Recent Advances in Synthesis, Modification, Characterization, and Applications of Carbon Dots

Although there is significant progress in the research of carbon dots (CDs), some challenges such as difficulty in large-scale synthesis, complicated purification, low quantum yield, ambiguity in structure-property correlation, electronic structures, and photophysics are still major obstacles that hinder the commercial use of CDs. Recent advances in synthesis, modification, characterization, and applications of CDs are summarized in this review. We illustrate some examples to correlate process parameters, structures, compositions, properties, and performances of CDs-based materials. The advances in the synthesis approach, purification methods, and modification/doping methods for the synthesis of CDs are also presented. Moreover, some examples of the kilogram-scale fabrication of CDs are given. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio, the laser pulse width, and the average molar mass of the polymeric precursor. Surface passivation also has a significant influence on the particle sizes of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs. The synchrotron near-edge X-ray absorption fine structure (NEXAFS) test has been proved to be a useful tool to explore the correlation among structural features, photophysics, and emission performance of CDs. Recent advances of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications were reviewed in this article. Furthermore, the roles of CDs, doped CDs, and their composites in these applications were also demonstrated.


Introduction
CD is a zero-dimensional (0D) quasi-spherical carbon nanomaterial with a size ranging from 1 to 10 nanometers. Compared with traditional quantum dots, such as metal or semiconductor-based QDs, carbon-based QDs (such as carbon QDs, graphene QDs, and polymer QDs) have excellent biocompatibility and low cytotoxicity. CDs have some unique properties, such as facile synthesis, ease of functionalization, excellent water dispersibility, excellent light-harvesting, sharp fluorescent emission spectrum, up-conversion, tunable photoluminescence properties, effective electron transfer, and good photostability. Therefore, CDs have been applied to a wide range of applications, including bioimaging, biomedical and chemical sensing, therapy, optoelectronics, solar cells, fertilizer, separation, security authentication, food packing, and co-catalyst for environmental remediation applications. The continuous efforts devoted to CDs-related research have significantly explored some important knowledge about CDs-based materials and improved their performance in different fields. However, some serious challenges, such as inadequate and complicated purification, difficulty in large-scale production, low quantum yield, and ambiguity

Top-Down Approach
Various physical and chemical methods were applied to break the bulk carbon structures for the preparation of CDs by using the top-down methods. CDs with high crystallinity and intact structures can be obtained by the top-down methods, especially the graphene quantum dots (GQDs) with typical graphitic structures.

Laser Ablation
Laser ablation is a facile and rapid process for the synthesis of CDs in which a laser irradiates carbon targets to remove material from the solid surface. Li et al. [10] fabricated twin graphene quantum dots (GQDs) by the electric-field-assisted temporally shaped femtosecond laser ablation liquid of the graphene dispersion. GQDs with similar size but different oxygen contents and crystallinity were obtained via adjusting the electric field intensity and the pulse delay of the femtosecond laser. Shen et al. [11] synthesized GQDs by the femtosecond laser ablation process in liquid. The laser-induced graphene is the carbon source. Moreover, by incorporating ammonia water into the graphene dispersion, N-doped graphene quantum dots can be prepared for the Fe 3+ sensing application. Zhang et al. [12] studied the synthesis of carbon quantum dots by using the pulsed laser ablation method, which can effectively inhibit Aβ42 peptide aggregation and reduce the cytotoxicity of the Aβ42 peptides. The graphite target pellet was irradiated by a pulsed Nd:YAG laser to ablate the pellet. Kaczmarek et al. [13] synthesized fluorescent carbon dots (CDs) by a nanosecond laser ablation of a graphite target immersed in polyethyleneimine and ethylenediamine and the separation by a dialysis process. Most of the molecular fluorophores are associated with carbogenic nanostructures for the ablation of polyethyleneimine, while, in the case of ethylenediamine, free fluorescent molecules are the dominant products. The ablated particles (including atoms, ions, and atom clusters) were heated to a high temperature, and they interacted and reacted with the surrounding liquid molecules in the cavitation bubbles. Then nanoparticles formed during the plasma-cooling period.

Electrochemical Method
Zhou et al. [14] reported the mass production (9.36 g per batch) of N-doped carbon quantum dots (N-CQDs) by an electrochemical setup, using prebaked carbon as the anode and ammonium bicarbonate as the electrolyte. The process with an efficient utilization rate of raw material (90.18 wt.%) and a high conversion (1.87 g (N-GQDs) g (PCA) −1 ) is promising and practical for the fabrication of N-CQDs. Zhou et al. [15] reported an electrochemical exfoliation process for the synthesis of GQDs. The solid-liquid interfacial exfoliation technique was carried out in a two-electrode cell. Carbon fiber bundles, Ti mesh, and H 2 SO 4 were used as the anode, cathode, and electrolyte, respectively. Danial et al. [16] reported the production of a suspension of graphene quantum dots (GQDs) by using an electrochemical exfoliation process that involved an electrolyte solution containing citric acid and NaOH, and pristine graphite rods-based electrode.

Bottom-Up Approach
CDs prepared by bottom-up methods have amorphous carbon cores, many surface functional groups, and doping sites. Many researchers synthesized the QDs by the hydrothermal/solvothermal approach. The thermal decomposition of precursors in aqueous or organic solution at temperatures typically between 100 and 250 • C. CDs can also be prepared by microwave-assisted hydrothermal/solvothermal processes. Compared with traditional hydrothermal/solvothermal methods, the microwave approaches can effectively reduce the reaction time for the CDs synthesis [17]. The crystallinity, size, and solubility of the CDs can be tuned by changing the power, reaction temperature, reaction time, and types of precursors. Precursors can be citric acid, saccharides, ethylene glycol, and even the abundantly available biomass, such as waste cotton linter [18], olive solid wastes [19], orange juice [20], bitter apple peel [21], shrimp egg [22], muskmelon peel [23], ginkgo leaves [24], eggshell membranes [25], extract of waste tea [26], egg yolk [27], pine applications in food chemistry. Graphene quantum dots (GQDs) prepared by the ultrasonic process were incorporated on the surface of amine-functionalized Si nanoparticles (SiNPs) through the amide linkage to make the GQDs-SiNPs nanocomposite [48]. The GQDs-SiNPs/GC electrode was proved to have high activity for glutathione sensing.

Fabrication of Kilogram-Scale CDs
CDs have attracted a lot of attention due to their useful merits. However, the complicated synthesis and purification processes limit their large-scale synthesis and wide application. After a lot of effort devoted to the research, there are recent breakthroughs in developing scale-up CD synthesis processes. An aldol condensation method at ambient temperature and pressure was proposed by Li et al. [49]. It is a low-cost and effective method for the large-scale synthesis of CDs, which can produce 1.083 kg CDs in 2 h. Meanwhile, CDs can be functionalized by doping with nitrogen or sulfur/nitrogen. Ji and coworkers [50] reported the fabrication of kilogram-scale CDs from o-phenylenediamine with a high yield (over 96%) by the facile purification process at a low cost (0.1 dollar/g). Figure 1 shows the synthesis process of C-dots606 (CDs with emission at 606 nm) from o-PDA at a kilogram scale (1.104 kg) with high yield by a combined microwave-hydrothermal process. The optical properties of CDs can be adjusted via protonation-deprotonation. A red shift of 47 nm for the fluorescent emission wavelength of CDs can be achieved by protonation. The photoluminescent QE of CDs can be enhanced by three times via deprotonation. Li et al. [51] reported a convenient green kilogram-scale preparation of fluorescent CQDs from poplar leaves through a hydrothermal method. The throughput of CQDs reaches a high level of 1.4975 kg per pot. A cost-efficient and robust approach for the largescale preparation of CDs was developed by Fang et al. [52] through microwave-assisted carbonization of inexpensive industrial surfactants. Four surfactants were selected as precursors to fabricate CDs on a kilogram scale, which also presented no limitation for further enlargement.

Purification of Carbon Dots
The products obtained from the reaction usually contain the mixtures of some CD fractions, molecular intermediates, and several side products. It was reported that many impurities might be formed when CDs were synthesized from waste materials or biomass. The byproducts formed during the synthesis of CDs may change their fluorescent emission properties [53]. Thus, the separation and purification of these products are critical for fabricating CDs nanomaterials with acceptable purity and consistency [54]. CDs can be  [50], 2022, Elsevier).

Purification of Carbon Dots
The products obtained from the reaction usually contain the mixtures of some CD fractions, molecular intermediates, and several side products. It was reported that many impurities might be formed when CDs were synthesized from waste materials or biomass. The byproducts formed during the synthesis of CDs may change their fluorescent emission properties [53]. Thus, the separation and purification of these products are critical for fabricating CDs nanomaterials with acceptable purity and consistency [54]. CDs can be purified by several processes, including centrifugation, dialysis, filtration, electrophoresis, column chromatography, and high-performance liquid chromatography [55][56][57]. Centrifugation and syringe filtration cannot effectively remove the unreacted precursors and reaction by-products. Dialysis is a widely used method for the purification of CDs by removing unreacted products and nanoparticles that are smaller than the expected CDs [58,59]. Selecting the dialysis membrane with the appropriate pore size is important to achieve pure CDs and prevent product loss. Chen et al. [60] reported that there was no standard for the dialysis González-González et al. [61] performed the purification of CDs via the ultrafiltration and dialysis processes by using the Spectra-Por ® Float-A-Lyzer ® G2 with a membrane of 3.5-5 kDa. A Dionex ICS-1600 Ion Chromatography System was used after the dialysis process to ensure the complete purification of CDs. Jia et al. [62] used the dialysis bags (14000 Dalton cutoff) to remove contaminations and unreacted species.
For the non-metallic atom-doped CDs, elements close to C in the periodic table, such as B, N, S, and P, can be incorporated as dopants into the carbon frameworks [82]. N-doped and S-doped CDs are widely studied among the reported non-metallic atom-doped CDs. N-doping is commonly used for the synthesis of doped CDs because of the comparability of the valence electrons and radius of the N atom, allowing effective tuning of optical and electronic properties of CDs [83]. The S atom has similar electronegativity but has a larger atomic size than the C atom. Hence, the S atom exhibits easier electronic transition than that of the C atom, allowing S doping to effectively improve the properties of CDs [84,85]. The doped CDs can be used as fluorescent probes for the sensing of various analytes [86]. The doped CDs were also applied to other applications, such as fluorescence imaging [87,88] and magnetic resonance imaging [89].

Surface Modification
The properties of CDs were affected by the types of surface functional groups. The surface-modified CDs can be used for light-emitting devices, drug releasing, chemical sensing, targeting, and extracting analytes [90]. The formation of surface functional groups such as hydroxyl, carboxyl, amine, and amide can impart higher dispersing stability of CDs in many aqueous media and solvents. It helps to improve the performance catalysis, sensing, and bio-related applications of CDs-based materials by facilitating their interaction with pollutants, analytes, and biological species. Moreover, the characteristics of graphene quantum dots can be tuned by functionalizing their edge structures. The surface modification of CDs can be achieved by (1) the intermolecular interactions, namely complexation, chelation, electrostatic interactions, and loading polymers (such as propionylethylenimineco-ethylenimine [91] and polyethylene-glycol [92]); and (2) the covalent modification of carboxyl, amino, and hydroxy groups on CDs. Tachi et al. [93] successfully elevated the quantum yield (QE) of graphene quantum dots (GQDs) by restricting the vibration and rotation of the functional groups at the edges of GQDs through an esterification reaction with benzyl alcohol. The graphene-stacked structures participated in the π-π Polymers 2022, 14, 2153 7 of 40 interaction with adjacent aromatic rings of the benzylic ester, impeding the nonradiative recombination process in GQDs. Meierhofer et al. [94] reported the synthesis of two types of CDs by the reaction between citric acid and diaminopyridine (DAP) or polyethylenimine (PEI). The CDs consisted of hybridized carbon cores and surface functional groups ( Figure 2). The chemical nature of the functional groups (such as hydroxyl, carboxyl, amino, epoxy, and amides) depends on the types of precursors, reaction conditions (including solvent, reactant ratio, time, temperature, pH, etc.), and purification processes (including centrifugation, column chromatography, and dialysis). The selected precursor decides the types and amounts of surface groups and fluorophores of CDs, changing their optical and pH-responsive properties.
face modification of CDs can be achieved by (1) the intermolecular interactions, namely complexation, chelation, electrostatic interactions, and loading polymers (such as propionylethylenimine-co-ethylenimine [91] and polyethylene-glycol [92]); and (2) the covalent modification of carboxyl, amino, and hydroxy groups on CDs. Tachi et al. [93] successfully elevated the quantum yield (QE) of graphene quantum dots (GQDs) by restricting the vibration and rotation of the functional groups at the edges of GQDs through an esterification reaction with benzyl alcohol. The graphene-stacked structures participated in the π-π interaction with adjacent aromatic rings of the benzylic ester, impeding the nonradiative recombination process in GQDs. Meierhofer et al. [94] reported the synthesis of two types of CDs by the reaction between citric acid and diaminopyridine (DAP) or polyethylenimine (PEI). The CDs consisted of hybridized carbon cores and surface functional groups ( Figure 2). The chemical nature of the functional groups (such as hydroxyl, carboxyl, amino, epoxy, and amides) depends on the types of precursors, reaction conditions (including solvent, reactant ratio, time, temperature, pH, etc.), and purification processes (including centrifugation, column chromatography, and dialysis). The selected precursor decides the types and amounts of surface groups and fluorophores of CDs, changing their optical and pH-responsive properties. Figure 2. The preparation of CD1 and CD2 by the solvothermal pyrolysis of citric acid and amine precursors PEI and DAP, respectively (reprinted with permission from [94], 2020, American Chemical Society).

Process Parameters
(1) Microwave-assisted process: incubation time and precursor ratio: Xu et al. [95] synthesized the nitrogen and sulfur (N,S-CDs)-co-doped CDs via a microwave-assisted process, using cystine as a source for C, N, and S. CDs with high solubility in an aqueous solution can be applied for the turn-on fluorescence Hg(II) sensing in lake water and spiked tap samples. The precursor ratio and incubation time have been optimized to achieve high fluorescence of CDs.
(2) Laser ablation synthesis: laser-pulse width Hu et al. [96] investigated the influence of laser-pulse width on the morphology and size of CDs prepared by a laser ablation method. The results revealed that the nucleation and growth of synthesized particles were tuned by changing the pulse width of the laser. After being refluxed in HNO3 and the surface passivation with organic molecules (PEG1500N or poly(propionylethyleneimine-co-ethyleneimine (PPEI-EI)) [97], the obtained CDs exhibit bright emitting properties. Compared with a short-pulse-width laser, Figure 2. The preparation of CD1 and CD2 by the solvothermal pyrolysis of citric acid and amine precursors PEI and DAP, respectively (reprinted with permission from [94], 2020, American Chemical Society). Xu et al. [95] synthesized the nitrogen and sulfur (N,S-CDs)-co-doped CDs via a microwave-assisted process, using cystine as a source for C, N, and S. CDs with high solubility in an aqueous solution can be applied for the turn-on fluorescence Hg(II) sensing in lake water and spiked tap samples. The precursor ratio and incubation time have been optimized to achieve high fluorescence of CDs.

Key Factors
(2) Laser ablation synthesis: laser-pulse width Hu et al. [96] investigated the influence of laser-pulse width on the morphology and size of CDs prepared by a laser ablation method. The results revealed that the nucleation and growth of synthesized particles were tuned by changing the pulse width of the laser. After being refluxed in HNO 3 and the surface passivation with organic molecules (PEG1500N or poly(propionylethyleneimine-co-ethyleneimine (PPEI-EI)) [97], the obtained CDs exhibit bright emitting properties. Compared with a short-pulse-width laser, using a long-pulse-width laser for the laser ablation process exhibited better control of the size and morphology of CDs.

Average Molar Mass of Polymeric Precursor
Jiang et al. [98] synthesized a series of CDs from polyethylene glycol (PEG) precursors with various molecular weights. The resulting CDs showed low cytotoxicity and amazing performance for cellular bioimaging. They found that CDs were formed from reactions of PEG via the "oxidized decomposition-crosslinking-carbonization" process ( Figure 3a). The emission wavelengths and surface functional groups of CDs samples prepared by various PEGs were similar. However, the molecular weight of the PEG precursors has a great influence on the fluorescence intensities, thermal stability, particle sizes, and element ratio of CDs. It provides a useful and convenient way for tuning the properties of CDs. These CDs exhibited low cytotoxicity and excellent cell-imaging performance (Figure 3b).

Average Molar Mass of Polymeric Precursor
Jiang et al. [98] synthesized a series of CDs from polyethylene glycol (PEG) precursors with various molecular weights. The resulting CDs showed low cytotoxicity and amazing performance for cellular bioimaging. They found that CDs were formed from reactions of PEG via the "oxidized decomposition-crosslinking-carbonization" process ( Figure 3a). The emission wavelengths and surface functional groups of CDs samples prepared by various PEGs were similar. However, the molecular weight of the PEG precursors has a great influence on the fluorescence intensities, thermal stability, particle sizes, and element ratio of CDs. It provides a useful and convenient way for tuning the properties of CDs. These CDs exhibited low cytotoxicity and excellent cell-imaging performance ( Figure 3b).

Surface Passivation Effect
Kang et al. [99] reported the synthesis of amino-functionalized GQDs (FGQDs) for Fe 3+ sensing application, using graphite and polypyrrole (PPy) as the carbon and nitrogen precursors, respectively. Compared with pristine GQDs, FGQDs exhibited a narrow size distribution and small particle sizes due to the "surface passivation effect" of the polypyrrole surfactant. When the pulse laser is exposed to the target (such as graphite flakes) during the pulsed laser ablation process, the partial decomposition of the graphite flakes, ethanol, and PPy. [100] lead to the formation of C, H, O, and N precursors. Then the FGQDs formed due to the high surface energy of precursors. The aggregation of FGQDs can be effectively suppressed via the loading of PPy because PPy and FGQDs have a positive charge and negative polarity, respectively [101]. The formation of PPy passivation layers around FGQDs leads to the smaller size and a narrower particle size distribution of FGQDs (surface passivation effect). The molecular weight of PEG precursors had a large influence on the fluorescence, size, surface chemistry, bioimaging performance, and thermal stability of CDs (reprinted with permission from [98], 2022, Elsevier).

Surface Passivation Effect
Kang et al. [99] reported the synthesis of amino-functionalized GQDs (FGQDs) for Fe 3+ sensing application, using graphite and polypyrrole (PPy) as the carbon and nitrogen precursors, respectively. Compared with pristine GQDs, FGQDs exhibited a narrow size distribution and small particle sizes due to the "surface passivation effect" of the polypyrrole surfactant. When the pulse laser is exposed to the target (such as graphite flakes) during the pulsed laser ablation process, the partial decomposition of the graphite flakes, ethanol, and PPy. Ref. [100] lead to the formation of C, H, O, and N precursors. Then the FGQDs formed due to the high surface energy of precursors. The aggregation of FGQDs can be effectively suppressed via the loading of PPy because PPy and FGQDs have a positive charge and negative polarity, respectively [101]. The formation of PPy passivation layers around FGQDs leads to the smaller size and a narrower particle size distribution of FGQDs (surface passivation effect).

Purification by Preparative Column Chromatography
CDs with a distribution of structures may be obtained because different reaction pathways between the precursors are possible during the hydrothermal, solvothermal, or microwave synthesis process. Usually, the synthesized CDs dispersion is purified by filtration and dialysis processes. Essner et al. [102] found that the CDs were not properly dialyzed in a lot of CDs-related papers. Their results revealed that the ion-sensing properties of CDs are much better than that of the side products, indicating that the CDs should be adequately purified to improve their performance.
Furthermore, it takes a longer time to purify the CDs during the dialysis process. Hinterberger et al. [103] reported the purification and separation of CDs solutions into various fluorescent species (free fluorophores, fluorophores bound on CDs, and low-fluorescent carbon particles without fluorophores) by the preparative column chromatography. The fabrication of purified CDs by the chromatography process exhibits some advantages, such as discovering the exact internal structure of CDs, improving CD properties for industrial applications, and removing toxic precursor/side-products to enhance the biocompatibility of CDs. In addition, Michaud et al. [104] reported the complete separation of N-CDs and other components by a two-step gradient chromatography method, including column chromatography and high-performance liquid chromatography processes.

Factors Affecting the Properties/Performance of CDs
The strong and tunable fluorescence of CDs has been widely investigated because fluorescence enables the use of CDs in biomedicine, optics, catalysis, and sensing applications. Factors affecting the properties or performance of CDs are worth studying, including the size-dependent fluorescence, concentration-dependent multicolor luminescence, solvation effects, electronic structures, and photophysics analysis.

Concentration-Dependent Multicolor Luminescence
The development of color-tunable fluorescent materials is quite important for lightemitting diode applications. He et al. [105] fabricated CDs by the hydrothermal method, using humic acid precursor. The CDs exhibited concentration-dependent multicolor luminescence in deionized water, N,N-dimethylformamide, and formic acid upon the UV light (λ = 365 nm) excitation ( Figure 4a

Solvation Effects (Polarity-Sensitive Fluorescence Effect)
Wang et al. [106] reported the preparation of red-emitting CDs (BNCDs), using citric acid and 1,1 -binaphthyl-2,2 -diamine via a solvothermal process. The shift of UV absorption and PL emission to a longer wavelength was observed when the polarity of solvent increased. BNCDs can be used for the sensitive detection of trace water in organic solvents because of their solvent-dependent (polarity-sensitive) fluorescence. Moreover, BNCDs can also act as a fluorescent probe for the Fe 3+ and F − sensing by using an "on-off-on" fluorescence mechanism ( Figure 5).  The polarity-sensitive fluorescence effect and Fe 3+ /F − sensing by "on-off-on" fluorescence mechanism (reprinted with permission from [106], 2022, Elsevier).

Size and Surface States
Hu et al. [107] found that PL properties of CDs were determined by their sizes and surface states. However, the photocatalytic activities only depend on their surface states. High upward band bending caused by surface COOH and C=O groups of CQDs leads to efficient separation of photoexcited carriers ( Figure 6). The size of CDs can be tuned by changing the concentration of HNO3 for the oxidative refluxing process, while the surface states of CDs can be changed by selective reduction with NaBH4. Figure 5. The polarity-sensitive fluorescence effect and Fe 3+ /F − sensing by "on-off-on" fluorescence mechanism (reprinted with permission from [106], 2022, Elsevier).

Size and Surface States
Hu et al. [107] found that PL properties of CDs were determined by their sizes and surface states. However, the photocatalytic activities only depend on their surface states. High upward band bending caused by surface COOH and C=O groups of CQDs leads to efficient separation of photoexcited carriers ( Figure 6). The size of CDs can be tuned by changing the concentration of HNO 3 for the oxidative refluxing process, while the surface states of CDs can be changed by selective reduction with NaBH 4 . Zhu et al. [108] investigated the origin of the surface-related emissions and the mechanism through the surface oxidation and reduction treatment of the pristine CDs. The blue, green, and red fluorescence originated from the carbogenic core's intrinsic state emission, the n → π* transitions of surface C=N and C=O groups, respectively. For CDs modified by the oxidation (o-CDs), the C=O groups increase, while the C=N groups dis- Zhu et al. [108] investigated the origin of the surface-related emissions and the mechanism through the surface oxidation and reduction treatment of the pristine CDs. The blue, green, and red fluorescence originated from the carbogenic core's intrinsic state emission, the n → π* transitions of surface C=N and C=O groups, respectively. For CDs modified by the oxidation (o-CDs), the C=O groups increase, while the C=N groups disappear, owing to the oxidation by HNO 3 . On the contrary, the C=O groups in the CDs modified by the reduction (r-CDs) almost disappear due to the reduction by NaBH 4 .

Electronic Structures and Photophysics Analysis of CDs
The synchrotron near edge X-ray absorption fine structure (NEXAFS) test is a useful tool to find out the electronic states and photophysics of materials [109][110][111]. The synchrotron NEXAFS test was utilized for the analysis of CDs-based materials. Pschunder et al. [112] studied the correlation between tunable emission performance and intricate structural features of N,B-doped CDs by the synchrotron NEXAFS tests, X-ray photoelectron spectroscopy (XPS), and steady-state and time-resolved optical spectroscopy. The simultaneous coexistence of B with N influences the electron delocalization and energylevel alignments, which eventually alter the overall photophysics of the co-doped CDs ( Figure 7). The XPS analysis of N-doped CDs suggests an aromatic core domain and the N atoms containing edge. However, the core N atoms covalently bonded to neighboring atoms are predominant in the N,B-doped CDs. The N K-edge NEXAFS tests also confirm this result, revealing the significantly enhanced planar σ* resonance of the N,B-doped CDs. The co-doping of B and N inhibited the formation of green emitting fluorophores. Moon et al. [113] found that the NEXAFS results revealed that N-GCDs had a high density of graphitic structures such as sp2-hybridized carbon and tiny amounts of defect. Bokare et al. [114] studied the mechanism of forming C-dots and GQDs with different functionality and size by XPS and NEXAFS measurements. In situ X-ray absorption spectroscopy was utilized by Ren et al. [115] to analyze the electronic structures of water molecules and CDs in an aqueous dispersion. They found that the and N doping and core graphitization improved the hydrogen bonding and electron transfer ability of CDs with surrounding water molecules.

Degradation of Organic Toxicants
The polluting of freshwater sources is a significant threat to providing safe drinking water. Due to unsustainably high development initiatives in construction activities, in-

Organic Dyes
The polluting of freshwater sources is a significant threat to providing safe drinking water. Due to unsustainably high development initiatives in construction activities, industrial emissions, and agricultural activities [116], inorganic toxicants such as heavy metals, deposits, and nutrients, and organic toxicants such as dyes, pesticides, pharmaceutical drugs, and endocrine disruptors, are deposited into the environment. The vast majority of these toxicants, mainly inorganic toxicants, will remain in the environment, leading to toxicant bioaccumulation and entry into the food matter. Finally, it has a toxic effect on higher trophic level organisms, which can be lethal. Photocatalysts have been developed for environmental cleanup in recent times [117,118]. CDs have nontoxicity, chemical inertness, photoinduced electron transfer, excellent biocompatibility, and customizable photoluminescence behavior patterns. Since CDs are frequently made from eco-friendly materials, they are inexpensive and environmentally friendly at reducing waste generation.
CDs have better catalytic performance with enhanced photocatalytic effectiveness and shorter degradation times. CDs were made by a simple one-pot solvothermal process, using ethanol and glyoxal as a precursor (0.26). In the sunlight, indoors, and under dark conditions, the degradation efficiency of indigo carmine (IC) can reach 91% [119]. The Cornus walteri leaves were used to synthesize G-CDs that acted as a photocatalyst for the degradation of organic dyes [120]. A hydrothermal procedure was used to make carbon dots containing Fe and N (Fe,N-CDs) as Fenton-like catalysts for the degradation of the MB dye. About 100% of the MB dye (20 mg L −1 ) was degraded within 60 min [120]. The TiO 2 nanoparticles decorated with a microalgae-based carbon dots (MCDs) (TiO 2 -MCDs) composite degraded the MB dye more efficiently than pristine TiO 2 . The MCDs in TiO 2 -MCDs act as the reservoirs for trapping photogenerated electrons and as photosensitizers for enhancing visible-light absorption [121]. Wang et al. [122] prepared the CDs-based porous europium micro-networks (CDs@P-Eu-MNs). The CDs can alter the morphology of CDs@P-Eu-MNs and result in a huge variety of porous structures. The incorporation of CDs can improve the activity of photocatalysts by increasing both the light absorption and the separation of photogenerated carriers. The visible-light-driven activity of the CDs/TNs (TNs-TiO 2 sheets) photocatalysts was significantly higher than that of bare TNs [123]. The remarkable stability of CDs/TNs was explored by completing repeated Congo red (CR) degradation for five cycles (photocatalytic degradation rate reached 85.9% after irradiation for 120 min). The CDs/MoS 2 /p-C 3 N 5 composites had outstanding photocatalytic degradation ability (93.51%) toward methylene blue.
CQDs-modified Sb 2 WO 6 nanosheets (CQDs/Sb 2 WO 6 ) exhibited a visible-to-nearinfrared (Vis-NIR) light-responsive property [124]. The photocatalytic degradation efficiency of RhB by the composite photocatalyst is roughly seven times better than that of Sb 2 WO 6 . The results of quenching experiments, DFT calculations, and electron spin resonance spectrometry revealed that the hydroxyl radical (•OH) played a dominant role in the photocatalysis reaction. Figure 8 shows the proposed photocatalytic degradation mechanism by the CQDs/Sb 2 WO 6 photocatalyst. The conversion of the long-wavelength light from 550 to 850 nm to the short-wavelength light from 440 to 500 nm was achieved by introducing CQDs, realizing the indirect use of NIR irradiation. CQDs-loaded Sb 2 WO 6 exhibited higher •OH production and photoexcited carrier utilization.
Sb2WO6. The results of quenching experiments, DFT calculations, and electron spin resonance spectrometry revealed that the hydroxyl radical (•OH) played a dominant role in the photocatalysis reaction. Figure 8 shows the proposed photocatalytic degradation mechanism by the CQDs/Sb2WO6 photocatalyst. The conversion of the long-wavelength light from 550 to 850 nm to the short-wavelength light from 440 to 500 nm was achieved by introducing CQDs, realizing the indirect use of NIR irradiation. CQDs-loaded Sb2WO6 exhibited higher •OH production and photoexcited carrier utilization. Figure 8. Proposed photocatalytic degradation mechanism by the CQDs/Sb2WO6 photocatalyst (reprinted with permission from [124], 2022, Elsevier).

Possible Mechanism
The nitrogen-doped carbon dot-modified ZnO composite (N-CDs@ZnO) photocatalyst shows significantly better activity (MB dye degradation efficiency > 99%, 60 min irradiation) than pristine ZnO photocatalysts (75%, 60 min irradiation). The improved photocatalytic activity resulted from increased UV-light absorption and hindered the recombination of photoexcited electron-hole pairs. The hindered carrier recombination may be due to effective electrons trapping by N-CDs. Moreover, loading N-CDs also helped to solve the photocorrosion problem of ZnO in the N-CDs@ZnO photocatalysts. N-CDs were decorated on the ZnO surface and created a complex structure providing the access to photogenerated charge transfer upon light irradiation. The combination of adsorbed O2 with the electrons in N-CDs leads to the formation of oxygen radicals. Then N-CDs with the up-conversion properties convert the long-wavelength light to the short-wavelength light that can excite ZnO to form separate electrons and holes. During the degradation process of the MB dye, the π-π* interaction between N-CDs and the MB dye can enhance Figure 8. Proposed photocatalytic degradation mechanism by the CQDs/Sb 2 WO 6 photocatalyst (reprinted with permission from [124], 2022, Elsevier).

Possible Mechanism
The nitrogen-doped carbon dot-modified ZnO composite (N-CDs@ZnO) photocatalyst shows significantly better activity (MB dye degradation efficiency > 99%, 60 min irradiation) than pristine ZnO photocatalysts (75%, 60 min irradiation). The improved photocatalytic activity resulted from increased UV-light absorption and hindered the recombination of photoexcited electron-hole pairs. The hindered carrier recombination may be due to effective electrons trapping by N-CDs. Moreover, loading N-CDs also helped to solve the photocorrosion problem of ZnO in the N-CDs@ZnO photocatalysts. N-CDs were decorated on the ZnO surface and created a complex structure providing the access to photogenerated charge transfer upon light irradiation. The combination of adsorbed O 2 with the electrons in N-CDs leads to the formation of oxygen radicals. Then N-CDs with the up-conversion properties convert the long-wavelength light to the short-wavelength light that can excite ZnO to form separate electrons and holes. During the degradation process of the MB dye, the π-π* interaction between N-CDs and the MB dye can enhance the degradation of MB by the N-CDs@ZnO photocatalyst [125]. The nitrogen-rich carbon nitride (p-C3N5) had exceptional electronic properties. The CDs-modified co-catalyst MoS 2 can significantly improve the photocatalytic activity of p-C3N5 by increasing the transfer rate of photoexcited electrons [126]. The photogenerated electrons in the CB of p-C3N5 are collected and stored by CD particles, transferred across the interface to a MoS 2 cocatalyst, leading to enhanced separation of electron-hole pairs. The electrons transfer to MoS 2 nanosheets and interact with the adsorbed H + and evolve H 2 . The holes in the VB of p-C 3 N 5 are consumed by the reacting with SO 3 2− and S 2− . CDs attached on MoS 2 act as the electron acceptors, facilitating charge-transfer efficiency and improving the photocatalytic H 2 production activity of the composite photocatalyst. Organic pollutant dyes such as RhB, MB, CR, and fuchsine were removed by using the newly developed Z-scheme C 3 N 4 -NS/CD/FeOCl ((C 3 N 4 -NS) g-C 3 N 4 nanosheets) photocatalysts [127]. The complete removal of RhB can be achieved within 60 min. The photocatalytic activity of C 3 N 4 -NS/CD/FeOCl for the removal of RhB, CR, MB, and fuchsine was about 39.7, 15.2, 26.9, and 20.9 times that of pristine C 3 N 4 . The C 3 N 4 -NS/CD/FeOCl sample also displayed high stability. The chemical scavenging studies revealed that the •O 2 •− , •OH, and h + played significant roles in the photocatalytic degradation reaction. The modified CDs-BiSbO 4 composite was used for the degradation of RhB effectively up to 90% [128]. The CDs with excellent up-conversion properties can successfully convert long wavelengths (550-900 nm) to short wavelengths (320-500 nm). CDs acted as the electron sink, reducing the recombination of photogenerated carriers originating from the BiSbO 4 nanomaterials. When O 2 molecules are deposited on the CDs surface, the electrons in the interlayer will be drawn toward O 2 , providing a perfect platform for activating the molecular oxygen.

Pharmaceutical Pollutant Removal
The increasing persistence of active pharmaceutical residues in water matrices, such as anti-inflammatory medicines (diclofenac, naproxen, indomethacin, etc.) and antibiotics (tetracycline, ciprofloxacin, norfloxacin, etc.), has become a global problem [129]. These harmful substances have negative impacts on the entire living ecosystem and deplete biodiversity. As a result, the development of effective solutions and preventative strategies is urgently needed to reduce the pharmaceutically active developing contaminants from the environmental matrices in a long-term manner. As a result, CD-based photocatalysts are gaining traction as a viable decontamination option for pharmaceutical pollutants. CDs play a vital role in the performance of photocatalysts, due to their unique up-conversion property, electron transfer capabilities, and effective separation of photogenerated carriers [130]. Table 1 listed the precursors, synthesis methods, target pollutants, active species, degradation efficiencies, and roles of CDs for the removal of various organic pollutants.

Treatment of Inorganic Toxicant
The fabrication of CDs from microcrystalline cellulose (MCC) is described in a scalable synthetic approach [138]. Elimination of hazardous Cr 6+ from wastewater was used to test the effectiveness of the produced CDs. Under sunshine illumination, CDs made from cellulose eliminated 20 ppm of Cr 6+ in about 120 min. During the control test in dark surroundings, no Cr 6+ removal was observed by the cellulose material as reference samples. With a half-life of 26 min, Cr 6+ elimination follows pseudo-first-order dynamics. Furthermore, cyclic voltammetry research confirmed the removal of Cr 6+ from wastewater. A photocatalytic Z-scheme TiO 2 -CDs/polyaniline electrode was constructed to improve photocatalytic activity [139]. The light adsorption was enhanced after loading carbon dots and PANI (polyaniline). Simultaneous carbamazepine degradation (44.67%) and Cr 6+ reduction (11.94%) was observed because of the decreased bandgap and increased photocurrent density. There is a noticeable improvement in the performance of the carbamazepine degradation (from 77.63% to 83.29%) and Cr 6+ reduction (from 23.70% to 25.68%). The freeradical •OH (78.63%) is the main active species in the degradation of carbamazepine (CBZ) into small molecules by a process consisting of hydrolysis, dehydration, de-ketonization, deaminization, and ring-opened reactions.

CO 2 Reduction
CO 2 can be utilized as a substrate for the synthesis of fuels or high-value carbon compounds such as HCOOH/HCOO − , CO, CH 4 , C 2 H 4 , CH 3 OH, or C 2 H 5 OH. The speci-ficity toward the formation of certain target products is a significant obstacle for ensuring an effective reduction reaction of CO 2 , because of the possible formation of a lot of carbon-containing products, and the competitive side H 2 evolution rate (HER), which also decreases the performance of the entire reduction reaction process of CO 2 in aqueous media. Furthermore, the exceptional inertness and stability of CO 2 molecules pose thermodynamic and kinetic barriers to effective CO 2 activation and conversion. Q. Liang et al. [140] reported that, without using a sacrificial agent or any other photosensitizer, a CD-modified Co 3 O 4 /In 2 O 3 composite catalyst for effective CO 2 photoreduction achieves an excellent CO production activity of 2.05 mmol h −1 g −1 . The CD catalyst produces 3.2 times more CO than the Ru catalyst based on the same testing conditions, namely without the addition of triethanolamine (TEOA). The transient photovoltage (TPV) measurements explored the interface charge transfer kinetics, suggesting that the CDs participate in the electron and hole transfer procedures, stabilizing charge, and enriching H + for the effective reduction of CO 2 .
The oxygen vacancy defect results from a loss of oxygen atom from its relative position in the crystal lattice. The introduction of surface oxygen vacancy is a promising method to tune band structure, modify surface chemical states, and accelerate charge separation of photocatalysts. Xiong et al. [141] designed a photocatalyst GQDs/BWO 6-x , consisting of graphene quantum dots (GQDs) and surface Vo (oxygen vacancies) with decorated Bi 2 WO 6 (BWO). The GQDs/BWO 6-x showed improved photocatalytic conversion of CO from CO 2 , with a high yield (43.9 µmol g −1 h −1 ), i.e., 1.7 times greater than pristine BWO. The GQDs/BWO 6-x generated electrons showed a longer fluorescence lifetime than pristine BWO, indicating an excellent separation efficiency for photoexcited carriers. According to the DFT calculations, the electrons move to Vo-adjacent atoms from Vo-remote atoms of GQDs/BWO 6-x . The energy barrier calculation of GQDs/BWO 6-x and BWO revealed that a simple transition of *COOH to *CO is the rate-limiting step. The results of DFT calculations of molecular binding energy (BE) on the surface of photocatalysts were performed to demonstrate the CO 2 activation mechanism by spotting the adsorption energy for intermediates. The BE outlines and intermediate configurations are shown in Figure 9a,b. The CO 2 molecules adsorbed on BWO and GQDs/BWO 6-x were first hydrogenated to *COOH, then converted to *CO and *OH, and then rehabilitated to *CO and *H 2 O. The *CO intermediate bonded at the Vo-neighboring sites was desorbed from the surface of GQDs/BWO 6-x to produce CO (possibility (1)) or hydrogenated to produce *CHO (possibility (2)). The CO desorption energy for GQDs/BWO 6-x is comparable to Bi 2 WO 6 . Therefore, the surfaces of GQDs/BWO 6-x and BWO were capable of desorbing *CO to form free CO in a similar manner. Since GQDs/BWO 6-x has a lower *CHO generation energy than the bulk BWO, the *CO intermediate for GQDs/BWO 6-x is more easily protonated to CH 4 . The formation of *CO by the protonation of *COOH is the rate-determining step. In comparison with the conversion of *COOH-BWO to *CO-BWO, the transformation of *COOH-GQDs/BWO 6-x to *CO-GQDs/BWO 6-x has a lower energy barrier, indicating that the decoration by GQDs and Vo was advantageous for the transformation of *COOH to *CO.
fore, the surfaces of GQDs/BWO6-x and BWO were capable of desorbing *CO to form free CO in a similar manner. Since GQDs/BWO6-x has a lower *CHO generation energy than the bulk BWO, the *CO intermediate for GQDs/BWO6-x is more easily protonated to CH4. The formation of *CO by the protonation of *COOH is the rate-determining step. In comparison with the conversion of *COOH-BWO to *CO-BWO, the transformation of *COOH-GQDs/BWO6-x to *CO-GQDs/BWO6-x has a lower energy barrier, indicating that the decoration by GQDs and Vo was advantageous for the transformation of *COOH to *CO. On a carbon nitride-like polymer (FAT) adorned with CDs, Wang et al. [142] demonstrated that CO2 is reduced to methanol (CH3OH) with 100% selectivity using H2O as the only electron source and discovered that carbon dots could extract holes in FAT with almost 75% efficiency before they became unreactive due to entrapment using transient absorption spectroscopy. The removal of holes resulted in a higher density of photoelectrons, indicating that shorter-lived reactive electrons recombined less frequently. Lee et al. [143] rationally created a CD/TOH hybrid catalyst consisting of N-doped carbon dots (CD) and hollow TiO2 spheres (TOH). They used it for the photocatalytic reduction of CO2 to make CH4. According to electron microscopy images, the CD/TOH composite has a porous hollow spherical shape that is uniformly loaded with CD. Furthermore, the CD/TOH hybrid exhibits excellent light-harvesting, large surface area, good CO2 adsorption capabilities, and, most critically, improved separation of photogenerated carriers for CO2 photoreduction processes. As a result, the CD/TOH with 2 wt.% CD exhibits a high CH4 generation rate of 26.8 µmol h −1 g −1 , equal to 98% of CH4 selectivity over the competitive H2 generation reaction. Wang et al. [144] designed and fabricated a direct Z-scheme heterojunction composite CPDs/Bi4O5Br2, consisting of Bi4O5Br2 nanosheets and carbonized polymer dots. It effectively facilitates photogenerated carrier migration and separation efficiency while retaining more negative electron reduction potential of carbonized On a carbon nitride-like polymer (FAT) adorned with CDs, Wang et al. [142] demonstrated that CO 2 is reduced to methanol (CH 3 OH) with 100% selectivity using H 2 O as the only electron source and discovered that carbon dots could extract holes in FAT with almost 75% efficiency before they became unreactive due to entrapment using transient absorption spectroscopy. The removal of holes resulted in a higher density of photoelectrons, indicating that shorter-lived reactive electrons recombined less frequently. Lee et al. [143] rationally created a CD/TOH hybrid catalyst consisting of N-doped carbon dots (CD) and hollow TiO 2 spheres (TOH). They used it for the photocatalytic reduction of CO 2 to make CH 4 . According to electron microscopy images, the CD/TOH composite has a porous hollow spherical shape that is uniformly loaded with CD. Furthermore, the CD/TOH hybrid exhibits excellent light-harvesting, large surface area, good CO 2 adsorption capabilities, and, most critically, improved separation of photogenerated carriers for CO 2 photoreduction processes. As a result, the CD/TOH with 2 wt.% CD exhibits a high CH 4 generation rate of 26.8 µmol h −1 g −1 , equal to 98% of CH 4 selectivity over the competitive H 2 generation reaction. Wang et al. [144] designed and fabricated a direct Z-scheme heterojunction composite CPDs/Bi 4 O 5 Br 2 , consisting of Bi 4 O 5 Br 2 nanosheets and carbonized polymer dots. It effectively facilitates photogenerated carrier migration and separation efficiency while retaining more negative electron reduction potential of carbonized polymer dots (CPDs) and more positive hole oxidation potential of Bi 4 O 5 Br 2 . CPDs also facilitate the adsorption of CO 2 and COOH* intermediates and the desorption of product CO. Under the Xe lamp irradiation, the maximum CO generation of CPDs/Bi 4 O 5 Br 2 is 132.42 µmol h −1 g −1 , which is 5.43 times higher than that of Bi 4 O 5 Br 2 nanosheets. When the excitation wavelength is higher than 580 nm, CPDs with up-conversion capabilities can extend the light utilization range, resulting in better CO 2 conversion performance in composite materials.

Hydrogen Evolution
Photocatalytic H 2 production using semiconductor photocatalysts is an eco-friendly technology for solar energy conversion [145][146][147]. Ding et al. [148] reported the synthesis of a sequence of CDs decorated HCNS-C x (hollow g-C 3 N 4 spheres) with glucose and cyanamide as precursors. The single-step in situ thermal polymerization method was used to synthesize the HCNS-C x . CDs and g-C 3 N 4 were able to maintain a tight bond and enhance the separation of photogenerated carriers. The HER of HCNS-C 1.0 (2322 µmol g −1 h −1 ) was 1.8 times and 19 times higher than the HCNS and bulk g-C 3 N 4 , correspondingly. Figure 10. presents the proposed photocatalytic H 2 production mechanism for the HCNS-C 1.0 composite photocatalyst. CDs act as the photosensitizer that extend the light absorption wavelength of composite photocatalysts, resulting in more photoexcited carriers. CDs also facilitate the electron transport and improve the reduction of H + to form H 2 .
polymer dots (CPDs) and more positive hole oxidation potential of Bi4O5Br2. CPDs also facilitate the adsorption of CO2 and COOH* intermediates and the desorption of product CO. Under the Xe lamp irradiation, the maximum CO generation of CPDs/Bi4O5Br2 is 132.42 µmol h −1 g −1 , which is 5.43 times higher than that of Bi4O5Br2 nanosheets. When the excitation wavelength is higher than 580 nm, CPDs with up-conversion capabilities can extend the light utilization range, resulting in better CO2 conversion performance in composite materials.

Hydrogen Evolution
Photocatalytic H2 production using semiconductor photocatalysts is an eco-friendly technology for solar energy conversion [145][146][147]. Ding et al. [148] reported the synthesis of a sequence of CDs decorated HCNS-Cx (hollow g-C3N4 spheres) with glucose and cyanamide as precursors. The single-step in situ thermal polymerization method was used to synthesize the HCNS-Cx. CDs and g-C3N4 were able to maintain a tight bond and enhance the separation of photogenerated carriers. The HER of HCNS-C1.0 (2322 µmol g −1 h −1 ) was 1.8 times and 19 times higher than the HCNS and bulk g-C3N4, correspondingly. Figure 10. presents the proposed photocatalytic H2 production mechanism for the HCNS-C1.0 composite photocatalyst. CDs act as the photosensitizer that extend the light absorption wavelength of composite photocatalysts, resulting in more photoexcited carriers. CDs also facilitate the electron transport and improve the reduction of H + to form H2. A non-metallic photocatalyst based on CQDs/covalent triazine-based framework (CQDs/CTF) was made using an impregnation approach for photocatalytic hydrogen generation. In comparison to pristine CTF, the composite with 0.24% CQDs showed a threefold increase in hydrogen generation rate of 102 µmol g −1 h −1 . CQDs acted as the electron libraries, facilitating electron capture and promoting the separation of photogenerated carriers in CTF-1, according to photoluminescence and photoelectrochemical research. CQDs' excitation-independent up-conversion fluorescent properties gave the catalysts a wider range of visible-light responses and improved the efficiency of solar energy utilization [149]. Wang et al. [150] reported a CQDs-CdIn2S4 (CQDs/CIS) heterostructured composite. As the number of CQDs increased, the morphology of the hybrid sample changed from 3D octahedrons to 2D nanosheets. This unique 3D/2D structure and synergistic effects between CdIn2S4 and CQDs effectively boosted the active reaction sites of the composite, improving quantum yield and photogenerated electron pair separation efficiency. In particular, the CQDs/CIS composite photocatalyst had the highest H2 production activity of 956.79 µmol g −1 h −1 , which was 7.57 times higher than the pristine CdIn2S4. A non-metallic photocatalyst based on CQDs/covalent triazine-based framework (CQDs/CTF) was made using an impregnation approach for photocatalytic hydrogen generation. In comparison to pristine CTF, the composite with 0.24% CQDs showed a threefold increase in hydrogen generation rate of 102 µmol g −1 h −1 . CQDs acted as the electron libraries, facilitating electron capture and promoting the separation of photogenerated carriers in CTF-1, according to photoluminescence and photoelectrochemical research. CQDs' excitation-independent up-conversion fluorescent properties gave the catalysts a wider range of visible-light responses and improved the efficiency of solar energy utilization [149]. Wang et al. [150] reported a CQDs-CdIn 2 S 4 (CQDs/CIS) heterostructured composite. As the number of CQDs increased, the morphology of the hybrid sample changed from 3D octahedrons to 2D nanosheets. This unique 3D/2D structure and synergistic effects between CdIn 2 S 4 and CQDs effectively boosted the active reaction sites of the composite, improving quantum yield and photogenerated electron pair separation efficiency. In particular, the CQDs/CIS composite photocatalyst had the highest H 2 production activity of 956.79 µmol g −1 h −1 , which was 7.57 times higher than the pristine CdIn 2 S 4 .
CQDs modified TiO 2 composites were prepared to coproduce H 2 and arabinose with increased selectivity. As depicted in Figure 11 when CQDs/TiO 2 composites are exposed to light, electrons in the VB (valence band) are excited into the CB (conduction band), leaving h + (holes) in the VB. The CQDs partially trap the photogenerated electrons and then cause proton reduction to produce H 2 , whereas the remaining electrons react with the absorbed oxygen to produce radical •O 2 − . The photogenerated h + combines with the absorbed H 2 O to form radical •OH. The CQDs/TiO 2 composites with particular colored CQDs can significantly enhance the selectivity of glucose to arabinose conversion (75%) and the H 2 production activity (2.43 mmol h −1 g −1 ) [151].
CQDs modified TiO2 composites were prepared to coproduce H2 and arabinose with increased selectivity. As depicted in Figure 11 when CQDs/TiO2 composites are exposed to light, electrons in the VB (valence band) are excited into the CB (conduction band), leaving h + (holes) in the VB. The CQDs partially trap the photogenerated electrons and then cause proton reduction to produce H2, whereas the remaining electrons react with the absorbed oxygen to produce radical •O2 − . The photogenerated h + combines with the absorbed H2O to form radical •OH. The CQDs/TiO2 composites with particular colored CQDs can significantly enhance the selectivity of glucose to arabinose conversion (75%) and the H2 production activity (2.43 mmol h −1 g −1 ) [151]. Figure 11. Proposed reaction mechanism for glucose photoreforming (reprinted with permission from [151], 2022, Elsevier).
Xu et al. [152] proposed that the incorporation of nitrogen-doped carbon quantum dots (NCQDs) into composite catalysts not only improved photoabsorption but also allowed effective and rapid migration of photogenerated electrons to NCQDs. The photocatalytic H2 generation activity can be effectively improved because of reduced photogenerated carrier recombination. The photocatalytic H2 generation performance of composite photocatalysts was significantly enhanced (2306.1 µmol g −1 h −1 ), which was around seven times that of the sample without NCQDs.

Food Storage
The curcumin (Cur) carbon quantum dots (Cur-NRCQDs), as a photosensitizer, can improve the efficiency of reactive oxygen generation and antibacterial performance. Cur-NRCQDs can inactivate 100% Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) under xenon lamp irradiation at concentrations of 10 and 15 M, respectively. The reactive oxygen created by Cur-NRCQDs during photodynamic therapy may have disrupted the cell membrane, leading to leakage of the contents [153]. Lin et al. [154] prepared nanosized, spherical, neutral charge, fluorescent carbon dots with good water dispersibility using fish, ginger, onion, and garlic as carbon sources. The fish and ginger CDs contained lower sulfur elements than the onion and garlic CDs. The onion CDs exhibited antibacterial activity against Pseudomonas fragi (P. fragi), as well as antimicrobial activity against S. aureus and E. coli. Onion CDs had MIC of 2 mg mL −1 and MBC of 4 mg mL −1 , against P. fragi. The cell membrane and cell-wall integrity were damaged after the light irradiation to CDs, and extracellular alkaline phosphatase (AKP) and ATP activity increased, resulting in a decrease in cell viability and a change in cellular shape in P. fragi. These results reveal that onion CDs can be used as a bacteriostatic agent for aquatic products. Ma et al. [155] produced AgNPs by reducing a combination of CDs and silver (Ag) ions with sodium borohydride (NaBH4), employing CDs with numerous chemical groups Figure 11. Proposed reaction mechanism for glucose photoreforming (reprinted with permission from [151], 2022, Elsevier).
Xu et al. [152] proposed that the incorporation of nitrogen-doped carbon quantum dots (NCQDs) into composite catalysts not only improved photoabsorption but also allowed effective and rapid migration of photogenerated electrons to NCQDs. The photocatalytic H 2 generation activity can be effectively improved because of reduced photogenerated carrier recombination. The photocatalytic H 2 generation performance of composite photocatalysts was significantly enhanced (2306.1 µmol g −1 h −1 ), which was around seven times that of the sample without NCQDs.

Food Storage
The curcumin (Cur) carbon quantum dots (Cur-NRCQDs), as a photosensitizer, can improve the efficiency of reactive oxygen generation and antibacterial performance. Cur-NRCQDs can inactivate 100% Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) under xenon lamp irradiation at concentrations of 10 and 15 M, respectively. The reactive oxygen created by Cur-NRCQDs during photodynamic therapy may have disrupted the cell membrane, leading to leakage of the contents [153]. Lin et al. [154] prepared nanosized, spherical, neutral charge, fluorescent carbon dots with good water dispersibility using fish, ginger, onion, and garlic as carbon sources. The fish and ginger CDs contained lower sulfur elements than the onion and garlic CDs. The onion CDs exhibited antibacterial activity against Pseudomonas fragi (P. fragi), as well as antimicrobial activity against S. aureus and E. coli. Onion CDs had MIC of 2 mg mL −1 and MBC of 4 mg mL −1 , against P. fragi. The cell membrane and cell-wall integrity were damaged after the light irradiation to CDs, and extracellular alkaline phosphatase (AKP) and ATP activity increased, resulting in a decrease in cell viability and a change in cellular shape in P. fragi. These results reveal that onion CDs can be used as a bacteriostatic agent for aquatic products. Ma et al. [155] produced AgNPs by reducing a combination of CDs and silver (Ag) ions with sodium borohydride (NaBH 4 ), employing CDs with numerous chemical groups as ligands. The stable CD-AgNPs (carbondot-stabilized silver nanoparticles) give a higher antibacterial performance than AgNPs without CDs. The CD-AgNPs had an MIC of 100 µg mL −1 with 0.613 µg mL −1 Ag. CD-AgNPs have a broad-spectrum antibacterial activity since they inhibit the growth of six bacteria, namely S. aureus, L. monocytogenes, E. coli, Vibrio parahaemolyticus, S. typhimurium, and Shigella castellani.

Wound Healing
The quaternized carbon quantum dots (qCQDs) regained the weight of rats in wounds infections caused by mixed bacteria, significantly decreased the death of rats from a serious infection, and also improved the healing of infected wounds in rat models. Biosafety tests revealed that qCQDs had no apparent toxic or adverse reactions during the testing phase. The quantification proteomics analysis showed that qCQDs primarily acted on ribosomal proteins in S. aureus and downregulated the citrate cycle proteins in E. coli [156].
Wu et al. [157] reported that the levofloxacin (antibiotic medicine)-based carbon dots (LCDs) improved the antibacterial activity and reduced drug resistance. The results showed that LCDs have effective antibacterial properties due to the active groups of levofloxacin being preserved. LCDs had a dual-antibacterial mode that distorted microorganisms because of the reactive species and positive surface charge generation at the same time. The LCDs in vivo antibacterial effects were evaluated in the infected wounds with E. coli or S. aureus of ICR mice ( Figure 12). The abscess was visible in infected wounds after 48 h of infection with E. coli or S. aureus. When the infected wounds were treated with levofloxacin hydrochloride, LCDs, and normal saline, from day 1 to day 10, as illustrated in Figure 12. The scab appeared after treatment in the LCDs and levofloxacin hydrochloride (LC-HCl) groups, but in the negative control group, the exudates and pus remained. The exudates and pus remained in the negative control group after seven days of treatment, although the area of infected wounds in the LCD groups was substantially reduced compared to the LC-HCl groups. as ligands. The stable CD-AgNPs (carbon-dot-stabilized silver nanoparticles) give a higher antibacterial performance than AgNPs without CDs. The CD-AgNPs had an MIC of 100 µg mL −1 with 0.613 µg mL −1 Ag. CD-AgNPs have a broad-spectrum antibacterial activity since they inhibit the growth of six bacteria, namely S. aureus, L. monocytogenes, E. coli, Vibrio parahaemolyticus, S. typhimurium, and Shigella castellani.

Wound Healing
The quaternized carbon quantum dots (qCQDs) regained the weight of rats in wounds infections caused by mixed bacteria, significantly decreased the death of rats from a serious infection, and also improved the healing of infected wounds in rat models. Biosafety tests revealed that qCQDs had no apparent toxic or adverse reactions during the testing phase. The quantification proteomics analysis showed that qCQDs primarily acted on ribosomal proteins in S. aureus and downregulated the citrate cycle proteins in E. coli [156].
Wu et al. [157] reported that the levofloxacin (antibiotic medicine)-based carbon dots (LCDs) improved the antibacterial activity and reduced drug resistance. The results showed that LCDs have effective antibacterial properties due to the active groups of levofloxacin being preserved. LCDs had a dual-antibacterial mode that distorted microorganisms because of the reactive species and positive surface charge generation at the same time. The LCDs in vivo antibacterial effects were evaluated in the infected wounds with E. coli or S. aureus of ICR mice (Figure 12). The abscess was visible in infected wounds after 48 h of infection with E. coli or S. aureus. When the infected wounds were treated with levofloxacin hydrochloride, LCDs, and normal saline, from day 1 to day 10, as illustrated in Figure 12. The scab appeared after treatment in the LCDs and levofloxacin hydrochloride (LC-HCl) groups, but in the negative control group, the exudates and pus remained. The exudates and pus remained in the negative control group after seven days of treatment, although the area of infected wounds in the LCD groups was substantially reduced compared to the LC-HCl groups.  The sulfur (S) functionalized turmeric-derived carbon dots (S-CDs) showed high antibacterial and antioxidant action against mouse fibroblast L929 cells with minimal cytotoxicity [158]. The UV protection capabilities of the CDs-added film were increased without affecting the transparency of the pectin/gelatin film. The hydrophobicity, mechanical, and water vapor permeability of the film were all changed by the addition of CDs. Furthermore, the DPPH and ABTS techniques revealed that CDs-loaded pectin/gelatin films had high antioxidant properties. Moreover, the film with sulfur functionalized CDs demonstrated high antibacterial action against foodborne pathogenic bacteria such as E. coli and L. monocytogenes. S-CDs embedded pectin/gelatin films can be employed in food packaging-related applications to increase the shelf life of foods and assure food safety. Glucose carbon dots (GCDs) were made by utilizing glucose as the carbon source and doped with sulfur, nitrogen, and boron to improve their functioning [159]. The NGCD, in particular, has the highest antioxidant activity of all the GCD. The S-doped GCD (SGCD) and B-doped GCD (BGCD) showed higher antibacterial activity against L. monocytogenes and E. coli. Both bacterial strains are susceptible to the NGCD, which has the strongest antibacterial action. The NGCD possesses substantial antifungal activity against P. citrinum, Ammophilus fumigatus, R. rubra, and C. albicans, but the SGCD effectively inhibits the F. solani growth. When mouse fibroblast L929 cells are exposed to a high dosage of 500 g/mL for 72 h, 80% of cells remained alive, indicating the low toxicity of CDs. The Ag,NCQDs are efficient antimicrobials against E. coli and S. aureus [160]. Antibacterial activity was evaluated by using S. aureus and E. coli colonies, and the morphologies of the bacteria were investigated by using SEM. The inhibitory effect is sequenced in spread plate tests: NCQDs > Ag,NCQDs > Ag-CQDs > NCQDs > Ag solution. As a result, doping Ag into N-CQDs is a more effective technique to improve antibacterial activity than mixing N-CQDs and Ag. Furthermore, Ag,NCQDs have antibacterial activity against both bacteria colonies, whereas the NCQDs counterpart has an inhibitory effect solely on E. coli. The lowest Ag,NCQDs preventive concentrations against E. coli and S. aureus are 250 and 200 µg mL −1 , respectively.

Cell Imaging
The development of fluorescent tools can improve the ability to probe biological dynamics [161]. The CDs-HS18 was employed as the fluorescent dyes for cell imaging study in the Saccharomyces cerevisiae (S. cerevisiae) bacteria, Jasminum mesnyi Hance (JMH) leaf, onion skin, and microworms via in vivo and in vitro methods, respectively [162]. The fluorescence signals found in JMH are in the leaf veins, revealing the supply tendency to the tip from the petiole, which is constant with the movement of H 2 O in the leaves, indicating the distribution of CDs in the leaves is mainly driven by transpiration. The confocal laser scanning microscopy (CLSM) data show that multicolored fluorescent signals can be found in the pharynx, somatic cells, pseudocoelom, and digestive system of the microworms. However, the signal strength in the mouth cavity and the tail is lower than in other organs. Wang et al. [162] assume that CDs are ingested by microworms by ingestion. Due to the exceptional biocompatibility of CDs, CDs enter the gastrointestinal tract through the oral cavity and pharynx, spreading to the somatic cells and pseudocoelom, and eventually being expelled through the anus. Microworm deaths were not seen during the test, implying that the regular metabolic activities of microworms were maintained. Figure 13 presents the synthesis of Ca-, N-, and S-doped CDs (Mis-mPD-CDs) from plant Miswak and mPD and their application for cell-imaging and intracellular CR sensing. The negatively charged and green-emitting Mis-mPD-CD can enter and label bacteria, fungi, and animal cells for long periods with excellent biocompatibility and stability. The Mis-mPD-CDs were used to detect and image CR in living cells (C. albicans, A549, S. aureus, and E. coli) and zebrafish. Mis-mPD-CDs were proved to be useful for the quantitative sensing of CR in real samples of industrial wastewater and fish tissues, as well [163]. The level of folic acid in the human body can be a useful indicator for evaluating the body's normal physiological activities and can provide information about cell growth and reproduction [164]. A high level of FA can cause a variety of diseases. The fluorescence spectra of N-CDs were quenched after the addition of folic acid due to the synergistic effects of the static quenching mechanism and internal filtering effect (IFE). Within the folic acid concentration range of 0-200.0 M, the LOD was 28.0 nM (S/N = 3) under optimal conditions. Furthermore, N-CDs were used to sense folic acid in the real samples, including urine and fetal bovine serum, with a quantitative addition recovery rate of 99.6%-100.7%. The experimental results revealed that N-CDs exhibit low toxicity, excellent cell imaging performance, and quantitative folic acid analysis. In the daylight, aqueous dispersions of Morus nigra CDs (M-CDs) exhibit a brownish-yellow color and cyan-blue light emission under UV light irradiation [165]. M-CDs show characteristic excitation-dependent emission with a high QY of 24%. M-CDs exhibit a high QY of 24% in a characteristic excitation-dependent emission manner. M-CDs were used as fluorescent sensors for the sensitive and selective Fe 3+ sensing via fluorescence quenching, with a detection ranging from 5 to 30 µm, and an LOD of 0.47 µM. Furthermore, M-CDs were applied to stain human colon cancer (HTC-116) cells for cell viability and microscopic analysis. The M-CDs-conjugated HTC-116 cells emitted blue, green, and red light when excited through 405, 488, and 555 nm filters, respectively. An efficient and environmentally friendly process for the preparation of nitrogen-doped carbon dots (N-CDs) was reported, using chicken waste Galli Gigerii Endothelium Corneum (GGEC) as a precursor [166]. Surprisingly, N-CDs can be used as a sensor for the selective and sensitive detection of nitroimidazoles, such as metronidazole, tinidazole, ornidazole, and secnidazole, using internal filtration effect (IFE) and static quenching mechanisms. N-CDs were proved to be effective for detecting nitroimidazoles in some real samples (e.g., chicken, plasma, and tablets). Moreover, N-CDs also showed great potential in the bioimaging application. The Kiwifruit-peel carbon dots (KFP-CDs) were successfully prepared from kiwi fruit peels without using a capping/passivation agent [167]. KFP-CDs were useful for the fabrication of novel fluorescent inks. When exposed to UV light, the images and words were instantly visible. Furthermore, when evaluated for the cell-imaging application in human cell lines, KFP-CDs are biocompatible and have low cytotoxicity. The findings suggest that KFP-CDs can be used as a cell labeling agent for the in vitro imaging of cancer cells and normal cells.
Most CDs that can discriminate between live and dead cells exhibit excitation-wavelength-dependent fluorescence and low photoluminescence quantum yields [168]. It may cause problems such as possible fluorescence overlap with the other fluorescent probe. Meanwhile, it is not feasible for dual-color live/dead staining. Therefore, developing CDs with high photoluminescence quantum yields and excitation-wavelength-independent The level of folic acid in the human body can be a useful indicator for evaluating the body's normal physiological activities and can provide information about cell growth and reproduction [164]. A high level of FA can cause a variety of diseases. The fluorescence spectra of N-CDs were quenched after the addition of folic acid due to the synergistic effects of the static quenching mechanism and internal filtering effect (IFE). Within the folic acid concentration range of 0-200.0 M, the LOD was 28.0 nM (S/N = 3) under optimal conditions. Furthermore, N-CDs were used to sense folic acid in the real samples, including urine and fetal bovine serum, with a quantitative addition recovery rate of 99.6-100.7%. The experimental results revealed that N-CDs exhibit low toxicity, excellent cell imaging performance, and quantitative folic acid analysis. In the daylight, aqueous dispersions of Morus nigra CDs (M-CDs) exhibit a brownish-yellow color and cyan-blue light emission under UV light irradiation [165]. M-CDs show characteristic excitation-dependent emission with a high QY of 24%. M-CDs exhibit a high QY of 24% in a characteristic excitationdependent emission manner. M-CDs were used as fluorescent sensors for the sensitive and selective Fe 3+ sensing via fluorescence quenching, with a detection ranging from 5 to 30 µm, and an LOD of 0.47 µM. Furthermore, M-CDs were applied to stain human colon cancer (HTC-116) cells for cell viability and microscopic analysis. The M-CDs-conjugated HTC-116 cells emitted blue, green, and red light when excited through 405, 488, and 555 nm filters, respectively. An efficient and environmentally friendly process for the preparation of nitrogen-doped carbon dots (N-CDs) was reported, using chicken waste Galli Gigerii Endothelium Corneum (GGEC) as a precursor [166]. Surprisingly, N-CDs can be used as a sensor for the selective and sensitive detection of nitroimidazoles, such as metronidazole, tinidazole, ornidazole, and secnidazole, using internal filtration effect (IFE) and static quenching mechanisms. N-CDs were proved to be effective for detecting nitroimidazoles in some real samples (e.g., chicken, plasma, and tablets). Moreover, N-CDs also showed great potential in the bioimaging application. The Kiwi-fruit-peel carbon dots (KFP-CDs) were successfully prepared from kiwi fruit peels without using a capping/passivation agent [167]. KFP-CDs were useful for the fabrication of novel fluorescent inks. When exposed to UV light, the images and words were instantly visible. Furthermore, when evaluated for the cell-imaging application in human cell lines, KFP-CDs are biocompatible and have low cytotoxicity. The findings suggest that KFP-CDs can be used as a cell labeling agent for the in vitro imaging of cancer cells and normal cells.
Most CDs that can discriminate between live and dead cells exhibit excitation-wavelengthdependent fluorescence and low photoluminescence quantum yields [168]. It may cause problems such as possible fluorescence overlap with the other fluorescent probe. Meanwhile, it is not feasible for dual-color live/dead staining. Therefore, developing CDs with high photoluminescence quantum yields and excitation-wavelength-independent emission becomes an important task. Excitation-wavelength-independent sulfur-doped CDs (S-CDs) were used to demonstrate that the S-CDs could distinguish dead cells from live cells for fungal, bacterial, and animal cells. Yu et al. [168] found that S-CDs could reach the interior of the dead cells, allowing the visualization of these cells. On the other hand, live cells cannot be stained by S-CDs because S-CDs cannot enter the live cells. Moreover, compared with the commercial live/dead staining dye propidium iodide, S-CDs showed better photostability and biocompatibility, indicating a promising future in cell viability assessment and cell-imaging applications.

CDs for Pollutant Sensing
Compared with conventional analysis techniques, fluorescence-based sensing has attracted attention because it has some advantages, including great sensitivity, rapid response time, easy operation, low cost, and efficiency [169,170]. Among the various fluorescent carbon nanomaterials, CDs have captivated scientists' interest due to their ability to combine the essential selective receptors on their surface. Due to the ease with which CDs can be surface modified to employ a wide range of intrinsic functional groups, they can rapidly improve their selectivity against specific targets. Theoretically, any fluorescence changes attributed to the concentration of various analytes, such as wavelength, intensity, anisotropy, or longevity, have the possibilities to be used as sensors.
Ji et al. [171] studied the N-doped carbon dots mediated by cetyltrimethylammonium bromide (CTAB/NCDs). The spatial structure created by CTAB/NCDs can selectively collect Hg 2+ and cause fluorescence quenching by interacting with the surface functional groups of NCDs. The N-doped carbon dots (NCDs) were made in a hydrothermal synthesis. The NCDs were discovered to offer potential as a fluorescence sensor for detecting Hg 2+ [172]. Static quenching of NCDs by Hg 2+ could be a viable detection technique. The findings suggested that an NCDs-based sensor could detect Hg 2+ in a real beverage sample. Zhang et al. [173] reported that red fluorescent InP/ZnS quantum dots (InPQDs), MOFs (ZIF-8), and blue-fluorescent carbon dots (CDs) were merged into nanosensor CDs/InPQDs@ZIF-8 for the successive optical detection of Hg 2+ , utilizing an in situ synthesis approach. The nano-low-sensor's detection limit and its high specificity and accuracy meet the requirements for safe Hg 2+ regulating and checking in environmental and drinking water. Furthermore, color recognition and processing software put on a smartphone may detect Hg 2+ in real time and at a high rate. The CDs doped with Eu 3+ ions (Eu-CDs) were produced hydrothermally, utilizing citric acid and urea as precursors and Eu(NO 3 ) 3 as a europium source by Correia et al. [174]. The Eu 3+ ions are strongly linked to the carboxylate groups on the CDs' surfaces and incorporated into the carbon core's nanographene network. CDs doped with Eu 3+ have higher diameters than CDs that are not doped, but they are split into smaller sp 2 carbon domains. Hg 2+ and Ag + greatly quench the CDs' luminescence, but other cations had no effect. Depending on the ion's nature, the quenching mechanism varies greatly. The blue emission of CDs is affected by the presence of Ag + . In the case of Hg 2+ , the blue emission of CDs and the red emission of Eu 3+ are quenched. Shen et al. [175] used Shewanella oneidensis MR-1 to make fluorescent carbon dots (CDs@MR-1) via a hydrothermal procedure to detect Hg 2+ and tetracycline water samples. The limits of detection for Hg 2+ and tetracycline were 0.43 and 0.21 µg·mL −1 , respectively. The internal filtration effect (IFE) causes the fluorescence quenching for tetracycline. On the other hand, along with IFE, dynamic quenching and static quenching mechanisms are involved in detecting Hg 2+ . The CDs@MR-1 can also identify Gram-positive bacteria from Gram-negative bacteria ( Figure 14). An Fe/N-doped CDs (CDBFe) catalytic amplification Apt method was developed for sensitive and fast SERS/RRS/Abs tri-mode sensing of ultra-trace Pb(II) ions [176]. There is an excellent linear relationship between the SERS intensity and Pb(II) concentration between 1.3 and 16 pM (picomole). Moreover, As(III) and Hg(II) can also be monitored by this assay. A dual-emission ratio fluorescent sensor, N-CDs/R-CDs@ZIF-8, was prepared by mixing N-doped blue fluorescence carbon dots and red fluorescence carbon dots in the environment of ZIF-8 [177]. The addition of Pb 2+ has a great effect on inhibiting the fluorescence of N-CDs, while showing little influence on the fluorescence of R-CDs. As a result, N-CDs/R-CDs@ZIF-8 can act as a ratiometric fluorescence probe for Pb 2+ sensing. The functionalized-GQD (F-GQD) was prepared by the edge functionalization of graphene quantum dots (GQDs) by 2,6-diaminopyridine molecules and served as a fluorescent nanoprobe [178]. The F-GQD is a suitable pH sensor in the 2-6 pH range because its fluorescence is very sensitive to the pH of the medium. With the addition of Pb 2+ , F-GQD exhibits fluorescence turn-on performance. The fluorescence enhancement could be due to nanodot aggregation caused by Pb 2+ . A fluorescent NCDs (nitrogen-doped carbon dots) sensor can detect the presence of divalent heavy metal ions. The detection limits of Pb 2+ and Cu 2+ can reach 3 and 15 ppb, respectively [179]. Additionally, the UV absorbance spectra of NCDs were used to detect Pb 2+ in a dose-dependent manner.
Cr 6+ is a toxic heavy metal that bacteria cannot decompose or consume. As a result, the development of a highly selective and sensitive method to detect Cr 6+ is urgently needed. Wang et al. [162] designed the fluorescence CDs-HS18, prepared from Ureibacillus thermosphaericus, using a hydrothermal method. The CDs-HS18-based fast and selective sensing platform displayed an outstanding linear relation for Cr 6+ between 0 and 9 µM, with a limit of detection of 36 nM. A majority of reported Cr 6+ sensors are based on the fluorescence quenching method, whereas the one reported by Chen et al. [180] is performed by the fluorescence enhancement mechanism. Results of TEM, FTIR, X-ray photoelectron spectroscopy, and XRD analysis revealed that Cr 6+ could increase the probe's fluorescence. The detection phenomenon was established by conferring the affiliation between the quenching efficiency of CDs-Kan fluorescence intensity and the concentration of Cr 6+ [181].
N-and-S-co-doped carbon dots (NSCDs) can act as fluorescent probes to detect Cr 3+ . These NSCDs exhibited strong fluorescence that was quickly quenched by Cr 3+ even in the presence of other competing ions, demonstrating that NSCD probes have outstanding selectivity and anti-interference ability. The mechanism of fluorescence quenching of NSCD by Cr 3+ ion may result from the increased non-radiation emission due to the deteriorated NSCD surface. These non-toxic and biocompatible NSCDs could be used to detect Cr 3+ in living cells [182]. An orange-emission carbon dots (OCDs) were created by using a hydrothermal method. According to TEM analysis, the new OCDs were similar in size, with an An Fe/N-doped CDs (CDBFe) catalytic amplification Apt method was developed for sensitive and fast SERS/RRS/Abs tri-mode sensing of ultra-trace Pb(II) ions [176]. There is an excellent linear relationship between the SERS intensity and Pb(II) concentration between 1.3 and 16 pM (picomole). Moreover, As(III) and Hg(II) can also be monitored by this assay. A dual-emission ratio fluorescent sensor, N-CDs/R-CDs@ZIF-8, was prepared by mixing N-doped blue fluorescence carbon dots and red fluorescence carbon dots in the environment of ZIF-8 [177]. The addition of Pb 2+ has a great effect on inhibiting the fluorescence of N-CDs, while showing little influence on the fluorescence of R-CDs. As a result, N-CDs/R-CDs@ZIF-8 can act as a ratiometric fluorescence probe for Pb 2+ sensing. The functionalized-GQD (F-GQD) was prepared by the edge functionalization of graphene quantum dots (GQDs) by 2,6-diaminopyridine molecules and served as a fluorescent nanoprobe [178]. The F-GQD is a suitable pH sensor in the 2-6 pH range because its fluorescence is very sensitive to the pH of the medium. With the addition of Pb 2+ , F-GQD exhibits fluorescence turn-on performance. The fluorescence enhancement could be due to nanodot aggregation caused by Pb 2+ . A fluorescent NCDs (nitrogen-doped carbon dots) sensor can detect the presence of divalent heavy metal ions. The detection limits of Pb 2+ and Cu 2+ can reach 3 and 15 ppb, respectively [179]. Additionally, the UV absorbance spectra of NCDs were used to detect Pb 2+ in a dose-dependent manner.
Cr 6+ is a toxic heavy metal that bacteria cannot decompose or consume. As a result, the development of a highly selective and sensitive method to detect Cr 6+ is urgently needed. Wang et al. [162] designed the fluorescence CDs-HS18, prepared from Ureibacillus thermosphaericus, using a hydrothermal method. The CDs-HS18-based fast and selective sensing platform displayed an outstanding linear relation for Cr 6+ between 0 and 9 µM, with a limit of detection of 36 nM. A majority of reported Cr 6+ sensors are based on the fluorescence quenching method, whereas the one reported by Chen et al. [180] is performed by the fluorescence enhancement mechanism. Results of TEM, FTIR, X-ray photoelectron spectroscopy, and XRD analysis revealed that Cr 6+ could increase the probe's fluorescence. The detection phenomenon was established by conferring the affiliation between the quenching efficiency of CDs-Kan fluorescence intensity and the concentration of Cr 6+ [181].
N-and-S-co-doped carbon dots (NSCDs) can act as fluorescent probes to detect Cr 3+ . These NSCDs exhibited strong fluorescence that was quickly quenched by Cr 3+ even in the presence of other competing ions, demonstrating that NSCD probes have outstanding selectivity and anti-interference ability. The mechanism of fluorescence quenching of NSCD by Cr 3+ ion may result from the increased non-radiation emission due to the deteriorated NSCD surface. These non-toxic and biocompatible NSCDs could be used to detect Cr 3+ in living cells [182]. An orange-emission carbon dots (OCDs) were created by using a hydrothermal method. According to TEM analysis, the new OCDs were similar in size, with an average particle size of 4-7 nm [183]. The fluorescence of OCDs was effectively quenched by Cr 3+ . Notably, OCDs were effectively used for cell-level fluorescence detection of Cr 3+ ions. N-doped carbon quantum dots (N-CQDs) were created by a top-down method, i.e., •OH radical opening of fullerene with H 2 O 2 in a basic environment with ammonia for two different reaction times [184]. N-CQDs were tested for metal ion detection in aqueous solutions and during bioimaging. They exhibited a shift in Cu 2+ and Cr 3+ selectivity at a greater extent of -NH 2 functionalization.
The copper-doped CDs (Cu-CDs) prepared by a pyrolysis approach were found to have peroxidase-mimicking activity, which reduced the fluorescent intensity of the Cu-CDs-mediated o-phenylenediamine (OPD) oxidized to a fluorescent 2,3-diaminophenazine (DAP) in the presence of H 2 O 2 via the generation of •OH radicals. The Cr 6+ species were reduced to Cr 3+ by using the reductant 2,2 -azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS). Moreover, the fluorescent intensity of DAP is proportional to the Cr 3+ content within the range of 5 × 10 −6 to 1.5 × 10 −4 M. It can be used to determine Cr 6+ and Cr 3+ levels in water samples ( Figure 15) [185].
Polymers 2022, 14, x FOR PEER REVIEW 26 of 42 average particle size of 4-7 nm [183]. The fluorescence of OCDs was effectively quenched by Cr 3+ . Notably, OCDs were effectively used for cell-level fluorescence detection of Cr 3+ ions. N-doped carbon quantum dots (N-CQDs) were created by a top-down method, i.e., • OH radical opening of fullerene with H2O2 in a basic environment with ammonia for two different reaction times [184]. N-CQDs were tested for metal ion detection in aqueous solutions and during bioimaging. They exhibited a shift in Cu 2+ and Cr 3+ selectivity at a greater extent of -NH2 functionalization. The copper-doped CDs (Cu-CDs) prepared by a pyrolysis approach were found to have peroxidase-mimicking activity, which reduced the fluorescent intensity of the Cu-CDs-mediated o-phenylenediamine (OPD) oxidized to a fluorescent 2,3-diaminophenazine (DAP) in the presence of H2O2 via the generation of •OH radicals. The Cr 6+ species were reduced to Cr 3+ by using the reductant 2,2′-azino-bis (3-ethylbenzothiazoline-6-sulfonate) (ABTS). Moreover, the fluorescent intensity of DAP is proportional to the Cr 3+ content within the range of 5 × 10 −6 to 1.5 × 10 −4 M. It can be used to determine Cr 6+ and Cr 3+ levels in water samples ( Figure 15) [185]. Fluorescent CQDs synthesized from crab-shell waste by a hydrothermal process exhibited excellent Cd 2+ sensing and antibacterial activity. CQDs exhibited bright green fluorescence and exorbitant quenching of Cd 2+ ions in aqueous media under UV light irradiation [186]. Wang et al. [187] reported gold nanoclusters (AuNCs) as fluorescence molecules with red emission, while N-and-S-co-doped CDs (N,S-CDs) were reported as signaling molecules. The fluorescence quenching is caused by the fluorescence resonance energy transfer (FRET) effect between N,S-CDs and AuNCs. The added Cd 2+ can interact with AuNCs to form complexes and interact with the O atoms in the carboxyl and hydroxyl groups of N,S-CDs, resulting in a stable complex, causing N,S-CDs/AuNCs to aggregate, and then resulting in enhanced fluorescence. The addition of Cd 2+ led to increased fluorescence intensities of N,S-CDs (F435) and AuNCs (F630).
Wu et al. [188] synthesized green fluorescence CDs (GCDs) by using 1,4-dihydroxyanthraquinone. The GCDs based sensor is used as a dual-mode visual sensor based on "off-on" fluorescence analysis to detect Cu 2+ and glyphosate. Turn-off fluorescence was observed when adding Cu 2+ to GCDs, indicating a strong binding interaction between Fluorescent CQDs synthesized from crab-shell waste by a hydrothermal process exhibited excellent Cd 2+ sensing and antibacterial activity. CQDs exhibited bright green fluorescence and exorbitant quenching of Cd 2+ ions in aqueous media under UV light irradiation [186]. Wang et al. [187] reported gold nanoclusters (AuNCs) as fluorescence molecules with red emission, while N-and-S-co-doped CDs (N,S-CDs) were reported as signaling molecules. The fluorescence quenching is caused by the fluorescence resonance energy transfer (FRET) effect between N,S-CDs and AuNCs. The added Cd 2+ can interact with AuNCs to form complexes and interact with the O atoms in the carboxyl and hydroxyl groups of N,S-CDs, resulting in a stable complex, causing N,S-CDs/AuNCs to aggregate, and then resulting in enhanced fluorescence. The addition of Cd 2+ led to increased fluorescence intensities of N,S-CDs (F435) and AuNCs (F630).
Wu et al. [188] synthesized green fluorescence CDs (GCDs) by using 1,4-dihydroxyanthraqu inone. The GCDs based sensor is used as a dual-mode visual sensor based on "off-on" fluorescence analysis to detect Cu 2+ and glyphosate. Turn-off fluorescence was observed when adding Cu 2+ to GCDs, indicating a strong binding interaction between GCDs and Cu 2+ ions. While turn-on fluorescence was observed after adding glyphosate to GCDs/Cu 2+ , these studies suggest that Cu 2+ has more binding sites to glyphosate than other competing pesticides. A GCDs-based smart-sensing membrane was applied to detect glyphosate on vegetable surfaces. The QY of NCDs can be elevated by liquid-liquid extraction and purification. NCDs have the potential to be an excellent multifunctional sensing platform for Cu 2+ and tetracycline antibiotics (TCs) such as oxytetracycline (OTC), tetracycline hydrochloride (TC), demeclocycline hydrochloride (DMC), doxycycline hydrochloride (DC), and minocycline hydrochloride (MC). Compared to other coexisting metal ions and antibiotics, this platform responded differently to Cu 2+ and TCs. IFE and static quenching were identified as the fluorescent quenching mechanisms of Cu 2+ and TCs to NCDs. The platform was successfully applied to detect Cu 2+ and TCs in real samples, including water, milk, and urine [189]. The precursors, synthesis methods, quantum yields, target analytes, linear ranges, and detection limits of various CDs for the sensing applications are summarized in Table 2.

Possible Applications and Roles of Various CDs
Up to the present, the possibility of the use of CDs in a wide range of applications has been reported, including bioimaging, catalysis, energy, sensing, therapy, fertilizer, separation, security authentication, food packing, and flame retardant. Due to the unique structure, adjustable chemical, physical, optical, and electronic properties of CDs, they can play different roles in diverse applications. The roles of CDs in these applications are also summarized in Table 3.   Hsd-CPDs surface contains bioactive moieties of apocynin, and guaiacol binds with the proteins of enterovirus A71 (EV-A71), thus blocking the viral attachment in neonatal mice.

Summary and Future Perspective
The unique properties of CDs, such as good biocompatibility, high photostability, excellent light-harvesting, up-conversion, effective electron transfer, and bandgap narrowing, make the CDs promising nanomaterials for applications in many fields. This review mainly focused on the recent progress of CDs in bioimaging, sensing, therapy, energy, fertilizer, separation, security authentication, food packing, flame retardant, and co-catalyst for environmental remediation applications. In addition, key factors affecting the properties and performance of CDs were listed in this review. The properties and performance of CDs can be tuned by some synthesis parameters, such as the incubation time and precursor ratio of the microwave-assisted process, the laser pulse width of the laser ablation synthesis, and the average molar mass of polymeric precursor. Surface passivation also has a significant influence on the particle size of CDs. Moreover, some factors affect the properties and performance of CDs, such as the polarity-sensitive fluorescence effect and concentration-dependent multicolor luminescence, together with the size and surface states of CDs.
Future efforts about CDs-based nanomaterials can focus on improving their fabrication, purification, and characterization technology, as well as expanding versatile applications and realizing structure/composition/property correlation and basic fluorescence mechanism. Some opportunities and challenges for future study are listed as follows.
(1) Commercial-scale fabrication: A lot of researchers have been devoted to the study of CDs and expanding their use in a wide range of applications. However, the complicated synthesis process and low yield hinder the commercial-scale fabrication and industrial application of CDs. Developing robust large-scale synthetic methods for high-quality CDs is critical to their applications. Although some progress in the fabrication of kilogram-scale CDs has been reported in the past three years (see Section 2.1.3), the scale-up synthesis of CDs still faces some challenges, such as low-cost synthesis procedures, batch-to-batch consistency, and efficient purification techniques. In the future, we should develop low-cost synthesis procedures, efficient purification techniques, and novel technology for reducing the by-products and improving the reproducibility of CDs concerning their size and quantum yield.
(2) Surface functional groups: During the synthesis, various functional groups (amine, carboxylic acid, and hydroxyl groups) can be generated on the surface of CDs. The appropriate control of these functional groups is quite important for elevating the sensing and catalysis performance of CDsbased materials. For example, appropriate surface functional groups support the efficient attachment of CDs co-catalyst on the catalyst's surface, thereby providing more active sites for improving the photocatalytic activity of the composite photocatalyst. Compared with bare CDs, functionalized CDs with excellent fluorescence properties and various functional groups exhibit better selectivity and sensitivity for detecting specific analytes. In the future, robust surface modification to generate reproducible surface functional groups on CDs will be a crucial factor for the real-world application of CDs.
(3) Structure/composition-property correlation: Specific hurdles should be overcome for the application of functional CDs in different fields. For example, fluorescent sensing needs CDs-based probes with a high quantum yield. Biocompatibility is the primary requirement for nanomedicine applications. The good electrical conductivity and morphology of CDs and close contact between CDs and catalysts are essential for catalytic applications. Since the certain property of CDs is critical for some specific applications, it is important to tune the CDs' properties. Therefore, exploring the composition-property correlation or the key factors for certain properties of CDs will be an important topic for future work.
(4) Photoluminescence property: Stable fluorescence is required for the LED-based application. Environment-dependent or analyte-dependent fluorescent response is critical to sensor applications. The development of CDs with long-wavelength-emitting properties and up-conversion luminescence is significant for exploring useful sensors. Hence, the mechanism of photoluminescence, the technology for tuning the light-emissive wavelength of CDs, and the improvement of the fluorescence quantum yield should be studied in the future.

(5) Characterization techniques:
It was reported that the existence of CDs in most cases could not be confirmed by XRD spectra, due to the low crystallinity and high dispersion of CDs [8]. In the future, the use of appropriate characterization methods is quite important for the research of CDs-based materials. For example, the presence of CDs in the photocatalyst composites can be determined by measuring their chemical compositions (by FTIR and XPS), morphology (TEM), and properties (UV-Vis spectra, photoluminescence spectra, photocurrent, and electrochemical impedance spectroscopy). Moreover, the synchrotron NEXAFS test is a useful tool to find out the electronic states and photophysics of CDs-based materials (Section 3.2.4). The correlation between emission performance and intricate structural features of doped CDs can be analyzed by the synchrotron NEXAFS tests, X-ray photoelectron spectroscopy, and steady-state and time-resolved optical spectroscopy [112].