5.4. Other
The unique properties resulting from the aggregation of CDs are leading to applications in new areas. As shown in figure 7G, Wu et al.[117] developed to use special hydrophobic CDs with rotatable surface groups to build the aggregation-induced emission active glycol CDs polymer gel, which could be interfacially bonded to an elastomer to prepare an anisotropic bilayer soft actuator. When the actuator was put in water, glycol will spontaneously diffuse out of the gel layer, leading to a color change from blue dispersive fluorescence to red aggregation-induced emission and shape deformation, whilst the large surface tension gradient promotes voluntary motion. Based on these principles, an artificial soft swimming robot with octopus-shape/color covariation and directional swimming motion was reported.
6. Summary and perspective
As an exciting new type of luminescent carbon-based nanomaterial, CDs received a lot of research attention in the past few years. However, there still many mysteries about the formation process, structure, classification, and luminescence of CDs. Many reports suggest that aggregation plays an important role in CDs, from formation to application, and understanding aggregation processes at different levels it will help resolve many of the mysteries around CDs. In this review, we aimed to highlight the role aggregation in the synthesis of different types of CDs, whilst also highlighting special luminescence properties of CDs that emerge in the aggregate state, including aggregation induced fluorescence quenching and quench-resistant research, aggregation-induced emission, and aggregation-induced room temperature phosphorescence. Finally, the special applications resulting from aggregation of CDs were summarized, including anti-counterfeiting, optoelectronic, bioimaging, and others.
Potential future development directions related to the aggregation of CDs include, but are not limited to, the following
(1) Clarify the structure of CDs. The ambiguity of CDs structure remains a limitation in the field of CDs research. Ideally, bottom-up synthesis methods will be discovered that allow CDs to be fabricated in a rational manner (with accurate size, composition control and structure), thereby allowing better understanding of these variables and aggregation phenomena affect performance. Understanding the precise structure of CDs will help solve many current controversies, including the classification of CDs and luminescence mechanisms, whilst greatly expanding the range of applications.
(2) Shape control adjustment. Shape and size play powerful part in determining the properties of most nanomaterials, but the morphology of the CDs remains difficult to adjust. Some of the above reports have shown that shape adjustment of GQDs and CQDs can be achieved using customized aggregation methods, but much more work is needed in this area. The shape regulation of CPDs remains very difficult. Taking inspiration from the synthesis of inorganic nanoparticles, template and micellar methods are worthwhile strategies to try for morphology-engineering of CDs.
(3) The reasons why some CDs possess aggregation-induced luminescence is unclear, prompting a exact structure description and mechanism analysis. If the mechanism can be properly explained, CDs with particular luminescence phenomena can be more easily customized for more advanced applications.
Despite the above-mentioned challenges, CDs have a bright future. We hope to open this review stimulates wider research linked to understanding aggregation phenomena in CDs.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (52122308, 21905253, 51973200, 52103239), the Natural Science Foundation of Henan (202300410372).