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Sun et al. Chem Synth 2023;3:16 https://dx.doi.org/10.20517/cs.2022.45 Page 5 of 17

encapsulated into the cavity of WP5 and form host-guest complexes. In contrast with the individual ICG
solution, the higher photothermal conversion efficiency was observed for the WP5⊃ICG complex under 808
nm laser irradiation, indicating that ICG can be prevented from aggregation or degradation by forming
host-guest assembly. The cellular experiments further confirmed the improved photothermal cancer
therapeutic effect of WP5⊃ICG.

Au nanomaterials are common agents for photothermal therapy [43,44] . However, their low photothermal
conversion efficiency in NIR range due to their small size hinders their application in photothermal therapy.
While upon aggregation, the plasmonic coupling interaction between adjacent Au nanospecies can enhance
their NIR absorption and photothermal conversion efficiency . Inspired by the above fact, Cheng et al.
[45]
prepared ferrocene (Fc)-capped Au nanoparticles and cucurbit[7]uril (CB[7])-capped Fe O NPs, with
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average diameters of 14.04 nm and 18.53 nm, respectively [Figure 1B] . For individual Au-Fc or Fe O -
[46]
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CB[7], the photothermal conversion efficiency was limited to 9.1% or 8.7%. However, owing to the host-
guest interaction between Fc and CB[7], intensive aggregation occurred with the simultaneous presence of
Au-Fc and Fe O -CB[7] and the supramolecular aggregates exhibited significantly enhanced absorption
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around 808 nm. The temperature raising of Au-Fc/Fe O -CB[7] host-guest assemblies upon 808 nm laser
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irradiation was demonstrated to be as high as 35 °C and the corresponding photothermal conversion
efficiency was increased to 36.7%, which is ~4 times higher than that of individual Au-Fc or Fe O -CB[7].
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When applied in biological systems, the Fe O -CB[7] was accumulated in tumor tissue by an external
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magnetic field followed by the injection of Au-Fc. Contributed by the host-guest interaction, the Au-Fc
displayed directional movement towards Fe O -CB[7], which eventually led to the formation of host-guest
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based aggregates and enhanced photothermal conversion efficiency at tumor sites. This concept was further
strengthened by Gao et al. . In this case, the -CD-modified Au nanoparticles and adamantane (Ada)-
[47]
modified Au nanoparticles were first decorated with Escherichia coli outer membrane vesicles, respectively
[Figure 1C]. With the bacterial-mimicking nature, the Au nanoparticles were easily phagocytosed by the
immune cells in vivo. After the degradation of the Escherichia coli outer membrane vesicles by immune
cells, the Au nanoparticles were released and the host-guest interaction between -CD and Ada led to their
aggregation and enhanced photothermal conversion efficiency.
In other cases, the unique microenvironment of tumor site [48-50] can drive the reorganization of host-guest
assembly, which will then affect the photothermal conversion efficiency of host-guest assembly-based
agents. Spermine, which overexpresses in some types of tumors (e.g., breast cancer), was employed as
external stimuli for the photothermal performance enhancement of host-guest assemblies. Wang et al.
prepared polydopamine (PDA) coated nanocages (AuNCs), followed by the conjugation of a small peptide,
[51]
Phe-Phe-Val-Leu-Lys (FFVLK), on the PDA shell via covalent bonds [Figure 2] . In the presence of CB[7],
the terminal Phe unit of the FFVLK was incorporated into the cavity of CB[7] via host-guest interaction,
resulting in the nanoassemblies of Au@PDA-FFVLK-CB. When accumulated in breast tumor tissue, due to
the strong interaction between the over-expressed spermine and CB[7], the CB[7] was displaced from the
Au@PDA-FFVLK, leaving the exposure of FFVLK. Owing to the intermolecular interaction of FFVLK via
hydrogen bonds, hydrophobic interaction and π-π interaction, the self-aggregation of AuNCs occurred. As a
result of the enhanced plasmonic coupling effect of AuNCs aggregates, the enhancement of photothermal
conversion efficiency and photothermal therapeutic effect was achieved.

Host-guest interaction-induced enhancement of photothermal conversion efficiency in NIR-II
Constructing photothermal agents that function in NIR-II range is of great significance. The common
strategy to achieve this goal lies in the extension of the -conjugation of specific compounds . However, the
[52]
above process requires elaborate design and excellent synthetic skills. To extend the NIR absorption of
photothermal agents into NIR-II range in a more flexible manner, Tang et al. set an example by exploiting