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).