Carbon Quantum Dots Optical: Comparison
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Please note this is a comparison between Version 2 by Rita Xu and Version 1 by Mattia Bartoli.

Carbon quantum dots are the materials of a new era with astonishing properties such as high photoluminescence, chemical tuneability and high biocompatibility.

  • carbon quantum dots
  • nanomaterials
  • optical properties

1. Introduction

Carbon materials have undergone several revolutions during the last century [1]. Several exciting discoveries have brought carbon science beyond the traditional use of graphite and carbon black. The rise of nanostructured carbon allotropes such as fullerene [2], carbon nanotubes (CNTs) [3] and graphene [4] has paved the way for a new era in material science. Curiously, fullerenes and CNTs were first observed by accident. This also happened for an entire new class of materials: carbon quantum dots (CDs). In 2004, Xu and co-workers [5] worked on the purification of oxidized CNTs using electrophoretic techniques and isolated a highly fluorescent fraction. This was the first description of CDs, even if a proper and clear identification had to wait until the work of Sun et al. [6]. Since then, CDs have gained great attention from the scientific community due to their high photoluminescence, solubility and size [7,8][7][8]. CDs have emerged as an easier to produce and handle alternative to inorganic quantum dots due to the fast and easy preparation routes [9] and several appealing features such as biocompatibility [10], solubility [11] and optical properties [12]. CDs have been tested for several kinds of advanced biological applications such as bioimaging [13[13][14],14], drug and gene delivery [15,16,17][15][16][17] and theranostics [18,19][18][19]. Furthermore, CDs can be used for other applications such as catalysts [20[20][21][22][23][24],21,22,23,24], sensing [25[25][26],26], environmental remediation [27,28][27][28] and filler for composite production [29].
The scientific community uses the term CDs to describe a quite heterogeneous set of nanoparticles with different chemical and optical features and only two common characteristics: nanometric size and high yield photoluminescence [30]. This loose definition is one of the issues related with carbon dots together while the relation between their properties and their chemical structure represents a major challenge in the field of a rational design of CDs [31]. These unfulfilled tasks hindered a proper critical approach to CD synthesis, and aimed to optimize their interactions with biological systems or to maximize their exploitation for targeting applications [32,33][32][33].

2. CDs’ Optical

Optical Properties

The most interesting and appealing characteristic of CDs is the great intensity of their fluorescence emission [62][34] that originates from quantum confinement (QC) occurring when the exciton’s Bohr radius is bigger than the average size of CDs [63][35]. The nanometer size of carbon dots does not allow the formation of crystal-like conduction, and the valence bands and the electronic level are discrete although somewhat broadened. The HOMO–LUMO gap increases when the CD size decreases, leading to the emission of photons in the UV region with an improvement in the quantum yield (QY) [64][36]. The fluorescence emission for CDs with π-domains of larger size (i.e., GQDs, CQNDs) is mainly due to conjugated π-electrons of the aromatic carbon domains [65][37]. Larger π-domains reduced the HOMO–LUMO gap and red shifted the fluorescence emission peak [62][34]. The same consideration holds for all xGQDs, although some adjustments are needed because of the presence of different heteroatoms. Considering CQNDs, Yu et al. [66][38] investigated another interesting effect on fluorescence coming from the core/shell size ratio and surface residues suggesting the fluorescence emission mechanism reported in Figure 1.
Molecules 28 02772 g004
Figure 1. Scheme of the fluorescence emission of CQNDs based on different core/shell size ratios and surface residues.
CDs’ core-related fluorescence is not the only emission mechanism as surface states also play a role. The great complexity of surface defects and related states is reflected by the various simultaneously active mechanisms such as excitation-dependent luminescence and pleochroism. As reported by Yan et al. [62][34], shell surface defects are capture centers for excitons and promote radiative relaxation from excited states to the ground state, leading to multicolor emissions, the red-shift being determined by the oxidation degree [67][39]. Du et al. [68][40] suggested that shell functional groups bonded to the edge of honeycomb carbon fragments (i.e., hydroxyl, carbonyl, carboxylic residues and their heteroatom derivative) are at the origin of the visible fluorescence in xGCDs. The authors hypothesized that amides play a major role in the blue emission while carboxylic derivatives induce a redshifted emission. CPDs fluorescence is closely related to the shell surface fluorescence of xGQDs due to the absence of a proper graphitic core. Accordingly, the aromatic clusters are dispersed and connected to each other through sp3 orbitals and are deemed the source for fluorescence emission [69][41]. As a matter of fact, the energy levels and electron cloud distribution of π and π* states in the aromatic domains could be influenced by their interaction with σ and σ* states of sp3 carbon surrounding [46][42]. Nevertheless, the great variability of CPDs requires a more detailed discussion that also considers structural and chemical features to provide a comprehensive picture of fluorescence emission. CPD fluorescence could arise from fluorophore residues retained or formed during the synthesis [70][43] of a heteroatoms rich regions. As reported by Qu et al. [71][44], nitrogen-enriched CPDs showed an increment of QY while other elements such as sulphur [72][45], boron [73][46] or fluoride [74][47] could increase the HOMO–LUMO gap, enhance the charge transfer or stabilize the fluorescence emission in a wide pH range, respectively. Nonetheless, CPDs could be fluorescent even without the presence of aromatic domains due to the crosslink enhanced emission effect. This phenomenon was firstly described by Tao et al. [75][48] by comparing the fluorescence emission of poly(ethylenimine) with CPDs-derived materials produced by the degradation and crosslinking of the polymer. The authors reported a magnification of fluorescence emission and they suggested that this was due to the formation of a rigid structure able to forbid the intramolecular rotations promoting the fluorescence emission. This was supported by further studies, indicating that the mobility reduction of the covalent bond due to steric hindrance or supramolecular interactions could further improve the fluorescence of CPDs [76,77][49][50]. Several mechanisms of CD chemiluminescence constitute the pillars of numerous different applications. CD fluorescence could be used for two main different applications, one based on its quenching and one based on its magnification. CD fluorescence quenching represents a profitable analytical tool for the detection of metals [78,79][51][52] and organic species [80][53]. The mechanisms of fluorescence quenching are multiple and are related to the interactions between CDs and the quencher molecules. The quenching mechanisms are (i) dynamic quenching [81][54], (ii) static quenching [82][55], (iii) photoinduced electron transfer [83][56], (iv) Förster resonance energy transfer [84][57], (v) Dexter energy transfer [85][58], (vi) inner filter effect [86][59] and surface energy transfer [87][60]. The dynamic fluorescence quenching is due to a diffusion quencher while CDs are in their excited state. The fluorescence dynamic quenching of CDs is temperature-sensitive and can be used for the detection of both inorganics and organic compounds [88][61]. Gonçalves et al. [89][62] attributed the CDs’ quenching observed in the presence of Hg(II) to the dynamic quenching effect due to the intensity of the phenomena without supporting it with more detailed investigations. A similar consideration was reported by Wang et al. [90][63], suggesting that the simultaneous dynamic and static fluorescence quenching. Contrary to dynamic one, static fluorescence quenching occurs through the formation of a ground state complex between the quencher and CDs [91][64]. The static quenching of CD fluorescence is tentatively exploited for the detection of inorganics that are forming stable complex species with CDs. Xu et al. [92][65] reported static quenching for poly(ethylene glycol)(PEG)-coated CPDs in the presence of Pt(IV) and Au(III). The metal–shell interactions promoted a great quenching of native fluorescence emission intensity without altering the fluorescence decay pathways. Authors suggested that it was due to static quenching involving the absence of electron–hole radiative recombination. Alternatively, to monitoring fluoresce quenching, Gong et al. [93][66] prepared quenched CPDs by creating stable complexes with Fe(III). The quenched CPDs were used to detect ascorbic acid due the formation of iron ascorbate, removing the quencher from CPDs with a corresponding enhancement of fluorescence emission. Wang et al. [94][67] proved the ability of CQNDs to form stable complexes on the shell with several metal ions through complex formation with nitrogen-based residues. The interactions between metals and CDs could also be destructive as reported by Song et al. [95][68]. The authors showed that, in presence of oxygenated water, Fe(II) could promote a Fenton reaction with the chemical oxidation of CDs. Photoinduced electron transfer quenching occurs on distances greater than 10 nm between the quencher and CDs [96][69], while the Förster resonance energy transfer quenching takes place for distances lower than 10 nm and involves dipole–dipole interactions [97][70]. These three phenomena were investigated as possible new routes to convert photons into electrons for the production of molecular devices [98,99][71][72]. Nonetheless, Förster resonance energy transfer has found an interesting application in the development of a dual species probe composed by CDs and metals clusters [100][73]. This approach allowed to quench CD fluorescence and then reactivate it when metal clusters are removed by the presence of analytes. The distance between the CDs and the fluorescence-quenching mechanism is also affected by the condition of the CD system. Yoo and co-workers [101][74] investigated the effect of dilution on GQDs with a controlled carbonization degree. As stated above, the fluorescence emission of GQDs is mainly due to π-conjugated domain of the core but the surface states play a relevant role in promoting the quenching aggregated state. This dual role of the π-conjugated domain in CDs offers a challenge to the prediction of photoluminescence (PL) characteristics in the solid-state. By tuning the core–shell ratio and the aromatic domains size, the authors proved that the CDs with a higher core–shell ratio of π-conjugated domain are dominated by a core emission in the concentrated solution with the total suppression of surface emission. Dexter energy transfer quenching occurs on a similar scale but it is based on orbital overlap [102][75]. The inner filter effect was originally considered as an error in fluorescence measurements, but it was not. It is a particular phenomenon due to the overlap between the excitation or emission spectrum of CDs and the absorption spectrum of quencher [103][76]. Surface energy transfer quenching takes place more in quantum dots that in CDs and it is due to interactions between surface plasmons and the orbital system of a fluorophore [104][77]. Nonetheless, it can also occur between CDs and small metal clusters [105][78]. An interesting phenomenon related to CD fluorescence is the excitation-dependent emission and its origin is debated in several studies [106,107][79][80]. A relevant study was conducted by Krishnaiah et al. [108][81] using a top–down approach to hydrothermally convert blue grass into CPDs. Authors reported the remarkable redshift of a fluorescence emission peak from 370 to 470 nm using a wavelength ranging from 280 up to 400 nm. The authors hypothesized that the red shift occurred due to n→π* transitions between non-bonding molecular orbitals of carbonyl and carboxylic groups on the CPDs shell. This was in good agreement with the modelling of PCDs’ structures reported by Mintz et al. [109][82] and with the nitrogen doping detected. The performances of PCDs were reasonable, with a quantum yield of 7% and the ability of detecting of Mn(II) and Fe(III). Surprisingly, the authors detected a loss of fluorescence quenching in the presence of both Pb(II) and Cd(II) suggesting the poor complexability of these cations by the functionality residues of the shell. The excitation-dependent emission is rather challenging due to the violation of the Kasha–Vavilov rule [110][83] and a first attempt to provide a theoretical explanation was provided by Khan et al. [111][84]. The authors investigated the fluorescence of CQNDs by a nanosecond time resolving spectroscopy reporting a significant energy redistribution and a relaxation among the emitting states. The authors proved that the inhomogeneous broadening and red shift of the emission peak were due to a system of energy sub-states and not caused by a quantum confinement of a different-sized CQNDs [112][85] and only weakly related to the oxidation of shell edges. This last point is still debated and the other authors suggested that the red shift was mainly related to the shell oxidation degree [113][86]. Experimental results are inconclusive and we believe that both interpretations are possible even if they reached opposite conclusions. This is related to the variability in CQNDs structures and unique features that make it impossible to provide a unique explanation for the emission-dependent red shift and require a two-limit scenario as mentioned above, which is able to describe all the CQND materials produced using different routes. CDs could also show another photoluminescence mechanism: phosphorescence [7]. Phosphorescence originates from the intersystem crossing from the lowest excited singlet state level to the triplet state and from radiative decay from the lowest excited triplet state to the ground state one. Particularly, PCDs are quite effective to exploit phosphorescence due to the crosslinking between aromatic domains and supramolecular interactions, RTP can be easily achieved through appropriate design without additional matrices, due to covalently cross-linked frameworks, polymer chains and supramolecular interactions [114][87]. Tao et al. [115][88] systematically studied the PCD phosphorescence, showing that the crosslinked matrix suppressed the nonradiative transitions due the covalent bonding of the neighboring emissive centers. Accordingly, the energy level distributions reduced the energy gap compared with the precursors promoting the formation of triplet states. Furthermore, the complex network of hydrogen bonds contributed to the system rigidity, decreasing the nonradiative relaxation. Phosphorescence was also observed in xGQDs: Song et al. [116][89] exploited the UV phosphorescence of CQNDs by reducing the graphitic domains or reducing the mobility of CD domains by adding sodium isocyanate crystals. The authors proved that the electron transition from the px to the sp2 orbital of the nitrogen generated an orbital angular momentum able to populate the triplet state. At the same time, the encapsulation of CQNDs reduced the energy dissipation triplet excitons reaching a phosphorescence lifetime of up to 16 ms. Similar results could be achieved by entrapping CQNDs into the silica [117][90] or polymeric matrix [118][91]. Knoblauch et al. [119][92] followed a different route to achieve phosphorescence CDs based on tailoring with bromine. Bromine-doped CDs had accessible triplet states exploiting the phosphorescence in liquid media at room temperature. A key factor that must be considered for the preparation of phosphorescent CDs is the stability. As discussed in the review work of Wei et al. [120][93], long exposure to UV light induced a loss of shell residues leading to the quenching of phosphorescence. Hu et al. [121][94] faced this issue by tailoring the CD surface with silicon residues, prolonging the lifetime under hard UV light up to 200 h. Another relevant optical property of CDs is their chemiluminescence [122][95]. Chemiluminescence is produced during chemical reactions in which intermediate radical species decompose to form electronically excited species and deactivate themselves. CD chemiluminescence originates from the fluorophore residues that act as emitting centers, as first reported by Lin et al. [123][96]. CD chemiluminescence has found plenty of applications in optoelectronics and catalytic processes related to the various structures of CDs available with unique fluorophores and energy levels [124][97]. Interestingly, Xu et al. [125][98] reported the tunability of chemiluminescence response by tuning the oxidation degree of CDs. The authors proved that highly oxidized CDs showed a high fluorescence emission with chemiluminescence while the opposite was true for poorly oxidized CDs. The authors concluded that fluorescence was related to the core states for photon absorption with the shell acting as a long wavelength trap providing nonradiative recombination centers. Conversely, chemiluminescence was related to the radicals formed in the shell surface states.

References

  1. Ajayan, P.M. The nano-revolution spawned by carbon. Nature 2019, 575, 49–50.
  2. Kroto, H.W.; Heath, J.R.; O’Brien, S.C.; Curl, R.F.; Smalley, R.E. C60: Buckminsterfullerene. Nature 1985, 318, 162–163.
  3. Radushkevich, L.; Lukyanovich, V.Á. O strukture ugleroda, obrazujucegosja pri termiceskom razlozenii okisi ugleroda na zeleznom kontakte. Zurn. Fisic. Chim. 1952, 26, 88–95.
  4. Novoselov, K.S.; Fal′ko, V.I.; Colombo, L.; Gellert, P.R.; Schwab, M.G.; Kim, K. A roadmap for graphene. Nature 2012, 490, 192–200.
  5. 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.
  6. Sun, Y.-P.; Zhou, B.; Lin, Y.; Wang, W.; Fernando, K.S.; Pathak, P.; Meziani, M.J.; Harruff, B.A.; Wang, X.; Wang, H. Quantum-sized carbon dots for bright and colorful photoluminescence. J. Am. Chem. Soc. 2006, 128, 7756–7757.
  7. Liu, J.; Li, R.; Yang, B. Carbon dots: A new type of carbon-based nanomaterial with wide applications. ACS Cent. Sci. 2020, 6, 2179–2195.
  8. Zheng, C.; Tao, S.; Yang, B. Polymer–Structure-Induced Room-Temperature Phosphorescence of Carbon Dot Materials. Small Struct. 2023, 2200327.
  9. Sagbas, S.; Sahiner, N. Carbon dots: Preparation, properties, and application. In Nanocarbon and Its Composites; Elsevier: Amsterdam, The Netherlands, 2019; pp. 651–676.
  10. Park, Y.; Kim, Y.; Chang, H.; Won, S.; Kim, H.; Kwon, W. Biocompatible nitrogen-doped carbon dots: Synthesis, characterization, and application. J. Mater. Chem. B 2020, 8, 8935–8951.
  11. Sanni, S.O.; Moundzounga, T.H.; Oseghe, E.O.; Haneklaus, N.H.; Viljoen, E.L.; Brink, H.G. One-Step Green Synthesis of Water-Soluble Fluorescent Carbon Dots and Its Application in the Detection of Cu2+. Nanomaterials 2022, 12, 958.
  12. Lim, S.Y.; Shen, W.; Gao, Z. Carbon quantum dots and their applications. Chem. Soc. Rev. 2015, 44, 362–381.
  13. Wang, Y.; Hu, A. Carbon quantum dots: Synthesis, properties and applications. J. Mater. Chem. C 2014, 2, 6921–6939.
  14. Lin, L.; Rong, M.; Luo, F.; Chen, D.; Wang, Y.; Chen, X. Luminescent graphene quantum dots as new fluorescent materials for environmental and biological applications. TrAC Trends Anal. Chem. 2014, 54, 83–102.
  15. Calabrese, G.; De Luca, G.; Nocito, G.; Rizzo, M.G.; Lombardo, S.P.; Chisari, G.; Forte, S.; Sciuto, E.L.; Conoci, S. Carbon dots: An innovative tool for drug delivery in brain tumors. Int. J. Mol. Sci. 2021, 22, 11783.
  16. Zhao, C.; Song, X.; Liu, Y.; Fu, Y.; Ye, L.; Wang, N.; Wang, F.; Li, L.; Mohammadniaei, M.; Zhang, M. Synthesis of graphene quantum dots and their applications in drug delivery. J. Nanobiotechnol. 2020, 18, 142.
  17. Zhang, W.; Chen, J.; Gu, J.; Bartoli, M.; Domena, J.B.; Zhou, Y.; Ferreira, B.C.; Kirbas Cilingir, E.; McGee, C.M.; Sampson, R.; et al. Nano-carrier for gene delivery and bioimaging based on pentaetheylenehexamine modified carbon dots. J. Colloid Interface Sci. 2023, 639, 180–192.
  18. Ross, S.; Wu, R.-S.; Wei, S.-C.; Ross, G.M.; Chang, H.-T. The analytical and biomedical applications of carbon dots and their future theranostic potential: A review. J. Food Drug Anal. 2020, 28, 677.
  19. Mishra, V.; Patil, A.; Thakur, S.; Kesharwani, P. Carbon dots: Emerging theranostic nanoarchitectures. Drug Discov. Today 2018, 23, 1219–1232.
  20. Hutton, G.A.; Martindale, B.C.; Reisner, E. Carbon dots as photosensitisers for solar-driven catalysis. Chem. Soc. Rev. 2017, 46, 6111–6123.
  21. Chen, B.B.; Liu, M.L.; Huang, C.Z. Carbon dot-based composites for catalytic applications. Green Chem. 2020, 22, 4034–4054.
  22. Yu, J.; Song, H.; Li, X.; Tang, L.; Tang, Z.; Yang, B.; Lu, S. Computational studies on carbon dots electrocatalysis: A review. Adv. Funct. Mater. 2021, 31, 2107196.
  23. Zhao, F.; Li, X.; Zuo, M.; Liang, Y.; Qin, P.; Wang, H.; Wu, Z.; Luo, L.; Liu, C.; Leng, L. Preparation of photocatalysts decorated by carbon quantum dots (CQDs) and their applications: A review. J. Environ. Chem. Eng. 2023, 11, 109487.
  24. Sendão, R.M.; Esteves da Silva, J.C.; Pinto da Silva, L. Applications of Fluorescent Carbon Dots as Photocatalysts: A Review. Catalysts 2023, 13, 179.
  25. Lo Bello, G.; Bartoli, M.; Giorcelli, M.; Rovere, M.; Tagliaferro, A. A Review on the Use of Biochar Derived Carbon Quantum Dots Production for Sensing Applications. Chemosensors 2022, 10, 117.
  26. Sun, X.; Lei, Y. Fluorescent carbon dots and their sensing applications. TrAC Trends Anal. Chem. 2017, 89, 163–180.
  27. Tran, N.-A.; Hien, N.T.; Hoang, N.M.; Dang, H.-L.T.; Van Quy, T.; Hanh, N.T.; Vu, N.H.; Dao, V.-D. Carbon dots in environmental treatment and protection applications. Desalination 2023, 548, 116285.
  28. Yang, X.; Sun, J.; Sheng, L.; Wang, Z.; Ye, Y.; Zheng, J.; Fan, M.; Zhang, Y.; Sun, X. Carbon dots cooperatively modulating photocatalytic performance and surface charge of O-doped g-C3N4 for efficient water disinfection. J. Colloid Interface Sci. 2023, 631, 25–34.
  29. Chen, J.; Xiao, G.; Duan, G.; Wu, Y.; Zhao, X.; Gong, X. Structural design of carbon dots/porous materials composites and their applications. Chem. Eng. J. 2021, 421, 127743.
  30. Jing, H.H.; Bardakci, F.; Akgöl, S.; Kusat, K.; Adnan, M.; Alam, M.J.; Gupta, R.; Sahreen, S.; Chen, Y.; Gopinath, S.C. Green Carbon Dots: Synthesis, Characterization, Properties and Biomedical Applications. J. Funct. Biomater. 2023, 14, 27.
  31. Banerjee, A.; Batabyal, S.K.; Pradhan, B.; Mohanta, K.; Bhattacharjee, R.R. Future perspectives of carbon quantum dots. In Carbon Quantum Dots for Sustainable Energy and Optoelectronics; Elsevier: Amsterdam, The Netherlands, 2023; pp. 473–479.
  32. Zhang, R.; Hou, Y.; Sun, L.; Liu, X.; Zhao, Y.; Zhang, Q.; Zhang, Y.; Wang, L.; Li, R.; Wang, C. Recent advances in carbon dots: Synthesis and applications in bone tissue engineering. Nanoscale 2023, 15, 3106–3119.
  33. 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.
  34. Yan, F.; Sun, Z.; Zhang, H.; Sun, X.; Jiang, Y.; Bai, Z. The fluorescence mechanism of carbon dots, and methods for tuning their emission color: A review. Microchim. Acta 2019, 186, 583.
  35. Connerade, J.P. A review of quantum confinement. In AIP Conference Proceedings; American Institute of Physics: College Park, MD, USA, 2009; pp. 1–33.
  36. Zhu, S.; Song, Y.; Zhao, X.; Shao, J.; Zhang, J.; Yang, B. The photoluminescence mechanism in carbon dots (graphene quantum dots, carbon nanodots, and polymer dots): Current state and future perspective. Nano Res. 2015, 8, 355–381.
  37. Li, S.-Y.; He, L. Recent progresses of quantum confinement in graphene quantum dots. Front. Phys. 2022, 17, 33201.
  38. Yu, J.; Liu, C.; Yuan, K.; Lu, Z.; Cheng, Y.; Li, L.; Zhang, X.; Jin, P.; Meng, F.; Liu, H. Luminescence Mechanism of Carbon Dots by Tailoring Functional Groups for Sensing Fe3+ Ions. Nanomaterials 2018, 8, 233.
  39. Sachdev, A.; Matai, I.; Gopinath, P. Implications of surface passivation on physicochemical and bioimaging properties of carbon dots. RSC Adv. 2014, 4, 20915–20921.
  40. Du, J.; Wang, H.; Wang, L.; Zhu, S.; Song, Y.; Yang, B.; Sun, H. Insight into the effect of functional groups on visible-fluorescence emissions of graphene quantum dots. J. Mater. Chem. C 2016, 4, 2235–2242.
  41. Liang, Q.; Ma, W.; Shi, Y.; Li, Z.; Yang, X. Easy synthesis of highly fluorescent carbon quantum dots from gelatin and their luminescent properties and applications. Carbon 2013, 60, 421–428.
  42. Tao, S.; Zhu, S.; Feng, T.; Xia, C.; Song, Y.; Yang, B. The polymeric characteristics and photoluminescence mechanism in polymer carbon dots: A review. Mater. Today Chem. 2017, 6, 13–25.
  43. Zhu, S.; Zhao, X.; Song, Y.; Lu, S.; Yang, B. Beyond bottom-up carbon nanodots: Citric-acid derived organic molecules. Nano Today 2016, 11, 128–132.
  44. Qu, D.; Zheng, M.; Zhang, L.; Zhao, H.; Xie, Z.; Jing, X.; Haddad, R.E.; Fan, H.; Sun, Z. Formation mechanism and optimization of highly luminescent N-doped graphene quantum dots. Sci. Rep. 2014, 4, 5294.
  45. Saberi, Z.; Rezaei, B.; Ensafi, A.A. Fluorometric label-free aptasensor for detection of the pesticide acetamiprid by using cationic carbon dots prepared with cetrimonium bromide. Microchim. Acta 2019, 186, 273.
  46. Jiang, K.; Sun, S.; Zhang, L.; Lu, Y.; Wu, A.; Cai, C.; Lin, H. Red, green, and blue luminescence by carbon dots: Full-color emission tuning and multicolor cellular imaging. Angew. Chem. 2015, 127, 5450–5453.
  47. Pan, L.; Sun, S.; Zhang, A.; Jiang, K.; Zhang, L.; Dong, C.; Huang, Q.; Wu, A.; Lin, H. Truly Fluorescent Excitation-Dependent Carbon Dots and Their Applications in Multicolor Cellular Imaging and Multidimensional Sensing. Adv. Mater. (Deerfield Beach Fla.) 2015, 27, 7782–7787.
  48. Tao, S.; Song, Y.; Zhu, S.; Shao, J.; Yang, B. A new type of polymer carbon dots with high quantum yield: From synthesis to investigation on fluorescence mechanism. Polymer 2017, 116, 472–478.
  49. Zhu, S.; Song, Y.; Shao, J.; Zhao, X.; Yang, B. Non-conjugated polymer dots with crosslink-enhanced emission in the absence of fluorophore units. Angew. Chem. Int. Ed. 2015, 54, 14626–14637.
  50. Liu, S.G.; Luo, D.; Li, N.; Zhang, W.; Lei, J.L.; Li, N.B.; Luo, H.Q. Water-soluble nonconjugated polymer nanoparticles with strong fluorescence emission for selective and sensitive detection of nitro-explosive picric acid in aqueous medium. ACS Appl. Mater. Interfaces 2016, 8, 21700–21709.
  51. Zu, F.; Yan, F.; Bai, Z.; Xu, J.; Wang, Y.; Huang, Y.; Zhou, X. The quenching of the fluorescence of carbon dots: A review on mechanisms and applications. Microchim. Acta 2017, 184, 1899–1914.
  52. Hu, C.; Lin, T.-J.; Huang, Y.-C.; Chen, Y.-Y.; Wang, K.-H.; Lin, K.-Y.A. Photoluminescence quenching of thermally treated waste-derived carbon dots for selective metal ion sensing. Environ. Res. 2021, 197, 111008.
  53. Xiong, Y.; Schneider, J.; Ushakova, E.V.; Rogach, A.L. Influence of molecular fluorophores on the research field of chemically synthesized carbon dots. Nano Today 2018, 23, 124–139.
  54. Lee, H.; Su, Y.-C.; Tang, H.-H.; Lee, Y.-S.; Lee, J.-Y.; Hu, C.-C.; Chiu, T.-C. One-pot hydrothermal synthesis of carbon dots as fluorescent probes for the determination of mercuric and hypochlorite ions. Nanomaterials 2021, 11, 1831.
  55. Rajendran, S.; Ramanaiah, D.V.; Kundu, S.; Bhunia, S.K. Yellow Fluorescent Carbon Dots for Selective Recognition of As3+ and Fe3+ Ions. ACS Appl. Nano Mater. 2021, 4, 10931–10942.
  56. Wang, L.; Jana, J.; Chung, J.S.; Hur, S.H. Glutathione modified N-doped carbon dots for sensitive and selective dopamine detection. Dye. Pigment. 2021, 186, 109028.
  57. Fu, Y.; Huang, L.; Zhao, S.; Xing, X.; Lan, M.; Song, X. A carbon dot-based fluorometric probe for oxytetracycline detection utilizing a Förster resonance energy transfer mechanism. Spectrochim. Acta Part A Mol. Biomol. Spectrosc. 2021, 246, 118947.
  58. Zhang, H.; Liu, J.; Wang, B.; Liu, K.; Chen, G.; Yu, X.; Li, J.; Yu, J. Zeolite-confined carbon dots: Tuning thermally activated delayed fluorescence emission via energy transfer. Mater. Chem. Front. 2020, 4, 1404–1410.
  59. Li, G.; Fu, H.; Chen, X.; Gong, P.; Chen, G.; Xia, L.; Wang, H.; You, J.; Wu, Y. Facile and sensitive fluorescence sensing of alkaline phosphatase activity with photoluminescent carbon dots based on inner filter effect. Anal. Chem. 2016, 88, 2720–2726.
  60. Liu, S.; Zhao, N.; Cheng, Z.; Liu, H. Amino-functionalized green fluorescent carbon dots as surface energy transfer biosensors for hyaluronidase. Nanoscale 2015, 7, 6836–6842.
  61. Sharma, S.; Umar, A.; Sood, S.; Mehta, S.C.; Kansal, S.K. Photoluminescent C-dots: An overview on the recent development in the synthesis, physiochemical properties and potential applications. J. Alloys Compd. 2018, 748, 818–853.
  62. Gonçalves, H.; Esteves da Silva, J.C.G. Fluorescent Carbon Dots Capped with PEG200 and Mercaptosuccinic Acid. J. Fluoresc. 2010, 20, 1023–1028.
  63. Wang, F.; Hao, Q.; Zhang, Y.; Xu, Y.; Lei, W. Fluorescence quenchometric method for determination of ferric ion using boron-doped carbon dots. Microchim. Acta 2016, 183, 273–279.
  64. Fraiji, L.K.; Hayes, D.M.; Werner, T. Static and dynamic fluorescence quenching experiments for the physical chemistry laboratory. J. Chem. Educ. 1992, 69, 424.
  65. Xu, J.; Sahu, S.; Cao, L.; Bunker, C.E.; Peng, G.; Liu, Y.; Fernando, K.A.S.; Wang, P.; Guliants, E.A.; Meziani, M.J.; et al. Efficient Fluorescence Quenching in Carbon Dots by Surface-Doped Metals—Disruption of Excited State Redox Processes and Mechanistic Implications. Langmuir 2012, 28, 16141–16147.
  66. Gong, J.; Lu, X.; An, X. Carbon dots as fluorescent off–on nanosensors for ascorbic acid detection. RSC Adv. 2015, 5, 8533–8536.
  67. Wang, S.; Deng, G.; Yang, J.; Chen, H.; Long, W.; She, Y.; Fu, H. Carbon dot- and gold nanocluster-based three-channel fluorescence array sensor: Visual detection of multiple metal ions in complex samples. Sens. Actuators B Chem. 2022, 369, 132194.
  68. Song, Y.; Zhu, S.; Xiang, S.; Zhao, X.; Zhang, J.; Zhang, H.; Fu, Y.; Yang, B. Investigation into the fluorescence quenching behaviors and applications of carbon dots. Nanoscale 2014, 6, 4676–4682.
  69. Doose, S.; Neuweiler, H.; Sauer, M. Fluorescence Quenching by Photoinduced Electron Transfer: A Reporter for Conformational Dynamics of Macromolecules. ChemPhysChem 2009, 10, 1389–1398.
  70. Sahoo, H. Förster resonance energy transfer—A spectroscopic nanoruler: Principle and applications. J. Photochem. Photobiol. C Photochem. Rev. 2011, 12, 20–30.
  71. Wang, X.; Cao, L.; Lu, F.; Meziani, M.J.; Li, H.; Qi, G.; Zhou, B.; Harruff, B.A.; Kermarrec, F.; Sun, Y.-P. Photoinduced electron transfers with carbon dots. Chem. Commun. 2009, 25, 3774–3776.
  72. Narayanan, R.; Deepa, M.; Srivastava, A.K. Förster resonance energy transfer and carbon dots enhance light harvesting in a solid-state quantum dot solar cell. J. Mater. Chem. A 2013, 1, 3907–3918.
  73. Miao, S.; Liang, K.; Kong, B. Förster resonance energy transfer (FRET) paired carbon dot-based complex nanoprobes: Versatile platforms for sensing and imaging applications. Mater. Chem. Front. 2020, 4, 128–139.
  74. Yoo, H.J.; Kwak, B.E.; Kim, D.H. Competition of the roles of π-conjugated domain between emission center and quenching origin in the photoluminescence of carbon dots depending on the interparticle separation. Carbon 2021, 183, 560–570.
  75. Guillet, J. Polymer Photophysics and Photochemistry; John Wiley & Sons: New York, NY, USA, 1985.
  76. Chen, S.; Yu, Y.-L.; Wang, J.-H. Inner filter effect-based fluorescent sensing systems: A review. Anal. Chim. Acta 2018, 999, 13–26.
  77. Vaishnav, J.K.; Mukherjee, T.K. Long-range resonance coupling-induced surface energy transfer from CdTe quantum dot to plasmonic nanoparticle. J. Phys. Chem. C 2018, 122, 28324–28336.
  78. Amjadi, M.; Abolghasemi-Fakhri, Z.; Hallaj, T. Carbon dots-silver nanoparticles fluorescence resonance energy transfer system as a novel turn-on fluorescent probe for selective determination of cysteine. J. Photochem. Photobiol. A Chem. 2015, 309, 8–14.
  79. Liu, X.; Xu, Z.; Cole, J.M. Molecular Design of UV–vis Absorption and Emission Properties in Organic Fluorophores: Toward Larger Bathochromic Shifts, Enhanced Molar Extinction Coefficients, and Greater Stokes Shifts. J. Phys. Chem. C 2013, 117, 16584–16595.
  80. Araneda, J.F.; Piers, W.E.; Heyne, B.; Parvez, M.; McDonald, R. High Stokes Shift Anilido-Pyridine Boron Difluoride Dyes. Angew. Chem. Int. Ed. 2011, 50, 12214–12217.
  81. Krishnaiah, P.; Atchudan, R.; Perumal, S.; Salama, E.-S.; Lee, Y.R.; Jeon, B.-H. Utilization of waste biomass of Poa pratensis for green synthesis of n-doped carbon dots and its application in detection of Mn2+ and Fe3+. Chemosphere 2022, 286, 131764.
  82. Mintz, K.J.; Bartoli, M.; Rovere, M.; Zhou, Y.; Hettiarachchi, S.D.; Paudyal, S.; Chen, J.; Domena, J.B.; Liyanage, P.Y.; Sampson, R.; et al. A deep investigation into the structure of carbon dots. Carbon 2021, 173, 433–447.
  83. Del Valle, J.C.; Catalán, J. Kasha’s rule: A reappraisal. Phys. Chem. Chem. Phys. 2019, 21, 10061–10069.
  84. Khan, S.; Gupta, A.; Verma, N.C.; Nandi, C.K. Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots. Nano Lett. 2015, 15, 8300–8305.
  85. Demchenko, A.P.; Dekaliuk, M.O. The origin of emissive states of carbon nanoparticles derived from ensemble-averaged and single-molecular studies. Nanoscale 2016, 8, 14057–14069.
  86. Thiyagarajan, S.K.; Raghupathy, S.; Palanivel, D.; Raji, K.; Ramamurthy, P. Fluorescent carbon nano dots from lignite: Unveiling the impeccable evidence for quantum confinement. Phys. Chem. Chem. Phys. 2016, 18, 12065–12073.
  87. Li, W.; Zhou, W.; Zhou, Z.; Zhang, H.; Zhang, X.; Zhuang, J.; Liu, Y.; Lei, B.; Hu, C. A universal strategy for activating the multicolor room-temperature afterglow of carbon dots in a boric acid matrix. Angew. Chem. 2019, 131, 7356–7361.
  88. Tao, S.; Lu, S.; Geng, Y.; Zhu, S.; Redfern, S.A.T.; Song, Y.; Feng, T.; Xu, W.; Yang, B. Design of Metal-Free Polymer Carbon Dots: A New Class of Room-Temperature Phosphorescent Materials. Angew. Chem. Int. Ed. 2018, 57, 2393–2398.
  89. Song, S.-Y.; Liu, K.-K.; Cao, Q.; Mao, X.; Zhao, W.-B.; Wang, Y.; Liang, Y.-C.; Zang, J.-H.; Lou, Q.; Dong, L.; et al. Ultraviolet phosphorescent carbon nanodots. Light Sci. Appl. 2022, 11, 146.
  90. Liang, Y.-C.; Gou, S.-S.; Liu, K.-K.; Wu, W.-J.; Guo, C.-Z.; Lu, S.-Y.; Zang, J.-H.; Wu, X.-Y.; Lou, Q.; Dong, L.; et al. Ultralong and efficient phosphorescence from silica confined carbon nanodots in aqueous solution. Nano Today 2020, 34, 100900.
  91. Jia, J.; Lu, W.; Gao, Y.; Li, L.; Dong, C.; Shuang, S. Recent advances in synthesis and applications of room temperature phosphorescence carbon dots. Talanta 2021, 231, 122350.
  92. Knoblauch, R.; Bui, B.; Raza, A.; Geddes, C.D. Heavy carbon nanodots: A new phosphorescent carbon nanostructure. Phys. Chem. Chem. Phys. 2018, 20, 15518–15527.
  93. Wei, X.; Yang, J.; Hu, L.; Cao, Y.; Lai, J.; Cao, F.; Gu, J.; Cao, X. Recent advances in room temperature phosphorescent carbon dots: Preparation, mechanism, and applications. J. Mater. Chem. C 2021, 9, 4425–4443.
  94. Hu, G.; Xie, Y.; Xu, X.; Lei, B.; Zhuang, J.; Zhang, X.; Zhang, H.; Hu, C.; Ma, W.; Liu, Y. Room temperature phosphorescence from Si-doped-CD-based composite materials with long lifetimes and high stability. Opt. Express 2020, 28, 19550–19561.
  95. Molaei, M.J. A review on nanostructured carbon quantum dots and their applications in biotechnology, sensors, and chemiluminescence. Talanta 2019, 196, 456–478.
  96. Lin, Z.; Xue, W.; Chen, H.; Lin, J.-M. Classical oxidant induced chemiluminescence of fluorescent carbon dots. Chem. Commun. 2012, 48, 1051–1053.
  97. Shen, C.-L.; Lou, Q.; Liu, K.-K.; Dong, L.; Shan, C.-X. Chemiluminescent carbon dots: Synthesis, properties, and applications. Nano Today 2020, 35, 100954.
  98. Xu, Y.; Wu, M.; Feng, X.Z.; Yin, X.B.; He, X.W.; Zhang, Y.K. Reduced carbon dots versus oxidized carbon dots: Photo-and electrochemiluminescence investigations for selected applications. Chem.—Eur. J. 2013, 19, 6282–6288.
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