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).