Graphene belongs to two dimension carbon nanomaterial composed of a layer of carbon atoms arranged in sp2 hybrid orbitals with honeycomb lattice. The π electrons in graphene exhibit notable delocalization effect doe to its large planar conjugate structure. Graphene shows many amazing electronic or electrical properties, such as room temperature quantum hall effect, free transport properties, high carrier mobility, low resistivity, excellent optical properties and mechanical properties. Unfortunately, different from most other two-dimensional materials, the graphene of large size bears zero band gap and semi-metallic property, which hinders its application in the fields of photoelectric device, semiconductor and so forth. Accordingly, how to open the band gap of graphene and transform it from semi-metallic material to semiconductor material has aroused numerous interests. Currently, there are two known approaches to open the band gap of graphene. One is to to break the π electronic conjugate system of graphene by chemical doping. The other is to tailor graphene into nanorods, nanosieves, or quantum dots based on their quantum effects. Graphene quantum dots (GQDs) are segments of two-dimensional graphene with plane size less than 100 nm. Thanks to the quantum confined effect and boundary effect, GQDs are endowed with special physical and chemical properties, and they are also semiconductor materials with band gap. GQDs are superior to conventional semiconductor quantum dots, owing to their low toxicity, favorable water solubility, low chemical activity, satisfactory biocompatibility and stable fluorescence properties. Besides, GQDs possess monatomic planar conjugate structure and large specific surface area, and the oxygen groups on the GQDs surface can provide active site for the binding of foreign molecules. Consequently, GQDs show a broad prospect of application in solar cells, optoelectronic devices, biological medicine and other fields. The synthesis approaches of GQDs can be divided into top-down and bottom-up methods. The top-down method mainly includes strong acid oxidation, hydrothermal/solvothermal reactions, electrochemical oxidation, and so forth, which feature good water solubility and is prone to be functionalized. The bottom-up method is mainly divided into controllable organic synthesis and carbonization reaction. As for the former, GQDs with uniform size and shape, and precise carbon atom number can be obtained, but it suffers from complicated synthtic process, long reaction time and low production rate. While the latter is the common reaction for the synthesis of GQDs, although the size and structure of the products can be hardly to control, and the obtained GQDs usually present polydispersity. This paper comprehensively introduces the diverse synthesis approaches of GQDs, and provide detailed comments on these approaches. At the same time, the reaction mechanism of important or novel methods are expounded. In addition, emphasis is put on the applications of GQDs in the field of biosensors. Finally, the research and development of the GQDs in the future is proposed. © 2019, Materials Review Magazine. All right reserved.
