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 a 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.