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.