Carbon Dots: A New Type of Carbon-Based Nanomaterial with Wide Applications

Abstract

Carbon dots (CDs), as a new type of carbon-based nanomaterial, have attracted broad research interest for years, because of their diverse physicochemical properties and favorable attributes like good biocompatibility, unique optical properties, low cost, ecofriendliness, abundant functional groups (e.g., amino, hydroxyl, carboxyl), high stability, and electron mobility. In this Outlook, we comprehensively summarize the classification of CDs based on the analysis of their formation mechanism, micro-/nanostructure and property features, and describe their synthetic methods and optical properties including strong absorption, photoluminescence, and phosphorescence. Furthermore, the recent significant advances in diverse applications, including optical (sensor, anticounterfeiting), energy (light-emitting diodes, catalysis, photovoltaics, supercapacitors), and promising biomedicine, are systematically highlighted. Finally, we envisage the key issues to be challenged, future research directions, and perspectives to show a full picture of CDs-based materials.

Figure 1

Classification of CDs: including graphene quantum dots (GQDs), carbon quantum dots (CQDs), and carbonized polymer dots (CPDs), and their main preparation approaches.

Figure 2

Syntheses and optical properties of CDs. Synthesis and PL spectra of (a) red emissive CPDs and (b) multicolor CPDs. Optical properties of (c) multicolor CPDs and (d) CQDs. (e) Synthesis and PL spectra of deep red emissive CPDs. Images reproduced with permission from refs (, , , , and 2). Copyright 2018 WILEY-VCH, 2016 American Chemical Society, 2018 WILEY-VCH, 2018 Springer Nature, and 2020 WILEY-VCH.

Figure 3

Optical applications of CDs. (a) Zn-doped CPDs were used for EDTA and Zn²⁺ sensing. (b) Schematic in practical application and images of CPDs combined with medical cotton cloth at different pH values. (c) Differentiation of various cancerous cells from normal cells by CPDs. (d) Acidophilic CPDs for distinguishing four bacteria via fluorescent images. (e) Multicolor RTP CDs for digital information encryption. (f) RTP CPDs for graphic security and digital information encryption. (g) Time-resolved information security using GQDs-based long-lived RTP/TADF. Images reproduced with permission from refs (, , , , , , and 28). Copyright 2018 American Chemical Society, 2019 WILEY-VCH, 2017 Royal Society of Chemistry, 2019 Royal Society of Chemistry, 2019 WILEY-VCH, 2018 WILEY-VCH, and 2020 WILEY-VCH.

Figure 4

Applications of CDs in catalysis and CLEDs. (a) Illustration of the synthesis of CoFe-CDs/CoFeRu@C catalysts and electrocatalytic OER/HER applications. (b) Illustration of the PEC mechanism on the CPDs-{001}TiO₂/Ti photoelectrode. (c) Device structure and energy levels of CPDs-based electroluminescent LEDs. (d) CQDs-based LED structure, energy diagram, and performance characterization. Images reproduced with permission from refs (, , , and 20). Copyright 2020 Elsevier, 2020 Elsevier, 2011 Royal Society of Chemistry, and 2018 Springer Nature.

Figure 5

Applications of CDs in supercapacitors and rechargeable batteries. (a) Illustration of conversion process. (b) Porous carbon derived from CPDs and polyacrylamide hydrogels. (c) Schematic representation of the formations of graphene-rich petal-like rutile TiO₂. (d) Illustrations of the PEI-CPDs-modified cathode composite at different charge–discharge stages. Images reproduced with permission from refs (, , , and 140). Copyright 2018 WILEY-VCH, 2019 WILEY-VCH, 2016 WILEY-VCH, and 2019 WILEY-VCH.

Figure 6

Application of CDs in bioimaging. (a) Confocal images of unfixed and fixed HeLa cells stained by CPDs, Hoechst, or SYTO RNASelect. (b) Biofilms of different microorganism species stained by CPDs. (c) Deep red emissive CPDs for stomach imaging. (d) In vivo imaging of nude mice with intravenous injection of deep red emissive CPDs at different time points. (e) Real-time ex vivo imaging of nude mice with intravenous injection of red emissive CPDs in phosphate buffer saline solution at different time points. Images reproduced with permission from refs (, , , , and 19). Copyright 2018 American Chemical Society, 2017 Royal Society of Chemistry, 2018 WILEY-VCH, 2020 WILEY-VCH, and 2018 WILEY-VCH.

Figure 7

Biomedical applications of CDs. (a) Structure of CPDs-decorated C₃N₄ nanocomposites and schematic diagram of 630 nm light-driven water splitting-enhanced PDT. (b) Schematic illustration of the Mn-CPDs assembly for enhanced PDT. (c) Effectiveness and monitoring of the tumor-targeted therapy based on FA-CPDs in vivo. (d) In vivo study of GQDs-based nanomaterials with A549 tumor-bearing BALB/c nude mice. (e) Illustration of the synthesis of Met-CPDs and their antibacterial activity against Porphyromonas gingivalis. (f) Illustration of the synthesis of Cur-CPDs and their antiviral applications. Images reproduced with permission from refs (, , , , , and 186). Copyright 2016 American Chemical Society, 2018 WILEY-VCH, 2018 WILEY-VCH, 2019 American Chemical Society, 2017 Royal Society of Chemistry, and 2019 WILEY-VCH.