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.