Near-Electric-Field Tuned Plasmonic Au@SiO₂ and Ag@SiO₂ Nanoparticles for Efficient Utilization in Luminescence Enhancement and Surface-Enhanced Spectroscopy

Application of core–shell plasmonic nanostructures in fluorescence enhancement and surface-enhanced Raman scattering (SERS) strongly depends on their near-field electrodynamical environments. A nonradiative energy transfer takes place between fluorescent molecules and surface plasmon when they are too close. However, for a dielectric shell, the SERS intensity of analytes decreases exponentially beyond 2 nm thickness. Although electromagnetic-field enhancement due to surface plasmon still occurs at longer distances from the metal core, it needs a proper design of the composite nanostructure to exploit this advantage, and an optimal distance between the metal-core and analyte/fluorescent molecule still seems necessary. We analyze, both theoretically and experimentally, the near-electric-field (NEF) distributions in the proximity of the core–shell and shell–medium interfaces of Au@SiO2 and Ag@SiO2 core–shell structures immersed in common dispersing media such as air, water, and DMSO to investigate the effects of surrounding medium and particle geometry on them. Through Mie-based theoretical calculations, we demonstrate that the NEF distributions near core–shell and shell–medium interfaces depend not only on the geometrical parameters, but also on the dielectric constant gradient at these interfaces. For each of the dispersion media and a wide range of metal-core radii, we calculate the optimum shell thickness for obtaining the maximum near-field enhancement at the core–shell and shell–medium interfaces, the essential requirements for applying these nanostructures in fluorescence enhancement and SERS. Theoretically obtained results have been qualitatively verified with experiments.