Preparation of Au–Ag coreshell nanoparticles and application of bimetallic sandwich in surface-enhanced Raman scattering (SERS)

Ag-coating Au colloidal nanoparticles have been prepared by Ag deposition on Au core via the chemical reduction of AgNO3 by hydroquinone. The thickness of Ag shell depends on the Au–Ag molar ratio and the reducing time, which were monitored by the ultraviolet–visible (UV–vis) spectrometry and transmission electron microscopy (TEM). A Raman-active molecules, 4-mercaptobenzoic acid (MBA), are adsorbed onto the surface of 16 nm-diameter Au core, and then a 3 nm-thick Ag shell coats the MBA modified Au core to form a Au/MBA/Ag sandwich structure. Surface-enhanced Raman scattering (SERS) spectra show that, in the sandwich structure, 3 nm-thick Ag shell can effectively enhance the SERS signal of MBA. This is probably caused by the electromagnetic coupling of the Au–Ag double metallic layers.