In situ infrared study of adenine adsorption on gold electrodes in acid media

In situ external reflection infrared reflection spectroscopy (IRRAS) and surface-enhanced infrared reflection absorption spectroscopy under attenuated total reflection conditions (ATR-SEIRAS) have been used to study the adsorption of adenine at Au(1 1 1) and gold thin-film electrodes in 0.1 M HClO4 solutions in deuterium oxide and in water. The existence of protonated and deprotonated adenine molecules at the electrode surface has been proved from the comparison of the spectra for adsorbed adenine with those obtained for dissolved adenine. In all cases, characteristic band frequencies are observed for the enhanced ring stretching band in D2O and also the bending band of the amino group in H2O. Both signals appear at higher wavenumbers for the protonated species. In acid media, deprotonation of adsorbed adenine is induced by increasing positive potentials and/or adenine coverage leading to the same adsorbed species as previously reported in basic media, namely chemically adsorbed adenine with a tilted orientation of the molecular plane on the electrode. Co-adsorption of water and perchlorate anions is detected in the case of the acid media, indicating the involvement of these species in the stabilization of the organic film. Time-resolved experiments at negative potentials and low adenine concentrations have shown also the physical adsorption of protonated adenine and have allowed the monitoring of the adsorption of the protonated and unprotonated forms as a function of time. Adsorption of unprotonated adenine is slower but is favored at the equilibrium at the electrode surface even if the pH of the media is three units lower than the pKa of adenine in solution.