Metal-Doped Carbon Dots as Fenton-like Catalysts and Their Applications in Pollutant Degradation and Sensing

Abstract

Metal-doped carbon dots (CDs) have become one of the most popular catalytic materials for Fenton-like reactions, mainly due to their low production cost, minimal toxicity, and high catalytic efficiency. Theses reactions not only provide an efficient decontamination method for the degradation of organic pollutants in wastewater but also demonstrate a wide range of sensing applications. Metal doping introduces new catalytically active centres, which increase the binding selectivity to the reactants and offer an additional advantage of improved catalytic degradation and sensing activity. The metal-doped CDs optimise the electronic structure of pristine CDs, thereby enhancing their catalytic properties and reaction rates. These enhancements make them an attractive option for water treatment and sensor design. The objective of this review is to provide a comprehensive overview of the current research progress in the utilisation of metal-doped CDs as Fenton-like reaction catalysts for the degradation of pollutants and sensing applications. This review examines the advantages of metal-doped carbon dots in terms of catalytic efficiency, selectivity, and application scope and discusses the potential challenges and future research directions. The aim is to promote further the sustainable application and green development of CD technology in environmental governance and analytical chemistry.

1. Introduction

In 2004, fluorescent fragments were accidentally discovered during the purification of carbon nanotubes [1]. The formal concept of carbon dots (CDs) was introduced in 2006 [2]. CDs are zero-dimensional optically active carbon-based sphere-like nanomaterials with a size less than 10 nm [3]. They have attracted increasing attention because of their water solubility, environmental friendliness, low toxicity, low cost, wide availability of raw materials, and high biocompatibility compared to traditional semiconductor quantum dots [4]. Their unique electronic, fluorescent, photoluminescent, chemiluminescent, and electro-chemiluminescent properties have been widely used in CD-based sensors to detect a range of analytes, such as metallic ions, small molecules, biomolecules, and environmental pollutants [5,6]. In addition, inspired by their excellent optical properties, CDs can be applied to LEDs, and their low toxicity promotes their development in bioimaging, photothermal therapy, and other biomedicine-related fields [7]. Figure 1 summarises the unique properties of CDs.

CDs are still a relatively new concept in the field of nanotechnology, with numerous potential applications. Recently, doping heteroatoms to CDs has been developed as an effective approach to regulate their physicochemical properties [8]. Both metal and non-metal atoms have been introduced to CDs, which causes atomic orbital overlapping, electronegativity differences, or surface functionalisation, which could occur between the doping atoms and carbon atoms. As a result, this doping strategy can tune their chemical composition and electronic structure, thus improving the intrinsic properties of CDs [8]. Nitrogen is the most common doping element as it enhances the electron cloud density of CDs, modifies the electron energy level, and increases the probability of radiative absorption and quantum yields [9].

As CDs possess a π-conjugated system with sp²/sp³ hybridisation, they display outstanding electron transfer properties, making them suitable for enhancing the catalytic property of different nanomaterials. However, the catalytic activities of pristine CDs are low due to the lack of active sites. Surface functionalisation is a common way to improve the catalytic performance of CDs as it can promote reactant adsorption, charge separation, and redox activity [10]. Doping CDs is an effective approach to enhance their electrical conductivity and charge transport ability, leading to improved catalytic activity and selectivity for specific chemical reactions [11]. Compared with non-metal doping atoms, metal atoms generally have larger atomic radii and provide flexibility in accepting and donating electrons, therefore enhancing the overall catalytic properties of CDs [11]. Interactions between metal doping and surface functional groups not only change the nature of their electronic structures but also create new synergistic effects. Compared with traditional metal catalysts at the nanoscale, such as nanoparticles (NPs), metal-doped CDs have the advantages of excellent aqueous solubility, large surface area, low cost, simple preparation, and easy access to raw materials; thus, they have become highly sought-after catalytic materials [12].

A Fenton reaction refers to the catalytical decomposition of hydrogen peroxide (H₂O₂) by ferrous ions (Fe²⁺) to produce hydroxyl radicals (·OH) and other reactive oxidising substances [13]. Due to their strong oxidising capacities, Fenton reactions are widely used in the treatment of refractory organic pollutants [14]. However, homogeneous Fenton reactions have the disadvantages of low utilisation of H₂O₂, strict pH requirement, and generation of iron sludge [15]. Refinements such as using heterogeneous catalysts [16], adding reducing agents [17], and adopting electro-Fenton [18] and photo-Fenton [19,20] mechanisms have been made to address the above issues.

Using CDs as heterogeneous catalysts for traditional Fenton processes can increase reaction kinetics, maintain catalyst reactivity, and reduce the use of H₂O₂. Dyes such as methyl orange, rhodamine B, p-nitrophenol and other complex molecules have been successfully degraded using CD-catalysed Fenton-like reactions [21]. Due to the presence of abundant oxygen-containing groups on the surface of CDs, they can act as electron mediators or catalysts and react with H₂O₂, a strong oxidant capable of capturing electrons [22]. In addition, the introduction of metal ions into CDs is likely to modify the physicochemical properties and has the possibility of creating more active sites on the surface, which would greatly increase the efficacy of CDs acting as heterogeneous Fenton catalysts [23].

Although there is considerable literature on the properties of metal-doped CDs, few focus on the impact of metal doping on the properties of CD and their application as Fenton-like catalysts. The main objectives of this review are as follows: (1) to compare metal doped CDs with non-metal doped CDs in terms of catalytic activity; (2) to identify how different dopants impact the catalytical properties of CDs; (3) to discuss how metal dopants enhance advanced oxidation processes (AOP) during the Fenton like reaction and what factors (e.g., pH; temperature; metal elements and their valence; H₂O₂ dosage) influence the reaction; (4) to summarise metal-doped CDs used for sensing applications based on Fenton-like reactions. In detail, we first compare the advantages and disadvantages of metal-doped CDs and non-metal doped CDs and highlight the importance of using metal-doped CDs as catalysts for Fenton-like reactions. Next, the synthesis methods for metal-doped CDs, as well as carbon and metal precursors used in the synthesis, are reviewed. In addition, the catalytic properties of metal-doped CDs with different doping elements, especially Cu, Zn, Fe, and Mg, are summarised, with particular emphasis on the effect of metal-doping on the electronic structures, band gap, and surface states of the CDs. Subsequently, advancements in metal-doped CDs as Fenton-like catalysts and their applications in pollutant degradation and sensing are reviewed. Finally, the potential challenges and future research directions in this field are discussed.

2. Metal-Doped CDs vs. Non-Metal Doped CDs

Doping is known as an effective way to modulate the physiochemical properties of CDs [24]. Non-metals, such as nitrogen [25], sulfur [26], boron [27], phosphorus [28], selenium [29], silicon [30], and halogen elements like fluorine [31] and chlorine [32], have been successfully incorporated into CDs. Compared to non-metallic atoms, metal ions, particularly transition metal ions, have more electrons and unoccupied orbitals, which are favourable for changing electronic structures associated with the energy gap of CDs, offering more active sites, and improving functionalities. Metallic doping elements mainly include Cu [33,34], Fe [20,35], Zn [36,37], and Mg [38,39]. Rare earth elements such as La, Gd, Yb, and Eu [40,41] have also been reported. Table 1 summarises the advantages and disadvantages of metal- and non-metal-doped CDs.

In terms of preparation, metal-doped CDs require the addition of metal precursors, such as metal nitrates and chlorides, or metal NPs, which involve more intricate procedures and high cost. In contrast, non-metal dopants like N, S, and P are commonly found in a variety of natural materials, including abundant resources like plant biomass, food waste, and other organic materials. This makes the synthesis of non-metal-doped CDs more cost-effective, sustainable, and environmentally friendly. The biocompatibility of non-metal-doped CDs is significantly enhanced due to the absence of metal ions, which can induce toxicity. In addition, the presence of abundant surface functional groups serves to enhance the stability of the substances in an aqueous environment, thereby facilitating subsequent surface modification processes. However, non-metal-doped CDs generally have low quantum yield and limited fluorescence emission wavelengths, especially in the red and near-infrared regions. In the red and near-infrared regions, the low energies of the photons necessitate precise level matching of the materials to efficiently achieve radiative composites. In such cases, metal doping can provide considerable advantages. The incorporation of metal ions increases the capacity to donate electrons and offers more empty orbitals, thereby changing the electronic structures of pristine CDs, improving their optical properties. The electronic contributions, empty orbitals, and atomic radii of metal dopants work together to change the structures and properties of CDs [42]. For example, incorporating Cr ions into CDs provides excitation-dependent fluorescent characteristics and a high fluorescence quantum yield [43]. Han et al. used Cr-doped CDs as fluorometric sensors, which demonstrated a wider linear fluorescence range, lower detection limits, and greater sensitivity for sensing p-nitrophenol in water than non-doped CDs [43]. Khare et al. [44] prepared Zn-doped CDs, in which the Zn dopant redshifted the emission peak towards the long-wavelength region and reduced radiation recombination. As a result, the Zn-doped CDs demonstrated ultra-bright excitation-independent red emissions along with excellent solubility and stability in aqueous media as well as excellent photostability [44]. The enhanced optical properties of metal-doped CDs make them more suitable for applications that require a high optical response, such as fluorescence imaging and photocatalysis [11]. In comparison to non-metal elements, metal doping has been demonstrated to enhance the charge transfer between the graphene matrix and metal ions in CDs, increasing their capacity to modulate charge density, electron distribution, and structure. This alteration has the potential to markedly improve the catalytic and photovoltaic properties of the materials, making them suitable for the photocatalytic degradation of pollutants and energy conversion applications [8]. Furthermore, metal doping has the potential to increase the number of catalytically active sites in CDs, and the electrons in the outer orbital of the metal ion can effectively combine with the functional groups of the reactants to improve the catalytic properties of the CDs [45,46]. Therefore, metal-doped CDs demonstrated high efficiency and feasibility in the field of environmental protection, especially in pollutant degradation [47].

Table 1. Advantages and disadvantages of metal-doped and non-metal-doped CDs.

	Advantages	Disadvantages	References
Non-metal doped CDs	Cost-effective and environmentally friendly precursors and synthesis methods. Excellent photostability in the ultraviolet and visible light regions. Non-toxicity and biocompatibility.	Relatively low quantum yield. Limited wavelength range of fluorescence emission, especially a notable deficiency in the near-infrared region. Limited catalytic performance due to the absence of active centres. Relatively low electrical conductivity and poor electron transport properties.	[8,9,42,48]
Metal-doped CDs	Ability to introduce new intermediate electronic states, improving the quantum yield and fluorescence of CDs. Wide emission wavelengths covering the UV, visible, and even near-infrared light regions. Capability to modify conductivity and charge transfer efficiency. Enhanced catalytic and photovoltaic properties.	Biotoxicity and environmental contamination due to the introduction of certain metal ions. More intricate synthesis procedures involving higher cost. Compromised stability in certain environments, potentially leading to the leaching or loss of metal ions and subsequent degradation of properties. Photobleaching and diminished optical stability in certain instances, particularly during prolonged use under high-intensity light exposure.	[34,49,50,51]

Table 1. Advantages and disadvantages of metal-doped and non-metal-doped CDs.

	Advantages	Disadvantages	References
Non-metal doped CDs	Cost-effective and environmentally friendly precursors and synthesis methods. Excellent photostability in the ultraviolet and visible light regions. Non-toxicity and biocompatibility.	Relatively low quantum yield. Limited wavelength range of fluorescence emission, especially a notable deficiency in the near-infrared region. Limited catalytic performance due to the absence of active centres. Relatively low electrical conductivity and poor electron transport properties.	[8,9,42,48]
Metal-doped CDs	Ability to introduce new intermediate electronic states, improving the quantum yield and fluorescence of CDs. Wide emission wavelengths covering the UV, visible, and even near-infrared light regions. Capability to modify conductivity and charge transfer efficiency. Enhanced catalytic and photovoltaic properties.	Biotoxicity and environmental contamination due to the introduction of certain metal ions. More intricate synthesis procedures involving higher cost. Compromised stability in certain environments, potentially leading to the leaching or loss of metal ions and subsequent degradation of properties. Photobleaching and diminished optical stability in certain instances, particularly during prolonged use under high-intensity light exposure.	[34,49,50,51]

3. Synthesis of Metal-Doped CDs

3.1. Raw Materials

The most common carbon precursors for CD synthesis are synthetic organic reagents with rich carbon contents, such as polyamines (e.g., ethylenediamine), polyvinylpyrrolidone, citric acid, polyethyleneimine, etc. [52]. In addition, a wide range of natural carbon sources have been used, such as fruits and vegetables, including pear, blueberry, pineapple, green leaves, mushrooms, broccoli, etc. These natural products not only provide a rich source of carbon but are also low-cost and non-toxic [53,54]. With the increasing demand for circular economy, waste materials are preferentially used as CD precursors, especially biomass residues, such as food wastes, which are by-products of food processing rich in soluble and insoluble carbohydrates (e.g., lemon peel, watermelon rind, etc.) [55,56]. Considering the increasing generation of non-biomass wastes and their non-biodegradable nature, plastics, sludge, wastepaper, waste kitchen chimney oil, and waste soots (e.g., kerosene fuel soot and candle soot) have also been converted into valuable CDs [57].

Metal precursors are essential to introduce metal doping into CDs. Inorganic metal salts like nitrates and chlorides have been widely used in hydrothermal and microwave processes due to their high aqueous solubility and low price. Water-soluble chloroauric acid [58] and gadopentetic acid [59] are also reported to introduce Au and Gd, respectively. Some metal precursors with less solubility, such as oxides, or metals themselves are also used, including Mg(OH)₂ [60] and Pt [61]. To increase the contact area during the reaction, the insoluble precursors in their nanoparticle forms are favourable due to their large specific surface area. Ferrocene is an organometallic compound with the formula Fe(C₅H₅)₂. It is insoluble in water but soluble in organic solvents, such as benzene and ether, and decomposes in strong acids such as concentrated sulfuric acid and dilute nitric acid Therefore, ferrocene is a suitable metal precursor for solvothermal methods [62]. Some biomolecules with metal components have also been employed as metal precursors to increase the biocompatibility of CDs; for example, metal-loaded horse spleen ferritin [63], as well as vitamin B12, which contains mineral cobalt [64].

3.2. Synthesis Methods

Undoped CDs can be prepared in two main ways: “top-down” and “bottom-up”, starting from bulk materials and molecular precursors, respectively. In the “top-down” approach, CDs are usually obtained by decomposing larger carbon-based structures; in contrast, the “bottom-up” approach refers to the carbonisation, pyrolysis, or hydrothermal/solvothermal/microwave treatment of small organic molecules or natural biomass to obtain CDs.

Top-down methods are not suitable for the synthesis of metal-doped CDs because they usually start with large pieces of carbon material (e.g., graphite, carbon nanotubes, or graphene) which are then fragmented by physical or chemical means into nano-sized CDs (via arc discharge, laser stripping, etc.). In this process, it is difficult to introduce metal ions or atoms into the structure of the CDs, particularly to dope them uniformly into the lattice or surface of the CDs. Therefore, metal-doped CDs are generally prepared by bottom-up methods (such as hydrothermal, solvothermal, or pyrolytic methods), where metal precursors and the carbon source are mixed at the molecular level to ensure uniformity of doping and strong bonding [65].

Table 2. Bottom-up synthesis methods for metal-doped CDs and their advantages and disadvantages.

Synthesis Methods	Method Description	Advantages	Disadvantages	References
Hydrothermal synthesis	Reactions in high-temperature aqueous solutions under a high vapour pressure.	Simple, low-cost, and highly efficient. Suitable for natural and biocompatible precursors.	Safety concerns as high temperatures and high pressure are required.	[66,67]
Chemical oxidation	Carbonisation of small organic molecules with strong oxidising acids.	Suitable for a wide source of raw materials. Simple operation.	Violent reactions. Environmental impact related to strong oxidising acids.	[68]
Microwave assisted synthesis	Rapid heating of carbon precursors using microwaves.	Simple operation. Low-cost and fast.	Uneven particle size, making separation and purification difficult.	[69,70]
Ultrasound assisted synthesis	Using energy generated by ultrasound to dehydrate, polymerise, and carbonise the precursors.	Simple operation.	Difficult to control particle size.	[71]
Pyrolysis	High-temperature reaction involving carbonisation.	Abundant sources of raw material. Low-cost.	Purification is difficult.	[72]
Solvent thermal synthesis	Synthesis at elevated temperature and high pressure in organic solvents.	Simple operation, Abundant sources of raw material.	Safety concerns and difficulty in controlling particle size.	[73]

list the commonly used bottom-up methods to prepare metal-doped CDs, together with their advantages and disadvantages.

Among these methods, hydrothermal synthesis offers better control over the size, morphology, and purity of CDs and is more suitable for preparing metal-doped CDs for catalytic and sensing applications. Its principle is to prepare an aqueous solution containing carbon and metal precursors and organic ligands and adjust the pH, temperature, and pressure to control crystal growth [67]. Synthesis is generally conducted in a closed system above 100 °C and at a pressure greater than 1 atmosphere. The crystal structure of CDs is important as it influences their specific solubility and adsorption abilities.

4. Effects of Metal Doping on the Catalytic Properties of CDs

Doping metal ions can modulate the electronic structure and surface features of the CDs, thereby having a significant impact on their catalytic properties. Figure 2 illustrates the electronic transition mechanism of metal-doped carbon dots compared to that of undoped CDs.

Generally, CDs are made of sp^2_and sp^3_types of carbon [24]. Amorphous carbon, diamond-like, graphite-graphite oxide, and crystalline carbon are interrelated as the core structure of CDs. Two types of electronic transitions are present in carbon cores, i.e., π-π* transition involving aromatic sp² carbons (aromatic C=C bonds, UV bands below 300 nm) and n-π* transition of the C=O bond (UV bands from 300 nm to 400 nm) [24]. The n-π* process has a smaller energy gap compared to π-π*, and less light is required for excitation with an emission of longer-wavelength light [10]. The absorption bands above 400 nm originate from the surface state transition, depending on functional groups, edge states, and defects on the surface.

The π-π* transition is associated with the excitation of conjugated π electrons from the ground state to the excited state of an aromatic ring structure (C=C double bond) in a carbon core. Metal doping can alter the π-electron distribution of the aromatic structure in CDs. If the metal ion disrupts the integrity of the π-conjugated system by introducing electronic defects, this can lead to an increase in the π-π* band gap and a blue shift in the absorption and emission spectra. If the metal ion conjugates with the π electrons and stretches the π electron system, this can lead to a decrease in the band gap and a red shift in the absorption and emission spectra. In addition, metal doping can cause defects on the CD surface, and this affects the distribution and energy of the π-electrons, which can weaken the radiative transitions and enhance the non-radiative ones. If the doped metal induces the localisation of the π-π* states, this can enhance the radiative transition and lead to an increase in fluorescence intensity.

The n-π* transition is usually associated with the lone pair electrons (n state) migrating from surface-oxygen-containing functional groups (e.g., carbonyls, hydroxyls, carboxyl) into unoccupied π* antibonding orbitals of these groups in CDs. After doping with metal ions, coordination bonds can form with surface-oxygen-containing groups, as the lone pair of electrons interacts with the d-orbitals of the metal. If the lone pair of electrons interacts strongly with the metal ions, the energy gap can be increased, leading to a blue shift in the UV spectrum. If the metal induces localisation of the lone pair, the transition energy may decrease, resulting in a red shift in the spectrum. In addition, the dopant metal can introduce additional energy levels between the n-state and the π* orbitals, leading to a multistage transition process in which the original single-stage n-π* lepton can be suppressed or enhanced, depending on the type and concentration of the dopant metal.

In metal-doped CDs, the n-π* and π-π* transitions do not undergo independent changes but are interrelated. The introduction of metal doping results in alterations to the overall electronic structure of the CDs, potentially affecting both the n-state and the π-state. Consequently, the n-π* and π-π* energy gaps may experience simultaneous narrowing (red-shift) or enlargement (blue-shift), depending on the specific metal employed and its interactions with the carbon core and the surface functional groups.

The following section discusses the four most common metal doping elements and how their involvements influence the catalytic properties of CDs.

4.1. Cu Dopants

As evidenced in the literature, copper is one of the most widely used metal dopants in CDs [34,74]. The outermost 4s energy region of Cu ions is only half filled, with the remaining half of the conduction band remaining vacant. This allows for the transfer of a significant number of electrons, plus metallic bonding, which contributes to the high conductivity observed in copper. It is reported that Cu-N dopants promote the electron transfer and photooxidation reactions of CDs. As a result, the photocatalytic efficiency of CDs in the photooxidation of 1,4-dihydro-2,6-dimethylpyridine-3,5-dicarboxylate was improved 3.5-fold after Cu doping [75]. The formation of bonds between copper and nitrogen has been shown to introduce new energy levels, known as intermediate band states, within the forbidden bands of the material. This development has the potential to extend the range of absorption of visible light by the material, facilitating the efficient transportation of photogenerated electrons. This, in turn, has been demonstrated to reduce the rate of electron-hole pair complexation. It may also act as a charge trapping site to extend the lifetime of the photogenerated carriers, allowing more electrons or holes to participate in the redox reaction, thus improving the catalytic efficiency. In Cu-doped CDs, copper atoms provide active sites that accelerate electron transfer and enhance sensitivity, enabling the rapid response and detection of glucose concentration. Furthermore, Cu, as a heteroatom, can enhance the electronic and surface chemical reactivities of the CDs. Duan et al. reported that the Cu-doped CDs enhanced peroxidase-like activity to horseradish peroxidase, maintaining catalytic stability in the pH range from 5.0 to 11.0 and in the temperature range from 20 °C to 80 °C [76]. The chelation ability of Cu ions in CDs with small molecules can also significantly enhance the selectivity of CDs for emerging pollutants. This chelation also enables the CDs to exhibit enhanced catalytic effects in degrading a wide range of pollutants, especially organic pollutants that are difficult to break down. For example, experiments conducted by Xing et al. demonstrated that Cu-doped CDs exhibited strong efficacy in the photocatalytic reduction of methyl blue [77]. Cardoso et al. demonstrated that Cu-doped CDs could significantly increase the ozonation rate of aqueous solutions of dyes at pH 7.0, achieving 90% efficiency for the azo dyes in about 6 min and for the anthraquinone dye in 30 min [34]. Moreover, Gao et al. [78] reported that the presence of Cu reduced the content of functional groups on the surface of CDs, especially carboxyl (O=C-O-) and amino (-NH₂) groups. Despite the reduction in the number of functional groups on the surface of CDs, the degree of carbonation was enhanced, and Cu²⁺ were enclosed within carbon cores through Cu-C and Cu-N bonds. The increased carbonation stabilised the structures of CDs, reducing the likelihood of decomposition or loss of activity during catalysis [78]. Thus, Cu-doped CDs display excellent photostability and chemical stability and retain their catalytic performance in complex environments. This property is of great importance in the context of environmental catalysis and pollutant detection, as it ensures that the materials remain effective and stable for extended periods of time. Additionally, copper ion doping has the potential to markedly enhance the density of catalytically active centres in CDs by influencing the charge distribution in the CDs. The presence of copper ions provides multivalent electron transfer pathways through interactions with redox species, thus greatly enhancing the catalytic efficiency of CDs [34]. During the Cu⁺/Cu²⁺ cycle, the hydroxyl radical (-OH) is generated by the reaction of Cu⁺ with H₂O₂, a strong oxidising agent capable of attacking the chemical bonds of the dye molecule non-selectively, leading to efficient degradation. Conversely, superoxide radicals (O₂₋-) are formed when Cu²⁺ accepts electrons and reacts with O₂, which can further oxidise intermediates and serve as precursors for -OH and H₂O₂. In contrast, hydrogen peroxide (H₂O₂) not only serves as a source of -OH, but also undergoes regeneration by O₂, ensuring continuous recycling within the system and thereby maintaining a steady supply of reactive oxygen species (ROS) throughout the reaction. These properties of Cu-doped CDs demonstrate great potential for photocatalytic degradation of organic pollutants, environmental remediation, etc. Finally, a significant number of Cu-doped CD systems demonstrate consistent performance across multiple cycles. This enhances the recyclability utility of Cu-doped carbon dots. Embedding Cu within the dot matrix typically yields a quasi-heterogeneous catalyst that can be filtered and reused. The durability of this catalyst is attributed to the stable Cu-C bonding within the CD, as well as the catalyst’s ability to withstand oxidising conditions. It has been observed that the leaching of Cu from the Cu-CD catalysts is often so low as to be undetectable. Similar studies have measured less than 1% leaching of Cu²⁺ even after 15 cycles in Cu-based Fenton catalysts. Furthermore, the study by Jin et al. [50] on the use of Cu-CDs in a Fenton-like RhB system also reports that the degraded filtrate is non-toxic to cells and mice, meaning that the catalysts do not release hazardous Cu ions into the solution.

4.2. Zn Dopants

Zn tends to lose electrons from its 4s orbitals rather than 3d orbitals during chemical reactions. This results in a reduced electron transfer energy, conferring a natural advantage for zinc doping. The incorporation of zinc ions can markedly enhance the photoluminescence intensity of CDs [79]. For example, He et al. [80] observed that the fluorescence quantum yield of undoped CDs was 37%, while Zn-doped CDs exhibited a quantum yield of 48%. Furthermore, fluorescence intensity gradual increased with elevated zinc doping concentrations, indicating that zinc doping in CDs exerts a substantial influence on their optical properties [80]. During the hydrothermal synthesis, Zn²⁺ from zinc acetate chelated with the carboxyl group in citric acid, allowing their excellent dispersion in CDs. Additionally, Zn may catalyse the growth of CDs, leading to a more complete CD structure [81]. Importantly, the band gap of CDs decreased after the incorporation of zinc ions, triggering a red shift and improved intensity of fluorescence. In a study comparing Zn-doped and undoped CDs, the band gap of undoped CDs was approximately 3.12 eV, whereas Zn-doped CDs exhibited a lower band gap, approximately 2.71 eV. The reason for the reduced band gap was due to the introduction of new conduction band energy levels and intermediate energy levels. The quantum yield of the prepared CDs also increased from 8.15% (undoped) to 10.34% (Zn-doped) [82]. Xu et al. [83] reported that zinc doping represented an effective passive agent that prevented the π-π stacking effect in graphene. This property reduced the generation of CD aggregates and increased the quantum yield of Zn-doped CDs [83]. The highly dispersed Zn-doped CDs also exhibited enhanced binding ability to pollutants, facilitating their degradation, thus demonstrating practical significance for environmental pollutant removal.

He et al. [80] conducted a detailed investigation of the forms and locations of the doped Zn in CDs and reported that most Zn²⁺ chelated with carboxyl groups on the CD surface to form Zn-O bonds. Some Zn²⁺ were present in the CD cores, and the remaining Zn²⁺ were reduced to zinc atoms and attached on the CD surface. These surface atomic Zn tend to be oxidised to ZnO [80]. The band gap of the ZnO on the surface also plays an important role in the optical properties of CDs; ZnO; a band gap above 3.2 eV is an excellent UV absorbent, and this property is useful in photo-Fenton reactions [84].

The major concern of using Zn-doped CDs is the potential toxicity of Zn ions [85]. In accordance with the guidelines set by the Australian Government, the level of dissolved organic matter in freshwater is typically sufficient to eliminate the toxicity of zinc because of the formation of less-toxic Zn complexes [84]. Furthermore, the toxicity of zinc can be reduced by increasing the acidity of the solution to a pH of approximately 8 [86]. Zinc-doped CDs are typically found in composite form and generally exhibit excellent cycling stability and sustainability. ZnO/CD composite catalysts utilised by Jung et al. [87] demonstrated a 96% degradation of methyl bromide within 30 min over five successive cycles. Further, the ZnO/CD composite catalysts exhibited a 96% degradation of methyl bromide over 30 min during five consecutive cycles [87]. Zinc is usually tightly bound to these nanomaterials (typically Zn-O or Zn-N complexes), and the study reported no significant zinc leaching during dye degradation. This phenomenon is partly attributable to the fact that zinc oxide or zinc-based substances are generally in a solid phase with carbon dots [87].

4.3. Fe Dopants

Similarly to Zn, Fe is a transition metal with an empty 3d orbital, and Fe doping significantly enhances the electron transfer efficiency of CDs. The superior charge transfer ability of Fe-doped CDs is not only favourable for fluorescence and sensing applications but also plays an important role in photocatalytic processes. Dang et al. [88] demonstrated that Fe doping offers substantial benefits for the photocatalytic reduction of carbon dioxide (CO₂) to methanol. The enhanced electron transfer rate enabled Fe-doped CDs to efficiently reduce CO₂ to methanol within a six-hour period, achieving a reaction rate 2.6 times higher than that of undoped CDs [88]. This study emphasises the potential of Fe doping in the field of photocatalysis, particularly for clean energy conversion and environmental remediation.

The main participant in the Fenton reaction is Fe. Thus, Fe-doped CDs also demonstrate excellent oxidising abilities through the reaction of H₂O₂ with ferrous ions [35]. Researchers have successfully developed Fe-CDs with an average size of approximately 3 nm. These Fe-CDs exhibit significantly enhanced peroxidase-like activity under near-infrared light (NIR) irradiation, producing more reactive oxygen species (ROS) and releasing heat. This synergistic effect has been demonstrated to facilitate the effective eradication of both Gram-positive (e.g., Staphylococcus aureus) and Gram-negative (e.g., Escherichia coli) bacteria. In addition, it was found that Fe-CDs excelled in promoting wound healing, as evidenced by their ability to inhibit bacterial infections, accelerate fibroblast proliferation, promote angiogenesis, and increase collagen deposition. The ultra-small size of Fe-CDs ensures their biocompatibility [89]. Fe-doped CDs were coated on the surface of highly crystalline HC-C₃N₅ to form an efficient photo-self-Fenton system. During the reaction, Fe-doped CDs serve as active sites, promoting light-induced charge separation and transfer electrons for the in-situ generation of H₂O₂ over HC-C₃N₅ [20]. In addition, the solid-phase Fe species (likely in the form of Fe-N) on the surface of CDs maximised the activation of in-situ generated H₂O₂ to produce highly oxidative OH and O²⁻ for photo-self-Fenton reaction. This research presents a novel photo-self-Fenton system based on Fe-doped CDs for efficient wastewater treatment [90].

The Fe species on the surface of CDs are more likely to be present in the form of iron oxide. The band gap for iron oxide NPs was found to be in the range of 1.9 to 2.5 eV, significantly lower than that of ZnO (above 3.0 eV). The intermediate band gap allows iron oxide to absorb visible light, facilitating electron excitation and migration in photocatalytic processes. In Fe-doped CDs, the Fe atoms are not present exclusively as iron oxides; rather, they are bound to the CD matrix in different valence states via interactions like ligand binding or surface adsorption through their 3d electron orbitals, thereby facilitating electron transfer, enhancing catalytic activity and enabling cycling between different valence states [91]. From a sustainability perspective, Fe-doped CD catalysts generally demonstrate favourable reusability over multiple treatment cycles. In one study, the sulphide removal activity of N, Fe-doped CD remained above 90% of the initial activity after four reuse cycles, with only a decrease (~66% efficiency) by the fifth cycle [92]. Soares et al. found that during heterophasic Fenton degradation, the iron supported in the carbon matrix showed ’iron-free leaching’ and maintained a high stability even when the catalyst was cycled [93].

4.4. Mg Dopants

Mg is an essential element for the human body, and Mg²⁺ is involved in several important physiological processes [94]. Mg-doped CDs display low toxicity, making them well-suited for biomedical applications. For example, Mg-doped CDs could induce osteoblast differentiation and bone matrix mineralisation in-vitro, showing great potential for future application in bone repair in combination with scaffold materials [95]. A fluorescent sensor based on Mg-doped CDs was developed to monitor arginine. Doping with Mg²⁺ effectively improves the fluorescence quantum yield and detection sensitivity of the CDs, and its water solubility and dispersibility enable stable operation in a water-based environment. The sensor demonstrated a linear detection range from 0.3 to 130 μmol/L and a detection limit of 0.15 μmol/L [39]. Additionally, when adenosine triphosphate (ATP) was added to the Mg-doped CD solution, the adenine, ribose and three phosphate groups on ATP could bind effectively to the functional groups on Mg-doped CDs to induce Mg²⁺-π and electrostatic interactions, thereby quenching the fluorescence of the sensor [39]. In most CD systems employed for ATP sensing (particularly doped metals such as Mg²⁺), it has been demonstrated that the bursting mechanism predominantly adopts a static form. This is attributed to the ability of Mg²⁺ to form stable complexes with the ATP phosphate structure. The complexation process gives rise to a non-fluorescent complex, resulting in a reduction in the intensity of the CDs’ fluorescence [39].

Theoretical calculations indicate that magnesium oxide (MgO) possesses a bandgap between 6.3 and 7.8 eV, which is one of the highest values observed among the four metal oxides that this review focused on [96]. However, MgO nanostructures exhibit a modified optical bandgap [94]. The energy gap observed in MgO nanoparticles (7 nm) is 2.8 eV, which is primarily attributed to strong quantum confinement effects and surface defects. These states contribute to narrowing the actual energy gap, while one-dimensional MgO nanostructures exhibit a marginally higher energy gap of 3.2 eV due to anisotropic confinement, potential surface chemistry differences, and strain effects. The band gap strongly depends on the particle size, allowing the possibility of band gap modulation [94]. Such modulation can achieve a balance between high electron transport efficiency and stabilised electrostatic interactions, particularly within complex biomolecular systems. This is of particular significance for nano-biomedical and molecular detection applications, as it not only enhances the stability of CDs but also improves their fluorescence emission, increasing their efficiency in sensing applications [97]. In addition, magnesium-doped CD demonstrated notable reusability, despite the varied functions of magnesium. Bhati et al. [98] observed that their r-Mg-N-CD catalysts exhibited minimal loss of activity after being recovered and reused for a minimum of five cycles. The magnesium doping process did not result in significant leaching issues, primarily due to the innocuous nature of Mg²⁺ at low concentrations, which typically remained within the carbon dots. The incorporation of magnesium in magnesium-doped carbon dots typically occurs through ionic complexes or surface functional groups [98].

Although metal-doped carbon dots (CDs) offer significant advantages in catalytic performance, their environmental safety—particularly the potential leaching of metal ions—remains a critical concern, especially in the context of sustainable water treatment. Studies have shown that the extent of metal ion release can vary depending on the dopant type, pH, and operational conditions. For instance, Zn-doped CDs may release Zn²⁺ in the range of 0.6–2.3 ppm under neutral to mildly acidic conditions (pH 5.0–7.0), while Cu-doped CDs have been reported to release up to 0.9 ppm of Cu²⁺ after multiple catalytic cycles. Although these concentrations are generally below acute toxicity thresholds, they may still exceed regulatory limits for drinking water or sensitive aquatic environments.

In practical applications, the amount of leaching depends on the dosage of catalyst and the treatment volume. For example, assuming 10 mg of metal-doped CDs are added to 1 L of wastewater, even at a doping level of 2 wt% and a worst-case scenario of 50% metal leaching, the resulting metal ion concentration would be approximately 1 ppm. In contrast, background levels of zinc in natural river water typically range from 0.00004 to 0.0016 ppm, and copper ranges from 0.0002 to 0.03 ppm. This suggests that without appropriate control, the use of metal-doped CDs could lead to a notable increase in local metal concentrations.

Encouragingly, several studies have demonstrated that metal-doped CDs can exhibit excellent structural stability and reusability with minimal leaching risk. For example, Bhati et al. showed that their r-Mg-N-CD catalyst maintained high catalytic activity over five consecutive cycles with negligible magnesium release, attributed to the stable incorporation of Mg²⁺ through ionic coordination or surface functional groups [98]. Similarly, Fe-doped CDs have shown strong cycling stability; one study reported over 90% sulfide removal efficiency during the first four cycles, with only a decrease to 66% in the fifth [92]. Soares et al. further demonstrated that in a heterogeneous Fenton system, Fe embedded within the carbon matrix did not leach into the solution, maintaining high catalytic stability [93].

Other systems, such as Zn- and Cu-doped CDs, also showed favourable long-term durability. Zinc is typically stabilised via Zn-O or Zn-N bonding, and leaching levels remained below 1 ppm throughout dye degradation experiments [87]. In Cu-based systems, less than 1% Cu²⁺ leaching was observed even after 15 catalytic cycles. Notably, Jin et al. confirmed that filtrates treated with Cu-CDs in RhB degradation showed no cytotoxicity to cells or mice, further confirming the absence of harmful Cu ion release [50].

5. CDs as Fenton-like Catalysts for Pollutant Degradation

Fenton and Fenton-like reactions are widely used in wastewater treatment for the degradation of organic pollutants. The basic principle is to break down and oxidise organic and inorganic pollutants into harmless compounds by generating a strong oxidant. Fenton reactions utilise H₂O₂ in the presence of Fe²⁺ catalyst to produce strong oxidants [19]. In Fenton-like reactions, metal ions other than Fe²⁺ perform a similar function to generate hydroxyl radicals. Common metal ions used in Fenton-like reactions include Cu, Mn, Co, Cr, etc. Metal-doped CDs are well-known for their effectiveness in removing recalcitrant organic contaminants [99]. In general, the identity of the metal dopant plays a critical role in the observed kinetics. Metal-doped carbon dots consistently enhance the kinetics of dye degradation, usually following first order reaction models. Rate constants from different studies show that undoped CDs achieve modest dye removal rates, while doping with metals (Ag, Pd, Cu, Fe, etc.) in most photocatalytic degradation experiments fits a pseudo-first order kinetic model. Pseudo-second order (PSO) kinetics are more commonly associated with adsorption-controlled processes. Metal-doped carbon dots generally exhibit higher rate constants (faster dye degradation) than their undoped counterparts due to improved light absorption, charge separation, or reactive radical generation.

Table 3. The catalytic performance of undoped and metal-doped CDs.

CD Type	Dye Target	pH	Degradation Efficiency (%)	Half-Life (t1/2, min)	k (min−1)	Normalised k (per mg CD/mM H2O2)	Experimental Conditions	References
Undoped CDs	Methylene Blue	6.8	42.0% (300 min)	68.5	0.0101	0.002 min−1·mg−1 (10 mg catalyst, 1 mM H2O2)	Daylight, 50 °C, 1 mM H2O2, 10 mg CD, MB = 0.25 mg/mL	[100]
Non-metal-doped CDs (N-CDs)	Rhodamine B	7.0	~55% (180 min)	—	~0.0078	~0.0015 min−1·mg−1 (estimated)	Visible light, room temp, 1 mM H2O2, ~10 mg CD, RhB = 0.25 mg/mL	[101]
Metal-doped CDs (Ag, Pd, Cu, Fe)	Methylene Blue	6.8	78–100% (≤300 min)	22.6–30.9	0.022–0.0316	~0.005–0.007 min−1·mg−1·mM−1	Daylight, 50 °C, 1 mM H2O2, 10 mg CD, MB = 0.25 mg/mL	[100,101]
Metal-doped CDs (Cu, Fe)	Rhodamine B/Orange II	7.0	90–98% (≤120 min)	2.3–21.9	0.0316–1.002	Up to 0.10 min−1·mg−1·mM−1	No light (ozonation) or visible light; room temp; 1–3 mM H2O2/O3; 5–10 mg catalyst; RhB/O-II = 0.1–0.2 mg/mL	[34]

states the catalytic performance of undoped and metal-doped CDs.

5.1. CDs Used in Combination with Traditional Fenton-like Catalysts

Some CDs are used in combination with traditional Fenton-like catalysts. For example, Co NPs coated with CDs were studied as a catalyst for the Fenton-like reaction to degrade polypropylene microplastics, and their catalytic performance was compared with those of uncoated Co NPs [102]. The functional groups on the CDs and the large surface areas of the CDs provide more opportunities to form hydrogen bonds with H₂O₂, enhancing the charge transfer in Co²⁺/Co³⁺ and the formation of peroxides on the surface [102]. In another report, Fe₃O₄@Cu₂O/CDs/N-doped CDs functioned as a heterogeneous photo-Fenton catalyst. The double CDs grown on the surface of Fe₃O₄@Cu₂O NPs exhibit visible and near-infrared light-responsiveness and enhanced catalytic activity [103].

5.2. Metal-Doped CDs as Fenton-like Catalysts

Figure 3 shows the use of metal-doped CDs to decolourise a polluted dye solution via Fenton-like reactions. The efficiency of hydrogen peroxide activation is a pivotal parameter that differentiates these nanocatalysts from conventional Fenton catalysts based on the Fe²⁺/Fe³⁺ system. In conventional Fenton reactions, the rate at which Fe²⁺ reacts with H₂O₂ to generate hydroxyl radicals has been widely characterised. In contrast, metal-doped discs (e.g., discs doped with iron, copper, zinc, or magnesium) activate H₂O₂ through a surface-mediated electron transfer process that is influenced by the type of dopant and the functional groups on the disc surface.

The presence of active sites (e.g., Cu-N or Fe-N bonds) in doped CDs has been shown to promote the formation of transient intermediate species, leading to more efficient generation of reactive oxygen species (ROS). Experimental studies have demonstrated that some metal-doped CDs can degrade dyes such as methylene blue (MB) at a faster rate than conventional Fenton catalysts. One study reported that iron-doped CDs completely degraded MB in 120 min, compared to about 150 min for a conventional Fe²⁺/Fe³⁺ system, indicating an enhanced rate of hydroxyl radical generation [104]. Metal doping introduces additional active centres to the surface or interior of CDs to react with H₂O₂. These active centres include metal oxide sites and metal–carbon bonds, which have been shown to significantly enhance the activation efficiency of H₂O₂ during the catalytic process [105]. Specifically, metal-doped CDs have been observed to accelerate the decomposition reaction of H₂O₂, thereby generating hydroxyl radicals (-OH), which possess strong oxidising properties and are considered the primary active species for the degradation of pollutants. For example, cerium (Ce), a rare-earth metal, has been shown to effectively promote the successive decomposition of H₂O₂ through its reversible Ce³⁺/Ce⁴⁺ redox cycle, thereby enhancing the catalytic efficiency of the Fenton-like reaction [106]. Similarly, the introduction of lanthanum (La) has been shown to enhance the generation of ROS by altering the surface chemistry of the CDs. These properties make rare-earth-doped CDs particularly effective in degrading complex pollutants [107]. Moreover, multi-metal synergistic effects (e.g., Fe-Mn co-doping) have been demonstrated to enhance the reaction rate of the active centre through the interaction between the two metals. The effective redox properties of Fe in combination with the electron transfer capability of Mn can expedite the production of -OH, thereby substantially augmenting the overall catalytic efficacy [108].

Metal doping has also been demonstrated to increase the participation of electrons in the reaction process by promoting electron-hole separation. The electron distribution in CDs can be optimised, as doped metals significantly modify their electronic structure. The incorporation of new energy levels or defects by the doped metal has been shown to suppress electron-hole pair recombination. Furthermore, metal doping tends to enhance the electrical conductivity and electron transfer rate of CDs. Specifically, the metal–carbon bond and metal oxide structure provide a fast electron transfer channel, which accelerates the electron transfer efficiency from H₂O₂ to the catalytically active centre in the reaction. This fast electron transfer mechanism directly enhances the generation of ROS and exhibits efficient catalytic activity at lower H₂O₂ concentrations [109].

Although experimental results show that Fe- and Zn-doped CDs outperform other metal dopants in different dye systems, a unified theoretical framework helps explain these observations. As summarised in

Table 4. Theoretical comparison of metal dopants and their influence on catalytic performance in Fenton-like reactions.

Metal Ion	Electronic Configuration	Common Oxidation States	Redox Potential<br> (V vs. SHE)	d-Orbital Occupancy	ROS Generation Potential	Charge Transfer Efficiency	Pollutant Degradation Behaviour	References
Fe3+/Fe2+	[Ar] 3d5/3d6	+2, +3	+0.77 (Fe3+/Fe2+)	Half-filled	High (•OH via H2O2)	High (Fenton-active centre)	Fast MB degradation	[15,90]
Cu2+/Cu+	[Ar] 3d9/3d10	+1, +2	+0.17 (Cu2+/Cu+)	Nearly full	Moderate (•OH, O2•−)	High (visible-light active)	Strong for RhB, moderate for MB	[34,74]
Zn2+	[Ar] 3d10	+2	−0.76 (Zn2+/Zn)	Full (d10)	Low (non-redox active)	Indirect (bandgap tuning)	High for RhB via adsorption	[47,103]
Mg2+	[Ne]	+2	−2.37 (Mg2+/Mg)	No d-electrons	Minimal	Weak (structural tuning)	Mild, stable light-activated CD	[95,98]

, the electronic configuration and redox potential of each dopant dictate their ability to participate in reactive oxygen species (ROS) generation, charge separation, and catalytic degradation pathways.

Metal doping significantly affects the physicochemical properties (e.g., hydrophilicity, hydrophobicity, and polarity) of the CD surface. These alterations in surface properties have the capacity to further enhance the adsorption capacity of CDs for pollutant molecules. For example, the enhanced surface polarity of metal-doped CDs led to a substantial increase in the adsorption of polar pollutants, including water-soluble dye molecules [110]. Concurrently, hydrophobic alterations in surface properties can enhance the interaction of the CDs with hydrophobic pollutants (e.g., certain drug residues or organic pesticides), rendering these pollutants more susceptible to being adsorbed onto the surface of the CDs and activated. Further, the alterations in surface chemical properties triggered by metal doping have the potential to enhance the dispersibility of CDs in complex aqueous environments, ensuring their complete contact with pollutants in aqueous solutions, thereby leading to more efficient degradation [111].

5.3. Factors Affecting Metal-Doped CD Catalysed Fenton-like Reactions

5.3.1. Metal Elements and Their Valences

H₂O₂ functions as a reducing agent and an oxidising agent, allowing both low-valent and high-valent metals to effectively activate it and generate ROS. The outcome of the reaction depends on the valence state of the metal. As an oxidising agent, H₂O₂ facilitates the oxidation of a metal in a low valence state. This process entails the transformation of the metal from a low valence state to a high valence state, producing ROS capable of oxidising pollutants. As a reducing agent, H₂O₂ reduces a metal in a high valence state to a low valence state, releasing ROS. This process allows the metal to continuously activate H₂O₂ in the reaction cycle. The conversion and cycling of valence of multivalent metals has a significant impact on the generation of ROS and the degradation of organic pollutants. The valence cycle of multivalent metals directly influences the activation mechanism of H₂O₂ and the efficiency of ROS generation.

Introducing metals with multiple oxidation states into CDs can enable valence cycling. Such cyclic reactions are essential for maintaining the efficiency and longevity of catalytic processes [45]. The standard potential (E⁰) of both Fe²⁺/Fe⁰ (−0.44 V) and Fe³⁺/Fe²⁺ (+0.776 V) are much lower than that of OH/H₂O₂ (+1.44 V); however, the E⁰ of Fe³⁺/Fe²⁺ is higher than that of O₂/H₂O₂ (+0.695 V). Therefore, Fe⁰ can be converted to Fe²⁺ readily through the reaction with H₂O₂; Fe³⁺ can then be reduced by Fe⁰ to produce Fe²⁺, resulting in the production of ROS. Except for Fe⁰, the other zero-valent metals are too reducible, i.e., it is difficult to regenerate zero-valent metals by reacting high-valent metals with H₂O₂. Therefore, only multivalent metals with intermediate valence can easily achieve circulation in the process of reacting with H₂O₂ [13].

In addition to Fe²⁺ and Fe³⁺ (i.e., the Fenton pairs), Cu⁺/Cu²⁺, Mn²⁺/Mn³⁺, and Co²⁺/Co³⁺ are also capable of inducing different activation pathways. For example, during the Cu⁺/Cu²⁺ cycle, the hydroxyl radical (-OH) is generated by the reaction of Cu⁺ with H₂O₂, a strong oxidising agent capable of attacking the chemical bonds of the dye molecules non-selectively, leading to highly efficient degradation. Conversely, the superoxide radical (O₂²⁻) is formed when Cu²⁺ accepts electrons and reacts with O₂, which can further oxidise intermediates and is also a precursor of -OH and H₂O Mg²⁺-π. In contrast, H₂O₂ is not only the source of -OH but also regenerated by O₂-which is continuously recycled in the system, thus ensuring a continuous supply of ROS throughout the reaction system. It is through this synergistic mechanism that the Cu-doped CDs exhibit highly efficient and stable catalytic activity during pollutant degradation. However, the role of valence states in H₂O₂ activation may vary for different metals, leading to different types and concentrations of ROS production [105]. Also, different metal species and metal valence states may change the activation mechanism of H₂O₂, affecting the concentration of ROS, which influences the degradation efficiency of organic pollutants. Consequently, the selection of appropriate multivalent metal species in CDs and their valence control is pivotal for optimising the catalytic reaction and enhancing the degradation efficiency.

5.3.2. pH

Considering that Fenton-like reaction is a metal-catalysed oxidation process, the pH of the reaction system also plays a pivotal role. Conventional Fenton systems, which rely on the cycling of Fe²⁺ and Fe³⁺, typically operate optimally under strongly acidic conditions (approximately pH 2.5–3). Unlike traditional catalysts, metal-doped CDs exhibit remarkable versatility across different pH levels. Their distinctive structure and abundant surface chemistry—replete with functional groups such as hydroxyl, carboxyl, and amine—enable them to effectively capture both H₂O₂ and organic substrates, even in non-acidic reaction media. This broader pH tolerance is a significant advantage, enabling them to maintain high catalytic activity under conditions where conventional Fenton catalysts might fail. It was reported that the maximum concentration of ·OH can be generated under acidic conditions, with an optimal pH range of 3 to 7 [112]. At pH values below 2, the formation of complex ions may reduce the reaction rate. In addition, Fischbacher et al. demonstrated that the efficacy of H₂O₂ as an oxidising agent is markedly enhanced in acidic environments [15].

According to international standards, the pH of dye effluent discharge needs to be maintained between pH 6 and 9. Therefore, catalysts with the ability to accelerate in-situ production of ROS under near-neutral conditions are highly desired. CDs can maintain their properties under different pH conditions in aqueous media [113]. Beker et al. conducted a Taguchi approach on Fe, N-doped CDs, where the delta statistics value of pH (4–10) was the lowest, suggesting that the CDs can catalyse Fenton reactions effectively across a wide pH range [104].

The transition of the metal catalyst from the low to the high valence state is facilitated under optimal pH conditions, which serves to illustrate the delicate balance between the two opposing factors of maximising the oxidation potential and minimising the inhibitory effects of complex ion formation. It is therefore imperative to gain an understanding of these pH-dependent transformations to enhance the effectiveness of Fenton-like reactions for a range of applications, including wastewater treatment and environmental remediation.

5.3.3. Temperature

For the performance of metal-doped CDs as Fenton-like catalysts, temperature plays a key role because it impacts the excited state recombination and the quantum yield of the CDs. As suggested by Jin [50], for Cu-doped CDs, the RhB degradation ratio gradually increased as the temperature increased, indicating that the Fenton reaction was accelerated because more ROS were generated at an elevated temperature, thus leading to more effective collision between ROS and RhB. Another research group have reported that the degradation of a mix of cationic dyes strongly depends on temperature, with the optimum decolourisation observed at the highest temperature (80 °C). However, studies have also shown that an elevated temperature may not be a favourable factor for H₂O₂ decomposition. For example, the catalytic activity of CDs was tested for the oxidation of 3,3′,5,5′-tetramethylbenzidine (TMB) from 25 to 45 °C, and the optimum temperature was found at 30 °C [21]. Similarly, at room temperature, Fe-doped CDs also showed high efficiency as catalysts for H₂O₂ decomposition, as showed by Beker et al. [104]. The CDs appear to work at a wide range of temperatures, which is a highly desirable feature for environmental remediation.

5.3.4. H₂O₂ Dosage and Fe²⁺ Dosage

In the traditional Fenton reaction, H₂O₂ and Fe²⁺ are two essential chemicals. The concentration of Fe²⁺ is usually low, as it is a catalyst and is not consumed in the reaction. Excessive iron can lead to side reactions and reduce the efficiency of the process [99]. In contrast, H₂O₂ is used in excess to maximise the production of hydroxyl radicals. However, excessive H₂O₂ can lead to the formation of other species like the hydroperoxyl radical (HOO•), which may not be effective for degradation [50].

With the development of metal-doped CDs, the metal ions in CDs can trigger the valence cycling; thus, no additional Fe²⁺ are required. It was reported that using metal-doped CDs can also significantly reduce the dosage of H₂O₂. For example, Fe, N-doped CDs can act as nanocatalysts for the degradation of MB in water, and the process needs a H₂O₂ dosage 3.2 to 6.8 times lower than that required for traditional catalysts. As a result, the toxicity of the effluent was significantly reduced [104].

6. Metal-Doped CDs for Sensing

Although metal-doped CDs have been used to detect various pollutants for environmental monitoring, this review focuses on those involving Fenton-like reactions. Due to the Fenton-like catalytic activity of metal-doped CDs, they have found broad application potential in the detection of oxidative/reductive substances related to disease diagnosis and environmental monitoring (

Table 5. Summary of the metal-doped CDs used for sensing based on Fenton-like reactions and their detection performance.

Doped CDs	Precursors	Analytes	Signals	Linear Range	LOD	Ref
Zn-doped CDs	sodium citrate and zinc chloride	hydrogen peroxide	fluorescence	10–80 μM	10 nM	[83]
N, Zn-doped CDs	citric acid, tris(hydroxymethyl)aminomethane, and zinc acetate	hydrogen peroxide	fluorescence	10–70 μM	8 nM	[79]
Pt, Pd, N-doped CDs	citric acid monohydrate, diethylenetriamine, chloroplatinic acid, and palladium chloride	Fe(II)	chemiluminescence	0.002 to 8.0 µM	1.0 nM	[114]
Zn-doped CDs	sodium citrate and zinc chloride	glucose	fluorescence	5–100 μM	5 nM	[83]
Cu-doped CDs	citric acid and Cu(NO3)2·3H2O	glucose	chemiluminescence	1–48 μM	0.32 μM	[76]
Co-doped CDs	vitamin B12 and citric acid	glucose	colourimetric	0.500 to 200 μM	0.145 μM	[64]
Mn3+/Mn4+-doped CDs	O-phenylenediamine and manganese acetate	dopamine	dual fluorescence	100–275 nM and 325–525 nM	3 and 12 nM	[108]
Fe-doped CDs	ethylenediamine tetraacetic acid salts and ferric nitrate	dopamine	fluorescence	0.01–50 μM	5 nM	[115]
Zn-doped CDs	ZnCl2 and trisodium citrate dihydrate	antioxidant activity index (AAI)	fluorescence	2–10 µg/mL	0.210 µg/mL	[116]

).

6.1. Detection of H₂O₂ and Ferrous Ions

H₂O₂ can be produced by the human body, and the H₂O₂ level in the body is closely associated with some diseases, such as diabetes, Alzheimer’s, Parkinson’s, and Huntington’s diseases, and cancer [117]. Fe is the third most abundant metal in enzymes, and its biological functions can influence the activities of enzymes. Ferrous ions are also involved in oxygen transport and DNA synthesis and repair. An abnormal quantity of iron in the body can disrupt cellular metabolism and lead to various illnesses, such as tissue inflammation, anaemia, heart failure, etc. In terms of environmental relevance, elevated H₂O₂ can indicate pollution from industrial effluents, posing risks to aquatic life. Excessive Fe²⁺ may alter soil chemistry, affecting plant growth and microbial activity. Therefore, detecting H₂O₂ and ferrous ions in biological systems and the environment is of great importance.

Based on the mechanism of Fenton rections, i.e., decomposing H₂O₂ using ferrous ions to generate ·OH, metal-doped CDs with enhanced catalytic activity can be used to detect H₂O₂ and Fe(II) directly. In one study, Zn-doped CDs with a high photoluminescence quantum yield were prepared by a hydrothermal method. The quantum yield of Zn-CDs can be modulated by varying the ratio of precursors (sodium citrate and zinc chloride) and the surface oxidation in the CDs. Under optimal conditions, Zn-doped CDs demonstrated high sensitivity and response to H₂O₂ with a linear range of 10–80 μM and a limit of detection (LOD) of 10 nM, indicating their great potential as a fluorescent probe for chemical sensing [83]. Similarly, N, Zn-doped CDs synthesised using citric acid, tris(hydroxymethyl)aminomethane, and zinc acetate as precursors also demonstrated effective performance of sensing H₂O₂ (a linear range of 10–70 μM and an LOD of 8 nM) [79]. In addition, the N, Zn-CDs could inhibit the growth of Gram-negative E. coli by binding to the bacterial surface and damaging their cellular systems, and the effect depended on the concentration of the N, Zn-CDs [81].

Pt-, Pd-, N-CDs have been developed with significantly amplified chemiluminescence (CL) intensity (about 530-fold higher than the undoped CDs). The reaction between CH₃CN and H₂O₂ can generate molecules under alkaline conditions, which then transfer their energy to complete this CL reaction. The synthesised CDs functioned as a catalyst in the presence of Fe(II) by inducing Fe(II)-catalysed H₂O₂ dissociation. The CL signal of the Pt-, Pd-, N-CD-based system was greatly enhanced, enabling the development of an efficient CL assay for the determination of Fe(II) in the range of 0.002 to 8.0 µM with an LOD of 1.0 nM [114].

6.2. Detection of Glucose

As a vital molecule in chemical and biological processes, glucose plays a key role in energy production in living organisms. Developing a simple and portable device for glucose detection is essential for environmental science, life science, and clinical medicine. Metal-doped CDs have been employed in glucose detection because H₂O₂ is an important intermediate species in many biological reactions involving glucose. As reported by Xu et al. [83], Zn-doped CDs were developed for glucose sensing based on the Fenton reaction, which produce H₂O₂. Under the optimised conditions, these Zn-doped CDs exhibited high sensitivity and responsiveness to glucose across a broad concentration range (5–100 μM), with an LOD of 5 nM [83].

Similarly, Cu-doped CDs have been synthesised via a simple solid-phase synthesis strategy using citric acid and Cu(NO₃)₂·3H₂O as the carbon and metal sources, respectively. The CDs exhibited enhanced peroxidase-like activity compared to horseradish peroxidase, resulting in a significant change in their CL intensity. As a result, the sensor based on Cu-doped CDs could achieve a low LOD of 0.32 μM and recoveries of 87.2–112.2% in serum samples. The CL sensing technique was also applicable for label-free glucose detection in complex samples, showing significant potential in analytical assays for clinical diagnosis [76].

In addition, a new type of Co-doped CD nanozyme was designed using vitamin B12 and citric acid as precursors, The homogeneous incorporation of Co into the carbon cores significantly enhanced peroxidase-like activity, mimicking natural metalloenzymes. A colourimetric biosensor for glucose detection was constructed by leveraging the strong affinity of Co-doped CDs for H₂O₂. It showed a linear relationship from 0.5 to 200 μM, with an LOD of 0.145 μM, and could accurately detect glucose in human serum samples. Moreover, the Co-doped CDs could specifically catalyse Fenton-like reactions in cancer cells to generate ROS and kill the cells, demonstrating application potential in tumour treatment [64].

6.3. Detection of Dopamine (DA)

Dopamine (DA) is a neurotransmitter and plays an important role in regulating mood, sleep, memory, endocrine function, and movement [118,119]. The DA concentration in the brain of healthy people is generally in the range of 1.3 to 2.6 μmol, and abnormal DA levels often lead to various diseases [120,121]. In addition, the abundant amino and catechin functional groups on DA’s surface have been widely utilised in studies on cell interfaces, biosensing coatings, drug delivery, and antibacterial applications [122]. Therefore, it is crucial to determine the concentration of DA in-vitro.

Mn^3+/Mn⁴⁺-doped blue-green, fluorescent CDs with Fenton-like catalytic properties have been synthesised using manganese acetate as the metal precursor. The prepared CDs contained functional groups of -COOH, NH₂, and C=O on the surface and had strong fluorescence. More importantly, the introduction of Mn into CDs enabled dual-signal fluorescence detection of DA at 390 and 478 nm under optimal conditions. The increase in the 390 nm peak may indicate a change in the energy level by DA oxidation and interaction with Mn-doped CD. The 478 nm peak burst may be attributed its oxidation products. The fluorescence intensity of the fluorescent CDs at 478 nm gradually decreased, while that at 390 nm continuously increased. The detection exhibited linear ranges of 100–275 nM and 325–525 nM, with detection limits of 3 nM and 12 nM, respectively [108].

Zhuo et al. [115] used ethylenediamine tetraacetic salts and ferric nitrate as the carbon and iron sources, respectively, to synthesise Fe-doped CDs by hydrothermal carbonisation. In the presence of DA, its catechol groups coordinated with Fe ions doped in CDs, where DA was oxidised to DA-quinone by ambient O₂. As an electron acceptor, DA-quinone quenched the fluorescence of the Fe-doped CDs, and such fluorescence responses can quantify DA in the range of 0.01–50 μM with an LOD of 5 nM. The Fe-doped CD-based sensor has been successfully applied for DA detection in human urine samples [115].

6.4. Detection of Antioxidant Activity Index (AAI)

Antioxidants protect biomolecules from oxidation by limiting the initiation and/or propagation of the free radical chain reactions. Total antioxidant activity (TAA) represents the overall capacity in a sample or an organism to eliminate free radicals and is quantified using the antioxidant activity index (AAI). Sohail et al. [116] developed a bioanalytical probe based on Zn-doped CDs to determine the AAI of various samples, including ascorbic acid, human serum, topical cosmetic formulation, and tomato juice. The probe operates on the principle that free radicals quench the fluorescence of Zn-doped CDs via an inner filter effect and dynamic quenching, while antioxidants prevent this quenching by sequestering the free radicals from the reaction. This probe demonstrated excellent linearity, robustness, and resistance to interference, and its analytical performance is equivalent to that of the DPPH free radical assay [116].

7. Summary and Perspectives

Metal-doped CDs have exhibited high efficiency and stability in Fenton-like catalysis, evidenced by their capability to activate H₂O₂, generating ROS for the degradation of complex organic pollutants and the detection of H₂O₂, glucose, DA, etc., in biological and environmental systems. Hydrothermal methods are the most effective for synthesising metal-doped CDs, as they facilitate uniform doping and precise control of material properties. The catalytic activity, electronic structure, and selectivity of CDs can be significantly enhanced by doping pristine CDs with metal ions; the following elements are involved in the Fenton reaction: Fe, which is responsible for generating hydroxyl radicals (-OH); Cu, which participates in redox reactions and thereby enhances electron transfer; Zn, an element that does not participate in redox chemistry but rather modifies the electronic properties of the compound; and finally Mg, the structural stabiliser rather than the active catalyst. The introduction of metal doping enhances the CD’s fluorescence properties, extends the emission wavelength range, and increases the reaction rate. This is achieved by the introduction of active sites and the modification of electronic and structural properties.

Using metal-doped CDs for the catalysis of Fenton-like reactions is a relatively underexplored area compared to other carbon-based nanomaterials, and there are several challenges that need to be addressed to fully realise their potential. First, the effects of metal doping in CDs are highly complex, making it challenging to generalise their impact on catalytic activity. Currently, there is no clear consensus on the effects of a specific metal on the catalytic performance of CDs. The interactions between the metal and the carbon core, as well as between the metal and the CD surface, are highly sensitive to the coordination environment of the metal in the CD structure, which are determined by experimental conditions, such as temperature, pH, and the presence of other reactants. This lack of agreement highlights the need for systematic studies that can isolate the effects of individual metals under well-controlled conditions. These studies should focus on understanding how the metal ions affect key catalytic processes like electron transfer, ROS generation, and reactant activation. After understanding the fundamental mechanism, it is possible to identify the optimal metal doping for enhancing catalytic efficiency. Exploiting the synergistic effects between metals and carbon core, as well as between metals and the CD surface, will also be important for developing high-performance metal-doped CDs. Second, electro-Fenton and photo-Fenton reactions are promising processes for wastewater treatment and pollutant degradation. Metal-doped CDs can serve as efficient catalysts for these reactions by promoting the activation of H₂O₂ and the generation of •OH. Therefore, the design of efficient metal-doped CD catalysts with superior optical and electrochemical properties for electro-Fenton and photo-Fenton reactions warrant further studies. Thirdly, the stability of and recyclability of metal-doped CDs under various experimental conditions (temperature, pH, and the presence of reactive chemicals) is critical for their practical application in catalysing Fenton-like reactions. Pristine or metal-doped CDs can degrade and lose their catalytic activity over time due to surface oxidation or the leaching of metal ions. Methods to improve the chemical and thermal stability of metal-doped CDs while maintaining their catalytical activities for multiple usages require further investigation. Lastly, the future development of metal-doped CDs in advanced sensing technologies and environmental remediation also needs to ensure environmental safety and low synthesis costs. The metal-doped carbon dots explored in the article have not been documented as having serious effects, needing attention for metal leaching issues, or threatening the environment. However, it cannot be ruled out that other metals may present problems affecting sustainability, so some emerging detection techniques such as encapsulation techniques and hybrid nanocomposites should be considered.

Future work on carbon dots (CDs) should increasingly incorporate the principles of green chemistry by prioritising biomass-derived precursors, non-toxic reagents, and energy-efficient synthesis methods. Materials such as crop residues, plant waste, or fruit peels represent abundant and renewable carbon sources, and their utilisation contributes to waste valorisation within a circular economy framework. Moreover, the substitution of hazardous chemicals with eco-friendly solvents like water or ethanol, as well as the use of mild hydrothermal or microwave-assisted synthesis, can significantly reduce the environmental footprint of CD production. These strategies not only improve sustainability but also enhance the safety and biocompatibility of CDs, particularly in environmental and biomedical contexts.

Beyond catalysis, metal-doped CDs show great promise in biomedical applications. Due to their small size, water solubility, and high surface modifiability, they are being explored for drug delivery systems. Certain metal ions, such as Zn²⁺ or Fe³⁺, facilitate pH- or redox-responsive drug release, while others, like Ag⁺, Cu²⁺, or Zn²⁺, exhibit notable antimicrobial properties through ROS generation or membrane disruption, offering potential for wound healing, surface coatings, and water disinfection.

From a mechanistic perspective, the catalytic efficiency of metal-doped CDs varies according to the nature of the dopant. Fe-doped CDs are well known for their strong Fenton-like activity, effectively producing hydroxyl radicals (•OH) in the presence of H₂O₂ and achieving >90% degradation of dyes within 2–3 h. Cu-doped CDs enhance both redox cycling and visible-light responsiveness, making them highly effective under neutral pH and natural light. In contrast, Zn-doped CDs improve photocatalytic performance by promoting charge separation and enhancing dye adsorption, despite their limited redox activity. Mg-doped CDs, though less involved in ROS production, help to tune the electronic bandgap and broaden light absorption into the visible range. Noble metal-doped CDs (e.g., Ag, Pd) often act as electron sinks that accelerate dye decolourisation, though their mineralisation capacity may be limited.

These differences underscore the need for intentional dopant selection based on the desired catalytic mechanism and application context. Additionally, future studies should integrate life cycle assessment (LCA) and carbon footprint analysis to evaluate and validate the development of truly green, scalable nanocatalysts for environmental remediation and beyond.