Characterizations and applications of thiol monolayers adsorbed at gold electrodes
The application of thiolate monolayers in electrochemical sensing systems involving redox amplification and the determination of nitric oxide has been examined. 10-mercaptosulfonate monolayers were formed at gold electrodes and characterized by infrared reflection absorption spectroscopy (IRRAS), x-ray photoelectron spectroscopy (XPS), and electrochemical reductive desorption (ERD). When dopamine was oxidized at the monolayer-coated electrode with ascorbic acid in solution, an amplification effect was noticed whose magnitude increased with ascorbic acid concentration. These sulfonate monolayer-coated electrodes showed potential in the selective determination of nitric oxide (NO) in the presence of an anionic interfering species. Also, other monolayer systems involving porphyrin moieties were investigated for electro-catalytic activity for the determination of NO.;Electrochemical and spectroscopic methods were utilized to elucidate thermodynamic and kinetic phenomena that take place during thiol adsorption at gold electrodes. ERD was used to determine the rates of film formation at annealed and unannealed electrode surfaces. When micromolar concentrations of thiol precursors were employed, full coverage was only obtained after an hour of immersion time. When data from annealed and unannealed surfaces were evaluated, a faster rate of formation was found for the unannealed surface, and the interrogation of processes at step and terrace sites of the unannealed surface revealed faster initial rates of adsorption at the step sites. Using the Langmuir adsorption isotherm as a first approximation, a DeltaG ads of -9.0 kcal mol-1 was calculated. Other ERD experiments demonstrated that butanethiolate monolayers show fine structure in their current-potential curves that can be attributed to heterogeneity in terrace width. Results from thin-layer visible spectroscopy showed a measurable pH change upon the adsorption of propanethiol from an aqueous solution containing chlorophenol red. The pH change was less than that expected for full monolayer coverage though ERD confirmed a theta > 0.9. Possible mechanisms for the monolayer adsorption process were examined in light of available thermodynamic and kinetic data.