NIR-Active Plasmonic Gold Nanocapsules Synthesized Using Thermally Induced Seed Twinning for Surface-Enhanced Raman Scattering Applications.

Hollow and porous core-shell nanostructures with defined interior nanogaps are of great significance in the field of surface-enhanced Raman scattering (SERS) applications because of the presence of intrinsic electromagnetic (EM) hot spots, multipolar resonances, and multiple facets. Further, nanomaterials having extinction in the near-infrared (NIR) region are particularly important for SERS and biomedical applications, and thus it is highly desirable to synthesize NIR-active plasmonic nanostructures. Herein, we report the synthesis of gold nanocapsules having a solid Au bead as core and a thin-porous rod-shaped shell with extinction in both NIR I and NIR II regions. Thermally induced twinned seeds were used for the silver-free synthesis of pentatwinned Au bead, which served as the foundation for the directed growth of Ag nanorods, which was finally converted to Au nanocapsules following galvanic replacement reaction (GRR). Detailed investigation was carried out to understand the effect of thermal treatment duration in the seed morphology and its subsequent growth to anisotropic Au beads. Ag overgrowth on Au beads yielded uniform Au-bead@Ag nanorods whose size can be tuned by varying the Ag precursor. Five different sized Au-bead@Ag nanorods were studied, and they were converted to Au nanocapsules following GRR. We explored the size-dependent SERS activity of the prepared Au nanocapsules along with their comparison with solid pentatwinned Au beads and found that the smallest sized Au nanocapsules were the best SERS performers. Finite-difference time-domain simulation revealed the presence of intense EM hot spots in the smallest sized Au nanocapsule and corroborated the experimental SERS data. Finally, we fabricated a simple flexible cellulose-based SERS substrate by using the smallest sized Au nanocapsules and investigated its SERS sensing ability for the detection of 2-napthalenethiol (2-NT), as a model analyte, and were able to achieve its detection down to 1 fM concentration.