Lighting up the gold nanoparticles quenched fluorescence by silver nanoparticles: a separation distance study

Abstract

Contrast ratios of “turn-on” fluorescence probes determine the detection sensitivity of fluorescence-based assays and detections. Conventional fluorescence turn-on methods are generally based on recovery of quenched fluorescence to its initial intensity. Here we designed a nano-platform in which the fluorescence intensity of a pre-quenched fluorophore was enhanced up to the level significantly higher than the intensity of the unquenched fluorophores by utilization of plasmon coupling enhanced fluorescence. In this approach, the fluorescence of rhodamine B isothiocyanate (RiTC), which was pre-quenched by gold nanoparticles (Au NPs), was subsequently enhanced by silver nanoparticles (Ag NPs) upon formation of Ag@SiO₂–Au–RiTC nanostructures. The thickness of SiO₂ spacer shell can be adjusted to tune the gap size between Au NPs and Ag NPs to control the plasmon coupling strength to achieve the optimum fluorescence enhancement. Among the series of Ag@SiO₂–Au–RiTC NPs with SiO₂ shell thickness of 4, 10, 13, 20, 25, 36, 43 and 50 nm, the optimum florescence of the coupled nanostructure was found to occur at silica shell thickness of 13 nm with its fluorescence intensity 101 times that of the Au–RiTC NPs, 4.4 times that of free RiTC. In addition to plasmon enhanced excitation efficiency and quantum yields due to metal–chromophore interactions, huge local electric field amplification at the gap region induced by plasmon coupling between Au NP and Ag NPs contributes additionally to further enhance the excitation efficiency to achieve the observed fluorescence enhancement. This plasmon coupling enhanced fluorescence concept should offer a new design strategy to give rise to a “turn-on” fluorescence responses with improved signal-to-background ratios.