Cyclic voltammetry and scanning electrochemical microscopy of ferrocenemethanol at monolayer and bilayer-modified gold electrodes

Ferrocenemethanol was chosen to study quantitatively the kinetics of the heterogeneous electron transfer at mono- and bilayer-modified gold electrodes. The monolayer was prepared by tetradecanethiol adsorption on the gold surface and the bilayer was formed by phospholipid adsorption on the first layer. The apparent electron transfer rate constants at the modified gold electrodes were measured by cyclic voltammetry (Tafel analysis) and from scanning electrochemical microscopy approach curves, as well as by simulations of the cyclic voltammograms. The values obtained by simulation based on a CEC mechanism were close to the values obtained by both other techniques. The results show that the surface coverage was high and depended on the thickness of the layer (0.993 for the monolayer and 0.9998 for the bilayer). The major consequence of this high coverage is a large decrease in the apparent rate constant for ferrocenemethanol oxidation that depended on the thickness of the modifying layer. The results also indicate that ferrocenemethanol could permeate to a small extent into the monolayer. The electron transfer at the bilayer is mainly governed by diffusion of the mediator through pinholes with electron transfer occurring at the free sites on the electrode rather than permeation.