摘要： Not only CDs, but many carbon allotropes showing graphitic structure, i.e., honeycomb-like arrangement of carbon atoms, have shown potential for various applications in electronic devices in biosensors and bioimaging agents. Some examples of these are 0D fullerene [274], diamond nanocrystals [123] and carbon dots (CDs), as well as 1D CNT [275], 2D graphene QD (GQDs) [276] and graphene [277]. However, it must be mentioned here that although CDs and GQDs have similar quantum-confined fluorescent carbon materials, CDs have been more used as biosensors and bioimaging agents [278]. Both CD and GQD are mainly composed of sp2 carbon, oxygen and nitrogen elements and other doped heteroatoms [24, 101]; the different spatial arrangements of carbon atoms exhibit distinctive physical and chemical properties [279]. Unlike GQD, the CDs do not have perfect crystal structures [42]. Moreover, CDs exhibit luminance at size < 10 nm, whereas GQDs have size up to 100 nm [280, 281] and are luminescent. Generally, CDs display strong optical absorption in the UV region, with a tail extending out into the visible range (Figure 5.1a), i.e., 280–360 nm. The absorption band could be regulated via surface passivation or modification. CDs show the excitation-dependent emission properties in wavelength and intensity (Figure 5.1b). Other properties that make CD more applicable for biomedical applications are their low cost, high quantum yield, abundant source, low cytotoxicity, and superior chemical and photo stability. Wang et al. [282] have evaluated the cytotoxicity of the CDs prepared by different combinations of precursor of CDs and molecules used for surface functionalization. It is found that in terms of their nanoscale structure and configuration CDs are not intrinsically cytotoxic. The cytotoxicity of CD depends on the selection of surface passivation molecules.