No abstract

We present results of the formation of 1,8-octanedithiol (ODT) monolayers on the Au(111) single-crystal surfaces by oxidative deposition from alkaline solutions under electrochemical control. Cyclic voltammetry shows the presence of two well-separated oxidative peaks that are assigned to the formation of the S-Au bond (peak A1) and the oxidation of the thiolate species to give the disulfide dimer either in solution or in the adsorbed state (peak A2). The formation of a disulfide species can take place with the participation of two neighboring ODT molecules or through the formation of a bilayer in the adsorbed state. The reductive desorption of the layers formed under these conditions gives us some information about its nature and allows us the choice of the experimental conditions to carry out a potentiostatic method to build the layers of ODT with determined properties. Electrochemical techniques, such as cyclic voltammetry, differential capacitance-potential, and chronocoulometry curves, are used to discriminate between the monolayer and bilayer formation. Moreover, XPS data are used to confirm the electrochemical results. It is concluded that very reproducible layers that contain mainly standing-up ODT molecules are formed by the potentiostatic method and that they are built in a shorter time than those formed by the spontaneous assembly from an ethanolic solution.

Near-monodisperse fractions of hexanethiol-protected gold nanoclusters (Au(25) and Au( 144)) have been obtained by using a variation of the two-phase Brust method based on constant low temperature (4 °C) and control of the stirring conditions during the synthesis procedure. The size and dispersity of the Au 25 (SC6) 18 and Au 144 (SC6) 59 nanoclusters are determined by electrochemical means. The two first oxidation steps for the molecule-like Au 25 (SC6) 18 clusters are quasi-reversible, and the electron transfer rate constants and diffusion coefficients have been determined by cyclic voltammetry and electrochemical impedance spectroscopy. The electrochemical behavior of these nanoclusters in different solvents can be explained by using the model described by Girault et al. (J.

Self-assembled monolayers (SAMs) of 6-mercaptopurine (6MP) have been prepared on a Au(111) single-crystal electrode by immersion of the metal surface in a 100 microM 6MP and 0.01 M HClO4 solution. The 6MP-SAM Au(111) single-crystal electrodes were transferred to the cell and allowed to equilibrate with the different aqueous working solutions before the electrochemical experiments. The influence of the solution pH was studied by cyclic voltammetry, double layer capacitance curves, and electrochemical impedance spectroscopy. The electrochemical behavior of the 6MP-SAM in acetic acid at pH 4 presents important differences in comparison to that obtained in 0.1 M KOH solutions. Cyclic voltammograms for the reductive desorption process in acid medium are broad and show some features that can be explained by a phase transition between a chemisorbed and a physisorbed state of the 6MP molecules. The low solubility of these molecules in acid medium could explain this phenomenon and the readsorption of the complete monolayer when the potential is scanned in the positive direction. The variation of the double-layer capacitance values in the potential range of monolayer stability with the pH suggests that the acid-base chemistry of the 6MP molecules is playing a role. This fact has been studied by following the variations of the electron-transfer rate constant of the highly charged redox probes as are Fe(CN)(6)-3/-4 and Ru(NH3)(6)+3/+2 as a function of solution pH. The apparent surface pKa value for the 6MP-SAM (pKa approximately 8) is explained by the total conversion of the different 6MP tautomers that exist in solution to the thiol species in the adsorbed state.