Electrochemical and Enzymatic Studies of Electron Transfer Mediation by Ferrocene Derivatives with Nafion-Glucose Oxidase Electrodes

Nafion-glucose oxidase-ferrocene electrodes have been used to study the electron transfer between an electrode and the active site of the enzyme. These electrodes were prepared by casting a mixture of Nafion and glucose oxidase, GOx, (both dissolved in methanol) at the surface of a platinum disk electrode. The ferrocene mediators were incorporated by either soaking Pt/Nafion and Pt/Nafion-GOx electrodes in an aqueous solutions of N′-dimethylaminomethylferrocene, DMAFc, or by depositing an aliquot of a ferrocene solution at the surface of the enzyme film electrode. The Nafion-GOx electrodes are characterized by a slower incorporation rate of the ferrocene than the plain Nafion electrode indicating that the affinity of Nafion for the ferrocene derivative is increased by the addition of the enzyme. Accordingly, the ion-exchange distribution coefficient is larger for plain Nafion (3.2×103) than for Nafion-GOx (7.2×102). The cyclic voltammetry of the Pt/Nafion-GOx electrode in the presence of glucose and in deaerated solution show a well developed catalytic wave indicating that the ferrocene acts as a mediator for the electron transfer between the enzyme and the platinum electrode. For a high glucose concentration (75 mM), the amperometric response of these electrodes increased and the apparent diffusion coefficient DMAFc decreased with an increase of DMAFc concentration in the enzymatic layer. These observations suggest that the glucose response is limited by the probability of an encounter between the reduced enzyme and the oxidized ferrocene rather than by the charge transfer between the reduced ferrocene and the electrode. The effective Michaelis constant, Km , for glucose and DMAFc were evaluated and values of 25 mM and 4 nmol/cm were obtained, respectively.