Dramatic Enhancement of Targeted Subcellular Raman Imaging via Synergetic Nanoscale Integration of Resonance and Surface Enhanced Raman Scattering Mechanisms

Surface-enhanced resonance Raman scattering (SERRS) can boost the sensitivity of Raman bioimaging through fine-tuning of both the electronic resonance of the Raman reporter molecule and localized surface plasmon resonance (LSPR) of conjugated plasmonic nanostructures to realize cooperative amplification in the overlapping spectral region of both resonances. Here, we report on the design of an azobenzene-based resonance Raman (RR) reporter having its electronic molecular resonance in the visible wavelength region, where it can readily overlap with the LSPR band of a silver core/gold shell nanoparticle. Furthermore, the reporter molecule is practically nonemissive to minimize autofluorescence contamination of the Raman signal. The density functional theory (DFT) calculations confirm charge redistribution upon the optical excitation that produces significant resonant enhancement of the Raman line assigned to stretching of the single C–N bond. This enhancement is associated with the bonds located at both ends of the central Azo group. At the same time, our analysis of the frontier molecular orbitals suggests that stretching of C–N bonds, residing on the tertiary amine group and located in proximity to the surface of the nanoparticle, contributes to the overall cooperative amplification of Raman signal via surface-enhanced Raman scattering (SERS) mechanism. We further demonstrate that our RR reporter exhibits significant selective intracellular internalization and high contrast, high-resolution, stable Raman imaging of rat brain glioma (C6) cells. Our integrated approach to bioimaging nanotechnology highlights the importance of selective tunability of the plasmonic substrate, excitation wavelength, and electronic resonance of the molecular reporter for real-time, high-resolution, and high-contrast bioimaging and biomolecular interaction analysis with SERRS.