Pyrene-based fluorescent ambidextrous gelators: Scaffolds for mechanically robust swnt-gel nanocomposites

Article abstract of DOI:10.1002/chem.201303401

With the rapid progress in the development of supramolecular soft materials, examples of low-molecularweight gelators (LMWGs) with the ability to immobilise both water and organic solvents by the same structural scaffold are very limited. In this paper, we report the development of pyrene-containing peptide-based ambidextrous gelators (AGs) with the ability to efficiently gelate both organic and aqueous solvents. The organo-and hydrogelation efficiencies of these gelators are in the range 0.7-1.1% w/v in various organic solvents and 0.5-5% w/v in water at certain acidic pH values (pH 2.0-4.0). Moreover, for the first time, AGs have been utilised to prepare single-walled carbonnanotube (SWNT)-included soft nanocomposites in both hydro-and organogel matrices. The influence of different non-covalent interactions such as hydrogen bonding, hydrophobic, p-p and van der Waals interactions in self-assembled gelation has been studied in detail by circular dichroism, FTIR, variable-temperature NMR, 2D NOESY and luminescence spectroscopy. Interestingly, the presence of the pyrene moiety in the structure rendered these AGs intrinsically fluorescent, which was quenched upon successful integration of the SWNTs within the gel. The prepared hydroand organogels along with their SWNT-integrated nanocomposites are thermoreversible in nature. The supramolecular morphologies of the dried gels and SWNT-gel nanocomposites have been studied by transmission electron microscopy, fluorescence microscopy and polarising optical microscopy, which confirmed the presence of three-dimensional self-assembled fibrillar networks (SAFINs) as well as the integrated SWNTs. Importantly, rheological studies revealed that the inclusion of SWNTs within the ambidextrous gels improved the mechanical rigidity of the resulting soft nanocomposites up to 3.8-fold relative to the native gels.

Full text of DOI:10.1002/chem.201303401

DOI: 10.1002/chem.201303401
Full Paper
&
Gels
Pyrene-Based Fluorescent Ambidextrous Gelators: Scaffolds for
Mechanically Robust SWNT–Gel Nanocomposites
Deep Mandal, Tanmoy Kar, and Prasanta Kumar Das*[a]
Abstract: With the rapid progress in the development of
supramolecular soft materials, examples of low-molecular-
weight gelators (LMWGs) with the ability to immobilise both
water and organic solvents by the same structural scaffold
are very limited. In this paper, we report the development of
pyrene-containing peptide-based ambidextrous gelators
(AGs) with the ability to efficiently gelate both organic and
aqueous solvents. The organo- and hydrogelation efficien-
cies of these gelators are in the range 0.7–1.1% w/v in vari-
ous organic solvents and 0.5–5% w/v in water at certain
acidic pH values (pH 2.0–4.0). Moreover, for the first time,
AGs have been utilised to prepare single-walled carbon-
nanotube (SWNT)-included soft nanocomposites in both
hydro- and organogel matrices. The influence of different
non-covalent interactions such as hydrogen bonding, hydro-
phobic, p–p and van der Waals interactions in self-assem-
bled gelation has been studied in detail by circular dichro-
ism, FTIR, variable-temperature NMR, 2D NOESY and lumi-
nescence spectroscopy. Interestingly, the presence of the
pyrene moiety in the structure rendered these AGs intrinsi-
cally fluorescent, which was quenched upon successful inte-
gration of the SWNTs within the gel. The prepared hydro-
and organogels along with their SWNT-integrated nanocom-
posites are thermoreversible in nature. The supramolecular
morphologies of the dried gels and SWNT–gel nanocom-
posites have been studied by transmission electron micros-
copy, fluorescence microscopy and polarising optical micros-
copy, which confirmed the presence of three-dimensional
self-assembled fibrillar networks (SAFINs) as well as the inte-
grated SWNTs. Importantly, rheological studies revealed that
the inclusion of SWNTs within the ambidextrous gels im-
proved the mechanical rigidity of the resulting soft nano-
composites up to 3.8-fold relative to the native gels.
Introduction
sis of new functional molecules with the objective of task-spe-
cific applications.[9] To this end, fluorescent supramolecular gels
have gained much interest due to their growing importance in
organic light-emitting diodes, photovoltaic cells, light-harvest-
ing systems and sensors.[10–13] Depending on their solvent im-
mobilisation nature, LMWGs are primarily categorised into two
classes: low-molecular-weight hydrogelators (LMHGs) and low-
molecular-weight organogelators (LMOGs).[6a,14] Moreover,
a third category, referred to as ambidextrous gelators (AGs), si-
multaneously act as both LMHGs and LMOGs.[15] Although
a large number of LMHGs and LMOGs have been designed
and synthesised during the last decade, reports on AGs are
very few. In this context, Miravet and Escuder reported on pyri-
dine-functionalised AGs for the preparation of nanostructured
supramolecular catalytic materials[15a] and Weiss and co-work-
ers developed AGs based on glucose-containing pyrenesulfo-
nyl derivatives.[15b] These AGs are gaining considerable interest
due to their ability to gelatinise solvents of extreme polarity.
However, despite the nature of gelators and their smart appli-
cations, most supramolecular gels usually suffer from poor me-
chanical strength.[16] Along with rapid structural modification,
researchers are trying to improve the mechanical rigidity of
the physical gel by developing novel soft nanocomposites.[17,18]
The interstitial space present in a self-assembled fibrillar net-
work (SAFIN) provides sufficient room for the inclusion of
exogenous material that can modulate the viscoelastic proper-
The self-assembly of small amphiphilic molecules to form di-
verse supramolecular architectures is a vibrant field of research
due to their versatile applications in areas such as drug deliv-
ery, tissue engineering, optical sensors, template materials and
enzyme-immobilisation matrices.[1–6] In this context, low-molec-
ular-weight gelators (LMWGs) are known to self-organise in
suitable solvents to form supramolecular structures with differ-
ent morphologies, such as fibres, rods and ribbons.[7] The pro-
cess of aggregation is primarily driven by non-covalent interac-
tions such as hydrogen bonding, p–p stacking, van der Waals
and electrostatic interactions, hydrophilic lipophilic balance
(HLB) and other supramolecular weak forces.[8] Over the last
few years, chemists have continuously indulged in the synthe-
[a] D. 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.201303401: Synthetic scheme for com-
pounds 1–3, NMR data of 1–3, TEM images of hydrogel 3 and its SWNT–
hydogel 3 composite, CD spectra of hydrogel 3, FTIR spectra of organogel
2 and its SWNT–organogel 2 composite, fluorescence spectra of organogel
2 and rheology of hydrogel 3.
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ties of gels.[18] Nanomaterials, such as carbon nanotubes (CNTs),
graphene oxide, silver and gold nanoparticles, could be suit-
able contenders for preparing such nanohybrids.[18] For exam-
ple, Ajayaghosh and co-workers included CNTs to reinforce
thixotropic organogels of oligo(p-phenylenevinylene).[17d] Re-
cently, Mandal and co-workers discovered a notable improve-
ment in the mechanical strength of a gel by incorporating
functionalised single-walled carbon nanotubes (f-SWNTs).[18d,i]
However, to date, the development of nanomaterial-included
hybrid soft materials from AGs in both the organic and water
phase has not been reported. To this end, our primary focus of
this work was to synthesise low-molecular-weight AGs that can
gelatinise both organic solvent and water (without structural
modification) as well as having the ability to include SWNTs
within the SAFIN of both hydro- and organogels.
Results and Discussion
The design of organo- and hydrogelators from the same struc-
tural scaffold instead of modifying a molecular framework is of
considerable importance as well as a challenging task.[15] More-
over, the design of new molecular scaffolds for gelation by the
introduction of judicious and task-specific functional moieties
is in high demand.[9] Accordingly, we have synthesised amphi-
philic molecules 1–3 by coupling pyrenebutyric acid and 3-(di-
methylamino)-1-propylamine to the N and C termini, respec-
tively, of three amino acids (Scheme 1). The extended aromatic-
ity of the pyrene (hydrophobic) unit provides the necessary p–
p stacking and hydrophobic interactions to enable their self-or-
ganisation in both water and organic solvents.[18a,b,e] Moreover,
pyrene derivatives can efficiently disperse super-amphiphobic
(otherwise insoluble in both aqueous and organic solvents)
carbon nanomaterials that might support the stable integra-
tion of SWNTs within the SAFIN of a gel.[18a,b,e] In addition, the
intrinsic fluorescence of pyrene can be exploited to develop
light-harvesting systems and sensors.[12,13] The linker amino
acids have a strong propensity to form intermolecular hydro-
gen bonding, which is a crucial factor for self-aggregation.[8]
We have used three different amino acids, l-alanine, l-phenyl-
alanine and l-tryptophan, which show a gradual change in
character from aliphatic to aromatic. The hydrophilic 3-(di-
methylamino)-1-propylamine was attached to the C terminus
of the amino acids to enhance the solubility of the molecules
in different solvents and also to tune their solubilisation in
water at different pH.
We report herein the design and development of pyrene-
containing amino acid derivatives 1–3 that act as ambidex-
trous gelators (Scheme 1). We have used pyrenebutyric acid as
the hydrophobic unit and 3-(dimethylamino)-1-propylamine as
We investigated the hydrogelation efficiency of the three
newly developed compounds 1–3 (Scheme 1) in phosphate–
HCl buffer in the pH range of 2.0–5.0 (Table 1). Compound 1
having l-alanine as linker showed weak hydrogelation ability
Table 1. Minimum gelation concentrations (MGCs) of 1–3 in water at dif-
ferent pH and at 258C.
Gelator
MGC [% w/v]
pH 3.0 pH 4.0
pH 2.0
pH 5.0
Scheme 1. Structures of gelators 1–3.
1
2
3
S[a]
0.65
1.1
5
0.5
Ins
5
0.6
Ins
Ins[b]
Ins
Ins
the pH-sensitive hydrophilic unit and combined them with
amino acid linkers (l-alanine, l-phenylalanine and l-trypto-
phan). Interestingly, molecules 1–3 efficiently gelate both or-
ganic solvents and water at acidic pH (pH 2.0–5.0). The gelators
exhibit excellent hydro- and organogelation ability, showing
minimum gelation concentrations (MGCs) of 0.5–1.1% w/v in
water and 0.7–5.0% w/v in organic solvents. We have success-
fully incorporated SWNTs into both organo- and hydrogel ma-
trices to form stable SWNT–gel nanocomposites. Rheological
studies indicated an approximately 3.8-fold enhancement in
the mechanical strengths of the SWNT–gel nanohybrids rela-
tive to the native gels. Importantly, the pyrene moiety provid-
ed the gel with excellent fluorescent character.
[a] S=Soluble. [b] Ins=Insoluble.
with an MGC of 5.0% w/v at pH 3.0 and 4.0. However, com-
pound 1 was soluble in water at pH 2.0 and insoluble in water
at pH 5.0. The formation of the hydrogel within a narrow pH
range likely arises from the variation in HLB (the degree to
which a molecule is hydrophilic or lipophilic) due to protona-
tion/deprotonation of the terminal N,N-dimethylamine;[19a] the
tertiary amine mostly exists in the protonated form (ammoni-
um ion) at pH 2.0 and easily dissolves in water, whereas the
same tertiary amine is insoluble in water at a relatively high
pH (pH 5.0) because most of the molecules exist in its depro-
tonated form. The increase in the hydrophobicity of the ter-
tiary amine results in its insolubility in water. Only within
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a narrow pH window (3.0–4.0, weak acidic medium) does the
N-terminal amine possess a suitable HLB through protonation/
deprotonation for the self-assembled hydrogelation of 1.[19b] In
addition, the hydrogen-bonding units in the amide linkage, p–
p stacking and hydrophobic interactions might have facilitated
the self-aggregated gelation of 1 in water with a high MGC
value. However, it is evident that the HLB for 1 is not optimal
above pH 4.0.
As described previously, the application of these supra-
molecular gels often suffers as a result of their poor mechani-
cal rigidity. Of many exogenous agents, the inclusion of carbon
nanotubes (CNTs) would be most suitable for improving the
strength of these physical gels due to the comparable dimen-
sions of SWNTs and gel fibres.[18d] The integration of SWNTs
within an entangled gel network may further enhance the me-
chanical strength by amplifying the intertwined criss-cross ar-
rangement. On this basis, we incorporated pristine SWNTs in
hydro- and organogels of 1–3 at their corresponding MGCs.
The required amount of SWNTs and gelators (at their MGCs)
were taken together in the respective solvent, probe-sonicated
for 10 min and then left to stand for 30 min. In the absence of
the gelators, the pristine SWNTs remained in a bundled state in
water and organic solvents. The formation of the SWNT–gel
nanohybrid was confirmed from the “stable-to-inversion” of
the glass vial. We monitored the optimum integration ability of
the SWNTs within the hydro- and organogels of 1–3 by varying
the SWNT concentration (0.025–0.6% w/v) without compromis-
ing the gelation efficiency. Hydrogels 1 and 3 maximally ac-
commodate 0.1 and 0.25% w/v SWNTs, respectively, at their
MGCs (5.0% w/v for 1 at pH 3.0 and 1.1% w/v for 3 at pH 2.0;
Table 3). Importantly, hydrogel 2 has the best SWNT inclusion
In the case of 2, we introduced an aromatic amino acid,
l-phenylalanine instead of l-alanine, with the objective of en-
hancing the p–p interactions between the gelators.[8j,15f] As ex-
pected, the gelation efficiency notably improved, with MGCs
of 0.5 and 0.6% w/v (Table 1) at pH 3.0 and 4.0, respectively.
This decrease in MGC is probably due to additional p–p inter-
actions between the aromatic (phenyl) rings in 2. As a conse-
quence, we considered that extended aromaticity possibly
could lead to more efficient hydrogelation of the pyrene-based
amine.[18c,e,j] Thus, we introduced l-tryptophan comprising an
extended aromatic system (indole moiety) as the linker amino
acid. Surprisingly, tryptophan-containing compound 3 was
found to be insoluble in water at pH 3.0 and above. However,
it showed efficient hydrogelation (MGC=1.1% w/v) at pH 2.0.
Presumably, compound 3 has lost the critical HLB required for
solubilisation/gelation in water above pH 2.0.
We also monitored the gelation of 1–3 in different aliphatic
and aromatic organic solvents (Table 2). All three compounds
have the ability to efficiently gelatinise numerous aromatic or-
ganic solvents, for example, benzene, toluene, xylene and
Table 3. Maximum accommodation of pristine SWNTs in gels of 1–3 at
their MGCs at 258C.
Gelator
[SWNT] in hydrogel
[% w/v]
[SWNT] in organogel (in toluene)
[% w/v]
1
2
3
0.1[a]
0.5[b]
0.25[c]
0.1
0.25
0.2
Table 2. Minimum gelation concentrations (MGCs) of 1–3 in different or-
ganic solvents at 258C.
[a] MGC of 1: 5% w/v at pH 3.0. [b] MGC of 2: 0.5% w/v at pH 3.0.
[c] MGC of 3: 1.1% w/v at pH 2.0.
Gelator
MGC [% w/v]
1
2
3
n-hexane
Ins[a]
S[b]
Ins
S
S
0.75
0.70
0.70
1.30
S
Ins
S
S
1.66
1.25
1.5
1.55
5.0
n-hexanol
acetic acid
benzene
toluene
m-xylene
ability, there being a maximum 0.5% w/v of pristine SWNTs in
its gel network. Similarly, the organogels of 1–3 in toluene also
showed SWNT inclusion ability; SWNT–organogel composites
were formed that maximally include 0.1, 0.25 and 0.2% w/v
SWNTs, respectively (Table 3). Presumably the hydrophobic in-
teraction between the SWNT surface and the extended aroma-
ticity of the pyrene ring assisted the dispersion of the SWNTs
and subsequently their integration within the gel network of
ambidextrous LMWGs. To the best of our knowledge, this is
the first example of the successful inclusion of SWNTs within
both hydro- and organogels of the same gelator molecule to
form a stable SWNT–gel soft nanocomposite without affecting
the stability of the gel.
S
1.25
1.20
1.25
1.85
S
chlorobenzene
nitrobenzene
[a] Ins=Insoluble. [b] S=Soluble.
chlorobenzene. The MGCs of the three gelators varied in the
range 0.7–1.8% w/v. Thus, all three compounds exhibit ambi-
dextrous gelation ability, importantly, without the alteration of
their structures. However, these compounds did not gelate the
tested aliphatic solvents. The best organogelation ability was
noted in toluene with MGCs of 1.20, 0.70 and 1.25% w/v, re-
spectively, for 1–3. Of the three gelators, 2 always showed
better gelation efficiency than the other two gelators in differ-
ent organic solvents (Table 2). Thus, it may be inferred that the
presence of an appropriate aromatic residue, such as l-phenyl-
alanine in 2, achieves the optimally required HLB for the most
efficient hydro- and organogelation.
The native hydrogels, organogels and SWNT–gel nanocom-
posites are thermoreversible in nature and also stable at room
temperature for several months. The temperature at which the
gel converts into a solution is known as the gel melting tem-
perature or gel-to-sol transition temperature (Tgel).[17,20] In ac-
cordance with the literature, the Tgel values of the gelators in-
creased with increasing gelator concentration (Figure 1).[17,18d]
For gelator 1, the Tgel at its MGC (5.0% w/v at pH 3.0) was
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Figure 2. TEM images of a) hydrogel 2 (0.5% w/v; inset shows a magnified
image of the gel fibres), b) SWNT (0.1% w/v)–hydrogel 2 (0.5% w/v), c) or-
ganogel 2 (0.7% w/v in toluene) and d) SWNT (0.1% w/v)–organogel 2
(0.7% w/v in toluene). Scale bars= a) 0.2 mm, inset a) 50 nm, b) 0.5 mm, c)
0.5 mm, d) 1 mm.
Figure 1. Variation of the gel-to-sol transition temperature (Tgel) of hydrogels
of 1–3 and their SWNT–hydrogel composites (SWNTs fixed at 0.1% w/v) with
increasing gelator concentration.
608C, which increased to 718C at 7.5% w/v. A similar trend in
Tgel was observed for hydrogel 2 (from 758C at 0.5% w/v at
pH 3.0 to 888C at 3.0% w/v) and hydrogel 3 (from 658C at
1.1% w/v at pH 2.0 to 778C at 4.5% w/v). The increase in Tgel
with increasing gelator concentration is possibly due to the in-
creased cross-linking between the self-assembled gel fibres.
We also monitored the Tgel of SWNT–hydrogel soft nanohybrids
with increasing concentration of gelators at fixed SWNT con-
centration (0.1% w/v). In every instance, the Tgel was always
higher for the soft nanocomposite than for the native gel
dicates the fibre to have a diameter of 15–20 nm and a length
of several micrometres (inset, Figure 2a). The SWNT–hydrogel 2
hybrid (0.1% w/v SWNT and 0.5% w/v of 2) exhibits a criss-
crossed dense network with different types of fibres (Fig-
ure 2b);[5d,18d,21] the network fibres are thicker (ca. 60–70 nm) in
the SWNT–hydrogel 2 than for the native gel fibres. Notably,
the TEM image of the SWNT–hydrogel 2 nanocomposite indi-
cates that the integrated SWNTs do not affect the supramolec-
ular self-assembly formed by the native gelator. Similarly, the
TEM images of organogel 2 (in toluene) and its SWNT–organo-
gel 2 nanocomposite show highly intertwined fibrous net-
works as well as the SWNT integrated within the network of
the organogel (Figure 2c,d). It is evident from Figure 2c that or-
ganogel 2 initially forms thin fibres of diameter 20–30 nm and
that further cross-linking of these thin fibres leads to the for-
mation of thick fibres of varying diameter. In the case of the
SWNT-included organogel 2, the morphology of the composite
is quite changed, and the fibre diameter appears to be thicker
than the native gel fibres (Figure 2d).[5d,18d] Analogous observa-
tions were found in the case of gelator 3 and its SWNT–hydro-
gel nanocomposites (see Figure S1 in the Supporting Informa-
tion). Thus, TEM studies confirmed the integration of pristine
SWNTs within the SAFIN of supramolecular hydro- and organo-
gels.
(Figure 1). Interestingly, the SWNT–hydrogel
2 composite
shows an almost 88C higher Tgel (Tgel =838C for SWNT–2) at its
MGC than its native gel (Tgel =758C for hydrogel 2). With fur-
ther increases in the concentration of 2, the Tgel of the SWNT–
hydrogel 2 soft nanocomposite increased to 1048C at 3% w/v
of 2 and 0.1% w/v SWNT. This Tgel is 168C higher than the cor-
responding Tgel of the native hydrogel 2 at the same concen-
tration. Similarly, we monitored the Tgel of organogel 2 (in ben-
zene) and its SWNT-included nanohybrid with increasing con-
centration of gelator. In this case the Tgel also increased with in-
creasing gelator concentration and at every instance the ob-
served Tgel for the SWNT–organogel 2 composite was always
higher than its native organogel (see Table S1 in the Support-
ing Information).[18d,i] It is possibly the physical cross-linking of
gel networks with exfoliated SWNTs that results in the increase
in the gel melting temperature (Tgel) of supramolecular soft
nanocomposites.
Fluorescence and polarising optical microscopy
The fibrous morphology as well as the intrinsic fluorescence of
the gelator was investigated by fluorescence microscopy.
Again, we have taken the dried hydrogel of compound 2 as
representative. A blue light-emitting cross-linked fibrous net-
work was observed upon exposing the sample to UV light (Fig-
ure 3a).[18c,f] This fibrillar network is in agreement with the en-
tangled SAFIN observed in the TEM image. The entangled fi-
brous network of 2 (xerogel prepared from toluene) exhibits
birefringence under polarising optical microscopy (POM; Fig-
Morphological studies
The supramolecular morphologies of individual gels as well as
the SWNT-integrated gels were observed by transmission elec-
tron microscopy (TEM). The TEM image of native hydrogel 2
(taken as a representative example because of its superior ge-
lation efficiency) at its MGC (0.5% w/v) shows an entangled 3D
SAFIN (Figure 2a). The magnified image of the same sample in-
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of an a-helix structure (which has generally been observed for
helical peptide structures) by its pattern but not by the posi-
tions of the double minima.[17a,22] Therefore this a-helical type
of arrangement for 2 in the self-assembled state possibly leads
to the formation of the fibrillar network structures that are evi-
dent in the corresponding microscopic images. The positive
peak at a longer wavelength (284 nm) in the CD spectra of 2
might be due to the n–p* transition involved in the amide
bond of 2.[19b] This peak was also found to increase with in-
creasing concentration of gelator, which indicates the supra-
molecular association of the chiral compound. On the other
hand, the CD spectrum of gelator 3 shows a negative peak at
198 nm along with a small negative peak at 227 nm (see Fig-
ure S2 in the Supporting Information). In all cases, the forma-
tion of a higher-ordered structure is derived from the supra-
molecular aggregation of individual gelators, which show an
increase in peak intensity with increasing gelator concentra-
tion. To understand the chiral supramolecular aggregation of
hydrogelator 2, a variable-temperature CD experiment was
performed by increasing the temperature gradually from 20 to
808C at a fixed gelator concentration (0.1% w/v). With increas-
ing temperature, the CD peak intensity gradually decreased
but the overall shape of the spectra remained similar to that
observed at 208C (Figure 4b). Thus, there is a notable loss of
the higher-ordered chiral aggregation at higher temperature
(808C). On lowering the temperature again to 208C, the gela-
tor molecule underwent self-assembly, as indicated by the ob-
served CD spectrum, which is almost equivalent to that of the
initial spectrum recorded at 208C. These variable-temperature
CD spectra clearly delineate the thermoreversibility and supra-
molecular chiral aggregation of the self-organised gelators
generated by non-covalent self-assembly.
Figure 3. a) Fluorescence microscopic image of dried hydrogel 2 and
b) POM image of dried organogel 2 (in toluene).
ure 3b), which demonstrates the high degree of molecular ar-
rangement in the gel state.[15d,17c]
Circular dichroism study
CD spectroscopy provided information on the dependence of
supramolecular aggregation on the molecular structures of the
gelators.[22] The CD spectra of hydrogelator 2 show a negative
cotton effect with double minima at 233 and 242 nm (Fig-
ure 4a). These peaks become more distinct with increasing
concentration of hydrogelator from 0.005 to 0.05% w/v. The
presence of double minima in the CD spectra is a characteristic
FTIR spectroscopy
FTIR spectroscopy is a powerful technique for studying the in-
fluence of different non-covalent interactions during the self-
aggregation of the gelators.[5d,17a] FTIR spectra were recorded
for pyrene derivatives 1 and 2 in the non-self-assembled state
in chloroform as well as in the gel state in D2O at pH 3 and as
dried organogels from toluene (Figure 5). The FTIR spectra of
gelators 1 and 2 in chloroform show transmittance bands at
3283–3313, 1655–1658 and 1507–1511 cmÀ1 that are character-
istic of non-hydrogen-bonded amide nNÀH (amide A), amide
nC=O (amide I) and dNÀH (amide II), respectively (Figure 5a,b).
However, in the gel state in D2O, the transmittance signals of
nNÀH (amide A) appear as a broad band at around 3253–
3530 cmÀ1 rather than as a distinct peak. In addition, in D2O,
the nC=O (amide I) and dNÀH (amide II) bands shift to 1637–
1640 and 1538–1541 cmÀ1, respectively. The dried organogel
(xerogel) of 1 and 2 in toluene shows nNÀH (amide A), amide
nC=O (amide I) and dNÀH (amide II) signals at 3273–3283,
1633–1636 and 1538–1541 cmÀ1, respectively. This shift in
stretching frequencies in the gel state clearly indicates strong
intermolecular hydrogen bonding of the C=O bond to the
amide NÀH of another molecule.[9g,17a,19b] Interestingly, the FTIR
spectrum of the nanocomposite SWNT–organogel 2 (toluene)
Figure 4. a) CD spectra of hydrogel 2 at varying concentrations (% w/v) in
water at pH 3.0 and b) temperature-dependent CD spectra recorded for
a heating and cooling cycle of hydrogel 2 (0.1% w/v).
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1
Figure 6. a) Variable-temperature H NMR spectra of 2 (0.7% w/v) in C6D6
and b) 2D NOESY spectra of 2 (1.3% w/v) in CDCl3 with 50% C6D6.
show NMR signals in the region d=6.89–6.43 ppm at room
temperature (258C). With a gradual increase in temperature to
358C, the amide protons shifted to lower d values of 6.86 and
6.29 ppm because the compound is still in the gel state at this
temperature.[17a,22c] As the temperature was increased to 458C
or even higher (55–658C), above the Tgel of 2 at 0.7% w/v (see
Table S1 in the Supporting Information), the NMR peaks of the
amide protons gradually shifted upfield to d=6.60 and
6.08 ppm at 658C (Figure 6a). At this higher temperature
(above the Tgel), the intermolecular hydrogen bonding between
the amide protons is destroyed resulting in a gel-to-sol transi-
tion.[17a] This variable-temperature NMR study confirmed the
presence of intermolecular hydrogen bonding between adja-
cent molecules through a -C=O···HÀN- linkage in the gel state.
Other intermolecular interactions operating between gelator
molecules were investigated by 2D nuclear Overhauser effect
spectroscopy (NOESY).[17a] For this experiment, we used gelator
2 (1.3% w/v) in CDCl3 (non-self-assembled state) but also in
the presence of 50% C6D6 (self-assembled state). As expected,
no off-diagonal cross-peak was recorded for the non-gelated
state of 2 in CDCl3. However, in 50% C6D6, several off-diagonal
cross-peaks can be observed between aromatic protons
(pyrene and l-phenylalanine) and the benzylic and chiral pro-
tons of the amino acid (Figure 6b). Moreover, other off-diago-
nal cross-peaks delineated the interaction between chiral pro-
tons and benzylic protons as well as between the amide pro-
tons and methylene protons of the tertiary amine chain. This
suggests that a through-space interaction between the pro-
Figure 5. FTIR spectra of a) gelator 1 and b) gelator 2 in D2O and toluene in
the gel state and in chloroform in the non-self-assembled state.
shows similar transmittance signals to that of its native or-
ganogel in toluene, which suggests that the intermolecular
non-covalent interactions between the gelator molecules
remain unperturbed even after the integration of SWNTs
within the gel matrix (see Figure S3 in the Supporting Informa-
tion).
NMR spectroscopy
We were also eager to know the other types of non-covalent
interactions that facilitate the self-assembly of these ambidex-
trous gelators. The preceding FTIR analysis showed the in-
volvement of intermolecular hydrogen bonding to be one of
the primary factors in self-assembled gelation.[17a,22c] To gain an
insight into the orientation at the molecular level of gelator
molecules in the hydrogen-bonding-induced self-assembly,
a variable-temperature 1H NMR experiment was conducted
with 2 (0.7% w/v) in C6D6. The two amide protons (Figure 6a)
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tons of two adjacent molecules also plays a crucial
role in self-assembled gelation.
Fluorescence study
Fluorescent spectroscopy is another important tool
for scrutinising the molecular interactions in self-as-
sembled gelation.[9g,17] At a very low concentration of
2 (0.001% w/v, 500 times lower than its MGC), the
gelator shows a strong emission peak at 396 nm
along with two other peaks at 377 and 417 nm upon
exciting the aqueous solution at 340 nm (Figure 7).
The gelator molecule is evidently in its non-self-as-
sembled state as the observed emission spectrum is
characteristic of a non-aggregated pyrene moiety. In
the self-assembled state (MGC=0.5% w/v at pH 3.0),
the gelator 2 exhibits a strong excimer peak at
Figure 8. a) Fluorescence spectra of hydrogel 2 and SWNT–hydrogel 2. Photographs of
b) hydrogel 2, c) SWNT–hydrogel 2 composite, d) hydrogel 2 upon exposure to UV light
and e) SWNT–hydrogel 2 upon exposure to UV light.
tubes within the SAFIN of the gel (Figure 8b,c), possibly
through interactions between the pyrene and aromatic back-
bone of the SWNTs. Moreover, a notable quenching of the fluo-
rescence intensity was observed for SWNT–hydrogelator 2
([SWNT]=0.0002% w/v) in the solution state, showing a similar
spectra to that of only hydrogelator 2 (0.001% w/v; see Fig-
ure S5 in the Supporting Information). Hence, the interaction
between gelator and SWNT also exists in solution. In addition,
we also observed a bright-blue emission of the hydrogel 2
upon irradiation with UV light (Figure 8d). This observation
clearly establishes the excellent intrinsic fluorescence of the hy-
drogelator. The well-known fluorescence quenching ability of
SWNTs as well as its integration within the gel network were
once again confirmed from the notably reduced intensity of
blue light after inclusion of SWNTs in the hydrogel (Figure 8e).
Figure 7. Fluorescence spectra of hydrogel 2 (at pH 3.0) in the solution and
gel states.
Rheology
467 nm (Figure 7).[18c] This clearly indicates self-aggregated di-
merisation between the pyrene moieties of the gelator mole-
cules in the gel state through strong p–p interactions. Also,
the corresponding redshift in the fluorescence emission
maxima is due to the presence of strong intermolecular inter-
actions between the LMWG molecules in the gel state. The
fluorescence spectrum of organogelator 2 (in toluene) at
a very low concentration (0.005% w/v) shows two peaks at
397 and 418 nm, however, at 0.6% w/v (gelator concentration
close to its MGC of 0.7% w/v), gelator 2 shows a high-intensity,
redshifted excimer peak at 464 nm (see Figure S4 in the Sup-
porting Information) analogous to that observed for the corre-
sponding hydrogel. Thus, a similar mode of self-assembled ge-
lation takes place in the organogelation through the dimerisa-
tion of the pyrene group. Furthermore, we also studied the
fluorescence of the SWNT-included hydrogel 2 (Figure 8a). In-
triguingly, a significant quenching of the excimer emission in-
tensity of pyrene was observed in the SWNT–hydrogel 2 nano-
composite. Quenching of the pyrene emission intensity in the
SWNT–hydrogel 2 hybrid confirms the integration of the nano-
It would be intriguing to find out how the mechanical strength
of the ambidextrous gels changed upon the successful integra-
tion of SWNTs within the soft materials. Rheological studies
provide information on the fluidity and rigidity of viscoelastic
materials such as gels.[5d,18d,i] Two primary parameters associat-
ed with viscoelasticity are the storage modulus (G’) and loss
modulus (G“). The storage modulus (G’) refers to the ability of
a deformed material to restore its native form whereas the loss
modulus (G”) represents the flow behaviour of the material
under applied stress.[23] For viscoelastic materials like gels,
G’>G“ (G’ and G”ꢀw0; w=angular frequency), and in the sol
state, G“>G’ (G’ꢀw2 and G”ꢀw). Initially we studied the me-
chanical rigidity of hydrogels 2 and 3 at their respective MGCs
(0.5 and 1.1% w/v, respectively) by an oscillatory frequency
sweep experiment in which G’ and G“ were recorded as a func-
tion of angular frequency (in the range 0.1–300 radsÀ1) at
a fixed strain of 0.01% (Figure 9 and Figure S5 in the Support-
ing Information). The plots of G’ and G” versus angular fre-
quency show the characteristic signature of viscoelastic materi-
als.[18d,23] Of the hydrogels, 2 exhibits a higher value of G’ (ca.
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native gel. In contrast, the SWNT–hydrogel 3 nano-
composite shows a value of G’ (G’ of nanocom-
posite=1299 Pa) comparable to its native gel (G’ of
native gel=1133 Pa) at its MGC (see Figure S6 in the
Supporting Information).
Conclusion
We have developed pyrene-containing fluorescent
ambidextrous gelators that efficiently gelatinise or-
ganic solvents as well as aqueous media at suitable
pH. The crucial role of the HLB in self-assembled ge-
lation was studied by sensible alteration of the
amino acid residue present in the gelator. The mech-
anism of gelation by the involvement of different
supramolecular interactions has been exemplified
through spectroscopic and microscopic studies. Im-
portantly, SWNTs can be successfully incorporated
into both the hydro- and organogel matrices of the
synthesised AGs to produce stable SWNT–gel nano-
hybrids, which have rarely been reported. Significant-
ly, the inclusion of SWNTs within the hydro- and or-
ganogel matrices resulted in a remarkable reinforce-
ment of the mechanical rigidity of the composites
compared with the native gels. Furthermore, the flu-
orescent nature of the native gels and SWNT-includ-
ed nanohybrids should find immense importance in
the field of materials science as well as in biomedicinal applica-
tions.
Figure 9. Plots of G’ and G“ of hydrogel 2 and the SWNT–hydrogel 2 composite with
SWNT concentrations of 0.05 and 0.1% w/v and gelator concentrations of 0.5 and 1.0%
w/v at pH 3.0 as a function of angular frequency at a fixed strain of 0.01%.
1108 Pa), probably due to its superior gelation efficiency and,
importantly, at a much lower concentration (0.5% w/v) com-
pared with hydrogel 3. Interestingly, upon inclusion of 0.05%
w/v pristine SWNTs into hydrogel 2 at its MGC (0.5% w/v), the
value of G’ increased approximately 3.5-fold (ca. 3865 Pa) com-
pared with the value of G’ (ca. 1108 Pa) of the native gel. Such
a remarkable increment in rigidity supports the mechanical re-
inforcement of SWNT–hydrogel nanocomposites through profi-
cient integration of pristine SWNTs within the SAFIN of 2. Sub-
sequently, we further increased the gelator 2 concentration
(1.0% w/v) as well as the SWNT concentration (0.1% w/v) and
measured the viscoelasticity of the resulting soft composite.
We found the highest value of G’ (5115 Pa) for the 0.1% w/v
SWNT-included nanocomposite, which is almost 3.8-fold higher
than that of the native gel of 2 at 1.0% w/v with G’=1337 Pa
(Figure 9). A remarkable improvement in the mechanical rigidi-
ty was also noted for the SWNT–organogel 2 composite with
0.7% w/v 2 and 0.1% w/v SWNTs (G’=13986 Pa) compared
with that of the native organogel 2 (G’=4380 Pa, Figure 10).
Hence, the enhanced cross-linking between the SWNTs and
the SAFIN of 2 leads to better mechanical resistance of the
soft nanocomposites. The difference between the two visco-
elastic moduli (DG=G’–G“) is considered a measure of the su-
premacy of the elastic nature of the material over its fluidity.[18c]
In this study, the value of DG (2496 Pa) for the SWNT–hydrogel
2 nanocomposite ([SWNT]=0.05% w/v with hydrogel 2 at its
MGC of 0.5% w/v) is notably higher than that of the native hy-
drogel 2 (DG=780 Pa at the MGC of 0.5% w/v). This indicates
that the elastic property dominates over its viscous nature in
the case of the SWNT–hydrogel 2 hybrid with respect to the
Experimental Section
Materials
All amino acids, tert-butoxycarbonyl (Boc) anhydride, dicyclohexyl-
carbodiimide (DCC), 4-(N,N-dimethylamino)pyridine (DMAP), 1-hy-
droxybenzotriazole (HOBT) and solvents were purchased from SRL,
Figure 10. Plots of G’ and G“ of organogel 2 and the SWNT–organogel 2
composite at its MGC (0.7% w/v in toluene) as a function of angular fre-
quency at a fixed strain of 0.01%.
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India. Trifluoroacetic acid (TFA), thionyl chloride and sodium hy-
droxide were procured from Spectrochem, India. Pyrenebutyric
acid and 3-(dimethylamino)-1-propylamine were bought from
Sigma. All deuteriated solvents for NMR and FTIR experiments
were obtained from Aldrich Chemical Co. TLC was performed on
output. The solution was then left undisturbed for 30 min. The
composite material with pristine SWNTs at a concentration of 0.1%
w/v formed a nanohybrid gel that was stable to the inversion of
the glass vial. Similarly, the concentrations of the gelators and
SWNTs were varied to prepare SWNT–gel nanocomposites with dif-
ferent SWNT and gelator concentrations.
1
Merck precoated silica gel 60-F254 plates. H NMR spectra were re-
corded with an Avance 500 MHz (Bruker) spectrometer. Mass spec-
trometric data were acquired by the electron spray ionisation (ESI)
technique on a Q-tof-micro quadruple mass spectrometer (Micro-
mass). Probe sonication was performed by using an Omni Sonic
Ruptor 250 instrument. Bath sonication was performed with a Tel-
sonic Ultrasonics bath sonicator.
Determination of gel-to-sol transition temperature (Tgel
)
The gel-to-sol transition temperature (Tgel) was determined by plac-
ing the gel-containing glass vial in a thermostatted oil bath and
slowly the temperature was elevated at a rate of 28CminÀ1. The
Tgel was refers to the temperature (Æ0.58C) at which the gel
melted and started to flow.
Synthetic procedure
All the compounds were synthesised following well-established
peptide chemistry (see Scheme S1 in the Supporting Informa-
tion).[5d,17a,22c]
Transmission electron microscopy
TEM experiments were performed on a JEOL JEM 2010 high-resolu-
tion microscope operated at an accelerating voltage of 200 kV.
Dilute solutions of gel were placed on a 300 mesh carbon-coated
copper grid and dried for few hours under vacuum before imag-
ing.
Synthesis of 1–3
The C-terminal end of the l-amino acid (l-alanine, l-phenylalanine
and l-tryptophan) was protected by esterification by stirring
a methanolic mixture of the amino acid with dropwise addition of
thionyl chloride. The ester-protected amino acid was then coupled
with pyrenebutyric acid through at the N terminus by using DCC
(1.1 equiv), DMAP (1.1 equiv) in the presence of HOBT (1 equiv) in
dry dichloromethane (DCM). The reaction mixture was initially kept
on an ice-bath and then allowed to warm to room temperature
and stirred for 12 h. The resultant reaction mixture was then fil-
tered and the filtrate was concentrated in a rotary evaporator. The
residue was then extracted with ethyl acetate, washed sequentially
with 2m HCl, 1m sodium carbonate and brine to neutrality. The or-
ganic part was dried over anhydrous sodium sulfate and concen-
trated. The pyrenebutyric acid coupled ester-protected amino acid
was then de-esterified by stirring the compound in a methanolic
solution of NaOH at room temperature for 12 h. It then underwent
work up with 1m HCl. The pyrenebutyric acid coupled amino acid
was subjected to another coupling reaction with 3-(dimethylami-
no)-1-propylamine by using DCC, DMAP and HOBT in DCM, similar-
ly to the procedure described above. After filtration of the reaction
mixture it was thoroughly washed with sodium carbonate and
brine to neutrality. The solution was stored over dry sodium sulfate
and the volume reduced on a rotary evaporator. The concentrated
liquid was then subjected to column chromatography by using
60–120 mesh silica gel with methanol/chloroform as the eluent.
The column-purified materials were then ready for use in the gela-
tion experiment.
Fluorescence microscopy
The gel (20 times diluted from the MGC) was placed on a glass mi-
croscope slide, dried and observed under a fluorescence micro-
scope (Olympus BX-61) with 100ꢁ magnification.
Polarising optical microscopy
POM images with of a dried sample prepared from toluene placed
on a glass microscope slide were taken by using an Olympus U-
TV1X-2 microscope.
Circular dichroism
CD spectra of an aqueous solution of gelator at varying concentra-
tions were recorded by using a quartz cuvette of 1 mm pathlength
in a JASCO J-815 CD spectropolarimeter.
FTIR measurements
FTIR spectra of the gelators in toluene, D2O and CHCl3 solution
were recorded with a Perkin–Elmer Spectrum 100 FT-IR Spectrome-
ter by using KBr pellets, a CaF2 cell and a KBr cell, respectively.
Variable-temperature NMR and 2D NOESY measurements
Preparation of gels
Variable-temperature 1H NMR spectra of 2 were recorded on an
Avance 300 MHz (Bruker) spectrometer at a concentration of 0.7%
w/v in C6D6 at variable temperature (25–658C). 2D NOESY spectra
of 2were recorded on an Avance 500 MHz (Bruker) spectrometer at
a concentration of 1.3% w/v in CDCl3 and in a mixed solvent of
C6D6 and CDCl3 (1:1, v/v).
The requisite amount of compound and phosphate–HCl buffer
(pH 2.0–5.0) or organic solvent were taken in a screw-capped vial
with an internal diameter (i.d.) of 10 mm and slowly heated to dis-
solve. The solution was then cooled slowly (undisturbed) to room
temperature. The gelation was checked by the “stable-to-inversion”
test of the aggregated material in the glass vial.
Preparation of SWNT–gel soft nanohybrids
Fluorescence spectroscopy
Pristine SWNTs (1 mg) were precisely weighed in the same screw-
capped glass vial with internal diameter of 10 mm. An aqueous/or-
ganic solution of amphiphile (1 mL) having the required concentra-
tion was added and tip sonicated for 10 min at 30% power
The emission spectra of the hydrogels were recorded on a Varian
Cary Eclipse luminescence spectrometer upon exciting the gelator
solution (in aqueous media and in toluene) at 340 nm (lex). The ex-
citation and emission slit width were both 5 nm.
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The rheological experiments were performed in cone and plate ge-
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Anton Paar MCR 302 instrument. The native gels and the SWNT–
gel composites were scooped on to the rheometer plate so that
there was no air gap with the cone. The frequency sweep experi-
ment was performed as a function of angular frequency (0.1–
300 radsÀ1) at a fixed strain of 0.01% at 258C, and the storage
modulus (G’) and the loss modulus (G“) were plotted against angu-
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Acknowledgements
P.K.D. is thankful to the Department of Science and Technology,
India (SR/S1/OC-25/2011) for financial assistance. D.M. and T.K.
acknowledge the Council of Scientific and Industrial Research,
India for research fellowships.
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Keywords: fluorescence
nanocomposites · self-assembly
·
gels
·
mechanical rigidity
·
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Received: August 29, 2013
Revised: November 7, 2013
Published online on December 11, 2013
Chem. Eur. J. 2014, 20, 1349 – 1358
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