Research on quantum dots has attracted significant attention since the Nobel Prize in Chemistry was awarded to scientists in this field. As a special type of quantum dots, fluorescent carbon quantum dots (CDs) have excellent fluorescence and controllable surface chemical properties, which can be applied in biological medicine fields such as bioimaging and diagnosis and treatment of diseases. CDs are fluorescent nanomaterials with a size of less than 10 nm. Their preparation methods are diverse and simple, precursors are widely available, optical properties are stable, and photobleaching resistance is strong. Compared with inorganic quantum dots, they do not contain heavy metals, so they have lower biotoxicity and higher biocompatibility, and have great application potential in the biomedical field. The surface of CDs is rich in functional groups, which determine their physical, chemical, and fluorescent properties, including quantum yield, emission wavelength, aggregation-induced emission/quenching, fluorescence lifetime, biocompatibility, and special material response. One of their most important properties is luminescence, which comprises a variety of mechanisms, including surface-controlled luminescence, cross-linked enhanced emission effect, quantum size effect, and carbon core-controlled luminescence. These mechanisms interact with each other to influence the CD fluorescence effect. Reasonable regulation of the CD luminescence properties, such as fluorescence wavelength and intensity, is of great significance in disease diagnosis. At the same time, through regulation of the surface functional groups of CDs, the scavenging ability of ROS free radicals can be adjusted. Therefore, CDs have great application potential in the diagnosis and treatment of tumors and inflammation.

Based on recent literature reports, this study introduces and summarizes in detail the application of CDs in the field of biomedicine and their related mechanisms and characteristics. First, in terms of biological imaging, CD nanostructures enter cells through endocytosis and exocytosis and disperse in the cytoplasm or specifically in some organelles. As a result, we can clearly observe the physiological activities of body structures such as micro vessels and brain tissue with the help of confocal microscopy or other instruments [Figs. 2(a) and (b)]. The design and preparation of CDs with near-infrared fluorescence wavelength and high quantum yield result in higher resolution and tissue penetration ability, which is advantageous for in vivo imaging [Figs. 2(c) and (d)]. On this basis, we introduce the application of CDs in disease diagnosis. Compared with other diagnosis methods, the application of CDs is less traumatic, which opens up broad prospects in disease diagnosis. CDs with pH-sensitive luminescence characteristics can be used as potential imaging reagents for pH monitoring [Fig. 3(a)]. Fluorescence enhancement strategies based on nitrogen-doped CDs induced by nicotinamide adenine dinucleotide can be used to monitor tumor occurrence and provide early warning of tumor formation [Fig. 3(b)]. CDs can also produce selective responses to some biomarkers, resulting in changes in fluorescence signals, which can play a role in monitoring the occurrence and development of diseases through the detection of cytochrome C in human serum samples [Fig. 3(c)] and creatine kinase (CK), an important biochemical indicator of heart injury [Fig. 3(d)]. Finally, CDs with rich surface states can interact with the body in a variety of chemical reactions. CDs designed and prepared with specific structures can be effective in disease treatment, mainly in the following three areas: 1) tumor treatment, 2) antibacterial and antiviral treatment, and 3) anti-inflammatory treatment. A novel CD for the treatment of glioblastoma was synthesized using metformin and gallic acid precursors [Fig. 4(a)], whereas Fe single-atom nano cases with high pyrrole nitrogen content and ultra-small CD support were prepared using a phenazoline mediated ligand-assist strategy [Fig. 4(b)]. CDs effectively inhibit the growth of tumor cells through synergistic chemical kinetics and photothermal effects. The synthesized quaternary ammonium CDs have positive charge properties and can retain antibiotic precursor active groups [Figs. 4(c) and (d)], leading to effective antimicrobial activity. The surface of CDs has many oxidized chemically active groups, such as phenolic hydroxyl, which react easily with oxidizing substances, thus playing antioxidant and anti-inflammatory roles. Anti-inflammatory and antioxidant CDs are prepared through precursor optimization, solvent extraction, and use of broccoli as a biological feedstock [Fig. 4(e)]. CDs can also be designed as nano-enzymes to perform anti-inflammatory and antioxidant functions [Fig. 4(f)]. At the end of the paper, we outline the challenges faced by CDs in biomedical applications. First, for efficient application of CDs in biomedical fields, various preparation conditions need to be considered comprehensively to achieve accurate control of their key properties. Second, the conversion of CDs into actual products still faces challenges, and further solutions are needed to promote their production and commercialization. Finally, the regulation and standardization of CDs are becoming increasingly important. To solve the above problems, the development and application of CD preparation technology should be promoted.

CDs are being widely used in the field of biomedicine, including bioimaging and diagnosis and treatment of diseases (Table 1). Their excellent optical, physical, and chemical properties provide them with obvious advantages in the field of biomedicine, including safety, light stability, easy access, and performance tunability. In the future, the large-scale and diversified development of CDs can be further enhanced to promote their industrial production, commercialization, and application in real life.