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 scatterin g (SE(R)RS). Control of the chemistry at the colloid surface is essent ial to realize the potential of this method. This study is aimed at un derstanding the surface chemistry in aqueous solutions, characterizing the nature of the final surface, and developing a robust method for S E(R)RS analysis at the surface. An optimized procedure for the reducti on of silver nitrate with trisodium citrate is described. Visible abso rption and photon correlation spectroscopies of colloid formation indi cate 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 n m) and polydisperse. Subsequent heating initially provides a less poly disperse mixture of 20-30 and 40-50 nm particles and finally an approx imately monodisperse distribution of smaller particles (similar to 27 nm). Solution NMR studies of the colloidal suspension indicate the pre sence of citrate and its decomposition products, acetoacetic acid and formate in solution throughout colloid formation. Raman scattering fro m aggregated aliquots of colloid indicates two forms of citrate depend ing on the stage of preparation, but neither acetoacetic acid nor form ate is detected as being adsorbed at the silver surface. The final, ap proximately monodisperse particles are believed to be stabilized by a surface layer of silver citrate, with pendant negative groups. The col loids are stable for over 2 months. The SE(R)RS effect requires contro lled aggregation of the colloid. The aggregation process is generally induced by the addition of acid or activating ions, for example, Cl- o r I-. Aggregation with acid (HNO3) and with poly(L-lysine) and ascorbi c acid are compared. The poly(L-lysine) method is more effective, enha ncing the monodispersity of colloidal aggregates. The reproducibility of SERRS (relative standard deviation (RSD) <5%.) is acceptable for an alytical purposes, whereas that from aggregation with acid (HNO3) (RSD = 18.5%) is not. Futhermore, at low analyte concentrations, SE(R)RS f rom 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 f or trace analysis of anionic, neutral, and cationic species of using r eagents which alter surface charge and dielectric constant are illustr ated.