Off-Resonant Gold Superstructures as Ultrabright Minimally Invasive Surface-Enhanced Raman Scattering (SERS) Probes

Surface-enhanced Raman scattering (SERS) tags that serve as exogenous contrast agents for SERS-based bioimaging are comprised of size- and shape-controlled plasmonic nanostructures. For maximum SERS activity and image contrast, the localized surface plasmon resonance (LSPR) wavelength of SERS tags based on individual nanostructures must match with the excitation wavelength (typically in the near-infrared (NIR) therapeutic window, i.e., 650–900 nm). However, under the resonant excitation, these SERS tags typically exhibit very high photothermal conversion efficiency, resulting in excessive heat that can perturb or even damage the biological species being imaged. Here, we demonstrate bioenabled synthesis of a novel class of ultrabright SERS probes with built-in and accessible electromagnetic hotspots formed by densely packed satellite nanoparticles grown on a plasmonic core. Through the rational choice of the shape of the core, the LSPR wavelength of Au superstructures can be tuned to be either off- or on-resonant with the NIR excitation without sacrificing their high SERS activity. Consequently, the photothermal efficiency of these ultrabright SERS tags can be tuned to realize either contrast agents with minimal heating and perturbation or multifunctional theranostic agents that can image and photothermally kill the targeted cells.