The striking influence of SWNT-COOH on self-assembled gelation

Article abstract of DOI:10.1039/c2cc16567h

A miniscule amount of f-SWNTs remarkably improved (～17-fold) the gelation efficiency of amphiphilic molecules by triggering the formation of interconnecting self-assembled fibrillar networks (SAFIN) in supramolecular gelation.

Full text of DOI:10.1039/c2cc16567h

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COMMUNICATION
The striking influence of SWNT–COOH on self-assembled gelationw
Subhra Kanti Mandal, Tanmoy Kar, Dibyendu Das and Prasanta Kumar Das*
Received 22nd October 2011, Accepted 8th December 2011
DOI: 10.1039/c2cc16567h
A
miniscule amount of f-SWNTs remarkably improved
(B17-fold) the gelation efficiency of amphiphilic molecules by
triggering the formation of interconnecting self-assembled fibrillar
networks (SAFIN) in supramolecular gelation.
Carbon nanotubes (CNTs) have gained enormous interest in
multiple disciplines owing to their high aspect ratio, electrical
conductivity, and extraordinary mechanical properties.1 One
such emerging field is the development of CNT based soft
composites having superior properties compared to their
constituents. Attempts have been made to hybridize these
carbonaceous nanomaterials with polymeric/molecular gels.2,3
Typically, the main prerequisite for gelation is the formation
of SAFIN where solvents flow get arrested primarily due to
the capillary forces.4,5 Until now, CNTs have been mostly used
as exogenous materials that reinforce the existing supramole-
cular network. It would be more intriguing if CNTs could
influence the formation of SAFIN, which plays a crucial role
in supramolecular gelation. The possibility of CNTs to assist
in gelation is quite sensible considering their nanoscale dimen-
sion. A dispersed single walled carbon nanotube (SWNT)
should offer similar capillary forces and networks analogous
to the SAFIN. However, the major hurdle with CNTs is their
inherent tendency to laterally stick with each other making
them repulsive towards solvents of extreme polarities. To this
end, we have shown efficient dispersion of SWNTs in water
using L-alanine based amphiphiles.6 It will be exciting to find
molecular structure that would facilitate the CNT dispersion
and consequently the debundled CNTs will trigger the for-
mation of SAFIN leading to development of efficient gel.
The present work highlights the pivotal role of acid function-
alized SWNTs (SWNT–COOH or f-SWNTs) in the formation
of SAFIN resulting in gelation at significantly low minimum
gelation concentration (MGC) compared to only amphiphiles
(1–5, Fig. 1). The presence of only 0.005% w/v f-SWNTs
remarkably improved the gelation ability of 1 in toluene.
Previously, we found that dipeptide based amphiphiles (1–5,
Fig. 1) with the C-16 alkyl tail at N-terminus and free –COOH
at the C-terminus are poor to moderate gelators having MGC,
1.0–5.0% w/v, in toluene (Table 1).7 At first we attempted to
Fig. 1 Structure of amphiphiles (1–5).
develop CNT-incorporated supramolecular gel by dispersing
pristine SWNTs (1 mg) in toluene solution (1 mL) of 1–5 at
their MGC. These amphiphile–SWNT mixtures were probe
sonicated and then allowed to cool at room temperature. Gel
formation did not take place rather pristine CNTs precipitated
out from toluene. To improve the solvophilicity of CNTs, we
used an f-SWNT instead of its pristine precursor. Under same
experimental conditions,
a well-dispersed f-SWNT was
obtained using 1, comprising of L-valine and L-tryptophan.
Strikingly the flow behaviour of this CNT-dispersion slowly
got restricted with simultaneous entrapment of the f-SWNT
resulting in a self-supporting gel, verified by stable to inversion
of a glass vial (Fig. S1a–c, ESIw). The MGC of 1 (5.0% w/v)
notably reduced to 0.3% w/v upon inclusion of the 0.1% w/v
f-SWNT (Table 1). A stable CNT–gel composite was formed
at B17-fold lower concentration than its MGC. To the best of
our knowledge, this is the first instance where self-assembled
gelation was influenced to such a great extent by SWNTs. The
dispersion of nanotubes was investigated by recording UV-vis-
NIR spectra of the f-SWNT–1 hybrid. Uniform dispersion of
Table 1 Minimum gelation concentration (MGC, % w/v) of amphiphiles
(1–5) in toluene at varying concentrations of SWNT–COOH (% w/v)
MGC (% w/v)
Amphiphiles
SWNT–COOH (% w/v)
1
2
3
4
5
Department of Biological Chemistry, Indian Association for the
Cultivation of Science, Jadavpur, Kolkata–700 032, India.
E-mail: bcpkd@iacs.res.in; Fax: +91-33-24732805
w Electronic supplementary information (ESI) available: Synthesis and
characterization of amphiphiles, UV-vis-NIR and PL spectra, Tgel plots,
DSC, rheological experiments. See DOI: 10.1039/c2cc16567h
0.0
5.0
0.5
0.5
0.4
0.3
5.0
5.0
5.0
5.0
1.5
1.0
1.0
1.0
0.7
0.5
2.5
2.5
2.5
2.5
0.75
1.75
1.75
0.25
0.25
0.15
0.005
0.02
0.05
0.1
c
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SWNT–COOH (0.01% w/v) in the presence of 1 (0.3% w/v)
was confirmed from the distinctive van Hove singularities
between 550–900 nm and 1200–1600 nm (S22 and S11 transition,
respectively, Fig. S1d, ESIw).3 These peaks arise from interband
transitions between the mirror image spikes in the density of
states of the individualized f-SWNTs. The influence of dispersed
nanotubes on self-assembled gelation was examined from field
emission scanning electron microscopic (FESEM) and trans-
mission electron microscopic (TEM) images of the gel–nano-
composite as well as that of the gelator and f-SWNT in toluene.
The FESEM image of 1 at 0.3% w/v (B17 times lower than
MGC) showed very few discrete fibers without any inter-
connecting network (Fig. 2a). The suspended f-SWNT in
toluene exhibited the presence of bundles having diameters
B40 to 50 nm (Fig. 2b). Interestingly, when merely 0.005% w/v
interconnecting SAFIN which was otherwise missing individual
components. This SAFIN progressively become denser with
increasing concentration of nanotubes resulting in superefficient
gelation for the f-SWNT–1 hybrid. Hence, f-SWNTs even at a
very low concentration markedly influence the self-assembled
gelation possibly by matching p–p and van der Waals inter-
actions between the f-SWNT and aromatic residue as well as the
hydrophobic chain of amphiphiles.8
At this point, we were intrigued to find out the influence of
f-SWNTs on the gelation of other amphiphiles (2–5). The
MGC of 2 (5% w/v) featuring L-alanine and L-phenylalanine
decreased by 3-fold to 1.5% w/v (Table 1) in the presence of
0.1% w/v f-SWNTs. However, the effect of f-SWNT is not as
significant as it was seen for 1. In the case of 3 (MGC, 1.0% w/v)
having L-valine (larger side chain substitution) and L-phenyl-
alanine, B2-fold improvement in gelation (MGC, 0.5% w/v)
was observed upon inclusion of 0.1% w/v f-SWNTs. In this
regard, earlier it was reported that alteration of the amino acid
sequence in the dipeptides plays a decisive role in modulating
the gelation due to the variation in hydrophilic–lipophilic
balance (HLB).7 Accordingly, we used 4 (MGC, 2.5% w/v)
where the amino acid sequence was exactly reversed to that of 3.
Here too the MGC reduced by B3.3 fold (0.75% w/v) in the
presence of 0.1% w/v f-SWNTs. Thus, alteration in aliphatic
amino acid could not provide any additional effect in the
CNT-triggered gelation. Also the influence of f-SWNTs on
the gelation of 2–4 was not striking as in 1. Presumably, the
reduced aromaticity of L-phenylalanine could not facilitate the
p–p interaction with f-SWNTs as efficiently as by the extended
aromatic ring of L-tryptophan in 1. Importance of p–p interaction
inspired us to exploit gelator 5 comprising of L-phenylalanine and
L-tryptophan. The gelation efficiency of 5 (MGC 1.75% w/v)
improved by B12 fold to 0.15% w/v (Table 1) in the presence of
0.1% w/v f-SWNTs. Hence, the influence of f-SWNTs is note-
worthy in self-assembled gelation of both 1 and 5 mainly due to
the favourable p–p interaction between the extended aromatic
ring of the gelator and the f-SWNT backbone.8
f-SWNTs were incorporated into toluene solution of
1
(at 0.3% w/v), the resulting nanocomposite exhibited entangled
fibrillar networks with a diameter of B50–70 nm (Fig. 2c) and
length of several micrometres. These networks became more
densely packed when the amount of SWNT–COOH was
increased to 0.01% w/v (Fig. 2d) presumably due to cross-
linking between f-SWNTs and gel fibers, leading to the
formation of SAFIN. Similarly, the TEM image of 1 (0.3% w/v)
showed the presence of thin fibers at very low density (Fig. 3a).
Also the bundled f-SWNT was observed in the corresponding
TEM image (Fig. 3b). However, the TEM image of 1 (0.3% w/v)
and f-SWNT (0.01% w/v) hybrid showed densely packed
interconnecting fibers (60–80 nm, Fig. 3c).
These spectroscopic and microscopic findings clearly delineate
that the amphiphile 1 is facilitating the dispersion of f-SWNTs in
toluene. But more importantly the minute amount of dispersed
f-SWNTs triggered the self-assembly that led to the formation of
The preceding results prompted us to vary the concentra-
tions of f-SWNT and amphiphile for determining the least
amount of nanotubes required in CNT-triggered gelation as
indicated in the microscopic investigations. The lowest
amount of f-SWNTs at which the gelation of 1 took place
was as low as 0.005% w/v with MGC of 0.5% w/v. An order
of magnitude enhancement in gelation ability in the presence
of such a low amount of CNTs is of great importance. Stable
gel formation was not observed below 0.1% w/v f-SWNTs for
2 and 4. In the case of 3, a minimum amount of f-SWNTs
needed for CNT-induced gelation was 0.05% w/v where MGC
decreased to 0.7% w/v. Interestingly for 5, only 0.02% w/v
f-SWNT was required to immobilize the gelator solution
(MGC 0.25% w/v), which is 7-fold lower than the reported
MGC. As a whole, notable decrease in MGC even at a very
low concentration of f-SWNTs confirmed the striking influence
of SWNT–COOH in triggering the self-assembled gelation. The
nanohybrid prepared using 3 and 5 also exhibited van Hove
singularities in UV-vis-NIR spectra (Fig. S1d, ESIw) indicating
uniform dispersion of CNTs. The formation of intertwined
fibrillar network was observed in the FESEM images of
f-SWNT–5 hybrid (f-SWNT (0.005% w/v)–5 (0.3% w/v)) with
Fig. 2 FESEM images of (a) 1, (b) f-SWNT, (c) f-SWNT (0.005% w/v)–1,
(d) f-SWNT (0.01% w/v)–1, (e) f-SWNT (0.005% w/v)–5, (f) f-SWNT
(0.03% w/v)–5 hybrids in toluene. [gelator] = 0.3% w/v.
Fig. 3 TEM images of (a) 1 (0.3% w/v), (b) f-SWNT, (c) f-SWNT
(0.01% w/v)–1 (0.3% w/v), (d) f-SWNT (0.02% w/v)–5 (0.3% w/v) in
toluene.
c
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a fiber diameter of B60–90 nm (Fig. 2e). Moreover at fixed
gelator concentration the fibers became more tightly packed with
increase in the amount of f-SWNTs from 0.005 to 0.03% w/v
(Fig. 2f). The TEM image of f-SWNT–5 hybrid gel also revealed
interconnected SAFIN (Fig. 3d) in the presence of CNTs.
The involvement of f-SWNTs in self-assembled gelation
through p–p interaction was further probed from the lumines-
cence spectra of 1 and 5. The concentrations of gelator and
f-SWNT were sequentially varied to determine their individual
influence on gelation. Upon excitation at 280 nm, the emission
intensity of the indole moiety at 340 nm was initially found to
increase with the gelator/nanotube up to a threshold concen-
tration and then gradually decreased with broadening of the
peak (Fig. S2–S5, ESIw).3,5,7 The emission intensity increased
till the gelators were in the non-self-assembled state and
followed by quenching at concentrations higher than 0.025
and 0.01% w/v for 1 and 5, respectively, as the planar
aromatic rings of amphiphiles and CNT backbone get closer
to each other due to the initiation of self-assembly. This
quenching effect was more prominent when the concentration
of f-SWNT was increased keeping the gelator amount fixed
(0.3 and 0.1% w/v for 1 and 5, respectively). This fluorescence
quenching was observed at a very low concentration of
f-SWNT (0.0025 and 0.005% w/v for 1 and 5, respectively),
which strongly suggests the integration of the f-SWNT
(a fluorescent quencher) with the aromatic ring of the gelator
through p–p interaction.3
Fig. 4 Plot of G/ and G// of the f-SWNT (0.1% w/v)–1 composite as a
function of angular frequency (0.01%) at varying gelator amounts
(0.5–5% w/v).
more rigid composite (Fig. S11, ESIw). Analogous increase in G/
was observed for 5 at varying f-SWNT amounts (Fig. S12, ESIw).
This result clearly shows that f-SWNTs not only improved
the gelation ability but also enhanced mechanical strength of
hybrids through matching integration of f-SWNTs within the
self-assembled network.
In conclusion, an extremely low amount of f-SWNTs can
remarkably influence the self-assembled gelation by instigating
the formation of interconnecting SAFIN resulting in super-
efficient mechanically strong gels.
We are grateful to DST, India, for financial assistance and
CSIR for fellowships to SKM, TK and DD.
All the f-SWNT–organogel hybrids were thermoreversible.
The gel-to-sol transition temperatures (Tgel) increased with
gelator concentration (1, 4, and 5, Fig. S6, ESIw) in the presence
of 0.1% w/v f-SWNTs. Prominent endothermic peaks for
f-SWNT–1 gel (1.0% w/v 1 and 0.1% w/v f-SWNT) at 73.6 1C
(Fig. S7, ESIw) and for f-SWNT–5 gel (0.75% w/v 5 and
0.1% w/v f-SWNT) at 77.4 1C (Fig. S8, ESIw) in differential
scanning calorimetry (DSC) were comparable to that of visually
observed Tgel at 71 1C and 79 1C, respectively.7 Importantly, at
fixed concentration of the gelator (0.7% w/v), Tgel steadily
improved even with small increase in f-SWNTs (Fig. S9, ESIw).
Such prominent increase in Tgel indicates the physical reinforcing
of the self-assembled gel network with f-SWNTs.
Notes and references
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Thus, the mechanical strength of the f-SWNT–gel compo-
sites was investigated by rheological studies.9 For gelator 1
alone, though the storage modulus (G/) was higher than the
loss modulus (G//) over the entire angular frequency range
(0.1–200 rad sÀ1) at a fixed strain of 0.01%, the signals were
noisy because of poor strength of the organogel (Fig. S10,
ESIw).3 Interestingly in the presence of 0.1% w/v f-SWNTs at
0.5% w/v of 1 (10 fold lower than its MGC), both G/ and G//
(G/ > G//) showed a plateau region under the same experi-
mental conditions which is a characteristic feature of viscoelastic
materials (Fig. 4).9 At fixed f-SWNT (0.1% w/v) almost B3
fold increase in G/ (450 Pa) was observed when gelator concen-
tration was increased to 1% w/v, which further increased to
600 Pa at 3% w/v of 1. Most strikingly the G/ of the f-SWNT–1
composite was found to be B6 fold higher (B25 000 Pa) than
that of the gel (1) alone at 5% w/v (Fig. 4 and Fig. S10, ESIw).
Such notable increase in G/ indicates the mechanical reinforce-
ment of the f-SWNT–gel composite. The G/ also enhanced with
inclusion of higher f-SWNT (0.05 and 0.1% w/v) resulting in a
7 T. Kar, S. Debnath, D. Das, A. Shome and P. K. Das, Langmuir,
2009, 25, 8639; S. Dutta, D. Das, A. Dasgupta and P. K. Das,
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