Journal of Nanomaterials & Molecular NanotechnologyISSN: 2324-8777

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Research Article, J Nanomater Mol Nanotechnol Vol: 2 Issue: 7

Synthesis of Poly(vinyl alcohol)-Silver Nanocomposites and Effect of CTAB on their Morphology

Shaeel Ahmed AL-Thabaiti1, Abdullah Yousif Obaid1 and Zaheer Khan2*
1Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O. Box 80203, Jeddah- 21413, Saudi Arabia
2Nanoscience Research Laboratory, Department of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi -110025, India
Corresponding author : Dr. Zaheer Khan
Nanoscience Research Laboratory, Department of Chemistry, Jamia Millia Islamia (Central University), Jamia Nagar, New Delhi, India
E-mail: [email protected]
Received: July 08, 2013 Accepted: November 29, 2013 Published: December 03, 2013
Citation: AL-Thabaiti SA, Obaid AY, Khan Z (2013) Synthesis of Poly(vinyl alcohol)-Silver Nano-composites and Effect of CTAB on their Morphology. J Nanomater Mol Nanotechnol 2:7. doi:10.4172/2324-8777.1000135

Abstract

Synthesis of Poly(vinyl alcohol)-Silver Nanocomposites and Effect of CTAB on their Morphology

Nacre-like film of silver-polyvinyl alcohol (PVA-water soluble biologically friendly polymer) nanocomposites is prepared via chemical reduction method, using tyrosine as a catalytic and reducing agent. Preparing of silver-PVA ordered layered nanocomposites by this method is done for the first time. The Ag- nanoparticles were characterized using UV-Vis spectroscopy, which shows an absorption band at 400-475 nm confirming the formation of nanoparticles. The results accord the formation of silver nanoclusters in conjunction with nanoparticles. The Ag nanoparticles were adsorbed in the PVA matrix through electrostatic interactions. PVA capped silver nanoparticles with or without cetyltrimethylammonium bromide, CTAB lead to the formation of nanostructures of various geometric shapes and sizes. Viscosity of the CTAB-PVA mixture was also determined. The role of PVA composition on the optical properties of silver nanoparticles has been examined and discussed.

Keywords:

Keywords

Nano-Composites; Poly(vinylalcohol); Tyrosine; Reduction

Introduction

Considerable attention has been devoted to the synthesis, characterization and application (to the components of conducting metal polymer systems, nonlinear optical materials, selective and active catalysts) of polymer-, block copolymer-, triblock polymer-, biodegradable polymer-, phospholipids-nanoparticles and/or nanocomposities of silver and gold [1-6] because their distinct optical, electrical and catalytic properties in the fields of bioengineering, photonics, and electronics [7-10]. Various methods (chemical reduction, γ-irradiation, laser irradiation, sonochemical, microwave and photochemical reduction ) and two techniques such as ex-situ, nanomaterials are first produced by soft-chemistry routes and then dispersed into polymeric matrices, and in-situ, metal particles are generated inside a polymer matrix, have been used to prepare metalpolymer nanocomposites. Chemical reduction and in-situ technique are the simplest and the most commonly used bulk-solution synthetic method for uniform dispersion of nanoparticles and/or nanocomposites in polymers ability of larger scale synthesis [11-15]. The chemical reduction method is convenient, valid and economic to follow in an appropriate medium in presence of capping agent. The preparation and characterization of silver- nanocomposites (poly(vinyl alcohol)-, poly(acrylamide)- poly(acrylonitrile)-, poly(imide)-, poly(methyl methacrylate-, chitosan- Ag) [16-21] by chemical in-situ reduction method has been the subject of a large number of investigations. Guni et al. prepared the poly(vinyl alcohol)-capped colloidal Ag–TiO2 and Ag-TiO2 nanocomposites and deterime their antibacterial activity against E. coli and Bacillus subtilis [22]. Many polymer thin films of noble metal nanoparticles, Au, Ag and Cu, have been prepared by reducing polymer metal complex films [23-26]. All these methods are based on the reduction of metal ions that are dispersed in polymer matrices. The use of silver organic complexes (silver carboxylate [27] and silver alkylcarbamate [28] has also been reported as a chemical reduction method of obtaining silver nanoparticles and conductive silver tracks and coatings.
Poly(vinyl alcohol) is a water soluble, synthetic, fully degradable, odorless and nontoxic polymer. Commercial PVA is widely used in textile, paper, pharmacy, and cosmetics [29]. It also exhibits an antimicrobial activity by binding and/or adsorbed on to the positively charged surface of silver. Generally, formaldehyde, alcohol, hydrazine sodium-potassium titrate, sodium borohydride, trisodium citrate, etc. used to the preparation of silver metal nanocomposites by aqueous-phase reduction of silver salt in the presence of stabilizer at elevated temperature. It has been established that the amount and molecular weight of the stabilizer, reaction temperature and/or dose of irradiation have significant impact on the morphology of silver nano particles. Amino acids, structural unit of proteins or enzymes also play an important role in determining the morphologies of the noble metal nanocomposites [30,31]. Our aim is to provide a simple one-step chemical reduction method to the preparation of water soluble nanoparticles of Ag, Au and MnO2 using bio-molecules and CTAB as a reducing- and shape-directing agents, respectively [32-34]. Tyrosine is a non-essential most easily oxidized amino acid having three potential coordination sites (NH2, -COOH, and –OH) with a polar side group, is involved in many protein oxidations. Molecular products of tyrosine oxidation include dityrosyl units, which are involved in protein cross-linking and dopaquinone units). It has been recognized that amino and carboxyl functional groups of amino acids undergo chemical transformations while the side chain remains intact. Amino acids with aromatic side chain were oxidized more rapidly than the alkyl side chain amino acids. To the best of our knowledge, there are no reports about the use of tyrosine-Ag+ redox system in the growth of PVA-induced silver composites. In the present paper we have synthesized the water soluble nanocomposites of silver-PVA. The nanocomposites thus formed were characterized using conventional techniques, UV-Vis spectroscopy, Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM).

Experimental Section

Preparation of Silver nanocomposites
All glassware was washed by ultrasonication in a mixture of ultra pure water and non-ionic detergent and dried prior to use. The required amount of AgNO3 (99 %, Merck India) was dissolved in double distilled and de-ionized water to give a solution of 0.01 mol dm-3 solution. Tyrosine (0.01 mol dm-3) was used as reducing agent. Stock PVA (BDH, England grade powder, 99-100 % hydrolyzed) solutions were prepared by slow drop by drop addition of PVA to deionized water whilst rapidly stirring to avoid the aggregation of PVA. To 20 cm3 of AgNO3 solution (total volume=50 cm3) was added to PVA solution (=0.1 g dm-3) over a period of 30 min with constant stirring at room temperature. We did not observe the appearance of any characteristic color due to the surface plasmon resonance which ruled out the reduction of Ag+ ions into the Ag0 by PVA under our experimental conditions and PVA not acted as a reducing agent. Therefore, tyrosine (BDH, 99 %; standard solution of tyrosine was prepared by dissolving the requisite amount of tyrosine in minimum quantity of HCl and then diluted to the desired concentration with deionized water) was used as a reducing agent. It was found that reaction mixture ([Ag+]=40.0×10-4 mol dm-3 and [PVA] (=0.1 g dm-3) turns yellow in presence of [tyrosine]=20.0×10-4 mol dm-3. The yellow color confirms the formation of silver nanoparticles [4,10].
Methods of characterization
Silver nanoparticles and the composites were characterized by UV-Vis, SEM and TEM. For this purpose Shimadzu UV-Vis spectrophotometer (model UV-1800, Japan) with 1 cm path length quartz cuvettes which could scan from 175 nm to 3300 nm was used. Diluted solution of nanoparticles was filled into the quartz cuvette to obtain the spectrum. Scanning electron microscope (SEM) (QUANTA FEG 450, FEI Company, Eindhoven, The Netherland) was used to determine the morphologies of the silver particles. The samples were mounted on the stub and coated with a thin film of gold before observations and keep the material as thin as possible (the coating layer particles as small as possible in order to accurately image of the sample surface). Transmission electron micrographs were obtained by using Hitachi 7600 with an accelerating voltage of 120 kV. All TEM images samples were prepared by drop casting of nanoparticles solution onto a carbon-coated copper TEM grid, followed by air drying at room temperature.
Viscosity measurements
In order to any type of complexation between CTAB and PVA in the aqueous solution [35], viscosities are determined using an Ubbelohde viscometer thermostated at room temperature. The relative viscosities (ηr) of reaction mixtures were determined against the water by using the relation:
where η1 and d1 represent the viscosity and the density of the reaction mixture, η0 and d0 are the viscosity and the density of water at the experimental temperature and t1 and t0 are the time of flow for the mixed volume for the reaction mixture and water, respectively. Density corrections were not made, as these were negligible. Therefore, η0 was calculated by the ration of t1 and t0 (Table 1).
Table 1: Relative viscosities (ηr) as a function of [PVA]a.

Results and Discussion

Synthesis of Ag- nanocomposites
Addition of a tyrosine solution into a solution of PVA, Ag+ ions only and PVA-Ag+ results in formation of yellowish-white turbidity and a gradual color change of the precursor solution from pale yellow to brown color for Ag+ ions and PVA-Ag+ solutions, respectively, suggests the formation of Ag-nanoparticles. The reduction of Ag+ to Ag0 is accomplished by ejection of side chain hydroxyl proton of the tyrosine (vide infra). The Ag+-PVA and tyrosine-PVA solutions did not show any characteristic absorption on the electronic spectra. As shown in Figure 1, UV- vis spectra of the nanocomposites reveal surface plasmon resonance band at 425 to 475 nm due to silver nanoparticles formation. On the other, spectra were also recorded in presence of shape-directing CTAB, a broad SRP absorption band centered at 450 nm as shown in Figure 2. Interestingly, a comparison between the Figures 1 and 2 clearly shows a red shift and significant damping effect was observed in the intensity of the plasmon peak in presence of PVA. These features are associated with the better capping and tighten effect of PVA than that of CTAB. It was well known that polymers, as protecting agents, are very effective to inhibit the agglomeration of particles. It was also observed that the sharpness and peak height of the plasmon band decrease with the addition of [PVA] which might be due to the adsorption of PVA acid onto the surface of metallic silver particles, which in turn, increases the Fermi level of particles [7]. The neutral nucleophiles and neutral stabilizing polymers have strong effect on the plasmon absorption band of silver and/or metal nanometer particles and donate the electron density to the particles via lone pairs of –OH electrons [36].
Figure 1: Absorption spectra of silver sol in presence of [PVA]=0.1 g dm-3. Reaction conditions: [Ag+]=40.0×10-4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, Temperature=30°C.
Figure 2: Absorption spectra of silver sol in presence of [CTAB]=10.0×10-4 mol dm-3. Reaction conditions: [Ag+]=40.0×10-4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, Temperature=30°C.
Morphology and role of PVA
The surface morphology of synthesized PVA–Ag nanocomposite was analyzed using the SEM technique. The SEM images of resulting nanocomposites shows that the different layers of particles are arranged and formed the nacre-like PVA-Ag nanocomposites, which has somewhat spherical nanoparticles (Figure 3A). It is visible from the image that the Ag-nanoparticles are inconsistently distributed leads to the formation of bundles of Ag-nanoparticles in presence of CTAB (Figure 3B). An important role of the ability of the PVA –OH groups to coordinate Ag+ ions and its nano particles, thus providing favorable microenvironment for their capping, has been reported [37]. The nacre-like metallic nano silver is a compact two-dimensional film but not a systematic arrangement of silver nanoparticles (array film). Despite the fact that the film is not a uniform two dimensional film at microscopic level, it can be shown that the film is made up of a large number of quantum dots, small and big silver nanoparticle clusters (Figure 3A). The sizes of these nanoparticle clusters are from 5 to 25 nm. Damping effect of PVA might be due to the formation of ion-pair complex between the lone pairs of PVA HO- group and Ag+ ions whereas such type of complexation was not possible with CTAB. As a result, the reduction of Ag+ into Ag0 inhibited but not totally prevent, which in turn, decreases the nucleation rates. The metallic Ag0 converted into Ag2 + ( Ag+ + Ag0 → Ag2 +) and finally Ag4 2+ formed after dimerization (Ag2 + + Ag2 + → Ag4 2+ ; stable species of silver cluster for a long time in presence of a polyanion even under air and growth stops at the stage of this species [37]). The plasmon absorption peak shifts to a higher wavelength with the increase of aggregation of the particles, which happened with increasing the concentration of [PVA] (Figure 4). Significant additions of PVA also altered the viscosity, and hence, the mobility of Ag+ in solution. This change in mobility of Ag+ may be the reason for the decrease in the nucleation rates, leads to the formation of nacre-like Ag-PVA nanocomposites after slow association and/or adsorption of Ag4 2+ with PVA through electrostatic and van der walls forces (Figure 3A). Therefore, the reaction mechanism is actually a complex interplay of the various components. Thus, the nacre-like morphology of Ag- PVA nanocomposites may be explained in terms of PVA adsorption (although highly schematic) onto the surface of Ag- nanoparticles (Scheme 1).
Figure 3: Scanning electron micrograph (SEM) of silver nanospheres in presence of PVA=0.1 g dm-3 (A) and CTAB=10.0×10-4 mol dm-3 (B). Reaction conditions: [Ag+]=40.0×10-4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, Temperature=30°C.
Figure 4: Absorption spectra of CTAB-stabilized silver sol in presence of [PVA]=0.1 g dm-3 (Inset-0.3 g dm-3). Reaction conditions: [Ag+]=40.0×10- 4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, [CTAB]=10.0×10-4 mol dm-3, Temperature=30°C.
Scheme 1: Formation of ion-pair complex between PVA and Agnanoparticles.
Growth of Ag-nanoparticles in presence of CTAB-PVA reaction mixture
It is well known that CTAB acted as a stabilizing- complexing- and shape-directing agent during the nucleation and growth processes silver nanoparticles under different experimental conditions [38,39]. To confirm whether the PVA was capable to inhibit the growth rate of Ag-nanoparticles, spectra were also recorded in presence of CTAB-PVA reaction mixture during the formation of silver sol. As can be seen in Figure 4 (typical example), as the PVA was added, the absorbance of silver sol formation decreases, shape of the spectra entirely changed and a sharp peak developed at 450 nm instead of a broad absorption (Figure 2). Interestingly, the CTAB stabilized Ag-nanoparticles were formed faster than in the CTAB + PVA containing reaction mixture. For example, a visual observation indicates that nucleation and growth was started in about 1h (Figure 2), but needed ca. 2h in presence of [PVA] (Figure 4). Presence of PVA in the reaction mixture sterically inhibits particle nucleation and growth. As a result, the size of the particles is lower in presence of PVA. The results indicate that the presence of PVA has a dramatic effect in diminishing the absorbance of silver sol (Figure 1).
The reaction is very sensitive to small concentration of PVA. Interestingly, the nucleation and growth are very sensitive to small concentration of PVA, a concentration of ≥ 1.0 g dm-3 being enough to attain ca. 80% inhibition of reaction (Figure 5). We did not observe any significant effect of higher [PVA] on the reaction rates. It seems appropriate to consider the role of CTAB, a cationic surfactant; it forms spherical micelles at lower concentration and increases the local concentration of tyrosine through hydrophobic and electrostatic interactions. Micellar surfaces are water rich and polarities of micelle-water interfaces are lower than that of bulk water. The micelle is a porous cluster with deep water filled cavities. Therefore, the presence of Ag+ ions in the reaction site of CTAB micelles cannot be ruled out completely. As a result, electron transfer between tyrosine and Ag+ occurs in the Stern layer of CTAB micelles. Different techniques (surface tension, viscosity, conductance and cloud point measurements) have been used to study the interaction between CTAB and PVA, it has been also established that surfactantpolymer complex or polymer-nucleated micelle was found during the surfactant-polymer interactions [40]. No appreciable change of ηr was found with [PVA] at fixed [CTAB]=10.0 ×10-4 mol dm-3 (Table 1) which indicate no strong evidence for the complexation between CTAB and PVA. Thus, we may safely conclude that possibility of the reactants solubilization/incorporation into the CTAB micelles is diminished but not totally prevent due to the presence of PVA at the surface of micelles. The possibility of PVA-CTAB ion-pair formation cannot be ruled out and solubilization/incorporation of tyrosine into the CTAB micelles diminished but not totally prevent due to the presence of PVA at the surface of micelles [35]. Our results seem to suggest that there is a competition between tyrosine and PVA to solubilize and /or incorporate into the Stern layer by electrostatic or chemical forces. On the other hand, different shape of UV-vis. spectra indicates that adsorption of PVA on the surface of silver nanoparticles. As a result, the effective concentrations of tyrosine decreases which, in turn, decrease the all processes (nucleation, growth and adsorption) involved in the Ag-nanoparticles formation. Another way we can say that PVA is more hydrophobic than the tyrosine, it has greater tendency of solubilization into the micelles. The PVA will probably be located in the Stern layer. Tyrosine incorporated hydrophobically into the junctures region of Stern-Palisade layer due to the presence of phenyl group and remaining part solubilize in the outermost region of the positive head group of CTAB micelles.
Figure 5: Plots showing the inhibitory effect of [PVA] on the SRP band of CTAB-stabilized silver nanoparticles after 1h (Inset - after 4h). Reaction conditions: [Ag+]=40.0×10-4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, [CTAB]=10.0×10-4 mol dm-3, Temperature=30°C.
Figure 6 shows the TEM of the particles in presence of CTAB. Particles are roughly spherical, poly-dispersed, and aggregated with small-sized of diameter ca. 25. It has been established that an aqueous surfactant solution has three components: surfactant monomers, micellar aggregates, and monomers absorbed as a film at the interface. The surfactant is in dynamic equilibrium among all these components [40]. Surfactant monomers rapidly join and leave micelles (as micelles is not a fixed entities and have a transient character), and the aggregation number represents only an average over time. Thus, we may safely conclude that the poly-dispersed nature of resulting Ag-nanoparticles would be due the transient character of the CTAB micelles. The broadness of the peak indicates the poly-dispersed distribution of the nanoparticles in the CTABPVA matrix, which is confirmed by the TEM images. These results are in good agreement to the observations of Porel et al. regarding the synthesis and optical limiting of free-standing embedded polymer film highly monodisperse silver nanoparticles [41,42]. The nanoparticles are protected, stabilized, and /or capped by a thin layer of tyrosine along with the CTAB (Figure 6B). Thus we may safely conclude that the morphology of the silver nanoparticles changes spherical to anisotropic structures followed by an aggregation in presence of shape-directing cationic CTAB surfactant.
Figure 6: TEM images of silver nanospheres in presence of CTAB at two magnification scale. Reaction conditions: [Ag+]=40.0×10-4 mol dm-3, [tyrosine]=20.0×10-4 mol dm-3, [CTAB]=10.0×10-4 mol dm-3, Temperature=30°C.
The most important and interesting findings of the present observations are the abrupt change in the morphology of silver nanoparticles from nacre-like layered nanofilms to spherical-bunched polydispersed in presence of PVA and CTAB, respectively (Figures 3 and 6). Different role of PVA and CTAB might be due to the different nature of PVA and CTAB. Firstly, PVA may be used as a solvent but CTAB formed micelles at a specific concentration. Secondly, electrostatic interaction of Ag+ ions and Ag-nanoparticles with the lone-pairs of -OH groups of PVA and solubilization or incorporation of both reactants (Ag+ ions and tyrosine) into the junctures region of Palisade- Stern-layer of CTAB micelles through hydrophobic interactions would be responsible for the different morphology. Micelles concentrates the reactants, decreases the surface area, which in turn, nucleation rates increases in case of CTAB and small size Agnanoparticles would be formed in comparison to PVA.

Conclusion

Silver–PVA nano-composite nacre-like arranged and layered films were built during the reduction of Ag+ ions in to Ag0 by tyrosine and the Ag0 coordinated in the PVA matrix with a homogeneous dispersion. Increasing concentrations of the PVA in the formulation has no effect on the position and nature of the surface plasmon resonance band. The Ag-nanoparticles gets coated with the PVA which improves its stability and functionality. Significant changes were noted in the surface morphology and homogeneous distribution of Ag-nanoparticles was observed in presence of CTAB. Morphological studies using SEM provided interesting information about the formation of nanoparticles in the form of bundles.

Acknowledgments

This research work was funded by the Deanship of Scientific Research (DSR), King abdulaziz Univeristy, Jeddah, under grant number (171/130/1433). The authors, therefore, acknowledge with thanks DSR for technical and financial support.

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