Investigating the metal-enhanced fluorescence on fluorescein by silica core-shell gold nanoparticles using time-resolved fluorescence spectroscopy

Abstract

Fluorescein silica core–shell nanoparticles, gold@silica@fluorescein were synthesized to study the effects of metal-enhanced fluorescence (MEF) on fluorescein in neutral and basic ethanol solutions. The interaction between gold nanoparticles and fluorescein with silica shells of different thicknesses as spacers was investigated by time-resolved fluorescence spectroscopy. Only the emission spectra of fluorescein overlapped with the surface plasmon resonance of gold nanoparticles, which is regarded as an example for the study of the emission-enhancement of MEF. The fluorescence spectra of GNP@silica@FiTC were red-shifted from those of silica@FiTC and FiTC in solution. And, the emission curves had biexponential decay, indicating multiple pathways for relaxation of the excited fluorophore. The values of fluorescence enhancement factor (EF) were slightly greater in neutral ethanol solution than in basic solution. The lowest silica shell thickness synthesized, ≈12 nm, had the largest fluorescence EF ≈ 2; this enhancement decreased as the shell thickness increased. At thickness 12 nm, the lifetime of the short component in the emission decay was 60–70 ps and its amplitude close to 90%. Based on the obtained lifetimes, ratios of component amplitudes, and proposed kinetic model, the rate constants for the process of the energy transfer between fluorophores and gold nanoparticles were attained. These rate constants for energy transfer displayed a dependence on the thickness of the silica shell, d-n, where n = 1.3–2.6.