Silver-promoted gelation studies of an unorthodox chelating tripodal pyridine-pyrazole-based ligand: Templated growth of catalytic silver nanoparticles, gas and dye adsorption

Article abstract of DOI:10.1039/c3nj01334k

A novel tripodal pyridine-pyrazole based tris amide ligand, namely, N ¹,N³,N⁵-tris(3-(pyridin-2-yl)-1H-pyrazol-5-yl) benzene-1,3,5-tricarboxamide was synthesized. The ligand acts as a promising low molecular weight gelator that gels silver salts in presence of water. The gels formed were found to be highly stable to heat and stress. Moreover, silver ions not only act as a gelling agent but also participate in nanoparticulate formation. The three-dimensional ligand-silver gel matrix serves as an excellent platform for the growth of these silver nanoparticles. The gels were characterized thoroughly using IR spectroscopy, UV-visible spectroscopy, SEM, TEM and rheological studies. TEM pictures revealed the nanoparticles to be attached exclusively onto the gel fibres confirming template formation. The silver nanoparticles not only provide high strength and stability to the gel network but also show excellent catalytic properties in reducing 4-nitrophenol and Methylene Blue dye in presence of sodium borohydride. In addition to the catalytic properties, xerogels displayed gas and dye adsorption properties. This journal is the Partner Organisations 2014.

Full text of DOI:10.1039/c3nj01334k

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Silver-promoted gelation studies of an unorthodox
chelating tripodal pyridine–pyrazole-based ligand:
templated growth of catalytic silver nanoparticles,
gas and dye adsorption†
Cite this: New J. Chem., 2014,
38, 2470
Satirtha Sengupta, Arijit Goswami and Raju Mondal*
A novel tripodal pyridine–pyrazole based tris amide ligand, namely, N1,N3,N5-tris(3-(pyridin-2-yl)-1H-
pyrazol-5-yl)benzene-1,3,5-tricarboxamide was synthesized. The ligand acts as
a promising low
molecular weight gelator that gels silver salts in presence of water. The gels formed were found to be
highly stable to heat and stress. Moreover, silver ions not only act as a gelling agent but also participate
in nanoparticulate formation. The three-dimensional ligand–silver gel matrix serves as an excellent
platform for the growth of these silver nanoparticles. The gels were characterized thoroughly using
IR spectroscopy, UV-visible spectroscopy, SEM, TEM and rheological studies. TEM pictures revealed
the nanoparticles to be attached exclusively onto the gel fibres confirming template formation. The
silver nanoparticles not only provide high strength and stability to the gel network but also show
excellent catalytic properties in reducing 4-nitrophenol and Methylene Blue dye in presence of sodium
borohydride. In addition to the catalytic properties, xerogels displayed gas and dye adsorption
properties.
Received (in Montpellier, France)
29th October 2013,
Accepted 31st January 2014
DOI: 10.1039/c3nj01334k
www.rsc.org/njc
to the above functional groups, one or more metal coordinating
sites which may favour metallogelation,17–22 a special class of
Introduction
Over the past two decades, rapid development in the field of supramolecular gels. The use of metals in the self-assembling
supramolecular gel chemistry has been observed. Supramolecular process, not only strengthen gels via crosslinking, but also
gels constructed from low molecular weight gelators (LMWGs)1–5 offers two huge advantages over organogels; firstly, stable
act as soft materials which find useful applications in the field coordination bonds are involved in the metallogel formation
of drug delivery,6–8 tissue engineering,9,10 nano-particle growth which can impart thermal stability and high porosity in the
and its stabilization,11,12 light harvesting materials13–15 and resulting supramolecular network23,24 and secondly, with a
optoelectronic displays.16 Entrapment of sufficiently large proper choice of metals these metallogels can be tuned to render
amounts of solvent molecules inside the network is the essence magnetic, redox, spectroscopic and catalytic properties.25–28
of gel formation. This particular supramolecular prerequisite Such applications are clearly beyond the scope of metal-free
makes the design of LMWGs a challenging task and requires a organogels. Nevertheless, the aforesaid functional groups are
proper choice and correct arrangement of functional groups equally important in the metallogel formation as they provide
such as urea, thiourea, amides, phenyl, naphthyl, etc. Inclusion extra dimensionality, which is crucial, to the initially formed
of such functional groups often facilitates solvent entrapments one- or two-dimensional metal complex giving rise to three-
and subsequently formation of supramolecular gels by self- dimensional supramolecular network entrapping solvent mole-
assembling via hydrogen bonding, hydrophobic interactions, cules within them.
p–p stacking, and van der Waal’s interactions etc. Another
One of the intriguing applications of supramolecular gels,
popular yet emerging approach is to incorporate, in addition though rare, is that they can in situ act as template for
nanoparticle growth. In other words, the three-dimensional,
Department of Inorganic Chemistry, Indian Association for the Cultivation of
Science, 2A & 2B Raja S. C. Mullick Road, Jadavpur, Kolkata 700 032, India.
E-mail: icrm@iacs.res.in; Fax: +91 33 24732805; Tel: +91 33 24734971
intertwined fibrous network that constitutes the gel can in fact
supply the platform for growth of metal and semi-conductor
nanoparticles.29–33 However, in order to have any meaningful
application, these nanoparticles should not attach to the net-
work very strongly such that they become practically non-
displaceable. Slow and controlled release of these nanoparticles
† Electronic supplementary information (ESI) available: Picture of metallogels,
SEM image of hydrogel, FT-IR spectra, Job’s plot, SEM, TEM and EDX images, plots
of elastic modulus and viscous modulus of silver tetrafluoroborate and triflate gels
and probable structures of the gel network. See DOI: 10.1039/c3nj01334k
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out of the gel matrix into the reaction medium is often water/DMF mixture. In addition to nitrate, gels were also obtained
necessary for them to participate in chemical reactions. Notably, with other non-coordinating silver counteranions such as
ꢀ
silver nanoparticles are of great importance as they show catalytic, BF4 and CF3SO3ꢀ. The formation and strength of these silver
optical and antibacterial properties.34–37 Particles generated from metallogels were highly dependent not only on the concen-
gels offer an advantage over other nanoparticles in that their size tration of the silver salts used but also on the ratio of DMF–
and shape can be controlled and tuned by varying their synthesis water solvent mixture. The presence of water was found to be
media and techniques.
crucial for gel formation as gel strength progressively increased
We report herein, a novel tripodal pyridine–pyrazole based on increasing the water/DMF mixture ratio from 1 : 4 to 4 : 1.
tris-amide molecule N1,N3,N5-tris(3-(pyridin-2-yl)-1H-pyrazol- Ironically, the gelation experiment cannot be performed in
5-yl)benzene-1,3,5-tricarboxamide (TPPBT) which is capable of water alone as a small amount of DMF was necessary to
gelating various silver salts in aqueous media. Pyrazole based dissolve the ligand. Interestingly, gels formed from DMF-rich
ligands are of immense importance not only biologically as they solvent systems took longer time and an extended period of
are potent anti-bacterial and anti-fungal agents38–40 but also sonication to form fully, but the water-rich gels formed instantly,
chemically as the pyrazole group has inbuilt-hydrogen bonding thereby suggesting the role of water in stabilizing the coordination
donating and accepting sites as well as sites for metal coordi- sphere of the gelator molecule through hydrogen bonding inter-
nation. Incorporation of pyridine moiety adjacent to pyrazole actions. Our system actually represents a classic example which
facilitates chelation as the incoming metal can simultaneously emphasizes the role of covalent interactions in bringing strength
bind to both pyridine and pyrazole nitrogens. This is necessary to the resultant gel network. Actually, the covalent interactions of
in order to avoid the precipitation of a solid polymeric coordi- silver with the ligand is so strong that it imparts not only a high
nation polymer which is quite probable if a divergent ligand is tensile strength to the gel network (as suggested from rheological
used. Our recent results further confirm the usefulness of studies) but also high thermal stability supported by the fact that
converging ligands on metallogel formation.1 Thus by incor- the gels did not show any signs of weakening even when heated
porating a convergent juxtaposed pyridine–pyrazole group we beyond the boiling point of the solvent.
are restricting the formation of a solid coordination polymer
There exists a unique relationship between the stoichio-
and enhancing the chances of gel formation. Furthermore, the metry and photochromatic behaviour of the gels i.e. after a
pyrazole moiety can have a direct influence on gelation by certain concentration they change colour when exposed to
virtue of its hydrogen bonding ability which can entrap solvent light. Gels formed with 0.5 equivalents or 1 equivalent of silver
molecules. In this regard it is worth mentioning that an array of nitrate with respect to the ligand did not show any visual
tripodal ligands has already been reported in the literature that change of colour over a period of few months. However, the
supports organo- and metallogelation,41–45 most of them, how- gel obtained using 1.5 equiv. of silver showed colour trans-
ever, employing pyridine as the metal binding site. We present formation from light yellow to very light brown over a period of
here the first example where a convergent chelating pyridine– few days, thereby suggesting formation of silver nanoparticles.
pyrazole based tris-amide ligand has been utilized in metallo- On increasing the concentration to 2 equiv. of silver nitrate, the
gelation. The ligand, in addition to forming metallogels with rate of formation of nanoparticles was so increased that the
silver, serves as an excellent scaffold for growth of silver change of colour was visible within four hours of gel formation.
nanoparticles. The nanoparticles so formed were found to Moreover with increase in the concentration of silver salts, the
reduce catalytically 4-nitrophenol and Methylene Blue dye in colour gradually intensified indicating formation of a large
presence of sodium borohydride in aqueous medium. In addi- number of nanoparticles. Solvents too have a major role in
tion to the catalytic activity, gas adsorption and dye adsorption nanoparticle formation as gel obtained from a DMF-rich solvent
studies have also been performed which is seldom reported system showed more intense colour as compared to a water-rich
with gel materials.
solvent system. This may be due to the fact that the rate of
formation of nanoparticulate silver is dependent on the rigidity of
the three-dimensional gel network as the gels formed in water
rich solvents are much stronger than that formed in DMF-rich
solvent. As expected, the rate of nanoparticle formation is light-
Results and discussion
The ligand was prepared by the reaction of 1,3,5-benzene- dependent and can be delayed by storing the samples in the dark.
tricarbonyl trichloride with 3{5}-amino-5{3}-(pyrid-2-yl)-1H-
pyrazole in presence of triethylamine. The ligand was found
IR spectroscopy
to be soluble in hot THF, hot dioxane, DMF and DMSO and only Hydrogen bonding interaction of the amide groups plays a major
slightly soluble in other common organic solvents. A 1 wt% part in inducing a self-assembling process which was probed
(w/v) solution of the ligand prepared in DMF/water (3 : 2) using infrared spectroscopy. The ligand itself shows a strong
mixture yielded a weak and opaque hydrogel suggesting forma- absorption band at 1669 cmꢀ1 and a broad band at 3417 cmꢀ1
tion of larger particles, as supported by scanning electron corresponding to CQO stretching and N–H stretching vibrations
microscopy (Fig. S2, ESI†), due to a high degree of aggregation. of the amide group, respectively, while the band for N–H bending
Addition of silver nitrate gave instantaneously transparent, vibration appears at 1542 cmꢀ1 (see Fig. S3 in the ESI†). These
stronger hydrogel at an optimum solvent composition of 4 : 1 bands get shifted to 1663, 3411 and 1548 cmꢀ1, respectively,
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in the xerogel prepared from 1 eq. silver nitrate. Further low- proton thereby confirming their participation in the hydrogen
ering of the CQO and N–H stretching vibrations to 1660 and bonding interactions with each other and with the solvent
3409 cmꢀ1, respectively, coupled with an increase of N–H molecules. The aromatic and pyridine ring protons showed
bending vibration to 1550 cmꢀ1 was observed for 1.5 eq. silver an upfield shift with a slightly broadened signal indicating
nitrate which did not show any further shifts after increasing metal coordination and p–p stacking of the rings’ involvement
the concentration to 2 eq. silver nitrate. The results clearly in the gelation process.
explain the optimum composition of a 2 : 3 ligand–metal ratio
as the threshold concentration as complete shifts of the peaks
UV-Visible spectroscopy
were observed at 1.5 eq. metal concentration. The decrease in
the N–H stretching frequency and increase in N–H bending
frequency in the metallogel suggests that hydrogen bonding
with solvent molecules via N–H is increased in the metallogel.
Moreover, the stretching frequencies of the pyrazole and pyridine
C–N which appear at 1598 and 1491 cmꢀ1 in the ligand decreased
to 1590 and 1469 cmꢀ1, respectively, in the 1.5 eq. metallogel,
thereby suggesting metal coordination to both pyridine and
pyrazole nitrogens. In addition to the above mentioned peaks,
a strong absorption band due to nitrate appears at 1382 cmꢀ1 in
the xerogel.
The absorption properties of the ligand and its metal complexes
were analysed using UV-visible spectroscopy (Fig. S1, ESI†).
A 1 ꢁ 10ꢀ5 M solution of the ligand prepared in DMF/water
(1 : 1) showed two distinct absorption peaks at 252 and 285 nm
corresponding to p–p* transitions the intensity of which
increased at first and then decreased gradually on addition of
aliquots of silver nitrate without any significant shift in the
peak positions. A broad peak also appeared centered around
330 nm probably due to intraligand charge transfer (ILCT)
which showed an increase in intensity upon addition of silver
nitrate. Moreover, Job’s plot analysis revealed a 2 : 3 binding of
the ligand with the metal (Fig. 1c and Fig. S4 in the ESI†). This
opens up two possibilities in which the ligand can act as a six
coordinating site by chelating with both pyridine and pyrazole
nitrogens or show a triple coordinating nature by binding with
only the three pyridine nitrogens. As a result, the metal will
show a tetra-coordinate nature by binding pseudo-tetrahedrally
or a two coordinate nature by binding linearly. The UV-visible
spectrum of the gel prepared using 1.5 eq.of silver nitrate
revealed a small hump at around 450 nm which increased in
NMR spectroscopy
1H NMR spectroscopic studies (Fig. 2) were performed with the
ligand and its 1.5 eq. silver nitrate xerogel in DMSO-d6 to
ascertain the role of metal, p–p stacking and hydrogen bonding
interactions of TPPBT in the gelation process. The N–H signals
due to pyrazole and amide showed a slight upfield shift with a
Fig. 2 1H NMR spectra of TPPBT (red) and TPPBT + 1.5 eq. silver nitrate
broadened signal, typically observed for a hydrogen bonded xerogel (black).
Fig. 1 UV-Visible absorption spectra of (a) ligand titrated with aliquots of silver nitrate (runs of 0.2 upto 3 eq.), (b) gels obtained with 1, 1.5, 2.0, 2.5 equiv. silver
nitrate after four weeks in the dark showing silver nanoparticle formation and (c) Job’s plot analysis of TPPBT with silver nitrate in DMF/H2O (1 :1) medium.
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intensity as the concentration of silver nitrate increased 0.01 to 200%, keeping the frequency constant at 1 Hz and the
(Fig. 1b). The band can be assigned to a typical Mie plasmon temperature at 298 K (Fig. 3). In all the gel samples, the elastic
resonance excitation from the Ag nanoparticles.46 With the modulus (G0) was found to be higher than the viscous modulus
increase in concentration of silver nitrate, more silver nano- (G00) by ca. one order of magnitude, as suggestive of gel-like
particles are generated, which results in an increase in the materials. A fresh sample of the gel prepared from 1.0 eq.of
intensity and broadness of the band. The formation of silver silver revealed a yield stress of 45 Pa which increased to 54 Pa
nanoparticles was further confirmed by a powder X-ray diffrac- for 1.5 eq.of silver but decreased to 50 Pa on increasing the
tion technique (Fig. 6). Four distinct peaks at 2y values of 38.1, silver concentration to 2.0 equiv. These results further validate
44.3, 64.4 and 77.31 were observed corresponding to reflections the 2 : 3 ratio of ligand and metal as the optimum gelation
of (111), (200), (220) and (311) lattice planes of face centered concentration i.e. the condition at which the gel is found to be
cubic silver, respectively.
the strongest. Interestingly, with the growth of silver nano-
Even though the 2 : 3 ligand–metal ratio is optimum for particles in 2.0 equivalent silver nitrate gel, the yield stress was
gelation as all the coordinating sites of the ligand are fully found to increase to 160 Pa. This fact was more pronounced for
saturated, still very slight darkening of the gel sample prepared silver tetrafluoroborate gel where the yield stress was found to
using 1.5 eq.of silver reveals the generation of a smaller amount be as high as 200 Pa while the silver trifluoromethanesulfonate
of silver nanoparticles. This establishes the fact that even when gel shows a similar value as that of its nitrate congener (Fig. S6,
the ligand is fully coordinated to silver and no excess silver is left ESI†). These results indicate that the nanoparticles formed
unbound, formation of nanoparticles is inevitable as leaching out impart an extra tensile strength to the resulting gel network
of silver from the gel actually strengthens the resulting gel which in turn provides stability to the resultant gel.
network by occupying the voids in the networks. Thus our system
Scanning electron microscopy (SEM) and transmission electron
microscopy (TEM)
represents a rare example where silver ions not only dictate
gelation but also participate in nanoparticle formation.29,46,47
In order to study the morphology of the silver metallogel as well
as to confirm the formation of silver nanoparticles, scanning
Rheological studies
In order to probe the effect of silver nanoparticles on the electron microscopy (SEM) and transmission electron micro-
strength of the resulting gel network stress, sweep rheometry scopy (TEM) studies were performed (Fig. 4). The gel prepared
was performed in which the viscous and elastic moduli were with 1.5 eq.silver nitrate revealed a fibrous intertwined network
measured as a function of increasing the strain amplitude from of infinite length with spherical particles attached onto the
Fig. 3 Plots of elastic modulus (G0, blue) and viscous modulus (G00, cyan) of silver nitrate metallogel as a function of applied stress for fresh (a) 1 equiv.
(b) 1.5 equiv. (c) 2.0 equiv. gel and (d) 2.0 equiv. gel after nanoparticle formation.
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Fig. 4 TEM images of metallogels (clockwise from left) with 1 equiv. silver nitrate, 2.0 equiv. silver nitrate, SAED of silver nanoparticles and silver
nanoparticles prepared from 1.5 equiv. silver nitrate.
fiber (Fig. S5a and b in the ESI†). With increase in concen- the metal was fully saturated. Moreover, as the concentration of
tration of silver nitrate to 2 equivalent, the number as well as silver is reduced below that of the optimum concentration of
size of spherical particles increased as evident from the deeper 1.5 equiv, nanoparticles did not form. Indeed, a gel sample
and richer colour of the gel samples. Thus the size of the prepared from 1 eq. silver nitrate did not show any noticeable
nanoparticles can be tuned and controlled effectively by varying change in colour even after keeping it under light for several
the concentration of the silver nitrate. This may be due to the months which was further confirmed by UV-visible spectro-
fact that as the concentration of silver is increased, the excess scopy which showed no plasmon band generally observed due
or unbound silver accelerates the rate of formation of nano- to silver nanoparticles. As evident from Fig. 4 the particles were
particles which in turn results in the generation of bigger found attached exclusively onto the gel fibers, thereby ascer-
particles. However, formation of nanoparticles was not com- taining the fact that the ligand actually acts as a template for
pletely dependent on the concentration of excess silver present the growth of these nanoparticles. The size of all the particles
in the medium as silver did leach out of the gel network (which varied from 2–10 nm the majority of which being in the range
may be due to the dynamic nature of the coordination bonds) of 3–5 nm. Selected area electron diffraction (SAED) study
prepared using 1.5 equivalent silver nitrate slowly even though revealed the crystalline nature of these spherical particles.
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The first four circular electron diffraction lines arise due to nanoparticles have been explored in the literature.29,32,35,48
reflections from the (111), (200), (220), (311) lattice planes of Metal catalysed reduction of aromatic nitro compounds to
the face-centered cubic (fcc) silver. Moreover, an energy dis- aromatic amines in presence of hydrogen or metal hydrides is
persive X-ray (EDX) spectrum is indicative of the presence of of great importance industrially49 as is used in the prepara-
silver in these particles (Fig. S5c in the ESI†).
tion of paracetamol, a potent analgesic and antipyretic from
Thus based on the above mentioned facts, there exist two 4-nitrophenol. Thus, we tested the catalytic role of silver
possibilities of gel formation where we can propose two models nanoparticles synthesized by exploring the reduction of
in which either the silver can pseudo-tetrahedrally chelate to two 4-nitrophenol to 4-aminophenol using NaBH4 as the reaction
pyridine–pyrazole moieties of two different TPPBT molecules or can be monitored using UV-visible spectroscopy (Fig. 5a and b).
the metal can bind only to pyridine nitrogen leaving the pyrazole In absence of sodium borohydride, 4-nitrophenol shows an
group to participate in solvent entrapment through hydrogen absorption maximum at 320 nm which red-shifts to 403 nm
bonding giving rise to one- or two-dimensional network (Fig. S7 on addition of sodium borohydride due to the formation of
in the ESI†). Self-assembling via p–p stacking of the benzene ring 4-nitrophenolate ion in presence of alkaline medium. It is
or argentophilic interactions among the silver centers can further noteworthy here that in the absence of any catalyst, the
supply the extra dimensionality necessary to form gel, giving rise to reduction of 4-nitrophenol by sodium borohydride is very slow.
a three-dimensional network. The amide groups participate in On addition of hydrogel containing silver nanoparticles to the
hydrogen bonding with the solvent molecules by entrapping them reaction medium, a gradual decrease in intensity of 403 nm
within the entangled network and stabilizing the counter-anions. peak with time with a concomitant development of a new peak
Generation of silver nanoparticles, because of their multi- centred around 295 nm was observed within half an hour of the
purpose applications, has attracted a great deal of attention in catalyst addition due to formation of 4-aminophenol. The
recent times. Controlled and subsequently targeted release of these presence of an isosbestic point between the two absorption
nanoparticles remained the prime focus for most of these research bands further guarantees the presence of only the two reaction
works. In the present context, we have identified a prerequisite species, p-nitrophenol and p-aminophenol. It is to be noted
condition to generate nanoparticulate silver. Our system exhibits a that sodium borohydride was used in excess so it remained
unique relationship between a metal to ligand ratio and nano- essentially constant throughout the reaction. The reaction
particle formation. As evident from the above results, the nano- could also be monitored visually as the yellow colour of the
particles are formed only when an optimal metal to ligand ratio of initial reaction mixture gradually faded away with time to a
1.5 is achieved, with all the coordinating sites of silver being finally bleached colourless solution of 4-aminophenol.
occupied. In other words, for this particular system 1.5 eq.of silver
In order to study the catalytic rate, it is assumed that the
represent the threshold concentration for nanoparticle synthesis. catalytic cycle with respect to 4-nitrophenol follows pseudo-
Looking at the other side of the coin, it also represents the minimum first-order kinetics. As a result, the ratio of absorbance (At) of
concentration below which nanoparticles are not formed. Thus our 4-nitrophenol at time t to the absorbance (A0) at time t = 0 must
system not only functions as a stable reservoir for silver nano- be equal to the concentration ratio of Ct/C0 of 4-nitrophenol.
particles but can be envisaged as functioning as an ‘‘on/off’’ switch Thus the kinetic equation for the catalytic reduction can be
by which we can generate and regulate silver nanoparticles at will.
written as50
dCt/dt = ꢀkappꢂCt, or ln(Ct/C0) = ln(At/A0) = ꢀkappꢂt
Catalytic studies
An array of reactions involving catalytic reduction of nitro com- where Ct is the concentration of 4-nitrophenol at time t and kapp is
pounds with sodium borohydride in presence of noble metal the apparent rate constant. From the plot of ln (At/A0) versus time,
Fig. 5 (a) Sequential UV-Vis absorption spectra of 4-nitrophenol solution (0.05 mM) after adding aqueous solution of sodium borohydride (5 mM) in
the presence of Ag nanoparticles, (b) plot of ln(At/A0) versus time corresponding to the reduction of 4-nitrophenol by Ag nanoparticles at 25 1C,
(c) sequential UV-Vis spectra for aqueous Methylene Blue solutions by adding sodium borohydride solution (5 mM) in presence of 10 mg silver
nanoparticles containing hydrogel.
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decreased with time until they completely disappeared with
simultaneous bleaching of the blue colour. The silver nano-
particles attached hydrogel thus act as an electron relay system
which functions by transferring electrons from the donor
borohydride to the acceptor dye molecules. Finally, the reusability
of the silver nanoparticles embedded hydrogel was tested by
decanting the reduced solutions, washing the catalyst with
water and repeating the same reactions over several runs. Even
though the reduction rate decreased, the catalyst did not show
any loss in efficiency which suggests the silver nanoparticles
are sufficiently stable in presence of the hydrogel and hence
will find important applications in the field of catalysis.
Gas adsorption and dye adsorption studies
In order to probe the porous nature of the gel network, N2 adsorp-
tion and dye adsorption studies were performed on xerogel prepared
using 1.5 eq. silver nitrate. The xerogel was activated with the
following method prior to the sorption experiment. 100 mg of
xerogel was soaked in 1 : 1 DCM–methanol mixture and was
heated in vacuum at 60 1C for 12 h. Afterwards the xerogel was
outgassed for 16 h at 80 1C and the N2 adsorption measurement
was performed at liquid nitrogen temperature (77 K) within a
Fig. 6 Powder XRD patterns of silver nanoparticles of xerogels prepared
using 2 eq. silver nitrate.
a good linear correlation was obtained which validates the fact pressure range of 0 to 1 atm. As supported by the isotherm (Fig. 7a),
that the reaction indeed follows pseudo-first-order kinetics and the xerogel exhibits a moderate N2 uptake of B29 cc gꢀ1 with a
the apparent rate constant kapp can be calculated from the considerable BET surface area of B46.56 m2 ꢀ1. The Barrett–
g
above equation. The kinetic rate constant was calculated to be Joyner–Halenda (BJH) pore size distribution (Fig. 7b) shows most of
7.8 ꢃ 1.5 ꢁ 10ꢀ3 sꢀ1 at 25 1C.
the pores lying between B2.5 to B6.5 nm suggesting a char-
In another experiment, we tested the catalytic reduction acteristic mesoporous nature of the xerogel.
of Methylene Blue dye in presence of sodium borohydride
Dyes, especially azo-based compounds, which are used for
(Fig. 5c). Methylene Blue (MB) is actually a redox indicator industrial purposes are a potent water pollutant which affects
which is blue in an oxidising environment but turn colourless aquatic ecosystems. The physico-chemical processes employed
when reduced due to the formation of Leucomethylene Blue. to treat dye-laden wastewater are not only expensive but also
The reduction process however could not be carried out with require a pretreatment step. Adsorption techniques, on the
sodium borohydride alone in absence of any suitable catalyst as other hand, provide a much cheaper alternative to dye removal
the reaction is very slow. So our hydrogel containing silver nano- if the adsorbent is inexpensive sans the pretreatment step.
particles was employed to study and monitor the reduction In view of the above mentioned facts, we tested our xerogel for
process using UV-visible spectrometry. In an aqueous medium, the adsorption of toxic dye, Methyl Orange. The ligand is unique
MB shows an absorption peak at 663 nm due to n–p* transition in having hydrophilic groups like amide, pyrazole moieties
and a broad shoulder at 615 nm. In presence of nanoparticles which can interact with water molecules and hydrophobic
containing hydrogel, the intensity of the peaks gradually groups such as phenyl, pyridyl rings which can interact with
Fig. 7 (a) N2 adsorption–desorption isotherm of silver nitrate xerogel, (b) BJH pore distribution for silver nitrate xerogel.
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Fig. 8 (a) UV-Visible spectra of Methyl Orange solution after addition of silver nitrate xerogel taken at 5 min time interval, (b) plot of the amount of dye
adsorbed per gram of xerogel at any time versus contact time, (c) plot of percentage dye removal as a function of time and (d) noticeable colour change
illustration (from orange to colourless) of Methyl Orange after 4 hours due to dye adsorption by xerogel.
the dye molecules which are the pre-requisite conditions for serves as an excellent platform for the growth of silver nano-
effective dye removal. Even though dendritic polymers,51 clays52 particles thus acting as a template for nanoparticle synthesis.
and siliceous materials53 have been employed for dye removal, Our system represents a rare example where silver both acts as a
supramolecular gels also provide an effective and non-expensive gelling agent as well as participates in nanoparticle formation.
way for dye adsorption owing to their large surface area and The silver nanoparticles, in addition to providing high stability
porous nature. In a typical experiment, 4 mg of xerogel was to the resultant gel network, show excellent catalytic properties.
soaked in 2 ml of dye solution prepared by dissolving 3 mg of the The xerogel constructed from 1.5 eq. silver nitrate showed gas
dye in 100 ml water and the adsorption rate was monitored adsorption and dye adsorption properties. The fact that silver
(at 465 nm) periodically (every five minutes) using UV-visible ions can leach out of the gel matrix supports the interesting role
spectroscopy (Fig. 8a). A sharp decrease in the absorption of the ligand in providing a platform and a very novel method for
intensity was observed within the first 5 min and adsorption successful generation and stabilization of silver nanoparticles
was complete (i.e. reached equilibrium) within 4 h of commence- which can further be released in a controlled manner into the
ment of the experiment. The xerogel was found to be highly reaction medium in order to participate in various catalytic
effective in dye adsorption with a dye removal percentage of 94% reactions.
(Fig. 8b) and adsorption capacity of 14.18 mg gꢀ1 (Fig. 8c).
Experimental
Conclusions
1,3,5-Benzenetricarbonyl trichloride was commercially available
In conclusion, we have synthesized a novel tripodal pyridine– (Aldrich) and used without further purification. FT-IR spectra
pyrazole based chelating tris-amide molecule which is capable were performed on a Nicolet MAGNA-IR 750 spectrometer with
of gelating silver salts in presence of water. The gels were found the KBr pellets containing the samples. 1H spectrum was recorded
to be highly stable towards heat and stress. The gel matrix on Bruker spectrometers operating at 400 MHz in DMSO-d6 solvent.
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Mass spectra were collected from Micromass Q-Tof Micro
TEM. A drop of 10ꢀ3 M gel solution was placed on a 300 mesh
instrument and UV-visible studies were performed in Perkin carbon-coated copper grid, dried under vacuum and mounted
Elmer Lambda 950 UV/VIS instrument. The elemental analyses on JEOL JEM 2010 high-resolution microscope instrument for
were carried out using a Perkin–Elmer 2400 Series-II CHN TEM imaging operating at an accelerating voltage of 200 kV.
analyser. EPR spectra were recorded on a JEOL instrument.
Spectral studies. UV-Visible studies were performed on Perkin
Electron microscopic studies were made using a JEOL, JMS- Elmer Lambda 950 UV-vis-NIR scanning spectrophotometer taking
6700F field emission scanning electron microscope (FESEM) 1 ꢁ 10ꢀ5 M ligand solution prepared in 1 : 1 volume ratio of DMF/
and JEOL JEM 2010 high-resolution microscope instrument for water and steps of measured aliquots of the corresponding silver
TEM images. Rheology experiments were performed in SDT Q salts were added sequentially, keeping the metal salt solution
Series Advanced Rheometer AR 2000.
without the ligand as background reference.
3{5}-Amino-5{3}-(pyrid-2-yl)-1H-pyrazole
Acknowledgements
The compound was prepared according to a previously reported
procedure.54
RM gratefully acknowledges Science and Engineering research
Board (SERB) (Project No. SR/S1/IC-65/2012) India, for financial
assistance. S.S.G. and A.G. are thankful to CSIR, India for
Senior Research Fellowships. S.S.G. also acknowledges Anindita
Das, Polymer Science Unit, for her assistance in Rheology
experiments, picture of the metallogels, SEM image of hydrogel,
FT-IR spectra, Job’s plot, SEM, TEM and EDX images, plots of
elastic modulus and viscous modulus of silver tetrafluoroborate
and triflate gels and probable structures of the gel network.
N1,N3,N5-Tris(3-(pyridin-2-yl)-1H-pyrazol-5-yl)benzene-
1,3,5-tricarboxamide (TPPBT)
1,3,5-Benzenetricarbonyl trichloride (0.796 g, 3 mmol) was
dissolved in dry acetonitrile and to it was added an acetonitrile
solution of 3{5}-amino-5{3}-(pyrid-2-yl)-1H-pyrazole (1.44 g, 9 mmol),
giving an immediate yellow precipitate. The reaction mixture was
refluxed overnight, cooled and filtered. The precipitate was washed
with acetonitrile and dried in vacuum. To the suspension of the
hydrochloride salt in methanol was added excess of triethylamine
and stirred at rt overnight. The precipitated solid was filtered,
washed with water, acetone, dried and purified from methanol–
chloroform (1 : 1). Yield 1.0 g (52%). Elem. Anal. for C33H24N12O3,
Calcd: C, 62.26; H, 3.80; N, 26.40; found: C, 62.54; H, 3.78; N, 26.19.
1H NMR spectra/d (ppm) (400 MHz, DMSO-d6): 13.27 (s, 3H,
pz NH), 11.11 (s, 3H, amide NH), 8.78 (s, 3H, aromatic CH), 8.64
(d, 3H, py C–H), 7.9 (dd, 3H, py C–H), 7.37 (m, 6H, py C–H), 7.32
(s, 3H, pz C–H). IR (KBr, cmꢀ1): 3417 (s, amide N–H), 3265
(br, pyrazole N–H), 1669 (s, CQO), 1598 (s, pyrazole C–N), 1546
(s, N–H amide), 1491 (s, pyridine C–N), 1307 (s), 1251 (s), 1000 (s),
966 (s), 783 (s).
References
1 S. Sengupta and R. Mondal, Chem.–Eur. J., 2013, 19, 5537–5541.
2 L. E. Buerkle and S. J. Rowan, Chem. Soc. Rev., 2012, 41,
6089–6102.
3 J. W. Steed, Chem. Commun., 2011, 47, 1379–1383.
4 J. H. van Esch, Langmuir, 2009, 25, 8392–8394.
5 P. Dastidar, Chem. Soc. Rev., 2008, 37, 2699–2715.
6 G. John, B. V. Shankar, S. R. Jadhav and P. K. Vemula,
Langmuir, 2010, 26, 17843–17851.
7 A. R. Hirst, B. Escuder, J. F. Miravet and D. K. Smith, Angew.
Chem., Int. Ed., 2008, 47, 8002–8018.
8 Z. Yang and B. Xu, J. Mater. Chem., 2007, 17, 2385–2393.
9 L. E. Beurkle, H. A. von Recum and S. J. Rowan, Chem. Sci.,
2012, 3, 564–572.
Gelation test. To 10 mg of the ligand dissolved in 0.2 ml of
DMF kept in a screw-capped sample vial, was added 0.8 ml of
the metal salt solution. Instant gels were obtained which were
tested by the ‘‘stable-to-inversion of the vial’’ method.
Rheological studies. The rheology experiment was done with 10 W. T. Truong, Y. Su, J. T. Meijer, P. Thordarson and F. Braet,
a cone and plate measuring system with the plate having a Chem.–Asian J., 2011, 6, 30–42.
diameter of 40 mm and a cone angle of 41. 1 wt% (w/v) of both 11 J. H. Lee, S. Kang, J. Y. Lee and J. H. Jung, Soft Matter, 2012,
the copper chloride and copper bromide gel were prepared, 8, 6557–6563.
sonicated and left undisturbed overnight at room temperature 12 M.-O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft Matter,
prior to the measurement. The distance between the cone 2011, 7, 2412–2418.
and the plate was so adjusted to remove any air gaps between 13 A. Dawn, T. Shiraki, S. Haraguchi, S. Tamaru and S. Shinkai,
them. The gel was transferred on the plate and a stress sweep Chem.–Asian J., 2011, 6, 266–282.
experiment was done on the gels by applying stress from 14 S. Banerjee, R. K. Das and U. Maitra, J. Mater. Chem., 2009,
0.1–1000 Pa at a constant temperature of 25 1C. The storage 19, 6649–6687.
modulus (G0) and the loss modulus (G00) were plotted against 15 A. Ajayaghosh, V. K. Praveen and C. Vijayakumar, Chem. Soc.
oscillatory stress.
Rev., 2008, 37, 109–122.
16 S. Diring, F. Caramel, B. Donnio, T. Dintzer, S. Toffanin,
R. Capelli, M. Muccini and R. Ziessel, J. Am. Chem. Soc.,
2009, 131, 18177–18185.
Microscopic studies
SEM. A drop of 0.2 wt% of gel solution was placed on a glass
cover slip, dried under vacuum and mounted on JEOL-6700F 17 A. Y-Y. Tam and V. W-W. Yam, Chem. Soc. Rev., 2013, 42,
microscope instrument for SEM imaging.
1540–1567.
2478 | New J. Chem., 2014, 38, 2470--2479
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

View Article Online
Paper
NJC
18 O. Kotova, R. Daly, C. M. G. dos Santos, M. Boese, P. E. 36 W. R. Li, X. B. Xie, Q. S. Shi, H. Y. Zeng, Y. S. Ou-Yang and
Kruger, J. J. Boland and T. Gunnlaugsson, Angew. Chem.,
Int. Ed., 2012, 51, 7208–7212.
19 W. J. Gee and S. R. Batten, Chem. Commun., 2012, 48,
4830–4832.
20 R. E. Bachman, A. J. Zucchero and J. L. Robinson, Langmuir,
2012, 28, 27–30.
Y. B. Chen, Appl. Microbiol. Biotechnol., 2010, 85, 1115–1122.
37 S. Porel, S. Singh, S. S. Harsha, D. N. Rao and T. P.
Radhakrishnan, Chem. Mater., 2005, 17, 9–12.
38 O. Prakash, D. K. Aneja, P. Lohan, K. Hussain, S. Arora,
C. Sharma and K. R. Aneja, Med. Chem. Res., 2012, 21,
2961–2968.
21 Y. He, Z. Bian, C. Kang and L. Gao, Chem. Commun., 2011, 39 O. Prakash, R. Kumar and V. Parkash, Eur. J. Med. Chem.,
47, 1589–1591. 2008, 43, 435–440.
22 M-O. M. Piepenbrock, G. O. Lloyd, N. Clarke and J. W. Steed, 40 R. Aggarwal, V. Kumar, P. Tyagi and S. P. Singh, Bioorg. Med.
Chem. Rev., 2010, 110, 1960–2004. Chem., 2006, 14, 1785–1791.
23 A. Mallick, E.-M. Schon, T. Panda, K. Sreenivas, D. D. Diaz 41 T. Nakagawa, M. Amakatsu, K. Munenobu, H. Fujii and
and R. Banerjee, J. Mater. Chem., 2012, 22, 14951–14963. M. Yamanaka, Chem. Lett., 2013, 42, 229–231.
24 S. Samai and K. Biradha, Chem. Mater., 2012, 24, 1165–1173. 42 S. Banerjee, N. N. Adarsh and P. Dastidar, Soft Matter, 2012,
¨
25 J. Zhang, X. Wang, L. He, L. Chen, C. Y. Su and S. L. James,
New J. Chem., 2009, 33, 1070–1075.
8, 7623–7629.
43 B. Li, L. Tang, L. Qiang and K. Chen, Soft Matter, 2011, 7,
26 T. W. Assenmacher, H. Peterlik, R. Weisbarth, M. Nieger
963–969.
¨
and K. H. Dotz, Angew. Chem., Int. Ed., 2007, 46, 6368–6371. 44 Y.-R. Liu, L. He, J. Zhang, X. Wang and C.-Y. Su, Chem.
27 W. Weng, J. B. Beck, A. M. Jamieson and S. J. Rowan, J. Am.
Chem. Soc., 2006, 128, 11663–11672.
28 S. Kawano, N. Fujita and S. A. Shinkai, J. Am. Chem. Soc.,
2004, 126, 8592–8593.
29 J. H. Lee, S. Kang, J. Y. Lee and J. H. Jung, Soft Matter, 2012,
8, 2557–2563.
30 M.-O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft
Matter, 2011, 7, 2412–2418.
Mater., 2009, 21, 557–563.
45 D. Bardelang, F. Camerel, R. Ziessel, M. Schmutz and
M. J. Hannon, J. Mater. Chem., 2008, 18, 489–494.
46 M.-O. M. Piepenbrock, N. Clarke and J. W. Steed, Soft
Matter, 2011, 7, 2412–2418.
47 J. Nanda, B. Adhikari, S. Basak and A. Banerjee, J. Phys.
Chem. B, 2012, 116, 12235–12244.
48 S. Wunder, F. Polzer, Y. Lu, Y. Mei and M. Ballauff, J. Phys.
Chem. C, 2010, 114, 8814–8820.
31 F. Muench, M. Rauber, C. Stegmann, S. Lauterbach,
U. Kunz, H.-J. Kleebe and W. Ensinger, Nanotechnology, 49 A. M. Tafesh and J. Weiguny, Chem. Rev., 1996, 96, 2035–2052.
2011, 22, 415602–415610.
32 F. M. Kelly and J. H. Johnston, ACS Appl. Mater. Interfaces,
2011, 3, 1083–1092.
33 S. Dutta, A. Shome, S. Debnath and P. K. Das, Soft Matter,
2009, 5, 1607–1620.
50 K. S. Shin, J.-Y. Choi, C. S. Park, H. J. Jang and K. Kim, Catal.
Lett., 2009, 133, 1–7.
51 M. Arkas, D. Tsiourvas and C. M. Paleos, Chem. Mater., 2005,
17, 3439–3444.
˘
52 A. Gu¨rses, Ç. Dogar, M. Yalçın, M. Açıkyıldız, R. Bayrak and
34 K. D. O. Santos, W. C. Elias, A. M. Signori, F. C. Giacomelli,
S. Karaca, J. Hazard. Mater., 2006, 131, 217–228.
H. Yang and J. B. Domingos, J. Phys. Chem. C, 2012, 116, 53 A. Sayari, S. Hamoudi and Y. Yang, Chem. Mater., 2005, 17,
4594–4604. 212–216.
35 S. Saha, A. Pal, S. Kundu, S. Basu and T. Pal, Langmuir, 2010, 54 C. M. Pask, K. D. Camm, C. A. Kilner and M. A. Halcrow,
26, 2885–2893. Tetrahedron Lett., 2006, 47, 2531–2534.
This journal is ©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014
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