The interaction between graphene quantum dots grafted with polyethyleneimine and Au@Ag nanoparticles: Application as a fluorescence “turn-on” nanoprobe
Graphical abstract
The fluorescence emission of graphene quantum dots (GQDs) grafted with polyethyleneimine (PEI) was “turned off” upon interaction with Au@Ag nanoparticles, and was “turned-on” in the presence of biothiols, resulting in a sensitive and selective fluorescence nanoprobe.
Introduction
Graphene quantum dots (GQDs) are carbon-based nanomaterials with excellent photoluminescence properties [1], hence have drawn a lot of attention for use as optical and fluorescence nanoprobes [2]. The photoluminescence properties of GQDs are determined by physical and chemical interactions taking place at their surfaces and edges. Such interactions may lead to either fluorescence quenching or enhancement [3], [4], [5].
The interactions of GQDs with biomolecules such as DNA, amino acids, cytochrome c, biothiols and melamine have been reported [6], [7], [8], [9], [10]. Also, GQDs have been grafted with polymers such as polyethylene glycol (PEG) and polyethyleneimine (PEI) to modify their surface and edge states in order to introduce desired functionality [11], [12]. PEI is an organic polymer that has a high density of amino groups that can be protonated to give polycations [13]. The grafting of GQDs with PEI introduces positive charges on the surface of GQDs [11] and is an excellent surface passivation agent; hence it is employed in this work. However, to date the fabrication of a nanosensor based on GQDs grafted with polymers has not been reported. In this study, we report for the first time on the fabrication of a fluorescence “turn-on” nanoprobe composing of GQDs-PEI and Au@Ag core-shell nanoparticles for sensing. The fluorescence of the grafted GQDs is “turned off” by Au@Ag nanoparticles (NPs), and “turned on” by analytes. In this work we use biothiols as test analytes due to the presence of thiol groups. Strong metal-thiol (Ag-S and Au-S) bonds are expected to be formed between the biothiols and the Au@Ag nanoparticles.
Core-shell and alloy bimetallic nanoparticles are interesting because they provide opportunities to tune the optical properties of individual metal nanoparticles [14], [15]. The bimetallic core-shell nanoparticles with combined properties of individual metallic elements are more interesting than single metallic nanoparticles [16].
The design and synthesis of nanoensembles or nanoprobes capable of binding and sensing biological molecules (biomolecules) selectively has attracted much attention in recent years because of the fundamental roles played by biomolecules in human systems and in chemical and environmental processes [17]. Among the biologically important molecules, biothiols are of particular interest due to the fact that biothiols (such as l-cysteine (Cys), dl-homocysteine (Hcys) and glutathione (GSH)) (which are used as examples in this work) are key players in cellular functions and take part in various redox reactions reversibly. The optical and fluorescence techniques developed so far for the detection of biothiols include; bovine serum albumin (BSA)-Ag nanoclusters, graphene oxide-Ru-DNA complex, GQDs-Hg2+ blend (not suitable for bio-applications due to Hg2+), G-quadruplex, carbon nanodots, semiconductors QDs (CdTe, CdSe/ZnSe) and metal nanoparticles (Au NPs) [18], [19], [20], [21], [22], [23], [24], [25]. Most of these fluorimetric methods involve fluorescence ‘turn-off’ and/or cannot be used for biological applications due to toxicity issues. To date, the detection and quantitation of biothiols (Cys, Hcys and GSH) by fluorescence ‘turn-on’ (which is more reliable than “turn-off”) are few. The decrease in fluorescence (“turn off”) may not be only due to the effects of the test analytes, hence is less reliable. Fluorescence “turn on” designs for biomolecules (including thiols) detection have been widely explored in the literature [26], [27], [28], [29], but not employing GQDs/Au@Ag system reported in this work.
Section snippets
Materials
Natural graphite powder (<20 μm), branched polyethyleneimine (PEI), hydrogen tetrachloroaurate (HAuCl4·3H2O, 99.9%), silver nitrate (AgNO3), l-cysteine, dl-homocysteine, glutathione (reduced), cytochrome c, histidine, lactic acid, bovine serum albumin (BSA), lysozyme, tyrosine, sodium borohydride, Rhodamine 6 G and dialysis membrane tubing (MWCO 1.5 kDa) were obtained from Sigma-Aldrich. Tryptophan was obtained from Fluka, glycine was purchased from SAAR Chem. All other chemicals were of
TEM images
TEM image (Fig. 1A) of GQDs-PEI shows the morphology and size distribution of the GQDs grafted with PEI. The GQDs-PEI are monodispersed with an overall quasi-spherical morphology with particles size ranging from 7 to 15 nm, with an average of 10.5 nm which is in close agreement with the average size obtained from DLS experiment (Fig. S1A). It could be observed from the TEM image that the GQDs-PEI were not aggregated, which is ascribed to PEI as a surface passivation agent [38]. The particle size
Conclusion
Au@Ag core-shell nanoparticles were employed to quench the fluorescence emission of the cationic polymer grafted GQDs. However, the fluorescence emission of GQDs-PEI in the GQDs-PEI@Au@Ag core-shell blend was restored reversibly in the presence of biothiols as test analytes. Based on this phenomenon, and the modulation of the restored GQDs-PEI emission as a function of the respective biothiols concentrations, we have developed a new method for biothiols detection. This method offers the highest
Acknowledgements
This work was supported by the Department of Science and Technology (DST) and National Research Foundation (NRF), South Africa through DST/NRF South African Research Chairs Initiative for Professor of Medicinal Chemistry and Nanotechnology (UID 62620) as well as Rhodes University/DST Centre for Nanotechnology Innovation, Rhodes University, South Africa.
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