Mesoporous Ag nanocubes synthesized via selectively oxidative etching at room temperature for surface-enhanced Raman spectroscopy

Silver nanocubes enriched with 100 facets have been extensively used for surface-enhanced Raman scattering. Herein, we report a new water-phase synthesis method for well-defined Ag nanocubes with tunable sizes via a two-step procedure at room temperature. First, irregularly shaped Ag nanoparticles (INPs) were prepared by reducing silver ammonia solution using ethylal. Second, the agglomerated INPs were selectively etched with HNO₃ and Cl^- to yield 100 facet-rich mesoporous Ag nanocubes. The mechanism of Ag-nanocube formation and growth was investigated in detail by elucidating the involved chemical reactions and physical changes at each step during the synthesis. The addition of Cl^- anions was responsible for facilitating Ag nanoparticle growth by removing surface-adsorbed AgCl⁺ species, thereby eliminating inter-particle repulsive forces. This agglomeration was found crucial for the subsequent selective oxidation of Ag nanoparticles because the protective agent used, polyvinylpyrrolidone (PVP), was the most effective one for adsorption on the surfaces of Ag nanoparticles of size greater than approximately 50 nm. Importantly, mesopores were found inside the Ag nanocubes; this can be attributed to the unavoidable imperfect packing during the agglomeration of INPs. The newly prepared Ag nanocubes were further used to enhance the Raman signal of rhodamine 6G, which is capable of reducing the detection limitation to 10^-16 mol·L^-1.