SURFACE-ENHANCED RAMAN-SCATTERING STUDY ON ADSORPTION STRUCTURES OF 9-METHYLADENINE AT SILVER ELECTRODE SURFACES

Surface-enhanced Raman scattering (SERS) spectra are measured for 9-methyladenine (9-MA), its N1-protonated form (9-MAH+), and the deuterated analogues of 9-MAH+ adsorbed on silver electrodes as a function of pH and electrode potential (E). The surface spectra observed for the adsorbate on the electrode in 0.1 mol/L KCl at E = 0.2 V (vs Ag/AgCl) and at pH values 2.8 and 1.0 prove that the adsorbate exists as the protonated form taking a nearly flat orientation to the electrode surface. When E is stepped to -0.4 V at pH 2.8, the adsorbate is converted to 9-MA with an end-on orientation; thus, the potential sweep results in a deprotonation process as well as an orientational change. At E = -0.2 V the coadsorbed chloride ions neutralize the positive charge of the adsorbate and stabilize the protonated form on the surface; on stepping E to -0.4 V, the chloride ions are desorbed from the surface, resulting in the increase in an electrostatic interaction between the adsorbate and the electrode surface; the increase causes the deprotonation. On the other hand, when E is swept from -0.2 to -0.4 V at pH 1.0, the adsorbate, which keeps the protonated state taking on a flat orientation to the surface, gives rise to prominent bands due to a C8-H out-of-plane bending (1033 cm-1) and a CH3 out-of-plane rocking (953 cm-1) vibration. The protonated species is more stable at pH 1.0 than the species at pH 2.8, and consequently the desorption of the chloride ions does not cauuse the deprotonation but induces a direct interaction between the adsorbate and the surface, which gives rise to the prominent out-of-lane vibrations.