Electrochemical Rectification at a Monolayer-Modified Electrode

An electrochemical current rectifier based on a ferrocene-monolayer-modified electrode in contact with an electrolyte solution containing sodium hexacyanoferrate(II) (sodium ferrocyanide) was constructed and evaluated. Rectification was accomplished via mediated ferrocyanide oxidation by electrogenerated ferricenium in monolayers of (10-mercapto-N-decyl)ferrocenecarboxamide with dodecanethiol and (7-mercapto-N-heptyl)ferrocenecarboxamide with nonanethiol on gold electrodes. The reverse reaction of ferricyanide reduction by immobilized ferrocene is thermodynamically disfavored, thereby providing the basis for rectification. A model proposed to describe the rectification process is reminiscent of earlier treatments of electron-transfer mediation at electrodes modified with redox polymers, with the important difference that, in the present system, long-range electron transfer across the monolayer replaces electron hopping through the polymer layer as a possible rate-limiting step. Steady-state current−voltage curves corresponding to mediated ferrocyanide oxidation were recorded and analyzed using the proposed model to give values for k0, the standard rate constant for electrochemical oxidation/reduction of immobilized ferrocene groups, and kcross, the rate constant for reaction of ferricenium ions in the monolayer with ferrocyanide ions in solution. Mean k0 values of 2700 ± 1000 s-1 for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system and 90 000 ± 60 000 s-1 for the (7-mercapto-N-heptyl)ferrocenecarboxamide/nonanethiol system obtained by analysis of steady-state current−voltage data are to be compared with values of 1400 and 51 000 s-1 for the same systems obtained by the fast-scan cyclic voltammetry method. The kcross value of 1.1 × 108 M-1 s-1 obtained from analysis of steady-state current−voltage data for the (10-mercapto-N-decyl)ferrocenecarboxamide/dodecanethiol system is in good agreement with a value calculated using the Marcus expression with literature values for the electron self-exchange rate constants for ferrocene/ferricenium and ferrocyanide/ferricyanide, indicating that the reactivity of ferricenium ions in the monolayer is similar to that expected for ferricenium in free solution.