a gel, even after intensive heating or sonication. TEM images
of these solutions showed the presence of mixed spherical and
fibrilar nanostructures (Fig. S10, ESIw). This implies that the
vesicles formed in pure water by the assembly of the folded
GAP are partially protected from the unfolding action of the
MeOH. These observations suggested that the formation of
the hydrogel is a result of a delicate interplay between hydro-
phobic and hydrophilic intermolecular interactions, which are
possible only in the unfolded conformation of 1.12,15,16
The thermal stability of the supramolecular hydrogel was
also studied as a function of the gelator content and the
amount of water added to the MeOH (Fig. 2B and C).
Interestingly, the stability of the gel depended on both variables,
being maxima at different ratios for every case. The suitable
comparison of the plots suggests that the stability of the gel is
more sensitive to changes in the solvent composition (Fig. 2B)
than to the gelator content (Fig. 2C). Besides, for the most
suitable MeOH: water ratios, the stability of the hydrogels
remains constant at different gelator contents above 0.5% w/v
(see Fig. 2C, blue and green plots). These results suggest that
the hydrogel of 1 is formed by the synergic action of hydro-
phobic and H-bonding interactions and thus, a possible
enthalpy/entropy compensation effect could be operating,
leading to some irregular trends in the temperature dependence.
The hydrogel of 1 can be also disassembled by changes in the
pH (Fig. 2A). Thus, simple addition of an acid (HCl) to the gel
produced the transition to a solution (see Fig. S21w, ESIw, for
snapshots of the acid-promoted gel disassembly) despite the
hydrogel being perfectly stable to the addition of a base
(Fig. 2A). The SEM images of the disassembled gel in acidic
medium showed the presence of large microspheres, suggesting a
cationic surfactant-like behavior (Fig. 2A). On the other hand,
according to the stability of the gel in basic medium, its SEM
micrographs still showed a fibrilar network (Fig. 2A). Thus, the
simple compound 1 can be considered as a stimulus-responsive
(temperature, pH and solvent) room-temperature hydrogelator.
In summary, we report the efficient design of a simple GAP
able to self-assemble in aqueous medium. In pure water, 1
self-assembles into vesicles at both acidic and neutral pH. The
addition of water to a methanolic solution of 1 spontaneously
forms room-temperature stable hydrogels, which can be
disassembled by heating or protonation. Very importantly,
the alcoholic co-solvent needed for the formation of the
hydrogel can be substituted by the more bio-compatible
EtOH.w For instance, compound 1 at 1% wt/v concentration
forms a stable gel (Tg = 53 1C) in 90 : 10 H2O : EtOH, a
mixture commonly used for drug formulation and biological
assays (see ESIw) and thus foreseeing the potential bionano-
technological applications of 1.
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This work was supported by the Spanish MICINN
(CTQ2009-14366-C02) and UJI-Bancaixa (P1-1B-2009-59).
J. R. thanks MICINN for personal financial support (FPU
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2212 Chem. Commun., 2012, 48, 2210–2212
This journal is The Royal Society of Chemistry 2012