Concentration dependence of the mechanism of glucose oxidation at gold electrodes in alkaline media

Abstract

The catalytic oxidation of glucose in alkaline solution at a gold electrode was studied by cyclic voltammetry at a rotating disc electrode as a function of glucose concentration and rotation velocity. Concentrations < 2 m M produced mass transport limited currents at a potential of ca. 0.4 V versus SHE with n = ca. 8. Concentrations > 30 m M produced currents limited by a kinetic step involved in the electron transfer reaction with n = ca. 2. From comparison of the voltammetric responses of glucose and glucose derivatives, the mass transport limited reaction was believed to proceed by the pairing of the enediol conformation of glucose, via hydrogen bridges, to the catalytic hydrous gold oxide, followed by oxidative cleavage of the enediol and oxidation of the primary hydroxyl group. The kinetically limited reaction for large glucose concentrations was concluded to effect only the oxidation of the aldehyde or hemi-acetal group producing gluconic acid or gluconolactone. The rate determining step for this process was concluded to be the transfer of an electron from an adsorbed carbohydrate radical species to the electrode. The concentration dependence of the mechanism of glucose oxidation is explained by a decrease in the number of interactions between each glucose molecule and the catalytic hydrous gold oxide as the glucose concentration is increased.