Figure 5 . (A) Solvothermal route towards triangular CQDs.
Reproduced with permission.[86] Copyright 2018,
Springer. (B) Schematic illustration of the in-situ synthesis of CQDs.
Reproduced with permission.[87] Copyright 2022,
American Chemical Society. (C) Schematic illustration of the synthesis
of annular CQDs. Reproduced with permission.[88]Copyright 2021, Wiley-VCH. (D) Scheme showing inverse micelle formation
and carbonization process during carbon nanorod synthesis. Reproduced
with permission.[89] Copyright 2019, American
Chemical Society. (E) Proposed three-step mechanism for the aggregation
behavior of GQDs with and without the addition of salt under acidic
(protonation) and alkaline (deprotonation) conditions. Reproduced with
permission.[90] Copyright 2017, American Chemical
Society.
For small organic fluorophore molecules, aggregation is often prevented
by introducing steric hindrance or through molecular conformation
control. For example, by connecting bulky rings, spiral kinks, and
dendritic wedges covalently to aromatic cores, intermolecular π–π
stacking interactions can be avoided.[28, 93]Similar methods have also been used to restrain the aggregation induced
fluorescence quenching of CDs.[94] Park et al.[95] exhibited that surface functionalization of
GQDs with hexadecylamine, poly(ethylene glycol), and polyhedral
oligomeric silsesquioxane could reduce π−π stacking-induced the
fluorescence quenching of GQDs (Figure 6A). Li et al.[96] prepared carbon nanorings using elaborate
precursors and synthesis processes that effectively suppressed
aggregation induced fluorescence quenching.
In addition, embedding CDs in the matrix is further approach to prevent
aggregation. Commonly used substrates are polymers, acid molecules
condensates, inorganic salts, organic-inorganic hybrid materials, and
mesoporous or layered materials.[97] Park et al.[98] incorporated GQDs within a boron oxynitride
matrix, with aggregation induced fluorescence quenching being
significantly suppressed. Zhou et al. [99]reported strongly luminescent CDs@silica composite gels, with the gel
matrix hindering collision between high concentrated CDs (Figure 6B).