Figure 6 (a) Temperal gel-sol-gel cycle driven by ATP. (b) ATP-driven gel temperoal swelling. Copyright (2021) Chinese Chemical Society.
3.1.4.H2O2-driven temporary gels. H2O2 is a highly active and strong oxidant. Redox-active species such as thiol and ferrocene can be rapidly oxidized by H2O2 and then gradually reduced by some stronger reductants. Thus, it can create a reaction network that is able to drive dissipative molecular self-assembly or create temporary materials. For example, Z. Guan and coworkers presented a H2O2-driven temporary hydrogel formed by a thiol-containing precursor. [57] (Figure 7a) The H2O2 oxidation resulted in a disulfide-linked dimmer (CSSC). It self-assembled into the fiber-like structure in aqueous solution, yielding a highly transparent hydrogel. A stronger reductant, dithiothreitol was used as the deactivator. It slowly decomposed the CSSC into CSH and destroyed the temporary hydrogel. Similarly, B. J. Ravoo and coworkers realized a H2O2-driven temporary gel-sol-gel process. [58] As shown in Figure 7b, the hydrogel was formed via the supramolecular crosslink between two grafted polyacrylic acids which are grafted with β-cyclodextrin (β-CD) and ferrocene (Fc), respectively. Upon the catalysis of horseradish peroxidase (HRP), H2O2destroyed the supramolecular interaction by oxidizing Fc into ferrocenium ion (Fc+), dissolving hydrogel quickly. D-glucose was chosen as a deactivator in this reaction network. It reduced Fc+ to Fc in the presence of GOx. The enzymatic reduction restored the supramolecular crosslink and rebuilt the hydrogel within a few hours. It can be found that the addition and consumption of H2O2, an active oxidant, creates a transient oxidizing environment that allows the redox-responsive precursor to temporally aggregate or crosslink to form material. Correspondingly, the reaction network driven by the reactant has also been successfully constructed. The reductant-driven temporal materials will be introduced in the following sections.