Hydrophobic End-Modulated Amino-Acid-Based Neutral Hydrogelators: Structure-Specific Inclusion of Carbon Nanomaterials

Article abstract of DOI:10.1002/chem.201504888

Hydrophobic end-modulated l-phenylalanine-containing triethylene glycol monomethyl ether tagged neutral hydrogelators (1-4) are developed. Investigations determine the gelators' structure-dependent inclusion of carbon nanomaterials (CNMs) in the self-assembled fibrillar network (SAFIN). The gelators (1, 3, and 4) can immobilize water and aqueous buffer (pH3-7) with a minimum gelator concentration of 10-15mg mL^-1. The hydrophobic parts of the gelators are varied from a long chain (C-16) to an extended aromatic pyrenyl moiety, and their abilities to integrate 1 D and 2 D allotropes of carbon (i.e., single-walled carbon nanotubes (SWNTs) and graphene oxide (GO), respectively) within the gel are investigated. Gelator1, containing a long alkyl chain (C-16), can include SWNTs, whereas the pyrene-containing 4 can include both SWNTs and GO. Gelator3 fails to incorporate SWNTs or GO owing to its slow rate of gelation and possibly a mismatch between the aggregated structure and CNMs. The involvement of various forces in self-aggregated gelation and physicochemical changes occurring through CNM inclusion are examined by spectroscopic and microscopic techniques. The distinctive pattern of self-assembly of gelators1 and 4 through J- and H-type aggregation might facilitate the structure-specific CNM inclusion. Inclusion of SWNTs/GO within the hydrogel matrix results in a reinforcement in mechanical stiffness of the composites compared with that of the native hydrogels.

Full text of DOI:10.1002/chem.201504888

DOI: 10.1002/chem.201504888
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&
Neutral Hydrogels |Hot Paper|
Hydrophobic End-Modulated Amino-Acid-Based Neutral
Hydrogelators: Structure-Specific Inclusion of Carbon
Nanomaterials
Pritam Choudhury, Deep Mandal, Sayanti Brahmachari, and Prasanta Kumar Das*[a]
Chem. Eur. J. 2016, 22, 5160 – 5172
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Abstract: Hydrophobic end-modulated l-phenylalanine-con-
taining triethylene glycol monomethyl ether tagged neutral
hydrogelators (1–4) are developed. Investigations determine
the gelators’ structure-dependent inclusion of carbon nano-
materials (CNMs) in the self-assembled fibrillar network
(SAFIN). The gelators (1, 3, and 4) can immobilize water and
aqueous buffer (pH 3–7) with a minimum gelator concentra-
tion of 10–15 mgmLÀ1. The hydrophobic parts of the gela-
tors are varied from a long chain (C-16) to an extended aro-
matic pyrenyl moiety, and their abilities to integrate 1D and
2D allotropes of carbon (i.e., single-walled carbon nano-
tubes (SWNTs) and graphene oxide (GO), respectively) within
the gel are investigated. Gelator 1, containing a long alkyl
chain (C-16), can include SWNTs, whereas the pyrene-con-
taining 4 can include both SWNTs and GO. Gelator 3 fails to
incorporate SWNTs or GO owing to its slow rate of gelation
and possibly a mismatch between the aggregated structure
and CNMs. The involvement of various forces in self-aggre-
gated gelation and physicochemical changes occurring
through CNM inclusion are examined by spectroscopic and
microscopic techniques. The distinctive pattern of self-as-
sembly of gelators 1 and 4 through J- and H-type aggrega-
tion might facilitate the structure-specific CNM inclusion. In-
clusion of SWNTs/GO within the hydrogel matrix results in
a reinforcement in mechanical stiffness of the composites
compared with that of the native hydrogels.
Introduction
nanomaterial-integrated gel, with the objective of improving
its physicochemical properties. Hybrid gels (containing nano-
materials) with improved mechanical stiffness are finding appli-
cations in the area of supercapacitors, nanoelectronics, photo-
voltaic devices, chemical sensors, biomedicine, and so on.[7a,8,9]
Exogenous nanomaterials such as carbon nanotubes (CNTs),
graphene, graphene oxide (GO), silver nanoparticles, gold
nanoparticles, and so on, have been included in the interstitial
spaces of SAFINs, and aided advantageous changes in their
properties.[10,11] However, it is difficult to predict the inclusion
of carbon nanomaterials (CNMs) within the fibrillar network;
there may be a dependence on the physical dimensions of the
CNMs as well as on the structure of the amphiphilic gelator.
Reports on the structure dependence of specific integration in
the gelator of different CNMs within SAFINs are scarce. Distinc-
tive structural and self-aggregation properties of the gelator
would play a vital role in the integration of CNMs within a hy-
drogel. Hence, it would be highly intriguing to design neutral
hydrogelators, as well as to find a correlation between molecu-
lar-structure-dependent aggregation and selective inclusion of
CNMs within the hydrogel matrix.
The development of self-assembled materials has been an ex-
panding research area in the last few decades because of their
versatile physicochemical properties as well as their mounting
applications in diverse areas including drug delivery, sensors,
template materials, and so forth.[1–6] Low-molecular-weight ge-
lators (LMWGs) are small organic molecules that have an ability
to restrict the mobility of solvents through the formation of
a self-assembled fibrillar network (SAFIN). Gelation of small
molecules is an outcome of a balanced combination of various
interactions such as hydrogen bonding, p–p stacking, van der
Waals forces, and so on.[7] An optimal balance between hydro-
phobicity and hydrophilicity of the amphiphile play the key
role in the formation of SAFINs.[6] In recent years, the develop-
ment of neutral hydrogels at physiological salinity has gained
importance owing to its suitability for bio-medicinal applica-
tions.[8] The design of a neutral hydrogelator, devoid of any
charged moiety, and which can form a gel in pure water or
buffer solutions at neutral pH, is a very tricky task. In this con-
text, the presence of a strong hydrophilic moiety (without any
charged residue) in the gelator’s structure would be the key
factor toward the rational design of a neutral hydrogelator. In
addition, a hydrophobic moiety and a residue capable of par-
ticipating in intermolecular hydrogen bonding need to be in-
cluded within the gelator’s structure to maintain the required
hydrophilic–lipophilic balance (HLB) for self-assembled gela-
tion.
Herein, we report the rational design of neutral hydrogela-
tors composed of hydrophobic terminal-group-modulated l-
phenylalanine with triethylene glycol monomethyl ether (TEG)-
tagged hydrophilic moiety (1–4, Figure 1). Amphiphiles 1, 3,
and 4 were found to form self-assembled hydrogels in water
and aqueous buffer solutions of varying pH values ranging
from 3.0–7.0. Amphiphile 2 self-aggregated to form a gel in
a DMSO/water mixture (1:3, v/v). The hydrophobic end of the
gelator molecule was judiciously varied from a long alkyl chain
(C-16) to an extended aromatic pyrenyl moiety to monitor the
selective CNM inclusion behavior of the hydrogels. The long
chain (C-16)-containing gelator 1 efficiently disperses and in-
cludes 1D allotropes of carbon, single-walled carbon nano-
tubes (SWNTs) within the gel matrix. Notably, the pyrene-con-
taining gelator 4 was able to integrate both 1D and 2D allo-
tropes of carbon (i.e., SWNTs and GO) in its SAFIN. The self-ag-
gregation behavior of the amphiphilic molecules and inclusion
of different carbon nanomaterials within the SAFINs were stud-
ied by spectroscopic and microscopic means. The mechanical
Simultaneously with rapid structural advancement, research-
ers are trying to build up soft nanocomposites composed of
[a] P. Choudhury, D. Mandal, S. Brahmachari, Prof. P. K. Das
Department of Biological Chemistry
Indian Association for the Cultivation of Science
Jadavpur, Kolkata, 700032 (India)
E-mail: bcpkd@iacs.res.in
Supporting information for this article can be found under http://
dx.doi.org/10.1002/chem.201504888. This includes the synthetic schemes of
1–4, characterization of gelators 1–4 (1H NMR, mass spectrometry, CHN
data), UV/Vis absorbance spectra and XRD spectra of GO, TEM image of
the dried hydrogel 1, CD spectra of 1 and 4 in absence and presence of
SWNT/GO, FTIR spectra of xerogels of 1, 4, 1-SWNT, 4-SWNT, and 4-GO.
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also contributed to the gelation through possible intermolecu-
lar hydrogen bonding.[5,10a,b,j]
We investigated the hydrogelation efficiency of the neutral
amphiphiles 1–4 in water, aqueous phosphate buffer of pH 3–
7, and DMSO-water mixture (1:3, v/v) (Table 1). Initially, amphi-
phile 1, containing the C-16 alkyl chain, formed a gel in water
with a minimum gelation concentration (MGC) of 15 mgmLÀ1.
Table 1. Minimum gelation concentrations (MGCs) of 1–4 in different
aqueous media.
Solvents
1
2
3
4
[mgmLÀ1
]
[mgmLÀ1
]
[mgmLÀ1
]
[mgmLÀ1
]
Milli-Q water
pH 7
pH 6
pH 5
pH 3
15.0
15.3
15.1
Sol[a]
Sol
Ins[b]
Ins
Ins
Ins
12.0
10.0
14.0
11
11
Sol
Sol
Sol
10.0
11.0
9.4
8.8
Sol
DMSO-water (1:3, v/v) Sol
10.5
[a] Sol=Soluble, [b] Ins=Insoluble.
Figure 1. Structures of the amphiphilic gelators 1–4.
The formation of the gel was confirmed from the “stable-to-in-
version” state of the glass vial (diameter 10 mm). With the ex-
pectation of improving the MGC, we incorporated an addition-
al l-phenylalanine in amphiphile 2, which might aid gelation
through its extra amide bond and additional aromatic residue.
To our surprise, 2 did not form a gel in water. However, it
showed efficient gelation in the DMSO-water mixture (1:3, v/v)
at MGC=10 mgmLÀ1. Presumably, amphiphile 2 has lost the
critical HLB required for self-aggregation in water owing to the
presence of an extra phenyl ring compared with 1. Subse-
quently, instead of the C-16 alkyl chain, we switched to an aro-
matic hydrophobic unit, that is, naphthalene-containing am-
phiphile 3, keeping all other structural components the same
as in 1. Amphiphile 3 showed hydrogelation ability in water at
MGC=14 mgmLÀ1, but at a very slow rate. It took almost five
days to form a gel that was stable to inversion in the glass vial.
Generally, it was found that the presence of an extended aro-
matic ring in the amphiphile’s structure facilitates self-assem-
bled gelation. Hence, we introduced a pyrenyl moiety instead
of naphthalene in amphiphile 4 with the aim of improving the
gelation efficiency. Amphiphile 4 was found to gelate in water
with improved efficiency (MGC=10 mgmLÀ1). The decrease in
MGC compared with 1 and 3, (which gelate in pure water) is
probably caused by the additional p–p stacking interaction be-
tween pyrenyl moieties.[7a,10b] We also monitored the gelation
abilities of 1–4 in phosphate-HCl buffer of pH 3.0–7.0 (Table 1).
It was found that amphiphiles 1, 3, and 4 can form hydrogels
at pH 6.0 and 7.0, whereas amphiphile 2 shows gelation ability
only at pH 3.0. Additionally, amphiphile 4 can also form a hy-
drogel at pH 5.0. Presumably, the tertiary nitrogen present at
the terminus of the gelator motif gives rise to the pH-respon-
sive hydrogelation behavior through protonation at different
pH values.[10b] All the amphiphiles remain soluble in DMSO,
and amphiphiles 2 and 4 can immobilize DMSO-water solvent
mixture (1:3, v/v) to form gels at MGC values of 10 and
stiffness of the gel and CNM–gel composite was monitored by
rheological measurements.
Results and Discussion
The development of a neutral amphiphilic molecule with hy-
drogelation ability is of high importance. Moreover, such a gela-
tor’s structure-dependent inclusion of CNMs within the hydro-
gel is a very interesting research domain, which has not been
well explored. In this present study, we developed neutral ge-
lators that form hydrogels in water and aqueous buffer, and
tried to understand the influence of the gelator structure on
specific CNM integration in its SAFIN. Accordingly, the hydro-
gelators (1–4, Figure 1, Schemes S1 and S2, Supporting Infor-
mation) were synthesized by coupling between the C-terminus
of l-phenylalanine and the twin-chain hydrophilic triethylene
glycol monomethyl ether (TEG) moiety attached to 2,2’-(ethyl-
enedioxy)bis(ethylamine). The N-terminus of the l-phenylala-
nine was coupled through an amide linkage by different hy-
drophobic groups such as C-16 (1), an additional l-phenylala-
nine linked with C-16 (2), naphthalene (3), and pyrene (4). The
hydrophobicity of the gelator was judiciously varied from
a long alkyl chain to an extended aromatic group to study
their aggregation pattern as well as their influence on CNM in-
clusion. With the aim of developing a neutral gelator, it is es-
sential to choose a specific hydrophilic moiety that is devoid
of any charged unit as well as being able to maintain the opti-
mal HLB required for hydrogelation. Hence, we chose the TEG
moiety as the hydrophilic terminus. The hydrophilic TEG-substi-
tuted 2,2’-(ethylenedioxy)bis(ethylamine) group enhanced the
solubility of the gelator in aqueous solvents, and the hydro-
phobicity of C-16 to pyrene might have maintained the neces-
sary HLB for self-assembled gelation. The linker amino acid
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10.5 mgmLÀ1, respectively. Among the four amphiphiles, 4
showed a better gelation efficiency in pure water and in aque-
ous buffer solutions, possibly owing to the presence of the ex-
tended aromatic ring (pyrenyl moiety), which may facilitate ad-
ditional p–p interactions. Thus, with variation in the hydropho-
bic unit of the gelator’s structure, the gelation efficiency also
modulated, presumably owing to the alteration in the overall
HLB of the amphiphile.
On the basis of the observed gelation, we chose hydrogela-
tors 1, 3, and 4 for inclusion of various CNMs within the hydro-
gel matrix. Gelator 2 was not considered for this study because
of its inefficiency in forming a hydrogel in pure water as well
as in pH 7.0 phosphate-HCl buffer solution. We selected two
types of CNMs for integration within the hydrogel: a 1D allo-
trope of carbon [single-walled pristine carbon nanotubes
(SWNTs)] and a 2D allotrope of carbon [graphene oxide
(GO)].[11] For effective inclusion of these CNMs in the gel
matrix, they should be well dispersed in the aqueous medium
in the presence of the amphiphile. Accordingly, we first tried
to disperse the amphiphobic pristine SWNTs using amphi-
philes 1, 3, and 4.[10b] Briefly, SWNTs (1 mg) were put in amphi-
phile solution (4 mL, 2.5 mgmLÀ1) and subjected to tip sonica-
tion followed by bath sonication. The suspension was then
centrifuged at 6000 rpm for 30 min. The supernatant was col-
lected, and the amount of dispersed SWNTs was calculated
using the calibration plot prepared with sodium dodecylbenze-
nesulfonate (SDBS).[8c] Amphiphiles 1 and 4 showed SWNT dis-
persion abilities of 190 and 210 mgmLÀ1, respectively. On the
other hand, amphiphile 3 showed moderate SWNT dispersion
(55 mgmLÀ1), probably owing to its slow rate of self-assembly.
Upon confirmation of the SWNT dispersion abilities of 1, 3, and
4, amphiphiles were taken at the respective MGCs (15 mgmLÀ1
for 1 and 3, and 10 mgmLÀ1 for 4) in water and sonicated
after mixing with required amount of SWNTs. The mixtures
were then left to stand for 30 min. The formation of the SWNT-
gel composite was tested through the “stable-to-inversion”
test of the glass vial (Figure 2). SWNTs were successfully includ-
ed in the hydrogel matrices of 1 and 4, giving 1-SWNT and 4-
SWNT nanohybrids, respectively, whereas 3 showed poor
SWNT inclusion owing to its slow rate of gelation (Figure 2). In
the absence of any amphiphiles, SWNTs remain insoluble in
water.
Figure 2. Inclusion and precipitation of SWNTs and GO in hydrogels of 1, 3,
and 4.
SWNTs with the gel fibers. The efficient dispersion of SWNTs in
aqueous solutions of amphiphiles 1 and 4 might have facilitat-
ed the successful inclusion of SWNTs within the gel matrix. At
this instant, we attempted to include the 2D allotrope of
carbon, GO, within the SAFIN of the hydrogel. Accordingly, GO
was synthesized from graphite powder through a modified
Hummer’s method and characterized as described in the Ex-
perimental Section (Figure S1, Supporting Information).[12a] The
freshly prepared GO and gelators (1, 3, and 4) at their MGCs
were put in milli-Q water and probe-sonicated for 10 min at
40% power output. The resulting mixtures were then allowed
to cool at room temperature. Intriguingly, in the presence of
GO, gelator 1 lost its hydrogelation ability, and GO precipitated
out from the solution (Figure 2). Similarly, amphiphile 3 was
also unable to include GO in its matrix (Figure 2). Interestingly,
only hydrogel 4 at its MGC (10 mgmLÀ1 in water) formed the
GO-included hydrogel 4 (4-GO) nanocomposite. The highest in-
clusion ability of GO within hydrogel 4 was noted to be
5 mgmLÀ1(Figure 2). It is probable that the pyrenyl moiety
present in gelator 4 aided the inclusion of GO through hydro-
phobic and p–p interactions to form the 4-GO composite.
It is apparent from the above observations that CNM inclu-
sion ability within the hydrogel matrix is not the same for all
the gelators. Gelator 1 containing the C-16 long chain was able
to include the 1D allotrope of carbon (SWNTs) in its SAFIN, but
failed to integrate the 2D allotrope (GO). Gelator 3 was unable
to integrate either SWNTs or GO. Presumably, the naphthalene
unit in gelator 3 does not provide the required hydrophobicity
to facilitate its interaction with the aromatic backbone of
SWNTs and GO. Also, there could be a mismatch between the
aggregated structure of 3 and the CNMs. Gelator 4 showed an
ability to include both the 1D (SWNTs) and 2D (GO) allotropes
of carbon. The successful integration of SWNTs and GO within
the hydrogel 4 matrix might be attributed to the additional p–
p interaction of gelator 4 originating from the pyrenyl group.
The ultrathin honeycomb-like planar structure of GO and the
aromatic backbone of SWNTs may fit between the extended ar-
We monitored the maximum inclusion amount of SWNTs
within hydrogels 1 and 4 by varying the SWNT concentration
(0.25–6.0 mgmLÀ1, Table 2) without compromising the stability
of the gel. Hydrogels 1 and 4 maximally accommodate 3.0 and
5.0 mgmLÀ1 SWNTs, respectively, at their MGCs. The successful
inclusion of SWNTs in the entangled gel network may be at-
tributed to the comparable morphology and dimensions of
Table 2. Maximum accommodation of pristine SWNTs and GO in hydro-
gels of 1 and 4 at their MGCs at 258C.
Gelator [SWNT] in hydrogel [mgmLÀ1
]
[GO] in hydrogel [mgmLÀ1
]
1
4
3.0
5.0
–
5.0
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omatic ring (pyrenyl moiety) through p–p stacking as well as
van der Waals’ interactions, leading to the successful formation
of the 4-GO/SWNT nanocomposite.[10f,12b] Thus, modulation of
the hydrophobicity in the gelators’ structures leads to struc-
ture-dependent inclusion of different CNMs in the gel matrices.
Determination of gel-to-sol transition temperature (Tgel)
The native hydrogels and SWNT/GO-gel nanocomposites were
thermoreversible in nature and also stable at room tempera-
ture for several months. The temperature at which the gel
transforms to solution is known as the gel melting tempera-
ture or gel-to-sol transition temperature (Tgel).[10b,j] For hydro-
gels 1, 3, and 4 (at their MGC in water), the Tgel values were
found to be 57, 61, and 658C, respectively, and for hydrogel 2
(at MGC in DMSO-water, 1:3 v/v) Tgel was 568C. Interestingly,
the Tgel values of the CNMs-included hydrogel (at MGCs) nano-
hybrids were found to be higher than those of the native gels.
In the case of 1-SWNT ([SWNT]=3 mgmLÀ1), 4-SWNT
([SWNT]=5 mgmLÀ1), and 4-GO ([GO]=5 mgmLÀ1), the mea-
sured Tgel values were 65, 73, and 758C, respectively. The incre-
ment in Tgel values of the SWNT/GO-gel nanocomposites might
be caused by the enhancement in crosslinking fibers between
SAFIN and CNMs in the gel matrix. Consequently, a higher
energy/temperature was required to break the self-assembly of
the CNM-gel composites.
Microscopic studies
The supramolecular morphologies of the individual gels as
well as SWNT/GO-integrated gels were investigated by trans-
mission electron microscopy (TEM). The TEM images of native
hydrogels 1, 3, and 4 in water and 2 in DMSO-water (1:3, v/v)
medium showed entangled fibrillar networks with fibers of 20–
50 nm in thickness (Figure 3a–d). The helical nature of the gel
fibers of hydrogel 1 was observed upon magnification of the
TEM image (Figure S2, Supporting Information). Incorporation
of SWNTs in hydrogel 1 led to a change in the morphology of
the 1-SWNT composite (Figure 3e). The extent of the criss-
crossed intertwined network increased in the aggregated
structure of the 1-SWNT composite compared with that of
native gel 1.[10a] In the case of the 4-SWNT composite (Fig-
ure 3 f), a denser fibrillar network was observed, with thicker
fibers (diameter 70–80 nm) than those of the native hydro-
gel 4.[10b] Upon inclusion of GO in hydrogel 4 (Figure 3g),
a very thin layered structure of GO sheet interlinked with gel
fibers in a 3D network was noted.[11b] The coexistence of the
intertwined fibrillar network of gelator 4 and the entrapped
GO sheet was also seen clearly from the scanning electron mi-
croscopy (SEM) image of the 4-GO composite (Figure 3h). In-
terestingly, the diameter of the fibers was comparable in the
native gel and the 4-GO nanohybrid, which indicates that in-
clusion of GO in the gel matrix did not perturb the self-assem-
bly of 4.[11b] The microscopic images of the native gel and
CNMs-gel composite revealed the gelator’s successful, struc-
ture-dependent inclusion of different CNMs within the SAFIN
of the hydrogel.
Circular dichroism (CD) study
CD spectroscopy is known to provide information about the
self-aggregation pattern of small-molecular-mass gelators.[13]
We have found different CD spectral patterns for the self-as-
sembly of hydrogelators of 1, 3, and 4 (Figure 4). The CD spec-
trum of 1 (1 mgmLÀ1 in milli-Q water) showed a negative
cotton effect with double minima at 202 and 213 nm. The
presence of double minima (202 and 213 nm) in the CD spec-
tra indicates a-helicity in its self-aggregation (by its pattern
but not by the position of the double minima).[10b,13] The helical
morphology of 1 is in concurrence with the corresponding
TEM image of hydrogel 1 (Figure S2, Supporting Information).
On the other hand, self-assembly of gelator 3 (1 mgmLÀ1 in
milli-Q water) showed a b-sheet pattern (positive band at
Figure 3. TEM images of hydrogels 1–4 (a–d, respectively). Insets of (a) and (d) show photographic images of hydrogels 1 and 4, respectively. TEM images of
e) 1-SWNT, f) 4-SWNT, and g) 4-GO composites. h) SEM image of 4-GO.
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Figure 4. CD spectra of amphiphilic gelators 1, 3, and 4.
222 nm and negative band at 231 nm, Figure 4) in its CD spec-
tra.[13] In the case of gelator 4 (1 mgmLÀ1 in milli-Q water), the
CD spectra showed a positive band at 226 nm and negative
band at 207 nm. This type of CD spectral pattern corroborates
with b-turn-like self-aggregation.[13d] Hence, the hydrophobic
end modulation in the gelator’s structure resulted in the for-
mation of a distinctly different pattern of self-assembly for am-
phiphiles 1, 3, and 4. Presumably, these structure-dependent
distinctive self-aggregation patterns of the hydrogelators
helped to integrate different CNMs in the gel matrix in a selec-
tive manner. Moreover, the CD spectral patterns of 1 and 4 in
the presence of SWNTs/GO (1-SWNT, 4-SWNT, and 4-GO com-
posites) remain unaltered with a slight decrease in CD signal
intensity (Figure S3, Supporting Information). This observation
revealed the unperturbed arrangement of the supramolecular
network formed by 1 and 4 upon inclusion of CNM in the hy-
drogel matrix.
Figure 5. FTIR spectra of a) 1 and b) 4 in the respective solvents.
of amphiphiles 1 and 4 affected the self-assembly of the gela-
tors. Interestingly, for the xerogel of the SWNT-incorporated
hydrogel 1, the amide I and amide II peaks were found to be
in a similar region, 1643 and 1520 cmÀ1, respectively (Figure S4,
Supporting Information) to that of the native gel in D2O. In the
case of the 4-GO composites, the peaks appeared at 1644 and
1542 cmÀ1 (Figure S4, Supporting Information), whereas 4-
SWNT showed analogous transmittance signals to its native hy-
drogel 4. Hence, it is evident that the integration of 1D and
2D carbon nanomaterials within the gel matrix did not perturb
the intermolecular noncovalent interactions between gelators
in the self-assembled state.[11]
FTIR spectroscopy
The influence of different kinds of noncovalent interactions
during the gelation process was investigated from FTIR spectra
of 1 and 4 recorded in the non-aggregated state (in chloro-
form) as well as in the gel state (in D2O, Figure 5).[10a,b] The FTIR
spectrum of 1 in CHCl3 showed transmittance peaks at approx-
imately n=3292, 1643, and 1515 cmÀ1, which originated from
nNÀH (amide A), amide nC (amide I), and nNÀH (amide II) vibra-
NMR spectroscopy
The 1H NMR study provides information about the participation
of various interacting forces such as intermolecular hydrogen
bonding, hydrophobic interactions, and so on, between the
self-aggregating molecules.[10b,j] Accordingly, we performed
temperature-dependent (Figure 6a and b) and solvent-depen-
=
O
tions, respectively (Figure 5a). These peaks shifted to approxi-
mately n=3377–3458 (broad band), 1617, and 1549 cmÀ1 in
D2O. Analogous results were found for gelator 4, for which the
transmittance peaks nNÀH (amide A)=3300 cmÀ1, amide nC
1
dent H NMR experiments (Figure 6c and d) for gelators 1 and
1
4. The temperature-dependent H NMR spectra of 1 (Figure 6a)
=
O
(amide I)=1643 cmÀ1, and nNÀH (amide II)=1532 cmÀ1 were
in the gel state (in D2O) at 258C showed that all the aromatic
protons of the phenyl ring are broad in nature in the region
d=6.95–7.13 ppm. This broadening of the aromatic proton
may have occurred because the hydrophobic interaction be-
tween the phenyl rings and the aromatic proton became more
shielded. With increasing temperature, the interaction gradual-
shifted to nNÀH (amide A)=3352–3471 cmÀ1 (broad band), nC
=
O
(amide I)=1609 cmÀ1, and nNÀH (amide II)=1558 cmÀ1, respec-
tively, upon changing the solvent from CHCl3 to D2O (Fig-
ure 5b). This shift in stretching and bending frequencies in
D2O indicates the involvement of intermolecular hydrogen
bonding between the carbonyl (C=O) and amide NÀH (i.e., C= ly decreased, and more sharp and downfield-shifted (d=7.49–
O···NÀH) during the self-assembled gelation. Now it is impor-
7.63 ppm at 858C) aromatic protons were observed. Similar ob-
servations were made in the case of 4 in D2O (Figure 6b):
tant to know whether inclusion of SWNTs/GO in the hydrogels
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molecular hydrogen bond and the p–p stacking was ruptured.
As a consequence, downfield shifts of aromatic and amide pro-
tons were observed at d=7.14–7.98 ppm and d=8.12–
8.16 ppm, respectively, with increasing sharpness in the peak
intensities. Interactions between aromatic rings during the ge-
lation process were further investigated through solvent-de-
pendent 1H NMR experiments. In [D6]DMSO, the aromatic
proton of gelator 1 (molecularly dissolved state) showed
a sharp peak in the region d=7.25–7.30 ppm (Figure 6c). The
D2O content of the system was increased gradually from 20 to
100%. As a result, the NMR signals of the aromatic protons
became broadened, along with a lowering of the d value in
the region 6.96–7.14 ppm. Similar observations were found in
the case of gelator 4 (Figure 6d), for which an upfield shift of
aromatic protons was observed with widening of peaks upon
increasing the D2O content. In [D6]DMSO, the lack of noncova-
lent interactions makes the gelator 1 molecules non-interactive
with each other for self-assembly. With a gradual increase in
D2O content, initiation of self-aggregation occurred, facilitating
the p–p interaction between the phenyl rings of the l-phenyl-
alanine moiety. Similarly, in the case of gelator 4, there was no
p–p stacking with the pyrenyl moiety in [D6]DMSO, and hence,
a characteristic aromatic proton was observed in the region
d=7.13–7.33. With a gradual increase in D2O content, effective
p–p stacking is initiated, resulting in the shielding of aromatic
protons and a notable diminishing of the peak intensities.[10b,j]
The abovementioned spectroscopic and microscopic experi-
ments delineate the participation of several noncovalent inter-
actions in the self-assembled hydrogelation of 1 and 4.
UV–visible spectroscopy
The aggregation pattern of small-molecule gelators was also
investigated through a solvent-dependent UV/Vis spectroscopy
study. The aggregation behavior of molecules is mainly catego-
rized by two types: H-type and J-type.[14] Molecules may self-
assemble through parallel plane-to-plane stacking to form
a sandwich-type array, which is referred to as H-type aggrega-
tion.[14a] Alternatively, they may form a head-to-tail arrange-
ment (end-to-end stacking), referred to as J-type aggregation.
In UV/Vis spectra, H-aggregates with side-by-side alliance of
molecules show a blueshifted absorption, whereas J-aggre-
gates with the head-to-tail alignment exhibit a redshifted ab-
sorption from that of the monomer units.[14] At this stage, we
were keen to monitor the self-aggregation type of gelators 1
and 4. As gelator 1 was devoid of any fluorescent moiety in its
structure, we hydrophobically tagged one fluorescent probe,
8-anilino-1-naphthalenesulfonic acid (ANS), in 1 (ANS-1). The
hydrophobic probe ANS is supposed to interact with the hy-
drophobic part of 1 and to localize itself at the hydrophobic
domain of the aggregate.[10j] Gelator 4 comprised an intrinsical-
ly UV-active pyrenyl moiety, so no external fluorophore was
needed to follow its UV/Vis spectra. We recorded the UV/Vis
spectra of ANS-1 ([1]=0.005 mgmLÀ1, [ANS]=1”10À6 m) and 4
(0.005 mgmLÀ1) (Figure 7) in various solvents from non-gelat-
ing (DMSO) to a gelating one (water). ANS-1 showed a broad
band at 367 nm in DMSO, which was redshifted to 380 nm
1
Figure 6. Temperature-dependent H NMR spectra of a) 1 and b) 4. Solvent-
dependent 1H NMR spectra of c) 1 and d) 4.
a low-intensity broad peak was found for the aromatic region
in the gel state at 258C. The amide proton (NÀH) of gelator 4
appeared at d=7.6 ppm in the gel state, and the amide pro-
tons are very much shielded owing to its participation in inter-
molecular hydrogen bonding with the C=O group. The aromat-
ic pyrenyl protons were also broadened owing to efficient p–p
stacking interactions. With increasing temperature, the inter-
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were blueshifted to 325 and 340 nm, respectively. This sug-
gests that the self-assembly occurred through H-type aggrega-
tion, that is, sandwich-type parallel layer formation (Fig-
ure 8).[14a] This distinctive parallel layer-by-layer orientation of
hydrogelator 4 probably aided successful GO inclusion in the
hydrogel 4 matrix compared with that in other hydrogels. Con-
sequently, it may be inferred that J-type aggregation helped to
integrate only the 1D allotrope of carbon, and that H-type self-
aggregation would be suited well for incorporation of both 1D
and 2D allotropes of carbon (Figure 8).
Fluorescence study
Hydrophobic interaction is a much-discussed phenomenon in
self-assembled hydrogelation. To this end, we performed a fluo-
rescence spectroscopy study using 8-anilino-1-naphthalenesul-
fonic acid (ANS) during the hydrogelation of 1 (Figure 9a). ANS
has an emission maximum (lmax) at 509 nm in water upon exci-
tation at 365 nm. The fluorescence intensity of ANS increased
in the presence of 1, and was enhanced gradually as the con-
centration of 1 was increased from 0.015 to 0.6 mgmLÀ1. The
increase in fluorescence intensity was accompanied by a blue-
shift from 509 to 479 nm (Figure 9a). Notably, the lmax intensity
decreased with a blueshift at 470 nm upon a further increase
in the gelator concentration to 1.5 mgmLÀ1. The above results
indicate that upon self-assembly of 1, ANS started to experi-
ence a hydrophobic environment. However, the fluorescent
probe experienced a less hydrophobic environment in the gel
state than in the intermediate state of gelation.[10j,15] Thus, the
gelation process seemed to occur via an intermediate state of
self-assembly to which ANS had a superior binding affinity
compared with that in the gel state. In the case of gelator 4, at
a concentration of 0.01 mgmLÀ1 (ꢀ1000 times lower than
MGC), a strong emission maximum was found at 397 nm,
along with other peaks at 378 and 419 nm, upon excitation at
340 nm (Figure 9b). At this concentration, the molecule is evi-
dently in a non-self-assembled state, in accordance with the
characteristic emission spectrum of 4. Upon increasing the ge-
lator concentration to 0.1 mgmLÀ1, a new excimer peak was
formed at a higher wavelength (l=469 nm), possibly because
of the p–p interactions between the pyrene moieties in the
self-assembled state.[10b,11b] Also, the bathochromic shift in fluo-
rescence emission maximum is attributed to the strong inter-
molecular hydrogen bonding between the LMWG molecules.
Furthermore, we recorded the fluorescence spectra of CNM-in-
cluded hydrogel 4 (0.5 mgmLÀ1), keeping the SWNT and GO
concentrations fixed at 0.5 and 0.075 mgmLÀ1, respectively
(Figure 9c). Quenching of fluorescence intensity was evident
from the emission spectra of both the SWNT- and GO-included
gel nanohybrid compared with that of the native hydrogel.
The reduced intensity in the fluorescence spectra of the 4-
SWNT and 4-GO hybrids confirmed the successful integration
of nanotubes and nanosheets within the SAFIN of the gel. The
possible interaction between the hydrophobic pyrene moiety
and aromatic backbone of SWNT/GO resulted in the quenching
of fluorescence intensity.[10b,11b] Moreover, the decrease in fluo-
rescence emission intensity can also be observed visually. The
Figure 7. UV/Vis spectra of a) ANS-tagged gelator 1 and b) native gelator 4
in different solvents.
upon changing the solvent to milli-Q water (Figure 7a). This
redshift clearly indicates the J-type aggregation pattern of
1 (Figure 8).[14b,c] Presumably, this distinct J-type self-assembly
of gelator 1 assisted spiral coiling in the gel fibers, as observed
from the TEM image (Figure S2, Supporting Information). Inter-
estingly, gelator 4 (0.005 mgmLÀ1) in non-gelating solvent
(DMSO) showed a sharp band of pyrene at 328 and 345 nm at-
tributed to p–p* transitions (Figure 7b). Upon changing the
solvent system from DMSO to water, the p–p* transition bands
Figure 8. Pictorial representation of the self-assembly of hydrogels 1 and 4
and inclusion of SWNTs and GO.
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between the hydrophobic interior of the aggregates thanks to
its rod-like shape and hydrophobic nature.[16a] The higher
steady-state anisotropy (r) value indicates the restricted move-
ment of DPH in various aggregated structures.[16b,c] We used
this technique to understand the gelation mechanism of 1. It
is observed that at a low concentration of 1 (0.20 mgmLÀ1) in
water, DPH showed a relatively large r value (0.13, Table 3). At
Table 3. Steady-state fluorescence anisotropic (r) values of DPH for gela-
tor 1 in water.
Concentration of gelator 1 [mgmLÀ1
]
r
0.20
0.30
0.60
1.50
3.00
0.13
0.10
0.10
0.10
0.10
a lower concentration of the amphiphile (below or around the
critical micellar concentration, cmc), DPH molecules aggregate
with each other, showing a higher anisotropy.[10j,16d] However,
the r value remained unaltered (r=0.10, Table 3) with increas-
ing concentrations of 1 (0.30–3.0 mgmLÀ1, Table 3). Presum-
ably, above the cmc, DPH molecules become solubilized in the
hydrophobic interior of the micelles with lower microviscosi-
ty.[10j] Notably, in the case of a micellar solution of common sur-
factant, cetyltrimethylammonium chloride (CTAC), within the
concentration range 5–100 mm, the r value was found to be
0.06. The higher anisotropy for 1 (r=0.10) compared with that
for CTAC (r=0.06) indicates that the aqueous solution of 1 is
a more viscoelastic system. The microviscosity of the solution
of 1 is higher, probably because of the formation of elongated
micelles (rod- or worm-like).[10j] On the basis of the above ob-
servation, it may be inferred that the formation of the hydrogel
of 1 (MGC=15 mgmLÀ1) is processed by changing the topolo-
gy from elongated micelles to a fibrillar network upon increas-
ing the concentration of 1. We were unable to perform the
fluorescence anisotropy experiment with gelator 4 because it
contains the intrinsically fluorescent pyrene unit, which inter-
feres in the anisotropy experiment as the excitation wave-
length of pyrene is comparable with that of DPH (lex =
370 nm).
Figure 9. Fluorescence spectra of a) ANS ([ANS]=1”10À5 m) tagged with 1,
and b) 4 with various concentrations. c) Fluorescence quenching of 4 upon
inclusion of SWNTs and GO within the self-assembly. Photographic images of
d) 4, 4-SWNT, and 4-GO upon irradiation with a UV torch.
Rheology
At this point, it would be fascinating to determine how the
stiffness of the gel changed upon addition of CNM to the gel
matrix. Rheological studies provide information on the fluidity
and rigidity of viscoelastic materials such as gels.[10a] Two major
parameters related to viscoelasticity are the storage modulus
(G’) and the loss modulus (G’’). The storage modulus G’ indi-
cates the ability of a deformed material to restore its native
form, whereas the loss modulus G’’ represents the flow behav-
ior of the material under applied stress.[10a,b] For viscoelastic
materials 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). Ini-
bright blue emission of the native hydrogel 4 upon UV irradia-
tion (365 nm) was reduced significantly upon inclusion of
SWNTs/GO in the gel matrix (Figure 9d).
Steady-state fluorescence anisotropy
Steady-state fluorescence anisotropy is an excellent tool for
measuring the fluidity of a system using a fluorescence probe.
The most common probe used to measure fluorescence aniso-
tropy is 1,6-diphenyl-1,3,5-hexatriene (DPH), which intercalates
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tially, we studied the mechanical stiffness of native hydrogels 1
and 4 at their respective MGCs (15 and 10 mgmLÀ1, respective-
ly) through an oscillatory frequency-sweep experiment, in
which G’ and G’’ were recorded as a function of angular fre-
quency (in the range 0.1–600 radsÀ1) at a fixed strain of 0.01%
(Figure 10). Native hydrogels 1 and 4 exhibited G’ values of
410 and 585 Pa, respectively. Interestingly, upon inclusion of
pristine SWNTs into hydrogel 1, the value of G’ increased two-
fold (G’=805 Pa) compared with the native gel at SWNT con-
centration of 1.5 mgmLÀ1. On the other hand, the GO-included
(1.5 mgmLÀ1) hydrogel 4 showed a mere 1.06-fold improve-
ment in G’ value (G’=622 Pa). However, 4-GO with 3 mgmLÀ1
GO content showed approximately 1.7-fold (G’=988 Pa) en-
hanced mechanical rigidity compared with that of the native
hydrogel. The above results indicate that the enhanced cross-
linking between SWNTs/GO and the SAFIN led to better me-
chanical resistance of the soft nanocomposites.
tively). The J-type and H-type self-aggregation patterns of hy-
drogelators 1 and 4, respectively, might have facilitated the
structure-dependent inclusion of CNMs within the hydrogel.
Hence, the modulation of the hydrophobic terminus of the ge-
lator has a notable influence on its self-aggregation as well as
its integration of CNMs of different dimensions. Moreover,
rheological studies of the hydrogels and SWNT/GO-gel compo-
sites show that the inclusion of SWNTs/GO within the hydrogel
improves the mechanical stiffness of the resulting soft nano-
composites compared with that of the native hydrogel.
Experimental Section
Materials
l-Phenylalanine, naphthalene-1-acetic acid, dicyclohexylcarbodi-
imide (DCC), 4-N,N-(dimethyl)aminopyridine (DMAP), 1-hydroxyben-
zotriazole (HOBT), di-tert-butyldicarbonate (BOC anhydride), silica
gel 60–120 and 100–200 mesh, and solvents were purchased from
SRL, India. Trifluoroacetic acid (TFA) and sodium hydroxide pellets,
potassium carbonate, and thionyl chloride were bought from Spec-
trochem, India. Palmitic acid, 1-pyrenebutyric acid, 2,2’-(ethylene-
dioxy) bis(ethylamine), triethylene glycol monomethyl ether (TEG),
tosyl chloride, single-walled carbon nanotubes (SWNTs), graphite
powder, CDCl3, [D6]DMSO, and D2O were procured from Sigma. All
deuterated solvents for NMR and FTIR experiments were obtained
from Aldrich Chemical Co. Thin-layer chromatography was per-
formed on Merck pre-coated silica gel 60-F254 plates. Lyophilization
was performed in a Virtis 4 KBTXL-75 freeze-drier. Mass spectro-
metric data were acquired through the electron spray ionization
(ESI) technique on a Q-tof-micro quadrupole mass spectrometer
(Micromass). 1H NMR spectra were recorded with an AVANCE
500 MHz (Bruker) spectrometer. Elemental analyses were per-
formed with
a PerkinElmer 2400CHN analyzer. High-resolution
transmission electron microscopy (TEM) images were taken on
a JEOL JEM2010 high-resolution microscope operating at 200 kV.
Field-emission scanning electron microscopy (SEM) was performed
with a JEOL-6700F microscope. The UV/Vis absorption spectra
were recorded on a PerkinElmer Lambda 25 spectrophotometer.
Fluorescence spectra were recorded with a Varian Cary Eclipse lu-
minescence spectrometer, and FTIR spectra were recorded on
a Perkin–Elmer Spectrum 100 spectrometer. XRD measurements
were made with a Seifert XRD 3000P diffractometer, with CuKa ra-
diation (a=0.15406 nm) with a voltage and current of 40 kV and
30 mA, respectively. Probe sonication and bath sonication were
performed with an Omni Sonic Ruptor 250 and Telsonic Ultrasonic
bath sonicator, respectively.
Figure 10. Plots of G’ and G’’ of a) hydrogel-1 and 1-SWNT and b) hydrogel-4
and 4-GO.
Conclusions
In brief, TEG -attached neutral hydrogelators containing l-phe-
nylalanine tailored with suitable hydrophobic residues were
synthesized. These gelators self-assembled to form hydrogels
in water and aqueous buffer solutions (pH 3–7). The critical
role of HLB in self-assembled gelation was analyzed by altering
the hydrophobic residue from a long chain (C-16) to an ex-
tended aromatic pyrenyl group. Moreover, the involvement of
various interactions in the gelation and the structure-depen-
dent inclusion of 1D and 2D allotropes of carbon nanomateri-
als within the SAFIN was investigated through spectroscopic
and microscopic studies. Interestingly, the C-16-containing ge-
lator 1 can successfully include the 1D allotrope of carbon
(SWNTs), whereas the pyrene-containing gelator 4 includes
both 1D and 2D allotropes of carbon (SWNTs and GO, respec-
Synthesis of amphiphilic gelators 1–4
All the amphiphiles were synthesized by following well established
peptide chemistry (Schemes S1 and S2, Supporting Information).
The carboxylic acid group of the l-phenylalanine was converted to
a methyl ester. The ester-protected amino acid was coupled with
palmitic acid with DCC (1 equiv), DMAP (cat.), and HOBt (1 equiv)
in dry DCM (CH2Cl2). The ester-protected amide was purified by
column chromatography on 60–120 mesh silica gel (methanol/
chloroform). The product was then hydrolyzed by treating with 1N
NaOH solution (1.1 equiv) in MeOH for 6 h with stirring at room
temperature. The reaction mixture was concentrated on a rotary
evaporator, and then diluted with water. The aqueous mixture was
washed with diethyl ether and subsequently acidified with a 1N
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aqueous HCl solution, and the produced carboxylic acid was ex-
tracted with ethyl acetate. This acid was coupled with mono-Boc-
protected 2,2’-(ethylenedioxy)bis(ethylamine) by treating it with
DCC (1 equiv), DMAP (cat.), and HOBT (1 equiv) in dry CH2Cl2. The
product was purified by column chromatography on 100–
200 mesh silica gel (methanol/chloroform). The purified product
was subjected to deprotection of Boc for 3 h by stirring with tri-
fluoroacetic acid (TFA; 4 equiv) in dry DCM. The solvent was re-
moved on a rotary evaporator and the mixture was taken in ethyl
acetate (EtOAc). The EtOAc part was washed thoroughly with 10%
aqueous sodium carbonate (Na2CO3) solution followed by brine to
neutrality. The organic part was dried over anhydrous sodium sul-
fate and concentrated to obtain the corresponding amine. The
product was purified by column chromatography on 100–
200 mesh silica gel (methanol/chloroform as eluent). In the final
step, the pure amine was subjected to nucleophilic substitution
under reflux conditions in dry acetonitrile (CH3CN) in the presence
of anhydrous K2CO3 (2.5 equiv) at 708C, to obtain the hydrophilic
triethylene glycol monomethyl ether (TEG)-substituted product. Ac-
cordingly, the reaction mixture was filtered, and the desired prod-
uct (1) was purified by column chromatography on 100–200 mesh
silica gel (methanol/chloroform). A similar protocol was applied to
synthesize 2–4 using C-16-chain-protected l-phenylalanine, naph-
thalene-1-acetic acid, and 1-pyrenebutyric acid (Schemes S1 and
S2, Supporting Information).1H NMR spectroscopic and mass spec-
troscopic analysis of 1, 2, 3, and 4 are provided in the Supporting
Information.
ed in water by sonication and again centrifuged at 20000 rpm.
This suspension/centrifugation process was repeated twice. Finally
the semi-solid residue was collected and freeze-dried in vacuum to
obtain GO. The formation of GO was further characterized by UV/
Vis spectroscopy and X-ray diffraction (XRD).
Preparation of SWNT/GO-included gel nanocomposites
The required amount of SWNTs was weighed in a screw-capped
glass vial (i.d. 10 mm). An aqueous solution (1 mL) of amphiphile (1
and 4) of the required concentration was added, and the mixture
was tip-sonicated for 10 min at 30% power output. The solution
was then left undisturbed for 30 min to form a nanohybrid gel (1-
SWNT and 4-SWNT) that was stable to the inversion of the glass
vial. A similar protocol was followed for the preparation of the GO-
included nanocomposite (4-GO). The required amount of GO was
weighed and an aqueous solution of amphiphile 4 (1 mL) of the
desired concentration was added and again tip-sonicated for
10 min at 40% power output. The solution was kept carefully at
room temperature for half an hour to prepare the soft nanocom-
posite (4-GO) having restricted mobility of the solvent.
Microscopic study
Field-emission SEM images were obtained on a JEOL-6700F micro-
scope. 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 performed on a JEOL JEM2010 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.
Preparation of hydrogels
The requisite amounts of 1, 3, and 4 were taken in screw-capped
vials (internal diameter, i.d., of 10 mm) and heated slowly to dis-
solve in Milli-Q water and in various phosphate-HCl buffer solu-
tions (pH 3.0–7.0). The gelation ability of 2 was tested in DMSO-
H2O (1:3, v/v). The solution was allowed to cool slowly (undis-
turbed) to room temperature. The gelation was checked through
the “stable-to-inversion” test of the aggregated materials in the
glass vials.
Circular dichroism
CD spectra of aqueous solutions of gelators 1, 3, and 4 at varying
concentrations were recorded by using a quartz cuvette of 1 mm
path length in a JASCO J-815 CD spectropolarimeter.
FTIR measurements
Determination of gel-to-sol transition temperature (Tgel
)
FTIR measurements were performed with gelators in the non-self-
assembled state in CHCl3, in the gel state in D2O, and in dried con-
dition for hydrogels 1 and 4 as well as for SWNT/GO nanocompo-
sites (1-SWNT, 4-SWNT, and 4-GO) at room temperature. All the ex-
periments were performed with a PerkinElmer Spectrum 100FTIR
spectrometer using KBr pellets (for CHCl3 solutions and xerogels)
or a 1 mm CaF2 cell (for D2O gels).
The gel-to-sol transition temperature (Tgel) was recorded by gradu-
ally increasing the temperature (at a rate of 28CminÀ1) in a thermo-
statted oil bath in which the hydrogel-containing glass vial (i.d.
10 mm) was placed. The temperature (Æ0.58C) at which the gel
liquified and started to flow under gravitation is referred to as Tgel
.
Synthesis of graphene oxide (GO)
1
Temperature- and solvent-dependent H NMR measure-
Graphene oxide (GO) was synthesized from graphite powder
(<30 mm) following the modified Hummer’s method. Graphite
powder (0.5 g) was dispersed in concentrated H2SO4 (20 mL),
sodium nitrate (0.5 g) was added to the dispersion, and this was
then cooled to 08C. Subsequently, KMnO4 (1.5 g) was added slowly
to the mixture so that the temperature remained below 208C, and
mixed thoroughly. The resulting solution was transferred to
a water bath at 358C and stirred for 30 min. The temperature was
raised to 908C during the addition of water (30 mL), and was main-
tained for a further 15 min. The whole solution was then mixed in
warm water (80 mL). To this solution, 30% H2O2 (0.5 mL) was
added to reduce the residual permanganate. The solution was fil-
tered and washed thoroughly with warm water. The solid was dis-
persed in distilled water (100 mL) by sonication. This dispersion
was centrifuged at 3000 rpm for 15 min. The residue was suspend-
ments
1
Temperature-dependent H NMR spectra of 1 and 4 were recorded
on an Avance 300 MHz (Bruker) spectrometer at a concentration of
15 and 10 mgmLÀ1, respectively, in D2O at various temperatures
(25–858C). Solvent-dependent 1H NMR spectra of 1 and 4 were
also recorded at the MGC for both 1 and 4 in various solvent ratios
of D2O and [D6]DMSO.
UV/Vis spectroscopy
The UV/Vis spectra of ANS-doped gelator 1 and native gelator 4
were recorded on a PerkinElmer Lambda 25 spectrophotometer by
varying the solvent from a non-gelating (DMSO) to a gelating one
(Milli-Q water).
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Frederix, K. I. Anderson, B. R. Saunders, R. V. Ulijn, Biomater. Sci. 2015, 3,
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Fluorescence spectroscopy
The emission spectra of ANS and aqueous solutions of ANS-doped
hydrogel 1 were recorded with a Varian Cary Eclipse luminescence
spectrometer. The probe molecules were added to the aqueous
solutions of amphiphiles at various concentrations at room tem-
perature. ANS stock solution was prepared in MeOH; from this
super stock solution, the required amount of ANS solution was
added to the gelators so that the final concentration of ANS solu-
tion was 1”10À5 m. The ANS solution was excited at 360 nm (lex).
A fluorescence study of gelator 4 was performed by recording the
spectra of the amphiphilic solution excited at 340 nm (lex) at vari-
ous gelator concentrations. The excitation and emission slit widths
were both 5 nm.
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Steady-state fluorescence anisotropy study
The steady-state fluorescence anisotropy study was performed
using the fluorescent probe DPH in an aqueous solution of surfac-
tant with a Varian Cary Eclipse luminescence spectrometer. A stock
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the final DPH concentration at 2”10À6 m. The aqueous solutions
were excited at 370 nm for DPH, and the corresponding emission
spectra were recorded at 450 nm. The excitation and emission slit
widths were 2.5 and 5 nm, respectively. The fluorescence aniso-
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Equation (1), in which IVV and IVH are, respectively, the fluorescence
intensities of the emitted light polarized parallel and perpendicular
to the excited light, and G=IVV/IVH is the instrumental grating
factor.
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r ¼ ðIVVÀGIVHÞ=ðIVV þ 2GIVH
Þ
ð1Þ
The fluorescence measurements were performed at 258C.
Rheology
Rheological experiments were performed in cone and plate geom-
etry (diameter 40 mm) on the rheometer plate using an Anton
Paar MCR302 instrument. The native gels (1 and 4) and the SWNT-
gel (1-SWNT) and GO-gel (4-GO) composites were scooped on the
rheometer plate so that there was no air gap with the cone. Fre-
quency sweep experiments were performed as a function of angu-
lar frequency (0.1–200 radsÀ1) at a fixed strain of 0.01% at 258C,
and the storage modulus (G’) and loss modulus (G’’) were plotted
against angular frequency (w).
Acknowledgements
PKD thanks the Department of Science and Technology, India
(SR/S1/OC-25/2011) for financial assistance. PC, DM, and SB ac-
knowledge the Council of Scientific and Industrial Research,
India for Research Fellowships.
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Keywords: amphiphiles · carbon nanomaterials · hydrogels ·
neutral gelators · self-assembly · soft nanocomposites
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Received: December 4, 2015
Published online on February 24, 2016
Chem. Eur. J. 2016, 22, 5160 – 5172
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5172
ꢀ 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim