Optimization of Ag-Coated Polystyrene Nanosphere Substrates for Quantitative Surface-Enhanced Raman Spectroscopy Analysis

The optical properties of Ag-coated polystyrene nanospheres can be varied by changing the vapor deposition angle theta. When theta < 55 degrees, a sharp absorbance spectrum is observed, and for theta > 55 degrees, broad band absorbance is achieved. The difference in optical absorbance is due to the coverage and shape change of the Ag coating on nanospheres with multiple oriented domains, as demonstrated by finite-difference time-domain simulations. The localized surface plasmon resonance of substrates coated at small theta can be tuned closely to the excitation wavelength, and these samples can serve as good surface-enhanced Raman scattering (SERS) substrates. The polystyrene vibrational peak at Delta nu = 1004 cm(-1), which is inherent to the substrate, can be used as an internal standard. By normalizing the SERS peak of an analyte by this peak intensity, the SERS intensity ratio is independent of the substrate fabrication conditions. Suchan internal standard is also shown to be effective for revealing the adsorption kinetics for rhodamine 6G and 4-mercaptophenol. Thus, metal films on polystyrene nanospheres can be a simple platform for quantitative SERS measurements.