Electrochemical Impedance Spectroscopy study of the adsorption of adenine on Au(111) electrodes as a function of the pH

Abstract

The adsorption of the three adenine forms involved in two acid-base equilibria on Au(111) electrodes is studied by Electrochemical Impedance Spectroscopy. The experiments are performed in solutions of pH values 1, 7.5 and 12, at which the cationic, the neutral and the anionic adenine forms are present, respectively. Two adsorption models are adopted, both based on the theory of Frumkin and Melik-Gaykazyan for the adsorption process, but one of them takes also into account a deprotonation step preceding the adsorption step. The same frequency dependence of the impedance data is observed in the three pH media. The Nyquist admittance plots suggest a mixed kinetic control, by adsorption and by diffusion. The analysis of the electrode impedance or admittance as a function of the frequency according to the adsorption models provides the values of adsorption resistance, Rad, adsorption Warburg coefficient, σad and adsorption capacitance, Cad as a function of potential. From the σadvs E plot obtained at pH 1 an equilibrium constant for adenine deprotonation at the vicinity of the electrode surface of c.a. 0.03 is calculated, which is three orders of magnitude higher than the equilibrium constant in the bulk solution. The different potential dependency of the adsorption rates at the three pH values is discussed on the basis of a Frumkin isotherm for the adsorption of adenine and a Butler-Volmer type potential dependence for the adsorption rate constant.