3.3. Temporary nanoreactors
The cell membrane system provides a specific local environment in living
cells to protect some chemical reactions against external interference.
The synthesis efficiency of the biomolecules was significantly improved.
The well-organized and efficient biosynthesis resulted from the cell
membrane system encourages chemists to develop synthetic nanomaterials
that can provide a mesoscopic chamber with special polarity and stereo
hindrance to change the yield and selectivity of the chemical reaction.
In recent decades, a large number of such materials, called
nanoreactors, were reported. [67-70] The yield and selectivity of
many organic reactions in the nanoreactor were remarkably improved
compared with the reaction in the homogeneous phase. In addition,
nanoreactors were also used for controlling the growth of inorganic
nanoparticles. The size of the nanoparticles synthesized in the
nanoreactors was restricted because the amounts of the reactants
encapsulated in each nanoreactor are limited. Therefore, it is an
effective approach to obtaining nanoparticles with a uniform size.
However, removing the nanoreactor after the completion of the synthesis
remains a quite critical challenge. It greatly constraints the
collection and purification of the product. Hence, it will be ideal to
find a nanoreactor that can spontaneously disintegrate after the
reaction completes. In 2016, L. J. Prins and coworkers attempted to
fabricate a conceptual self-dissociable nanoreactor with the aid of
fuel-driven DSA. They found a cationic surfactant,
C16TACN·Zn2+, can be micellized with
the addition of ATP. [26] (Figure 12a) The micelles formed created a
nonpolar microdomain that promoted the reaction between two hydrophobic
reactants (C8-SH and NBD-Cl) in aqueous solution.
Furthermore, the potato apyrase in the solution slowly decomposed ATP to
AMP and phosphoric acid, resulting in the dissociation of the micellar
nanoreactor. The lifetime of the nanoreactor can be well controlled by
the amount of ATP added. The increase in the ATP concentration enhanced
the yield of the reaction between C8-SH and NBD-Cl. The
similar temporary nanoreactor can also be realized by the DSA of
polymers. In 2021, A. Walther, J. Boekhoven et al. applied the EDC, as
the chemical fuel, and poly(ethylene
glycol)-b -poly(styrene-alt-maleic acid) (BCP1), as the precursor,
to fabricate a polymeric temporary nanoreactor. [71] (Figure 12b)
The reaction network was the same as their previous work.[34] As an
example, a Diels-Alder reaction between two hydrophobic reactants
successfully proceeded in this temporary nanoreactor. It was also found
that the larger fuel concentrations resulted in higher reaction yields
due to the longer sustained encapsulation of the nanoreactor. In the
meantime, H. Zhao and coworkers presented another EDC-driven polymeric
temporary nanoreactor. [72] The reactor was formed by the DSA of
poly(ethylene glycol)-b -polyacrylate (PEG-PAE), which yielded by
the EDC-catalyzed esterification of poly(ethylene
glycol)-b -polyacrylic acid at pH 6.5. While the ester slowly
hydrolyzed in such a weakly acidic environment resulting in the
dissociation of the nanoreactor. The authors employed UV-induced
anthracene dimerization as a model to study the nanoreactor. They not
only found that the nanoreactor significantly increased both the yield
and rate of the dimerization in aqueous solution but also provided
evidence that the fuel-driven temporary nanoreactor could effectively
prevent the product from being trapped, further enhance the inversion of
the reactants and the utilization of the reactor.
3.4. Temporary Self-healable materials
In the above discussion, it can be found that the concept of fuel-driven
DSA or temporary material can be extended to improve the properties of
existing functional materials. Another example is self-healing
materials. The kinetic stability of intrinsic self-healing materials was
usually unsatisfactory since the existence of dynamic cross-linking. It
will lead to the undesired merging of intact materials. However, the
dynamic bonds or non-covalent interactions between the polymer chains
are essential for self-healing, making it difficult to develop a
synthetic material that has excellent dynamic stability and self-healing
ability simultaneously. In 2020, X. Wang and coworkers provided a strat
egy to create kinetically stable hydrogels with self-healing ability
enlightened by the concept of fuel-driven temporary material. [73]
As shown in Figure 13a, the hydrogel was synthesized by crosslinking
4-armed histidine-modified polyethylene glycol with
Co3+. A small amount of GOx was mixed during the
preparation of hydrogel. It was found that the Co3+cross-linked hydrogel has high kinetic stability without the undesired
fusion due to the strong coordinate interaction. But it cannot
spontaneously heal after cutting into two completely separated pieces.
However, healing can be realized by bringing the damaged surfaces
together with the addition of a mixture of ascorbic acid and glucose.
The healing efficiency can reach 90% after 24 h. It is because that
Co3+ was rapidly reduced to Co2+ by
ascorbic acid, weakening the coordinate interaction and causing a local
dissolution on the fracture surfaces. It will facilitate the
recombination of the two separated pieces. More importantly, the
dissolution was temporary since the production of
H2O2 by GOx-catalytic glucose oxidation.
The regeneration of Co3+ would solidify the
combination resulting in a high self-healing efficiency. This work
exhibited that replacing permanent self-healing with fuel-driven
temporary self-healing smartly avoided the aporia of maintaining
kinetically stability.