Formation and Electrochemical Characterization of Self-Assembled Monolayers of Thioctic Acid on Polycrystalline Gold Electrodes in Phosphate Buffer pH 7.4

Abstract

Formation of self-assembled monolayers (SAMs) of thioctic acid (TA) on polycrystalline gold electrodes has been investigated. The Au electrodes are polished, first mechanically and subsequently electrochemically, until the roughness of the surface is minimal. The best TA SAMs are formed onto these electropolished electrodes from stirred solutions, while a potential E = 0.0 V or E = +0.2 V versus saturated calomel reference electrode (SCE) is applied during the adsorption process. Alternating current (ac) impedance methods are used to monitor the formation of the SAMs and to characterize these. Potentiostatically formed TA SAMs have better characteristics than those formed at open circuit. The TA SAMs are stable in 100 mM phosphate buffer pH 7.4, and when protected from light, also in buffered 2 mM hexacyanoferrate(II/III) solution. However, when the hexacyanoferrate(II/III) solutions are exposed to light, the TA SAMs are not stable. Probably, damaging of the SAM proceeds via etching of the gold surface by photochemically released cyanide ions. Using potential pulses (+1.6, 0.0, −0.8 V versus SCE, each 0.1 s) for 15 min in the flow (0.5 mL min-1), the TA SAM can be removed completely, recovering the initial state of the gold electrode, as deduced from potential-step experiments. This way TA SAMs can be formed and removed repeatedly and reproducibly on a single electrode.