Electrocatalytic oxidation of ascorbic acid at [osmium(2,2′-bipyridyl)2-(poly-4-vinylpyridine)10Cl]Cl modified electrodes; implications for the development of biosensors based on osmium-containing redox relays

Abstract

The oxidation of ascorbic acid at electrodes modified with the redox polymer [Os(bipy)₂(PVP)₁₀Cl]Cl (where bipy = 2,2′-bipyridyl and PVP = poly-4-vinylpyridine) has been studied. The reaction proceeds electrocatalytically at 0.250 V versus SCE by mediation via the Os^II/Os^III redox couple immobilized in the polymer film. The reaction is negative first order with respect to proton concentration and positive first order with respect to ascorbic acid concentration. At pH3.0, limiting currents are diffusionally controlled and the kinetic regime may be classified as an S_k″ mechanism. The second order rate constant for the surface electrocatalytic reaction k″ at pH 4.5 is 7.8 ± 0.4 × 10^–4dm³mol^–1 cm s^–1, while at pH 7.0 k″ is 9.1 ± 0.6 × 10^–4 dm³ mol^–1 cm s^–1. At pH ⩽2.0 ascorbic acid permeates the polymer film and reacts at the mediator sites within the polymer film giving an L_k mechanism, while at potentials >0.4 V versus SCE the reaction proceeds both at the underlying electrode and with the Os^III mediator sites within the polymer film. The modified electrode has been characterized as an electrochemical sensor for ascorbic acid in flow injection systems.