Influence of coverage in the surface-enhanced Raman scattering of cytosineand 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 derivat ives 1-methylcytosine and 1,5-dimethylcytosine when adsorbed on Ag colloids . The correspondence found between the surface-coverage, Cl-, and pH effect s 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 observ ed when adding Cl- or at acidic pH. The effect induced by OH- ions at alkal ine 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 displaceme nt 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 pre sence or absence of Cl- indicates the influence of the halide on the morpho logy of the metal particles. This reveals a strong effect of the halide mai nly through the electromagnetic mechanism. However, the differences observe d in the spectral pattern of first layer SERS spectrum and the multilayer o ne may be attributed to a considerable contribution of the charge transfer mechanism to the SERS effect, which is more efficient in the first layer. ( C) 2001 Elsevier Science B.V. All rights reserved.