Antimicrobial alginate PVA silver nanocomposite

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ORIGINAL PAPER
Antimicrobial alginate/PVA silver nanocomposite
hydrogel, synthesis and characterization
Hossein Ghasemzadeh & Fereshteh Ghanaat
Received: 7 August 2013 /Accepted: 2 January 2014
# Springer Science+Business Media Dordrecht 2014
Abstract Superabsorbent hydrogel-silver nanocomposite
based on poly(vinyl alcohol) (PVA) and sodium alginate
(Na-Alg) was prepared using free radical polymerization in
the presence of acrylamide (AAm) monomer. The reactions
were conducted under normal atmospheric conditions, using
ammonium persulfate (APS) as an initiator and methylene
bisacrylamide (MBA) as a crosslinking agent. The effect of
reaction parameters such as MBA, AAm, and APS concen-
tration as well as Na-Alg/PVA weight ratio on the water
absorbency and the gel content of the hydrogels were studied.
Evidence of grafting was obtained by comparing the FT-IR
spectra and the TGA of the initial substrates with that of the
superabsorbent hydrogel. Furthermore, Ag nanoparticles were
synthesized in a green synthesis process. Highly stable silver
nanoparticles were obtained with the hydrogel networks as
nanoreactor via in situ reduction of silver nitrate by using
sodium borohydride as a reducing agent. The hydrogel silver
nanocomposite was fully characterized by using scanning
electron microscopy (SEM), transmission electron microsco-
py (TEM), and thermogravimetric analysis (TGA). The effect
of cross-link density, and Na-Alg/PVA weight ratio on the
loading and the size of nanoparticles were studied. The anti-
bacterial activity of the silver nanocomposite hydrogel was
investigated as well.
Keywords Nanocomposite . Hydrogel . Sodium alginate .
Polyvinyl alcohol . Acrylamide
Introduction
Hydrogels are cross-linked polymer chains that can absorb,
swell and retain aqueous fluids up to thousands of times of
their own weights [1–3]. Hydrogels have a variety of func-
tional properties that have been exploited in a number of
applications including biosensors [4–6], drug delivery sys-
tems [7–10], tissue engineering [11, 12], wound dressings
[13–15], contact lenses [16], etc. Hydrogels containing hydro-
philic groups can be used to produce nanoparticles because of
free special space among the crosslinked networks in the
swollen state that can behave as nano reactor [17–19]. Nano
metal systems have been considered as a novel class of mate-
rials for catalytic [20–23], electronic [24, 25], biomedical and
the others application [26–28]. Silver nanoparticles have
unique electric, optical, catalytic and particularly antimicrobi-
al properties, which are well investigated mainly in colloidal
systems [29, 30]. Furthermore, despite of some potential risks
to human health [31], a wide range of industrial and medical
applications such as water treatment, bio-sensing, and treat-
ment of disease was reported in recent years [28, 32, 33]. The
appropriate functional groups of polymers can be used for the
controlled synthesis of the nanoparticles. Natural polymers
have been of great interest during the past decades for the
design of new hydrogel systems [34, 35]. Sodium alginate is a
polyanionic linear copolymer of 1,4-linked α-l-guluronic acid
and β-D-mannuronic acid residues which has unique biocom-
patibility, biodegradability, and non-toxicity [36–38]. Poly(-
vinyl alcohol) (PVA) is a water-soluble none-ionic poly hy-
droxy polymer which has been extensively used in biomedical
and pharmaceutical applications due to its non-toxic, biocom-
patible and biodegradable properties [39, 40]. Moreover,
Polyvinyl alcohol (PVA) is one of the well-known polymer
gel that has been used in numerous biomedical applications,
such as artificial organs, drug delivery, and wound dressings,
due to its good biocompatibility [41–44]. It is expected that a
H. Ghasemzadeh (*)
Department of Chemistry, Imam Khomeini International University,
P.O. Box 288, Qazvin, Iran
e-mail: hoghasemzadeh@gmail.com
F. Ghanaat
Department of Chemistry, Islamic Azad University, Karaj Branch,
P.O. Box 31485-313, Karaj, Iran
J Polym Res (2014) 21:355
DOI 10.1007/s10965-014-0355-1
new kinds of hydrogel as a nanoreactor with improved struc-
ture can be obtained by the combination of sodium alginate as
a polyanionic and PVA as a none-ionic polyhydroxy polymer.
In this work, a series of superabsorbent-silver nano-
composites based on sodium alginate and polyvinyl alcohol
was synthesized via graft polymerization of acrylamide
(AAm) in aqueous medium using methylenebisacrylamide
(MBA) as a crosslinking agent and ammonium persulfate
(APS) as an initiator. Grafting of acrylamide onto sodium
alginate [45] and PVA [46] has been reported previously.
The effects of reaction variables, such as MBA, AAm, and
APS concentration as well as alginate/polyvinyl alcohol
weight ratio on the water absorbency and the gel content of
the superabsorbent hydrogel were investigated. Evidence of
grafting was obtained by comparing the FT-IR and TGA
spectra of the initial substrates with that of the superabsorbent
hydrogel. Highly stable silver nanoparticles were prepared
within the hydrogel network via in situ reduction of silver
nitrate using sodium borohydride as a reducing agent. The
formation of silver nanoparticles was confirmed with scan-
ning electron microscopy (SEM), energy dispersive x-ray
(EDX) analysis, and transmission electron microscopy
(TEM). The effect of the cross-link density, and Na-Alg/
PVAweight ratio on the loading and the size of nanoparticles
as well as antibacterial properties of the nanocomposite were
studied.
Experimental
Materials
The polysaccharide sodium alginate was purchased from
Merck and used without further purification. PVA was pur-
chased from Kardan Shimi Co. The molecular weight and
degree of saponification of PVAwas 70,000 and greater than
98 %, respectively. MBA, APS, AAm, sodium borohydride
(from Merck), and silver nitrate (from Aldrich) were used as
received. All other chemicals were of analytical grade. Dis-
tilled water was used to prepare the solutions.
Hydrogel synthesis
(Na Alg-PVA)-g-PAAm hydrogel were prepared by free rad-
ical polymerization method. For this purpose, certain amounts
of PVA (0.3–3.0 g) and Na-Alg (3.0–0.3 g) were dissolved in
30 ml distilled water in a 500 mL three-neck reactor equipped
with a mechanical stirrer while stirring (200 rpm). The reactor
was immersed in a thermostated water bath preset at 80 °C.
After homogenizing, APS (dissolved in 5 ml distilled water),
was added to the reaction mixture. After 5 min, MBA (dis-
solved in 5 ml distilled water) and then AAm, were added to
the reaction mixture. The water bath was heated and kept at
80 °C for 30 min. The time at which a rapid increase in
viscosity was observed and the gel build up on the surface
of the solution, was measured as the gelation time of the
polymer solution. After polymerization, the produced hydro-
gel was cooled and then dewatered by the addition of 200 mL
ethanol. After 48 h, the product was filtered, and washed with
fresh ethanol (2×50 mL) and then dried at 50 °C in oven for
12 h. The final product was stored away from moisture, heat
and light.
Preparation of hydrogel-silver nanocomposites
For preparation of hydrogel-silver nanocomposite, dry hydro-
gel samples were equilibrated in water for 3 days. Then, the
completely swollen hydrogels were transferred to another
beaker containing 100 mL of 5 mM AgNO3 aqueous solu-
tions. In this solution, the silver ions are exchanged from
solution into the hydrogel networks space. After 24 h, the
silver salt loaded hydrogels were transferred into a beaker
containing 100 mL of 0.1 M NaBH4 aqueous solution and
were allowed for 3 h at room temperature to reduce the silver
ions into silver nanoparticles. The dark brown color of the
hydrogel indicated the formation of silver nanoparticles. The
product were filtered, washed with fresh ethanol and dried in
oven at 50 °C for 10 h.
Swelling measurements
The equilibrium swelling (g/g) was measured according to a
conventional “tea bag” method [47]. The particlesizes of the
dry hydrogel were 40 to 60 meshes (250–400 μm). The tea
bag (100 mesh nylon screen) containing an accurately
weighed powdered sample (0.3±0.01 g) was immersed en-
tirely in 200 ml distilled water and was allowed to soak for 3 h
at room temperature. The tea bag was hung up for 10 min to
remove the excess water. The equilibrium swelling (ES) was
calculated triple times according to Eq. 1:
ES g=gð Þ ¼ W 2−W 1
W 1
ð1Þ
where W1 and W2 are the weights of dry and swollen gel,
respectively. So, the absorbency was calculated as grams of
water per gram of the hydrogel (g/g).
Swelling kinetics
To investigate the rate of absorbency of the hydrogel, certain
amount of the sample (0.3±0.01 g) with average particle sizes
between 40 and 60 meshes (250 – 400 μm) was poured into a
weighed tea bag and immersed in 200 ml distilled water. At
355, Page 2 of 14 J Polym Res (2014) 21:355
consecutive time intervals, the water absorbency of the hydro-
gel was measured according to the earlier mentioned method.
Gel and Sol content determination
Tomeasure the gel and sol content, the accurately weighed dry
hydrogels were dispersed in distilled water to swell complete-
ly. Then the swollen hydrogel were filtered and washed with
distilled water frequently. The samples dewatered in excess
ethanol for 48 h, and dried at 50 °C for 12 h until the hydrogels
reached to a constant weight. The gel content was defined as
the following equation:
Gel %ð Þ ¼ Wd
Wi
� 100 ð2Þ
whereWd is the weight of dried hydrogel after extraction andWi
is the initial weight of the hydrogel. The soluble content (sol)
was calculated as the weight loss of the initial crude product.
Determination of silver ion loading in the hydrogel matrix
The accurately weighed dried hydrogel samples, particle sizes
40 to 60 meshes, were dispersed in distilled water to swell
completely. The hydrogel samples were equilibrated with
water for 3 days. Then, the completely swollen hydrogels
were transferred to a beaker containing 100 mL of AgNO3
aqueous solutions, with various concentrations for 24 h. Then
the hydrogel samples were filtered and the concentration of
silver ion in the remaining solution was determined by atomic
absorption spectroscopy.
Antimicrobial assays
Antimicrobial activities of the hydrogel silver nano composite
are assessed using the minimum inhibitory concentration
(MIC) which leads to inhibition of bacterial growth. The
MIC is read after 24 h of incubation. The minimum bacteri-
cidal concentration (MBC) is examined by inoculation of the
samples to agar medium without antimicrobial agents. The
MBC is determined as the lowest concentration that inhibits
the visible growth of the used bacterium.
Morphologies of the hydrogels
Scanning electron microscopy (SEM) and energy dispersive
x-ray (EDX) analysis of the dry hydrogels was performed to
visualize the hydrogel pores. The microstructure of the hydro-
gel was imaged using a scanning electron microscope, SEM,
(Philips, XL30) operated at 25 kV after coating the samples
with the gold film. For transmission electron microscopy
(TEM) studies, a Zeiss - EM10C transmission electron
microscope operating at 80 kV was employed. The samples
were homogenized by ultrasonic homogenizer in ethanol and
water solvents for 20 min. Then, the samples of the hydrogels
were prepared on a 300 mesh carbon-coated copper grid.
FT-IR analysis
FT-IR spectra of the samples in the form of KBr pellets were
recorded using a Bruker Tensor 27 FT-IR spectrophotometer.
Atomic absorption analysis
The amount of silver ion loaded in the hydrogel matrix deter-
mined by a Varian model 220Z spectrophotometer. The influ-
ence of sodium alginate/PVA, initial concentration of MBA,
and silver nitrate on the extent of silver ion loading was
studied by this method.
Thermal analysis
Thermal Analysis was performed using a thermogravimetric
analyzer (Perkin Elmer Pyris Diamond TG-DTA) and a differ-
ential scanning calorimeter (Perkin Elmer Pyris DiamondDSC).
Results and discussion
Synthesis, characterization and mechanistic aspects
The hydrogels were prepared by free radical polymerization in
distilled water under atmospheric condition. Graft polymeri-
zation of acrylamide (AAm) onto sodium alginate and poly
(vinyl alcohol) were carried out in the presence of MBA as a
crosslinking agent, and ammonium persulfate (APS) as an
initiator. It is supposed that the persulfate decompose on
heating and produces sulfate anion-radicals that abstract hy-
drogen atoms from the hydroxyl groups of sodium alginate
and Poly (vinyl alcohol). The active centers on polymer
chains, radically initiate the polymerization of acrylamide
(AAm) which lead to a graft polymer. Since a crosslinking
agent (MBA) is present, the copolymer comprises a
crosslinked structure (Fig. 1).
Synthesis of hydrogel–Ag nanoparticle composites
Recent researches demonstrate that composite hydrogels are
becoming most promising as nano reactors for in situ synthe-
sis of small size nanoparticles [48]. It is reported that the
control of the nanoparticles sizes can be achieved by modify-
ing the hydrogel network architectures by varying the amount
of monomer, cross-linker, and functionality of the gel net-
works [49, 50]. In this work, Ag nanoparticles were synthe-
sized in a green synthesis process which utilizes water as an
J Polym Res (2014) 21:355 Page 3 of 14, 355
environmentally benign solvent and polysaccharides as a cap-
ping agent. The completely swollen hydrogels were loaded
with the AgNO3 solution and then it reduced in aqueous
solution of NaBH4 (Fig. 2). The silver nanoparticles are
immobilized throughout the hydrogel networks due to a
strong localization of the Ag+ ions within the hydrogel net-
work. This is caused by the complexation of the Ag+ ions by
either the hydroxyl groups of polyvinyl alcohol, the hydroxyl
and the carboxylate groups of polysaccharide, and/or the amid
groups of AAm. The rest of silver ions occupied free spaces of
the hydrogel network. The well-dispersed nanoparticles in the
hydrogel networks were obtained after optimizing the amount
of crosslinker and Na-Alg/PVAweight ratio. The prepared Ag
nanoparticles in the hydrogel networks are highly stable and
did not show any signs of aggregation, even after storage for
several months. Similar results were reported previously for
semi-interpenetrating networks of pluronic and poly
(acrylamide) hydrogel [51].
Fourier transform infrared spectroscopy analysis
Infrared spectroscopy was carried out to confirm the chemical
structure of the superabsorbent hydrogel and nano composite.
The FT-IR spectra of the initial substrates and the graft poly-
mer are depicted in Fig. 3. Figure 3a shows the characteristic
absorption bands of sodium alginate at around 1,574 cm−1 and
1,416 cm−1, assigned to the asymmetric and symmetric
stretching modes of the carboxylate anion. A band at
1,032 cm−1 was attributed to the stretching vibrations of C-
O-C group. Figure 3b shows the characteristic absorption
bands of PVA at around 1,097 cm−1 attributed to C-O
stretching and bending modes. The broad band at 3,100–
3,600 cm−1 was due to stretching of the hydroxyl groups of
the PVA. A new absorption peaks appeared at 1,663 cm−1 in
the spectrum of the hydrogel may be attributed to the C = O
stretching mode of amide groups of grafted acrylamide,
Fig. 3d. In addition, a new absorption peak in the spectrum
of the hydrogel is appeared at 1,112 cm−1, in comparison to
the spectrum of the physical mixture of Na-Alg, PVA, and
PAAm hydrogel. The stretching band of -NH overlapped with
the -OH stretching band of the alginate and PVA. These
informations give direct evidence that the PAAm chains have
grafted on the macromolecular chains, and the –CONH2 exist
in the grafting polymer.
The characteristic features of the spectrum of hydrogel-
silver nano-composite were almost similar to those of the
spectrum of the hydrogel but the O–H stretching peak of the
hydrogel-silver nano-composite has less intensity than the O–
H stretchingpeak of the hydrogel. The lower intensity of the
O–H stretching peak indicate that the interaction also probable
between the reduced Ag nanoparticles and the hydroxyl
groups of PVA, or the hydroxyl and the carboxylate groups
of Na-Alg. Similar interactions between nano-Ag and special
group attached to the polymer has been reported [52].
Effect of crosslinker concentration
The effect of the amount of crosslinker on the water absor-
bency and the gel content values of (Na Alg-PVA)-g-PAAm
hydrogel are shown in Fig. 4. In these reactions, the Alg/
PVAweight ratio in the initial mixture was chosen to be 3/1.
This is a well known behavior that swelling decreases with
increasing the concentration of crosslinker. Higher
crosslinker concentration produce more crosslinked points
in polymer chains and increases the density of crosslinking,
which results in less swelling. Similar observations have
been reported in the literature [53]. According to the data in
Fig. 4, a sample of 0.04 mol/L of MBA, provides an
optimum values of swelling and the gel content. At lower
than the optimum values, the gel content is diminished
substantially because of the formation of very loosely
crosslinked networks, resulting in highly swollen hydrogels
with very low gel strength. In addition, the time of gelation
was found to be 3.0, 2.3, 1.5, and 0.4 min for the superab-
sorbent hydrogel with the amount of initial MBA of 0.1,
0.2, 0.3, 0.4 g, respectively.
Effect of APS concentration
APS concentration can affect the swelling and gel content
of the hydrogel, as well as the rate of polymerization. The
effect of APS concentration on the swelling and gel content
of the hydrogel were studied by verifying the concentration
of APS from 11 to 45 mmol/L. As depicted in Fig. 5,
swelling is increases with increasing the initiator concen-
tration up to 27 mmol/L, but it is decreases with further
increase in concentration of APS. Increase in APS concen-
tration, led to increase the concentration of free radicals
produced by decomposition of APS, causes to higher graft
polymerization extent, so that the water absorbency of the
hydrogel increases. Subsequent decrease in swelling is
originated from an increase in terminating step reaction
via bimolecular collision which in turn, causes to enhance
crosslinking density.
This possible phenomenon is referred to as “self
crosslinking” by Chen and Zhao [54]. In addition, the
decrease in the water absorbency and the gel content
may be attributed to free radical degradation of the Na-
Alg substrate which can take place at high APS levels.
Similar behaviors has been observed for the degradation
of other polysaccharides such as carrageenan [55] and
chitosan [56, 57] with persulfate. The time of gelation
was found to be 4.5, 2.5, 0.55, and 0.35 min for the
superabsorbent hydrogel with the amount of initial APS
355, Page 4 of 14 J Polym Res (2014) 21:355
of 0.1, 0.2, 0.3, 0.4 g, respectively. So, the higher APS
concentration causes a higher rate of polymerization.
Effect of monomer weight ratio
The presence of the hydrophilic groups in polymer
chains results in the increase of the swelling. The swell-
ing capacity of the hydrogels prepared with various
ratios of the monomer, is shown in Fig. 6. The initial
increase in the water absorbency of the Alg/PVA-g-
polyAAm hydrogels with increasing of the AAm weight
ratio can be attributed to the hydrophilic character of
carboxamide groups in the grafted AAm. The subse-
quent decrease in water absorbency of the hydrogel
after an initial increase can be attributed to (a) prefer-
ential homopolymerization over graft copolymerization,
and (b) the enhanced chance of chain transfer to mono-
mer molecules.
The gel content is affected by degree of monomer
grafting achieved during polymerization. The gel content
increases with increasing the amount of monomer con-
centration. The time of gelation was found to be 5.5,
3.4, 2.4, and 2.2 min for the superabsorbent hydrogel
NaAlg
HO
COONa
OH
n
S2O8
2-
heat
SO4-
NaAlg
O
COONa
O
n
CONH2n
H
N
H
N
O O
NH
NH
O
O
NaAlg
O
COONa
CONH2
O
n
CONH2
N
aA
lg
O COONa
H2NOC
H2NOC
n-1
n-1
O
n
Fig. 1 Proposed mechanistic
pathway for the synthesis of (Na
Alg-PVA)-g-PAAm hydrogel
AlgNa + PVA
MBA/AAm
APS
AgNO3
NaBH4
Ag+ Ag
Fig. 2 Schematic representation of the Ag nanoparticles formation in the
hydrogel network
J Polym Res (2014) 21:355 Page 5 of 14, 355
with the 1.5, 2.5, 3.5, and 4.0 g of AAm, respectively.
So, the higher monomer concentration causes a higher
rate of polymerization.
Effect of drying time
Drying is an important step in nearly every hydrogel
production process. The polymerization reaction often
continues during drying process and this stage may be
crucial in determining the residual monomer levels in
the final product. The effect of drying time on swelling
capacity of the crosslinked Alg/PVA-g-poly AAm was
studied by varying the time of drying from 10 to 50 h
at 80 °C (Fig. 7). As shown in this figure, the water
absorbency and gel content of the hydrogels are in-
creases with increasing the drying time up to 20 h and
30 h, respectively. Then, they are decreased with a
further increase in the time of drying. The maximum
absorbency (127.3 g/g) is obtained at 20 h drying.
The initial increase in the swelling value can be attributed
to formation of a porous structure as a result of removal of
solvent, and the degradation or cleavage of MBA as a
crosslinker under heating. The subsequent decrease in swell-
ing may be resulted from over crosslinking of polymeric
chains which can be occurred under heating due to the reac-
tion of unreacted species present in the hydrogel. In addition,
over crosslinking and thermal degradation also may be re-
sponsible for slightly changes in the gel content.
5001000150020002500300035004000
Tr
an
sm
it
ta
n
ce
 (%
)
Wavelength (cm-1)
Na Alg PVA
Physical mixture Hydrogel
Silver nanocomposite
Fig. 3 FT-IR spectra of NaAlg (a), PVA (b), Physical mixture of Na Alg,
PVA, and PAAm hydrogel (c), (Na Alg-PVA)-g-PAAm hydrogel (d), and
hydrogel–silver nanocomposite (e)
Fig. 4 Effect of crosslinker concentration on the swelling capacity and
the gel content of (Na Alg-PVA)-g-PAAm hydrogel. Na-Alg/PVAweight
ratio = 3, AAm weight ratio = 0.4, APS = 25 mmol/L, at 80 °C
Fig. 5 Effect of initiator concentration on the swelling capacity and the
gel content of (NaAlg-PVA)-g-PAAm hydrogel. Grafting conditions: Na-
Alg/PVAweight ratio = 3, AAmweight ratio = 0.4,MBA= 0.04mol/L, at
80 °C
Fig. 6 Effect of monomer weight ratio on the swelling capacity and the
gel content of (NaAlg-PVA)-g-PAAm hydrogel. Grafting conditions: Na-
Alg/PVA weight ratio = 3, MBA = 0.04 mol/L, APS = 25 mmol/L, at
80 °C
355, Page 6 of 14 J Polym Res (2014) 21:355
Effect of drying temperature
Drying temperature is another important factor in the produc-
tion of superabsorbent hydrogels. Figure 8, demonstrates the
effect of the drying temperature on the swelling behavior and
the gel content of the Alg/PVA-g-polyAAm hydrogels.
It shows that the water absorbency and the gel content of
the hydrogels decreases with increasing the reaction tempera-
ture up to 200 °C. Decreasing in the swelling can be attributed
to the degradation of the polysaccarid, and thermal
crosslinking of polyacrylamide that can occurs at high tem-
perature. The polysaccharide “main chain” is expected to be
degraded at high temperature. The disconnection decreases
the polysaccarid molecular weight (MW), and as a result, the
swelling of the hydrogel reduces. Moreover, thermal
crosslinking of polyacrylamide can also reduce the swelling.
Thermal crosslinking of amide groups at high temperature has
been reported for the poly (methyl methacrylate-co-N,N-
dimethylaminopropylacrylamide) [58]. Decreasing in the gel
content may be attributed to the degradation of crosslinker in
the hydrogel structure [59], and the degradation of the sodium
alginate which started at above 100 °C as shown in TGA
diagram.
Effect ofreaction temperature
To study the influence of temperature on the water absorbency
and gel content of the Alg/PVA-g-polyAAm hydrogels, the
reactions were carried out at different temperatures, ranging
from 65 to 90 °C. As shown in Fig. 9, water absorbency of the
hydrogels is increased by increasing the temperature up to
85 °C and then it is decreased. At 85 °C, maximum water
absorbency (137 g/g) was obtained. Increase in the water
absorbency and the gel content could be attributed to the
following factors: increase in the number of free radicals
Fig 7 Effect of drying time on the swelling capacity and the gel content
of (Na Alg-PVA)-g-PAAm hydrogel. Grafting conditions: Na-Alg/PVA
weight ratio = 3, AAm weight ratio = 0.4, MBA = 0.04 mol/L, APS =
25 mmol/L, at 80 °C
Fig 8 Effect of drying temperature on the swelling capacity and the gel
content of (Na Alg-PVA)-g-PAAm hydrogel. Grafting conditions: Na-
Alg/PVAweight ratio = 3, AAm weight ratio = 0.4, MBA = 0.04 mol/L,
APS = 25 mmol/L, 80 °C
Fig 9 Effect of reaction temperature on the swelling capacity and the gel
content of (Na Alg-PVA)-g-PAAm hydrogel. Grafting conditions: Na-
Alg/PVAweight ratio = 3, AAm weight ratio = 0.4, MBA = 0.04 mol/L,
APS = 25 mmol/L
Fig. 10 Effect of Alg/PVAweight ratio on the swelling capacity and the
gel content of (Na Alg-PVA)-g-PAAm hydrogel. Grafting conditions:
MBA = 0.04 mol/L, AAm weight ratio = 0.4, APS = 25 mmol/L, 80 °C
J Polym Res (2014) 21:355 Page 7 of 14, 355
formed on the polymer backbone, and the higher collision
probability with the macroradicals.
However, decreasing in water absorbency beyond 85 °C
may be attributed to the higher radical chain termination at
higher temperatures. The loss of gel content at higher temper-
ature may be attributed to the rapidly gelation of the system
that prevents the monomers and polysaccharide to fully par-
ticipate in the crosslinking polymerization. The time of gela-
tion was found to be 9.2, 7.2, 4.1, 2.4, 1.3, and 1.0 min for
reaction temperatures of 65, 70, 75, 80, 85, and 90 °C, respec-
tively. So, higher temperature leads to a higher reaction rate
during polymerization.
Effect of Alg/PVAweight ratio
The swelling capacity of the hydrogels with various weight
ratios of Alg/PVA is shown in Fig. 10. With increase the
amount of Na-Alg, water absorbency is increased but the gel
content is decreased. The time of gelation was found to be 9.0,
6.0, 4.5, 2.5, and 1.4 min for Alg/PVA weight ratio of 0.20,
0.37, 1.0, 3.0, and 5.0, respectively.
The increase in absorbency value may be attributed to
higher hydrophilicity of sodium alginate polysaccharide
rather than PVA. The sol content of the hydrogel increases
with increasing the Alg/PVAweight ratio. The higher algi-
nate content in the reaction mixture results in higher vis-
cosity which prohibits the reactant movement and effective
collision. Such behaviors are reported by other investiga-
tors [60]. This lead to rapidly gelation of the system and as a
consequent, the gel content is reduced at high Alg/PVA
weight ratio. The low gel content in overall can be due to
the presence of the atmospheric oxygen that inhibiting the
free radical polymerization and preventing a perfect gel
network formation [61].
Silver ion loading in the hydrogel matrix
The effect of verifying Ag+ ion concentration, initial MBA
concentration, and Alg/PVA weight ratio on the loading was
studied. It was obtained that the Ag+ ion loading increases
with increasing the silver ion concentration (ranged from 1 to
15 mM), as shown in Table 1. Also, loading was less in the
samplewithmoreMBA content. These results suggest that the
silver ion loading in the hydrogel matrix decreases with in-
creasing the cross-linke density, as illustrated in Table 2.
Moreover, the result indicated that the silver ion loading
increases with increasing Alg/PVA weight ratio, as shown in
Table 3. This may be because the silver ion interacts more
strongly with sodium alginate chains compared to PVA, lead-
ing to higher loading of silver ions. Another reason for in-
crease of loading is that the water absorbency increases with
increasing Alg/PVAweight ratio.
Scanning electron microscopy analysis
Morphology of the hydrogels was studied using scanning
electron microscopy (SEM). Figure 11a–c indicates the SEM
images of Alg/PVA-g-polyAAm hydrogel synthesized with
various weight ratios of Alg/PVA. It can clearly be seen that
with increase in the amount of Na-Alg, the porosity is in-
creased. Figure 11d–e, indicates the SEM image of the
hydrogels with different amount of crosslinker. The porous
structure of the hydrogels can be because of the using ethanol
as a nonsolvent during drying step. As reported by Omidian
et al. the low surface tension of alcohol prevents the porous
structure from collapse during drying [62]. With increase the
amount of crosslinker, the porosity is increased. This may be
attributed to the presence of small amount of air in the reaction
mixture. The structure porosity is arising from gelation stage.
At higher MBA concentration, a short gelation time is oc-
curred. As a result, air bubbles could not escape from the
viscose reaction mixture and thus they were trapped and
created foamy structures. Similar behavior was reported for
polymerization of partially neutralized acrylic acid in the
presence of carbon dioxide gas [63]. SEM image of silver
nanoparticle hydrogel composites are shown in Fig. 11f. The
Table 1 Effect of initial concentration of AgNO3 on the loading
Initial concentration of
AgNO3 (ppm)
MBA
(gr)
Alg/PVA
weight ratio
Loading
(ppm)
Loading
(%)
98.4 0.25 3.0/1.0 49.4 50.2
417.2 0.25 3.0/1.0 187.2 44.9
822.3 0.25 3.0/1.0 271.1 32.9
1382.3 0.25 3.0/1.0 373.5 27.0
Table 2 Effect of initial MBA concentration on the loading
MBA
(gr)
Initial concentration of
AgNO3 (ppm)
Alg/PVA
weight ratio
Loading
(ppm)
Loading
(%)
0.1 417.2 3.0/1.0 210.0 50.3
0.2 417.2 3.0/1.0 196.3 47.0
0.3 417.2 3.0/1.0 185.2 44.3
0.4 417.2 3.0/1.0 146.2 35.0
Table 3 Effect of Alg/PVAweight ratio on the loading
Alg/PVA
weight ratio
MBA (gr) Initial concentration
of AgNO3 (ppm)
Loading
(ppm)
Loading
(%)
1.0/3.0 0.25 417.2 130.2 31.2
2.0/2.0 0.25 417.2 178.2 42.7
3.0/1.0 0.25 417.2 187.2 44.9
355, Page 8 of 14 J Polym Res (2014) 21:355
size and morphology of the nanoparticles could be controlled
by modifying the networks of the hydrogels, i.e. by varying
the crosslink density, and functionalization. To control the size
of nanoparticles in the hydrogel, we prepared hydrogels with
various weight ratios of Alg/PVA. Figure 14g–i, indicates the
effect of various weight ratios of Alg/PVA on the size of silver
nanoparticles in the hydrogel. With increase the amount of
Na-Alg, the size of silver nanoparticles increases. The reason
may be attributed to the fact that with increase the amount of
Na-Alg, the porosity is increased. Increase in the porosity
provides a pathway for the reducing agents to enter into the
pores more easily to produce the higher size of nanoparticles.
As a result, the growth of the nanoparticles in the networks is
increased. Figure 11j–k demonstrates the effect of the MBA
concentration on the size of silver nanoparticles in the hydro-
gel networks.
The result indicates that highly denser hydrogel networks,
with a high MBA content, produce lower sized silver nano-
particles. The denser networks would help to stabilize and
regulate the size of nanoparticles. On the other hand, lower
MBA-crosslinked gel networks provide enough free space to
grow greater nano-sized particles.
a b c
d e f
g h
j k
i
Fig. 11 SEM photographs of Alg/PVA-g-polyAAm hydrogels with var-
iousweight ratios of Alg/PVA a0.2, b1.0, c3.0 (AAmweight ratio = 0.4),
and with different amount of crosslinker in the network d 0.015 mol/L,
and e 0.06 mol/L. SEM photographs of hydrogel–Ag nanoparticles
composites synthesized under optimized conditions f. SEM photographs
of hydrogel–Ag nanoparticles composites with various weight ratios of
Alg/PVA g 0.2, h 1.0, i3.0, and with various weight ratios of initial
crosslinker concentration, j 0.015 mol/L, and k 0.06 mol/L
J Polym Res (2014) 21:355 Page 9 of 14, 355
Such behavior was reported previously [64]. The silver
nanoparticles formed in the cross-linked networks are nearly
spherical, highly dispersed, and low size in nanometer. The
network not only helps to controlling the size of the nanopar-
ticles, but also provides better stabilization of them for longer
periods. A similar observation was reported [65]. Table 4, lists
the average chemical composition of hydrogel–Ag nanoparti-
cle composites determined by energy dispersive x-ray (EDX)
analysis.
Transmission electron microcopy (TEM) was used to find
out the size of silver nanoparticles. The hydrogel nanocom-
posite samples were dispersed in distilled water and ethanol
with an ultrasonic homogenizer. TEM micrographs reveal the
presence of nearly spherical and well-separated Ag nanopar-
ticles in the extracted Ag nanoparticles with diameters in the
range of 4–10 nm, as shown in Fig. 12.
Thermal analysis
TGA method was employed to thermally characterize the
hydrogel and nanocomposite in comparison with the intact
polymers. Figure 13, indicates that about 8–10 % weight loss
was observed below 130 °C in the TGA curve of the sodium
alginate and hydrogel. This was attributed to the removal of
absorbed water. According to the TGA curves, the values
related to the sodium alginate, and PVA around 300–
400 °C are lower than the values related to the hydrogel.
The values of T50 are 269 °C, 309 °C, and 349 °C for the
sodium alginate, PVA, and the hydrogel, respectively. It is
well-known that the ionic structure in the network may act
as heat barriers and as a consequence, enhance the overall
thermal stability of the hydrogel. Another factor in en-
hancement of the thermal stability of the hydrogels is the
cross-linking. Evidence of grafting was obtained by com-
paring the TGA of the physical mixture containing poly-
acrylamide hydrogel, sodium alginate and PVA with TGA
of Alg/PVA-g-polyAAm superabsorbent hydrogel. As
shown in Fig. 13, silver nano-particles exhibits higher
Table 4 EDX analysis
of hydrogel–Ag nano-
particles composites
Elemental composition Wt% At%
Na 1.74 7.01
S 4.23 12.23
Ag 94.03 80.76
Fig 12 TEM micrographs of
hydrogel–Ag nanocomposite;
dispersed in ethanol (a,c) and
water (b,d) with different
magnification, 125000× for a,b
and 160000 × for c,d, respectively
Fig 13 TGA Curves of a PVA, b (Na-Alg-PVA)-g-PAAm hydrogel, c
physical mixture of polyacrylamide hydrogel, sodium alginate and PVA,
dNa-Alg, and c silver nano composite at heating rate of 20 °C/min and N2
purge
355, Page 10 of 14 J Polym Res (2014) 21:355
thermal stability than the hydrogel. Similar behavior has
been reported [13]. The char yield of hydrogels containing
Ag nano particle was 52 % when heated up to 750 °C,
whereas for the hydrogel without Ag nanoparticles was
only 7 %.
Swelling kinetics
In practical applications, not only a higher swelling capacity is
required, but also a higher swelling rate is needed. It has been
suggested that the swelling kinetics of the superabsorbent is
significantly influenced by the factors such as swelling capac-
ity, size distribution of particles, specific size area and com-
position of polymer [66]. Figure 14, demonstrates that the
dynamic swelling behavior of the Alg/PVA -g-polyAAm hy-
drogel is substantially affected by concentration of the
crosslinker. Initially, the rate of the water absorbency sharply
increases and it then begins to level off. The equilibrium
swelling was achieved after 5 min.
A power law behavior is obvious from Fig. 14. The data
may be well fitted with a “Voigt-based model” Eq. 3 [67]:
St ¼ Se 1−et=τ
� �
ð3Þ
where St (g/g) is the degree of swelling at time t, Se is the
equilibrium swelling (power parameter, g/g), t is time (min)
for swelling St, and τ (min) stands for the “rate parameter”.
The rate parameter was found to be 67, 62, and 65 s, for the
superabsorbent hydrogel with initial crosslinker concentration
of 0.016, 0.033, and 0.070 mol/L, respectively. Figure 15,
represents the dynamic swelling behavior of the optimally
prepared Alg/PVA-g-polyAAm hydrogel and nanocomposite
hydrogel in distilled water. The rate parameters for superab-
sorbent hydrogel and silver nano composites with particle
sizes 250–400 μm, were found to be 54, and 45 s, respective-
ly. The appropriate “rate parameter” of these hydrogels put
them in the fast-swelling hydrogels category. Since the τvalue
is a measure of swelling rate (i.e., the lower the τ value, the
higher the rate of swelling), it can be used for comparative
evaluation of the rate of water absorption of hydrogels on the
condition that the particle size of the comparing samples are
the same or, at least, in the same range.
Antibacterial activity
The antibacterial activity of the nanocomposite hydrogels was
investigated on two model microorganisms, i.e., with gram
negative, Escherichia coli and gram positive, Staphylococcus
aureus cultures in nutrient agar medium. Table 5, exhibited the
minimal inhibitory concentration (MIC), and the minimal
bactericidal concentration (MBC) of the hydrogel–silver
nanocomposite. The antibacterial activity is resulting mainly
due to the release of silver nanoparticles from the hydrogel.
Also, it is reported that the bactericidal efficiency is affected
by the type of the microorganism, as well as the size of the
nano particles. Table 5, demonstrates that gram positive S.
aureus is more resistant to silver nanoparticles compared to
gram negative E. coli. Such behavior has been reported pre-
viously. Kim et al. [68] reported greater biocidal efficiency of
Fig 14 The swelling kinetics of superabsorbent hydrogel with different
concentration of crosslinker in distilled water
Fig 15 The swelling kinetics of the optimized sample and nanocompos-
ite hydrogel in distilled water
Table 5 MIC (μg ml−1) andMBC (μg ml−1) of the silver nanocomposite
hydrogels for E. coli and S. aureus
Culture ATCC PTCC MIC
(μg ml−1)
MBC
(μg ml−1)
MBC/MIC ratio
E. coli 8739 1330 31.25 125 4
S. aureus 6538 1112 250 1000 4
J Polym Res (2014) 21:355 Page 11 of 14, 355
the silver nanoparticles for E. coli, and attributed it to differ-
ence in cell wall structure between gram negative and gram
positive microorganisms. The MBC/MIC ratio is a parameter
that reflects the bactericidal capacity of the analyzed com-
pounds. The MBC/MIC ratio of the hydrogel–silver nano
composites for E. coli and S. aureus is achieved 4. A good
correlation is observed between the MIC and MBC values
across the two cultures. The results showed that the silver
nanocomposite hydrogels have very good antibacterial activ-
ity on gram-positive and gram-negative micro organisms.
Conclusion
Alg/PVA-g-polyAAm hydrogels were synthesized in high-
ly practical situation by graft polymerization of AAm onto
sodium alginate and PVA under normal conditions of at-
mosphere. The effect of reaction parameters, such as initial
concentration of crosslinker and initiator, reaction temper-
ature, and Alg/PVA weight ratio was studied on the swell-
ing behavior and the gel content of the superabsorbent
hydrogels. The effect of initial crosslinker concentration
and Alg/PVA weight ratio on the size of nanoparticles was
investigated. The result indicates that with increase the
amount of Na-Alg, the size of silver nanoparticles in-
creases. In addition, highly denser hydrogel networks, with
a high MBA concentration, produces lower sized of silver
nanoparticles. The silver ion loading in the hydrogel matrix
decreases with increasing in cross-linker density. In addi-
tion, the silver ion loading increases with increasing Alg/
PVA weight ratio. TEM images indicate that well defined
silver nanoparticles produced in the hydrogels. TGA dem-
onstrate that the silver nano-particles dramatically enhance
the thermal stability of the hydrogel. The hydrogels have an
appropriate “rate parameter for swelling”. The silvernano-
composite hydrogel has shown very good antibacterial ac-
tivity on gram-positive and gram-negative micro organ-
isms. The Alg/PVA-g-polyAAm hydrogel silver nanocom-
posites can be suitable for many different applications in
many fields i.e. in biological systems, wound dressing,
catalysis, and water purification.
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355, Page 14 of 14 J Polym Res (2014) 21:355
	Antimicrobial alginate/PVA silver nanocomposite hydrogel, synthesis and characterization
	Abstract
	Introduction
	Experimental
	Materials
	Hydrogel synthesis
	Preparation of hydrogel-silver nanocomposites
	Swelling measurements
	Swelling kinetics
	Gel and Sol content determination
	Determination of silver ion loading in the hydrogel matrix
	Antimicrobial assays
	Morphologies of the hydrogels
	FT-IR analysis
	Atomic absorption analysis
	Thermal analysis
	Results and discussion
	Synthesis, characterization and mechanistic aspects
	Synthesis of hydrogel–Ag nanoparticle composites
	Fourier transform infrared spectroscopy analysis
	Effect of crosslinker concentration
	Effect of APS concentration
	Effect of monomer weight ratio
	Effect of drying time
	Effect of drying temperature
	Effect of reaction temperature
	Effect of Alg/PVA weight ratio
	Silver ion loading in the hydrogel matrix
	Scanning electron microscopy analysis
	Thermal analysis
	Swelling kinetics
	Antibacterial activity
	Conclusion
	References