ELECTROCHEMICAL RECTIFICATION AT A MONOLAYER-MODIFIED ELECTRODE

An electrochemical current rectifier based on a ferrocene-monolayer-mo dified electrode in contact with an electrolyte solution containing so dium hexacyanoferrate(II) (sodium ferrocyanide) was constructed and ev aluated, Rectification was accomplished via mediated ferrocyanide oxid ation 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 revers e reaction of ferricyanide reduction by immobilized ferrocene is therm odynamically disfavored, thereby providing tile basis for rectificatio n. A model proposed to describe the rectification process is reminisce nt of earlier treatments of electron-transfer mediation at electrodes modified with redox polymers, with the important difference that, in t he present system, long-range electron transfer across the monolayer r eplaces electron hopping through the polymer layer as a possible rate- limiting step, Steady-state current-voltage curves corresponding to me diated ferrocyanide oxidation were recorded and analyzed using the pro posed model to give values for k(0), the standard rate constant for el ectrochemical oxidation/reduction of immobilized ferrocene groups, and k(cross), the rate constant fur reaction of ferricenium ions in the m onolayer with ferrocyanide ions in solution. Mean k(0) values of 2700 +/- 1000 s(-1) for the ercapto-N-decyl)ferrocenecarboxamide/dodecaneth iol system and 90 000 +/- 60 000 s(-1) for the mercapto-N-heptyl)ferro cenecarboxamide/nonanethiol system obtained by analysis of steady-slat e current-voltage data are to be compared with values of 1400 and 51 0 00 s(-1) for the same systems obtained by the fast-scan cyclic voltamm etry method, The k(cross) value of 1.1 x 10(8) M(-1) s(-1) obtained fr om analysis of steady-state current - voltage data for the ercapto-N-d ecyl)ferrocenecarboxamide/dodecanethiol system is in good agreement wi th a value calculated using the Marcus expression with literature valu es for the electron self-exchange rate constants for ferrocene/ferrice nium and ferrocyanide/ferricyanidel indicating that the reactivity of ferricenium ions in the monolayer is similar to that expected for ferr icenium in free solution.