Figure 4 (a) An alkali-driven
temporal organogel. Copyright
(2019) John Wiley and Sons. (b) A KOH-driven temporal supramolecular
chiral G-quadruplex hydrogels. Copyright (2021) John Wiley and Sons.
3.1.2. Temporary gels based on pH-responsive gelator.pH-responsive gels are a sort of typical stimuli-responsive material
which has been widely reported in recent decades. [38-41] They are
thought to be good candidates for controlled drug delivery [42,43]
or gel actuators. [42, 44-46] The key to fabricating a pH-responsive
gel is to introduce a group, which is a weak acid (or base) or can be
broken in presence of H+ (or OH-),
to the 3D molecular network of the gel. It is therefore expected that
combining a pH-responsive gel with a fuel-driven reaction network in
which the pH value can temporarily change is an applicable strategy for
creating a non-equilibrium temporary gel material. According to this
hypothesis, some temporary gels based on pH-responsive gelators have
been reported. In 2019, D. J. Adams and coworkers presented a temporary
supramolecular gel by applying an amino-containing gelator. [47] As
shown in Figure 4a, the hydrochloride of the gelator was well-soluble in
water. To initiate the gelation, urease, and urea were added.
NH3 yielded by their autocatalytic ureolysis rapidly
enhanced the pH to 9.1–9.2, resulting in the deprotonation of the
gelator. A semitransparent hydrogel can be obtained within 5 min.
However, the increase in pH triggered the base-catalyzed saponification
reaction of methyl formate, which was added together with the urease and
urea. The formic acid generated by the saponification led to a slow
decrease of pH to 6 or even 4. It will redissolve the gelator and
destroy the hydrogel. The stiffness, viscosity, and lifetime can be
enhanced by both increasing the concentration of urea and decreasing the
amount of methyl formate. This work provided a simple example of
pH-regulated temporal gel. Similarly, J. Li and coworkers reported a
temporary supramolecular chiral G-quadruplex hydrogel realized by
another pH-feedback reaction network. (Figure 4b) [48] Herein the pH
was enhanced by directly adding KOH. It combined the guanosine and
5-fluorobenzoxaborole, yielding a benzoxaborolate complex that can
self-assemble into a helical G-quadruplex, ultimately leading to the
formation of a highly transparent hydrogel. The recovery of pH in this
work was achieved by the hydrolysis of 1,3-propanesultone. It decomposed
the gelator and damaged the temporary hydrogel. In addition to the
alkalis such as NH3 and KOH, acid-induced temporal pH
decrease has also been used to promote the formation of temporary gels.
For example, A. Walther and coworkers applied citric acid as a chemical
fuel to initiate the DSA of a pH-responsive dipeptide precursor.
[24] (Figure 5a) The precursor immediately self-assembled into
twisted nanofibrils and thereafter gelation occurred. The automatic pH
recovery is caused by urase and urea premixed with the precursor. The
rapid pH drop activated the urease that can catalyze the hydrolysis of
urea into CO2 and NH3, resulting in the
disassembly of the nanofibrils and the collapse of the hydrogel.
Furthermore, the acid-driven temporal deformation of hydrogel based on
the same principle has also been reported recently. X. Wang et al.
developed a bilayer hydrogel actuator consisting of a tertiary
amine-containing, pH-responsive layer, and a urease-containing
non-responsive layer. [49] (Figure 5b) Upon the addition of acidic
urea solution, a significant bend can be observed due to the swelling of
the pH-responsive layer. Meanwhile, urease in the other layer would
catalyze the hydrolysis of the urea. The resulting NH3led to the deswelling of the automatic relaxation of the actuator. It
can be found that the key to constructing the above temporary gel
material is to find a deactivator that can slowly and spontaneously
self-decompose to neutralize the pH jump caused by the addition of
alkali or acid. However, in some special cases, the pH can also be
recovered by direct decomposition of the acidic/alkaline chemical fuel
itself. For example, A. Quintard and coworkers presented a temporary gel
that formed under the drive of CCl3COOH.
[50] CCl3COOH
protonated the precursor and led to its self-assembly and gelation.
While the basic moieties in the reaction mixture, as a deactivator,
catalyzed the decomposition of CCl3COOH into volatile
chloroform and carbon dioxide. Unsurprisingly, the temporary gel
gradually dissolved as the CCl3COOH was consumed.