Self-Assembled Monolayer of Monomercaptoundecahydro-closo-dodecaborate on a Polycrystalline Gold Surface

In this work, we present an electrochemical study of the boron cage monomercaptoundecahydro-closo-dodecaborate [B₁₂H₁₁SH]^2- in solution and in a self-assembled monolayer over a polycrystalline gold electrode. Cyclic voltammetry of the anion [B₁₂H₁₁SH]^2- in solution showed a shift in the peak potentials related to the redox processes of gold hydroxides, which evidences the interaction between the boron cage and the gold surface. For an Au electrode modified with the anion [B₁₂H₁₁SH]^2-, cyclic voltammetry response of the probe Fe(CN)₆^3-/Fe(CN)₆^4- showed a ΔEp value typical for a surface modification. Electrochemical impedance spectroscopy presented R and C_dl values related to the formation of a self-assembled monolayer (SAM). A comparison of electrochemical responses of a modified electrode with thioglycolic acid (TGA) reveals that the boron cage [B₁₂H₁₁SH]^2- diminishes the actives sites over the Au surface due to the steric effects. Differential capacitance measurements for bare gold electrode and those modified with [B₁₂H₁₁SH]^2- and (TGA), indicate that bulky thiols enhance charge accumulation at the electrode-solution interface. In addition to electrochemical experiments, DFT calculations and surface plasmon resonance measurements (SPR) were carried out to obtain quantum chemical descriptors and to evaluate the molecular length and the dielectric constant of the Boron cage. From SPR experiments, the adsorption kinetics of [B₁₂H₁₁SH]^2- were studied. The data fit for a Langmuir kinetic equation, typical for the formation of a monolayer.