pH-dependent binding interaction between functionalized thiol monolayers and electroactive ions

The charge transfer kinetics at ionizable thiol monolayer-modified gold electrodes was investigated as a function of the redox-couple composition and the solution pH by potential scan voltammetry and electrochemical impedance spectroscopy. The adsorption of indigotetrasulfonate and chromate onto a 4-aminothiophenol monolayer, at low pH values, resulted primarily from an electrostatic attraction between the protonated terminal amino groups and the anions. An increase in the solution pH, however, raised the apparent heterogeneous rate constant of indigotetrasulfonate at a 4-aminothiophenol-modified electrode. This pH dependence, which was opposite to that observed at a 1-butanethiol-modified electrode, can be ascribed to a change in the potential difference between the gold electrode and monolayer surface. A large change in the heterogeneous rate constant with the pH implies that in addition to electrostatic interaction, a specific intermolecular force contributes to the interactions between the 4-aminothiophenol monolayer and the redox species.