Surface-enhanced Raman scattering inside Au@Ag core/shell nanorods

The design and synthesis of plasmonic nanoparticles with Raman-active molecules embedded inside them are of significant interest for sensing and imaging applications. However, direct synthesis of such nanostructures with controllable shape, size, and plasmonic properties remains extremely challenging. Here we report on the preparation of uniform Au@Ag core/shell nanorods with controllable Ag shells of 1 to 25 nm in thickness. 1, 4-Aminothiophenol (4-ATP) molecules, used as the Raman reporters, were located between the Au core and the Ag shell. Successful embedding of reporter molecules inside the core/shell nanoparticles was confirmed by the absence of selective oxidation of the amino groups, as measured by Raman spectroscopy. The dependence of Raman intensity on the location of the reporter molecules in the inside and outside of the nanorods was studied. The molecules in the interior showed strong and uniform Raman intensity, at least an order of magnitude higher than that of the molecules on the nanoparticle surface. In contrast to the usual surface-functionalized Raman tags, aggregation and clustering of nanoparticles with embedded molecules decreased the surface-enhanced Raman scattering (SERS) signal. The findings from this study provide the basis for a novel detection technique of low analyte concentration utilizing the high SERS response of molecules inside the core/shell metal nanostructures. As an example, we show robust SERS detection of thiram fungicide as low as 10^-9 M in solutions.