Laser-based two-step synthesis of Au-Ag alloy nanoparticles and their application for surface-enhanced Raman spectroscopy (SERS) based detection of rhodamine 6G and urea nitrate

•Surfactant free silver and gold nanoparticles were synthesized using pulsed laser ablation in liquid.•The Au-Ag alloy was synthesized by irradiating Ag and Au nanoparticles mixture with a nanosecond pulsed laser beam.•Correlation between irradiating laser pulse energy and irradiation time on Au-Ag alloy formation was demonstrated.•The SERS study of Rhodamine 6G using Au-Ag alloy nanoparticles as a substrate resulted in significant Raman enhancement.•The Au-Ag alloy nanoparticles-based SERS substrates were able to detect low concentrations (10-4 M) of urea nitrate. The synthesis of Au-Ag alloy nanoparticles (NPs) by laser beam exposure of a mixture of monometallic nanoparticle colloids is described. The monometallic Ag and Au NPs are first synthesized in water using pulsed laser ablation. Subsequently, the mixture of Ag and Au nanoparticles is irradiated with a nanosecond pulsed laser beam. The kinetics of Au-Ag alloy formation is investigated by adjusting the laser parameters such as laser wavelength, fluence, and exposure time. The alloying process is found to be strictly dependent on the laser wavelength. No alloy formation is detected for 1064 nm laser, while 532 nm laser is found to be effective for alloy formation. Moreover, the extent of alloy formation increases with the rise of fluence and irradiation time of 532 nm laser. The optimum laser fluence and exposure period are 400 mJ/cm2 and 20 min, respectively. In line with the calculations based on the Drude model and quasi-static theory, the wavelength of the SPR peak of alloy NPs showed nonlinear dependence on the concentrations of Ag and Au nanoparticles. In order to study the effectiveness of the AgAu alloy nanoparticles as a SERS substrate, Rhodamine 6G (R6G) and urea nitrate, a well-known ingredient of homemade explosives, are employed as probe molecules. The characteristic peaks of R6G can be detected at concentrations as low as 1 nM. Similarly, 100 µM urea nitrate concentrations are successfully detected.