CHARACTERIZATION OF THE SURFACE OF A CITRATE-REDUCED COLLOID OPTIMIZED FOR USE AS A SUBSTRATE FOR SURFACE-ENHANCED RESONANCE RAMAN-SCATTERING

Citrate-reduced colloids can be used to give reproducible, sensitive, and selective analysis by surface-enhanced (resonance) Raman scattering (SE(R)RS). Control of the chemistry at the colloid surface is essential to realize the potential of this method. This study is aimed at understanding the surface chemistry in aqueous solutions, characterizing the nature of the final surface, and developing a robust method for SE(R)RS analysis at the surface. An optimized procedure for the reduction of silver nitrate with trisodium citrate is described. Visible absorption and photon correlation spectroscopies of colloid formation indicate that the initial reduction of Ag-I to Ag-0 occurs within 2 min of citrate addition, and the initial particles formed are large (60-80 nm) and polydisperse. Subsequent heating initially provides a less polydisperse mixture of 20-30 and 40-50 nm particles and finally an approximately monodisperse distribution of smaller particles (similar to 27 nm). Solution NMR studies of the colloidal suspension indicate the presence of citrate and its decomposition products, acetoacetic acid and formate in solution throughout colloid formation. Raman scattering from aggregated aliquots of colloid indicates two forms of citrate depending on the stage of preparation, but neither acetoacetic acid nor formate is detected as being adsorbed at the silver surface. The final, approximately monodisperse particles are believed to be stabilized by a surface layer of silver citrate, with pendant negative groups. The colloids are stable for over 2 months. The SE(R)RS effect requires controlled aggregation of the colloid. The aggregation process is generally induced by the addition of acid or activating ions, for example, Cl- or I-. Aggregation with acid (HNO3) and with poly(L-lysine) and ascorbic acid are compared. The poly(L-lysine) method is more effective, enhancing the monodispersity of colloidal aggregates. The reproducibility of SERRS (relative standard deviation (RSD) <5%.) is acceptable for analytical purposes, whereas that from aggregation with acid (HNO3) (RSD = 18.5%) is not. Futhermore, at low analyte concentrations, SE(R)RS from both the analyte and the citrate layer are observed on aggregation with nitric acid. However, SE(R)RS is only observed from the analyte on aggregation with poly(L-lysine) and ascorbic acid. The advantages for trace analysis of anionic, neutral, and cationic species of using reagents which alter surface charge and dielectric constant are illustrated.