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A study on synthesis and properties of Ag nanoparticles immobilized polyacrylamide hydrogel composites
Materials Chemistry and Physics 103 (2007) 278–282
A study on synthesis and properties of Ag nanoparticles immobilized polyacrylamide hydrogel composites P. Saravanan ∗ , M. Padmanabha Raju, Sarfaraz Alam Defence Materials and Stores Research and Development Establishment, DMSRDE (P.O.), G.T. Road, Kanpur 208013, India Received 18 October 2005; received in revised form 27 February 2006; accepted 18 February 2007
Abstract Synthesis of Ag nanoparticles containing polyacrylamide (PAm) hydrogel composites was performed by free-radical cross-linking polymerization of acrylamide monomer in an aqueous medium containing Ag+ ions. The Ag nanoparticle/PAm composites exhibit faint yellow colour and are found to stable under ambient conditions, without undergoing oxidation. TEM micrographs reveal the presence of nearly spherical and well-separated Ag nanoparticles with diameters in the range of 4–7 nm. UV–vis studies apparently show the characteristic surface plasmon band at ∼415 nm, for the existence of Ag nanoparticles within the hydrogel matrix. The effect of varying Ag+ ion concentration within the PAm hydrogels on the amount of formation of Ag nanoparticles, as well as on the bulk properties of hydrogel nanocomposites such as equilibrium swelling, optical and electrical properties are studied. The Ag/PAm hydrogel nanocomposites have higher swelling ratio and lower electron transfer resistance than its corresponding conventional hydrogel. © 2007 Elsevier B.V. All rights reserved. Keywords: Ag nanoparticle; Hydrogel; Polyacrylamide; Solution-state polymerization
1. Introduction The synthesis of hydrogel composites containing metal nanoparticles has gained considerable attention, owing to their interesting optical, electrical and catalytic properties [1]. Hydrogels are networked structures of polymer chains cross-linked to each other and surrounded by an aqueous medium. The stability of the gel structure is due to delicate balance of hydrogen bonds and the degree of shrinking and swelling is highly dependent on the factors such as temperature [2], pH [3], pressure [4] and electric fields [5]. The swelling and shrinking properties exhibited by the hydrogel nanocomposites are currently being exploited in a number of applications including microfluidic flow [6], muscle-like actuators [7], biosensors [8,9], drug delivery [10] and switchable electronics [11,12]. The synthetic methods for metal nanoparticles immobilized hydrogel composites reported in the literature involved the preparation of nanoparticles and hydrogels separately and then
∗ Corresponding author. Present address: Defence Metallurgical Research Laboratory, Kanchanbagh P.O. Hyderabad 500058 Tel.:+91 40 24586820; fax: +91 40 24340884. E-mail address: [email protected] (P. Saravanan).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.025
physically combining two, or mixing pre-made nanoparticles with a hydrogel precursor followed by gelation [8,13,14]. Willner and co-workers [12] prepared cross-linked polyacrylamide (PAm) hydrogels on Au-wire electrodes by the electropolymerization of acrylamide monomer in presence of ZnCl2 and N,N -metheylene-bis-acrylamide. Au nanoparticles were later introduced into the hydrogels by breathing mechanism, whereby the shrunken hydrogel was allowed to swell in an aqueous Au nanoparticle solution and the structure was then reshrunk in acetone. Mayer et al. [15] reported the preparation of polyoxometalate (POM)-ferrogels by the radical copolymerization of acrylamide at 70 ◦ C in an aqueous dispersion containing pre-made ␥-Fe2 O3 using POM as cross-linker and potassium peroxodisulfate as radical initiator. In this context, herein we report a simple route for preparing PAm hydrogel nanocomposites containing homogeneously dispersed Ag nanoparticles. This method is to be distinguished from others, where the nanocomposites are prepared with hydrogel templates containing metal-reactive thiol groups [16]. In their work, hydrogel was first synthesized by cross-linking polymerization of N-isopropylacrylamide and co-monomers containing thiol as a functional group, that may form complexes with Au3+ ions. The functionalized hydrogel matrix was then modulated to the formation of colloidal Au nanoparticles, after the addition of
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Fig. 1. Scheme showing the preparation of Ag/PAm hydrogel nanocomposite.
reductant NaBH4 . In contrast, the synthetic approach employed here requires neither any sophisticated instrument nor any metalreactive functional group for the preparation of Ag nanoparticle containing PAm hydrogel composites. Solution-state polymerization of acrylamide monomer into PAm hydrogel was carried out in aqueous medium containing Ag+ ions. The Ag+ ions functionalized PAm hydrogel matrix was then hydrolyzed to yield colloidal Ag nanoparticles within the hydrogel network. The reaction scheme is illustrated in Fig. 1. 2. Experimental 2.1. Materials All the chemicals were of analytical grade and used without further purification. Monomer acrylamide and cross-linker N,N -methylene-bisacrylamide were purchased from Aldrich (Milwaukee, WI). Initiator ammonium persulfate and NaOH were purchased from Samir Tech-Chem Pvt. Ltd. (Vadodara, India). AgNO3 was purchased from Qualigens (Mumbai, India).
2.2. Synthesis of PAm hydrogels The synthesis of PAm hydrogel was performed as follows. A weighed amount of metal precursor AgNO3 dissolved in double distilled water was taken in a three-necked flask, equipped with a mechanical stirrer, thermometer and nitrogen inlet. Subsequently, desired quantity of monomer acrylamide and cross-linker N,N -methylene-bisacrylamide (MBAm) were added to the solution containing Ag+ ions. The mixture was then heated slowly until the temperature rises to 70 ◦ C, then a catalytic amount of radical initiator ammonium persulfate (APS) was added. Stirring continued maintaining the constant temperature and the gel formation (gelation) occurred exactly after 30 min of polymerization. The obtained gels were removed from the flask and washed several times with deionized water to remove unreacted reactants. Blank PAm gels were prepared in the similar manner without using metal precursor.
2.3. Synthesis of Ag/PAm hydrogel nanocomposites The synthesis of Ag/PAm hydrogel nanocomposites was accomplished by hydrolyzation of Ag+ ions functionalized PAm hydrogel matrix in 5 wt% of NaOH aqueous solution. The reduction of Ag+ ions into colloidal Ag nanoparti-
cles was allowed to proceed overnight and the complete of reduction was marked by change in colour from transparent to faint yellow colour, indicating the formation of colloidal Ag nanoparticles within the hydrogel network. The resulting nanocomposite gels were washed extensively with deionized water and kept in distilled water to reach equilibrium swelling. Similarly, Ag/PAm hydrogel nanocomposites with different molar ratios of monomer/Ag+ ion were prepared by maintaining constant MBAm and APS concentration and the reaction parameters are listed in Table 1.
2.4. Characterization A JEOL 3010 transmission electron microscope operating at 300 kV was employed for TEM studies. TEM samples of hydrogels were prepared on a 300 mesh carbon-coated copper grid; by placing it on a piece of filter paper and one drop of water containing ground hydrogel particles was applied to the grid with the help of a pipette. The filter paper served to wick away excess solution. For the UV–vis studies, swollen hydrogels were filled in 1 cm path-length quartz cuvette and the spectra were recorded on a Perkin-Elmer Lambda 900 spectrometer using deionized water for background correction. Faradaic impedance measurements were carried out by using ac impedance spectroscopy systems of EG&G Instruments, which consists of potentiostat (Model 283) coupled with frequency response analyzer (Model 1025). Impedance spectra were directly recorded over the swollen hydrogels using stainless steel plates as electrodes in the frequency range 100 kHz–10 mHz with an alternate voltage of 5 mV.
3. Results and discussion Polyacrylamide is crystalline as well as membrane-forming hydrophilic polymer and it can undergo gelation reaction in aqueous medium, due to cross-linking of the polymer [17]. It is well known that the radical initiator APS decomposes at 70 ◦ C according to the equation: 70 ◦ C
(NH4 )2 S2 O8 −→2SO4 • + 2NH4 ↓ The resulting radicals initiate the polymerization of the acrylamide monomer and simultaneously, Ag+ ions are functionalized with the hydrogel matrix. On hydrolyzation, the Ag+ ions inside the gel matrix are modulated into colloidal Ag nanoparticles. In the present study, X-ray photoelectron spectroscopy
Table 1 Properties of as-prepared Ag/PAm hydrogel nanocomposites with different molar ratios of monomer/Ag+ ion Sample
Molar ratios of monomer/Ag+ ion
Mean particle size (nm)
Degree of hydration (H)
Resistance of swollen hydrogels (k)
PAm Ag/PAm1 Ag/PAm2 Ag/PAm3
1:0 1:0.1 × 10−2 1:0.2 × 10−2 1:0.3 × 10−2
– 5.0 ± 1.7 4.8 ± 1.0 5.4 ± 0.8
90 140 147 150
4.77 0.35 0.21 0.13
APS as initiator and MBAm as cross-linker with concentrations of each 1 mmol and a reaction temperature of 70 ◦ C were the other parameters used for preparing the above samples.
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was not employed to confirm the presence of Ag+ ions within the hydrogel matrix after hydrolysis; however, our observation seems to suggest that complete reduction of Ag+ ions into Ag could be possible, on hydrolyzation. The Ag/PAm composite gels prior to hydrolysis, when stored under ambient conditions turned to black, whereas composite gels after hydrolysis even after 3 months in storage exhibit faint yellow colour. This indicates that the hydrolyzed Ag/PAm gels did not undergo any oxidation and were free from Ag+ ions. Further, UV–vis studies on the aged Ag/PAm hydrogels in water (for 3 months), as well as on the NaOH aged hydrogels (for 2 days) showed any significant change in the optical properties; indicating that size and content of the Ag nanoparticles were unaltered. Probably, above facts supports our assumption, i.e. on hydrolyzation, all the Ag+ ions functionalized with the hydrogel matrix are modulated into colloidal Ag nanoparticles. The matrix was gel-like and therefore attempts to characterize the particles by X-ray diffraction failed due to the broadening of the peaks. A representative selected area electron diffraction pattern obtained for the Ag/PAm3 nanocomposite is shown in Fig. 2a. All the four rings could be indexed to the diffraction peaks corresponding to the planes: (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of face-centered cubic Ag [18]. UV–vis spectra of the blank PAm gel as well as of the Ag/PAm composite gels before and after hydrolysis are shown in Fig. 3a. No absorption was found both in the case of blank PAm gel and Ag/PAm2 gel before hydrolysis, whereas an absorption band at ∼415 nm was observed for the hydrolyzed Ag/PAm2 gel and this is ascribed
Fig. 3. (a) UV–vis spectra of blank PAm hydrogel (· · ·), Ag/PAm2 hydrogel prior to hydrolysis (- - -), and after hydrolysis (—). (b) Spectral evolution of Ag/PAm nanocomposites corresponding to different molar ratios of monomer/Ag+ ion. (1) 1:0.1 × 10−2 ; (2) 1:0.2 × 10−2 and (3) 1:0.3 × 10−2 .
Fig. 2. (a) Selected area electron diffraction pattern and (b) histogram of Ag nanoparticles within the Ag/PAm3 hydrogel nanocomposite.
to the characteristic surface plasmon resonance of Ag nanoparticles. The appearance of the plasmon band indicated that the colloidal Ag nanoparticles have formed within the PAm matrix on hydrolysis. As can be seen in Fig. 3b, the surface plasmon bands display a symmetrical configuration and the intensity of plasmon band increases with the increase of Ag+ ion concentration. In general, the intensity enhancement of absorption band results from the increase of metal particle size, combining with the band shift [19,20]. However, in our study, no band shift was observed with the increase in concentration of Ag+ ion. This demonstrates that the size of Ag nanoparticles did not vary on changing the Ag+ ion concentration and the intensity enhancement is due to the larger number of Ag particles created with the increase in metal precursor concentration. These conclusions can be corroborated with the TEM results. The microstructures of the obtained nanocomposite materials were imaged by TEM. Fig. 4a–d depict the morphology of nanocomposites obtained with different molar ratios of
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Fig. 4. TEM micrographs of Ag/PAm hydrogel nanocomposites (a) PAm; (b) Ag/PAm1; (c) Ag/PAm2 and (d) Ag/PAm3. The scale bars in all cases correspond to 50 nm.
monomer/Ag+ ion. As can be seen in all the images (except for the blank PAm hydrogel, i.e. Fig. 4a, the polymeric networks are clearly embedded with well-separated monodisperse Ag particles. The average sizes of Ag nanoparticles estimated from the TEM analysis are presented in Table 1 and a typical histogram showing size distribution of Ag nanoparticles within the sample Ag/PAm3 is shown in Fig. 2b. A comparison of the morphologies indicate that with the increase in concentration of the metal precursor, we get hydrogels containing more number of particles with no distinctive change in particle shape and diameter. This indicates that the number of silver nuclei increased with the increase in metal precursor concentration, leading to increasing particle number of Ag in the hydrogel network; confirming the analysis of the above UV–vis absorption studies. Further, on increasing the concentration of Ag+ , a mass of separated Ag nanoparticles with controlled size was observed and this is due the ability of polymer hydrogels in regulating the particles toward a nanoscopic regime, as demonstrated in previous studies [21,22]. The influence of Ag nanoparticles on the electron transfer resistance of the hydrogel composites was examined by Faradaic impedance spectroscopy. The resistance of the swollen
nanoparticle free gel was determined to be ∼4.7 k, while the resistances of Ag/PAm1, Ag/PAm2 and Ag/PAm3 nanocomposite gels were 0.35, 0.21 and 0.13 k, respectively. The low resistance is attributed to the electrical communication between the Ag nanoparticles and hydrogels. Fig. 5 shows impedance spectra for the swollen Ag/PAm hydrogel composites and it can be seen that the interfacial electron transfer resistance decreases as the amount of Ag nanoparticle increases in the hydrogel; implying enhanced conductivity of the hydrogel matrix. In addition, we also observed that the presence of Ag nanoparticles inside the PAm hydrogels has significant influence on the equilibrium swelling properties. Equilibrium swelling of the blank and Ag/PAm hydrogels are determined as follows. Gels were dried for 1 day at room temperature and then dried in vacuum at 60 ◦ C for 24 h. After determining the weights of dried samples, the samples were equilibrated in deionized water for 3 days at room temperature and then weighed again. The equilibrium swelling or the degree of hydration (H) is defined as the weight of water uptake (g) per unit mass (g) of dried polymer [Ws/Wp − 1], where Ws is the weight of the swollen state at room temperature and Wp is that of dried state [23]. Compared with blank PAm hydrogels, the Ag/PAm hydrogels display
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the hydrogel matrix, on increasing the concentration of Ag+ ions. Compared with non-Ag-containing PAm gel, the Ag/PAm nanocomposite hydrogels display remarkably enhanced bulk properties of equilibrium swelling and electron transfer resistance, which are dependent on the amount of Ag nanoparticles present in the hydrogel network. Acknowledgements Authors thank Prof. C.N.R. Rao for giving permission to avail the TEM facility at JNCASR, Bangalore and Dr. A.V. Ramesh Kumar, DMSRDE, Kanpur, for helping with the impedance measurements. References
Fig. 5. Faradaic impedance spectra showing the decrease of electron transfer resistance with respect to increase in Ag+ ion concentration.
remarkably higher degree of hydration as indicated in Table 1. An enhanced degree of hydration of about 1.6 times higher than that of the blank PAm hydrogels (H = 90) is observed for the Ag/PAm nanocomposite hydrogels. The significant increase in the degree of hydration is attributed due to the presence of surface charge of Ag colloidal nanoparticles. It is expected that immobilization of nanoparticles within a hydrogel should result in an afflux of water to balance the osmotic pressure buildup, which causes the hydrogel to swell and this is known as the Donnan effect of polyelectrolyte gels [24]. The degree of osmotic swelling is therefore dependent on the amount of immobilized charges, which is proportional to the surface area of the Ag nanoparticles. Consequently, higher osmotic pressure inside the Ag/PAm composite gels would result from the discrete Ag nanoparticles, which possess more surface area at a fixed Ag+ ion concentration. 4. Conclusions A solution-state polymerization technique for the synthesis of cross-linked Ag/PAm hydrogel nanocomposites is demonstrated in the present study. Nanocomposites exhibit a characteristic surface plasmon band at ∼415 nm for the existence of Ag nanoparticles within the hydrogel and these gels are found to be stable under ambient conditions. Results obtained with TEM and UV–vis studies indicate that more number of Ag nanoparticles with no distinctive change in morphology is formed within
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A study on synthesis and properties of Ag nanoparticles immobilized polyacrylamide hydrogel composites P. Saravanan ∗ , M. Padmanabha Raju, Sarfaraz Alam Defence Materials and Stores Research and Development Establishment, DMSRDE (P.O.), G.T. Road, Kanpur 208013, India Received 18 October 2005; received in revised form 27 February 2006; accepted 18 February 2007
Abstract Synthesis of Ag nanoparticles containing polyacrylamide (PAm) hydrogel composites was performed by free-radical cross-linking polymerization of acrylamide monomer in an aqueous medium containing Ag+ ions. The Ag nanoparticle/PAm composites exhibit faint yellow colour and are found to stable under ambient conditions, without undergoing oxidation. TEM micrographs reveal the presence of nearly spherical and well-separated Ag nanoparticles with diameters in the range of 4–7 nm. UV–vis studies apparently show the characteristic surface plasmon band at ∼415 nm, for the existence of Ag nanoparticles within the hydrogel matrix. The effect of varying Ag+ ion concentration within the PAm hydrogels on the amount of formation of Ag nanoparticles, as well as on the bulk properties of hydrogel nanocomposites such as equilibrium swelling, optical and electrical properties are studied. The Ag/PAm hydrogel nanocomposites have higher swelling ratio and lower electron transfer resistance than its corresponding conventional hydrogel. © 2007 Elsevier B.V. All rights reserved. Keywords: Ag nanoparticle; Hydrogel; Polyacrylamide; Solution-state polymerization
1. Introduction The synthesis of hydrogel composites containing metal nanoparticles has gained considerable attention, owing to their interesting optical, electrical and catalytic properties [1]. Hydrogels are networked structures of polymer chains cross-linked to each other and surrounded by an aqueous medium. The stability of the gel structure is due to delicate balance of hydrogen bonds and the degree of shrinking and swelling is highly dependent on the factors such as temperature [2], pH [3], pressure [4] and electric fields [5]. The swelling and shrinking properties exhibited by the hydrogel nanocomposites are currently being exploited in a number of applications including microfluidic flow [6], muscle-like actuators [7], biosensors [8,9], drug delivery [10] and switchable electronics [11,12]. The synthetic methods for metal nanoparticles immobilized hydrogel composites reported in the literature involved the preparation of nanoparticles and hydrogels separately and then
∗ Corresponding author. Present address: Defence Metallurgical Research Laboratory, Kanchanbagh P.O. Hyderabad 500058 Tel.:+91 40 24586820; fax: +91 40 24340884. E-mail address: [email protected] (P. Saravanan).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.02.025
physically combining two, or mixing pre-made nanoparticles with a hydrogel precursor followed by gelation [8,13,14]. Willner and co-workers [12] prepared cross-linked polyacrylamide (PAm) hydrogels on Au-wire electrodes by the electropolymerization of acrylamide monomer in presence of ZnCl2 and N,N -metheylene-bis-acrylamide. Au nanoparticles were later introduced into the hydrogels by breathing mechanism, whereby the shrunken hydrogel was allowed to swell in an aqueous Au nanoparticle solution and the structure was then reshrunk in acetone. Mayer et al. [15] reported the preparation of polyoxometalate (POM)-ferrogels by the radical copolymerization of acrylamide at 70 ◦ C in an aqueous dispersion containing pre-made ␥-Fe2 O3 using POM as cross-linker and potassium peroxodisulfate as radical initiator. In this context, herein we report a simple route for preparing PAm hydrogel nanocomposites containing homogeneously dispersed Ag nanoparticles. This method is to be distinguished from others, where the nanocomposites are prepared with hydrogel templates containing metal-reactive thiol groups [16]. In their work, hydrogel was first synthesized by cross-linking polymerization of N-isopropylacrylamide and co-monomers containing thiol as a functional group, that may form complexes with Au3+ ions. The functionalized hydrogel matrix was then modulated to the formation of colloidal Au nanoparticles, after the addition of
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Fig. 1. Scheme showing the preparation of Ag/PAm hydrogel nanocomposite.
reductant NaBH4 . In contrast, the synthetic approach employed here requires neither any sophisticated instrument nor any metalreactive functional group for the preparation of Ag nanoparticle containing PAm hydrogel composites. Solution-state polymerization of acrylamide monomer into PAm hydrogel was carried out in aqueous medium containing Ag+ ions. The Ag+ ions functionalized PAm hydrogel matrix was then hydrolyzed to yield colloidal Ag nanoparticles within the hydrogel network. The reaction scheme is illustrated in Fig. 1. 2. Experimental 2.1. Materials All the chemicals were of analytical grade and used without further purification. Monomer acrylamide and cross-linker N,N -methylene-bisacrylamide were purchased from Aldrich (Milwaukee, WI). Initiator ammonium persulfate and NaOH were purchased from Samir Tech-Chem Pvt. Ltd. (Vadodara, India). AgNO3 was purchased from Qualigens (Mumbai, India).
2.2. Synthesis of PAm hydrogels The synthesis of PAm hydrogel was performed as follows. A weighed amount of metal precursor AgNO3 dissolved in double distilled water was taken in a three-necked flask, equipped with a mechanical stirrer, thermometer and nitrogen inlet. Subsequently, desired quantity of monomer acrylamide and cross-linker N,N -methylene-bisacrylamide (MBAm) were added to the solution containing Ag+ ions. The mixture was then heated slowly until the temperature rises to 70 ◦ C, then a catalytic amount of radical initiator ammonium persulfate (APS) was added. Stirring continued maintaining the constant temperature and the gel formation (gelation) occurred exactly after 30 min of polymerization. The obtained gels were removed from the flask and washed several times with deionized water to remove unreacted reactants. Blank PAm gels were prepared in the similar manner without using metal precursor.
2.3. Synthesis of Ag/PAm hydrogel nanocomposites The synthesis of Ag/PAm hydrogel nanocomposites was accomplished by hydrolyzation of Ag+ ions functionalized PAm hydrogel matrix in 5 wt% of NaOH aqueous solution. The reduction of Ag+ ions into colloidal Ag nanoparti-
cles was allowed to proceed overnight and the complete of reduction was marked by change in colour from transparent to faint yellow colour, indicating the formation of colloidal Ag nanoparticles within the hydrogel network. The resulting nanocomposite gels were washed extensively with deionized water and kept in distilled water to reach equilibrium swelling. Similarly, Ag/PAm hydrogel nanocomposites with different molar ratios of monomer/Ag+ ion were prepared by maintaining constant MBAm and APS concentration and the reaction parameters are listed in Table 1.
2.4. Characterization A JEOL 3010 transmission electron microscope operating at 300 kV was employed for TEM studies. TEM samples of hydrogels were prepared on a 300 mesh carbon-coated copper grid; by placing it on a piece of filter paper and one drop of water containing ground hydrogel particles was applied to the grid with the help of a pipette. The filter paper served to wick away excess solution. For the UV–vis studies, swollen hydrogels were filled in 1 cm path-length quartz cuvette and the spectra were recorded on a Perkin-Elmer Lambda 900 spectrometer using deionized water for background correction. Faradaic impedance measurements were carried out by using ac impedance spectroscopy systems of EG&G Instruments, which consists of potentiostat (Model 283) coupled with frequency response analyzer (Model 1025). Impedance spectra were directly recorded over the swollen hydrogels using stainless steel plates as electrodes in the frequency range 100 kHz–10 mHz with an alternate voltage of 5 mV.
3. Results and discussion Polyacrylamide is crystalline as well as membrane-forming hydrophilic polymer and it can undergo gelation reaction in aqueous medium, due to cross-linking of the polymer [17]. It is well known that the radical initiator APS decomposes at 70 ◦ C according to the equation: 70 ◦ C
(NH4 )2 S2 O8 −→2SO4 • + 2NH4 ↓ The resulting radicals initiate the polymerization of the acrylamide monomer and simultaneously, Ag+ ions are functionalized with the hydrogel matrix. On hydrolyzation, the Ag+ ions inside the gel matrix are modulated into colloidal Ag nanoparticles. In the present study, X-ray photoelectron spectroscopy
Table 1 Properties of as-prepared Ag/PAm hydrogel nanocomposites with different molar ratios of monomer/Ag+ ion Sample
Molar ratios of monomer/Ag+ ion
Mean particle size (nm)
Degree of hydration (H)
Resistance of swollen hydrogels (k)
PAm Ag/PAm1 Ag/PAm2 Ag/PAm3
1:0 1:0.1 × 10−2 1:0.2 × 10−2 1:0.3 × 10−2
– 5.0 ± 1.7 4.8 ± 1.0 5.4 ± 0.8
90 140 147 150
4.77 0.35 0.21 0.13
APS as initiator and MBAm as cross-linker with concentrations of each 1 mmol and a reaction temperature of 70 ◦ C were the other parameters used for preparing the above samples.
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was not employed to confirm the presence of Ag+ ions within the hydrogel matrix after hydrolysis; however, our observation seems to suggest that complete reduction of Ag+ ions into Ag could be possible, on hydrolyzation. The Ag/PAm composite gels prior to hydrolysis, when stored under ambient conditions turned to black, whereas composite gels after hydrolysis even after 3 months in storage exhibit faint yellow colour. This indicates that the hydrolyzed Ag/PAm gels did not undergo any oxidation and were free from Ag+ ions. Further, UV–vis studies on the aged Ag/PAm hydrogels in water (for 3 months), as well as on the NaOH aged hydrogels (for 2 days) showed any significant change in the optical properties; indicating that size and content of the Ag nanoparticles were unaltered. Probably, above facts supports our assumption, i.e. on hydrolyzation, all the Ag+ ions functionalized with the hydrogel matrix are modulated into colloidal Ag nanoparticles. The matrix was gel-like and therefore attempts to characterize the particles by X-ray diffraction failed due to the broadening of the peaks. A representative selected area electron diffraction pattern obtained for the Ag/PAm3 nanocomposite is shown in Fig. 2a. All the four rings could be indexed to the diffraction peaks corresponding to the planes: (1 1 1), (2 0 0), (2 2 0) and (3 1 1) of face-centered cubic Ag [18]. UV–vis spectra of the blank PAm gel as well as of the Ag/PAm composite gels before and after hydrolysis are shown in Fig. 3a. No absorption was found both in the case of blank PAm gel and Ag/PAm2 gel before hydrolysis, whereas an absorption band at ∼415 nm was observed for the hydrolyzed Ag/PAm2 gel and this is ascribed
Fig. 3. (a) UV–vis spectra of blank PAm hydrogel (· · ·), Ag/PAm2 hydrogel prior to hydrolysis (- - -), and after hydrolysis (—). (b) Spectral evolution of Ag/PAm nanocomposites corresponding to different molar ratios of monomer/Ag+ ion. (1) 1:0.1 × 10−2 ; (2) 1:0.2 × 10−2 and (3) 1:0.3 × 10−2 .
Fig. 2. (a) Selected area electron diffraction pattern and (b) histogram of Ag nanoparticles within the Ag/PAm3 hydrogel nanocomposite.
to the characteristic surface plasmon resonance of Ag nanoparticles. The appearance of the plasmon band indicated that the colloidal Ag nanoparticles have formed within the PAm matrix on hydrolysis. As can be seen in Fig. 3b, the surface plasmon bands display a symmetrical configuration and the intensity of plasmon band increases with the increase of Ag+ ion concentration. In general, the intensity enhancement of absorption band results from the increase of metal particle size, combining with the band shift [19,20]. However, in our study, no band shift was observed with the increase in concentration of Ag+ ion. This demonstrates that the size of Ag nanoparticles did not vary on changing the Ag+ ion concentration and the intensity enhancement is due to the larger number of Ag particles created with the increase in metal precursor concentration. These conclusions can be corroborated with the TEM results. The microstructures of the obtained nanocomposite materials were imaged by TEM. Fig. 4a–d depict the morphology of nanocomposites obtained with different molar ratios of
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Fig. 4. TEM micrographs of Ag/PAm hydrogel nanocomposites (a) PAm; (b) Ag/PAm1; (c) Ag/PAm2 and (d) Ag/PAm3. The scale bars in all cases correspond to 50 nm.
monomer/Ag+ ion. As can be seen in all the images (except for the blank PAm hydrogel, i.e. Fig. 4a, the polymeric networks are clearly embedded with well-separated monodisperse Ag particles. The average sizes of Ag nanoparticles estimated from the TEM analysis are presented in Table 1 and a typical histogram showing size distribution of Ag nanoparticles within the sample Ag/PAm3 is shown in Fig. 2b. A comparison of the morphologies indicate that with the increase in concentration of the metal precursor, we get hydrogels containing more number of particles with no distinctive change in particle shape and diameter. This indicates that the number of silver nuclei increased with the increase in metal precursor concentration, leading to increasing particle number of Ag in the hydrogel network; confirming the analysis of the above UV–vis absorption studies. Further, on increasing the concentration of Ag+ , a mass of separated Ag nanoparticles with controlled size was observed and this is due the ability of polymer hydrogels in regulating the particles toward a nanoscopic regime, as demonstrated in previous studies [21,22]. The influence of Ag nanoparticles on the electron transfer resistance of the hydrogel composites was examined by Faradaic impedance spectroscopy. The resistance of the swollen
nanoparticle free gel was determined to be ∼4.7 k, while the resistances of Ag/PAm1, Ag/PAm2 and Ag/PAm3 nanocomposite gels were 0.35, 0.21 and 0.13 k, respectively. The low resistance is attributed to the electrical communication between the Ag nanoparticles and hydrogels. Fig. 5 shows impedance spectra for the swollen Ag/PAm hydrogel composites and it can be seen that the interfacial electron transfer resistance decreases as the amount of Ag nanoparticle increases in the hydrogel; implying enhanced conductivity of the hydrogel matrix. In addition, we also observed that the presence of Ag nanoparticles inside the PAm hydrogels has significant influence on the equilibrium swelling properties. Equilibrium swelling of the blank and Ag/PAm hydrogels are determined as follows. Gels were dried for 1 day at room temperature and then dried in vacuum at 60 ◦ C for 24 h. After determining the weights of dried samples, the samples were equilibrated in deionized water for 3 days at room temperature and then weighed again. The equilibrium swelling or the degree of hydration (H) is defined as the weight of water uptake (g) per unit mass (g) of dried polymer [Ws/Wp − 1], where Ws is the weight of the swollen state at room temperature and Wp is that of dried state [23]. Compared with blank PAm hydrogels, the Ag/PAm hydrogels display
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the hydrogel matrix, on increasing the concentration of Ag+ ions. Compared with non-Ag-containing PAm gel, the Ag/PAm nanocomposite hydrogels display remarkably enhanced bulk properties of equilibrium swelling and electron transfer resistance, which are dependent on the amount of Ag nanoparticles present in the hydrogel network. Acknowledgements Authors thank Prof. C.N.R. Rao for giving permission to avail the TEM facility at JNCASR, Bangalore and Dr. A.V. Ramesh Kumar, DMSRDE, Kanpur, for helping with the impedance measurements. References
Fig. 5. Faradaic impedance spectra showing the decrease of electron transfer resistance with respect to increase in Ag+ ion concentration.
remarkably higher degree of hydration as indicated in Table 1. An enhanced degree of hydration of about 1.6 times higher than that of the blank PAm hydrogels (H = 90) is observed for the Ag/PAm nanocomposite hydrogels. The significant increase in the degree of hydration is attributed due to the presence of surface charge of Ag colloidal nanoparticles. It is expected that immobilization of nanoparticles within a hydrogel should result in an afflux of water to balance the osmotic pressure buildup, which causes the hydrogel to swell and this is known as the Donnan effect of polyelectrolyte gels [24]. The degree of osmotic swelling is therefore dependent on the amount of immobilized charges, which is proportional to the surface area of the Ag nanoparticles. Consequently, higher osmotic pressure inside the Ag/PAm composite gels would result from the discrete Ag nanoparticles, which possess more surface area at a fixed Ag+ ion concentration. 4. Conclusions A solution-state polymerization technique for the synthesis of cross-linked Ag/PAm hydrogel nanocomposites is demonstrated in the present study. Nanocomposites exhibit a characteristic surface plasmon band at ∼415 nm for the existence of Ag nanoparticles within the hydrogel and these gels are found to be stable under ambient conditions. Results obtained with TEM and UV–vis studies indicate that more number of Ag nanoparticles with no distinctive change in morphology is formed within
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