Robust fibrillar nanocatalysts based on silver nanoparticle-entrapped polymeric hydrogels

Article abstract of DOI:10.1016/j.apcata.2014.06.026

Robust fibrillar network of hydrogels entrapped with silver nanoparticles were prepared by in situ polymerization of acrylic acid and reduction of silver nitrate using amidodiol as cross-linking cum reducing agent under ambient conditions. Silver nanoparticle-entrapped polyacrylic acid-amidodiol hydrogels (SPAGs) were characterized by UV-visible spectroscopy, SEM, TEM, XRD and rheology. Further, catalytic activities of SPAGs were studied using dyes such as methylene blue, rhodamine 6G and crystal violet. Effects of temperature and pH on the reduction process were studied. The activation energy for the reduction process was calculated and found to be decreased by ～10 kJ/mol in the SPAG-catalysed reaction. In addition, SPAGs showed excellent reusability, easy separation, high rate constant and absence of induction period. All these results suggested SPAGs as a promising catalyst for the reduction of organic molecules.

Full text of DOI:10.1016/j.apcata.2014.06.026

Applied Catalysis A: General 483 (2014) 31–40
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Robust fibrillar nanocatalysts based on silver nanoparticle-entrapped
polymeric hydrogels
Rohini Kuttiplavil Narayanan, Sudha Janardanan Devaki∗, Talasila Prasada Rao
Chemical Sciences and Technology Division, CSIR – National Institute for Interdisciplinary Science and Technology, Thiruvananthapuram 695019, India
a r t i c l e i n f o
a b s t r a c t
Article history:
Robust fibrillar network of hydrogels entrapped with silver nanoparticles were prepared by in situ poly-
merization of acrylic acid and reduction of silver nitrate using amidodiol as cross-linking cum reducing
agent under ambient conditions. Silver nanoparticle-entrapped polyacrylic acid–amidodiol hydrogels
(SPAGs) were characterized by UV–visible spectroscopy, SEM, TEM, XRD and rheology. Further, catalytic
activities of SPAGs were studied using dyes such as methylene blue, rhodamine 6G and crystal violet.
Effects of temperature and pH on the reduction process were studied. The activation energy for the
reduction process was calculated and found to be decreased by ∼10 kJ/mol in the SPAG-catalysed reac-
tion. In addition, SPAGs showed excellent reusability, easy separation, high rate constant and absence of
induction period. All these results suggested SPAGs as a promising catalyst for the reduction of organic
molecules.
Received 17 April 2014
Received in revised form 11 June 2014
Accepted 24 June 2014
Available online 2 July 2014
Keywords:
Catalyst
Kinetics
Morphology
Swelling
Dyes
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
properties. Silver-catalysed reactions such as Diels Alder reaction,
epoxidation, hydrogenation of aldehydes, hydroformylation, car-
In recent years, there has been considerable interest in polymer-
dispersed metal nanocomposites, particularly polymeric hydrogel
metal nanocomposites which respond to various stimuli in the bulk
phase. They were used in responsive artificial muscles, switches,
memory devices, catalysis, tissue engineering, etc. [1–5]. Hydro-
gels are three-dimensional polymeric fibrillar networks which can
imbibe large amount of water with stimuli-responsive swelling
and de-swelling properties. Covalent and non-covalent interactions
such as hydrogen bonding, ␲–␲ stacking, and van der Waal’s inter-
actions are involved during the organization of these molecules into
3D fibrillar architectures [6–8].
The unique properties of metal nanoparticles are different from
their bulk counter parts and they find promising applications in
the areas such as nanoelectronic devices, catalysis, sensors, bio-
chemical tagging reagents, optical switches and so forth [9–12].
Among the metal nanoparticles, silver is easily reducible and
has high excitation efficiency, surface plasmon resonance energy
in the visible region and unique electrical, optical and catalytic
properties. They are potential candidates in catalysis because of
their high surface area and size and shape-dependent catalytic
bonylation, reduction of dyes, nitrophenol, etc. have been reported
earlier [13–17]. Metal nanoparticles tend to aggregate during the
catalytic process due to their van der Waal’s forces of attraction
and high surface area. A wide range of capping agents such as block
copolymers, polysaccharides, latex particles, hydrogels and surfac-
tants have been used to enhance the stability and dispersion of
metal nanoparticles [18–20]. Among these stabilizers, polymeric
hydrogels with three-dimensional fibrillar networks are receiving
importance. The hydrophilic ionic groups present in the hydrogel
network can potentially serve as nanoreactors for the synthesis of
metal nanoparticles and provide cage effect for its stabilization.
Moreover, the fibrillar network can speed up the diffusion of metal
ions through the hydrogel networks. Apart from this, the size and
shape of metal nanoparticles could be tuned by controlling the
amount of monomer, cross-linker and metal ions [21–23].
Polymeric hydrogel metal nanocomposites are viable catalysts
because they prevent aggregation of nanoparticles, and the loosely
bound dynamic structure will give easy access to the nanopar-
ticles. One of the disadvantages of such systems is their poor
mechanical stability. The mechanical and chemical properties of
hydrogel can be improved by incorporating inorganic clays, cel-
lulose, carbon nanotubes, etc. [24,25]. Mechanical stability of the
hydrogels can be build up by increasing the crosslinking den-
sity using rigid rod-like functional molecules as crosslinker. In the
present work, we have used amidodiol(1,6-bis(hydroxy butyra-
mido)hexane) as the crosslinking agent which is obtained by the
∗
Corresponding author. Tel.: +91 471 2515316.
E-mail addresses: rohinikn007@gmail.com (R. Kuttiplavil Narayanan),
sudhajd2001@yahoo.co.in (S. Janardanan Devaki), tprasadarao07@gmail.com
(T. Prasada Rao).
http://dx.doi.org/10.1016/j.apcata.2014.06.026
0926-860X/© 2014 Elsevier B.V. All rights reserved.

32
R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
aminolysis of ␥-butyrolactone with hexamethylene diamine. Ami-
dodiol is endowed with two terminal hydroxyl groups connected
through a hexyl linked two amide groups. They are expected to
exhibit extensive hydrogen bonding to form intermolecular phys-
ical crosslinks with polyacrylic acid. The networks present in the
gel may trap silver nanoparticles and also enhance its proxim-
ity to reagents. In this context, preparation of robust hydrogel
network entrapped with silver nanoparticles with high mechan-
ical stability and catalytic activity is attracting significance. In
this paper, we are reporting a novel strategy for the preparation
of silver nanoparticle-entrapped robust polymeric hydrogel and
demonstrated its efficient catalytic activity for the reduction of
dyes. SPAGs were characterized by SEM, TEM, XRD, rheology and
UV–visible spectroscopy. Further, efficiency of catalytic activity of
these robust hydrogel was studied for the reduction of organic dyes
using sodium borohydride as the reducing agent.
by periodically weighing the hydrogel. The swelling percentage of
SPAGs was calculated using the following equation:
mt − m0
S (%) =
× 100
m0
Here, m0 is the initial mass and mt is the mass of swollen gel at
time t.
2.5. Dye reduction studies
Aqueous solutions of cationic dyes such as rhodamine
6G (240 mg/L), methylene blue (160 mg/L) and crystal violet
(200 mg/L) were prepared. The dye removal studies were carried
out by taking 20 mL of each dye solution, 2 mL of 10 mM sodium
borohydride solution and 20 mg of SPAG catalyst. The progress of
the reaction was monitored by measuring the periodic decrease in
the UV absorbance with small aliquots of solution from the reaction
mixture. Effect of silver concentration, pH and temperature on the
catalytic reduction was studied.
2. Experimental
2.1. Materials
2.6. Characterization techniques
Hexamethyelene diamine, rhodamine 6G (Sigma–Aldrich), ␥-
butyrolactone (Fluka, Germany), isopropanol, acrylic acid, silver
nitrate, sodium borohydride, ammonium persulphate (E-Merk,
India), methylene blue, crystal violet (Nice Stains, India).
FT-IR spectroscopic measurements were made with a fully com-
puterized Nicolet Impact 400D FT-IR spectrophotometer. Materials
were mixed thoroughly with potassium bromide. All spectra were
corrected for the presence of moisture and carbon dioxide in the
optical path. The experiments were performed for a scan of 45
times and with a resolution of 4 cm−1. X-ray diffraction studies
were done with X-ray diffractometer (Philips X’pert Pro) with Cu
K␣ radiation (ꢀ ∼ 0.154 nm) employing X’celarator detector and a
monochromator at the diffraction beam side. The d-spacing of the
nanocomposite was calculated from the angular positions 2ꢁ of
the observed d001 reflection peaks based on the Bragg’s formula
nꢀ = 2d sin ꢁ, where ꢀ is the wavelength of the X-ray beam and ꢁ is
the diffraction angle. Averaged 2ꢁ was used with the 2ꢁ resolution
of 0.002◦ from 2◦ to 70◦. For SEM measurements, samples were sub-
jected for thin gold coating using a JEOL JFC-1200 fine coater and the
probing side was inserted into JEOL JSM-5600 LV scanning electron
microscope. TEM measurements were carried out using FEI (TEC-
NAI G2 30 S-TWIN) with an accelerating voltage of 100 kV. For TEM
measurements, the samples were casted on a carbon-coated copper
grid and dried in vacuum at room temperature before observation.
Optical properties of the gels were studied by absorption spectra
in the range 200–700 nm using UV–visible spectrophotometer (Shi-
madzu model 2100). Rheological measurements of the sample were
conducted with Anton Paar Physica MCR 150 rheometer with par-
allel stainless steel plates of 50 mm diameter, and the gap between
parallel plates was set 1 mm.
2.2. Preparation of amidodiol(1,6-bis(hydroxy
butyramido)hexane)
Amidodiol(1,6-bis(hydroxybutyramido)hexane) was prepared
by the aminolysis of ␥-butyrolactone using hexamethylene
diamine. Typical procedure is as follows: a solution of 0.2 mol
(17.2 g) of ␥-butyrolactone in 10 mL of isopropanol was taken
in a conical flask and cooled in ice bath at 5 ◦C. A solution of
11.62 g (0.1 mol) of hexamethylene diamine in isopropanol was
added dropwise with stirring for 2 h (800 rpm) and kept overnight.
Formation of the product was confirmed using TLC in a 7:2:1 ben-
zene, methanol and triethylamine mixture. White crystalline solid
formed was filtered and washed several times with isopropanol and
then dried in vacuum. Further, it was recrystallized from 150 mL of
methanol and acetone mixture. Scheme for the synthesis of ami-
dodiol was shown in the supporting information (Scheme S1).
2.3. Preparation of silver nanocluster-entrapped polyacrylic
acid-amidodiol (SPAG)
Acrylic acid (1 mL) and 10% amidodiol (1 mL) in water was mixed
with 0.2 mL of 0.02 M silver nitrate. Two drops of 10% ammonium
persulphate was added to the mixture. The content was shaken well
and kept at room temperature for 10–20 min for gel formation. It
was washed several times with distilled water to remove residual
monomers and ions. The prepared gel was kept at 70 ◦C for 12 h
to ensure the complete reduction of silver nanoparticles. Experi-
mental details of preparation of SPAGs are given in the supporting
information (Table S1). Hydrogel was also prepared in the absence
of silver nitrate and is designated as PAG.
3. Results and discussion
3.1. Preparation and characterization of silver
nanoparticle-entrapped polyacrylic acid-amidodiol hydrogels
(SPAGs)
SPAGs were prepared by in situ reduction of Ag+ ions and poly-
merization of acrylic acid at room temperature using ammonium
per sulphate (APS) as radical initiator and 1,6-bis(hydroxy butyra-
mido)hexane (amidodiol) as reductant cum physical crosslinking
agent as shown in Scheme S2. Amidodiol exhibit extensive hydro-
gen bonding with acrylic acid and increases the crosslinking density
and functionality of the hydrogel because of the presence of two
terminal hydroxyl groups and two amide groups. The amide group
can also interact with metal nanoparticles imparting more stabil-
ity to the hydrogel nanocomposite. Amidodiol is expected to impart
rigidity for the prepared hydrogel and also acting as reducing agent
2.4. Swelling studies
Definite amount of SPAG hydrogels were accurately dried and
transferred into 20 mL of water in a beaker at room temperature for
24 h. Sample was taken out at definite time intervals and the water
present on the surface of the swollen hydrogel was removed by
soft pressing the sample between the folds of a filter paper before
weighing. The uptake of water with respect to time was calculated

R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
33
characteristic amide bands were observed at 1680, 1553 and
1450 cm−1 [32,33]. FTIR spectra of PAA exhibited a broad band at
3600 cm−1 is attributed due to the OH stretching. The absorption
band at 3100 cm−1 may be due to the
C
H stretching vibration
of the residual unsaturation. The band observed at 1714 cm−1 cor-
responds to O stretching vibration of the carboxylic acid [34].
C
The band at 1399 cm−1 matches with C H stretching vibrations
which are distinct to the varying microenvironments in PAA. The
broad band due to the hydroxyl and NH group observed at 3500
and 3200 cm−1 in PAG and SPAG reveals the formation of exten-
sive hydrogen bonding interaction among OH and NH groups
present in the hydrogels. The
C
O band shifted to 1700 cm−1 in
PAG and 1698 cm−1 in SPAG attributing from the hydrogen bond
formation between PAA and amidodiol and the weak coordination
bond between the oxygen of
C O and silver. The band observed
at 2930 cm−1 in both PAG and SPAG is attributed to the stretching
vibrations of CH2 group present in the amidodiol moiety. The
bands at 1630–1550 and 1450 cm−1 correspond to the NH
bending vibrations of the amide group. Thus, the observed shift
in the positions of bands in PAG and SPAG on comparison with
PAA revealed the formation of extensive hydrogen bonding among
acrylic acid and amidodiol and also the stabilization of silver
nanoclusters in the fibrillar networks of SPAGs. Further, formation
of silver nanoparticle-entrapped fibrillar network was confirmed
by UV–visible spectroscopy in combination with microscopic
observations. Thus, the combination of amidodiol with PAA form
physically crosslinked network through extensive intermolecular
hydrogen bonding and the charged carboxyl and hydroxyl groups
present in the network which are expected to stabilize the silver
nanoparticles via cage effect.
Fig. 1. FT-IR spectra of AMD, PAA, PAG and SPAG.
for the reduction of metal ions. Earlier from our group, amidodiol
was used for the preparation of liquid crystalline materials [26].
SPAGs were prepared by varying the concentration of silver nitrate
at room temperature and the details are given in Table S1. The
dispersed Ag+ accelerates the generation of radical anion from per-
sulphate ion and initiates the polymerization of acrylic acid at room
temperature [27]. Simultaneously, the homodispersed Ag+ ions
were reduced by reductive particles, amidodiol or solvated elec-
trons present in the system. The primary products are silver atoms
(Ag0) which may combine in various fashions to produce progres-
sively larger silver clusters of silver atoms and silver nanoparticles
as shown in the supporting information (Fig. S1) [28,29]. Thus,
acrylic acid is polymerized to polyacrylic acid (PAA) in the pres-
ence of radical initiator APS and simultaneously silver ions present
in the system are reduced to silver atoms [30,31]. These reactions
were observed as a transformation of the system from colourless
low viscous liquid to light yellowish transparent gel as shown in
the photograph given in Fig. S2. The functional groups such as two
hydroxyl and two amide groups present in amidodiol can establish
extensive hydrogen bonding interaction with PAA to form fibrillar
networks. Polymerization reactions were progressed with much
faster rate in the presence of silver ions than in its absence which
may be due to the catalytic activity of silver ions.
3.2. Rheology
Mechanical stability of hydrogels was studied by measuring
viscoelastic properties by rheometry. Rheological properties were
measured under frequency sweep in the linear viscoelastic region
using a parallel-plate sensor. Rheological studies were conducted
for PAG hydrogels with different loading of amidodiol and are
shown in Fig. S3. The value of storage modulus increases with
increase in the percentage of amidodiol in PAG gel is confirmed
that the rod-like amidodiol molecules possess both hydroxyl and
amide functional groups and can effectively crosslink with poly-
acrylic acid which enhances the strength of the gel. Further loading
of amidodiol will decrease the swelling percentage of PAG and
hence the optimum loading of amidodiol is fixed as 10%. Storage
modulus/loss modulus was plotted against angular frequency and
the profile of PAG, SPAG2 and SPAG4 are shown in Fig. 2a. It was
The various interactions present in SPAGs were studied by FTIR
spectroscopy. FTIR spectra of amidodiol (AMD), PAA, PAG and
SPAG are shown in Fig. 1. FTIR spectra of amidodiol showed a band
at 3300 cm−1 which is attributed to the OH and NH stretching
vibrations. The band at 2896 cm−1 corresponds to the stretching
vibrations of CH2 group present in the amidodiol moiety. The
Fig. 2. Effect of silver on the viscoelastic properties of PAG, SPAG2 and SPAG4. (a) Angular frequency versus storage modulus (Gꢀ) of PAG ( ), SPAG2 ( ) and SPAG4 ( ) and
loss modulus (Gꢀꢀ) of PAG ( ), SPAG2 ( ) and SPAG4 ( ). (b) Angular frequency versus complex viscosity profile (ꢂ) of PAG, SPAG2 and SPAG4.

34
R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
observed that at lower frequency region, the variation of Gꢀ and Gꢀꢀ
is independent of angular frequency where the applied frequency
does not affect considerably the three-dimensional structure of the
gels which is the signature for true gel formation. The gap between
storage and loss modulus is in the order PAG < SPAG2 < SPAG4. This
showed that as the amount of silver ion increases, the elastic mod-
ulus is also enhanced due to the high extent of crosslinking density
present in SPAGs. The gap between the storage and loss modu-
lus for SPAG4, SPAG2 and PAG are observed as 3470, 2580 and
2338 Pa, respectively, revealing more solid-like nature of SPAG gel.
Complex viscosity variation with angular frequency is shown in
Fig. 2b. Generally, all the rheograms showed a power law thinning
region at lower frequency region. As moving from PAG to SPAG4,
complex viscosity showed hike in the values. SPAGs showed an
increased storage modulus, and complex viscosity values on com-
parison with PAG are due to the crosslinking effect of silver ions
present in the SPAGs. The initial decrease in viscosity with increase
in frequency can be attributed due to the change in the periodicity
of the chains present in gels with increasing frequency. The strength
of the SPAGs were observed to be higher (6 × 103 Pa) compared to
PAG (3 × 103 Pa) due to higher extent of crosslinking in the presence
of silver ions.
Fig. 3. Normalized solid state UV–visible spectra of SPAGs with varying the concen-
tration of silver nitrate.
distorted spherical and cuboidal nanoparticles (Fig. 4a and b). The
SEM picture showed a strong contrast between silver nanoparticles
and polymeric matrix, reflecting the diverse atomic composition of
film components. The absorption maxima of SPAG3 showed a broad
band at ∼373 nm. The SEM picture of the same showed mixture
of cubes and needles (Fig. 4c). Surface plasmon resonance energy
(SPRE) of SPAG4 is observed as a bimodal spectrum with two max-
ima appeared at 316 and 488 nm. It was reported that frequencyand
intensity of SPRE depends on the distribution of polarization charge
across the nanoparticles, which is determined by the size and shape
of the particles [35,36]. The SEM picture of SPAG4 showed bar/rod-
like morphology and the same is shown in Fig. 4d. Thus, the two
UV–visible maxima observed are resulting from the polarization
of these nanobars along their long axis (longitudinal plasmon
resonance) and width (transverse plasmon resonance). Both
SPREs are quite broad due to the polydispersity in the length and
3.3. Optical and morphological analysis
Optical properties of silver nanoparticles were studied by
recording UV–visible spectra. Spectra for SPAGs prepared with
varying concentration of silver nitrate are given in Fig. 3. It has been
observed that the nature and shape of the band is changed with the
amount of silver ions used in the preparation. This can be qualita-
tively related to the shape and size distribution of the formed silver
nanoparticles. UV–visible spectra of SPAG1 and SPAG2 exhibited
a sharp well-defined UV band maximum at 306 and 308 nm. An
SEM picture of SPAG1 and SPAG2 exhibited narrow distribution of
Fig. 4. SEM images of SPAGs with varying the concentration of silver nitrate showing the formation of nanospheres and cubes in (a) SPAG1, (b) SPAG2, (c) mixture of cubes
and needles in SPAG3 and (d) nanobars and rods of SPAG4.

R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
35
Fig. 5. TEM images of (a) SPAG1, (b) SPAG2, (c) SPAG3, (d) SPAG4 and (e) EDS spectrum of SPAG2.
width of the silver particles. The TEM images of SPAGs are shown
in Fig. 5. SPAG1 and SPAG2 exhibited distorted cuboidal and
spherical geometry and are shown in Fig. 5a and b. The TEM images
of SPAG3 (capsules) and SPAG4 (nanobars and rods) are shown
in Fig. 5c and d, respectively. Thus, it could be inferred from the
morphological studies that on increasing the amount of silver
nitrate, an obvious evolution in the shape and size of the silver
nanoparticles from spheres to cubes to bars were observed. Typical
EDS spectrum confirming the presence of silver nanoparticles in
the SPAG2 system is shown in Fig. 5e.
broad peaks in the XRD, and the crystalline peak of silver nanopar-
ticle is diminished. The XRD patterns of PAG, SPAG1, SPAG2, SPAG3
and SPAG4 are shown in Fig. S4. The broad peak appeared around
2ꢁ = 17◦ in PAG and SPAGs corresponds to the d-spacing of 3.5 A
˚
and is closely related to the lateral distance between the repeating
length of polyacrylic acid units as reported earlier [37]. Diffrac-
tograms of SPAG1, SPAG2, SPAG3 and SPAG4 exhibited diffraction
maxima at 2ꢁ = 30.38◦, 44◦ and 65◦ corresponds to the (111), (200)
and (220) plane of silver crystallites. These maxima correspond to
the characteristic diffraction peaks of face-cantered cubic lattice
of the crystalline silver as reported in the literature [38,39]. As we
move from SPAG1 to 4, the characteristic reflection peaks of sil-
ver become more intense revealing the formation planes with high
surface energy.
Formation of silver nanoparticles and their crystalline states in
the hydrogel were further confirmed by XRD. SPAG gels are not
completely crystalline and it is having an amorphous character
because of the polyacrylic acid chains present. Hence they exhibit

36
R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
Fig. 6. (a) SEM images of SPAG2 after swelling (pH ∼ 7.5), (b) swelling percentage versus varying the concentration of silver at pH ∼ 7.5 and (c) swelling of SPAG2 at different
pH (2–10).
3.4. Swelling studies
3.5. Catalytic reduction of organic dyes
Swelling properties of SPAGs were conducted in distilled water
(pH ∼ 7.5) as per the procedure given in the experimental section. It
was observed that the presence of silver nanoparticles in the hydro-
gel caused an enhancement in the swelling index. The increase
in the degree of hydration was attributed to the presence of sur-
face charge of silver nanoparticles. The repulsion between silver
nanoparticles with identical surface charges results in an expan-
sion of the hydrogel network, which, in turn, allows more water
molecules to diffuse into the gel [40,41]. Morphological changes
during swelling were studied by SEM and the swelled image of
SPAG2 is shown in Fig. 6a. It exhibited highly porous network estab-
lished by high swelling of the network. The bright portion of the
SEM image in the fibrillar network revealed the presence of silver
nanoparticles. The percentage of swelling index with the increas-
ing amount of silver at pH ∼ 7.5 is given in Fig. 6b. Swelling studies
were also carried out under different pH conditions (pH 2–10) and
are shown in Fig. 6c. An enhancement in the swelling index was
observed with increase in the pH value due to the salt formation of
the free acrylic acid. The increase in the swelling ratio is expected
on the basis of acrylic acid going from the non-ionized state to the
ionized state at pH greater than 4.2. Upon neutralization of the car-
boxylate groups, the mutual repulsion between these groups causes
the molecule to adopt an expanded configuration, leading to higher
swelling of the matrix. Swelling studies were also carried out at
different temperature (303–343 K) and are shown in Fig. S5a. The
swelling percentage increases with increase in temperature is due
to the increase in the entropy and internal energy of the solution
which lead to a fast diffusion process. The SEM images of SPAG2
at 313 and 343 K are given in Fig. S5b and c. Morphological anal-
ysis revealed that at higher temperature, more number of silver
nanoparticles are exposed to the surface of hydrogel and enhances
its efficiency for the reduction process and will be describing in the
latter part of this paper.
Catalytic property of SPAGs was investigated for the reduction
of organic dyes such as rhodamine 6G (R6G), crystal violet (CV) and
methylene blue (MB) using NaBH4 as the reducing agent. Reduction
of dyes with NaBH4 is incomplete in the absence of catalyst. The
absorbance, chemical formula, molar mass and colour index num-
ber for R6G, CV and MB are given in Table S2. The choice of dyes
was based on the factors, i.e., the dyes possess different colours
in the oxidized and reduced states and the absorption maxima of
the dyes do not overlap with the surface plasmon band of metal
nanoparticles which is employing as a catalyst. In the present sys-
tem, the absorption maxima of silver nanoparticles was observed
around 430 nm and the maxima for R6G, CV and MB is 525, 590 and
665 nm, respectively. The absorption maxima of these bands are
distinct from the SPRE of silver nanoparticles present in the SPAGs
and hence the precise determination of the catalytic behaviour of
the silver nanoparticles is viable.
The catalytic reduction was carried out at room temperature and
at pH ∼ 7.5 and was monitored using UV–visible spectroscopy. The
decrease in the intensity of UV absorption maxima was noted peri-
odically and the representative diagrams showing the UV–visible
absorption spectra at various intervals of time for MB, R6G and CV
in the presence and absence of SPAG are shown in Fig. 7. The kinet-
ics of the reduction reaction was studied using first-order kinetic
equation. The first-order kinetic plot is obtained by plotting ln(a/a0)
versus time, where a is the absorbance at time t and a0 is the initial
absorbance as shown in Fig. 8. The rate constant was calculated
from the slope of the line. The rate constants of the uncatal-
ysed reaction are 0.00014 0.000015, 0.000075 0.0000024 and
0.000142 0.00001 s−1 for R6G, MB and CV, respectively. The
rate constants calculated for the SPAG2-catalysed reaction are
0.0047 0.0002, 0.0037 0.00016 and 0.0048 0.0002 s−1 for
R6G, MB and CV, respectively. The increased value for the rate
constant in the presence of SPAG suggested an enhancement in the

R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
37
Fig. 7. The dye reduction studies of MB, R6G and CV in the absence and presence of SPAG2 catalysts (dye concentration: R6G (240 mg/L), MB (160 mg/L), CV (200 mg/L),
amount of catalyst: 20 mg, temperature: 30 ◦C, pH: 7.5).
rate of reaction. Here, the hydrogel network effectively stabilizes
the silver nanoparticles and prevent aggregation of silver particles
during the reaction. The colour of the dye was not retaining in
the gel and is confirmed the complete reduction of dye. Effect of
silver concentration on the dye reduction process was monitored.
Effect of silver concentration on the dye reduction process was
monitored. The rate constant values for SPAG1 (spherical), SPAG2
(distorted cubic), SPAG3 (capsules) and SPAG4 (nanorods) are
0.002 0.0001, 0.0047 0.0002, 0.0088 0.00023, 0.024 0.001
for R6G, 0.0011 0.0003, 0.0037 0.00016, 0.0074 0.0002,
0.015 0.0021 for MB, 0.0023 0.0002, 0.0048 0.0002,
0.0089 0.00014, 0.031 0.0012 for CV, respectively. As we
move from SPAG1 to SPAG4, there is an increase in the number of
silver nanoparticles as evidenced from the UV–visible spectral area
Fig. 8. The kinetic plots of MB, R6G and CV for (a) uncatalysed reaction and (b) SPAG-catalysed reaction.

38
R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
representation of decrease in the activation energy in the
SPAG-catalysed reduction of R6G is given in Fig. S7.
3.7. Effect of pH
Effect of pH on the reduction process was studied at room tem-
perature by varying pH from 2 to 10 and is shown in Fig. S8. The
change in the pH of the solution can change the surface charges of
the absorbent and can promote or depress the rate of reaction. Here,
the rate of reaction is increased with increase in the pH of dye solu-
tion. At lower pH, most of the COOH groups of the hydrogel were
not ionized. At higher pH, the COOH groups in the SPAG hydrogel
segments dissociate to form COO and this generates an electro-
static repulsion among the adjacent ionized groups of the polymer
networks which causes an expansion of the polymer chain within
the hydrogel network. When the hydrogel was swelled, it may pos-
sibly imbibe large amount of dye containing water molecules and
hence the surface area of the catalyst is enhanced. This enhance-
ment in the surface area may favour for the adsorption of more
amount of dye, and the increased rate of dye reduction process
was observed. We could not observe any change in the set pH dur-
ing the reduction process revealing hydrogels are stable during the
experimental condition.
Fig. 9. Arrhenius plot of R6G, MB and CV (dye concentration: R6G (240 mg/L), MB
(160 mg/L), CV (200 mg/L), amount of catalyst: 20 mg, temperature: 30–70 ◦C, pH:
7.5).
which facilitates the reduction of dye molecules. Apart from these
observations, the anisotropic shape with high-energy exposed
crystalline planes of the silver particles present in SPAG3 and
SPAG4 as shown in the XRD patterns also contribute for the high
efficiency in the reduction process conducted in presence of SPAG3
and SPAG4.
The rate of the reaction is compared with the literature reports
[15,16,42] and SPAGs showed a high catalytic efficiency even with
high concentration of dye (160 mg/L MB) and the value of rate
constant is found to be 0.0037 s−1 (0.22 min−1) with SPAG2 and
0.015 s−1 (0.9 min−1) with SPAG4 which is higher than previous
reported values.
3.8. Reusability and storage stability
Reusability of the SPAGs for the reduction process was checked
by taking SPAG2 as model gel. Experiments were performed at
room temperature and at pH ∼ 7.5 for 10 cycles and is shown in Fig.
S9. There was no loss in the catalytic efficiency as evidenced from
the same rate of reaction even after 10 cycles, which suggests that
SPAG as an efficient catalyst for the reduction of dyes. Long-term
stability of the catalyst was monitored by measuring the efficiency
of catalyst up to 6 months after storing the gel at 4 ◦C in a sealed
cover. The storage stability of the SPAG for the reduction process
was checked with SPAG2 at room temperature and at pH ∼ 7.5. The
graph showing the variation of the rate constant with time (up to
6 months) for the SPAG2-catalysed reduction of MB, R6G and CV is
given in Fig. S10.
3.6. Effect of temperature
Effect of temperature on the catalytic reduction of dyes was
studied using SPAG2 and is shown in Fig. S6. The temperature
experiments were conducted in the range of 303–343 K and at the
pH of ∼7.5. The rate of reduction showed an enhancement with
increase in temperature. The temperature-dependent swelling of
SPAG2 exhibited an increase in the swelling index. The degree
of swelling of hydrogel increases at high temperature due to the
fast diffusion of dye molecules to the surface of hydrogel [43,44].
Morphological observation of these gels under SEM showed more
number of silver nanoparticles on the surface (Fig. S5), suggesting
an enhancement in the rate of reaction. The high temperature low-
ers the activation energy for the reaction and increases the rate of
the reaction. These factors contribute for the enhancement in the
rate of reduction reaction at higher temperature.
3.9. Mechanism of dye reduction by SPAG
Mechanism for the reduction of dye can be speculated from
the various experimental results. Morphological and spectroscopic
studies showed that SPAG hydrogels consist of silver nanoparti-
cles entrapped on fibrillar networks which consist of functional
groups such as carboxyl, hydroxyl and amide. Morphological stud-
ies revealed that silver particles are well exposed on the surface
of these swollen fibrils and also present inside the fibrils. Dye
molecules are electrophilic and NaBH4 is nucleophilic compared
to silver nanoparticles. The proposed mechanism for the SPAG-
catalysed reduction of dyes with sodium borohydride as reducing
agent is shown in Fig. 10. Generally, the rate of reduction process
depends on the potential difference between the donor and accep-
tor molecules. Silver nanoparticles are negatively charged in the
presence of NaBH4 by the following equation [45]:
Activation energy during the reduction process was compared
by calculating Ea values using Arrhenius equation:
Ea
/RT
k = A e−E
,
ln k = ln A −
a
RT
where k is the rate constant, A is the pre-exponential factor, Ea
is the activation energy, R is the gas constant (8.314 J mol−1 K−1
BH4− + Agn + 3H2O → BO33− + Agn8− + 10H+
)
and T is the temperature. A straight line is obtained on plotting ln k
versus 1/T for the SPAG-catalysed reaction as shown in Fig. 9. The
activation energy is calculated from the slope and is 54, 45 and
44 kJ/mol for MB, R6G and CV, respectively. The activation energy
measured for the uncatalysed reduction is 66, 55 and 56 kJ/mol
for MB, R6G and CV, respectively. The apparent reduction in
the activation energy (ꢃEa = Ea of uncatalysed reaction −Ea of
catalysed reaction) is approximately 11 kJ/mol. The schematic
These negatively charged silver particles may become more
accessible to the positively charged dye molecules due to elec-
trostatic interaction. Thus, the electron transfer between dye
−
molecules and BH4 through the silver nanoparticles becomes
viable and enhances the rate of reaction. The observed enhance-
ment in the rate of reduction revealed the efficiency of the
catalytic effect of SPAG. The rate of the reduction of dyes increased

R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
39
Fig. 10. Mechanism for the catalytic reduction of dyes in the presence of SPAG.
when moving from SPAG1 to SPAG4. The UV–visible spectral area
increases when sample changes from SPAG1 to SPAG4 due to the
increased number of silver particles. This facilitates the reduction
of dye molecules. Apart from these observations, it is attributed to
the change in the size, shape and crystalline state of silver nanopar-
ticles. Morphological observation of SPAGs made under SEM and
TEM showed variation in the shape and size of the silver parti-
cles: It exhibited spherical-, rod- and capsule-type particles. XRD
results of SPAG3 and SPAG4 suggested the formation planes of
(220) and (200) with enhanced intensity compared to SPAG1 and
SPAG2 revealing high surface energy. This may also enhance the
efficiency of the catalyst. The high viscoelastic strength observed
during rheological measurements suggested its operational and
storage stability. Even after 10 successive uses, the shape of the cat-
alyst and the efficiency of catalysis remains intact. Thus, the SPAGs
with silver nanoparticle-entrapped fibrillar network of the hydro-
gel allow for the easy diffusion of the solute molecules and enhance
the potential difference between nucleophile and electrophile in
the reduction process.
constant encouragement and support. We are also thankful to
Mr. P. Guruswamy, Mr. M. Chandran, Ms. Lucy Paul and Mr. Kiran
Mohan for XRD, SEM and TEM analyses.
Appendix A. Supplementary data
Supplementary data associated with this article can be
found, in the online version, at http://dx.doi.org/10.1016/j.apcata.
2014.06.026.
References
[1] C. Wang, N.T. Flynn, R. Langer, Adv. Mater. 16 (2004) 1074.
[2] S. Patrick, G. Schmidt, J. Colloids Interface Sci. 287 (2009) 1.
[3] R. Yoshida, H. Ichijo, T. Hakuta, T. Yamaguchi, Macromol. Rapid Commun. 16
(1995) 305.
[4] L.-C. Dong, A.S. Hoffman, J. Control. Release 15 (1991) 141.
[5] R. Fuhrer, E.K. Athanassiou, N.A. Luechinger, W.J. Stark, Small 5 (2009) 383.
[6] Y. Zhao, H. Su, L. Fang, T. Tan, Polymer 46 (2005) 5368.
[7] S. Ramachandran, Y. Tseng, Y.B. Yu, Biomacromolecules 6 (2005) 1316.
[8] Y. Xing, E. Cheng, Y. Yang, P. Chen, T. Zhang, Y. Sun, Z. Yang, D. Liu, Adv. Mater.
23 (2011) 1117.
[9] S.J. Li, S. Gong, Adv. Funct. Mater. 19 (2009) 2601.
4. Conclusion
[10] Y. Wu, J. Xiang, C. Yang, W. Lu, C.M. Lieber, Nature 430 (2004) 704.
[11] D. Adams, B. Captain, L.J. Zhu, J. Am. Chem. Soc. 126 (2004) 3042.
[12] R.C. Jin, Y.W. Cao, C.A. Mirkin, K.L. Kelly, G. Schatz, Science 294 (2000) 1901.
[13] R. Xu, D. Wang, J. Zhang, Y. Li, Chem. Asian J. 1 (2006) 888.
[14] S. Jana, S.K. Ghosh, S. Nath, S. Pande, S. Praharaj, S. Panigrahi, S. Basu, T. Endo,
T. Pal, Appl. Catal. A: Gen. 313 (2006) 41.
[15] Z.-J. Jiang, C.-Y. Liu, L.-W. Sun, J. Phys. Chem. B 109 (2005) 1730.
[16] Y. Zheng, A. Wang, J. Mater. Chem. 22 (2012) 16552.
[17] Y. Lu, P. Spyra, Y. Mei, M. Ballauff, A. Pich, Macromol. Chem. Phys. 208 (2007)
254.
Robust silver nanoparticle-entrapped hydrogel composites
were prepared by a simple strategy of in situ reduction and
polymerization approach. Morphological studies revealed the for-
mation of nanospheres, nanocubes and nanorods depending on the
amount of silver nitrate. Catalytic efficiency of SPAGs was stud-
ied for the reduction of organic dyes using UV–Vis spectroscopic
method. It showed an enhancement in the catalytic efficiency with
increase in pH and temperature. Kinetic studies showed that within
15 min, MB, R6G and CV with concentration of 150–250 mg/L could
be reduced completely at room temperature. Activation energy for
the reduction process in the SPAG-catalysed reaction is found to be
reduced by ∼10 kJ. High rate constant, high reusability, easy separa-
tion, good storage life and the absence of induction period suggest
SPAG as an efficient catalyst for the reduction of organic molecules
and the process is rapid, efficient and economical.
[18] W. Oppermann, B.B. Lindemann, Polymer Gels, vol. 833, American Chemical
Society, 2002, pp. 37–50 (Chapter 3).
[19] S.-H. Yu, H. Colfen, J. Mater. Chem. 14 (2004) 2124.
[20] L. Berti, A. Alessandrini, P. Facci, J. Am. Chem. Soc. 127 (2005) 11216.
[21] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80–82.
[22] B. Wiley, T. Herricks, Y. Sun, Y. Xia, Nano Lett. 4 (2004) 1733–1739.
[23] Y. Murali Mohan, K. Vimala, T. Varsha, K. Varaprasad, B. Sreedhar, S.K. Bajpai,
K.J. Mohana Raju, J. Colloids Interface Sci. 342 (2010) 73–82.
[24] C. Zhou, Q. Wu, Y. Yue, Q. Zhang, J. Colloids Interface Sci. 353 (2011) 116–123.
[25] L.-W. Xia, X.-J. Ju, J.-J. Liu, R. Xie, L.-Y. Chu, J. Colloids Interface Sci. 349 (2010)
106.
[26] J.D. Sudha, C.K.S. Pillai, J. Polym. Sci. A: Polym. Chem. 41 (2003) 335.
[27] D.A. House, Chem. Rev. 62 (1962) 185.
[28] J.L. Marignier, J. Belloni, M.O. Delcourt, J.P. Chevalier, Nature 344 (1985) 317.
[29] A. Henglein, E. Janata, A. Fojtik, J. Phys.Chem. 96 (1992) 4734.
[30] Y. Lu, Y. Mei, M. Schrinner, M. Ballauff, M.W. Möller, J. Breu, J. Phys. Chem. C
111 (2007) 7676.
Acknowledgements
We thank CSIR Network project and KSTEC for the financial
support. We also thank Dr. Suresh Das, Director, CSIR-NIIST, TVM,
and Dr. A. Ajayaghosh, Dean AcSIR, CSIR-NIIST, TVM, for their
[31] Z. Zhang, M. Han, J. Mater. Chem. 13 (2003) 641.
[32] J.D. Sudha, J. Polym. Sci. A: Polym. Chem. 38 (2000) 2469.

40
R. Kuttiplavil Narayanan et al. / Applied Catalysis A: General 483 (2014) 31–40
[33] C.K.S. Pillai, K.Y. Sandhya, J.D. Sudha, M. Saminathan, J. Phys. 61 (2003) 417.
[34] S. Tan, C.T. Williams, J. Phys. Chem. C 117 (2013) 18043.
[35] H. Itoigawa, T. Kamiyama, Y. Nakamura, J. Non-Cryst. Solids 220 (1997) 210.
[36] K.L. Kelly, E. Coronado, L.L. Zhao, G.C. Schatz, J. Phys. Chem. B 107 (2002) 668.
[37] X. Xu, Y. Yin, X. Ge, H. Wu, Z. Zhang, Mater. Lett. 37 (1998) 354.
[38] J.Q. Hu, Q. Chen, Z.X. Xie, G.B. Han, R.H. Wang, B. Ren, Y. Zhang, Z.L. Yang, Z.Q.
Tian, Adv. Funct. Mater. 14 (2004) 183.
[40] E.M. Ahmed, F.S. Aggor, J. Appl. Polym. Sci. 117 (2010) 2168.
[41] P. Saravanan, M. Padmanabha Raju, S. Alam, Mater. Chem. Phys. 103 (2007)
1278.
[42] M. Zhu, C. Wang, D. Menga, G. Diao, J. Mater. Chem. A 1 (2013) 2118.
[43] A. Kostic´, B. Adnadjevic´, A. Popovic´, J. Jovanovic´, J. Serb. Chem. Soc. 72 (2007)
1139.
[44] A.M. Saeed, Int. J. Adv. Biolog. Biomed. Res. 1 (2013) 1614.
[45] T. Ung, L.M. Liz-Marzán, P. Mulvancy, J. Phys. Chem. B 103 (1999) 6770.
[39] W.C. Zhang, X.L. Wu, H.T. Chen, Y.J. Gao, J. Zhu, G.S. Huang, P.K. Chu, Acta Mater.
56 (2008) 2508.