Pristine carbon-nanotube-included supramolecular hydrogels with tunable viscoelastic properties

Article abstract of DOI:10.1002/chem.201300302

Research investigations involving pristine carbon nanotubes (CNTs) and their applications in diversified fields have been gathering enormous impetus in recent times. One such emerging domain deals with the hybridization of CNTs within hydrogels to form soft nanocomposites with superior properties. However, till now, reports on the inclusion of pristine CNTs within low-molecular-weight hydrogels are very scarce due to their intrinsic feature of remaining in the bundled state and strong repulsive behavior to the aqueous milieu. Herein, the synthesis of a series of amino acid/dipeptide-based amphiphilic hydrogelators having a quaternary ammonium/imidazolium moiety at the polar head and a C16 hydrocarbon chain as the hydrophobic segment is reported. The synthesized amphiphiles exhibited excellent hydrogelation (minimum gelation concentration (MGC) ≈0.7-5 % w/v) as well as single-walled carbon nanotube (SWNT) dispersion ability in aqueous medium. Interestingly, the dispersed SWNTs were incorporated into the supramolecular hydrogel formed by amphiphiles with an imidazolium moiety at the polar end through complementary cation-π and π-π interactions. More importantly, the newly synthesized hydrogelators were able to accommodate a significantly high amount of pristine SWNTs (2-3.5 % w/v) at their MGCs without affecting the gelating properties. This is the first time that such a huge amount of SWNTs has been successfully incorporated within hydrogels. The efficient inclusion of SWNTs to develop soft nanocomposites was thoroughly investigated by spectroscopic and microscopic methods. Remarkably, the developed nanocomposites showed manifold enhancement (≈85-fold) in their mechanical strength compared with native hydrogel without SWNTs. The viscoelastic properties of these nanocomposites were readily tuned by varying the amount of incorporated CNTs. Tubes included: Amino acid/dipeptide-based amphiphilic hydrogelators with an imidazolium head group incorporate large amounts of single-walled carbon nanotubes (SWNTs) without compromising their intrinsic properties. Remarkable enhancement (ca. 85-fold) in the mechanical rigidity of the resulting soft nanocomposite is observed, which can be modulated by varying the amount of incorporated SWNTs (see figure). Copyright

Full text of DOI:10.1002/chem.201300302

FULL PAPER
Hydrogels
S. K. Mandal, T. Kar,
P. K. Das* ..................... &&&& — &&&&
Pristine Carbon-Nanotube-Included
Supramolecular Hydrogels with
Tunable Viscoelastic Properties
Tubes included: Amino acid/dipeptide-
based amphiphilic hydrogelators with
an imidazolium head group incorpo-
rate large amounts of single-walled
carbon nanotubes (SWNTs) without
compromising their intrinsic proper-
ties. Remarkable enhancement (ca. 85-
fold) in the mechanical rigidity of the
resulting soft nanocomposite is
observed, which can be modulated by
varying the amount of incorporated
SWNTs (see figure).
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DOI: 10.1002/chem.201300302
Pristine Carbon-Nanotube-Included Supramolecular Hydrogels with
Tunable Viscoelastic Properties
Subhra Kanti Mandal, Tanmoy Kar, and Prasanta Kumar Das*[a]
Abstract: Research investigations in-
volving pristine carbon nanotubes
(CNTs) and their applications in diver-
sified fields have been gathering enor-
mous impetus in recent times. One
such emerging domain deals with the
hybridization of CNTs within hydrogels
to form soft nanocomposites with supe-
rior properties. However, till now, re-
ports on the inclusion of pristine CNTs
within low-molecular-weight hydrogels
are very scarce due to their intrinsic
feature of remaining in the bundled
state and strong repulsive behavior to
the aqueous milieu. Herein, the synthe-
sis of a series of amino acid/dipeptide-
C16 hydrocarbon chain as the hydro-
phobic segment is reported. The syn-
thesized amphiphiles exhibited excel-
lent hydrogelation (minimum gelation
concentration (MGC) ꢀ0.7–5% w/v)
as well as single-walled carbon nano-
tube (SWNT) dispersion ability in
aqueous medium. Interestingly, the dis-
persed SWNTs were incorporated into
the supramolecular hydrogel formed by
amphiphiles with an imidazolium
moiety at the polar end through com-
plementary cation–p and p–p interac-
tions. More importantly, the newly syn-
thesized hydrogelators were able to ac-
commodate a significantly high amount
of pristine SWNTs (2–3.5% w/v) at
their MGCs without affecting the gelat-
ing properties. This is the first time
that such a huge amount of SWNTs
has been successfully incorporated
within hydrogels. The efficient inclu-
sion of SWNTs to develop soft nano-
composites was thoroughly investigated
by spectroscopic and microscopic
methods. Remarkably, the developed
nanocomposites showed manifold en-
hancement (ꢀ85-fold) in their me-
chanical strength compared with native
hydrogel without SWNTs. The viscoe-
lastic properties of these nanocompo-
sites were readily tuned by varying the
amount of incorporated CNTs.
Keywords: amphiphiles · hydrogels ·
mechanical properties · nanotubes ·
soft nanocomposites
based
amphiphilic
hydrogelators
having a quaternary ammonium/imida-
zolium moiety at the polar head and a
Introduction
tion of gelators by a combination of supramolecular interac-
tions, such as hydrogen-bonding, p–p stacking, and van der
Waals interactions, led to the formation of a self-assembled
fibril network (SAFIN), the macroscopic outcome of which
is a molecular gel.[7] The interstitial space present within the
SAFIN could be suitable to accommodate exogenous mate-
rials of comparable nanoscale dimension. For instance, Rao
and co-workers have used triphenylenevinylene-based LMW
organogelators to host various functionalized CNTs and
boron nitride nanotubes.[8] Similarly, Ajayaghosh and co-
workers included CNTs for reinforcing mechanically unsta-
ble thixotropic organogels of oligo (p-phenylenevinylene).[9]
However, reports on the inclusion of pristine CNTs within
LMW hydrogels are really scarce. The extreme insolubility
of CNTs in aqueous environments is the prime obstacle in
incorporating this carbonaceous nanomaterial within supra-
molecular hydrogels. The strong intertubular van der Waals
interaction influences amphiphobic CNTs to exist in the
bundled state. Thus, incorporation of this water-insoluble
nanotube within LMW hydrogels still remains a challenging
task. At the same time, development of CNT–hydrogel com-
posites is of immense importance because of their potential
applications in electronics and biomedicine, in particular
tissue engineering (cellular microenvironments have a broad
range of storage modulus, 0.1–107 kPa)[10a,b] through modu-
lating the viscoelastic properties of the soft materials. In this
The carbon nanotube (CNT), a quasi-one-dimensional allo-
trope of carbon, has received immense interest in diversified
research domains owing to its unique properties, such as
thermal, optoelectronic, mechanical, and physical fea-
tures.[1,2] Recently, CNTs are finding importance as prefer-
red constituents for the development of nano-bioconjugates
due to their huge surface area and chemical inertness.[3] In
this context, attempts have been made to develop superior
hybrid materials by amalgamation of this amazing nanoma-
terial (CNTs) with low-molecular-weight (LMW) gels.[4]
Such LMW gels are a fascinating class of supramolecular
materials comprising a small number of gelator molecules
that immobilize a large volume of solvent.[5,6] Self-aggrega-
[a] S. K. Mandal, T. Kar, Prof. P. K. Das
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata 700032 (India)
Fax : (+91)33-24732805
E-mail: bcpkd@iacs.res.in
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/chem.201300302. It contains synthetic
schemes for compounds 1–7, their characterization and dispersion
data, FTIR spectra of 5 and 6, fluorescence spectra of 7, and rheology
data of 5–7.
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Table 1. MGC (% w/v) and maximum accommodation of SWNTs (% w/
v) at the MGC.
regard, Fukushima et al. showed for the first time the effi-
cient exfoliation of pristine single-walled carbon nanotubes
(SWNTs) upon grinding with imidazolium-based ionic liq-
uids (ILs) at room temperature.[10] The presence of imidazo-
lium cations within the molecular structure resulted in de-
bundling of SWNTs due to cation–p and other noncovalent
interactions.[10] This feature of the imidazolium moiety
makes it a potential candidate for bringing extremely hydro-
phobic SWNTs into the aqueous milieu. Imidazolium ILs
are also known for their solvating properties for a range of
solvents, including water.[11] To this end, we thought to
design a new class of cationic amphiphilic hydrogelators
comprising an IL-like structural scaffold that would facili-
tate the exfoliation of SWNTs in water, and the physical
cross-linking between the dispersed CNTs and the SAFIN
would lead to the formation of SWNT-included hydrogel.
Herein, we report the development of amino acid/pep-
tide-based cationic amphiphiles comprising either a quater-
nary ammonium or imidazolium group at the polar head
and a C16 alkyl tail at the C terminus (1–7, Scheme 1).
These amphiphiles exhibited moderate to excellent hydroge-
lation ability (minimum gelation concentration (MGC)=
0.7–5% w/v, Table 1). Interestingly, the gelators with the
Amphiphile
MGC
Maximum accommodation of SWNTs at MGC
1
2
3
4
5
6
7
S[a]
5.0
4.0
1.5
5.0
1.0
0.7
–
–
–
–
2.0
2.0
3.5
[a] S=solution.
imidazolium moiety at the polar head (5–7, Scheme 1) were
able to accommodate the hydrophobic SWNTs within their
hydrogel networks, which resulted in the formation of
SWNT-included hydrogels, unlike those amphiphiles with a
quaternary ammonium group at their head (1–4, Scheme 1).
The amino acid residue was altered to find out its influence
on the gelation efficiency. Interestingly, the imidazolium-
based hydrogelators were capable of accommodating a large
amount of SWNTs (2–3.5% w/v) at their MGC without
compromising their individual properties. Notably, rheologi-
cal studies indicated the remarkable (ꢀ85-fold) improve-
ment in the mechanical strength of the pristine CNT-includ-
ed soft nanocomposites compared with the corresponding
native gel, and it can be easily modulated by varying the
amount of SWNTs.
Results and Discussion
The water immobilization ability of small molecules comes
from a crucial balance between hydrophilicity and lipophi-
licity.[12] Noncovalent interactions, such as hydrogen-bond-
ing, p–p stacking, electrostatic, and van der Waals interac-
tions, among the gelator molecules play an essential role in
the formation of stable hydrogels.[7,12] Alongside the devel-
opment of nanomaterials included soft composites are of
great importance because of their huge prospects in materi-
als science to biomedicine.[1–3]
Among many nanomaterials, the CNT is preferred to be
included within the SAFIN of a supramolecular gel because
of its similar physical dimension to that of the gel fiber and
also its propensity to become cross-linked within a fibrillar
network.[8–10] In this regard, LMW amphiphiles have been
exploited for dispersion of this superhydrophobic material
(SWNTs) in water by forming CNT-included self-assemblies.
Recently, we reported efficient dispersion of SWNTs by
amino acid-based amphiphiles with a quaternary ammonium
group as the polar head and a C16 alkyl tail as the hydro-
phobic segment.[13] Among the different amphiphiles, com-
pound 1 (Scheme 1), with the smallest aliphatic side-chain
residue l-alanine at the polar head, exhibited the maximum
SWNT dispersion efficiency of ꢀ87% in aqueous media
(Figure S1 in the Supporting Information).[13] However, am-
phiphile 1 was itself a nongelator. We were curious to inves-
tigate whether dispersed SWNTs in combination with the
Scheme 1. Structures of amphiphiles 1–7. All the amino acids used were
l-amino acids.
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self-assemblies of amphiphile 1 could immobilize water. To
develop a CNT hybrid gel, first pristine SWNTs (1 mg) were
added to an aqueous solution of 1 (1 mL 5% w/v) with
probe sonication for 8 min at 30% power output and then
allowed to cool at room temperature. However, no gelation
was observed for the self-assemblies of SWNTs and 1
(termed SWNT-1), which was tested by inversion of the vial.
The absence of a fibril network in the case of nongelating
amphiphile 1 possibly could not yield the complementary
fusion with dispersed CNTs.
An amphiphile having self-assembled hydrogelation abili-
ty would have been a rational choice to incorporate debun-
dled CNTs within its matrix. Consequently, we used amphi-
phile 2 comprising l-phenylalanine as the amino acid resi-
due with a C16 hydrocarbon chain at the C terminus and a
quaternary ammonium moiety as head group at the N termi-
nus. Compound 2 exhibited poor hydrogelation ability
(MGC=5.0% w/v, Table 1) and moderate CNT dispersion
ability of 57.6% (Figure S1, Supporting Information) in
aqueous solution.[13,14] An attempt was made to develop the
gel nanohybrid following a similar procedure to that illus-
trated above. However, the inherent gelation property of 2
was found to deteriorate in the presence of SWNTs, and
CNTs precipitated out from aqueous medium. Presumably,
incorporation of hydrophobic SWNTs within the supramo-
lecular hydrogel might have perturbed the hydrophilic–lipo-
philic balance (HLB) of the resulting hybrid.[13] A similar
result was observed in the case of a dipeptide-based amphi-
phile (3) containing the aliphatic amino acid residue l-ala-
nine, and aromatic amino acid residue l-phenylalanine (dis-
persion efficiency ꢀ49% and MGC=4.0% w/v). This indi-
cates that the resultant van der Waals and p–p stacking in-
teractions arising from 3 were still not sufficient to accom-
modate SWNTs within the hydrogel matrix.
At this point, we thought that the presence of another ar-
omatic moiety within the gelatorꢁs structure may provide
the complementary HLB and p–p interaction that is re-
quired for the development of supramolecular soft nano-
composites.[6c,12] This can be accomplished in two different
ways, firstly by introducing another aromatic amino acid res-
idue in the peptide backbone and secondly by incorporating
an aromatic ring at the polar head. Accordingly, we synthe-
sized amphiphilic dipeptide molecule 4 containing two l-
phenylalanine residues. Introduction of an additional aro-
matic amino acid improved the hydrogelation ability
(MGC=1.5% w/v, Table 1) of gelator 4 by ꢀ3.5-fold rela-
tive to that of gelator 2, possibly through enhanced p–p
stacking. The SWNT exfoliation efficacy of 4 was ꢀ47%
(Figure S1, Supporting Information). An analogous proce-
dure was followed for gelator 4 to prepare CNT-included hy-
drogel composites. Here too, CNT-included gel formation
did not take place and the nanotubes separated out from
the water. The added aromaticity in the peptide backbone
of the gelatorꢁs structure (4) was not adequate to accommo-
date pristine SWNTs within its SAFIN.
acid, we introduced an imidazolium moiety at the polar
head of the amphiphile, which also has the additional ability
to exfoliate CNTs. This feature is of great advantage as one
can “design” soft materials directly from the resulting com-
posite. The presence of an imidazolium moiety would in-
crease the aromaticity of the amphiphile and also retain its
cationic character. Accordingly, we synthesized amphiphile 5
by substituting the quaternary ammonium moiety of 2 with
an imidazolium group connected through a methylene unit
at the N terminus. The MGC of 5 was found to be same
(5% w/v, Table 1) as that of 2. An attempt was made to de-
velop soft nanocomposites involving SWNTs and hydrogel
of 5 under the same experimental conditions as mentioned
above. Initially, well-dispersed SWNTs (dispersion efficiency
53%, Figure S1, Supporting Information) were obtained by
using 5 and importantly the flow behavior of this CNT dis-
persion slowly became restricted with simultaneous entrap-
ment of the SWNTs, which resulted in a self-supporting
SWNT-5 hydrogel (as proven by the inverted vial tests). The
imidazolium head group in the gelatorꢁs structure (5) clearly
played a crucial role in the exfoliation as well as inclusion of
CNTs within the gel matrix by means of cation–p or p–p in-
teractions.[10] An aqueous dispersion of SWNT-5 might have
assisted synergistically in the molecular ordering of amphi-
philes that resulted in the pristine SWNT (0.1% w/v)-incor-
porated hydrogel. Improving the content of CNTs within
the hydrogel will be more befitting in modulating the viscoe-
lastic properties of soft nanocomposites. Interestingly, the
presence of the imidazolium moiety facilitated the pristine
SWNT inclusion maximum up to 2% w/v (Table 1) within a
hydrogel of 5 at its MGC without compromising its hydroge-
lation ability.
To exploit the CNT debundling ability of the imidazolium
moiety and consequently the complementary fusion of exfo-
liated SWNTs with the SAFIN of the hydrogel, the quater-
nary ammonium head groups of 3 and 4 were substituted by
an imidazolium moiety to synthesize 6 and 7 (Scheme 1), re-
spectively. In the case of 6, the MGC improved by fourfold
(1% w/v, Table 1) relative to gelator 3, and 7 showed two-
fold enhancement in gelation efficacy (MGC=0.7% w/v)
with respect to 4. The additional hydrogen-bonding and p–p
interactions between the imidazolium residues play a crucial
role in improving the gelation efficacy. The nanotube disper-
sion efficiency of 6 and 7 was found to be 60 and 50% (Fig-
ure S1, Supporting Information), respectively. A similar
method was used to prepare the SWNT-included hydrogel
composites and in both cases pristine CNT-included hydro-
gel formation was observed at 0.1% w/v SWNTs at the
MGC. Similar to 5, a maximum 2% w/v SWNT accommoda-
tion was observed within a hydrogel of 6 at its MGC (1.0%
w/v). Most significantly, the maximum SWNT inclusion abil-
ity was enhanced up to 3.5% w/v (Table 1) for amphiphile 7
at its MGC without losing its gelation ability. To the best of
our knowledge, this is the first example in which such a high
amount of SWNTs was included within a physical gel. The
preceding observations clearly indicate that the p-electronic
interaction between the SWNT surface, aromatic ring of
Hence, we decided to include an aromatic moiety at the
polar end. Instead of including an extra aromatic amino
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amino acids, and imidazolium moiety assisted in the efficient
CNT exfoliation and thereby facilitated its physical integra-
tion within the SAFIN of the hydrogel.
The native hydrogels and SWNT composites were ther-
moreversible in nature and were stable at room temperature
for several months. The temperature at which the gel con-
verts into a solution is known as the gel melting temperature
or gel-to-sol transition temperature (Tgel). In concurrence
with the literature, the Tgel values of the gelators were im-
proved with increasing gelator concentration (Figure 1).[6c,12]
UV/Vis-NIR spectra: The exfoliation of pristine SWNTs
was investigated by recording UV/Vis-NIR spectra. Homo-
geneously dispersed aqueous solutions of pristine SWNTs
(0.1% w/v) in the presence of amphiphiles (Figure S1, Sup-
porting Information) at their MGC were used for the ex-
periments. Efficient dispersion of SWNTs was evident from
the characteristic van Hove singularities in the absorption
spectra (Figure 2) of SWNT-6 and SWNT-7 nanohybrids dis-
Figure 2. UV/Vis-NIR spectra of the SWNT (0.1% w/v)–amphiphile dis-
persions (6: 1.0% w/v, 7: 0.7% w/v).
persed in water. The well-resolved peaks around 550–
900 nm (S22 transition) and 1200–1600 nm (S11 transition)
arise from the interband transition between the mirror
spikes in the density of states of individualized SWNTs, thus
revealing the good-quality dispersion in the amphiphilic ge-
lator solution.[15]
Figure 1. Variation of gel-to-sol transition temperature (Tgel) of native
gels and soft nanocomposites (keeping SWNT concentration fixed at
1.0% w/v) with varying gelator concentration (in % w/v).
For amphiphile 5, Tgel at its MGC (5.0% w/v) was 418C,
which was enhanced up to 528C at 10% w/v. Similar en-
hancements in Tgel values were observed for gelators 6 (from
608C at 1.0% w/v to 808C at 2.5% w/v) and 7 (from 588C
at 0.7% w/v to 708C at 2.5% w/v). The enhancement of Tgel
with gelator concentration indicates that the self-assembly
in the gel state is guided by physical cross-linking between
gel fibers. The SWNT accommodation ability of hydrogels
varied from 0.1 to 3.5% w/v depending on the gelator. Thus,
we measured the Tgel of soft nanocomposites with varying
concentration of gelators at 1% w/v SWNTs. Here also the
trends of enhancement in Tgel values with varying gelator
concentration remain identical (Figure 1). Importantly, in all
the instances notable enhancement in Tgel values was ob-
served, which were sufficiently higher than that of native
gels. In the presence of 1.0% w/v pristine SWNTs the Tgel of
5 was enhanced from 41 to 568C at the MGC, which further
increased to 698C at 10% w/v 5. Analogous results were ob-
served for SWNT-6 and SWNT-7. In both cases the Tgel sig-
nificantly increased to ꢀ1188C in the presence of 1% w/v
SWNTs at 2.5% w/v gelator. Inclusion of SWNTs within the
hydrogels makes the fibrillar network much stronger
through the physical cross-linking of the SAFIN with exfoli-
ated CNTs and consequently a higher temperature was re-
quired for the disintegration of supramolecular soft nano-
composites.
Microscopic studies: Field-emission scanning electron micro-
scopy (FESEM) was performed to gain visual insight into
the supramolecular arrangement of hydrogels and soft nano-
composites. SWNTs suspended in water were found to be in
an aggregated state (Figure 3a), whereas the xerogel of
native hydrogel 6 at its MGC (1.0% w/v) showed an entan-
gled fibril network with fibers 80–100 nm in thickness (Fig-
ure 3b). Water molecules were entrapped within the fibrous
morphology and formed an efficient hydrogel. Interestingly,
upon incorporation of 0.1% w/v SWNTs into the same hy-
drogel (1% w/v) of 6, the resulting nanocomposites exhibit-
ed densely packed networks (Figure 3c) with fiber diameter
similar to that of the native gelators. The SAFIN became
denser when the doped SWNT amount was increased to
0.25% w/v (Figure 3d). Complementary fusion of the dis-
persed nanotubes with the gel fibers made the fibril network
denser and thicker than the native hydrogel.[4b,16] Similarly,
hydrogel 7 at the MGC (0.7% w/v) showed a three-dimen-
sional (3D) intertwined fibrillar network with a diameter of
30–50 nm (Figure 3e). Some of the fibers were associated
with each other to form thicker fibers of size approximately
80–100 nm. Importantly, SWNT-7 composite (0.1% w/v
SWNTs and 0.7% w/v 7) exhibited an interconnected dense
fibrous morphology with fiber diameter 35–50 nm (Fig-
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Figure 3. FESEM images of a) pristine SWNTs, b) xerogel of 6 (1.0% w/v), c) SWNT (0.1% w/v)-6 (1.0% w/v), d) SWNT (0.25% w/v)-6 (1.0% w/v),
e) xerogel of 7 (0.7% w/v), f) SWNT (0.1% w/v)–7 (0.7% w/v).
ure 3 f), which further established the physical integration of
SWNTs within the gel matrix.
each other. Notably, the gel fiber diameter remained unal-
tered in both 6 and SWNT-6 composite, which evidently in-
dicates that the integrated SWNTs did not affect the supra-
molecular assembly formed by the native gelator. A similar
observation was made in the case of gelator 7 and its CNT
hybrid nanocomposite. In the case of 7 (with two l-phenyla-
lanine residues), a fibrillar network was formed by inter-
twined fibers of size approximately 30–50 nm (Figure 4e) at
its MGC (0.7% w/v). In this case also, two markedly differ-
ent kinds of fibers were found in SWNT-7 nanocomposites
(0.1% w/v SWNTs and 1.0% w/v 7). The diameters of
native gel fibers remained same but CNTs with a diameter
of 10–20 nm were distinctly found (Figure 4 f) within the gel
network. Microscopy studies confirmed the integration of
pristine SWNTs with the SAFIN of supramolecular hydro-
gels.
The presence of an interconnected morphology within the
individual gels as well as soft nanocomposites was further
confirmed by transmission electron microscopy (TEM). Pris-
tine SWNT suspended in water (in the absence of gelator)
was found to be in a bundled state (Figure 4a). TEM images
of native hydrogel 6 at its MGC showed 3D networks with
fiber diameter of 85–120 nm and length of several microme-
ters (Figure 4b). The SWNT-6 nanocomposites (0.1% w/v
SWNTs and 1.0% w/v 6) exhibited entangled, dense net-
works with different types and diameters of fibers (Fig-
ure 4c). Further magnification of the same sample con-
firmed the presence of two distinctly different kinds of
fibers within the network. Possibly the fibers with diameters
85–125 nm were of the gel fibers and with diameters 25–
35 nm were of CNTs (Figure 4d) that were cross-linked with
Figure 4. TEM images of a) pristine SWNTs, b) xerogel of 6 (1.0% w/v), c) SWNT (0.1% w/v)-6 (1.0% w/v), d) magnified view of SWNT (0.25% w/v)-6
(1.0% w/v), e) xerogel of 7 (0.7% w/v), f) SWNT (0.1% w/v)-7 (0.7% w/v). In (d) and (f), white arrows indicate the presence of gel fibers and the black
arrows show the CNTs.
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FTIR spectroscopy: FTIR spectroscopy is a powerful tool
for investigating the involvement of different noncovalent
interactions during the self-assembly of the gelators. FTIR
spectra of all the imidazolium-based amphiphiles were re-
corded at the gel state (MGC) in D2O, and in chloroform
for the non-self-assembled state of gelators. In chloroform
the imidazolium-based gelators 5–7 showed distinctive trans-
mittance bands at 3430–3444, 1652–1657, and 1527–
1535 cmÀ1, which are characteristic of non-hydrogen-bonded
at the native gel (7) in D2O was recorded at 3410 cmÀ1 for
amide nN H (amide A), 1631 cmÀ1 for amide nC=O
À
(amide I), and 1546 cmÀ1 for dN H (amide II) (Figure 5b).
À
The nanocomposite also showed identical transmittance sig-
nals, which suggest that the intermolecular noncovalent in-
teractions between the gelator molecules remain unpertur-
bed even after the inclusion of CNTs within the gel
matrix.[17]
À
À
amide nN H (amide A), amide nC=O (amide I), and dN H
(amide II), respectively (Figure 5a, Figure S2 in the Sup-
Circular dichroism: The supramolecular chirality originating
from the self-aggregation of chiral monomer during the
process of self-assembled hydrogelation was investigated by
circular dichroism (CD) spectroscopy. The aqueous solution
of hydrogelator 7 (taken as representative) showed a nega-
tive peak at 224 nm and a positive peak at 203 nm at 0.01%
w/v (Figure 6a). The pattern of the peaks in the spectra is
Figure 5. FTIR spectra of a) 7 in D2O in gel state and in chloroform, and
b) xerogel of 7 and SWNT-7 composite.
porting Information). However, at the gel state in D2O, the
transmittance signals were shifted to 3410–3416 cmÀ1 for
À1
À
amide nN H (amide A), 1630–1638 cm for amide nC=O
Figure 6. CD spectra of a) 7 with varying concentration in water, and b) 7
and SWNT-7 composite solution in water.
(amide I), and 1542–1549 cmÀ1 for dN H (amide II). This
À
shift of stretching frequencies at the gel state indicates the
involvement of the carbonyl (C=O) bond in intermolecular
À
hydrogen bonding with the amide N H of another molecule.
similar to that generally found (but not by exact positions of
peaks) for the b-sheet secondary structure (positive peak at
197 nm and negative peak at 218 nm) of proteins. The ob-
served Cotton effect at the amide absorption region (i.e., at
220–225 nm) could be attributed to the characteristic p–p*
transition of the amide bond, sensitive to coupling with
neighboring amide.[12] The particular spectral trend con-
firmed that the self-assembled structure of 7 in water is
mostly governed by b-sheet arrangement of the peptide
backbone. Moreover, the increase in peak intensity with in-
crement in gelator concentration up to 0.05% w/v (Fig-
Again the antisymmetric (nas) and symmetric (ns) CH2
stretching frequencies observed at 2927 and 2854 cmÀ1, re-
spectively, in chloroform for the imidazolium-based gelators
were shifted to 2918 and ꢀ2849 cmÀ1 in D2O. The decrease
in fluidity of the hydrophobic chains due to the formation of
strong self-assembled aggregates through van der Waals in-
teraction was evident from this particular shift in the CH2
stretching frequency.[12] Interestingly, the FTIR spectrum of
the D2O xerogel of gelator SWNT-7 composite was found to
be similar to that of its native hydrogel in D2O. The IR peak
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ure 6a) also suggested a highly ordered arrangement of
chiral planes at the supramolecular level. Inclusion of
SWNTs (0.01% w/v) into the gelator (7) solution (0.05% w/
v) led to a decrease in the intensity of the CD signal, al-
though the peak position and spectral pattern remained un-
altered (Figure 6b). This observation further established the
interaction of CNTs with the supramolecular chiral environ-
ment of the gelator and also the undisturbed arrangement
(b sheet) of the supramolecular network in the soft nano-
composite.[18]
Luminescence studies: Participation of hydrophobic interac-
tion among the amphiphilic molecules in self-assembled hy-
drogelation is one of the most important parameters. To in-
vestigate the involvement of hydrophobic interaction in the
gelation of the present amphiphilic molecules, we recorded
the fluorescence spectra of 8-anilino-1-naphthalenesulfonic
acid (ANS), a hydrophobic fluorescent probe for hydro-
gels 6 and 7 and their SWNT composites in water. We
doped ANS (1ꢂ10À5 m) in the aqueous amphiphilic solution
and excited the mixture at 360 nm. In the case of gelator 6
at a concentration of 0.001% w/v, the emission intensity of
the fluorescent probe was recorded at 478 nm (Figure 7a).
The peak intensity was found to increase further with gela-
tor concentration up to 0.0075% w/v, which was far below
the MGC of 6. The increment in the fluorescence intensity
of ANS is possibly due to the enhanced hydrophobicity in
its microenvironment. Above this threshold concentration
(i.e., 0.0075% w/v), the fluorescence intensity gradually de-
creased with broadening of the peak. It is evident that the
process of gelation passed through an intermediate state for
which the hydrophobic environment experienced by ANS
was a maximum compared to the higher gelator concentra-
tion.[19] Similar luminescence behavior was observed in the
case of hydrogel 7 (Figure S3a, Supporting Information),
for which the peak intensity increased up to 0.0075% w/v
and then decreased with further increase in gelator concen-
tration. Fluorescence studies were also performed for the
SWNT-6 and SWNT-7 nanohybrids keeping the SWNT con-
centration fixed at 0.0025% w/v. In both cases, the fluores-
cence peak intensity notably decreased at comparable con-
centration to that in the absence of CNTs but followed iden-
tical trends (initially increase in intensity up to 0.0075% w/v
followed by a decrease) with increase in gelator concentra-
tion (Figure 7b, S3b, Supporting Information). The quench-
ing of ANS emission intensity was more prominent when
the CNT amount was varied at a fixed gelator concentration
of 0.0075% w/v. In both nanocomposites, beyond 0.0025%
w/v pristine SWNTs, the emission intensity of ANS dropped
dramatically (Figure 7c, Figure S3c in the Supporting Infor-
mation) with increase in SWNT concentration (0.025% w/v
for SWNT-6 and 0.0075% w/v for SWNT-7). This significant
decrease in peak intensity in the presence of a minute
amount of SWNTs resulted from the interactions between
gelator molecules and pristine CNTs known for their intrin-
sic fluorescence quenching ability. The observed results also
strongly confirmed that the p–p interaction between the ar-
Figure 7. Luminescence spectra of ANS (1ꢂ10À5 m) with a) varying con-
centration of gelator 6 (from 0.001 to 0.1% w/v) in water, b) varying con-
centration of gelator 6 (from 0.001 to 0.05% w/v) at a fixed SWNT con-
centration of 0.0025% w/v in water, and c) varying concentration of pris-
tine SWNTs (from 0.001 to 0.025% w/v) at a fixed gelator concentration
of 0.0075% w/v in water.
omatic backbone of SWNTs and the aromatic ring of gelator
molecules resulted in the inclusion of CNTs within the hy-
drogel through physical cross-linking.
Rheological studies: The spectroscopic and microscopic
studies envisaged the encounter of hydrogels and CNTs to
develop a stable and superior system. This led us to an obvi-
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Nanotube-Included Supramolecular Hydrogels
FULL PAPER
ous eagerness regarding the
mechanical strength of the de-
veloped nanohybrid. Rheologi-
cal studies provide important
information about the fluidity
and rigidity of viscoelastic ma-
terials. The two primary pa-
rameters are the storage mod-
ulus (G’), which corresponds
to the ability of the deformed
materials to store energy, and
the loss modulus (G’’), which
represents the flow behavior of
the material under applied
stress. For viscoelastic materi-
als such as gels, G’>G’’ (G’
and G’’ꢀw0; w=angular fre-
quency) and in the sol state
G’’>G’ (G’ꢀw2 and G’’ꢀw).
Initially, we studied the me-
chanical rigidity of hydro-
gels 5–7 at their respective
MGCs (0.7–5% w/v) by oscil-
latory frequency sweep experi-
ments in which G’ and G’’
were noted as a function of an-
gular frequency (in the range
of 0.1–200 radsÀ1) at a fixed
strain of 0.01%.[8a,20] All the
native gels showed higher G’
than G’’ over the entire fre-
quency range but the G’ values
were considerably low (in the
range of ꢀ60–150 Pa), thus in-
dicating
poor
mechanical
strength (Figure S4, Support-
ing Information).[4a,8a] Also, the
plot of G’ and G’’ exhibited
the characteristic signature of
viscoelastic
materials.[4b,16a]
Among the hydrogels, 7 exhib-
ited higher G’ (ꢀ150 Pa) be-
cause of its superior gelation
efficiency and importantly at a
much lower concentration
(0.7% w/v) compared with the
hydrogels of 5 (5.0% w/v) and
6 (1.0% w/v).
Figure 8. Plot of G’ and G’’ of a) SWNT-5 composite with varying SWNT concentration at fixed gelator
amount (5.0% w/v), b) SWNT-6 composite with varying SWNT concentration at fixed gelator amount (1.0%
w/v), and c) SWNT-7 composite with varying SWNT concentration at fixed gelator amount (0.7% w/v) as a
function of angular frequency at a fixed strain of 0.01%.
Interestingly, upon incorporation of 0.1% w/v pristine
SWNTs into hydrogel 5 at its MGC (5% w/v), the G’ value
increased by ꢀ3.5-fold (ꢀ250 Pa) relative to the G’
(ꢀ70 Pa) of native gel at the MGC (Figure 8a). The G’ of
SWNT-5 was further improved to 835 Pa (by ꢀ11-fold)
when the CNT content increased to 1% w/v. The mechani-
cal strength of SWNT-5 remarkably improved by ꢀ22-fold
(ꢀ1550 Pa) when 1.5% w/v pristine SWNTs were incorpo-
rated within hydrogel 5. Such a huge increment in rigidity
confirmed the mechanical reinforcement of pristine SWNT–
gel nanocomposites through efficient physical integration of
pristine SWNTs within the SAFIN of the hydrogel. Howev-
er, no further increase in G’ was observed upon inclusion of
2% w/v SWNTs (the maximum CNT accommodation abili-
ty) within the hydrogel of 5 at its MGC value. The above
findings suggest that such a high SWNT content possibly
started to impose heterogeneity on the soft nanocomposite
structure that compromised the rigidity of the gel nanohy-
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P. K. Das et al.
brid.[21] Analogous trends were found in the rheological
properties of SWNT-6 and SWNT-7 nanocomposites. The
presence of 0.1% w/v SWNTs in hydrogel 6 at its MGC
(1.0% w/v) enhanced the G’ by about tenfold (ꢀ800 Pa)
relative to that of native gel (ꢀ80 Pa; Figure S4, Supporting
Information), which was further enhanced to 1733 Pa
(ꢀ20 times) and ꢀ3050 Pa (ꢀ37-fold) when 1 and 1.5% w/v
SWNTs were included within the hydrogel, respectively (Fig-
ure 8b). Here also the rigidity (G’ꢀ2330 Pa) of the hybrid
gel decreased at its maximum CNT content (2.0% w/v),
which was still significantly higher than that of the native
gel at the MGC. Similarly, in the case of the hydrogel of 7,
the mechanical strength improved by eightfold (1180 Pa) at
0.1% w/v content of SWNTs whereas the G’ was enhanced
by a maximum of ꢀ30-fold (4450 Pa) when 2% w/v SWNTs
were incorporated within the hydrogel at its MGC (Fig-
ure 8c). Here too, with further increment in CNT content
up to its highest accommodation level (3.5% w/v), the G’
decreased to ꢀ3300 Pa. Thus, we can systematically tune the
mechanical properties of the hydrogel by varying the
amount of SWNTs incorporated within the hydrogels.
In continuation of our effort to improve the mechanical
strength of supramolecular gels, we further measured the
viscoelastic parameters of soft nanocomposites by varying
both the gelator and SWNT concentration. Interestingly, the
nanocomposite prepared by incorporating 1.5% w/v SWNTs
within 2% w/v gelator 6 showed a remarkable enhancement
of ꢀ85-fold in G’ value (ꢀ15700 pa) relative to the native
hydrogel containing 2% w/v gelator (Figure S5 in the Sup-
porting Information, and Figure 9). Similarly, an approxi-
mately 70-fold increment in G’ was observed for SWNT-7
composite with gelator concentration of 2% w/v and CNT
content of 3.5% w/v (Figure S5 in the Supporting Informa-
tion, and Figure 9). With further variation of the gelator and
SWNT concentration, the G’ was increased up to a maxi-
mum value of 22300 Pa for SWNT-6 (2.0% w/v SWNTs–
3.0% w/v 6) and 28400 Pa for SWNT-7 (3.5% w/v SWNTs–
3.0% w/v 7; Figure 9), which is the highest G’ value report-
ed for nanocomposites developed by using LMW hydrogel
and pristine SWNTs. Such a noteworthy enhancement in the
rigidity of the gel nanocomposites was observed due to the
complementary interactions between SWNTs and the
SAFIN of the hydrogel through the imidazolium moiety as
well as the long hydrocarbon chain of the gelating amphi-
philes.
Conclusion
In summary, we have successfully developed pristine CNT-
incorporated low-molecular-weight hydrogel nanocompo-
sites by using imidazolium-based gelator molecules. A de-
tailed structure–property investigation was carried out by ju-
dicious alteration of the amino acid residues to find out
their influence in modulating the gelation and SWNT incor-
poration ability. The maximum amount of SWNTs that was
accommodated (3.5% w/v) within the hydrogel without
compromising the intrinsic features of either of the compo-
nents is the highest reported to date. The successful integra-
tion of SWNTs into the self-assembled gel was characterized
by various spectroscopic and microscopic techniques. Most
importantly, the inclusion of SWNTs within the hydrogel
matrix resulted in a remarkable enhancement in the me-
chanical rigidity of the soft nanocomposites. Furthermore,
the tunable viscoelastic properties of the nanohybrid gel
would find importance in the field of materials science and
also in biomedicine applications, including tissue engineer-
ing.
Experimental Section
Materials: All amino acids, tert-butoxycarbonyl (Boc) anhydride, dicyclo-
hexylcarbodiimide (DCC), 4-N,N-(dimethyl)aminopyridine (DMAP), 1-
hydroxybenzotriazole (HOBT), and solvents were purchased from SRL,
India. Trifluoroacetic acid (TFA), chloroacetic acid, thionyl chloride, and
sodium hydroxide were procured from Spectrochem, India. 1-Methylimi-
dazole was bought from Sigma. All deuterated solvents for NMR and
FTIR experiments and 8-anilino-1-naphthalenesulfonic acid (ANS) for
fluorescence spectroscopy were ob-
tained from Aldrich Chemical Co.
Thin-layer chromatography was per-
formed on Merck precoated silica
gel 60-F254 plates. 1H NMR spectra
were recorded on an AVANCE
500 MHz (Bruker) spectrometer.
Mass spectrometric data were ac-
quired by the electrospray ionization
technique on a Q-Tof-micro quadru-
pole mass spectrometer (Micromass).
Elemental analyses were performed
on a Perkin–Elmer 2400 CHN ana-
lyzer. Probe sonication was per-
formed using an Omni Sonic Ruptor
250 apparatus. Bath sonication was
performed with a Telsonic Ultrasonics
bath sonicator. A Sorvall RC 6 instru-
ment was used for centrifugation.
Synthetic procedure: All the amphi-
philes were synthesized by following
Figure 9. Plot of G’ of SWNT-6 and SWNT-7 composites with varying gelator and SWNT concentrations.
very well established peptide chemis-
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Nanotube-Included Supramolecular Hydrogels
try (Scheme S1, Supporting Information).
FULL PAPER
lowed by bath sonication for 2 h and again tip sonicated for 10 min. The
dispersed solution was then centrifuged at 2300 g for 90 min. The SWNT
dispersion efficiency of the amphiphiles was calculated from the observed
absorbance value at 550 nm, which was fitted in the standard calibration
plot prepared by using sodium dodecylbenzenesulfonate. The experimen-
tal errors were in the range of 3–5% in triplicate experiments.
Synthesis of 1 and 2: Boc-protected l-amino acids were coupled with n-
hexadecylamine by using DCC (1 equiv) and a catalytic amount of
DMAP in the presence of HOBT (1 equiv) in dry dichloromethane. The
reaction mixture was initially kept in an ice bath and then allowed to
come to room temperature and stirred for 12 h. The resultant reaction
mixture was then filtered and the filtrate was concentrated in a rotary
evaporator. The whole residue was then extracted in ethyl acetate and
the product was washed with 2m HCl, brine, 1m sodium carbonate, and
brine to neutrality. The organic part was dried over anhydrous sodium
sulfate and concentrated. Boc-protected amide was then purified through
column chromatography using 60–120 mesh silica gel and acetone/hexane
as the eluent. Column-purified materials were then subjected to depro-
tection of the Boc moiety by TFA (4 equiv) in dry dichloromethane.
After 2 h of stirring, solvents were removed on a rotary evaporator and
the reaction mixture was diluted with ethyl acetate. The ethyl acetate
part was washed thoroughly with 10% aqueous sodium carbonate solu-
tion followed by brine to neutrality. The organic part was dried over an-
hydrous sodium sulfate and concentrated to give the corresponding
amines. The primary amines (1 equiv) thus obtained were quaternized
with excess iodomethane using anhydrous potassium carbonate and a cat-
alytic amount of 18-crown-6 ether in dry DMF for 2 h. The reaction mix-
ture was extracted in ethyl acetate and washed with 5% aqueous sodium
thiosulfate solution and water. The concentrated part was then subjected
to column chromatography using 60–120 mesh silica gel and methanol/
chloroform as the eluent. The column-purified material was crystallized
from methanol/diethyl ether to obtain solid quaternized iodide salt. The
iodide salt thus obtained was subjected to ion exchange on an Amberlite
Ira-400 chloride ion-exchange resin column followed by crystallization
from diethyl ether to give the pure chlorides. The overall yield was ꢀ60–
65%.
UV/Vis-NIR study: The nanohybrid in water obtained after sonication
was used for UV/Vis-NIR experiments. In this case, a background correc-
tion was performed with the aqueous solutions of the respective amphi-
philes and the spectra were recorded using a Varian Cary 5000 spectro-
photometer.
Preparation of soft nanocomposites: Pristine SWNTs (1 mg) were weigh-
ed in the same screw-cap glass vial with i.d. of 10 mm. An aqueous solu-
tion (1 mL) of amphiphile having the required concentration was added
and tip sonicated for 10 min at 30% power output. The solution was then
kept undisturbed for 30 min. The composite material with pristine SWNT
concentration 0.1% w/v formed a nanohybrid gel, which was stable to in-
version of the glass vial. Similarly, varying amounts of gelators and
SWNTs were used to obtain nanocomposites with different SWNT and
gelator concentrations.
Determination of gel-to-sol transition temperature: The gel-to-sol transi-
tion temperature (Tgel) was determined by placing the gel-containing
glass vial (i.d. 10 mm) in a thermostatted oil bath and slowly raising the
temperature at a rate of 28CminÀ1. The Tgel was defined as the tempera-
ture (Æ0.58C) at which the gel melted and started to flow.
Microscopic study: Field-emission scanning electron microscopy
(FESEM) images were obtained on a JEOL-6700F microscope. A drop
of gel (at the MGC) was placed on a piece of cover slip and dried for a
few hours under vacuum before imaging. TEM experiments were per-
formed on a JEOL JEM 2010 high-resolution microscope operated at an
accelerating voltage of 200 kV. A dilute solution of gel was placed on a
300 mesh carbon-coated Cu grid and dried for a few hours under vacuum
before imaging. The hydrogelators were negatively stained with uranyl
acetate (2% w/v).
Synthesis of 3 and 4: The amino acid coupled with n-hexadecylamine at
the C terminus and with free N terminus (after deprotection of the Boc
moiety) was prepared as described above. This amine was further cou-
pled with another Boc-protected amino acid by using DCC, HOBT, and
a catalytic amount of DMAP in dry dichloromethane at room tempera-
ture for 12 h. The product was purified in a similar way to that men-
tioned earlier and also the Boc group was removed with TFA. The dipep-
tide-based primary amine was then quaternized by using the same proce-
dure as that described above. The column-purified material was crystal-
lized from methanol/diethyl ether to obtain solid quaternized iodide salt.
The iodide salt was subjected to ion exchange on an Amberlite Ira-400
chloride ion-exchange resin column followed by crystallization from di-
ethyl ether to give the pure chlorides. The overall yield was ꢀ55–60%.
FTIR measurements: FTIR measurements were performed with gelators
in a nongelated state in CHCl3 and in the gel state in D2O at their corre-
sponding MGCs at room temperature. All the experiments were carried
out in a Perkin–Elmer Spectrum 100 FTIR spectrometer using KBr pel-
lets (for CHCl3 solutions) and a 1 mm CaF2 cell (for D2O gels).
Circular dichroism: CD spectra of the aqueous solution of gelator 7 and
the aqueous dispersion of SWNT-7 with varying concentrations were re-
corded by using a quartz cuvette of 1 mm path length in a JASCO J-815
CD spectropolarimeter.
Fluorescence spectroscopy: The emission spectra of ANS and aqueous
solutions of hydrogels were recorded on a Varian Cary Eclipse lumines-
cence spectrometer. The probe molecules were added to aqueous solu-
tions of the amphiphiles at varying concentrations. ANS stock solution
was prepared in MeOH and from this super stock solution the required
amount of ANS solution was added to gelators so that the final concen-
tration of ANS solution was 1ꢂ10À5 m. The ANS solution was excited at
lmax =360 nm.
Synthesis of 5: l-Phenylalanine coupled with n-hexadecylamine at the
C terminus and with free N terminus (after deprotection of Boc) was ob-
tained as described earlier. This amine was further coupled with chloro-
acetic acid by using DCC, HOBT, and a catalytic amount of DMAP in
dry dichloromethane at room temperature for 12 h. The product was pu-
rified in a similar way to that mentioned earlier. The quaternization of
the coupled product with 1-methylimidazole was performed in dry ace-
tone under reflux conditions for 24 h with a yield of isolated product of
60%.
Rheology: Rheological experiments were carried out in cone and plate
geometry (diameter 40 mm) on the rheometer plate using an Anton Paar
MCR 302 instrument. The native gels and the SWNT–gel composites
were scooped on the rheometer plate so that there was no air gap with
the cone. Frequency sweep experiments were performed as a function of
angular frequency (0.1–200 radsÀ1) at a fixed strain of 0.01% at 258C
and the storage modulus (G’) and the loss modulus (G’’) were plotted
against angular frequency (w).
Synthesis of 6 and 7: The same procedure was followed to prepare dipep-
tide-based amines with an n-hexadecyl long chain as that described for
the synthesis of 3 and 4. The dipeptide amines were then reacted with
chloroacetic acid by following the standard protocol of DCC coupling.
Then the coupled product obtained was heated at reflux with 1-methyli-
midazole in dry acetone for 24 h. The yield of the products was 65–70%.
Preparation of gel: The required amount of amphiphile was placed in a
screw-capped vial with internal diameter (i.d.) of 10 mm and slowly
heated to dissolve in water. The solution was then allowed to cool slowly
(undisturbed) to room temperature. The gelation was checked by “stabil-
ity to inversion” of the aggregated material in the glass vial.
Acknowledgements
Quantification of the SWNT dispersions by the cationic amphiphiles (1–
7): Pristine SWNTs (1 mg) were added to an aqueous solution (4 mL) of
the respective amphiphiles (2.5 mgmLÀ1). The aqueous dispersion of
SWNTs was first tip sonicated for 10 min (at 40% power output) fol-
P.K.D. is grateful to the Department of Science and Technology, India
(SR/S1/OC-25/2011) for financial assistance. T.K. and S.K.M. acknowl-
edge the Council of Scientific and Industrial Research, India, for research
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Received: January 26, 2013
Revised: June 7, 2013
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