Influence of coverage in the surface-enhanced Raman scattering of cytosine and its methyl derivatives on metal colloids: chloride and pH effects

In this work we study the influence of surface-coverage, chloride and pH on the surface-enhanced Raman scattering (SERS) of cytosine and their derivatives 1-methylcytosine and 1,5-dimethylcytosine when adsorbed on Ag colloids. The correspondence found between the surface-coverage, Cl−, and pH effects on the SERS of these molecules points out the importance of a first layer effect to account for the changes in intensity and spectral pattern observed when adding Cl− or at acidic pH. The effect induced by OH− ions at alkaline pH is the opposite to that observed in the presence of the halide and H+, and is rather related to a multilayer profile produced by the displacement of the adsorbate from the surface induced by the adsorption of OH− ions. A different response of the Ag colloid to the excitation wavelength in presence or absence of Cl− indicates the influence of the halide on the morphology of the metal particles. This reveals a strong effect of the halide mainly through the electromagnetic mechanism. However, the differences observed in the spectral pattern of first layer SERS spectrum and the multilayer one may be attributed to a considerable contribution of the charge transfer mechanism to the SERS effect, which is more efficient in the first layer.