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).