Effect of different mechanisms of surface binding of dyes on the surface-enhanced resonance Raman scattering obtained from aggregated colloid

Surface-enhanced resonance Raman scattering (SERRS) is crucially dependent on the little understood process of the binding of the adsorbate to the active surface. To aid in characterizing the effect of this process, SERRS from aggregated silver colloid of four dyes expected to bond to the surface in different ways was recorded. A comparison of the effect of aggregating the colloid with sodium chloride and sodium nitrate showed that the nitrate was more effective as an aggregating agent in each case. Rhodamine 6G is believed to attach to the surface through polar bonds. At slightly above the predicted concentration for monolayer coverage by the dye of the silver surface, and with sodium chloride as an aggregating agent, a time-dependent adsorption process was identified from a comparison of fluorescence and Raman scattering efficiencies. The silver chloride formed on the surface was postulated to displace the dye, reducing the effective enhancement, but once formation of the silver chloride layer was complete Rhodamine 6G re-adhered to the surface and the Raman scattering efficiency increased. An azo dye with two terminal phosphoric acid groups gave consistent SERRS with nitrate aggregation but was displaced from the surface with chloride aggregation. The most consistent SERRS enhancement was achieved with an azo dye with a benzotriazole group which complexes directly and strongly to the silver surface, regardless of which aggregation method was used. Finally, an azo dye which could bond to the surface in different ways gave one of two different spectra, depending on the type and concentration of aggregating agent, dye concentration and time. Although the absolute scattering efficiencies of the dyes cannot be compared owing to the different frequencies of the resonance maxima, the maximum enhancement caused by aggregation was different for each dye and greatest for the benzotriazole-containing azo dye at the particular concentration studied. The study showed that dyes specifically designed to adhere to the surface in a manner not affected by the aggregant improve the reproducibility of SERRS and that the complex results often obtained from multifunctional ligands can be explained at least in part by changes in the chemistry of surface adhesion.