The adsorption of mercaptosuccinic acid (MSA) on gold(111) surface in 0.1 M NaOH aqueous solution was examined with electrochemical methods and potential dependent in situ FTIR spectroscopy. Differential capacitance and chronocoulometry results indicated the multistep adsorption, proceeding with a slow rate. Gibbs isotherm was determined to evaluate the MSA Gibbs excess and to record the chronocoulometry curve for the MSA monolayer of known surface coverage, transferred by horizontal touch (HT) technique. The potential dependence of electrosorption valency (charge number per adsorbed molecule) was determined and it was found that its value is higher than unity for potentials above pzc. This result suggests that adsorption takes place not only through the gold-sulfur bond formation, but the interactions with the metal substrate via the carboxylate group is also very likely. The orientation of the MSA molecules on gold in contact with aqueous NaOH as a function of the potential was studied by RATIO method, using surface enhanced infrared reflection absorption (SEIRA) spectroscopy. An effort to extract the tilt angle separately for each of two MSA carboxylate groups was made. A schematic representation of MSA molecule adsorbed on Au, defining the determined tilt angle (γ) for one of the groups is shown in Figure 1. The model of MSA molecule adsorbed with one COO^- group (closer to the sulfur atom) directed towards the gold substrate and the second one exposed to the solution was proposed. MSA coated gold was successfully employed for electrostatic attachment of cytochrome c, preserving electroactivity of the protein, as verified with SEIRA spectro-electrochemical experiment. A deduced binding site of the cytochrome c by MSA molecules appears to be different than for monocarboxylate terminated alkanethiolates monolayers.

Figure 1

Schematic representation of mercaptosuccinate adsorbed on Au substrate (caution – the SH bond cleavage is not included) with the marked tilt angle γ and directions of the transition moments corresponding to the symmetric (blue arrow) and asymmetric (red arrow) carboxylate stretch.

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