Carbon Quantum Dots and Graphene Quantum Dots: Comparison
View Latest Version
Please note this is a comparison between Version 2 by Camila Xu and Version 1 by Manpreet Kaur.

Carbon quantum dots (CQDs) are small carbon NPs with a size less than 10 nm having excellent conductivity, chemical stability, environmental friendliness, high photostability, broadband optical absorption, low toxicity, photobleaching resistance, high surface area, and ease of modification. Graphene quantum dots (GQDs) are two-dimensional nanocrystals composed of small graphene particles with lateral diameters less than 100 nm. 

  • carbon quantum dots
  • graphene quantum dots
  • sensors

1. Introduction

Carbon-based materials have gained immense attention for environmental decontamination owing to their appealing physicochemical characteristics viz large specific surface area, tunable structure, low toxicity, and outstanding stability. Quantum dots (QDs) are semiconductor nanoparticles (NPs) of a size less than 10 nm, confined inside spatial dimensions with quantized energy states [1,2][1][2]. The atoms in the QDs are arranged similarly to those in bulk materials, however because of 3-dimensional truncation, there are more atoms on their surfaces. QDs have exceptional luminescent and electrical features, including a narrow emission, broad and continuous absorption spectrum, and great light stability [3,4][3][4].
Semiconductor-based QDs are highly effective inorganic fluorescent probes, having long-term photobleaching resistance. In terms of biocompatibility, chemical inertness, and low cytotoxicity, carbon-based QDs viz. carbon quantum dots (CQDs) and graphene quantum dots (GQDs) outperform the typical semiconductor QDs such as CdSe dots, ZnO dots and others [5,6,7,8][5][6][7][8]. They utilize easily available natural resources as raw materials, and have a number of advantages, including wavelength-dependent luminescence emission, ease of synthesis, and bioconjugation [9,10][9][10]. Their optical characteristics are improved by the non-zero band gap. They also exhibit edge effects caused by the existence of a functional group on their surface. In the case of carbon-based QDs, a broad photoluminescence emission with strong excitation wavelength dependency and broad absorption bands are seen [11].

2. CQDs and GQDs

CQDs are small carbon NPs with a size less than 10 nm having excellent conductivity, chemical stability, environmental friendliness, high photostability, broadband optical absorption, low toxicity, photobleaching resistance, high surface area, and ease of modification. The photoluminescent properties of CQDs are their most notable feature [20,21][12][13]. CQDs have high fluorescence stability, which indicates that the fluorescence emission intensity can remain constant even after a long time of continuous stimulation. CQDs are an appealing option in sensor applications because of their unique properties [22][14]. Due to many carboxyl groups on their surface, CQDs have outstanding potential to either contain appropriate chemically reactive groups for functionalization, or to connect with diverse polymeric, organic, biological, inorganics, or natural materials, for surface modification. Surface passivation promotes fluorescence, and surface functionalization improves solubility in both aqueous and non-aqueous media [23][15]. When there is a lot of oxygen in the air, it creates a new energy state inside the band gap of CQDs, which causes the diameter to shift [24][16]. Thus, CQDs as sensors have grabbed the interest of researchers [25][17]. GQDs are two-dimensional nanocrystals composed of small graphene particles with lateral diameters less than 100 nm. Single atomic layered GQDs containing only carbon are ideal. They are also known to include oxygen and hydrogen, as well as many layers having a thickness of less than 10 nm. GQDs have an adjustable band gap energy between 0 and 6 eV. Due to the quantum confinement effect, and conjugated edge effects, the band gap can be tailored by modifying the lateral size or surface chemical characteristics [26][18]. GQDs have exceptional optoelectronic capabilities, high biocompatibility, and low manufacturing costs, making them a viable alternative to well-known metal chalcogenides-based quantum dots. In addition, compared to typical semiconductor quantum dots, graphene has remarkable thermal and electrical conductivity because of the bonding below and above the atomic plane [27,28][19][20]. Tunable photoluminescence (PL), outstanding biocompatibility, remarkable spin property, exceptional UV-blocking capability, and high photo stability, are among the wide range of excellent properties of GQDs, along with other physical characteristics such as high surface area and surface grafting assistance through conjugated network [29][21]. As the pH of GQDs solution is increased from 1.0 to 11.0, a red shift from 522 to 575 nm is detected [30][22]. This is caused by deprotonation of oxygen-containing functional groups like carboxyl, hydroxyl, and epoxy. A peak at 626 nm is detected at pH levels of more than 11.0, giving it red florescence. They are quite sensitive to changes in pH and temperature, and even with a small change in temperature or pH, a rapid shift in fluorescence has been reported. Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images reveal monodispersed GQDs and CQDs [31,32][23][24] (Figure 1). Because of their distinctive photoluminescence properties and tendency to conjugate with various biomolecules, CQDs and GQDs have gained much interest in biomedical and bioanalytical applications. The development of sensitive bioanalysis systems for use in point-of-care testing would also benefit from the usage of CQDs and GQDs in conjunction with their unique optical properties. The simple accessibility of several types of carbon dots for therapeutic usage may mark a discernible improvement in their application. Excellent performance and excellent practicality have been demonstrated for the CQDs-based biomacromolecule detection in point-of-care testing for viruses [33][25]. Very recently, Li et al. [34][26] successfully used Gd3+ doped CQDs and specific antibody to detect SARS-CoV-2. Yuan et al. [35][27] utilized GQDs, gold nanoclusters that were conjugated with antibodies to detect pathogens.
Figure 1. (a) TEM; (b) HRTEM image of GQDs (Adapted from the Ref. [31][23] with the permission of catalysts); (c) TEM; and (d) HRTEM image of CQDs (Adapted from the Ref. [32][24] with the permission of ACS Applied Materials and Interfaces).

3. Difference between CQDs and GQDs

CQDs and GQDs differ in their structural properties. GQDs are crystalline and sp2 hybridized, while CQDs are amorphous and sp3 hybridized. Surface defects produce the fluorescence of CQDs, which have a diameter of less than 10 nm. Quantum confinement causes the fluorescence of GQDs, which range in size from 2 to 20 nm [36][28]. GQDs are distinguished from carbon dots (CDs) by the graphene sheets within the dots, which is less than 100 nm in size and just 10 layers thick. CDs are usually carbon nanoparticles that are quasi-spherical in shape [37][29]. GQDs are ideal spherical nanocrystals of sp2 carbon nanosheets, whereas other types of carbon-based dots, such as CQDs, have a more complex structure with a carbogenic core, which in some cases resembles graphene nanolayers coupled with amorphous carbon. Two types of carbogenic spherical structures: one with a crystalline core holding both sp2 and sp3 carbons; and the other with a disordered structural core containing predominantly sp3 carbons, can be present. Bandgap transitions are produced by conjugated domains of QDs, which resemble massive aromatic systems with substantial conjugation of certain electronic energy bandgaps for photoluminescence emissions. Such electronic transitions are caused by the absorption of light by a number of high-density electrons in the sp2 hybridized orbitals, which have a substantial absorption band in the ultraviolet region [38][30]. The synthesis of CQDs, GQDs, and their nanocomposites, is covered in the next two parts. Figure 2 shows different methods of synthesis of CQDs and GQDs.
Figure 2.
 Different methods for synthesis of CQDs and GQDs and their distinctive features.

References

  1. Sharma, G.; Kumar, A.; Naushad, M.; Kumar, A.; Al-Muhtaseb, A.H.; Dhiman, P.; Ghfar, A.A.; Stadler, F.J.; Khan, M.R. Photoremediation of toxic dye from aqueous environment using monometallic and bimetallic quantum dots-based nanocomposites. J. Clean. Prod. 2018, 172, 2919–2930.
  2. Singh, P.; Shandilya, P.; Raizada, P.; Sudhaik, A.; Rahmani-Sani, A.; Hosseini- Bandegharaei, A. Review on various strategies for enhancing photocatalytic activity of graphene-based nanocomposites for water purification. Arab. J. Chem. 2020, 13, 3498–3520.
  3. Reshma, V.G.; Mohanan, P.V. Quantum dots: Applications and safety consequences. J. Lumin. 2019, 205, 287–298.
  4. Hu, Z.M.; Fei, G.T.; Zhang, L.D. Synthesis and tunable emission of Ga2S3 quantumdots. Mater. Lett. 2019, 239, 17–20.
  5. Hagiwara, K.; Horikoshi, S.; Serpone, N. Luminescent monodispersed carbon quantum dots by a microwave solvothermal method toward bioimaging applications. J. Photochem. Photobiol. A Chem. 2021, 415, 113310.
  6. Ding, S.; Gao, Y.; Ni, B.; Yang, X. Green synthesis of biomass-derived carbon quantum dots as fluorescent probe for Fe3+ detection. Inorg. Chem. Commun. 2021, 130, 108636.
  7. Alaghmandfard, A.; Sedighi, O.; Tabatabaei Rezaei, N.; Abbas Abedini, A.; Malek Khachatourian, A.; 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.
  8. Yue, X.-Y.; Zhou, Z.-J.; Wu, Y.-M.; Li, Y.; Li, J.-C.; Bai, Y.-H.; Wang, J.-L. Application Progress of Fluorescent Carbon Quantum Dots in Food Analysis. Chin. J. Anal. Chem. 2020, 48, 1288–1296.
  9. Abd Rani, U.; Yong Ng, L.; Yin Ng, C.; Mahmoudi, E. A review of carbon quantum dots and their applications in wastewater treatment. Adv. Colloid Interface Sci. 2020, 278, 102124.
  10. Rahal, M.; Atassi, Y.; Alghoraibi, I. Quenching photoluminescence of Carbon Quantum Dots for detecting and tracking the release of Minocycline. J. Photochem. Photobiol. A Chem. 2021, 412, 113257.
  11. Wei Heng, Z.; Chan Chong, W.; Ling Pang, Y.; Hoon Koo, C. An overview of the recent advances of carbon quantum dots/metal oxides in the application of heterogeneous photocatalysis in photodegradation of pollutants towards visible-light and solar energy exploitation. J. Environ. Chem. Eng. 2021, 9, 105199.
  12. 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.
  13. Ahmad Nazri, N.A.; HidayahAzeman, N.; Luo, Y.; Ashrif, A.; Bakar, A. Carbon quantum dots for optical sensor applications: A review. Opt. Laser Technol. 2021, 139, 106928.
  14. Kumar Barman, M.; Patra, A. Current status and prospects on chemical structure driven photoluminescence behaviour of carbon dots. J. Photochem. Photobiol. C 2018, 37, 1–22.
  15. Ying Lim, S.; Shen, W.; Gao, Z. Carbon quantum dots and their applications. Chem. Soc. 2015, 44, 362–381.
  16. Yu, X.; Liu, J.; Yu, Y.; Zuo, S.; Li, B. Preparation and visible light photocatalytic activity of carbon quantum dots/TiO2 nanosheet composites. Carbon 2014, 68, 718–724.
  17. JafarMolaei, M. Principles, mechanisms, and application of carbon quantum dots in sensors: A review. Anal. Methods 2020, 12, 1266–1287.
  18. Zhu, J.; Tang, Y.; Wang, G.; Mao, J.; Liu, Z.; Sun, T.; Wang, M.; Chen, D.; Yang, Y.; Li, J.; et al. Green, rapid, and universal preparation approach of graphene quantum dots under ultraviolet irradiation. ACS Appl. Mater. Interfaces 2017, 9, 14470–14477.
  19. Zhang, N.; Zhang, L.; Ruan, Y.-F.; Zhao, W.-W.; Xu, J.-J.; Chen, H.-Y. Quantum-dots-based photoelectrochemical bioanalysis highlighted with recent examples. Biosens. Bioelectron. 2017, 94, 207–218.
  20. Zhou, X.; Gao, X.; Song, F.; Wang, C.; Chu, F.; Wu, S. A sensing approach for dopamine determination by boronic acid- functionalized molecularly imprinted graphene quantum dots composite. Appl. Surf. Sci. 2017, 423, 810–816.
  21. Gong, P.; Wang, J.; Hou, K.; Yang, Z.; Wang, Z.; Liu, Z.; Han, X.; Yang, S. Small but strong: The influence of fluorine atoms on formation and performance of graphene quantum dots using a gradient F-sacrifice strategy. Carbon 2017, 112, 63–71.
  22. Yuan, F.; Ding, L.; Li, Y.; Li, X.; Fan, L.; Zhou, S.; Fang, D.; Yang, S. Multicolor fluorescent graphene quantum dots colorimetrically responsive to all-pH and a wide temperature range. Nanoscale 2015, 7, 11727–11733.
  23. Chandra Biswas, M.; Islam, T.; Kumar Nandy, P.; Hossain, M. Graphene Quantum Dots (GQDs) for bioimaging and drug delivery applications: A review. ACS Mater. Lett. 2021, 3, 889–911.
  24. Fan, Q.; Li, J.; Zhu, Y.; Yang, Z.; Shen, T.; Guo, Y.; Wang, L.; Mei, T.; Wang, J.; Wang, X. Functional Carbon Quantum Dots towards Highly Sensitive Graphene Transistors for Cu2+ Ion Detection. ACS Appl. Mater. Interfaces 2020, 12, 4797–4803.
  25. Madurani, K.A.; Suprapto, S.; Syahputra, M.Y.; Puspita, I.; Masudi, A.; Rizqi, H.D.; Hatta, A.M.; Juniastuti, J.; Lusida, M.I.; Kurniawan, F. Recent development of detection methods for controlling COVID-19 outbreak. J. Electrochem. Soc. 2021, 168, 37511.
  26. Li, Y.; Ma, P.; Tao, Q.; Krause, H.J.; Yang, S.; Ding, G.; Dong, H.; Xie, X. Magnetic graphene quantum dots facilitate closed-tube one-step detection of SARS-CoV-2 with ultra-low field NMR relaxometry. Sens. Actuators B Chem. 2021, 337, 129786.
  27. Yuan, H.; Lin, J.-H.; Dong, Z.-S.; Chen, W.-T.; Chan, Y.K.; Yeh, Y.-C.; Chang, H.-T.; Chen, C.-F. Detection of pathogens using graphene quantum dots and gold nanoclusters on paper-based analytical devices. Sens. Actuators B Chem. 2022, 363, 131824.
  28. Kuo, W.-S.; Chen, H.-H.; Chen, S.-Y.; Chang, C.-Y.; Chen, P.-C.; Hou, Y.-I.; Shao, Y.-T.; Kao, H.-F.; Lilian Hsu, C.-L.; Chen, Y.-C.; et al. Graphene quantum dots with nitrogen-doped content dependence for highly efficient dual-modality photodynamic antimicrobial therapy and bioimaging. Biomaterials 2017, 120, 185–194.
  29. Feng, H.; Qian, Z. Functional carbon quantum dots: A versatile platform for chemosensing and biosensing. Chem. Rec. 2018, 18, 491–505.
  30. Zhu, S.; Meng, Q.; Wang, L.; Zhang, J.; Song, Y.; Jin, H.; Zhang, K.; Sun, H.; Wang, H.; Yang, B. Highly photoluminescent carbon dots for multicolor patterning, sensors, and bioimaging. Angew. Chem. 2013, 125, 4045–4049.
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