Reduced state carbon dots as both reductant and stabilizer for the synthesis of gold nanoparticles
Introduction
Carbon dots (CDs), a new kind of fluorescent nanoparticles, were discovered serendipitously by researchers purifying single-walled carbon nanotubes in 2004 [1]. Subsequently, this fluorescent nanoparticles which were dubbed CDs [2] by Sun et al. have attracted extensive attentions. Compared conventional organic dyes with semiconductor quantum dots, CDs possess superior properties such as low toxicity, excellent biocompatibility, and high stability against photobleaching [2], [3], [4], [5]. Owing to above advantages, CDs were applied in many fields. CDs have been applied in biolabeling and bioimaging [2], [6], [7], [8]. Chen and co-workers [9] and Qu and co-workers[10] fabricated photovoltaic devices and light-emitting diodes with CDs. Shi et al. [11] reported that CDs could catalyze the oxidization of 3,3′,5,5′-tetramethylbenzidine (TMB) by H2O2, CDs also could be used as visible-light photocatalysts [12]. Zhou applied CDs in detecting mercury ion (Hg2+) [13]. However, as an excellent electron acceptor and electron donor [14], CDs has promising potential to be an oxidizing or reducing agent, which have seldom been reported.
Gold nanoparticles (AuNPs) have been widely used in many areas, including catalysis [15], [16], detection [17], [18], therapy [19]. AuNPs were usually prepared by reducing chloroauric acid (HAuCl4) with a variety of reducing agents, such as citrate [20], sodium borohydride [21]. Alcohols were also often acted as reducing agents for synthesis of metal nanomaterials [22], [23], [24], but the reducing mechanism of different alcohols was different. For instance, gold, silver and palladium nanomaterials could be prepared by the reducing ability of the oxyethylene groups of poly(ethylene glycols) (PEG) [25]. The reducing mechanism of ethylene glycol (EG) was demonstrated that EG generated glycolaldehyde (GA) at the first time, then GA reduced Ag(I) to form silver nanoparticles [26]. Xia et al. [27], [28] proposed that nanomaterials was prepared by using poly(vinyl pyrrolidone) which their ends were terminated with the hydroxyl group (–OH) and proved their reducing function derived from –OH.
Capping agent is another key factor of synthesis of nanomaterials and it can prevent nanomaterials from aggregating. Some capping agents such as thoils [20], oleic acid [29], hexadecyltrimethylammonium bromide [30] have been utilized to prepare metal and magnetic nanomaterials. But all of these are highly reactive chemicals which posing potential environmental and biological risks. As low-risk carbon nanomaterials, graphene oxide and graphene quantum dots have been applied in synthesis of metal nanomaterials acting as reducer or stabilizer [31], [32].
Recently, our group synthesized reduced state carbon dots (r-CDs) successfully, which could be oxidized by some oxidants like KMnO4 [33]. Taking the reducibility of r-CDs and the oxidizability of HAuCl4 into account, AuNPs are synthesized successfully by simply mixing r-CDs with HAuCl4. We find that r-CDs act as both reducer to reduce HAuCl4 and stabilizer to prevent AuNPs from aggregating. The obtained AuNPs are monodispersed with an average diameter of ca. 16.4 ± 3.8 nm. Thus, this exploration may open a new scope of the applications of the CDs, as both reducer and stabilizer, in synthesis of metal nanomaterials.
Section snippets
Materials
HAuCl4 was purchased from Aladdin Reagent Co., Ltd. (Shanghai, China). Lamp black was obtained from Anhui Jixi Hukaiwen Ink Industry Co. (Xuancheng, China). Sodium borohydride was purchased from Tianjin Huan Wei Fine Chemical Co., Ltd. (Tianjin, China). Dialysis bags (molecular weight cut off = 3500) were ordered from Shanghai Green Bird Science & Technology Development Co., Ltd (Shanghai, China). Unless otherwise stated, other reagents were of analytical grade and were used as received.
Characterization of r-CDs
As shown in Fig. 1, the r-CDs were spherical and monodisperse with an average diameter of 3.4 ± 0.8 nm (100 particles were measured). The r-CDs possessed a weak absorption peak at 280 nm (Fig. S1). And the maximum excitation and emission of r-CDs were 280 and 440 nm, respectively (Fig. S2). These results were in good agreement with the result reported previously by our group [33].
Synthesis of AuNPs with the r-CDs
As demonstrated before [33], r-CDs possessed reducibility, it could reduce some oxidant, such as KMnO4, K2Cr2O7.
Conclusions
We applied r-CDs to synthesize AuNPs successfully. It was confirmed that r-CDs acted as both reducer to reduce Au(III) and capping agent to keep AuNPs from aggregating. The reducibility of r-CDs may derive from the hydroxyl groups on surfaces of r-CDs. And the capping ability can be attributed to the hydrogen binding between AuNPs and hydroxyl groups. Although the AuNPs we prepared exhibit less excellent catalytic activity in the reduction of 4-NP with NaBH4, the AuNPs we prepared show higher
Acknowledgments
This work was supported by the National Key Scientific Program-Nanoscience and Nanotechnology (No. 2011CB933600), the National Natural Science Foundation of China (No. 21175110), the Natural Science Foundation Project of CQ CSTC (CSTC, 2010BB4004), and the Fundamental Research Funds for the Central Universities (XDJK2013A022).
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