Tip-Functionalized Au@Ag Nanorods as Ultrabright Surface-Enhanced Raman Scattering Probes for Bioimaging in Off-Resonance Mode

Surface-enhanced Raman scattering (SERS) performance of Au nanorods (AuNRs) can be enhanced by the tip adsorption of Raman molecules (RMs) through anisotropic polymer stabilization or through the embedding of RMs between AuNR cores and Ag shells of AuNR@RM@Ag composite particles. We propose a new strategy to design ultrabright SERS probes composed of high-aspect-ratio AuNRs with anisotropic Ag coatings, with preferred adsorption of RMs to open AuNR tips. Specifically, for 4-nitrobenzenethiol (NBT) concentrations above a threshold value c > c(tr), the fabricated Au@NBT@Ag particles had NBT-functionalized open Au tips, as well as anisotropic Ag shells grown on the AuNR sides. The SERS response of these probes with an optimal Ag shell was highest in the off-resonance mode, when the excitation wavelength was far from the plasmon resonance of the Au@NBT@Ag composites. Growing the Ag shell further to completely cover the AuNRs decreased the SERS enhancement. For biocompatibility and stability, the probes were additionally covered with a thin silica layer. Under optimal conditions, the probes demonstrated superstrong and superstable SERS spectra, as compared to those from common SERS tags (AuNRs, nanostars, and Au@Ag NRs) with surface-adsorbed NBT. The excellent SERS performance of the developed ultrabright probes is illustrated by single-particle detection of SERS spectra, Raman imaging of living cells, and deep tissue imaging.