Properties, synthesis and application of graphene quantum dots



Graphene quantum dots (gqds) is a new kind of carbon fluorescent material with graphene lamellae size less than 100nm and lamellae number less than 10. Generally speaking, graphene quantum dots are a group of carbon fluorescent materials and their derivatives with the same structure and performance, including graphene quantum dots, graphene oxide quantum dots and partially reduced graphene oxide quantum dots.            UV absorption properties of graphene quantum dots            Due to the C = C double bond structure in graphene quantum dots, π - π transition can occur, so it can absorb a large number of photons in the short wavelength range. Generally speaking, it will show a strong absorption peak in the range of 260-320nm of the UV absorption spectrum, with the tailing extending to the visible light range. At the same time, due to the influence of N - π transition, the shoulder peak of graphene QDs may appear in the range of 270-390nm. Moreover, due to the influence of surface modification functional groups and surface passivation, the position and shape of UV absorption peak will be affected.            Photoluminescence properties of graphene quantum dots            The luminescence performance of graphene quantum dots is the most important performance, and it is also the most widely studied and practical performance by researchers. Compared with spherical carbon quantum dots, graphene quantum dots with lamellar structure have more regular crystal structure, so they have higher fluorescence quantum yield.            Synthesis of graphene quantum dots            There are two methods to prepare graphene quantum dots: top-down and bottom-up.            Top down synthesis            The top-down method refers to the physical or chemical etching of large-scale materials into nano-sized graphene quantum dots, including solvothermal, electrochemical and chemical stripping.            Solvothermal method is one of many methods in the preparation of graphene quantum dots. The process can be divided into three steps: firstly, the graphene oxide is reduced to graphene nanoflakes by high temperature in vacuum; the graphene nanoflakes are oxidized and cut in concentrated sulfuric acid and nitric acid; finally, the graphene nanoflakes after oxidation are reduced to graphene quantum dots in solvothermal environment.            The technological process of the preparation of graphene quantum dots by electrochemical method can be summarized into three stages: the first stage is the induction period in which graphite is about to peel off to form graphene, and the color of electrolyte begins to change from colorless to yellow to dark brown; the second stage is the obvious expansion of graphite of anode; the third stage is that graphite flakes have peeled off from anode to form black together with electrolyte Color solution. In the second and third stages, precipitates were found at the bottom of the beaker. There is interaction between water and anion in ionic liquid in electrochemical reaction, so the shape and size distribution of products can be adjusted by changing the proportion of water and ionic liquid. The size of QDs prepared from electrolyte with high ion concentration is larger than that from electrolyte with low ion concentration.            The law of chemical stripping carbon fiber is that the carbon source is peeled layer by layer through chemical reaction to produce graphene quantum dots. Peng et al. Used resin based carbon fiber as carbon source, and then stripped the graphite stacked in the fiber through acid treatment. Graphene quantum dots can be obtained in one step, but their particle size is not uniform.            Bottom up synthesis            The bottom-up method is to prepare graphene quantum dots with small structural units as precursors through a series of mutual forces, mainly including solution chemistry, ultrasonic and microwave methods.            The solution chemical method is mainly to prepare graphene quantum dots by the solution chemical method of aryl oxidation condensation. The synthesis process is to use small molecules (3-iodo-4-bromoaniline or other benzene derivatives) polymer to gradually condense to prepare the precursor of PPD, then to prepare graphene group by oxidation reaction, and finally to prepare graphene quantum dots by etching.            According to the microwave law, carbohydrates (such as glucose, fructose, etc.) are used as carbon sources, because carbohydrates can form C = C after dehydration, thus forming the basic skeleton unit of graphene quantum dots. The hydrogen and oxygen elements in hydroxyl and carboxyl groups will be dehydrated and removed in hydrothermal environment, while the remaining functional groups will still be bonded on the surface of graphene quantum dots. They exist as "passivation layer", which can make graphene quantum dots have good water solubility and fluorescence properties.            Practical application of gqds            Graphene quantum dots have important potential applications in biology, medicine, new semiconductor devices and other fields. It can realize the single molecule sensor, and it may also give birth to the subminiature transistor or the on-chip communication using the semiconductor laser to make the chemical sensor, solar cell, medical imaging device or nano level circuit and so on.            Application in biosensor            Zhao et al. Developed a new kind of general signal conduction equipment, which uses fluorescence quenching caused by resonance transfer of luminescence energy between graphene and graphene quantum dots to detect human immunoglobulin G (higg) sensitively.            Application in biological imaging technology            The application of graphene quantum dots in biomedical research is based on a large number of reports that graphene can be used as a biosensor because of its large specific surface area. Graphene quantum dots not only have the properties of stable photoluminescence, low toxicity and good biocompatibility, but also can be a good imaging probe.                 报错