Journal of Nanomaterials
Volume 2019 (2019), Article ID 3451307, 19 pages
https://doi.org/10.1155/2019/3451307

Review Article

Carbon Dots: A Mystic Star in the World of Nanoscience

Biswajit Gayen,¹ Soubantika Palchoudhury,² and Joydeep Chowdhury³

¹Department of Chemistry, Dr. Sudhir Chandra Sur Degree Engineering College, JIS Group of College, 540 Dum Dum Road, Kolkata 700074, India
²Department of Civil and Chemical Engineering, University of Tennessee at Chattanooga, Chattanooga, Tennessee 37403, USA
³Department of Physics, Jadavpur University, 88, Raja S. C. Mallick Road, Kolkata 700032, India

Correspondence should be addressed to Joydeep Chowdhury; joydeep72_c@rediffmail.com

Received 15 March 2019; Accepted 17 June 2019; Published 11 December 2019

Academic Editor: Oscar Perales-Pérez

Copyright © 2019 Biswajit Gayen et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract.

Carbon dots (CDs) have received significant attention worldwide from the beginning of this century, and recently, it has bloomed in every branch of applied sciences. Because of their outstanding physical and chemical properties together with biocompatibilities, CDs find a wide spectrum of applications in drug delivery, explosive detection, chemical sensing, food safety, bioimaging, energy conversion, photocatalysis, etc. This brief review is focused on the synthesis of CDs and their applications. The photophysical properties of CDs are also discussed herewith.

1. Introduction

Carbon dots (CDs) have emerged as most precious gifts in nanotechnology because of their magical properties and applications. CDs are typically carbon nanoparticles, most of them with average diameter less than 10 nm [1, 2]. These materials are derived from organic compounds and are stable in aqueous media which is extremely significant in terms of biological points of view [3]. Surface engineering plays a significant role for CDs in diversified applications like explosive detection, chemical sensing, food safety, bioimaging, drug delivery, energy conversion, and photocatalysis. Photophysical and chemical properties of CDs vary dramatically by tuning their shapes and sizes and also by doping heteroatoms such as oxygen, nitrogen, phosphorus, sulfur, and boron [4]. Moreover, photostability, high quantum yield, biocompatibility, low toxicity, water solubility, good conductivity, and environmental friendliness of CDs receive additional advantages over other well-recognized quantum dots (QDs) like grapheme quantum dots (GQDs), metal oxides (ZnO, TiO₂), and inorganic QDs (ZnO-PbS, CdSe, CuInS/ZnS, and CuInS/ZnS). In fact, noncarbon QDs are not much graceful in their field of applications compare to CDs, because of their serious health and environmental issues [5, 6]. CDs can be synthesized from both natural and synthetic organic precursors. Synthetic methodologies that are very frequently used in this concern are microwave irradiation, hydrothermal treatments, ultrasonic irradiation, laser ablation, electrochemical, arc discharge, and pyrolysis [7]. This short review has been specifically focused on the synthetic methodologies of CDs and their wide applications in pure and applied sciences.

2. Discovery of CDs

CDs were discovered accidentally in 2004 at the time of purification of single wall carbon nanotubes (SWCNTs) by Xu et al. [8]. Two years later, in 2006, Sun et al. first synthesized stable photoluminescent carbon nanoparticles of different sizes and named them “carbon quantum dots” (CQDs) [9]. Within a year, water soluble CDs passivated with poly-propionylethylenimine-co-ethylenimine had been reported by Sun et al. The as-prepared CDs showed two-photon-induced luminescence spectra and were utilized to detect human breast cancer MCF-7 cells [10].

3. Architecture of CDs

CD is the youngest member in the family of nanoworld. They are commonly spherical in shape having average diameter less than 10 nm [3]. GQDs have only sp²-hybridized carbon framework whereas CDs are composed of both sp² and sp³ hybrid carbon networks [11]. Moreover, they can be easily functionalized with hydroxyl, carboxyl, carbonyl, amino, and epoxy groups over their surfaces thereby offering extra advantages for binding with both inorganic and organic moieties (Figure 1) [12]. The functionalities specifically allow the surfaces of CDs to espouse either with hydrophilic or with hydrophobic character which finally provide the necessary thermodynamic stabilities in different solvents especially in water [13]. In addition to these, bare carbon nanoparticles do not exhibit any kind of photoluminescent activities while their surface modifications lead to exhibit strong photoluminescent signals [7]. Surface modification of CDs by different functionalities, passivating agent and solvent, reflects a smart variation in their properties [14].

Figure 1: Generalized structure of CDs.

4. Synthesis

In the last few years, a number of facile synthetic methodologies have been developed for making CDs with varied functionalities and photophysical properties. Synthetic pathways for CDs are mainly classified into two categories: (a) “bottom-up” and (b) “top-down” approaches (Figure 2).

Figure 2: Different synthetic methodologies.

4.1. Bottom-Up Approach

In the “bottom-up” methodology, CDs are synthesized from small molecules by a microwave irradiation, hydrothermal, and pyrolysis method.

4.1.1. Microwave Irradiation Method

Microwave- (Mw-) assisted synthetic methodologies have recently received significant attention in the scientific community because of their time-saving, energy-efficient, and eco-friendly nature. In this methodology, carbonization of small organic molecule occurs by microwave heating within a very short period of time (Figure 3). In 2009, Zhu et al. first reported CDs with the aid of Mw methodology from carbohydrates with excellent photophysical properties within a very short period of reaction time [15]. Liu et al. synthesized photoluminescent CDs from glycerol and 4,7,10-trioxo-1,13-tridecylenediamine as a surface-passivating agent by a Mw irradiation method. They reported moderately high quantum yield (QY) of 12% due to the incorporation of amino groups (NH₂) over the surface of CDs (Figure 3) [16]. Feng et al. synthesized CDs with QY~46% by Mw irradiation from silkworm chrysalis [17]. The as-synthesized CDs were used in bioimaging due to their low toxicity and photoluminescent nature. Recently, Liu et al. synthesized photoluminescent CDs by a Mw heating method from citric acid, L-cysteine, and dextrin with high QY of 22%. As-synthesized CDs are reported to be photostable and were used in the detection of Cu²⁺ in drinking water [18]. Recently, Sun et al. synthesized N-ethylcarbazole functionalized CDs by a microwave method that exhibited very nice photoinduced redox properties [19].

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Figure 3: (a) Synthesis of CDs by a Mw irradiation method. (b) HR-TEM image of as-synthesized CDs as obtained from the Mw irradiation method: (i) 20 nm (size distribution histogram shown inset); (ii) 5 nm (magnification of a single nanoparticle shown inset) (reproduced with permission from [16]).

4.1.2. Hydrothermal Method

Hydrothermal methodology perhaps is the most promising method in recent years for the synthesis of CDs due to their nontoxic, environment friendly, low cost, and easy operational technique. In this methodology, a solution of organic precursors is sealed in a hydrothermal synthetic reactor where reaction occurs at high temperature and pressure (Figure 4(a)). In 2010, Zhang et al. first reported a one-pot hydrothermal method to make CD from ascorbic acid in the presence of ethanol as solvent. QY and average particle sizes of their synthesized CDs were 6.79% and ~2 nm, respectively [20]. Pang et al. reported synthesis of codoped nitrogen and sulfur in CDs (NS-CDs) derived from methionine by a hydrothermal method [21]. NS-CDs, as recovered in their synthesis, exhibit selective detections of heavy metal ions in water. Recently, Shen et al. reported synthesis of highly photoluminescent CDs from sweet potato as a natural source of carbon by a hydrothermal method with high QY. The as synthesized CDs utilized for detection of Fe³⁺ (Figure 4) [22]. In another report, Zhang et al. prepared surface-modified CDs by polyethylenimine from hyaluronic acid with very high QY 26%. The as-synthesized CDs used those photoluminescent CDs in tumor targeting and gene delivery [23].

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Figure 4: (a) Synthesis of CDs by a hydrothermal method from sweet potato; (b) sensing of Fe³⁺ ions by as-synthesized CDs.

4.1.3. Pyrolysis Method

Pyrolysis is a facile method to synthesize CDs from organic compounds by simple chemical reactions carried out at very high temperature in the presence of strong acid or alkali (Figure 5(a)). Martindale et al. synthesized CDs of average diameter ~6 nm by pyrolysis of citric acid at 180°C for generation of hydrogen fuel-utilizing solar energy [24]. Guo et al. synthesized stable CDs from hair (keratin) by a one-step pyrolysis method at 200°C for 24 hours of reaction time. They successfully recovered CDs and used their CDs in the detection of Hg²⁺ with higher sensitivity and selectivity [25]. Recently, Rong et al. synthesized highly photoluminescent nitrogen-doped CDs (N-CDs) derived from guanidinium chloride and citric acid by a pyrolysis method and fluorescence quenching observed in the presence of Fe³⁺ (Figure 5) [26]. N-CDs obtained by their synthesis were profoundly used in metal ion detections and in bioimaging.

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Figure 5: (a) Synthesis of CDs by a pyrolysis method; (b) fluorescence quenching in the presence of Fe³⁺.

4.2. Top-Down Approach

In the “top-down” methodology, CDs are synthesized by a laser ablation, electrochemical oxidation, and arc discharge method.

4.2.1. Electrochemical Method

The electrochemical method is used to synthesize ultrapure CDs from larger molecular matter like carbon nanotube, graphite, and carbon fiber by an electrolytic process where larger organic molecules are used as electrode in the presence of proper electrolytes (Figure 6). Zhou et al. first reported synthesis of CDs from multiwalled carbon nanotubes in the presence of tetrabutylammonium perchlorate as electrolyte [27]. Zheng et al. synthesized water soluble pure CDs by an electrochemical method using graphite as electrode in the presence of phosphate buffer at neutral pH. The as-prepared CDs were successfully applied as potential biosensor [28]. Li et al. prepared crystalline CDs by an electrochemical method from graphite. The as-prepared CDs exhibited size-dependent upconversion photoluminescence (PL) properties and are used in photocatalysis [29]. Recently, CD with polyaniline hybrid was synthesized by an electrochemical technique with high QY and purity. The as-synthesized CD-polyaniline composite reported to exhibit high capacitance and used in energy-related devices (Figure 6) [30].

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Figure 6: (a) Synthesis of CDs by an electrochemical method; (b) formation of CD-PANI hybrid composite from as-synthesized CDs.

4.2.2. Laser Ablation Method

The laser ablation technique has been widely used for making of CDs of varied sizes. In laser ablation route, complex organic macromolecules are exposed under laser radiation operated in CW or in pulsed mode and nanosized carbon particles are detached from the larger molecular structures (Figure 7(a)). Synthesis of CDs by a laser ablation technique was first reported by Sun et al. in 2006 from graphite powder [9]. They synthesized CDs upon laser excitation from a Nd:YAG (1064 nm, 10 Hz) source in an atmosphere of argon at 900°C and 75 kPa. Thongpool et al. synthesized CDs from bulk graphite in the presence of ethanol using a Nd:YAG laser of wavelength 1064 nm. The synthesized CDs showed a broad absorption spectrum peaked at 325 nm (Figure 7(b)) [31]. Recently, photoluminescent CDs of ~3 nm size have been synthesized by a laser irradiation technique from carbon glassy particles in the presence of polyethylene glycol 200. CDs so prepared are applied in bioimaging for cancer epithelial human cells [32].

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Figure 7: (a) Synthesis of CDs by a laser ablation method; (b) absorption spectrum of as-synthesized CDs (reproduced with permission from [31]).

4.2.3. Arc Discharge Method

CDs by an arc discharge method had been an accidental event. This method was first reported by Xu et al. during synthesis of SWCNTs [8]. Electrical discharge across two graphite electrodes results in the formation of small carbon fragment or CDs (Figure 8). Bottini et al. reported CDs derived from pristine and SWCNTs by means of an arc discharge method with bright PL in the violet-blue and blue-green region, respectively [33]. Recently, Boron- and nitrogen-doped QDs were synthesized by the arc discharge method from graphite. They used B₂H₆ for doping boron and NH₃ for nitrogen (Figure 8) [34].

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Figure 8: (a) Synthesis of quantum dots by an arc discharge method; (b) (i) TEM and (ii) HR-TEM image of as-synthesized quantum dots (reproduced with permission from [34]).

5. Photophysical Properties of CDs

CDs, as obtained by different synthetic methodologies and precursors, show excellent optical properties and exhibit absorption, photoluminescence (PL), chemiluminescence (CL), and phosphorescence spectra. In this section, the optical properties of CDs are described in brief.

5.1. Absorption Spectroscopy (UV-Vis)

CDs generally exhibit broad optical absorption maxima in the ultraviolet (UV) region (250-350 nm) together with weak absorption tail in the visible region of UV-Vis spectra [35]. The absorption peaks appear at around ~240 nm are due to π-π electronic transition of C=C bonds, and peaks at around ~340 nm are owed to n-π transition from C=O bonds present as functionalities (Figure 9(a)) [36]. Surface engineering can modify the corresponding absorption spectra of CDs that can alter their emission spectra also [37]. Doping of heteroatom can also regulate absorption spectra for doped CDs by altering % heteroatom because of their alteration in the π-π energy level [38]. Surface defects ingrained in CDs are considered to be responsible for broad spectral features in their absorption spectra [39]. Moreover, carbonyl and amino functionalities promote red shifts of band maxima in UV-Vis spectra due to the variations in HOMO-LUMO energy levels of CDs because of fictionalizations (Figure 9(b)) [40].

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Figure 9: (a) UV-Vis absorption spectrum of CDs; (b) variation of HOMO-LUMO energy gap of CDs with functionalities (reproduced with permission from [36, 40]).

5.2. Photoluminescence Spectroscopy

CDs show excellent PL behaviors. Appearance of photoluminescent peaks () and their intensities depend on excitation wavelengths (). Multiple PL spectra thus can easily be attained from single CD (Figure 10(a)). The PL maximum for CDs commonly appears in the blue and green region of the spectra (Figure 10(b)) [41]. PL behavior of CDs can be controlled by varying the initial precursors and synthetic methodologies and by surface engineering [42]. Interestingly, PL that generates from CDs almost remains unchanged under irradiation for long time [43] vis-à-vis to organic dyes that are susceptible to photobleaching [8]. Sun et al. reported CDs with an average particle size ~1.54 nm. The as-synthesized CDs exhibit PL spectra with peak maxima centred at ~460 nm, 540 nm, and 620 nm upon excitations with 380 nm, 460 nm, and 540 nm wavelengths, respectively [44]. Wang et al. reported N-CDs derived from m-aminobenzoic acid with highly intense PL maxima, peak appeared at 415 nm. They further reported amine-passivated CDs and used them as biosensors for the sensing of Fe³⁺ and pH [45]. Sun et al. reported CDs passivated with oligomeric ethylene glycol diamine (PEG1500N). These passivated CDs exhibit photoinduced electron transfer (PET) reactions from quenching of its PL in the presence of 4-nitrotoluene, 2,4-dinitrotoluene, Ag⁺, and N,N-diethylaniline (DEA) [46].

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Figure 10: (a) PL spectra of CDs with variation of excitation wavelengths (); (b) excitation and PL emission spectra of CDs in the blue-green region (reproduced with permission from [41]).

5.2.1. Upconversion PL

Few CDs exhibit upconversion PL. During upconversion, peak maxima in emission spectra appear at shorter wavelengths even upon excitation with longer wavelengths. Upconversion PL of CDs are effective in terms of higher efficiency and their inherent ability to penetrate within deep tissue without causing any damage [47, 48]. Gude reported citric acid- and tyrosine-based N-CDs and showed upconversion PL by irradiation above 500 nm of (Figure 11(a)) [49]. Cui et al. synthesized green photoluminescent CDs from ammonium citrate and ammonia. These CDs exhibited upconversion behavior and were used for HeLa cell imaging [50]. Recently, Gogoi et al. reported CDs derived from citric acid by the hydrothermal method which showed bright upconverted PL. The as-synthesized CDs, when exited with near infrared (NIR) radiation, PL spectra appeared in the UV-Vis region (Figure 11(b)) [51].

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Figure 11: Upconversion PL spectra of CDs as obtained: (a) excitation wavelength () above 500 nm exhibited peak maxima at around ~420 nm; (b) above 900 nm (NIR region) exhibited green PL at 530 nm (reproduced with permission from [49, 51]).

5.3. Phosphorescence Spectroscopy

CDs are reported to exhibit phosphorescence spectra. Carbonyl groups present on the spherical surface of CDs offer exited triplet state which are responsible for phosphorescence in CDs [52]. Deng et al. reported room temperature phosphorescence (RTP) of CDs derived from disodium salt of EDTA (ethylene diamine tetraacetic acid) in polyvinyl alcohol (PVA) by forming CD-PVA composite. They obtained phosphorescence peak at 500 nm having extended lifetime of 380 ms by exiting the sample with 325 nm (Figure 12) [53]. Li et al. synthesized CDs from citric acid and urea by the hydrothermal method which showed excellent PL behaviors. Subsequently, they prepared CD-CA powder by centrifuging CD and cyanuric acid (CA) mixture. The aqueous solution of CD-CA powder so prepared is reported to emit green phosphorescent light that can be observed even with the naked eye at room temperature [54].

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Figure 12: (a) Mechanism for room temperature phosphorescence showed by CD-PVA matrix; (b) visibility of invisible written words by CDs by UV light (reproduced with permission from [53]).

5.4. Chemiluminescence Spectroscopy

Chemiluminescence (CL) of CDs is another feature which is not explored too much as yet and endowed with wide prospects for future research. Lin et al. first reported CL of CDs where they obtained intense CL in the presence of KMnO₄ and cerium (IV) ion. Hole generated within the matrix of CD by various oxidants combine with electron and release energy as CL [55]. Zhao et al. reported CL from CD derived from glucose and PEG₁₅₀₀ as a surface-passivating agent (Figure 13(a)). The as-synthesized CDs exhibited CL in the presence of dissolved oxygen present in an aqueous alkali medium [56]. Recently, Zhang et al. reported an enhancement of CL intensity for the luminol-K₃Fe(CN)₆ system when in the presence of CDs at alkaline pH. The as-fabricated CD composite material is used for the detection of anticancer drug 2-methoxyestradiol (Figure 13(b)) [57].

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Figure 13: (a) Mechanism for CL generated from CDs (b) enhance CL intensities diagram for (1) luminol-K₃Fe(CN)₆, (2) CDs-luminol-K₃Fe(CN)₆, and (3) CDs-luminol-K₃Fe(CN)₆-2-methoxyestradiol (reproduced with permission from [56, 57]).

5.5. Quantum Yield

QY is the most important parameter for light-emitting systems and provides an index for their future possibilities to be used as photodevices. It has been reported that high QY can be achieved by surface fabrication on CDs [58, 59]. CDs synthesize from common precursors containing electron-withdrawing groups (EWGs) like carboxylic and epoxy decrease electron density in CDs and produce relatively low QY [60]. Surface engineering can convert EWGs to electron-donating groups (EDGs) by easy chemical conversion without much more alteration in the basic shape of the carbon nanoparticles (Figure 14) [61]. Doping CDs with heteroatom is another alternative approach that can generate new CDs with high QY by changing the bandgap and electron densities [62, 63]. Initially, CDs originated from candle soot, graphite, and citric acid exhibited very low QY maximum up to 10%. Recently, researchers have achieved very high QY. Zhuo et al. synthesized CDs with high QY of 80% in an aqueous medium by using citric acid and glutathione as starting materials [64].

Figure 14: Tuning of QY for CDs (i) PL spectrum of quantum dots having carboxylic acid as functionality (red line) and (ii) having hydroxyl functionality (blue line) (reproduced with permission from [61]).

5.6. Spectroscopic Origin of CDs

Significant progress in synthetic methodologies has been noticed in the last few years while explanation about the photophysical properties especially the PL of CDs has not been clearly understood until now. The PL behaviors of CDs are now being explained by (a) the bandgap transition and (b) the surface defect model.

5.6.1. Bandgap Transition Model

Quantum confinement effect (QCE) is thought to be responsible for PL in CDs. The size of CDs plays an important role for PL of CDs as because QCE are size-dependent phenomenon [65]. Li et al. reported alkali-assisted electrochemical fabrication of CDs varying size 1.2 nm to 3.8 nm. The as-prepared CD exhibited blue PL with emission maximum ~450 nm. The emission wavelengths can be easily tuned from blue to green, green to yellow, and red by altering sizes of CDs (Figure 15(a)) [29]. Theoretical modelling studies showed PL spectrum peak positions, intensities varied with variation of their particle sizes [66]. Sk et al. claimed higher electron delocalization of electron in the sp²-hybridized cyclic network due to the presence of varied functionalities and doped heteroatom reduced bandgap energy by virtue of the QCE and resulted red-shifted PL spectra (Figure 15(b)) [67].

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Figure 15: Tuning of PL for CDs: (a) (i) visible colours observed by the naked eye for different CDs; (ii) PL spectra for blue, green, yellow, and red CDs showed by red, black, green, and blue lines (b) with sp²- and sp³-hybrid functionalities (reproduced with permission from [29, 67]).

5.6.2. Surface Defect Model

PL exhibited by CDs are also envisaged due to surface defects formed in CDs which helps to capture excitations. Surface defects in CDs are normally generated due to surface oxidation, presence of functionalities, and doping of heteroatoms [68]. Surface oxidations lead to higher surface defects, resulting in larger number of emissive sites within CDs (Figure 16(a)) [39, 69, 70]. Enhanced photoluminescent property has been explained by doping of heteroatom that altered electron density in the cyclic structure of CDs (Figure 16(b)) [71] and multiphoton PL mechanism followed by anti-Stoke transitions (Figure 16(c)) [49]. Chen et al., Hu et al., and other groups noticed significant variation of PL spectra of CDs which had been attributed to surface defects resulting in the alteration of bandgap energies [69]. Ding et al. observed red-shifted PL of CDs from 440 to 625 nm by rising oxygen percentage in CD matrix [70]. Tetsuka et al. reported similar phenomenon where they varied nitrogen content by functionality of primary amine [72].

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Figure 16: Tuning of PL for CDs: (a) alternation of PL intensities and peak positions by increasing surface oxidation; (b) peak positions by increasing the percentage of doping heteroatoms; (c) Stokes and anti-Stokes PL diagram for CDs (reproduced with permission from [49, 70, 71]).

6. Applications of CDs

6.1. Detection of Toxic Chemicals in Food

Serious health issues are destined due to the presence of toxic materials like heavy metal ions [73], pesticides [74], antibiotics [75], preservatives [76], and nonbiodegradable chemicals over a certain permissible limit in food staff [77]. CD-based PL quenching sensors are successfully employed for ultrasensitive detections of heavy and highly poisonous metal ions like Hg²⁺ (Figure 17(a)) [73, 78], Cr⁶⁺ [79], Pb²⁺ [80], Cu²⁺ [81], Al³⁺ [82], and Co²⁺ [83]. In most cases, linear relationships between quenching of PL intensity of CDs and concentration of metal ion present in samples have been observed [84]. Along with metal ions, anions like F⁻ [85], PO₄³⁻ [86], I⁻ [87], and HClO₄⁻ [88] can also be detected by utilizing CD-based “off-on” logic gate sensor. These types of CD-based sensor are very much selective and independent over the presence of other ions (Figure 17(b)) [73, 89].

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Figure 17: Detection of Hg²⁺ composite-CDs: (a) PL spectral intensities decrease with increasing of Hg²⁺ concentration; (b) selectivity CDs on Hg²⁺ over the presence of other metal ions; (c) a typical linear plot between PL intensities and glucose concentrations (reproduced with permission from [73, 100]).

Excessive use of veterinary drugs in poultries can create severe health issues in animal bodies. Antibiotic used in poultries may leave antibiotic residues above danger level in animal-derived food products like in milks, eggs, and meats [90]. Residue corresponding to antibiotics was determined by CD-based composite sensor where either PL quenching (turn off) or enhancement (turn on) was observed. Antibiotics or their residues like tetracycline [91], cephalexin [92], ciprofloxacin [93], norfloxacin [94], oxytetracycline, and chlortetracycline [95] have been detected from raw milk, egg, meat, and human urine sample. Estrogen drugs those were used in animals, birds, for fast growth can also be traced out by CD-based sensor very effectively [96].

The presence of bacteria like Escherichia coli [97], Bacillus subtilis, Listeria monocytogenes [98], and salmonella typhimurium [99] in food is also detected by composite CDs. Other food additives like sugar, vitamin, amino acid, and different dyes used in food items are detected by means of CD-based sensor (Figure 17(c)) [100–102].

6.2. Detection of Explosives

Detection and monitoring of explosives have drawn worldwide attention [103] in concern with national and international security [104]. Picric acid, trinitrotoluene (TNT), and dinitrotoluene (DNT) are very commonly recognized explosives whose presence even in trace concentrations proves life threatening to mankind [105]. Thus, the detections of these chemicals are a challenging task for researchers. Technologies which are normally used for their detections are very expensive. Interestingly, CDs or CD-based composites are proved useful for their detections. There are several reports found where CDs utilized for sensitive detection of explosive with better efficiency and low-cost technology [106, 107]. Zhang et al. reported amino-containing surface-fabricated CDs that have the potential to detect TNT even at ultralow concentrations by the PL quenching technique [106]. Tb-CD-composites constructed with CDs and rare earth metal terbium (Tb) are employed for screening of picric acid ranging from 500 nM to 100 mM (Figure 18) [107].

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Figure 18: (a) Schematic presentation for determination of picric acid; (b) decrease of PL spectral intensities with increasing picric acid concentrations (reproduced with permission from [107]).

6.3. Chemical Sensors

CD-based logic gate sensors are extensively used in the detection of different chemicals. YES, INH, NOT, NOR, AND, integrative NOR and INH, and integrative IMP plus NOR plus AND logic gates have been developed to sense different chemicals [108]. Jana et al. reported a composite logic gate sensor (CD-MnO₂) for sensing low concentration of NaAc and H⁺, respectively (Figure 19(a)) [109]. Interestingly, PL active N-CDs derived from D-(+)-glucose and spermine selectively interacted with right-handed B-DNA and modified it to left-handed Z-DNA under physiological salt conditions (Figure 19(b)) [110]. CDs derived from chitosan [111] and anchored with metal [112] find wide potential applications in biosensings, in cellular imagings, and in drug deliveries. Metal ion like Hg²⁺ ion and organic compound like glutathione have been simultaneously determined by CD-based logic gate sensor up to micromolar limit [113].

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Figure 19: (a) Schematic presentation of a CD-based logic gate sensor; (b) conversion of B-DNA to Z-DNA by CDs (reproduced with permission from [109, 110]).

6.4. Surface Enhance Raman Spectroscopy (SERS)

SERS has now been established as an elegant analytical technique to detect molecules at ultrasensitive concentrations down to a single molecule detection limit [114–116]. However, SERS spectra as obtained from dielectric molecular assay and surface plasmon materials are not so much effective due to lack of proper interaction of the organic molecules with metal or metal complexes. Composite CDs remove such difficulties by behaving as mediator between active metal surfaces and the probe molecules. Wang et al. reported N-CDs which acted as catalyst for the formation of gold nanoparticles by the reaction between HAuCl₄ and H₂O₂. They quantitatively determined SO₄²⁻ concentration by means of SERS intensity measurement (Figure 20) [117]. Bhunia et al. reported a composite-SERS active film fabricated with polydimethylsiloxane (PDMS), CD, and silver nanoparticles. Such substrate exhibited SERS spectra, much well resolved than that reported for conventional SERS-active dyes as well as for bacterial samples [118]. Recently, Zhao et al. reported Ag-CD composites obtained by combination of N-CDs and Ag nanoparticle. The as-synthesized composites were used as a logic gate sensor and exhibited potential SERS activities [119].

Figure 20: Determination of SO₄²⁻ by SERS intensity measurement (reproduced with permission from [117]).

6.5. Drug Delivery, Bioimaging, and Biosensing

CDs are extensively utilized in health and medicinal chemistry for drug deliveries and bioimagings and in the development of efficient biosensors [13] because of their excellent biocompatibility, water solubility, nontoxic, photoluminescent, and high photostability. Most of the nanomaterials reported as drug carriers have not shown any PL activities whereas CDs acted as super drug carrier because of their emissive nature which helped in tracing drugs in normal cell as well as abnormal (disease affected) cell. Zheng et al. reported CD modified with oxaliplatin and used it as an efficient cancer drugs [120]. Composite-CD released oxaliplatin within cell in an optimal condition of cell environment and exhibited their activities on malignant cells. Then, free CDs with their PL behavior assisted to carry out complete study about cancer cell. Recently, Shu et al. synthesized a composite assembly of curcumin with ionic liquid-based CDs which showed its efficacy as anticancer drug having excellent drug-carrying potential, high cell-penetration power, and high drug-loading capacity. The composite exhibited 69.2% drug loading and 87.5% cell viability on HeLa cells [121].

In vitro and in vivo bioimaging study is a very essential tool for clinical purposes for the detection and monitoring of disease-affected abnormal cell in animals. CDs derived from arginine have been successfully utilized in vitro cytotoxicity on different cells like MCF-7, HeLa, and NIH 3T3 with very high cell viability above 90% [122]. Recently, brighter fluorescent spectra of CDs doped with ZnS inorganic salt have been reported by Sun et al. These CDs are efficiently applied in vivo cell imaging study in mice [123]. Huang et al. reported modified CDs with dye ZW800 that exhibited excellent photoluminescent properties and were utilized in in vivo imaging to understand the circulation of blood and in tumor detection [124]. Recently, Singh et al. reported CD-based drug carrier. They showed that their drug carrier interacted with cytosine-rich single-stranded DNA phosphoramidate linkage and released drug due to change in electrostatic interaction with DNA at optimum pH condition (Figure 21) [125].

Figure 21: (a) Schematic pictorial presentation for encapsulation of drugs.

6.6. Photocatalyst

Pure or composite CDs both can absorb light with a broad wavelength range and are used for different photocatalytic activities. Generation H₂ fuel from water by utilizing CD-based photocatalysts are the most promising research topic in recent years. Sun et al. recently reported photodecomposition of CO₂ by utilizing doped CDs with Au as photocatalyst [126]. Liu et al. reported a low cost and environmentally friendly CDs-C₃N₄ composite catalyst which successfully produced H₂ by splitting of water [127]. BiVO₄- and CD-containing composites have also been employed as photocatalyst for degradation of organic pollutants and water splitting (Figure 22) [128]. PEG1500N-functionalized CDs coated with Au have the potential to convert most active components of greenhouse gas, CO₂, into formic or acetic acid [129]. A number of toxic chemicals can be damaged or better converted to less toxic chemical by CD-based photocatalyst [130]. Further, simple CDs can be used as photocatalyst in normal organic oxidation-reduction reaction. Li et al. reported that oxidation of benzyl alcohol to benzaldehyde can be achieved with a very high (92%) conversion rate and excellent selectivity (100%) by means of CD as photocatalyst [131].

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Figure 22: (a) A schematic pictorial diagram for H₂-fuel cell by splitting of water and probable mechanism; (b) amount of H₂ and O₂ generated with time in exposure of light (reproduced with permission from [128]).

6.7. Solar Energy Conversion

Doped CDs [14] introduce a new age in scientific world for the development of solar cells (SCs) by fabricating with CDs in the presence of other materials [132, 133]. In concern of pollution, development of eco-friendly solar cell free from any toxic chemical carries an extra importance. Carolan et al. first reported this achievement to form a completely eco-friendly composite SC device. Their device worked on a FRET-based mechanism (Figure 23) [134, 135] where doped CDs acted as photoactive as well as photoanode layer [136, 137]. Wang et al. reported N-CD-perovskite-based composite that acted as SCs with better efficiency in comparison to normal perovskite SCs [138]. Yang et al. reported unique SCs that worked in very low light even at night [139].

Figure 23: Schematic presentation of solar cell based on CDs (reproduced with permission from [134]).

7. Future Perspectives and Conclusion

This review is focused on the recent progress of CDs in terms of their rational synthesis, surface engineering, and recent applications. Thousands of new synthetic methodologies of CDs are being reported every year, albeit simple and high yields still remain a challenge to their preparations in large scale. Despite of innumerable publications, there are no unequivocal theoretical explanations on the photophysical properties of CDs as yet that leave hence a bright scope of research for the physicists and physical chemists in the near future. Due to its biocompatibility, CDs are expected to replace inorganic quantum dots and will find wide applications in bioimaging and the preparation of logic gate biosensors. In addition, eco-friendly light-harvesting devices constructed with composite CDs are proving their unique presence in energy conversion sector. However, developmental works for low cost and better efficient light-harvesting devices are urgently needed in the future.

Conflicts of Interest

There is no conflict of interest.

Acknowledgments

The authors express their thanks to the Jadavpur University, Department of Higher Education, Science & Technology and Biotechnology (DHESTB, Govt. of West-Bengal) for the financial support through the research projects (Project Sanction No.: 202 (Sanc.)/ST/P/S&T/16G-22/2017), Council of Scientific and Industrial Research (CSIR) (Project Sanction No.: 03 (1437)/18/EMR-II)), and Science and Engineering Research Board (SERB) (File no. EMR/2017/000901, Diary No. SERB/F/10748/2018-2019).

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