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 × 10−7 M 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 10−5–10−3 M range and the other in the 10−3–10−1 M range.