Microwave–hydrothermal synthesis of fluorescent carbon dots from graphite oxide
Introduction
Various nanostructured carbon materials, such as carbon nanotubes [1], graphene [2], nanofibers [3], fullerenes [4] and nanodiamonds [5] have been developed. Because of their potentially superior performance, these carbon nanomaterials have been applied in a broad range of fields [6]. Recently, a novel carbon nanomaterial, fluorescent carbon dots (CDots), has exhibited huge potential in applications including biological labeling, bioimaging and life sciences [7], [8], [9], [10], [11], [12]. CDots possess excellent fluorescent properties compared with fluorescent semiconductor quantum dots, such as strong fluorescence, high photostability against photobleaching and blinking, broad excitation spectra, narrow and tunable emission spectra [12]. In particular, brighter fluorescence (i.e., increased fluorescence quantum yield) is observed after passivation by covalent binding of organic groups [13], [14]. In addition, CDots are biocompatible, small in size and low in molecular weight, and have low toxicity, making them superior to quantum dots [11], [12].
Many precursors, including graphite [15], [16], [17], [18], carbon nanotubes [19], [20], an ammonium carboxylate salt [21], [22], carbon soot [23], [24] and carbohydrates [14], [25] have been used to prepare CDots. In general, fluorescent CDots are successfully obtained by methods involving laser irradiation [13], [18], thermal decomposition [21], [22], electrochemistry [15], [16], [17], [20] or heating under reflux in nitric acid [11], [14], [23], [24]. Sun et al. [13] prepared highly luminescent CDots by a complex method combining laser ablation, heating under reflux in nitric acid and prolonged passivation. Thermal decomposition methods can directly produce functionalized CDots, but require a suitable carbon source [21], [22]. Several research groups have reported various innovative routes for achieving CDots through electrochemical oxidation of graphite electrodes in buffer solution or ionic liquids [15], [16], [17]. The CDots consisting of multiple colors synthesized by oxidation of carbon soot in nitric acid consistently suffer from a low quantum yield [23]. More recently, it was found that microwave pyrolysis of glucose in polyethylene glycol can produce fluorescent CDots in just several minutes [25]. Although various precursors and methods have been developed for preparing CDots, unfortunately, most of the CDots produced display weak fluorescence. On the other hand, the fluorescence of CDots was enhanced after surface passivation with amino-terminated polymers, but the accrescent size of modified CDots confined its further applications. In general, the synthesis of fluorescent CDots to date has many drawbacks, such as long and complex processes, strict experimental conditions and producing CDots with low quantum yields. Rapid, convenient and efficient methods are still required for producing fluorescent CDots for various applications.
Microwave-assisted techniques have shown significant progress in their application to materials synthesis because of rapid heating and the resulting dramatic increase in reaction rate [26], [27]. Moreover, there are many examples of nanoparticles being prepared by hydrothermal methods [28]. The combination of both routes is known as microwave–hydrothermal synthesis, which has recently been used to prepare nanomaterials [29], [30], [31].
In this paper, microwave-assisted heating under reflux and microwave–hydrothermal routes with graphite oxide (GO) as a precursor were used to prepare fluorescent CDots. The CDots attained via these routes exhibit higher quantum yields and absorption, and longer lifetimes than CDots prepared by heating under reflux in nitric acid. An added advantage is that microwave-assisted techniques are simple and rapid compared with heating under reflux in acid for extended periods.
Section snippets
Chemicals
Specpure graphite was purchased from Tianjin Guangfu Fine Chemical Research Institute (China), candle soot was collected from smoldering candles, lamp black was purchased from Anhui Jixi Hukaiwen Ink Industry Co. (China), and fluorescein was purchased from Shanghai SSS Reagent Co., Ltd. (China). Ultrapure water was prepared with a Milli-Q-Plus system (18.2 M Ω) and used throughout the experiments. All other reagents were used as received.
Preparation of GO
GO was synthesized from specpure graphite powder according
Optimization of the precursor material
Four kinds of carbon materials, candle soot, lampblack, conductive carbon black and GO were used to prepare fluorescent CDots by heating under reflux in nitric acid. Specpure graphite was also used as a precursor but unfortunately no CDots were obtained because graphite hardly reacted with nitric acid heated under reflux. What’s more, GO aqueous solution showed no fluorescence without reacting with nitric acid. The CDots yields from different carbon materials are low (GO CDots 0.75–1%, other
Conclusions
Four precursor materials and three synthetic routes have been used to prepare fluorescent CDots. Of the precursors investigated, GO was the most suitable, producing CDots that exhibited the best fluorescent properties. Microwave-assisted routes decreased the reaction time and enhanced the fluorescent properties of CDots because of microwave effects. Of the synthetic routes investigated, the microwave–hydrothermal method is best suited for the preparation of CDots. This reaction took just 30–45
Acknowledgments
The authors thank the National Key Scientific Program – Nanoscience and Nanotechnology (Nos. 2006CB933100, and 2011CB933600), the Fundamental Research Funds for the Central Universities (No. XDJK 2009C079), the Foundation of Southwest University (No. SWNUQ2005007), and the 211 Project of Southwest University (the Third Term) for financial supports.
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