Concentration dependence of the SERS of some water-soluble cationic porphyrins on an electrochemically prepared silver surface

Surface-enhanced Raman scattering (SERS) spectra, taken with 632.8 nm excit ation, are reported for the water-soluble tetrakis(3-N-methylpyridyl)porphy rin chloride [TMPyP(3)], tetrakis(4-N-methylpyridyl)porphyrin chloride [TMP yP(4)] and tetrakis(4N,N,N-trimethylanilinium)porphyrin chloride (TMAP) usi ng an electrochemically prepared Ag surface. Known amounts of the samples w ere placed on the surface as aqueous solutions and the water was allowed to evaporate. It was found that the spectra vary as increasing amounts of the compounds are placed on the surface to give surface concentrations in the approximate range 80-0.07 pmol mm(-2). We suggest that the reason for the c hanges is that the orientations of the porphyrin macrocyclic molecules chan ge, i,e, the porphyrin molecules lie flat on the Ag surface at low concentr ation but are arranged edge-on, perpendicularly or slightly tilted, on the surface at higher concentrations. Copyright (C) 2000 John Wiley & Sons, Ltd .