Specific Response of Quinone Redox at 2-Aminoethanethiol Modified Electrode

Abstract

Self-assembled monolayers of alkanethiols on electrode surface have been the subjects of intensive studies in both fundamental interests of surface chemistry and their technological significance. The self-assembly is one of the most widely used techniques to construct ordered molecular structures and to control surface characteristics, due to its facile procedure. 2-aminoethanethiol has been used as an anchor to introduce functional molecules to electrode surfaces as it has a reactive amino group and its carbon chain is short. We have found that a 2-aminotehanethiol modified Au electrode shows a specific electrochemical response to p-hydroquinone in aqueous solution. That is, a peak separation (ΔE p) in the CV appeared to be very large at the first potential cycle, but it drastically decreased upon successive potential cycling. We have been studying condition where the specific response takes place and its mechanism. Figure 1A shows typical CVs of an aminoethanethiol modified Au disk electrode in a 0.1 M sodium perchlorate solution containing 5 mM hydroquinone at 100 mV s-1. The ΔE p for quinone redox was found to be 530 mV for the first cycle, indicating a slow electron transfer (Fig. 1A curve a). After application of +0.9 V vs. Ag/AgCl for 30 min at which hydroquinone is oxidized, the ΔE p largely decreased to 120 mV (Fig. 1A curve b) and the CV overlapped with that of a bare electrode. On the other hand, application if –0.5 V was applied, the CV remained virtually constant. Interestingly, when the electrode was left in the solution at open circuit or –0.5 V for 3 h, the ΔE p increased to the original value (530 mV). These results indicate that the changes in the ΔE p is reversible and not originating from peeling of the film, and the increase can be attributed to an interaction between aminoethanethiol and hydroquinone, whereas the decrease is due to oxidation of hydroquinone. A Au electrode modified with 2-dimethylanimoethanethiol showed similar behavior (Fig. 1B), indicating that Schiff base formation from amino group of 2-aminoethanethiol and p-quinone is not the dominant cause of the changes in the electrochemical response. At the current stage, we speculate two possible mechanisms for the changes in the electrochemical response. The first one is changes in structure of aminoethanethiol due to changes in an interaction with quinone, although it is known that short alkyl chains on the electrode surface do not block electron transfer. The second one is a chemical reaction between hydroquinone and aminoethanethiol, which produces derivatives with slow electrode reaction rate constant. We currently investigate those two possibilities with STM measurements for an aminoethanethiol modified electrode before and after the electrolysis and with NMR measurements for the reaction. Figure 1