Preparation of nanostructures by orthogonal self-assembly of hydrogelators and surfactants

Article abstract of DOI:10.1002/anie.200704609

Social networking: A variety of novel multicompartment nanostructures has been easily formed by combining the supramolecular aggregative properties of surfactants with low-molecular-weight hydrogelators that are based on 1,3,5-cyclohexyltricarboxamide (see picture, AA=amino acid). The resulting structures include self-assembled interpenetrating networks. (Figure Presented)

Full text of DOI:10.1002/anie.200704609

Angewandte
Chemie
DOI: 10.1002/anie.200704609
Self-Assembly
Preparation of Nanostructures by Orthogonal Self-Assembly of
Hydrogelators and Surfactants**
Aurelie Brizard, Marc Stuart, Kjeld van Bommel, Arianna Friggeri, Menno de Jong, and
Jan van Esch*
Nature, through its ubiquitous examples of complex nano-
objects constructed by the self-assembly of molecular com-
ponents, in particular lipids and proteins, can deliver the
topologically controlled compartmentalized architectures
that are essential to high-end physiological functions.[1] The
formation of natural systems is driven not only by comple-
mentary intermolecular interactions, but also emerges from
the inherent incompatibility of components that leads to
(micro)phase separation, as manifested in protein folding and
biological membranes. Microphase separation has been
extensively exploited in the self-assembly of block copoly-
mers,[2,3] however, there are only a few examples in which the
controlled phase separation of distinct components has been
utilized to fabricate artificial nanostructured assemblies.
These examples include, phase-separated polymer systems,[4]
bilayers of hydrocarbon and perfluorinated phospholipids,[5]
low-molecular-weight hydrogelators in liquid-crystalline
phases[6] or with surfactant micelles[7] as well as nanocap-
sules[8] Despite the progress in supramolecular chemistry,[9]
man-made self-assembled structures can not yet compete with
the level of complexity and functionality of natural systems.[10]
Herein we show that the orthogonal self-assembly of multiple
components, that is, the independent formation within a
single system of different supramolecular structures, each
with their own characteristics, is a versatile and powerful
approach towards the formation of novel and more complex
architectures. These architectures include self-assembled
Scheme 1. Schematic representation of hydrogelators, which self-
assemble preferentially in one dimension. Light blue regions corre-
spond to hydrophilic groups and dark blue areas to hydrophobic
interpenetrating networks and vesicle configurations that
coexist with hydrogel fibers (Scheme 1).
entities (AA=amino acids). Chemical structures of hydrogelators 1–4
based on 1,3,5-cyclohexyltricarboxamide as well as a representation of
the orthogonal self-assembly strategy using hydrogelators and surfac-
tants to create more complex and compartmentalized structures are
also shown.
[*] Dr. A. Brizard, Prof. Dr. J. van Esch
Self-Assembling Systems, Delft Chem Tech
University of Delft
Surfactants are typical small molecules that offer an
interesting starting point for the fabrication of architectures
through orthogonal self-assembly. They are essential compo-
nents of cell membranes and have numerous technological
applications. Despite their large structural diversity, surfac-
tants lead to a limited range of supramolecular architectures,
including spherical or rodlike micelles, bilayers, vesicles, and
inverted micelles.[11] Another attractive class of self-assem-
bling units are the low-molecular-weight gelators. Although
they too lead to a similar range of supramolecular structures,
gelators can also self-assemble into highly organized and
responsive fibrillar architectures.[12] This property is the
source of the considerable current interest in the develop-
ment of smart materials.[13]
Julianalaan 136, 2628BL Delft (The Netherlands)
Fax: (+31)15-27-84289
E-mail: j.h.vanesch@tudelft.nl
Homepage: http://www.dct.tudelft.nl/sas
Dr. M. Stuart
Groningen Biomolecular Sciences and Biotechnology Institute
University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
Dr. K. van Bommel, Dr. A. Friggeri, Dr. M. de Jong
BioMaDe Technology Foundation
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
[**] We thank M. Wijnhold for technical assistance. W. Browne is
acknowledged for proofreading this manuscript. This work was
supported by Nanoned/STW and the Netherlands Organization for
Scientific Research (NWO).
In this study, low-molecular-weight hydrogelator mole-
cules based on 1,3,5-cyclohexyltricarboxamide were used.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
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They were chosen because they can self-assemble into 1D
arrays that are stabilized by both hydrogen-bonding and
hydrophobic interactions (Scheme 1).[14] Similar interactions
serve to stabilize protein secondary structures which are
stable in the presence of weakly interacting surfactants.[15]
These hydrogelators can be functionalized at the periphery
with different solvophilic or pH-sensitive groups (Scheme 1),
thus giving access to modular architectures and properties.
Thermoreversible hydrogels of 1–4 can be prepared easily by
cooling warm solutions, which typically contain 0.1–1 w/v%
of the gelator, to below their gel-sol phase transition temper-
ature (Tgs) thus leading to their spontaneous assembly into
quasi-one-dimensional fibers, which in turn form a three-
dimensional network.
The compatibility of hydrogel formation by 1–4 with
various types of surfactants was investigated by dissolving 1–4
at T> Tgs in solutions of the surfactant and subsequently
examining the samples for gelation after they had been cooled
to room temperature. Transparent, thermoreversible gels of
1–4 were obtained in the presence of the spherical micelle
forming surfactants alkyl trimethylammonium bromide
(CnTAB, n = 12, 14, 16) or sodium dodecyl sulfate (SDS), at
concentrations below and above their critical micelle concen-
tration (CMC). Precipitation occurred only when 1 was
combined with SDS, presumably because of strong electro-
static interactions. Gel formation by 1–4 was also observed in
combination with other surfactant aggregate morphologies.
For example, cetyltrimethylammonium tosylate (CTAT) first
forms spherical micelles just above its CMC (0.2 mm),[16] but
progressively transforms into elongated entangled micelles
upon increasing the surfactant concentration (ca. 20 mm).[17]
Figure 1. a) DSC heating and cooling traces of an aqueous dispersion
of DPPC (4 mm) in the absence of 1 (black dotted line) or in the
presence of 1 (blue solid line, 4 mm); b) Molar fraction of 1 incorpo-
rated in the fibers (Xf), at a fixed concentration ([1]=4 mm) deter-
mined by 1H NMR spectroscopy as a function of the concentration of
C12TAB (spherical micelles), CTAT (cylindrical micelles), and DODAB
(vesicles) concentrations.
Incorporation of CTAT at concentrations below and above lipid bilayers is not disturbed by the hydrogelator molecules,
the CMC, thus in the spherical and elongated micelle regimes,
with 1–4 resulted in the formation of transparent, thermor-
eversible gels. Gelation by 1–4 took place even in the
presence of zwitterionic lipids such as dioleoylphosphocho-
line (DOPC), dimyristoylphosphatidylcholine (DMPC), and
dipalmitoylphosphatidylcholine (DPPC), that form bilayer
vesicles.
thus indicating that they are not incorporated into the
bilayers.
Conversely, the behavior of the hydrogelators was exam-
ined for possible interactions with the amphiphiles. Gels of 1–
4 are characterized by high Tgs, which is indicative of strong
intermolecular interactions.[14b] Although the properties of
the surfactant aggregates appear to remain unchanged upon
incorporation into a gel network, the presence of surfactant in
the hydrogels led to a significant destabilization of the gel
network, which was manifested by a decrease in the Tgs value
upon increasing the surfactant concentration (see Supporting
Information). Interestingly, this destabilization was depen-
dent on the morphology of the surfactant aggregate: overall,
lipid bilayers destabilized the gel network less than spherical
micelles. To understand this result, the molar fraction of
hydrogelator molecules in solution (Xm) and in the solid fibers
These results show that gel formation by 1–4 is compatible
with surfactants and lipids at the macroscopic level. The
interdependence of the self-assembly of 1–4 and surfactants at
the supramolecular level was examined by looking at the
effect hydrogelators had on the self-assembly properties of
the surfactants and vice versa. For micelle-forming surfac-
tants, the CMC is a characteristic property to monitor because
it is expected to change if interactions of the surfactants with
other components occur.[18] Interestingly, the CMC value for
C14TAB or SDS, as determined by conductimetry measure-
ments, did not change in the presence of hydrogels of 1 and 2,
respectively. In the case of bilayer-forming lipids and surfac-
tants, the transition of the lipid physical state from the
ordered Lb phase to the disordered liquid-crystalline La phase
is very sensitive to interaction with foreign molecules.[19] The
differential scanning calorimetry (DSC) traces (Figure 1a)
show no significant differences in either the shape or position
of the heating and cooling traces of DPPC, either with or
without hydrogelator 1 at below and above the critical
gelation concentration.[20] Evidently, the physical state of the
1
(Xf = 1ÀXm) were measured by H NMR spectroscopy as a
function of surfactant concentration. In the presence of
surfactants that form spherical micelles, the fibrous fraction
remained constant at Xf ꢀ 0.95 below the CMC (15 mm for
C12TAB), but decreased progressively above the CMC. This
difference indicates a partial solubilization of the gel fibers by
the micelles leading to macroscopic destabilization of the gel
network. In the presence of CTAT, which first forms spherical
micelles that evolve into cylindrical micelles, the Xf value
passes through a minimum just above the CMC and then
increases again upon formation of cylindrical micelles above
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approximately 20 mm. Evidently, the morphological transi-
tion from spherical to cylindrical micelles leads to the
expulsion of hydrogelator molecules from the surfactant
aggregates and favors the formation of separate self-assem-
bled architectures. This behavior most probably results from
stronger surfactant–surfactant interactions, which accompany
the increase of their packing parameters,[11] as compared to
surfactant–hydrogelator interactions. This conclusion is sup-
ported by the Xf value measured upon addition of dioctade-
cyldimethylammonium bromide (DODAB) vesicles, which
remained constant at around 95%. This finding indicates no
substantial dissolution of hydrogelator molecules by the
bilayers, which is in excellent agreement with the unchanged
melting temperature (Tm) measured by DSC. These results
demonstrate clearly that the mutual interference of the self-
assembly of, on the one hand, gelator molecules and, on the
other hand, surfactants and lipids, not only depends on their
hydrophobicity, but importantly also on the morphology of
the surfactant or lipid aggregate. In the case of cylindrical
micelles and bilayer aggregates the two self-assembly pro-
cesses are truly orthogonal, most probably because of
stronger interactions between surfactants in the latter type
of aggregates compared to spherical micelles. This finding
holds particular relevance to natural systems, where lipid
bilayers are one of the predominant structures that coexist
with other self-assembled architectures.
The formation of new supramolecular architectures
through orthogonal self-assembly of hydrogelators and sur-
factants by the cooling of isotropic solutions was investigated
by cryo-transmission electron microscopy (cryo-TEM). Fig-
ure 2a shows the formation of well-defined fibers with
diameters of about 5 nm (typical for a hydrogel of 1), which
was unaffected by the presence of micelle-forming surfactants
such as C14TAB at below or above its CMC, although fiber
ends were more frequently observed. At an increased
magnification, spherical micelles encapsulated in a fibrillar
network were observed when 1 was combined with C14TAB
above its CMC (Figure 2a inset). This observation indicates
that gel fibers of 1 and micelles of C14TAB can coexist, even if
a fraction of the hydrogelator monomers is dissolved by
spherical micelles. When this strategy was extended to CTAT
surfactants and 1 or 2, the coexistence of gel fibers and
cylindrical micelles were observed by cryo-TEM experiments.
Figure 2b shows elongated, stiff fibers with diameters of 7–
20 nm formed by 2 (left side). In the background one can
distinguish unambiguously the presence of highly entangled
cylindrical micelles with diameters of a few nanometers. To
the best of our knowledge, this system represents the first
example of interpenetrating networks based on orthogonal
self-assembly of two different components: both nanostruc-
tures evolve from noncovalent interactions between units that
self-assemble and disassemble independently. Interestingly,
these self-assembled interpenetrating networks display new
viscoelastic properties, which are unattainable when using the
individual components alone. When we combined these two
types of networks, an intermediate and adjustable rheological
response was observed: by progressively increasing the
[hydrogelator]/[surfactant] ratio, the samples transform
from viscoelastic systems to more solidlike samples without
any macroscopic phase separation.
Inspired by the remarkable functionality of cytoskeletons
and extracellular matrixes in nature, another new and
interesting application for our system could be as a fibrous
network coexisting with bilayer vesicles. Cryo-TEM studies
revealed that the typical fibrous networks of 1–4 remain intact
in the presence of bilayer-forming surfactants and lipids such
as DODAB and DOPC. Reciprocally, vesicles of DODAB
and DOPC are stable in the presence of gel fibers (Figures 3a
and b). These observations are in excellent agreement with
the DSC and NMR spectroscopy studies described above, and
together they show unambiguously that the molecular
components do not interact significantly. However, their
self-assembled structures influence each other at a supra-
molecular level: the confined space provided by the pores of
Figure 3. Cryo-TEM images of unilamellar a) DOPC vesicles coexisting
with a network of well-defined fibers of 1 with a high aspect ratio;
b) DODAB vesicles with thicker fibers of 3; c) and d) DOPC vesicles
deformed by the growth of fibers of 1 directly contained in their
aqueous compartment.
Figure 2. Cryo-TEM images of fibrous networks of a) 1 (4 mm) coexist-
ing with spherical micelles of C14TAB at 20 mm; b) 2 (4 mm) in the
presence of CTAT at 100 mm (cylindrical micelles). Scale bar is
100 nm.
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the gel network forces the vesicles to accommodate their
shape because of the stress exerted on their walls (see arrows
in Figures 3a and b ).
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what extend our strategy could be exploited to gelate the
aqueous compartment of vesicles. To this end, DOPC vesicles
were prepared in the presence of 1, which can be reversibly
switched from the gel state (basic conditions) to the liquid
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incompatible to avoid coassemble) which should give access
to a broad and unexplored range of nanostructures exclu-
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Received: October 5, 2007
Published online: February 13, 2008
[21] As a contraction of gel and liposomes.
Keywords: amphiphiles · gels · micelles · self-assembly · vesicles
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