SERS substrates of Ag nanoparticles prepared by a magnetron sputtering method and the activation by halide solutions

Surface enhanced Raman spectroscopy (SERS) is a powerful tool of detecting trace molecules, but the decay of Ag-based nanostructures in the SERS activity is one of problems in applications. In this work, a magnetron sputtering method is used to prepare Ag nanoparticles (NPs) on glass substrates, which exhibit a pretty good reproducibility and a detection limit of rhodamine-6G as low as 1×10⁻¹² M. By exposing the Ag NPs to ambient air, the decay of SERS activity is investigated and the inactive SERS substrates are activated by halide solutions, in which the solution of 1×10⁻³ M KCl exhibits the best activation effects. It is found that the modification of halide solutions enables the Ag NPs to undergo a process of dissolution and recrystallization and thus leads to the removal of oxides and contaminations and the activation of SERS activity. Besides the re-formed surfaces of Ag NPs, the variation in the size distribution has important impacts on the Raman enhancement of activated SERS substrates. Statistical analysis reveals that the best hot spots probably appear on the Ag NPs with the sizes close to ~ 45 nm and the density is in association with the type and concentration of halide solution.