5.3. Bioimaging
The bioimaging applications of CDs are severely hampered by aggregation-induced quenching, which result in weak fluorescent images and nonuniform staining. Accordingly, CDs that can stain various kinds of bacteria or cells effectively and real-time without concentration-dependent PL quenching are required. Solid-state fluorescent CDs have been prepared to prevent aggregation-induced quenching through dispersion in solid matrices.[118-121] However, many results show that this method has many limitations. Chen et al. [34]synthesized a self-quenching-resistant CDs powder utilizing PVA and ethylenediamine, which had a relatively low quantum yield of 35%. Yang et al. [115] showed that polyethyleneglycol-functionalized CDs possess weak staining capability (Figure 7E). This is due to the resistance of polyethyleneglycol to protein and reduced interactions between the biological cells and CDs. CDs encapsulated in solid matrices during the synthesis or post-preparation may reduce the bio-affinity interactions between biological cells and CDs and diminish the biocompatibility of the materials. Therefore, aggregation fluorescent CDs without solid matrices are necessary for bioimaging. Zhang et al. [32]fabricated self-quenching-resistant solid-state PL CDs without any solid matrices. The multicolor CDs were successfully applied to rapid staining of representative bacterial species, including acid-fast bacteria, gram-positive, and gram-negative. In addition, some pathogenic bacteria could be stained rapidly within 1 min through the smear staining method without any incubation, which is also applicable to the use of liquid incubation methods. Wang et al. [116] prepared concentration-dependent fluorescent tunable CDs that can be used as nanoprobes for Fe3+ detection and multicolor cell imaging (Figure 7F).