A New Way of Using ac Voltammetry To Study Redox Kinetics in Electroactive Monolayers

Abstract

A new protocol for analyzing variable-frequency ac voltammetry (acv) data to obtain electron-transfer rate constants for redox-active groups immobilized in monolayer films on electrodes is described. The protocol involves plotting the ratio of the peak current to the background current as a function of the log of frequency and then fitting the plot to a calculated curve obtained using the Randles equivalent circuit model. The fitting procedure is particularly straightforward since it involves just two adjustable fitting parameters and can usually be performed manually in a spreadsheet. Advantages of the new method include the following: (1) it requires fewer adjustable fitting parameters than the conventional impedance analysis performed in the complex-plane format; (2) because there are fewer fitting parameters, there is less chance of the fit yielding a false convergence; (3) nonideal effects associated with a distribution of rate constants are apparently revealed in a particularly intuitive and easily interpreted manner; (4) data analysis can be accomplished without the need for information on the phase of the ac current signal; and (5) analysis can be performed on extremely small voltammetric peaks that would be difficult if not impossible to analyze in the complex-plane format due to the overwhelming effects of background currents. Electron-transfer rate constants obtained by this method for ferrocene oxidation/reduction in mixed monolayers of N-(mercaptopentadecyl)ferrocenecarboxamide (FcCONH-C15-SH) and 16-mercaptohexadecanol (HO-C16-SH) on gold were in the range of 9−13 s-1, in good agreement with values obtained using a complex nonlinear least-squares method to fit data in the conventional complex-plane format and also with previously reported values for this system obtained by analysis of fast-scan dc cyclic voltammetry data. Analysis of the redox kinetics for an ensemble of just 1.8 fmol or 1 × 109 molecules of ferrocene is also demonstrated.