VOLTAMMETRY OF REDOX-ACTIVE GROUPS IRREVERSIBLY ADSORBED ONTO ELECTRODES - TREATMENT USING THE MARCUS RELATION BETWEEN RATE AND OVERPOTENTIAL

A treatment of linear sweep voltammetry for redox-active groups irreversibly immobilized on electrodes is presented with use of the Marcus theory of electrode kinetics to relate rate constants to overpotential. The present treatment extends an earlier treatment of the same problem (Laviron, E. J. Electroanal. Chem. 1979, 101, 19) that used the Butler-Volmer theory to relate rate constants to overpotential. The behavior predicted in the present treatment matches that of the earlier treatment for very high reorganization energies; however, for reorganization energies below about 2.0 eV, voltammograms are predicted to be broader and peak potentials are in most cases predicted to shift further from E degrees, than in the earlier treatment. These effects are most pronounced at high overpotentials and at high sweep rates. Voltammetric data acquired over a wide range of sweep rates for ferrocene oxidation/reduction in self-assembled monolayers of N-(15-mercaptopentadecyl)ferrocenecarboxamide coadsorbed with 16-mercaptohexadecanol onto gold were analyzed using the present treatment. Predictions of broader voltammograms and greater shifts in peak potential with increasing sweep rate were fully realized in the experimental data. A protocol based on fitting peak potential vs log (sweep rate) data to predictions from theory is suggested as the preferred means of analyzing voltammetric data. Fitting was accomplished using a simplex algorithm with the heterogeneous electron-transfer self-exchange rate constant, k(0), and the reorganization energy, lambda as fitting parameters. Values of k(0) = 7.0 s(-1) and lambda = 0.87 eV, and k(0) = 6.6 s(-1) and lambda = 0.87 eV, were obtained from fits to two independent voltammetric data sets for two separate ferrocene-containing monolayers. These k(0) and lambda values are in excellent agreement with those in a recent report (Chidsey, C. E. D. Science 1991, 251, 919) on a related system studied by potential-step amperometry.