Advances in Smart Nanomaterials: Environmental PerspectiveView this Special Issue
Review Article | Open Access
Biswajit Gayen, Soubantika Palchoudhury, Joydeep Chowdhury, "Carbon Dots: A Mystic Star in the World of Nanoscience", Journal of Nanomaterials, vol. 2019, Article ID 3451307, 19 pages, 2019. https://doi.org/10.1155/2019/3451307
Carbon Dots: A Mystic Star in the World of Nanoscience
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.
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 . 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 . 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 . 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. . Two years later, in 2006, Sun et al. first synthesized stable photoluminescent carbon nanoparticles of different sizes and named them “carbon quantum dots” (CQDs) . 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 .
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 . GQDs have only sp2-hybridized carbon framework whereas CDs are composed of both sp2 and sp3 hybrid carbon networks . 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) . 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 . 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 . Surface modification of CDs by different functionalities, passivating agent and solvent, reflects a smart variation in their properties .
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).
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 . 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) . Feng et al. synthesized CDs with QY~46% by Mw irradiation from silkworm chrysalis . 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 . Recently, Sun et al. synthesized N-ethylcarbazole functionalized CDs by a microwave method that exhibited very nice photoinduced redox properties .
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 . Pang et al. reported synthesis of codoped nitrogen and sulfur in CDs (NS-CDs) derived from methionine by a hydrothermal method . 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) . 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 .
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 . 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 . 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) . N-CDs obtained by their synthesis were profoundly used in metal ion detections and in bioimaging.
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 . 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 . 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 . 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) .
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 . 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)) . 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 .
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 . 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 . 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) .
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 . 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)) . Surface engineering can modify the corresponding absorption spectra of CDs that can alter their emission spectra also . Doping of heteroatom can also regulate absorption spectra for doped CDs by altering % heteroatom because of their alteration in the π-π energy level . Surface defects ingrained in CDs are considered to be responsible for broad spectral features in their absorption spectra . 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)) .
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)) . PL behavior of CDs can be controlled by varying the initial precursors and synthetic methodologies and by surface engineering . Interestingly, PL that generates from CDs almost remains unchanged under irradiation for long time  vis-à-vis to organic dyes that are susceptible to photobleaching . 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 . 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 . 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) .
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)) . Cui et al. synthesized green photoluminescent CDs from ammonium citrate and ammonia. These CDs exhibited upconversion behavior and were used for HeLa cell imaging . 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)) .
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 . 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) . 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 .
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 . 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 . 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)) .
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 . 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) . 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 .
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 . 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)) . Theoretical modelling studies showed PL spectrum peak positions, intensities varied with variation of their particle sizes . 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)) .
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 . 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))  and multiphoton PL mechanism followed by anti-Stoke transitions (Figure 16(c)) . 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 . Ding et al. observed red-shifted PL of CDs from 440 to 625 nm by rising oxygen percentage in CD matrix . Tetsuka et al. reported similar phenomenon where they varied nitrogen content by functionality of primary amine .
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 , pesticides , antibiotics , preservatives , and nonbiodegradable chemicals over a certain permissible limit in food staff . 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+ , Pb2+ , Cu2+ , Al3+ , and Co2+ . In most cases, linear relationships between quenching of PL intensity of CDs and concentration of metal ion present in samples have been observed . Along with metal ions, anions like F- , PO43- , I- , and HClO4-  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].
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 . 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 , cephalexin , ciprofloxacin , norfloxacin , oxytetracycline, and chlortetracycline  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 .
The presence of bacteria like Escherichia coli , Bacillus subtilis, Listeria monocytogenes , and salmonella typhimurium  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  in concern with national and international security . Picric acid, trinitrotoluene (TNT), and dinitrotoluene (DNT) are very commonly recognized explosives whose presence even in trace concentrations proves life threatening to mankind . 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 . 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) .
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 . Jana et al. reported a composite logic gate sensor (CD-MnO2) for sensing low concentration of NaAc and H+, respectively (Figure 19(a)) . 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)) . CDs derived from chitosan  and anchored with metal  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 .
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) . 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 . 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 .
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  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 . 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 .
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% . 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 . 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 . 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) .
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 . Liu et al. reported a low cost and environmentally friendly CDs-C3N4 composite catalyst which successfully produced H2 by splitting of water . BiVO4- and CD-containing composites have also been employed as photocatalyst for degradation of organic pollutants and water splitting (Figure 22) . PEG1500N-functionalized CDs coated with Au have the potential to convert most active components of greenhouse gas, CO2, into formic or acetic acid . A number of toxic chemicals can be damaged or better converted to less toxic chemical by CD-based photocatalyst . 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 .
6.7. Solar Energy Conversion
Doped CDs  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 . Yang et al. reported unique SCs that worked in very low light even at night .
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.
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).
- G. A. M. Hutton, B. C. M. Martindale, and E. Reisner, “Carbon dots as photosensitisers for solar-driven catalysis,” Chemical Society Reviews, vol. 46, no. 20, pp. 6111–6123, 2017.
- J. Wang and J. Qiu, “A review of carbon dots in biological applications,” Journal of Materials Science, vol. 51, no. 10, pp. 4728–4738, 2016.
- S. Y. Lim, W. Shen, and Z. Gao, “Carbon quantum dots and their applications,” Chemical Society Reviews, vol. 44, no. 1, pp. 362–381, 2015.
- M. Tuerhong, Y. Xu, and X. B. Yin, “Review on carbon dots and their applications,” Chinese Journal of Analytical Chemistry, vol. 45, no. 1, pp. 139–150, 2017.
- J. Geys, A. Nemmar, E. Verbeken et al., “Acute toxicity and prothrombotic effects of quantum dots: impact of surface charge,” Environmental Health Perspectives, vol. 116, no. 12, pp. 1607–1613, 2008.
- R. Wang, K.-Q. Lu, Z.-R. Tang, and Y.-J. Xu, “Recent progress in carbon quantum dots: synthesis, properties and applications in photocatalysis,” Journal of Materials Chemistry A, vol. 5, no. 8, pp. 3717–3734, 2017.
- M. Farshbaf, S. Davaran, F. Rahimi, N. Annabi, R. Salehi, and A. Akbarzadeh, “Carbon quantum dots: recent progresses on synthesis, surface modification and applications,” Artificial Cells, Nanomedicine, and Biotechnology, vol. 46, no. 7, pp. 1331–1348, 2018.
- X. Xu, R. Ray, Y. Gu et al., “Electrophoretic analysis and purification of fluorescent single-walled carbon nanotube fragments,” Journal of American Chemical Society, vol. 126, no. 40, pp. 12736-12737, 2004.
- Y.-P. Sun, B. Zhou, Y. Lin et al., “Quantum-sized carbon dots for bright and Colorful photoluminescence,” Journal of American Chemical Society, vol. 128, no. 24, pp. 7756-7757, 2006.
- L. Cao, X. Wang, M. J. Meziani et al., “Carbon dots for multiphoton bioimaging,” Journal of the American Chemical Society, vol. 129, no. 37, pp. 11318-11319, 2007.
- V. Georgakilas, J. A. Perman, J. Tucek, and R. Zboril, “Broad family of carbon nanoallotropes: classification, chemistry, and applications of fullerenes, carbon dots, nanotubes, graphene, nanodiamonds, and combined superstructures,” Chemical Reviews, vol. 115, no. 11, pp. 4744–4822, 2015.
- C. Ding, A. Zhu, and Y. Tian, “Functional surface engineering of C-dots for fluorescent biosensing and in vivo bioimaging,” Accounts of Chemical Research, vol. 47, no. 1, pp. 20–30, 2014.
- J. Zuo, T. Jiang, X. Zhao, X. Xiong, S. Xiao, and Z. Zhu, “Preparation and application of fluorescent carbon dots,” Journal of Nanomaterials, vol. 2015, Article ID 787862, 13 pages, 2015.
- F. Yuan, S. Li, Z. Fan, X. Meng, L. Fan, and S. Yang, “Shining carbon dots: synthesis and biomedical and optoelectronic applications,” Nano Today, vol. 11, no. 5, pp. 565–586, 2016.
- H. Zhu, X. Wang, Y. Li, Z. Wang, F. Yang, and X. Yang, “Microwave synthesis of fluorescent carbon nanoparticles with electrochemiluminescence properties,” Chemical Communications, vol. 34, no. 34, pp. 5118–5120, 2009.
- C. Liu, P. Zhang, F. Tian, W. Li, F. Lib, and W. Liu, “One-step synthesis of surface passivated carbon nanodots by microwave assisted pyrolysis for enhanced multicolor photoluminescence and bioimaging,” Journal of Materials Chemistry, vol. 21, no. 35, pp. 13163–13167, 2011.
- J. Feng, W.-J. Wang, X. Hai, Y.-L. Yu, and J.-H. Wang, “Green preparation of nitrogen-doped carbon dots derived from silkworm chrysalis for cell imaging,” Journal of Materials Chemistry B, vol. 4, no. 3, pp. 387–393, 2016.
- Q. Liu, N. Zhang, H. Shi et al., “One-step microwave synthesis of carbon dots for highly sensitive and selective detection of copper ions in aqueous solution,” New Journal of Chemistry, vol. 42, no. 4, pp. 3097–3101, 2018.
- X. Ren, W. Liang, P. Wang et al., “A new approach in functionalization of carbon nanoparticles for optoelectronically relevant carbon dots and beyond,” Carbon, vol. 141, pp. 553–560, 2019.
- B. Zhang, C. -Y. Liu, and Y. Liu, “A Novel One‐Step Approach to Synthesize Fluorescent Carbon Nanoparticles,” European Journal of Inorganic Chemistry, vol. 2010, no. 28, pp. 4411–4414, 2010.
- Y. Pang, H. Gao, S. Wu, and X. Li, “Facile synthesis the nitrogen and sulfur co-doped carbon dots for selective fluorescence detection of heavy metal ions,” Materials Letters, vol. 193, pp. 236–239, 2017.
- J. Shen, S. Shang, X. Chen, D. Wang, and Y. Cai, “Facile synthesis of fluorescence carbon dots from sweet potato for Fe3 + sensing and cell imaging,” Materials Science and Engineering C, vol. 76, pp. 856–864, 2017.
- M. Zhang, X. Zhao, Z. Fang et al., “Fabrication of HA/PEI-functionalized carbon dots for tumor targeting, intracellular imaging and gene delivery,” RSC Advances, vol. 7, no. 6, pp. 3369–3375, 2017.
- B. C. M. Martindale, G. A. M. Hutton, C. A. Caputo, and E. Reisner, “Solar hydrogen production using carbon quantum dots and a molecular nickel catalyst,” Journal of American Chemical Society, vol. 137, no. 18, pp. 6018–6025, 2015.
- Y. Guo, L. Zhang, F. Cao, and Y. Leng, “Thermal treatment of hair for the synthesis of sustainable carbon quantum dots and the applications for sensing Hg2+,” Scientific Reports, vol. 6, no. 1, p. 35795, 2016.
- M. Rong, Y. Feng, Y. Wang, and X. Chen, “One-pot solid phase pyrolysis synthesis of nitrogen-doped carbon dots for Fe3+ sensing and bioimaging,” Sensors and Actuators B: Chemical, vol. 245, pp. 868–874, 2017.
- J. Zhou, C. Booker, R. Li et al., “An electrochemical avenue to blue luminescent nanocrystals from multiwalled carbon nanotubes (MWCNTs),” Journal of American Chemical Society, vol. 129, no. 4, pp. 744-745, 2007.
- L. Zheng, Y. Chi, Y. Dong, J. Lin, and B. Wang, “Electrochemiluminescence of water-soluble carbon nanocrystals released electrochemically from graphite,” Journal of American Chemical Society, vol. 131, no. 13, pp. 4564-4565, 2009.
- H. Li, X. He, Z. Kang et al., “Water‐Soluble Fluorescent Carbon Quantum Dots and Photocatalyst Design,” Angewandte Chemie, vol. 49, no. 26, pp. 4430–4434, 2010.
- Z. Zhao and Y. Xie, “Enhanced electrochemical performance of carbon quantum dots-polyaniline hybrid,” Journal of Power Sources, vol. 337, pp. 54–64, 2017.
- V. Thongpool, P. Asanithi, and P. Limsuwan, “Synthesis of carbon particles using laser ablation in ethanol,” Procedia Engineering, vol. 32, pp. 1054–1060, 2012.
- C. Doñate-Buendia, R. Torres-Mendieta, A. Pyatenko, E. Falomir, M. Fernández-Alonso, and G. Mínguez-Vega, “Fabrication by laser irradiation in a continuous flow jet of carbon quantum dots for fluorescence imaging,” ACS Omega, vol. 3, no. 3, pp. 2735–2742, 2018.
- M. Bottini, C. Balasubramanian, M. I. Dawson, A. Bergamaschi, S. Bellucci, and T. Mustelin, “Isolation and characterization of fluorescent nanoparticles from pristine and oxidized electric arc-produced single-walled carbon nanotubes,” The Journal of Physical Chemistry B, vol. 110, no. 2, pp. 831–836, 2006.
- S. Dey, A. Govindaraj, K. Biswas, and C. N. R. Rao, “Luminescence properties of boron and nitrogen doped graphene quantum dots prepared from arc-discharge-generated doped graphene samples,” Chemical Physics Letters, vol. 595-596, pp. 203–208, 2014.
- S. N. Baker and G. A. Baker, “Luminescent carbon nanodots: emergent nanolights,” Angewandte Chemie International Edition, vol. 49, no. 38, pp. 6726–6744, 2010.
- A. N. Emam, S. A. Loutfy, A. A. Mostafa, H. Awad, and M. B. Mohamed, “Cyto-toxicity, biocompatibility and cellular response of carbon dots–plasmonic based nanohybrids for bioimaging,” RSC Advances, vol. 7, no. 38, pp. 23502–23514, 2017.
- S. Kim, S. W. Hwang, M.-K. Kim et al., “Anomalous behaviors of visible luminescence from graphene quantum dots: interplay between size and shape,” ACS Nano, vol. 6, no. 9, pp. 8203–8208, 2012.
- X. Li, S. P. Lau, L. Tang, R. Ji, and P. Yang, “Sulphur doping: a facile approach to tune the electronic structure and optical properties of graphene quantum dots,” Nanoscale, vol. 6, no. 10, pp. 5323–5328, 2014.
- L. Bao, Z. L. Zhang, Z. Q. Tian et al., “Electrochemical tuning of luminescent carbon nanodots: from preparation to luminescence mechanism,” Advanced Materials, vol. 23, no. 48, pp. 5801–5806, 2011.
- Y. Li, H. Shu, X. Niu, and J. Wang, “Electronic and optical properties of edge-functionalized graphene quantum dots and the underlying mechanism,” The Journal of Physical Chemistry C, vol. 119, no. 44, pp. 24950–24957, 2015.
- O. Adedokun, A. Roy, A. O. Awodugba, and P. S. Devi, “Fluorescent carbon nanoparticles from Citrus sinensis as efficient sorbents for pollutant dyes,” Luminescence, vol. 32, no. 1, pp. 62–70, 2017.
- S. Zhu, Q. Meng, L. Wang et al., “Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging,” Angewandte Chemie, vol. 52, no. 14, pp. 3953–3957, 2013.
- X. Li, H. Wang, Y. Shimizu, A. Pyatenko, K. Kawaguchi, and N. Koshizaki, “Preparation of carbon quantum dots with tunable photoluminescence by rapid laser passivation in ordinary organic solvents,” Chemical Communications, vol. 47, no. 3, pp. 932–934, 2011.
- C. Sun, Y. Zhang, S. Kalytchuk et al., “Down-conversion monochromatic light-emitting diodes with the color determined by the active layer thickness and concentration of carbon dots,” Journal of Material Chemistry C, vol. 3, no. 26, pp. 6613–6615, 2015.
- R. Wang, X. Wang, and Y. Sun, “One-step synthesis of self-doped carbon dots with highly photoluminescence as multifunctional biosensors for detection of iron ions and pH,” Sensors and Actuators B: Chemical, vol. 241, pp. 73–79, 2017.
- X. Wang, L. Cao, F. Lu et al., “Photoinduced electron transfers with carbon dots,” Chemical Communications, no. 25, pp. 3774–3776, 2009.
- Z. Wang, H. Liao, H. Wu, B. Wang, H. Zhao, and M. Tan, “Fluorescent carbon dots from beer for breast cancer cell imaging and drug delivery,” Analytical Methods, vol. 7, no. 20, pp. 8911–8917, 2015.
- Q. Liu, B. Guo, Z. Rao, B. Zhang, and J. R. Gong, “Strong two-photon-induced fluorescence from photostable, biocompatible nitrogen-doped graphene quantum dots for cellular and deep-tissue imaging,” Nano Letters, vol. 13, no. 6, pp. 2436–2441, 2013.
- V. Gude, “Synthesis of hydrophobic photoluminescent carbon nanodots by using L-tyrosine and citric acid through a thermal, oxidation route,” Belilstein Journal of Nanotechnology, vol. 5, pp. 1513–1522, 2014.
- Y. Cui, C. Zhang, L. Sun, Z. Hu, and X. Liu, “Simple and efficient synthesis of strongly green fluorescent carbon dots with upconversion property for direct cell imaging,” Particle & Particle Systems Characterization, vol. 32, no. 5, pp. 542–546, 2015.
- S. Gogoi and R. Khan, “NIR upconversion characteristics of carbon dots for selective detection of glutathione,” New Journal of Chemistry, vol. 42, no. 8, pp. 6399–6407, 2018.
- Y. Wang and A. Hu, “Carbon quantum dots: synthesis, properties and applications,” Journal of Materials Chemistry C, vol. 2, no. 34, pp. 6921–6939, 2014.
- Y. Deng, D. Zhao, X. Chen, F. Wang, H. Song, and D. S. Deng, “Long lifetime pure organic phosphorescence based on water soluble carbon dots,” Chemical Communications, vol. 49, no. 51, pp. 5751–5753, 2013.
- Q. Li, M. Zhou, M. Yang, Q. Yang, Z. Zhang, and J. Shi, “Induction of long-lived room temperature phosphorescence of carbon dots by water in hydrogen-bonded matrices,” Nature Communications, vol. 9, no. 1, p. 734, 2018.
- Z. Lin, W. Xue, H. Chen, and J. -M. Lin, “Classical oxidant induced chemiluminescence of fluorescent carbon dots,” Chemical Communications, vol. 48, no. 7, pp. 1051–1053, 2012.
- L. Zhao, F. Di, D. Wang et al., “Chemiluminescence of carbon dots under strong alkaline solutions: a novel insight into carbon dot optical properties,” Nanoscale, vol. 5, no. 7, pp. 2655–2658, 2013.
- M. Zhang, Y. Jia, J. Cao et al., “Carbon dots-enhanced luminol chemiluminescence and its application to 2-methoxyestradiol determination,” Green Chemistry Letters and Reviews, vol. 11, no. 4, pp. 379–386, 2018.
- M. Krysmann, A. Kelarakis, P. Dallas, and E. Giannelis, “Formation mechanism of carbogenic nanoparticles with dual photoluminescence emission,” Journal of American Chemical Society, vol. 134, no. 2, pp. 747–750, 2012.
- S. C. Ray, A. Saha, N. R. Jana, and R. Sarkar, “Fluorescent carbon nanoparticles: synthesis, characterization, and bioimaging application,” Journal Physical Chemistry C, vol. 113, no. 43, pp. 18546–18551, 2009.
- K. P. Loh, Q. Bao, G. Eda, and M. Chhowalla, “Graphene oxide as a chemically tunable platform for optical applications,” Nature Chemistry, vol. 2, no. 12, pp. 1015–1024, 2010.
- L.-L. Li, J. Ji, R. Fei et al., “A Facile Microwave Avenue to Electrochemiluminescent Two‐Color Graphene Quantum Dots,” Advanced Functional Materials, vol. 22, no. 14, pp. 2971–2979, 2012.
- Y. Zeng, D. Ma, W. Wang et al., “N, S co-doped carbon dots with orange luminescence synthesized through polymerization and carbonization reaction of amino acids,” Applied Surface Science, vol. 342, pp. 136–143, 2015.
- Q. Xu, P. Pu, J. Zhao et al., “Preparation of highly photoluminescent sulfur-doped carbon dots for Fe(III) detection,” Journal of Materials Chemistry A, vol. 3, no. 2, pp. 542–546, 2015.
- Y. Zhuo, H. Miao, D. Zhong, S. Zhu, and X. Yang, “One-step synthesis of high quantum-yield and excitation-independent emission carbon dots for cell imaging,” Materials Letters, vol. 139, pp. 197–200, 2015.
- W. Kwon, Y. -H. Kim, C. -L. Lee et al., “Electroluminescence from graphene quantum dots prepared by amidative cutting of tattered graphite,” Nano Letters, vol. 14, no. 3, pp. 1306–1311, 2014.
- G. Eda, Y. Y. Lin, C. Mattevi et al., “Blue photoluminescence from chemically derived graphene oxide,” Advanced Materials, vol. 22, no. 4, pp. 505–509, 2010.
- M. A. Sk, A. Ananthanarayanan, L. Huang, K. H. Lim, and P. Chen, “Revealing the tunable photoluminescence properties of graphene quantum dots,” Journal of Materials Chemistry C, vol. 2, no. 34, pp. 6954–6960, 2014.
- L. Bao, C. Liu, Z. -L. Zhang, and D. -W. Pang, “Photoluminescence‐Tunable Carbon Nanodots: Surface‐State Energy‐Gap Tuning,” Advanced Materials, vol. 27, no. 10, pp. 1663–1667, 2015.
- S. Hu, A. Trinchi, P. Atkin, and I. Cole, “Tunable photoluminescence across the entire visible spectrum from carbon dots excited by white light,” Angewandte Chemie, vol. 54, no. 10, pp. 2970–2974, 2015.
- H. Ding, S. -B. Yu, J. -S. Wei, and H. -M. Xiong, “Full-color light-emitting carbon dots with a surface-state-controlled luminescence mechanism,” ACS Nano, vol. 10, no. 1, pp. 484–491, 2016.
- L. Guo, J. Ge, W. Liu et al., “Tunable multicolor carbon dots prepared from well-defined polythiophene derivatives and their emission mechanism,” Nanoscale, vol. 8, no. 2, pp. 729–734, 2016.
- H. Tetsuka, R. Asahi, A. Nagoya et al., “Optically Tunable Amino‐Functionalized Graphene Quantum Dots,” Advanced Materials, vol. 24, no. 39, pp. 5333–5338, 2012.
- L. Zhou, Y. Lin, Z. Huang, J. Ren, and X. Qu, “Carbon nanodots as fluorescence probes for rapid, sensitive, and label-free detection of Hg2+ and biothiols in complex matrices,” Chemical Communications, vol. 48, no. 8, pp. 1147–1149, 2012.
- J. Hou, G. Dong, Z. Tian et al., “A sensitive fluorescent sensor for selective determination of dichlorvos based on the recovered fluorescence of carbon dots-Cu(II) system,” Food Chemistry, vol. 202, pp. 81–87, 2016.
- Y. Feng, D. Zhong, H. Miao, and X. Yang, “Carbon dots derived from rose flowers for tetracycline sensing,” Talanta, vol. 140, pp. 128–133, 2015.
- C. Lei, X. Zhao, S. Jiao et al., “A turn-on fluorescent sensor for the detection of melamine based on the anti-quenching ability of Hg2+to carbon nanodots,” Analitical Methods, vol. 8, no. 22, pp. 4438–4444, 2016.
- L. Shi, Y. Li, X. Li et al., “Controllable synthesis of green and blue fluorescent carbon nanodots for pH and Cu2+ sensing in living cells,” Biosensors and Bioelectronics, vol. 77, pp. 598–602, 2016.
- V. Roshni and D. Ottoor, “Synthesis of carbon nanoparticles using one step green approach and their application as mercuric ion sensor,” Journal of Luminescence, vol. 161, pp. 117–122, 2015.
- J. Shen, S. Shang, X. Chen, D. Wang, and Y. Cai, “Highly fluorescent N, S-co-doped carbon dots and their potential applications as antioxidants and sensitive probes for Cr (VI) detection,” Sensors and Actuators B: Chemical, vol. 248, pp. 92–100, 2017.
- Y. Liu, Q. Zhou, J. Li, M. Lei, and X. Yan, “Selective and sensitive chemosensor for lead ions using fluorescent carbon dots prepared from chocolate by one-step hydrothermal method,” Sensors and Actuators B: Chemical, vol. 237, pp. 597–604, 2016.
- Y. Wang, W. Wu, M. Wu et al., “Yellow-visual fluorescent carbon quantum dots from petroleum coke for the efficient detection of Cu2+ ions,” New Carbon Materials, vol. 30, no. 6, pp. 550–559, 2015.
- Y. Kim, G. Jang, and T. S. Lee, “Carbon nanodots functionalized with rhodamine and poly(ethylene glycol) for ratiometric sensing of Al ions in aqueous solution,” Sensors and Actuators B: Chemical, vol. 249, pp. 59–65, 2017.
- X. Wen, L. Shi, G. Wen et al., “Green and facile synthesis of nitrogen-doped carbon nanodots for multicolor cellular imaging and Co2+ sensing in living cells,” Sensors and Actuators B: Chemical, vol. 235, pp. 179–187, 2016.
- M. Zhou, Z. Zhou, A. Gong, Y. Zhang, and Q. Li, “Synthesis of highly photoluminescent carbon dots via citric acid and Tris for iron(III) ions sensors and bioimaging,” Talanta, vol. 143, pp. 107–113, 2015.
- X.-Y. Sun, L.-L. Wu, J.-S. Shen et al., “Highly selective and sensitive sensing for Al3+ and F− based on green photoluminescent carbon dots,” RSC Advances, vol. 6, no. 99, pp. 97346–97351, 2016.
- J. Xu, Y. Zhou, G. Cheng, M. Dong, S. Liu, and C. Huang, “Carbon dots as a luminescence sensor for ultrasensitive detection of phosphate and their bioimaging properties,” Luminescence, vol. 30, no. 4, pp. 411–415, 2015.
- Z. Li, H. Yu, T. Bian et al., “Highly luminescent nitrogen-doped carbon quantum dots as effective fluorescent probes for mercuric and iodide ions,” Journal of Materials Chemistry C, vol. 3, no. 9, pp. 1922–1928, 2015.
- B. Yin, J. Deng, X. Peng et al., “Green synthesis of carbon dots with down- and up-conversion fluorescent properties for sensitive detection of hypochlorite with a dual-readout assay,” Analyst, vol. 138, no. 21, pp. 6551–6557, 2013.
- Y. Zhang, P. Cui, F. Zhang et al., “Fluorescent probes for "off-on" highly sensitive detection of Hg2+ and L-cysteine based on nitrogen-doped carbon dots,” Talanta, vol. 152, pp. 288–300, 2016.
- J.-W. Zhou, X.-M. Zou, S.-H. Song, and G.-H. Chen, “Quantum dots applied to methodology on detection of pesticide and veterinary drug residues,” Journal of Agricultural and Food Chemistry, vol. 66, no. 6, pp. 1307–1319, 2018.
- X. Yang, Y. Luo, S. Zhu, Y. Feng, Y. Zhuo, and Y. Dou, “One-pot synthesis of high fluorescent carbon nanoparticles and their applications as probes for detection of tetracyclines,” Biosensors Bioelectronics, vol. 56, pp. 6–11, 2016.
- F. Akhgari, N. Samadi, K. Farhadi, and M. Akhgari, “A green one-pot, synthesis of nitrogen and sulfur co-doped carbon quantum dots for sensitive and selective detection of cephalexin,” Canadian Journal of Chemistry, vol. 95, no. 6, pp. 641–648, 2017.
- S. Wang, Y. Wang, K. Yang, Y. Zhong, X. Yang, and Z. Chen, “Synthesis of carbon dots originated from hydroxypropylmethyl cellulose for sensing ciprofloxacin,” Analytical Science, vol. 33, no. 10, pp. 1129–1134, 2017.
- J. Hua, Y. Jiao, M. Wang, and Y. Yang, “Determination of norfloxacin or ciprofloxacin by carbon dots fluorescence enhancement using magnetic nanoparticles as adsorbent,” Mikrochimica Acta, vol. 185, no. 2, pp. 1–9, 2018.
- J. Song, J. Li, Z. Guo et al., “A novel fluorescent sensor based on sulfur and nitrogen co-doped carbon dots with excellent stability for selective detection of doxycycline in raw milk,” RSC Advances, vol. 7, no. 21, pp. 12827–12834, 2017.
- C. Zhao, Y. Jiao, L. Zhang, and Y. Yang, “One-step synthesis of S, B co-doped carbon dots and their application for selective and sensitive fluorescence detection of diethylstilbestrol,” New Journal of Chemistry, vol. 42, no. 4, pp. 2857–2864, 2018.
- I. P.-J. Lai, S. G. Harroun, S.-Y. Chen, B. Unnikrishnan, Y.-J. Li, and C.-C. Huang, “Solid-state synthesis of self-functional carbon quantum dots for detection of bacteria and tumor cells,” Sensors and Actuators B: Chemical, vol. 228, pp. 465–470, 2016.
- D. Zhong, Y. Zhuo, Y. Feng, and X. Yang, “Employing carbon dots modified with vancomycin for assaying Gram-positive bacteria like _Staphylococcus aureus_,” Biosensors Bioelectronics, vol. 74, pp. 546–553, 2015.
- R. Wang, Y. Xu, T. Zhang, and Y. Jiang, “Rapid and sensitive detection of Salmonella typhimurium using aptamer-conjugated carbon dots as fluorescence probe,” Analytical Methods, vol. 7, no. 5, pp. 1701–1706, 2015.
- H. Wang, Y. Xie, S. Liu et al., “Presence of fluorescent carbon nanoparticles in baked lamb: their properties and potential application for sensors,” Journal of Agricultural and Food Chemistry, vol. 65, no. 34, pp. 7553–7559, 2017.
- R. Purbia and S. Paria, “A simple turn on fluorescent sensor for the selective detection of thiamine using coconut water derived luminescent carbon dots,” Biosensors and Bioelectronics, vol. 79, pp. 467–475, 2016.
- H. Xu, X. Yang, G. Li, C. Zhao, and X. Liao, “Green synthesis of fluorescent carbon dots for selective detection of tartrazine in food samples,” Journal of Agricultural and Food Chemistry, vol. 63, no. 30, pp. 6707–6714, 2015.
- S. S. R. Dasary, D. Senapati, A. K. Singh, Y. Anjaneyulu, H. Yu, and P. C. Ray, “Highly sensitive and selective dynamic light-scattering assay for TNT detection using p-ATP attached gold nanoparticle,” ACS Applied Materials & Interfaces, vol. 2, no. 12, pp. 3455–3460, 2010.
- S. Kumar, N. Venkatramaiah, and S. Patil, “Fluoranthene based derivatives for detection of trace explosive nitroaromatics,” Journal of Physical Chemistry C, vol. 117, no. 14, pp. 7236–7245, 2013.
- W. E. Tenhaeff, L. D. McIntosh, and K. K. Gleason, “Synthesis of Poly(4‐vinylpyridine) Thin Films by Initiated Chemical Vapor Deposition (iCVD) for Selective Nanotrench‐Based Sensing of Nitroaromatics,” Advanced Functional Materials, vol. 20, no. 7, pp. 1144–1151, 2010.
- L. L. Zhang, Y. J. Han, J. B. Zhu, Y. L. Zhai, and S. J. Dong, “Simple and sensitive fluorescent and electrochemical trinitrotoluene sensors based on aqueous carbon dots,” Analytical Chemistry, vol. 87, no. 4, pp. 2033–2036, 2015.
- B. B. Chen, Z. X. Liu, H. Y. Zou, and C. Z. Huang, “Highly selective detection of 2,4,6-trinitrophenol by using newly developed terbium-doped blue carbon dots,” Analyst, vol. 141, no. 9, pp. 2676–2681, 2016.
- W.-S. Zou, Q.-C. Zhao, W.-L. Kong et al., “Multi-level fluorescent logic gate based on polyamine coated carbon dots capable of responding to four stimuli,” Chemical Engineering Journal, vol. 337, pp. 471–479, 2018.
- J. Jana, T. Aditya, M. Ganguly, and T. Pal, “Carbon dot-MnO2 FRET system for fabrication of molecular logic gates,” Sensors and Actuators B, vol. 246, pp. 716–725, 2017.
- L. Y. Feng, A. D. Zhao, J. Ren, and X. Qu, “Lighting up left-handed Z-DNA: photoluminescent carbon dots induce DNA B to Z transition and perform DNA logic operations,” Nucleic Acids Research, vol. 41, no. 16, pp. 7987–7996, 2013.
- X. Liu, J. Pang, F. Xu, and X. Zhang, “Simple approach to synthesize amino-functionalized carbon dots by carbonization of chitosan,” Scientific Reports, vol. 6, no. 1, p. 31100, 2016.
- D. Chowdhury, N. Gogoi, and G. Majumdar, “Fluorescent carbon dots obtained from chitosan gel,” RSC Advances, vol. 2, no. 32, pp. 12156–12159, 2012.
- Z. Bai, F. Yan, J. Xu et al., “Dual-channel fluorescence detection of mercuric (II) and glutathione by down- and up-conversion fluorescence carbon dots,” Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy, vol. 205, pp. 29–39, 2018.
- S. Saha, M. Ghosh, B. Dutta, and J. Chowdhury, “Silver coated gold nanocolloids entrapped in organized Langmuir-Blodgett Film of stearic acid: Potential evidence of a new SERS active substrate,” Applied Surface Science, vol. 362, pp. 364–373, 2016.
- S. D. Roy, M. Ghosh, and J. Chowdhury, “Near-Field Response on the Far-Field Wavelength-Scanned Surface-Enhanced Raman Spectroscopic Study of Methylene Blue Adsorbed on Gold Nanocolloidal Particles,” Journal of Physical Chemistry C, vol. 122, no. 20, pp. 10981–10991, 2018.
- V. Živanović, Z. Kochovski, C. Arenz, Y. Lu, and J. Kneipp, “SERS and cryo-EM directly reveal different liposome structures during interaction with gold nanoparticles,” Journal of Physical Chemistry Letters, vol. 9, no. 23, pp. 6767–6772, 2018.
- L. Wang, C. Li, Y. Luo, and Z. Jiang, “Preparation of highly catalytic N-doped carbon dots and their application in SERS sulfate sensing,” Materials, vol. 11, no. 9, p. 1655, 2018.
- S. K. Bhunia, L. Zeiri, J. Manna, S. Nandi, and R. Jelinek, “Carbon-dot/silver-nanoparticle flexible SERS-active films,” ACS Applied Materials and Interfaces, vol. 8, no. 38, pp. 25637–25643, 2016.
- Y. Su, B. Shi, S. Liao, J. Zhao, L. Chen, and S. Zhao, “Silver nanoparticles/N-doped carbon-dots nanocomposites derived fromSiraitia Grosvenoriiand its logic gate and surface-enhanced raman scattering characteristics,” ACS Sustainable Chemistry & Enggineering, vol. 4, no. 3, pp. 1728–1735, 2016.
- M. Zheng, S. Liu, J. Li et al., “Integrating oxaliplatin with highly luminescent carbon dots: an unprecedented theranostic agent for personalized medicine,” Advanced Materials, vol. 26, no. 21, pp. 3554–3560, 2014.
- Y. Shu, J. Lu, Q.-X. Mao et al., “Ionic liquid mediated organophilic carbon dots for drug delivery and bioimaging,” Carbon, vol. 114, pp. 324–333, 2017.
- H. Fu, Z. Ji, X. Chen et al., “A versatile ratiometric nanosensing approach for sensitive and accurate detection of Hg2+ and biological thiols based on new fluorescent carbon quantum dots,” Analytical Bioanalytical Chemistry, vol. 409, no. 9, pp. 2373–2382, 2017.
- S. T. Yang, L. Cao, P. G. Luo et al., “Carbon dots for optical imaging in vivo,” Journal of American Chemical Society, vol. 131, no. 32, pp. 11308-11309, 2009.
- X. Huang, F. Zhang, and L. Zhu, “Effect of injection routes on the biodistribution, clearance, and tumor uptake of carbon dots,” ACS Nano, vol. 7, no. 7, pp. 5684–5693, 2013.
- S. Singh, A. Mishra, R. Kumari, K. K. Sinha, and M. K. Singh, “Carbon dots assisted formation of DNA hydrogel for sustained release of drug,” Carbon, vol. 114, pp. 169–176, 2017.
- L. Cao, S. Sahu, and P. Anilkumar, “Carbon nanoparticles as visible-light photocatalysts for efficient CO2 conversion and beyond,” Journal of American Chemical Society, vol. 133, no. 13, pp. 4754–4757, 2011.
- J. Liu, Y. Liu, N. Liu et al., “Metal-free efficient photocatalyst for stable visible water splitting via a two-electron pathway,” Science, vol. 347, no. 6225, pp. 970–974, 2015.
- K.-H. Ye, Z. Wang, J. Gu et al., “Carbon quantum dots as a visible light sensitizer to significantly increase the solar water splitting performance of bismuth vanadate photoanodes,” Energy & Environmental Science, vol. 10, no. 3, pp. 772–779, 2017.
- J. Wang, S. Sahu, S. K. Sonkar et al., “Versatility with carbon dots from overcooked BBQ to brightly fluorescent agents and photocatalysts,” RSC Advances, vol. 3, no. 36, pp. 15604–15607, 2013.
- H. Wang, Z. Wei, H. Matsui, and S. Zhou, “Fe3O4/carbon quantum dots hybrid nanoflowers for highly active and recyclable visible-light driven photocatalyst,” Journal of Materials Chemistry A, vol. 2, no. 38, pp. 15740–15745, 2014.
- H. Li, R. Liu, S. Lian, Y. Liu, H. Huang, and Z. Kang, “Near-infrared light controlled photocatalytic activity of carbon quantum dots for highly selective oxidation reaction,” Nanoscale, vol. 5, no. 8, pp. 3289–3297, 2013.
- C. Piliego, L. Protesescu, S. Z. Bisri, M. V. Kovalenko, and M. A. Loi, “5.2% efficient PbS nanocrystal Schottky solar cells,” Energy & Environmental Science, vol. 6, no. 10, pp. 3054–3059, 2013.
- V. Gupta, N. Chaudhary, R. Srivastava, G. D. Sharma, R. Bhardwaj, and S. Chand, “Luminscent graphene quantum dots for organic photovoltaic devices,” Journal of American Chemical Society, vol. 133, no. 26, pp. 9960–9963, 2011.
- D. Carolan, C. Rocks, D. B. Padmanaban, P. Maguire, V. Svrcek, and D. Mariotti, “Environmentally friendly nitrogen-doped carbon quantum dots for next generation solar cells,” Sustainable Energy & Fuels, vol. 1, no. 7, pp. 1611–1619, 2017.
- T. Forster, “Intermolecular energy migration and fluorescence,” Annals of Physics, vol. 437, no. 2, pp. 55–75, 1948.
- X. Yan, X. Cui, B. Li, and L. -S. Li, “Large, solution-processable graphene quantum dots as light absorbers for photovoltaics,” Nano Letters, vol. 10, no. 5, pp. 1869–1873, 2010.
- P. Mirtchev, E. J. Henderson, N. Soheilnia, C. M. Yip, and G. A. Ozin, “Solution phase synthesis of carbon quantum dots as sensitizers for nanocrystalline TiO2solar cells,” Journal of Materials Chemistry, vol. 22, no. 4, pp. 1265–1269, 2012.
- Y. Wang, J. Zhang, S. Chen, H. Zhang, L. Li, and Z. Fu, “Surface passivation with nitrogen-doped carbon dots for improved perovskite solar cell performance,” Journal of Materials Science, vol. 53, no. 12, pp. 9180–9190, 2018.
- J. Yang, Q. Tang, Q. Meng et al., “Photoelectric conversion beyond sunny days: all-weather carbon quantum dot solar cells,” Journal of Materials Chemistry A, vol. 5, no. 5, pp. 2143–2150, 2017.
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.