In situ Fourier transform infrared reflection absortion spectroscopy study of adenine adsorption on gold electrodes in basic media

In situ Fourier transform infrared reflection absortion spectroscopy (FT-IRRAS) has been used in the external (SNIFTIRS method) and the internal (ATR-SEIRAS) reflection configurations to determine the pH influence, in the neutral and basic range, on the adsorption of adenine on Au(111) and gold nanofilm electrodes from D2O and H2O solutions. In D2O solutions, the main adsorbate band around 1640 cm(-1), due to a ring stretching mode, shows different characteristics in the spectra collected at pH values at which the neutral and the basic adenine forms predominate in solution. The analysis of these differences, in comparison with the respective spectra of adenine in solution, permits us to conclude that both forms of adenine can adsorb chemically. The high sensitivity of the ATR-SEIRAS method has been used to analyze the contribution to the spectra of each form of adsorbed adenine as a function of the pH of the solution. The pKa(2) obtained for the adsorbed species from this analysis is almost coincident with the pKa(2) reported for adenine in solution, indicating that the coordination to the electrode and the second acid-base equilibrium involves different atoms of the adenine molecule. This result confirms the previously proposed adsorption model for adenine, implying the bonding of adenine to the electrode by the amine nitrogen (N-10) and either the ring nitrogens N-1 or N-7, while the second acid-base equilibrium of adenine involves the ring nitrogen N-9. Comparison of the 3400-3600 cm(-1) region of the ATR-SEIRAS spectra of adenine obtained in H2O solutions at different pH values, which corresponds to the characteristic-OH stretching mode of the interfacial water molecules, permits us to discard the co-adsorption of water molecules in neutral and basic media, contrary to the case of adenine adsorption from acid media. (C) 2014 Elsevier Ltd. All rights reserved.