Blue-emitting carbon quantum dots: Ultrafast microwave synthesis, purification and strong fluorescence in organic solvents
Graphical Abstract
Carbon quantum dots (CQDs) synthesized by microwave hydrothermal method from L-cystine were purified by dialysis in water and direct phase-transfer into organic solvents. The as-obtained CQDs in the basic reaction mixture show green excitation-independent fluorescence around 505 nm with a maximum excitation wavelength of 421 nm. However, the CQDs after purification by dialysis display an excitation-dependent blue fluorescence, the maximum emission intensity focuses on the position of 365 nm excitation and 450 nm emission. Particularly, the CQDs purified by phase-transfer show two strong blue fluorescence peaks respectively focusing around 422–434 nm and 400–410 nm in organic solvents with a maximum excitation at 370–372 nm.
Introduction
In recent years, carbon quantum dots (CQDs) have attracted great interest due to their outstanding advantages such as photo stability, chemical stability, biocompatibility, low toxicity and low cost as compared with other optical materials [1], [2], [3], [4], [5], [6], [7], [8], [9], so they are extensively studied in the fields of light-emitting materials [10], [11], [12], [13], sensors [14], [15], [16], [17], [18], bio-imaging [19], [20], [21] and labeling [22], as well as promoters for solar cells [23].
Currently, multiple color CQDs have been synthesized by “top-down” and “bottom-up” strategies. In the former case, CQDs are produced from large precursors such as nanosized carbon [24], active carbon [25], [26], [27] and powder graphite [28]. The “bottom-up” strategies have more selections in carbon resources such as amino acid [29], [30], proteins [31], [32], peptides [29], sugars [33], [34], fruit juices [35], [36], [37], plant gums [38], [39] or hydrophilic polymers [40]. Among the “bottom-up” strategies pyrolysis carbonization has exhibited advantages of operation simplicity, high efficiency and so on [14], [34], [41], [42], [43]. For example, high-quality CQDs can be synthesized by hydrothermal methods at 300 °C from hydrophilic compounds [29], at 160–220 °C from orange pericarp extraction [44], or at 120 °C from orange juice [35]; meanwhile, hydrophobic CQDs can be obtained by pyrolysis of ascorbic acid in ethanol under reflux [45]. Besides, Xiong and his coworkers reported a solvent-controlled synthesis method for CQDs with a wide color gamut and a narrow emission peak in solvothermal reactions at 210 °C [46]. Particularly, the microwave hydrothermal carbonization can provide series of CQDs toward miscellaneous applications by low cost, simple reaction equipments [30], [31], [32], [33], [38], [47], so they are considered as fast and efficiency techniques to face the challenge of large-scale producing [37], [38], [48], [49], [50], [51]. Many carbon-containing small molecules and polymers can be used as presursors in the microwave methods including citric acid [48], [52], glycerol [50], [53], polyethylene glycols [54], amino acids [30], [55], [56], [57], saccharides [33], [58], [59], [60], nature polysaccharides [38], [49], [61], [62], [63], peptide and polyamine [64], [65]. Yu and Wang prepared highly green-fluorescent CQDs from phthalic acid and triethylenediamine [66]; Liu and Wang et al. reported water-soluble luminescent carbon dots as a biocompatible fluorescent ink from citric acid and urea [67]; Zhang and his coworkers synthesized crystalline red- or green- emitting CQDs in glycerol or ethylene glycol from ammonium citrate tribasic and formamide [68].
However, the purification of the obtained CQDs is still a critical problem till now. First, filtration and centrifugation can only remove aggregates or large particles and cannot provide a really pure sample [38], [39], [48], [49], [60], [61], [62], [63]. Second, though dialysis [30], [50], [57], [64], [65], [66] and silica gel chromatography [68] have been used for purification, they cannot adopt to a scalable amount. So that, large-scale producing of the CQDs were usually not purified [37], [51], and in this case the CQDs factually were suspended in a mixture solution or dispersed in a mixture powder of raw materials and by-products, and such impurity will severely limit their application. In this paper, CQDs were synthesized by microwave hydrothermal method from L-cystine, then dialysis and direct phase-transfer were introduced to purify the as-obtained CQDs, respectively. The CQDs before purification showed excitation-independent green fluorescence with pH-responsive emission intensity. However, after purification by dialysis in water or phase-transfer into organic solvents (also can be named as extraction) they emitted blue fluorescence: (1) the purified CQDs by dialysis showed excitation-dependent fluorescence from 410 nm to 500 nm as the excitation wavelength changed from 310 to 430 nm; (2) the purified CQDs by phase-transfer showed two blue emission peaks respectively around 422–434 nm and 400–410 nm in different organic solvents with largely enhanced emission intensity. The surface states of the different CQDs were analyzed to explain the difference of fluorescence property.
Section snippets
Synthesis of the CQDs
CQDs were synthesized from L-cystine by microwave hydrothermal method [30]. In detail, 1.0 g L-cystine (99.5%, Aladdin, China) was dissolved in 10 mL NaOH solution (0.95 mol/L), followed by an ultrasonication of the system to be clear. Then, the reaction system was controlled at 150 °C for 30–90 s under stirring in an intelligent microwave chemical synthesizer (XH-200A, 800 W, Xianghu Technologies, Beijing, China). The samples were obtained after a naturally cooling to room temperature.
pH-dependent fluorescence measurement
The
Synthesis of CQDs by microwave hydrothermal carbonization
In this work, CQDs were synthesized by microwave carbonization of a carbon source L-cystine [30]. The reactant concentration was relatively high: approximately, L-cystine was 0.417 mol/L and NaOH as a catalyst was 0.95 mol/L. The color of the obtained systems changed from light yellow to dark brown as prolonging the carbonization time in the microwave synthesizer [30]. The obtained samples showed green fluorescence as irradiated by 365 nm light, so the fluorescence emission spectra of these
Conclusions
In this work, CQDs were synthesized by the ultrafast microwave hydrothermal carbonization. The as-obtained CQDs in an aqueous basic mixture solution display pH response, excitation-independent fluorescence with the maximum excitation/emission wavelengths of 421/505 nm. After purification by dialysis in water, the emission intensity dramatically decrease, and the maximum excitation/emission wavelengths shift to 365/450 nm with excitation-dependent fluorescence. Whereas, after purification by
CRediT authorship contribution statement
Jie Zhu: Data curation, Investigation, Methodology, Writing - original draft. Chunxing Wu: Investigation, Methodology, Writing - original draft. Yongmei Cui: Formal analysis, Software. Dongxiang Li: Conceptualization, Funding acquisition, Supervision, Writing - original draft, Writing - review & editing. Yaojun Zhang: Data curation, Software, Investigation. Jie Xu: Methodology, Resources. Chunfang Li: Project administration, Supervision, Validation, Resources, Writing - review & editing. Shahid
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the National Natural Science Foundation of China (NNSFC 21975139), and the Natural Science Foundation of Shandong Province (ZR2017MB042), China.
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