Carbon nanomaterials had extensive studied and received great attention since the discovery of Buckminster fullerene [1]. Carbon dots (CDs) consist, graphene, carbon quantum dots (CQDs), carbon nanotubes (CNTs), carbon nano onions (CNOs) and now it’s a new, important fluorescent carbon nanomaterial with below than 10 nm in size. These carbon nano materials had strong luminescent property and highly soluble in aqueous medium [2]. An experiment designed to understand the procedure of the preparation of long chain carbon moieties interstellar space, took the place of fullerene. The characterization of carbon nanotubes (CNTs) was made after this [3]. After a year of the discovery of CNT, another member of carbon family, carbon nano onion (CNO) or carbon quantum dot (CQD) was discovered [4], an ideal member of the carbon allotropes, which made up of concentric graphitic shells [5]. It was first synthesized and obtained by Iijima in 1980 [6], in vacuum deposited amorphous carbon films by high resolution TEM (Fig. 1). After discovery CNO, Ugarte observed a transformation of CNT to CNO [4]. CNO had a quasi-spherical onion like structure composed of concentric graphitic layers with an inner core, which either hollow or encapsulated with metal. Because of the ideal structure, potential applications of CNO for lubricants [7, 8], magnetic storage materials [9], devices [10] and electrochemical capacitors [11]. Carbon quantum dots (CQDs) had amorphous sp²-conjugated nanocrystalline core with sp²/sp³ hybridization and consist highly abundant oxygen groups (like carboxyl, hydroxyl, aldehyde etc) on the surface [12], which consider those nano materials as water soluble CQDs (Fig. 2). They had wonderful electron donor-acceptor induced photoluminescence and fluorescence properties. Due to photoluminescence property, critical to re-size and change the surface chemical groups of carbon dots [13]. Doping with other elements also used to increase their properties [14]. Carbon quantum dots or carbon dots, consider as next generation material in nanomaterial technology which had unique optical and electronic properties.

In comparison of other quantum dot semiconductors, organic agents, and other fluorescent sensors, carbon quantum dots (CQD) had very interesting properties like unique emissions with various fluorescence excitation, chemical compositions, cheap and simple synthesis, functionalization of surface and modification, photo-chemical stability [2], which proved CQDs or CDs could be apply in different technical, medical applications. Carbon dots easily played favourable roles in applications due to their simple and cheap synthesis, easily water soluble and biocompatible [15-18]. Those fluorescent carbon  nano materials attract the interest of researchers due to their wide variety of applications like bioimaging, bioanalysis, sensor, drug delivery, photocatalysis, optoelectronics, electrocatalysis, fuel cell and many more [19].

Many fluorescent quantum dots like graphene quantum dots (GQDs) [20-24], polymer dots (PDs) [25, 26], carbon nanotube (CNT) dots [27, 28], nano diamonds (NDs) [29-33], and CQDs synthesised previously. According to density of states (DOS), quantum dots had distinct materials with compact atoms. When the size of QD particles decreased upto below the size, bandgap energy increased. Because of electron-hole radiative recombination, effect of quantum-confinement did not relevant to carbon dots. Carbon quantum dots consist small size with high surface defects. Size-dependent optical properties because of various surface defect and surface to volume ratio [34]. Here, we described the synthesis, characterization and properties of carbon quantum dots with their application in various areas.

Fig.1. Spherical particles of graphitic carbon nano-onions found in the amorphous carbon films, (a – d). (scale bar 2 nm) [6]

Fig.2. Structure of carbon quantum dots (CQDs) [35]. 

2. Synthesis of nano carbon quantum dots (CQDs):

Carbon quantum dots (CQDs), serendipitously determined in the  process of single wall carbon nano tubes (SWCNTs) purification [36]. Suspension of SWCNTs on gel electrophoresis, differentiated into three important classes of materials, included a fast moving band of high fluorescent material which showed  size dependent fluorescent properties. It was  first discovery of carbon nanoparticles or carbon dots. CDs contain contained lower amount of carbon with high oxygenated groups, also called carbogenic nanodots [17]. Baker et al. described different synthesis process and properties of carbon nanoparticles. Synthesis of CDs divided into two classes of methods; top-down and bottom-up. Top down method included synthesis of nanoparticles from large carbon structure. And other hand, in bottom-up method CDs prepared by molecular precursor.

2.1 Top-down methods:

Firstly, fluorescence nano materials CDs stable graphene sheets, carbon fibres, CNTs, CNOs, carbon soots etc, classified as following methods:

(i) Arc-discharge method: Bottini et al. [37] used electric arc technology to synthesized luminescence CDs through pristine CNTs and oxidized (with nitric acid) CNTs.

(ii) Laser ablation method: Sun's et al. [38] synthesized CDs via Q-switched Nd : YAG laser ablation at 900 °C and 75 kPa, by graphite and cement mixture hot pressing. Carbon dot’s surface were passivated via diamine-terminated oligomeric PEG_1500N with passing acid; which attached with organic moieties on the surface of CDs obtained (Fig. 3a).

(iii) Hydrothermal cutting method: Pan et al. [39] synthesized CDs via  hydrothermal method, in it cutting graphene sheets into 10 nm sized functionalized graphene quantum dots. These functionalized GQDs exhibited blue fluorescence which induced through edge effect.

(iv) Chemical oxidation method: Zhou et al. [40] reported that carboxylic group attached onto carbon nano materials or GQDs surface, synthesized through oxidation reaction under UV irradiation, in between graphene oxide (GO) and Fenton reagent (Fe²⁺/Fe³⁺/H₂O₂).

(v) Electrochemical oxidation method: Zhou et al. described [41], synthesis of  CD through multiwall carbon nanotubes which fabricated by electrochemical oxidation method (Fig. 3b). 

Fig.3. Schematic view of synthesis of (a) fluorescent carbon nano dots (CDs) via laser ablation and attached with PEG on the surface [38]; (b) GCDs synthesized by exfoliation in ionic liquid [42]. TEM image of graphene quantum dots and illumination the aq. Solution of carbon dots by UV lamp.

Fig. 4. Schematic view of bottom-up synthesis of carbon dots with different temperature and relationship between different products [43].

2.2 Bottom – up methods:

CDs were developed by bottom – up methods by self-assembling molecular precursors, which used to control and well-defined properties of molecular weights, particle sizes, morphologies, and optical. Oxygenated, carboxy(-COOH) and hydroxyl (-OH) groups, used as carbon precursors and amine, amido groups used as nitrogen precursors. The fluorescence colour and quantum yield dependent upon ratio of carbon and nitrogen precursors. According to different synthesis methods, bottom-up classified into  following  categories.

(i) Thermal decomposition: Wang et al. [44] reported, fluorescent, amorphous CDs prepared by coordinating solvent. It attached via methoxy silyl groups which exhibited high quantum yield 47% and 0.9 nm diameter.

(ii) Combustion: Vinci et al. [45] described a low-cost process synthesis of carbon nano particles based on soots and it also obtained from combustion of paraffin oil.

(iii) Hydrothermal synthesis: Yang et al. described the amino attached fluorescent CDs obtained from chitosan via  hydrothermal carbonization [46].

(iv) Microwave synthesis: Liu et al. described the preparation of carbon dots through mechanism of one-step microwave assisted and polyethyleneimine attached onto surface of carbon materials [47] (Fig. 4).

(v) Supported synthesis: Bourlinos et al. described, carbon dots (CDs) obtained from ion-exchanged NaY zeolite via thermal oxidation and as resultant spherical carbon nano dots or particles attached onto the zeolite surfaces [48].

3. Properties of Carbon Dots (CDs)

3.1 Structures

Carbon dots had graphitic in-plane lattice spacing 0.18–0.24 nm and 0.32 nm spaces of interlayer (Fig. 5a). CDs varied on raw materials and synthetic method and  composed of carbon crystalline cores similar to sp² carbon and amorphous clusters (Fig. 5b) [49-51]. The crystallinity degree of carbon dots lesser than graphene quantum dots, and some CDs had diamond like  sp³ hybridisation of carbon [52]. According to Raman spectroscopy, two peaks observed around 1350 and 1600 cm⁻¹, indicate disorder of sp² carbon and crystalline graphitic carbon, respectively [51, 53]. At framework of carbon, different functional groups attached onto the surface of CDs via surface passivation or functionalization. It protect the surface of CDs and enhance their luminance property .

3.2 Fluorescence

The origin of photoluminescence emission of carbon nanoparticles had centre of interest. Depending upon the experimental and theoretical results three factors responsible for the origin of PL emission, as follows: (1) the presence of triplet ground state of carbene (σ¹π¹) at zigzag sites [39, 54]; (2)  

Fig. 5. (a) HR TEM images of carbon dots (CDs); (b) schematic view of carbon dots with sp² carbon [53].

the presence of triplet ground state of carbene (σ¹π¹) at zigzag sites [39, 54]; (2) the presence of localized finite – sized sp² clusters within sp³ matrix, lead to confinement of π – electrons. Radiative recombination of electron – hole pairs in sp² clusters, rise the PL emission [54-56]; (3) Surface passivation of carbon moiety by organic molecules or by hydrophilic groups which introduced by covalent functionalization or by oxidation reaction respectively. In presence of oxygenated functional groups, suggested for observed PL emission from carbon nanoparticles [38, 57], as like surface – oxidized silicon nanocrystal [58]. the oxygen containing functional groups can passivate the surface by extended hydrogen bonding [59, 60].

Presently, mechanism of photoluminescence property for CDs/CQDs, received much attention and interest for applications in different fields. Regarding photoluminescence property of CDs, there many theories had proposed, like nanoscale dependence, dependence of excitation wavelength and dependence of surface group. Li's group [61] reported a electrochemical synthesis CDs within 1.2–3.8 nm size range and resultant size-dependent photoluminescence carbon nanoparticles; small size nano carbon dots (1.2 nm) get emitted under UV light (350–450 nm), medium-size carbon dots (1.5–3 nm) get emission spectra at 400– 700 nm wavelength and large size carbon dots (3.8 nm) get emission spectra at NIR range. Bao group [62] reported the size of CDs observed through electrochemical methods with various potential and at constant excitation wavelength and decreasing size of CDs the emission spectra showed  red-shift [61]. Here, the degree of surface oxidation, CDs had essential and much important  in  photo-luminescence properties. Red fluorescent carbon dots had higher potential, exhibited high oxygenated surface states. Thus, red shift of emission peak responsible for surface states.

In chemical processes, the surface of carbon dots, modified through attached oxygenated functional groups and the photoluminescence property depend on it, which cause of higher quantum yield with high emission spectra observed  respectively. At high surface oxidation degree, increase the surface defects and red shift emission observed. Ding et al. [63] described, the preparation of carbon dots via hydrothermal process with their PL property, which showed a relation in between the surface of CDs, with degree of oxidation. Zheng et al. [64] synthesized, the green colour fluorescence carbon dots

 Fig. 6. Schematic view of photoluminescence (a) CQDs; (b) carbon nano dots as fluorescent nanodots [65].

Thus, the fluorescence property of CDs, depend upon quantum confinement effects; i.e., the size of CDs decrease, the energy gap between the valence shell and conduction band enhanced and the emission spectra also decreased. According to previous studies, the fluorescence of carbon dots didn’t cause of single factor but it raised because of combine of various causes like size, surface passivation, functional groups, and heteroatoms [62].

According to Fig. 6, PL emission of carbon quantum dots occur when trap states present in the bandgap [65]. The PL properties of CDs or CQDs attracted interest due to their applications like fluorescent imaging, nanocarriers drug delivery and controlled release method, analytes detection, biosensors, optical/electrochemical sensors, light- emitting diodes, energy conversion and storage, electro and photocatalysis [66].

3.3 Surface Passivation and Doping

Pristine carbon dots, called as undoped carbon dots. After the initial synthesis, it had exposed carbon and oxygen sites [53]. Burlinos et al. reported, the functionalization of CDs through one-step pyrolysis, which had thermal decomposition process, citric acid mix with different amines. In it, citrate provided the carbon core, whereas the amines were attached as functional groups on the CDs [67]. Yang group described a method for large-scale synthesis of heavy metal-doped fluorescence CDs [68]. Hetero-atom (N, S, or Se) based doped CQDs obtained through one-step hydrothermal reduction and in situ doping treatment. Heavy metal-doped CQDs had  1 - 6 nm size with their photoluminescence property at various emission wavelength which depended on electro-negativity of heteroatom (Fig. 7). Thus, N- and S doped carbon dots were highly sensitive for detection of Cu²⁺ and Hg²⁺, respectively [68].

Pristine graphene material had zero-band gap which important for optoelectronic properties including their fluorescence properties [69]. Chemical functionalization of graphene changed the band gap and it shifts at their Fermi level [70]. If graphene doped with more electronegative elements like nitrogen, a blue shift emission observed, on the other hand if it

Fig. 7. Relation between electronegativity of heteroatoms and emission wavelength (λ_em) of doped carbon dots [68].

Fig. 8. Schematic view of microwave bottom-up method for synthesis of green-GQDs and blue-GQDs [71].

doped with less electronegative elements like  sulfur, a red shift fluorescence observed [68]. In general, after attachment of oxygenated groups like epoxy, hydroxyl functional groups, on surface of sp²-hybridized carbon material, wide bandgap observed. N-doped carbon dots (N-CDs) prepared with organic molecules like hydrazine, urea, hexamethylenetetramine, diethyl amine, ethanolamine, and ethylenediamine, which increased electron density and reduced the work function of the CDs, cause of blue shift emission. Umrao group described a sequential bottom up method to synthesized blue and green fluorescent GQDs; g-GQDs and b-GQDs (Fig. 8) [71]. Green fluorescent of g-GQDs formed at initial stage and final product was blue fluorescent b-GQDs, observed respectively. It showed only one emission peak at 433 nm. Thus, two-step microwave irradiation method decreased size of graphene quantum dots.

4. Application:

4.1 Chemical sensing

Among many application fields of carbon quantum dots, chemical sensing one of the interesting use. Heavy metal detection like Hg²⁺ utmost important due to harmful effect at environment and human health. Due to non-toxic, water soluble, high photo and chemical stability property, carbon quantum dots used for chemical sensing.

Firstly, CQDs used as chemical sensor for specific detection of Hg²⁺ in aq. solutions [72-77] and live cells [78]. Goncalves et al. described about the emissions of CQDs for detection of Hg²⁺ [79, 80]. Yan et al. described, Hg²⁺–CQD composite use to detect Hg²⁺ ion within live cells [78]. They synthesized two types of CQDs; (CQD-1) with 1,2-ethyldiamine and (CQD-2)N-(b-aminoethyl)-g-aminopropyl which had high quantum yield upto 65.5 and 55.4%, respectively and also describe the  quenching of CQD-1 and CQD-2 through Hg²⁺ (Fig. 9). Furthermore, carbon nano materials used as highly sensitive fluorescence moiety for sensing of minute amount of Hg²⁺ in both aqueous medium and living cell. Some fluorescence were quenched but after adding strong chelating agent like EDTA, it could be recovered, which proved those CQDs had reversible fluorescent moieties [78]. Similarly, other elements like Cu²⁺ [81], Fe³⁺ [82], Pb²⁺ [83], Cr(VI) [84] and Ag⁺ [85] also detected by carbon quantum dots.

Along with the sensing of metal ions, CQDs also able to detect pH [85, 86], C₂O₄^2- [87], PO₄^3- [88], CN^- [89], F^- [90], S^2- [91], ClO^- [92], I^- [93] and NO₂ gas [94]. Moreover, small organic molecules like ascorbic acid [84]; and 4-nitrophenol [95], quercetin, 2,4-dinitrophenol and 2-amino-3,4,8-trimethyl-3H-imidazo [4,5 f] quinoxaline [96] also sensed by CQDs via fluorescence spectra. CQDs exhibited good chemi-luminescence [97] and electrochemiluminescence [98] property. thus, many groups developed for chemi-luminescent sensing for NO₂^- [99] and Co²⁺ [100] and electro chemi-luminescent assays for minute amount of penn-POM/CNO hybrid composite, detected toxic metanil yellow food colour upto 3.83 nmol ml^-1 (Fig. 10) [102].

Fig. 9. Comparison of UV-visible (black) and fluorescence spectra of (a) CQD-1 and (b) CQD-2 in absence (blue) and presence (red) of Hg²⁺ in aq. solution; fluorescence spectra of (c) CQD-1 and (d) CQD-2 in aqueous solution with presence of 20 mM concentration of different metal ions at 360 nm wavelength [78].

Fig. 10. Fluorescence change of various concentration of metanil yellow food colour in presence of constant 3.43 x 10^-6 mol ml^-1 aq. solution of the Ln-POM/CNO nanocomposite [102].

4.2 Bioimaging

CQDs/CDs were chemically and photochemically stable including had interesting optical properties as well as environmentally friendly. Thus, CQDs  used in biological systems with vivo and vitro system [103]. Central core of carbon were non-toxic but because of surface passivating on  surface CQDs showed cytotoxicity also [104]. After using CQDs, did not observed any abnormality in harvested organs [105]. In bio sensing, different amount of CQDs used for treating cell viability. At 1.8 mg ml^-1 concentration of CQDs cell viability greater than 95%. This showed that CQDs were highly biocompatible than other semiconductor quantum dots [106].

Organic dye composite with CQDs were highly effective luminescent moiety for H₂S. With FRET method, in the presence of minute amount of H₂S, blue to green fluorescence observed [107]. Previously described that H₂S able to penetrate cell membrane by diffusion process [108]. Here, fluorescence microscope used for observation property of CQDs with organic dye composite and it also changed physiology of H₂S in living cells. According to Fig. 11, the fluorescence images of HeLa and L929 cells, incubated with composite of CQDs before and after treated with H₂S. The intracellular fluorescence of organic dye composite with CQD stained cells and exposed to H₂S within 30min at 37 °C, green fluorescence observed, which showed organic dye composite with CQDs had fluorescence for sense different level of H₂S in live cells [107].

It was well known, CQDs able to exhibited multicolour emissions, which were used as labelling agents and also allowed chemists to use different excitation and emission wavelengths [109]. According to Fig. 12, the emissions property of CQDs were clearly visible [110]. At various wavelengths light were emitted with excitation of different wavelengths. HepG2 cells incubated with 4,7,10-trioxa-1, 13-tridecanediamine-passivated CQDs, which showed multicolour

Fig. 11. Emission images of live cells which treated with organic dye composite with CQDs, before (A, D) and after incubating (B, E), in presence of 100 mM H₂S (C, F) [107]

Fig. 12. (a) Absorption and fluorescence emission spectra of carbon quantum dots at various excitation wavelength; fluorescent images of MCF-10A cells which treated with CQDs and excite on (b) at UV, (c) at blue and (d) at green light [110].

emissions after excitation at various wavelengths [111]. Synthesis of CQDs from juice of  sugar cane showed various emissions, at various excitation wavelength and also observed different colour fluorescence images in bacterial and yeast cells [103]. Ray and co -worker described that surface passivation not necessary for high emission, which necessary for cell imaging [112]. Here, CQDs synthesized from thermal combustion of soots with acid treatment and also translocate Ehrlich ascites carcinoma cells [112].

Sarkar et al. reported fluorescence images of life-cycle of Drosophila malenogaster of  through CQDs [59].

4.3 Biosensing

Carbon quantum dots also used in biosensing for antibodies and gene-recovery fragments. CQDs specially used as

Fig. 13. Schematic view of nucleic acid LFA (NALFA) [113].

 
fluorescent labels for immunoassays. According to Posthuma-Trumpie and co-workers [113], CQDs use as lateral flow and microarray immunoassays. It were cheap in coast but more stable and more sensitive thus used as fluorescent labels. CQDs were highly sensitive as fluorescence moiety in lateral flow assays (LFAs) [114]. Carbon quantum dots also showed sensitivity upto picomolar range [115]. Nucleic acid LFA (Fig. 13), differentiate as the biosensing labels on amplicons which identified their unique antibodies respectively and emission signals also observed. CQDs also used as fluorescent moiety for identify small bio molecules like anti-bacterial, anti-viral moieties, including combat those bacterial and viral diseases [116].

 4.4 Nanomedicine

Carbon quantum dots had great attraction in the field of nanomedicine because of its non-toxic nature [117]. it was demonstrated by toxicity experiments in mice. Where it injected through CQDs in vain, which observed upto four weeks and resultant after four weeks, internal organs and their functions did not affected, respectively. CQDs had bio-comparability property which supported to prothrombin time assays in plasma. Thus, it showed that CQDs did not lead for blood coagulation [110].

Bechet et al. demonstrate the application of CQDs in photodynamic therapy. It were clinical treatment usually used in tumours [118]. In tumour tissue, it used as photosensitizers, with a specific wavelength, and also generate singlet oxygen species which cause of cell death. Carbon quantum dots had inhibition effect on MCF-7 and MDA-MB-231 cancer cells (Fig. 14) [110]. CQDs also generate reactive oxygen species (ROS) which make it an ideal photosensitizers [116, 119], including also able to combat highly infectious bacterial and viral strains in presence of light and air to cure dangerous diseases such as New Delhi metallo-betalactamase-1 (NDM-1) producing Enterobacteriaceae (Fig. 15) [116] and novel corona virus. Besides that, CQDs also used as nano drug carriers and fluorescent tracers, which able to regulate release of drugs. Sarkar et al. described carbon nano material or CQDs as nanocarrier for alzimer via pH dependent the “open and close” target system [120].

Fig. 14. Fluorescence images of mice with tumour [76].

Fig. 15. Under visible light, rGO activity on Enterobacter sp.: in presence of air (a) absence of rGO, (b) with presence rGO, (a and b); in argon (c) absence of  rGO and (d) presence of rGO, (c and d) [116].

Fig. 16. Under visible light, demonstration of catalytic mechanism of TiO₂ – CQD nanocomposites [61].

4.5 Photocatalysis

Now present days, photocatalytic methods get high interest as green alternatives in organic synthesis [121-125]. The very famous and important photocatalyst TiO₂ which use for removing of organic pollutants and also used in water splitting and generation of H₂ [125]. Due to bandgap of TiO₂ exist in UV region (3.0–3.2 eV) and TiO₂ used only less than 5% of sunlight. Nanocomposite of CQDs-TiO₂ used the full spectrum of sunlight that increased efficiency, in this experiment methylene blue (MB) used as ideal compound (Fig. 16). Li et al. reported that under visible light, CQDs-TiO₂ nanocomposites, degraded complete MB (50 mg mL^-1) within 25 min as photocatalyst, where only MB degraded upto 5% by TiO₂ [61].

4.6 Electrocatalysis

These days, production of energy for example fuel cells and fuel production, oxygen reduction reaction (ORR) and oxygen
evolution reaction (OER) attracts interest by scientists. Electrocatalysts generally used to increase kinetic activity of ORR. Unfortunately, platinum-based electrocatalysts which were high in cost, forced scientists to find out replacement of platinum, or non-platinum-based electrocatalysts for better efficiency of electrocatalytic than platinum-based electrocatalysts. Small sized CQDs, which had good electrical conductivity and stability, made it ideal electrocatalytic materials for ORR. Previously, doped nitrogen atoms with carbon nano materials play an important role to enhanced their electrocatalytic activities toward ORR [126]. Li and co-workers [127] described, the synthesis of N-CQDs with oxygenated functional groups through electrochemical methods, electrocatalytically active. Potential of ORR was -0.16 V (vs. Ag/AgCl), which resemble to electrocatalysts of commercial platinum-based (Fig. 17).

Later, similar observations get by Yan group [128] and Liu group [129] with nitrogen doped CQDs, which prepared by same methods. In comparison between absence of CQDs and nitrogen doped CQDs (N-CQDs), enhanced electrocatalytic property of nitrogen doped CQDs were related with nitrogen doping effect in presence of methanol. The observed electrocatalytic activity were responsible for nitrogen with graphene [130]. Zhu et al. described CQDs and its electrocatalytic activity, which synthesized by natural biomass – soy milk [131]. It had similarity with N-CQDs for obtained oxygen and enhanced electrochemical reduction reaction.

Fig. 17. Spectra of cyclic voltammetry of (a) nitrogen doped CQD/graphene; (b) commercial Pt/C on GC electrode at N₂-saturated 0.1 M KOH, O₂-saturated 0.1 M KOH and O₂-saturated 3 M CH₃OH solutions [127].

Fig. 18. Demonstration of Gram plants after 10 days, (a) plants grown (v-1 to v-5) treated with non-toxic wsCNTs; without treated blank (v-6) (b) after 10 days, plant growth measurements of gram plants (b-1, b-2) without wsCNTs, (b-3, b-4) treated with 200 μL wsCNTs and (b-5) treated with 100 μL of wsCNTs  respectively [132].

4.7 Agriculture

Water soluble CQDs,  take a very important part in field of agriculture, where supply of water difficult and the supply of water crucial which requires maximum conservation. Sarkar and group [132] developed wsCNTs from mustard oil and described, the effect of water soluble CNTs in plant growth of gram seeds (Cicer arietinum) with or without conventional fertilizers. It had short life cycle and their vascular bundles arranged in rings. Continuously used non - toxic wsCNTs enhance the growth rate of plant. Firstly, plant seeds were grown without CNTs in 5.0 ml distilled water.  Then, 100 μL wsCNT used in 5.0 ml distilled water; next, 200μL of wsCNT  used in 5.0 ml double distilled water. Monitored the whole system upto 10 days in comparison of  root length, shoot length, number of roots and uptake of water by gram plants. Results showed the enhancement in plant growth (Fig. 18) [132] .

5. Conclusion

Since from the discovery of carbon nano materials had numerous, cheap and efficient way to prepare fluorescent CQDs. The defects of CQDs played an important role in fluorescence emissions at excitation of different wavelength. The physicochemical properties of fluorescent CQDs, developed highly sensitive sensing property with their chemical stability. CQDs take an important part in analytical and biological science. Applications of CQDs in the field of sensing and bioimaging increased the sensitivity and selectivity. CQDs, an ideal nano material for biomedicine for their use in imaging and nanomedicine. The non - toxic and chemically stable CQDs had great advantages for vivo biomedical applications. It also had ability to develop new diagnostics, therapeutics and preventives which can be use in diagnose and treat cancer and other serious diseases. CQDs also used  in drug delivery which more prominent than nano spheres made by biodegradable polymers [133]. CQDs able to generate ROS, which could be combat harmful and dangerous bacterial and viral strains like NDM-1 producing Enterobacteriaceae and corona virus. It also used in photocatalytic and electrocatalytic applications including fuel cell, hydrogen evolution, water splitting etc. CQDs also used in food industry for adulteration of toxic food colours at nano level. It also helpful in agriculture for increase plant growth.

Although, carbon quantum dots already proved the it take a very impotent part in nanotechnology for development of assays, sensors, bioimaging agents, drug carriers, phototherapy, photocatalysis and electrocatalysis, agriculture, food industry, combat pathogens, water treatment etc. it also take part in the area of bioimaging and biomedical. Thus, CQDs recognised by researchers as materials science, synthetic chemistry, drug delivery, nanomedicine and clean energy. Research on cheap and non-toxic CQDs also proved as ideal implement and bioinformatics for diagnosis, prevention and treatment of dangerous diseases like cancer, Alzheimer, diabetes etc pathogenic diseases NDM -1, corona etc, which did not have any cure; including energy storage and conversion, food industry. 

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