Adenine Adsorption at Single Crystal and Thin-Film Gold Electrodes: An In Situ Infrared Spectroscopy Study

Adenine adsorption at gold electrodes is studied in neutral sodium fluoride solutions by cyclic voltammetry and in situ infrared spectroscopy. External reflection measurements were performed with Au(111) and Au(100) single crystal electrodes in D2O solutions, whereas surface enhanced infrared reflection absorption spectroscopy experiments under attenuated total reflection conditions were carried out with sputtered gold thin-film electrodes, both in D2O and H2O solutions. The results clearly show the specific adsorption of the molecule providing significant surface enhancement of the strong ring stretching band and the scissoring mode of the amino group. On the basis of the results, an adsorption model is proposed in which the molecular plane and the C-N(H-2) bond are tilted on the electrode surface. Coordination is proposed to take place via the N(amino) atom upon sp(3) hybridization and the N-7 atom of the ring. The model applies irrespective of the surface crystallographic orientation of the electrode or the applied potential. The influence of the reconstructed or the unreconstructed state of the surface is inferred from the analyses of the spectra recorded while scanning the potential in opposite directions.