Abstract: Au-Ag alloy nanoparticles with different molar ratios were prepared by using sodium citrate as the reducer, and the size was increased via the NH2OH\5HCl-growth method. One plasmon band between that of monometallic Ag and Au observed in their UV-Vis spectra indicated the formation of Au-Ag alloy nanoparticles. TEM results showed diameters about 60 nm and homogeneous color of the alloy nanoparticles without clear core-shell contrast. By using thiophenol as the probe molecules, SERS studies were performed. The results indicated the SERS effect was dependent on the composition and size of the alloy nanoparticles. At the certain sizes of nanoparticles, the SERS intensities were increased gradually with the increase of gold content except Au₂₅Ag₇₅. As Au₂₅Ag₇₅ were smaller than Ag, the size effect made the SERS intensity of Au₂₅Ag₇₅ lower. While aggregating way of Au₅₀Ag₅₀ and Au₇₅Ag₂₅ after adding TP molecules were gradually close to Au, and the surface plasma resonance of aggregates approached to 1064 nm, so SERS of TP at higher Au content was controlled by surface plasma resonance enhancement of aggregates.

Key words: Au-Ag alloy nanoparticles, SERS, Thiophenol