Abstract
This paper presents the fluorescence manipulation of Rhodamine-6G (R6G) due to Au nanoparticles (Au-NPs) covered by pristine graphene and hydrogen-terminated graphene. By taking florescence signals of R6G on a quartz substrate as the standard reference, we observe an ∼fourfold increase in fluorescence intensity of R6G on bare Au-NPs deposited on the quartz substrate. However, this enhancement reduces to ∼1.8-fold when Au-NPs are covered by H-terminated graphene. In the case of Au-NPs covered by pristine graphene, the fluorescence of R6G is significantly quenched by a factor of ∼7.6-fold. The resulting fluorescence level can be attributed to the local field enhancement from Au-NPs and the quenching effect of graphene in the Au–graphene hybrid nanostructure, which are confirmed by our controlled experimental and simulation results. Our work reveals that the surface modification of metal NPs by graphene materials would bring a great impact on fluorescence, providing a simple approach for artificially manipulating fluorescence for specific molecular sensing, detecting, and imaging.
Highlights
A maximum of ∼fourfold increase was found on bare Au nanoparticles (Au-NPs), with the enhancement decreasing to ∼1.8-fold by H-terminated graphene film covered Au-NPs, and the fluorescence was significantly quenched off ∼7.6-fold by pristine graphene film covered Au-NPs
Regarding Au-NPs on the quartz substrate, the enhancement can be attributed to the energy transfer through the overlap of the localized surface plasmon resonance (LSPR) band and the emission spectrum of R6G
The high resistivity of graphene after subjecting it to plasma hydrogenation allows the strong local EM fields among Au-NPs to penetrate the graphene, enhancing the fluorescence of R6G adsorbed on the film
Summary
A great deal of fundamental investigation and technological innovation have arisen based on the Purcell effect, which stated that fluorescence emission could be altered by resonant coupling to the external electromagnetic (EM) environment. Enhanced and suppressed fluorescence emission have been observed from luminescent materials coupled to various entities possessing a strong EM field, such as optical cavities, photonic crystals, and metal nanostructures. Numerous devices based on altered fluorescence emission have been demonstrated, including surface plasmon enhanced LEDs, single-photon sources, and bio-sensors and chemical sensors. Enhancement of fluorescence is of great interest because it can provide a higher detection sensitivity and signal-to-noise ratio for fluorescence analysis and imaging. On the other hand, conditional quenching of fluorescence can be effectively applied for other forms of sensing, such as selective quenching or negative sensing. In this sense, the ability of artificially enhancing or quenching the fluorescence signals is greatly beneficial for specific fluorescence-based applications. Graphene is chemically inert and stable single atom thick and has proven to be the best candidate for hybridizing with metal nanostructures.30 In such a metal–graphene hybrid system, graphene can be considered the thinnest protective film for metal plasmonic substrates, especially for Ag-, Cu-, and Al-NPs, as these are oxidized in air (impose negative effects on fluorescence signals).. Graphene itself has proven to be an efficient quencher of fluorescence, and its quenching efficiency can be controlled by defect engineering; it is thereby possible to extend the range of fluorescence manipulation in a metal–graphene hybrid system For this purpose, here, we employ pristine graphene and hydrogen-terminated graphene to modify the surface of Au-NPs deposited on top of quartz substrates and successfully manipulate the fluorescence of Rhodamine-6G (R6G) from enhancement (∼fourfold) to quenching (∼7.6-fold). This work may be helpful in understanding the effect of graphene materials on the local EM fields generated at the edges and corners of metal NPs and provide a simple pathway for fluorescence manipulation
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