Novel Graphene-Gold Hybrid Nanostructures Constructed via Sulfur Modified Graphene: Preparation and Characterization by Surface and Electrochemical Techniques
Graphical abstract
Graphene nanosheet-gold nanoparticles (GNs-AuNPs) hybrid has been fabricated from sulfur-modified graphene nanosheets (S-GNs) impregnated with HAuCl4 as Au precursor. Application of the GNs-AuNPs hybrid in electrochemical biosensing was demonstrated by immobilization of glucose oxidase as a model on the surface of GCE-ATP-GNs-AuNPs, and then, using it for sensing of glucose.
Introduction
Graphene nanosheets (GNs), a novel generation of carbon-based nanosheets with a sp2 hybrid carbon network, has inspired much theoretical and experimental application compared to other carbon materials due to its exceptional electronic, optical, high charge mobility and large surface area properties [1], [2], [3], [4], [5], [6]. These carbon nanosheets have been exhibited as the foundation for various technologies such as nanoelectronic [7], fuel cell [8], hydrogen storage [9], batteries [10], nanosensors, and nanomeidicine [11], [12], [13]. The GNs can play an important role in improving the biosensor properties due to its large specific area and excellent electron transfer [14], [15], [16]. However, due to some inherent disadvantages, such as its chemical inertness and easy aggregation in aqueous solution, the application of GNs has been limited [17], [18]. Therefore, the production of a homogeneous suspension of individual GNs in aqueous solution is important in application of GNs [14], [19], [20]. Chemical modification is one the most common and widely used way to introduce the spacers to GNs and improve their solubility or dispersibility [15], [21].
To explore application potential of graphene-based sensors and biosensor, robust methods for producing, modification and functionalization of GNs surface with metal nanoparticles are of great important [22], [23], Metal nanoparticle-graphene hybrids have attracted much attention in recent years due to their great promise in various applications [9], [24], Among graphene-based hybrid materials, graphene nanosheet-gold nanoparticles (GNs-AuNPs) hybrids are the most interesting studied subjects [25], [26]. Attachment of catalytic nanoparticles, such as gold and platinum, could further increase the electrocatalytic activity of these nanosheets [27], [28]. In addition, decorating by metal nanoparticles increases the distance between the GNs and inhibits adhesion of the resulting GNs in dry state [29]. However, due to hydrophobic nature, pristine GNs do not provide enough anchoring sites for decorating metal nanoparticles.
There are many imaginative methods for decorating metal nanoparticles onto GNs, each offering varying degrees of control of particle size and distribution. However, most of these methods use of organic linker (spacers) [30], [31], [32]. Typically, in-situ reduction of gold salt on graphene oxide has been employed to obtain GNs-AuNPs hybrids [33]. Dharuman have used a poly (amidoamine) dendrimer as a linker for adhesion of AuNPs to fabricate label free electrochemical DNA hybridization sensor [34].
Still, there are some significant challenges in synthesis and develop of graphene-based hybrid materials [35], [36], [37]. For example: (i) Controlling the uniformity of metal nanoparticle decorating the GNs surface, in particular, the strength of the attachment between the organic and the inorganic nanostructure. (ii) Finding appropriate conditions for modification of the flat and rigid structure of GNs. The harsh conditions, usually used to overcome the high-energy barriers associated with intralayer conjugating and interlayer van der-Waals force, can be destructive for reactants. On contrary, if moderate reaction conditions are used, the yield of the reactions is usually low. (iii) Covalent functionalization generally changes the hybridization of GNs carbon atoms from sp2 to sp3 which disrupts the band-to-band transitions of π electrons and causes loss of the desired properties of GNs, such as their high mobility of electron and remarkable mechanical properties [38], [39]. Therefore, it is important to modify the surface of GNs by proper techniques, enabling to control distribution and density of the metal nanoparticles on the substrate, and also avoid deterioration of the electronic network of GNs [40].
The GNs surface can be modified by covalent attachment of functional groups or non-covalent adsorption of functional molecules onto the surface. Yang et al. found that non-covalently functionalized CNT surfaces interact strongly with Pt nanoparticles through the formation of Pt-S bonds, resulting in very high Pt nanoparticle loadings, with both good dispersion and a narrow size distribution [41]. Wang et al. have recently shown that elemental sulfur-graphene composites can be prepared through heating a mixture of GNs and elemental sulfur. They used the synthesized composite as cathode for lithium-sulfur batteries [42]. Nazar prepared sulfur-graphene composites with sulfur as high as 87 wt% using a simple one pot, scalable method [43].
Another challenge is a control over orientation and three-dimensional distribution of metal nanoparticle-graphene hybrids, which is a necessity and of prime importance for many applications [36]. To addresses this problem, layer-by-layer (LBL) assembly which is simple, efficient and allowing precise control over the structure and thickness of the films in nanoscale, has been followed [35], [44].
Here, we present, for the first time direct thiolation of GNs by physically mixing elemental sulfur with GNs to synthesize highly dispersed AuNPs with narrow size distribution on GNs. The modification is based on sulfur impregnation of GNs from toluene solution followed by a melt-coat step. The AuNPs are supported on the GNs by the wet impregnation-thermal reduction method. Morphological characteristics of GNs-AuNPs hybrids are investigated by X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, surface Raman spectroscopy (SRS), X-ray photoelectron spectroscopy (XPS), and high resolution transmission electron microscopy (HRTEM). Then, a simple strategy is proposed for construction of GNs-AuNPs hybrids assembly on glassy carbon electrode (GCE) surface by electrografting process via ATP based on LBL method to get GCE-ATP-GNs-AuNPs.
A combination of analytical methods including cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) is used to completely characterize the electrocatalytic properties of GCE-ATP-GNs-AuNPs modified surface for reduction of oxygen, where excellent electrocatalytic activity was observed. To further demonstrate the electrocatalytic efficiency and applicability of the modified surface, the glucose oxidase (GOx) was used as a model, immobilized on the GCE-ATP-GNs-AuNPs and successfully tested for recognition of glucose in the presence oxygen.
Section snippets
Material and Reagents
Graphite powder, HAuCl4·3H2O, 4-Aminothiophenol (ATP), elemental sulfur, Glucose oxidase (GOx), (from Aspergillus Niger 20,000 units/g, EC 1.1.3.4) and other chemicals were of analytical grade obtained from commercial sources (Sigma® or Merck®), and used without further purification. All solutions were prepared with double-distilled water. Stoke solutions of glucose (10 mM) were prepared in 0.1 M PBS, pH 7.4, and stored overnight at 4 °C before use. Additionally, about 0.5 mg of GNs-AuNPs hybrid
Synthesis of GNs-AuNPs Hybrid
Scheme 1 shows a representation of the process performed by this paper. The GNs was prepared according to the literature (see Section 2.2). A thin layer of sulfur on GNs (S-GNs) is used as a mediator that acts to absorb and bind the gold ions precursors from solution phase into the GNs, and additionally confines their particle growth during reduction to result in ultrafine dispersed AuNPs. Sulfur exhibits a strong affinity for owing the Au, and thus, functions as a gold ions trap [54]. The gold
Conclusion
We have shown here the ability to synthesize a novel hybrid nanomaterial based on sulfur-modified graphene, which generates AuNPs anchoring sites on the inert surface of the GNs and enables high Au dispersion on the support surface without damaging the intrinsic conductivity of the GNs. The GNs-AuNPs hybrid with uniform distribution of AuNPs and particle size of less than 10 nm was synthesized by thermal treatment of the Au-impregnated sulfur-modified graphene. The stability of AuNPs attached to
Acknowledgment
The authors gratefully acknowledge the University of Isfahan for financial supports and research facilities. Also, thanks to Dr. Razgar Ahmadi for his help in preparation of hybrids in the initial steps of this work. We also acknowledge technical assistance of the University of Isfahan, Central Laboratory, in obtaining the XRD and FTIR data.
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