A Review on Carbon Dots: Synthesis, Characterization and Its Application in Optical Sensor for Environmental Monitoring

The development of carbon dots (CDs), either using green or chemical precursors, has inevitably led to their wide range application, from bioimaging to optoelectronic devices. The reported precursors and properties of these CDs have opened new opportunities for the future development of high-quality CDs and applications. Green precursors were classified into fruits, vegetables, flowers, leaves, seeds, stem, crop residues, fungi/bacteria species, and waste products, while the chemical precursors were classified into acid reagents and non-acid reagents. This paper quickly reviews ten years of the synthesis of CDs using green and chemical precursors. The application of CDs as sensing materials in optical sensor techniques for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, was also discussed in this review. This profound review will offer knowledge for the upcoming community of researchers interested in synthesizing high-quality CDs for various applications.


Introduction
Fluorescent carbon dots (CDs), also known as carbon quantum dots (CQDs) or carbon nanodots (CNs), have drawn a great deal of attention in recent years, owing to their high photostability, excellent water solubility, tuneable fluorescence and optical properties, low toxicity, good biocompatibility, and environmental friendliness [1]. To date, a variety of precursors have been utilized to prepare CDs through "bottom-up" or "top-down" approaches [2]. Among these approaches, hydrothermal/carbonization treatment is frequently applied for the preparation CDs because of the outstanding advantages, such as high yield, simple manipulation, easy control, uniform products, lower air pollution, low energy consumption and so on [3]. Despite these advantages, CDs cannot be produced without the presence of starting materials, also known as precursors. In this regard, the development of green and chemical synthesis methods for producing high fluorescent CDs has gathered the focus of researchers. The green synthesis methods by means of green precursors of synthesis involves the usage of inexpensive or recycled materials, while the chemical synthesis methods involve toxic chemical reagents or organic solvents as precursors. CDs synthesized using these methods usually contain a certain amount of oxygen, hydrogen, and nitrogen. With all these options to prepare CDs under certain experimental conditions, it is no surprise that differential fluorescence properties of CDs were easily acquired. In addition, surface passivation (generally involves functional groups such as amine groups and hydroxyl groups) and the heteroatom doping (n-type doping: nitrogen, phosphorus, sulfur, and chlorine or p-type doping: boron) approach is another factor that paves the way to efficiently improve the fluorescent properties, quantum yield,

Fruits
Orange juice and watermelon peels were among the very first precursors used as a carbon source to synthesize CDs. The studies reveal the as-prepared CDs have different fluorescence properties, due to the presence of different chemical groups in the raw materials and synthesis method. The synthesized CDs from orange juice showed the quantum yield of 26% at an emission of 441 nm, whereas a 7.1% quantum yield was demonstrated by watermelon peel-derived CDs at an emission of 490-580 nm [9,10]. In 2014, Du et al. reported renewable wastes of sugar cane bagasse as a new precursor for fluorescent CDs [11]. It, thus, demonstrated that such bagasse-derived CDs could function as highly effective fluorescent sensing probes for labeling and imaging in biomedical applications.
Lemon peels, prunus avium extract, cornstalk, corn bract, dried lemon peels, pulpfree lemon juice, citrus lemon peels, citrus sinensis peels, etc. are another precursor that have been used in the development of CDs, as shown in Table 1  . Among these precursors, acidic fruits, such as lemon peels, lemon juice, and citrus sinensis, were frequently chosen. This is because the juice extract is rich in sucrose, glucose, fructose, citric acid, and ascorbic acid, while the peels are mainly composed of proteins, fibers, and less of oils and antioxidants. Consequently, CDs from juice extract exhibit higher fluorescence properties than peels, due to the high acid and sugar contents that provide a considerable amount of carbon and hydrogen elements. A comparison study of the different precursors used in hydrothermal synthesis was then carried out to understand the role of citric acid [30]. It was found that lemon juice has higher photoluminescence (PL) emission than ripe lemon juice and orange juice. This result is because lemon juices have higher concentrations of citric acid than orange juices. Moreover, the decrease in PL emission of ripe lemon juice was due to a significant decrease in the grade of constituents and destroyed their surface structure.

Vegetables
The exploration of new carbon sources that possess abundant reserve, green, simple and high quality CDs has drawn tremendous attention in the area of kitchen waste, such as vegetables. Various green and non-green vegetables, such as celery leaves, sweet pepper, lemon grass, tomato, carrot, rose-heart radish, turmeric, cinnamon, red chili, black pepper, hongcaitai, cauliflower, kelp, tomato, crown daisy leaves, cabbage, cherry tomatoes, scallion leaves, and red beet, have been reported, as shown in Table 2 [42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57][58]. Both green and non-green vegetables have many properties and their own advantages. For instance, the family of green vegetables, such as celery leaves, lemon grass, hongcaitai, kelp, cabbage, crown daisy leaf, and scallion, often contain many organic compounds, such as organic acids, amides, amino acids, proteins, saccharides, carbohydrates, chlorophyll, etc., which can bring good physical and chemical properties to CDs. Meanwhile, non-green vegetables, such as tomato, red chili, turmeric, black pepper, cinnamon, and red beet, are plants rich in various bioactive compounds of lycopene, capsaicin, curcumin, piperine, and cinnamaldehyde, respectively, which enable their application in the biomedical field. It was reported that bioactive compounds will partially remain inside or at the surface of the CDs after the hydrothermal process, leading to different photoluminescent and biomedical properties. Vasimalai et al. (2018) have demonstrated the uses of cinnamon, red chili, turmeric, and black pepper as CD precursors for biomedical applications [48]. They found that black pepper CDs have the highest quantum yield of 43.6% due to the various functional groups present in the sample, namely O-H, C-H, C-O-N, C=O, C-O, and C-N vibrational stretching peaks. All in all, the CDs prepared using celery leaves have contributed to the highest quantum yield of 53%. It was discovered that celery leaves contain abundant folic acid with affluent -COOH and the addition of L-glutathione as N, Sdopant has enriched their surface groups, which are beneficial for improving the quantum yield of CDs [42].

Flowers
Flowers such as Selenicereus grandifloras, water hyacinth, Osmanthus fragrans, rose flowers, and Tagetes erecta have also shown promise as carbon precursors, which were subsequently used to bind pesticides and metal ions. From Table 3, we can observe that there is an increase in quantum yield from 3.8% in 2019 to 63.7% in 2021. This significant improvement was attained by the presence of surface-active organic groups of C-N, -NH, and -OH, as shown in the FTIR analyses [59][60][61][62][63].   In another study, Shekarbeygi et al. (2020) evaluated the effect of rose pigments (blue, red, and yellow) and the effect of their extraction methods (aqueous and alcohol) on optical properties of the synthesized CDs [62]. The results indicated that the quantum yields obtained for all the CDs were not affected by the rose pigments and extraction methods, as the yields were almost the same. However, such rose pigments and extraction methods affected the CDs' thermal stability and emission wavelengths. In the case of thermal stability, the fluorescence intensities were reduced in both aqueous and alcoholic CDs, with an increase in temperature ranging from 17 to 57 • C, yet aqueous CDs showed a higher decreasing rate than alcoholic CDs. As for the emission wavelengths, the alcohol extract had a larger wavelength than the aqueous extract, which may be due to the change in dielectric constant of solvent and the presence of more phenolic groups in the alcoholic extract. Thus, this work showed that CDs prepared with alcoholic extract with yellow petals has higher stability and a longer emission wavelength with better quantum yield than the others.
Due to those health benefits, the synthesized CDs could serve simultaneously in the fluorescent sensing of metal ions and as contrast agents for live cells [64][65][66]. Nonetheless, more efforts are still needed to explore the effect of reaction temperature, time, and pH, despite the remarkable use of carbon precursors. The work of Dager et al. (2019) witnessed complete carbonization of fennel seeds (hydrocarbon converted to the graphitic structure) when the reaction temperature increased to the highest temperature at 500 • C for 3 h [70]. It was also found that by heating the sample longer (for 5 h) did not result in any significant change in either PL or crystallinity. Furthermore, when the synthesized CDs were dispersed in water at different pHs ranging from 3 to 13, the emission intensity gradually increased from acidic to basic media and eventually decreased when the reaction pH was higher than 9. Kentucky bluegrass Ethylenediamine Hydrothermal TEM-9 nm λ em -370-470 nm λ ex -280-400 nm 7% 2022 [78] Leaves are recognized as the most excellent carbon precursors by reason of exhibiting higher quantum yields than seeds and stem. Most interestingly, the Calotropis procera leaf-derived CDs provided an excellent quantum yield (71.95%) without any toxic agents or surface passivation chemicals [73]. It is noteworthy that an excellent quantum yield resulted from the functional groups (OH, N-H, C=O, and C=N bond) derived from the carbon precursors itself. Furthermore, the synthesized CDs have a strong peak around 320 nm in the UV-Vis spectra of CDs (Figure 2), implying that it was spawned by the n → π* transition of C=O bonds over the surface of CDs. nm in the UV-Vis spectra of CDs (Figure 2), implying that it was spawned by the n → π* transition of C=O bonds over the surface of CDs.

Crop Residues
Crop residues, such as sago waste, palm kernel shell, and wheat straw, are another potential carbon source used in the green preparation of CDs (Table 5). In the year of 2014, Tan et al. successfully demonstrated the conversion of sago waste into fluorescent CDs via thermal pyrolysis without any surface passivation [79]. The heating temperature of pyrolysis was found to alter the degree of carbonization of bulk sago waste into carbonaceous residues. It was observed that heating treatments of lower than 400 • C can lead to incomplete carbonization, while higher temperatures can cause severe decomposition of the organic structures in sago waste into ashes, which leads to the loss of the fluorescing property. The optimum temperature of carbonization was found to be 400 • C. However, the synthesized CDs may suffer from low quantum yields due to the absence of solvent. Solvents, especially solvents with high boiling points, play an important role in providing better carbonization rates for the formation of CDs. Of these, controlling the reaction conditions by adding solvent is of particular importance. Thus, to date, diethylene glycol [80], ultrapure water and ethanol [81], and deionized water [82] have been used as solvents to improve carbonization efficiency. The results showed that the reaction of diethylene glycol (DEG) on palm kernel shell-derived CDs had the highest quantum yield of 44%. This advancement is due to the higher boiling point of DEG than the others, thus allowing a more complete reaction for the formation of CDs at higher temperatures.

Fungi/Bacteria Species
The conversion of fungi/bacteria species into a value-added product such as carbonaceous nanomaterials has contributed to the green and sustainable improvement. Algal blooms [83], yogurt [84], enokitake mushroom [85], microalgae biochar [86], mushroom [87], agarose waste [88], and Shewanella oneidensis [89] have been previously chosen as a carbon resource (Table 6). A study conducted by Pacquiao et al. (2018) demonstrated the increment of quantum yield of 11% to 39% upon passivation with tetraethylenepentamine in the presence of 5% v/v sulfuric acid [85]. The XPS spectrum revealed the presence of small signals of nitrogen and sulfur at 399 eV (N 1s ) and 168 eV (S 2p ), respectively, thus confirming the successful heteroatoms doping on CDs and sulfuric acid as a reagent. Furthermore, the effect of temperatures (180,200, and 250 • C) and reaction times (4, 6, and 8 h) on the quantum yield was evaluated, yielding the optimum temperature and reaction time at 250 • C for 4 h. The research, thus, concluded that higher temperatures and shorter reaction times can produce higher quantum yields. Even so, CDs prepared from agarose waste-derived CDs without using other passivation chemicals exhibited the highest quantum yield of 62% [88]. This quantum efficiency can be attributed to the presence of different functional groups (-C=O, -OH, and N-H) on the surface of CDs, in addition to being related to the surface defects and the particle size of CDs.

Waste Products
There are ongoing efforts to improve the photoluminescent properties while reducing the production cost and protecting the environment. Of the different plant and fungi sources, the reuse of waste (Table 7), such as frying oil [90], biocrude oil [91], polystyrene [92], tea powders [93], expired milk [94], coal powder [95], papers [96], polyolefin [97], polypropylene plastic waste [98], bike soot [99], etc. [100][101][102][103][104][105][106][107][108][109][110][111], has become one of the hottest scientific research topics nowadays for the development of CDs. Among these materials, plastic waste and heavy oil products offer a promising precursor, as they have produced high quantum yields of more than 60%. In the case of plastic waste, cup-derived CDs have displayed the highest quantum yield value of 65% at 310 nm compared to bottles (64%) and polybags (62%) [108]. The high quantum yield of CDs is highly dependent on the carbonyl groups in CDs, revealing the presence of NH 2 . Furthermore, all the three prepared CDs displayed a slight red shift in the emission peak, with respect to the excitation wavelength. These behavioral variations were mainly due to the presence of epoxy and hydroxyl functional groups in the CDs prepared from the plastic waste, which resulted in creating new energy levels between n-π* gaps. Due to this reason, the energy gaps of the CDs between the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) became reduced with the increasing degree of surface oxidation, thereby contributing to the enhanced fluorescence property ( Figure 3). Ma et al. (2021) observed that CDs synthesized with asphalt as the precursor have the highest fluorescence quantum yield compared to heavy oil, light deasphalted oil (LDAO), and heavy deasphalted oil (HDAO) [110]. The reason is that the CDs from asphalt had the highest aromatic carbon ratio and the lowest naphthenic carbon ratio and alkyl carbon ratio. In addition, the asphalt had higher molecular weight, more oxygen, nitrogen and sulfur contents, and higher carbon/hydrogen atomic ratio than the other precursors. This result was further confirmed by the presence of the doublet γ peak and (002) peak in the X-ray diffraction pattern of asphalt, which indicated that the asphalt had a great quantity of graphite-like ordered structures.   Recently, the precursors with higher molecular weight, higher heteroatoms and higher aromatic structures were shown to be more conducive to the production of CDs. Ma et al. (2021) observed that CDs synthesized with asphalt as the precursor have the highest fluorescence quantum yield compared to heavy oil, light deasphalted oil (LDAO), and heavy deasphalted oil (HDAO) [110]. The reason is that the CDs from asphalt had the highest aromatic carbon ratio and the lowest naphthenic carbon ratio and alkyl carbon ratio. In addition, the asphalt had higher molecular weight, more oxygen, nitrogen and sulfur contents, and higher carbon/hydrogen atomic ratio than the other precursors. This result was further confirmed by the presence of the doublet γ peak and (002) peak in the X-ray diffraction pattern of asphalt, which indicated that the asphalt had a great quantity of graphite-like ordered structures.

CDs Synthesized from Chemical Precursors
Although many natural precursors have been used in the preparation of low-cost CDs, chemical precursors are still being explored to this day to produce high quantum yields of CDs. This section reviews two types of reaction chemical precursors used in CD preparation, namely acid and non-acid reagents, which were briefly summarized in Tables 8 and 9.

Acid Reagents
Since 2012, carbon sources based on acid reagents, such as citric acid, phosphoric acid, acetic acid, folic acid, ascorbic acid, sodium citrate, ascorbic acid, glutamic acid, malonic acid, maleic anhydride, boric acid, pyrogallic acid, phthalic acid, 3-aminobenzeneboronic acid, succinic acid, p-aminosalicylic acid, diethylenetriamine-pentacetate acid, maleic anhydride, DL-thioctic acid, sulfamic acid, tartaric acid, 2-aminoterephthalic acid, transaconitic acid, dehydroabietic acid, dithiosalicylic acid, etc., have been intensively employed in the production of CDs . Dong et al. (2013) first compared the fluorescence properties of the following three types of CDs: (i) O-CDs synthesized from citric acid only, (ii) N-CDs synthesized from citric acid and glycine, and (iii) N,S-CDs synthesized from citric acid and L-cysteine [113]. It was observed that the doping of nitrogen into O-CDs can introduce a new kind of surface state (labelled as the N-state). Electrons trapped by the new formed surface states are able to facilitate a high yield of radiative recombination. Although the quantum yield of N-CDs was found to be higher than that of O-CDs, the fluorescence spectra were still broad and excitation-dependent. In such cases, the introduction of sulfur atoms into CDs could lead to more significant enhancement, offering higher yields and excitation-independent emission. The introduced sulfur atoms seem to be able to eliminate the O-states and enhance the N-state, meaning that the original surface states are almost neglected in the N,S-CDs. However, when the authors tuned the ratio of L-cysteine into citric acid, from 1 g:2 g into 0.125 g:2 g, the quantum yield of N,S-CDs decreased from 73% to 37% and the emission wavelength was dependent on excitation and yielded a red-shift from 415 to 540 nm under the excitation wavelength of 375 to 480 nm. Interestingly, when the excitation wavelength was set lower than 375 nm, the emission wavelength was excitation independent. Although the aforementioned CDs were highly enhanced by nitrogen and sulfur co-doping, the chemistry involved therein is extremely challenging due to the involvement of oxygen, let alone the interference from defects. Therefore, until single nitrogen doping-sources, such as urea, ethylenediamine, PEG diamine, melamine, etc., have been widely reported, as shown in Table 8.
On the other hand, citric acid as the carbon source was commonly used in the chemical synthesis of CDs. Citric acid has considerable advantages over other acid reagents, such as being relatively cheap, more sour, less harmful to the environment, and readily available in large commercial quantities. Consequently, two highest fluorescence quantum yields were attributed to the reflux reaction of citric acid and diethylenetriamine [133] and hydrothermal reaction of citric acid and ethylenediamine [176] at 82.40% and 85.69%, respectively. As expected, ethylenediamine as N-doping agents plays a major role in improving the fluorescence properties in CDs. By way of example,  have reported the influence of three different polyethylenic amine molecules, i.e., ethylenediamine (EDA), diethylenetriamine (DETA), and tetraethylenepentamine (TEPA) with a combination of citric acid on photoluminescence performance [129]. The results showed that the CDs-EDA has the highest PL quantum yield at 69.3%, followed by CDs-DETA (68%) and CDs-TEPA (33.4%). The results imply that the increasing presence of cyclic imines (C=N) with the enhancement of conjugated π-domains in CDs imparts superior PL efficiency.
There is another study that reported the fabrication of CDs from different nitrogen sources, such as ethylenediamine (e), hexamethylenetetramine (h), and triethanolamine (t) [130]. From the X-ray photoelectron spectroscopy analyses, the ratios of carbon to nitrogen in these CDs were determined, namely, 83:17 for e-CDs, 86.14 for h-CDs, and 96:4 for t-CDs. Because of the high nitrogen content in e-CDs, the quantum yield was found to be highest at 53%. The amount of nitrogen can be typically correlated with a high photoluminescence quantum yield and confirms similar features in the absorption spectra (a shoulder at 234 nm and a broad peak at 340 nm) between e-CDs and pure citrazinic acid, the basic unit from the presumed class of fluorophores. However, at wavelengths longer than 400 nm, pure citrazinic acid has no absorption characteristic, unlike e, h and t-CDs, as shown in Figure 4a. The presence of broad absorption in this energy is commonly assigned to surface states related to the functional surface groups in CDs, which form low-energy sub-band gaps within the n−π* band gap. luminescence quantum yield and confirms similar features in the absorption spectra (a shoulder at 234 nm and a broad peak at 340 nm) between e-CDs and pure citrazinic acid, the basic unit from the presumed class of fluorophores. However, at wavelengths longer than 400 nm, pure citrazinic acid has no absorption characteristic, unlike e, h and t-CDs, as shown in Figure 4a. The presence of broad absorption in this energy is commonly assigned to surface states related to the functional surface groups in CDs, which form lowenergy sub-band gaps within the n−π* band gap.  Furthermore, Chang et al. (2017) reported that the phosphoric acid (H3PO4) or sucrose solution alone could not produce a significant fluorescence emission under the excitation of 423 nm [137]. However, when H3PO4 and sucrose solutions were mixed and incubated in the oven, the fluorescence light was emitted at the excitation wavelength of 423 nm. It is thought that the mild heating of H3PO4 has the power to hydrolyze sugar into simple carbon-rich molecules. Of note, to deactivate the carbonization process, the authors then added sodium hydroxide into CDs, but this appeared to cause a high salt content that was  [137]. However, when H 3 PO 4 and sucrose solutions were mixed and incubated in the oven, the fluorescence light was emitted at the excitation wavelength of 423 nm. It is thought that the mild heating of H 3 PO 4 has the power to hydrolyze sugar into simple carbon-rich molecules. Of note, to deactivate the carbonization process, the authors then added sodium hydroxide into CDs, but this appeared to cause a high salt content that was not favourable to many applications. Hence, acetone with a 1:1 volume-to-volume ratio was used to overcome this problem, resulting in two phases, as shown in Figure 4b. The highly ionic salt species remained in the water mother-liquor, while the CDs that were less ionic were partitioned into the acetone phase. The removal of CDs from acetone can be easily achieved via evaporation, since acetone is quite volatile. Based on this, the fluorescence signal of CDs was found to be increased by approximately 10%.
Recently, other carbon sources derived from amino acids, namely arginine, lysine, histidine, cysteine, and methionine have been reported [168,[193][194][195]. This is because of their several advantages, for instance, non-toxic, biocompatible, and eco-friendly nature. Most importantly, each of the amino acids have different numbers of -COOH, -NH, and -SH functional groups, which can effectively improve the optical and chemical properties of CDs. Upon mixing these amino acids with citric acid via the microwave method, N and/or S-doped CDs were successfully synthesized in a very short time for the first time, which only took 1-4 min [168]. The reaction mechanism was proposed to include the following two steps: (i) reaction of carboxylic acid of citric acid with the -NH and -SH groups of amino acids via condensation polymerization and then the esterification of citric acid with amino acids, (ii) carbonization of these polymer clusters during irradiation by total dehydration to generate N and/or S doped CDs under microwave exposure. The research concluded that only cysteine-CDs were found to possess the highest quantum yield of 89.5% upon the N and S atom containing CDs.
The highest quantum yield of 93% was achieved when sodium citrate dihydrate and urea were used as precursors [179]. Sodium citrate dihydrate plays an important role as a self-assembly trigger for a carbon-based structure, due to the intermolecular H-bonding, while urea acts as nitrogen-doping precursors, which both are responsible for the increase in quantum yield. In another study, instead of using common synthesis methods such as oxidation, combustion, and hydrothermal, Ji et al. (2021) established another method, which is simpler and timesaving. Briefly, 0.34 g of L-cysteine, 0.15 g of urea, and 8.5 g of diphosphorus pentoxide were mixed, followed by the addition of 6 mL of water with a rapid stirring process [193]. These CDs were then embedded in in polyvinyl alcohol (PVA) gel substrate to form a smooth and high fluorescent CD/PVA film for further use in hexavalent chromium detection.

Non-Acid Reagents
Carbon sources based on non-acid reagents, namely graphite oxide [198], poly(ethylene glycol) [199,200], 3-(3,4-dihydroxyphenyl)-L-alanine [201], ethanolamine [202], polyimide [203], polyethyleneglycol bis(3-aminopropyl) [204], ethanol [205], 3bromophenol [206], azidoimidizole [207], activated carbon [208], glucose [209], Nmethylethanolammonium thioglycolate [209], chitosan [210], chlorophyll [211], microcrystalline cellulose [212], o-phenylenediamine [213], carbon paper [214], papain [215], dextrose solution [216], etc. [217][218][219][220][221][222][223][224][225][226][227][228][229][230], have also been reviewed, as shown in Table  9. 3-Bromophenol is the first insecticide adopted to be a potential precursor, thus suggesting that carbon sources are not limited in typical chemical reagents with high purity or low toxicity [206]. In this study, the effect of mixed solvent of ethanol and water ratio (v/v) such as 1:9, 3:7, 5:5, 7:3 and 9:1 and the effect of temperatures such as 150, 160, 170 °C were evaluated. The results indicate that both much thicker organic solvents and much higher carbonized temperatures are in favour of the formation of C-dots featuring mono-dispersion, high yield, and smaller size. The proposed mechanism behind the fabrication of insecticide-based CDs was verified by the occurrence of aromatic C-Br and Br anion, Csp2 C-C peak and Csp3 C-C peak found on the XPS spectrum. The three possible main classes of the reaction that happened among the 3-bromophenol molecules are given in Figure 5. Efforts to develop and improve the fluorescence of the CDs have been made toward polymer compounds, including synthetic and natural polymers. Thanks to the toxicity of synthetic polymers, natural polymers extracted from plants have been used as a carbon source with the characteristics of being environment-friendly, non-toxic, low price, good biocompatibility, and stable chemical properties. For this reason,  reported microcrystalline cellulose as a carbon precursor to prepare a facile and low-cost one-step hydrothermal treatment of CQDs [212]. The modification of these CQDs with ethylenediamine not only allowed nitrogen atoms on its surface but also introduced them into the carbon nuclear lattice to obtain better optical properties and higher quantum yield than CQDs. Fortunately, nitrogen doped CQDs (NCQDs) have a fluorescence emission Efforts to develop and improve the fluorescence of the CDs have been made toward polymer compounds, including synthetic and natural polymers. Thanks to the toxicity of synthetic polymers, natural polymers extracted from plants have been used as a carbon source with the characteristics of being environment-friendly, non-toxic, low price, good biocompatibility, and stable chemical properties. For this reason,  reported microcrystalline cellulose as a carbon precursor to prepare a facile and low-cost one-step hydrothermal treatment of CQDs [212]. The modification of these CQDs with ethylenediamine not only allowed nitrogen atoms on its surface but also introduced them into the carbon nuclear lattice to obtain better optical properties and higher quantum yield than CQDs. Fortunately, nitrogen doped CQDs (NCQDs) have a fluorescence emission intensity of 7.1 times stronger than the CQDs, and it can be explained by the reduction of epoxy, ether, and carboxyl groups on the surface of NCQDs via nucleophilic reactions with amidogen, thus forming a few non-radiative recombination centers in NCQDs. The amination and/or amidation also create radiative recombination centers, such as amine (-C-N-) and amide groups (R 2 -NCOR), which belong to electron-donating groups. These electron-donating groups and π-electrons that existed in the -C=N group have contributed to the improvement of the fluorescence intensity of a substance. All in all, these NCQDs successfully provided the highest quantum yield of 55%.
The synthesis of CDs using non-acid reagents can be further explored by attempting to use three different aniline compounds, i.e., 1,2,4,5-benzenetetramine tetrahydrochloride, 1,2,4-benzenetriamine dihydrochloride, and o-phenylenediamine, as carbon sources and ethanol as a solvent [217]. In an approach where more amino groups and a high nitrogen content are present in CDs-benzenetetramine tetrahydrochloride, the excitation and emission wavelengths all exhibited a redshift with an increase in the CD particle size. The quantum yield also greatly increased up to 30.2%. Additionally, the fluorescence intensity of the CDs was observed to gradually increase with the increasing amount of raw material from 0.005 g to 0.025 g. It is important to note that the number of amino groups and particle size of CDs have a great influence on the emission wavelength, while the amount of raw material influences the color and intensity of the fluorescence. In another study, the CDs were synthesized using three types of aldehydes, such as glutaraldehyde, nitrobenzaldehyde and benzaldehyde, via the solvothermal method [225]. The resulting CDs were then composited with chitosan, functionalized by a DNA probe and utilized as a detecting platform for rapid detection of microRNA-21. The function of CDs is further classified as a cross-linking agent and fluorescent donor, while chitosan plays an important role in the preparation of three-dimensional hydrogel frames.

Application of CD-Based Optical Sensor for Environmental Monitoring
The incorporation of CDs in optical sensors for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, are reviewed below (Figure 6). This review aims to survey state-of-the-art CD-based optical sensors with their sensing performance, namely range of detection, limit of detection, and linear correlation coefficient.

Application of CD-Based Optical Sensor for Environmental Monitoring
The incorporation of CDs in optical sensors for environmental monitoring, including the detection of heavy metal ions, phenol, pesticides, and nitroaromatic explosives, are reviewed below (Figure 6). This review aims to survey state-of-the-art CD-based optical sensors with their sensing performance, namely range of detection, limit of detection, and linear correlation coefficient.

Heavy Metal Ions
Environmental pollutions caused by toxic heavy metal ions (HMI) have resulted in the development of different optical sensors using nanoscale materials [231][232][233][234][235][236][237]. Of late, CD-based optical sensors have garnered tremendous research interest, which is summarized chronologically in Table 10. Herein, an optical fibre sensor for Hg 2+ and Cu 2+ detection was developed using CDs functionalized with poly(ethylene glycol) and Nacetyl-l-cysteine [238]. This sensor has quite interesting analytical potential, as it allows for higher linear coefficients when detecting Hg 2+ in aqueous solutions. Furthermore, the concentration range of Hg 2+ was successfully lowered to femtomolar detection using a fluorescent sensor [199]. The outstanding nature of the sensing performance can be probably attributed to the Hg 2+ ions, which have stronger affinity toward carboxylic

Heavy Metal Ions
Environmental pollutions caused by toxic heavy metal ions (HMI) have resulted in the development of different optical sensors using nanoscale materials [231][232][233][234][235][236][237]. Of late, CD-based optical sensors have garnered tremendous research interest, which is summarized chronologically in Table 10. Herein, an optical fibre sensor for Hg 2+ and Cu 2+ detection was developed using CDs functionalized with poly(ethylene glycol) and N-acetyll-cysteine [238]. This sensor has quite interesting analytical potential, as it allows for higher linear coefficients when detecting Hg 2+ in aqueous solutions. Furthermore, the concentration range of Hg 2+ was successfully lowered to femtomolar detection using a fluorescent sensor [199]. The outstanding nature of the sensing performance can be probably attributed to the Hg 2+ ions, which have stronger affinity toward carboxylic groups on the surface of CDs and stronger quenching effect on the PL of the prepared CDs.
On the other hand, Co 2+ and Cu 2+ ions have been detected using CD-based composites. In the presence of CTAB to PEG-passivated CDs, a long fluorescence lifetime with higher emission efficiency was noticed due to the rapid production of •OH radicals from the Co(II)-H 2 O 2 -OH − system, which is responsible for the highly selective response of the present chemiluminescence system towards Co 2+ ions [200]. On the other hand, the electrochemiluminescence of the oxidized CDs/K 2 S 2 O 8 system resulted in higher quenching to 20 nM of Cu 2+ compared to 1 µM of Pb 2+ , Ni 2+ , Mn 2+ , Fe 2+ , and Co 2+ [201]. Another HMI of interest is Fe 3+ , which have been detected three times in a row in the year of 2014 using diverse optical sensors [116,117,119]. Among them, the phenolic hydroxyl of N-doped CDs exhibited higher sensitivity toward Fe 3+ quenching, with the linearity range of 0.01−500 µM and detection limit of 2.5 nM [116]. The reason is that the hydroxyl group has a good binding affinity for Fe 3+ , leading to the splitting of d orbital of Fe 3+ and consequently causing significant fluorescent quenching. Similarly, nitrogen-doped CQDs were used as an effective fluorescent sensing platform for label-free sensitive detection of Hg 2+ ions in an ultrapure water solution with a detection limit of 0.23 µM [118]. Due to the excellent sensitivity and selectivity of the Hg 2+ sensor, this sensor was further applied to the determination of Hg 2+ in tap water and real lake water samples. The results showed that the PL intensity (excited at 360 nm) decreased gradually with the increasing concentration of Hg 2+ in both tap water and lake water from 5 to 50 µM.
In other work, the sensing potential of fluorescent CDs synthesized through the pyrolysis method was evaluated for Cu (II), Cr (II), Co (II), Ni (II), Al (III), Ca (II), Pb (II), Zn (II), Sn (II) and Hg (II) ions [79]. Among all the metal ions tested, Cu (II) and Pb (II) have the highest sensitivity, while Zn (II), Hg (II) and Ca (II) have the lowest sensitivity. The reason for this relatively low sensitivity might be due to the diamagnetic properties of Zn (II), Hg (II) and Ca (II) ions, which eliminate the paramagnetic quenching mechanism with the CDs.
With the accelerating demand of mercury in the skin lightening industry, a high number of optical techniques for Hg 2+ have been reported [15,23,54,65,89,122,124-128, 133,142,143,152,153,160,178,207]. Notably, fluorescence techniques are the most employed. Altogether, there are several phenomena about the quenching effect of CDs to Hg 2+ , which are as follows: (i) electron transfer from excited nitrogen and sulfur atoms in CDs to Hg 2+ (nonradiative recombination); (ii) strong binding between Hg 2+ and S atoms or carboxylate or hydroxyl groups from CDs; and (iii) Hg 2+ has a larger ionic radius and polarization; therefore, deformation happened more easily when it interacted with the nitrogen and sulfur atoms. Fortunately, the Hg 2+ can be detected as low as 2.6 nM using CD-labeled oligodeoxyribonucleotide (ODN) and quenched by graphene oxide [122]. In this study, CD-ODN acted as the energy donor and molecular recognition probe and GO served as the fluorescence resonance energy transfer acceptor.
Due to the concerns about improving and protecting living organisms, more optical detection methods that utilize CDs as a sensing element have been developed to detect Fe 2+/3+ [13,22,38,40,72,78,87,124,135,142,151,159,166,169,219,226], Cu 2+ [53,66,97,141,227,239], Pb 2+ [65,87,181,240], Cr 6+/4+ [23,85,193,241], and Co 2+ [51]. For instance, Shamsipur et al. (2018) prepared green-emitting CDs instead of common blue-emitting CDs for selective and sensitive detection of Fe 3+ [159]. This green CDs approach could detect Fe 3+ in the range of 0.05-10.0 µM, with a detection limit of 0.0137 µM. The detection limit was found to be significantly lower than the maximum level of Fe 3+ (0.54 µM) allowed in drinking water by the U.S. Environmental Protection Agency.  invested in interesting research to detect low concentrations of Pb 2+ ions [181]. An electrochemiluminescence (ECL) aptasensor was prepared by modifying a glassy carbon electrode (GCE)/NCQDs with amino aptamers by NHS/EDC linkage for selective and sensitive detection of Pb 2+ . As shown in Figure 7, the binding of the hairpin aptamer with Pb 2+ forms the G-quadruplex, and thus exposes the amino group to the NCQDs, causing significant changes in ECL intensity. The results revealed that the GCE/NCQDs/aptamer sensor can detect Pb 2+ quickly and accurately, providing the lowest detection limit of 18.9 pM. Importantly, the proposed aptasensor did not suffer from interference and it had excellent stability. NCQDs, causing significant changes in ECL intensity. The results revealed that the GCE/NCQDs/aptamer sensor can detect Pb 2+ quickly and accurately, providing the lowest detection limit of 18.9 pM. Importantly, the proposed aptasensor did not suffer from interference and it had excellent stability. The fabrication of CDs with industrial/agricultural waste has proven to be useful as a detection probe for metal ions. A study conducted by Yan et al. (2020) demonstrated that the synthesized N-CDs from crown daisy leaves had abundant surface functional groups that could selectively and sensitively detect Cu 2+ at a very low detection limit of 1.0 nM by the fluorescent quenching effect [53]. This result is due to the selective complexation interaction between Cu 2+ and carboxyl and amino groups of the N-CDs.
On the other hand, improving the detection performance of hexavalent chromium (Cr 6+ ) even at the nanomolar level has become an enduring concern of researchers. Rajendran and Rajendiran (2018) reported that the CQDs can be effectively used as fluorescent probes for the detection of Cr 6+ at the nanomolar level [23]. This research reported strong fluorescence quenching with increasing Cr 6+ concentration, resulting in an improved detection limit of 10 nm. Nevertheless, the detection limit of Co 2+ was found to be higher at the micromolar level despite using CD-based optical sensors [51]. The proposed sensor was then applied in the analysis of real river water samples with a recovery value of 95.0-106.8% and a relative standard deviation below 5.3%, indicating that the synthesized CDs had great application prospects in Co 2+ detection.  The fabrication of CDs with industrial/agricultural waste has proven to be useful as a detection probe for metal ions. A study conducted by Yan et al. (2020) demonstrated that the synthesized N-CDs from crown daisy leaves had abundant surface functional groups that could selectively and sensitively detect Cu 2+ at a very low detection limit of 1.0 nM by the fluorescent quenching effect [53]. This result is due to the selective complexation interaction between Cu 2+ and carboxyl and amino groups of the N-CDs.
On the other hand, improving the detection performance of hexavalent chromium (Cr 6+ ) even at the nanomolar level has become an enduring concern of researchers. Rajendran and Rajendiran (2018) reported that the CQDs can be effectively used as fluorescent probes for the detection of Cr 6+ at the nanomolar level [23]. This research reported strong fluorescence quenching with increasing Cr 6+ concentration, resulting in an improved detection limit of 10 nm. Nevertheless, the detection limit of Co 2+ was found to be higher at the micromolar level despite using CD-based optical sensors [51]. The proposed sensor was then applied in the analysis of real river water samples with a recovery value of 95.0-106.8% and a relative standard deviation below 5.3%, indicating that the synthesized CDs had great application prospects in Co 2+ detection.

Phenols
The existence of different forms of phenolic in petrochemical engineering, printing, food processing, and dyeing has been continuously reported as the most harmful pollutant due to its high toxicity and low biodegradability [242]. Table 11 summarizes the chronological order of the detection of phenolic compounds using optical sensors. Among the reported studies, 2-4-6-trinitrophenol, also called picric acid, is the most common pollutant monitored by means of fluorescent sensor [72,112,114,134,136,150,170,221]. It is well known that 2-4-6-trinitrophenol is a representative electron-deficient nitroaromatic, due to the electron-withdrawing nature of the three nitro groups. From the perspective of the sensor performance, Niu et al. (2013) explored the feasibility of using amine-capped CDs for the detection of 2-4-6-trinitrophenol in aqueous and ethanol solutions [114]. For the detection of 2-4-6-trinitrophenol in ethanol, the amine-capped CDs demonstrated obvious fluorescent quenching and superior sensitivity to 2-4-6-trinitrophenol in aqueous solution. Since water is much more polar than ethanol, the detection is, therefore, driven by an electrostatic interaction between 2-4-6-trinitrophenol and CDs.
Generally, there are three possible mechanisms causing fluorescence quenching between the phenolic compounds and proposed CDs, which are as follows: (i) electron transfer; (ii) fluorescence energy transfer; and (iii) inner filter effect. Amid all the mechanisms, fluorescence energy transfer (FET) is a dominant quenching pathway due to the electronrich nature of the CDs and the electron-deficient aromatic group of the phenol [115,134,243]. In contrast to FET, the electron transfer process became possible for phenol fluorescence quenching [112,150,204]. Notably, electron transfer in the excited state of CDs can occur from the lowest unoccupied molecular orbital (LUMO) to the LUMO of phenols, if possible. In addition, there are certain rules that are required to fulfil FET quenching. The rules are as follows: (i) the absorption spectrum of the quencher (phenols) should overlap with the emission spectrum fluorophore and (ii) there is a change in the fluorescence lifetime of CDs before and after the addition of the quencher. Furthermore, the inner filter effect (IFE) is also considered important and has been applied as a developing novel fluorescent assay in recent studies [34,136,170,218,219,226]. This approach offers more simplicity and expediency because IFE-based sensors use fluorophore and the receptor directly rather than chemically interacting with each other.
In another study, Xue et al. (2018) found that phenol can form hydrogen bonds with the carboxyl group on the surface of the fluorescence CQDs, which will facilitate the fluorescence quenching of the CQDs in the excited state, leading to the high sensitivity and selectivity of CQDs for the detection of phenol [95]. In addition, the fluorescence quenching effect can be explained by either a dynamic or static process. Interestingly, a static quenching effect between N-CDs and tannic acid found in the works of [167] can be proved by several measurements, which are as follows: (i) a decrease in the fluorescence lifetime; (ii) a rate that is constant more than 1.0 × 10 10 M −1 s −1 ; and (iii) a blue-shift of the absorption peak of CDs. Wang et al. (2019) compared the selectivity of the CDs@MIP and CDs@MIP to 4-nitrophenol, which resulted in good selectivity of CDs@MIP on 4-nitrophenol, due to the hydrogen bonding interactions, leading to the charge transfer from CDs to 4-nitophenol [165]. There is still much room for further development of CD-based optical sensors for the detection of phenolic compounds, despite achieving a high quantum yield and good sensing performance. For instance, Saravanan et al. (2020) successfully synthesized N@CDs from m-phenylenediamine as a single source of carbon and nitrogen to detect 2-4-6-trinitrophenol [221]. The fluorescent N@CD probe showed high selectivity toward 2-4-6-trinitrophenol relative to other nitro explosives, such as 4-nitrophenol, 4-amino-3-nitrophenol, 2,5-dinitrophenol, phenol, 2-chlorophenol, 3-nitrophenol, and 4-amino-triphenol.    In addition, ultra-weak chemiluminescent (CL) systems have become one of the focuses of the increasing attention on the determination of tannic acid [173]. Interestingly, tannic acid could dramatically suppress the CL intensity of the FNCDs-H 2 O 2 -K 3 Fe(CN) 6 system with an excellent linear response (R 2 = 0.9971) and was further applied for the determination of tannic acid in a red wine sample. The recovery of 98.9-102.1% was obtained, demonstrating its reliability and application potential in real sample analysis. Recently, the aggregated N,S-CDs as a dual-excitation ratiometric fluorescent probe have proven to be an effective approach for the quantitative determination of chlorogenic acid [184]. Apart from dual excitation,  observed the dual-color fluorescence emission of ultraviolet and red light when detecting 4-nitrophenol using CDs@PDA [226]. The detection limits obtained for the ultraviolet and red-light emission were 3.44 µM and 7.29 µM, respectively. In addition, 2-4-6-trinitrophenol has been detected using the fluorescence method with a detection limit of 0.11 µM in the concentration range of 0.3-3.3 µM [72]. The possible mechanism for fluorescence quenching of N-CDs by 2-4-6-trinitrophenol is explained by the electrostatic interaction or hydrogen bonding that occurs between the nitrogen groups of CDs and phenolic group of 2-4-6-trinitrophenol. To be exact, the electrostatic interactions occur when a large number of nitro groups with electron-withdrawing characteristics on benzene interact with nitrogen-containing functional groups of CDs. Hence, it can be concluded that nitro groups play a vital role in fluorescent quenching.

Pesticides
Pesticides are chemical compounds that are used to kill pests. Generally, there are six classes of pesticides according to their target species, i.e., insecticides, herbicides, rodenticides, bactericides, fungicides, and larvicides. The largest and most widely used in crop protection are insecticides [244]; however, they are classified as an extremely toxic class of chemical compounds by the World Health Organization (WHO).
Organophosphorus (OP) compounds, such as methyl parathion [121,138], paraoxonethyl [137], dichlorvos [245], malathion [245], ethion [245], paraoxon [154,155,211], chlorpyrifos [63,96,246], methyl-paraoxon [180], diazinon [50,62], and quinalphos [247], [63], are widely reported insecticides in the development of CD-based optical sensors. Oddly, most OP pesticides are always selected as inhibitors for sensors based on enzyme activity. According to Hou et al. (2016), dichlorvos can inhibit the activity of acetyl cholinesterase (AChE), which can turn the fluorescence of CDs off again [245], which is similar to the detection of paraoxon [155]. However, in a study of , only OP of chlorpyrifos effectively inhibited H 2 O 2 production by destroying the acetylcholinesterase activity, thereby increasing the fluorescence of C-dots after being initially turned off by Fe 3+ [96]. Moreover, the proposed sensor exhibits a low selectivity against OP. Therefore, a new facile fluorescence probing based on novel N-doped carbon dots and methyl parathion hydrolase (MPH) was developed to detect methyl parathion selectively [138]. The use of MPH in this study helps to degrade OP compounds and has many advantages over other enzymes, such as less background noise and high turnover number, which is favourable for sensitive detection.
Another class of insecticide is carbamates. Li and co-workers have been devoted to detecting carbaryl in the presence of acetylcholinesterase (ACh) and choline oxidase (ChOx) by using N-CQDs, S-CQDs, and co-doped N, S-CQD-based photoluminescence sensors [209]. The results demonstrated that high sensitivity originates from the N and S dopants, which offer stronger capacities to adsorb hydrogen peroxide (H 2 O 2 ) and generate local states to trap hot electrons, promoting the electron transfer to H 2 O 2 , and thus resulting in the strong quenching of the CQD fluorescence.
Apart from the insecticide organophosphate, insecticides neonicotinoids and organochlorine have become a target pesticide. Mandal et al. (2019) have reported a simple fluorescencebased method using CDs for the detection of commonly used insecticides, such as imidacloprid and lindane [246]. The fluorescence emission of the CDs was found to be enhanced in the presence of the increasing concentration of imidacloprid; however, the emission was quenched in the presence of lindane. The interactions between CDs and insecticides were mainly driven by the high reactive -NO 2 and -NH groups in imidacloprid that directly bind to the surface functionality amino groups of CDs and by the H-bond or week ionic interaction or covalent binding between the free surface groups of the CDs and lindane, followed by the substitution reaction. In addition, the researchers also expanded the potential of CDs in sensing unclassified pesticides, namely tetradifon [246]. The leaving groups (-Cl) of tetradifron were found to possessstrong covalent binding with the surface amine group of the CDs with a high affinity. Nonetheless, the results showed that tetradifron has a higher detection limit than imidacloprid, but offers a lower detection limit than lindane. This phenomenon can be attributed to the binding affinity of all the different target pesticides. Their binding affinity can be arranged in the following decreasing order: imidacloprid > tetradifron > lindane.
Until 2021, the fluorescence response of CDs towards herbicides [50,60,213,220,246] and fungicides [73,182,183] has also been explored. It is reported that with increasing concentrations of herbicides, i.e., atrazine [213], pretilachlor [60], glyphosate [50], and fungicide thiophanate methyl [183], this has resulted in an increase in PL emission intensity (turn-on). This fluorescence 'turn-on' model was dynamic quenching, whereas, when the concentrations of trifluralin [220], pyrimethanil [182], and isoprothiolane [73] were increased, the fluorescence emission of CDs was found to decrease, leading to static quenching. Supchocksoonthorn et al. (2021) suggested that the fluorescent quenching is closely related to the inner filter effect, due to the π-π interaction between the large aromatic core of the CDs and anilinopyrimidine unit of pyrimethanil and between their polar functional groups [182].
Furthermore, as an improvement to the method with a single fluorescence signal, Zhu et al. (2022) developed a new type of CD-functionalized core-shell nanospheres as a ratiometric fluorescence to detect λ-cyhalothrin (LC), a typical pyrethroid insecticide. Figure 8 shows the main binding reactions in the preparation of the CD-functionalized core-shell nanospheres. Here, silica nanoparticles play a role in preventing the contact between m-CDs and the dispersion solvent so the m-CDs could act as a reliable reference background; o-CDs act as a fluorescence signal; 3-aminopropyrtriethoxy silane (APTES) as a functional monomer to connect with LC through hydrogen bonding; ionic liquids (IL) as a cationic surfactant, which are attached to the system by electrostatic interaction to enhance the fluorescence of the core-shell nanospheres; and tetraethyl orthosilicate (TEOS) is used to form a layer of imprinted silicon shell on the surface of m-CDs@SiO 2 . Inspired by the role of materials, these newly formed core-shell nanospheres managed to achieve lower detection limits, excellent reusability, and better precision than those of other methods reported in this work [230]. All the proposed CD-based optical sensors for the detection of pesticides are tabulated orderly in Table 12. other methods reported in this work [230]. All the proposed CD-based optical sensors the detection of pesticides are tabulated orderly in Table 12.

Explosive Compounds
Military-grade explosives such as trinitroluene (TNT), dinitroaniline (DNT), are still a major worldwide concern in terms of terror threat and environmental impact. The only method that detects explosive compounds using CDs is a fluorescent sensor. For instance, Zhang et al. (2015) used N-rich CDs for 2,4,6-trinitrotoluene determination [120]. Apparently, TNT is a typical electron deficient nitro compound, which can selectively interact with amine group on CDs, leading to charge transfer from fluorescent CDs to the aromatic rings of TNT and consequently quenching the fluorescence emission of CDs strongly.
Dai and co-workers found that fluorescence sensors are well suited for detecting nitroaromatic compounds, such as 2,4-dinitrotoluene (DNT), as they can quench the emission of the excited species [132]. In this work, static quenching occurred with an increasing concentration of DNT, as CDs formed stable charge-transfer complexes with DNT molecules. Considering that the selectivity of those sensors were relatively low because some metal ions or other structure analogy can quench those fluorescent nanoparticles, Xu et al. introduced molecularly imprinted polymers (MIPs) to improve the selectivity [131]. However, this causes poor sensitivity because of the highly cross-linked nature of MIPs. Therefore, a novel strategy using periodic mesoporous silica particles as the imprinting matrix and using amino-CDs directly as a "functional monomer" was proposed to improve the sensitivity of M-MIPs@CDs.
In addition, Campos et al. (2016) first demonstrated the fluorometric sensing of 4-chloro-2,6-dinitroaniline in an aqueous solution using carbon quantum dots coated with PAMAM-NH 2 [208]. The presence of amine groups in the PAMAM-NH 2 dendrimer results in the enhanced fluorescence intensity and quantum yield from 3.94% to 6.93%. Moreover, it is interesting that with an increase in pH starting from 2.25, the fluorescence intensity increases drastically and then become constant from pH 6 to 11. The reason for the constant fluorescence intensity may be due to the buffering effect of the surface groups (-COOH and -NH 2 mainly).
Next, a selective and sensitive detection of TNT explosive residues based on CDmodified optical sensors has been developed and rapidly elevating until 2022 [99,139,196,214,248]. The research concluded that the possible mechanism for TNT detection corresponds to the Jackson-Meisenheimer (JM) complex. Particularly, functionalizing CDs with amino groups can selectively form a JM complex with typical electron-deficient groups in TNT via charge transfer during a nucleophilic aromatic substitution reaction, which consequently causes fluorescence quenching. In addition, the selectivity of the sensor against TNT was investigated by Devi's group [214]. The proposed sensor showed high selectivity for TNT over other weaker Lewis acids, such as dinitrotoulene (DNT), 2,4-dinitrotoluic acid (TA), nitrobenzene (NB), and p-nitrophenol (NP). It can be concluded that the JM complex formed by TNT is more stabilized due to more resonance structures; more nitro groups on the TNT aromatic ring than that formed by DNT, TA, NB, and NP. All the proposed CD-based optical sensors for the detection of explosive compounds are tabulated orderly in Table 13.  [196] Where N-rich CNDs: nitrogen-rich carbon nanodots, M-MIPs@CDs: mesoporous structured molecularly imprinted polymers capped carbon dots, CQDs@PAMAM-NH 2 : carbon quantum dots functionalized with amine groups by PAMAM dendrimer, EDA: ethylenediamine, CDs and Fe@SiO 2 -NH 2 : carbon dots and the magnetism of amino-functionalized magnetic core-shell nanomaterial, PEI-CQDs: polythylenimine capped carbon quantum dots.

Concluding Remarks
An enormous research effort has been devoted to the development of CDs using green and chemical precursors. The reason behind the interest in this fabrication is its fluorescent properties that make it continuously competitive to produce bright and high-quality CDs. This paper has presented the recent progress in the field of CDs, focusing on the green and chemical precursors and fluorescent properties. To the best of our knowledge, this is the first review paper that highlights each of the precursors used in the CDs with the values of the fluorescence quantum yield.
Among the available green precursors, CDs produced by plants are the most popular source because it is a more stable and straightforward process to scale up. For instance, the CDs synthesized from leaves, i.e., Calotropis procera leaves, without any addition of surface passivation have the highest quantum yield of 71.95% under the hydrothermal carbonization process, as shown in Figure 9a. It can be observed that the leaves have shown great potential as carbon sources due to their carbon-, hydrogen-, oxygen-, nitrogenrich composition, which can eliminate the need of surface-chemical passivating agents. Other factors that contributed to the excellent photoluminescence properties include (i) heteroatom doping, such as sulfur, nitrogen, phosphorus, and boron; (ii) heating temperature and time; (iii) type and ratio of solvent; and (iv) pH of media. Even so, the quantum yields obtained from the green precursors were still low compared to the chemical precursors, as depicted in Figure 9b. It was shown that the thermal pyrolysis treatment of sodium citrate dihydrate and urea led to green fluorescence CDs with a quantum yield as high as 93%. As expected, CDs synthesized from acid reagents possess higher quantum yields due to the existence of carboxyl groups. The greater the number of carboxyl groups in the carbon source, the more amino groups of urea that will conjugate onto the surface of CDs. quality CDs. This paper has presented the recent progress in the field of CDs, focusing on the green and chemical precursors and fluorescent properties. To the best of our knowledge, this is the first review paper that highlights each of the precursors used in the CDs with the values of the fluorescence quantum yield.
Among the available green precursors, CDs produced by plants are the most popular source because it is a more stable and straightforward process to scale up. For instance, the CDs synthesized from leaves, i.e., Calotropis procera leaves, without any addition of surface passivation have the highest quantum yield of 71.95% under the hydrothermal carbonization process, as shown in Figure 9a. It can be observed that the leaves have shown great potential as carbon sources due to their carbon-, hydrogen-, oxygen-, nitrogen-rich composition, which can eliminate the need of surface-chemical passivating agents. Other factors that contributed to the excellent photoluminescence properties include (i) heteroatom doping, such as sulfur, nitrogen, phosphorus, and boron; (ii) heating temperature and time; (iii) type and ratio of solvent; and (iv) pH of media. Even so, the quantum yields obtained from the green precursors were still low compared to the chemical precursors, as depicted in Figure 9b. It was shown that the thermal pyrolysis treatment of sodium citrate dihydrate and urea led to green fluorescence CDs with a quantum yield as high as 93%. As expected, CDs synthesized from acid reagents possess higher quantum yields due to the existence of carboxyl groups. The greater the number of carboxyl groups in the carbon source, the more amino groups of urea that will conjugate onto the surface of CDs.
Although the highest quantum yield results have been achieved using chemical precursors (acid reagents), the high toxicity of this precursor may hinder their potential for diverse applications. Therefore, future development should firstly focus on synthesizing CDs from green precursors, particularly plant parts and waste products. In addition, it is highly necessary to fabricate green precursors consisting of hydroxyl, carboxyl and amino groups to avoid the use of toxic chemical passivating agents. Furthermore, this will allow for the development of high-quality and non-toxicity CDs, finally paving the way towards their practical application in various fields.  Although the highest quantum yield results have been achieved using chemical precursors (acid reagents), the high toxicity of this precursor may hinder their potential for diverse applications. Therefore, future development should firstly focus on synthesizing CDs from green precursors, particularly plant parts and waste products. In addition, it is highly necessary to fabricate green precursors consisting of hydroxyl, carboxyl and amino groups to avoid the use of toxic chemical passivating agents. Furthermore, this will allow for the development of high-quality and non-toxicity CDs, finally paving the way towards their practical application in various fields.