Size Tunable Au@Ag Core–Shell Nanoparticles:\nSynthesis and Surface-Enhanced Raman Scattering Properties

Abstract

We\ndescribe a simple and efficient methodology for the aqueous\nsynthesis of stable, uniform, and size tunable Au@Ag core–shell\nnanoparticles (NPs) that are stabilized by citrate ions. The synthetic\nroute is based on the stepwise Ag reduction on preformed Au NPs. The\nfinal size of the core–shell NPs and therefore their optical\nproperties can be modulated at least from 30 to 110 nm by either tuning\nthe Ag shell thickness or changing the size of the Au core. The optical\nproperties of the Au@Ag core–shell NPs resemble those of pure\nAg NPs of similar sizes, which was confirmed by means of Mie extinction\ncalculations. We additionally evaluated the surface-enhanced raman\nscattering\n(SERS) enhancing properties of Au@Ag core–shell NP colloids\nwith three different laser lines (532, 633, and 785 nm). Importantly,\nsuch core–shell NPs also exhibit a higher SERS efficiency than\nAg NPs of similar size under near-infrared excitation. The results\nobtained here serve as a basis to select Au@Ag core–shell NPs\nof specific size and composition with maximum SERS efficiency at their\nrespective excitation wavelengths for SERS-based analytical and bioimaging\napplications.