded via an intermediate charge-transfer complex of the reduced polymer with the oxidized coenzyme. The complex dissociated more rapidly into the oxidation products as the reduced polymer protonated. Thus, the rate constant for the cross-exchange reaction rose with a decrease in pH. For NADH oxidation, the polyelectrolyte exhibited an electrocatalytic activity higher than the monomeric dye because of a stronger oxidizing power of the. second oxidized form of the polymer.