Poly(vinyl alcohol) mediated synthesis of silver nanoparticles
Nandita Narayan, Asit Kumar Pramanick, Suprabha Nayar and Arvind Sinha
National Metallurgical Laboratory
(Council of Scientific and Industrial Research)
Jamshedpur 831 007, India
Abstract
Silver nanoparticles were synthesized using poly (vinyl alcohol) as reducing as well as a capping
agent to reduce the steps and parameters involved in the synthesis which enable us to control the
size of particles. The dimensions, morphology, stability and optical properties of synthesized
silver nanoparticles manifested a dependence on polymer concentrations. The silver
nanoparticles were characterized by UV-visible spectrophotometer, dynamic light scattering
(DLS), transmission electron microscope (TEM) and atomic force microscopy (AFM).
Keywords
Silver nanoparticles, Poly (vinyl alcohol), Reducing agent, Zeta potential
Introduction
Organic-inorganic nanocomposites have attracted immense attention because of their potential to
combine the features of polymeric materials with those of inorganic materials. Colloidal silver
nanoparticles synthesized in a polymer matrix have wide applications as biosensors,
antimicrobial agents, catalysts and in new generation light weight electronic devices [1]. A
battery of techniques is available in the literature to synthesize silver nanoparticles in aqueous as
well as in non aqueous medium [2-7]. The general philosophy of the synthesis of metal
nanoparticles from its salt solution is based on using a reducing agent in presence of a capping
agent. Capping agents keep the nanoparticles away from agglomeration besides modifying their
morphology as well [8 - 10]. Few examples include the work of Rita et al [11], who reported the
synthesis of silver nanoparticles using hydroquinone and sodium citrate as reducing agent with
neutral polymers poly (vinyl-pyrrolidone) and poly (vinyl alcohol) as stabilizers. Soloman et al
[12] synthesized Ag nanoparticles by reducing silver nitrate with sodium borohydride without
using any surfactant leading to aggregation. Kim et al. [13] chose various silver salts as starting
material and examined the effect of initial precursor on the rate of nanoparticle formation. They
found that in the presence of AgBF4, AgPF6 and AgClO4 the initially fast rate was reduced after
sometime, whereas in the case of silver nitrate (AgNO3) the reaction rate was slower but
constant. Prasad and co-workers [14] used sophorolipids for the synthesis and stabilization of
silver nanoparticles. On the contrary to in situ reduction of silver ions in aqueous solution, a
polymer matrix mediated reduction of silver ions has been found more suitable for the synthesis
of polymer-silver nanocomposite particles for various biomedical applications [15]. Matrix
mediated synthesis of metal nanoparticles, a derivative of biomineralization can be termed as
biomimetic synthesis and it is reported to yield nanoparticles with stringent control over shape
and size [16]. PVA has been widely used in the biomimetic synthesis of nanoparticles. In case of
silver nanoparticles various groups have reported in situ synthesis of silver nanoparticles in PVA
with and without another reducing agent [17-19]. clemensen et al have studied the effect of silver
ion concentration on PVA mediated synthesis of silver nanoparticles [20]. However no body, so
far has reported effect of PVA concentration on the synthesis of metal nanoparticles. As PVA
plays a dual role of capping as well as reducing agent its concentration is likely to have more
significant effect on the dimension, stability and optical properties of silver nanoparticles. In
order to address this issue, present manuscript describes the in situ synthesis of silver
nanoparticles using three different PVA concentrations (4%, 5% and 6%) and correlates the same
with the morphological, topographic, colloidal stability and optical properties of synthesized
silver nanoparticles.
Experimental
Materials - Silver nitrate (AgNO3) was purchased from Nice chemicals Ltd. and PVA (MW
95000, degree of hydrolysis 90%) from Acros Organics.
MethodA simple one step reaction of silver nitrate with PVA molecule is used to prepare silver–PVA
nanocolloid. Aqueous solutions of PVA were prepared by dissolving PVA in distilled water with
continuous stirring and heating at 80o C of concentrations (4%, 5%and 6%). The solutions were
kept at room temperature until the bubbles disappeared. Then equal volume of silver nitrate
solution (1M) added dropwise in PVA (4%,5%and 6%) over a magnetic stirrer at 60o – 70o to
reduce Ag+ to Ag0. The silver nanoparticles disperse in PVA molecule and color of nanocolloid
turns light brown. The color of solutions obtained with various PVA concentrations is slightly
different. The sample was cooled to room temperature just after the reaction then we
characterized them.
UV-Vis Absorption spectrophotometer
Absorption spectra of samples were recorded using Cary 50 Bio UV-Vis spectrophotometer by
VARIAN.
Dynamic Light Scattering Study (DLS)
The hydrodynamic diameter and zeta potential of nanoparticles were estimated with the help of a
Zetasizer (Malvern).
Transmission Electron Microscopy (TEM)
The dimensions of in situ synthesized silver nanoparticles in PVA were studied using
Transmission electron microscope (TEM, CM 20,CX Philips at 160kv) by putting a thousand
times diluted sample on a carbon coated copper grid.
Atomic Force Microscopy (AFM)
The topography of silver nanoparticles were observed using Atomic force microscopy (SPI3800
N, Seiko, Japan).
Results and Discussion
The above synthesis provided silver colloidal solutions of light brown colour whose
contrast seems to be decreasing with an increase in polymer concentration. Mechanism of the
synthesis of silver nanoparticles using PVA is well documented in the literature [21]. PVA is
known to have several active -OH groups capable of absorbing Ag+ ions through secondary
bonds and steric entrapment. A reaction of Ag+ with PVA leading to its association with polymer
and in situ reduction can be expressed as:
>R−OH + Ag+ → >R−O−Ag + H+
>R−O−Ag →−R=O + Ag0
>R−OH + Ag+ →−R=O + Ag0 + H+
Where , - R=O represents a monomer in a partially oxidized PVA at the reaction surface while
H+ is an acid by product HNO3. A characteristic -C=O stretching band occurs in “ –R=O ” in
infra-red spectrum at 1730 cm−1[21]. With increasing PVA concentration number of –OH group
increases and silver nanoparticles got more strongly capped by PVA which resulted in decrease
in absorbance.
The color of colloidal silver is due to a phenomenon known as surface plasmon
resonance. In silver nanoparticles the conduction band and valence band lie very close to each
other in which electrons move freely. These free electrons give rise to a surface plasmon
resonance absorption band occurring due to the collective oscillation of electrons of silver
nanoparticles in resonance with the light wave. The UV-Vis absorption spectra of the silver
nanoparticles dispersed in 4%, 5% and 6% PVA is shown in figure.1. The absorption peak is
obtained in the visible range at 440nm, 425nm and 420nm for the 4%, 5% and 6% PVA
respectively. We observed a blue shift with increasing concentration of PVA that shows a
reduction in the particle size. Absorbance is directly proportional to concentration of particles
and here it is decreasing with the increasing concentration of PVA. This implies that at higher
concentration of PVA, particles are more strongly capped which reduces their UV absorption
capability. DLS analysis of colloidal systems, provides particle size including the ligand shell (
PVA in present case) and gives hydrodynamic diameter & zeta potential value. Figure 2 shows
their variation with PVA concentration. It has been observed that hydrodynamic diameter
decreases with increasing concentration of PVA and is found in good agreement with the UV-
visible results. Zeta potential is directly proportional to stability of colloid and it is decreasing
with increasing PVA concentration. It is index of the magnitude of the interaction between
colloidal particles and tells us about stability of colloid. If all the particles in a colloid have a
large negative or positive zeta potential then they will tend to repel each other and there will be
very less chances of the particles to come together. Colloidal dispersion in aqueous media carries
an electric charge. Origin of this charge depends upon the nature of the particles and surrounding
medium. In our system, Ag+ is surrounded by OH- active group of PVA, and a negatively
charged surface is developed around Ag+ is shown in figure.3. To maintain the stability of the
colloidal system the repulsive force between the particles must be dominant. Polymer added in
system adsorb onto the particle surface, preventing the particle surfaces coming into close
contact to keep particles separated by steric repulsion. Zeta potential is measured by
electrophoresis technique. Electrophoresis is the movement of charged particle relative to liquid
it is suspended in, under the influence of an applied electric field. The velocity of particle in a
unit electric field is called electrophoretic mobility. Zeta potential is related to electrophoretic
mobility by the well known Henry equation :UE = 2ε Z f (ka)/ 3η
Where,
UE = electrophoretic mobility,
Z = zeta potential
f (ka) = Henry’s function
Electrophoretic mobility is inversely proportional to viscosity of the medium and viscosity is
directly proportional to PVA concentration, so with increasing concentration of PVA
electrophoretic mobility decreases. Zeta potential is directly proportional to electrophoretic
mobility that’s why we can say that according to Henry equation zeta potential decreases with
increasing PVA concentration and stability of colloid decreases.
Figure 4 represents the variation of hydrodynamic diameter and absorbance with PVA
concentration and absorbance decreases with increasing PVA concentration that means capping
property increases. Figure 5 represents the relation between diameter, wavelength and PVA
concentration. Wavelength and diameter both decreases showing that there is a blue shift with
decreasing particle size. Reasons for this phenomenon could be the fact that the rate of reaction
is directly proportional to the concentration of reactant according to ‘Law of mass action’ so rate
of reaction increases with PVA concentration. As the rate increases, the silver ions are consumed
faster thus leaving less possibility for particle size growth. The rate of nucleation also decreased
as a result of the addition of the polymer, because the polymer chains present in the solution
interfere with particle formation leading to enhanced steric stabilization.
TEM studies of the silver nanoparticles revealed irregular morphology of the particles in
the size range 5nm – 6nm. All the three samples revealed almost similar shape and size and
hence a representative bright field microstructure is depicted in the figure 6. A dilution of silver
nanoparticles might be responsible inorder of non obtaining distinguishable microstructural
features of the nanoparticles in different PVA concentration. In contrast to TEM studies,
topography of the silver nanoparticles by AFM has exhibited a more clear dependence of
topographic features on PVA concentration. Figures 7a, b and c showing AFM images of silver
nanopartcles in 4%, 5% and 6% respectively. AFM topographs exhibit a size range of 30 nm - 60
nm for 4 % PVA while 25 nm – 50 nm and 15 nm – 40 nm for 5% and 6% PVA respectively. As
expected the measurements made by AFM are systematically higher than one obtained by other
techniques (DLS and TEM), however 6% PVA–silver system did manifest a phase separation of
PVA tubules (Fig.7c). Phase separation of PVA may be well correlated with the destability of the
colloidal system as has already been confirmed by a systematic reduction in zeta potential by
increasing PVA concentration from 4% to 6%. Higher values of silver nanoparticle’s dimensions
by AFM may be attributed to the extended force fields associated with PVA capped silver
nanoparticles.
Conclusions
The in situ reduction of silver ions in PVA matrix is although an attractive process having
industrial potential to scale up, however, our study clearly demonstrates that PVA concentration
plays a major role in determining the dimensions as well as the stability of the silver colloidal
solution. Hence, a proper optimization is must to develop silver colloids of narrow size
distribution.
Acknowledgement
Authors express their sincere thanks to Dr. Shashi Singh, Scientist, CCMB Hyderabad for her
support in characterization of silver colloids. Nandita expresses her gratitude towards Council of
Scientific and Industrial Research for the Diamond Jubilee Research Fellowship. Department of
Science and Technology, Government of India, is also acknowledged under Indo-Bulgarian
international project.
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