A NEW WAY OF USING AC VOLTAMMETRY TO STUDY REDOX KINETICS IN ELECTROACTIVE MONOLAYERS

A new protocol for analyzing variable-frequency ac voltammetry (acv) d ata to obtain electron-transfer rate constants for redox-active groups immobilized in monolayer films on electrodes is described. The protoc ol involves plotting the ratio of the peak current to the background c urrent as a function of the log of frequency and then fitting the plot to a calculated curve obtained using the Randles equivalent circuit m odel. The fitting procedure is particularly straightforward since it i nvolves just two adjustable fitting parameters and can usually be perf ormed manually in a spreadsheet. Advantages of the new method include the following: (1) it requires fewer adjustable fitting parameters tha n the conventional impedance analysis performed in the complex-plane f ormat; (2) because there are fewer fitting parameters, there is less c hance of the fit yielding a false convergence; (3) nonideal effects as sociated with a distribution of rate constants are apparently revealed in a particularly intuitive and easily interpreted manner; (4) data a nalysis can be accomplished without the need for information on the ph ase of the ac current signal; and (5) analysis can be performed on ext remely small voltammetric peaks that would be difficult if not impossi ble to analyze in the complex-plane format due to the overwhelming eff ects of background currents. Electron-transfer rate constants obtained by this method for ferrocene oxidation/reduction in mixed monolayers of N-(mercaptopentadecyl)ferrocenecarboxamide (FcCONH-C-15-SH) and 16- mercaptohexadecanol (HO-Cls-SH) on gold were in the range of 9-13 s(-1 ), in good agreement with values obtained using a complex nonlinear le ast-squares method to fit data in the conventional complex-plane forma t and also with previously reported values for this system obtained by analysis of fast-scan de cyclic voltammetry data. Analysis of the red ox kinetics for an ensemble of just 1.8 fmol or 1 x 10(9) molecules of ferrocene is also demonstrated.