CORRELATION BETWEEN SURFACE-ENHANCED RAMAN-SCATTERING AND ABSORBENCY CHANGES IN SILVER COLLOIDS - EVIDENCE FOR THE CHEMICAL ENHANCEMENT MECHANISM

Colloidal Ag suspensions including adsorbed pefloxacin, an antibacteri al agent, were studied by photon correlation spectroscopy (PCS), zeta potential measurements UV-vis absorption, and surface enhanced Raman s cattering(SERS). These techniques provide complementary information ab out the particle size and state of aggregation as well as charge (re)d istributions and adsorbate (re)orientations in the electrical double l ayer, When the pefloxacin concentration was varied, it was found that the SERS enhancement correlates with the absorbance in the visible reg ion, i.e., at the excitation wavelengths of 514 and 632 nm. This corre lation also holds for those Ag sols in which absorbance changes are in duced upon addition of Ca(NO3)(2) and CaCl2 as well as for the time-de pendent absorbance changes of the individual sol preparations. Since t hese changes are not paralleled by variations of the particle size or aggregation, it was concluded that the origin of the visible absorptio n band is adsorbate-specific, The analysis of the zeta-potential measu rements as well as of the SERS spectra indicates that the shape and th e maximum of the visible absorption band depend on the orientation of the adsorbate and/or the detailed structure of the surface complex. Th us, this band is assigned to an Ag- adsorbate charge transfer transiti on so that the correlation between its strength and the SERS enhanceme nt can readily be understood in terms of the so-called chemical mechan ism.