A New Type of SERS Tags: Au@Ag Core/Shell Nanorods with Embedded Aromatic Molecules

Abstract

SERS tags are a new class of nanoprobes consisting of metal nanoparticles with adsorbed Raman active molecules. The brightness of a single tag depends on the Raman cross section of the reporter molecules and on the enhancing properties of plasmonic particles. Here, nine types of composite AuNR(core, 81 × 25 nm)@R@Ag(shell, 14 nm) nanorods (NRs) with thiolated aromatic molecules (R, nine different molecules) embedded between metal layers were synthesized. The Raman efficiency of reporters was evaluated by measuring normal Raman spectra in ethanol. The surface-enhanced Raman scattering (SERS) was evaluated for molecules adsorbed on Au nanorods before and after Ag shell growth. The coverage of the functionalized Au nanorods with an Ag shell results in an increase in SERS enhancement factor by two orders of magnitude, from 2.5 × 10⁴ for the initial Au NRs to 8.5 × 10⁶ for composite particles. Finite-difference time-domain (FDTD) simulations showed that the ultrahigh enhancement of Raman scattering inside composite nanorods can be explained by the enhancement of the EM field at the boundary of metal layers. Thus, for fabricated composite particles, we have a specific variant of EM SERS enhancement at the boundary of Au/Ag metals, which is different from the local field enhancement near plasmonic particles.