Carbon dots (CDs) as a new type of carbon-based nanomaterials, due to its diverse physicochemical properties and good biocompatibility, unique optical properties, low cost, eco-friendliness, rich functional groups (such as amino, hydroxyl, carboxyl), high stability and electron mobility and other advantages, in recent years has attracted extensive research interest.
In the latest review published by Professor Yang Bai et al. of Jilin University, on the basis of analyzing the formation mechanism, micro and nano structure and property characteristics of CDs, a comprehensive review is carried out on the classification of CDs, and the synthesis methods and optical properties of CDs are introduced, including strong absorption, photoluminescence and phosphorescence. In addition, recent significant advances in various application areas, including optics (sensors, anti-counterfeiting), energy (light-emitting diodes, catalysis, photovoltaics, supercapacitors), and promising biomedicine, have been systematically emphasized. Finally, the authors look ahead to the key issues, future research directions, and prospects to present the full picture of CDs-based materials. The paper was published in the journal ACS Central Science under the title "Carbon Dots: A New Type of Carbon-Based Nanomaterial with Wide Applications."
Thesis Link:
https://pubs.acs.org/doi/10.1021/acscentsci.0c01306
1. Introduction
Carbon-based materials play an important role in the development of materials science. From traditional industrial carbon (such as activated carbon, carbon black) to new industrial carbon (such as carbon fiber, graphite) and graphene, carbon nanotubes and other new carbon nanomaterials, the basic research and application of carbon-based materials has always been a hot spot in the fields of chemistry and materials. However, macroscopic carbon materials lack suitable band gaps and are difficult to become an effective fluorescent material. Carbon dots (CDs) are an emerging star in the carbon family, which has attracted much attention due to its excellent tunable photoluminescence (PL), high quantum yield (QY), low toxicity, small size, good biocompatibility and rich low-cost sources, and has important application prospects in biomedicine, catalysis, catalysis and other fields.
Figure 1. Classification of CDs: including graphene quantum dots (GQDs), carbon quantum dots (CQDs) and carbide polymer dots (CPDs) and their main preparation methods.
2. Optical properties
Absorption: CDs prepared from different carbon sources or different synthesis methods have different adsorption behaviors. However, they typically exhibit strong absorption in the ultraviolet (UV) region (200−400 nm), with the tail extending into the visible light range, where the absorption band is assigned to the π-π* transition of the C=C bond or the n−π* transition of the C=O/C=N bond.
Photoluminescence: Compared with traditional cadmium/lead-containing quantum dots, rare earth nanomaterials, organic dyes and other fluorescent materials, CDs have the advantages of good photostability, high QY, low toxicity, rich inexpensive sources, and good biocompatibility, and have important applications in various fields.
Phosphorescence: Room temperature phosphorescence (RTP) is one of the attractive properties of CDs, which is produced by two key processes: (i) intersystem crossover (ISC) from the lowest excited state (S1) to the triplet state (Tn) and (ii) the radiation transition from the lowest excitation triplet state (T1) to the ground state (S0).
Figure 2. Synthesis of CDs and their optical properties. (a) Synthesis and PL spectra of red-emitted CPDs and (b) Polychromatic CPDs (c) Optical properties of multicolor CPDs and (d)CQDs (e) Synthesis and PL spectra of crimson-emitted CPDs.
3. Optical applications
Sensors: CDs are widely used in the detection of various analytes in the environment or biological system due to their inherent fluorescence properties, high sensitivity, fast response, low cost, and simple preparation method.
Information encryption: Information encryption helps protect valuable things from being copied. Compared with traditional anti-counterfeiting technologies such as fluorescent printing and plasma anti-counterfeiting labels, CDs are environmentally friendly, easy to operate, simple in design, and low in cost, providing a wise choice for anti-counterfeiting data and encryption applications.
Figure 3. Optical applications of CDs. (a) Zinc-doped CPDs are used for sensing EDTA and Zn2+. (b) Schematics and images of the practical application of CPDs and medical cotton at different pH values. (c) CPDs differentiate from normal cells into various cancer cells. (d) Methods for identifying four bacteria by fluorescence images of eosinophilic CPDs. (e) Multi-color RTP discs for the encryption of digital information. (f) RTP CPDs for graphics security and encryption of digital information. (g) Time-resolved information security for long-life RTP/TADF based on GQDs
4. Energy applications
Catalysis: CDs have been proposed as photocatalysts, electrocatalysts, and photocatalysts because of their different structures, properties, and uses. CDs-based light-emitting diodes (CLEDs): As an emerging fluorescent material, CDs are expected to replace expensive rare earth phosphors and toxic metal semiconductor quantum dots due to their richness, adjustable luminous color, low cost, and environmental friendliness. Typically, CDs can be used as luminescent materials or active layers in electroluminescent devices.
Solar cells: CDs have been widely studied in solar cells (SCs), and due to their unique optical properties, abundant functional groups (e.g., amino groups, hydroxyl groups, carboxyl groups) and high electron mobility, they have been found to improve efficiency in various jobs. Supercapacitors: Supercapacitors have the characteristics of fast charge and discharge speed, high power density, long cycle life, and low energy density, which limit their practical application in the field of energy storage. CDs are hybridized with other carbon materials, polymers, or metal oxides to improve the electrochemical properties of supercapacitors.
Rechargeable batteries: Rechargeable batteries are considered to be one of the most efficient energy storage technologies that connect renewable energy production and consumption.
5. Biomedical applications
Biological imaging: Biological imaging is a technique that can directly visualize biological events in real time, non-invasively through detectors and detectors.
Phototherapy: Phototherapy, including photodynamic therapy (PDT) and photothermal therapy (PTT), is a non-invasive treatment that, with the help of photosensitizers, converts irradiated light into reactive oxygen species and heat, inducing local apoptosis of cancer cells.
Drug/gene delivery: In addition to anticancer phototherapy, CDs can combine imaging tools with drugs or genes to form image-guided nanobreeds to improve delivery efficiency or provide benefits in treatment strategies.
Nanomedicine: In addition to being used as a carrier, CDs also have antibacterial, anti-cancer, antiviral, antioxidant and other therapeutic effects.
Figure 4. Application of CDs in biological imaging. (a) Confocal images of unfixed and fixed HeLa cells stained with CPDs, Hoechst, or SYTO-RNASelect. (b) CPDs staining of biofilms of different microbial species. (c) Crimson emission CPDs are used for gastric imaging. (d) Intravenous injection of dark red radial CPDs in nude mice at different time points. (e) Real-time imaging of naked mice injected with red radial CPDs intravenously at different time points.
6. Current Challenges and Outlook
The preparation method of CDs is simple, environmentally friendly, the preparation methods are diverse, the photoelectric properties are excellent, the cost is low, and it has good biocompatibility, which makes it widely used in optics, energy, biomedicine and other fields. CDs have made great progress in synthetic strategy, structure, properties, mechanism research and application development. These encouraging findings suggest that CDs can provide many exceptional opportunities to study new phenomena and new properties observed in a multidisciplinary environment, although there are still many key questions to be addressed. (Text: Ai Xin Jue Luo Xing)
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