CQDs were first discovered in 2004 during the washing process of single-walled CNT [8]. CQDs are emerging categories of carbon-based nanomaterials with many unique properties and advantages. Due to their size at the nanometer level, carbon nanodots exhibit significant quantum confinement effects, i.e., their electronic struc­ture and optical properties change significantly. This makes CQDs have tunable fluorescence properties, such as emission wavelength, luminescence intensity, and fluorescence lifetime; they also have good photostability, which can maintain their fluorescence performance under prolonged illumination, and are not easy to fade out or degrade, which makes them suitable for applications such as long-term tracking and micro-imaging; and they are biocompatible with better biocompatibility, and have a lower toxicity to organisms and cells. This makes them have a wide range of potential applications in the biomedical field, such as biomarkers, drug delivery and bioimaging, etc. Meanwhile, CQDs are relatively simple to synthesize and can be prepared by pyrolysis of carbon sources, chemical oxidation or laser stripping. In addition, their surfaces are easy to be functionalized and modified, and different functional groups can be introduced through chemical reactions to achieve specific application requirements.