ION-INDUCED SERS OF DYE MOLECULES ON VERY SMALL SILVER COLLOIDS IN AQUEOUS-SOLUTION

We report strong surface-enhanced Raman spectra (SERS) from cationic dye molecules adsorbed on radiolytically prepared very small and isolated silver colloids in aqueous solution. The colloids show a very sharp plasmon absorption band at 380 nm and were ''electromagneticaliy SERS-inactive'' at excitation wavelengths used in the experiments. ''Chemically caused'' SERS of dye molecules adsorbed on these colloids was obtained only when silver, barium, or manganese ions were brought to the surface of the colloidal silver particles which was indicated by a small red shift of the plasmon absorption band. The chemical enhancement due to metal ions strongly depended on the different anions of the dye molecules and was very strong for I- whereas chemically caused SERS was not found for dyes with Cl- and C2H5SO4- as anions. Different mechanisms which can be responsible for appearance of a chemical SERS enhancement on the colloidal silver particles will be considered.