Surface‐enhanced Raman scattering (SERS) of random silver or gold particle arrays on aminodextran‐coated polystyrene beads

Citrate and/or chemically modified aminodextran (Amdex) SERS bands were detected from silver particle-Amdex-polystyrene (PS) beads either as singlets or small clusters, prepared by reaction between aqueous silver nitrate, Amdex-coated PS beads, and aqueous sodium citrate solution. However, citrate on individual silver or gold particles of the same size was not detected with 633-nm excitation. SERS of dyes such as rhodamine 6 G was readily observed on individual silver particles or on the gold beads. Three distinct modes of citrate/modified-Amdex binding to the silver particle-Amdex-PS beads were characterized: (1) exchangeable citrate bound through one or more carboxylate groups—citrate could be washed away with H2O or D2O; displaced by successively greater concentrations of R6G; or squeezed out mechanically by applying the pressure of a second coverslip on top of a droplet of bead suspension; (2) nonexchangeable Amdex bound to silver via a primary alcoholate ion of oxidized, glutaraldehyde-cross-linked Amdex; and (3) nonexchangeable oxidized Amdex bound to silver via carboxylate group(s) but with H/D exchangeable alcohol hydrogen atom. That oxidation of the glucopyranose residues of Amdex on PS beads had taken place by Ag(I) as silver nitrate in aqueous solution to split out carboxylate groups was first identified from their characteristic vibrational modes in the SERS spectra of colloidal silver suspensions. Minimum amplification factors for (R6G on Ag)/(R6G on Au) with 633-nm excitation were estimated as 1 × 1015/1 × 1011. An extra SERS enhancement of about 1000-fold due to multiplicity of metal particles per bead was included. Copyright © 2005 John Wiley & Sons, Ltd.