Correlation between surface enhanced Raman scattering and absorbance changes in silver colloids. Evidence for the chemical enhancement mechanism

Colloidal Ag suspensions including adsorbed pefloxacin, an antibacterial agent, were studied by photon correlation spectroscopy (PCS), zeta potential measurements UV-vis absorption, and surface enhanced Raman scattering(SERS). These techniques provide complementary information about the particle size and state of aggregation as well as charge (re)distributions and adsorbate (re)orientations in the electrical double layer, When the pefloxacin concentration was varied, it was found that the SERS enhancement correlates with the absorbance in the visible region, i.e., at the excitation wavelengths of 514 and 632 nm. This correlation also holds for those Ag sols in which absorbance changes are induced upon addition of Ca(NO3)(2) and CaCl2 as well as for the time-dependent absorbance changes of the individual sol preparations. Since these changes are not paralleled by variations of the particle size or aggregation, it was concluded that the origin of the visible absorption band is adsorbate-specific, The analysis of the zeta-potential measurements as well as of the SERS spectra indicates that the shape and the maximum of the visible absorption band depend on the orientation of the adsorbate and/or the detailed structure of the surface complex. Thus, this band is assigned to an Ag- adsorbate charge transfer transition so that the correlation between its strength and the SERS enhancement can readily be understood in terms of the so-called chemical mechanism.