ARTICLE IN PRESS
Radiation Physics and Chemistry 76 (2007) 1333–1336 www.elsevier.com/locate/radphyschem
Radiolytic synthesis of Ag-poly(BIS-co-HEMA-co-IA) nanocomposites Z. Kacˇarevic´-Popovic´a,, S. Tomic´b, A. Krkljesˇ a, M. Mic´ic´a, E. Suljovrujic´a a
Vincˇa Institute of Nuclear Sciences, Mike Alasa 12, P.O. Box 522, 11001 Belgrade, Serbia Faculty of Technology and Metallurgy, Belgrade University, Karnegijeva 4, P.O. Box 3503, 11120 Belgrade, Serbia
b
Abstract Ag-poy(BIS-co-HEMA-co-IA) nanocomposites are prepared via in situ reduction of silver salt embedded in swollen polymer gels by employing gamma irradiation. Hydrogels based on 2-hydroxyethyl methacrylate, itaconic acid and four types of poly(alkylene glycol) acrylate or methacrylate (Bisomers) were previously prepared using gamma irradiation. The nanocomposites are characterized by using UV–vis, swelling measurements and thermal analysis. Evolution of plasmon absorption detected by UV–vis spectrophotometry indicated generation of Ag nanoparticles in polymer hydrogels. Altering the structure of the hydrogels did not lead to alternation of the position of the absorption maximum. The bulk property of equilibrium swelling is dependent on the presence of the Ag nanostructures. The initial thermal stability of the polymer is slightly increased due to presence of silver as nanofiller. r 2007 Elsevier Ltd. All rights reserved. Keywords: Gamma irradiation; Ag nanoparticles; Hydrogels; HEMA
1. Introduction Nanocomposite materials consisting of noble metal nanoparticles embedded in synthetic polymer hydrogels have attracted attention due to applications in catalysis, photonics, electronics, optics and biomedicine. The strong optical absorption and scattering of noble metal nanoparticles is due to an effect called localized surface plasmon resonance of the electrons in the conduction bands. This effect enables their use in electronic and optoelectronic devices and the development of novel biomedical applications (Aslan et al., 2005). Particularly the Ag nanoparticles have a power to cure different diseases caused by bacteria, fungi and viruses, (Panacek et al., 2006). Therefore Ag nanoparticles are a highly promising class of nanomaterials for new biomedical applications. Although gamma irradiation has proven to be a powerful tool for synthesis and modification of materials (Marinovic-Cincovic et al., 2003), not so many studies have been reported concerning the radiolytic formation Corresponding author. Tel.: +381 11 2453986; fax: +381 11 2447382.
E-mail address:
[email protected] (Z. Kacˇarevic´-Popovic´). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.02.026
of metal nanoparticles in hydrogel matrix (Kumar et al., 2005). The radiolytic method is particularly suitable for generating metal particles in solution. The radiolytically generated species, solvated electrons and secondary radicals, exhibit strong reducing ability, so that metal ions are reduced at each encounter. On the other hand, swollen gels contain liquid-filled cavities, which were anticipated to serve as micro reactors for synthesizing nanoparticles (Krkljes et al., 2007). In the present work, the radiolytic formation of Ag nanoparticles in poly(2-hydroxyethyl methacrylate) (PHEMA)-based hydrogels, previously obtained by gamma radiolysis, was investigated. PHEMA and related hydrogels as a matrix component is selected because they have been considered for a variety of medical applications (Young et al., 1998). A PHEMA hydrogel is inerted into the normal biological processes shows resistance to degradation, is permeable to metabolites and is not absorbed by the body. Copolymerization of hydroxyethyl methacrylate with polyethylene glycol (PEG)-based polymers can be used to improve the biocompatibility of hydrogels. Moreover, itaconic acid (IA) provides polymer chains with carboxylic side groups,
ARTICLE IN PRESS Z. Kacˇarevic´-Popovic´ et al. / Radiation Physics and Chemistry 76 (2007) 1333–1336
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which are highly hydrophilic and are able to form hydrogen bonds with corresponding groups (Tasdelen et al., 2004). 2. Experimental 2.1. Materials
2.3. Characterization of nanocomposites
2-Hydroxyethyl methacrylate (HEMA) (Aldrich), itaconic acid (IA) (Aldrich), and poly(alkylene glycol) (meth) acrylates, i.e., Bisomers (BIS1-PEA6, BIS2-PPM5S, BIS3PEM63P, BIS4-PPM63E) (Laporte Chemical) and ethylene glycol dimethacrylate (EGDMA) (Aldrich) were used as reactants in the synthesis of the hydrogels. The monomers were distilled, under vacuum, prior to use. The general chemical structure of Bisomers is given in Fig. 1. Water from Millipore Milli-Q system was used. AgNO3 and 2-propanol were products of MERK. Ar gas was of high purity (99.5%). 2.2. Preparation of the nanocomposites The gels were polymerized by gamma irradiation radical copolymerization. The monomers were dissolved in 10 ml of water/ethanol mixture (1:1, by volume). The HEMA/ BIS(1, 2, 3 or 4)/IA mole ratios were 70/28/2. EGDMA was added to the reaction mixture in the amount of 0.5 mol% with respect to the total number of moles of monomers. It was found that a small amount of EGDMA can effectively facilitate the cross-linking of the methacrylate monomers during low-dose rate gamma irradiation and hence improves the cross-linking efficiency (Tomic et al., 2006). The reaction mixture was degassed prior to polymerization and placed between two glass plates, sealed with a PVC spacer. The monomer solutions were irradiated in a 60 Co radiation source, under ambient conditions, at a dose rate of 0.5 kGy h1, to absorbed dose of 25 kGy. In order to remove any unreacted chemicals, the hydrogels were immersed in deionized water, which was changed every day, for 1 week. Ag nanocomposites were prepared by swelling the crosslinked polymer samples with water solutions of 3.9 103 mol dm3 AgNO3 and 0.2 mol dm3 2-propanol for 24 h. Swelling of Ar-saturated gels was carried out in tightly closed containers for 24 h at room temperature in the dark; longer swelling period had no effect. Only gels R H2C
free of voids were employed. Gamma irradiation was performed in 60Co radiation facility, at room temperature at a dose rate of 14 kGy h1 until achieving complete reduction of Ag+ ions (total dose up to 8 kGy). The gels were cut into discs (5 mm in diameter, 1 mm thickness) and dried at room temperature to constant mass.
C
C
O
R1
O
H
n
O Bisomers Fig. 1. General chemical structure of Bisomers (R ¼ H or CH3; R1 ¼ ethylene or propylene groups; n is the number of groups).
Perkin-Elmer Lambda 5 spectrophotometer was used to record UV–vis spectra. The equilibrium degree of swelling was determined gravimetrically. The xerogel discs were immersed in an excess distilled water, to obtain equilibrium swelling at 25 1C. The degree of swelling (q) was calculated from the following Eq. (1): q¼
Wt W0 , W0
(1)
where W0 and Wt are the weights of the xerogel at time 0 and of the swollen hydrogel at time t, respectively. Dynamic swelling experiments were also investigated. The thermal properties of nanocomposites obtained by drying the Ag-poly(BIS-co-HEMA-co-IA) hydrogels in vacuum oven at 40 1C for 24 h were analyzed by heating samples of 3–5 mg in platinum sample holder at 10 1C min1 in nitrogen (26 ml min1), from ambient temperature to 550 1C, in a Perkin–Elmer TGS-2 instrument. 3. Results and discussion The primary radicals and molecules produced in water upon gamma irradiation are shown on the right-hand side of Eq. (2) H O ! e ð2:7Þ; OHd ð2:7ÞHd ð0:6Þ; H ð0:45Þ; H O ð0:7Þ. 2
aq
2
2
2
(2) The numbers in parentheses represent the respective G values. In the presence of alcohol, the OHd and Hd radicals abstract hydrogen from the alcohol to produce an alcohol radical (Kumar et al., 2005). Under the stated experimental conditions, the reduction of Ag+ ions takes place by the strongly reducing hydrated electrons and alcohol radicals nAgþ þ n eaq =CH3 Cd OHCH3 ! ðAgÞn :
(3)
After irradiation a yellow colored gel was obtained for all compositions. Reduction of Ag+ in hydrogel matrix yielded the typical surface plasmon of Ag particles with no broad absorptions at wavelengths longer than the particle plasmon band, as shown in Fig. 2a–c. Fig. 2 depicts the appearence of the crystallite surface plasmon in four different types of poly(Bis-co-HEMA-co-IA) hydrogels. Irradiation of AgNO3-loaded hydrogels resulted in a strong sharp absorption centered at 405–410 nm. No significant change in the UV–vis characteristics of the Ag
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Table 1 Equilibrium degree of swelling (qe), kinetic parameters (k) i (n) and diffusion coefficient (D) for poly(BIS-co-HEMA-co-IA) hydrogels and Ag-poly (BIS-co-HEMA-co-IA) nanocomposites, in pH 7.40
4 P(BIS1/HEMA/IA) λmax= 410,4 nm P(BIS2/HEMA/IA) λmax= 405,6 nm P(BIS3/HEMA/IA) λmax= 408,8 nm
Absorbance
3
Sample pH
2
BIS1 BIS2 BIS3 BIS4
1
300
400
500 600 Wavelenght (nm)
D 107(cm2 s1)
k
Ag +Ag Ag +Ag Ag +Ag Ag
+Ag
4.72 4.15 3.28 2.28
0.172 0.137 0.109 0.108
0.751 0.710 0.467 0.343
0.67 0.64 0.63 0.62
0.51 0.50 0.46 0.45
0.45 0.44 0.41 0.40
0.17 0.15 0.13 0.13
2.48 2.37 2.06 1.96
poly(BIS2-co-HEMA-co-IA) Ag - poly(BIS2-co-HEMA-co-IA)
700 80
P(BIS4/HEMA/IA)-hydrogel P(BIS4/HEMA/IA)-xerogel λmax= 415 nm
3
Absorbance
7.40 7.40 7.40 7.40
n
qe
100
0
2
Residual mass (%)
a
1335
60
40
20 1 0 100 0 300
400
500 600 Wavelenght (nm)
700
4 P(BIS1/HEMA/IA)-hydrogel λmax= 410 nm P(BIS1/HEMA/IA)-xerogel λmax= 415 nm
Absorbance
3
2
1
0 300
400
500 600 Wavelenght (nm)
700
Fig. 2. The UV–vis absorption spectra of Ag nanoparticles formed in the poly(BIS-co-HEMA-co-IA) hydrogels. (a) Influence of hydrogel composition. (b) Influence of drying procedure for poly(BIS4-co-HEMA-co-IA) hydrogel. (c) Influence of nanoparticles environment.
nanoparticles formed in these three hydrogels with compositions was observed (Fig. 2a). The sharp absorption pattern indicates that the particle size distribution is quite narrow. In the case of the reduction carried out in poly
200
300 400 500 Temperature (οC)
600
700
Fig. 3. Thermogravimetric curves in nitrogen for poly(BIS2-co-HEMAco-IA) xerogel and Ag–poly(BIS2-co-HEMA-co-IA) nanocomposite.
(BIS4-co-HEMA-co-IA) hydrogel, the formation of broad absorption at a wavelength of 415 nm for the particle plasmon band was observed, as shown in Fig. 2b. The absorption pattern of the xerogel of the same sample obtained by the evaporation of the water under vacuum showed the sharp absorption centered at the same wavelength. The complete reduction of Ag+ ions occurred after the drying procedure. On the other hand, in the dry polymer matrix (xerogel) of other polymer compositions, as shown in Fig. 2c, the signal not only increased in intensity compared to the hydrogel, but also shifted as a consequence of change in dielectric properties of surrounding environment (Krkljes et al., 2007). The swelling properties of hydrogels in deionized water at 25 1C are shown in Table 1. The qe values are in the range from 2.28 to 4.72. The hydrogel containing pure EG acrylate (poly(BIS1-co-HEMA-co-IA)), with the acrylate residue in the main chain and EG dangling chains, shows the highest swelling due to the highest hydrophilicity. As for the copolymers with methacrylic Bisomers, better swelling was obtained for the sample containing pure PG and shorter dangling chains (poly(BIS2-co-HEMA-co-IA)) than for those with mixed EG/PG units in the longer dangling chains (poly(BIS3-co-HEMA-co-IA) and poly (BIS4-co-HEMA-co-IA)). Introducing Ag nanoparticles in
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hydrogels leads to lower swelling degrees and kinetic parameters. According to previous investigations by FTIR spectroscopy, an interaction between Ag nanoparticles with the polymer matrix takes place over the OH groups (Krkljes et al., 2006). Probably due to these interactions the swelling properties of the hydrogels are changed. The thermogravimetric analysis of the Ag-poly(BIS-coHEMA-co-IA) nanocomposites showed decomposition profile starting at about 260 1C and continuing till about 430 1C, Fig. 3. This shows that the thermal stability of the polymer is slightly increased due to presence of silver as nanofiller, but the overall degradation pattern is not changed. 4. Conclusion In this work, a new type of Ag-hydrogel nanocomposite was synthesized from 2-hydroxyethyl methacrylate (HEMA), itaconic acid (IA) and four different poly(alkylene glycol) (meth)acrylate components (Bisomers). It was shown that Ag ions could be efficiently reduced by gamma irradiation in poly(BIS-co-HEMA-co-IA) hydrogel matrixes to obtain new types of Ag-hydrogels nanocomposites. UV–vis spectra indicated that the particle dimensions are in the nanometer range. Altering the structure of the hydrogels did not lead to alternation in the position of the plasmon absorption maximum. The bulk property of the equilibrium swelling of the hydrogels decreased due to the presence of the Ag nanostructures. The thermal stability of the polymer matrix is slightly increased due to presence of silver as nanofiller.
Acknowledgments This work has been supported by the Ministry for Science and Environment Protection of the Republic of Serbia (Grant nos. 142066, 145072, 141013).
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