Removal of toxic dyes from aqueous medium using adenine based bicomponent hydrogel

Article abstract of DOI:10.1039/c2ra22988a

By utilizing hydrogen bonding and π-π stacking interactions, we have demonstrated the construction of three dimensional adenine based gel networks due to the self assembly with complementary tricarboxylic acid derivatives which were designed and unambiguously characterized with the help of NMR, HRMS, and FTIR. Upon cooling the homogeneous aqueous solution of adenine and tricarboxylic acid, it formed hydrogels which were thermoreversible in nature and characterized by various instrumental techniques such as OM, FESEM, TEM, AFM, FL, XRD, FT-IR, rheology etc. Networks of belts in the hydrogel were clearly observed and the dimension of belt depended on the tricarboxylic acid used. The intermolecular hydrogen bonds which were considered to be the driving force for the formation of stable gel were confirmed by FT-IR studies. In spite of the absence of symmetry either in bpca or adenine, these two moieties surprisingly produced gels and it was due to the symmetrical position of complementary interaction sites between adenine and tricarboxylic acids. The mechanical strength of the hydrogel network as revealed by rheological study depended on the tricarboxylic acid used in the two-component systems and also on the composition of fixed pair. These kind of hydrogels have potential to be utilized as inexpensive materials for the treatment of waste water containing organic dyes (methylene blue, rhodamine 6G and crystal violet) that are widely used in textile as well as dye industries.

Full text of DOI:10.1039/c2ra22988a

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Removal of toxic dyes from aqueous medium using
adenine based bicomponent hydrogel3
Cite this: RSC Advances, 2013, 3,
1902
Pradip K. Sukul and Sudip Malik*
By utilizing hydrogen bonding and p–p stacking interactions, we have demonstrated the construction of
three dimensional adenine based gel networks due to the self assembly with complementary tricarboxylic
acid derivatives which were designed and unambiguously characterized with the help of NMR, HRMS, and
FTIR. Upon cooling the homogeneous aqueous solution of adenine and tricarboxylic acid, it formed
hydrogels which were thermoreversible in nature and characterized by various instrumental techniques
such as OM, FESEM, TEM, AFM, FL, XRD, FT-IR, rheology etc. Networks of belts in the hydrogel were clearly
observed and the dimension of belt depended on the tricarboxylic acid used. The intermolecular hydrogen
bonds which were considered to be the driving force for the formation of stable gel were confirmed by FT-
IR studies. In spite of the absence of symmetry either in bpca or adenine, these two moieties surprisingly
produced gels and it was due to the symmetrical position of complementary interaction sites between
adenine and tricarboxylic acids. The mechanical strength of the hydrogel network as revealed by
rheological study depended on the tricarboxylic acid used in the two-component systems and also on the
composition of fixed pair. These kind of hydrogels have potential to be utilized as inexpensive materials for
the treatment of waste water containing organic dyes (methylene blue, rhodamine 6G and crystal violet)
that are widely used in textile as well as dye industries.
Received 4th October 2012,
Accepted 26th November 2012
DOI: 10.1039/c2ra22988a
www.rsc.org/advances
valent interactions is more appealing to generate new
functional materials that are achievable by nanometer-scale
Introduction
manipulation.17,18 The incorporation of multi noncovalent
interactions can increase the stability and robustness of the
ordered superstructures,19 and simultaneously the reversible
and dynamic nature of these noncovalent interactions can
allow supramolecular materials to respond to external
stimuli.20 Utilizing the facile approach, well-defined nanos-
tructures such as nanofibers, nanoribbons, helical structures,
nanotubes, etc., have been created.21–23
Recently, supramolecular gels have received considerable
attention both as a case of self-assembly and phase separation,
and in creating ‘‘smart’’ materials.1–3 The gel state is
characterized by its solid-like properties despite being largely
liquid in composition.4,5 Conventional polymeric gels are
formed by cross-linked covalent polymers which are able to
swell and trap many times their own weight in solvent.6 In
contrast, supramolecular gels are generally formed from low-
molecular weight organic compounds which utilize noncova-
lent interactions to self-assemble into supramolecular net-
works capable of trapping solvent. Because such gels are held
together by multiple weak and therefore reversible interac-
tions, supramolecular gels are a particularly responsive and
tunable form of soft matter. The abnormal changes in viscosity
carried out by gelation can be triggered by a vast range of
stimuli such as changes in temperature, sonication,7 oxida-
Among the family of gels, the growth of low molecular
weight hydrogels (LMWH) has advanced in many fields
ranging from cosmetic products to pharmaceuticals to gene
delivery, due to their characteristics of biocompatibility and
biodegradation. In addition, the development of an ever-
increasing spectrum of functional hydrogelators continues to
broaden the versatility of hydrogel applications. LMWH now
play a critical role in many tissue engineering scaffolds,
biosensor and biological microelectro-mechanical systems,
and drug carriers.24 In contrast to single component organo
gel systems, new gelation systems comprising of two compo-
nents have been developed.14,25,26 In these systems, both
components that serve as gelators are held together by
noncovalent interactions. The most important feature of the
two-component systems is the ease with which the properties
and structures of the gel can be modulated by changing the
molar ratio of the two components or by changing one of the
tion,8 pH,9 addition of anions,10 and irradiation with light.11
A
wide variety of chemical species have been shown to exhibit
gelation including surfactants,12 sugars,13 fatty acids,14 and
amino acids,15 amongst many others.4,5,16 The bottom-up
fabrication of ordered superstructures using specific nonco-
Polymer Science Unit, Indian Association for the Cultivation of Science, 2A & 2B Raja
S. C. Mullick Road, Jadavpur, Kolkata-700032, India. E-mail: psusm2@iacs.res.in
Electronic supplementary information (ESI) available. See DOI: 10.1039/
3
c2ra22988a
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two components. Excellent examples of a dendritic building
block based on an amino acid in combination with an
aliphatic diamine have been reported by Smith et al.26, who
studied the properties and structures of the resulting gel.
Adenine, a purine nucleobase, is an important naturally
occurring nitrogen heterocycle present in nucleic acids DNA
and RNA. Several reports are present in the literature
describing adenine metal coordinated crystals and adenine
derivatives used for molecular recognition.27 In this study, we
report a two-component gel system in which three acid groups
and aromatic cores act as hydro gelators in the process of self-
assembly. The compositions of the two components have a
pronounced effect on the gel properties and on rheological
properties. However, to the best of our knowledge, almost no
studies have been concerned about adenine based hydrogels
except one recent example from our group.28 Making adenine
based hydrogels and subsequent potential application of these
gels will be interesting.
some harmful effects, such as heartbeat increase, vomiting,
shock, cyanosis etc. Methods of dye removal from industrial
waste waters could require many processes such as biological
treatment, coagulation, flotation, electrochemical techniques,
adsorption, and oxidation. Within these methods, adsorption
is more preferred due to high efficiency, easy handling, and
availability of different adsorbents. Hydrogels possessing
different functional groups have been investigated for this
purpose.29 Regarding this it should be mentioned here that
gelator molecules containing both hydrophobic and hydro-
philic groups can adsorb the dye molecules very efficiently. As
our moieties fulfil the above criteria, the produced hydrogel
has been successfully applied for the purification of dye
contaminated water.
Experimental section
Materials and instruments
Textile, paper, plastics, and cosmetic industries use a wide
variety of dyes to colour their products and discharge large
amount of effluents including dyes which are very toxic and
could cause serious ecological problems. These dyes can cause
4-Bromoacetophenone, K2S2O7, 5-amino isophthalic acid,
bis(pinacolato)diborane, and PdCl2 were purchased from
Aldrich. NaNO2, KOAc and all other chemicals and solvents
Fig. 1 (a) Synthesis scheme of biphenyl-3,4’,5-tricarboxylic acid (bpca). (b) Synthetic scheme of 1,3,5-tris(4-carboxyphenyl)benzene (tpca). (c) Chemical structure of
adenine (ADN). (d) Representative photo of inverted solution and gel of ADN/bpca 11 (0.41% w/v) [inner diameter of the glass tube is 10 mm].
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were purchased from Merck. HPLC grade water from Spectrum
was purchased and used as gelling medium.
Dimethyl-5-iodobenzene-1,3-dicarboxylate (2)
A solution of sodium nitrite (8.28 g, 0.12 mol) in water (150
mL) was added to a suspension of 1,3-dimethyl-5-aminoben-
zene-1,3-dicarboxylate (1) (25.11 g, 0.12 mol) in 20% HCl (75
mL) at -5 uC. Toluene (200 mL) and then a solution of
potassium iodide (40.32 g, 0.48 mol) in water (100 mL) were
slowly added to the suspension. After the addition, the
reaction mixture was stirred for 12 h and afterward refluxed
for 1 h. The organic layer was separated and washed three
times with water, dried over MgSO4, filtered, and concentrated
in vacuum. The crude product was purified by column
chromatography (silica gel, ethyl acetate/hexane, 3 : 7 v/v)
and recrystallized from methanol, giving 2 as light-brown
crystals. Yield 44%.1H NMR (300 MHz, CDCl3): d = 3.95 (s, 6H),
8.53 (d, 2H), 8.61 (t, 1H) ppm. 13C NMR (300 MHz, CDCl3): d =
52.6, 93.4, 129.8, 132.3, 142.4, 164.7 ppm.
Synthesis
All synthesis procedures for tricarboxylic acids were carried
out according to Fig. 1a, b. All the compounds were fully
characterized by mass spectrometry, 1H-NMR spectroscopy
(300 MHz), and 13C-NMR spectroscopy (300 MHz).
Synthesis of 1,3-dimethyl-5-aminobenzene-1,3-dicarboxylate
(1)
5-amino isophthalic acid (2.0 g, 11.04 mmol) was dissolved in
methanol (50 mL). Concentrated sulfuric acid (2 mL) was
added slowly and the solution was refluxed for 24 h. After
removing the solvent, the residue was dissolved in chloroform
(150 mL) and washed twice with saturated bicarbonate, and
the solvent was removed under reduced pressure to receive the
desired product 1 as a white powder (yield: 95%). 1H NMR (300
Biphenyl-3,49,5-tricarboxylic acid (bpca)
MHz, CDCl3): d 3.90 (s, 6H), 7.51 (s, 2H), 8.04 (s, 1H) ppm. 13
C
A mixture of dimethyl 5-iodobenzene-1,3-dicarboxylate (2) (320
mg, 1.00 mmol), bis(pinacolato)diborane (279 mg, 1.10 mmol),
potassium acetate (294 mg, 3.00 mmol), PdCl2(dppf) (24 mg,
NMR (300 MHz, CDCl3): d 52.4, 119.8, 120.7, 131.6, 146.8,
166.6 ppm. HRMS: calcd for C10H11NO4 209.20, found 210.01
(MH+), 231.9 (MNa+).
Fig. 2 (a) Optical image, (b) optical image under polarized light and (c) fluorescence image of dried gel under excitation at 380 nm [ADN : bpca = 1 : 1, (0.41% w/v),
a slice of gel was cast on a glass slide and dried under air prior to taking image].
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0.03 mmol), and dried DMF (6 mL) was stirred at 80 uC for 2 h.
After cooling at room temperature, methyl-4-bromobenzoate
(640 mg, 2.00 mmol), PdCl2(dppf) (24 mg, 0.03 mmol), and CsF
(456 mg, 3.0 mmol; dissolved in 2.5 mL of water) were added.
The mixture was stirred at 80 uC overnight and afterward
extracted several times with diethyl ether. The organic layer
was dried with MgSO4, and the solvent was removed and the
crude product was purified by column chromatography (silica
Fig. 3 (a,b) FE-SEM images of ADN/bpca11 dried gel, (c,d) ADN/tpca11 dried gel. TEM images of ADN/bpca11 dried gel (e,f). AFM topographs of dried gel cast on a
freshly cleaved mica surface, (g) ADN/bpca11 gel, 0.41% w/v, (h) ADN/tpca11 gel, 0.53% w/v, and (i) corresponding height profile.
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gel, ethyl acetate/hexane, 3 : 7 v/v). Subsequently it was
hydrolyzed as follows. A mixture of biphenyl-3,49,5-methyltri-
carboxylate (0.96 g, 2.5 mmol), THF (40 mL), and NaOH (1.6 g,
40 mmol) dissolved in water (40 mL) was refluxed for 1 h, the
organic solvent was removed under reduced pressure, and the
aqueous solution was refluxed again for 4 h. The reaction
mixture was cooled and acidified with 50% H2SO4 to get the
precipitate which was filtered, washed and dried to obtain
bpca as a white powder. Yield 56%. 1H NMR (300 MHz, DMSO-
d6): d = 7.86–7.87 (d, 2H), 8.04–8.06 (d, 2H), 8.41 (s, 2H), 8.48
(s, 1H), 13.28 (s, 3H) ppm. 13C NMR (300 MHz, DMSO-d6): d =
127.1, 129.47, 130.4, 131.4, 132.2, 140, 142.4, 166.3, 167 ppm.
MS (MALDI-TOF) calcd 286.05 found 309.1 [M + Na]+. FTIR:
n-BuLi in hexane (10.5 mL, 16.3 mmol) was added dropwise. A
red to light-green precipitation of the aryl lithium derivative
was formed. Predried gaseous carbon dioxide was passed into
the mixture at 260 uC to give a colorless precipitate of the
lithium salt. The mixture was allowed to warm and was
quenched with 50% aqueous acetic acid. The solid product
was filtered and recrystallized from acetic acid to give 0.8 g
1
(33%) of white microcrystals. H NMR (DMSO-d6): d = 8.06 (s,
12 H), 8.09 (s, 3 H) ppm, 13C NMR (300 MHz, DMSO-d6): d =
125.6, 127.4, 129.9, 130.0, 140.8, 143.8, 167.2 ppm. MS
(MALDI-TOF) calcd 438.1 found 439.2 [M + H]+. FTIR: 3071,
2985, 1691, 1608, 1419, 1318, 1294, 1245 cm21
.
Microscopy
3262, 1739, 1695, 1610, 1455, 1394, 1279 cm21
.
The morphology of the gel was observed through an optical
microscope (Leitz, Biomed) under perfectly crossed polarizers
and taking the picture through a digital camera (Leica D-LUX
3). FESEM experiments were performed by placing a small
portion of gel samples on a microscope cover glass. Samples
were dried in air and vacuum successively and coated with
platinum prior to observation. Micrographs were recorded by
using a Jeol Scanning Microscope JSM-6700F. TEM images
were taken using a JEOL electron microscope operated at an
accelerating voltage of 200 kV and these samples were
prepared through placing the gel-phase material on a TEM
grid (300 mesh carbon-coated Cu grid). AFM topologies were
recorded using atomic force microscopy (Veeco, model
AP0100) in noncontact mode at a tip resonance frequency of
300 kHz. Samples for the imaging were prepared by drop
casting the solution on freshly cleaved mica surface at the
required concentrations at ambient conditions.
Synthesis of 1,3,5-tri(4-bromophenyl)benzene (3)
4-Bromoacetophenone (10 g, 50.25 mmol), 0.5 mL of H2SO4(c)
and K2S2O7 (15 g, 59 mmol) were heated at 180 uC for 14 h
under nitrogen atmosphere. The resulting crude solid was
cooled to room temperature and refluxed in 50 mL of dry EtOH
for 1 h and then cooled to room temperature. The solution was
filtered and the resulting solid was refluxed in 50 mL of H2O,
giving a pale-yellow solid that was filtered. The crude product
was dried under vacuum and recrystallized in CHCl3 . Yield:
70%. 1H NMR (300 MHz, CDCl3): d = 7.53 (d, 6H), 7.60 (d, 6H),
7.68 (s, 3H) ppm. 13C NMR (300 MHz, CDCl3): d = 122.2, 125.1,
129,132.2, 139.7, 141.6 ppm.
Synthesis of 1,3,5-tris(4-carboxyphenyl)benzene (tpca)
1,3,5-Tri(4-bromophenyl)-benzene (3 g, 5.52 mmol) was dis-
solved in 40 mL of anhydrous THF under N2 atmosphere. The
stirred solution was cooled to 260 uC and a 1.6 M solution of
Scheme 1 Plausible modelling for the formation of different size of belt for different gel and their growth in the longitudinal direction.
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Fluorescence microscopy and spectroscopy
temperature to Tgel (93 uC), the stable gel is transformed into
sol phase, and the clear solution is finally obtained. The
process could be reversed to form the hydrogel by cooling the
solution to room temperature. The cycle has been repeated
many times and the gelation ability is not affected, indicating
a fully thermoreversible nature.
Optical microscope images (Fig. 2a, b) show the formation
of a fibrillar network like structure with each fibre having a
length of several millimetres. These long fibers are formed due
to extended intermolecular H-bonding between ADN and
tricarboxylic acids. The dried gels prepared by the deposition
of a gel sample followed initially by drying in air and finally in
vacuum are observed under an electron microscope. As shown
in Fig. 3, the field emission scanning electron microscope (FE-
Gel-phase material was placed on a glass microscope slide,
dried, and examined under
a fluorescence microscope
(OLIMPUS BX-61) in 40X magnification at excitation 360 nm.
Fluorescence spectral studies of ADN/bpca and ADN/tpca gel
samples prepared in a sealed cuvette were carried out in a
Horiba Jobin Yvon Fluoromax 3 instrument. The gel samples
were directly prepared in a quartz cell of 1 cm path length.
Fluorescence excitation acquisition and emission acquisition
were taken in slit width of 2/2 nm and scan rate was 0.2 s.
XRD study
The WAXS studies of the dried gels were performed by a Seifert
X-ray diffractometer (C-3000) using nickel filtered Cu-Ka
radiation with
a parallel beam optics attachment. The
instrument was operated at a 35 kV voltage and a 30 mA
current and was calibrated with standard silicon sample. The
samples were scanned from 2h = 2u at the step scan mode (step
size 0.03u, preset time 2 s), and the diffraction pattern was
recorded using a scintillation counter detector.
Rheology
Rheological experiments were performed with an AR 2000
advanced rheometer (TA Instruments) using cone plate
geometry in a Peltier plate. The plate diameter was 40 mm,
with a cone angle of 2 degrees. Two types of experiments were
performed: (i) by frequency sweep and (ii) by strain sweep
methods. The frequency sweep experiments were made at 25
uC at constant 1% strain. The strain sweep experiment was
performed at 25 uC at a constant frequency of 2 rad s21
.
FTIR spectroscopy
FT-IR spectra of bpca, tpca and dried gels were recorded using
KBr pellets of samples in an FTIR-8400S instrument
(Shimadzu). All sample were diluted with KBr and pelletized
prior to the experiment. Two independent experiments were
performed for each sample.
Results and discussion
Molecules chosen here for gelation studies were adenine
(ADN), 1,3,5-tris(4-carboxyphenyl)benzene (tpca),30 Biphenyl-
3,49,5-tricarboxylic acid (bpca).31 These two tricarboxylic acids
were synthesized according to Fig. 1a, b and characterized by
1H-NMR, HRMS, and FTIR. All these components have three
complementary binding sites through hydrogen bonding
…
…
either CLO H–N or C–O–H N. Both of the above two acids
have the strong possibility to form gel with adenine through
supramolecular interactions. The gelation ability is primarily
observed by a ‘‘stable to inversion of the test-tube’’ method
and it will be later proved by rheology. Appropriate amount of
tpca/bpca and ADN were taken into water at room temperature
and these mixtures were heated to form transparent liquid,
then cooled to room temperature to observe whether immobile
gel formed or not in the inverted test-tube. It has shown in the
Fig. 1d that the complete gelations of tpca/bpca with adenine
have occurred in water within 5 min. With the increase of the
Fig. 4 (a) Fluorescence excitation (filled symbol) and emission spectra (vacant
symbol) of pure ADN, pure tpca and ADN/tpca gel at room temperature
[pathlength 10 mm]. lex for the emission spectra of ADN = 280 nm, tpca = 370
nm, and ADN/tpca gel = 370 nm. (b) Fluorescence excitation (filled symbol) and
emission spectra (vacant symbol) of pure ADN, pure bpca and ADN/bpca gel. lex
for the emission spectra of ADN = 280 nm, bpca = 340 nm, and ADN/bpca gel =
340 nm.
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SEM) images demonstrate the network of belts that are formed
by self-assembled nanofibers for all gels. Careful investiga-
tions of these images at higher magnification (Fig. 3a–d) reveal
that belts having a width of 20–100 nm are then assembled to
form fatty belts having width of above 1–2 mm and a length of
several micrometers. Belts like network structures are
observed for both gels having different molar ratio of adenine
and tricarboxylic acids.32 The formations of such belt like
morphology is due to the long range inter molecular hydrogen
bonding interaction between ADN and tricarboxylic acids. It is
very interesting that ADN/tpca produces slim belts whereas
ADN/bpca forms fatty belts in all compositions. ADN/tpca
could self-assemble into nanoscale fibrous structures with
regular fiber diameters of ca. 20–100 nm whereas ADN/bpca
shows fibers having large diameter (500–600 nm). The
formation of such different size belts for the two different
gels can be explained from Scheme 1. As there is large
between two benzene ring in case of tpca, extended long-
itudinal p–p stacking is restricted for the ADN-tpca gel system
and a slim belt is formed. However, for the ADN-bpca system
the probability of non-planarity of the benzene plane is much
lower and the extended longitudinal p–p stacking that occurs
is responsible for fatty belt formation. Transmission electron
microscopy (TEM) images of dried gels prepared directly on a
carbon coated Cu-grid reveal the formation of supramolecular
nanobelts (50–60 nm in width) which have created a three-
dimensional network (Fig. 3e, 3f). Therefore hydrogelation has
occurred by immobilizing water molecules in the nanospaces
of the three-dimensional networks. AFM topology (Fig. 3g, 3h)
clearly reveals the presence of belt like morphology in the
dried gel. Individual belts are bundled up to form fatty belts
having large width of y1 mm. The AFM height image in
Fig. 3h, 3i shows the clear layer by layer assembly of the belts.
probability for the flipping of the single bond (Csp2–Csp2
)
Fig. 6 (a) WAXS of pure adenine (ADN), pure tpca, ADN/tpca11, ADN/tpca13,
and ADN/tpca31 dried gel, all the gels were caste on glass slide and dried under
air. (b) WAXS of pure adenine (ADN), pure bpca, ADN/bpca11, ADN/bpca13,
and ADN/bpca31 dried gel, all the gels were cast on a glass slide and dried
under air.
Fig. 5 (a) FTIR plot of pure adenine (ADN), pure tpca, ADN/tpca11, ADN/
tpca13, and ADN/tpca31 dried gel. (b) FTIR plot of pure adenine (ADN), pure
bpca, ADN/bpca11, ADN/bpca13, and ADN/bpca31 dried gel.
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As the optical density of gel at any composition is above the
instrumental limit of a UV-Vis spectrophotometer, we have
performed the fluorescence excitation and emission spectro-
scopy of the pure compounds in water and their hydrogels are
shown in Fig. 4. Only adenine in water produces the excitation
and emission peaks at 297 nm and 395 nm respectively,
whereas only tpca produces the excitation and emission peaks
at 348 nm and 407 nm respectively. bpca exhibits the
excitation and emission peaks at 328 nm and 385 nm
respectively. However, after making the gel by mixing ADN
with tpca/bpca solutions in 1 : 1 ratio, excitation and emission
spectra generate new peaks at 379 nm and 436 nm for ADN-
tpca, and 341 nm and 405 nm for ADN-bpca gel, respectively.
The red shift of excitation and emission peaks for tpca/bpca
occurs after gel formation and it is due to the definite
arrangement of adenine in the gel. To investigate the
luminescence properties of the gel, fluorescence microscopy
of all the dried gels has performed. Under the microscope, it
clearly generates blue fluorescent gel networks upon excitation
at 380 nm (Fig. 2c).
A low-molecular-weight gelator forms self-assembled nano-
fibers or nanotapes in a supramolecular gel through hydrogen
bonding and p–p interactions, which can be analyzed by FTIR
spectroscopy. FTIR spectra of tpca, bpca, adenine and gels are
presented in Fig. 5. Typical IR stretching bands arising from
the non-hydrogen bonded acid groups in tpca/bpca are
observed around 1692/1739 cm21, whereas the same in the
dried gels have appeared around 1672/1699 cm21 respectively.
So the carboxylic acid groups show a large shifting of
frequency, from 1692 to 1672 cm21 and 1739 to 1699 cm21
,
that indicates the strong intermolecular hydrogen bonding in
gel state.33 Similarly for adenine, two strong bands at around
1672 cm21(CLN) and 1603 cm21 (R–NH2 bending) and a weak
band at 3354 cm21 (nN–H, amine ADN) appeared in the pure
Fig. 7 (a) Rheological studies (frequency sweep) of hydrogels of different molar ratio at room temperature of ADN/tpca system. (b) Rheological studies (frequency
sweep) of hydrogels of different molar ratio at room temperature of ADN/bpca systems. (c) Rheological studies (strain sweep) of hydrogels of different molar ratio at
room temperature of ADN/tpca systems. (d) Rheological studies (strain sweep) of hydrogels of different molar ratio at room temperature of ADN/bpca systems, (filled
symbol indicates G9 and corresponding vacant symbol indicates G99).
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state. Whereas in the gel state the first two bands are shifted to
a lower frequency (1587 cm21 and 1546 cm21 for ADN/tpca gel
or 1604 cm21 and 1520 cm21 for ADN/bpca gel), surmising the
reduction of electron density on ring due to its involvement of
hydrogen bonding with tpca/bpca. This result clearly indicates
hydrogen-bonding-induced hydrogelation of adenine with
(frequency of oscillation at time ‘t’). In the sol state, G99(v) .
G9(v) (G9 y v2 and G99 y v; v the angular frequency) and in
the gel state G9(v) . G99(v) (G9 and G99 y vu), demonstrating
the dominant elastic behavior of the system.10,35 All three gels
(Fig. 7a, b) have been subjected to frequency sweep measure-
ments under a constant strain of 1% and it is apparent from
Fig. 7a that in the frequency range of 1 to 500 rad s21, ADN/
tpca 1 : 1 and ADN/tpca 1 : 3 gels show elastic storage moduli,
G9, which are always greater than the associated loss moduli
(G99) in the said frequency zone whereas similar for ADN/tpca
3 : 1 in the frequency range 1 to 350 rad s21. In Fig. 7b for
ADN/bpca 1 : 1, it is from 1 to 500 rad s21, for ADN/bpca 1 : 3,
1 to 250 rad s21, and for ADN/bpca 3 : 1, it is lowest, only 1 to
50 rad s21. It indicates that the viscoelastic behaviours of these
gels are dominated by an elastic nature and are dependent on
the composition. This kind of mechanical behaviour is
generally a characteristic feature observed for soft materials.
However, it is also clear from Fig. 7a, b that the 3 : 1
composition gives the least stable gel for both systems.
Similarly, strain sweep measurements (Fig. 7c, d) under a
constant frequency 2 rad s21 reveal that G9 is not always
greater than G99 in the entire range of the strain sweep. At a
lower value of strain, G9 is greater than G99. After a certain
strain is applied to the gel, it is transformed to a sol as
indicated by reversal of G9 and G99 in the plot.
tpca/bpca similar to
a common hydrogelator previously
reported.26b,26e From the FTIR spectroscopy, it can be
surmised that adenine and tpca/bpca are self-assembled into
nanofibers through a number of hydrogen-bonding interac-
tions. In our previous report, it was observed that only 1,3,5-
benzene tricarboxylic acid (1,3,5-B) formed a gel among three
isomers of benzene tricarboxylic acids.28 It was due to the
symmetrical position of complementary interaction sites
between adenine and 1,3,5-B. In the present instance, bpca,
as well as adenine, does not have molecular symmetry, still
these two produce gel in water. The symmetrical position of
hydrogen bonding interaction sites have oriented these two
moieties to produce short fibers which are entangled together
to form a network of belts which ceases the flow of water upon
tilting the gel tube. The intermolecular hydrogen bonds are
the driving force for the formation of stable gel.
X-ray powder diffraction has been frequently used for
ascertaining the molecular packing of gelator molecules in gel
networks. Fig. 6a, b show the XRD pattern of ADN, tpca and
bpca in powdered state and those in dried gel states, which
had been prepared from the gels of ADN/tpca/water. Clearly,
the XRD traces are all characterized by a group of well-resolved
reflections showing that the samples are all crystalline in
nature. The ADN/tpca dried gels in the Fig. 6a indicates the
presence of the main diffraction peaks (2h = 5.5u, 11.6u, 14.7u,
17.2u, 20.6u, 24.7u). The corresponding d-values are 1.6, 0.76,
0.60, 0.50, 0.40, and 0.36 nm respectively, and the interlayer
distance is 1.6 nm. Further examination of any trace of ADN/
bpca dried gel (Fig. 6b) indicates the presence of the main
diffraction peaks (2h = 4.6u, 9.1u, 13.7u, 18.3u, 22.9u, 27.6u). The
corresponding d-values are 1.9, 0.97, 0.64, 0.48, 0.38, and 0.32
nm respectively that follow a ratio of 1 : 1/2 : 1/3 : 1/4 : 1/5 : 1/
6, indicating layer stacking of assemblies34 and the interlayer
distance is 1.9 nm.
Rheological studies of gels are an important technique to
prove the formation of gel as gelation involves a transition
from a viscous fluid (sol state) to a semi-solid state (gel). The
viscoelastic behaviour of a system is generally characterized by
the storage modulus (G9) and loss modulus (G99). In an
oscillatory amplitude sweep experiment, G9 represents the
ability of the deformed gel to restore its original geometry (i.e.
measurement of elastic behaviour in solid-like gel), and G99
indicates the tendency of a material to flow under stress (i.e.
measurement of fluidity in gel). The dynamic modulus of
hydrogel system can be described by the relation between the
oscillatory stress and strain
so
where, G9 (storage modulus) =
so
cos d
eo
G99 (loss modulus) =
‘eo & do’ represent the amplitude of strain and stress. ‘d’ is
the phase difference between them and is equal to ‘‘vt’’
sin d
eo
Fig. 8 Photographs of the hydrogels before and after the removal of dyes (a)
methylene-blue, (b) rhodamine-6G, and (c) crystal violet.
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Fig. 9 UV-Vis spectra of dye solution kept contact with ADN-tpca hydrogel (a) methylene blue, (b) rhodamine 6G and (c) crystal violet after indicated hours.
Adsorption of dye molecules from their respective aqueous
solution was monitored by UV-visible spectroscopy and dye
uptake capacities of the hydrogels were determined by batch
kinetic sorption experiments. The adsorption capacity, Q
(milligram dye per gram gelator), of gelators was calculated
by using the following expression:
dyes. The process of dye uptake by the hydrogel is shown in
Fig. 8, which shows photographs of MB, Rh6G, and CV
removal for the two sets of hydrogels, ADN-tpca, and ADN-
bpca after 48 h. For the measurement of the kinetics of dye
extraction by the gelators, hydrogels were first prepared and
then these gels were kept in contact with the aqueous
solution of dyes. Fig. 9 (c) shows the maximum number of CV
dye molecules can be removed from water within 48 h.
Similarly the times taken for the removal of dyes like MB and
Rh6G dyes are 32 h and 40 h respectively. Plots of lmax as a
function of time for the two gel systems of the three different
dyes are presented in Fig. 10 which reveals a significant
difference in both the relative dye extraction and maximum
dye uptake for the gel systems. From Fig. 11, it is observed
ðCi-CeÞV
Qðmg=gÞ~
m
where, Ci and Ce are the initial and equilibrium
concentrations of dye (mg L21), respectively, V is the volume
of the solution added (L) and m is the amount of gelator (g).
To study the adsorption properties of these specific gelators,
we chose three dyes (methylene blue (MB), rhodamine 6G
(Rh6G) and crystal violet (CV)) which have absorption bands
above 450 nm for the determination of the relative dye
uptake of the gelators. In this regard, these hydrogels were
very efficient to adsorb three different aqueous solutions of
that the ADN-tpca gel system is
a more efficient bi-
component gel system for the absorption of dye than the
ADN-bpca system. It is also noticeable that the dye adsorp-
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Fig. 10 Plots of lmax vs. time (a) methylene-blue absorption at lmax = 664 nm, (b) rhodamine-6G absorption at lmax = 524 nm, and (c) crystal violet absorption at lmax
= 590 nm. Corresponding structures of the dyes are attached with the graphs, respectively.
tion efficiency of CV is very large (98.2%) compared to those
of the other two dyes, MB (76.1%) and Rh6G (82.4%) for the
Conclusion
In the present manuscript, we have reported the gelation
ADN-tpca gel system (Table 1). This may be due to the
abilities and physical properties of adenine with two different
hydrophobic interaction between the gelators and dye
tricarboxylic acids. Upon cooling the pre-warmed solution of
molecules in a proper way. These results indicate that our
adenine and 1,3,5-tris(4-carboxyphenyl)benzene (tpca) or
hydrogels are efficient to remove toxic dye molecules from
biphenyl-3,49,5-tricarboxylic acid (bpca) in water, hydrogels
water.
were formed which were thermo-reversible in nature and were
characterized with the help of various instrumental techniques
such as OM, FESEM, HRTEM, AFM, FL. XRD, FT-IR, rheology
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Fig. 11 (a) Graphical representation of the percentage of adsorption of dyes into the hydro gel matrix (computed from Fig. 9). (b) Schematic representation of the dye
adsorption by the hydro gel fibers.
etc. The entangled network composed of belts in the hydrogel
was observed and the dimensions of the belt depended on the
tricarboxylic acid used. The intermolecular hydrogen bonds
which were considered to be the driving force for the
formation of a stable gel were confirmed by FT-IR studies. In
spite of the absence of symmetry either in bpca or adenine,
these two moieties surprisingly have produced gels and it is
due to the symmetrical position of complementary interaction
sites. The mechanical strength of the hydrogel network as
revealed by rheological study depends on the tricarboxylic acid
used in the two-component systems and also on the composi-
tion of the fixed pair. These hydrogels were able to sequester
over 0.6 mg of dye per mg of gelator from aqueous solution,
with the ADN-tpca gel achieving a higher level of dye
adsorption for crystal violet dye than for other dyes. These
structure property relationship studies hold the future
promise for the realization of inexpensive and highly effective
purification systems for waste water.
Table 1 Adsorption data for hydrogels using various dyes
Wavelength (nm) of
Initial aqueous concentration
Quantity of the adsorbed dye in mg per mg of the gelators
light absorption maximum of the corresponding dyes
Name of dye
of dye solution only
[mM] (mg mL21
)
ADN-tpca gel
ADN-bpca gel
Methylene blue 664
1.0 (0.319)
1.0 (0.479)
1.0 (0.407)
6.34
7.21
7.86
6.02
7.18
7.48
Rhodamine 6G
Crystal violet
524
590
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