Electrochemical properties of the 2-mercaptohydroquinone monolayer on a gold electrode. Effect of solution pH, adsorption time and concentration of the modifying solution

Abstract

The redox properties of an adsorbed mercaptohydroquinone (H 2QSH) monolayer on a gold electrode in both buffered and unbuffered solutions over a very wide pH range (0–12.7) are investigated in detail and are compared with those of free quinone (Q)-hydroquinone (H 2Q) species in solution at an unmodified gold electrode. Reversible waves due to the redox process of the immobilized Q-H 2Q groups are observed. The redox potentials vary with pH with a slope of −60 mV pH −1, showing that the reaction proceeds as a 2-electron-2-proton reaction. The redox potentials of the surface-attached H 2QSH are more negative than those of free H 2Q in solution. The redox behavior becomes complex in neutral unbuffered solution and two redox waves are observed. Several possibilities are considered for the origins of these redox waves based on the results of detailed experiments including sweep rate dependence and the effect of proton concentration. The effect of the adsorption time and the concentration of H 2QSH in the modifying solution on the redox potential of the adsorbed H 2QSH monolayer on a gold electrode are also studied. The number of adsorbed molecules increases as the concentration of H 2QSH becomes higher and the dipping time becomes longer. The redox potential of the adsorbed H 2QSH is affected by both the adsorption time and the concentration of H 2QSH in the modifying solution and becomes more negative as the coverage of H 2QSH increases.