AuBr2 –‑Engaged Galvanic Replacement for Citrate-Capped Au–Ag Alloy Nanostructures and Their Solution-Based Surface-Enhanced Raman Scattering Activity

This work demonstrates that AuBr2 – can serve as a suitable AuI precursor in the galvanic replacement reaction with Ag nanoparticles (NPs), especially for preparing citrate-capped Au–Ag alloy nanostructures. A cost-effective recipe for stable AuBr2 – solution was presented by the reduction of AuBr4 – derived from the ion exchange between AuCl4 – and optimal amount of Br–. This recipe successfully suppressed the disproportionation of Au+ without the aid of concentrated salt solution, ensuring the colloidal stability of the citrate-capped Ag NPs during the replacement reaction. A comparative study was then performed on the structural evolution of citrate-capped Ag NPs during the replacement reaction with AuBr2 – and AuCl4 –. Uniform Au–Ag nanoshells were gradually formed when AuBr2 – was employed, while porous Au–Ag nanoframes are generated by using AuCl4 –. Moreover, the as-obtained AuBr2 – solution can also be extended to the synthesis of complex nanostructures, such as Au@Au–Ag nanorattles. Finally, an improved solution-based Raman spectroscopic technique was established to precisely characterize the surface-enhanced Raman scattering (SERS) properties of these Au–Ag alloy nanostructures. It is found that, under the same amount of Au ions, the Au–Ag alloy nanoshells exhibit stronger SERS activity than the porous and fragile Au–Ag nanoframes. This AuBr2 –-based strategy nicely complements the galvanic replacement reaction and provides a promising opportunity to synthesize chemically stable and surface accessible Au–Ag alloy nanostructures for biomedical applications.