Colloid Size Effect of Silver Clusters to Surface-Enhanced Raman Scattering

Abstract

Surface-enhanced Raman scattering (SERS) has attracted considerable attention as a probing technique because of its high sensitivity. It is known that a very strong enhancement is obtained from particular sites, the so-called hot spots, that may be junctions between nanoparticles (NPs). Recently, a simple method to fabricate silver colloid clusters, which have hot spots, for SERS has been developed.1 The enhancement was relatively high as about 1x10^6. However, the size of silver colloid particles consisting of the clusters was not optimized. In this study, we have studied the size effect of silver colloid particles consisting of the silver colloid clusters to the SERS enhancement. Silver seed solution containing silver seed NPs whose size was ranged from 7 to 12 nm was prepared by adding sodium borohydride solution into silver nitrate solution containing sodium citrate. The isotropic growth of Ag seed NPs was initiated by adding growth solutions containing the appropriate amount of silver nitrate and sodium ascorbate as a reducing agent into the seed solution. By adjusting the volume ratio of the seed solution to the growth solution, the size of silver NPs was controlled. Using this method, three silver colloid solutions containing silver NPs whose diameters were 21, 28, and 32 nm were prepared. Silver clusters for SERS were prepared by a three-step process: immobilizing silver NPs on poly(4-vinyl pyridine) (PVP) films by dipping a cover glass coated with PVP into a silver colloid solution, adsorbing target molecules (adsorbates) on the immobilized particles, and then immobilizing silver NPs again. The morphology of the SERS substrates was examined by scanning electron spectroscopy and characterized by UV-vis spectroscopy. The size and shape of silver NPs were examined by transmission electron microscopy. The size of silver colloid clusters was increased with prolonging the second dipping time. The surface plasmon absorption band due to the longitudinal mode was red-shifted with increasing the second dipping time. The SERS intensity measured by excitation with 514.5 nm laser light was critically affected by the second dipping time. The highest SERS intensity was measured from the substrate whose absorption maximum due to the longitudinal mode was near 535 nm. The SERS substrate made of 28 nm silver nanoparticles showed higher enhancement than that made of 21 or 32 nm silver nanoparticles. The highest enhancement factor was about 4.8 x10^7, which is 48 times higher than the reported value, 1x10^6.