Bimetallic Au/Ag Core-Shell Superstructures with Tunable Surface Plasmon Resonance in the Near-Infrared Region and High Performance Surface-Enhanced Raman Scattering.

Due to the larger surface area and the synergistic effects between two noble metals, the bimetallic superstructures exhibit enhanced distinctive optical, catalytic, and photothermal performances and surface-enhanced Raman scattering (SERS) "hot-spot" effect, and thus have attracted great interest in various applications. Compared with the common Pd, Pt hierarchical structures coated onto Au nanoparticles (NPs), easily synthesized via fast autocatalytic surface growth arising from intrinsic properties of Pd and Pt metals, precisely controlling the hierarchical Ag growth onto Au NPs is rarely reported. In our present study, the reducing agent dopamine dithiocarbamate (DDTC) was covalently capped onto the first metal core (Au) to delicately control the growth model of the second metal (Ag). This results in heterogeneous nucleation and growth of Ag precursor on the surface of Au nanorods (NRs), and further formation of cornlike bimetallic Au/Ag core-shell superstructures, which usually cannot be achieved from traditional epitaxial growth. The thickness of the hierarchical Ag shell was finely tuned in a size range from 8 to 22 nm by simply varying the amount of the ratio between Ag ions and DDTC capped on Au NR core. The tunable Ag shell leads to anisotropic bimetallic Au/Ag core-shell superstructures, displaying two distinctive plasmonic resonances in the near-infrared region (NIR). In particular, the longitudinal surface plasmon resonance exhibits a broadly tunable range from 840 to 1277 nm. Additionally, the rich hot spots from obtained Au/Ag superstructures significantly enhance the SERS performance.