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

Abstract

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@ SiO 2 and Ag@SiO 2 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.

Robust one-pot synthesis of citrate-stabilized Au@CeO2 hybrid nanocrystals with different thickness and dimensionality

“…Remarkably, for the thicker shells measured, the SPR shift reaches a maximum value where it saturates. This phenomenon, widely reported for the other similar systems, relates to the maximum distance to which the local field extends from NP's surface [27,28]. Similarly, the broadening of the plasmon band as the CeO2 shell thickness increases can be associated with the non-compact nature of the CeO2 shell coating.…”

“…This assumption determines the effective dielectric constant of the system, which ultimately affects the extent on the SPR red-shifts. (SERS) requires the study of the optimum shell thickness for obtaining the maximum near-field enhancement at the core-shell and shell-medium interfaces [28]. Moreover, the plasmonic enhancement is particularly interesting for the study of plasmon resonances at their fundaments, including nonlinear optical processes such as second harmonic generation [31].…”

Seed-Mediated Growth of Ag@Au Nanodisks with Improved Chemical Stability and Surface-Enhanced Raman Scattering

“…As can be seen from Figure 6a, the substrate prepared with Ag@Au nanodisks exhibits much enhanced Raman signals in comparison to the Raman signal intensities obtained from their solution-casted counterpart. Apart from the effect of dispersion media (air in the first case and water in the second case), 64 the substrate-based assembly of plasmonic nanostructures seems to be of help in producing uniform and more intense “hot spots” which are responsible for the enhancement of Raman signal of the analytes. The Ag@Au nanodisks revealed highly intense Raman signals in comparison to the Ag NPLs.…”

Application of Gold Nanoparticle to Plasmonic Biosensors

“…Similar to these approaches, various studies were performed to verify the effect of spatial variation by controlling spacing within shell thickness. For example, Montaño-Priede et al demonstrated that the maximum fluorescence enhancement of quantum dots (QDs, CdSe) was observed when a silica shell between a GNP core and QDs was approximately 10 nm; however, both the reduction and the increase of shell thickness diminished the enhancement effect of the fluorescence either by energy transfer from the QDs to GNP or a larger separation between the CdSe QDs and GNP core, respectively ( Figure 3 D) [ 53 ].…”