Introduction of Quantum Dots
Quantum dots (QDs) are nanomaterials with special optical and electronic properties. Owing to their various advantages, such as size-tunable luminescence, high photostability, and strong fluorescence quantum yield, quantum dots have been widely used in display, bioimaging, solar cells, and medical diagnostics. With the rapid development of quantum dot technology, the applications of quantum dots in modern science and technology are becoming more and more extensive, which also means more potential environmental and health risks.
Quantum Dot Products List
Basic Properties and Applications of Quantum Dots
The physicochemical properties of quantum dots have great application potential in different areas. First of all, quantum dots are usually 2–10 nanometers in diameter. The optoelectronic properties of quantum dots are size- and shape-dependent. For instance, when the quantum dots are 5-6 nm in diameter, the light they emit is orange or red, while when the quantum dots are 2-3 nm in diameter, the light they emit is blue and green. This size-dependent optical property makes it possible to tune the colors of quantum dots by controlling their size. In display technology, QDs provide richer colors and an optimal color gamut with brightness and high resolution. Quantum dot light-emitting diodes (QLEDs) have been widely used in television sets and mobile phone screens. In addition, quantum dots are applied in biomedical cell imaging, disease diagnosis, and drug delivery. Quantum dots have been developed as optical labels in biomedical research due to their high brightness and stability. In many biological applications, quantum dots show significant advantages over traditional organic dyes in biomedical imaging. In the field of energy, quantum dot solar cells have high photoelectric conversion efficiency and have been used to develop new thermoelectric materials. Quantum dots have also shown great potential in environmental monitoring and detection, such as the detection of heavy metals, organic pollutants, etc.
Green Synthesis Methods of Quantum Dots
Green synthesis methods are referred to as technologies that use renewable resources to synthesize QDs without using toxic chemicals and less energy. At the same time, it can reduce the environmental burden and improve the biocompatibility and stability of the material.
Biological synthesis: non-toxic and environmentally friendly QDs are synthesized by using plants, microorganisms, or fungi as reducing agents. For example, by using plant extracts (walnut oil, lemon juice, pomegranate peel, etc.) to synthesize CdSe quantum dots can not only reduce the use of organic solvents, but also make quantum dots have better purity and stability. Moreover, by controlling the reaction conditions, the size and optical properties of quantum dots can be precisely controlled by biosynthesis.
Hydrothermal method: synthesis is carried out at low temperature in aqueous solution, without the need for high temperature and pressure, which reduces energy consumption and environmental pollution. The hydrothermal method is suitable for synthesizing water-soluble quantum dots. For example, the CdTe/CdS/ZnS quantum dots prepared by the hydrothermal method can be modified with hydrophilic groups (hydroxyl groups, amino groups, etc.) on the surface to further improve their biological system application. In addition, the hydrothermal method can reduce the toxicity of quantum dots by adding biocompatible coatings (chitosan, peptides, etc. ).
Mechanochemical method: QDs are synthesized by grinding reaction, without using high temperature and pressure, which is a low energy, high efficiency synthesis method. The mechanochemical method is suitable for the synthesis of carbon quantum dots (CQDs). The surface of CQDs can be functionalized (polyethylene glycol (PEG), hyaluronic acid (HA), etc.) to further improve the biomedical application potential.
Cadmium-Free Quantum Dots
To avoid the use of heavy metals, many researchers have been dedicated to exploring new types of cadmium-free quantum dot materials to replace Cd-based QDs. These materials not only have excellent optical properties but also have higher biocompatibility and lower toxicity.
InP/ZnS quantum dots: InP/ZnS quantum dots are a kind of cadmium-free quantum dot material with excellent optical properties and biocompatibility. InP/ZnS quantum dots can be coated on the surface with ZnS shell to improve its stability and fluorescence efficiency. Moreover, InP/ZnS quantum dots can also be modified on the surface (such as thiol, carboxyl group modification) to improve its biomedical application.
ZnSe/ZnS quantum dots: ZnSe/ZnS quantum dots have excellent photostability and biocompatibility and are suitable for biomedical applications. The surface can be coated with biocompatible coatings (such as chitosan, peptides) to reduce toxicity. In addition, ZnSe/ZnS quantum dots can also be modified on the surface (such as polyethylene glycol (PEG), hyaluronic acid (HA)) to improve their potential for biomedical application.
Carbon quantum dots (CQDs): CQDs is a kind of zero-dimensional nanomaterial composed of organic materials, with low toxicity, high fluorescence, and excellent biocompatibility. The synthetic methods of CQDs are mainly divided into two types of top-down and bottom-up. The top-down method (such as laser ablation, chemical oxidation, arc discharge) obtains CQDs by breaking down the original large carbon structure into nanoscale materials. This method can prepare CQDs with high purity and controllable size and morphology, but the equipment is relatively complex and the energy consumption is high. The bottom-up method (such as microwave-assisted carbonization, thermal decomposition, solvothermal synthesis) is more economical and flexible, but many methods are still based on synthetic precursors and high-energy consumption. In recent years, green synthesis (such as biomass, fruit peel, crop straw, renewable resources) has gradually become the mainstream, which can reduce environmental pollution and improve production efficiency.
Silicon quantum dots: Silicon quantum dots (QDs) have ultra-low toxicity and are suitable for biomedical applications and environmental monitoring. It can be coated on the surface with ZnS shell or be coated with biocompatible coatings (such as chitosan, peptides) to improve stability and targeting. In addition, it can be modified on the surface (such as PEG, HA) to improve the potential of biomedical application.
Surface Modification and Functionalization
Surface modification is another crucial process to improve QDs' performance and decrease their toxicity. Surface coating or the addition of biocompatible coatings can improve the fluorescence properties, stability, and biocompatibility of QDs.
Surface coating: The QD surface may be coated with another semiconductor material (e.g., ZnS or CdS) to form a core/shell structure that enhances the stability and fluorescence efficiency of QDs. For example, CdSe/ZnS core-shell quantum dots have high quantum yields and excellent photostability. Surface coating also can reduce QDs' toxicity by introducing biocompatible coatings (e.g., chitosan and peptides).
Bioplasmic coating: The addition of biocompatible coatings (e.g., chitosan, peptides, and hyaluronic acid (HA)) can significantly enhance the biocompatibility and targeting properties of quantum dots. For example, HA-modified quantum dots can be used for targeted imaging and drug delivery applications. Biocompatible coatings can also enhance the dispersibility and stability of quantum dots in biological systems by modifying their surface charge and hydrophilicity.
Surface modification techniques: Several surface modification techniques have been developed, including thiol conjugation, sulfhydryl conjugation, and polymer coating. For example, sulfhydryl conjugation takes advantage of the strong complexation between metal ions and sulfhydryl groups on the quantum dot surface to couple thiol carboxylic acids to the quantum dot shell, enhancing its hydrophilicity. Sulfhydryl conjugation can also improve QDs' stability by introducing multiple sulfhydryl molecules but may decrease their fluorescence efficiency.
Functionalization: The addition of functionalized molecules (e.g., peptides, antibodies, and nucleic acids) can further enhance the targeting and application potential of quantum dots. For example, folic acid-modified quantum dots can target cancer cells for imaging and drug delivery applications. Functionalization can also improve the dispersibility and stability of QDs in biological systems by modifying their surface charge and hydrophilicity.
