Surfactant size effect on surface-enhanced Raman scattering intensity from silver nanoparticles.

We report on the synthesis of two types of Ag nanoparticles (NPs) and the influence of adsorbed surfactant size on the NP surface for surface-enhanced Raman scattering (SERS) signals. Both particles were of similar size and morphology but were covered by surfactants of different sizes; one surfactant was sodium citrate (molecular weight: 258) and the other was sodium polyacrylate (molecular weight: 2100). For SERS measurement, 4-mecapobenzoic acid and 4-naphthalene thiol as Raman-active dyes were immobilized on the surface of each AgNP. The signals from Raman-active dyes on AgNPs covered with citrate displayed 10 times higher intensity than those from polyacrylate-stabilized AgNPs. Elemental analysis (EA) revealed that the average weight percentage of sulfur is 0.94 wt% and 0.12 wt% for citrate-stabilized and polyacrylate-stabilized AgNPs, respectively. The sulfur content difference was attributed to the size of the existing surfactant influencing the ligand exchange by steric hindrance and subsequently the amount of sulfur content of the particles. These experimental results suggest that the size of initial surfactant should be taken into account when synthesizing a metal particle for enhancing SERS signal.