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.