You Don't Learn That in School: An Updated Practical Guide to Carbon Quantum Dots

Abstract

Carbon quantum dots (CQDs) have started to emerge as candidates for application in cell imaging, biosensing, and targeted drug delivery, amongst other research fields, due to their unique properties. Those applications are possible as the CQDs exhibit tunable fluorescence, biocompatibility, and a versatile surface. This review aims to summarize the recent development in the field of CQDs research, namely the latest synthesis progress concerning materials/methods, surface modifications, characterization methods, and purification techniques. Furthermore, this work will systematically explore the several applications CQDs have been subjected to, such as bioimaging, fluorescence sensing, and cancer/gene therapy. Finally, we will briefly discuss in the concluding section the present and future challenges, as well as future perspectives and views regarding the emerging paradigm that is the CQDs research field.

Keywords: biosensors; carbon quantum dots; chemosensors; separation methods; surface modifications; synthesis.

Figure 1

Chemical structures of IPCA (a) and TPDCA (b).

Figure 2

Representation of CQD hydrothermal synthesis, CQDs under UV illumination and practical application. Reprinted from ref. [33], with permission from Elsevier.

Figure 3

(a) Formation of CQDs from citric acid and o-phenylenediamine; (b) formation of DAP by oxidative dimerization of o-phenylenediamine. Reprinted from ref. [38], with permission from Elsevier.

Figure 4

Heteroatom doping progress timeline.

Figure 5

Schematic representation of the conjugation of amino acids on the CQDs surface. Reprinted with permission from ref. [82]. Copyright 2018 American Chemical Society.

Figure 6

Diagram portraying common characterization methods of CQDs.

Figure 7

(a–c) SEM of CQDs; (d) and (e) TEM of CQDs; (f) CQDs size distribution; (h) XRD of CQDs. Reproduced with permission from ref. [105], published by Springer Open, 2020.

Figure 8

(a) and (c) UV-Vis absorption spectra of CQDs obtained at reaction temperature 60 and 240 °C; (b) and (d) PL spectra demonstrating independent and dependent behavior. Reproduced with permission from ref. [111], published by Scientific Reports, 2014.

Figure 9

FTIR spectra of three CQDs fractions obtained from the same synthesis. Reprinted from ref. [114], with permission from Elsevier.

Figure 10

Schematic correlation between CQDs’ physical and chemical properties and possible applications in various fields.

Figure 11

(A) In vivo imaging and (B) ex vivo imaging of a mice after intravenous injection of CQDs. Reproduced with permission from ref. [122], published by Springer Open, 2020.

Figure 12

Underlying principle for the sensing of dopamine and alpha lipoic acid. Reprinted from ref. [147], with permission from Elsevier.

Figure 13

Photographs of the prepared LSC under ambient illumination (a–c) and one Sun illumination, 100 mW/cm² (d–g). Reprinted from ref. [172], with permission from Elsevier.

Figure 14

Schematic representation of the interaction between CQDs and dyes during photocatalytic degradation. Reprinted from ref. [114], with permission from Elsevier.

Figure 15

Proposed mechanisms for viral replication inhibition of HCoV-229E-Luc infection by CQDs, through the interference with protein S receptor interaction (a) and by the inhibitory influence on the replication of viral RNA genome. Reproduced with permission from ref. [193], published by American Chemical Society, 2019.

Figure 16

Intracellular uptake of EPA-CQDs (A) and PEI-CQDs (B) by normal HeLa cells (Cont) and by cells at three cell cycle phases (G0/G1, S, and G2/M), cultured in media with and without serum. Reprinted from ref. [222], with permission from Elsevier.