Abstract
Silver colloids of uniform shape and size are prepared by a two-step reduction. Small silver particles form initially by the rapid reduction of silver nitrate with sodium citrate at 100°C and then grow at 92°C. The reaction processes and resulting silver colloids are characterized by transmission electron microscopy, ultraviolet–visible absorption spectrophotometry, zeta-potential measurements, and Ag+ concentration analysis. The surface-enhanced Raman scattering (SERS) activity of the silver colloids is then investigated, using crystal violet (CV) as a SERS probe. The silver colloids exhibit uniform shape and size and stable SERS activity. The average size of the silver particles is 47 nm (14% relative standard deviation), while the average sizes of the silver colloids prepared at 100°C and 92°C are 41 (30%) and 71 nm (33%), respectively.
Highlights
Surface-enhanced Raman scattering (SERS) was first observed from pyridine adsorbed on a roughened silver electrode by Fleischmann et al [1]
crystal violet (CV) is used as a SERS probe, and the silver particles exhibit high SERS activity compared with the colloids prepared at 100°C or 92°C
Silver colloids are prepared in a two-step reduction, without capping agents
Summary
Surface-enhanced Raman scattering (SERS) was first observed from pyridine adsorbed on a roughened silver electrode by Fleischmann et al [1]. SERS has since attracted much attention because of its high sensitivity to low analyte concentrations. The sensitivity enhancement factor is on the order of 105 to 106 but can reach 1014 to 1015 [2]. Previous SERS studies of bulk samples have elucidated the SERS mechanisms and shown that electromagnetic and chemical enhancements operate simultaneously. The former is due to surface plasmon resonance (SPR) confined to the near-surface region, while the latter is due to surface active sites or charge transfer between the adsorbate and substrate [3,4]. Regardless of electromagnetic or chemical enhancement, active substrates and adsorbed analytes (mono- or multi-layers) are both necessary for producing high-quality SERS spectra. Substrate activity depends on the scale of its features (10 to 100 nm) and its morphology [5,6,7]
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