Abstract
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 overlapped with the plasmon resonance of gold nanoparticle. The enhancement factor for fluorescence displaying maximum at spacer separation ~ 10 nm is 2.4, 3.8, 4.6, and 5.5 for diameter 45, 65, 80, and 100 nm Au@SiO2 NPs, respectively. Biexponential decay of emission is observed for small thickness of spacer indicating multiple pathways for relaxation of the excited states. Both time constant τ1 and τ2 are consistently increased with increased separation of silica spacer. The fast component has the most amplitude at short spacer thickness and large NP sizes. The biexponential decay is explained that the back energy transfer of the bright modes of GNPs to fluorophore is nonnegligible. For 100 nm GNPs, we obtain that rate constant for energy transfer from RB to GNP is 9×106 to 2.0×1010 s-1 (bright + dark modes) for separation 5 nm to 45 nm, displaying a dependence to the separation of silica shell d-n with n 2.5. The backward rate constant is 3.5×109 to 4.9×109 s-1 for separation 5 nm to 18 nm.