Carbon quantum dots: Preparation, optical properties, and biomedical applications

As an excellent fluorescent nanomaterial, carbon quantum dots (CQDs) have the properties of small molecular weight, small particle size, high fluorescence stability, no optical flicker, wide excitation spectrum, continuous, long adjustable emission wavelength, good biocompatibility and low toxicity. At present, it has been widely used in biological detection, in vivo and in vitro imaging, drug delivery and diagnostic drugs. This review presents a broad overview of recent progress in synthesis, optical properties, and biomedical applications of CQDs. The synthesis methods of CQDs are first introduced, followed by an extensive discussion of its purification, surface functionalization methods and stability. Furthermore, the optical properties of CQDs are presented, with an emphasis on photoluminescence mechanism and the relationship between synthesis conditions and optical properties. This review's much focus is given to the biomedical applications of CQDs, including detection, imaging (in vivo and in vitro imaging) and drug delivery (targeted therapy, photodynamic and photothermal therapy, and immunological and gene therapy). Finally, we cover the key issues and future prospects/advances that CQDs still have to deal with in the biomedical sector as well as possible solutions.