SERS-active Ag, Au and Ag-Au alloy nanoparticles obtained by laser ablation in liquids for sensing methylene blue

We have synthesized Ag-Au nanoparticles by laser ablation in liquids using five different targets: 100% Ag, 80%Ag/20%Au, 50%Ag/50%Au, 20%Ag/80%Au and 100% Au (weight percentages). We used ethanol and methylene blue solutions in ethanol as the liquid media. The nanoparticles were mostly spherical with diameters 15, 19, 18, 23 and 11 nm, respectively. When alloyed targets were used, the resulting nanoparticles were completely alloyed forming solid solutions as evidenced by UV-vis Spectroscopy and Scanning Transmission Electron Microscopy. The obtained nanoparticles were employed to study the SERS effect of the methylene blue molecule. All the samples showed good SERS activity, however the ones composed of pure silver showed the greatest Raman signal enhancement. Finally, pure Ag nanoparticles were used for sensing methylene blue at different concentrations. While almost no signal can be discerned from the Raman spectrum when no particles are used at a concentration of methylene blue of 1 × 10−2 M (∼3000 ppm), when Ag nanoparticles are used one can observe the characteristic peak of the molecule at concentrations as low as 1 × 10−10 M (∼3 × 10−5 ppm).