Amplified light scattering and emission of silver and silver core–silica shell particles

Side versus forward light scattergrams, and fluorescence (488 nm excitation) intensity versus particle count histograms were gathered for bare, R6G-coated, and silica–R6G-coated silver particles of 150–200 nm diameter, one-by-one by flow cytometry. Fluorescence emission intensity of the composite particles monotonically increased and then reached a plateau with greater R6G concentrations, as measured by flow cytometry. Fluorescence amplification factors of up to 3.5 × 10 3 were estimated by reference to measurements on core–shell particles with silica instead of silver cores. Huge surface enhanced Raman scattering (SERS) intensities, at least 10 14-fold greater than normal Raman scattering intensities, were observed with 633 nm excitation for molecules such as rhodamine 6G (R6G) on the same single particles of silver. Although routine transmission (TEM) and scanning (SEM) electron microscopies showed gross structures of the bare and coated particles, high-resolution field emission scanning electron microscopy (FE-SEM), revealed Brownian roughness describing quantum size and larger structures on the surface of primary colloidal silver particles. These silver particles were further characterized by extinction spectra and zeta potentials. Structural and light scattering observations that are reported herein were used to tentatively propose a new hierarchical model for the mechanism of SERS. The light scattering of small particles of silver in the range, 100 to 200 nm in diameter, can be distinctly observed by conventional flow cytometer. It was also possible to detect fluorescence emission intensity from rhodamine 6G on silver particles or on silver–silica core–shell particles.