A top-down chemical approach to tuning the morphology and plasmon resonance of spiky nanostars for enriched SERS-based chemical sensing

Abstract

Gold-silver (AuAg) alloys, an important noble bimetallic system, have received significant attention due to their chemical stability and unique properties for use in catalysis, surface-enhanced Raman scattering (SERS) and biomedical applications. We report a chemical approach to reconstruct the morphology of AuAg spiky nanostars using additional metal ions, into nanostructures that exhibits enriched SERS-based chemical sensing. To achieve this, we developed a two-step reduction process involving the simultaneous reduction of gold and silver followed by a successive reduction process. First, AuAg nanostars were synthesized by reducing Au and Ag using a one-pot method. Secondly, Au and Ag ions were separately deposited on the as-prepared AuAg nanostars and reduced. Successive reduction of Ag onto the AuAg nanostar resulted in a layered core-shell nanostructure with minimal mixing of the two metals. The deposition of Au ions onto the AuAg nanostars yielded maximally mixed alloyed nanostructures. There was no significant change in the morphology of the spiky nanostars with the use of high or low Ag feed concentrations or low Au concentrations. However, a new star-like nanostructure was formed as a result of the successive reduction of Au ions at a high feed concentration. The properties evaluated by SERS studies reveals that the as-prepared maximally mixed random alloy outperforms the minimally mixed layered core-shell nanostructures.