Structural Characterization of (3-Mercaptopropyl)sulfonate Monolayer on Gold Surfaces

We have investigated the structure of (3-mercaptopropyl)sulfonate (MPS) monolayer self-assembled onto gold surfaces by quartz crystal microbalance with energy dissipation monitoring (QCM-D) and various electrochemical methods. QCM-D experiments show that the MPS monolayer behaves as a thin rigid film with a surfacic mass of 166 ng cm^-2. Interfacial capacitance measurements demonstrate that the MPS monolayer is a rather open structure that can be penetrated by the ionic species of the phosphate buffer electrolyte. From MPS reductive desorption experiments, MPS surface concentration corresponds to 4.6 x 10^-10 mol cm^-2, which represents 60% of the coverage reported for a densely packed thiol monolayer. Despite this low packing density, oxidation of catechol is strongly inhibited leading to voltammograms that are free of diffusional contribution. This unique behavior has been exploited to show that the MPS monolayer covers the entire gold surface with a surface coverage at least equal to q 0.9981, which means a very low number of MPS-free pinholes and/or defects. Kinetics of electron transfer toward soluble redox species has been studied using catechol as a neutral hydrophilic probe, but also ferrocyanide as hydrophilic anion and ferrocenemethanol as neutral hydrophobic molecule. It is proposed that the MPS monolayer provides a high kinetic barrier toward permeation of these species and that electron transfer mainly occurs by electron tunneling through the MPS monolayer.