English

• 1. Introduction

  In recent years, rapid development across various industries has led to severe environmental pollution. Among these, the discharge of wastewater containing a large amount of highly toxic, persistent, and difficult-to-degrade pollutants poses a threat to both ecological environments and human health [1-4]. Therefore, the development of efficient techniques to solve water pollution is an urgent problem for researchers [5]. Traditional quantum dots, due to their excellent optical properties, have been widely used for the detection and removal of pollutants from wastewater. However, traditional quantum dots often contain cadmium-based Ⅱ-Ⅵ and lead-based Ⅳ-Ⅵ elements, which can cause secondary pollution during wastewater treatment and lack environmental friendliness [6, 7]. Therefore, there is a need for further development of environmentally friendly materials to achieve the efficient treatment of water pollution.

  In 2004, while purifying single-walled carbon nanotubes synthesized by arc discharge using electrophoresis, Xu et al. [8] accidentally obtained a water-soluble and fluorescent carbon nanoparticle, which is known as carbon dots (CDs) [9]. CDs are zero-dimensional carbon particles with a size smaller than 10 nm and quasi-spherical morphology, with a lattice structure similar to graphitized carbon, primarily composed of sp² and sp³ hybridized carbon atoms [10]. Compared to traditional quantum dots, CDs exhibit excellent biocompatibility, photoluminescence (PL), low toxicity, chemical inertness, and photo-induced electron transfer properties, making them promising alternative materials to quantum dots [11, 12].

  The preparation cost of CDs is low and there are numerous methods available for their synthesis. Theoretically, most carbon-containing materials can be used to prepare CDs. Small molecules such as activated carbon, citric acid, amino acids, p-phenylenediamine, chitosan [13-18], as well as heterocyclic compounds [19], biomass (sodium alginate [20], guar gum [21], pullulan [22]), waste (acrylate waste fibers [23]), and organic macromolecules (starch [24]), have all been used as carbon sources for the preparation of CDs. Fig. 1 summarizes the carbon sources reported in the preparation of CDs over the past decade (Fig. 1). Utilizing these abundant materials for CDs synthesis significantly reduces production costs. The synthesis methods for CDs can be classified into top-down and bottom-up approaches based on the carbon source [25], among which hydrothermal/solvothermal method, microwave method, and pyrolysis methods are the most widely applied [26-28]. There are also emerging methods for preparing CDs in recent years, including the magnetic hyperthermia method [29], ball milling method [30]. By selecting appropriate carbon sources and synthesis methods, CDs with rich surface functional groups can be prepared, thereby imparting specific properties to the synthesized CDs. At present, the synthesis methods for CDs have matured to a certain extent, although the PL mechanisms of CDs remain controversial, but in recent years, researchers have reached a certain degree of consensus, suggesting that the PL emission mechanisms of CDs include surface states, carbon core states, molecular states, and Crosslink‐Enhanced Emission (CEE) [31-34].

  Figure 1

  

  Figure 1.  Carbon sources for preparing CDs in recent ten years.

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  In recent years, there have been numerous reviews on the preparation and application of CDs. For instance, Emam [35] has conducted a comprehensive review of the synthesis and biomedical applications of PL CDs, utilizing biopolymers for their production. Besides focusing on various biological-based carbon sources such as polysaccharides, starch macromolecules, kitchen wastes, lignin macromolecules, and chitosan as precursors for CDs, the review also covers their biomedical applications in areas like sensing and transfecting properties, drug delivery, and bone tissue engineering. In 2024, the research team [36] published another review on the chemical properties and potential applications of CDs derived from polysaccharides. This review particularly concentrated on the applications of CDs in the areas of soil fertilization, metal detection, textile finishing, and biological activities. It also summarized the preparation methods of CDs published in recent years, including the top-down and bottom-up strategies. The author believed that the PL mechanism of CDs is not yet fully understood, but it involves different mechanisms such as quantum size effect, surface defect state, and molecular-like states. In another review, Omer et al. [37] summarized the application of CDs-based fluorescence detection methods in pharmaceutical analysis. The article studied the sensing mechanisms of fluorescence detection, including static quenching, dynamic quenching, hydrogen bonding, inner filter effect, fluorescence resonance energy transfer, and turn-off-on. At the same time, the review also investigated other detection techniques based on CDs for drug analysis, such as colorimetry, electrochemiluminescence, surface-enhanced Raman spectroscopy, and surface plasmon resonance.

  At present, due to the various excellent properties of CDs, including elemental-doped CDs and CDs-based composite materials, they have been widely and successfully applied in the detection and removal of pollutants in wastewater. However, the existing reviews on the preparation of CDs and their applications in wastewater treatment are quite limited and incomplete. Therefore, differing from the previously mentioned reviews [35-37], this review, considering the scarcity of comprehensive overviews on the role of CDs in wastewater treatment, emphasizes the versatility of CDs in this field. Since the preparation methods and PL properties of CDs directly affect their applications in the detection and removal of pollutants in wastewater treatment, this review also highlights the diversity and comprehensiveness of CDs preparation, alongside the widely accepted PL mechanisms.

  Based on the above considerations, this review comprehensively summarizes the synthesis methods of CDs, their PL mechanisms, and their applications in wastewater treatment (Fig. 2). The paper first introduces the structure and classification of CDs to understand the different classifications of CDs and their structures. And then describes the PL mechanisms of CDs, including surface states, carbon core states, molecular states, and CEE. In addition, this article focuses on the synthesis methods of CDs, summarizing the research progress of CDs synthesis methods in recent years. Finally, it outlines the applications of CDs in wastewater treatment, including sensing, removal of inorganic pollutants, and degradation of organic pollutants. Building upon this foundation, the challenges and prospects of CDs development are discussed.

  Figure 2

  

  Figure 2.  A summary of the synthesis process, classification, PL mechanisms, and applications of CDs.

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  2. Structure and classification of CDs

  CDs are commonly defined as quasi-zero-dimensional carbon nanoparticles with bright fluorescence. CDs are composed of carbon cores and passivated surface (Fig. 3a). The carbon cores form the skeleton of the CDs, typically < 10 nm in size. The carbon cores can be sp² hybridized graphene fragments, carbon nanoparticles composed of both sp² and sp³ hybridized carbons, or even nanoparticles constructed from non-conjugated polymers. Generally, CDs form passivated surface by bonding with other groups [10].

  Figure 3

  

  Figure 3.  (a) Schematic diagram of the structure of CDs. Reproduced with permission [10]. Copyright 2019, Royal Society of Chemistry. (b) The classification of CDs. Reproduced with permission [44]. Copyright 2022, Springer Nature. (c) Different edge types of GQDs. Reproduced with permission [46]. Copyright 2020. Royal Society of Chemistry. (d) Schematic diagram of the structure of CQDs. Reproduced with permission [11]. Copyright 2022, John Wiley and Sons.

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  CDs can be classified based on different carbon cores, surface groups, and properties [38]. However, researchers have different viewpoints on the classification of CDs. Cayuela's group [39] divided CDs into three types: Graphene quantum dots (GQDs), carbon quantum dots (CQDs), and carbon nanodots (CNDs), and Liu's group [40] had the same viewpoint. Hu [10], Yao [41], and Chen et al. [42] also divided CDs into three categories, which differ from the previous two teams by dividing CDs into GQDs, CNDs, and carbonized polymer dots (CPDs). In 2019, Xia et al. [38] classified CDs into four categories: GQDs, CQDs, CNDs, and CPDs, which was unanimously recognized by Chung [43], Zhai [11], and Ðorđević [44] (Fig. 3b). And most researchers believe that CDs can be divided into four categories. Li et al. [45] proposed a new perspective on the classification of CDs. They believed that graphitic carbon nitride quantum dots (CNQDs) should also be considered as a type of CDs. Therefore, they classified CDs into five types. However, this classification method is not universal.

  GQDs are a disk-shaped form of graphene sheets with a carbon core composed of a single layer or a few layers of graphene. Their lateral dimensions exceed their longitudinal dimensions, and various types of chemical functional groups are attached to the edges. The chemical characteristics of these functional groups determine the optical physics and photochemical properties of GQDs (Fig. 3c). A fundamental feature of GQDs is the presence of the graphene lattice, which exhibits high crystallinity with a large sp² hybridized carbon structure [44, 46]. CQDs exhibit a quasi-spherical shape with a multi-layered graphite structure where the lateral and longitudinal dimensions are similar (Fig. 3d). As a result, they possess crystallinity arising from the graphene lattice and interlayer stacking [11, 34, 44]. CNDs are also spherical with chemical functional groups on their surface. The carbon cores are composed of sp²/sp³ hybridized carbon or amorphous carbon, exhibiting no distinct crystallinity [10, 11]. CPDs exhibit a quasi-spherical shape and are a type of carbon nanoparticles. They possess a polymer structure or a polymer/carbon hybrid structure, with abundant polymer chains on the surface and rich functional groups. They are formed through the polymerization, carbonization, and crosslinking of small molecules or linear polymers, resulting in a slightly carbonized carbon core [47-49].

  3. Photoluminescence mechanisms

  In recent years, CDs have attracted widespread attention in scientific research due to their unique optical properties and potential applications in multiple fields. However, the PL mechanism of CDs is not fully understood, which limits their applications in high-performance optical materials. Recent studies have been dedicated to revealing the PL mechanism of CDs. After years of exploration, the currently recognized PL mechanisms of CDs include surface state [25, 50], carbon core state [51], molecular state [52-54], and CEE (Fig. 4) [55, 56].

  Figure 4

  

  Figure 4.  The four currently recognized PL mechanisms for CDs. Surface state. Reproduced with permission [25]. Copyright 2023, Elsevier. Carbon core state. Reproduced with permission [51]. Copyright 2014, Royal Society of Chemistry. Molecular state. Reproduced with permission [52]. Copyright 2022, Springer Nature. CEE. Reproduced with permission [55]. Copyright 2015, John Wiley and Sons.

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  The surface state PL mechanism of CDs is closely related to the surface properties of CDs. Factors such as surface functional groups, heteroatom doping, and surface defects significantly influence the PL properties of CDs. Zhang et al. [25] systematically investigated the formation process of multiple luminescent centers in CDs by controlling the solvent thermal reaction temperature. The research found that CDs synthesized at lower reaction temperatures (140-CDs) exhibited smaller particle sizes (3–4 nm) and emitted predominantly green-yellow light, while CDs synthesized at higher reaction temperatures (180-CDs) showed larger particle sizes (8–9 nm) and enhanced red emission and near-infrared (NIR) emission (Fig. 5a). The chemical structure of CDs was analyzed by XPS, FTIR, and NMR spectroscopy. The results indicate that with the increase of solvent thermal treatment temperature, the nitrogen content in CDs gradually increases, while the carbon and oxygen content undergo changes. This demonstrated that the variation in the surface chemical structure of CDs regulates their PL properties. This research group [50] further attributed the red emission to oxygen-containing surface groups at the surface state through steady-state and time-resolved spectroscopic data and density functional theory (DFT) calculations. These findings provide important insights into understanding the PL mechanism of CDs and help in the rational design of CDs for different PL applications.

  Figure 5

  

  Figure 5.  (a) Mechanisms of PL in the surface state of CDs. Reproduced with permission [25]. Copyright 2023, Elsevier. (b) Calculated emission wavelength using Time-Dependent DFT method in vacuum as a function of the diameter of GQDs. Reproduced with permission [51]. Copyright 2014, Royal Society of Chemistry. (c) Mechanisms of PL in the molecular state of CDs prepared by oPD. Reproduced with permission [52]. Copyright 2022, Springer Nature. (d) Diagram of three categories of CEE in polymers or NCPDs. Reproduced with permission [55]. Copyright 2015, John Wiley and Sons.

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  The carbon core state (also known as quantum size effect) PL mechanism is related to the sp²-hybridized carbon domains within CDs. The size and structure of sp²-hybridized carbon domains directly influence the emission wavelength and intensity. Through DFT and Time-Dependent DFT calculations, Sk's team [51] revealed that the PL properties of GQDs can be sensitively tuned by various factors, including the GQDs size, edge structure, shape, chemical functional groups, defects, and heteroatom hybridization carbon networks. The research found that the PL of GQDs composed of heteroatom hybridized carbon networks was essentially determined by embedded isolated small sp² clusters, which were separated by sp³ carbon atoms. The PL emission wavelength of GQDs exhibited a red shift with the increase in the size of the sp² conjugated domains, meaning that the larger the diameter, the longer the emitted wavelength. By calculating the emission wavelength of zigzag-edged GQDs of different diameters, it was found that size variations can tune fluorescence from deep ultraviolet to NIR emission, exhibiting significant size-dependent emission (Fig. 5b).

  Molecular state PL involves organic fluorescent molecules or their aggregates bound to CDs. These fluorescent groups in molecular states significantly contribute to the PL properties of CDs. Organic fluorescent groups are preserved during the carbonization process, potentially embedded within the carbon framework or attached to the surface of CDs. In recent years, researchers have provided strong evidence for the molecular state PL mechanism by isolating small molecular fluorescent groups from the process of CD preparation. Li et al. [52] synthesized red-emitting CDs from o-phenylenediamine (oPD) and catechol (CAT) using a solvent-free method and identified 5, 14-dihydroquinoxalino[2, 3-b] phenazine (DHQP) as the fluorescent molecule of CDs (Fig. 5c). The study found that CDs and DHQP exhibited similar optical responses in solvent effects, pH value influence, and time-resolved PL characteristics, indicating that the PL mechanism of CDs was similar to the molecular state fluorescence mechanism of DHQP. Additionally, thermal gravimetric analysis (TGA) revealed differences in the thermal stability between CDs and DHQP, suggesting the absence of free DHQP in CDs. Through ¹H NMR spectroscopic analysis, researchers speculated that DHQP molecules might be embedded into the core structure of conjugated CDs via sp³ hybridization, while a portion was also connected to the surface of CDs. Fluorescent groups can be isolated not only from aromatic molecule-derived CDs but also from aliphatic molecule-derived CDs. For instance, in the process of preparing CDs using citric acid as the carbon source, Song's research group [53] isolated a bright blue fluorescent group, imidazo[1, 2-a]pyridine-7-carboxylic acid (IPCA), and demonstrated their contribution to the formation of molecular state PL. Additionally, Kasprzyk et al. [54] during the microwave-assisted pyrolysis of citric acid to prepare CDs, formed a green fluorescent group named 4-hydroxy-1H-pyrrolo[3, 4-c]pyridine-1, 3, 6(2H, 5H)-trione (HPPT).

  The CEE effect refers to the phenomenon of enhanced PL through crosslinking interactions in conjugated CPDs and non-conjugated polymer dots (NCPDs). This effect can be achieved by restricting the movement of the fluorescent groups, which suppresses non-radiative transitions and thereby enhances PL. During the investigation of the PL enhancement mechanism in NCPDs, Zhu et al. [55] discovered that they did not contain traditional fluorophores. Instead, the PL performance was enhanced through the immobilization of sub-fluorophores (such as C=O, C=N, N=O) in the polymer chains, leveraging the CEE effect. The CEE effect was a physical phenomenon where the vibration and rotation of sub-fluorophores were restricted through chemical cross-linking or physical confinement, thereby increasing the probability of radiative transitions and enhancing PL. The PL enhancement mechanisms in NCPDs mainly included covalent bond CEE, rigidity aggregated CEE, and supramolecular interaction CEE (Fig. 5d). Covalent bond CEE enhanced PL by forming strong and directional covalent bonds, while rigidity aggregated CEE restricted non-radiative transitions through the rigid structure of physical aggregates. Furthermore, supramolecular interaction CEE involved non-covalent interactions such as hydrogen bonds and van der Waals forces, which also limited the motion of sub-fluorophores to some extent, thus enhancing PL. The CEE effect has been recognized as one of the important influencing factors for the high PL quantum yield (PLQY) of CPDs and is considered the primary PL mechanism of CPDs.

  The PL mechanism of CDs is multifaceted, encompassing surface state, carbon core state, molecular state, and CEE. A thorough understanding of these mechanisms can guide the synthesis and applications of CDs. For instance, by selecting different precursors, controlling synthesis conditions (such as temperature, solvent, time.), and post-treatment (such as oxidation, reduction, doping), CDs with specific PL properties can be synthesized. Additionally, theoretical calculations and simulations serve as important tools for understanding the PL mechanism of CDs, aiding in the prediction and design of novel CDs materials to meet the demand for high-performance PL materials in areas such as bioimaging, sensing, optoelectronic devices.

  4. Synthesis of CDs

  Since the discovery of CDs, a variety of methods have been reported for their preparation (Fig. 6). These methods can be divided into top-down and bottom-up methods according to the perspective of carbon precursor [25]. Top-down methods typically involve etching, oxidation, and other means to obtain CDs from bulk solids (such as graphene, carbon nanotubes, carbon black) or biomass. On the other hand, bottom-up methods primarily use small organic molecules (amino acids, glucose, phenylenediamine, citric acid, etc.) or polymers as carbon sources, undergoing processes like dehydration, polymerization, carbonization, surface passivation, etc. to form CDs. Each method for preparing CDs will be described in detail below.

  Figure 6

  

  Figure 6.  Visualization activities of preparation timelines for CDs in recent years.

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  4.1 Top-down strategies

  4.1.1   Arc discharge method

  The arc discharge method is a technique used to prepare CDs by generating arc discharge between two graphite electrodes in an arc chamber filled with inert gas, typically helium or argon. Arc discharge generates extremely high heat, leading to localized vaporization and decomposition of the anode graphite material, resulting in the production of carbon vapor, and rapid cooling occurring at the cathode. The rapid cooling and condensation process resulted in the formation of small carbon clusters, and these condensed carbon clusters undergo further nucleation and growth, ultimately forming CDs with sizes typically smaller than 10 nm [57, 58].

  In 2004, Xu et al. [8] used 3.3 mol/L HNO₃ to oxide arc-discharge soot and extracted it with NaOH to obtain a suspension. And subsequently, they employed gel electrophoresis to obtain carbon nanomaterials emitting blue, green, and yellow light, which were later named carbon dots (CDs) [9]. Although the maximum PLQY obtained for CDs was only 1.6%, the arc discharge method holds milestone significance in the first discovery of CDs. After that, Dey's team [57] utilized a two-step method involving arc discharge and chemical cutting to prepare B/N-doped GQDs. In the first step, in the presence of boron powder or diborane vapor as the B source, and NH₃ as the N source, B- and N-doped graphene were generated by using a gas-phase arc discharge method. The second step is to chemically cut the B/N-doped graphene to obtain B/N-CDs (Fig. 7a). The PLQY of BCD-1, BCD-2, and N-CDs are 2.3%, 2.5%, and 8.7%, respectively. Compared to the work of Xu's group [8], there has been a significant improvement in PLQY, potentially applicable in energy technologies.

  Figure 7

  

  Figure 7.  (a) Synthetic strategy of B/N doped CDs. Reproduced with permission [57]. Copyright 2014, Elsevier. (b) Schematic illustration of experimental setup. Reproduced with permission [61]. Copyright 2011, Royal Society of Chemistry. (c) Schematic diagram of mechanism of DMSO-CQD. Reproduced with permission [63]. Copyright 2020, Elsevier. (d) The schematic diagram of twin GQDs prepared by ETLAL. Reproduced with permission [66]. Copyright 2021, Elsevier.

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  The arc discharge method is an effective approach for preparing CDs, featuring simple operation and resulting in CDs with excellent fluorescence. However, the CDs produced by this method often exhibit lower purity and a broader size distribution. Additionally, the process requires a high input of electrical energy, limiting its widespread application.

  4.1.2   Laser ablation method

  Laser ablation is a method for the synthesis of carbon dots (CDs) based on the interaction of high-energy laser with a carbon source. In this process, a high-powered laser is focused on a target material containing carbon, leading to rapid vaporization and fragmentation of the target. The intense laser irradiation induces ablation in the carbon-containing material, generating a plasma rich in carbon that flows over the surface of the sample, leading to sample ionization [59].

  Sun and co-workers [9] first utilized the laser ablation of a carbon target method to prepare brightly luminescent carbon dots. Initially, a mixture of graphite powder and cement is subjected to hot pressing to form carbon targets. Subsequently, in the presence of water, carbon targets are ablated by laser at 900 ℃ and 75 kPa using argon as a carrier gas. Furthermore, the resulting product is refluxed in a 2.6 mol/L HNO₃ aqueous solution for 12 h. Finally, the acid-treated carbon particles are passivated with PEG1500 N or PPEI-EI, resulting in brightly luminescent CDs. The prepared CDs can achieve a maximum PLQY of up to 10% under an excitation wavelength of 400 nm. Although the prepared CDs exhibit relatively high fluorescence, the preparation process is complex, and the application involves strong acids, which may cause environmental pollution. Therefore, Hu et al. [60] reported a one-step synthesis method of fluorescent carbon nanoparticles (CNPs) by laser irradiation. In this method, the graphite powder was dispersed in PEG200 N to form a suspension, and then the suspension was irradiated by a Nd: YAG pulsed laser for 2 h, and further centrifuged to obtain luminescent CNPs. This method has the advantages of one-step completion, simple operation, and easy industrialization. Li's research group [61] then used the same method (Fig. 7b) to prepare tunable photoluminescent CQDs. In the same year, Hu et al. [59] synthesized CQDs with average sizes of approximately 3, 8, and 13 nm by one-step laser ablation method, using graphite flakes in a polymer as the precursor. The prepared CQDs exhibited excitation-dependent and size-dependent photoluminescence. The PLQY of CQDs depended on their size distribution. Their size distribution can be controlled by adjusting the pulse width of the laser. The results indicated that laser with a longer pulse width was more suitable for the synthesis of nanostructures with different sizes and morphologies. A nanosecond near-infrared pulse laser was also employed to ablate amorphous thin carbon films to obtain CQDs and nanodiamonds (NDs). The characteristic size of CQDs ranged from 20 nm to 300 nm. The size of NDs ranged from 30 nm to 500 nm. The obtained CQDs and NDs can be used for designing miniaturized chemical, biosensors and microfluidic devices [62]. To further improve the PLQY, Cui et al. [63] synthesized CQDs from a low-cost carbon cloth, passivated by DMSO molecules, using a novel dual-beam ultrafast pulsed laser ablation technique. The process of this method involved simultaneously irradiating carbon cloth with dual laser beams, having specific wavelength, pulse width, and pulse energy. This action triggered a Coulomb explosion, leading to the formation of a high-temperature plasma plume. The carbon cloth irradiated by the laser broke due to the absorption of high energy, forming sp² carbon domains. Meanwhile, DMSO molecules decomposed under high temperatures into oxygen and sulfur-containing substituents, ultimately resulting in the formation of CQDs with a passivated surface. Compared to the single-beam laser ablation method, the dual-beam laser increased the probability of ablation in the same area, resulting in more uniform CQDs with a PLQY of up to 35.4% (Fig. 7c). This made them widely applicable in cell bioimaging. Not only CQDs but also GQDs can be prepared by laser ablation. Such as in 2018, Calabro's research group [64] utilized laser ablation and chemical oxidation methods to synthesize GQDs and compared the properties of the products obtained by the two methods. Compared to graphene quantum dots synthesized by chemical oxidation (CO-GQDs) with an average diameter of 4.1 nm, lattice spacing of 0.21 nm, and an average particle height of 6.3 Å, graphene quantum dots produced by laser ablation (LA-GQDs) exhibit smaller sizes. The LA-GQDs had an average diameter of 1.8 nm, lattice spacing of 0.31 nm, and an average particle height of 3.5 Å, approximately equivalent to a monolayer graphene thickness. XPS and FT-IR characterizations revealed that CO-GQDs tended to contain carboxyl groups, with a higher sp² carbon fraction. In contrast, LA-GQDs tended to have hydroxyl groups, showing a higher sp³ carbon fraction. Photoluminescence (PL) spectra indicated that LA-GQDs underwent a blue shift, attributed to the influence of particle size and surface functional groups. Further confirmation through TCSPC determined that the luminescent characteristics were mainly influenced by the nature of surface functional groups, with a relatively minor impact from the intrinsic size effect. Compared to chemical oxidation, liquid-phase laser ablation was a cleaner and faster one-step method for producing GQDs. Composite materials of GQDs can also be synthesized using laser ablation method. Hameed et al. [65] synthesized GQDs and gold nanoparticles (AuNPs) in deionized water medium using graphite particles and gold foil as precursors, employing the pulsed laser ablation in liquid (PLAL) method. The prepared GQDs had an average size of 7–9 nm. Two types of nanoparticles were mixed together in a colloidal solution, and laser irradiation was applied to prepare the AuNPs@GQDs nanocomposite material. The author found that the addition of GQDs to AuNPs resulted in a redshift in optical measurements through characterization. Moreover, the AuNPs@GQDs nanocomposite material exhibited a core-shell structure with AuNPs as the core and GQDs as the shell. The addition of GQDs to AuNPs was found to enhance the generation of reactive oxygen species (ROS), facilitating the ability to kill cancer cells in photodynamic therapy. Additionally, they exhibited antibacterial properties. In order to obtain GQDs with better characteristics, Li et al. [66] reported a method for preparing twin GQDs using electric-field-assisted temporally-shaped femtosecond laser ablation liquid (ETLAL). By adjusting the electric field and femtosecond laser parameters, they achieved controlled preparation of twin GQDs. Their average particle size was 2–3 nm. The entire process was completed in three stages: Plasma, cavitation bubbles, and chemical modification (Fig. 7d). The introduction of an electric field and the delayed femtosecond laser pulses can rapidly control the directional movement of cavitation bubbles and induce collision crystallization of nanoparticles at higher temperatures and pressures. These two factors were crucial for the formation of twin GQDs. The combined action of laser ablation and electric field can generate oxygen-containing functional groups, promoting the formation of GQDs with surface oxygen functional groups and defects. The paper presented a rapid, efficient, and environmentally friendly method for the preparation of twin GQDs with crystalline and defect engineering.

  One-step laser ablation eliminates the initial complex steps in the preparation of CDs and is a fast method for their synthesis. This method can also precisely control the size of the CDs, resulting in a more uniform product, and the preparation process can be directly laser irradiated carbon source to generate CDs, no additional chemical reagents are required. However, the use of laser ablation method requires relatively high equipment and energy, cannot achieve large-scale production of CDs, so laser ablation method has not been widely used.

  4.1.3   Chemical oxidation method

  Chemical oxidation method is a synthesis strategy that employs strong acids such as HNO₃, H₂SO₄, or weak acids like H₂O₂ as oxidants to oxidize and cut large-sized carbon sources into small-sized CDs. Typically, surface passivation is employed to introduce oxygen-containing functional groups onto the surface of CDs, imparting them with excellent water solubility and photoluminescent properties.

  In 2007, Liu and colleagues [67] first utilized chemical oxidation method to prepare fluorescent CNPs by treating candle soot. They refluxed candle soot in 5 mol/L HNO₃ and purified it using polyacrylamide gel electrophoresis (PAGE) to obtain water-soluble fluorescent CNPs with a particle size of approximately 1 nm. A PLQY < 2% limited their applications. In order to enhance PLQY, Ray's team [68] refluxed carbon soot in 5 mol/L HNO₃ for 12 h, followed by centrifugation after cooling. The collected light yellow supernatant exhibited green fluorescence under UV irradiation (Fig. 8a). They further mixed the supernatant with chloroform and ethanol, centrifuged it at 800 rpm using a high-speed centrifuge, evaporated to dryness to remove chloroform and ethanol, and finally dissolved it in water, obtaining CNPs with a particle size ranging from 2 nm to 6 nm. The final product achieved a PLQY of 3%, making it suitable for applications in bioimaging. PLQY was still low and needed to be further improved. In the same year, natural gas soot was also utilized as a carbon source to prepare carbon nanoparticles through chemical oxidation method. Unfortunately, the obtained carbon nanoparticles had a PLQY of only 0.43% [69]. Surface passivation was considered an effective means to enhance PLQY. Peng [70], Qiao [71], and Wang [72] utilized carbohydrates (Fig. 8b), activated carbon, and carbon soot as carbon sources, respectively, etching with HNO₃ to obtain a single carbon nanoparticle, and then passivating the carbon nanoparticle with end amine compounds (such as TTDDA or PEG1500 N) to obtain luminescent CDs. The PLQY of CDs obtained by passivation can reach up to 60%, which had important applications in the field of bioimaging. Shen et al. [73] further oxidized graphene oxide (GO) with HNO₃ and cut it into small pieces, then surface passivation with PEG1500 N and finally reduction with hydrazine hydrate to obtain GQDs. The prepared GQDs exhibited bright blue emission at neutral pH. More importantly, they found that the prepared GQDs had up-conversion PL properties when the excitation wavelength was 600–800 nm and the emission wavelength was 390–468 nm. And the energy of the up-conversion emitted light and excited light is almost constant, about 1.1 eV. Due to the deep tissue penetration capability of long excitation wavelengths, up-conversion PL was more conducive to achieving in vivo imaging. The oxidation temperature had great influence on the structure and properties of the products. Such as Peng et al. [74] acidified carbon fiber (CF) to prepare GQDs. GQDs synthesized at the reaction temperature of 120 ℃ had a size distribution of 1–4 nm and a height of 0.4–2 nm, which was equivalent to 1–3 layers of graphene (Fig. 8c). Changing the reaction temperature could induce GQDs to emit blue, green, and yellow light. This might arise from variations in the size and properties of the GQDs. According to the Fast Fourier Transform (FFT) patterns, the prepared GQDs had edges with a Zigzag pattern. GQDs with such Zigzag edges can offer specific electronic or magnetic connections. The previous articles all used strong acid HNO₃ to oxidize and cut bulk carbon sources to prepare CDs. However, during this process, potentially toxic gases such as NO₂ and N₂O₄ may be generated, increasing environmental problems. Therefore, Jiang et al. [75] used GO as the carbon source and under the synergistic action of 30% H₂O₂ as a mild oxidizing agent and ammonia solution, obtained single or double-layered amine-functionalized GQDs. The final products with fluorescent multicolor properties were separated using a Sephadex G-25 gel column. The single-layered GQDs had a particle size of approximately 7.5 nm with a PLQY of 4.4%. Despite the relatively low PLQY, the authors discovered their inherent anti-chlamydial properties and found them can be used as an anti-mycoplasma regime agent to expand biomedical applications. Similarly, Yan's team [76] investigated a method for synthesizing water-soluble carbon CQDs based on starch chemical oxidation. In the first step, the starch was carbonized for 4 h under high temperature and pressure to obtain the carbon source; In the second step, the carbon source was put into the mixed oxidizer composed of distilled water, HAc and H₂O₂ through the steps of ultrasound, reflux, filtration, neutralization, concentration, dialysis and drying to obtain light yellow CQDs. The prepared CQDs had a particle size distribution in the range of 5–8 nm, with a PLQY of 11.4%. At the excitation wavelength of 330 nm, CQDs emitted bright blue fluorescence. The prepared CQDs exhibited excellent optical stability and biocompatibility. H₂O₂, as an oxidizing agent, could generate highly oxidative species such as hydroxyl radicals. These oxidative species were effective in oxidizing and cutting large carbon sources. The use of H₂O₂ as an oxidizing agent could avoid the generation of toxic gases and environmental hazards. However, the processes involving H₂O₂ mentioned earlier were generally time-consuming.

  Figure 8

  

  Figure 8.  (a) Steps in the preparation of CDs from carbon soot. Reproduced with permission [68]. Copyright 2009, American Chemical Society. (b) The preparation process of fluorescent CDs. Reproduced with permission [70]. Copyright 2009, American Chemical Society. (c) The method of cutting CF into GQDs through oxidation. Reproduced with permission [74]. Copyright 2012, American Chemical Society.

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  Chemical oxidation method for preparing CDs had the advantage of a rich variety of available carbon sources, and the process did not require expensive equipment, making it relatively simple to operate. Surface modification of CDs could be achieved through passivation. However, the excess oxidizing agent needed alkaline substances for neutralization, resulting in increased energy consumption. Additionally, the use of strong acids could lead to a certain degree of environmental pollution. Hence, Chemical oxidation method had not been widely used.

  4.1.4   Electrochemical method

  The top-down electrochemical method usually uses conductive carbon material as anode and carbon source to prepare CDs by electrochemical oxidation etching.

  In 2007, Zhou et al. [77] reported a method for preparing CDs through electrochemical treatment of multiwalled carbon nanotubes (MWCNTs). The study utilized MWCNTs deposited by chemical vapor deposition as the anode, platinum wire as the counter electrode, and Ag/AgClO₄ as the reference electrode. CDs were electrochemically synthesized in a 0.1 mol/L acetonitrile solution of tetrabutylammonium perchlorate (TBAP) as the electrolyte. With a scan rate of 0.5 V/s and a potential range of −2.0 V to +2.0 V, as the reaction time increased, MWCNTs were gradually etched, leading to the formation of CDs entered the electrolyte. The color of the electrolyte changed from colorless to yellow and finally to deep brown. The authors predicted that TBA cations inserted into the interstices of MWCNTs, disrupting the tube structure near defects during the electrochemical cycling process, resulting in the generation of CDs. The prepared CDs achieved a PLQY of 6.4%. This work proposed an electrochemical method for preparing CDs for the first time, which provided a new approach for the preparing of CDs with better properties. Afterward, graphite rod [78] was also used as a carbon source and working electrode, coupled with a platinum mesh as the counter electrode, an Ag/AgCl as the reference electrode, and the phosphate buffer solution (PBS) as the electrolyte, successfully preparing CDs with an average diameter of 2 nm (Fig. 9a). Within a certain scanning rate and potential range, the generated CDs were observed to exhibit electrochemiluminescence (ECL). During the potential cycling process, a reduced state (R^•−) could be formed at negative potentials, while an oxidized state (R^•+) could be formed at positive potentials. The electron transfer annihilation between the reducing and oxidizing species led to the formation of the excited state (R*), and the emission of an ECL signal occurred when the excited state transitioned back to the ground state (Fig. 9b). Due to the ECL of CDs, they hold broad application prospects in the development of novel biosensors and display devices. Before this, researchers were more inclined to study the PL of CDs. This article investigated the ECL of CDs for the first time, with the potential to broaden the applications of CDs in other fields. Later, some researchers [79-81] used graphite rods as anode and cathode to prepare CDs in an electrochemical device composed of alkaline electrolyte. The authors prepared CDs by electrochemical oxidation of graphite at a constant potential voltage of 5v. Under normal temperature conditions, the formed CDs dispersion was colorless, but gradually became bright yellow (Fig. 9d). Through spectral characterization, it was speculated that the color change was caused by oxidation of CDs [81]. They found that the current density affected the PL color distribution of CDs. Through comparative experiments, They found that the alkaline environment is a key factor in the formation of CDs. In the experiments, they discovered that changing the current density led to a change in the PL color of CDs, with lower current density increasing the emission of longer-wavelength or warm-colored CDs. At the same time, experimental and theoretical calculations confirmed the size-dependent PL of CDs. The size of CDs increased from 1.2 nm to 3.8 nm, and the color of PL changed from blue to red (Fig. 9c) [79]. The prepared CDs exhibited up-conversion characteristics (Fig. 9c), high electron acceptability and hydrogen bond catalytic activity [79-81]. Based on these excellent properties, composite photocatalysts can be formed with TiO₂ or SiO₂ to achieve efficient utilization of full spectrum sunlight, thus solving environmental and energy problems [79]. In addition, the hydrogen bond catalytic activity of CDs could be significantly enhanced under visible light irradiation, thus accelerating the aldol reaction [80]. At the same time, ferric ion detection and biological imaging can be realized by using the properties of PL [81]. The synthesis of CDs can be precisely controlled by adjusting the current density and electrode potential. In 2011, Bao and co-workers [82] utilized carbon fiber as the working electrode and carbon source, while Pt foil and silver wire served as the counter electrode and reference electrode, respectively, to electrochemically prepare CDs in acetonitrile with 0.1 mol/L TBAP. The sizes of CDs obtained at different potentials were different, but they were all spherical and dispersed uniformly. The electrode potential played a crucial role in controlling the size of CDs, with higher potentials resulting in smaller-sized CDs (Fig. 9e). The authors speculated that the formation and detachment of CDs on the carbon fiber surface were influenced by the application of potential and the intercalation of electrolyte into the layered graphite structure of carbon fiber. The PL spectra of the prepared CDs showed excitation-dependent characteristics. And under the same wavelength excitation, CDs prepared at different potentials exhibited distinct emission peak positions and intensities. The study discussed the influence of size and surface oxidation degree on the PL of CDs, confirming that surface states were the main mechanism of the luminescence of CDs. Moreover, as the surface oxidation degree increased, the emission wavelength of CDs underwent a redshift. This research not only assisted in the preparation of CDs with controllable size and long-wavelength emission but also provided insights into the PL mechanism of CDs. Graphene films could also be used as working electrodes to prepare CDs by pairing with PBS as electrolyte [83]. The sizes of the synthesized CDs were about 3–5 nm, the height was about 1–2 nm, and the green light was emitted when the UV lamp was excited at 365 nm. Raman spectroscopy, XPS, and FT-IR showed that the surface of CDs contains more hydroxyl, carboxyl, and carbonyl oxygen-rich groups, which made them water-soluble. The LOMO level of CDs was determined to be 4.2–4.4 eV by electrochemistry. The prepared CDs had water solubility, high specific surface area, high electron mobility, and adjustable band gap, and had great potential as electron acceptors in photovoltaic devices. In the second year, the team [84] replaced the electrolytes from the previous work [83] with acetonitrile solution of TBAP to generate N-GQDs in situ. In the experiment, they observed that with an increase in the number of scanning cycles, the color of the electrolyte changed from colorless to yellow. In comparison to graphene film, the XPS spectra of the products revealed the presence of an N 1s peak, with an N/C atomic ratio of approximately 4.3%, indicating the successful synthesis of N-GQDs. The N-GQDs had particle sizes ranging from 2 nm to 5 nm, with a height of 1–2.5 nm, equivalent to 1–5 layers of graphene. N-GQDs/graphene film composite materials were prepared using a hydrothermal method, and their electrocatalytic performance was investigated. Experimental results indicated that the composite material exhibited excellent conductivity, making it suitable for use as an electrocatalyst in the oxygen reduction reaction (ORR). In order to reduce costs, Hu et al. [85] prepared N-CDs using a coal-based rod as the working electrode and carbon source. The N-CDs were further synthesized into a composite catalyst for use as an electrocatalyst in ORR, significantly reduced the costs. In principle, carbon materials with excellent conductivity can be used as both the carbon source and working electrode for the electrochemical synthesis of CDs. For instance, the screen printed carbon electrodes had also been employed in the electrochemical preparation of CDs [86].

  Figure 9

  

  Figure 9.  (a) The device diagram of CDs prepared by electrochemical method. Reproduced with permission [78]. Copyright 2009, American Chemical Society. (b) Schematic illustration of the ECL and PL mechanisms in CNCs. Reproduced with permission [78]. Copyright 2009, American Chemical Society. (c) Optical images of CQDs of different sizes under 365 nm UV light and Upconverted PL properties of CQDs. Reproduced with permission [79]. Copyright 2010, John Wiley and Sons. (d) Schematic diagram of electrochemical preparation of CDs by graphite electrode and color change at room temperature. Reproduced with permission [81]. Copyright 2016, Royal Society of Chemistry. (e) Dependence of size of CDs on applied potentials. Reproduced with permission [82]. Copyright 2011, John Wiley and Sons.

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  CDs prepared by electrochemical method using conductive carbon materials as carbon source and working electrode have good water solubility in general, and the size and PL characteristics of CDs can be controlled by adjusting the current density and potential. However, this method usually requires a high energy input, making the preparation high cost, and in the process of preparing CDs may produce more waste liquids, resulting in a certain impact on the environment.

  4.1.5   Pyrolysis method

  Top-down pyrolysis is a simple and rapid method to prepare nano-CDs by dehydrating, polymerizing, and carbonizing bulk carbon sources at a suitable temperature.

  Biomass was a commonly used carbon source in the pyrolysis method for preparing CDs. For instance, in 2012, Hsu et al. [87] utilized coffee grounds in the pyrolysis process to synthesize CDs. Specifically, during the heating process, small molecules in the coffee grounds underwent dehydration, polymerization, and carbonization, forming small carbon nuclei. These nuclei further grew by solute diffusion to the particle surface, resulting in the eventual formation of CDs. The average particle size of the CDs synthesized by pyrolysis was 5 nm, with a PLQY of 3.8%. By weight measurement, 1 g of coffee grounds could produce 120 mg of CDs, with a yield of approximately 12%. The PL of the CDs exhibited excitation-dependent behavior, and they demonstrated salt resistance and photostability. The generated CDs can be utilized as probes for cell imaging. In the same year, Zhou et al. [88] employed watermelon peel as a carbon source to synthesize CDs. The authors subjected watermelon peel to carbonization through pyrolysis at 220 ℃ for 2 h, followed by ultrasonication, filtration, and dialysis to obtain CDs with a particle size smaller than 2 nm and a PLQY of 7.1%. These CDs exhibited strong blue luminescence, a long fluorescence lifetime, and excellent stability in a broad pH range and at higher salt concentrations. This method enabled the large-scale production of water-soluble CDs, and the CDs could be applied for cell imaging. Some studies found that the temperature and reaction time of the pyrolysis process had an impact on the generation of CDs [28, 89]. In 2013, Zhu et al. [89] utilized the pyrolysis method to prepare CDs from plant leaves. The plant leaves were subjected to pyrolysis at temperatures of 250, 300, 350, and 400 ℃ for 2 h each. Subsequently, CDs were collected through cooling, centrifugation, and filtration. Under 365 nm ultraviolet light, the synthesized CDs exhibited a blue color (Fig. 10a). They concluded that excessively high or low temperatures were not suitable for CDs production. Low-temperature pyrolysis failed to achieve complete carbonization of the plant leaves, while high temperatures led to excessive oxidation of CD particles, causing damage to their surface and internal structure. The optical properties of CDs prepared at lower temperature and higher temperature would decrease. The use of low-temperature plasma and microwave irradiation techniques further enhanced the PL intensity of CDs. In another study, waste frying oil (WFO) was used as a carbon source, and CDs were synthesized with the assistance of concentrated sulfuric acid. The addition of concentrated sulfuric acid accelerated the dehydration and carbonization processes of the carbon source. The CDs prepared in this reaction were highly pH-sensitive, showing a linear relationship between PL intensity and pH (3–9) (Fig. 10b). These CDs demonstrated advantages such as high PLQY, high photostability, and low cytotoxicity, making them suitable for applications in the field of biological imaging. This study also revealed an interesting finding: Both reaction temperature and time were crucial parameters for CDs synthesis, and these two factors complemented each other. The effect of reaction temperature was similar to that of Zhu [89]. Additionally, reaction time also played a significant role, and at lower temperatures, extending the reaction time did not lead to complete carbonization of WFO. CDs with good optical properties could be synthesized only when pyrolysis at a suitable temperature and reaction time [28]. The CDs introduced above were mostly used in cell imaging, biomedicine, and fluorescence detection. Actually, CDs prepared from biomass can also be applied in the photocatalytic degradation of organic pollutants. Firstly, corn cob was pyrolyzed to prepare CDs, and then Bi₂S₃/CDs composite material was synthesized using a hydrothermal method for the photocatalytic degradation of methylene blue (MB) and tetracycline hydrochloride (TC). Through experiments, it was found that compared to the photocatalysis of Bi₂S₃ alone, the efficiency of photocatalytic degradation of both organic compounds significantly increased with Bi₂S₃/CDs. The authors proposed that the enhancement of photocatalytic activity in the composite material may be attributed to three points: the lower bandgap of the composite material, allowing for a wider spectrum absorption; CDs acting as electron acceptors inducing efficient electron transfer; and the large surface area of the composite material promoting the generation of reactive species ^•O₂⁻ and h⁺(Fig. 10c) [90]. Other biomass sources had also been used to prepare CDs. For instance, hair [91] and Zingiberis rhizoma [92] had been employed as carbon sources for the pyrolysis-based synthesis of CDs. The prepared CDs exhibited PL properties that are correlated with excitation, pH values, and solvent conditions. The surface of CDs contained numerous functional groups, providing them with excellent dispersibility in water and many polar organic solvents. They could be applied in various fields such as biomedical applications for pain relief, anti-counterfeiting labels, luminescent inks, and flat-panel displays [91, 92].

  Figure 10

  

  Figure 10.  (a) Diagram of synthesis of CDs from plant leaves and the PL enhancement phenomenon. Reproduced with permission [89]. Copyright 2013, Royal Society of Chemistry. (b) Linear relationship between PL intensity and pH value. Reproduced with permission [28]. Copyright 2014, Elsevier. (c) Possible mechanism of photocatalytic degradation of MB and TC. Reproduced with permission [90]. Copyright 2017, Royal Society of Chemistry.

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  Pyrolysis of biomass is a commonly employed method for the preparation of CDs. This method is characterized by its simplicity, wide availability of carbon sources, low cost, and the absence of the need for toxic or passivating chemicals, making it a green synthesis approach. However, CDs produced through this method often exhibit a broad size distribution, and the mechanism of the formation of CDs is relatively complex.

  4.1.6   Microwave method

  The top-down microwave method is a technique for carbonization of large carbon sources or biomass under microwave irradiation, resulting in the formation of CDs.

  In 2012, Li et al. [93] prepared CDs from GO under acidic conditions (HNO₃ and H₂SO₄) using microwave irradiation. Specifically, GO was mixed with the two acids in a certain ratio, and the mixture was then subjected to microwave heating (240 W) for 3 h Subsequently, the resulting mixture underwent cooling, ultrasonication, neutralization, filtration, and dialysis to obtain yellow-green PL g-GQDs with a PLQY of 11.7%. The pre-dialysis mixture was further reduced using NaBH₄, followed by a series of post-treatment steps, resulting in blue PL b-GQDs with a PLQY of 22.9% (Fig. 11a). Due to the transition between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), the prepared CDs exhibited excitation-dependent behavior. They observed ECL emission in the prepared CDs, and based on this characteristic, potential applications in biosensors were suggested. This method used two strong acids, and excessive acids could cause environmental pollution, which was not conducive to large-scale CDs preparation. Then, Hai and co-workers [94] utilized GO as a carbon source to prepare CDs in an acid-free medium. The PLQY of the obtained CDs was 21.1%, exhibiting excitation independence, pH insensitivity, salt resistance, and photobleaching resistance. This material can be applied for cell imaging. In comparison to the work by Li and colleagues [93], this method reduced the use of acids, making it a relatively environmentally friendly preparation method. However, the use of GO as a carbon source for preparing CDs presents some complexity, as it required initial processing of large carbon sources such as graphite sheets or graphite powder to obtain GO, followed by microwave operations to obtain CDs. In order to streamline the operational steps, in 2016, Feng et al. [95] used silkworm chrysalis as a carbon source to prepare nitrogen-doped CDs through a microwave method. They calculated that nitrogen doping significantly increased the PLQY of the CDs up to 46%, and the CDs exhibited excellent PL and biocompatibility, making them suitable for cell imaging. This preparation process was simple, as it did not require pre-processing to obtain the carbon source, eliminating unnecessary operational steps. Additionally, no other chemical reagents needed to be added during the preparation process, making it a green synthesis method for CDs. Based on the work of Hai [94] and Feng [95], it was discovered that heteroatom doping can significantly enhance PLQY, which is conducive to expanding the applications of CDs. Other biomass such as palm kernel shell [96] was also used as a carbon source for the microwave preparation of CDs. The authors found that both the microwave irradiation time and the concentration of the dopant would influence the optical performance of the CDs. CDs obtained after 90 s of microwave irradiation were highly dispersed. The prepared CDs exhibited excellent properties such as salt resistance, UV light stability, and repeatability. Leveraging the fluorescence quenching effect, they were successfully applied for the detection of Cu²⁺, with an extremely low detection limit of only 0.05 mmol/L. The mechanism of CDs formation had not been discussed in the previous microwave synthesis of CDs. In order to understand its mechanism, Olmos-Moya et al. [97] used orange peels as a carbon source to prepare CDs with an average particle size of 1.16 nm, blue PL, and high water dispersibility through a microwave-assisted hydrothermal method (Fig. 11b). The formation mechanism may involve the condensation, polymerization, carbonization, and surface functionalization of organic molecules present in orange peels (Fig. 11c).

  Figure 11

  

  Figure 11.  (a) Preparation route diagram of GQDs. Reproduced with permission [93]. Copyright 2012, John Wiley and Sons. (b) Diagram of microwave-assisted synthesis of CDs. Reproduced with permission [97]. Copyright 2022, Elsevier. (c) Possible formation mechanism of CDs prepared from orange peel. Reproduced with permission [97]. Copyright 2022, Elsevier.

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  The advantages of microwave synthesis for preparing CDs lie in its simplicity, efficiency, and the ability to increase yield while reducing reaction time, facilitating uniform dispersion of CDs. However, the method may be cumbersome when dealing with large carbon sources, and further optimization may be required for achieving specific properties of CDs in certain cases.

  4.1.7   Ultrasonic method

  Utilizing the pressure difference and shear forces generated by ultrasound cavitation, certain chemical bonds in the carbon precursor are broken, leading to in situ carbonization and passivation, thereby forming CDs.

  In 2014, Park et al. [98] utilized ultrasonic method to prepare water-soluble green CDs from food waste. The food waste/ethanol solution was subjected to 40 kHz ultrasound treatment for 45 min, followed by centrifugation, filtration, and drying to obtain the CDs. The synthesis mechanism involved four stages: Dehydration, polymerization, carbonization, and passivation. Initially, organic molecules from food waste underwent dehydration, polymerization, and carbonization to form single-crystal carbon nuclei. Subsequently, solutes diffused to the single-crystal carbon nuclei, leading to their growth and ultimately resulting in highly functionalized CDs. Approximately 120 g of CDs could be synthesized from every 100 kg of food waste, achieving large-scale CD synthesis. The synthesized CDs exhibited excellent photostability and low cytotoxicity, presenting broad prospects for applications in biomedical and seed germination fields. In another study, Zhu et al. [99] utilized graphene prepared by chemical vapor deposition (CVD) with inherent defects, making it prone to fragmentation, for the synthesis of CDs through ultrasonic treatment. Specifically, they first grew a monolayer of graphene on a copper foil. Subsequently, they etched the graphene with (NH₄)₂S₂O₈ solution and subjected the graphene fragments to 10 min of ultrasonic treatment. Finally, blue PL CDs were obtained through centrifugation and extraction (Fig. 12a). Recently, Huang et al. [100] employed a one-step ultrasonic method to synthesize CDs. They subjected a mixture of dried albizia flowers and acetone to ultrasonic treatment using an ultrasonic processor under 600 W power conditions. The resulting mixture was purified to obtain CDs emitting near-infrared light. Subsequently, the CDs were encapsulated into polymer dots (Pdots) (Fig. 12b) using nano-precipitation technology, and these Pdots were successfully applied for the detection of Cu²⁺ in biological systems. Ultrasonic method could not only prepare green, blue, and red-emitting CDs but also synthesize white-emitting CDs. In this process, polyimide resin (as the carbon source) and ethylenediamine (EDA) were added to deionized water and subjected to ultrasonic treatment for 3 h. The addition of a silane coupling agent (KH570) was found to enhance the PLQY of the generated CDs to 28.3%. The prepared white-emitting CDs were used as white fluorescent powders for constructing LEDs and fluorescent inks [101]. Cigarette ash was also employed as a carbon source for the ultrasound-assisted synthesis of functionalized CDs in the past. The resulting CDs exhibited characteristics such as uniform size, stable fluorescence, low cytotoxicity, and biocompatibility, making them suitable for applications in the field of bioimaging [102].

  Figure 12

  

  Figure 12.  (a) Schematic diagram of the synthesis of CDs using defective CVD graphene. Reproduced with permission [99]. Copyright 2016, Elsevier. (b) Schematic diagram of the preparation of Pdots. Reproduced with permission [100]. Copyright 2022, Elsevier. (c) PL spectra of CDs at 250 nm < λ_ex < 530 nm. Reproduced with permission [13]. Copyright 2015, Elsevier. (d) Preparation route of P-CDs/Ni-MOL composites. Reproduced with permission [104]. Copyright 2022, Elsevier.

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  Ultrasound-assisted synthesis is a method for preparing CDs that offers mild reaction conditions, a short reaction time, and simple experimental procedures. However, this approach requires specialized ultrasound equipment, thereby increasing experimental costs. Researchers need to choose appropriate synthetic methods based on specific needs.

  4.1.8   Ball milling method

  Ball milling method is a technique that utilizes mechanical force to crush and mix materials. In the process of preparing CDs through ball milling, carbon sources, and other reagents are subjected to high-intensity impacts and friction from the balls inside the ball mill. This results in the fragmentation of the carbon source and its reaction with other reagents, leading to the formation of nano-sized CDs.

  In 2015, Wang et al. [13] prepared high-performance CDs through high-energy ball milling method. The authors ball-milled a mixture of activated carbon (AC), KOH, and stainless-steel balls using a ball mill at 500 rpm for 50 h The ball-milled mixture was dissolved in deionized water, forming a uniform suspension through ultrasound, and finally, the suspension was purified by ultrafiltration, resulting in CDs with a PLQY of 7.6%. The PL behavior of the CDs was investigated by varying the excitation wavelength (250 nm < λ_ex < 530 nm). Experimental findings revealed that the CDs prepared by this method exhibited dual-wavelength PL emission peaks. The first PL emission peak was independent of λ_ex, with the maximum PL intensity occurring at an emission wavelength of 490 nm. The second PL emission peak showed excitation dependence and underwent a redshift with increasing λ_ex (Fig. 12c). The dual-wavelength PL emission properties were mainly attributed to the surface states and size effects of the CDs. Additionally, the prepared CDs exhibited dual-wavelength up-conversion PL properties in the excitation range of 550 nm < λ_ex < 690 nm and dual-wavelength ECL characteristics. The unique dual-wavelength emission properties made them promising for a wide range of applications in sensors, catalysis, and biomedical fields. Several years later, Youh et al. [30] utilized conductive carbon black (selected for its small particle size and well-graphitized crystallinity, making it an excellent carbon source for preparing CDs) and Na₂CO₃ as raw materials to prepare CDs with blue PL characteristics by dry ball milling method. In this preparation process, zirconium oxide balls were used instead of stainless-steel balls, with the remaining operational steps similar to Wang's work [13]. The resulting CDs had an average size of 3.0 nm, a narrow size distribution, and a PLQY of 2.23%. The authors speculated on the mechanism of CDs preparation through ball milling. They believed that Na₂CO₃ played a key role in enhancing the functionalization and grinding efficiency of CDs. On one hand, Na₂CO₃ as a reaction medium, underwent chemical reactions during grinding, increasing the number of oxygen-containing functional groups on the surface of CDs. This not only enhanced the water solubility of CDs but also simultaneously achieved the functionalization of CDs. On the other hand, during the ball milling process, Na₂CO₃ acted as a grinding medium, promoting collisions and friction between the target materials, thus enhancing the grinding efficiency. By optimizing parameters such as raw material ratios and processing time, both PLQY and yield could be increased, contributing to a narrower particle size distribution. CDs prepared by ball milling carbon black can be applied in fields such as bioimaging [30] and drug delivery [103]. In the previously mentioned study, heteroatom doping was highlighted for its ability to enhance the PLQY of CDs. Additionally, heteroatom doping could cause the prepared CDs to form defect states, reduce the electron transport resistance, form a new band gap, and enhance light absorption, which was favorable for photocatalysis. Based on this understanding, Ma et al. [104] employed a ball milling-assisted hydrothermal method to prepare phosphorus-doped carbon dots (P-CDs) using red phosphorus (RP) as the phosphorus source and lignin as the precursor. A mixture of a specific amount of lignin, RP, ethanol, and zirconium oxide balls was ball-milled for 8 h Subsequently, a small amount of the powder was added to deionized water with 0.3 g NaOH, heated at 180 ℃ in a high-pressure reaction vessel for 5 h, and finally purified to obtain P-CDs. The prepared P-CDs were then combined with Ni-MOL synthesized through a hydrothermal method to form the composite material P-CDs/Ni-MOL (Fig. 12d). This composite material was successfully applied in the photocatalytic degradation of tetracycline (TC). Under visible light irradiation, the composite material exhibited a TC degradation rate of up to 98.98% within 120 min. Later, coffee grounds [105] were also employed as a carbon source for the ball milling preparation of CDs. Three types of CDs with a PLQY of 3%−4% were prepared using coffee grounds: carbon dots derived from coffee powder (CFCDs), carboxylic acid-functionalized carbon dots (CA-CFCDs), and nitrogen-containing carbon dots (N-CFCDs). The latter two types of CDs were achieved by adding oxygen/nitrogen-containing small molecules during the ball milling process. Modified carbon dots exhibited excellent dispersibility compared to unmodified CFCDs. Embedding CA-CFCDs into microgel particles enabled sensitive detection of Fe³⁺.

  The advantages of the ball milling method include simple operation, cost-effectiveness, and environmental friendliness. However, there are also some drawbacks, such as the potential for excessively long ball milling times and lower PLQY.

  4.2 Bottom-up strategies

  4.2.1   Hydrothermal/solvothermal method

  The hydrothermal/solvothermal method is one of the most commonly used approaches for preparing CDs in recent years. Its preparation principle involves subjecting a mixture of carbon source and solvent to high temperature and pressure treatment in a high-pressure reactor, inducing the precursor to undergo reactions. And then the CDs with special properties are obtained through cooling, filtration, and purification steps. The types of carbon source and solvent both influence the properties of the synthesized CDs. In recent years, various types of carbon sources such as ascorbic acid [106, 107], curcumin and folic acid [108], phenylenediamine [14, 109-111] and citric acid [112, 113] have been successfully applied in the hydrothermal/solvothermal method for preparing CDs. And CDs prepared using this method have been successfully applied in various fields such as fluorescence probes, bioimaging, WLEDs, fingerprint recognition, photocatalysis, and others.

  In 2010, Zhang et al. [106] first synthesized CDs using a simple hydrothermal/solvothermal method. Specifically, l-ascorbic acid was used as the carbon source, with water and ethanol as solvents. The mixture of carbon source and solvent was reacted at 180 ℃ for 4 h in a high-pressure reactor. Finally, after purification, CDs with a particle size of 2 nm and a PLQY of 6.79% were obtained. Years later, Liu's research group [107] also utilized ascorbic acid as the carbon source and ethanol as solvent to prepare CDs via a hydrothermal/solvothermal method. The mixture of carbon source and solvent was introduced into a polytetrafluoroethylene high-pressure reactor and reacted at 160 ℃ for 1 h. Afterward, CDs were obtained through purification. They developed a novel ratio fluorescent probe based on the prepared CDs and red fluorescent ruthenium bipyridine [Ru(bpy)₃]²⁺, and constructed a smartphone platform for sensitive and quantitative analysis of F⁻. In another study on the hydrothermal/solvothermal preparation of CDs, both o-phenylenediamine and dopamine were used simultaneously as carbon and nitrogen sources to enhance the photostability of the synthesized CDs. Nitrogen doping resulted in CDs with a PLQY of up to 33.96%. The presence of Fe³⁺ led to selective fluorescence quenching of the prepared CDs, enabling the design of a sensitive detector for Fe³⁺ in living cells. Additionally, the CDs were successfully applied in bioimaging and LEDs [109]. The fluorescence properties of CDs could be optimized by changing the functional groups of carbon source [114], the position of carbon source functional groups [110], types of solvent [111, 112], reaction temperature, and time [115]. In 2021, Zhao et al. [114] optimized the fluorescence properties of CDs by simply changing the functional groups of the carbon source (Fig. 13a). The optimized CDs exhibited a PLQY of 10.1%. Moreover, by adjusting the pH to 1, 2, or 3, they achieved color changes in the fluorescence, producing red, brown, and green emission. Inspired by this, they successfully developed molecular signal lights and applied CDs in the fields of anti-counterfeiting and information encryption. The dithiosalicylic acid (DTSA) was dissolved in acetic acid solvent, and subsequently, o-phenylenediamine, m-phenylenediamine, and p-phenylenediamine were added respectively. After stirring, the mixed solution was transferred into a high-pressure reactor and heated at 180 ℃ for 10 h. After cooling, the reaction mixture was reintroduced into boiling water, followed by filtration and drying to obtain red, green, and blue-emitting CDs. Their PLQY could reach 20.77%. The prepared multi-color CDs could be used in white LEDs and fingerprint recognition. This research work adjusted the emission color of CDs by simply altering the position of the functional groups of the carbon source [110]. In the same year, Li's team [111] prepared blue, green, red, and white-emitting CDs using a hydrothermal/solvothermal method by altering the type of solvent. Simply put, using p-phenylenediamine as the carbon source, a mixture of amides (formamide, DMF, dimethylacetamide (DMAC), and formamide with DMF) and sodium hydroxide as the solvent, the mixture of carbon source and the four solvents was heated at 180 ℃ for 12 h, yielding blue, green, red, and white-emitting CDs. Their PLQYs reached 61.6%, 41.3%, 29.1%, and 19.7%, respectively. The prepared CDs exhibited excellent biocompatibility and could be applied in the field of bioimaging. Additionally, the white-emitting CDs demonstrated outstanding performance in WLEDs. An's team [112] also confirmed that changing the solvent can tune the emission color of the synthesized CDs. They used citric acid as the carbon source and various alkylamines as solvents to prepare multicolor solid-state fluorescent CDs via hydrothermal/solvothermal method. Meng et al. [115] utilized p-phenyldiacetonitrile and terephthalaldehyde as precursors to prepare blue, green, and red-emitting CDs with diameters of 2.5 nm, 3.8 nm, and 4.6 nm, respectively, via a hydrothermal/solvothermal method. This was facilitated by the edge -CN groups promoting curvature in different lengths of π-conjugated carbon quantum belts, thus forming CDs with different diameters. Specifically, p-phenyldiacetonitrile and terephthalaldehyde were dissolved in deionized water, sonicated for 10 min, and then the mixed solution was transferred to a polytetrafluoroethylene-lined high-pressure reactor. The reaction was conducted at 120 ℃, 140 ℃, and 160 ℃ for 1 h, 1.5 h, and 2 h, respectively. After cooling, further centrifugation and purification yielded blue, green, and red-emitting CDs. The yields reached 37%, 50%, and 62% respectively, with PLQYs of 38%, 46%, and 30%. By weighing, they found that CDs were produced on a gram scale, making this method suitable for industrial-scale production. Moreover, the prepared CDs hold vast application prospects in WLEDs. Another study [116] altered the commonly used purification methods, such as centrifugation and dialysis, by employing silica gel column chromatography to purify the synthesized CDs. In this research, glutathione was mixed with formamide, heated at 160 ℃ for 4 h via solvent thermal treatment, and then purified using silica gel column chromatography to obtain CDs with a diameter of 3 nm. In order to reduce the cost of preparing CDs, Chen et al. [26] utilized refractory organic pollutant, Reactive Red 2, as a carbon source to synthesize CDs using a hydrothermal/solvothermal method. The synthesized CDs had an average diameter of 2.43 nm. The prepared CDs were applied in bioimaging. This method did not require strong acids or other oxidants, making it an environmentally friendly approach for CDs synthesis. Furthermore, using organic pollutants as carbon sources not only saves costs but also turns waste into a valuable resource. In recent years, our research group developed a method for synthesizing CDs using a hydrothermal/solvothermal method based on deep eutectic solvents (DES). The preparation process consisted of two steps. Firstly, p-phenylenediamine and polyethylene glycol 400 (PEG400) were mixed at a molar ratio of 1:38, stirred at 110 ℃ for 40 min, and then vacuum-dried at 60 ℃ for 2 h to obtain PEG-DES. Secondly, a certain amount of PEG-DES was mixed with absolute ethyl alcohol solvent and added to a high-pressure reactor, heated at 210 ℃ for 12 h. After cooling to room temperature, the mixture was filtered using a 0.22 µm organic filter membrane, yielding orange-emitting CDs with an average diameter of 2.95 nm, a fluorescence lifetime of 8.95 ns, and a PLQY of 14.47%. Furthermore, by exploiting the selective fluorescence quenching effect of model naphthenic acids (NAs) on CDs, quantitative detection of NAs was achieved (Fig. 13b) [14]. Using the same method, our research group synthesized nitrogen-rich N-CDs using 1H-Benzotriazole and choline chloride as precursors and anhydrous ethanol as the solvent. The PLQY of N-CDs was 25%. Nitrogen doping facilitated the enhancement of PLQY in the preparation of CDs. The prepared N-CDs were utilized for simultaneous detection and treatment of CO²⁺ [117]. Hydrothermal/solvothermal method could also be utilized to prepare CDs using large organic molecules as carbon sources. For instance, in 2023, Kang et al. [118] utilized polyacrylic acid (PAA) as a carbon source to synthesize tunable room-temperature phosphorescent CDs. Firstly, PAA was dissolved in deionized water, and then a certain amount of HCl was added. The mixed solution was then added to a high-pressure reactor and heated at 200 ℃ for 10 h. After cooling, filtration was carried out using a 0.22 µm membrane, followed by dialysis for 48 h to remove impurities and excess HCl. Finally, CDs were obtained by freeze-drying (Fig. 13c). During the preparation process, carboxyl groups existed in the form of carboxyl dimer association and isolated carboxyl. Hydrogen bonds were easily formed between two carboxyl groups, generating triplet energy levels. Therefore, phosphorescent emission from the triplet excited state to the singlet ground state could be observed, which was the fundamental reason for the room-temperature phosphorescence exhibited by the prepared CDs.

  Figure 13

  

  Figure 13.  (a) Schematic diagram of tuning CDs fluorescence properties by changing the functional groups of carbon sources. Reproduced with permission [114]. Copyright 2021, American Chemical Society. (b) Preparation process of CDs and mechanism of NAs detection. Reproduced with permission [14]. Copyright 2023, Elsevier. (c) Schematic diagram of the preparation of CDs. Reproduced with permission [118]. Copyright 2023, John Wiley and Sons.

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  Hydrothermal/solvothermal method only requires high pressure reactor equipment, the equipment cost is low, and the carbon source required for preparation is very rich. In the preparation process, the operation is simple, the preparation process is highly controllable, and the properties of CDs can be accurately controlled by adjusting the reaction conditions, carbon sources, solvents, etc. The entire reaction takes place in a sealed high-pressure reactor, preventing the release of toxic substances into the environment. This method is an economical, environmentally friendly, green, and efficient method for synthesizing CDs, and has been widely used in recent years.

  4.2.2   Microwave method

  The bottom-up microwave method can rapidly generate high temperatures within small molecular carbon sources, leading to the decomposition of the carbon source into carbon atoms or small carbon clusters. Subsequently, these carbon atoms or clusters gradually aggregate into larger carbon nanostructures, ultimately forming CDs.

  In 2009, Zhu's team [119] first employed the microwave method to prepare CDs with high fluorescence intensity, good water solubility, and photobleaching resistance. Specifically, they initially dissolved a certain amount of PEG-200 and glucose in distilled water, forming a homogeneous, transparent mixture solution. Subsequently, the mixture solution was heated in a 500 W microwave oven for 2–10 min. During the heating process, the colorless mixture solution turned yellow, and eventually dark brown, confirming the formation of CDs. CDs prepared by this method not only exhibited PL properties but also ECL properties, with their ECL luminescence mechanism being the same as that of Zheng's work [78]. Microwave preparation of CDs not only significantly reduced the preparation time but also their stable PL and ECL endowed them with potential applications in biosensing. Since then, glucose had become one of the most commonly used carbon sources for preparing CDs by microwave method. For example, Tang et al. [120] prepared CDs with an average particle size of 1.65 nm and a PLQY of 7%−11% by microwave-assisted hydrothermal method using glucose as carbon source. The author analyzed the formation mechanism of CDs by a series of characterization methods. First, glucose molecules were dehydrated by heat to form carbon core rich in C=C; Secondly, the glucose molecules were dehydrated at the edge of the carbon core, forming new C=C, promoting the growth of the carbon core, and the high pressure generated under hydrothermal conditions makes the newly generated C=C orderly, so that the generated CDs had a crystal structure; Finally, the functional groups located on the surface of the carbon core became the surface passivation layer of the CDs (Fig. 14a). Deep ultraviolet (DUV) emission at 303 nm was observed for the first time at 197 nm excitation wavelength for the generated CDs. This work provided a new idea for developing DUV photonic devices. For another example, Arroyave's team [121] also used glucose as a carbon source to prepare CDs by microwave method and analyzed the detailed structure of the prepared CDs. Based on the high solubility of CDs in polar solvents, the types and possible structures of CDs were determined by solution-state NMR spectra and theoretical calculations (density functional theory and time-dependent functional theory). The characterization and calculation results showed that polymer dots (PDs), carbon dots polymer (PCDs), or a combination of both exist in the prepared CDs (Fig. 14b). PDs contained only a polymer structure, while PCDs were formed by small carbonized core surrounded by a polymer structure. It was worth noting that the purification step was critical in the process of preparing CDs. CDs must be purified by filtration, dialysis, and column chromatography before being characterized. In this paper, the structure of CDs prepared by microwave method was studied in detail for the first time through characterization and theoretical calculation. This study provided a new approach for the development of structural characterization of CDs synthesized by other methods and other different carbon sources. Nitrogen-doped CDs could also be prepared by microwave method by selecting glucose as carbon source and adding an appropriate amount of nitrogen source (such as urea). R Thara et al. [122] used this method to prepare N-doped CDs with blue fluorescence with PLQY of 14.9%. Specifically, a certain amount of glucose and urea was dissolved in distilled water, and then the mixture was irradiated by microwave for 15 min. In the process of irradiation, the water was evaporated to form a grayish-white precipitate, and N-CDs were obtained after the grayish-white precipitate was charred. The product was further dissolved in distilled water and purified by ultrasonic treatment, filtration, and centrifugation. The PL of the prepared N-CDs was excitation dependent, and excitation at 300–340 nm led to hyperchromic shift, and PL was time and light stable. The prepared N-CDs could selectively and sensitively detect tetracycline with dual sensors (fluorescence sensing and electrochemical sensing) and could be used for visible light catalytic degradation of tetracycline. In addition to glucose, citric acid was another carbon source commonly used for preparing CDs by microwave method. Kasprzyk and Qu's team [54, 123] prepared nitrogen-doped CDs with green fluorescence and PLQY of 14% using citric acid as carbon source and urea as nitrogen source. In the specific process, a certain amount of citric acid and urea were dissolved in distilled water, and the mixed solution was placed in an open container and heated in a 750 W microwave oven for 4–5min. During the heating process, the mixed solution was changed from colorless to brown and finally to a dark brown cluster solid. Then the generated solid was heated in a vacuum oven at 60 ℃ for 1 h to remove impurities. The solid was dissolved in distilled water, and the large particles were removed by centrifugation to obtain CDs with good water solubility. The prepared CDs exhibited excitation related PL. The green fluorescence was found to be due to the formation of HPPT by NMR. The prepared CDs had low toxicity, good biocompatibility and excellent PL properties, so it could be used in the fields of new fluorescent ink and information encryption. However, the PLQY of nitrogen-doped CDs was not very high, which made it easy to limit the application of CDs. Therefore, researchers shifted their goal to prepare CDs doped with other hybrid elements in order to obtain higher PLQY. Therefore, Zheng et al. [124] prepared solid-state Si-CDs with PLQY up to 65.8% by microwave-assisted hydrothermal method. Specifically, citric acid was used as carbon source, KH-792 as distance barrier chains and Si source, and the initial reaction molar ratio was 1:5, dissolved in deionized water, and the mixture was heated at 180 ℃ in Monowave 300 for 5 min, then cooled, filtered and dried to obtain the product. The addition of KH-792 could provide steric long chains and prevent aggregation-induced quenching by maintaining an appropriate distance between Si-CDs particles, thus achieving high PLQY fluorescence emission. The author investigated the effect of reaction temperature on the PL properties of Si-CDs. Too low temperature led to incomplete carbonization and silication, while too high temperature led to serious carbonization, resulting in particle growth and aggregation. Therefore, it was very important to select the suitable temperature. The PL of Si-CDs exhibited excitation independence and appeared bright blue under ultraviolet light. The application of Si-CDs in fluorescent films, WLEDs and backlight displays was realized by combining the film forming characteristics and photothermal stability of Si-CDs (Fig. 14c). High PLQY could also be achieved by multi-element doping. In 2016, Choi et al. [125] used microwave method to prepare BN-CDs with an average diameter of 2.8 nm, using citric acid as carbon source, ethylenediamine as nitrogen source, and boric acid as boron source. The highest PLQY was 80.8%. For comparison, N-CDs were prepared at the same time. Under ultraviolet irradiation, BN-CDs showed bright blue emission, and the emission spectrum showed an excitation-independent behavior. But the emission spectra of N-CDs showed excitation dependence. Through a series of spectral analysis and theoretical calculation, it was concluded that the abundant graphite structure and uniform surface states in the core of BN-CDs were the reasons for the enhancement of PL in BN-CDs. In addition, single-molecule spectral analysis showed that a single BN-CD showed a high PL compared to N-CD, which was caused by an increase in the number of photons emitted by each BN-CD. The CDs prepared by microwave method mentioned above do not mention multicolor luminescence, but it can also be prepared by simple microwave method. Fu et al. [126] prepared solid-state fluorescent multi-color carbon dots (SFM-CDs) using a simple microwave method. The authors irradiated a mixture of citric acid (carbon source) and l-cysteine (sulfur source and nitrogen source) and NaOH or KCl (used to adjust the structure of CDs) in a microwave oven for 3–5 min. During the microwave heating process, the microwave was stopped at regular intervals and the mixture was stirred to promote a uniform reaction. After microwave irradiation, the product was cooled to room temperature and dispersed in water. After dialysis for 8 h, blue, yellow and red luminescent CDs with PLQY of 54.68%, 17.93% and 2.88% were obtained. Finally, three kinds of color emission CDs were used as phosphors to prepare multi-color LEDs. WLEDs were prepared by combining three kinds of CDs as phosphors, and cold, neutral and warm WLEDs were formed by controlling different proportions. In another study, glutaraldehyde (GA) as a crosslinking agent was added to poly(ethylenimine) (PEI) solution. By controlling the initial molar ratio of GA to PEI and heating at 180 ℃ in a microwave oven for 15 min, multicolored luminescent CDs were obtained, which can be adjusted from blue to yellow in the wavelength range of 464–556 nm. The prepared fluorescent CDs can be used as fluorescent probes for cell imaging and detection of Co²⁺[127]. Interestingly, CDs prepared by microwave method could also achieve dual detection through fluorescence quenching-recovery. Wang et al. [128] obtained red emission CDs from p-phenylenediamine through microwave treatment. Glutathione (GSH) could quench the red fluorescence of the prepared CDs, and the quenched fluorescence could be recovered by increasing the temperature (Fig. 14d). Thus, red-emitting CDs could be used for "on-off-on" fluorescence sensing of GSH and temperature measurement through fluorescence quenching and recovery processes, respectively. In a recent study, yellow fluorescent chitosan-based CDs were prepared by microwave method. In the specific preparation process, chitosan was dissolved in acetic acid solution to obtain chitosan-acetic acid solution. Then a certain amount of o-phenylenediamine was added to the chitosan-acetic acid solution. The solution was mixed evenly with ultrasound for 20 min. The mixed solution was heated in a 700 W microwave oven for 40 min. During the reaction, 10 mL water was added to the reaction system every 5 min. After the reaction was complete, the mixed system was transformed into a solid powder, which was then dissolved in deionized water after cooling, and then further centrifuged to remove the precipitation and finally obtained chitosan-based CDs. The CDs prepared by this method had time stability, high salt stability, and light stability, which provided conditions for Al³⁺ detection. By cultivating zebrafish embryos, it was confirmed that the prepared CDs had low toxicity and biocompatibility, and could be applied in the field of bioimaging (Fig. 14e) [27].

  Figure 14

  

  Figure 14.  (a) Schematic diagram of the preparation of CDs by MAH method. Reproduced with permission [120]. Copyright 2012, American Chemical Society. (b) The preparation route of CDs and the possible structure of PCDs. Reproduced with permission [121]. Copyright 2021, Elsevier. (c) Synthesis and application of Si-CDs. Reproduced with permission [124]. Copyright 2017, Royal Society of Chemistry. (d) Schematic illustration of preparing red-emitting CDs for fluorescent "on-off-on" sensing. Reproduced with permission [128]. Copyright 2016, John Wiley and Sons. (e) The preparation and applications of CDs. Reproduced with permission [27]. Copyright 2024, Elsevier.

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  Microwave method is a simple and fast method to prepare CDs, which can shorten the preparation time to a few minutes. This is because the microwave has a strong penetration ability and can heat the carbon source to produce high temperature in a very short time to react. The CDs prepared have excellent luminescence properties, and the carbon source of CDs prepared by the bottom-up microwave method is very rich, which has become one of the most commonly used methods to prepare CDs like hydrothermal/solvothermal method.

  4.2.3   Plasma method

  Plasma is a state of matter typically formed when a gas is energized to high energy states. Various methods can generate plasma, including ionization, photoionization, radiative ionization, and collisional ionization. Ionization generally refers to the ionization of atoms or molecules in a gas through heating or electron bombardment, forming positively charged ions and negatively charged electrons, and positive and negative charges interact to produce high temperatures. In the process of preparing CDs by plasma method, ionization is often used to produce plasma. Carbon sources are evaporated into a gaseous state within a high-temperature plasma, and then further polymerizes and grows to eventually form CDs.

  Li's team [129] proposed a new strategy for preparing CDs - plasma method. A certain amount of acrylamide was placed in the plasma reactor, and treated in the air plasma generated by the plasma reactor under 150 W conditions for 10 min, resulting in dark brown powder. The dark brown powder was dispersed in ethanol and subjected to ultrasound, followed by centrifugation to remove non-fluorescent substances. Finally, the supernatant was filtered using an ultrafiltration membrane to remove impurities, obtaining blue-emitting CDs with particle sizes ranging from 3 nm to 4 nm and a PLQY of 6%. The prepared CDs were successfully applied in WLEDs, and had a higher color rendering index than traditional WLEDs, showing wide prospects in optoelectronic devices. Later, Wang et al. [15] reported a method for synthesizing CDs using an atmospheric pressure micro-plasma anode device. This method differed from the work of Li [129]. A certain amount of citric acid and ethylenediamine were dissolved in deionized water to form a mixed solution, which was then subjected to micro-plasma treatment in an "H"-shaped micro-plasma device. The entire reaction took place under a discharge voltage of 2500 V, a current of 6 mA, and an atmosphere of argon gas. After micro-plasma treatment for 30 min, followed by dialysis, blue fluorescent CDs with a particle size distribution of 1–3 nm and a PLQY of 5.1% were obtained (Fig. 15a). By comparison, it was found that a large number of oxide species were generated at the plasma anode, providing favorable conditions for the preparation of CDs. The authors further investigated the formation mechanism of CDs, suggesting that CDs were formed through condensation reactions between citric acid and ethylenediamine, and micro-plasma could accelerate the rate of condensation reactions, thereby speeding up the formation rate of CDs. The prepared CDs exhibited excitation and pH-dependent PL, thus could be used as pH sensors. Importantly, the synthesized CDs were first used to detect Uranium, with a detection limit of 0.71 ppm, demonstrating good selectivity and sensitivity. Sugars could also serve as carbon sources for the plasma method to prepare CDs. Treepet et al. [130] used three simple sugars as carbon sources to prepare blue-emitting Fru-CDs, Glu-CDs, and Suc-CDs using the solution plasma (SP) method (Fig. 15b). Specifically, sugar aqueous solutions were added to a glass reactor and subjected to plasma treatment for 20 min using a double-pulse power supply. During the synthesis process, the effects of the molecular structure of the carbon source (fructose, glucose, and sucrose), solution pH (2, 5, and 8), and drying method (freeze-drying at 80 ℃ for 48 h and heat-drying at 80 ℃ for 10 h) on the properties of the prepared CDs were investigated. The results showed that only blue-emitting CDs could be generated at pH 5 and 8 and under heat-drying conditions. Through characterization analysis, the roles of each influencing factor were obtained. The molecular structure of the carbon source and the plasma determined the formation and orderliness of the carbon core (Fru-CDs exhibited the highest degree of order). Heat-drying led to surface oxidation of CDs, converting O-H to C=O functional groups. Among all the prepared CDs, Fru-CDs-5 synthesized at pH 5 exhibited the strongest fluorescence intensity, with a PLQY of 9.16%. This may be attributed to the more organized carbon core structure and the presence of more C=O functional groups on the surface. Based on its selectivity and sensitivity to Fe³⁺, Fru-CDs-5 was successfully applied for the detection of Fe³⁺.

  Figure 15

  

  Figure 15.  (a) Schematic of preparing CDs with the atmospheric-pressure micro-plasma method. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry. (b) Schematic diagram of the process of synthesizing CDs from three sugars using SP. Reproduced with permission [130]. Copyright 2022, Elsevier.

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  From the examples above, it can be observed that plasma method for synthesizing CDs can significantly reduce reaction time, enabling rapid production of CDs. Furthermore, the morphology, size, and optical properties of CDs can be controlled by adjusting the parameters of the plasma reactor. However, CDs prepared using this method typically exhibit lower PLQY, which limits the application of CDs. Additionally, plasma method requires high-energy input resulting in higher preparation costs, which limits the large-scale use of this method.

  4.2.4   Electrochemical method

  In contrast to the top-down electrochemical method for preparing CDs, the bottom-up electrochemical method typically employs non-carbon electrodes as the anode and cathode, and the solution containing organic carbon source as electrolyte to prepare CDs.

  In 2011, Li et al. [131] prepared fluorescent CDs using an electrochemical method with ethanol as the precursor. Using Pt rods as cathode and anode and sodium hydroxide solution dissolved with ethanol as electrolyte, the electrolyte was electrochemically treated at room temperature for 4 h without reference electrode. Finally, the brown solution with CDs was obtained by dialysis and filtration. The prepared CDs had a diameter of 3–7 nm, a PLQY of 4%, and a fluorescence lifetime of 6.74 ns. The authors speculated that the formation of CDs underwent four stages: ethanol decomposition, polymerization, growth, and oxidation (Fig. 16b). The high concentration of NaOH used as an activator for ethanol was identified as a key factor in ethanol decomposition during the preparation process. The CDs exhibited PL with different colors under various excitation wavelengths (Fig. 16a), along with up-conversion luminescence properties and pH-sensitive PL characteristics. These properties made them suitable for applications in novel fluorescent probes and biomedical fields. However, their work faced challenges such as low PLQY and low yield. Later, Niu et al. [132] employed a mixture of acetonitrile and the ionic liquid BMIMPF₆ as the electrolyte, with Pt sheets serving as the working and auxiliary electrodes, to electrochemically synthesize CDs. During the electrolysis process, as the electrolysis reaction time extended, the color of the electrolyte gradually changed from colorless to yellow and eventually turned deep brown. Due to the presence of positively charged BMIM⁺ ions, the resulting CDs appeared as precipitates at the cathode. The CD's precipitate could also be dispersed in water, turning it yellow. TEM analysis revealed that the average size of the CDs was 3 nm. Under 365 nm ultraviolet light, the CDs emitted bright blue fluorescence. By adjusting the volume ratio of acetonitrile to the ionic liquid, the PLQY was enhanced to 13.3%. The prepared CDs were successfully applied in cell imaging and the detection of iron ions. This work effectively solved the problem of low PLQY. Until 2023, Michenzi and co-workers [133] employed a bottom-up electrochemical method to synthesize CDs and successfully applied them in catalyzing the Knoevenagel condensation reaction (Fig. 16c). The catalytic activity of CDs not only enhanced the yield of the condensation product (up to 98%) but also allowed for recyclability. The authors initially obtained an Aqueous solution (AQS) from orange peel waste (OPW) through the hydrothermal carbonization (HTC) process. Subsequently, using AQS as the precursor, Pt wires as both the anode and cathode and under alkaline conditions, they electrochemically synthesized constant potential and constant current CDs. The synthesis of CDs exhibited a very high yield, reaching up to 37%. This method utilized biomass as a raw material to obtain the precursor for electrochemical synthesis of CDs, reducing costs, effectively solving the problem of low yield, and presenting an environmentally friendly and green preparation method.

  Figure 16

  

  Figure 16.  (a) Fluorescence microscope images of CDs under different excitation. Reproduced with permission [131]. Copyright 2011, Royal Society of Chemistry. (b) Formation mechanism of CDs. Reproduced with permission [131]. Copyright 2011, Royal Society of Chemistry. (c) Synthesis and catalytic application of CDs. Reproduced with permission [133]. Copyright 2023, Elsevier.

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  4.2.5   Pyrolysis method

  The bottom-up pyrolysis method is similar to the top-down pyrolysis method in that the carbon source (small molecules) undergoes carbonization and polymerization reactions under high temperature conditions, followed by purification steps (filtration, centrifugation, dialysis, electrophoresis, column chromatography, etc.) to obtain CDs. The morphology, size, and optical properties of CDs can be adjusted by controlling pyrolysis temperature, time, and reaction atmosphere.

  In 2008, Bourlinos and co-workers [134] used pyrolysis method to prepare carbon dots for the first time. The authors obtained organophilic and hydrophilic CDs by pyrolyzing citrate salts or 4-aminoantipyrine. The prepared CDs had an average particle size of 7 nm and exhibited excitation independence within the 400–500 nm excitation range, with a PLQY of 3%. Although the PLQY in this study was very low, it provided a new insight into the preparation of CDs. Inspired by Bourlinos [134], Ye et al. [135] obtained nitrogen-doped CDs also by pyrolyzing citrate. In specific experimental procedures, a beaker containing ammonium citrate was placed in an oven at 180 ℃ for 2 h to obtain CDs. In order to purify the CDs, the following steps were taken: Centrifugation for 15 min, dialysis for 12 h, PVDF membrane filtration, and drying in a vacuum oven at 60 ℃ for 24 h These steps resulted in high-purity solid CDs. The CDs exhibited high solubility in distilled water. CDs had found widespread applications in various fields such as bioimaging, biosensing, drug delivery, catalysis, optoelectronic devices. This research work, apart from these applications, applied the prepared CDs as efficient corrosion inhibitors for carbon steel and experimentally demonstrated a corrosion inhibition efficiency of up to 94.7%. This study serves as an inspiration for expanding the applications of CDs. The preparation process of the pyrolysis method was mostly similar, where the carbon source was directly calcined at high temperatures to obtain CDs. Equal masses of PMA, BTA, and TPA were mixed with a certain amount of boric acid separately, then calcined at 220 ℃ in a muffle furnace for 10 min, resulting in CDs with PLQY of 49%, 64%, and 78% and emitting light in light blue, purple, and deep blue colors, respectively. Importantly, the prepared CDs also exhibited room temperature phosphorescence, showcasing broad potential applications in areas such as anti-counterfeiting and information encryption [136]. Various methods could also be employed to assist in the synthesis of CDs with specific properties. Nugroho's team [137] first prepared CDs through a hydrothermal synthesis method, then used citric acid as a carbon source to prepare GQDs via pyrolysis method at temperatures ranging from 220 ℃ to 240 ℃. During the process, the prepared CDs were added to the mixture and thoroughly stirred with NaOH to obtain CDs@GQDs composite materials with shared properties of CDs and GQDs (Fig. 17a). Characterization analysis revealed that the particle size of CDs@GQDs was approximately 2.7 nm, with a PLQY of 15%, higher than that of individual CDs and GQDs. Although the PLQY was still not very high, the composite material was successfully applied in the detection of Cr⁶⁺, demonstrating very high selectivity and sensitivity.

  Figure 17

  

  Figure 17.  (a) Schematic diagram of the preparation process of CDs@GQDs for Cr⁶⁺ detection. Reproduced with permission [137]. Copyright 2024, Elsevier. (b) Preparation and internalization of CNPs for LDs imaging. Reproduced with permission [138]. Copyright 2021, American Chemical Society.

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  Due to its abundant carbon sources, low cost, and simple operation, the bottom-up pyrolysis method for preparing CDs has become relatively widespread in recent years. However, it also has some issues, such as relatively low PLQY and the need for cumbersome post-treatment, so there is still a need to improve these existing problems to further expand the application of the pyrolysis method for preparing CDs.

  4.2.6   Chemical oxidation method

  The bottom-up chemical oxidation method utilizes small molecules as carbon sources, triggering the gradual oxidation of the carbon source through oxidizing agents such as acids, ultimately leading to the formation of CDs. In 2021, Liu et al. [138] utilized o-phenylenediamine (oPD) as a precursor and concentrated HNO₃ as an oxidizing agent to prepare CPDs through a chemical oxidation method. The specific experimental procedure involved adding diluted concentrated HNO₃ to the oPD aqueous solution, conducting a polymerization reaction at room-temperature (RT) for 4 h, adjusting the pH to neutral with NaOH, and finally obtaining CPDs through centrifugation, filtration, and chromatographic column separation. The synthesized CPDs exhibited uniform size, with dimensions smaller than 10 nm, and high dispersion. Additionally, CDs were prepared using a high-temperature (HT) hydrothermal method. Both carbon nanodots (CNPs) were collectively applied for monitoring and tracking lipid droplets (LDs). CNPs selectively illuminated LDs inside cells, producing high-quality images. CDs demonstrated good intracellular retention capability, enabling the long-term tracking of LDs outside cells, and providing insights into the development of long-term cell imaging probes (Fig. 17b). Although the carbon dots produced by this method have advantages such as amphiphilicity, high photostability, and long half-life, similar to top-down chemical oxidation methods, the preparation process involved the use of strong oxidizing HNO₃, which might pose environmental risks. The excessive HNO₃ required neutralization with strong base, leading to increased reagent costs and potential further environmental pollution. Therefore, it was advisable to avoid the use of strong acids. Another research group employed FeCl₃ as an oxidizing agent and p-phenylenediamine as a carbon source to prepare near-infrared-emitting CDs through oxidative polymerization. The prepared CDs exhibited a tunable solvent-color effect, providing a new perspective for multicolor applications. This method avoided the use of strong acids, making it a more environmentally friendly approach for the preparation of CDs [139].

  4.2.7   Template method

  The principle of preparing CDs by template method is to form CDs of specific shape and size by using the size of the limited space of template molecules through the interaction of carbon source and template molecules. Limited by the size of template molecules, the size of CDs prepared by template method is easier to control and more uniform size distribution.

  In 2009, Liu et al. [140] used silica as a template to prepare CDs with particle sizes ranging of 1.5–2.5 nm for the first time. Initially, they utilized amphiphilic triblock copolymer F127-functionalized silica colloidal spheres as carriers and soluble phenolic resin as a carbon precursor to prepare satellite-like polymer/F127/silica composite materials via an aqueous route. Subsequently, the template composite materials were subjected to high-temperature treatment and further acid etching to produce nanoscale CDs. Finally, water-soluble multicolor luminescent CDs were obtained through simple surface passivation (Fig. 18a). The template method allowed for effective control of the aggregation of generated CDs and is expected to become a potential bioimaging agent providing single-molecule resolution. In another study, silica was also used as a template for preparing CDs. Mesoporous silica spheres were obtained using an impregnation method as nanoreactors, with citric acid as the carbon source and NaOH as the etchant, resulting in CDs with particle sizes ranging from 1.5 nm to 2.5 nm and a PLQY of approximately 23%, emitting blue light. Additionally, the prepared CDs exhibited up-conversion fluorescence, making them potentially useful for synthesizing novel photocatalysts [141]. Apart from silica, mesoporous molecular sieves can also be used as templates. Phenol or aniline as carbon sources were filled into the SBA-15 template. After pyrolysis at 200 ℃ in air, CDs were formed in the SBA-15 channels, and they were extracted by ethanol dissolution. The SBA-15 after removing the CDs could be reused after annealing treatment (Fig. 18b). The prepared CDs exhibited high yield (up to 65%) and PLQY of 88%. The preparation process did not generate any waste of resources and did not involve hazardous chemicals, making it an environmentally friendly synthesis method [142]. Several methods could be chosen to collaboratively prepare CDs, and it was also possible to combine bottom-up and top-down approaches to synthesize CDs. For instance, Chernyak's team [143] employed a combination of bottom-up (template pyrolysis) and top-down (oxidation) methods to prepare CDs. They grew graphene nanoflakes (GNF) with a size of 20 nm on MgO templates (obtained through prolonged precipitation of magnesium oxalate, followed by calcination at 500 ℃) and then oxidized the GNF with HNO₃ treatment. Nitrogen-doped CDs were obtained through centrifugal dialysis. By varying precursor types, synthesis temperature, and oxidation time, the color of the CDs could be changed from green to orange.

  Figure 18

  

  Figure 18.  (a) Synthesis process diagram of multicolor photoluminescent CDs prepared by template method. Reproduced with permission [140]. Copyright 2009, John Wiley and Sons. (b) Synthesis process of CDs using SBA-15 as template. Reproduced with permission [142]. Copyright 2023, Elsevier.

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  The advantages of template synthesis of CDs lie in the ability to precisely control their morphology, size, and structure, thereby enhancing yield and fluorescence performance. However, the disadvantage is the complexity of the preparation process, requiring precise control of conditions and template selection, which may lead to resource wastage.

  4.2.8   Aldol condensation method

  Aldol condensation is a method to form carbon skeleton structure by polycondensation of aldol molecules under acidic or alkaline conditions, and finally to obtain CDs by separation and purification.

  In 2015, Hou's team [144] first employed the aldol condensation method to prepare CDs. They mixed a certain amount of NaOH with acetone and stirred vigorously for 1 h Subsequently, the mixture was left in ambient air, at a specific temperature and pressure for 120 h The pH of the mixture was adjusted to neutral using dilute HCl, followed by centrifugation, washing, and evaporation at 100 ℃ for 12 h, resulting in powdered CDs. The authors proposed a possible mechanism for the reaction. They suggested that, with the assistance of NaOH, acetone underwent aldol reactions, producing unsaturated ketones. These unsaturated ketones then underwent polymerization reactions to form polymer-like products with extended carbon chains. The coiling and entanglement of these products led to the generation of numerous CDs (Fig. 19a). Dehydration and hydroxylation reactions might also occur as side reactions during the process, resulting in a mixture of oligomers as products. Furthermore, the authors treated the mixture left standing for 120 h in an Ar atmosphere to obtain a 3D structure carbon material. This material exhibited excellent electrochemical performance, including an ultra-long cycle life and ultra-fast rate, making it a promising candidate for commercial sodium-ion battery (SIBs) electrode materials. Later, this research group achieved the preparation of kilogram-scale CDs through the aldol condensation method. The authors used ethanol and NaOH as raw materials, and under optimal reaction conditions, obtained 1.083 kg of CDs within 2 h. Additionally, by introducing urea and cysteine into the reaction, the preparation of nitrogen-doped carbon dots (NCDs) and sulfur/nitrogen co-doped carbon dots (NSCDs) was realized. These materials exhibited excellent cycling stability and rate performance in potassium-ion batteries [145].

  Figure 19

  

  Figure 19.  (a) The formation mechanism of CDs. Reproduced with permission [144]. Copyright 2015, John Wiley and Sons. (b) Diagram of CDs synthesis mechanism. Reproduced with permission [148]. Copyright 2019, Royal Society of Chemistry.

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  The aldol condensation method is a novel, simple, and low-energy-consuming approach for the preparation of CDs. This method not only provides an efficient and economical means for the large-scale synthesis of CDs but also expands the potential applications of CDs in functionalized carbon materials.

  4.2.9   Ultrasonic method

  Ultrasonic method can also be employed for the bottom-up synthesis of CDs. Precursors placed in the ultrasonic field generate small vacuum bubbles and undergo cavitation under the action of ultrasonic waves. These cavitation effects result in high-speed collision of liquid jets, disaggregation, and powerful fluid dynamic shear forces, accelerating the chemical reactions of precursors and enabling the rapid preparation of CDs.

  In 2011, Li et al. [146] first utilized a one-step ultrasonic method to synthesize fluorescent nano CDs. In a typical experiment, activated carbon was mixed with an H₂O₂ solution to form a turbid suspension. The suspension was subjected to ultrasonic treatment for 2 h, filtered through a 2 nm pore size membrane, and H₂O₂ was evaporated, and water-soluble CDs were obtained by dissolving in deionized water. The prepared CDs exhibited multi-color luminescent properties, emitting different fluorescence under various excitation wavelengths. Due to their multiphoton activity, the synthesized CDs also demonstrated up-conversion fluorescence characteristics. These properties made them suitable for applications in novel fluorescent labeling, optical imaging, and biomedical. In a later study, Zhou et al. [147] employed citric acid as a carbon source and 1, 2-phenylenediamine as an N-doping agent to synthesize yellow-emitting CDs using an ultrasonic method. The average particle size of the prepared CDs was 2 nm, with a PLQY of 1%. The synthesized CDs were then embedded into sodium polyacrylate and successfully applied in 3D printing. CDs as a lube can also be prepared by ultrasonic method [148]. A certain proportion of citric acid (CA), urea, and polyethylene glycol (PEG) were mixed, and then ultrasonic treated at room temperature for 60 min. CDs-based lube was obtained. The principle involved the dehydration carbonization of CA and urea, forming nitrogen-doped carbon core. Subsequently, the surface of the carbon core was coated with PEG, creating a surface passivation layer (Fig. 19b). Scaling up the mass of each reactant in the same proportion allowed for the large-scale preparation of CDs-based lube. This method was green, efficient, and high-yielding, providing a new avenue for the development of lube.

  4.2.10   Microfluidic method

  The principle of microfluidic methods for preparing CDs involves controlling chemical reactions within microchannels on a microfluidic chip. This enables precise mixing and reaction of carbon sources and surface-modifying agents at the microscale, efficiently synthesizing fluorescent CDs.

  In 2014, Lu et al. [149] first reported the use of microreactors as a research tool for CDs. Specifically, the authors initially screened hundreds of reaction conditions (carbon precursor, solvent, dopant, etc.) to obtain tunable maximum PL. Subsequently, by precisely controlling the reaction time, CDs at different developmental stages were obtained. Glucose was dissolved in formamide to obtain the stock solution, which was then pumped into a polytetrafluoroethylene capillary at a constant rate using a syringe pump. The capillary was heated to 180 ℃ in an oil bath with different residence times, and the final product was purified through vacuum distillation and dialysis to obtain CDs. The relationship between PL emission and the developmental stages of CDs was investigated. The conclusion was drawn that PL was primarily associated with surface defect states, rather than the composition, carbonization degree, or morphology size. This study elucidated the microreactor synthesis system, providing a promising tool for the rapid screening of reaction conditions for CDs. It also held significant theoretical research value. The continuous synthesis of full-spectrum fluorescent CDs can be achieved through microfluidic method. Shao et al. [150] dissolved citric acid (CA) and urea separately in deionized water to obtain transparent solutions. These two transparent solutions were pumped into a microchannel by two parallel-flow pumps. After reaction for 20 min, the products were washed with acetone and methanol to obtain CDs. Experimental characterization and DFT calculations indicated that the number of -NH₂ groups determined the energy gap of CDs, consequently influencing the fluorescence emission wavelength of CDs. This microfluidic method not only significantly reduced the reaction time but also produced more uniform and smaller CDs. More importantly, it enabled the preparation of full-spectrum fluorescent CDs. The synthesized CDs were successfully applied in Fe³⁺ detection and in vitro bioimaging. Microfluidic method can also be employed for the top-down synthesis of GQDs [151]. A graphite-water suspension was driven through micro Z-shaped channels by a high-pressure pump from the feed tank. The maximum flow velocity within the microchannels can reach 400 m/s, applying shear force to solid particles. Subsequently, graphite flakes were exfoliated into graphene sheets, further breaking down into nanoscale GQDs (Fig. 20a). The prepared GQDs had a diameter of 2.7 nm, thickness ranging from 2 nm to 4 nm, equivalent to 2–4 layers of graphene. PL properties exhibit excitation independence, with a PLQY of 1.32%. This method was environmentally friendly as it did not involve acid oxidation precursors. Unfortunately, the PLQY of prepared CDs was relatively low. To further improve the low PLQY problem, Rao and co-workers [152] fabricated an ultrasonic microreactor based on biomimetic leaf veins and applied it to prepare CDs (Figs. 20b and c). Aqueous ammonium citrate precursor and N, N-dimethylformamide (DMF) modifier were injected into the microreactor at a constant flow rate. Under the dual effects of ultrasound cavitation and high temperature-high pressure, the precursor solution rapidly underwent condensation and carbonization. The product was collected and CDs were obtained. The experiments revealed that by enhancing the intensity of the ultrasound field, the PLQY of CDs could be increased to 83.1%. Moreover, by adjusting key parameters such as precursor concentration, temperature, and flow rate, the preparation of multi-color spectral CDs could be achieved, with emission spectra covering the entire visible light range.

  Figure 20

  

  Figure 20.  (a) Microfluidic device diagram, Z-shaped channel, and GQDs formation process. Reproduced with permission [151]. Copyright 2016, American Chemical Society. (b) Schematic diagram of ultrasonic microreactor. Reproduced with permission [152]. Copyright 2023, Elsevier. (c) Structure design and key parameters of biomimetic leaf vein microchannel in microreactor. Reproduced with permission [152]. Copyright 2023, Elsevier.

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  Microfluidic method offers high controllability and repeatability, allowing for the directed synthesis and modulation of the optical properties of CDs. It provides a novel approach for the preparation of CDs with specific application characteristics. However, this method for synthesizing CDs has a relatively low PLQY and a relatively small yield, limiting its application.

  PL is one of the most attractive features of CDs. The brightness of PL is quantitatively reflected by the value of PLQY, which is deeply influenced by the synthesis methods and the carbon sources. Table 1 summarizes the PLQY of CDs prepared by different synthesis methods and carbon sources.

  Table 1

  

  Table 1.  PL quantum yields of CDs synthesized by different carbon sources and synthesis methods.

  DownLoad: CSV

  Synthesis methods	Carbon sources	LLQY (%)	Ref.	Synthesis methods	Carbon sources	PLQY (%)	Ref.
  Arc discharge method	Soot	1.6	[8]	Microwave method	Graphene oxide	22.9	[93]
  	Graphene	8.7	[57]		Graphene oxide	21.1	[94]
  Laser ablation method	Carbon target	10	[9]		Silkworm chrysalis	46	[95]
  	Graphite flakes	12.2	[59]		Palm kernel shell	43.07	[96]
  	Carbon cloth	35.4	[63]		Glucose	14.9	[122]
  	Graphene nanosheets	26.75	[66]		Citric acid	15	[54]
  Chemical oxidation method	Candle soot	1.9	[67]		Citric acid	65.8	[124]
  	Carbon soot	3	[68]		Citric acid	80.8	[125]
  	Activated carbon	12.6	[71]		Citric acid	54.68	[126]
  	Carbon soot	60	[72]		p-Phenylenediamine	15	[128]
  	Graphene oxide	7.4	[73]	Ultrasonic method	Food waste	2.85	[98]
  	Starch	11.4	[76]		Albizia flowers	24.7	[100]
  	p-Phenylenediamine	7	[139]		Oligomer polyamide resin	28.3	[101]
  Electrochemical method	Multiwalled carbon nanotubes	6.4	[77]	Ball milling method	Activated carbon	7.6	[13]
  	Graphite rods	12	[79]		Carbon black	2.23	[30]
  	Graphite electrode	11.2	[81]		Spent coffee grounds	3.71	[105]
  	Ethanol	4	[131]	Hydrothermal/ Solvothermal method	l-Ascorbic acid	6.79	[106]
  	Acetonitrile	13.3	[132]		p-Phenylenediamine	61.6	[111]
  Pyrolysis method	Coffee grounds	3.8	[87]		p-Phenylenediamine	14.47	[14]
  	Watermelon peel	7.1	[88]		NADES	25	[117]
  	Plant leaves	16.4	[89]	Plasma method	Acrylamide	6	[129]
  	Hair	17	[91]		Citric acid	5.1	[15]
  	Zingiberis rhizoma	5.2	[92]	Template method	Resols	14.7	[140]
  	Houttuynia cordata and citric acid	15	[137]		Citric acid	23	[141]

  5. Applications in wastewater treatment

  5.1 Sensing

  CDs possess excellent optical properties, including high fluorescence intensity, tunable colors, and light resistance. Utilizing optical characteristics such as fluorescence enhancement, fluorescence quenching, and resonance energy transfer of CDs, they can be used either alone or in combination with other materials for detecting heavy metal ions and organic pollutants in wastewater.

  In the field of heavy metal detection, researchers have successfully utilized CDs and their composite materials to detect a variety of ions such as Co(Ⅱ) [153], Pb(Ⅱ) [154], Cr(Ⅵ) [155], Hg(Ⅱ) [156, 157], Cu(Ⅱ) [158], Ag(Ⅰ) [159], and Pd(Ⅱ) [160].

  Wu et al. [153] prepared amino-modified carbon quantum dots (NH₂-CQDs) through a combination of electrochemical and ultrasonic methods. They utilized NH₂-CQDs as a fluorescence probe, based on the fluorescence quenching effect, to achieve highly sensitive detection of Co(Ⅱ). The method exhibited a linear relationship between Co(Ⅱ) concentration in the range of 50 nmol/L to 40 µmol/L, with a detection limit (LOD) reaching 12 nmol/L. Our research group [117] used natural deep eutectic solvents (NADES) as precursors and synthesized nitrogen-rich carbon quantum dots (N-CQDs) via a hydrothermal method. These N-CQDs were successfully applied for the detection and treatment of trace amounts of Co(Ⅱ) under high-salt conditions. The study not only achieved linear detection of Co(Ⅱ) in the range of 5–250 µmol/L with a LOD of 1.2269 µmol/L but also demonstrated a remarkable 99.98% removal efficiency of Co(Ⅱ), providing an efficient and environmentally friendly strategy for practical industrial wastewater treatment. Sebastian et al. [154] developed a functionalized DTPAN-fn-GQDs for sensitive detection of Pb(Ⅱ), achieving a linear detection range of 0.5–40 nmol/L and a LOD of 0.25 nmol/L. Gong et al. [161] devised a dual-emission nanohybrid based on NCDs-AuNCs, serving as a ratio fluorescence probe for Pb(Ⅱ) monitoring. This nanohybrid exhibited dual emission peaks at 460 nm and 625 nm under a single excitation wavelength of 360 nm. Quantitative detection of Pb(Ⅱ) was achieved by monitoring the ratio change between the two fluorescence emission peaks. The sensor demonstrated good selectivity and anti-interference capability in real water samples and enabled visual semi-quantitative determination of Pb(Ⅱ) using a paper-based sensor. Roy et al. [155] developed a portable fluorescence probe PV/BH@CD for the selective detection of Cr(Ⅵ) in wastewater. The probe operates based on the photoinduced electron transfer (PET) mechanism, demonstrating extremely high selectivity and sensitivity (with a LOD of approximately 66 nmol/L), and remains stable under different pH values and water quality conditions. To further enhance the sensitivity of Cr(Ⅵ) detection, Zhao et al. [162] proposed a carbon dot sensing system combined with a masking agent. They successfully constructed a new method for highly selective detection of Cr(Ⅵ) by using strong-emission carbon dots (CE-CDs) and a specific masking agent DMPS. The linear range extends up to 500 µmol/L, with a low LOD of 23 nmol/L. In another study, CDs were utilized as fluorescence probes for simultaneous detection of Cr(Ⅵ) and Hg(Ⅱ) [16]. Using 3, 5-dihydroxybenzoic acid and l-arginine as precursors via a hydrothermal method, CDs with spectral selectivity were synthesized. These CDs exhibit two distinct emission centers with fluorescence peaks at 354 nm and 460 nm, corresponding to optimal excitation wavelengths of 295 nm and 330 nm, respectively. Based on the inner filter effect, Cr(Ⅵ) can quench the excitation light at 295 nm for CDs and absorb the emitted fluorescence at 354 nm, enabling the detection of Cr(Ⅵ). On the other hand, Hg(Ⅱ) caused static quenching through electrostatic interactions with surface groups of CDs, significantly reducing the fluorescence intensity at 460 nm. This sensing strategy achieved the detection of both heavy metal ions, demonstrating its enormous potential in the field of environmental metal analysis. In earlier studies, CDs were used for the standalone detection of Hg(Ⅱ). Li et al. [156] utilized N-CDs derived from orange juice for the rapid detection of Hg(Ⅱ), demonstrating excellent selectivity and sensitivity. This method had been proven effective for detecting Hg(Ⅱ) in wastewater. Zhao et al. [158] successfully synthesized nitrogen-doped red-emitting carbon dots (N-CDs) using a hydrothermal method. N-CDs exhibited rapid and highly selective recognition capability for Cu(Ⅱ), with a LOD of 45.87 µmol/L and linearity observed in the range of 45–70 µmol/L. To further enhance the sensitivity of Cu(Ⅱ) detection, Bai et al. [163] combined the fluorescence properties of CDs with the specific adsorption function of an ion-imprinted polymer (IIP) to develop CD@Cu-IIP-A composite materials for Cu(Ⅱ)detection. Experimental results showed that CD@Cu-IIP-A can quantitatively analyze Cu(Ⅱ) in the range of 80–780 µmol/L, with a LOD of 3.17 µmol/L. In the field of Ag(Ⅰ)detection, teams led by Tang [159] and Jia [164] had both developed an optical sensing platform based on smartphones. The prepared CDs achieved sensitive detection of Ag(Ⅰ) using the RGB analysis software of smartphones, with a LOD of 56 nmol/L (Fig. 21a) [164]. This research provided an efficient, convenient, and real-time method for on-site monitoring of Ag(Ⅰ), opening up new avenues in the field of environmental pollutant detection. In another study, Gao et al. [160] developed a fluorescence probe based on L-CQDs for detecting Pd(Ⅱ). Pd(Ⅱ) can quench the fluorescence of L-CQDs, exhibiting a good linear relationship within the concentration range of 5–40 µmol/L, with a LOD of 0.1 µmol/L. In addition to the detection of the aforementioned ions, fluorescence-based sensors using CDs had also been utilized for detecting other types of ions such as Fe(Ⅲ) [165-168], Zn(Ⅱ) [169], U(Ⅴ) [15], F⁻ [170].

  Figure 21

  

  Figure 21.  (a) Illustration of the synthetic procedure of the or-CDs and their application in detection of Ag⁺ in the fluorescent and smartphone-assisted colorimetric dual-mode manner. Reproduced with permission [164]. Copyright 2023, Elsevier. (b) Schematic diagram of multifunctional sensor for detection of FQs and His based on fluorescence Y-CDs. Reproduced with permission [172]. Copyright 2018, American Chemical Society.

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  In recent years, CDs have not only demonstrated their potential in the detection of heavy metal ions but have also been successfully applied to the detection of organic compounds in wastewater, such as pharmaceutical wastewater containing antibiotics, petrochemical industry wastewater, and other industrial wastewater.

  Rahal et al. [171] prepared CQDs using citric acid and thiourea as precursors through a microwave-assisted method. The prepared CQDs were used for the selective detection of minocycline (MC), showing a good linear relationship when the concentration of MC ranged from 0.28 µmol/L to 30.39 µmol/L, with a LOD reaching 91 nmol/L. The authors also discussed the fluorescence quenching mechanism, and experimental characterization resulted along with theoretical calculations determined that the quenching mechanism conforms to the inner filter effect (IFE). The occurrence of IFE was attributed to the overlap between the absorption spectra of CQDs (as the emitter) and MC (as the absorber). Researchers were particularly excited that CDs could not only achieve sensitive detection of single substances but also dual detection. For instance, Lu et al. [172] constructed CDs fluorescence sensors capable of simultaneously detecting fluoroquinolones (FQs) and histidine (His). These sensors (Y-CDs) emitted bright yellow fluorescence, and the addition of FQs caused quenching of Y-CDs fluorescence. The authors examined the sensitivity of Y-CDs to three types of FQs (NOR, CIP, and OFX), and the results showed that Y-CDs exhibited good linear responses to FQ concentrations, with linear ranges of 0.05−50 µmol/L, 0.2−25 µmol/L, and 0.4−10 µmol/L, and LOD of 17, 35, and 67 nmol/L, respectively. The authors were surprised to find that introducing His into the quenched fluorescence system (Y-CDs-FQs) restored fluorescence. This was because FQs had a stronger affinity for His, and upon adding His, FQs would bind with His, causing FQs to detach from the surface of Y-CDs and thus restore the fluorescence of Y-CDs. The linear range obtained from experiments for His was 0.05−10 µmol/L, with a detection limit for His of 35 nmol/L (Fig. 21b). Other antibiotics such as ciprofloxacin [173] and oxytetracycline [174] had also been successfully detected with high efficiency and sensitivity. In the detection of organic pollutants in petrochemical industrial wastewater, our research group [175] synthesized natural deep eutectic solvents (NADES) using 5-ATZ and PEG400 as precursors. Based on NADES, N-CQDs were synthesized using a one-step hydrothermal method, with a PLQY of 7.22%. The prepared N-CQDs exhibited excellent fluorescence properties and stability, showing good responsiveness to naphthenic acids (NAs) and displaying a linear relationship within the NAs concentration range of 0.03−0.09 mol/L. The LOD was 0.000588 mol/L. N-CQDs were successfully applied to the detection of NAs in actual wastewater. In order to further enhance the sensitivity of detecting NAs, our research group [14] developed near-infrared PEG-CQDs based on polyethylene glycol (PEG-400) based deep eutectic solvents (DES). Compared to previous work [175], PEG-CQDs emitted orange fluorescence, with a PLQY increased to 14.47%. They exhibited stronger penetration capability for NAs, and the detection limit for NAs reached 0.16 mmol/L, effectively improving the detection sensitivity. In subsequent studies, other types of organic pollutants, such as phenol [176], catechol [177], toluene [178], and xylene [178] were also successfully detected using CDs. As an organic pollutant produced by the military industry and explosive manufacturing process, TNT will also cause a certain degree of pollution to the water environment, so it is necessary to detect and control TNT. Tian et al. [179] synthesized CDs using citric acid as the carbon source and ethylenediamine as the nitrogen source via a hydrothermal method. The CDs were further surface-passivated to obtain CDs@NH₂ solution. The prepared CDs@NH₂ exhibited excellent selectivity and sensitivity towards TNT, making them suitable for trace detection of TNT. They showed a good linear relationship (R² = 0.997) within the TNT concentration range of 0–1 µmol/L, with a detection limit of 0.213 µmol/L. Table 2 summarizes the applications of CDs/CDs composite materials in the field of sensing.

  Table 2

  

  Table 2.  The applications of CDs/CDs composite materials in sensing based on different precursors.

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  Detector	Carbon sources	Synthesis method	Fluorescence probe	PLQY (%)	Linear range	LOD	Ref.
  Co(Ⅱ)	Graphite rods	Electrochemical method	NH2-CQDs	—	50 nmol/L-40 µmol/L	12 nmol/L	[153]
  Co(Ⅱ)	NADES	Hydrothermal	N-CQDs	25	5–250 µmol/L	1.2269 µmol/L	[117]
  Pb(Ⅱ)	Graphite powder	—	DTPAN-fn-GQD	40	0.5–40 nmol/L	0.25 nmol/L	[154]
  Pb(Ⅱ)	Citric acid	Hydrothermal	NCDs-AuNCs	—	0.86–20 µmol/L	0.68 µmol/L	[161]
  Cr(Ⅵ)	—	Hydrothermal	PV/BH@CD	—	0–20 µmol/L	66 nmol/L	[155]
  Cr(Ⅵ)	Citric acid	Hydrothermal	CE-CDs	72	0–500 µmol/L	23 nmol/L	[162]
  Cr(Ⅵ) Hg(Ⅱ)	l-Arginine	Hydrothermal	CDs	20	0.1–2 µmol/L 0.4–5 µmol/L	0.024 µmol/L 0.084 µmol/L	[16]
  Hg(Ⅱ)	Orange juice	Hydrothermal	N-CDs	31.7	4–32 µmol/L	—	[156]
  Cu(Ⅱ)	p-Phenylenediamine	Hydrothermal	N-CDs	2.16	45–70 µmol/L	45.87 µmol/L	[158]
  Cu(Ⅱ)	Tryptophan o-Phenylenediamine	Hydrothermal	CD@Cu-IIP-A	—	80–780 µmol/L	3.17 µmol/L	[163]
  Ag(Ⅰ)	Lycium ruthenicum	Hydrothermal	N-CDs	21.8	0.7–36 µmol/L	59 nmol/L	[159]
  Ag(Ⅰ)	p-Phenylendiamine Sulfanilic acid	Hydrothermal	or-CDs	18.4	0.8–10 µmol/L and 10–100 µmol/L	56 nmol/L	[164]
  Pd(Ⅱ)	LLC	Hydrothermal	L-CQDs	5.32	5–40 µmol/L	0.1 µmol/L	[160]
  Fe(Ⅲ)	Sweet potato roasting residues	Pyrolysis	SPCDs	53	25.0–250 µmol/L	13.96 µmol/L	[165]
  Fe(Ⅲ)	Diphenylsemicarbazide	Hydrothermal	Zn/Co-NCDs	38.6	0.03–20.00 µg/mL	0.010 µg/mL	[166]
  Fe(Ⅲ)	o-Phenylenediamine	Microwave-assisted	N-CDs	20.64	1–70 µmol/L	0.1011 µmol/L	[167]
  Fe(Ⅲ)	Zinc gluconate	Hydrothermal	C-dots	8.3	0–200 µmol/L	1.9 µmol/L	[168]
  Zn(Ⅱ)	Citric acid	Pyrolysis	C-dots	—	0.1–2.0 µmol/L	6.4 nmol/L	[169]
  U(Ⅴ)	Citric acid	Microplasma	CDs	5.1	0–75 ppm	0.71 ppm	[15]
  F−	Glucose	Microwave	Zr(CDs–COO)2EDTA	—	0.10–10 µmol/L	0.031µmol/L	[170]
  Minocycline	Citric acid	Microwave-assisted	CQDs	30.8	0.28–30.39 µmol/L	91 nmol/L	[171]
  NOR CIP OFX His	o-Phenylenediamine	Hydrothermal	Y-CDs	22.6	0.05–50 µmol/L 0.2–25 µmol/L 0.4–10 µmol/L 0.05–10 µmol/L	17 nmol/L 35 nmol/L 67 nmol/L 35 nmol/L	[172]
  Ciprofloxacin	Osmanthus fragrans leaves	Hydrothermal	MIPs@CdTe/CDs@SiO2	—	0–60 nmol/L	0.0127 nmol/L	[173]
  Oxytetracycline	Stalk of cherry tomatoes	Hydrothermal	Stalk-Trp CDs@Eu	30.77	6–100 µmol/L	0.018 µmol/L	[174]
  NAs	NADES	Hydrothermal	N-CQDs	7.22	0.03–0.09 mol/L	0.588 mmol/L	[175]
  NAs	p-Phenylenediamine	Hydrothermal	PEG-CQDs	14.47	0.5–15 mmol/L	0.16 mmol/L	[14]
  Catechol	Citric acid	Hydrothermal	N-CQDs	9.47	2–40 µmol/L	0.65 µmol/L	[177]
  Toluene Xylene	PEG-800	Hydrothermal	α-CD/CA/CDots	—	—	3.7 mg/L 4.9 mg/L	[178]
  TNT	Citric acid	Hydrothermal	CDs@NH2	32.25	0–1 µmol/L	0.213 µmol/L	[179]

  5.2 Removal of inorganic pollutants

  Inorganic pollutants include heavy metals and non-metal inorganic toxins (such as nitrates, cyanides, fluorides, and N/S oxides). Additionally, radioactive isotopes generated from military or nuclear leaks (such as uranium, lanthanum, and americium) also fall under inorganic pollutants. These pollutants are highly toxic and non-biodegradable. Even at low concentrations in the environment, they can pose serious threats to ecosystems and human health. Therefore, the removal of inorganic pollutants is an inevitable task. In recent years, researchers have developed composite materials based on CDs as adsorbents applied to the removal of heavy metal ions (HMIs) and radioactive isotopes.

  Most CDs composite materials have a relatively large specific surface area, abundant surface functional groups, and active sites, endowing them with strong adsorption capabilities for inorganic pollutants in water. According to relevant reports, adsorption is mainly achieved through surface complexation [180-182], ion exchange [183-185], electrostatic attraction [180, 181, 184, 186], physical adsorption [187], hydrogen bonding [188, 189] and precipitation [185, 190]. Surface complexation refers to the formation of stable complexes through coordination bonds between HMIs and the functional groups on the adsorbent surface (such as N, O, S-containing functional groups). During the contact between CDs composite materials and HMIs, ion exchange may also occur, which is influenced by solution pH, ion concentration, and electronegativity. Electrostatic attraction refers to the mutual attraction between charged HMIs and active sites on the adsorbent with opposite charges, and this mechanism is directly related to pH and the electronegativity of HMIs. HMIs can diffuse to the surface of the adsorbent through physical adsorption, and then enter the pores of the adsorbent through internal diffusion. The larger the specific surface area of the adsorbent, the stronger the adsorption capacity. When the adsorbent contains O-H or N-H, hydrogen bonds can be formed with the O atoms in metal oxyanions, expressed as O-H···O or N-H···O. Finally, some HMIs can form precipitates after contacting the adsorbent to achieve adsorption and removal.

  In the removal of HMIs, Chen et al. [180] reported a method based on fluorescence carbon dots (NCDs) cross-linked cellulose nanofiber/chitosan (CNF/CS) interpenetrating network hydrogel (NCDs-CNF/CSgel) for detecting and adsorbing Cu(Ⅱ) and Cr(Ⅵ) ions in water. The hydrogel was prepared using a low-temperature hydrothermal method and solvent replacement technique, showing high adsorption capacities for Cu(Ⅱ) and Cr(Ⅵ) (148.30 mg/g and 294.46 mg/g, respectively) and exhibiting good pH responsiveness. The study indicated that the main mechanism of Cu(Ⅱ) adsorption was the complexation between Cu(Ⅱ) and surface functional groups of hydrogels. Meanwhile, Cr(Ⅵ) was adsorbed onto the hydrogel via electrostatic attraction, with a portion of the adsorbed Cr(Ⅵ) being complexed with the hydrogel functional groups and another portion being reduced to Cr(Ⅲ). Additionally, it was observed that copper hydroxide and chromium hydroxide were loaded on the NCDs-CNF/CSgel (Fig. 22a). In another study, You et al. [190] successfully developed CCMg ternary composite material. This material exhibited significant efficiency in the enrichment and fixation of Cd(Ⅱ) and Cu(Ⅱ) in wastewater. Through a series of experimental techniques and DFT calculations, the researchers revealed the interface coupling and removal mechanisms between CCMg and HMIs. The adsorption performance of CCMg was evaluated using pseudo-first-order and pseudo-second-order kinetic models. The results indicated that the pseudo-second-order model better fit the experimental data, suggesting that the adsorption process may involve chemical adsorption. Furthermore, by fitting with Langmuir and Langmuir-Freundlich isotherm models, the researchers determined the maximum theoretical adsorption capacities of CCMg for Cd(Ⅱ) and Cu(Ⅱ) to be 988.4 mg/g and 656.3 mg/g, respectively. The results of the cyclic experiments with CCMg composite material showed that it maintained high removal efficiency even after multiple cycles of use, demonstrating good durability and reusability. From recent years' research, it can be summarized that CDs composite materials as adsorbents have achieved the adsorption and removal of various HMIs, including Cu(Ⅱ) [163, 186], Cr(Ⅵ) [191], Cd(Ⅱ) [186, 187], Co(Ⅱ) [191], Ni(Ⅱ) [192], Pb(Ⅱ) [193, 194], Fe(Ⅲ) [182], and Hg(Ⅱ) [189]. The processes in these studies were generally similar to the work of Chen [180] and You [190], and the prepared composite materials exhibited very high adsorption capacities and removal efficiencies for HMIs. Additionally, As has become a global focus due to its high toxicity, tendency for accumulation, and non-biodegradability. Recently, Lin et al. [188] successfully synthesized magnetic CDs with imine functional groups (FeCD-NH) and amino functional groups (FeCD-NH₂) by finely controlling the surface chemical properties of the adsorbent and utilized them for the adsorption and removal of As(Ⅴ). FeCD-NH, owing to its unique imine functional groups, exhibited outstanding adsorption performance for As(Ⅴ), with a removal rate of 97.80% within 5 min, significantly better than FeCD-NH₂. Advanced characterization techniques such as FTIR, XPS, and DFT calculations were employed to reveal that FeCD-NH achieved ultra-fast adsorption of As(Ⅴ) through a dual-configuration hydrogen bonds synergistic mechanism (Fig. 22b).

  Figure 22

  

  Figure 22.  (a) Adsorption mechanisms of NCDs-CNF/CSgel for Cu(Ⅱ) and Cr(Ⅵ). Reproduced with permission [180]. Copyright 2022, Elsevier. (b) Different adsorption mechanisms of As(Ⅴ) on FeCD-NH₂ and FeCD-NH. Reproduced with permission [188]. Copyright 2023, Elsevier. (c) Adsorption mechanisms of LDO-C for U(Ⅵ). Reproduced with permission [185]. Copyright 2018, Elsevier.

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  U(Ⅵ) is one of the primary components in nuclear reactions and is a radioactive element. If released into the environment in nuclear power plants or military applications, it can have harmful effects on humans and ecological balance. Therefore, the removal of U(Ⅵ) is of practical significance. Yao et al. [185] successfully synthesized a novel type of layered double oxides/carbon dots nanocomposites (LDO-C) and demonstrated their high efficiency in the removal of U(Ⅵ) and ²⁴¹Am(Ⅲ). The study utilized a simple calcination method to synthesize LDO and LDO-C and explored their adsorption behavior towards U(Ⅵ) under different conditions through a series of experiments. The research found that the adsorption capacity of LDO-C for U(Ⅵ) was significantly higher than that of LDO, with a maximum adsorption capacity of 354.2 mg/g. This improvement in performance was attributed to the higher specific surface area and abundant surface oxygen functional groups of LDO-C. XPS and EXAFS analysis revealed that the adsorption mechanism of U(Ⅵ) on the surface of LDO-C mainly included surface complexation, electrostatic interactions, ion-exchange, and precipitation (Fig. 22c). Additionally, LDO-C exhibited good adsorption performance for ²⁴¹Am(Ⅲ), indicating that the material not only effectively removes U(Ⅵ) but also has the potential to remove other radioactive elements. In another study, Mahmoud et al. [194] developed GQDOs-Ba nano-biosorbents for the rapid and efficient removal of La(Ⅲ) from water. The adsorption capacity for La(Ⅲ) reached 1500 µmol/g within 15 s. These two studies provided new strategies and theoretical foundations for the immobilization of radioactive pollutants. Table 3 represents the applications of various CDs/CDs composite materials in the removal of inorganic pollutants.

  Table 3

  

  Table 3.  CDs/CDs composite materials for the removal of inorganic pollutants.

  DownLoad: CSV

  Adsorbents	Target inorganics	Adsorption isotherm	Adsorption kinetics	Efficiency (% or mg/g)	Adsorption mechanism	Ref.
  NCDs-CNF/CSgel	Cu(Ⅱ) Cr(Ⅵ)	Langmuir	pseudo-second-order	148.30 mg/g 294.46 mg/g	Complex reaction, Electrostatic attraction, Reduction reaction	[180]
  CD@Cu-IIP-A	Cu(Ⅱ)	Langmuir	pseudo-second-order	85.6 mg/g	—	[163]
  CCMg	Cd(Ⅱ) Cu(Ⅱ)	–	–	988.4mg/g 656.3 mg/g	Cd(Ⅱ): hydrogen/dative bonds Cu(Ⅱ): precipitation	[190]
  N, S-CQDs@Fe3O4@HTC	Cu(Ⅱ) Cd(Ⅱ)	–	–	99.90% 85.08%	Electrostatic attraction, Surface complexation	[186]
  LFPC	Cd(Ⅱ)	Freundlich	–	98%	Physical adsorption	[187]
  C-dot/ChNC	Cr(Ⅵ) Co(Ⅱ)	Langmuir	–	155 mg/g 152 mg/g	Chemical binding	[191]
  β-CD(CA)/QDs	Ni(Ⅱ), Cu(Ⅱ), Cd(Ⅱ), Pb(Ⅱ)	Langmuir-Freundlich	pseudo-second-order	243 mg/g, 229 mg/g, 228 mg/g, 222 mg/g	—	[192]
  SA@PEI-CDs	Pb(Ⅱ)	Freundlich	pseudo-second-order	380.39 mg/g	Surface complexation, Cation exchange, Electrostatic attraction, Intra-particle diffusion	[184]
  Fluorescent aerogels	Pb(Ⅱ)	Langmuir	pseudo-second-order	183 mg/g	Chelation and coordination	[193]
  FSH	Pb(Ⅱ)	Langmuir	pseudo-second-order	265.9 mg/g	Electrostatic attraction, Ion exchange, Complexation	[181]
  MAR@poly(TAPA)-CD	Fe(Ⅲ)	Langmuir	pseudo-second-order	32.9 mg/g	Chelation and coordination, Electron-transfer	[182]
  CB-50%	Hg(Ⅱ)	Langmuir	pseudo-second-order	290.70 mg/g	Coordination bonds, Hydrogen bonds	[189]
  FeCD-NH	As(Ⅴ)	Langmuir	pseudo-second-order	97.8%	Dual-configuration hydrogen bonds	[188]
  LDO-C	U(Ⅵ) 241Am(Ⅲ)	Langmuir	pseudo-second-order	354.2 mg/g 95.4%	Surface complexation, Electrostatic interactions, Ion-exchange, Precipitation	[185]
  PECQDs/MnFe2O4	U(Ⅵ)	Freundlich	pseudo-second-order	194 mg/g	Cation exchange	[183]
  GQDOs-Ba	Pb(Ⅱ) La(Ⅲ)	Langmuir	pseudo-second-order	98.5%−99.8% 94.6%−96.2%	Chemical binding	[194]

  These studies not only offer effective materials for the removal and recovery of HMIs and radioactive isotopes but also provide a deep understanding of the interaction mechanisms between the materials and HMIs (or radioactive isotopes) through a combined approach of theoretical calculations and experiments. These composite materials hold broad prospects for application in the field of water treatment.

  5.3 Degradation of organic pollutants

  In recent years, with the rapid development of various industries, water pollution has become a serious environmental issue. Especially, organic pollutants in wastewater (such as antibiotics [195], chemical dyes, and organic pesticides [196]), due to their corrosiveness, high toxicity, and difficulty in degradation, urgently require efficient methods for their removal from wastewater. CDs, due to their excellent electron transfer induction properties, have become star materials for the degradation of organic pollutants. Currently, researchers have successfully applied CDs to the degradation of antibiotics, dyes, and other types of organic pollutants.

  Tetracycline (TC) is widely used in the aquaculture, animal husbandry, and medical industries. However, as an antibiotic, TC has also become one of the main pollutants in water bodies. Therefore, some researchers have focused their work on the efficient removal of TC. Zhang et al. [197] reported a novel cellulose nanofiber-modified CDs/ZIF-8 composite hydrogel (CZCH) that not only achieved efficient detection of TC but also exhibited excellent adsorption capacity towards TC, with a maximum adsorption capacity of up to 810.36 mg/g. Meanwhile, Abdurahman et al. [198] prepared CN/CQD/BiOCl_0.75Br_0.25 composite material by modifying CQDs and BiOCl_xBr_1-x on graphitic carbon nitride (CN). This composite material showed outstanding photocatalytic degradation performance towards TC, with the highest degradation rate reaching 84.3% under optimal reaction conditions. In order to further enhance the removal efficiency of TC, The teams of Thara [122] and Xie [199] respectively designed N-doped carbon dots and CDs/TiO₂ for photocatalytic degradation of TC. Experimental results demonstrated that after 70 min of reaction, CDs/TiO₂ can achieve complete degradation of TC (Fig. 23a) [199]. These two studies investigated the degradation performance of both individual CDs and CDs composite materials towards TC. By comparison, although pure CDs can efficiently remove TC under visible light irradiation (greater than 95%), their degradation rate of TC can be further improved after forming composite materials with TiO₂, opening up new possibilities for the application of CDs.

  Figure 23

  

  Figure 23.  (a) Photocatalytic behavior of TC by CDs/TiO₂. Reproduced with permission [199]. Copyright 2024, Elsevier. (b) Predicted mechanism of the CV dye degradation. Reproduced with permission [210]. Copyright 2022, Springer Nature.

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  Ciprofloxacin (CIP), another type of antibiotic, was widely used in the treatment of bacterial infections, but its excessive use and discharge also posed threats to human health and the ecological environment. Therefore, there was an urgent need to develop an efficient method for removing CIP. Mou's team [200] developed a novel two-dimensional composite material doped with CDs, named BiOCl/NGQDs, and applied it to the photocatalytic degradation of CIP. Through experiments, the authors found that compared with pure BiOCl, the composite material with optimized NGQDs content exhibited a significant enhancement in photocatalytic degradation capability for CIP under visible light irradiation. The composite material achieved a degradation efficiency of 82.5% for CIP within 60 min, far exceeding the 34.9% achieved by pure BiOCl. In addition to TC and CIP antibiotics, CDs materials were also applied in the photocatalytic degradation of other types of antibiotics such as quinoline [201], norfloxacin [202], and AmB [203], with degradation efficiencies exceeding 90%. These studies indicated that CDs showed tremendous potential in the field of antibiotic degradation, providing new directions for future environmental protection and pollution control.

  Another common type of organic pollutant in wastewater is dyes, including RhB [3, 204, 205], MB [206-208], CV [209, 210], MO [211-214], MG [17], and others. These dyes have been widely used in various industries, and their extensive use and discharge inevitably lead to water environmental pollution. Therefore, the removal of various dyes from wastewater is also an inevitable issue.

  In exploring efficient methods for removing RhB, CDs have attracted attention due to their unique optical properties and good photocatalytic activity. The study by Wang et al. [204] demonstrated the efficient performance of CDs in removing RhB by developing a novel multifunctional adsorbent, CAC@CDs-BPEI. This adsorbent combined the advantages of activated carbon and modified CDs, achieving rapid adsorption and efficient removal of RhB through its unique honeycomb-like structure and high specific surface area. Ibarbia et al. [205] further investigated the photocatalytic degradation capability of GQDs under visible light irradiation for RhB. They found that the chemical composition, crystal structure, and bandgap of GQDs significantly influence their photocatalytic activity. Particularly, GQDp with larger visible light absorption and stronger negative surface charge exhibited higher RhB degradation efficiency, achieving a degradation rate of 55% after 6 h of photocatalysis. Although pure CDs were applied in the photocatalytic degradation of RhB, the overall degradation rate remained relatively low. To address this issue, Chen et al. [3] first prepared CQDs through chemical oxidation and then synthesized CQDs/TiO₂ nanocomposites to enhance the activity of TiO₂ in photocatalytic RhB removal. By introducing CQDs into TiO₂, visible light absorption was realized, generating more electron-hole pairs and enhancing photocatalytic performance, resulting in an increase in RhB degradation rate to 91.28%.

  CDs also demonstrated tremendous potential in removing organic pollutants such as MB. Yadav et al. [206] synthesized boron and phosphorus co-doped CDs using hydrothermal and microwave-assisted methods for the photocatalytic degradation of MB. The study found that microwave-assisted synthesized CDs exhibited higher efficiency in MB degradation, achieving a removal rate of 92.3%, attributed to their higher PLQY and surface charge characteristics. In two other studies, both individual CDs and CDs-based composite materials showed efficient photocatalytic activity against MB, with degradation rates reaching 100%. This was attributed to the improvement of electron transfer efficiency by CDs, reducing the recombination of photogenerated charge carriers, thereby achieving efficient removal of MB [207, 208].

  In the removal of CV, CDs could achieve efficient adsorption and photocatalytic degradation of CV either individually or in the form of composite materials [209, 210]. Nitrogen-modified carbon dots (NCQDs) synthesized through a green hydrothermal method from Cucurbita pepo extract exhibited significant photocatalytic degradation activity towards CV dye under visible light irradiation. In the presence of 1 mL H₂O₂, NCQDs could achieve 99.9% degradation of CV dye within 180 min. The degradation mechanism of CV dye involved NCQDs generating electron-hole pairs under visible light irradiation and promoting electron transfer between NCQDs and CV through their surface functional groups, reducing the recombination of electron-hole pairs. On the other hand, electrons and holes reacted with dissolved oxygen and water to generate ^•O₂⁻ and ^•OH, respectively, participating in the oxidative decomposition of the dye, achieving efficient photocatalytic degradation of CV (Fig. 23b) [210].

  Methyl Orange (MO), as a typical azo dye, frequently appears in wastewater due to its widespread use in the textile industry, posing potential threats to the environment and ecology. To effectively remove MO, researchers have explored various methods, including photocatalytic technologies. For example, Li et al. [211] prepared PDs-TiO₂ nanocomposites by grafting polymer dots (PDs) onto TiO₂, resulting in PDs-TiO₂ nanocomposites with Ti-O-C bonds. PDs-TiO₂ exhibited excellent photocatalytic activity under visible light irradiation, with a photocatalytic rate constant 3.6 times higher than that of pure TiO₂. This was mainly attributed to the introduction of PDs, which not only enhanced the light absorption capacity of TiO₂ but also promoted rapid electron transfer through π-conjugated structures and Ti-O-C bonds, significantly improving the photocatalytic degradation efficiency of MO. In another study, Monje et al. [212] synthesized CDs via acid etching and used them to prepare magnetite-decorated nanocomposites. These composites exhibited efficient degradation capability towards MO through photo-Fenton treatment under near-neutral pH conditions, with a degradation rate of up to 98%. To achieve complete degradation of MO, N-CDs prepared from plants and fruits via hydrothermal methods could be applied for efficient MO removal. Experimental results showed that the prepared materials achieved a degradation rate of 100% for MO, achieving complete removal of MO [213, 214].

  Recently, researchers have utilized green synthesis methods to prepare novel photocatalysts to enhance the photocatalytic degradation efficiency of harmful dyes such as Malachite Green (MG). Vijeata et al. [17] synthesized highly fluorescent N-CDs using Azadirachta Indica leaves via pyrolysis method. The prepared N-CDs not only had a high PLQY of 42.3% but also achieved a removal rate of 98.25% for MG within 70 min of light exposure. This study indicated that photocatalysts prepared via green synthesis methods not only effectively remove harmful dyes from water bodies but also possessed good photostability and selectivity.

  CDs also achieved efficient photocatalytic degradation of several dyes such as RhG [215], RR120 [216], and CR [217], with degradation rates ranging from 80% to 97.8%. Through several examples, it can be seen that CDs have tremendous potential in dye degradation. However, further optimization of synthesis conditions, improvement of CDs' stability and reusability, and deeper exploration of the mechanism of CDs in photocatalytic degradation are still needed to realize their practical application in environmental management.

  In addition to the degradation of antibiotics and dyes two major organic pollutants. CDs have also been successfully applied to the removal of other types of organic pollutants such as phenol [218], PNP [219], BzP [220], and BaP [221]. Tian et al. [218] significantly improved the activity and stability of the photocatalyst by combining N-CQDs with Ag₂CO₃. This was attributed to the influence of N-CQDs on the growth of Ag₂CO₃ crystals, resulting in reduced crystal size and increased specific surface area, thereby enhancing the photocatalytic performance. Under optimal reaction conditions, N-CQDs/Ag₂CO₃ achieved a photocatalytic degradation efficiency of 55% for phenol. To further enhance the photocatalytic activity, researchers focused on constructing heterojunctions, mostly using CDs to build heterojunctions with g-C₃N₄ [[219], [220]]. Yang et al. [219] directly combined CDs with g-C₃N₄ to construct CDs/g-C₃N₄ (CDs/CN) heterojunctions. This heterojunction served as an efficient photocatalyst for the photocatalytic degradation of PNP, achieving removal rates exceeding 95% under optimal reaction conditions. It was found that the addition of CDs could increase the specific surface area of g-C₃N₄, attract photogenerated electrons from g-C₃N₄, prolong the lifetime of photogenerated electrons, promote the separation of photogenerated electron-hole pairs, and broaden the response range to visible light. Free radical capture experiments confirmed that the hole (h⁺) and ^•O₂⁻ were the key active species in the photocatalytic process. In another study, Tian et al. [220] successfully constructed a CQDs-mediated Z-scheme g-C₃N₄–CQDs/BiVO₄ heterojunction by introducing CQDs between g-C₃N₄ and BiVO₄. This structure significantly improved the photocatalytic degradation efficiency of BzP under visible light irradiation. As an electron mediator, CQDs facilitated the transfer of electrons from g-C₃N₄ to BiVO₄ and enhanced the separation of photogenerated charges, thereby enhancing the redox capability of the photocatalyst. Table 4 displays the degradation efficiency of organic pollutants by CDs/CDs composite materials synthesized from various precursors.

  Table 4

  

  Table 4.  The degradation efficiency of organic pollutants by CDs/CDs composite materials prepared from different precursors.

  DownLoad: CSV

  Materials	Precursors	Synthesis method	Target organics	Degradation method	Reaction condition	Degradation Efficiency	Ref.
  CZCH	Citric acid	Hydrothermal	TC	Adsorption	—	810.36 mg/g	[197]
  CN/CQD/BiOCl0.75Br0.25	Citric acid	Hydrothermal	TC	Photo-degradation	Xe lamp light, C(TC) = 10 mg/L, C(CN/CQD/BiOCl0.75Br0.25) = 0.1 g/L, t = 30 min	84.3%	[198]
  N-doped carbon dots	d-glucose, Urea	Microwave	TC	Photo-degradation	C(TC) = 22 mg/L, C(N-doped carbon dots) = 800 µg/mL, t = 10 min	> 95%	[122]
  CDs/TiO2	Citric acid, Ethylenediamine	Hydrothermal	TC	Photo-degradation	C(TC) = 20 mg/L, pH 5, t = 70 min	100%	[199]
  BiOCl/NGQDs	Citric acid, Urea	Hydrothermal	CIP	Photo-degradation	t = 60 min	82.5%	[200]
  LFC0.5/G-CQDs	Citric acid, Urea	Hydrothermal	Quinoline	Photo-degradation	Visible light, C(quinoline) = 20 mg/L, C(LFC0.5/G-CQDs) = 0.5 g/L, t = 120 min	91%	[201]
  CQDs/Bi2MoO6	—	Hydrothermal	Norfloxacin	Photo-degradation	C(norfloxacin) = 20 mg/L, C(CQDs/Bi2MoO6) = 0.8 g/L, t = 30 min	99%	[202]
  CD@MIL-88 B(Fe)	—	Hydrothermal	AmB, Nap	Photo-degradation	C(CD@MIL-88 B(Fe)) = 0.2 g/L	92%, 90%	[203]
  CAC@CDs-BPEI	Citric acid	Hydrothermal	RhB	Adsorption	t = 180 min	1734.55 mg/g	[204]
  GQDp	Carbon black	Chemical oxidation	RhB	Photo-degradation	t = 6 h	55%	[205]
  CQDs/TiO2	Starch	Chemical oxidation	RhB,	Photo-degradation	UV-light irradiations, C(CQDs/TiO2) = 20 mg, C(RhB) = 5 mg/L, t = 120 min	91.28%,	[3]
  CDs	Citric acid	Hydrothermal and microwave-assisted	MB	Photo-degradation	pH 11	92.3%	[206]
  Si-CQD	Rice husk	Hydrothermal	MB	Photo-degradation	C(MB) = 10 ppm, t = 120 min	100%	[207]
  CQDs/CuFe2O4	Citric acid, Urea	Microwave	MB	Photo-degradation	Xenon lamp, C(CQDs/CuFe2O4) = 10 mg/L, pH 7, t = 180 min	99.86%	[208]
  YMFenAp@CD	Yerba mate	Pyrolysis	CV	Adsorption	t = 45 min	3.18 mmol CV/gads	[209]
  NCQDs	Zucchini	Hydrothermal	CV	Photo-degradation	Visible light, C(CV) = 10 ppm, t = 180 min	99.9%	[210]
  PDs-TiO2	PVA	Hydrothermal	MO	Photo-degradation	UV–vis light irradiation, t = 8 h	96.7%	[211]
  CDs	YM	Chemical oxidation	MO	Photo-degradation	Halogen lamp irradiation, C(MO) = 8.5 ppm, C(CDs) = 100 ppm, pH 6.2, t = 7 h	98%	[212]
  N-CDs	Opuntia Ficus Indica	Hydrothermal	MO	Photo-degradation	t = 12 min	100%	[213]
  CQDs	Cordia myxa L	Hydrothermal	MO, MB, CV, EBT	Photo-degradation	UV-light irradiations, C(pollutant) = 0.015 mmol/L, m(CQDs) = 0.3 mg, t = 45–60 min	100%	[214]
  N-CDs	Azadirachta Indica leaves	Pyrolysis	MG	Photo-degradation	Mercury lamp irradiation, m(N-CDs) = 20 mg, t = 70 min	98.25%	[17]
  GQDs	GO	Hydrothermal	RhG	Photo-degradation	Sunlight irradiation, m(GQDs) = 50 mg, t = 80 min	80%	[215]
  CQDs	Fish scales	Hydrothermal	RR120	Photo-degradation	C(RR120) = 15 mg/L, C(CQDs) = 20 mg/L, t = 120 min	97.8%	[216]
  CQDs	Rubber seed shells	Microwave	CR	Photo-degradation	t = 100 min	> 90%	[217]
  N‐CQDs/Ag2 CO3	Citric acid, Urea	Hydrothermal	Phenol	Photo-degradation	Xe lamp light, C(phenol) = 0.02 g/L, m(N‐CQDs/Ag2CO3) = 0.02 g, t = 150 min	About 55%	[218]
  CDs/CN	Citric acid, Urea	Pyrolysis	PNP	Photo-degradation	Xenon lamp, t = 35 min	95.9%	[219]
  g–C3N4–CQDs/BiVO4	Citric acid, Ethylenediamine	Hydrothermal	BzP	Photo-degradation	Xenon lamp, C(BzP) = 10 mg/L, m(g–C3N4–CQDs/BiVO4) = 0.1 g, t = 150 min	85.7%	[220]
  CDs/C11-Fe3O4	Citric acid, Urea	Microwave heating	BaP	Adsorption	t = 20 min	76.23 ng/mg	[221]

  In conclusion, CDs, as a novel carbon-based nanomaterial, have significant application advantages and research potential in the degradation of organic compounds. In the future, further exploration of modification methods for CDs can be pursued to achieve more efficient and stable catalytic performance, thereby contributing to environmental protection and sustainable development.

  6. Summary and prospect

  This review comprehensively covers the classification, structure, and preparation methods of CDs, highlighting the hydrothermal/solvothermal method for its easy operation, cost-efficiency, and eco-friendliness, yielding CDs with high PLQY. The PL mechanisms of CDs, attributed to surface, carbon core, molecular states, and CEE, are widely accepted. CDs' unique PL characteristics, along with their chemical stability and electron transfer properties, position them as a viable alternative for wastewater treatment applications.

  Despite the advances, challenges remain in the scalability of CDs production, necessitating the development of methods amenable to industrial application. Precision in CDs synthesis is paramount, with properties being highly dependent on synthesis conditions such as raw materials, temperature, time, and solvents. Understanding the formation mechanisms is crucial for tailoring CDs with desired attributes.

  Various techniques, such as femtosecond transient absorption, steady-state/transient PL, ultrafast excitation state imaging, in-situ PL, and high-pressure absorption, are utilized to elucidate the PL mechanisms of CDs [49, 222]. Further exploration is needed to better understand the PL emission details of CDs. Combining optical physics characterization techniques with theoretical calculations and computer modeling can be a direction for research. Additionally, there's a need for more studies on room-temperature phosphorescence and up-conversion PL to expand CDs' applications in fields like anti-counterfeiting and near-infrared imaging.

  In wastewater treatment applications, CDs face issues of performance degradation and regeneration challenges, underscoring the need for a deeper understanding of their interaction with pollutants. Transitioning from laboratory to industrial scale involves overcoming hurdles related to cost, efficiency, and long-term environmental impact, which requires thorough documentation and research on ecological sustainability.

  Looking forward, the continued evolution of CDs research hinges on refining structural characterization methods and theoretical insights to enhance our grasp of their PL mechanisms. With careful control over synthesis and structure, CDs can be optimized for broader applications. The collective efforts of the research community are expected to steer CDs towards a promising future in environmental management and beyond.

  Declaration of competing interest

  The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

  Acknowledgments

  This work was supported by the Natural Science Foundation of Hebei Province (No. E2022208046), National Science Foundation of China (No. 52004080), Key project of National Natural Science Foundation of China (No. U20A20130), Key research and development project of Hebei Province (No. 22373704D), 2023 Central Government Guide Local Science and Technology Development Fund Project (No. 236Z1812 G).

  
  
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• 

  Figure 1  Carbon sources for preparing CDs in recent ten years.

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  Figure 2  A summary of the synthesis process, classification, PL mechanisms, and applications of CDs.

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  Figure 3  (a) Schematic diagram of the structure of CDs. Reproduced with permission [10]. Copyright 2019, Royal Society of Chemistry. (b) The classification of CDs. Reproduced with permission [44]. Copyright 2022, Springer Nature. (c) Different edge types of GQDs. Reproduced with permission [46]. Copyright 2020. Royal Society of Chemistry. (d) Schematic diagram of the structure of CQDs. Reproduced with permission [11]. Copyright 2022, John Wiley and Sons.

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  Figure 4  The four currently recognized PL mechanisms for CDs. Surface state. Reproduced with permission [25]. Copyright 2023, Elsevier. Carbon core state. Reproduced with permission [51]. Copyright 2014, Royal Society of Chemistry. Molecular state. Reproduced with permission [52]. Copyright 2022, Springer Nature. CEE. Reproduced with permission [55]. Copyright 2015, John Wiley and Sons.

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  Figure 5  (a) Mechanisms of PL in the surface state of CDs. Reproduced with permission [25]. Copyright 2023, Elsevier. (b) Calculated emission wavelength using Time-Dependent DFT method in vacuum as a function of the diameter of GQDs. Reproduced with permission [51]. Copyright 2014, Royal Society of Chemistry. (c) Mechanisms of PL in the molecular state of CDs prepared by oPD. Reproduced with permission [52]. Copyright 2022, Springer Nature. (d) Diagram of three categories of CEE in polymers or NCPDs. Reproduced with permission [55]. Copyright 2015, John Wiley and Sons.

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  Figure 6  Visualization activities of preparation timelines for CDs in recent years.

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  Figure 7  (a) Synthetic strategy of B/N doped CDs. Reproduced with permission [57]. Copyright 2014, Elsevier. (b) Schematic illustration of experimental setup. Reproduced with permission [61]. Copyright 2011, Royal Society of Chemistry. (c) Schematic diagram of mechanism of DMSO-CQD. Reproduced with permission [63]. Copyright 2020, Elsevier. (d) The schematic diagram of twin GQDs prepared by ETLAL. Reproduced with permission [66]. Copyright 2021, Elsevier.

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  Figure 8  (a) Steps in the preparation of CDs from carbon soot. Reproduced with permission [68]. Copyright 2009, American Chemical Society. (b) The preparation process of fluorescent CDs. Reproduced with permission [70]. Copyright 2009, American Chemical Society. (c) The method of cutting CF into GQDs through oxidation. Reproduced with permission [74]. Copyright 2012, American Chemical Society.

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  Figure 9  (a) The device diagram of CDs prepared by electrochemical method. Reproduced with permission [78]. Copyright 2009, American Chemical Society. (b) Schematic illustration of the ECL and PL mechanisms in CNCs. Reproduced with permission [78]. Copyright 2009, American Chemical Society. (c) Optical images of CQDs of different sizes under 365 nm UV light and Upconverted PL properties of CQDs. Reproduced with permission [79]. Copyright 2010, John Wiley and Sons. (d) Schematic diagram of electrochemical preparation of CDs by graphite electrode and color change at room temperature. Reproduced with permission [81]. Copyright 2016, Royal Society of Chemistry. (e) Dependence of size of CDs on applied potentials. Reproduced with permission [82]. Copyright 2011, John Wiley and Sons.

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  Figure 10  (a) Diagram of synthesis of CDs from plant leaves and the PL enhancement phenomenon. Reproduced with permission [89]. Copyright 2013, Royal Society of Chemistry. (b) Linear relationship between PL intensity and pH value. Reproduced with permission [28]. Copyright 2014, Elsevier. (c) Possible mechanism of photocatalytic degradation of MB and TC. Reproduced with permission [90]. Copyright 2017, Royal Society of Chemistry.

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  Figure 11  (a) Preparation route diagram of GQDs. Reproduced with permission [93]. Copyright 2012, John Wiley and Sons. (b) Diagram of microwave-assisted synthesis of CDs. Reproduced with permission [97]. Copyright 2022, Elsevier. (c) Possible formation mechanism of CDs prepared from orange peel. Reproduced with permission [97]. Copyright 2022, Elsevier.

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  Figure 12  (a) Schematic diagram of the synthesis of CDs using defective CVD graphene. Reproduced with permission [99]. Copyright 2016, Elsevier. (b) Schematic diagram of the preparation of Pdots. Reproduced with permission [100]. Copyright 2022, Elsevier. (c) PL spectra of CDs at 250 nm < λ_ex < 530 nm. Reproduced with permission [13]. Copyright 2015, Elsevier. (d) Preparation route of P-CDs/Ni-MOL composites. Reproduced with permission [104]. Copyright 2022, Elsevier.

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  Figure 13  (a) Schematic diagram of tuning CDs fluorescence properties by changing the functional groups of carbon sources. Reproduced with permission [114]. Copyright 2021, American Chemical Society. (b) Preparation process of CDs and mechanism of NAs detection. Reproduced with permission [14]. Copyright 2023, Elsevier. (c) Schematic diagram of the preparation of CDs. Reproduced with permission [118]. Copyright 2023, John Wiley and Sons.

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  Figure 14  (a) Schematic diagram of the preparation of CDs by MAH method. Reproduced with permission [120]. Copyright 2012, American Chemical Society. (b) The preparation route of CDs and the possible structure of PCDs. Reproduced with permission [121]. Copyright 2021, Elsevier. (c) Synthesis and application of Si-CDs. Reproduced with permission [124]. Copyright 2017, Royal Society of Chemistry. (d) Schematic illustration of preparing red-emitting CDs for fluorescent "on-off-on" sensing. Reproduced with permission [128]. Copyright 2016, John Wiley and Sons. (e) The preparation and applications of CDs. Reproduced with permission [27]. Copyright 2024, Elsevier.

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  Figure 15  (a) Schematic of preparing CDs with the atmospheric-pressure micro-plasma method. Reproduced with permission [15]. Copyright 2015, Royal Society of Chemistry. (b) Schematic diagram of the process of synthesizing CDs from three sugars using SP. Reproduced with permission [130]. Copyright 2022, Elsevier.

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  Figure 16  (a) Fluorescence microscope images of CDs under different excitation. Reproduced with permission [131]. Copyright 2011, Royal Society of Chemistry. (b) Formation mechanism of CDs. Reproduced with permission [131]. Copyright 2011, Royal Society of Chemistry. (c) Synthesis and catalytic application of CDs. Reproduced with permission [133]. Copyright 2023, Elsevier.

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  Figure 17  (a) Schematic diagram of the preparation process of CDs@GQDs for Cr⁶⁺ detection. Reproduced with permission [137]. Copyright 2024, Elsevier. (b) Preparation and internalization of CNPs for LDs imaging. Reproduced with permission [138]. Copyright 2021, American Chemical Society.

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  Figure 18  (a) Synthesis process diagram of multicolor photoluminescent CDs prepared by template method. Reproduced with permission [140]. Copyright 2009, John Wiley and Sons. (b) Synthesis process of CDs using SBA-15 as template. Reproduced with permission [142]. Copyright 2023, Elsevier.

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  Figure 19  (a) The formation mechanism of CDs. Reproduced with permission [144]. Copyright 2015, John Wiley and Sons. (b) Diagram of CDs synthesis mechanism. Reproduced with permission [148]. Copyright 2019, Royal Society of Chemistry.

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  Figure 20  (a) Microfluidic device diagram, Z-shaped channel, and GQDs formation process. Reproduced with permission [151]. Copyright 2016, American Chemical Society. (b) Schematic diagram of ultrasonic microreactor. Reproduced with permission [152]. Copyright 2023, Elsevier. (c) Structure design and key parameters of biomimetic leaf vein microchannel in microreactor. Reproduced with permission [152]. Copyright 2023, Elsevier.

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  Figure 21  (a) Illustration of the synthetic procedure of the or-CDs and their application in detection of Ag⁺ in the fluorescent and smartphone-assisted colorimetric dual-mode manner. Reproduced with permission [164]. Copyright 2023, Elsevier. (b) Schematic diagram of multifunctional sensor for detection of FQs and His based on fluorescence Y-CDs. Reproduced with permission [172]. Copyright 2018, American Chemical Society.

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  Figure 22  (a) Adsorption mechanisms of NCDs-CNF/CSgel for Cu(Ⅱ) and Cr(Ⅵ). Reproduced with permission [180]. Copyright 2022, Elsevier. (b) Different adsorption mechanisms of As(Ⅴ) on FeCD-NH₂ and FeCD-NH. Reproduced with permission [188]. Copyright 2023, Elsevier. (c) Adsorption mechanisms of LDO-C for U(Ⅵ). Reproduced with permission [185]. Copyright 2018, Elsevier.

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  Figure 23  (a) Photocatalytic behavior of TC by CDs/TiO₂. Reproduced with permission [199]. Copyright 2024, Elsevier. (b) Predicted mechanism of the CV dye degradation. Reproduced with permission [210]. Copyright 2022, Springer Nature.

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  Table 1.  PL quantum yields of CDs synthesized by different carbon sources and synthesis methods.

  Synthesis methods	Carbon sources	LLQY (%)	Ref.	Synthesis methods	Carbon sources	PLQY (%)	Ref.
  Arc discharge method	Soot	1.6	[8]	Microwave method	Graphene oxide	22.9	[93]
  	Graphene	8.7	[57]		Graphene oxide	21.1	[94]
  Laser ablation method	Carbon target	10	[9]		Silkworm chrysalis	46	[95]
  	Graphite flakes	12.2	[59]		Palm kernel shell	43.07	[96]
  	Carbon cloth	35.4	[63]		Glucose	14.9	[122]
  	Graphene nanosheets	26.75	[66]		Citric acid	15	[54]
  Chemical oxidation method	Candle soot	1.9	[67]		Citric acid	65.8	[124]
  	Carbon soot	3	[68]		Citric acid	80.8	[125]
  	Activated carbon	12.6	[71]		Citric acid	54.68	[126]
  	Carbon soot	60	[72]		p-Phenylenediamine	15	[128]
  	Graphene oxide	7.4	[73]	Ultrasonic method	Food waste	2.85	[98]
  	Starch	11.4	[76]		Albizia flowers	24.7	[100]
  	p-Phenylenediamine	7	[139]		Oligomer polyamide resin	28.3	[101]
  Electrochemical method	Multiwalled carbon nanotubes	6.4	[77]	Ball milling method	Activated carbon	7.6	[13]
  	Graphite rods	12	[79]		Carbon black	2.23	[30]
  	Graphite electrode	11.2	[81]		Spent coffee grounds	3.71	[105]
  	Ethanol	4	[131]	Hydrothermal/ Solvothermal method	l-Ascorbic acid	6.79	[106]
  	Acetonitrile	13.3	[132]		p-Phenylenediamine	61.6	[111]
  Pyrolysis method	Coffee grounds	3.8	[87]		p-Phenylenediamine	14.47	[14]
  	Watermelon peel	7.1	[88]		NADES	25	[117]
  	Plant leaves	16.4	[89]	Plasma method	Acrylamide	6	[129]
  	Hair	17	[91]		Citric acid	5.1	[15]
  	Zingiberis rhizoma	5.2	[92]	Template method	Resols	14.7	[140]
  	Houttuynia cordata and citric acid	15	[137]		Citric acid	23	[141]

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  Table 2.  The applications of CDs/CDs composite materials in sensing based on different precursors.

  Detector	Carbon sources	Synthesis method	Fluorescence probe	PLQY (%)	Linear range	LOD	Ref.
  Co(Ⅱ)	Graphite rods	Electrochemical method	NH2-CQDs	—	50 nmol/L-40 µmol/L	12 nmol/L	[153]
  Co(Ⅱ)	NADES	Hydrothermal	N-CQDs	25	5–250 µmol/L	1.2269 µmol/L	[117]
  Pb(Ⅱ)	Graphite powder	—	DTPAN-fn-GQD	40	0.5–40 nmol/L	0.25 nmol/L	[154]
  Pb(Ⅱ)	Citric acid	Hydrothermal	NCDs-AuNCs	—	0.86–20 µmol/L	0.68 µmol/L	[161]
  Cr(Ⅵ)	—	Hydrothermal	PV/BH@CD	—	0–20 µmol/L	66 nmol/L	[155]
  Cr(Ⅵ)	Citric acid	Hydrothermal	CE-CDs	72	0–500 µmol/L	23 nmol/L	[162]
  Cr(Ⅵ) Hg(Ⅱ)	l-Arginine	Hydrothermal	CDs	20	0.1–2 µmol/L 0.4–5 µmol/L	0.024 µmol/L 0.084 µmol/L	[16]
  Hg(Ⅱ)	Orange juice	Hydrothermal	N-CDs	31.7	4–32 µmol/L	—	[156]
  Cu(Ⅱ)	p-Phenylenediamine	Hydrothermal	N-CDs	2.16	45–70 µmol/L	45.87 µmol/L	[158]
  Cu(Ⅱ)	Tryptophan o-Phenylenediamine	Hydrothermal	CD@Cu-IIP-A	—	80–780 µmol/L	3.17 µmol/L	[163]
  Ag(Ⅰ)	Lycium ruthenicum	Hydrothermal	N-CDs	21.8	0.7–36 µmol/L	59 nmol/L	[159]
  Ag(Ⅰ)	p-Phenylendiamine Sulfanilic acid	Hydrothermal	or-CDs	18.4	0.8–10 µmol/L and 10–100 µmol/L	56 nmol/L	[164]
  Pd(Ⅱ)	LLC	Hydrothermal	L-CQDs	5.32	5–40 µmol/L	0.1 µmol/L	[160]
  Fe(Ⅲ)	Sweet potato roasting residues	Pyrolysis	SPCDs	53	25.0–250 µmol/L	13.96 µmol/L	[165]
  Fe(Ⅲ)	Diphenylsemicarbazide	Hydrothermal	Zn/Co-NCDs	38.6	0.03–20.00 µg/mL	0.010 µg/mL	[166]
  Fe(Ⅲ)	o-Phenylenediamine	Microwave-assisted	N-CDs	20.64	1–70 µmol/L	0.1011 µmol/L	[167]
  Fe(Ⅲ)	Zinc gluconate	Hydrothermal	C-dots	8.3	0–200 µmol/L	1.9 µmol/L	[168]
  Zn(Ⅱ)	Citric acid	Pyrolysis	C-dots	—	0.1–2.0 µmol/L	6.4 nmol/L	[169]
  U(Ⅴ)	Citric acid	Microplasma	CDs	5.1	0–75 ppm	0.71 ppm	[15]
  F−	Glucose	Microwave	Zr(CDs–COO)2EDTA	—	0.10–10 µmol/L	0.031µmol/L	[170]
  Minocycline	Citric acid	Microwave-assisted	CQDs	30.8	0.28–30.39 µmol/L	91 nmol/L	[171]
  NOR CIP OFX His	o-Phenylenediamine	Hydrothermal	Y-CDs	22.6	0.05–50 µmol/L 0.2–25 µmol/L 0.4–10 µmol/L 0.05–10 µmol/L	17 nmol/L 35 nmol/L 67 nmol/L 35 nmol/L	[172]
  Ciprofloxacin	Osmanthus fragrans leaves	Hydrothermal	MIPs@CdTe/CDs@SiO2	—	0–60 nmol/L	0.0127 nmol/L	[173]
  Oxytetracycline	Stalk of cherry tomatoes	Hydrothermal	Stalk-Trp CDs@Eu	30.77	6–100 µmol/L	0.018 µmol/L	[174]
  NAs	NADES	Hydrothermal	N-CQDs	7.22	0.03–0.09 mol/L	0.588 mmol/L	[175]
  NAs	p-Phenylenediamine	Hydrothermal	PEG-CQDs	14.47	0.5–15 mmol/L	0.16 mmol/L	[14]
  Catechol	Citric acid	Hydrothermal	N-CQDs	9.47	2–40 µmol/L	0.65 µmol/L	[177]
  Toluene Xylene	PEG-800	Hydrothermal	α-CD/CA/CDots	—	—	3.7 mg/L 4.9 mg/L	[178]
  TNT	Citric acid	Hydrothermal	CDs@NH2	32.25	0–1 µmol/L	0.213 µmol/L	[179]

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  Table 3.  CDs/CDs composite materials for the removal of inorganic pollutants.

  Adsorbents	Target inorganics	Adsorption isotherm	Adsorption kinetics	Efficiency (% or mg/g)	Adsorption mechanism	Ref.
  NCDs-CNF/CSgel	Cu(Ⅱ) Cr(Ⅵ)	Langmuir	pseudo-second-order	148.30 mg/g 294.46 mg/g	Complex reaction, Electrostatic attraction, Reduction reaction	[180]
  CD@Cu-IIP-A	Cu(Ⅱ)	Langmuir	pseudo-second-order	85.6 mg/g	—	[163]
  CCMg	Cd(Ⅱ) Cu(Ⅱ)	–	–	988.4mg/g 656.3 mg/g	Cd(Ⅱ): hydrogen/dative bonds Cu(Ⅱ): precipitation	[190]
  N, S-CQDs@Fe3O4@HTC	Cu(Ⅱ) Cd(Ⅱ)	–	–	99.90% 85.08%	Electrostatic attraction, Surface complexation	[186]
  LFPC	Cd(Ⅱ)	Freundlich	–	98%	Physical adsorption	[187]
  C-dot/ChNC	Cr(Ⅵ) Co(Ⅱ)	Langmuir	–	155 mg/g 152 mg/g	Chemical binding	[191]
  β-CD(CA)/QDs	Ni(Ⅱ), Cu(Ⅱ), Cd(Ⅱ), Pb(Ⅱ)	Langmuir-Freundlich	pseudo-second-order	243 mg/g, 229 mg/g, 228 mg/g, 222 mg/g	—	[192]
  SA@PEI-CDs	Pb(Ⅱ)	Freundlich	pseudo-second-order	380.39 mg/g	Surface complexation, Cation exchange, Electrostatic attraction, Intra-particle diffusion	[184]
  Fluorescent aerogels	Pb(Ⅱ)	Langmuir	pseudo-second-order	183 mg/g	Chelation and coordination	[193]
  FSH	Pb(Ⅱ)	Langmuir	pseudo-second-order	265.9 mg/g	Electrostatic attraction, Ion exchange, Complexation	[181]
  MAR@poly(TAPA)-CD	Fe(Ⅲ)	Langmuir	pseudo-second-order	32.9 mg/g	Chelation and coordination, Electron-transfer	[182]
  CB-50%	Hg(Ⅱ)	Langmuir	pseudo-second-order	290.70 mg/g	Coordination bonds, Hydrogen bonds	[189]
  FeCD-NH	As(Ⅴ)	Langmuir	pseudo-second-order	97.8%	Dual-configuration hydrogen bonds	[188]
  LDO-C	U(Ⅵ) 241Am(Ⅲ)	Langmuir	pseudo-second-order	354.2 mg/g 95.4%	Surface complexation, Electrostatic interactions, Ion-exchange, Precipitation	[185]
  PECQDs/MnFe2O4	U(Ⅵ)	Freundlich	pseudo-second-order	194 mg/g	Cation exchange	[183]
  GQDOs-Ba	Pb(Ⅱ) La(Ⅲ)	Langmuir	pseudo-second-order	98.5%−99.8% 94.6%−96.2%	Chemical binding	[194]

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  Table 4.  The degradation efficiency of organic pollutants by CDs/CDs composite materials prepared from different precursors.

  Materials	Precursors	Synthesis method	Target organics	Degradation method	Reaction condition	Degradation Efficiency	Ref.
  CZCH	Citric acid	Hydrothermal	TC	Adsorption	—	810.36 mg/g	[197]
  CN/CQD/BiOCl0.75Br0.25	Citric acid	Hydrothermal	TC	Photo-degradation	Xe lamp light, C(TC) = 10 mg/L, C(CN/CQD/BiOCl0.75Br0.25) = 0.1 g/L, t = 30 min	84.3%	[198]
  N-doped carbon dots	d-glucose, Urea	Microwave	TC	Photo-degradation	C(TC) = 22 mg/L, C(N-doped carbon dots) = 800 µg/mL, t = 10 min	> 95%	[122]
  CDs/TiO2	Citric acid, Ethylenediamine	Hydrothermal	TC	Photo-degradation	C(TC) = 20 mg/L, pH 5, t = 70 min	100%	[199]
  BiOCl/NGQDs	Citric acid, Urea	Hydrothermal	CIP	Photo-degradation	t = 60 min	82.5%	[200]
  LFC0.5/G-CQDs	Citric acid, Urea	Hydrothermal	Quinoline	Photo-degradation	Visible light, C(quinoline) = 20 mg/L, C(LFC0.5/G-CQDs) = 0.5 g/L, t = 120 min	91%	[201]
  CQDs/Bi2MoO6	—	Hydrothermal	Norfloxacin	Photo-degradation	C(norfloxacin) = 20 mg/L, C(CQDs/Bi2MoO6) = 0.8 g/L, t = 30 min	99%	[202]
  CD@MIL-88 B(Fe)	—	Hydrothermal	AmB, Nap	Photo-degradation	C(CD@MIL-88 B(Fe)) = 0.2 g/L	92%, 90%	[203]
  CAC@CDs-BPEI	Citric acid	Hydrothermal	RhB	Adsorption	t = 180 min	1734.55 mg/g	[204]
  GQDp	Carbon black	Chemical oxidation	RhB	Photo-degradation	t = 6 h	55%	[205]
  CQDs/TiO2	Starch	Chemical oxidation	RhB,	Photo-degradation	UV-light irradiations, C(CQDs/TiO2) = 20 mg, C(RhB) = 5 mg/L, t = 120 min	91.28%,	[3]
  CDs	Citric acid	Hydrothermal and microwave-assisted	MB	Photo-degradation	pH 11	92.3%	[206]
  Si-CQD	Rice husk	Hydrothermal	MB	Photo-degradation	C(MB) = 10 ppm, t = 120 min	100%	[207]
  CQDs/CuFe2O4	Citric acid, Urea	Microwave	MB	Photo-degradation	Xenon lamp, C(CQDs/CuFe2O4) = 10 mg/L, pH 7, t = 180 min	99.86%	[208]
  YMFenAp@CD	Yerba mate	Pyrolysis	CV	Adsorption	t = 45 min	3.18 mmol CV/gads	[209]
  NCQDs	Zucchini	Hydrothermal	CV	Photo-degradation	Visible light, C(CV) = 10 ppm, t = 180 min	99.9%	[210]
  PDs-TiO2	PVA	Hydrothermal	MO	Photo-degradation	UV–vis light irradiation, t = 8 h	96.7%	[211]
  CDs	YM	Chemical oxidation	MO	Photo-degradation	Halogen lamp irradiation, C(MO) = 8.5 ppm, C(CDs) = 100 ppm, pH 6.2, t = 7 h	98%	[212]
  N-CDs	Opuntia Ficus Indica	Hydrothermal	MO	Photo-degradation	t = 12 min	100%	[213]
  CQDs	Cordia myxa L	Hydrothermal	MO, MB, CV, EBT	Photo-degradation	UV-light irradiations, C(pollutant) = 0.015 mmol/L, m(CQDs) = 0.3 mg, t = 45–60 min	100%	[214]
  N-CDs	Azadirachta Indica leaves	Pyrolysis	MG	Photo-degradation	Mercury lamp irradiation, m(N-CDs) = 20 mg, t = 70 min	98.25%	[17]
  GQDs	GO	Hydrothermal	RhG	Photo-degradation	Sunlight irradiation, m(GQDs) = 50 mg, t = 80 min	80%	[215]
  CQDs	Fish scales	Hydrothermal	RR120	Photo-degradation	C(RR120) = 15 mg/L, C(CQDs) = 20 mg/L, t = 120 min	97.8%	[216]
  CQDs	Rubber seed shells	Microwave	CR	Photo-degradation	t = 100 min	> 90%	[217]
  N‐CQDs/Ag2 CO3	Citric acid, Urea	Hydrothermal	Phenol	Photo-degradation	Xe lamp light, C(phenol) = 0.02 g/L, m(N‐CQDs/Ag2CO3) = 0.02 g, t = 150 min	About 55%	[218]
  CDs/CN	Citric acid, Urea	Pyrolysis	PNP	Photo-degradation	Xenon lamp, t = 35 min	95.9%	[219]
  g–C3N4–CQDs/BiVO4	Citric acid, Ethylenediamine	Hydrothermal	BzP	Photo-degradation	Xenon lamp, C(BzP) = 10 mg/L, m(g–C3N4–CQDs/BiVO4) = 0.1 g, t = 150 min	85.7%	[220]
  CDs/C11-Fe3O4	Citric acid, Urea	Microwave heating	BaP	Adsorption	t = 20 min	76.23 ng/mg	[221]

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