Near-Field Engineering of Silica-Shelled Plasmonic Nanoparticles toward Enhanced Near-Infrared Fluorescence

Near-infrared (NIR) fluorescent molecules often suffer from low quantum yields, limiting their applications in sensing and imaging. Their fluorescence can be enhanced by coupling with plasmonic nanoparticles exhibiting resonances in the same spectral region. However, this approach requires precise nanoscale control over the plasmonic near-field. Here, we combine scanning transmission electron microscopy with electron energy-loss spectroscopy (STEM-EELS) and boundary element electromagnetic simulations to engineer silica-coated anisotropic plasmonic nanoparticles. We synthesised three types of nanoparticles, gold nanorods (AuNRs), gold bipyramids (AuBPs), and silver nanorods (AgNRs) with tunable NIR plasmonic resonances. Silica serves as a dielectric spacer, enabling precise control over the distance between the nanoparticle surface and a potential molecular emitter. To assess their suitability for fluorophore attachment, we investigated the dependence of the electric near-field on silica shell thickness. High-resolution STEM-EELS mapping revealed that thin silica shells (4.4 nm for AuNRs and 3.1 nm for AuBPs) allow most of the near-field to extend beyond the dielectric layer. For AgNRs, a thicker shell of 7.0 nm was required to prevent oxidation while maintaining near-field accessibility. Electromagnetic simulations further evaluated the optimal shell thickness and the dependence of fluorescence enhancement on particle geometry, material, fluorophore internal quantum yield, and fluorophore-metal separation. Together, these results provide practical design guidelines for optimizing particle morphology and sub-10 nm silica shells in plasmonic nanostructures for fluorescence enhancement.