Correlation of Size and Surface-Enhanced Raman Scattering Activity of Optical and Spectroscopic Properties for Silver Nanoparticles

The primary focus of this research is to investigate the potential correlations between the absorption band and surface-enhanced Raman scattering (SERS) activity of solution-based silver nanoparticles and the morphology and size of the silver nanoparticles. Silver nanoparticles were synthesized using sodium borohydride reduction methods. The silver nanoparticles were tested for SERS activity using a highly SERS-active compound, trans-1,2-bis(4-pyridyl)ethylene (BPE). Scanning transmission electron microscopy (STEM) was used to analyze the size and shape of the Ag nanoparticles. The STEM, SERS, and UV-vis data have been investigated in order to make correlations between SERS activity, size, and shape of the nanoparticles and their corresponding lambda(max) or surface plasmon (SP) band. The data suggest that as particle size increases, SERS activity decreases. Thus, using the sodium borohydride reduction and solution-based SERS studies, there appears to be an optimal nanoparticle size that is obtained at a lambda(max) of 390 rim. Highly SERS-active nanoparticles had an average particle size of approximately 15 nm and a lambda(max) at 390 nm. At this wavelength and hence average particle size, SERS activity is the strongest. The correlation between SERS activity, wavelengths associated with the surface plasmon band, and the size of the nanoparticles in solution diverges from most of the work which focuses on nanoparticles on solid-substrate surfaces. Although solid-substrate work has focused primarily on the coupling between nanoparticles and substrates on immobilized surfaces and their profound influence on SERS activity, solution-based studies are needed to understand this coupling and its influence on SERS activity in solution. SERS of solution-based nanoparticles provides a significant opportunity to probe the SERS mechanism in solution while also providing, an alternative for using SERS in chemical analyses more suited for solution, such as probes in living cells or as diaonostic tests. Herein we combine UV-vis spectroscopy with high-resolution electron microscopy analysis for the study of the effects of size and lambda(max) of Ag nanoparticles on SERS enhancement.