Coal based carbon dots: Recent advances in synthesis, properties, and applications

Carbon dots are zero-dimensional carbon nanomaterials with quantum confinement effects and edge effects, which have aroused great interests in many disciplines such as energy, chemistry, materials, and environmental applications. They can be prepared by chemical oxidation, electrochemical synthesis, hydrothermal preparation, arc discharge, microwave synthesis, template method, and many other methods. However, the raw materials’ high cost, the complexity and environmental-unfriendly fabrication process limit their large-scale production and commercialization. Herein, we review the latest developments of coal-based carbon dots about selecting coal-derived energy resources (bituminous coal, anthracite, lignite, coal tar, coke, etc.) the developments of synthesis processes, surface modification, and doping of carbon dots. The coal-based carbon dots exhibit the advantages of unique fluorescence, efficient catalysis, excellent water solubility, low toxicity, inexpensive, good biocompatibility, and other advantages, which hold the potentiality for a wide range of applications such as environmental pollutants sensing, catalyst preparation, chemical analysis, energy storage, and medical imaging technology. This review aims to provide a guidance of finding abundant and cost-effective precursors, green, simple and sustainable production processes to prepare coal-based carbon dots, and make further efforts to exploit the application of carbon dots in broader fields.


SYNTHESIS AND PROPERTIES OF COAL-BASED CDS
According to the degree of carbonization and processing technology, the precursors of coal-based CDs can be mainly divided into two categories: raw coals including bituminous coal, anthracite and lignite; and coal derivatives including coke and coal tar.

2.1
Bituminous coal Ye et al. reported the fabrication of GQDs from the bituminous coal (b-GQDs) by a facile one-step wet chemistry route ( Figure 1A), generally including mixed acid (sulfuric acid [H 2 SO 4 ] and nitric acid [HNO 3 ]) oxidation, stirring, oil bath heating (100 • C), and neutralization (sodium hydroxide). [66] The size of the crystalline hexagonal GQDs was 2.96 ± 0.96 nm, the thickness was 1.5-3 nm, and the intensity ratio of defect signal (D) to graphitic order (G) peak (I D /I G ) was 1.55 ± 0. 19. With the rise of oxidation temperature to 120 • C, the size of GQDs decreased to 2.30 ± 0.78 nm. The b-GQDs exhibited size-and pH-dependent PL in aqueous solutions ( Figure 1B, C). The smaller the size of GQDs, the blue shift of emission wavelength. The stronger the acidity of GQDs solution, the blue shift of emission peak occurred. The stronger the alkalinity of GQDs solution, the redshift of emission peak took place. By applying high frequency (239.2 and 336 GHz) electron spin resonance spectroscopy, they found four distinct magnetic defect centers in the prepared b-GQDs. [68] Two of them were intrinsic carbon-core defects (broad signal: width = 697 [ (Figure 2). [80] It was found that with the decrease of coalification degree, the yield of Coal A decreased from 56.30% to 14.66%. When the potential was cycled between −1.5 and +1.8 V (pH 7), the electroluminescence (ECL) performance of GQDs was significantly improved by adding S 2 O 8 2− . Zhang and coworkers also reported the fabrication of GQDs containing a large number of hydroxyl (-OH), carboxyl (-COOH), aldehydes, carbonyls (-C = O) and ether bond (C-O-C) functional groups from bituminous coal by chemical oxidation of mixed acid (H 2 SO 4 and HNO 3 ), and the size of b-GQDs ranged from 2 to 3.2 nm. [81] Recently, Hower et al. synthesized b-CQDs with a size distribution of 2-12 nm by an ultrasonic-assisted wet-chemical method in the presence of hydrogen peroxide (H 2 O 2 ). [82] The quantum yield (QY) of b-CQDs was 0.53%, and the obtained b-CQDs emitted bright blue fluorescence at 365 nm excitation wavelength. F I G U R E 1 A, Schematic illustration of the synthesis of b-GQDs; the relationship between PL emission wavelength and the size of b-GQDs (B) and pH (C). Reproduced with permission. [66] Copyright, 2013, Springer Nature F I G U R E 2 Treatment procedures of coal samples. Reproduced with permission. [80] Copyright 2014, Royal Society of Chemistry

Anthracite coal
Coal, especially anthracite with a high degree of coalification, contains many sp 2 microcrystals, which is easy to be exfoliated and surface modified by chemical oxida-tion. In order to rapidly separate these different-sized carbon domains, Ye and co-workers tuned the band gaps of anthracite coal-based GQDs (a-GQDs) in two ways: one was to change the pore size of cross-flow ultrafiltration membrane ( Figure 3A), and the other was to control the chemical oxidation temperature directly. [83] In the first method, it was found that with the increase of corresponding hydrodynamic diameters from 10 ± 2.5 to 76 ± 18 nm, the average size of a-GQDs increased from 4.5 ± 1.2 to 70 ± 15 nm, and the QY of the obtained a-GQDs decreased from 1.1% to 0.38%. The study of photophysical properties of the separated a-GQDs showed that the maximum PL emission peak was redshifted from ∼520 to ∼620 nm, the a-GQDs solutions emitted light across the majority of the visible spectrum from green (∼2.4 eV) to orange-red (∼1.9 eV) regions, under 365 nm UV light irradiation. The relationship between the bandgap and a-GQDs size or molecular mass cutoff was summarized in Figure 3B. In the second method, the average size of a-GQDs decreased from 54 ± 7.2 to 7.6 ± 1.8 nm with the rise of oxidation temperature from 50 to 150 • C in mixed acid (H 2 SO 4 and HNO 3 ), and the temperature effect in bandgap engineering of a-GQDs was summarized in Figure 3C. Meanwhile, Qiu's group reported the synthesis of anthracite coal-based CDs (a-CDs) with controllable structure by a combined technique of carbonization and HNO 3 F I G U R E 3 A, Schematic illustration of the separation of GQDs by cross-flow ultrafiltration; the relationship between energy bandgap and membrane pore size (B) and synthesis temperature (C). Reproduced with permission. [83] Copyright, 2015, American Chemical Society oxidation. [84] The average size of a-CDs raised from 1.96 ± 0.73 to 3.10 ± 0.80 nm and the average height of a-CDs improved from 1.04 to 1.42 nm, when the carbonization temperature of anthracite coal increased from 0 to 1500 • C. The nano-scale sp 2 carbon domains and surface defects resulted in two different emission modes (the short-wavelength emission at about 420 nm and longwavelength emission at about 505 nm). In 2017, the same team reported the preparation of fluorescent nitrogendoped CDs (N-CDs) from Taixi anthracite by a simple, green and sustainable one-pot solvothermal method in the presence of dimethylformamide (DMF). [85] The as-synthesized N-CDs emitted blue fluorescence at the excitation wavelength of 365 nm with singlet oxygen generation of 19% and QY of 47%, and the average size was 4.7 nm. Besides the complex and toxic strong acid oxidation method (e.g., HNO 3 , H 2 SO 4 , hydrofluoric acid and potassium permanganate), Hu et al. had reported a simple, efficient and green method for the first time, to synthesize a-CDs with size distribution ranging from 1 to 3 nm. [86] This technique took the advantage of hydroxyl radicals (•OH) generated by the thermal excitation of H 2 O 2 to attack the carbon atoms of nanoparticles containing hydroxyl and epoxy groups to form a-CDs ( Figure 4A). It was found that long reaction time resulted in stronger fluorescence intensity ( Figure 4B).

Lignite
Thiyagarajan et al. applied ethylenediamine as both the reaction solvent and surface passivator to prepare structurally controllable and surface-functionalized carbon nanomaterials from lignite by reflux, microwave radiation, and laser ablation methods. [87] They found that the photoexcitation of the carbogenic nucleus (π-π*) produced an electron-hole (e-h) pair, which relaxed and recombined at the diverse surface defect sites resulting in PL. The photophysical behavior could be observed only when the particle size was less than 10 nm. In 2017, Manoj's group reported the preparation of fluorescent organic semiconductor dots with adjustable particle size and surface function through chemical oxidation, followed by centrifugation, dialysis, and heating. [88] It was found that the lateral size of the nanocarbon fractions (Residue (LC1), the Supernatant (LC2), and the Permeate (LC3)) from lignite were 10-23, 10-17, ∼5 nm, the thickness of them were 5-10, 4-8, 1-2 nm, and the I D /I G were 1.36, 0.72, 0.64, respectively. The maximum emission wavelength of LC1, LC2, and LC3 were 515, 517, 447 nm at the excitation wavelength of 440, 440, 320 nm. These nano fragments could be used to detect Cu 2+ as low as 0.0089 nm in water. Later, the same group reported the fabrication of LC1C, LC2C, LC3C from LC1, LC2, and LC3 by chloroform (CHCl 3 ) reflux and F I G U R E 4 A, Schematic illustration of the forming of CDs. B, time-dependent fluorescence emission spectra of 2-hydroxyterephthalic acid formed by the reaction of terephthalic acid with •OH generated by reaction system under the heating at 80 • C (left) and the time-dependent fluorescence intensities of 2-hydroxyterephthalic acid at 426 nm (right). Reproduced with permission. [86] Copyright 2016, Elsevier Ltd direct oxidation with concentrated HNO 3 . [89] They found that the band gaps of the LC1C, LC2C, LC3C were 2.90, 2.50, and 3.40 eV from the UV visible analysis by employing Tauc Plot. The size distribution of LC1C was 3-4 nm, the average size was 3.91 ± 0.32 nm, and the LC2C and LC3C nanostructures were stacking and agglomeration.
With the improvement of excitation wavelength from 280 to 500 nm, the fluorescence spectra of all samples showed a redshift, and the maximum excitation wavelength of LC1C, LC2C, LC3C were 320, 340, 320 nm. The as-prepared carbon nanomaterials showed pH independence, which made them stable under any conditions. Liu et al. reported a green and facile fabrication method of CQDs from lignite adopting ozone as the oxidant to replace strong oxidizing acids. [90] It was found that the as-synthesized quantum dots had a yield of 35% with a size distribution of 2 to 4 nm. These quantum dots emitted strong blue fluorescence under 345 nm UV light irradiation and were able to detect Fe 3+ as low as 0.26 µmol L −1 .

Coal derivatives
In recent years, numerous researches have focused on preparing CDs from low-priced and abundant coal derivatives. Coal tar pitch (CTP), a side product of coking industry, has a unique molecule structure, consisting of an aromatic nucleus and multiple side chains bonding on this graphene-like nucleus, which is very similar to the structure of GQDs. Hu et al. prepared a multiple color and concentration-dependent CQDs from CTP powder in a mixture of formic acid and H 2 O 2 without heating ( Figure 5). [91] When the concentration of CDs decreased from 3.0 to 0.03 g L −1 , the fluorescence emission wavelength shifted to red. Under 365 nm excitation, the QY changed from 5.4% to 10.1%, which firstly increased and then decreased, and reached the maximum at 0.06 g L −1 (up to 10.1%). The yield of CDs was 49%. By contrast, the solid sample exhibited a wide absorption under UV visible light with the strongest absorption peak at around 440 nm and emitted bright red light under white light excitation. Liu and co-workers reported the preparation of GQDs with high solubility and strong fluorescence in aqueous solution from CTP by H 2 O 2 oxidation under mild conditions (100 • C for 2 hours in reflux). [92] The particle size distribution of GQDs-1 (15% H 2 O 2 oxidation) and GQDs-2 (30% H 2 O 2 oxidation) were 1.7 ± 0.4, 2.3 ± 0.7 nm, and the average thicknesses were 1-2, 1.9 ± 0.7 nm, respectively. Both GQDs-1 and GQDs-2 showed an excitation-dependent PL behavior. The yield and QY of CQDs-1 were 80% and 2.37%, and the optimum excitation and emission wavelengths were 325 and 445 nm. The as-synthetized CQDs emitted blue fluorescence under 325 nm excitation. Wang et al. reported the preparation of CQDs by direct carbonization of dispersed carbonaceous microcrystals from mesophase pitch (MP) without any strong oxidants ( Figure 6). [93] With the rise of Reproduced with permission. [91] Copyright 2017, Royal Society of Chemistry F I G U R E 6 Schematic illustration of the fabrication of CQDs from MP. Reproduced with permission. [93] Copyright 2018, Royal Society of Chemistry carbonization temperature from 430 • C to 450 • C, the particle size of CDs increased from 2.2 ± 0.37 to 4.5 ± 0.51 nm, the height improved from 2.0 to 4.5 nm, and the QY varied from 67% to 87%. CQD440 (carbonization temperature at 440 • C) emitted a strong blue fluorescence when exposed to 365 nm UV light in ethanol, exhibited luminescence behavior independent of excitation, and there were three fixed emission peaks at 413, 435, and 460 nm.
Meanwhile, Zeng's group reported the preparation of the GQDs from coke by a one-step electrochemical stripping method (Figure 7). [94] By adjusting the current density and content of coke and water in the electrolyte solution, the structure of GQDs could be precisely adjusted to obtain multicolor fluorescent GQDs Also, the asprepared G-GQDs could be further reduced by sodium borohydride to form new GQDs and emitted bright blue fluorescence (B-GQDs). Feng and Zhang had published an environmentally friendly and simple one-step chemical oxidation (H 2 O 2 ) method to prepare fluorescent CQDs from coke for white light-emitting devices. [95] The QY of the as-produced CQDs was 9.2%, and the composition of the blue-green-red spectral was up to 48%. The obtained CQDs emitted blue fluorescence under 365 nm illumination and exhibited excitation-dependent PL emission F I G U R E 7 Schematic illustration of the synthetic process of multicolor GQDs from coke. Reproduced with permission. [94] Copyright 2018, Elsevier Ltd behavior, with an average size of 6.5 nm. Geng et al. reported the preparation of CQDs from coal tar by a costeffective solvothermal method (200 • C for 12 hours in the toluene solution). [96] The as-made CQDs emitted strong and stable orange fluorescence at 605 nm excitation. The size of the CQDs was 1.5-4.5 nm, and the I D /I G was 1.029. The QY of the as-fabricated CQDs was 29.7% in toluene solution. After the modification of the liposomes, the CQDs became hydrophilic and showed a redshift of fluorescence to 640 nm under UV light. The QY of the liposome-CQDs was 10.7%, and lipsome-CQDs showed stable pH-independent behavior. Quinoline Insoluble (QI) residues existed in the form of solid carbon particles in coal tar. Kundu et al. reported the fabrication of high fluorescence CDs from the QI particles of coal tar by adding different oxidants in 2020. [97] The average size of the CD1 (H 2 O 2 oxidation) and CD2 (H 2 SO 4 and HNO 3 oxidation) were ∼1 and ∼5 nm, and the corresponding QY were 1% and 2%. The maximum excitation and emission wavelengths of CD1 were at 335 and 440 nm, and the utmost excitation and emission wavelengths of CD2 were at 490 and 578 nm.

Energy-related applications
Ye and co-workers primarily assembled boron (B)-and nitrogen (N)-doped GQDs on graphene adopting a hydrothermal method ( Figure 8A). [98] The as-fabricated , and large superficial area, making it an excellent electrocatalyst for the oxygen reduction reaction (ORR). They found that the ORR activity was significantly affected by the doping time and dopant concentration. BN-GQD/G-30 exhibited the most positive initial potential and maximum current density in the whole potential range, possessed an almost complete four-electron ORR process (n = 3.93) and a large kinetic current (J K = 11.1 mA cm −2 ) ( Figure 8B, C). The activity of the hybrid material (with ∼15 mV more positive onset potential) in alkaline medium was even higher than commercial Pt/C. Zhang and co-workers had developed a template-assisted assembly method to fabricate the high performance hierarchical porous carbon nanosheets (HPCNs) for supercapacitor electrodes adopting coal-based GQDs as the building unit ( Figure 9). [81] The obtained HPCNs showed an interconnected, loosely packed graphene-like structure, with a specific surface area of 1332 m 2 g −1 , well-organized pore distribution, good conductivity, rich active centers, and sufficient ion migration channels. Under the optimized conditions, the specific capacitance of HPCN-1:3 (0.5 g GQDs and 1.5 g magnesium hydroxide) was 200 F g −1 (1 A g −1 ), . Reproduced with permission. [98] Copyright, 2014, American Chemical Society F I G U R E 9 Schematic description for the preparation of the HPCNs. Reproduced with permission. [81] Copyright 2017, Elsevier Ltd and the capacity remained at 112 F g −1 (100 A g −1 ). A small amount of potassium hydroxide (KOH) was added for in-situ chemical activation in the preparation process, further improving its energy storage performance. AHPCN-1:0.25 (0.5 g GQDs and 0.125 g KOH) showed that the capacity improved to 230 F g −1 at 1 A g −1 , while the capacity remained at 170 F g −1 at 100 A g −1 , the capacitance retention rate was 74%. All samples showed excellent cycle stability, even after 10,000 cycles at 10 A g −1 , there was no obvious capacity degradation. This research indicated that GQDs were ideal precursors for the preparation of advanced energy storage materials. This technology was expected to open up new ways for the clean and efficient use of coal resources.
Wang et al. reported the preparation of graphene nanosheets (GNs) with outstanding stability and super capacity from inertinite and vitrinite rich coal. The capacitance and resistance of inertinite-based GNs were greater than those of vitrinite-based GNs. [99] Zhang et al. reported the preparation of coal-based GQDs/α-Fe 2 O 3 nanocomposites with excellent cycling and rate capability by electrodeposition. [100] By adjusting the electrolytic solvent ratio (V DMF :V Water = 2:1), the antler-shaped α-Fe 2 O 3 nanoparticles were prepared on a nickel substrate, and then GQDs/α-Fe 2 O 3 composites were fabricated by twostep electrodeposition using GQDs solution as the electrolyte. The electrochemical test results demonstrated that the specific capacitance of GQDs/α-Fe 2 O 3 composite could F I G U R E 1 0 Schematic of the mechanism of the photocatalytic CO 2 reduction with water on NH 2 -CNPs. Reproduced with permission. [102] Copyright 2018, Royal Society of Chemistry reach 1582.5 mAh g −1 at the current density of 1A g −1 . After 110 cycles, the charge-discharge specific capacitance was still as high as 1320 mAh g −1 , indicating that the cycle performance had been improved. In addition, the specific capacity of GQDs/α-Fe 2 O 3 composite could reach 1091 mAh g −1 at high current density (5 A g −1 ), which exhibited its ideal rate performance.

Catalytic applications
At present, the best way to solve the problem of increasing carbon dioxide (CO 2 ) emissions and energy shortage is to use solar radiation to reduce CO 2 into organic fuels catalytically. Maimaiti's group reported CDs/TiO 2 nanoparticles' preparation with a diameter of 30-50 nm. [101] They found that the CDs/TiO 2 nanoparticles connected with C = C, C-OH/C-O-C, C = O, Ti-O and other functional groups, and the bandgap decreased from 3.2 eV of pure TiO 2 to 2.9 eV of the as-prepared CDs/TiO 2 . The removal rate of COD Cr was 81.2%, and most organic compounds (except nitrogen heterocyclic compounds) in the black liquor were degraded after 3 hours of treatment. They synthesized N, S co-doped ammoniated coal-based carbon nanoparticles (NH 2 -CNPs) via thionyl chloride chlorination and ethylenediamine passivation. [102] The bandgap of NH 2 -CNPs reduced from 2.42 eV of CNPs to 2.02 eV, and the products of CO 2 /H 2 O 2 photocatalytic reduction by NH 2 -CNPs were methanol (CH 3 OH), carbon monoxide (CO), ethanol (C 2 H 5 OH), hydrogen (H 2 ) and methane (CH 4 ) ( Figure 10). The total amount of products was 807.56 µmol g −1 cat., and the content of CH 3 OH was 618.7 µmol g −1 cat. after reaction for 10 hours. The same group also fabricated a cuprous oxide (Cu 2 O)-based composite photocata-lyst composed of Cu 2 O/CNPs. [103] 3 OH in the fifth cycle of the photocatalyst was still 90.6% of the initial yield, indicating that the photocatalyst's stability and reusability were excellent. Meanwhile, they prepared Ag/CDs composite nanoparticles by adopting a simple silver mirror reaction to attach CDs to the silver surface in situ. [104] The composite material's surface contained a large number of oxygen groups and a mesoporous interface formed by the accumulation of uniform particles, which were advantageous to a good adsorption capacity for CO 2 . Within the measurement range, Ag/CDs containing 16% of CDs exhibited the highest catalytic activity. After 10 hours of light, the yield of CH 3 OH from photocatalytic CO 2 reduction reached 17.82 µmol, which was almost three times higher than that of pure Ag catalyst.
Recently, Liu and co-workers had successfully prepared an efficient, reusable and stable CTP/TiO 2 composite photocatalyst from CTP by a simple and environmental friendly one-step solvothermal method ( Figure 11). [105] The CTP/TiO 2 nanocomposites had a relatively uniform shape with an average size of 5.17 ± 0.95 nm, the degradation efficiency of CTP/TiO 2 nanocomposites increased from 41.0% to 91.1% with the content of CTP varied from 1% to 5%, and the photocatalytic degradation rate of CTP/TiO 2 nanocomposites was 23 times higher than that of pure TiO 2 at the optimum content of CTP. The possible chemical reactions during the degradation of Rhodamine B (RhB) were as follows: Different from other coal-based CQDs, lignite-derived CQDs showed obvious concentration-dependent redshifted absorption, remarkable aggregation-induced diverse color emissions and intense orange-red solidphase fluorescence. Yu et al. synthesized the CQDs/TiO 2 nanocomposites by an in situ method. [106] According to optimize the loading ratio of CQDs, they found that CQDs/TiO 2 containing 2 wt.% of CQDs exhibited the best photocatalytic degradation of RhB (close to 95%) under visible light. The photoelectrochemical characterization of CQDs/TiO 2 confirmed that the improvement of CQDs/TiO 2 photocatalytic performance was due to the heterostructure interface between CQDs and TiO 2 , which reduced the bandgap of TiO 2 , changed the absorption boundary, and significantly accelerated electrons and holes. The separation of holes made CQDS become electron donors, providing more electrons for the photocatalytic process.

Sensing applications
The coal-based CDs exhibit low toxicity and significant fluorescence characteristics, which can be applied as a probe for ion detection in environmental protection and biological analysis. Qiu's group reported that the CDs reduced by sodium borohydride (r-CDs) was an effective fluorescence sensing material, which could detect Cu 2+ with a detection limit of 2.0 nM. [84] It was found that Cu 2+ exhibited the highest selectivity for the fluorescence quenching of r-CDs. In contrast, this selectivity hardly changed in the presence of other metal ions. In 2019, Zhang et al. fabricated a-CQDs with uniform size distribution and a diame-ter of 3.2 ± 1.0 nm by ultrasonic physical cutting method, which also could be used to detect Cu 2+ . [107] Xu and coworkers reported that the N, P, and S co-doped coal-based CDs could detect Pb 2+ as low as 0.75 µM, with a linear dynamic range of 1-20 µM. [108] Hu et al. synthesized CDs with dual PL peaks adopting diammonium phosphate as a modifier, which could be used to determine the pH distribution and change in complex systems containing interfering metal ions. [109] Manoj's group reported the preparation of pH-independent CDs from lignite, an unlabeled and effective probe for selective detection of glucose ions with a detection limit of 0.125 mM. [89]

Biomedical applications
To estimate whether liposome-CQDs could be used as biological probes, Geng et al. studied the evolution of liposome-CQDs activity in 24 and 48 hours at doses of 20, 40, 60, 80 and 100 mg L −1 adopting HeLa cells as the model ( Figure 12). [96] After injection of these solutions, the cytoplasm showed enhanced fluorescence around the nucleus. The excitation wavelengths of the confocal microscopic images were 405, 488, and 546 nm, and the detection wavelengths were in the range of 420-480, 505-540, and 550-620 nm ( Figure 12A-D). By taking tumor-bearing nude mice as an animal model, they found that the strong fluorescence signals (emission wavelength at 620 nm) could be clearly observed in the tumor area where liposome-CQDs were injected at 580 nm excitation after 0.5 hours of injection ( Figure 12E). The liposome-CQDs accumulated in the tumor area, and the obvious fluorescence could be observed compared with other tissues after 1 hours of injection ( Figure 12F). When the imaging dose was 40 mg L −1 , the cell survival rate remained above 92% after 24 hours of incubation, and there was no obvious cytotoxicity at low doses ( Figure 12G). The research demonstrated that the fluorescent liposome-CQDs exhibited good biocompatibility and was suitable for the biomedical field. It could be used as an excellent in vitro and in vivo imaging marker materials. Meanwhile, Wang and Co-Workers reported a Reproduced with permission. [96] Copyright 2017, Royal Society of Chemistry one-pot hydrothermal method for synthesizing amine and sulfur-based co-functional GQDs. [110] It was found that the as-synthesized GQDs showed super stability and attractive two-photon fluorescence properties. The as-produced GQDs had a two-photon absorption cross-section of 31000 GM, which greatly exceeded most conventional fluorescent materials. Non-cytotoxic GQDs exhibited negligible photothermal effects under 808 nm femtosecond laser irradiation, suitable for the long-term two-photon imaging and observation. These findings opened up new possibilities for the applications of the two-photon fluorescent GQDs in various biological fields. Kang et al. firstly developed a green and facile pulsed laser ablation in liquid to prepare graphene oxide quantum dots (GOQDs) with excellent optical properties from the abundant and low-cost coal. [111] They found that the pancreas cancer cells (PanC-1) effectively retained the original morphology after incubation with GOQDs, and showed a green PL. When the GOQD concentration increased from 0.1 to 5 mg mL −1 , the PanC-1 cell survival rate was still more than 85%, indicating that the unmodified GOQDs with low toxicity and good biocompatibility was a kind of promising biological imaging material. Furthermore, some researchers take interests in developing antioxidants that can treat diseases associated with excessive oxidative stress, and the efficacy is comparable to that of metalloenzymes. Nilewski and co-workers, [112] reported the preparation of coal-derived GQDs and their polyethylene glycol (PEG) functionalized derivatives as effective antioxidants ( Figure 13A). It was found that the onset voltage of aGQDs and bGODs were close to −0.2 V, covering a wide range with a maximum peak at −1.5 V, demonstrating that the antioxidants of aGQDs and bGQDs were stronger than GO. According to the initial peak potential, the as-fabricated antioxidants' strength was almost the same as that of the hydrophilic carbon clusters (HCCs) ( Figure 13B). Compared with bGQDs, aGQDs protected more cells at a lower concentration, and the data were as follows: aGQD versus bGQD (2 mg L −1 ): 87.1 ± 18.3 versus 75.7 ± 11.62, P = .042; aGQD versus bGQD (4 mg L −1 ): 104.5 ± 14.8 versus 77.7 ± 12.1, P < .0001; aGQD versus bGQD (8 mg L −1 ): 102.2 ± 11.1 versus 88.8 ± 9.6, P = .0079 ( Figure 13C, D). Recently, Ghorai et al. reported that GQDs were internalized into the cell-matrix and nucleus of breast cancer cells, which could effectively introduce chemotherapeutic drugs into specific tumor targeting sites, and exhibited great potential applications in tumor treatment. [79]

Applications in polymer composites
Owing to a large number of polar functional groups, coalbased GQDs can be applied in polymer composites without any surface modification. Kovalchuk et al. used polyvinyl alcohol (PVA) as the matrix polymer and GQDs as the raw material to prepare the luminescent polymer PVA/GQDs nanocomposite by the casting method. [113] They found that the as-fabricated composite films achieved high optical F I G U R E 1 3 A, Schematic illustration of PEG-GQDs from coal. B, CV of GO, HCCs, aGQDs + GO, and bGQDs + GO. (C) and (D) cell viability following different treatments. Reproduced with permission. [112] Copyright, 2019, American Chemical Society transparency (78-91%), and the nanoparticles showed good dispersibility under the GQDs concentration of 1%-5% in the PVA. The composite material exhibited a concentration dependence. With the improvement of GQDs content, the PL intensity gradually increased. When the GQDs content was 10%, the material's PL intensity reached the largest ( Figure 14). These obtained PVA/GQDs nanocomposites had a wide PL emission spectrum, covering most of the visible light range, which could be utilized in light-emitting diodes (LEDs), flexible electronic displays, and other optoelectronic applications. Polyacrylonitrile electrospun carbon nanofiber fabrics (ECNFs) are considered to be provided with great potential in many fields. However, the finite strength and flexibility greatly hinder their practical applications without optimization. Zhu et al. reported the preparation of tough, flexible and hydrophobic ECNFs by crosslinking coal-based GQDs ( Figure 15A). [114] It was found that the strength, flexibility, and hydrophobicity of ECNFs were significantly improved by simply adding coal-based GQDs to the spinning solution. As the content of coal-based GQDs increased, the hydrophobicity of ECNFs gradually improved. ECNF-1.0 (0.4 g GQDs and 0.4 g ECNFs) showed the best hydrophobicity and super lipophilicity ( Figure 15B). The ECNFs with coal-based GQDs exhibited outstanding strength and flexibility and could be twisted or folded into other shapes. After unfolding, the fabric could be restored to its original state without any cracks ( Figure 15C, D). However, the ECNF-0 derived from the F I G U R E 1 4 Photograph (A), PL spectra (B) of the PVA and PVA/GQD composite films. Reproduced with permission. [113] Copyright 2015, American Chemical Society F I G U R E 1 5 A, Schematic of the ECNF preparation procedure. B, ECNF-1.0 paperboat. C, before and (D) after being twisted. E, photograph of ECNF-0 after slight compression. F, the Young's modulus and (G) the stress-strain curves of the ECNFs. Reproduced with permission. [114] Copyright 2017, Elsevier Ltd pure polyacrylonitrile (PAN) displayed special fragile (Figure 15E). The maximum tensile stress of ECNF-1.0 was about 2.2 Mpa, and the Young's modulus was about 70 MPa ( Figure 15F, G).

CONCLUSIONS, PROSPECTS, AND CHALLENGES
In this article, we reviewed the latest progress of coal-based CDs synthesis and their potential applications in the fields of energy, environment, biomedical, and polymer composites fields. There is an obvious trend that researchers are drawing more attention on the preparation of CDs from inexpensive and abundant coal derivatives (coke, coal pitch, coal tar, etc.) by green, simple, and efficient methods. In particular, H 2 O 2 chemical oxidation represents a promising, environmentally friendly and safe way for the massive preparation of CDs. By adjusting the band gaps and heteroatom (N, P, S, and B) doping, the electroluminescence, photoluminescence and absorption characteristics of CQDs/GQDs can be effectively controlled, owing to their quantum confinement effect and edge effect. The ability to produce coal-based CDs on a large scale can significantly contribute to the long-term feasibility development of the applications mentioned above, and commercialize the green and cost-effective CDs, which will benefit people's lives. Making full use of these new nanomaterials, the research of CDs has quickly become the focus of people's attention. To this end, several knowledge gaps need to be resolved: (1) To further understand the relationship between the structure and properties of CDs, such as the influence of the shape, functional groups, and defects of CDs on the properties. (2) To further develop an environmentally friendly, safe, efficient, and industrially feasible synthetic method of CDs and improve CDs' yield and QY.
(3) Deepening the mechanism of coal-based CDs in related application fields and exploring potential application possibilities in other areas.