Study of the Interaction between Gold Nanoparticles and Rose Bengal Fluorophores with Silica Spacers by Time-Resolved Fluorescence Spectroscopy

The interaction between gold nanoparticles (GNPs) with silica shell as spacer, Au@SiO2 NPs, and fluorophore rose bengal (RB) is studied using time-resolved spectroscopy. Varied sizes of GNPs with controlled thickness of silica shell were synthesized to investigate the effects on metal-enhanced fluorescence. Fluorophore RB covalently connected to prefunctionlized silica surface has spectral overlap with the plasmon resonance of the gold nanoparticle. The enhancement factor for fluorescence displaying a maximum at spacer separation similar to 10 nm is 2.4, 3.8, 4.6, and 5.5 for diameters 45, 65, 80, and 100 nm Au@SiO2 NPs, respectively. Biexponential decay of emission is observed for small thicknesses of spacer, indicating multiple pathways for relaxation of the excited states. Both time constants tau(1) and tau(2) are consistently increased with increased separation of the silica spacer. The fast component has the most amplitude at short spacer thicknesses and large NP sizes. The biexponential decay is explained by the back energy transfer of the bright modes of GNPs to fluorophore being nonnegligible. For 100 nm GNPs, we find that the rate constant for energy transfer from RB to GNP is 9 x 10(6) to 2.0 x 10(10) s(-1) (bright + dark modes) for separation 5-45 nm, displaying a dependence on the separation of the silica shell d(-n) with n 2.5. The backward rate constant is 3.5 x 10(9) to 4.9 x 10(9) s(-1) for separation 5-18 nm.