Effect of Substrate Optical Absorption on Surface-Enhanced Raman Spectrometry on Colloidal Silver

Quantitative analytical applications of surface-enhanced Raman spectrometry (SERS) on colloidal substrates are hindered by the experimental difficulties in measuring intensities with precision comparable to that of state-of-theart spectrochemical techniques. Several factors, both adsorbateand substrate-dependent, contribute to the poor reproducibility of SERS. Since the observed enhancement factor is related to the sample-induced aggregation of the colloid, the several possible aggregation deqees achieved when samples of identical adsorbate concentration are added to colloidal dispersions result in poor reproducibility. Large local concentrations of adsorbate when mixing sample and colloid cause variable crystal-growth rates and a range of final aggregation states. Crystal growth in colloidal systems is a dynamic process in nature, so the timing in the measurement process must be carefully controlled. On the other hand, the SERS spectrum of an adsorbed species corresponds to the spectrum of the first adsorbed monolayer. Subsequent layers of adsorbate undergoing enhancement due to long-range electromagnetic effects have the tendency to generate the ordinary Raman spectrum.1*2 As a result, competitive molecular adsorption by the limited amount of active sites on the metal surface may result in uncontrolled intensities for a given adsorbate. Several authors have discussed the effects of coadsorption of nitrate and borate on SERS intensities on colloidal silver.3-5 Differences in enhancement factors may also result from inhomogeneities in the distribution of molecules at the surface, their several possible orientations, and various adsorption energies.6 Temperature, pH, aging of solutions, and colloidal preparations as well as laboratory practices contribute certainly to the observed reproducibilities.3,7