Synthesis of AuAg@Ag core@shell hollow cubic nanostructures as SERS substrates for attomolar chemical sensing

Abstract

Electrum, the AuAg bimetallic alloy, has been an important system since ancient times to the present day, as these alloys have bi-functional properties lacking from their constituent metallic elements. We report a simple synthesis of a novel AuAg@Ag core@shell nanostructure (NS), where a bimetallic AuAg alloy hollow nanocube (HNC) core was coated with a monometallic Ag shell. The AuAg hollow cubic NSs were synthesized through a direct, one-pot synthetic protocol using Tween 80 micelles, a non-ionic surfactant, as a soft-template. The core@shell NSs were prepared from Ag and ascorbate ions under alkaline conditions in the presence of AuAg HNC as the core. The atomic percent of Ag in the core-shell NS was varied by using different amounts of the feed Ag ions during the synthesis. Depending on the Ag nanoparticle coating, the localized surface plasmon resonance (LSPR) peak of the AuAg@Ag core-shell hollow cubic NS underwent significant blue-shifting and narrowing. The necessity for such a AuAg alloy core and Ag-shell NS was demonstrated via surface-enhanced Raman scattering (SERS) studies performed using 4-mercaptothiophenol (4-MPh) as a probe molecule. Moreover, the sensitivity of the AuAg@Ag core-shell NSs as effective SERS substrates was revealed via concentration-dependent SERS studies on 4-MPh (up to 1 aM). In this work, the importance of electronic ligand effects associated with charge transfer from the AuAg alloy core through the Ag shell to the probe molecule has been highlighted using the commonly studied 4-aminothiophenol.