Introductions and Applications of Quantum Dots

Abstract: Quantum dots are semiconductor nanocrystalline materials with quantum confinement effects in three dimensions of space, also known as “artificial atoms.” The particle size of the quantum dot material is generally between 1-10 nm. Since the electrons and holes are quantum confined, the continuous band structure changes to a vertical energy level structure, thereby bringing a narrow luminescence spectrum (20-30 nm), as well as high purity, wide color gamut and other advantages. The origin of quantum dot technology dates back to the mid-1970s. It was originally developed to solve the global energy crisis. In today’s nano research field, quantum dot counting is one of the star materials. Compared with organic fluorescent materials, quantum dots have controllable emission wavelengths and research; high luminous efficiency, one-step excitation, multiple colors, strong fluorescence intensity, long-lasting anti-photobleaching, etc., which is an ideal substitute for organic fluorescent materials. Based on the above characteristics and advantages, the outstanding performance of quantum dots in life sciences and medicine has attracted wide attention. For example, hydrothermal preparation of CdSe quantum dots and their application in cell imaging, based on CdSe/ZnS quantum dots to construct dopamine fluorescence detection method and mitomycin fluorescence detection system, CdSe quantum dot synthesis and the application of heavy metal ion detection, graphene quantum dot preparation technology and the status quo of innovative resources. This paper focuses on the wide application and development prospects of quantum dots.

Keywords: quantum dots, CdSe/ZnS, fluorescence detection, development and application

The concept of quantum dots and its classification

 

Quantum dots are stable, water-soluble semiconductor nanoparticles composed of Group II-VI or Group III-V elements.

 

Quantum dots are composed of a core and a shell. The core generally uses CdX (X=S, Se, Te) as a material, and the shell portion is composed of other materials with different band gaps or a vacuum medium. Studies have shown that quantum dots can be prominent in biochemistry such as labeled peptide chains and deoxyribonucleotide long-chain polymers, which has attracted widespread attention from biologists, chemists, and physicists. Because its research direction involves many interdisciplinary subjects such as physics, chemistry, biomedicine, etc., quantum dots have considerable development and application prospects.

The application of quantum dots in the field of life science

Traditional fluorescent probes are a far cry from quantum dots as fluorescent probes. Because quantum dots have the advantages of wide laser spectrum, continuous distribution, and symmetric emission spectrum distribution, narrow width and rich color, quantum dots can solve many problems that traditional fluorescent probes cannot solve. However, the coupling problem that occurs with quantum dots of different materials is still the main problem that currently limits the development of quantum dots. Scientists predict that if the coupling problem of quantum dots of different materials is solved, the researcher can combine quantum dots as fluorescent probes with specific antibodies, and the fluorescent probes will be specific to different specific organelles in the cell. After sexual combination, different organelles are distinguished. Since a variety of semiconductor nanocrystals of different sizes can change according to the change of size, a series of marking systems with different wavelengths and different colors are produced, and the fluorescence intensity, high stability and durability make the semiconductor nanocrystals can withstand multiple times. Excited and maintained in the original state, it is convenient for the researcher’s long-term experimental observation, and the optical characteristics will not change significantly.

Fluorescence Resonance Energy Transfer (FRET) is a spectral analysis method that analyzes non-radiative energy transfer between fluorescent substances. Since its introduction, FRET has been widely used in various fields of agriculture, medicine, forensic science and scientific research. . In recent years, FRET technology has greatly improved in spatial resolution and sensitivity. Sensors designed according to FRET have been widely used in biological research fields, such as detecting conformational changes of biological macromolecules and interactions between biological macromolecules and the distance between biomolecules on the nanometer scale. Studies have shown that the optical properties of the energy donor and the assembly method of the sensor have a crucial impact on the detection performance of the FRET sensor. However, the traditional energy supply receptors (such as organic fluorescent dyes, biological materials, lanthanides, etc.) are fluorescent. Low intensity, low light stability, sensitive to the environment, poor biocompatibility, and high toxicity, easily affected by autofluorescence and stray light in the living body, resulting in the degradation of fluorescence intensity and the failure of biological detection to achieve the desired target. This limits the development and application of FRET sensors.

The FRET system in the near-infrared region has been constructed. The donor has a fluorescence emission peak at the near-infraredInP/ZnS quantum dot, and the acceptor is a near-infrared fluorescent dye Cy7. Make up for the shortcomings of traditional visible quantum dots in biological applications. At the same time, the concentration and cell microenvironment pH sensitivity of the system were tested. The results show that the fluorescence intensity of the FRET system can directly reflect the pH change of the cell microenvironment, and has high detection accuracy for the pH value of the system.

By studying the effect of different pH solutions on the FRET system, the results show that the Cy7 dye itself is not sensitive to pH. The sensitivity of the FRET system to pH is mainly due to the sensitivity of the quantum dots to pH, when the pH of the solution is at From 7 to 10 hours, the FRET system has a high FRET transfer efficiency. The cell test results show that the fluorescence signal of the FRET probe changes significantly with the change of the pH of the extracellular fluid, and can be used for detecting cancer cells in the biological microenvironment. At the same time, the obvious fluorescence signal of the FRET system in the extracellular fluid of breast cancer can be applied to the imaging of cancer cells, achieving the dual function of the FRET system.

This study provides a theoretical and experimental basis for the application of the FRET system as a sensitive probe for early diagnosis of cancer.

The application of quantum dots in medical drugs (taking CdSe and ZnS as examples)

Since quantum dots of different sizes can produce different mark colors, scientists can combine quantum dots of different sizes with different target cells to detect target cells for drug action, thereby screening drugs; it is known that centimeter-thickness of tissue, infrared rays can be Easy to penetrate and visible light can’t. Therefore, it is possible to mark the tissue with quantum dots that emit light in the infrared region, and perform medical diagnosis under the excitation of infrared light by the same detection principle as medical imaging. CdSeQDs is a kind of quantum dot which is relatively mature in research. Compared with other kinds of quantum dots, CdSeQDs has significant advantages. For example, under the same wavelength of light, the emission spectrum of CdSeQDs is 430 with different particle size. Adjustable in the range of -660nm, CdSeQDs has high fluorescence quantum yield, easy detection, mild synthesis conditions and short synthesis cycle. Therefore,CdSe quantum dots have long been widely concerned and studied.

Synthesis method of CdSeQDs

The morphology and structure of semiconductor quantum dots have a great influence on their inherent magnetic, electrical and optical properties. The properties of QDs prepared by different synthetic methods are different and their uses are different. Therefore, the synthesis of quantum dots has been subject to scientists’ wide attention. At present, organic phase synthesis and aqueous phase synthesis are two main synthetic methods. Organic phase synthesis of QDs is one of the most commonly used methods. Its development mainly includes two stages: organic metal method using organic metal as reaction precursor and green chemical method using inorganic metal as reaction precursor. The organometallic method can synthesize QDs which are monodisperse and have high stability, and the QDs prepared by this method have better fluorescence quantum yield and optical properties. However, the organometallic method also has many disadvantages, such as the use of raw materials is relatively expensive and has greater toxicity, the synthesis process is dangerous, prone to explosion, etc., which limits the further promotion and application of this method. Later, the researchers proposed a green chemical synthesis method, using high stability, low toxicity and low cost cadmium oxide as raw material instead of dimethyl cadmium as a precursor, mixed with Se solution to prepare CdSeQDs with excellent properties. Although this method reduces the reaction cost, the use of a more toxic organic phosphine solvent is retained during the reaction, so a greener synthesis method needs to be sought. The preparation of QDs by aqueous phase method overcomes the shortcomings of organic synthesis, but the method also has problems such as imperfect crystals, many surface defects and poor luminescence properties, so the aqueous phase synthesis method needs further improvement.

Surface functionalization of CdSeQDs

Although CdSeQDs have many unique and excellent physicochemical properties and optical properties, they also have some limitations, such as low biocompatibility, poor water solubility, and relatively high toxicity. In order to overcome these shortcomings, the surface of quantum dots needs to be encapsulated and modified. In recent years, various small molecules (including sulfur, polymer, DNA and other modifiers) have been reported to encapsulate modified CdSeQDs. The prepared QDs have good biocompatibility and are widely used in the detection of biological small molecules, disease diagnosis and treatment, biosensing, etc., and have broad application prospects in the field of biomedicine.

Protein package

Protein-encapsulated QDs have many important applications in the biomedical field, so proteins are used as a common encapsulant for the synthesis of QDs. Proteins can usually be modified to the surface of QDs by electrostatic attraction, covalent bond coupling, etc., where covalent bonding is the most common method. Covalent bond bonding is a combination of a functional group of a protein molecule and a functional group on the surface of a quantum dot to form a covalent bond. Since the protein has an amino group and a sulfhydryl group, it is easy to form a covalent bond with the amino group and the carboxyl group-modified QDs. The use of novel proteins (natural phytochelatins) bound to poorly water-soluble CdSe/ZnSQDs increases the water solubility of QDs, and the encapsulated QDs exhibit high colloidal stability and remain in aqueous solution, including its high quantum yield, capable of bioconjugation with different functional groups.

The following figure is a schematic diagram of fluorescence resonance energy transfer caused by self-assembly of GCN-mCherry and glutathione-coated CdSe/ZnS quantum dots.

Antibody package

An antibody is a glycoprotein capable of specifically binding an antigen. Attaching an antibody to a quantum dot enables specific binding of an antibody on the QDs to an antigen in the living body, thereby enabling qualitative and quantitative detection of biomolecules in the living body. When a disease occurs in an organ in a living body, it causes overexpression or decreased expression of certain molecules in the organ; therefore, antibody-modified QDs can detect tumor cells as fluorescence sensors in vivo. The binding method of antibody and quantum dot is divided into non-covalent binding and covalent binding, and non-covalent binding mostly adopts biotin-avidin linkage. Covalently bound to the surface functional group of the antibody and the quantum dot reacts with each other by the activation of dimethylaminopropyl and ethylcarbodiimide hydrochloride to form a covalent bond. The most commonly used combination is amino and carboxyl. The most commonly used combination is an amino group and a carboxyl group, an amino group and a thiol group, and an aldehyde group and a hydrazide.




 

Peptide wrap and DNA wrap

Peptides are often used to modify QDs as common biomolecules. The combination of peptides and quantum dots often utilizes the amino and sulfhydryl groups on the surface of the peptide to covalently bond with the amino and carboxyl groups on the surface of the quantum dot. In addition, the method of synthesizing small molecule-encapsulated QDs by biomimetic method is relatively simple and has been widely promoted. Usually, phospholipids are also used to wrap QDs.

DNA-functionalized QDs play an important role in biological applications. Single-stranded DNA recognizes complementary DNA/RNAs that form a normal helical structure through base-pair pairing, so DNA-functionalized QDs can detect viruses and bacteria. DNA fragments as well as small RNAs, DNA can also act as aptamers with high affinity to target molecules. These aptamer-functionalized quantum dots can be used to detect ions, proteins, or for disease diagnosis and cell imaging.

Construction of mitomycin fluorescence detection system based on CdSe/ZnS quantum dots

In the development of the past few decades, experts have studied the optimal reaction conditions of quantum dots in detail, and developed a new method for detecting mitomycin in biological fluid based on the fluorescence properties of CdSe/ZnS quantum dots. The intensity has a good linear relationship with the concentration of mitomycin. Under the optimal experimental conditions, the CdSe/ZnS quantum dots interacted with mitomycin, and the fluorescence intensity of the quantum dots was quenched with the increase of mitomycin concentration. The quenching mechanism causes photo-induced electron transfer to cause quantum dot fluorescence quenching. The detection method is simple in operation, high in sensitivity, and has good applicability to analysis of actual samples. In addition, based on CdSe/ZnS quantum dots, the study of dopamine fluorescence detection methods can be constructed, and corresponding results have been obtained.

 

Application of quantum dots in fingerprint visualization

In 2000, MENZEL in the United States reported for the first time the use of CdS quantum dots for the fingerprinting of cans on the surface of cans, creating a precedent for the application of quantum dots as a new material in fingerprint visualization. Subsequently, MENZEL et al. used PAMAM (polyamide) as a template to control the growth of CdS quantum dots encapsulated in dendrimers by the spatial threshold effect of dendrimers. The synthesized CdS/PAMAM polymer was diluted with methanol as solvent. After that, it was successfully used for the appearance of latent fingerprints on aluminum foil and polyethylene samples. They believe that functional groups such as amino or carboxyl groups on the surface of CdS/PAMAM can interact with fingerprint residues to deposit CdS/PAMAM onto the fingerprint lines, and the fingerprints appear through the fluorescence of CdS/PAMAM polymer under ultraviolet light. The method can be used for the identification of surface fingerprints of various objects (such as transparent tape, black plastic bags, tin foil, etc.) of different background colors.

The above is a comparison of fingerprint effects with CdS/PAMAM polymer and traditional visualization reagents. As you can see, the fingerprints displayed with CdS/PAMAM polymers are clearer(The picture on the left is CdS/PAMAM).

Application of quantum dots in improving solar cells

Solar cells, as the name implies, are devices that make full use of the energy of sunlight and convert solar energy into electrical energy through the photoelectric effect. Ordinary solar cells cannot fully utilize solar energy, and heat loss during conversion is large, resulting in defects in low photoelectric efficiency conversion rate. Scientists have used silicon semiconductor materials as a medium for solar and electrical energy conversion to develop common semiconductor materials for solar cells. After the silicon semiconductor material is irradiated by the sun, the internal electrons move freely to form a current. It is this special property of semiconductor materials that semiconductor solar cells can operate normally, but the results in actual production are not satisfactory. Scientists have learned that currents are generated only when these free electrons are moved to the electrodes. In fact, there are few free electrons moving to the electrodes, resulting in low conversion to solar energy. Therefore, ordinary semiconductor materials cannot completely convert solar energy into electrical energy.

Scientists envisaged that if the theory of quantum mechanics was introduced into the solar cells of ordinary semiconductor materials, it would be able to make up for the shortcomings of the actual power generation efficiency of ordinary semiconductor materials. Scientists conducted a large number of theoretical calculations, and the results were quite shocking to the people present. This kind of battery material is not only cheaper, but also can make full use of the sunlight energy while the consumption in the conversion process is minimal, so that the conversion rate of the solar cell to the photoelectric is doubled. Although the current super battery is still under development, whether it is as good as scientists expected and when it will be released to the public is still unknown. However, it is undeniable that quantum dot solar cells are becoming a new trend in research and development and are currently the most concerned research topic. It is believed that this super battery can completely solve the shortcomings of high energy consumption and low photoelectric conversion efficiency of ordinary semiconductor materials. Quantum dot solar cells will show excellent skills and bright prospects in future solar energy conversion and utilization.

Quantum dot sensitized solar cells are one of the quantum dot solar cells that have been developed so far. A quantum dot sensitized battery is a battery between a common semiconductor material solar cell and a super cell (quantum dot solar cell). So quantum dot sensitized cells cannot be called true quantum dot solar cells, it is just a transitional product. Quantum point solar cells in the true sense are still in the research and development stage. Quantum dot solar cells need to be studied in the way of photoelectric conversion, and the selection of medium is also necessary. The medium is the focus and hot spot of research on super batteries. The dielectric materials of super batteries must have ultra-high conversion rate and low cost, quantum dots. There is still a long way to go before solar energy research and development. As a transition product, quantum dot sensitized solar cells are based on dye-sensitized solar cells (DSSC) and are designed and developed using similar working principles. The quantum dot sensitized solar cell photoanode is composed of a quantum dot attachment and a photogenerated electron injection carrier, and the carrier is mainly composed of a binary semiconductor oxide.

The development prospect of quantum dots

As a new research field, quantum dots have attracted the attention of scientists in many fields in a short time. Scientists are actively involved in the study of quantum dots, which further demonstrates that the study of quantum dots is related to the advancement of science and technology and the development of science. Quantum dot technology is still in the early stage of theory, and scientists have found that quantum dots play an important role in life sciences and medicine. Scientists envision that if we can fully apply quantum dots to our lives, our lives will be more convenient and better. This is a perfect flaw for scientists and everyone. Scientists are fully devoted to the study of quantum dots, trying to put the application of quantum dots from theory to life and put them into practice. Quantum dot technology is a high-tech. Scientists have discovered the outstanding performance of quantum dots as fluorescent probes, the application of quantum dots in medical drugs, and the application of quantum dots in improving solar cells. Quantum dots are A little bit infiltrated into our lives. The research of quantum dots is still in its infancy. With the advancement of society and the continuous development of science and technology, the research scope of quantum dots is gradually expanding. As a new concept, quantum dots are well known. There are more mysteries in semiconductor microcrystals waiting for us to explore and discover. The study of quantum dots is moving at a strong pace, and the application of quantum dots is showing bright prospects.