A Catalytic 2-Aminoethanethiol Reduction on a Gold Electrode through a Sulfur-Gold Bond Formation

Abstract

In this note, we report a newly found catalytic reduction of a thiol compound on a gold electrode, which consequently catalyzes the reduction of protons. In a 2-aminoethanethiol solution, no reduction wave was measured with a glassy carbon electrode but a large cathodic wave was observed with a gold electrode indicating a catalytic reduction of a thiol compound at a gold surface. The chemistry involves the adsorption of a self-assembled monolayer (SAM) of 2-aminoethanethiol and its reductive desorption reaction. It was reported that an adsorbed self assembled monolayer (SAM) of a thiol compound on the gold electrode surface was reductively desorbed from the surface in alkaline solutions when an appropriate potential was applied.1-3 The desorbed thiolate anions were electro-deposited again on the gold surface, which is another route to form a thiol monolayer. 4 In other studies by Morin and coworkers, the amount oxidatively re-deposited was affected by the pH of the solution. 5,6 It was suggested that the amount of the redeposition by the oxidation decreases with the increasing solubility of the thiols because of the diffusion into the bulk of the solution.For this study, 2-aminoethanethiol (HSCH 2 CH 2 NH 3 + ) in a pH 6 aqueous solution, which has not been studied for the reductive desorption reaction, was used as a thiol compound. It has a short chain length with an ionic amino group and is easily dissolved in an aqueous solution. Compared to long chain alkanethiols, it gives less ordered monolayer. 7,8 It is widely employed to give base sites for enzyme layer formations 9-11 and its adsorption was characterized by XPS and electrochemical studies.12 According to the electrochemical and quartz crystal microbalance measurements of the present study, a catalytic reduction mechanism of a thiol compound is presented here. Figure 1A shows a comparison of the voltammograms with a bare gold plate electrode in a pH 6.0 phosphate buffer in the absence (the dashed line) and presence (the solid line) of 5.0 mM 2-aminoethanethiol. A reduction wave was observed at about −0.7 V in the solution of the 2-aminoethanethiol. In Figure 1B, when a glassy carbon electrode polished with 0.3 µm alumina powder was employed, almost the same two voltammograms in a pure supporting electrolyte (the dashed line) and in a 5.0 mM 2-aminoethanethiol solution (the solid line) were shown. No reduction of the thiol compound was observed with the glassy carbon electrode. The reduction of the 2-aminoethanethiol seems to be catalyzed on the gold electrode surface. Figure 2 is an electrochemical quartz crystal microbalance (EQCM) measurement in a pure supporting electrolyte with a gold electrode on a quartz crystal resornator. A SAM was previously formed on the gold surface by immersing in a 5.0 mM 2-aminoethanethiol solution for three hours. Figure 2A is a cyclic voltammogram as the potential scanning proceeded. In the first negative potential scan, a cathodic wave was shown at −0.63 V and a small anodic wave was shown at −0.58 V with the backward p...