Study of silver films over silica beads as a surface-enhanced Raman scattering (SERS) substrate for detection of benzoic acid

Surfaces prepared by vacuum depositing silver on silica nanospheres were investigated as surface-enhanced Raman scattering (SERS) substrates using benzoic acid as an analyte. The SERS sensitivity of the substrates was probed as a function of substrate parameters, such as silver-film thickness and silica-bead dimensions. The SERS signals were found to increase with an increase in silver-film thickness and silica-bead diameter until a maximum signal was reached. Additional increases in the film thickness or the bead diameter resulted in a decrease in the SERS signal. Maximum SERS response was obtained with a substrate made by vapor depositing a 6 nm thick coating of silver on a thin layer of silica beads with a nominal diameter of 565 nm. Scanning electron microscopy (SEM) of the substrates was performed to consider the effect of surface morphology on the SERS responses. Optimal SERS substrates exhibited a detection limit of 5 × 10−7M for adsorbed benzoic acid. A linear relationship was demonstrated between the SERS response and the concentration of benzoic acid with two different slopes, one in the 10−5–10−3M range and the other in the 10−3–10−1M range. Copyright © 2003 John Wiley & Sons, Ltd.