**Preface**

Carbon quantum dots can be designated as a new allotropic form of carbon materials. Carbon materials are ever-interesting arousing materials, and it is no wonder that the works related to two of the previously known allotropic forms of carbon, namely, fullerenes and graphene, have won two Nobel prizes. In 1996, Robert F. Curl Jr., Sir Harold Kroto and Richard E. Smalley were jointly awarded the Nobel prize in Chemistry for their discovery of fullerenes. Likewise, in 2010, Andre Geim and Konstantin Novoselov were jointly awarded the Nobel prize in Physics for their groundbreaking work on the two-dimensional carbon material called graphene. However, much remains to be explored in carbon materials research. The discovery of carbon dots, the newest allotrope of carbon, is no ordinary discovery. Unlike their predecessors, carbon dots are hydrophilic, water-soluble, nano-sized and spherical. They are also highly polar and functionalized. Carbon dots exhibit peculiar light absorption and emission properties. They are susceptible to tuning in terms of size, composition, surface functionality, light absorption and emission properties. They have found applications in almost all spheres of human activity. The applications of carbon dots in the fields of catalysis, electrocatalysis, photocatalysis, photoelectrocatalysis, medicine and materials science are well known. Many new applications are being unravelled. The Editors would like to express their gratitude to the research groups that have contributed their papers that formed the 12 chapters in this edited reprint. Grateful thanks are due to Mrs. Cathy Yang, the Editorial Manager, for her steadfast support to the Editors. Finally, we bow down before our Lord and Savior Jesus Christ for His all-sufficient grace that has enabled the successful completion of this endeavor.

Dedicated to "My LORD and my God, Jesus Christ. John 20:28" "My grace is sufficient for three. 2 Corinthians 9:12"

> **Indra Neel Pulidindi, Archana Deokar, and Aharon Gedanken** *Editors*

### *Review* **Synthetic Methods and Applications of Carbon Nanodots**

**Anjali Banger 1, Sakshi Gautam 1, Sapana Jadoun 2, Nirmala Kumari Jangid 1,\*, Anamika Srivastava 1, Indra Neel Pulidindi 3, Jaya Dwivedi <sup>1</sup> and Manish Srivastava 4,\***


**Abstract:** In the recent decade, carbon dots have drawn immense attention and prompted intense investigation. The latest form of nanocarbon, the carbon nanodot, is attracting intensive research efforts, similar to its earlier analogues, namely, fullerene, carbon nanotube, and graphene. One outstanding feature that distinguishes carbon nanodots from other known forms of carbon materials is its water solubility owing to extensive surface functionalization (the presence of polar surface functional groups). These carbonaceous quantum dots, or carbon nanodots, have several advantages over traditional semiconductor-based quantum dots. They possess outstanding photoluminescence, fluorescence, biocompatibility, biosensing and bioimaging, photostability, feedstock sustainability, extensive surface functionalization and bio-conjugation, excellent colloidal stability, eco-friendly synthesis (from organic matter such as glucose, coffee, tea, and grass to biomass waste-derived sources), low toxicity, and cost-effectiveness. Recent advances in the synthesis and characterization of carbon dots have been received and new insight is provided. Presently known applications of carbon dots in the fields of bioimaging, drug delivery, sensing, and diagnosis were highlighted and future applications of these astounding materials are speculated.

**Keywords:** carbon nanodots; synthesis; applications; surface functionality; biocompatibility; low toxicity; bioimaging; applications

#### **1. Introduction**

Nanoparticles are microscopic particles with a size range of 1–100 nm. During the past decade, considerable research was conducted on the fabrication and application of nanoparticles in many fields. Based on their unique properties, nanoparticles have a substantial impact in various industries, including health, cosmetics, energy, pharmaceuticals, and food.

Enormous work was completed in recent years to design nanostructured materials with specific characteristics that will ultimately influence their function and application. In this era of carbon nanotechnology, special emphasis is laid on the organic functionality of nanomaterials or organic nanomaterials, including graphene, carbon nanotubes, and fullerenes. Because of their biocompatibility, ease of fabrication, and fascinating features, especially their water solubility fluorescence emission, carbon nanodots with a size in the range of 1–10 nm have taken the central stage of materials research. Carbon nanodots (CDs) are known to have zero dimension with almost spherical geometry. This material has become a rising star in the field of luminescent nanomaterials [1]. Due to their desirable qualities, such as hydrophilicity, ease of functionalization, outstanding biocompatibility, bright luminescence, good solubility, high chemical inertness, and low toxicity, they are potent candidates for various applications in solar cells, biosensors [2–10], bioimaging, and optoelectronic devices, etc. CNDs exhibit many remarkable properties including outstanding photoinduced electron transfer, stable chemical

**Citation:** Banger, A.; Gautam, S.; Jadoun, S.; Jangid, N.K.; Srivastava, A.; Pulidindi, I.N.; Dwivedi, J.; Srivastava, M. Synthetic Methods and Applications of Carbon Nanodots. *Catalysts* **2023**, *13*, 858. https://doi.org/10.3390/ catal13050858

Academic Editor: Francisco José Maldonado-Hódar

Received: 30 December 2022 Revised: 9 March 2023 Accepted: 6 May 2023 Published: 9 May 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

inertness, low cytotoxicity [11–15], good biocompatibility, and efficient light harvesting. Dots made of carbon, such as carbon nanodots (CDs) and graphene quantum dots (GQDs), are a brand-new carbonaceous nanomaterial with zero dimensions [16–22]. Until now, a lot of work has been carried out, and substantial advances have been made in the synthesis and uses of carbon-based dots [23–27].

This class of carbon-based nanomaterials was initially found by the top-down approach of minimizing huge carbon nanomaterials, and has recently advanced at a startling rate. Their purity standards, classification, and fluorescence mechanisms have made an impact on the research community, as evident from the rapid pace of research and publications in this area. The emphasis of the review is mainly on the synthesis and applications of carbon nanodots. A schematic of the methods of synthesis of CNDs and their structure was depicted in Figures 1 and 2, respectively. They usually have the inner hybridization of sp2 and outer hybridization of sp3, and these hybridized structures tend to have functional groups containing oxygen atoms (such as -OH, -COOH, -CO, and many more).

**Figure 1.** Methods used for the synthesis of Carbon nanodots (1–10 nm).

Due to their excellent characteristics, CNDs are prospective replacement probes for bioimaging and bioassay [28–33]. So far, a number of methods have been investigated to synthesize CNDs, including thermal oxidation, chemical oxidation, and arc discharge, laser ablation of graphite, electrochemical synthesis, microwave synthesis, and ultrasonic methods [34–40]. The use of most of these techniques are, however, constrained since they sometimes require an expensive carbon source, intricate reactions, lengthy process time, and post-treatment steps. Therefore, there is a significant demand for synthetic methods that are easy to use, sustainable, and ecologically friendly for the mass production of high-quality CNDs [41].

**Figure 2.** Commonly available carbon nanodot structures: spherical particles, nanosheets of graphene, and amorphous structures. Reproduced with the permission from Ref. [41], 2019, American Chemical Society.

There are not many reviews focused explicitly on the synthetic pathways, characteristics, and uses of the CDs, despite the fact that they have all been thoroughly summarized elsewhere [42–45]. For improved clarity and understanding, a general evaluation of the most recent developments in the synthetic methods of CNDs is presented. Moreover, the advances in the understanding of the properties of carbon dots and the resulting applications were highlighted, and a new insight is provided by correlating the synthetic strategy, property, and application [46–50]. The organization of the review comprises a discussion of the approaches for the fabrication of carbon nanodots for various sustainable resources (including biomass), with an emphasis on cost-effectiveness and eco-friendliness. Afterward, the major applications of CNDs were highlighted, with an emphasis on bioimaging and photocatalysis.

#### **2. Novel Methods for the Synthesis of Carbon Nanodots**

During the last ten years, namely 2013–2023, many methods were developed to synthesize carbon nanodots with attractive features and applications in a specific field. These well-known CD synthesis techniques are typically categorized into "top-down" and "bottom-up" categories. The top-down techniques involve the exfoliation of nanomaterials chemically, laser ablation, electrical and chemical oxidation, arc discharge, and ultrasonic synthesis. Graphene quantum dots, or two-dimensional nanomaterials, are often produced via a "top-down" technique by exfoliating and cutting the macroscale framework of carbon species, such as carbon rods, tubes, graphite powder, activated carbon, carbon black, carbon soot, and carbon fibers possessing the graphene lattices. Top-down strategies typically demand a lengthy processing time, challenging reaction environments, and expensive materials and machinery [51–53], and these methods work well for the mass production of CNDs. On the contrary, the bottom-up method is used for producing carbonized polymer dots and carbon quantum dots (3D nanoparticles with spherical centers) by polymerizing molecular precursors, including glucose, sucrose, and citric acid, using processes such as chemical vapor deposition, plasma treatment, microwave pyrolysis, and solvothermal reactions, with a high degree of controllability. Of course, these methods are not flawless altogether [54].

Hawrylak et al. [55] synthesized carbon nanodots (CNDs) for the first time by surprise in 2004 in their effort to purify single-walled carbon nanotubes. Arc-discharge soot was used in the experiment as a source of carbon nanotubes. The components of the soot suspension were separated during the procedure using gel-electrophoresis, which exposed a brand-new band of fluorescent material. Currently, CNDs can be synthesized by chemical or physical processes. Thermal therapy, electrochemistry, acidic or hydrothermal oxidation, or ultrasonic treatment are examples of chemical processes. Arc discharge, plasma therapy, and laser ablation are examples of physical approaches [56]. The processes for synthesizing CNDs can be roughly categorized as top-down or bottom-up syntheses, as mentioned earlier [57–66]. Top-down techniques often include etching, intercalation, hydrothermal

or solvothermal cutting, chemical oxidation, and laser ablation to break down a larger carbon structure into progressively smaller pieces. In bottom-up methods, CNDs are synthesized by carbonizing organic precursors. These include dehydration with sulfuric acid, microwave pyrolysis with solvent mediation, and refluxing pyrolysis. Using solvothermal or direct thermal breakdown, organic precursors such as allotropic carbon forms, natural gas, and carbohydrates are transformed into CNDs using direct thermal or solvothermal breakdown [67]. To achieve uniformity, the final product can be processed through electrophoresis, chromatography, centrifugation, dialysis, or through some other processes [68]. The carbon precursor, preparation technique, and experimental circumstances have a significant impact on the shape and structure of CNDs. For instance, depending on the parent material, CNDs made using top-down techniques have a different size and shape (coal, graphite powder, or graphene nanosheets). UV radiation or hydrothermal treatments can be used in a single preparation technique such as etching, and both would yield different outcomes. CNDs produced from the top-down approach would typically have dimensions below 10 nm, a spherical or sheet-like shape, and a size below 3 nm. Data indicate that bottom-up synthesized CNDs may be layered with a size of less than 10 nm.

Arc discharge and laser ablation are likely the most well-known top-down techniques for producing carbon-based nanomaterials. The term "arc discharge" refers to producing a current between two electrodes, often graphite rods, which causes them to vaporize. As a result, soot is formed, which may contain various nanoparticles of carbon. In contrast, the technique of laser ablation comprises applying a pulse of laser energy to a solid surface, resulting in carbon nanomaterials. Laser ablation in solution (LAS) has attracted interest as a top-down, single-step method for producing nanomaterials quickly and affordably, as shown in Figure 3 [69].

**Figure 3.** General methods of synthesis carbon nanodots (CNDs: Bottom-up approach: CNDs were synthesized from smaller carbon units (small organic molecules) by applying energy (electrochemical/chemical, thermal. laser, microwave, etc.). The source molecules will get ionized, dissociated, evaporated, or sublimated and then condensed to form CNDs; Top-down approach: CNDs are synthesized by transformation of larger carbon structures into ultra-small fragments by applying energy (thermal, mechanical, chemical, ultrasonic, etc.). Reproduced with the permission from Ref. [69], 2019, Springer Nature.

Amorphous (a) nanoparticles (a-CDs) are often produced at relatively low temperatures (<300 ◦C), whereas graphitized (g) structures are produced at higher temperatures (g-CDs). The surface functional groups of the resulting nanoparticles are significantly influenced by the precursors used. The most prevalent surface functional groups are amine and carboxylate, using precursors such as citric acid or polyamines. Such functionality can be generated via post-synthetic functionalization reactions. At the beginning of the research in this area, researchers found that proteins were the suitable precursors for the synthesis of carbon nanodots because they were readily available, affordable, and capable of undergoing dehydration and decarboxylation reactions to produce CNDs with heteroatom doping [70,71]. N-doping is accountable for better photoluminescence quantum yields, red-shifted absorption, and more favorable optoelectronic features. Citric acid in combination with amino acids (arginine, Arg) was also explored, leading to the formation of carbogenic nanoparticles' molecular precursor, which benefits from its distinct reactive behavior and capacity to serve as a "passivating agent" or "capping agent" for the outer surface [72,73].

The hydrothermal approach, aided by microwave heating, is one among numerous potential synthesis processes. It has been widely employed for manufacturing a variety of carbon materials. Hydrothermal synthesis offers a minimal toxicological impact on materials and processes [74]. The use of hydrothermal conditions causes the reagent's solubility to rise or change, enhances their chemical and physical interactions, and makes it easier for the carbonaceous structures to form. The production of nanomaterials with higher amounts of carbon, such as graphitic carbon compounds, and nanotubes at higher temperatures is a reliable process. Microwave-assisted methods have also grown in popularity as a method for synthesizing nanomaterials. Issues with the conventional heating process used for preparing nanomaterials, such as the tendency for insoluble compounds to cause heterogeneous heating, leading to an increase in the size of nanomaterials, is solved by microwave heating [75,76]. Due to its high energy consumption efficiency, MW irradiation offers a safe, inexpensive, and practical mode of heating, producing higher yields of the desired products [77]. As a result, the MW-assisted hydrothermal approach, which combines the benefits of both MW and hydrothermal processes, has become essential for the production of carbon dots.

The carbonization of small molecule precursors is achieved in the bottom-up fabrication of carbon dots. The most commonly available methods for the fabrication of carbon dots via the bottom-up method include a mixture of molecules having nitrogen atoms (such as urea) and citric acid [78–84]. The pyrolysis of these molecular precursors in an autoclave or microwave forms a black nanopowder of CDs. These CDs are easily dissolved in water and have exceptional fluorescent characteristics. These CDs are capable of emitting blue [85], green [86], and red emissions depending on their surface properties and circumstances (excitation source of radiation) [87,88], albeit a thorough purification is frequently required to separate the carbon dots [89]. When it comes to top-down methods, the starting materials include carbon structures such as amorphous activated carbon, carbon fibers, graphite, nanotubes, and fullerenes that are physically or chemically fragmented to produce very small carbon nanoparticles [90–93]. One such instance is graphitic oxidation in an extremely acidic environment [94,95], enabling the surface to be functionalized and the bulk-precursor to be broken up, resulting in the optical characteristics that are typical of CDs. Top-down synthetic methods frequently produce CDs with lower quantum yields of emission. These top-down synthetic methods of CDs are far more complex and time-consuming. However, they enable better structural control and end-product purity [96,97].

#### *2.1. Sonochemical/Ultra-Sonic Fabrication of CDs*

Sonochemistry is exploited for the synthesis of nanostructured materials. In this acoustic activation technique, severe physical and chemical conditions are generated as a result of the use of high-intensity ultrasound [98–106]. As the molecular dimensions are smaller than acoustic wavelengths, the chemical features of the resulting materials are not a result of the interaction between the ultrasonic and chemical species in the liquid state. Sonochemical fabrication is the result of the high compression heating of gas and vapor, resulting in incredibly high temperature and pressure conditions [107].

For the first time, Zhuo et al. [108] described the synthesis of graphene quantum dots using the ultra-sonic exfoliation of graphene. A simple sonochemical approach for the production of extremely photoluminescent CDs was devised by Wei et al. [109] in 2014. High-intensity ultrasound forms collapsing bubbles that serve as microreactors and offer intense, momentary conditions ideal for the pyrolysis of carbon precursors. Sono-chemically produced CDs in the presence of surface passivation agents have a high quantum yield and outstanding photostability.

The ultrasonic technique has merits of being inexpensive and easy to operate for the synthesis of carbon dots. The ultrasound method involves alternate high-pressure and low-pressure waves, which cause small bubbles in liquid to form and break. Thus, by means of powerful hydrodynamic shear forces resulting from the cavitation of tiny bubbles, macroscopic carbon materials were reduced to nanoscale CDs. Generally, the ultrasonic power, reaction time, and solvent and carbon source ratio were altered to produce CDs with various properties.

Huang et al. [110] used a direct ultrasonic exfoliation method to produce the chlorineinfused graphene quantum dots. Park et al. [111] conducted a typical experiment in which they first produced water-soluble CQDs from food waste. From ethanol and food waste mixture, approximately 120 g of carbon dots with an average diameter of 2–4 nm can be produced. The benefits of the as-prepared CDs for in vitro bioimaging include photostability, low cytotoxicity, and good PL characteristics.

Jiang et al. [112] reported a one-pot eco-friendly fabrication of silver nanoparticles supported on carbon from carbon and silver nitrate liquid solution without additional capping or reducing agents. Simply altering the molar ratio of carbon nanodots and AgNO3 would change the size of the silver nanoparticles supported on carbon. The simple method used to create amorphous carbon-supported Ag NPs was significant because it is synthesized in the absence of reducing or capping agents. Moreover, the stability and electrical and catalytic activities of the silver nanoparticles for the electrocatalytic reduction of H2O2 were also enhanced. The great sensitivity and low detection limit of the Ag/C nanocomposites make them excellent non-enzymatic H2O2 sensors. The Ag/C nanocomposites acted as a non-enzymatic H2O2 sensor due to their high sensitivity and selectivity, as shown in Scheme 1.

Manoharan et al. [113] used an easy and affordable technique to turn coconut water into vivid eco-friendly fluorescent carbon nanodots. Coconut water, as an environmentally friendly and less expensive carbon precursor, is used to fabricate finely dispersible carbon nanodots with both the amorphous and nano-crystalline carbon-phase. High-resolution transmission electron microscopy was used to demonstrate the monodispersed CNDs' spherical shape, (4 ± 1 nm) nm particle size. FT-IR measurements revealed extensive surface functionalization. Using UV-visible absorption and photoluminescence spectroscopic techniques, the eco-friendly luminescent properties of the carbon nanodots were assessed. The fluorescence quantum yield of the carbon nanodots having a core of carbon with extensive surface functionalization was found to be 60.18%. These CNDs, fabricated from tender coconut water, are more favorable than those from other resources due to their stability, high quality, fast reaction rate, and fine dispersion.

Currently, microalgae productivity has been increasing worldwide. To take advantage of the situation, Choi et al. [114] set out to show that the aqueous-type biofriendly luminescent carbon nanodots (C-paints) could be successfully applied for enhancing the growth rate of microalgae, *Haematococcus pluviali*. A straightforward procedure of ultrasonic irradiation with the passivating agent, polyethylene glycol, was used to prepare C-paints. The end product, called a C-paint, has a carbonyl-rich surface, outstanding

particle size homogeneity, high water-solubility, photo-stability, fluorescence efficacy, and biocompatibility.

**Scheme 1.** Schematic view of the formation of carbon-supported silver nanoparticles. Reproduced with permission from Ref. [112]. 2014, Elsevier.

Using polymer dots enclosed in NIR emissive hydrophobic carbon nanodots, Huang et al. [115] proposed the first endoplasmic reticulum (ER) focused nearinfrared (NIR) nanosensor for detecting Cu2+ in biosystems. With a detection limit of 13 nM, this nanosensor with stable fluorescence can be utilized to quantify Cu2+ in a linear range from 0.25 to 9.0 M. It responded quickly to Cu2+ (120 s). In addition, compared to other metal ions and amino acids, the nanosensor's fluorescence fluctuations are extremely selective to Cu2+. Moreover, the developed nanosensor showed low cytotoxicity, superior biocompatibility, and ER targeting capability [116].

#### *2.2. Hydrothermal Synthesis*

The hydrothermal method for producing CDs is inexpensive and non-toxic. This is an easy method for creating carbon quantum dots compared to other synthetic approaches. Teflon lined stainless steel autoclaves are used as reaction vessels for the aqueous solution of the CD precursor and chemical agents. The autoclave is then placed in an air oven where the contents are hydrothermally reacted at a high pressure and high temperature to form the CDs [117–123]. Mehta et al. [124] developed a plant-based (sugarcane juice) source for producing luminescent carbon quantum dots, which are soluble in an aqueous medium with a size of less than 5 nm. These CQDs were used for the sensitive and specific detection of Cu2+. In their proposal, Lu et al. [125] suggested the production of carbon quantum dots via this method from pomelo peel having the size of less than 5 nm. The fabricated carbon

quantum dots, which had remarkable yield, were used for the sensitive detection of Hg2+ at lower concentrations for the examination of water samples collected from the lake.

Li et al. [126] successfully manufactured CDs in 2018 using a one-step hydrothermal process that was ecologically friendly, easy, and affordable. Oxidation resistance, stability, excellent solubility, and high quantum yield (18.67%) were all features of the CDs. It was discovered that a charge transfer procedure could cause picric acid (PA) to quench the CDs' early fluorescence. These CDs worked well as fluorescent probes to identify PA in our study. The technique was successfully used on actual and laboratory-derived tap water samples, and it was found to have beneficial properties, such as an outstanding selective nature, excellent sensitive nature, and lower detection limit of 10 nM.

Gao et al. [127] proposed a simple and inexpensive technique in which they showed the coupling of graphene quantum dots with carbon nitride (hexagonal-structure) via freeze-drying. The result showed enhanced photocatalytic activity, improved absorption in the visible region, and the effective separation of photon-generated electron-hole pairs. We have successfully completed the simple synthesis of B/N co-doped, fluorescent surface passivated carbon nanodots with a high quantum yield at a low cost, as reported by Jahan et al. [128]. Further employing these carbon dots results in the production of supramolecular moieties, which turns off fluorescence before being turned on by SR III.

For the first time, Soni et al. [129] have shown the precise source of light absorption and its emission of carbon nanodots. They demonstrated that molecular fluorophore, which is generally found in the fabrication mixture as a by-product, is the true source of the emission in red emissive carbon nanodots.

Using p-phenylenediamine and urea, Ding et al. [130] developed multiple multicoloremitting carbon nanodots via this process. After being purified using column chromatography, the carbon nanodots were obtained without excitation, causing fluorophores to show different colors. Along with a progressive increase in the red-shifted fluorescence emission, the oxidation on the surface of the carbon nanodots also became enhanced. The band width narrows as the oxygen content of the carbon nanodots' surface increases; as a result, the higher level of surface oxidation causes the red-shifted emission.

Bakier et al. [131] proposed a new turn-off fluorescent chemical sensor for the ultrasensitive detection of aniline in the liquid phase, via the formation of colloidal carbon nanodots supported on nitrogen. They demonstrated a susceptible fluorescent aniline liquid sensor based on incredibly tiny carbon dots supported on nitrogen (Figure 4).

**Figure 4.** Fluorescent chemical sensor for ultra-sensitive detection of aniline in the liquid phase, via formation of colloidal carbon nanodots supported on nitrogen. Reproduced with permission from Ref. [131]. 2021, Elsevier.

The carbon dots supported on nitrogen were produced using folic acid that had undergone ultrasonic processing at lower temperatures. They further discovered that the sensor's operation followed the static N-CD fluorescence quenching caused by electrostatic contact with aniline. The detection limit for aniline via any method was 3.75 nM (0.332 ppb), which is the sensor's detection limit. Furthermore, real sample analysis was investigated using the N-CDs' nano-probe with real tap water, and excellent results were obtained with 99.7–101% recovery (Figure 5). Hence, this proposal could prove to be helpful in developing a simple and environmentally benign nano-sensing process with excellent sensitivity, outstanding selectivity, and good quantitative value to monitor harmful aniline against the degradation of the environment [131].

**Figure 5.** Schematic view of the experimental procedure. Reproduced with permission from Ref. [131]. 2021, Elsevier.

#### *2.3. Carbonization/Pyrolysis*

In recent years, pyrolysis has emerged as a potent technique for producing fluorescent CDs by using precursors that are microscopic carbon structures. Short reaction times, minimal costs, simple procedures, the absence of any solvent, and high quantum yields are all benefits of this technology. Under high temperatures, the following basic processes of heating, dehydrating, degrading, and carbonization are essential for converting the molecules with organic carbon into carbon quantum dots. During the pyrolysis process, strong concentrations of alkali perform the cleavage of carbon initiators into carbon nanoparticles.

Ma et al. [132] produced nitrogen-doped graphene quantum dots by directly carbonizing ethylene diamine tetra acetic acid at 260–280 ◦C, and this study also offered a growth mechanism for GQDs. It is important to note that ion doping has been found to produce a variety of CQD kinds. Li and colleagues created chlorine-doped graphene quantum dots by using HCl and fructose as precursors in a typical experiment. The average size of the quantum dots was found to be 5.4 nm. They altered the color of the emission by alternating the excitation wavelength from 300 to 600 nm, and the color changed from blue to red, respectively [133]. The fluorescent carbon quantum dots were also made by Praneerad et al. [134] by carbonizing the durian peel biomass waste. The produced CQDs were used to develop a composite-based electrode that displayed a significantly greater specific capacitance value as compared to the electrode made of pure carbon. According to 135.Zhang et al. [135], the quantum dots of carbon with increased sulphur and nitrogen contents were created by carbonizing hair fiber combined with H2SO4 through sonication.

Gunjal et al. [136] used a straightforward carbonization process to create waste tea residue carbon dots from surplus and inexpensive kitchen waste biomass, so that it is cheaper, greener, and more environmentally friendly than previous techniques. As soon as they are created, waste tea residue carbon dots exhibit excitation dependent emission and are very stable in ionic media. Furthermore, due to the oxidative nature of the ion,

it has demonstrated excellent fluorescence quenching for ClO−. The fabricated sensor has the advantage in that it is highly sensitive and selective in comparison to the other 21 common interfering ions which were tested against it. Its detection limit is comparatively lower than other biomass-made carbon dots. Its quick rate of reaction enables easy and feasible ClO− detection in real samples with excellent precision and reliability. A simple method to covalently immobilize nanoscale carbon dots upon conducting carbon surface for sensing purposes is reported by Gutiérrez-Sánchez et al. [137]. The carbon nanodots (N-CD) containing amine functionalization on the surface can be electro-grafted upon the electrodes of carbon, where they are then readily covalently immobilized. They were made using a carbonization approach with microwave aid and cost-effective, biocompatible initiators, such as D-fructose as the primary carbon source and urea as the N-donor reagent, to produce peripheral enhanced nitrogen CD. It has been determined through various methods of analysis that the synthesized nanomaterial comprises regular-sized amorphous structures that glow blue when exposed to UV light. Through the relatively stable immobilization of nitrogen carbon dots onto the electrode surfaces through electrografting, hybrid electrodes with higher relative surface areas and enhanced electron transfer capacities are generated, holding great potential for electrochemical sensing. Because of their conductive nature, electrical properties, abundant edges sites, and high catalytic activity, N-CDs that are immobilized on carbon electrodes efficiently amplify the electro-chemiluminiscence (ECL) signal from the luminophore [Ru(bpy)3] 2+ in a taurine sensor.

#### *2.4. Electrochemical Synthesis*

The one-pot electrochemical method is used to controllably synthesize fluorescent or luminescent carbon nanodots (C-dots) from small molecular alcohols as a single carbon source for the first time. By adjusting the applied potential, it is possible to control the size of the resulting C-dots, which can then be used to image cells using luminescence microscopy.

A titanium tube cathode and a pure graphite loop electrode were assembled in the center, according to Pender et al. [138]. Distilled water was used for synthesizing both the electrolyte and the cathode while the anode was isolated from them by an insulating O-ring. Luminescent blue-colored carbon dots were broadly employed in pure water thanks to the use of electronic voltage and ultrasonic control, which eliminated the need for laborious cleaning. The amount yield was 8.9%, while the size of the synthesized C-dots was 2–3 nm. The C-dots offered good fluorescent properties and thermodynamic constancy in the aqueous phase. Fluorescent CDs were made from ethanol by electrochemical carbonization, according to Miao et al. [139]. The synthesizing procedure is easy, economical, and environmentally benign. The synthesized carbon dots were amorphous, spherical, and easily dispensable in water, making them ideal for analytical uses. A strong fluorescence intensity with a QY of 10.04% was attained in the absence of a surface passivation reagent. By identifying Fe3+ induced fluorescence quenching, carbon nanodots were successfully used for the Fe3+ test.

Keerthana and Ashraf [140] highlighted the hydrothermal carbonization approach for synthesizing carbon dots from chitosan. Chitosan was totally transformed into carbon dots, according to an analysis using UV-Visible spectroscopy. With one step microwave synthesis, Arvapalli et al. [141] were able to synthesize carbon nanodots that had remarkable selectivity and sensitivity for the detection of Fe (III) ions. The synthesized carbon nanodots exhibit excellent stability, high photoluminescence, and strong water solubility. Bright blue fluorescence from carbon nanodots was successfully internalized inside endothelial cells, and when the cells were nurtured with iron, the fluorescence quenching phenomenon was seen, demonstrating the possibility of sensing iron in living cells. The transfer of charge specifically between the carbon nanodots and iron was responsible for the fluorescence quenching of the carbon nanodots, and cyclic voltammetry experiments have further confirmed this.

Tyrosinase was immobilized on carbon-based nanoparticles and cysteamine (electrically active layer) covering the gold electrode in the small gold-epinephrine biosensor reported by Baluta et al. [142]. This sensor system made use of the differential pulse and cyclic voltammetry voltammetric methods to monitor the oxidation of norepinephrinetonorepinephrine-quinone via catalysis.

#### *2.5. Microwave-Assisted Synthesis*

The electromagnetic wave known as the microwave has a vast wavelength range of 1 mm to 1 m and is frequently employed in daily life and scientific study. Microwaves can also deliver high energy to breakdown the chemical bonds in a substrate, just like lasers can. It is believed that using a microwave to create CDs is an energy-efficient method. Additionally, the reaction time may be significantly reduced. The substrate is typically pyrolyzed and the surface functionalized during microwave-aided synthesis [143,144].

The CDs are synthesized more quickly using a green, economical microwave-aided method. For the creation of CDs, microwave irradiation can deliver consistent heat. For the first time, Li et al. [145] produced green-colored luminescent graphene quantum dots via the cleavage of graphene oxide sheets chemically in the presence of acids under microwave conditions. They have an emission peak of 500 nm when excited at 260 nm and 340 nm. For the first time, electrochemiluminescence has been observed from the graphene quantum dots and is highly applicable in imaging and bio-sensing.

Liu et al. [146] produced carbon dots under microwave conditions. They used glutaraldehyde as a cross-linking agent for the fluorescent system. The luminescent emissions of the carbon dots come in a range on the basis of the amount of glutaraldehyde used. The as-prepared carbon dots showed remarkable luminescent characteristics and were less toxic, highly stable, and water soluble.

A simple microwave-assisted hydrothermal was used to produce CDs from *Mangifera indica* leaves [147]. The resulting carbon dots were employed as temperature sensors inside the cells, had good biocompatibility, and strong photostability. To create carbon dots from raw cashew gum, Pires et al. [148] devised a heating method that is microwaveassisted and has dual steps. The carbon quantum dots have an average size of nearly 9 nm. The synthesis involves two steps: the first step is the partial depolymerization, i.e., autohydrolysis of the gum and production of 5-hydroxymethyl furfural, while the second step involves poly-condensation for the production of the polyfuranic structure, accompanied by carbonization and nucleation. The generated carbon quantum dots have been used in the cell imaging of live cells because they exhibit good biocompatibility and low cytotoxicity.

Simsek et al. [149] showed that under different physical conditions, a quick and onestep green synthesis of carbon nanodots from *Nerium oleander* leaves may be achieved using a household oven and a microwave-assisted hydro-thermal synthesizer (Figure 6). The effects of the synthesizer system, the kind of extract based on the plant extraction of the plant solvents, and the synthetic conditions, including the time of reaction, temperature of reaction, surface-passivation reagent inclusion into the reaction medium, physical and chemical properties, and optical characteristics of carbon dots, were examined.

Ren et al. [150] reported the synthesis of 5.6 nm-diameter N-doped graphene quantum dots using microwave-assisted heat. The resulting N-doped graphene quantum dots were used in metal ion detection and exhibit strong and constant blue fluorescence emission with an 8% quantum yield.

Sendao et al. [151] suggested a microwave method to synthesize blue-emitting carbon quantum dots and looked into photoluminescent emission features. They discovered that the synthesis technique created green-emitting molecular fluorophore that can hide the photoluminescent emission of the carbon dots. It is important to note that in the same solution, these fluorophores and the carbon dots do not function as different species with independent emissions. Instead, their interaction results in a hybrid luminescence which is seen. This method demonstrates that the reactive nature and the characteristics in the excited-state are indistinct in comparison to their individual characteristics. The impurities of the fluorescence generated from its formation have formed a critical drawback in the investigation of the photoluminescent property of the carbon quantum dots (Tables 1 and 2).

**Figure 6.** Schematic illustration of the fabrication process. Reproduced with permission from Ref. [149]. 2019, Elsevier.


**Table 1.** Advantages and disadvantages of different synthetic methods of carbon nanodots.

**Table 2.** Methods for the conversion of biobased and chemical feedstock into functionalized carbon nanodots.


#### **3. Applications of Carbon Dots (CDs)**

There are various applications which are associated with carbon dots. CDs also show a number of biomedical applications. The application of CDs is shown in Figure 7.

**Figure 7.** Application of Carbon dots.

#### *3.1. Sensing*

One of the most common and potentially significant uses of CDs is sensing [152–154]. Due to their superior optical qualities, high fluorescence sensitivity to the surrounding environment [155,156], and ability to function as effective electron donors [157–159], CDs are frequently suggested as detectors for a variety of harmful substances, including heavy metals such as mercury [160–162], copper, and iron [163–166]. To make CDs more sensitive to one or more of these analytes, persistent work is being conducted in this direction. Only a handful of studies, however, have attempted to examine the interactions of CDs with metal ions at a more fundamental level; for example, Goncalves and colleagues demonstrated that the fluorescence emissions of both CQD solution and CQDs immobilized in sol–gel are sensitive to the presence of Hg2+ [167]. In their study, laser-ablated and NH2-PEG200 and *N*-acetyl-L-cysteine-passivated CQDs were used as fluorescent probes. It was observed that the fluorescence intensity of the CQDs is efficiently quenched by micro molar amounts of Hg2+ with a Stern–Volmer constant of 1.3 × 105 <sup>M</sup><sup>−</sup>1. Therefore, judging from the relatively large magnitude of the Stern–Volmer constant [168], the quenching provoked by Hg2+ is probably due to the static quenching arising from the formation of a stable non-fluorescent complex between CQD and Hg2+. A substantial improvement in the sensitivity down to nanomolars was later realized by replacing the laser-ablated CQDs with N-CQDs. Again, static quenching is thought to be responsible for the quenching of fluorescence, but with a much larger Stern–Volmer constant of 1.4 × 107 <sup>M</sup>−1, two orders of magnitude higher than that of the previous system [169]. It was suggested that the presence of the nitrogen element in the N-CQDs, most probably -CN groups on the N-CQD surface, is responsible for the much-improved performance of Hg2+ sensing.

#### *3.2. Bio Imaging Probes*

An intriguing application of C dots is their use as a potential agent for in vivo and in vitro bioimaging of cells and species due to their photoluminescence, which is an important property of C dots [170–172]. The bioimaging of cells and tissues is an important part of the diagnosis of many diseases, particularly cancer. Various fluorescent systems for diagnostic purposes have been reported, ranging from organic and inorganic dyes to the most recent nanoparticle-based systems.

To be considered suitable for use as an imaging probe, a bioimaging agent must have excellent biocompatibility, a tunable emission spectrum, and be free of cytotoxicity. Rapid progress in implementing a new class of nanoparticles has resulted in a material that meets these criteria and can be used for both diagnostic and therapeutic purposes. Chemical functionalization is used to successfully conjugate the required drug molecule to the fluorescent nanoprobes for these theranostic applications. Sahu et al. [173] reported the synthesis of C dots from orange juice hydrothermal treatment. This was one of the first examples of making fluorescent C dots from readily available natural resources [174,175]. The C dots were non-cytotoxic and efficiently taken up by MG-63 human osteosarcoma cells for cellular imaging.

The "central dogma" states that genetic information flows from DNA to RNA to proteins. Researchers investigated the physiological activity of RNA during cancer research by using RNA dynamics in cellular functions and the real-time monitoring of their temporospatial distribution. The experiments were carried out using fluorescent carbon dots created by the one-pot hydrothermal treatment of o-, m-, or p-phenylenediamines with triethylenetetramine by Chen et al. [176]. Because carbon has excellent biocompatibility and negligible cytotoxicity, there has been a lot of interest in using carbon nanodots as bioimaging probes instead of other types of nanoparticles. C dots are ideal candidates for theranostic applications due to their ease of synthesis, acceptable emission spectra, high photostability, and lack of cytotoxicity.

Tao et al. [177] used a mixed acid treatment to create C dots from carbon nanotubes (CNTs) and graphite. Under UV light, the C dots emit a strong yellow fluorescence with no cellular toxicity. They also demonstrated in vivo bioimaging in the near-infrared region using a rat model, and this experiment exemplified the possibilities for the development of fluorescent imaging probes in both the ultraviolet (UV) and infrared (IR) range spectra.

#### *3.3. Photodynamic Therapy*

Photodynamic therapy is a relatively new advancement in biomedical nanotechnology that uses energy transfer to destroy damaged cells and tissues. This method is useful in dealing with cancer cells because it effectively targets and destroys malignant tissue while leaving normal, healthy tissue alone. This targeted destruction in photodynamic therapy can be accomplished with fluorescent C dots that have adequate photostability [178].

Shi et al. [179] used the hydrothermal method to create N-doped C dots from rapeseed flowers and bee pollen. The authors demonstrated that C dots had no cytotoxic effect up to a limiting concentration of 0.5 mg/mL after this successful large-scale synthesis. Human colon carcinoma cells were imaged successfully in this study, and the C dots were found to have good photostability and biocompatibility.

Wang et al. [180] reported C dot synthesis from the condensation carbonization of linear polyethylenic amine (PEA) analogues and citric acid (CA) of different ratios. The authors successfully demonstrated that the extent of conjugated π-domains with CN in the carbon backbone was correlated with their photoluminescence quantum yield. The main conclusion from this study is that the emission arises not only from the sp2/sp3 carbon core and surface passivation of C nanodots, but also from the molecular fluorophores integrated into the C dot framework. This work provided an insight into the excellent biocompatibility, low cytotoxicity, and enhanced bioimaging properties of N-doped C dots, which opens the possibilities for new bioimaging applications.

Bankoti et al. [181] fabricated C dots from onion peel powder waste using the microwave method and studied cell imaging and wound healing aspects. The C dots exhibited stable fluorescence at an excitation wavelength of 450 nm and an emission wavelength of 520 nm at variable pH, along with the ability to scavenge free radicals, which can be further explored for antioxidant activity. The radical scavenging ability leads to an enhanced wound healing ability in a full-thickness wound in a rat model.

#### *3.4. Photocatalysis*

There has been significant research interest in photocatalysts over the past decade due to the scenario of environmental safety and sustainable energy. The applications of nanomaterials for the efficient fabrication of photocatalysts made the journey fast and effective.

Ming et al. [182] successfully developed C dots using a one-pot electrochemical method that only used water as the main reagent. This is an extremely promising synthetic methodology because it is a green protocol that is also cost-effective, with good photocatalytic activity of C dots for methyl orange degradation.

Song et al. [183] devised a two-step hydrothermal method for the creation of a C dot– WO2 photocatalyst. The authors used this system to photocatalytically degrade rhodamine

B. It is worth noting that the reaction rate constant reported in this study is 0.01942 min<sup>−</sup>1, which is approximately 7.7 times higher than the catalytic rate using WO2 alone.

For photocatalytic hydrogen generation, a C-dot/g-C3N4 system was used. The authors created C dots from rapeseed flower pollen and hydrothermally incorporated them into g-C3N4. Under visible light irradiation, this system was able to photocatalytically generate hydrogen via sound with an output greater than that of bulk g-C3N4.

#### *3.5. Biological Sensors and Chemical Sensors*

There is great interest in using nanoparticles as biochemical sensors because C dots have been found to be useful in sensing chemical compounds or elements. Based on the properties of C dots, particularly their fluorescence properties and surface-functionalized chemical groups, various sensors for biological and chemical applications have been developed.

Qu et al. [184] developed ratiometric fluorescent nano-sensors using C dots in a single step of microwave-assisted synthesis. This research is significant in C-dot sensor research because the developed nanosensors are multi-sensory and can detect temperature, pH, and metal ions such as Fe (III). Because it can detect and estimate multiple metabolic parameters at the same time, this exciting feature is proving to be widely applicable in the biological environment. The sensory mechanism is non-cytotoxic and based on ratiometric fluorescence, which is a promising feature for future research.

Vedamalai et al. [185] developed C dots that are highly sensitive to copper (II) ions in cancer cells. They used a relatively simple hydrothermal synthesis method based on ortho-phenylenediamine (OPD). The orange color was caused by the formation of the Cu(OPD)2 complex on the surface of the C dots. Further investigation revealed that the C dots were highly water dispersible, photostable, chemically stable, and biocompatible.

Shi et al. [186] used C dots to detect Cu(II) ions in living cells as well. The hydrothermal pyrolysis of leeks resulted in blue and green fluorescent C dots. In a single step of hydrothermal carbonization, the C dots were modified with boronic acid using phenylboronic acid as the precursor. This C-dot-based sensor successfully detected blood sugar levels and demonstrated good selectivity with minimal chemical interference from other species [187].

Nie et al. [188] used a novel bottom-up method to develop a pH sensor out of C dots. This method yielded C dots with high crystallinity and stability. The procedure involved a one-pot synthesis with high reproducibility using chloroform and diethylamine. The authors were able to use the technique for cancer diagnosis after successfully implementing the pH detection of two C dots with different emission wavelengths.

Wang et al. [189] described an intriguing C-dot sensor for hemoglobin detection (Hb). The C dots were developed from glycine using an electrochemical method that included multiple steps, such as electro-oxidation, electro-polymerization, carbonization, and passivation. The authors successfully validated the sensitivity of Hb detection and discovered that the luminescence intensity varied inversely with Hb concentration in the 0.05–250 nM range.

#### *3.6. Drug Delivery*

Carbon dots' excellent biocompatibility and clearance from the body meet the requirements for in vivo applications. Carbon dots with rich and tunable function groups, such as amino, carboxyl, or hydroxyl, can carry therapeutic agents, resulting in theranosticnanomedicines [190–195]. The bright emission of carbon dots allows for the dynamic and real-time monitoring of drug distribution and response. Zheng et al. [196] used carbon dots synthesized through the thermal pyrolysis of citric acid and polyene polyamine to transport oxaliplatin, a platinum-based drug, because platinum-based drugs are the most effective anticancer drugs and are used in more than 50% of clinical cancer patients' chemotherapeutic treatments.

#### *3.7. Micro-Fluidic Marker*

The study of fluidic physics at the micro-scale is now best conducted using microfluidic systems. Because of their considerably higher surface-to-volume ratio, surface tension and viscosity dominate those of inertia, making the fluid easier to control. Static laminar flows and dynamic droplet formation are typical microfluidic situations. Both exhibit many advantages, including minimal reagent use, high sensitivity, and high output, which leads to a wide range of applications in bioassays, chemical reactions, drug delivery, etc. The majority of applications rely on the microfluidic circuit's ability to visualize fluid flow. However, the biocompatibility and cheap cost of the fluorescent materials currently in use cannot be balanced, which is a critical issue for microfluidic applications, particularly for bio-applications. Sun's colleagues used carbon dots, synthesized by heating glucose and urea in a microwave, to visualize microfluid flows for the first time to address this problem [197–199]. The scientists used carbon dots dissolved in the deionized water as a fluorescent marker to investigate the dynamics of the mixture of glycerol and deionized water. When the interface is ruptured by an electric field above a threshold, fast mixing occurs at the microscale. In addition to laminar flow, the authors also synthesized monodispersed droplets in a flow focusing system, where the continuous phase was mineral oil while the aqueous solution of carbon dots appeared as the dispersed phase. The diameter of the droplets will shrink because a higher capillary number results in a greater interfacial shear force. Additionally, the authors successfully demonstrated the multiple component droplet, merged droplet, and double emulsion, each of which has a distinct core-shell structure. To more accurately determine the speed of the flow field, luminescent seeding carbon dots were made via a mixture of carbon dots (liquid state) and polystyrene microparticles [200–202].

#### *3.8. Bioimaging*

Carbon dots have significant advantages over fluorescent organic dyes and genetically engineered fluorescent proteins, such as high PL quantum yield, photostability, and resistance to metabolic degradation, which endows them with enormous potential for use in bioapplications. While the toxicity testing of carbon dots is required before exploring their bioapplications, Yang et al. [203] used human breast cancer MCF-7 cells and human colorectal adenocarcinoma HT-29 cells (previously reported by other scientists, Yang modified and used it) to assess the in vitro toxicity of carbon dots synthesized by the laser ablation of graphite powder and cement with PEG1500N [204–206] as a surface passivation agent. All the observations of cell proliferation, mortality, and viability from both cell lines indicated that the carbon dots exhibited superior biocompatibility, even at concentrations as high as 50 mg/mL, which is much higher than the practical application demand, for example, in living cell imaging.

#### *3.9. Carbon Dots Chiral Photonics*

Chirality is essential in a number of practical application fields, such as chiral drug recognition, chiral molecular biology, and chiral chemistry [207–209]. As a result, as previously proposed by M. Va'zquez-Nakagawa et al. [210], chirality and carbon dots can be combined to form intriguing chiral optics based on carbon dots. The carbon dots used in their groundbreaking research were created by chemically exfoliating graphite with strong sulfuric and nitric acids. The carbon dots' surface carboxylic acid groups were subsequently converted to acid chlorides using thionyl chloride. When the acid chlorides and the (R) or (S)-2-phenyl-1-propanol reacted simultaneously, enantiomerically pure esters and chiral carbon dots were created (chiral molecular). Enantiomerically esters and chiral carbon dots were formed, and their formation was verified using 13C-NMR and FTIR spectroscopy. The presence of phenyl substituents was suggested by the appearance of peaks in the 13C–NMR. The recent work in this field is the most notable development in the chiral regulation of bioreactions for chiral carbon dots. Xin et al. [211] described the destruction of the cell walls of gram-positive and gram-negative bacteria via carbon dots in the presence of D-glutamic

acid, which resulted in the fatality of bacteria. In contrast, the carbon dots formed in the presence of L-glutamic acid demonstrated an insignificant effect on bacterial cells. This implied that antimicrobial nanoagents with chirality can be synthesized from carbon dots. The D-form and L-form of cysteine-based carbon dots were used to regulate the chirality of the enzyme. For instance, L-form cysteine carbon dots reduce the enzymatic activity while D-form cysteine carbon dots enhance the enzymatic activity of the enzyme. According to Li et al. [212], these cysteine-based carbon nanodots have the capacity to affect cellular energy metabolism. We anticipate that other chiral carbon dots-based applications will be investigated in the future [213], and that carbon dots with chirality will emerge as a novel but exciting topic because of their wide applications.

#### **4. Conclusions**

Carbon dots have drawn rigorous attention since they possess outstanding photoluminescence, fluorescence, biocompatibility, sensing and imaging, photostability, excellent colloidal stability, eco-friendly synthesis, low toxicity, and are cost-effective. In this review, widespread synthesis procedures have been discussed in detail, including bottom-up and top-down methods, along with biological and eco-friendly synthetic ways. This concludes numerous synthesizing routes that could be helpful to many scientific and research areas, since carbon nanodots can be easily synthesized for various applications. Earlier, the synthetic methods were limited because of unreliable quantum yields. However, in recent years, the synthesizing methods have seen a remarkable lift in yield, hence enhancing their use in different fields for varied applications. Despite many advancements in the field of carbon nanodots, there is still room for improvement in its synthetic methods. Several bio-related fields are left undiscovered and need special attention.

**Author Contributions:** Writing—original draft, A.B.; review and editing, S.G.; Funding acquisition, S.J.; investigation, formal analysis, data curation, N.K.J.: conceptualization, methodology, A.S.; Chemistry and English language editing, I.N.P.; methodology, data curation, J.D.; Supervision, M.S. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work is financially supported by the Department of Science & Technology—Fund for Improvement of S&T Infrastructure in Universities and Higher Educational Institutions (DST—FIST), India (Order No. SR/FST/CS-II/2022/252).

**Data Availability Statement:** Data is available upon request.

**Acknowledgments:** The authors are thankful to the Department of Chemistry, Banasthali Vidyapith for providing the necessary infrastructure. We are thankful to publishing houses, namely ACS and Elsevier, Springer Nature for providing copyright permissions for the figures used in this review article.

**Conflicts of Interest:** This research received no external funding.

#### **References**


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## *Review* **Carbon Quantum Dots: Synthesis, Structure, Properties, and Catalytic Applications for Organic Synthesis**

**Pradeep Kumar Yadav 1, Subhash Chandra 2, Vivek Kumar 3, Deepak Kumar <sup>4</sup> and Syed Hadi Hasan 3,\***


**Abstract:** Carbon quantum dots (CQDs), also known as carbon dots (CDs), are novel zero-dimensional fluorescent carbon-based nanomaterials. CQDs have attracted enormous attention around the world because of their excellent optical properties as well as water solubility, biocompatibility, low toxicity, eco-friendliness, and simple synthesis routes. CQDs have numerous applications in bioimaging, biosensing, chemical sensing, nanomedicine, solar cells, drug delivery, and light-emitting diodes. In this review paper, the structure of CQDs, their physical and chemical properties, their synthesis approach, and their application as a catalyst in the synthesis of multisubstituted 4H pyran, in azidealkyne cycloadditions, in the degradation of levofloxacin, in the selective oxidation of alcohols to aldehydes, in the removal of Rhodamine B, as H-bond catalysis in Aldol condensations, in cyclohexane oxidation, in intrinsic peroxidase-mimetic enzyme activity, in the selective oxidation of amines and alcohols, and in the ring opening of epoxides are discussed. Finally, we also discuss the future challenges in this research field. We hope this review paper will open a new channel for the application of CQDs as a catalyst in organic synthesis.

**Keywords:** carbon quantum dots; synthetic methods; fluorescence; optical properties; catalyst

#### **1. Introduction**

Recently, carbon-based nanomaterials such as graphene [1], fullerenes [2], nanodiamonds [3], carbon nanotubes (CNTs) [4], and carbon quantum dots (CQDs) have attracted great attention because of their distinctive structural dimensions, as well as their outstanding chemical and physical properties [5]. It was found that the preparation and separation of nanodiamonds are complicated, while other nanomaterials such as graphene, fullerenes, and CNTs do not display good water solubility and also do not exhibit strong fluorescence in the visible region. These limitations prevent their applications in different areas [6]. Although semiconductor quantum dots (SQDs) exhibit good fluorescence properties, because of the presence of heavy metals, they are toxic in nature. This prevents their biological applicationin biosensors, bio-imaging, and drug delivery. In contrast, fluorescent CQDs are nontoxic and, thus, have attracted enormous interest over other carbon-based nanomaterials [7]. Xu et al. in 2004 accidentally discovered CQDs using gel electrophoresis during the purification of single-walled carbon nanotubes [8]. However, the name CQDs was given by Sun et al. in 2006 during the synthesis of carbon nanomaterials of different sizes [9]. Subsequently, CQDs became rising stars among various carbonbased nanoparticles and are considered an extremely precious asset of nanotechnology. CQDs are also known as carbon nano-lights because of their strong luminescence properties [10]. CQDs have attractive features such as ease of synthesis, good water solubility, high photostability, high photoresponse, low cytotoxicity, facile surface functionalization,

**Citation:** Yadav, P.K.; Chandra, S.; Kumar, V.; Kumar, D.; Hasan, S.H. Carbon Quantum Dots: Synthesis, Structure, Properties, and Catalytic Applications for Organic Synthesis. *Catalysts* **2023**, *13*, 422. https:// doi.org/10.3390/catal13020422

Academic Editors: Indra Neel Pulidindi, Archana Deokar and Aharon Gedanken

Received: 31 December 2022 Revised: 10 February 2023 Accepted: 14 February 2023 Published: 16 February 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

good catalysis properties, and tunable excitation–emission [11–17]. Due to these characteristic properties, CQDs are widely utilized in photovoltaic devices, medical diagnosis, sensing, drug delivery, catalysts, photocatalysis, optronic devices, bio-imaging, laser, single electron transistors, solar cells, and LEDs [18–29]. However, very few reports have been investigated regarding the application of CQDs as a catalyst in photochemical water splitting [30] the preparation of substituted 4H pyran with indole moieties [31], azidealkyne cycloadditions [32], the degradation of levofloxacin [33], the selective oxidation of alcohols to aldehydes [34], the removal of Rhodamine B [35], the selective oxidation of amines and imine [36], high-efficiency cyclohexane oxidation [37], H-bond catalysis in Aldol condensations [38], intrinsic peroxidase-mimetic enzyme activity [39], and the ring opening of epoxides [40]. In this review paper, we explain the synthetic approach, structure, optical properties, and applications of CQDs as a catalyst. Finally, we also discuss their future prospects.

#### **2. Synthesis Approach**

Since the discovery of carbon quantum dots (CQDs), several convenient, cost-effective, size-controlled, and large-scale production approaches have been developed. For the synthesis of CQDs, two general categories, top-down and bottom-up, approaches are utilized (Figure 1). Even though CQDs synthesis is facile, there are definite challenges related to their synthesis, such as an aggregation of nanomaterials, the tuning of surface properties, and controlling the size and uniformity [41]. To adjust the functional groups present on the surface and achieve better CQDs performance, post-treatment can be conducted in both approaches. Quantum yields (QYs) of CQDs can be enhanced after surface passivation, which eliminates the emissive traps from the surface. CQDs doped with heteroatoms (N and P) or metals such as Au or Mg improve solubility and electrical conductivity [42]. Even though for the synthesis of CQDs, both the top-down and bottom-up approaches have been used, the environmentally and cost-effective bottom-up approach is most commonly used [43].

**Figure 1.** The typical approaches for the synthesis of CQDs.

#### *2.1. Top-Down Approach*

In a top-down approach, the larger carbon resources such as carbon nanotubes, fullerene, graphite, graphene, carbon soot, activated carbon, etc., are broken down into smaller constituents with the help of different techniques such as laser ablation and electrochemical and arch discharge [28,44–47]. Carbon structures with sp2 hybridization that lack efficient energy gaps or band gaps are commonly used as starting materials for topdown processes. Although the top-down approach is extremely helpful and suitable for microsystem industries, it has some limitations, such as the fact that pure nanomaterials cannot be obtained from the large carbon precursor; their purification is costly and also unable to accurately control the morphology and size distribution of CQDs [48].

#### 2.1.1. Laser Ablation Method

Sun and co-workers in 2006 first reported a laser ablation technique. In this technique, the CQDs are synthesized by irradiating a target surface with a high-energy laser pulse [9]. Recently, Li et al. synthesized ultra-small CQDs with uniform sizes by using the laser ablation method. They utilized fluorescent CQDs for cell imaging applications [49]. Cui and co-workers have also synthesized homogeneous CQDs by an ultrafast, highly efficient dual-beam pulsed laser ablation method for bio-imaging applications along with high QYs [50]. Buendia and co-workers also used laser ablation techniques to synthesize fluorescent CQDs for cell labeling [51]. The CQDs synthesized by this technique are usually non-fluorescent in nature, have heterogeneity in size, and have low quantum yield, which influences different potential applications of CQDs. Therefore, to increase the fluorescence properties and quantum yield, pre-treatments such as surface passivation (doping) and oxidation are required.

#### 2.1.2. Electrochemical Method

The electrochemical method was first described by Zhou and coworkers in 2007. They used tetra-butyl ammonium perchlorate solution as the electrolyte to fabricate the first blue luminescent CQDs from multiwall carbon nanotubes (CNTs) [52]. In this method, larger carbon precursors are cut down into smaller parts by electrochemical oxidation in the presence of a reference electrode. Zhao et al. prepared fluorescent carbon nanomaterial by electrochemical oxidation with the help of a graphite rod as a working electrode [53]. Subsequently, Zheng and colleagues developed water soluble CQDs with tunable luminescence using graphite as an electrode material and buffering the pH with phosphate [54]. Using the oxidation method, Deng and coworkers synthesized the CQDs from low-molecularweight alcohol. According to them, the most straightforward and convenient way to create CQDs is to conduct it under ambient pressure and temperature [55]. Hou and colleagues manufactured bright blue emitting CQDs in 2015 by treating urea and sodium citrate electrochemically in de-ionized water [56]. The electrochemical method has a few benefits; for example, it requires no surface passivation, is low cost, and has a simple purification process [42]. However, the limitation of this method is that for the synthesis of CQDs, it allows only a few little molecular precursors and has a tedious purification process. Therefore, it is the least frequently used technique [41].

#### 2.1.3. Arch Discharge Method

Fluorescent carbon quantum dots were first discovered by Xu and coworkers accidentally during the separation and purification of a single-wall carbon nanotube by the arch discharge method. In this process, nitric acid was used as an oxidizing agent to oxidize arch ash, which formed the different functional groups on the surface, due to which aqueous solubility increased. The QYs obtained were 1.66% at a 366 nm excitation wavelength [8]. An additional experiment demonstrated that the surface of CQDs was attached to hydrophilic carboxyl groups. In the discharge process, carbon particles of different sizes are produced. CQDs obtained using this method are highly water soluble, having a wide distribution of particle sizes. Furthermore, an electronic flash method was used to separate fluorescent

nanomaterials from neat carbon nanostructures and carbon nanostructures oxidized with nitric acid [57,58]. Zhang et al. synthesized CQDs with up-conversion fluorescence using arc-synthesized carbon by-products, and Hamid Delavariet al. synthesized CQDs by arc discharge in water [59,60]. However, CQDs synthesized by this technique have some impurities that are difficult to eliminate because of their complex composition [28].

#### *2.2. Bottom-Up Approach*

In a bottom-up approach, the smaller carbon resources such as amino acids, polymers, carbohydrates, and waste materials combine to form CQDs by a variety of techniques such as hydrothermal/solvothermal, combustion, pyrolysis, and microwave irradiation. In this method, the size and structure of CQDs depend on a variety of factors such as solvent, precursor molecular structures, and conditions of the reaction (temperature, pressure, reaction time, etc.). The conditions of the reaction are necessary, since they influence the reactants and the extremely casual nucleation and escalation procedure of CQDs. This approach strengthens the material chemistry because of its ease of operation, lower cost, and easier implementation for production in a large scale [61].

The precursor used for the synthesis of CQDs may be both chemical and biological, i.e., natural. The chemical precursors include glucose, sucrose, citric acid, lactic acid, ascorbic acid, glycerol, ethylene glycol, etc. [62–68]. The natural sources include Artocarpous lakoocha seeds, rice husks, Azadirachta indica leaves, pomelo peel, the latex of Ficus benghalensis, aloe vera, etc. (Figure 2) [69–72].

**Figure 2.** Chemical and biological precursors utilized for the synthesis of CQDs [61–72].

#### 2.2.1. Hydrothermal Method

The hydrothermal method was first reported by Zhang et al., for the synthesis of CQDs from the precursor L-ascorbic acid (carbon source) without any chemical action or other surface passivation. The average size of the synthesized CQDs was ~2 nm, and the QY obtained was 6.79%. They utilized four different solvents (water, ethyl acetate, acetone, and ethanol) for the synthesis of bright blue emission CQDs and observed that the water soluble CQDs were very stable at room temperature over 6 months. Additionally, the fluorescence intensity of CQDs was stable in a wide pH range and highly ionic salt conditions (2 M NaCl) [73]. In the hydrothermal process, the precursor molecules are dissolved in water, set aside in a Teflon-lined stainless steel autoclave, and placed in the hydrothermal chamber at high temperature and pressure for a few hours [66].The precursor

molecules utilized for the synthesis include proteins, polymers, amino acids, polyols, glucose, some wastes, and natural products [13,74]. In recent years, the hydrothermal method has attracted great attention around the world because of its single step, ease of operation, nontoxicity, low cost, and ecofriendliness. CQDs prepared from the hydrothermal treatment have a range of beneficial properties, such as being highly homogeneous, watersoluble, monodispersed, and photostable, having salt tolerance and a controlled particle size, and exhibiting an elevated QY with no surface passivation. Similar to the hydrothermal method, for the synthesis of CQDs, a solvothermal method is also utilized using ammonia, alcohol, and other organic and inorganic solvents as a substitute for water [63,75–77].

#### 2.2.2. Combustion Method

In 2007, Liu et al. first reported the combustion method to synthesize CQDs. This method involves oxidative acid treatments which aggregate smaller carbon resources into CQDs, enhance the aqueous solubility, and control the fluorescence properties. Liu and coworkers explained that candle ashes were obtained by partial combustion of a candle with aluminum foil and refluxing it in nitric acid solution. When the candle ashes were dissolved in a neutral medium followed by centrifugation and a dialysis method, the pure CQDs were obtained [78]. The CQDs synthesized by the combustion method had low QY but displayed good fluorescence without doping [70].

#### 2.2.3. Pyrolysis Method

The pyrolysis method is the thermal decomposition of the precursor at an elevated temperature (typically over 430 ◦C) and under pressure in the absence of oxygen. Additionally, the carbon precursor cleavages into nanoscale colloidal particles in the presence of an alkali and strong acid concentration as a catalyst. The advantageous properties of this method include practicability, repeatability, and simplicity, as well as having a high QY. However, it is challenging to separate small precursors from raw materials.

In 2009, Liu et al. first described a novel method for the preparation of CQDs through pyrolysis using resol (as a carbon source) and surfactant-modified silica spheres. The synthesized CQDs exhibited blue fluorescence and were amorphous, with sizes ranging from 1.5 to 2.5 nm, and the QY obtained was 14.7%. Moreover, the CQDs were stable in a broad pH range (pH 5–9) [79]. After that, several investigations were carried out for the preparation of CQDs using the pyrolysis method. Pan et al., in 2010, synthesized extremely blue fluorescent CQDs from ethylenediamine-tetraacetic acid (EDTA) salts using the pyrolysis method. The average size of the synthesized CQDs was 6 nm. The quantum yield (QY) obtained was 40.6% [80]. With the help of the pyrolysis of citric acid at 180 ◦C, Martindale and coworkers (in 2015) synthesized fluorescent CQDs with an average size of 6 nm, and at the excitation of 360 nm, the calculated QY was 2.3% [81]. Rong and coworkers in 2017 also prepared fluorescent N-CQDs by the pyrolysis of citric acid and guanidinium chloride without organic solvent, acid, alkali, or further modification and passivation, resulting in N-CQDs with a size of 2.2 nm and a QY of 19.2%. They utilized N-CQDs intensively in the detection of metal-ion (Fe3+) and in bio-imaging [82]. Lately, several CQDs were synthesized using the pyrolysis method and utilized in different fields [41,83,84].

#### 2.2.4. Microwave Irradiation Method

Microwave synthesis is a faster and cost-effective method for the synthesis of CQDs via microwave heating. Compared to other techniques, this is a simple and convenient method because it requires less time for the synthesis of CQDs, with an improved quantum yield. Zhu et al. first synthesized fluorescent CQDs under the microwave (500W) by heating poly (ethylene glycol) (PEG-200) and saccharide for 2–10 min [48]. This method is rapid, novel, green, and energy efficient in synthesizing CQDs. However, there are some limitations, such as difficulty in the separation procedure and purification, and that non-uniform particle sizes of CQDs restrict their prospective applications [85,86]. Recently, various investigations were carried out for the preparation of CQDs using microwave irradiation, utilizing them for different applications [87–91].

#### 2.2.5. Template Method

Bourlinos and coworkers first synthesized fluorescent CQDs using the template method [92]. The template method involves two steps: (i) The preparation of CQDs in the appropriate template or silicon sphere by calcinations. (ii) The etching process occurs to eliminate the supporting materials. Some advantageous properties of the template method are that it is straightforward, the equipment is easily obtainable, it is suitable for the surface passivation of CQDs, it prevents the particles from agglomerating, and it controls the size of CQDs. The disadvantageous property of the template method is the difficulty in the separation of the CQDs from the template, which may affect the purity, particle size, fluorescence property, and QY.

#### **3. Structure of CQDs**

Tang et al. reported that CQDs have core–shell structures which are either amorphous (mixed sp2/sp3) or graphitic crystalline (sp2), depending upon the extent of the occurrence of sp2 carbon in the core [93]. Graphitic crystalline (sp2) cores were reported by several researchers [94–96]. The size of cores is very small (2–3 nm), with a characteristic lattice spacing of ~0.2 nm [97]. The cores are categorized depending on the technique utilized for the synthesis and the precursors used, as well as other synthetic parameters (such as duration, temperature, pH, etc.) [98]. Generally, the graphitization (sp2) structure is obtained at over 300 ◦C reaction temperatures, while amorphous cores are obtained at lower temperatures, unless sp2/sp3-hybridized C is present in the precursor [99]. To determine the core structure of CQDs, various instrumental techniques such as Transmission Electron Microscopy (TEM) or High Resolution (HR) TEM, Scanning Electron Microscopy (SEM), Raman spectroscopy, and X-ray diffraction (XRD) are utilized. To measure the size and morphology of the CQDs, TEM or SEM are carried out [100]. The selected area electron diffraction (SAED) patterns reveal the amorphous or crystalline nature of CQDs [101]. The XRD pattern also determines the crystal structure of CQDs. The broad peak at 2θ 23◦ indicates the amorphous nature of CQD, while the occurrence of two broad peaks at 2θ 25◦ and 44◦ specifies a low-graphitic carbon structure analogous to (002) and (100) diffraction [102]. The general structure and presence of different functional groups on the surface of CQDs are determined using Fourier transform infrared (FT-IR) spectroscopy, X-ray photoelectron spectroscopy (XPS), elemental analysis (EA), and nuclear magnetic resonance (NMR) [103,104]. Using nitrogen sorption analysis, the surface area of the carbon nanoparticles is calculated [103]. To decide the optical properties and qualitative information regarding the presence of C=C and C=O in CQDs, UV-Vis absorption spectroscopy is carried out [105]. To determine the positive or negative charge on the surface of CQDs and the extent of the electrostatic interaction between them, zeta potential is conceded [106,107].

Figure 3 is the typical structure of carbon quantum dots (CQDs), which reveals the presence of different functional groups (such as carbonyl, carboxyl, hydroxyl, amino, etc.) on the surface of CQDs. The presence of these functional groups was confirmed by instrumental techniques such as FTIR and XPS [108].

**Figure 3.** Typical structure of CQDs with different functional groups on the surface.

#### **4. Optical Properties of Carbon Quantum Dots (CQDs)**

#### *4.1. Absorbance*

CQDs generally exhibit two absorption bands in the visible region around 280 nm and 350 nm, alongside a tail broadly in the UV region. Hu et al. reported that an absorption band at 280 nm is due to a pi-pi\* (π-π\*) transition of a C=C bond, and the one at 350 nm is due to an n-π\* transition of the C=O bond [109]. Figure 4 is the typical UV-visible absorption spectrum of fluorescent CQDs. The absorption properties of CQDs can be influenced by surface modification or surface passivation [110–113]. Depending on the raw precursor and synthesis methodology, the positions of these absorption bands are different to some extent. Doping in CQDs can also alter the absorption wavelength.

**Figure 4.** The UV-visible absorption spectrum of fluorescent CQDs.

The optical properties of CQDs can be customized by doping/co-doping with heteroatoms, functional groups, and surface passivation [114]. In the process of surface passivation, a slim insulating (protecting) layer of covering materials such as thiols, thionyl chloride,

spiropyrans, and oligomers (polyethylene glycol (PEG), etc.) is formed on the CQDs surface. The important functions of such types of protective layers are to shield CQDs from the adhesion of impurities and to provide stability [115]. CQDs with surface-passivating agents become extremely optically active, demonstrating considerable fluorescence from the visible to the near-IR region [116]. The quantum yields (QYs) of CQDs can also be enhanced up to 55–60% by surface passivation [114]. The absorbance of CQDs improved to longer wavelengths (350–550 nm) after surface passivation with 4,7,10-trioxa-1,13-tridecanediamine (TTDDA) [117]. Particle size is associated with the absorption wavelength. As the size of the CQDs increases, absorption wavelength also increases [118,119]. The CQDs are viable for covalent bonding with functionalizing agents [114]. Different functional groups such as amines, carboxyl, hydroxyl, carbonyl, etc., were introduced on the surface of CQDs by surface functionalization. The functionalized CQDs revealed good biocompatibility, high stability, outstanding photoreversibility, and low toxicity compared to undoped CQDs. The efficient technique to modify the CQDs absorption spectrum is doping/co-doping with heteroatoms (such as boron (B), nitrogen (N), fluorine (F), phosphorous (P), and sulfur (S)). The dopant adjusts the bandgap, electronic structure, and, consequently, the optical properties of CQDs by altering the π-π\* energy level (related through the core-sp2 carbon system) [120]. On increasing N-dopant concentration, a gradual increase in the band gap of the CQDs from 2.2 to 2.7 eV was observed [121]. In contrast, it was also found that the doping of N in CQDs results in a reduction in size [122]. The CQDs established innovative electronic states, resulting in a reduction in the bandgap of CQDs (about ~48–57%) [123]. Zuo et al. synthesized F-doped CQDs using a hydrothermal method which exhibited higher QYs and enhanced the electron transfer and acted as a superior photocatalyst [124].

#### *4.2. Photoluminescence*

The emission of light from a substance upon the absorption of light (photon) is called photoluminescence (PL). Photoluminescence includes two types, namely fluorescence and phosphorescence. Fluorescent materials emit absorbed light from the lowest singlet excited state (S1) to the singlet ground state (S0). This process is very fast and has a nanosecond lifetime. The transitions that occur among two electronic states in the fluorescence process are allowed because it has the same spin multiplicity. In contrast, in phosphorescence, the transition occurs from the lowest triplet excited state (T1) to singlet ground state (S0), i.e., a forbidden transition occurs according to the spin selection rule.

#### 4.2.1. Fluorescence

The fluorescence properties of CQDs have attracted great attention among researchers because of their several sensing and analytical applications. Numerous mechanisms have been reported to gain deep insight into the cause of fluorescence in CQDs [125–130]. Among them, the following two have been found more prominent. The first is that the fluorescence mechanism is due to band gaps' transitions arising from the π-conjugated domains (sp2 hybridized), which is similar to aromatic molecules employing definite energy band gaps in favor of absorptions and emissions [131]. The second cause of fluorescence is related to the surface defects, quantum size effect, carbon core state, surface passivation/functionalization effect, and different emissive traps on the surface of CQDs [132–134].

The main reason for the surface defects in CQDs is an unsymmetrical allocation of sp2- and sp3-hybridized carbon atoms, and the existence of heteroatoms such as B, N, P, and S [126,135]. When this surface defect is independently incorporated into the solid host, it creates surroundings similar to aromatic molecules. These molecules can attract UV light and display various color emissions [131,136]. CQDs show two types of emission, i.e., excitation-dependent emission (tunable emission) and excitation-independent emission. The tunable emission is due to the presence of various emission sites on the surface of CQDs along with particle size distribution; because of this, most CQDs exhibit tunable emissions [137]. The excitation-independent emission is due to the extremely ordered graphitic structure of CQDs [118]. CQDs exhibit extensive and unremitting excitation

spectra which are highly photostable and have steady fluorescence, in contrast to traditional organic dye [95,138,139].

#### 4.2.2. Phosphorescence

In CQDs, the phosphorescence property is also observed, which was first described by De et al. via dispersing CQDs to polyvinyl alcohol matrix at RT and exciting them with ultraviolet light. The maximum emission obtained was 500 nm, with an average lifetime of 380 ns at a 325 nm excitation [140]. Phosphorescence in CQDs arises when the singlet and triplet states of an aromatic carbonyl group in CQDs and polyvinyl alcohol matrix are close in energy to assist spin–orbit coupling, which increases the intersystem crossing (ISC). By using microwave synthesis, Lu et al. synthesized ultra-long phosphorescent carbon quantum dots (P-CQDs). When P-CQDs were excited at 354 nm, they displayed yellowgreen phosphorescence (525 nm) for up to 9 s. They concluded that as the pH increases, the phosphorescence intensity of P-CQDs gradually decreases. The reason is that protonation dissociates the hydrogen bonds and distresses the phosphorescent sources. By introducing the tetracyclines (TCs), the phosphorescence of P-CQDs was quenched. They applied P-CQDs as biological and chemical sensing and time-resolved imaging [141]. Figure 5 is the typical excitation (black line) and emission (red line) spectrum of fluorescent CQDs.

**Figure 5.** Excitation and emission spectrum of CQDs.

#### **5. Application of Carbon Quantum Dots as a Catalyst**

The presence of different functional groups such as -OH, -COOH, -NH2, etc., on carbon quantum dots' (CQDs) surface provides vigorous coordination sites to bind with transition metal ions. The CQDs doped with multiple heteroatoms might further improve the catalytic activity by encouraging electron transfer via interior interactions. The presence of more active catalytic reaction sites offered by CQDs and favorable charge transfers during the catalytic process is also responsible for the application of CQDs as a catalyst (Figure 6) [6,142–144].

**Figure 6.** Catalytic applications of CQDs.

#### *5.1. CQDs as a Catalyst for the Peroxidase-Mimetic Enzyme Activity*

Natural enzymes such as peroxidase can catalyze a variety of reactions with high catalytic activity and excessive surface specificity [145]. Because of this, they are broadly utilized in different fields such as the pharmaceuticals industry, medicine, agriculture, etc. [146]. However, they possess some limitations such as high cost, short storage life, rigorous storage conditions, and poor thermal stability [147]. Therefore, to point out these limitations, carbon-based nanomaterials were found very suitable for intrinsic peroxidase-mimetic catalytic activity. Yadav et al. have synthesized fluorescent CQDs from leaf extracts of neem (*Azadirachtaindica*) by using a one-pot hydrothermal method. The as-prepared Neem-Carbon Quantum Dots (N-CQDs) exhibited peroxidase-mimetics catalytic activity in an extensive pH range for the oxidation of peroxidase substrate 3,3 ,5,5 - tetramethylbenzidine (TMB) in the presence of hydrogen peroxide (H2O2). The peroxidase-mimetic catalytic activity of N-CQDs was confirmed by taking UV−visible absorption spectra of N-CQDs in the presence and absence of H2O2 with TMB in an acetate buffer. When the mixtures of TMB and N-CQDs were taken, no absorbance at 652 nm was observed, revealing no oxidation of TMB. Additionally, when the mixture of TMB and H2O2 reacted, a less intense peak at 652 nm was obtained, enlightening the partial oxidation of TMB with the existence of a partial blue color. Interestingly, in the presence of N-CQDs, TMB, and H2O2, the absorbance at 652 was found at a maximum, with the color changing from colorless to blue, revealing the complete oxidation of TMB. These results powerfully confirmed that N-CQDs act as a catalyst for peroxidase-mimetic activity. To determine the intermediate reaction, the active species trapping experiment with isopropyl alcohol (IPA) and methyl alcohol (MA) was carried out. The IPA and MA are hydroxyls radical (•OH) scavengers. When these scavengers were added to the oxidized blue-colored solution of TMB, a decrease in the absorption at 652 nm was observed, enlightening the incomplete oxidation of TMB because the IPA and MA consumed the •OH radical. This examination specifies that in the presence of N-CQDs, the •OH radicals were generated during a peroxidase-like catalytic reaction, which oxidized TMB via a one-electron transfer to produce a blue-colored solution. Additionally, the high surface area, small size, and presence of a negative-charge density on the N-CQDs surface were also responsible for this catalytic activity (Figure 7) [39].

**Figure 7.** Showing the oxidation of TMB along with H2O2in the presence of CQDs as a catalyst.

#### *5.2. CQDs as a Catalyst for Selective Oxidation of Alcohols to Aldehydes*

Aldehydes are highly demanded as a crucial intermediate for the production of an extensive range of materials, such as pesticides, toiletries, dyes, and perfumes, in the pharmaceuticals and agribusiness industries. The popular method for the synthesis of aldehydes is catalytic alcohol oxidation, but establishing an ecofriendly method with high-yield production and selectivity is still a major challenge for researchers [148–150]. Rezaie et al. developed a multifunctional tungstate-decorated CQDs base catalyst, A-CQDs/W, by using a one-pot hydrothermal technique, and utilized it for the oxidation of a variety of alcoholic substrates into analogous aldehydes with the help of H2O2 as an oxidant and an ultrasound effect as a green activation method. Before investigating the catalytic activity, the oxidizing potential of an amphiphilic multifunctional catalyst was examined, and they observed that A-CQDs/W were capable of oxidizing a wide range of alcoholic substances into corresponding aldehydes with 100% selectivity and above 95% yield. This achievement was because of the synergic effect among ultrasound irradiation and the suitable design of the catalyst. The proposed mechanism for this oxidation reaction firstly involves the reaction between H2O2 and A-CQDs/W, resulting in the production of bisperoxo tungstate, which is immobilized on A-CQDs via hydrophilic groups. This is able to diffuse into the organic alcoholic phase and trigger the oxidation reaction with the assistance of an ultrasound wave. Finally, aldehyde was fabricated after inserting the alcoholic ligand on A-CQDs/W, followed by a ligand exchange reaction [34,151,152].

#### *5.3. CQDs as a Catalyst for Selective Oxidation of Amine to Imine*

Imines are valuable for the preparation of biologically active molecules, such as oxazolidines, chiral amines, amides, nitrones, aminonitriles, and hydroxylamines. Additionally, β-lactams complexes are also synthesized using imine intermediates [153–155]. Several materials were used as a catalyst for the selective oxidation of amine to imine, but the carbon-based materials such as CQDs, mesoporous carbon, graphene oxide (GO), amorphous carbon, graphitic carbon nitride, and carbon nanotubes (CNTs) have been recognized as potential catalysts compared to conventional metal-based catalysts because of their relatively low cost and natural abundance [156–158].

Ye et al. prepared oxygen-rich carbon quantum dots (O-CQDs) from fullerenes (C60) and utilized them as nanocatalysts (metal-free) for the oxidation of amines to imine with

an excellent 98% yield. The mechanism behind this catalytic oxidation reveals that the molecular oxygen and amine molecules are trapped and activated by carboxylic functional groups present on the surface of CQDs, along with the unpaired electrons, resulting in the conversion of amine. For the oxidative coupling of amine to imine, the catalytic performance of O-CQDs was further improved by heat treatment. The aerobic oxidation of amines was probably because of the occurrence of several carboxyl functional groups, which coupled with spins of π-electrons from the atoms situated at the surface of O-CQDs [36].

#### *5.4. CQDs as a Catalyst in the Synthesis of Multisubstituted 4H Pyran with Indole Moieties*

Indole scaffolds have attracted much attention among researchers because of their applications in the field of pharmacology, such as antihypertensive, antiproliferative, anticholinergic, antifungal, cardiovascular, optimal inhibitory, antibacterial, antiviral, and anticonvulsant activities [159–161]. Additionally, there are some pharmaceutically significant compounds and natural products which have anticancer, hypoglycemic, antiinflammatory, antipyretic, and antitumor properties, and contain indole scaffolds in their structures [162,163]. 4H-pyrans are an important family of oxygen-containing heterocyclic compounds with a wide spectrum of biological properties such as antioxidant, anticoagulant, diuretic, spasmolytic, anti-anaphylactic, and anticancer activities [164,165]. Rasooll et al. synthesized a novel heterogeneous nano-catalyst from CQDs and phosphorus acid moieties by using ultrasonic irritation followed by a hydrothermal method and named it CQDs–N(CH2PO3H2)2. The instrumental techniques such as transmission electron microscopy (TEM), energy-dispersive X-ray (EDX) spectroscopy, X-ray diffraction (XRD), FT-IR spectroscopy, scanning electron microscopy (SEM), fluorescence, and thermogravimetric (TG) analysis were utilized to characterize this catalyst. An efficient catalyst, CQDs–N (CH2PO3H2)2, was effectively applied for the preparation of 2-amino-6-(2-methyl-1H-indol-3-yl)-4-phenyl-4H-pyran-3,5-dicarbonitriles, with the help of a variety of aromatic aldehydes, 3-(1H-indol3-yl)-3-oxopropanenitrile derivatives, and malononitrile. The principal advantages of this catalytic activity include fresh and mild reaction conditions, little reaction time, and the recycling of the catalyst.

The anticipated mechanism for this catalytic reaction is that, firstly, the acidic proton of CQDs–N(CH2PO3H2)2 activates the aldehyde group, followed by the reaction with malononitrile, and intermediate (I) is formed by the loss of one molecule of H2O. In the next step, 3-(1Hindol-3-yl)-3-oxopropanenitrile reacts with intermediate (I) to provide intermediate (II) following tautomerization. Finally, after intramolecular cyclization, the desired product is obtained from intermediate (II) with the loss of another molecule of H2O [31].

#### *5.5. As a Photocatalyst for High-Efficiency Cyclohexane Oxidation*

In the 21st century, the highly efficient and highly selective catalytic oxidation of cyclohexane under mild conditions is the principle objective of catalysis chemistry. Liu et al. synthesized fluorescent CQDs and gold (Au) nanoparticle composites (Au/CQDs composites). The CQDs were prepared through the electrochemical ablation method using graphite. A chemical reduction method was used to synthesize AuNPs by an aqueous solution of HAuCl4 and trisodium citrate, which resulted in a pink color immediately after the addition of the NaBH4 solution. When in the solution of CQDs, a HAuCl4 solution was added, and the solution turned red, revealing the formation of a composite (Au/CQDs composites). Interestingly, they utilized this composite as a tunable photocatalyst for the selective oxidation of cyclohexane to cyclohexanone with the help of an oxidant H2O2 (30%). The conversion efficiency was 63.8% and selectivity was over 99.9%. The mechanism involves enrichment in the absorption of light by surface plasma resonance of Au nanoparticles, the generation of active trapping oxygen species (HO·) through H2O2 decomposition, and interaction among CQDs and AuNPs under visible light [37].

#### *5.6. As a Catalyst for the Removal of Rhodamine B*

Preethi et al. prepared bluefluorescent CQDs from a natural carbon precursor (muskmelon peel) using a stirrer-assisted method. The synthesized CQDs were utilized as an excellent photocatalyst and a sonocatalyst for the degradation of Rhodamine B (RhB) dye. The efficiency of CQDs for the degradation of RhB is 99.11% in sunlight, with a degradation rate constant of 0.06943 min−<sup>1</sup> and 83.04% in ultrasonication. These results advocate that CQDs are an efficient catalyst for the breakdown of organic dyes in wastewater. The mechanism reveals the generation of •OH radicals during active species trapping experiment. •OH was confirmed by taking terephthalic acid (TA) as a scavenger. The dye molecules adsorbed on the surface of CQDs may be oxidized by these active species, ensuing in dye degradation [35].

#### *5.7. As a Catalyst in Azide-Alkyne Cycloadditions*

Liu and coworkers synthesized yellow light-emitting bio-friendly CQDs from Na2[Cu(EDTA)] by thermolysis. Cu(I)-doped fluorescent CQDs were utilized for catalyzing the Huisgen 1,3-dipolar cycloaddition among azides and terminal alkynes, the classical example of "click chemistry". The possible mechanism behind this catalytic property using these CQDs was projected to be the UV-induced split of excitons. First of all, the escape of electrons from the CQDs occurs, resulting in the formation of holes to compete with Cu(I), and at last, Cu(I) is released from the CQDs. The high biocompatibility of this nanocatalyst was confirmed by Hep-2 cells, revealing intracellular detection [32].

#### *5.8. As H-Bond Catalysis in Aldol Condensations*

Han and coworkers synthesized CQDs by an electrochemical etching method and utilized them as efficient heterogeneous nanocatalysts for H-bond catalysis in aldol condensations. The catalytic activity was excellent (89% yields), with visible light irradiation. Highly efficient electron-accepting capabilities, novel photochemical properties, and functional hydroxyl and carboxylic groups on the surface are responsible for such soaring catalytic activities of CQDs [38]. The catalytic efficiency of CQDs was high in visible light irradiation, and almost no conversion was observed in the absence of light. The CQDcatalyzed aldol condensation was greatly influenced by solvents. Han et al. used different solvents such as ethanol, tetrahydrofuran (THF), acetone, chloroform (CHCl3), and toluene. However, the highest yield (89%) was calculated when the solvent and reactant were acetone. These investigations exposed that CQDs acted as an outstanding catalyst for Aldol condensation. The mechanism revealed that the cationic or anionic intermediates were generated during catalytic reaction. The hydroxyl groups present on the CQDs edge act as extremely weak acids, which can form H-bonds with oxygenates [166,167]. Aldehydes and ketones, both reactants, were capable of forming H-bonds. They confirmed that the hydroxyl groups present on the surface of CQDs favor contact with aldehyde groups. When the reactions were carried out in the absence of a hydroxyl group, no product was obtained and free CQDs were unreactive. These results advocate that the capability of CQDs to intervene in reactions is through interfacial H-bond catalysis. In visible light irradiation, CQDs act as highly proficient electron acceptors and attract electrons from the O−H···O region, resulting in the development of a positive charge on hydrogen and oxygen, and the negative charge increases. This effect results in an increase in the s-character in the oxygen hybrid orbital, thereby leading to the strengthening of the O−H bond, which efficiently activates the C=O bond of the aldehyde group and accelerates the aldol condensation. Furthermore, the reaction-intermediate or transition-state species is stabilized by the enhanced O−H bonds, resulting in the highest yield of 89.4% [168,169].

#### *5.9. As a Catalyst for the Ring Opening of Epoxides*

In modern organic synthesis, acid catalytic reactions contribute a characteristic and imperative role [170]. Some carbon-based nanostructures such as sulfated-graphene/ tube/-active carbon materials have been utilized as acid catalysts in several catalytic

applications [171]. However, they possess some limitations, such as the requirement of sufficient surface functionalization, low efficiency, and complex synthesis steps [172]. As a result, the development of carbon materials-based acid catalysts with high efficiency that are light-driven or light-enhanced are still required. Keeping these in mind, Li et al. described the synthesis of CQDs based on a novel, photoswitchable solid acid catalyst. The CQDs were synthesized from a graphite rod using an electrochemical method, doped with hydrogen sulfate groups (S-CQDs). They utilized S-CQDs as light-enhanced acid catalysts, which catalyze the ring opening of epoxides in the presence of nucleophiles and solvents (methanol and other primary alcohols). The mechanism revealed that the additional protons are released from the ionization of the -SO3H group under visible light irradiation and, as a result, a stronger acid environment is offered for the opening reaction, and a higher yield as well as selectivity of the product is obtained compared to the process without light irradiation. The photoexcitation and charge separation in the CQDs create an electron-withdrawing effect from the acidic groups. The utilization of S-CQDs as visiblelight-responsive and convenient photocatalysts is a novel application of CQDs in green chemistry [40].

#### *5.10. As a Catalyst for the Degradation of Levofloxacin*

Levofloxacin (LEVO), also known as levaquin, is an important antibiotic medicine. Several bacterial infections such as pneumonia, acute bacterial sinusitis, urinary tract infection, *H. pypori*, and chronic prostatitis are treated by LEVO. It is also used to treat tuberculosis, pelvic inflammatory disease, or meningitis, along with other antibiotics [173,174]. However, the degradation of LEVO is typical. Although some techniques have been utilized for the degradation of LEVO, the degradation using CQD had not been discovered. Meng et al. synthesized CQDs@FeOOH nanoneedles for an efficient electro-catalytic degradation of LEVO. The CQDs were synthesized by a hydrothermal method from orange peels. With the help of a facile in situ growth method, the α-FeOOH was fabricated by using Fe2(SO4)3and H2O in 50 mL distilled water. Similarly, a CQDs@FeOOH electro-catalyst was prepared using the above method, except that 500 mL aqueous solutions of 0.5 g/L CQDs were used instead of 500 mL distilled water. By using CQDs@FeOOH, about 99.6% LEVO and 53.7% total organic carbon (TOC) could be competently removed after 60 min degradation. This high degradation performance for LEVO was due to the soaring mass transfer capability and the high % OH generation ability of the CQDs@FeOOH. Meng et al. proposed a possible LEVO degradation mechanism and also investigated the change in toxicity throughout LEVO degradation. The mechanism revealed the generation of both % OH and SO4% in LEVO degradation, but a dominant role was played by % OH. Liquid chromatography-mass spectrometry (LC-MS) results designated that the LEVO could be entirely decomposed by % OH under the de-piperazinylation, decarboxylation, and ring opening reaction. This novel work offers a proficient technique to reduce the quantity and toxicity of antibiotics in water [33].

#### *5.11. CQDs as Electrocatalyst*

CQDs are also utilized as electrocatalysts in hydrogen evolution reduction, oxygen evolution reaction, CO2 reduction reaction and oxygen reduction reaction. The large surface area, good conductivity and fast charge transfer process of CQDs are responsible for the electrocatalytic applications [6].

#### **6. Conclusions and Future Perspectives**

The present review paper discusses the structures, synthetic methods, optical properties, and applications of CQDs as a catalyst. The structure of CQDs includes core–shell, either graphitic (sp2) or amorphous (mixed sp2/sp3). CQDs are usually amorphous, having different functional groups such as amino, carboxyl, hydroxyl, etc. CQDs are synthesized by both the bottom-up and the top-down approach. The bottom-up method is better because it is ecofriendly and economically viable, but it has poor control over the size of CQDs. In contrast, the top-down methods are expensive. For the synthesis of CQDs, chemical as well as biological precursors are used. CQDs possess admirable optical properties and have superior water solubility, low toxicity, biocompatibility, and ecofriendliness. The optical properties and QYs are essential parameters for the applications of CQDs in the field of nanomedicine, biosensing, chemical sensing, bioimaging, solar cells, drug delivery, and light-emitting diodes. In this review paper, we have focused on the applications of CQDs as a catalyst in the degradation of levofloxacin, the selective oxidation of amines and alcohols, azide-alkyne cycloadditions, the synthesis of multisubstituted 4H pyran, the selective oxidation of alcohols to aldehydes, the removal of Rhodamine B, cyclohexane oxidation, the ring opening of epoxides, and intrinsic peroxidase-mimetic enzyme activity. The mechanism suggests that the catalytic activity might be due to the presence of more active reaction sites, favorable charge transfer, improved structure stability, and enhanced electronic conductivity.

However, during the last fifteen years, several investigations have been carried out on CQDs, and numerous challenges require being resolved for the extensive adoption of CQDs. (1) It is difficult to synthesize CQDs of a desired structure and size because of the requirement of accurate control over different synthesis parameters. Therefore, to powerfully control the core structure, a manufacturing process could be developed which helps increase QYs and the large-scale production of CQDs. (2) In many research papers, it has not been reported why the fluorescence QY of doped and co-doped CQDs are high in contrast to the un-doped CQDs. Thus, in the future, it is possible to realize the basic fluorescence mechanism in doped and co-doped CQDs. (3) Most doped and co-doped CQDs emit blue fluorescence. Hence, it is challenging for the researcher to synthesize multicolor emission CQDs and utilize them in different applications in the future. (4) To broaden the spectrum of CQDs, efforts must be made, particularly in the near-IR region, so that the applications of CQDs can be widespread, such as in organic bioelectronics. (5) CQDs possess some limitations such as low reactivity, poor stability, short lifetime, etc., which prevents them from promising to be a good catalyst. Therefore, in the future, it will be possible to overcome these shortcomings.

Compared to other applications of CQDs, very few studies have been reported on the application of CQDs as a catalyst in organic synthesis. In detail, theoretical and experimental studies are required to carefully design CQD-based catalysts with attractive catalytic action and durable operation stability. The applications of CQDs as a catalyst in organic synthesis signify the flexibility of CQDs in the most unpredicted areas. It is inspiring to see the applications of CQDs in green chemistry and clean energy production. It looks obvious that the future of CQDs remains promising.

**Author Contributions:** Conceptualization, P.K.Y.; methodology, P.K.Y. and S.C.; literature investigation, P.K.Y., S.C., V.K., D.K. and S.H.H.; writing—original draft preparation, P.K.Y.; writing—review and editing, P.K.Y. and S.C.; visualization, P.K.Y., S.C., V.K. and D.K.; supervision, S.H.H. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Data Availability:** Not applicable.

**Acknowledgments:** The authors are thankful for the Indian Institute of Technology, BHU, India, for encouraging and facilitating us in pursuing this research. The authors also give thanks to Department of Chemistry, Jagatpur P.G. College, affiliated to MGKV University Varanasi, India, for providing a conductive atmosphere for research activities.

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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### *Review* **The Behavior of Carbon Dots in Catalytic Reactions**

**Lerato L. Mokoloko 1,2,\*, Roy P. Forbes <sup>2</sup> and Neil J. Coville 2,\***


**Abstract:** Since their discovery in 2004, carbon dots (CDs), with particle sizes < 10 nm, have found use in various applications, mainly based on the material's fluorescent properties. However, other potential uses of CDs remain relatively unexplored when compared to other carbon-based nanomaterials. In particular, the use of CDs as catalysts and as supports for use in catalytic reactions, is still in its infancy. Many studies have indicated the advantages of using CDs in catalysis, but there are difficulties associated with their stability, separation, and aggregation due to their small size. This small size does however allow for studying the interaction of small catalyst particles with small dimensional supports, including the inverse support interaction. However, recent studies have indicated that CDs are not stable under high temperature conditions (especially >250 ◦C; with and without a catalyst) suggesting that the CDs may agglomerate and transform under some reaction conditions. The agglomeration of the metal in a CD/metal catalyst, especially because of the CDs agglomeration and transformation at high temperature, is not always considered in studies using CDs as catalysts, as post-reaction analysis of a catalyst is not always undertaken. Further, it appears that under modest thermal reaction conditions, CDs can react with some metal ions to change their morphology, a reaction that relates to the metal reducibility. This review has thus been undertaken to indicate the advantages, as well as the limitations, of using CDs in catalytic studies. The various techniques that have been used to evaluate these issues is given, and some examples from the literature that highlight the use of CDs in catalysis are described.

**Keywords:** carbon dots; thermal stability; metal support; heterogeneous catalysis

#### **1. Introduction**

Carbon allotropes are multifunctional materials, due to their unique physical and chemical properties. Carbon allotropes can be chemically modified by other elements via functionalization or doping, and they can also be used in combination with other materials to form carbon–carbon or metal–carbon composite materials [1]. The modification of carbon allotropes helps to enhance their properties, and also widens their spectrum of applications. Hence, carbon allotropes such as graphite, graphene, fullerene, carbon nanotubes (CNTs), carbon nanofibers (CNFs), carbon black (CB), carbon nano-onions (CNOs), carbon spheres (CSs) and carbon dots (CDs) have been successfully incorporated in fields such as nanomedicine, electronics, sensor fabrication and catalysis [1–4].

Carbon's many allotropes have shown great potential when used as a catalyst support, and the carbon can even act as a catalyst in its own right [1,5]. The enormous interest in carbon-based support materials is due to their surface chemistry, variable surface area and the porosity of the carbon [1,5]. The use of carbon allotropes is also enhanced by their electronic properties, which are influenced by their structure and the carbon atom valence. The electronic effects can promote a high dispersion of the supported (metal) catalyst, and also of surface defects, and this can enhance their capability for gas storage, adsorption and/or separation processes [5,6].

**Citation:** Mokoloko, L.L.; Forbes, R.P.; Coville, N.J. The Behavior of Carbon Dots in Catalytic Reactions. *Catalysts* **2023**, *13*, 1201. https:// doi.org/10.3390/catal13081201

Academic Editors: Indra Neel Pulidindi, Archana Deokar and Aharon Gedanken

Received: 28 June 2023 Revised: 26 July 2023 Accepted: 5 August 2023 Published: 11 August 2023

**Copyright:** © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

Heterogeneous metal catalysts usually comprise a metallic catalyst that is anchored or supported on the surface of a material that serves to enhance the surface area of the metal while also improving the metal stability during chemical reactions [7]. Support materials are important for immobilizing and anchoring the active metal catalyst, improving its dispersion and durability, and aiding it in avoiding deactivation during a catalytic reaction [8]. Many studies have shown that a catalytic reaction rate is influenced by the catalyst dispersibility [9]. Metal–support interactions (MSI) are an important factor to consider when selecting a catalyst support, as the support directly influences the dispersibility, sintering, reducibility, and overall performance of the catalyst. MSIs are effects measured by the physical and chemical interactions between the metal catalyst and the support [10–12]. The stronger the MSI, the more resistant the catalyst is to deactivation via sintering during a reaction [12,13]. Carbon, in its various allotropes, generally forms a weak MSI with a metal catalyst. However, the MSI can be improved by means of functionalizing or doping of the carbon surfaces with heteroatoms. These effects create defects in the carbon nanostructure, and also alter the electronic properties of the carbon allotrope. Consequently, the dispersibility of the metal catalyst is increased and metal sintering/deactivation is reduced by doping/functionalization [12].

Most carbon allotropes are relatively 'stable' under harsh reaction conditions; this makes them less susceptible to structural and/or chemical changes during catalysis [12]. This is important, because the structural disintegration of a catalyst support can lead to deactivation of a catalyst. The properties of carbon supports have been studied under different conditions using varying pH, solvents, temperatures, time, etc. For example, carbon allotropes have successfully been used as supports for titania in photocatalysis [14], platinum in electrocatalysis [15], and cobalt, iron and ruthenium in catalysis for fuel production under a wide range of reaction conditions [12].

Carbon dots (CDs) are a fairly new type of carbon nanomaterial. CDs are defined as zero-dimensional quasi-spherical carbon nanomaterials with particle sizes below 10 nm [16]. Their general structure consists of sp2- and sp3-type carbons, with a large number of functional groups or polymer chains attached to their surfaces [17–19]. Numerous reviews have summarized the potential of CDs in many traditional and emerging areas, such as photoluminescence (PL) and photoelectrochemical-driven sensing, catalysis, imaging, and biomedicine applications, where they have been shown to be superior to other carbon allotropes [16,20]. Further, CDs, unlike metal-based quantum dots, have a high water dispersibility, low toxicity, and good biocompatibility [21]. Thousands of papers have been written about CDs in terms of their synthesis, characterization and uses. The aim of this review is not to reproduce the information on the synthesis, applications, and the properties of CDs, which has been summarized in scores of reviews, but to indicate the transformation of CDs into other shaped materials, which is associated with temperature and the presence of catalysts.

CDs, like many carbon allotropes, have been used as both catalysts and catalyst 'supports,' and reviews on the use of CDs both as carbocatalysts and as composites with metal and metal oxide supports have been reported [22–24]. As is known, there are many advantages to using CDs and CD/metal complexes in catalysis, but there are also limitations on the use of these carbons. For example, in many of the studies it is not clear if the integrity of the carbon has been retained, and the degree to which metal sintering has taken place during their synthesis or in their catalytic reactions. Indeed, some studies have revealed that CDs and CD/metal materials, lose their shape/size during reaction, especially at high temperatures (>100 ◦C) and in the presence of easy-to-reduce metals. This has been observed in carbon–carbon coupling reactions [25], hydrogen evolution reactions [26] and hydrogenation reactions, where the CDs either decomposed or were transformed into other carbon structures [27,28].

In this review we have evaluated data on the use of CDs and CD/metal catalysts in catalysis. One of the advantages of using these two types of catalysts, relative to more typical catalyst systems, relates to the easy access to CDs which can be produced at low cost from any carbon source, including waste materials. Since the metal particles are either the same size or larger than the CDs, the terminology CD/metal has been used to describe the catalysts made from CDs and metal particles. We have not comprehensively reviewed all reports where CDs and CD/metal materials have been used in catalysis, but have rather chosen some typical literature reactions to highlight the challenges associated with the use of CDs in catalytic reactions. In particular, we wish to highlight the structural changes or destruction of the CDs that can occur during a reaction that can ultimately lead to metal agglomeration. The techniques that can be used to evaluate the stability of CDs, especially post reaction, are discussed with special reference to metal and carbon agglomeration.

#### **2. The Structure of Carbon Dots (CDs)**

#### *2.1. Types of CDs*

CDs are usually divided into four distinct subgroups. These are graphene quantum dots (GQDs), carbon quantum dots (CQDs), carbon nanodots (CNDs) and carbonized polymer dots (CPDs) [29–32] (see Figure 1). In the literature, these four types are usually called carbon dots (CDs), and generally they are all regarded as being similar in their catalytic (and many other) properties. However, this may not always be true. To date, this issue has not been explored, and there is evidence to suggest that the different CDs will react differently as a function of temperature and when in the presence of metals that can aid their decomposition [27]. In this review, the term CDs will be used for all carbon dots shown in Figure 1.

**Figure 1.** The proposed representative structures of a graphene quantum dot (GQD), carbon quantum dot (CQD), carbon nanodot (CND), and carbonized polymer dot (CPD). Adapted with permission from Wiley-VCH Verlag [33].

CDs must be differentiated from carbon nano-onions (CNOs) and onion-like nanocarbons (OLNCs), which are also in the nanometer range [34]. CNOs and OLNCs have structures with carbon layers similar to those found in a concentric-like onion structure (see Figure 2), while CDs (Figure 1) tend to have the carbon layers arranged as parallel graphene sheets (Figure 1). The CNOs/OLNCs are prepared at high temperatures (>500 ◦C), while CDs are typically prepared at T < 200 ◦C. This results in differences in their thermal stabilities that impact on their chemical properties.

**Figure 2.** Graphical representation showing the differences between (**a**) a carbon nano-onion (CNOs), and (**b**) an onion-like nanocarbon (OLCN).

There are two methods used to modify the structure/surface of CDs. One method is by functionalization, and this is described in more detail below. The other is by making the CDs with precursors in which reactants provide non-carbon atoms to the CD. This is referred to as doping. The most typical dopant is nitrogen, and the addition thereof in quantities ranging from 1 to 10% can lead to substantial changes in the surface chemistry of a carbon material, including CDs. Many CDs have also been doped with metal ions, and this modification can also impact their chemistry and catalytic reactions [35].

#### *2.2. Surface Properties of CDs*

CDs have many useful properties associated with their small size and functional groups. While the surface of the CD is important in correlating with their physical and chemical properties, the role of the core is still poorly understood [36]. Thus, most studies on the use of CDs that have been performed have related to their surface chemistry [37]. The surface of a CD contains many functional groups, and these groups are responsible for the catalytic activity of the CD. CDs are typically synthesized with both oxidizing and reducing groups that are used to bring about organic redox transformation reactions. These functional groups are also used to modify their PL spectra. Their surface chemistry can be modified by classical procedures associated with modifying any carbon surface. The surface modification can be achieved by covalent and non-covalent bonding of reactants with the CD surface, as reported in work by Yan et al. [38]. This type of modification is carried out to improve the properties of the CDs for a specific application such as biosensing, or metal and molecule detection [38]. Examples are shown in Figure 3.

**Figure 3.** Post-surface functionalization of different CDs via (**a**) amide coupling-type reaction (covalent bonding), (**b**) esterification (covalent bonding) and (**c**) pi–pi interactions (non-covalent bonding). Adapted with permission from [38], Copyright 2018, Springer Nature.

The surface charge on the CD can be modified by changing the functional groups. The starting precursors can be manipulated in order to obtain CDs that are either hydrophilic or hydrophobic [39]. Typically, CDs are synthesized using multifunctional organic chemicals, and this produces hydrophilic CDs. Similarly, hydrophobic CDs can be produced using aliphatic chemicals such as dodecylamine [39,40]. It is further noted that the surface charge on the CD can be modified by changing the functional groups, and this can be done without post modification of the CDs [41]. Thus, CDs with different surface charges [41,42] and polarities [40,43] have been reported. For example, most CDs are made with a negative charge (associated with COO− groups). An important approach to generate positively charged CDs is by modifying CD surfaces with ionic liquids (ILs) and then annealing the material at *ca.* 240 ◦C. The IL-covered CDs were then used to detect metal ions in solution [41] or to make inks [42].

CDs can also react with themselves. For example, CD–CD linkages have been achieved from CDs that were made from C60 fullerene (by a base reaction). The CDs self-assembled when they were freeze dried [44]. After annealing (600–800 ◦C), the materials were used in capacitor studies. The self-assembly was proposed to be achieved with ice crystals acting as a template [44]. Studies have also been carried out in which a chemical reaction between two different CDs produced CD assemblies with surface properties associated with the different CDs used. Zhou et al. made three different types of CDs, and under room temperature conditions and with different CD combinations, they were linked together via the functional groups on the CDs. In one instance, the CDs, after reaction with each other, gave nanostructures that were used as drug nanocarriers [45,46]. Many other examples have also been reported [27].

Self-assembly of CDs has been achieved by the use of tannic acid. Modification of the functional groups on the CDs with ionic liquids provided for a good interaction of the CDs, with the negative charges on the tannic acid allowing for formation of assembled species (in some cases, dCD > 100 nm), which were readily detected by Tyndall cone measurements and transmission electron microscopy (TEM) measurements [47]. Supramolecular organization of CDs after alkylation of amine-functionalized CDs [39] has led to a series of alkyl-functionalized CDs, which could be separated by chromatography and that formed an organized structure in the solid state. The new assemblies were used in nonlinear optical studies [40]. A recent extension of this work described the self-assembly of CDs made from chiral cholesteryl to make thermotropic liquid crystals with a range of architectures [43]. These self-assembled CDs could provide an entry into novel structures for use as both carbocatalysts and to make CD-metal catalysts.

A key property associated with a CD surface is its hydrophilicity, which has led to the extensive use of CDs in medicinal chemistry [48]. Another important property is their photoluminescence (PL), which has allowed them to be used in sensing devices [49]. Typically, addition of a metal reactant to the CD functional groups results in a decrease in their PL spectrum, and hence these types of experiments are usually conducted to detect metal ions within various media [50]. The changes in the PL spectra of CDs will be influenced, in a catalytic reaction, by the varying concentrations of products and reactants that could bind to the CD surface, thus allowing for the exploration of a reaction mechanism. The addition of polymers to a CD surface has been found to lead to improved PL properties [29].

Due to their small size (<10 nm), the separation and purification of CDs is not simple. Further, yields of the purified CDs are not always reported, and it is thus difficult to assess the usefulness of many synthetic strategies. While CDs have a small size, their surface area tends to vary, and can be lower than expected. The surface area can vary, for example, between 16.4 m2 g−<sup>1</sup> [51] and 1690 m<sup>2</sup> g−<sup>1</sup> [52].Thus, interaction with a metal ion or particle will be limited by this property. However, the many surface groups can be used to reduce metal ions to metal particles and in so doing lead to the formation of small metal particles, by limiting the site of the reduction. As expected, when CD surfaces, as in all carbons, are doped with N atoms, metal particle agglomeration is reduced [53].

The role of carbons as supports is limited by their reactions under oxygen, hydrogen or inert gases. In the presence of oxygen, most carbons will oxidize (to CO, CO2) below 600 ◦C [54,55], while under H2, the carbon can react to form CH4, typically at temperatures above 500 ◦C [56]. Under an inert atmosphere, surface groups on the carbon can be removed at temperatures dependent on the carbon-to-element bond. In the absence of a catalyst, and under an inert atmosphere, the carbon core can be stable to temperatures above 600 ◦C. CDs, because of their size, have a high surface-to-bulk (core) carbon atom ratio [57]. Thus, all the reactions listed above can be expected to be modified when CDs are used, in relation to reactions with larger carbon molecules.

#### *2.3. Synthesis of Carbon Dots and Their Application as Reducing Agents*

Many papers and review articles have been written on the synthesis of CDs and this topic will not be discussed in detail here. CDs can be synthesized by "top-down" procedures, by cutting down larger carbon allotropes such as graphene, fullerene and CNTs using strong oxidizing agents like sulphuric and nitric acid [58]. The CDs can also be prepared by the "bottom-up" process, and are generally made from precursors that contain functional groups that are typically retained from their synthesis precursors [29]. For example, CDs can be prepared from highly oxygenated starting materials such as ascorbic acid, sucrose, and citric acid [59,60], using a "bottom-up" synthesis approach (Figure 4). Further functionalization using a variety of methods can be carried out to advance the surface chemistry and other properties of the CDs [29,38]. The CD surface groups affect their overall chemical behavior, such as their electronic properties. These electronic properties have been exploited for oxidizing and reducing metals, and in this way generate CD/metal catalysts [24,61].

**Figure 4.** Schematic presentation of the "bottom-up" and "top-down" CDs synthesis procedures from oxygen-rich starting materials (highlighted in red) or by reacting 'large' carbon allotropes with oxidizing acids.

There are numerous studies that have been reported in which CD surface oxygen groups have been modified/reduced after the CDs have been synthesized. These are referred to as reduced CDs (r-CDs). Reducing agents used include NaBH4, ascorbic acid, sodium citrate, and hydrazine hydrate, with NaBH4 being the most effective reducing agent [62]. The r-CDs have been reported to have better luminescence properties than their pristine (more highly oxidized) CD counterparts [37]. These r-CDs have been used to reduce strong oxidizing agents such as KMnO4, KIO4 and K2Cr2O7. These reagents in turn have been used to selectively oxidize the O-H groups in r-CDs to C=O [63]. They have also been used to reduce metal ions, to generate metal catalysts.

#### **3. Carbon Dots in Catalysis**

The main role of any carbon in the field of catalysis is that of the carbon acting either as a catalyst (called carbocatalysis) or as a support for a heterogenous catalyst [22]. Because of their small size, CDs add this extra feature of support dimension to their use in catalysis, relative to other carbon supports.

#### *3.1. Applications of Carbon Dots-Based Catalysts*

Excellent reviews exist on the use of CDs as carbocatalysts [22–24,64,65] in thermal, photocatalytic and electrocatalytic applications. Many simple and classic chemical transformation have occurred in the presence of CDs (Figure 5) [24].

**Figure 5.** CD application in nano-organocatalysis and photocatalysis. (Adapted with permission from [24] Copyright 2020, American Chemical Society).

As will be seen in these reviews, much focus has been on their surface functionalization and use, rather than on morphology changes of the CDs observed after or during reactions. This is understandable, as in most instances very small amounts of catalyst are used, and re-use studies have indicated that the carbon 'catalyst' is stable after many reaction cycles. However, some studies have shown that the CDs can be converted into other morphologies, typically when reactions occur at high temperatures or in the presence of easily reduced metal ions. The difficulty in using smaller carbon particles (CDs) was hinted at in an early review on the use of nanocarbons in catalysis, viz. "However, it is useful to comment that it is not always proven that functionalized nanocarbons act as real catalysts; e.g., they are not consumed during the reaction" [66]. Some examples where post-reaction studies of CDs have been reported and have revealed CD conversion to other morphologies are given below.

#### - A limitation of using carbon dots in catalysis

The conversion of CDs into carbons with a different framework was noted in early studies on the synthesis of CNTs when the CDs were decomposed at high temperature in porous anodic aluminas [67]. Later studies showed that CDs, made from acetone, could be easily converted into carbons called porous carbon frameworks (PCFs) at high temperatures of between 400 ◦C and 800 ◦C [68]. Transmission electron microscopy (TEM), scanning electron microscopy (SEM) and Fourier-transform infrared spectroscopy (FT-IR) studies showed that conversion of the CD morphology occurred. A mechanism involving Na ions

interacting with the CDs was suggested, in which carbon nanosheets were formed by CD decomposition to carbon atoms which then self-assembled to form sheets; these materials were used in electrochemical studies [68]. The synthesis of P- and N-doped porous materials (NPCNs) was also achieved by adding the above CDs to amino trimethylene phosphonic acid, and heating to 800–1000 ◦C (Figure 6). The NPCNs and porous carbon frameworks (PCFs) were used in electrochemical studies that gave exceptional behavior as metal-free ORR catalysts [69].

**Figure 6.** TEM images of NPCN-900 (i.e., carbon heated to 900 ◦C) at (**a**) 0.2μm scale, (**b**,**c**) 100 nm scale, highlighting the nanoporosity and holes in their structure (**c**), as well as an (**d**) HRTEM (20 nm scale) image of the highlighted nanopores and holes (reproduced with permission from [69], 2017 Elsevier Ltd.).

The reaction of CDs made from acetaldehyde mixed with NaHPO4 at temperatures ranging from 400 ◦C to 900 ◦C gave P-doped carbon nanosheets (P-CNSs) [70]. TEM and SEM studies of the CDs (made without the P addition) and the P-CNSs showed that sheet-like materials had been made. These materials were studied for their electrochemical behavior. They showed good sodium ion storage when used as an anode material for sodium-ion batteries [70]. The thermal conversion of CDs made from sucrose or glucose [71,72] has been monitored, and the data clearly indicated a simple morphology change to a layered carbon material with graphene-like structure. These conversion reactions are described in Section 6. In summary, numerous examples have shown that CDs can readily convert to sheet-like materials under thermal conditions.

#### *3.2. Metals Doped into CDs and Metals Supported on CDs as Catalysts*

Metals salts can be added to CDs in two different ways: (i) during the CD synthesis to give metal-doped CDs [35] and (ii) after reaction, to give metal-supported CD materials. In the literature, the catalysts in which the metal is (i) on/in the surface of a CD, (ii) covered by a carbon layer or, (iii) on a carbon layered material made from a CD, have been given various names. In this review all will be referred to as a CD/metal material, to indicate that the CD is generally smaller than the metal particle.

Different types of CD/metal composites have been studied as catalysts in a variety of organic reactions, including carbon–carbon bond formation, oxidation, reduction, hydrogenation, heterocyclic synthesis, multi-component synthesis, and simple organic conversions under light- or mild-temperature conditions (≤100 ◦C). This data has been reviewed [22,23]. Furthermore, CDs have also been studied as catalysts in a variety of nano-organocatalytic and nano-photocatalytic reactions [24]. For example, Li et al. have reported on the use of novel nanocomposites made by doping CDs with a variety of metals (Cu, Zn, Co, Fe, etc.) to improve the optical and electronic properties of the CDs for use in photo-/electrocatalysis [35].

When considering data from the literature, it is clear that the mixture of metal ions and CDs at low temperatures leads to a range of possible chemical interactions. Thus, reports have shown that addition of metal ions leads to (i) coordination compounds with the CD surface [73], (ii) metal reduction by the CD surface groups [74], and even (iii) reduction of the metal by the carbon core [25]. These reactions are discussed below.

The ability of the CD core to reduce a metal is determined by the reducibility of the metal. Ellingham diagrams have been used to indicate the role of carbon in reducing bulk metal oxides to a metal, and some date indicating this are shown in Figure 7 [75]. Any metal oxide above the free energy of carbon in the diagram can be reduced by carbon. While it is expected that nano-sized metal oxides will have different phase diagrams and hence different free energy values from those in the Ellingham diagram, the differences will be small, and will allow for similar generalizations to be made. As can be noted, a reduction reaction can occur at T < 100 ◦C for some metals. Further, the addition of metal ions to carbon, where the metal appears below the carbon free energy line, will not be expected to bring about a reaction with carbon. Post analysis of reactions to support the above, where reported, suggest no reaction with the carbon CD core has occurred and that the CDs have retained their size/shape [35].

**Figure 7.** Ellingham diagram showing some metal oxides and carbon.

#### 3.2.1. High-Temperature Reactions of Metals Supported on CDs

Studies have shown that when high-temperature reactions (T > 400 ◦C) are used to react metal ions with large spherical carbon materials, the carbon itself acts as a reducing agent and in so doing gives a surface with the metal particle 'embedded' in the carbon. For example, the reaction of Co ions with carbon spheres (d = *ca*. 450 nm) under an inert gas, at T *ca.* 450 ◦C, produced Co/C catalysts with small reduced Co particles for use in the Fischer–Tropsch (FT) reaction (Figure 8). These particle sizes compared with the sizes of Co particles produced under H2 gas, but with a better ability to prevent Co agglomeration during the FT reaction [76]. The above suggests that the reduction of metal ions on carbon should be possible on smaller carbon spheres, the CDs. However, as the size of the carbon sphere is reduced, there comes a point at which the carbon and the metal particle will have similar sizes, and this could influence the resulting metal–carbon interaction.

**Figure 8.** An illustration showing the reaction of CoOx with a carbon sphere (cobalt oxide = ; cobalt metal = ).

Indeed, the study of reactions in which the support is comparable in size to the metal is referred to as inverse support catalysis (Figure 9) [77–79]. While this concept is well known when metal oxide supports are used (SiO2/TiO2/Al2O3), this concept has rarely been exploited with carbon as the support.

**Figure 9.** An illustration of (**a**) a conventional support; where small metal catalyst particles are dispersed on a large surface area support, and (**b**) an inverse support; small amounts of a support material are dispersed on the surface of a metal catalyst (forming "nano islands" around the metal).

A previous attempt was made to study catalysts made with Co and CDs (with similar small dimensions) in the Fischer–Tropsch reaction [28]. In the reaction (220 ◦C/10 bar pressure) the CDs were found to decompose, and this led to Co agglomeration. It is clear that both surface groups and the CD core were altered/removed in the reaction. The residual carbon support showed no CDs, and the CDs were completely transformed into a layered carbon material, as shown in Figure 10. The changes to the CDs were accompanied by the simultaneous reduction of the Co active metal phase. This data clearly illustrated how

the support material changed alongside the active metal during the reduction treatment. It is unclear if the Co metal dictates the changes to the CDs, or vice versa.

**Figure 10.** TEM image of CoX/CDs after reaction [28].

The data are consistent with the high-temperature studies of CDs in the absence of a catalyst (see [71]). This could limit the potential use of CDs as metal supports under reducing conditions, but may open up new ways of making metals supported on carbons with unexpected morphologies.

Other studies have also appeared in the literature in which similar observations have been made. CDs were prepared from citric acid and ethylenediamine. To the CDs was added nickel nitrate, and the mixture annealed under a nitrogen atmosphere at 300, 400, 500, and 600 ◦C for 3 h. The catalysts were used for the nitro-reduction of halogenated nitrobenzenes. The Ni@NCDs (NCD = nitrogen-doped CD) exhibited a nanosheet structure with Ni nanoparticles (6.88 nm) embedded in the NCDs, as observed in high-resolution TEM (HRTEM) images (Figure 11). Ni metal particles could be seen forming from NiO, even at 300 ◦C (detected by XRD studies) and the NiO had disappeared by 600 ◦C [80].

**Figure 11.** HRTEM image of Ni encompassed by carbon layers made from CDs (adapted with permission from American Chemical Society [80]).

Studies have also shown that CDs can be added to metal complexes to produce carbon covered metal oxide composites. Thus, the addition of CDs (made from acetone) added to TiCl3 (and CTAB) after annealing at 800 ◦C gave carbon-covered TiO2 (Figure 12). This provides an excellent method of producing total coverage of the TiO2 by carbon. The petal-like structures were used to 'store' sodium ions for use in battery studies [81].

**Figure 12.** (**a**) TEM and (**b**) HRTEM images of titania covered by carbon layers made from CDs. Adapted with permission from [81], 2016 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim, Germany).

These results are consistent with the data from Ellingham diagrams discussed above.

#### 3.2.2. Low-Temperature Reactions of Metals Supported on/in CDs

Many reactions have been studied on metal-CD composites in catalysis at temperatures lower than 150 ◦C. In these studies, the CDs act as a reducing agent for the reduction of a metal salt (and as a capping agent) that is to be used as a catalyst. Any pre-existing O-H groups on the CDs are sufficient to reduce metal salts and metal oxides into a lower oxidation state, and even to the metallic state. It is less likely that the carbon core itself acts as a reducing agent at these low temperatures, but this will be influenced by the metal under study. In general, very few post-reaction studies have been reported to evaluate any changes that may have occurred at the surface or to the carbon after the use of the CD in chemical reactions.

#### - Examples where the CD structure is retained

Since CDs can reduce metals, it is important to use appropriate reaction conditions and metals (or metal oxides) to make CD/metal composites. Examples where CDs have been loaded onto metals (or metal oxides) and retained their morphology are given below. For example, simple coordination of metal ions onto the CD surface has been shown by studies on single-atom (Fe) catalysts that were added to CDs [82]. The coordination of Fe3+ with the carboxyl groups on the CD was studied by the extended X-ray absorption fine structure (EXAFS) technique, and results clearly indicated that the metal had not been reduced and that the Fe was directly linked to the CD [73]. The 'coordination' of CDs with metal oxide particles has also been observed, e.g., with Fe3O4 particles [83].

Studies in which the surface groups on CDs have been used to reduce metal ions and then bind metal particles to the CDs are well known. Ferrocene, acetone and hydrogen peroxide were used to make a CDs/Fe3O4@CS product in a solvothermal reaction that showed CDs (3.9–9.8 nm) and Fe3O4 embedded in a carbon sphere (CS). The average diameter of the nanocomposite, which resembled a pomegranate fruit, was 451.9 nm (Figure 13). The HRTEM studies clearly showed the co-existence of the CDs and the FeOx particles. The CDs/Fe3O4@CS were used in peroxymonosulfate, persulfate and H2O2 studies, with and without visible-light illumination, and in ibuprofen degradation studies [84]. Interestingly, when commercial Fe3O4 particles and glucose/acetic acid (to make CDs) were heated at 140 ◦C for various time periods (4 h–18 h), the products formed showed CDs attached to the Fe3O4 particles [85]. In contrast, glutaric acid-functionalized Fe3O4 particles (14–20 nm) when reacted with CDs (2.5 nm) made from polyacrylamide at 270 ◦C gave Fe3O4 particles encapsulated by carbon layers [86].

**Figure 13.** (**a**) TEM and (**b**) HRTEM images of a CDs/Fe3O4@CS catalyst at different magnifications. Adapted with permission from [84], Copyright 2020 Elsevier.

CDs were produced from ethylenediamine and citric acid, and these were added to copper acetate solutions in different CD/Cu ratios. The photocatalytic reaction of copper acetate and CDs produced Cu/CDs (4–6 nm), as detected by HRTEM studies (Figure 14). The interaction of the Cu with the CD can be clearly seen in the image. The larger the CD/Cu ratio, the larger the particles that were formed. The Cu/CD catalysts were used for the photocatalytic hydrogen evolution from lactic acid solutions [87].

**Figure 14.** (**a**) TEM and (**b**) HRTEM images of Cu/CDs NPs (reproduced with permission from [87], Copyright 2017 Elsevier.

In another study, the polyoxometalate (POM) clusters, Na6[H2PtW6O24] and Na6[H2PtMo6O24], and carbon dots (CDs) were added together, and were used in the hydrogen oxidation reaction (HOR) in acid [88]. The CDs (*ca.* 5 nm) were made by electrolyzing graphite rods. The CDs improved the catalytic performance of the POM by enhancing the electron acquisition ability of Pt. The HRTEM image (Figure 15) clearly shows well-dispersed CDs on the POM.

**Figure 15.** HRTEM image of Na6[H2PtW6O24] with CDs observed on the surface. Adapted with permission from [88], Copyright 2021 Elsevier.

Lu et al. synthesized CDs from ethylene glycol by electrolyzing an electrolyte solution. The as-prepared CDs (particle size range: 2–4 nm) were then used to reduce HAuCl4 and AgNO3 to AuNPs and AgNPs, respectively, using a facile room temperature method [61]. HRTEM images showed the presence of metallic Au particles (12–14 nm). The particle size range for the Ag nanoparticles was 6 to 8 nm. In the study, analysis of the samples using FT-IR spectroscopy and X-ray photoelectron spectroscopy (XPS), before and after the reduction reaction, showed that the O-H group concentration on the CDs surface were significantly reduced, while the carbonyl group concentration increased post reduction. Additionally, the as-prepared metal nanoparticles showed good dispersibility, and this was associated with possible hydrogen bonding between the residual CD hydroxyl groups and the metal nanoparticles [61]. The CD-reduced metal catalysts were active in the colorimetric detection of H2O2 and glucose. Later, Yang et al. developed a AuPd bimetallic catalyst [89] using CDs prepared from ethylene glycol following the procedure reported by Lu et al. [61]. The reduction of the metals by CDs was also associated with the presence of the hydroxyl functional groups on the surface of the CDs. The prepared AuPd nanoparticles were tested for the catalytic reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP).

Another interesting study was conducted by Jin et al., who prepared four different types of CDs from sucrose, sucrose and acetylcholine chloride, sucrose and mercaptosuccinic acid, and sucrose and N-acetyl-L-cysteine as precursors to produce pristine CDs (with C and O functionalities only), N-doped CDs, S-doped CDs, and N–S doped CDs. The TEM results showed that doping the CDs, especially with S-functional groups, produced smallsized Ag nanoparticles [90]. The prepared AgNPs showed good antibacterial properties. CDs prepared from polyethyleneimine [91] and chitosan [92] have also been used in the preparation of stable AgNPs from AgNO3. XPS data of the Ag nanoparticles indicated the presence of C, possibly acting as a capping agent.

In another study, Pd nanoparticles (4.7 nm) supported on activated carbon were added to CDs (1.7–3.6 nm) made from citric acid and L-cysteine, to give N−S (nitrogen and sulfur) doped carbon quantum dots (N,S-CDs). The mixture was dried at 100 ◦C and the product used for the liquid-phase selective hydrogenation of ρ-chloronitrobenzene. HRTEM data

revealed that the Pd interacts with the CDs. (Figure 16) [93], and that coverage does not appear to be complete.

**Figure 16.** HRTEM image for Pd/N,S-CDs catalysts. Adapted with permission from [93], copyright 2019 American Chemical Society).

Liu et al. produced CDs made by the electrochemical ablation of graphite [94]. To a solution containing the CDs was added a HAuCl4 solution to give an Au nanoparticles/CDs catalyst used in the selective photocatalytic oxidation of cyclohexane. Similar studies were performed on Cu/CD and Ag/CD catalysts made from Ag or Cu ions, and CD mixtures. HRTEM studies showed an interaction between the CDs and the Au (and Cu and Ag) (Figure 17). This was confirmed by X-ray absorption spectroscopy experiments on the Au/CDs composites when the Au/CDs composites were exposed to visible light; EXAFS data indicated Au–C interactions [94].

**Figure 17.** HRTEM images of (**a**) Au/CD, (**b**) Ag/CD and (**c**) Cu/CD catalysts. Adapted with permission from [94], copyright 2014, American Chemical Society.

The above examples show that CDs make good reducing and stabilizing agents for different metals. In summary, it appears that in the examples described above, the CD-metal interaction involves (i) simple coordination chemistry and (ii) reduction of the metal by CD surface functional groups. Additionally, the metal nanoparticles produced usually have uniform size, are well dispersed and have good stability (stable for months, post-synthesis).


The examples below show studies in which the CDs have lost their morphology during reaction between a metal salt and the CD. In some studies, the layered carbon takes the shape of the formed metal nanoparticles. These CD-transformed carbons, containing metals, were used in further catalytic studies.

Ag nanoparticles with estimated particle sizes of 40 nm were produced by reducing AgNO3 in a solution containing CDs, at 50 ◦C for 5 min [95]. The CDs (~2–6 nm sizes), contained −OH and −NH2 surface groups, and it was believed that these groups were responsible for the reduction of the metal salt into metallic Ag. Post analysis of the CDs after reduction was reported. The resulting CDs-Ag nanocomposite had a core-shell structure (called Ag@CD), with the Ag nanoparticles encapsulated inside the CD layers. The thickness of this carbon shell was found to be about 2 nm (see Figure 18a). HRTEM data analysis confirmed the conversion of the Ag salt to metallic Ag. The Ag, surrounded by C, was successfully used for the catalytic oxidation of TMB in the presence of H2O2, the plasmon-enhanced-driven photocatalytic reaction of p-nitrothiophenol (PNTP) into 4,4 -dimercaptoazobenzene, and the catalytic-driven reduction of PNTP to PATP in the presence of NaBH4 [95].

**Figure 18.** (**a**) TEM images (**a1**) HRTEM image and (**a2**) C and Ag element mapping of Ag@CDs (copyright 2016, American Chemical Society [95]), and (**b**) Pd@CDs nanocomposite (copyright 2013, Royal Society of Chemistry [25]), showing that the CDs are transformed into thin shells that encapsulate the metal catalysts.

Interestingly, Dey at el. produced CDs of estimated particle sizes of 6.6 nm from clotted cream. The CDs were then mixed with H2PdCl4 and refluxed at 100 ◦C for 6 h. From this, a nanocomposite of Pd@CDs was formed, and the HRTEM revealed that the reduced Pd nanoparticles were encapsulated inside a ~3.8 nm carbon layer (Figure 18b). Reduction of the Pd salt to metallic Pd was confirmed by XRD data [25]. The Pd@CDs was tested for activity in the Heck and Suzuki-based coupling reactions, e.g., in the reaction of phenylboronic acid and bromobenzene to give biphenyl. Beyond4hreaction time, the catalyst deactivated. PVP was then added to the Pd@CDs to further enhance the dispersibility of the catalyst. This led to an improved conversion of biphenyl from 45% to 95% and a >4 h reaction time, presumably due to a limited agglomeration of the Pd [25].

A study by Zhang et al. showed that CDs produced from chitosan could reduce Rh3+ to its active catalytic form, Rh0 [96]. The CDs were synthesized using a microwaveassisted hydrothermal reaction of chitosan, and the obtained CD particle size was *ca.* 9.6 nm. The prepared CDs were then mixed with RhCl3·3H2O and allowed to react for 1 h at 120 ◦C. Post reaction, the sample was analyzed using XRD, and the data showed the presence of metallic Rh0. TEM results showed that the resulting Rh nanoparticles and CDs formed clustered structures, and their average particle sizes after synthesis at two different CD:Rh salt concentrations (4:1 and 6:1), were 23.4 nm and 27.8 nm, respectively. The TEM images recorded in the study are shown in Figure 19, and were reported to show

a close interaction of the Rh particles with the CDs. The prepared catalysts were used to hydrogenate polybutadiene (HTPB) and hydroxy-terminated butadiene-acrylonitrile (HTBN), and showed a high degree of hydrogenation at 80 ◦C [96]. The CDs also acted as a stabilizing agent during the reactions.

**Figure 19.** HRTEM images of Rh-CD composites prepared using (**a**) 4:1 and (**b**) 6:1 CDs:Rh salt (reproduced with permission from [96], copyright 2017 Elsevier).

CD precursors, when mixed with ZnCl2, led to the formation of graphitic sheet layered materials in which the Zn is said to link the CDs together. In the absence of Zn, CDs are formed (from citric acid/urea/autoclave at 180 ◦C). The sheet size was affected by the zinc/carbon ratio, where an increase in Zn produced larger graphene sheets [97].

In summary, it is still not clear as to how the conversion of CDs to the carbon sheets takes place. Loss of the functional groups must lead to a change in the CD morphology, but this in itself would not lead to an obvious stitching of the carbon layers. Also, at high temperatures, a competition will exist between carbon oxidation (by the metal oxide) and carbon stitching.

#### 3.2.3. Post Reduction of CDs as Metal Supports

Post reduction of a CD to give CDs that have been used to support metals have been reported. In the post-reduction process, C=O groups are converted to CH2OH groups (Figure 20) [62,63,98]. For example, Wang et al. obtained CDs by treating carbon black ("lampblack") in acid under reflux. The obtained CDs were further treated with NaBH4 to produce r-CDs with approximate particle sizes of 3.4 [98]. It was observed that the C=O groups found in the pristine CDs were reduced to C-OH groups. The r-CDs were used for the synthesis of Au metal nanoparticles. When HAuCl4 was mixed with r-CDs and heated at different temperatures (40, 60 and 80 ◦C) for 24 h [98], the obtained Au nanoparticles had averages sizes of 7 ± 2.1 nm, 16.4 ± 3.8 nm, and 15.9 ± 4.2 nm, respectively. Interestingly, it was observed that the r-CDs were oxidized back to CDs after reaction with the metal salt; the resulting CDs showed an increase in C=O peaks, as detected in the photoluminescence emission spectrum. No post synthesis of the CD-derived Au (e.g., by TEM analysis) was performed. The obtained Au nanoparticles were kept for 6 months without any aggregation. It was believed that the CDs acted as capping agents for the resulting Au nanoparticles, leading to their stability. However, there was no TEM evidence seen for a carbon layer. These nanoparticles were used for the catalytic oxidation of 3,3 ,5,5 -tetramethylbenzidine (TMB) by H2O2, and the CD-reduced Au nanoparticles showed superior performance to Au nanoparticles obtained after treating HAuCl4 with citrate.

**Figure 20.** Conversion of CDs to r-CDs by C=O reduction to CH2OH groups (adapted with permission from [62], copyright 2015 Royal Society of Chemistry).

Zhuo et al. synthesized CDs from cysteine using a microwave-assisted hydrothermal reaction. The CDs were then reduced using sodium borohydride [62]. The average particle sizes of the CDs and r-CDs were 2.0 and 2.3 nm, respectively. The FT-IR spectra of CDs showed a C=O peak at 1639 cm−1, which shifted to 1651 cm−<sup>1</sup> in the r-CDs, signifying a reduction of the C=O bonds. Additionally, the C=O peak at ~287.9 eV in the XPS was reduced significantly in size for the r-CDs sample, while the O-H peaks increased, further confirming the reduction of the CDs [62]. The reduced CDs (r-CDs) were then used to reduce AgNO3 and HAuCl4 to metallic Ag and Au [62]. The obtained r-CDs-Ag and r-CDs-Au materials had average particle sizes ranging between 6–10 nm and 2–3 nm, respectively.

In summary, the ability to pre-reduce CDs should permit an enhanced ability to reduce metal ions for catalytic reactions.

#### **4. Techniques to Evaluate CD and CD/Metal Transformations**

Many different techniques can be used to establish the structure and composition of CDs and CD/metal composites. However, of importance in CD studies is the post analysis of the carbons. As noted in the sections above, the CDs can be modified during reaction. Electron microscopy studies (TEM/SEM) are the most useful techniques to evaluate changes in the CDs, as the CDs typically have dimensions <10 nm. TEM analysis in particular provides a means of establishing if the spherical CDs change into layered materials or core/shell structures. However, it has been noted that CDs may sometimes not be seen in microscope images because of their low contrast, relative to the substrate used [92].

HRTEM has been used extensively to indicate these changes. Various spectroscopic techniques have also been used to determine the bulk and surface structure of CDs and the changes in the CDs with temperature after chemical reactions. These include infrared, photoluminescence, solid state NMR and Raman studies, as well as XPS studies. However, the changes that occur do not necessarily provide definitive data on the difference between CDs and their conversion to layered materials. Careful analysis does show changes in the Raman D and G band intensity ratios, as well as changes in NMR and IR data that relate to C=O/C-O/COOH ratios. XRD studies can follow the changes in the carbon by monitoring the carbon–carbon layers in the structure. The 002 peak in the PXRD data tends to be broad, but does shift as the structure is modified. The use of total X-ray scattering experiments enables the collection of data on the local structural order that exists within the CDs. This means that useful data can be extracted from nano-sized CDs, since a total scattering experiment is insensitive to structural disorder and insensitive to their small size [71,72].

Traditional laboratory-based XRD studies can follow the changes in the carbon by monitoring the carbon–carbon layers in the structure. The 002 peak in the XRD data tends to be broad, but does shift along the x-axis, as the structure is modified. Similarly, total X-ray scattering experiments provide data on the local structural order that exists within the CDs. This type of data contains both Bragg and diffuse X-ray scattering, which allows for a comprehensive analysis of the CD atomic structure. This means that useful information can be extracted via the pair distribution function (PDF) analysis of the data collected on nanosized CDs, which produce primarily diffuse scattering. This is due to the technique being insensitive to structural disorder and the small size inherent to CDs [71,72]. Accordingly, PDF analysis of high-energy synchrotron X-ray data typically produces information that is more accurate and provides a more representative description of the nanoscale structure of the CDs than what is currently achievable with standard laboratory-based X-ray equipment. Consider the radial functions that were extracted from synchrotron X-ray data that were collected on a set of CDs, each of which was calcined at different temperatures (Figure 21).

**Figure 21.** A comparison of the radial distribution function extracted from total scattering data collected on a set of CDs prepared as a function of calcination temperature.

The data in Figure 21 shows the general similarities of the CDs in this sample set up to r = 3Å. The peak at r = 1.38 Å is due to the carbon–carbon double bonds in the structure of the CDs. Beyond this point in the data, departures are seen, as the sample calcined at the highest temperature (700 ◦C) has better long-range order, with perturbations in the data that extend further afield than when compared to that of the sample calcined at lower temperatures (200, 250 and 400 ◦C). From this qualitative analysis of the data it is possible to show that an increased calcination temperature produced increasingly more crystalline CDs. By comparison, similar data collected on these samples using ordinary XRD data would typically have shown some evidence of these features with long data acquisitions. However, owing to its brilliance, the sensitivity of the data and speed with which they are generated at a synchrotron are unmatched.


A consideration of papers published in the area leads to some generalizations on metal-CD mixtures that can be made, and the role this will have in catalysis.


#### **5. Future Directions**

It is hoped that the review has presented some useful thoughts on the use of CDs as a metal support in catalysis. It is clear from the many reports and reviews in the literature that the area is a fruitful one for further studies.

There are a number of key issues that need to be addressed to provide an understanding of the interaction between CDs and metal, i.e., when the CD-metal is retained and when the CD converts to another morphology in the presence/absence of a metal.


The review also indicates that post analysis of CD/metal catalysts is needed; the current stability repeat studies do not necessarily give information on the morphology of the active catalyst.

#### **6. Conclusions**

The use of CDs and metal-CDs has been extensively reported in the literature. The data suggest that the CDs (as carbocatalysts), when studied at low temperatures (<150 ◦C) appear to retain their morphology in the reactions. It is possible that morphology changes could occur at the higher temperature, but lack of post-catalysis data has generally not allowed for this to be confirmed. Further, while the surface groups can be modified in the reaction, this does not appear to influence the carbon core. At higher temperatures, the CDs are converted to sheet-like carbons. These new carbons have also been studied as catalysts, e.g., in electrochemical reactions.

Metal-doped CDs also appear to act as classic carbocatalysts/metal catalysts in lowtemperature catalytic reactions. High-temperature studies could provide information on CD morphology changes influenced by the metal dopants.

The addition of metals to CDs to make different metal-CD catalysts appears to be dependent on the metal reducibility and the CD reduction ability. If high temperatures are used, the metal/metal salt can react with the carbon core and remove or change the morphology of the carbon.

The use of temperature/pressure/carbon functional groups to produce retention/ transformation of the CD structure thus provides exciting possibilities for further studies in this area of catalysis using CDs.

**Author Contributions:** Conceptualization: N.J.C. and L.L.M.; Data curation and formal analysis: N.J.C., L.L.M. and R.P.F.; Writing—first draft: L.L.M.; Funding—N.J.C.; Writing—review & editing: N.J.C., L.L.M. and R.P.F. All authors have read and agreed to the published version of the manuscript.

**Funding:** We wish to thank the University of the Witwatersrand, the DSI-NRF Centre of Excellence in Catalysis (c\*change) and the NRF for financial support.

**Acknowledgments:** We wish to also acknowledge the European Synchrotron Radiation Facility (ESRF) for provision of synchrotron radiation facilities under proposal number MA-5435, and we would like to thank Jonathan Wright for assistance and support in using beamline ID11 (DOI: 10.15151/ESRF-ES-1028463059).

**Conflicts of Interest:** The authors declare no conflict of interest.

#### **References**


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