A Review on the Synthesis of Carbon Dots and Their Applications in Environmental Analysis

Abstract

Carbon dots (CDs) have garnered attention for their potential applications across diverse fields. This is attributed to their characteristics, which include abundant raw material sources, uncomplicated surface modification, cost-effectiveness, excellent fluorescence, low toxicity, and biocompatibility. In this review, we have introduced the top-down and bottom-up synthesis techniques of CDs. Then, we discuss their physical, chemical, and optical features and focus on their diverse applications in environmental analysis, including metal ion sensing, contaminants detection, photocatalysts, and other aspect. We presented conclusions and future perspectives on the challenges of CDs. The review will provide insights into the evolving landscape of CD research and its pivotal role in advancing environmental analytical techniques.

1. Introduction

With the development of science and technology, a diverse array of novel materials continues to emerge constantly. In these materials, carbon dots (CDs) have appeared as a fascinating class of fluorescent nanomaterials with promising applications in various fields, such as bioimaging, sensors, anti-counterfeiting, and photocatalysis, showcasing their versatile potential [1,2,3,4,5,6,7]. The seminal work by Xu et al. in 2004 [8] showcased the preparation of single-walled carbon nanotubes utilizing gel electrophoresis purification techniques and arc discharge methods. This pioneering research led to the discovery of carbon nanoparticles with inherent fluorescent properties, sparking widespread interest and exploration in this field. For example, the rise of fluorescent carbon dots.

Generally, CDs are carbon-based nanoparticles that are found in aqueous solutions or other suspensions and can be categorized broadly into several types [1,9,10]. Graphene quantum dots (GQDs) consist of nanoscale graphite layers with surface and edge functional groups or interlayer defects, appearing anisotropic with a lateral size larger than their height. Their optical characteristics are mainly influenced by the size of π-conjugated structural domains and surface/edge structure. In contrast, carbon nanodots (CNDs) encompass two subclasses: carbon nanoparticles (CNPs) and carbon quantum dots (CQDs). CQDs and carbonated polymer dots (CPDs) typically feature spherical cores linked to surface moieties. The spherical cores of CQDs exhibit a multilayer graphite structure, with their photoluminescence properties primarily determined by intrinsic luminescence and the quantum confinement effect of their size. Most carbon dots consist of sp2/sp3 hybrid carbon nuclei with surface functional groups. As the size of CDs decreases to the nanoscale, quantum confinement effects become more prominent. Smaller-sized CDs confine the motion of charge carriers, leading to a quantization of energy levels and resulting in tunable photoluminescence properties. The size of CDs directly influences their bandgap energy [10].

These nanoparticles exhibit exceptional optical attributes, including tunable fluorescence emissions, and remarkable photostability, rendering them as prime candidates for cutting-edge imaging, sensing, energy conversion, and antibacterial applications [9,11,12,13,14,15]. The distinctive optical features of CDs stem from the quantum confinement effect and the presence of surface functional groups [16]. The quantum confinement effect engenders size-dependent optical characteristics, facilitating tunable fluorescence emissions across a broad spectral range [17]. Moreover, the surface functional groups, comprising hydroxyl, amino, and carboxyl groups, not only enhance the solubility and stability of CDs but also offer avenues for facile surface modifications, amplifying their versatility and applicability [18]. In particular, they have been applied to environmental analysis, including the detection of metals, organic pollutants, contaminant degradation, etc.

CDs possess remarkable potential in the realm of environmental applications due to their intrinsic characteristics [19]. Primarily, their unique optical properties play a pivotal role in their environmental utility [7,20,21]. With continuous and wide absorption spectra, CDs offer versatile options for sensing a broad range of pollutants and contaminants [16]. Their strong fluorescent activity enables sensitive detection methods, while their excellent photostability ensures reliable and long-lasting performance in environmental monitoring systems. Moreover, the highly tunable photoluminescence of CDs allows for tailored detection strategies, optimizing their use in various environmental analyses. Additionally, the outstanding optoelectronic performances of CDs enhance their effectiveness in advanced environmental sensing technologies, contributing to precise and efficient pollutant detection and monitoring efforts.

Further enhancing their applicability, CDs exhibit a diverse range of beneficial physicochemical properties attributable to the abundant surface functional groups and versatile synthetic approaches used in their production [22]. These surface functional groups not only enable facile functionalization for specific environmental applications but also contribute to the superior water dispersibility of CDs, essential for their integration into aqueous environmental systems. The ultra-small sizes of CDs, coupled with their large surface areas, enhance their reactivity and adsorption capabilities, making them effective tools for pollutant removal and environmental remediation processes. Additionally, the exceptional charge transport properties of CDs enable efficient electron transfer processes, facilitating rapid and accurate detection of environmental pollutants and contaminants.

Moreover, the cost-effectiveness of CDs, combined with their high biocompatibility and environmentally friendly nature, further underscores their suitability for environmental applications [23,24]. Their low cost makes them accessible for widespread use, while their biocompatibility ensures minimal adverse effects on biological systems, making them ideal for applications involving environmental monitoring in diverse ecosystems. The environmentally friendly attributes of CDs align with sustainable practices, emphasizing their potential to contribute to green technologies for environmental protection and conservation efforts [25,26,27].

These distinctive qualities collectively position CDs as versatile and effective tools for pollutant monitoring and removal, for the development of innovative water treatment membranes, and as antibacterial materials combating pathogenic bacteria in environmental settings. For example, Mohandoss et al. [28] synthesized 2–8 nm NFS-CDs with excellent stability and excitation-dependent fluorescence. The NFS-CDs showed strong selectivity and sensitivity for Hg²⁺ and exhibited a linear dynamic detection range from 0 to 10 × 10⁻⁶ M, with a lower detection limit of 18.0 and 67.5 × 10⁻⁹ M, respectively, at 490 and 580 nm. Long et al. [20] have reviewed the application of CDs in pollutant sensing, contaminant adsorption, membrane separation, and pollutant degradation, and as antimicrobial agents. Xiao et al. [29] underscore the broad potential of CDs in wastewater treatment and outlines future directions for CDs in this domain, emphasizing innovations in catalytic performance, cost efficiency, and practical applications. Hebbar et al. [22] have focused on the synthesis methods of CDs and discussed their use in the detection of heavy metals and various contaminants. With the evolving integration of nanotechnologies, CDs are poised to have a growing influence on environmental technologies. Hence, it is crucial to provide a thorough review of the latest CD advancements in environmental analysis. Thus, in this review, we will introduce the recent progress of synthesis for CDs and summarize the developments of CDs in different environmental analysis applications.

2. Synthesis of CDs

There exist numerous techniques for preparing CDs, which can be categorized into two main groups based on their preparation processes: top-down and bottom-up methods (Figure 1). The top-down approach involves the cutting or exfoliation of larger carbon-based materials like carbon powder, graphite rods, carbon nanotubes, and graphene, among others, to yield CDs with nanoscale dimensions [30,31]. The bottom-up method involves the assembly of numerous small organic carbon atoms to generate carbon nanoparticles. Typically, organic compounds or small molecule oligomers serve as the primary carbon sources [32]. The advantages and disadvantages of different methods of CD synthesis are compared in Table 1.

2.1. Top-Down

The top-down preparation methods encompass arc discharge, electrochemical etching, chemical oxidation, laser ablation, and so on. This method offers the advantage of precise control over the size, morphology, and structure of the fluorescent CDs; however, the synthesis process tends to be relatively intricate. For instance, Nagarajan et al. [34] reviewed the advancements made toward the applications of various top-down methods in synthesizing CDs with targeted applications. They have discussed the significant breakthroughs and major setbacks during the implementation of the top-down processes toward developing CDs. Liu et al. [35] prepared CDs with sizes of 4.0 ± 0.2 nm and high crystallinity by the electrochemical oxidation of a graphite electrode in alkaline alcohols. The CDs were applied to the detection of ferric ion, with a low limit of detection of 1.8 μM (10–200 μM linear ranges). Chao-Mujica et al. [36] reported fluorescent CDs (1–5 nm size) synthesized by submerged arc discharge in water. They discussed the simplicity, natural phase separation, and scalability of this method and explored the theory of formation of those nanostructures, then proposed a general model of formation. Torrisi et al. [37] obtained nanometric CDs by using the laser ablation of vegetable carbon in a liquid medium using a pulsed diode laser operating at 970 nm. The laser irradiated a charcoal target in a phosphate-buffered saline solution with neutral pH, resulting in carbon ablation and exfoliation with an ablation yield of approximately 5 ng per laser pulse. The synthesis produced small carbon dots and larger carbon nanoparticles in the solution. Upon irradiation with a 365 nm ultraviolet (UV) lamp, the CDs exhibited visible luminescence emissions at around 478 nm, displaying a characteristic blue-green color. UV-visible-near infrared spectrum (NIR) and Fourier transform infrared (FTIR) spectroscopies were utilized to assess the liquid’s transmittance and absorbance. Transmission electron microscopy (TEM) revealed that the synthesized CDs were crystalline, spherical, and with a mean size of approximately 1.5 nm. The luminescence properties and quantum yield (QY) of the CDs in a biocompatible dispersion were analyzed for potential applications in the biological and medical fields.

Table 1. Comparison of different synthesis methods for CDs.

Methods		Advantages	Disadvantages	Refs.
Bottom-up	Thermal decomposition	More time-saving, easy to operate, low cost, large-scale production	Wide size distribution	[38]
	Hydrothermal treatment	Cheap, eco-friendly, lack of toxicity, low cost	Low yield	[39]
	Microwave synthesis	Fast, low cost, eco-friendly	Poor size control	[40]
Top-down	Electrochemical/ chemical oxidation	High yield, high purity, low cost, control over size	Few small molecule precursors	[41]
	Arc discharge	Fabricate carbon NPs in various gases	Require complex purification	[36]
	Laser ablation	convenient size control and photolumicense property	Costly, sophisticated process	[42]
	Ultrasonic treatment	Convenient to break large carbon materials, good dispersion, low crystallinity	High energy cost	[1]

Table 1. Comparison of different synthesis methods for CDs.

Methods		Advantages	Disadvantages	Refs.
Bottom-up	Thermal decomposition	More time-saving, easy to operate, low cost, large-scale production	Wide size distribution	[38]
	Hydrothermal treatment	Cheap, eco-friendly, lack of toxicity, low cost	Low yield	[39]
	Microwave synthesis	Fast, low cost, eco-friendly	Poor size control	[40]
Top-down	Electrochemical/ chemical oxidation	High yield, high purity, low cost, control over size	Few small molecule precursors	[41]
	Arc discharge	Fabricate carbon NPs in various gases	Require complex purification	[36]
	Laser ablation	convenient size control and photolumicense property	Costly, sophisticated process	[42]
	Ultrasonic treatment	Convenient to break large carbon materials, good dispersion, low crystallinity	High energy cost	[1]

2.2. Bottom-Up

In the bottom-up synthesis approach, the process involves the gradual building of carbon nanoparticles from smaller organic molecules, enabling precise control over the composition and structure of the resulting carbon dots. By utilizing organic precursors, the bottom-up method offers versatility in tailoring the properties of the synthesized CDs to meet specific application requirements. Commonly employed sources in this method include citric acid, glucose, polyethylene glycol, urea, ionic liquids, and others. This approach is a relatively simple synthesis process. The selection of organic compounds as carbon sources allows for a diverse range of precursors, each contributing distinct characteristics to the resulting carbon nanoparticles. Various bottom-up synthesis techniques include hydrothermal methods (

Table 2. CDs synthesized using hydrothermal method.

Chemosensors	Precursors	Size (nm)	QYs (%)	Fluorescence	Refs.
CDs	o-phenylene-diamine, DL-Thioctic acid	3.3 ± 0.4	21.82	Yellow	[44]
SN-CCDs	Citric acid, thiourea	4.4	30.60	Bright blue	[45]
PC-CD	Water amaranth leaves, 1-pyrenecarboxaldehyde	7.75 ± 0.51	12.10	Red	[46]
N-CDs	C. retusus fruits	5 ± 2	9.00	Blue	[27]
MP-CQDs	Persimmon pulp	3.18 ± 0.69	8.39	Blue	[47]
N-CDs	citric acid, melamine	2.58	45.00	Blue	[48]
CQDs	green pomelo peel via	5.5	17.31	Blue	[49]
NSCQDs	potato, ammonium sulphate	5	16.96	Blue	[50]
CDs	black plum leaves	0.34	15.90	Greenish	[51]
N,S-CQDs	o-amino benzenesulfonic acid, Alkali lignin	4.86	23.68	Greenish	[52]
CQDs	Tagetes patula flowers	5.15	29.88	Bright blue	[53]
CDs	m-phenylenediamine, ethylenediamine	3.5 ± 0.7	25.50	Greenish	[54]
GCDs	Delonix regia leaves	6.75 ± 2.5	7.00	Red	[55]
N/SCDs	p-Phenylene-diamine, 2-mercapto-thiazoline	8.67		Bright green	[56]
R-CDs	o-phenylenediamine, 1,8-diamino-naphthalene	1.97 ± 0.24	11	Red	[57]
RCDs	tetra (4-carboxyphenyl) porphyrin, thiourea	5.6	26.7	Red	[58]
N-CQDs	citric acid, urea	2.74	9.60	Blue	[59]
N,S-CQDs	Thiourea, citric acid	30	33.00	Greenish	[60]
CQDs	5-dimethylamino methyl furfuryl alcohol, o-phenylene diamine	2.6 ± 0.75	12.00	Yellow	[61]
N,S-CQDs	o-Phenylene-diamine, methionine	4	21.00	Blue	[62]
N-DCCDs	desiccated coconut flour	8	32.00	Blue	[63]
CM-CDs	camel milk	<10	24.60	Blue	[64]

), solvothermal methods, microwave synthesis, and the template method [42]. Hydrothermal and solvothermal techniques utilize high-pressure and high-temperature aqueous environments to facilitate the synthesis of carbon dots. The hydrothermal carbonization method is the most common one. Certain factors of this method, such as reaction temperature, time, and precursor composition, can impact the size distribution and shape of the resulting CDs. For example, Mohandoss et al. [12] prepared FNS-CDs with a size of 4 nm via one-step hydrothermal synthesis using flufenamic acid as the fluorine and carbon source and thiourea as the nitrogen and sulfur source (Figure 2). The mixture solution of precursor substances was transferred to a Teflon-lined stainless-steel autoclave to heat for 8 h. The condition for the reaction was set at 160 °C. Then, the resulting light-yellow solution was centrifuged at 10⁴ rpm for half an hour to remove the precipitates. Furthermore, the solution was dialyzed for 24 h in a dialysis bag (molecular weight cutoff 1000). FNS-CDs exhibit good water solubility and excellent pH and ionic strength stability. Additionally, the undecorated FNS-CDs exhibited high selectivity and sensitivity as nano-sensors for Cu²⁺ (linear range of 1–25 μM, detection limit of 83.10 nM). The practical application was conducted in real water samples. Microwave synthesis harnesses microwave radiation to expedite the formation of carbon nanoparticles. This method presents several advantages for the efficient production of CDs, offering rapid reaction times, energy efficiency, controlled synthesis conditions, high yields, and enhanced properties. However, challenges such as equipment costs, scalability issues, non-uniform heating, the need for precise control, and safety considerations should be considered when utilizing microwave synthesis. Li et al. [43] prepared nitrogen-doped CDs by the microwave rapid preparation approach with lipase and guanidine hydrochloride as precursors within 6 min. The CDs exhibited high sensitivity detection of hematin, and the authors further developed a rapid and accurate detection method for hematin in human blood samples, based on photoluminescence change. The template method involves the use of templates to control the size and morphology of the synthesized carbon dots. This method offers advantages such as controlled size and shape, tailored properties, enhanced stability, ease of functionalization, and potential scalability. However, challenges including template removal, limited template options, potential contamination, template-induced defects, and time-consuming optimization should be considered when utilizing this approach. In summary, the bottom-up approach offers a straightforward and versatile means of producing carbon dots with tailored properties, making it a valuable strategy in the synthesis of these fluorescent nanomaterials for various applications.

3. Properties of CDs

Due to the diversity of raw materials and synthetic approaches, CDs have properties with a wide range, including water solubility, environmental friendliness, non-toxicity, tunable fluorescence, and high stability. Herein, the physical, chemical, and optical properties of CDs will be discussed, along with their potential impact on different applications.

CDs, a kind of carbon nanoparticle less than 10 nm in size, possess oxygenated functional groups on their surfaces that alter surface structures and sizes, leading to the quantum confinement effect [1]. They excel in charge transport, effectively trapping and transferring electrons to inhibit charge recombination [65]. This superior charge transport stems from the size-dependent optical properties of CDs, where charge carriers are in close proximity, usually below the carrier diffusion length. The surface passivation can enhance the photoluminescence properties of CDs by reducing surface defects and preventing non-radiative recombination of electron–hole pairs, leading to brighter and more stable fluorescence emissions. The edge-rich features of CDs make them ideal for photocatalysis, with reactions often occurring at edges rather than at basal planes. Functionalizing the surface of CDs can improve their chemical stability and solubility, protecting them from oxidation and degradation, which can extend their shelf life and enhance their performance in various applications. Their chemical inertness and robust aqueous stability further enhance their utility in nanotechnology and sensor development.

Although CDs can vary in physicochemical attributes, they often exhibit similar optical properties, especially concerning fluorescence and ultraviolet absorption [20,66]. Typically, CDs display robust ultraviolet absorption, extending into the visible spectrum, with peak absorption around 260–320 nm [16]. Chemical groups within CDs contribute to ultraviolet-visible absorption. Discrepancies in absorption spectra suggest differences in composition or structure among various hybrid derivatives. Most CDs demonstrate excitation-dependent fluorescence emissions, where longer excitation wavelengths result in red-shifted emissions [67,68]. Nevertheless, some CDs exhibit excitation-independent fluorescence. Understanding the fluorescence mechanism aids in regulating CDs’ fluorescence and physicochemical properties for optimized applications. Research efforts have focused on factors like surface state, quantum size effect, carbon-core and molecular states, surface passivation/functionalization, and conjugation, all influencing the fluorescence characteristics of CDs.

4. Application of CDs

4.1. Sensing Metal Ions

The detection of metal ions stands as a critical mission of global environmental pollution monitoring, prevention, and control efforts. Elevated levels of heavy metals in the environment can exert trace toxic effects on organisms, posing significant risks to human health [69]. Due to the photoluminescence and excitation-based emissions, and the quenching and reversal of quenching processes, CDs have emerged as invaluable sensors with diverse applications in metal ion detection [1,70,71,72,73]. Leveraging their exceptional optical properties and chemical stability, the functional groups on the surface of CDs form interactions with various metal ions, leading to distinctive changes in their fluorescence properties [71,72]. These changes often manifest as shifts in emission intensity, wavelength, or fluorescence lifetime in the presence of specific metal ions. At present, the mechanisms of CDs for metal ion detection are reported, including inner filter effect, electron transfer, aggregation-induced emission enhancement effect, fluorescence resonance energy transfer, dynamic quenching effect, aggregation-induced emission quenching effect, and static quenching effect [74].

By exploiting the unique interactions between CDs and metal ions, sensitive and selective detection methods can be developed for a wide range of heavy metal pollutants. For instance, Yoo et al. [75] reviewed the CDs as an effective fluorescent sensing platform for metal ion detection and discussed some synthetic methods for preparing CQDs (carbon quantum dots) and the progress in CD research, with an emphasis on their application in heavy metal sensing (Figure 3). Tang et al. [76] reported CDs that can act as sensors to detect chromium (Cr⁶⁺) and mercury (Hg²⁺). Waste polyester–cotton blended fabric was used as a carbon source, and CDs were synthesized by a thermal agent approach in the mixed solution of ethylene glycol and sulfuric acid. Grounded on the inner filter effect, the fluorescence quenching of CDs was induced to detect Cr₂O₇²⁻ with a low limit of detection (LOD). Wang et al. [77] reported nitrogen (N)-doped CDs (N-CDs) with high QY (60.51%). The CDs were prepared by the microwave method with citric acid (carbon source), urea (nitrogen source), and glycerol. They characterized N-CDs and found that N-CDs can perform as an excellent fluorescent probe for the detection of Hg²⁺ (detection limit is 0.08 µmol/L). Xu et al. [78] prepared an orange-red CD (OR-CD) with salicylic acid and 5-amino-1,1-phenanthroline and applied it to detect Cd²⁺. Raypah et al. [79] used a biomass from the root extract of Polyalthia bullata to synthesize CDs by the hydrothermal method. The CDs exhibited remarkable fluorescence and were employed as a nanoprobe for Fe³⁺ ions in real samples, including tap, farm, and lake water, yielding a limit of detection of 0.186 µM. With the diversification of CD preparation, more and more CDs are applied to metal ions detection, as listed in

Table 3. CDs used to detect different metal ions in past 3 years.

Metal Ion	Chemosensors	Precursors	Synthesis Technique	QYs (%)	Fluorescence	Linear Range (mM)	LOD (nM)	Refs.
Au3+	CQDs	Robinia hispida L. flowers	Hydrothermal	5.13	Blue	500–3500	400	[80]
Cr6+	Cu-CDs	Citric acid, urea, copper chloride	Hydrothermal	27.3	Blue	0–80,000	186	[81]
Cr6+	NCDs	Salicylic acid, o-phenylenediamine	Hydrothermal		Yellow	0–90,000	190	[82]
Cr6+	SN-CDs	Citric acid, thiourea	Hydrothermal	30.60	Bright blue	0–0.12	428	[45]
Cr6+	N,Zn- CDs	Zinc nitrate hexahydrate, citric acid monohydrate, 4-pyridinecarboxaldehyde	Microwave-assisted	13.60		0.000005–0.000135	0.47	[83]
Cr6+	N-CQDs	Natural Goji Berry	Hydrothermal		Greenish	0–0.04	160	[84]
Cr6+	R-CDs	o-phenylene-diamine, p-Acetylamino benzene sulfonyl azide	Solvothermal strategy	8.15	Red	0.004–0.04	80	[85]
Cd2+	PC-CD	Water amaranth leaves, 1-pyrenecarboxaldehyde	Hydrothermal	12.10	Red	0–0.07	15	[46]
Fe3+	N-CDs	Aegle Marmelos, urea	Microwave assisted synthesis	14.21	Blue	0.001–0.004	148	[24]
Fe3+	N-CDs	C. retusus fruits	Hydrothermal	9.00	Blue	0–0.002	70,000	[27]
Fe3+	MP-CQDs	Persimmon pulp	Hydrothermal	8.39	Blue	0–0.09	324	[47]
Fe3+	N-CDs	Peony seed residue, o-phenylene-diamine	Hydrothermal		Orange -red	0–0.05	20	[86]
Fe3+	N-CDs	citric acid, melamine	Hydrothermal	45.00	Blue	0.02–0.08	3180	[48]
Fe3+	N-CDs	citric acid, urea	Microwave assisted	18.10	Greenish	0.001–0.08	1000	[87]
Fe3+	CQDs	green pomelo peel via	Hydrothermal	17.31	Blue	0.0001–0.16	86	[49]
Fe3+	NSCQDs	potato, ammonium sulphate	Hydrothermal	16.96	Blue	0–0.5	260	[50]
Fe3+	CDs	black plum leaves	Hydrothermal	15.90	Greenish	0–0.08	130	[51]
Fe3+	CQDs	Tagetes patula flowers	Hydrothermal	29.88	Bright blue	0–0.004	320	[53]
Fe3+	CQDs	Castor leaves	Hydrothermal			0–300,000	19,100	[88]
Fe3+	N-CDs	Hedyotis diffusa willd	Hydrothermal	15.2	Blue	10–100	6.62	[89]
Fe3+	CDs	Polyalthia bullata	Hydrothermal	7.55	Blue	0–57,000	186	[84]
Fe2+	CNPs	1, 3, 5-trimesic acid, o-phenylenediamine	Solvothermal	15.17	Bright yellow	10–160	88	[90]
Fe2+	CDs	m-phenylenediamine, ethylenediamine	Hydrothermal	25.50	Greenish	0.033–1.044	16,000	[54]
Fe2+	CQDs	Mango leaves	Pyrolysis	18.20	Blue	0–0.01	620	[25]
Pb2+	GCDs	Delonix regia leaves	Hydrothermal	7.00	Red	0.01–0.18	3.3	[55]
Pb2+	R-CDs	Rice straw	Thermolysis	44.29	Blue	1000–100,000	110	[91]
Pb2+	GA-DMF CDs	Gallic acid, gallic acid	Hydrothermal		Strong green	30,000–120,000	7.15 × 105	[92]
Cu2+	N/SCDs	p-Phenylene-diamine, 2-mercapto-thiazoline	Hydrothermal		Bright green	0.005–0.4	215	[56]
Cu2+	N,S-CDs	Creatinine, thiourea, disodium edetate	Direct carbonization	60.5	Blue	400–2400	70	[93]
Cu2+	B-CDs	Citric acid, ethylenediamine	Solvothermal	10.28	Blue	250–10,000	180	[94]
Cu2+	N-CDs	Diethylenetriamine, citric acid	Microchannel	63.87		20–100	46	[95]
Cu2+	N-CQDs	citric acid, urea	Hydrothermal	9.60	Blue	0.0005–0.005	32	[59]
Cu2+	NCDs	Sida cordifolia root, triethylene tetraamine	Hydrothermal	17.80	Blue	0–0.13	110	[96]
Co2+						0.00001–0.00009	0.03
Co2+	L-CDs	emon juice, ethylenediamine	Amicroflow approach	25.40	Blue	0–0.1	318	[97]
Cr6+	CDs	3,5-dihydroxybenzoic acid, L-Arginine	Hydrothermal	20.00	Blue	0.0001–0.002	24	[98]
Hg2+						0.0004–0.005	84
Hg2+	N,S-CQDs	Thiourea, citric acid	Hydrothermal	33.00	Greenish	0.000005–0.00016	4	[60]
Hg2+	CQDs	5-dimethylamino methyl furfuryl alcohol, o-phenylene diamine	Hydrothermal	12.00	Yellow	0.015–0.1	5.2	[61]
Hg2+	N,Cl-CQDs	Triticum aestivum	Microwave synthesis	36	Blue	0–350	39	[99]
Hg2+	N-CDs	Citric acid, urea, glycerol	Microwave solvent	60.51	Blue	30,000–120,000	80	[77]
Hg2+	NCQDs	N-phenyl orthophenylenediamine, citric acid	Hydrothermal	50.5	Blue	0–2000	4.98	[100]
Hg2+	B-CDs	Tartaric acid, 1-amino-2-naphthol-4-sulfonic acid	Solvothermal method	34	Strong blue	1000–8000	175	[101]
Hg2+	CDs	Urea, citric acid	Hydrothermal	21.3	Bright Blue	0–500,000	8.2	[102]
Hg2+	M-NCDs	Urea, D-glucose	Microwave assisted	14.90	Blue	0.00003–0.0001	3.5	[103]
Hg2+	N,S-CQDs	o-Phenylene-diamine, methionine	Hydrothermal	21.00	Blue	0–0.018	120	[62]
Hg2+	N-DCCDs	desiccated coconut flour	Hydrothermal	32.00	Blue	0.0001–0.0002	2.4	[63]
Mn7+	CM-CDs	camel milk	Hydrothermal	24.60	Blue	0–0.5	580	[64]
Mn2+	CDs	citric acid, tris base	Microwave abetted	14.14	Blue	0.00001–0.00007	0.37	[104]
Cd2+	G-CDs	Pumpkin, acetone	Solvent heat	17.50	Greenish	0–0.0165	680	[105]
Cd2+	CDs	ammonium citrate, glutamic acid	Hydrothermal		Blue	0.001–0.025	13	[106]
Ag+	PEG-CDs	1-pyrenecarboxaldehyde, 0-phenylenediamine and PEG	Solid phase	73.90	Blue	0–0.2	0.114	[107]
Ag+	CQDs	Sugar cane molasses, L-cysteine/D-cysteine (L/D-Cys) and Rhodamine B(RhB)	Hydrothermal	6.33	Blue	22,000–220,000	140	[108]
Ce4+	NB-CDs	p-phenylenediamine, boric acid	Solvothermal	4.00	Orange	0–0.18	140	[109]
Al3+						0–0.1	1070

.

4.2. Sensing of Emerging Contaminants

The application of CDs in the sensing of organic pollutants represents a cutting-edge and promising avenue in environmental monitoring and analytical chemistry. CDs, a kind of carbon-based nanomaterial, have garnered significant attention due to their unique optical properties, excellent biocompatibility, and high chemical stability. These attributes make CDs well-suited for the detection and quantification of organic pollutants in various environmental matrices, ranging from water bodies to air samples [22]. The application of CDs as sensing platforms for organic pollutants capitalizes on their adsorption capacity for organic pollutants, which hinges on the chemical properties, the number of functional groups, and the specific surface area of CDs. The CDs interact selectively with target analytes, leading to discernible changes in their fluorescence properties. CDs serve as adsorbent materials for both cationic and anionic dyes, involving not only electrostatic interactions but also van der Waals forces in the adsorption mechanism [20]. For example, Chen et al. [110] prepared CDs by the one-step hydrothermal method and developed an optical sensing gadget with CDs to detect 2,4,6-trinitrophenol in samples of tap water and lake water. CDs can specifically discriminate 2,4,6-trinitrophenol in an aqueous medium, which is attributed to the synergistic effect of the inner filter effect and electron transfer. Additionally, they developed a portable sensing gadget, and TNP in tap water and lake water samples could be detected. Yaoping Hu and Zhijin Gao [111] converted organics in sewage sludge to CDs, which could serve as a sensitive and selective sensor to para-Nitrophenol. Due to fluorescence quenching, the linear detection range was from 0.2 to 20 μM, and a low detection limit was obtained at 0.069 μM. Tafreshi et al. [112] presented a sensing method for diazinon, glyphosate, and amicarbazone using plant-based CDs, and they obtained a detection limit of 0.25, 2, and 0.5 ng mL⁻¹, respectively. The results suggested that CDs are potential sensors for application in food safety and environmental monitoring. Ghosh et al. [113] synthesized CDs (QY = 63.7%) from Tagetes erecta flowers (TEF-CDs) by the solvo (hydro)-thermal carbonization method, which is highly selective towards chlorpyrifos and quinalphos. The fluorescence spectral changes showed a good linearity, with concentrations in the range of 0.05–100.0 μM for chlorpyrifos and 0.01–50.0 μM for quinalphos, and the limits of detection were 2.1 ng mL⁻¹ and 1.7 ng mL⁻¹, respectively. Moreover, the TEF-CDs can be applied to rapidly estimate, with a high accuracy, samples of rice and fruit. Swain et al. [114] prepared N, S-doped CDs (N, S-CDs) with Giloy stem by using the hydrothermal method and applied them as probes for the selective detection of toxic organic pollutants that include 4-nitrophenol (4-NP) and Congo red, with limits of detection of 380 and 62 nM, respectively. The mechanism was found to have an inner filter effect, and the excellent N, S-CDs can be used to detect 4-NP and CR in real tap and pond water. In addition, N, S-CDs were employed for the colorimetric and smartphone-assisted detection of Congo red. The advantages of smartphone-assisted detection include naked-eye visualization of the probe–analyte interactions, the use of a portable and easy-to-handle device, and the elimination of the need for an expensive spectrophotometer. By functionalizing CDs with specific recognition elements or surface modifications, tailored sensors can be developed to detect a wide range of organic pollutants with high sensitivity and selectivity.

Table 4. Carbon dots are used to detect emerging pollutants.

Target	Chemosensors	Precursors	Synthesis Technique	QYs (%)	Fluorescence	Linear Range (mM)	LOD	Refs.
4-nitrophenol	N,S-CDs	Giloy stem	Hydrothermal	7.2	Green	500–6000	380 nM	[114]
Congo red						10–1000	62 nM
4-nitrophenol	CDs	Pithecellobium dulce	Single-step carbonization	24	Blue	20–80	14 nM	[115]
4-nitrophenol	CDs	Crayfish shells	Hydrothermal method	10.68	Bright blue	0–50,000	160 nM	[116]
4-nitrophenol	CDs-MFMIP	Magnetic covalent organic frameworks, green persimmons	Hydrothermal	10.45	Blue	50–50,000	17.44 nM	[117]
4-nitrophenol	Cu-MOF/NGO	Cu(NO3)2·3H2O, trimesic acid	Solvothermal			500–100,000	35 nM	[118]
2-nitrophenol	C-dots	O-phenylenediamine, oleic acid	One-pot carbonization method.	19.5	Bright yellow	74–134	6.4 nM	[119]
4-nitrophenol							4.8 nM
2,4-dinitrophenol	Gl N,P-CDs	Diammonium hydrogen phosphate, ethanediamine, ganoderma Lucidum	Hydrothermal	11.41	Blue	0–30,000	73.03 nM	[120]
4-nitrophenol						0–30,000	68.09 nM
2,4,6-trinitrophenol	CDs	Dextrose, HCl	Ultrasonication	40	Bluish green	500–200,000	200 nM	[121]
2,4,6-trinitrophenol	NCDs	Pyridine, water	Solution plasma	61	Bright blue	0–50,000	10 nM	[122]
2,4,6-trinitrophenol	CDs	Pomegranate leaf	Hydrothermal	14.64	Blue	0–120,000	62 nM	[123]
2,4,6-trinitrophenol	CDs	4-(diethylamino)salicylaldehyde	Hydrothermal		Orange	1–100,000	480 nM	[110]
2,4,6-trinitrophenol	C-dots	Guanine	hydrotherma	26.8	Blue	0–30,000	58.5 nM	[124]
2,4,6-trinitrophenol	PTPEBP	Sodium dodecyl sulfate, potassium carbonate	Suzuki-miniemulsion polymerization	8.14		10,000–30,000	1070 nM	[125]
2,4,6-trinitrophenol	CDs	Sewage sludge	Microwave-assisted	21.7	Blue	200–20,000	69 nM	[111]
2-nitrophenol	CDs	Celery leaves glutathione	Hydrothermal	53	Blue	50–500	39 nM	[126]
3-nitrophenol							43 nM
4-nitrophenol							26 nM
Crystal violet	Se,N-codoped carbon dots	Selenourea o-phenylenediamine	Hydrothermal	16.7	Yellow	20–1600	7.3 nM	[127]
4-nitrophenol	CNDs	Apple seeds	Pyrolysis	20	Blue	50–53,000	13 nM	[128]
2,4,6-trinitrophenol	CDs	Abelmoschus Manihot Flowers	Hydrothermal	30.8	Blue	25–40,000	5 nM	[129]
2-nitrophenol	CQDs	Rosa roxburghii	Hydrothermal	24.8		80–40,000	15.2 nM	[130]
4-nitrophenol	N,S-codoped	Palm Shell powder triflic acid	Hydrothermal		Green	200–400	79 nM	[131]
DNP						500–850	165 nM
2,4,6-trinitrophenol						200–4000	82 nM
2,4,6-trinitrophenol	CTA-CDs	4,7,10-Trioxa-1,13-tridecanediamine, carboxymethyl cellulose sodium	hydrothermal treatment	87.3	Blue	0–100,000	704 nM	[132]
4-nitrophenol	CDs-WT	Escherichia coli	Hydrothermal	15.8	Blue	20–33,000	11 nM	[133]
4-nitrophenol	Aa N,P-CDs	Auricularia Auricula	Hydrothermal	7.25	Blue	0–37,500	198 nM	[134]
4-nitrophenol	Cl,N-CD	Shaddock peel HCl	High-temperature Carbonization and low-temperature concentrated acid (HCl) acidification	17.99	Blue	900–90,000	37.1 nM	[135]
4-nitrophenol	G-CDs	Cornus walteri leaves	Hydrothermal	18.3	Green	0–50,000	0.0175 nM	[136]
Paraoxon	CQDs	Chlorophyll	Hydrothermal			0.05–50 μgL−1	0.050 μgL−1	[137]
Chlorpyrifos	CDs	Burning ash of waste paper	Hydrothermal	20		0.01–1.0 μgL−1	3 ng mL−1	[138]
Glyphosate	CD/Fe3+	Sophora Japonica leaves	Hydrothermal	6.51	Blue	0.1–16 ppm	8.75 ppb	[139]
Thiabendazole	CDs	Rosemary leaves	Hydrothermal			0.03–0.73 μgL−1	8 ng mL−1	[140]
Diazinon	CDs	Cauliflower	Hydrothermal	43	Green	0.25–5000 μgL−1	0.25 ng mL−1	[112]
Chlorpyrios	TEF-CDs	Tagetes Erecta flower	Solvo/Hydrothermal	63.7	Blue	50–100,000	2.1 ng mL−1	[113]
Quinalphos						10–50,000	1.7 ng mL−1
Dichlorvos	F-CDs	Feather	Hydrothermal	2.4		6–60	3.8 nM	[141]
Trifluralin	CQDs	Cherry Tomatoes	Hydrothermal	9.7	Green	50–200,000	0.5 nM	[142]
Sudan I	SCQDs	Lignin	Hydrothermal	13.5	Green	0–40,000	120 nM	[143]

lists the application of the CDs used to detect emerging pollutants.

4.3. Photocatalysts

The previous sections discussed the use of CDs for metal and emerging contaminant detection. Apart from detection, environmental remediation is crucial for preserving natural resource quality. Various water treatment techniques like adsorption, membrane separation, ion exchange, and biological degradation have been developed to eliminate contaminants like dyes and antibiotics. However, these methods often exhibit low removal efficiencies and can lead to additional costs or secondary pollution. Advanced oxidation processes, including photocatalysis, have been prominent for decades, utilizing solar energy to catalyze reactions, enhancing kinetics, and maintaining mild reaction conditions. Photocatalysis involves light absorption, electron excitation and transfer, and electron–hole pair formation for oxidation and reduction reactions [144,145,146]. In cases where the catalyst has poor light absorption, photosensitizers are employed to enhance excitation. Choosing a system with the right bandgap is crucial for effective light absorption and delaying exciton recombination. Aligning the redox potentials of the photocatalyst and electron donors/acceptors or using appropriate dopants/composites are key strategies. Modifications like photosensitization using dyes and incorporating newer semiconductors, dopants, multi-layered systems, and metal promoters have been explored to make photocatalysis viable and scalable. The emergence of carbon dots has brought new possibilities. Studies have investigated substituting conventional photosensitizers with CDs, creating CD–metal oxide semiconductor hybrids, and using doped CDs to degrade contaminants. CDs have shown potential in CO₂ reduction, H₂ generation, SIWE, and treatment systems. CDs can enhance photocatalytic abilities through increased surface area, acting as electron mediators, extending absorption to visible light, suppressing exciton recombination, and improving cascade reaction rates for efficient contaminant degradation into simpler molecules like CO₂ and H₂O. For example, Xu et al. [147] prepared CDs with strong blue fluorescence. By characterizing the optical properties of CDs, it was suggested that there exist three types of photoelectrons with varying transition pathways. After adding hole scavengers, CDs could be applied to generate hydrogen by the photolysis of water. In the process, electrons and holes served as reducing and oxidizing agents, separately. Nian N. Mohammad [148] prepared P, N co-doped CDs using tire waste by the solvothermal method. They characterized the CDs and evaluated the efficacy of CDs in the photocatalytic degradation of different dyes, demonstrating notable success in degrading methyl orange within 8 h in the visible region. Sun et al. [149] prepared copper nanorods laden with coal-based N-CDs to execute the photoelectron catalytic degradation of wastewater and simultaneously produce hydrogen. In addition, compounds like aldehydes, phenols, acids, amines, ethers, alkanes, ketones, and esters in cotton pulp black liquor could be efficiently degraded within 1 h. Das et al. [150] synthesized green-emissive CQDs by a facile hydrothermal method of pear juice. The reaction was conducted in a 100 mL stainless-steel autoclave with a Teflon lining and thermolyzed for 36 h at 180 °C. The obtained product is an orange-brown, highly viscous liquid that exhibited superior photocatalytic activity under visible-light irradiation owing to the efficient light absorption, electron transfer, and separation of photogenerated charge carriers, facilitating ~99.5% degradation of methylene blue (MB) within almost 2 h (Figure 4). The main reactive species were determined to be holes and hydroxyl radicals, and a plausible mechanistic pathway was proposed on the basis of radical-trapping experiments. In addition, the as-prepared CQDs were also explored as photoluminescent probes for the selective and sensitive detection of Fe(III) and ascorbic acid.

4.4. Application of CDs in Other Aspects

In addition to metal ion detection, pollutant detection, and photocatalytic degradation, CDs can also be used for antimicrobial and membrane separation. With their unique physicochemical properties, including high surface area, excellent biocompatibility, and tunable surface chemistry, CDs offer novel opportunities for combating microbial infections and enhancing membrane separation efficiency.

Researchers have proposed various pathways of antimicrobial activity for CDs in recent years, including physical/mechanical destruction, ROS-induced effects, oxidative stress, photocatalysis, and inhibition of bacterial metabolism. CDs have been utilized for killing pathogens and bacteria, with mechanisms such as membrane damage being crucial. For example, CDs can disrupt bacterial cell walls, leading to cytoplasmic leakage and dysfunction, ultimately causing bacteriostatic and bactericidal effects [151,152]. Recent studies have shown that CDs can enter bacteria, disrupt cell walls, inhibit gene expressions, and lead to bacterial or fungal death. Additionally, positively charged CDs can interact with cell membrane components, destabilizing membranes and inhibiting membrane synthesis, resulting in bacterial death. For example, Nian N. Mohammad [148] prepared P, N co-doped CDs using tire waste by the solvothermal method. Then, CDs were applied for carrying out agar disk-diffusion assays against a spectrum of microorganisms, which revealed substantial inhibition zones against Methicillin-Resistant Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli. Furthermore, the mechanism was explored, and it was found that there was a powerful binding affinity of the prepared CDs with certain proteins associated with antibacterial activity, which suggests that the CDs effectively engage with the active sites of those proteins, indicating their potential as promising antibacterial agents.

CDs are promising nano-fillers in thin film nanocomposite (TFN) membrane fabrication due to their small size, aqueous stability, and surface functional groups. In the TFN manufacturing process, CDs are dispersed in the aqueous phase for interfacial polymerization to create TFN membranes. CDs enhance membrane properties like proton conduction, permeability, hydrophilicity, solute selectivity, and anti-fouling performance. Functionalized CDs and optimized concentrations further enhance TFN membrane performance. For instance, Song et al. [153] developed a TFN membrane with CDs for chlorine resistance by forming covalent bonds with TMC. These membranes exhibited stable performance with a 98.8% NaCl rejection rate and a 51.8% increase in permeate flux. CD-modified TFN membranes also showed improved water flux and dye rejection rates. CDs in the polymer matrix contributed to a negative ζ-potential, loose active layer structure, and enhanced water flux, making them valuable for applications like wastewater treatment and purification, with enhanced chlorine resistance.

5. Conclusions and Future Perspective

As a novel category of carbon-based fluorescent nanomaterials, CDs have garnered significant interest for their diverse source materials, exceptional physicochemical attributes, and distinct optical characteristics. This review comprehensively elucidates the top-down and bottom-up synthesis techniques of CDs; outlines their physical, chemical, and optical features; and delves into their versatile applications in environmental analysis, including metal ion sensing, contaminant detection, photocatalysts, antimicrobial activities, and membrane separation.

While the current methods for synthesizing CDs are diverse, challenges persist, including complex synthesis processes and the uncontrollable morphology and structure. Future advancements will aim to develop more efficient, controllable, and environmentally friendly synthesis approaches to enhance CD properties and broaden their applications in environmental analysis. To enhance CD development, the following are necessary: (1) Utilize artificial intelligence to accelerate material design and performance prediction. The optical properties of CDs can be predicted by machine learning. Then, CDs with the desired performance characteristics can be obtained more efficiently. (2) It is possible to mine the relationships between the various properties of CDs and the synthesis conditions via AI algorithms, which will promote the surface modifications of multifunctional CDs. (3) Computational calculations could be applied to the design of CDs for environmental applications, offering valuable insights into their structural, electronic, and optical properties. By employing various computational techniques, such as density functional theory (DFT), molecular dynamics simulations, and Monte Carlo methods, the behavior of CDs can be investigated and predicted at the atomic and molecular levels. (4) Drive the effective commercialization of environmentally friendly CDs and highlight the eco-friendly nature of CDs and their positive impact on environmental sustainability in marketing and branding efforts. Then, develop CDs with optimized properties for environmental analysis applications, such as high sensitivity, selectivity, and stability, and offer customizable CDs to meet the specific needs of environmental monitoring and analysis, including pollutant detection, water quality assessment, and air pollution monitoring.