Synthesis of Doped/Hybrid Carbon Dots: Comparison
View Latest Version
Please note this is a comparison between Version 4 by Vicky Zhou and Version 3 by Vicky Zhou.

Carbon dots (CDs) are a novel type of carbon-based nanomaterial that has gained considerable attention for their unique optical properties, including tunable fluorescence, stability against photobleaching and photoblinking, and strong fluorescence, which is attributed to a large number of organic functional groups (amino groups, hydroxyl, ketonic, ester, and carboxyl groups, etc.). In addition, they also demonstrate high stability and electron mobility. The doping of CDs with organic and inorganic atoms and molecules leads to their functionalization to obtain desired physical and chemical properties for biomedical applications. 

  • CDs
  • hybrid CDs
  • doped CDs
  • synthesis

1. Introduction

Carbon dots (CDs) are part of nanoscale carbon materials, including carbon nanotubes, graphene, fullerene, nano-diamonds, and nanofibers. Over the past few years, carbon and carbon-based nanomaterials have attracted significant scientific attention for their physical, chemical, and biological properties [1]. Carbon-based nanomaterials are highly biocompatible, have low toxicity, and have impressive thermal and mechanical properties, as well as the ability to be functionalized with a wide variety of organic and inorganic molecules [2]. Carbon nanoscale materials with strong fluorescence are commonly referred to as CDs because of their unique properties. CDs are also characterized by their high stability, reduced harmful activity, water solubility, and derivatization ability [3][4]. These properties enable them to be used in a number of disciplines, as presented in Figure 1. Doped or hybrid CDs consist of graphitic carbon and functional heteroatoms (nonmetal) attached with a carbonized core [5][6][7][8][9][10]. These are a relatively new type of nanomaterials and are considered to be promising for various applications [11]. Nanomaterials called CDs are also refer to polymer dots, carbon quantum nanodots, and graphene or graphene oxide quantum dots.
Nanomaterials 12 00898 g001
Figure 1. Overview of doped and hybrid CDs and showing the possible potential applications.
In general, doped CDs are composed of sp2 hybridized conjugated carbon as the core and a shell of organic molecules with functional groups such as COOH, C=O, NH, OH, C-O-C, C-N, SH, or polymer aggregates [1][3]. Moreover, doped CDs are also considered the newest form of zero-dimensional carbon with less than 10 nm diameter and excellent fluorescent properties [12][13][14]. CDs have attracted considerable attention since their discovery in 2006 largely due to their unique fluorescence properties. It has found many applications in arenas of bioimaging [15][16], cell imaging [15], dye degradation [17], chemiluminescence [18][19], solar cells [20], photocatalyst [21], nanoelectronics devices [4], Combat COVID-19 [22], photodynamic therapy [23] and gene delivery [24]. Unlike pristine CDs, much less attention was dedicated to developing doped, hybrid or functionalized CDs either by metals, polymers or other atoms and molecules. Most of the papers dealing with this issue are related to the doped and hybrid CDs because of the enhanced fluorescence resulting from this doping [25][26][27][28][29][30]. CD-based materials are mainly synthesized by either top-down or bottom-up techniques [31], as presented schematically in Figure 2. According to the top-down approach, CDs are prepared by chemical and physical methods, including laser ablation/passivation, chemical oxidation, and electrochemical synthesis [32]. In the bottom-up approach, CDs are made from molecular precursors using laser ablation [33], pyrolysis [34], combustion [34], hydrothermal process [15], microwave [35][36], precipitation method [13] and sonication [37]. In contrast to bottom-up methods, top-down methods are not preferred because they require longer synthesis times, higher costs, and harsh environmental conditions [4]. Nanomaterials 12 00898 g002
Figure 2. This diagram illustrates the top-down and bottom-up approaches to the synthesis of CDs.
Furthermore, the synthesis of doped CDs via top-down routes requires a separate step of doping and functionalization of the CD’s surface, whereas in the bottom-up methods, this is done simultaneously [38]. In addition to these two approaches, several others have been described, for example plasma treatment [39], cage-opening of fullerenes [40] and the solution chemistry synthesis [41]. These methods yield functionalized CDs in a wide range of sizes distribution and very hard to separate the well-dispersed CDs. There are also several other post-synthesis separation steps required, such as chromatography [42], dialysis [43], and gel electrophoresis [44]. Several non-metallic (nitrogen, sulfur, phosphorus, boron, fluorine) and metallic heteroatoms (Sn, Au, Ag, Zn, Ni, Mg, Mo, Gd, Cu, Ga, Fe, Co, Ca) were used as dopants. Some of them were found to affect the physicochemical characteristic of the doped CDs [4][45] The impregnation of metal or nonmetal ions may also change the electronic properties of doped CDs, designing the HOMO-LUMO energy band gap that determines the photo-optical properties of the doped CDs [6][46]. Doping with elements which are essential to the human body, such as calcium, iron, copper, manganese, copper and nickel made these doped CDs of special importance [10]. Doping with elements such as Au, Ag, Ga, Zn, and Fe are also of some importance for catalysis and biomedical applications. Compared to non-metal heteroatoms, metals are better electron donors, have a larger atomic radius and have a larger number of unoccupied orbitals [12][45]. Therefore, the charge density, as well as the charge transition between the graphene matrix and metallic ions, can be changed by doping the CDs with metallic ions [47]. Metal functionalized CDs have significantly more intense optical absorbance in the visible region than pristine CDs due to charge-transfer from the metal/metal-ion to the carbon or graphite [48]. As a result of the surface plasmonic resonance (SPR) of the metal nanoparticles, CDs also exhibit enhanced fluorescence properties [47]. Additionally, the presence of metal atoms in CDs results in an increase in their electrical conductivity as well as electron transfer and electron-acceptance capabilities, which facilitates electron transfer and increases their photocatalytic activity over pristine CDs [48][49]. Furthermore, CDs can act as an excellent reducing agent and reduce incorporated metal ions thus forming metal-doped CDs [50]. Conjugation of metal oxide nanomaterials with CDs or polymers improves their light-induced reactive oxygen species (ROS) production rate [5][51][52]. This enhancement cannot occur without the external stimuli of light. The light promotes up-conversion of the fluorescence characteristic, enhanced photo-induced electron transfer, and unique electron reservoir properties of the doped CDs. It also inhibits recombination of photo-induced electron-hole pairs [10][49][53]. All these effects occur due to the ROS generation. In addition, the increase in the concentration of ROSs enhances their photocatalytic antibacterial activities. Similarly, fluorescent CDs can be conjugated with magnetic nanoparticles to develop multimodal imaging platforms (optical and electromagnetic resonance). Ternary conjugates consisting of fluorescent CDs, NIR-responsive nanoparticles, and magnetic nanoparticles can be used in multimodal biomedical applications [10][49][53].

2. Synthesis of Hybrid and Doped CDs

Recently, several synthetic approaches for the synthesis of CDs and hybrid/doped CDs have been explored and reported. These approaches were aimed at improving the physical-chemical characteristics of the hybrid/doped CDs, such as increasing the fluorescence quantum yield and transparency and narrowing the band gap. These approaches are described herein. CDs can be synthesized by several methods, for example arc-discharge, laser ablation, sonochemical, microwave, hydrothermal and electrochemical methods and strong acid oxidation [4][54][55][56]. The first synthetic procedure was developed for the formation of pristine CDs by an arc discharge procedure [57][58][59]. It involved a polymerization reaction of small organic molecules followed by carbonization to form CDs. Generally, metal- and nonmetal-doped CDs have been classified into two groups: (i) top-down methods and (ii) bottom-up methods. Under the top-down synthesis method, larger carbon structures are broken down using chemical oxidation, discharge, the electrochemical oxidation of carbon, and ultrasonic methods [58][59]. However, this top-down approach has a number of disadvantages, such as harsh reaction conditions, expensive materials, and long reaction times [60][61][62]. As an alternative, a bottom-up approach involves converting carbon-based small molecules and structures into CDs of the desired size. Bottom-up methods include sonochemical, microwave, and hydrothermal thermal decomposition of organic molecules, pyrolysis of carbon-based materials, and solvothermal reaction synthesis [63]. Herein will discuss primarily the bottom-up approach to the synthesis of metal- and non-metal doped CDs using organic molecules and a variety of metal or non-metal precursors.

2.1. Synthesis of Metal-Doped CDs

Several bottom-up approaches have been extensively adopted for the creation of metal-doped CDs because of their low cost and better efficiency. CDs have been synthesized by solvothermal reactions of carbon-based materials and metallic salts to contain ions of silver [64], manganese [65], copper [49], magnesium [66][67], cobalt [68], zinc [69], gadolinium [28][70] and chromium [71]. As an example, aqueous GQDs (graphene quantum dots, which is the same as CDs) (0.1 mg/mL) were prepared by using an electrochemical cyclic voltammetry instrument [66]. Following this, a series of AgNO3 fresh aqueous solutions (0.1 mol/L) with measured volumes (5–60 mL) were added to CD (GQD) solutions (2 mL), respectively, and the above mixtures were stirred for 1 h to form a homogeneous solution of Ag-GQDs [64]. The resulting mixtures were then illuminated by UV light at the wavelength of 365 nm (18 W) from a distance of 20 mm for 3 h. When the photochemical reaction was completed, the color of the solutions transformed from yellow to brownish red (depending on the volume ratio of GQDs to Ag ions), indicating that Ag NPs had been formed [64]. The silver-doped CD (Ag-CD) nanoparticles on silica were engineered to form highly active surfaces enhanced by Raman scattering [64]. A number of researchers have also synthesized Zn-doped CDs for biomedical applications [69][72][73][74][75][76]. Dehydration, coordination, and carbonization are the steps required to synthesize Zn2+ doped CDs according to Cheng et al. [69]. In detail, Zn2+-doped CDs were prepared by dissolving citric acid, urea, and ZnCl2 in toluene. The resultant mixture was transferred to an autoclave hydrothermal reactor and heated at 200 °C for 12 h. It was then allowed to cool to room temperature. The brown solution obtained after the hydrothermal reaction contained the Zn ion-doped CDs. Evaporating the solvents led to the formation of solid samples, and the Zn2+ doped CDs showed excellent solubility in many organic solvents, as well as in water. In addition, the group applied the as-synthesized Zn2+ doped CDs with intense bright yellow PL to a variety of applications. Additionally, a study shows different metal-doped CDs (M = Ga, Sn, Zn, Bi, Au, and Ag) and carbon dot-decorated substrates [4]. CDs were doped with low melting point metals by sonicating the molten metal overlayered with polyethylene glycol 400 (PEG-400) (PEG-400) [14][77][78]. The formation and doping of the CDs occurred simultaneously, and the effects of several experimental parameters, such as reaction temperature (4–150 °C), sonication duration (3 min–3 h) and sonication amplitude (40–70%), [37] were studied [79]. Doping the CDs with metallic Ga, In, Sn, Pb, Zn, Au or Ag was performed by ultrasound irradiation without the use of any catalyst or post-acid/base treatment [12]. Ga-doped CDs were also decorated with Ga nanoparticles and given the terminology Ga@CDs@Ga nanoparticles. Unlike the other reported synthetic routes for Metal@CDs the sonochemical synthesis. Sonications is the only direct technique for the preparation of Metal@CDs. Likewise, Sn-doped CDs [80] was synthesized by sonochemical reaction of polyethylene glycol-400 and metallic Sn as a precursor [78][79]. It yielded both Sn@CDs@Sn nanoparticles and Sn@CDs. In a recent study, Xu et al., synthesized manganese-doped CDs (Mn-CDs) that demonstrated ultrahigh quantum yield of 54% for metal-doped CDs [76]. The Mn-CDs nanomaterials were prepared by a facile hydrothermal synthesis route using sodium citrate and manganese carbonate as the precursors and citric acid as an auxiliary agent. The materials displayed bright blue fluorescence and the capability to switch their luminescence on or off in the presence of compatible ions [76]. Taking advantage of the superior reversible fluorescence properties of the Mn-CD, Xu et al., developed a reusable universal flourescence particles for the nanomolar detection of a mercury ion in simulated polluted water samples [76]. Moreover, Duan et al., described a facile solid-phase synthesis method for the Cu-doped CDs nanomaterials using citric acid as the carbon source and Cu(NO3)2·3H2O as the dopant materials, respectively [49]. These Cu-CDs show superior peroxidase-like activity to horseradish peroxidase and were stable under a wide range of pH and temperatures [49].

2.2. Synthesis of Nonmetal-Doped CDs

The Doping of nonmetals in CDS can affect the overall electrical distribution and the relevant levels of electronic energy in the nonmetal-doped CDs [81]. CDs can be tuned to exhibit their desired fluorescent and other properties by doping foreign atoms [81]. In order to improve the properties of CDs for further applications, various nonmetals such as boron [82], nitrogen [83], phosphorous [84], sulfur [85], silicon [86], and halogens (F, Cl) [87] have been doped into the CDs, using either the “top-down” or “bottom-up” approaches. B, N, and P- doped CDs were recently synthesized by Kumar et al., using hydrothermal and sonochemical methods [45]. They synthesized N-doped carbon quantum dots (NCDs) from bovine serum albumin protein in water via a hydrothermal chemical reaction [15]. The involved the steps of fragmentation, aromatization, polymerization, and carbonization [88]. During the hydrothermal reaction (>175 °C) carbonization of the BSA polymer occurs leading to the formation of a final NCDs material with a complicated surface structure [88]. The same hydrothermal method was used to synthesize boron-doped and phosphorus-doped CDs [45]. A temperature-dependent mechanism for the synthesis of N-doped CDs has been proposed. These NCDs were used for doping/functionalization of hydroxyapatite (NCDs-HA) nanomaterials in water by stirring of hydroxyapatite (HA) with NCDs at room temperature [89]. A study presents a schematic illustration of doping NCDs with the HA particles. NCDs were used as the stabilizer in three different compositions of HA particles accommodate 1% to 5% NCDs [89]. Adding P and N did not improve the fluorescence properties of N/P-co-doped CDs, although their aqueous dispersibility increases significantly [90]. The purpose of S-doping in CDs is typically to enhance metal ion (Fe3+ ions, other heavy metal ion) detection sensitivity by 0–500 µM [91]. However, if the S atom is fixed to the ring of polythiophene, strong red-emitting light can be observed when the excitation wavelength is 543 nm [92]. Further, the B/ N/S-co-doped CDs demonstrate high-efficiency red emission at wavelength near 600 nm [92]. Even though a variety of nonmetals, such as N, P, S, and B, have been reported, there is still a strong demand for the development of highly efficient CDs in term of real application for the society [92]. Therefore, further exploration of other nonmetal doping systems is highly desirable. For example, although fluorine is not present in biological systems, the incorporation of F-containing grafts can increase the therapeutic efficacy of many drugs, improve the chemical stability of proteins, and enhance phase separation in both polar and nonpolar conditions [93]. Thus, by modifying CDs with the F, it may be possible to significantly tune their fluorescence properties including wavelengths and behavior in biological systems or biomedical applications [93]. Zuo et al., studied the F-doping strategy to dramatically lengthen the emission wavelength of CDs [94]. Using 1,2-diamino-4,5-difluorobenzene as the fluorine source and tartaric acid to improve aqueous solubility, they synthesized a kind of F-doped CD (F-CD) in a one-pot solvothermal process [94]. When compared with undoped CDs, the emission wavelength of F-CDs is redshifted by 50 nm [63][94][95][96]. Using an excitation wavelength of 480 nm, the F-CD emits intense yellow fluorescence, while the emitted red light is found upon excitation at 540 nm [94]. Additionally, the F-CDs provide highly sensitive detection of intracellular Ag+ as well as high imaging sensitivity of red blood cells in various cell systems [94]. The hydrothermal method was by Kalaiyarasan to synthesize phosphorus-doped CQDs (PCQDs) from phosphoric acid and Trisodium citrate (TSC) [97]. They investigated the effect of the reaction temperature, durationand precursor concentrations on the formation of PCQDs and the doping effect of phosphorus in CDs. and found that fluorometric probes for iron detection. Moreover, the P-CDs with high fluorescence QY have been examined for changes in the fluorescence intensities induced by the addition of Fe3+. These changes are a consequence of the formation of a stable fluorescent inactive complex (P-CDs-Fe3+), which may be useful for biomedical applications (Such as live cell imaging, Fe3+ detection in blood, urine) [97]. In addition, a highly sensitive fluorescent probe based on sulfur-doped carbon dots (S-CDs) was synthesized by Kamali et al., using a microwave irradiation method [36]. Using this probe, a strong blue emission was observed with 36% QY [36]. This class of nonmetal-doped CDs (B, N, P, S, F, etc) or co-doping two/three nonmetals was utilized as an effective fluorescent materials for live cell imaging owing to their excellent QY, good water dispersibility, tunable strong fluorescence, better biocompatibility and low cytotoxicity.

2.3. Nanohybrids of CDs with Metals and Metal Oxides

CDs-metal nanohybrids are prepared mainly by mixing CDs with dispersions of metallic nanoparticles followed by sonochemical reaction. Kumar et al., synthesized TiO2/Sn@CDs hybrid nanomaterials by adding TiO2 nanoparticles in the powder form into an aqueous solution of Sn@CDs and sonicating for 20 min, followed by drying at 80 °C for 12 h in a vacuum chamber [17]. Recently, Li et al., prepared Cu2O/CDs hybrid nanomaterials using ultrasonic reaction [98]. Kumar et al., also developed hybrid nanostructured CDs decorated with Fe3O4 for neuronal growth enhancement [13]. These Fe3O4 functionalized CDs nanospheres were dispersed in an aqueous solution by ultrasonication for 30 min for further study the physical, chemical and biological properties. Tang et al., prepared CDs-BiVO4 nanosphere by refluxing BiVO4 powder in an aqueous solution of synthesized CDs at 90 °C for three hours. Jiang et al., prepared gold-doped CDs (Au@CDs) or gold decorated CDs nanomaterials by using anisotropic gold nanomaterials via microwave assisted method [99]. Tiny gold clusters were found scattered within the skeletons of CDs or on the surfaces of CDs. Au@CDs effectively combine the properties of CDs and gold nanoclusters (AuNCs). In order to obtain the nanoprobes, antibodies were attached to the surface of Au@CDs for further biomedical applications, especially intracellular imaging of cancer-derived exosomes) [99][100].

3. Conclusions and Summary

The goal was to provide an overview of recent advances in metal and nonmetal doping of CDs, including synthesis approaches, physical-chemical properties, and biomedical applications. Furthermore, metal ion/nonmetal ion doping has been demonstrated to be a promising synthetic route to enhance the chemical, optical, electrical, and magnetic properties of doped CDs by modifying the electronic and atomic arrangement of the CD structure. Thus, doped CD-based nanomaterials have demonstrated remarkable performance in nano probing, sensing, imaging, drug delivery, and tissue engineering. Researchers have significantly improved the biomedical performance of nonmetal/metal-doped CDs by using innovative synthesis techniques. Due to their unique fluorescent properties, excellent biocompatibility, and high aqueous stability, doped C-dots can be a versatile material for biomedical and sensing applications.

References

  1. Lisik, K.; Krokosz, A. Application of Carbon Nanoparticles in Oncology and Regenerative Medicine. Int. J. Mol. Sci. 2021, 22, 8341.
  2. Zhao, Y.; Shi, L.; Fang, J.; Feng, X. Bio-nanoplatforms based on carbon dots conjugating with F-substituted nano-hydroxyapatite for cellular imaging. Nanoscale 2015, 7, 20033–20041.
  3. Alaghmandfard, A.; Sedighi, O.; Rezaei, N.T.; Abedini, A.A.; Khachatourian, A.M.; Toprak, M.S.; Seifalian, A. Recent advances in the modification of carbon-based quantum dots for biomedical applications. Mater. Sci. Eng. C 2021, 120, 111756.
  4. Kumar, R.; Kumar, V.B.; Gedanken, A. Sonochemical synthesis of carbon dots, mechanism, effect of parameters, and catalytic, energy, biomedical and tissue engineering applications. Ultrason. Sonochem. 2020, 64, 105009.
  5. El-Shabasy, R.M.; Elsadek, M.F.; Ahmed, B.M.; Farahat, M.F.; Mosleh, K.M.; Taher, M.M. Recent Developments in Carbon Quantum Dots: Properties, Fabrication Techniques, and Bio-Applications. Processes 2021, 9, 388.
  6. Feng, J.; Dong, H.; Pang, B.; Shao, F.; Zhang, C.; Yu, L.; Dong, L. Theoretical study on the optical and electronic properties of graphene quantum dots doped with heteroatoms. Phys. Chem. Chem. Phys. 2018, 20, 15244–15252.
  7. Li, Y.; Zhao, Y.; Cheng, H.; Hu, Y.; Shi, G.; Dai, L.; Qu, L. Nitrogen-Doped Graphene Quantum Dots with Oxygen-Rich Functional Groups. J. Am. Chem. Soc. 2012, 134, 15–18.
  8. Li, X.; Fu, Y.; Zhao, S.; Xiao, J.; Lan, M.; Wang, B.; Zhang, K.; Song, X.; Zeng, L. Metal ions-doped carbon dots: Synthesis, properties, and applications. Chem. Eng. J. 2021, 430, 133101.
  9. Liu, Y.; Wu, P.; Wu, X.; Ma, C.; Luo, S.; Xu, M.; Li, W.; Liu, S. Nitrogen and copper (II) co-doped carbon dots for applications in ascorbic acid determination by non-oxidation reduction strategy and cellular imaging. Talanta 2020, 210, 120649.
  10. Xu, J.; Tao, J.; Su, L.; Wang, J.; Jiao, T. A Critical Review of Carbon Quantum Dots: From Synthesis toward Applications in Electrochemical Biosensors for the Determination of a Depression-Related Neurotransmitter. Materials 2021, 14, 3987.
  11. Zhu, Z.; Zhai, Y.; Li, Z.; Zhu, P.; Mao, S.; Zhu, C.; Du, D.; Belfiore, L.A.; Tang, J.; Lin, Y. Red carbon dots: Optical property regulations and applications. Mater. Today 2019, 30, 52–79.
  12. Kumar, V.B.; Kumar, R.; Gedanken, A.; Shefi, O. Fluorescent metal-doped carbon dots for neuronal manipulations. Ultrason. Sonochem. 2019, 52, 205–213.
  13. Kumar, V.B.; Marcus, M.; Porat, Z.; Shani, L.; Yeshurun, Y.; Felner, I.; Shefi, O.; Gedanken, A. Ultrafine Highly Magnetic Fluorescent γ-Fe2O3/NCD Nanocomposites for Neuronal Manipulations. ACS Omega 2018, 3, 1897–1903.
  14. Nissan, I.; Kumar, V.B.; Porat, Z.; Makovec, D.; Shefi, O.; Gedanken, A. Sonochemically-fabricated @Ga nanoparticle-aided neural growth. J. Mater. Chem. B 2017, 128, 7756–7757.
  15. Kumar, V.B.; Sheinberger, J.; Porat, Z.; Shav-Tal, Y.; Gedanken, A. A hydrothermal reaction of an aqueous solution of BSA yields highly fluorescent N doped C-dots used for imaging of live mammalian cells. J. Mater. Chem. B 2016, 4, 2913–2920.
  16. Abu-Ghosh, S.; Kumar, V.B.; Fixler, D.; Dubinsky, Z.; Gedanken, A.; Iluz, D. Nitrogen-doped carbon dots prepared from bovine serum albumin to enhance algal astaxanthin production. Algal Res. 2017, 23, 161–165.
  17. Kumar, V.B.; Perkas, N.; Porat, Z.; Gedanken, A. Solar-Light-Driven Photocatalytic Activity of Novel TiO2 Catalyst. ChemistrySelect 2017, 2, 6683–6688.
  18. Teng, P.; Xie, J.; Long, Y.; Huang, X.; Zhu, R.; Wang, X.; Liang, L.; Huang, Y.; Zheng, H. Chemiluminescence behavior of the carbon dots and the reduced state carbon dots. J. Lumin. 2014, 146, 464–469.
  19. Amjadi, M.; Manzoori, J. Strong enhancement of the chemiluminescence of the cerium(IV)-thiosulfate reaction by carbon dots, and its application to the sensitive determination of dopamine. Mikrochim. Acta 2014, 181, 671–677.
  20. Gao, N.; Huang, L.; Li, T.; Song, J.; Hu, H.; Liu, Y.; Ramakrishna, S. Application of carbon dots in dye-sensitized solar cells: A review. J. Appl. Polym. Sci. 2020, 48443, 1–11.
  21. Akbar, K.; Moretti, E.; Vomiero, A. Carbon Dots for Photocatalytic Degradation of Aqueous Pollutants: Recent Advancements. Adv. Opt. Mater. 2021, 9, 2100532.
  22. Kotta, S.; Aldawsari, H.M.; Badr-Eldin, S.M.; Alhakamy, N.A.; Shadab; Nair, A.B.; Deb, P.K. Exploring the Potential of Carbon Dots to Combat COVID-19. Front. Mol. Biosci. 2020, 7, 616575.
  23. Chen, J.; Wu, W.; Zhang, F.; Zhang, J.; Liu, H.; Zheng, J.; Guo, S.; Zhang, J. Graphene quantum dots in photodynamic therapy. Nanoscale Adv. 2020, 2, 4961–4967.
  24. Hu, L.; Sun, Y.; Li, S.; Wang, X.; Hu, K.; Wang, L.; Liang, X.-J.; Wu, Y. Multifunctional carbon dots with high quantum yield for imaging and gene delivery. Carbon N. Y. 2014, 67, 508–513.
  25. Ngo, Y.-L.T.; Nguyen, P.L.; Jana, J.; Choi, W.M.; Chung, J.S.; Hur, S.H. Simple paper-based colorimetric and fluorescent glucose sensor using N-doped carbon dots and metal oxide hybrid structures. Anal. Chim. Acta 2021, 1147, 187–198.
  26. Peng, H.; Li, Y.; Jiang, C.; Luo, C.; Qi, R.; Huang, R.; Duan, C.-G.; Travas-Sejdic, J. Tuning the properties of luminescent nitrogen-doped carbon dots by reaction precursors. Carbon N. Y. 2016, 100, 386–394.
  27. Liu, X.; Zhang, N.; Bing, T.; Shangguan, D. Carbon Dots Based Dual-Emission Silica Nanoparticles as a Ratiometric Nanosensor for Cu2+. Anal. Chem. 2014, 86, 2289–2296.
  28. Hu, M.; Qi, J.; Ruan, J.; Shen, G. Highly Sensitive Detection of Glucose by a “Turn-Off-On” Fluorescent Probe Using Gadolinium-Doped Carbon Dots and Carbon Microparticles. J. Biomed. Nanotechnol. 2018, 14, 1117–1124.
  29. Nabid, M.R.; Bide, Y.; Fereidouni, N. Boron and nitrogen co-doped carbon dots as a metal-free catalyst for hydrogen generation from sodium borohydride. New J. Chem. 2016, 40, 8823–8828.
  30. Teng, X.; Ma, C.; Ge, C.; Yan, M.; Yang, J.; Zhang, Y.; Morais, P.C.; Bi, H. Green synthesis of nitrogen-doped carbon dots from konjac flour with “off–on” fluorescence by Fe3+ and l-lysine for bioimaging. J. Mater. Chem. B 2014, 2, 4631–4639.
  31. De, B.; Karak, N. Recent progress in carbon dot–metal based nanohybrids for photochemical and electrochemical applications. J. Mater. Chem. A 2017, 5, 1826–1859.
  32. Niu, Z.; Zhang, Y.; Zhang, Y.; Lu, X.; Liu, J. Enhanced electrochemical performance of three-dimensional graphene/carbon nanotube composite for supercapacitor application. J. Alloy. Compd. 2020, 820, 153114.
  33. Thongpool, V.; Asanithi, P.; Limsuwan, P. Synthesis of Carbon Particles Using Laser Ablation in Ethanol. Procedia Eng. 2012, 32, 1054–1060.
  34. Kang, C.; Huang, Y.; Yang, H.; Yan, X.F.; Chen, Z.P. A Review of Carbon Dots Produced from Biomass Wastes. Nanomaterials 2020, 10, 2316.
  35. Wang, D.; Zhu, L.; Mccleese, C.; Burda, C.; Chen, J.-F.; Dai, L. Fluorescent carbon dots from milk by microwave cooking. RSC Adv. 2016, 6, 41516–41521.
  36. Kamali, S.R.; Chen, C.-N.; Agrawal, D.C.; Wei, T.-H. Sulfur-doped carbon dots synthesis under microwave irradiation as turn-off fluorescent sensor for Cr(III). J. Anal. Sci. Technol. 2021, 12, 48.
  37. Kumar, V.B.; Porat, Z.; Gedanken, A. Facile one-step sonochemical synthesis of ultrafine and stable fluorescent C-dots. Ultrason. Sonochem. 2016, 28, 367–375.
  38. El-Shafey, A.M. Carbon dots: Discovery, structure, fl uorescent properties, and applications. Green Process. Synth. 2021, 10, 134–156.
  39. Park, S.Y.; Lee, C.Y.; An, H.-R.; Kim, H.; Lee, Y.-C.; Park, E.C.; Chun, H.-S.; Yang, H.Y.; Choi, S.-H.; Kim, H.S.; et al. Advanced carbon dots via plasma-induced surface functionalization for fluorescent and bio-medical applications. Nanoscale 2017, 9, 9210–9217.
  40. Vougioukalakis, G.C.; Roubelakis, M.M.; Orfanopoulos, M. Open-cage fullerenes: Towards the construction of nanosized molecular containers. Chem. Soc. Rev. 2010, 39, 817–844.
  41. Liu, Q.; Zhang, N.; Shi, H.; Ji, W.; Guo, X.; Yuan, W.; Hu, Q. One-step microwave synthesis of carbon dots for highly sensitive and selective detection of copper ions in aqueous solution. New J. Chem. 2018, 42, 3097–3101.
  42. Hinterberger, V.; Damm, C.; Haines, P.; Guldi, D.M.; Peukert, W. Purification and structural elucidation of carbon dots by column chromatography. Nanoscale 2019, 11, 8464–8474.
  43. Chen, C.-Y.; Tsai, Y.-H.; Chang, C.-W. Evaluation of the dialysis time required for carbon dots by HPLC and the properties of carbon dots after HPLC fractionation. New J. Chem. 2019, 43, 6153–6159.
  44. Kokorina, A.A.; Bakal, A.A.; Shpuntova, D.V.; Kostritskiy, A.Y.; Beloglazova, N.V.; De Saeger, S.; Sukhorukov, G.B.; Sapelkin, A.V.; Goryacheva, I.Y. Gel electrophoresis separation and origins of light emission in fluorophores prepared from citric acid and ethylenediamine. Sci. Rep. 2019, 9, 14665.
  45. Kumar, V.B.; Kumar, R.; Friedman, O.; Golan, Y.; Gedanken, A.; Shefi, O. One-Pot Hydrothermal Synthesis of Elements (B, N, P)-Doped Fluorescent Carbon Dots for Cell Labelling, Differentiation and Outgrowth of Neuronal Cells. ChemistrySelect 2019, 4, 4222–4232.
  46. Nie, H.; Li, M.; Li, Q.; Liang, S.; Tan, Y.; Sheng, L.; Shi, W.; Zhang, S.X. Carbon Dots with Continuously Tunable Full-Colour Emission and Their Application in Ratiometric PH Sensing. Chem. Mater. 2014, 26, 3104–3112.
  47. Slate, A.J.; Karaky, N.; Crowther, G.S.; Butler, J.A.; Banks, C.E.; McBain, A.J.; Whitehead, K.A. Graphene Matrices as Carriers for Metal Ions against Antibiotic Susceptible and Resistant Bacterial Pathogens. Coatings 2021, 11, 352.
  48. Zhang, Q.; Xu, W.; Han, C.; Wang, X.; Wang, Y.; Li, Z.; Wu, W.; Wu, M. Graphene structure boosts electron transfer of dual-metal doped carbon dots in photooxidation. Carbon N. Y. 2018, 126, 128–134.
  49. Duan, Y.; Huang, Y.; Chen, S.; Zuo, W.; Shi, B. Cu-Doped Carbon Dots as Catalysts for the Chemiluminescence Detection of Glucose. ACS Omega 2019, 4, 9911–9917.
  50. Raveendran, V.; Kizhakayil, R.N. Fluorescent Carbon Dots as Biosensor, Green Reductant, and Biomarker. ACS Omega 2021, 6, 23475–23484.
  51. Ji, C.; Zhou, Y.; Leblanc, R.M.; Peng, Z. Recent Developments of Carbon Dots in Biosensing: A Review. ACS Sens. 2020, 5, 2724–2741.
  52. Liu, B.; Zhuang, J.; Wei, G. Recent advances in the design of colorimetric sensors for environmental monitoring. Environ. Sci. Nano 2020, 7, 2195–2213.
  53. Lin, L.; Luo, Y.; Tsai, P.; Wang, J.; Chen, X. Metal ions doped carbon quantum dots: Synthesis, physicochemical properties, and their applications. TrAC—Trends Anal. Chem. 2018, 103, 87–101.
  54. Gayen, B.; Palchoudhury, S.; Chowdhury, J. Review Article Carbon Dots: A Mystic Star in the World of Nanoscience. J. Nanomater. 2019, 2019, 19.
  55. De Medeiros, T.V.; Manioudakis, J.; Noun, F.; Macairan, J.-R.; Victoria, F.; Naccache, R. Microwave-assisted synthesis of carbon dots and their applications. J. Mater. Chem. C 2019, 7, 7175–7195.
  56. Nekoueian, S.K.; Amiri, M.; Sillanpa, M.; Marken, F.; Boukherroub, R.; Sabine, S. Chem Soc Rev Carbon-based quantum particles: An electroanalytical and biomedical perspective. Chem Soc Rev. 2019, 48, 4281–4316.
  57. Xu, X.; Ray, R.; Gu, Y.; Ploehn, H.J.; Gearheart, L.; Raker, K.; Scrivens, W.A. Electrophoretic Analysis and Purification of Fluorescent Single-Walled Carbon Nanotube Fragments. J. Am. Chem. Soc. 2004, 126, 12736–12737.
  58. Khayal, A.; Dawane, V.; Amin, M.A.; Tirth, V.; Yadav, V.K.; Algahtani, A.; Khan, S.H.; Islam, S.; Yadav, K.K.; Jeon, B.-H. Advances in the Methods for the Synthesis of Carbon Dots and Their Emerging Applications. Polymers 2021, 13, 3190.
  59. Desmond, L.J.; Phan, A.N.; Gentile, P. Critical overview on the green synthesis of carbon quantum dots and their application for cancer therapy. Environ. Sci. Nano 2021, 8, 848–862.
  60. Gawande, M.B.; Goswami, A.; Felpin, F.-X.; Asefa, T.; Huang, X.; Silva, R.; Zou, X.; Zboril, R.; Varma, R.S. Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 2016, 116, 3722–3811.
  61. Sharma, A.; Das, J. Small molecules derived carbon dots: Synthesis and applications in sensing, catalysis, imaging, and biomedicine. J. Nanobiotechnol. 2019, 17, 92.
  62. Buglioni, L.; Raymenants, F.; Slattery, A.; Zondag, S.D.A.; Noël, T. Technological Innovations in Photochemistry for Organic Synthesis: Flow Chemistry, High-Throughput Experimentation, Scale-up, and Photoelectrochemistry. Chem. Rev. 2021, 122, 2752–2906.
  63. Liu, J.; Li, R.; Yang, B. Carbon Dots: A New Type of Carbon-Based Nanomaterial with Wide Applications. ACS Central Sci. 2020, 6, 2179–2195.
  64. Ge, J.; Li, Y.; Wang, J.; Pu, Y.; Xue, W.; Liu, X. Green synthesis of graphene quantum dots and silver nanoparticles compounds with excellent surface enhanced Raman scattering performance. J. Alloy. Compd. 2016, 663, 166–171.
  65. Sun, S.; Zhao, L.; Wu, D.; Zhang, H.; Lian, H.; Zhao, X.; Wu, A.; Zeng, L. Manganese-Doped Carbon Dots with Redshifted Orange Emission for Enhanced Fluorescence and Magnetic Resonance Imaging. ACS Appl. Bio Mater. 2021, 4, 1969–1975.
  66. Zhang, Z.; Fan, Z. Application of magnesium ion doped carbon dots obtained: Via hydrothermal synthesis for arginine detection. New J. Chem. 2020, 44, 4842–4849.
  67. Han, Y.; Chen, Y.; Wang, N.; He, Z. Magnesium doped carbon quantum dots synthesized by mechanical ball milling and displayed Fe3+ sensing. Mater. Technol. 2019, 34, 336–342.
  68. Huang, S.; Yang, E.; Yao, J.; Chu, X.; Liu, Y.; Zhang, Y.; Xiao, Q. Nitrogen, Cobalt Co-doped Fluorescent Magnetic Carbon Dots as Ratiometric Fluorescent Probes for Cholesterol and Uric Acid in Human Blood Serum. ACS Omega 2019, 4, 9333–9342.
  69. Cheng, J.; Wang, C.-F.; Zhang, Y.; Yang, S.; Chen, S. Zinc ion-doped carbon dots with strong yellow photoluminescence. RSC Adv. 2016, 6, 37189–37194.
  70. Du, F.; Zhang, L.; Zhang, L.; Zhang, M.; Gong, A.; Tan, Y.; Miao, J.; Gong, Y.; Sun, M.; Ju, H.; et al. Engineered gadolinium-doped carbon dots for magnetic resonance imaging-guided radiotherapy of tumors. Biomaterials 2017, 121, 109–120.
  71. Li, C.; Zheng, Y.; Ding, H.; Jiang, H.; Wang, X. Chromium(III)-doped carbon dots: Fluorometric detection of p-nitrophenol via inner filter effect quenching. Mikrochim. Acta 2019, 186, 384.
  72. Li, W.; Wu, M.; Jiang, H.; Yang, L.; Liu, C.; Gong, X. Carbon dots/ZnO quantum dots composite-based white phosphors for white light-emitting diodes. Chem. Commun. 2022, 58, 1910–1913.
  73. Khan, I.A.; Badshah, A.; Shah, F.U.; Assiri, M.A.; Nadeem, M.A. Zinc-Coordination Polymer-Derived Porous Carbon-Supported Stable PtM Electrocatalysts for Methanol Oxidation Reaction. ACS Omega 2021, 6, 6780–6790.
  74. Tammina, S.K.; Wan, Y.; Li, Y.Y.; Yang, Y. Synthesis of N, Zn-doped carbon dots for the detection of Fe3+ ions and bactericidal activity against Escherichia coli and Staphylococcus aureus. J. Photochem. Photobiol. B Biol. 2020, 202, 111734.
  75. Khare, P.; Bhati, A.; Anand, S.R.; Gunture; Sonkar, S.K. Brightly Fluorescent Zinc-Doped Red-Emitting Carbon Dots for the Sunlight-Induced Photoreduction of Cr(VI) to Cr(III). ACS Omega 2018, 3, 5187–5194.
  76. Xu, Q.; Su, R.; Chen, Y.; Sreenivasan, S.T.; Li, N.; Zheng, X.; Zhu, J.; Pan, H.; Li, W.; Xu, C.; et al. Metal Charge Transfer Doped Carbon Dots with Reversibly Switchable, Ultra-High Quantum Yield Photoluminescence. ACS Appl. Nano Mater. 2018, 1, 1886–1893.
  77. Kumar, V.B.; Gedanken, A.; Kimmel, G.; Porat, Z. Ultrasonic cavitation of molten gallium: Formation of micro- and nano-spheres. Ultrason. Sonochem. 2013, 21, 1166–1173.
  78. Kumar, V.B.; Natan, M.; Jacobi, G.; Porat, Z.; Banin, E.; Gedanken, A. as an antibacterial agent for the eradication of Pseudomonas aeruginosa. Int. J. Nanomed. 2017, 12, 725–730.
  79. Kumar, V.B.; Perelshtein, I.; Lipovsky, A.; Porat, Z.; Gedanken, A. The sonochemical synthesis of particles. RSC Adv. 2015, 5, 25533–25540.
  80. Kumar, V.B.; Tang, J.; Lee, K.J.; Pol, V.G.; Gedanken, A. In situ sonochemical synthesis of luminescent and hybrid @Sn anode for lithium-ion batteries. RSC Adv. 2016, 6, 66256–66265.
  81. Li, F.; Yang, D.; Xu, H. Non-Metal-Heteroatom-Doped Carbon Dots: Synthesis and Properties. Chem. A Eur. J. 2018, 25, 1165–1176.
  82. Bian, W.; Wang, X.; Wang, Y.; Yang, H.; Huang, J.; Cai, Z.; Choi, M.M.F. Boron and nitrogen co-doped carbon dots as a sensitive fluorescent probe for the detection of curcumin. Luminescence 2018, 33, 174–180.
  83. Du, F.; Jin, X.; Chen, J.; Hua, Y.; Cao, M.; Zhang, L.; Li, J.; Zhang, L.; Jin, J.; Wu, C.; et al. Nitrogen-doped carbon dots as multifunctional fluorescent probes. J. Nanoparticle Res. 2014, 16, 1–10.
  84. Kumar, R.; Kumar, V.B.; Marcus, M.; Gedanken, A.; Shefi, O. Element (B, N, P) doped carbon dots interaction with neural cells: Promising results and future prospective. In Proceedings of the Progress in Biomedical Optics and Imaging—SPIE, San Diego, CA, USA, 1 July 2019; Volume 10892.
  85. Naik, V.M.; Gunjal, D.B.; Gore, A.H.; Pawar, S.P.; Mahanwar, S.; Anbhule, P.V.; Kolekar, G.B. Quick and low cost synthesis of sulphur doped carbon dots by simple acidic carbonization of sucrose for the detection of Fe3+ ions in highly acidic environment. Diam. Relat. Mater. 2018, 88, 262–268.
  86. Liu, Y.; Cao, Y.; Bu, T.; Sun, X.; Zhe, T.; Huang, C.; Yao, S.; Wang, L. Silicon-doped carbon quantum dots with blue and green emission are a viable ratiometric fluorescent probe for hydroquinone. Mikrochim. Acta 2019, 186, 399.
  87. Song, H.; Yu, J.; Tang, Z.; Yang, B.; Lu, S. Halogen-Doped Carbon Dots on Amorphous Cobalt Phosphide as Robust Electrocatalysts for Overall Water Splitting. Adv. Energy Mater. 2022, 2102573.
  88. Khajuria, D.K.; Kumar, V.B.; Karasik, D.; Gedanken, A. Fluorescent Nanoparticles with Tissue-Dependent Affinity for Live Zebrafish Imaging. ACS Appl. Mater. Interfaces 2017, 9, 18557–18565.
  89. Khajuria, D.K.; Kumar, V.B.; Gigi, D.; Gedanken, A.; Karasik, D. Accelerated Bone Regeneration by Nitrogen-Doped Carbon Dots Functionalized with Hydroxyapatite Nanoparticles. ACS Appl. Mater. Interfaces 2018, 10, 19373–19385.
  90. Tong, X.; Cherif, M.; Zhang, G.; Zhan, X.; Ma, J.; Almesrati, A.; Vidal, F.; Song, Y.; Claverie, J.P.; Sun, S. N, P-Codoped Graphene Dots Supported on N-Doped 3D Graphene as Metal-Free Catalysts for Oxygen Reduction. ACS Appl. Mater. Interfaces 2021, 13, 30512–30523.
  91. Ding, H.; Wei, J.-S.; Xiong, H.-M. Nitrogen and sulfur co-doped carbon dots with strong blue luminescence. Nanoscale 2014, 6, 13817–13823.
  92. Wei, Z.; Lu, W.; Wang, X.; Ni, J.; Prova, U.H.; Wang, C.; Huang, G. Harnessing versatile dynamic carbon precursors for multi-color emissive carbon dots. J. Mater. Chem. C 2022, 10, 1932–1967.
  93. Dehnen, S.; Schafer, L.L.; Lectka, T.; Togni, A. Fluorine: A Very Special Element and Its Very Special Impacts on Chemistry. Organometallics 2021, 40, 3858–3864.
  94. Zuo, G.; Xie, A.; Li, J.; Su, T.; Pan, X.; Dong, W. Large Emission Red-Shift of Carbon Dots by Fluorine Doping and Their Applications for Red Cell Imaging and Sensitive Intracellular Ag+ Detection. J. Phys. Chem. C 2017, 121, 26558–26565.
  95. Muthamma, K.; Sunil, D.; Shetty, P. Carbon dots as emerging luminophores in security inks for anti-counterfeit applications—An up-to-date review. Appl. Mater. Today 2021, 23, 101050.
  96. Cui, S.; Wang, B.; Zan, Y.; Shen, Z.; Liu, S.; Fang, W.; Yan, X.; Li, Y.; Chen, L. Colorful, time-dependent carbon dot-based afterglow with ultralong lifetime. Chem. Eng. J. 2021, 431, 133373.
  97. Kalaiyarasan, G.; Joseph, J.; Kumar, P. Phosphorus-Doped Carbon Quantum Dots as Fluorometric Probes for Iron Detection. ACS Omega 2020, 5, 22278–22288.
  98. Lai, C.-W.; Hsiao, Y.-H.; Peng, Y.-K.; Chou, P.-T. Facile synthesis of highly emissive carbon dots from pyrolysis of glycerol; gram scale production of carbon dots/mSiO2 for cell imaging and drug release. J. Mater. Chem. 2012, 22, 14403–14409.
  99. Jiang, X.; Zong, S.; Chen, C.; Zhang, Y.; Wang, Z.; Cui, Y. Gold–carbon dots for the intracellular imaging of cancer-derived exosomes. Nanotechnology 2018, 29, 175701.
  100. Mitchell, M.J.; Billingsley, M.M.; Haley, R.M.; Wechsler, M.E.; Peppas, N.A.; Langer, R. Engineering precision nanoparticles. Nat. Rev. Drug Discov. 2021, 20, 101–124.
More
© Text is available under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/)
Video Production Service
Feedback