Carbon Quantum Dots (CQDs) are a new type of carbon nanomaterial, which is generally less than 10 nanometers in size and has good fluorescence properties. In recent years, CQDs have been applied in various fields and attracted widespread attention, and are expected to become a new alternative to traditional semiconductor quantum dots. Compared with traditional semiconductor quantum dots, CQDs not only have similar optical properties, but also have the advantages of low toxicity, environmental friendliness and simple synthetic route.
The origin of CQDs dates back to the research of fluorescent components in nanoparticles in 2004, and is now widely used in bioimaging, biosensing, drug delivery, photocatalysis, photovoltaics and optoelectronics. The structure of CQDs is different from that of traditional semiconductor quantum dots. It combines the optical/photonic properties of quantum dots and the electrical properties of sp2 carbon-based materials, and it has great potential in many fields.
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Synthesis Methods of Carbon Quantum Dots
1. Top-down method
The top-down method refers to the peeling or decomposition of smaller CQDs from bulk carbon materials such as graphene, carbon nanotubes, graphite, and fullerene. The common methods are laser ablation, electrochemical discharge and arc discharge. These methods usually need high temperature or complex conditions, but can prepare CQDs with high purity, size and morphology control. For example, laser ablation method uses a high-energy laser beam to decompose the bulk carbon material into nanometer particles, so as to obtain CQDs.
2. Bottom-up method
The bottom-up method refers to the synthesis of CQDs from small molecules (such as citric acid, amino acids and carbohydrates) by chemical reactions. The common methods are hydrothermal, combustion, microwave and pyrolysis. These methods are generally more economical and flexible, but the synthetic routes of many methods still rely on synthetic precursors and energy-intensive processes. For example, hydrothermal method can form CQDs by carbonizing small molecules under high temperature and pressure conditions.
3. Green synthesis method
In recent years, green synthesis methods have received increasing attention because of their environmental protection, low cost and strong reproducibility. Green synthesis methods use natural biomass (such as plants, algae, fruit shells, etc.) as precursors, and are prepared by hydrothermal, solvothermal or biosynthetic methods. For example, walnut oil, lemon juice, pomegranate peel and sucrose were used as carbon sources for hydrothermal synthesis, and the reaction conditions were optimized to obtain CQDs with unique physical and chemical properties.
4. Doping technology
The introduction of heteroatoms (such as nitrogen, sulfur and phosphorus) can regulate the optical properties and functionalization capabilities of CQDs. For example, nitrogen doping can enhance the photoluminescence of CQDs, while sulfur doping can improve its specific response. Doping technology not only improves the performance of CQDs, but also expands the application potential of CQDs in different fields.
5. Summary of synthetic routes
The synthetic methods have a great influence on the structure, fluorescence properties and application effects of CQDs. For example, the top-down method is generally used to prepare CQDs with high purity and controllable size, while the bottom-up method is more suitable for preparing CQDs with more complex structures and functionalization. In addition, doping and surface functionalization technology can further optimize the performance of CQDs, making CQDs show a wider application prospect in various fields.
Optical Properties of Carbon Quantum Dots
Fluorescence properties
Carbon quantum dots (CQDs) have attracted extensive attention in recent years due to their unique optical properties. It has a tunable fluorescence emission range, from blue light to red light, and exhibits good photostability and no flickering, which provides CQDs with broad application prospects in the fields of bioimaging, biosensing, and optoelectronic devices. The fluorescence emission wavelength of CQDs has been found to be closely related to its size, with smaller dots exhibiting shorter emission wavelengths, a phenomenon known as blue shift. In addition, the fluorescence intensity of CQDs is also influenced by the excitation wavelength, pH value, and surface state, resulting in obvious excitation dependence and environmental dependence.
The fluorescence quantum yield (QY) of CQDs is typically low, but its QY can be significantly improved through techniques such as surface passivation and doping. For example, the photoluminescence properties of CQDs can be enhanced by introducing oxygen-containing groups or performing surface oxidation treatment. CQDs also possess chemiluminescence (CL) and electrochemiluminescence (ECL) properties, endowing them with significant application value in high-sensitivity biosensing.
Surface functionalization
The surface functionalization of CQDs plays a crucial role in enhancing their performance and application potential. By surface modification, the water solubility, biocompatibility, and targeting ability of CQDs can be improved. For instance, introducing functional groups such as polyethylene glycol (PEG) or polyethyleneimine (PEI) can enhance the photostability of CQDs and exhibit better application prospects in the biomedical field. In addition, surface functionalization can also regulate the electronic structure and optical properties of CQDs by introducing metal ions or heteroatoms (such as nitrogen and sulfur atoms).
Photoluminescence mechanism
The luminescence mechanism of CQDs is mainly related to its surface state and defect structure. Currently, many theoretical models exist to explain it, including quantum confinement effect, surface defect recombination, and molecular state emission. Studies have shown that when the size of CQDs is smaller than the exciton Bohr radius, quantum confinement effects will be induced, thus affecting its electronic properties and optical properties. In addition, the surface defects of CQDs can lead to non-radiative transitions, resulting in a reduction in its fluorescence quantum yield. Therefore, the photoluminescence efficiency of CQDs can be effectively improved by surface passivation and defect control.
Application Fields of Carbon Quantum Dots
Bioimaging and biosensing
CQDs have been widely used in the fields of bioimaging and biosensing due to their high biocompatibility and low toxicity. In bioimaging, CQDs can be used as fluorescent probes for cell imaging, tumor detection, and in vivo imaging. For example, CQDs modified with PEG have achieved in vivo optical imaging of different organs, showing good biocompatibility and potential optical imaging applications. CQDs also show excellent performance in biosensing. Owing to their good photostability, multicolor emission, and high water solubility, CQDs have become ideal materials for the detection of biological molecules such as cellular iron, copper, nucleic acids, and pH.
Drug delivery system
CQDs have become a multifunctional carrier for drug delivery systems due to their good biocompatibility, chemical inertness, and strong fluorescence properties. By means of surface modification, CQDs can be used as drug carriers to realize targeted delivery and controlled release. For example, CQDs-Oxa composites can be used for diagnosis, treatment and monitoring of cancer treatment response, while DOX-loaded CQDs show potential application in cell imaging and cancer therapy. In addition, CQDs can also be used as an alternative to gold nanoparticles (AuNPs) to build multimodal platforms, including drug delivery, photothermal therapy, and bioimaging.
Photocatalysis and electrocatalysis
CQDs have exhibited great potential in the field of photocatalysis and can be used to address issues related to energy crises and environmental pollution. CQDs can be used as photocatalysts to participate in processes such as carbon dioxide conversion, solar water splitting, and dye degradation. For example, CQDs/Fe2O3 composites exhibited higher catalytic activity than Fe2O3 nanoparticles in the degradation of methanol and gaseous benzene, which was attributed to the capture of electrons by CQDs and the inhibition of electron-hole recombination. CQDs also show excellent performance in the field of electrocatalysis. Due to their small size, superconductivity, and fast electron transfer, they have excellent catalytic performance in oxygen reduction reaction (ORR), oxygen evolution reaction (OER), hydrogen evolution reaction (HER), and carbon dioxide reduction reaction (CO2RR).
Environmental governance
CQDs also have important applications in the field of environmental governance. They can be used for the detection and removal of heavy metal ions, such as Hg²⁺, Cu²⁺, and so on. In addition, CQDs can also be used in water purification and wastewater treatment to degrade organic pollutants by generating reactive oxygen species (ROS). Compared with traditional photocatalysts, CQDs have a wider visible light absorption range and tunable electronic properties, making them more efficient in environmental remediation.
Energy and electronic devices
CQDs also have broad application prospects in the fields of energy and electronic devices. In solar cells, CQDs can be used as electron mediators, photosensitizers, spectrum converters, and single photocatalysts, which can effectively utilize visible light to generate electrons and holes, and improve photocatalytic efficiency. In addition, CQDs also have important applications in the fields of LEDs and optoelectronic devices. Due to their excellent photoluminescence properties and short fluorescence lifetime, they have potential application value in visible light communications (VLC) and polymer solar cells (PSCs). In addition, CQDs can also be used to construct a multimodal imaging platform, which combines fluorescence, magnetic resonance, and photoacoustic imaging techniques to achieve comprehensive monitoring of diseases.
Carbon Quantum Dots Market
With the gradual development of green nanomaterials, carbon quantum dots are showing explosive growth in the market. The new report shows that the global carbon quantum dots market size is expected to reach $1 billion by 2030. The main reason for this is its wide range of applications in biomedicine, environmental monitoring, energy and electronic devices. As a new type of fluorescent nanomaterial, carbon quantum dots have become a potential replacement for traditional semiconductor quantum dots with advantages of low toxicity, good water solubility, tunable optical properties and environmentally friendly synthesis.
In terms of the market, carbon quantum dots are still in the early stages of commercial development, and several research institutions and companies have started its commercial development. For instance, some companies are exploring its application in biomedical imaging and drug delivery, while others are researching its potential in environmental governance and energy storage. Although the market is fiercely competitive, the unique advantages of carbon quantum dots, such as its tunable optical properties and good environmental compatibility, have become an irreplaceable advantage in multiple industries. Moreover, with the continuous optimization of its synthesis technology, the performance and scalability of carbon quantum dots are also being constantly improved, further promoting its commercialization process.
Future Challenges and Prospects of Carbon Quantum Dots
Although carbon quantum dots have many advantages, there are still some technical challenges in their practical applications. Firstly, the efficiency and stability of synthesis are the current research focus. Currently, there are many methods for the synthesis of carbon quantum dots, but many of these methods require complex equipment and high energy consumption, which limits their large-scale production. In addition, carbon quantum dots have low quantum yield, and the controllability of their structure and size still needs to be further optimized. Secondly, large-scale production is also a challenge. The synthesis process of carbon quantum dots requires complex chemical reactions and physical conditions, so it is currently difficult to achieve large-scale and low-cost production. Finally, functionalization and surface modification are also the focus of research. Although carbon quantum dots can improve their performance by surface passivation and functionalization, the key is how to achieve multifunctionality while maintaining biocompatibility, which is still an urgent problem to be solved.
Although carbon quantum dots have low toxicity, their long-term environmental behavior and bioaccumulation still need to be further studied. Although studies have shown that carbon quantum dots have high stability in water, their long-term effects in soil and organisms are still unclear. In addition, although the biodegradability and environmental friendliness of carbon quantum dots have been confirmed to a certain extent, their behavior under different environmental conditions still needs further study. Therefore, in the future, research should pay more attention to the environmental safety and ecological impact of carbon quantum dots to ensure their sustainability in practical applications.
In the future, research on carbon quantum dots will focus on the following directions. First of all, green synthesis will become a focus. By using renewable resources such as biomass, peels and crop residues, the impact on the environment can be reduced and a more economical and sustainable production method of nanomaterials can be provided. Secondly, multifunctionalization and intelligence will become an important development trend of carbon quantum dots. By introducing new functional groups and composite materials, carbon quantum dots can achieve a wider range of applications, such as intelligent control in drug delivery, bioimaging and environmental monitoring. Finally, industrialization will become the key to the future development of carbon quantum dots. With the continuous improvement of synthesis technology and the gradual expansion of market demand, carbon quantum dots will be industrialized in more fields and applications in the future, promoting their wide application in biomedicine, energy and environmental governance.
