Ethanol in aqueous hydrogen peroxide solution: Hydrothermal synthesis of highly photoluminescent carbon dots as multifunctional nanosensors
Introduction
With unique optical properties, low environmental hazard, excellent biocompatibility, and robust chemical inertness [1], [2], carbon dots (C-dots) are showing promise in a myriad of applications, such as bioimaging [3], [4], drug delivery [5], sensors [6], [7], [8], [9], [10], [11], optoelectronics [12], and photocatalysis [13], [14]. Owning to the significant progress in synthetic strategies, numerous routes including arc discharge [15], laser ablation [16], plasma treatment [17], electrochemical synthesis [18], [19], ultrasonic/microwave approach [20], [21], acidic/thermal oxidation [22], [23], [24], and hydrothermal technique [25], [26], [27], [28], [29], have been explored to fabricate C-dots from a variety of carbon precursors. However, most of these routes suffer from expensive equipments, harsh synthetic conditions, and tedious processes, leading to manufacturing difficulties and high costs for large scale production. Moreover, the resultant C-dots usually show low photoluminescence (PL) quantum yields (QYs), typically less than 10%, which impedes their practical applications. Surface passivation and heteroatoms doping are two effective ways to enhance the QYs of C-dots [16], [21], [30]. Nevertheless, surface passivation needs complex post-treatment steps, and heteroatoms doping often require special nitrogen-containing organic precursors which are toxic and difficult to obtain. Currently, facile and low-cost synthesis of highly photoluminescent C-dots (QYs > 30%) without additional surface passivation or heteroatoms doping is hardly achieved despite great efforts have been made.
Ethanol is one of the most common reagents in laboratory. It has been employed as a cheap and abundant carbon source for preparation of carbon nanotubes by catalytic chemical vapor deposition [31]. The production of graphene from ethanol based on solvothermal treatment and sonication has also been reported [32]. However, the synthesis of ethanol-derived C-dots is seriously limited [33], as much attention has focused on fabricating C-dots from graphene-based materials [13], [14], [15], [16], [18], [19], saccharides [12], [20], citric acid [21], [23], [25], and biomasses [8], [17], [26], [27], [28]. In the present work, we develop a novel one-step approach to synthesize highly fluorescent C-dots by hydrothermal oxidation of ethanol with an environmentally friendly oxidant, hydrogen peroxide (H2O2). Compared with other methods, our approach is more economic and much greener, because it only requires the most affordable and benign reagents, ethanol and H2O2, and no expensive equipments, severe synthetic conditions, or complicated procedures are demanded. The resulting C-dots are highly crystalline with abundant oxygenous functional groups. Their QY has been significantly improved to be 38.7% without further surface passivation or heteroatoms doping, much higher than the values of most reported doping-free and nitrogen-doped C-dots. It is remarkable that the C-dots can act as multifunctional fluorescence nanosensors to detect pH, temperature, and the concentration of hypochlorite ion (ClO−) in aqueous solution (Fig. 1). Importantly, a portable paper-based sensor has been prepared by simply coating the C-dots on the commercial filter paper, and visual fluorescence detection of pH by naked eyes is achieved under a UV lamp. Moreover, the feasibility of C-dots for ClO− assay in local tap water is presented.
Section snippets
Materials
Ethanol and hydrogen peroxide (30 wt%) were purchased from Nanjing Chemical Reagents Factory (Nanjing, China). Sodium hypochlorite solution was obtained from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). All other reagents and chemicals were of analytical grade and used as received. Deionized water produced through a Millipore water purification system (⩾18 MΩ, Milli-Q, Millipore) was used throughout the experiments.
Synthesis of C-dots
The C-dots were synthesized by hydrothermal treatment of ethanol in H2O2
Characterization of the C-dots
Hydrothermal treatment of ethanol (3 mL) in the presence of aqueous H2O2 solution (4 wt% in final 30 mL volume) at 180 °C for 12 h led to a yellow dispersion of C-dots. TEM image (Fig. 2a) shows that the C-dots are nearly spherical and well-dispersed. The average diameter of the C-dots is 4.8 nm and the size distribution accords well with Gaussian distribution by measuring 300 nanoparticles (inset of Fig. 2a). High-resolution TEM image (Fig. 2b) reveals clear lattice fringes with spacing of 0.21 nm,
Conclusions
In summary, we have developed a H2O2-assisted hydrothermal technique for synthesis of highly photoluminescent C-dots from ethanol that can serve as multifunctional nanosensors. The present work has several advantages over the previous studies. First, the carbon source used here is ethanol, which is the most common, abundant, and inexpensive regent in laboratory. Second, our synthetic method is environmentally friendly and facile, because in addition to ethanol, only a green oxidant, H2O2, is
Acknowledgements
This work was financially supported by the National Basic Research Program (2010CB732401), National Science Fund for Creative Research Groups (21121091), and National Natural Science Foundation of China (20875045).
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