Simple and efficient fabrication of dimers of silver colloidal particles for surface‐enhanced Raman scattering

Abstract

AbstractWe developed a simple and efficient method to fabricate surface‐enhanced Raman scattering (SERS) substrates consisting mainly of dimers of Ag colloidal particles. The SERS substrates were fabricated by immobilization of Ag colloidal particles on a cover glass coated with poly(4‐vinyl pyridine), and then adsorption of target molecules on the immobilized colloidal particles, and then second immobilization. To increase the surface charge of the colloidal particles, pristine silver sols were purified by centrifuging. Also, the colloidal particle diameter was optimized to achieve a high Raman signal enhancement from their dimers. When silver sols purified were used in the immobilizations, the dimers of Ag colloidal particles were formed dominantly with very few trimers and tetramers. The percentage of dimerization among Ag colloidal particles immobilized by the first immobilization was about 65%, 73%, and 77% when the second immobilization time was 10, 20, and 30 min, respectively. With an increase in the second immobilization duration, the peak intensity of the longitudinal mode of the dimers increased without any shift in its extinction peak wavelength (λmax) from 519 nm. By excitation with a 514.5 nm laser line, the highest enhancement was obtained when dimers were formed from the Ag colloidal particles whose average diameter was 28 nm. The fabricated substrates were very sensitive and stable. Also, they showed very good SERS spectra, with an average enhancement factor of ~6.2 × 107. The enhancement factor at the junction was estimated roughly to be 1.3 × 109. Our method could be used to measure SERS spectra of most molecules.