Preparation of saccharide-derived carbon dots and their applications in biomedical and environmental pollutant analysis

Carbon dots（CDs） are a new class of materials with a wide range of applications. CDs possessed the advantages of small size effect， remarkable photostability， low cytotoxicity， good biocompatibility， simple synthesis and surface modification， and abundant surface functional groups（hydroxyl groups， carboxyl groups， amino groups， etc.）. Currently， CDs show ultra-high potential in various fields. Carbohydrates are one of the most diverse and important classes of biomolecules in nature and can be modified at isomeric positions and hydroxyl functional groups. The saccharides are readily available carbohydrates in nature with non-toxicity and low permeability， providing an attractive and inexpensive starting material for the synthesis of CDs with specific properties and multifunctional applications. In recent years， the preparation of CDs from bio-based saccharide sources provides a new idea for CDs synthesis due to its advantages of low cost， renewable raw materials and green environment. Depending on the carbon source， the synthesis strategy of CDs can be broadly classified into top-down and bottom-up approaches. The top-down approach refers to the decomposition of larger carbon structures into nanoscale particles（e.g. graphene， carbon nanotubes， etc.）， while the bottom-up approach refers to the synthesis of CDs from smaller carbon units（e.g. saccharides， organic acids， etc.）. Using saccharides as carbon source， the methods for synthesis of CDs via bottom-up approach have been carefully summarized. In detail， bottom-up approach consisted of hydrothermal， microwave-assisted， ultrasonic and pyrolysis methods. The acquired saccharide-derived CDs had the properties of good water-solubility， low-toxicity， photostable and chemically stability. What’s more， the obtained CDs had broad application prospects in various fields， such as bioimaging， biosensing， drug/gene delivery and chromatographic analysis. In bioimaging， CDs have excellent optical properties and low toxicity， and can be used as fluorescent probes for real-time imaging in cells and tissues. In biosensing， the functional groups on the surface of CDs can specifically bind to the detected substances to achieve highly sensitive detection of biomolecules or ions. In drug/gene delivery， CDs can be used as carriers for efficient delivery of drugs or genes， reducing side effects and improving therapeutic effects. In chromatographic separation， CDs can be loaded on stationary phases and interact with compounds to achieve efficient separation of compounds. Furthermore， the saccharide-derived CDs showed good performances in the separation and analysis of new contaminants including nuclides， antibiotics， etc. Additionally， the saccharide-derived CDs exhibited outstanding properties for pharmaceuticals（alkaloids， nucleoside analogues， etc.） determination. Thus， CDs provide new tools for environmental monitoring and drug analysis. In the future， the research objectives in the preparation of CDs include the following： firstly， to continue to develop low-cost and simple preparation methods for the large-scale production of physically and chemically stable CDs. Secondly， the surface functional groups of CDs are further enriched in order to improve the interaction ability of CDs with target molecules or ions and to be able to endow CDs with more functionalities， such as targeting and responsiveness. In addition， a variety of heteroatoms（e.g.， nitrogen， boron， phosphorus， sulfur， etc.） are doped to improve the properties of CDs. In addition to the improvement of preparation protocols， expanding the application of CDs in chromatography and sensing analysis， as well as in-depth study of their mechanism of action still deserves attention.