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Significant effects of sodium acetate, an impurity present in poly(vinyl alcohol) solution on the radiolytic formation of silver nanoparticle
ARTICLE IN PRESS
Radiation Physics and Chemistry 77 (2008) 871–876 www.elsevier.com/locate/radphyschem
Significant effects of sodium acetate, an impurity present in poly(vinyl alcohol) solution on the radiolytic formation of silver nanoparticle Junhwa Shin, Yunhye Kim, Kiwon Lee, Youn Mook Lim, Young Chang Nho Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea Received 9 November 2007; accepted 13 December 2007
Abstract A silver nanoparticle (AgNPs) stabilizer, polyvinyl alcohol (PVA) generally contains a relatively large amount of sodium acetate (NaOAc) as an impurity (up to several weight percentages) as a result of a base-catalyzed hydrolysis of poly(vinyl acetate) (PVAc). In this study, the effects of NaOAc on the radiolytic formation of AgNPs in PVA solutions were studied by using UV/vis spectroscopy. Several AgNPs were prepared by g-ray irradiation using 60Co source at various doses in the presence of various amounts of NaOAc. The UV data of the AgNPs observed at around 410 nm show that more AgNPs are generally produced as the NaOAc concentration in the PVA solution increases. Furthermore, no significant absorption band of the AgNPs was observed when the purified PVA containing a very small amount of NaOAc (less than 3 10 4 M) was applied with 1 10 3 M AgNO3 up to 10 kGy. These results reveal that NaOAc present as an impurity in PVA, plays an important role in the radiolytic formation of AgNPs. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticles; g-Radiation; PVA; Sodium acetate
1. Introduction Nano-sized metallic materials are of great interest because they can exhibit extraordinary electrical and optical properties, which are distinctly different from those of bulk (Ozin, 1992). Many research efforts have been devoted to develop synthetic methods for metallic nanoparticles and one of these synthetic methods to prepare such nanoparticles is to use g-ray irradiation, which involves the reduction of metal salts in an aqueous solution by hydrated electrons and radicals arising from g-ray radiolysis of water (Marignier et al., 1985; Belloni et al., 1998; Henglein, 1989; Kumar et al., 2005; Temgire and Joshi, 2004). Nanoparticles formed by g-ray irradiation or other synthetic methods are likely to coalesce to form larger particle clusters; as a result, an appropriate nanoparticle stabilizer (generally, polymeric molecules such as PVA (Kumar et al., 2005; Temgire and Joshi, 2004; Bogle et al., Corresponding author. Tel.: +82 63 570 3060; fax: +82 63 570 3069.
E-mail address: [email protected] (Y.C. Nho). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.12.006
2006; Mahapatra et al., 2007), PVP (Shin et al., 2004; Zhang et al., 1996), PAA (Mostafavi et al., 1993; Xu et al., 1998), and PGA (Lin and Yang, 2005; Yu D.-G et al., 2007; Yu H et al., 2007, etc.) should be added to prevent a coalescence of the nanoparticles. The polymeric stabilizers need functional groups with a high affinity to nanoparticles to ensure an interaction with a nanoparticle surface and a sufficient chain length to protect a nanoparticle from coalescing with the next one. PVA is a water-soluble synthetic polymer that has been used in a wide range of industrial, medical, and food applications. Even though many research works have been performed with PVA, not much attention has been paid to NaOAc, a major impurity present in PVA. NaOAc remains in commercial PVA as a result of a base-catalyzed hydrolysis of poly(vinyl acetate) (PVAc) and it is reported that its contents sometimes reach several percentage points (Smirnov et al., 1967). Therefore, if raw PVA is applied without a purification to stabilize the AgNPs, NaOAc that is an impurity in PVA naturally remains in PVA solution. This impurity may not affect the formation of the AgNPs if structurally related sodium citrate (NaCit)
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(Badr and Mahmoud, 2006; Yu D.-G et al., 2007; Yu H et al., 2007) or other reducing agents (Chou and Ren, 2000; Khanna et al., 2005; Mbhele et al., 2003) are used to reduce the metallic salts such as AgNO3. However, it may play an important role in the nucleation and/or growth processes of the AgNPs if other physical sources such as g-ray are applied. As far as we are aware, no one has investigated and paid attention to the effects of NaOAc in PVA on a radiolytic formation of AgNPs so far, although structurally related NaCit has been widely used as a reducing agent and even a stabilizer (Henglein and Giersig, 1999). Here, we report our results obtained from UV/vis spectral characteristics and AgNPs prepared under various conditions including the NaOAc contents in PVA, g-ray dosages, Ag ion concentration, and other carboxylates. In this experiment, NaOAc contents in raw and purified PVA polymers were measured by 1H NMR spectroscopy (Shin et al., 2008). 2. Experimental part 2.1. Materials PVA polymers (Mw 85,000–124,000, 99+% hydrolyzed, ash content maximum 1.2%), silver nitrate (AgNO3), silver acetate (AgOAc), sodium acetate (NaOAc), and sodium citrate (NaCit) were obtained from Aldrich Chemical Co. and used as received. Triple distilled water was used as a reaction solvent in all the experiments. 2.2. Preparation of the PVA solutions containing various amounts of NaOAc To measure the NaOAc contents in the PVA samples, 1H NMR spectroscopy (JEOL, 500 MHz for 1H NMR) was utilized as described in our previous work (Shin et al., 2008). About 1 wt% PVA was dissolved in D2O at 100 1C and then subjected to 1H NMR analysis (relaxation time and the number of scans were fixed at 5 s and 32, respectively). The NMR data indicate that the raw PVA material applied in this study contains ca. 1.0 wt% of NaOAc as an impurity. To obtain PVA containing various amounts of NaOAc, a PVA raw sample was washed with triple distilled water (4–5 times weight of PVA) at room temperature. At a selected time, the washing water was exchanged with fresh distilled water via two decantations and this process was repeated for the given washing period. The purified PVA samples were placed in a 60 1C oven for at least 4 days to obtain dried PVA samples and the residual NaOAc content in a purified sample was measured by using NMR spectroscopy as described earlier. In this experiment, purified PVA polymers containing 0.5 (15 min washing), 0.3 (30 min washing), 0.1 (1 h washing), and 0.02 wt% (8 h washing) were prepared and used for the experiments. PVA polymers containing different amounts of NaOAc (1.0, 0.5, 0.3, 0.1 and 0.02 wt%) were then dissolved in distilled water
using an autoclave (120 1C, 1.5 kgf/cm2, 25 min) to prepare 2.5 wt% PVA solutions. The final NaOAc concentrations in the 2.5 wt% PVA solutions were 3 10 3, 1.5 10 3, 9 10 4, 3 10 4, and 6 10 5 M, respectively. 2.3. Preparation of AgNPs in PVA solution by g-irradiation and its characterization Generally, the PVA solutions containing 1.0 10 3 M Ag ions were prepared by an addition of AgNO3 to the 2.5 wt% PVA solutions and the mixture was used for the radiolytic formation of the AgNPs. For the study of an acetate anion effect, AgOAc instead of AgNO3, was added to the purified PVA solutions (prepared from PVA washed for 8 h and containing 6 10 5 M NaOAc). NaCit for the substitution of NaOAc, was added into a 2.5 wt% PVA solution containing purified PVA and AgNO3 to investigate the carboxylate effect. 0.5 mL of a 2.5 wt% PVA solution containing AgNO3 was put into the wells of a 48-well plate (solution height is about 0.5 cm) and irradiated by g-ray using 60Co source at various doses, typically 1–10 kGy (2.0 kGy/h dose rate). The irradiated samples in the plate were then subjected to UV scans in a spectral range of 300–700 nm (BioTek, PowerWave XS). Transmission electron microscopy (TEM) was used to observe the particle size of the AgNPs and for EDS analysis. A copper grid was dipped into the irradiated solution and allowed to dry inside an oven at 35 1C before an introduction to the TEM microscope. 3. Results and discussion Several PVA samples containing various amounts of NaOAc were prepared by washing raw PVA with distilled water to investigate the effects of NaOAc present in PVA as an impurity, during the radiolytic formation of AgNPs. A typical 1H NMR spectrum of PVA in D2O is illustrated in Fig. 1A. Methyl peak of NaOAc (CH3CO2Na) present as an impurity in PVA appeared at 1.77 ppm, as a sharp singlet. Fig. 1B shows the overlapped methyl peaks of NaOAc in the raw and washed PVA polymers, and their NaOAc content values (%) were obtained by a calculation of the peak intensity as described in our previous work (Shin et al., 2008). Based on this quantification and purification, PVA polymers containing 1.0, 0.5, 0.3, 0.1, and 0.02 wt% of NaOAc were prepared and then utilized as an AgNPs stabilizer during the radiolytic formation of AgNPs in this experiment. The pH values of 2.5 wt% PVA solutions prepared with raw PVA (pH6.2) are little higher than that of 8 h washed PVA (pH5.2), due to the presence of a base impurity (NaOAc). PVA solutions containing Ag ions were irradiated by g-ray using 60Co source and then submitted to TEM for the confirmation of an AgNPs formation. The TEM image in Fig. 2A shows the AgNPs prepared with AgNO3 (1 10 3 M), 2.5 wt% PVA, and NaOAc (3 10 3 M) at a dose of 5 kGy. Since PVA, which is structurally similar to
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Fig. 1. (A) A typical 1H NMR spectrum of PVA in D2O (B) The overlapped 1H NMR peaks of NaOAc in the raw and purified PVA polymers, and their NaOAc content values (%) obtained by the calculation of peak intensity.
Fig. 2. TEM image (A) and EDS analysis (B) of the AgNPs radiotically prepared in 2.5 wt% PVA solution containing AgNO3 (1 10 3 M) and NaOAc (3 10 3 M) at a dose of 5 kGy.
2-pronanol, could be also acted as an oxidizing radical scavenger, 2-propanol was not used in this experiment (Kumar et al., 2005). The TEM results revealed a particle size distribution ranging from 10 to 30 nm. An EDS spectrum of a selected particle shows that it solely consists of Ag (Fig. 2B). The absorption band around 410 nm due to a surface plasmon absorption band has mainly been studied and discussed for the NaOAc effects during the radiolytic formation of AgNPs in a PVA solution. The absorption bands of the UV/vis spectra of the AgNPs generated from 1 10 3 M AgNO3 solutions by g-ray for a total dose of 3, 5, and 10 kGy are illustrated in Fig. 3. The results show that the absorption bands, which are proportional to the number of non-aggregated AgNPs, increases at certain
ranges of the NaOAc concentration. No absorption bands were observed from 3 10 4 to 6 10 5 M NaOAc (prepared with PVA containing 0.1 and 0.02 wt% NaOAc, respectively) up to a 10 kGy irradiation condition, suggesting that the AgNPs are not formed at these lower concentrations of NaOAc. The solutions containing these lower concentrations of NaOAc were clear after the irradiations while other solutions containing higher concentrations of NaOAc became yellow or yellow-brown. The absorption bands of the solutions containing 3 10 3 and 1.5 10 3 M NaOAc (prepared with PVA containing 1 and 0.5 wt% NaOAc, respectively) revealed a similar band intensity and shape. For up to a 10-fold higher concentration of NaOAc (i.e. 3 10 2 M, prepared with raw PVA containing 1 wt% NaOAc and additional 9 wt% of NaOAc), similar absorption bands were observed (data not shown). From these results, we concluded that an equivalent or higher concentration of NaOAc than the concentration of AgNO3 (1 10 3 M) is required to facilitate in AgNPs formations under the applied conditions. Therefore, careful attention to the NaOAc concentration arising from an impurity present in a commercial PVA should be paid because the NaOAc contents in commercial PVA polymers are varied and the applied ratio of the PVA and AgNO3 is variable depending on the fabrication conditions. The absorption bands of AgNPs prepared in the presence of NaCit (1 10 3 M) at a radiation dose of 1, 3, 5, and 10 kGy are illustrated in Fig. 4A. NaCit as a substituent for NaOAc was added into 2.5 wt% purified PVA solution containing 6 10 5 M NaOAc. Since NaCit has three carboxylic groups in a molecule while NaOAc has one carboxylic group, 1 10 3 M NaCit solution is nearly equivalent to 3 10 3 M NaOAc in terms of the number of
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Fig. 4. UV/vis absorption spectra of the AgNPs radiotically prepared from (A) AgNO3 in the purified PVA solution containing NaCit at various doses and (B) from AgOAc in the purified PVA solution.
Fig. 3. UV/vis absorption spectra of the AgNPs radiotically prepared from 1 10 3 M AgNO3 in 2.5 wt% PVA solution containing various amount of NaOAc at a dose of 3 (A), 5 (B), and 10 kGy (C).
carboxylic groups. The absorption bands of the AgNPs prepared in the presence of 1 10 3 M AgNO3 and 1 10 3 M NaCit increased as the radiation dose increased up to 5 kGy as shown in Fig. 4A. Fig. 4B shows the absorption bands of the AgNPs prepared with AgOAc (1 10 3 M) instead of AgNO3, in the purified PVA solution (6 10 5 M NaOAc). The absorption bands of the AgNPs prepared with AgOAc were observed at 3 and 5 kGy while no absorption band of the AgNPs prepared with AgNO3 was observed. These results indicate that the absorption band intensities and shapes of the AgNPs are considerably influenced by not only the Ag ion content and the radiation dose but also carboxylate content in a solution. It is interesting to find that the absorption band
of 3 10 3 M NaOAc in Fig. 3 is similar to 1 10 3 M NaCit in Fig. 4A, while 9 10 4 M NaOAc in Fig. 3 is similar to 1 10 3 M AgOAc in Fig. 4B. This result suggests that the absorption band seems to be determined by the carboxylate content when same amount of Ag ion content and radiation dose are applied to prepare AgNPs. Fig. 5 reveals how a large amount of the Ag ion and the radiation dose are necessary to prepare AgNPs in a purified PVA solution (i.e. in the deficiency of NaOAc, 6 10 5 M). The results show that a 10–100 times more Ag ion content is necessary to create the absorption bands of the AgNPs at below 10 kGy (Fig. 5A and B). The absorption band of AgNPs generated from 1 10 3 M AgNO3 was observed at 50 kGy, indicating the formation of AgNPs in the purified PVA solution at a higher dose (Fig. 5C). The absorption bands generated from 1 10 1 M AgNO3 were red-shifted and broadened as a result of the AgNPs aggregation. So far, we demonstrated that NaOAc present in PVA as an impurity plays an important role in the radiolytic formation of AgNPs. This impurity can be involved in the nucleation and/or growth process of AgNPs as a structurally related sodium citrate does (Henglein and Giersig, 1999; Olson et al., 2006). There can be two possible mechanisms of the NaOAc effects during the radiolytic formation of AgNPs. Firstly, acetate ion from NaOAc interacts with Ag ion prior to a irradiation to form Ag+? OAc complex which could be more easily reducible by a
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ion on the radiolytic formation of AgNPs, we demonstrate that NaOAc present in PVA as a major impurity significantly promotes the radiolytic formation of AgNPs based on an observation by utilizing the UV/vis spectral characteristics of AgNPs prepared in PVA solutions containing various amounts of NaOAc. Even though many research works have been performed without careful consideration of NaOAc present in commercial PVA, it must be given attention since the NaOAc content in a commercial PVA polymer and in the final PVA concentration for an AgNPs are various. In this work, 1H NMR spectroscopy was successfully applied to determine the NaOAc content in PVA polymer. Acknowledgments This work was supported by the nuclear R&D Program from the Ministry of Science & Technology, Korea. References
Fig. 5. UV/vis absorption spectra of the AgNPs radiotically prepared in the purified PVA solution containing various amount of AgNO3 at 3 (A), 10 (B), and 50 kGy (C).
irradiation than Ag+? NO3. Olson et al. (2006) reported that a single-crystal structure of AgOAc consists of Ag2(OAc)2 dimer and the Ag(I)–Ag(I) in this dimer may promote the formation of AgNPs during a thermally induced reduction of Ag carboxylates. The proximity of two Ag(I)–Ag(I) atoms may promote a radiolytic formation of AgNPs too. Secondly, Ag nuclei (Agn or Agn+) formed by an irradiation is interacted (or stabilized) with acetate ion to form Agn(or Agn+) OAc complex as found in the capping action of citrate (Henglein and Giersig, 1999) and this complex may facilitate in a subsequent growth process to create larger particles. 4. Conclusion In conclusion, although a more detailed mechanistic study is needed to further elucidate this effect of an acetate
Badr, Y., Mahmoud, M.A., 2006. Manifestation of the silver nanoparticles incorporated into the poly vinyl alcohol matrices. J. Mater. Sci. 41, 3947–3953. Belloni, J., Mostafavi, M., Remita, H., Marignier, J.L., Delcourt, M.O., 1998. Radiation-induced synthesis of mono- and multi-metallic clusters and nanocolloids. New J. Chem. 22, 1239–1255. Bogle, K.A., Dhole, S.D., Bhoraskar, V.N., 2006. Silver nanoparticles: synthesis and size control by electron irradiation. Nanotechnology 17, 3204–3208. Chou, K.-S., Ren, C.-Y., 2000. Synthesis of nanosized silver particles by chemical reduction method. Mater. Chem. Phys. 64, 241–246. Henglein, A., 1989. Small-particle research: physicochemical properties of extremely small colloidal metal and semiconductor particles. Chem. Rev. 89, 1861–1873. Henglein, A., Giersig, M., 1999. Formation of colloidal silver nanoparticles: capping action of citrate. J. Phys. Chem. B 103, 9533–9539. Khanna, P.K., Singh, N., Charan, S., Subbarao, V.V.V.S, Gokhale, R., Mulik, U.P., 2005. Synthesis and characterization of Ag/PVA nanocomposite by chemical reduction method. Mater. Chem. Phys. 93, 117–121. Kumar, M., Varshney, L., Francis, S., 2005. Radiolytic formation of Ag clusters in aqueous polyvinyl alcohol solution and hydrogel matrix. Radiat. Phys. Chem. 73, 21–27. Lin, W.-C., Yang, M.-C., 2005. Novel silver/poly(vinyl alcohol) nanocomposites for surface-enhanced Raman scattering-active substrates. Macromol. Rapid Commun. 26, 1942–1947. Mahapatra, S.K., Bogle, K.A., Dhole, S.D., Bhoraskar, V.N., 2007. Synthesis of gold and silver nanoparticles by electron irradiation at 5–15 keV energy. Nanotechnology 18, 135602 (5pp). Marignier, J.L., Belloni, J., Delcourt, M., Chevalier, J.P., 1985. New micro aggregates of non noble metals and alloys prepared by radiation induced reduction. Nature 317, 344–345. Mbhele, Z.H., Salemane, M.G., van Sittert, C.G.C.E., Nedeljkovic´, J.M., Djokovic´, V., Luyt, A.S., 2003. Fabrication and characterization of silver-polyvinyl alcohol nanocomposites. Chem. Mater. 15, 5019–5024. Mostafavi, M., Delcourt, M.O., Picq, G., 1993. Study of the interaction between poly acrylate and silver oligomer clusters. Radiat. Phys. Chem. 41, 453–459. Olson, L.P., Whitcomb, D.R., Rajeswaran, M., Blanton, T.N., Stwertka, B.J., 2006. The simple yet elusive crystal structure of silver acetate and the role of the Ag–Ag bond in the formation of silver nanoparticles during the thermally induced reduction of silver carboxylates. Chem. Mater. 18, 1667–1674.
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Ozin, G.A., 1992. Nanochemistry: synthesis in diminishing dimensions. Adv. Mater. 4, 612–649. Shin, H.S., Yang, H.J., Kim, S.B., Lee, M.S., 2004. Mechanism of growth of colloidal silver nanoparticles stabilized by polyvinyl pyrrolidone in g-irradiated silver nitrate solution. J. Colloid. Interface Sci. 274, 89–94. Shin, J., Kim, Y., Lim, Y.M., Nho, Y.C., 2008. Removal of sodium acetate in poly(vinyl alcohol) and its quantification by 1H NMR Spectroscopy. J. Appl. Polym. Sci. 107, 3179–3183. Smirnov, L.V., Kulikova, N.P., Platonova, N.V., 1967. Infrared spectra of polyvinylalcohol. Polym. Sci. USSR A9, 2849–2856. Temgire, M.K., Joshi, S.S., 2004. Optical and structural studies of silver nanoparticles. Radiat. Phys. Chem. 71, 1039–1044.
Xu, X., Yin, Y., Ge, X., Wu, H., Zhang, Z., 1998. g-Radiation synthesis of poly(acrylic acid)-metal nanocomposites. Mater. Lett. 37, 354–358. Yu, D.-G., Lin, W.-C., Lin, C.-H., Chang, L.-M., Yang, M.-C., 2007. An in situ reduction method for preparing silver/poly(vinyl alcohol) nanocomposite as surface-enhanced Raman scattering (SERS)-active substrates. Mater. Chem. Phys 101, 93–98. Yu, H., Xu, X., Chen, X., Lu, T., Zhang, P., Jing, X., 2007. Preparation and antibacterial effects of PVA–PVP hydrogels containing silver nanoparticles. J. Appl. Polym. Sci. 103, 125–133. Zhang, Z., Zhao, B., Hu, L., 1996. PVP protective mechanism of ultrafine silver powder synthesized by chemical reduction processes. J. Solid State Chem. 121, 105–110.
Radiation Physics and Chemistry 77 (2008) 871–876 www.elsevier.com/locate/radphyschem
Significant effects of sodium acetate, an impurity present in poly(vinyl alcohol) solution on the radiolytic formation of silver nanoparticle Junhwa Shin, Yunhye Kim, Kiwon Lee, Youn Mook Lim, Young Chang Nho Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongeup-si, Jeollabuk-do 580-185, Republic of Korea Received 9 November 2007; accepted 13 December 2007
Abstract A silver nanoparticle (AgNPs) stabilizer, polyvinyl alcohol (PVA) generally contains a relatively large amount of sodium acetate (NaOAc) as an impurity (up to several weight percentages) as a result of a base-catalyzed hydrolysis of poly(vinyl acetate) (PVAc). In this study, the effects of NaOAc on the radiolytic formation of AgNPs in PVA solutions were studied by using UV/vis spectroscopy. Several AgNPs were prepared by g-ray irradiation using 60Co source at various doses in the presence of various amounts of NaOAc. The UV data of the AgNPs observed at around 410 nm show that more AgNPs are generally produced as the NaOAc concentration in the PVA solution increases. Furthermore, no significant absorption band of the AgNPs was observed when the purified PVA containing a very small amount of NaOAc (less than 3 10 4 M) was applied with 1 10 3 M AgNO3 up to 10 kGy. These results reveal that NaOAc present as an impurity in PVA, plays an important role in the radiolytic formation of AgNPs. r 2007 Elsevier Ltd. All rights reserved. Keywords: Ag nanoparticles; g-Radiation; PVA; Sodium acetate
1. Introduction Nano-sized metallic materials are of great interest because they can exhibit extraordinary electrical and optical properties, which are distinctly different from those of bulk (Ozin, 1992). Many research efforts have been devoted to develop synthetic methods for metallic nanoparticles and one of these synthetic methods to prepare such nanoparticles is to use g-ray irradiation, which involves the reduction of metal salts in an aqueous solution by hydrated electrons and radicals arising from g-ray radiolysis of water (Marignier et al., 1985; Belloni et al., 1998; Henglein, 1989; Kumar et al., 2005; Temgire and Joshi, 2004). Nanoparticles formed by g-ray irradiation or other synthetic methods are likely to coalesce to form larger particle clusters; as a result, an appropriate nanoparticle stabilizer (generally, polymeric molecules such as PVA (Kumar et al., 2005; Temgire and Joshi, 2004; Bogle et al., Corresponding author. Tel.: +82 63 570 3060; fax: +82 63 570 3069.
E-mail address: [email protected] (Y.C. Nho). 0969-806X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.radphyschem.2007.12.006
2006; Mahapatra et al., 2007), PVP (Shin et al., 2004; Zhang et al., 1996), PAA (Mostafavi et al., 1993; Xu et al., 1998), and PGA (Lin and Yang, 2005; Yu D.-G et al., 2007; Yu H et al., 2007, etc.) should be added to prevent a coalescence of the nanoparticles. The polymeric stabilizers need functional groups with a high affinity to nanoparticles to ensure an interaction with a nanoparticle surface and a sufficient chain length to protect a nanoparticle from coalescing with the next one. PVA is a water-soluble synthetic polymer that has been used in a wide range of industrial, medical, and food applications. Even though many research works have been performed with PVA, not much attention has been paid to NaOAc, a major impurity present in PVA. NaOAc remains in commercial PVA as a result of a base-catalyzed hydrolysis of poly(vinyl acetate) (PVAc) and it is reported that its contents sometimes reach several percentage points (Smirnov et al., 1967). Therefore, if raw PVA is applied without a purification to stabilize the AgNPs, NaOAc that is an impurity in PVA naturally remains in PVA solution. This impurity may not affect the formation of the AgNPs if structurally related sodium citrate (NaCit)
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(Badr and Mahmoud, 2006; Yu D.-G et al., 2007; Yu H et al., 2007) or other reducing agents (Chou and Ren, 2000; Khanna et al., 2005; Mbhele et al., 2003) are used to reduce the metallic salts such as AgNO3. However, it may play an important role in the nucleation and/or growth processes of the AgNPs if other physical sources such as g-ray are applied. As far as we are aware, no one has investigated and paid attention to the effects of NaOAc in PVA on a radiolytic formation of AgNPs so far, although structurally related NaCit has been widely used as a reducing agent and even a stabilizer (Henglein and Giersig, 1999). Here, we report our results obtained from UV/vis spectral characteristics and AgNPs prepared under various conditions including the NaOAc contents in PVA, g-ray dosages, Ag ion concentration, and other carboxylates. In this experiment, NaOAc contents in raw and purified PVA polymers were measured by 1H NMR spectroscopy (Shin et al., 2008). 2. Experimental part 2.1. Materials PVA polymers (Mw 85,000–124,000, 99+% hydrolyzed, ash content maximum 1.2%), silver nitrate (AgNO3), silver acetate (AgOAc), sodium acetate (NaOAc), and sodium citrate (NaCit) were obtained from Aldrich Chemical Co. and used as received. Triple distilled water was used as a reaction solvent in all the experiments. 2.2. Preparation of the PVA solutions containing various amounts of NaOAc To measure the NaOAc contents in the PVA samples, 1H NMR spectroscopy (JEOL, 500 MHz for 1H NMR) was utilized as described in our previous work (Shin et al., 2008). About 1 wt% PVA was dissolved in D2O at 100 1C and then subjected to 1H NMR analysis (relaxation time and the number of scans were fixed at 5 s and 32, respectively). The NMR data indicate that the raw PVA material applied in this study contains ca. 1.0 wt% of NaOAc as an impurity. To obtain PVA containing various amounts of NaOAc, a PVA raw sample was washed with triple distilled water (4–5 times weight of PVA) at room temperature. At a selected time, the washing water was exchanged with fresh distilled water via two decantations and this process was repeated for the given washing period. The purified PVA samples were placed in a 60 1C oven for at least 4 days to obtain dried PVA samples and the residual NaOAc content in a purified sample was measured by using NMR spectroscopy as described earlier. In this experiment, purified PVA polymers containing 0.5 (15 min washing), 0.3 (30 min washing), 0.1 (1 h washing), and 0.02 wt% (8 h washing) were prepared and used for the experiments. PVA polymers containing different amounts of NaOAc (1.0, 0.5, 0.3, 0.1 and 0.02 wt%) were then dissolved in distilled water
using an autoclave (120 1C, 1.5 kgf/cm2, 25 min) to prepare 2.5 wt% PVA solutions. The final NaOAc concentrations in the 2.5 wt% PVA solutions were 3 10 3, 1.5 10 3, 9 10 4, 3 10 4, and 6 10 5 M, respectively. 2.3. Preparation of AgNPs in PVA solution by g-irradiation and its characterization Generally, the PVA solutions containing 1.0 10 3 M Ag ions were prepared by an addition of AgNO3 to the 2.5 wt% PVA solutions and the mixture was used for the radiolytic formation of the AgNPs. For the study of an acetate anion effect, AgOAc instead of AgNO3, was added to the purified PVA solutions (prepared from PVA washed for 8 h and containing 6 10 5 M NaOAc). NaCit for the substitution of NaOAc, was added into a 2.5 wt% PVA solution containing purified PVA and AgNO3 to investigate the carboxylate effect. 0.5 mL of a 2.5 wt% PVA solution containing AgNO3 was put into the wells of a 48-well plate (solution height is about 0.5 cm) and irradiated by g-ray using 60Co source at various doses, typically 1–10 kGy (2.0 kGy/h dose rate). The irradiated samples in the plate were then subjected to UV scans in a spectral range of 300–700 nm (BioTek, PowerWave XS). Transmission electron microscopy (TEM) was used to observe the particle size of the AgNPs and for EDS analysis. A copper grid was dipped into the irradiated solution and allowed to dry inside an oven at 35 1C before an introduction to the TEM microscope. 3. Results and discussion Several PVA samples containing various amounts of NaOAc were prepared by washing raw PVA with distilled water to investigate the effects of NaOAc present in PVA as an impurity, during the radiolytic formation of AgNPs. A typical 1H NMR spectrum of PVA in D2O is illustrated in Fig. 1A. Methyl peak of NaOAc (CH3CO2Na) present as an impurity in PVA appeared at 1.77 ppm, as a sharp singlet. Fig. 1B shows the overlapped methyl peaks of NaOAc in the raw and washed PVA polymers, and their NaOAc content values (%) were obtained by a calculation of the peak intensity as described in our previous work (Shin et al., 2008). Based on this quantification and purification, PVA polymers containing 1.0, 0.5, 0.3, 0.1, and 0.02 wt% of NaOAc were prepared and then utilized as an AgNPs stabilizer during the radiolytic formation of AgNPs in this experiment. The pH values of 2.5 wt% PVA solutions prepared with raw PVA (pH6.2) are little higher than that of 8 h washed PVA (pH5.2), due to the presence of a base impurity (NaOAc). PVA solutions containing Ag ions were irradiated by g-ray using 60Co source and then submitted to TEM for the confirmation of an AgNPs formation. The TEM image in Fig. 2A shows the AgNPs prepared with AgNO3 (1 10 3 M), 2.5 wt% PVA, and NaOAc (3 10 3 M) at a dose of 5 kGy. Since PVA, which is structurally similar to
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Fig. 1. (A) A typical 1H NMR spectrum of PVA in D2O (B) The overlapped 1H NMR peaks of NaOAc in the raw and purified PVA polymers, and their NaOAc content values (%) obtained by the calculation of peak intensity.
Fig. 2. TEM image (A) and EDS analysis (B) of the AgNPs radiotically prepared in 2.5 wt% PVA solution containing AgNO3 (1 10 3 M) and NaOAc (3 10 3 M) at a dose of 5 kGy.
2-pronanol, could be also acted as an oxidizing radical scavenger, 2-propanol was not used in this experiment (Kumar et al., 2005). The TEM results revealed a particle size distribution ranging from 10 to 30 nm. An EDS spectrum of a selected particle shows that it solely consists of Ag (Fig. 2B). The absorption band around 410 nm due to a surface plasmon absorption band has mainly been studied and discussed for the NaOAc effects during the radiolytic formation of AgNPs in a PVA solution. The absorption bands of the UV/vis spectra of the AgNPs generated from 1 10 3 M AgNO3 solutions by g-ray for a total dose of 3, 5, and 10 kGy are illustrated in Fig. 3. The results show that the absorption bands, which are proportional to the number of non-aggregated AgNPs, increases at certain
ranges of the NaOAc concentration. No absorption bands were observed from 3 10 4 to 6 10 5 M NaOAc (prepared with PVA containing 0.1 and 0.02 wt% NaOAc, respectively) up to a 10 kGy irradiation condition, suggesting that the AgNPs are not formed at these lower concentrations of NaOAc. The solutions containing these lower concentrations of NaOAc were clear after the irradiations while other solutions containing higher concentrations of NaOAc became yellow or yellow-brown. The absorption bands of the solutions containing 3 10 3 and 1.5 10 3 M NaOAc (prepared with PVA containing 1 and 0.5 wt% NaOAc, respectively) revealed a similar band intensity and shape. For up to a 10-fold higher concentration of NaOAc (i.e. 3 10 2 M, prepared with raw PVA containing 1 wt% NaOAc and additional 9 wt% of NaOAc), similar absorption bands were observed (data not shown). From these results, we concluded that an equivalent or higher concentration of NaOAc than the concentration of AgNO3 (1 10 3 M) is required to facilitate in AgNPs formations under the applied conditions. Therefore, careful attention to the NaOAc concentration arising from an impurity present in a commercial PVA should be paid because the NaOAc contents in commercial PVA polymers are varied and the applied ratio of the PVA and AgNO3 is variable depending on the fabrication conditions. The absorption bands of AgNPs prepared in the presence of NaCit (1 10 3 M) at a radiation dose of 1, 3, 5, and 10 kGy are illustrated in Fig. 4A. NaCit as a substituent for NaOAc was added into 2.5 wt% purified PVA solution containing 6 10 5 M NaOAc. Since NaCit has three carboxylic groups in a molecule while NaOAc has one carboxylic group, 1 10 3 M NaCit solution is nearly equivalent to 3 10 3 M NaOAc in terms of the number of
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Fig. 4. UV/vis absorption spectra of the AgNPs radiotically prepared from (A) AgNO3 in the purified PVA solution containing NaCit at various doses and (B) from AgOAc in the purified PVA solution.
Fig. 3. UV/vis absorption spectra of the AgNPs radiotically prepared from 1 10 3 M AgNO3 in 2.5 wt% PVA solution containing various amount of NaOAc at a dose of 3 (A), 5 (B), and 10 kGy (C).
carboxylic groups. The absorption bands of the AgNPs prepared in the presence of 1 10 3 M AgNO3 and 1 10 3 M NaCit increased as the radiation dose increased up to 5 kGy as shown in Fig. 4A. Fig. 4B shows the absorption bands of the AgNPs prepared with AgOAc (1 10 3 M) instead of AgNO3, in the purified PVA solution (6 10 5 M NaOAc). The absorption bands of the AgNPs prepared with AgOAc were observed at 3 and 5 kGy while no absorption band of the AgNPs prepared with AgNO3 was observed. These results indicate that the absorption band intensities and shapes of the AgNPs are considerably influenced by not only the Ag ion content and the radiation dose but also carboxylate content in a solution. It is interesting to find that the absorption band
of 3 10 3 M NaOAc in Fig. 3 is similar to 1 10 3 M NaCit in Fig. 4A, while 9 10 4 M NaOAc in Fig. 3 is similar to 1 10 3 M AgOAc in Fig. 4B. This result suggests that the absorption band seems to be determined by the carboxylate content when same amount of Ag ion content and radiation dose are applied to prepare AgNPs. Fig. 5 reveals how a large amount of the Ag ion and the radiation dose are necessary to prepare AgNPs in a purified PVA solution (i.e. in the deficiency of NaOAc, 6 10 5 M). The results show that a 10–100 times more Ag ion content is necessary to create the absorption bands of the AgNPs at below 10 kGy (Fig. 5A and B). The absorption band of AgNPs generated from 1 10 3 M AgNO3 was observed at 50 kGy, indicating the formation of AgNPs in the purified PVA solution at a higher dose (Fig. 5C). The absorption bands generated from 1 10 1 M AgNO3 were red-shifted and broadened as a result of the AgNPs aggregation. So far, we demonstrated that NaOAc present in PVA as an impurity plays an important role in the radiolytic formation of AgNPs. This impurity can be involved in the nucleation and/or growth process of AgNPs as a structurally related sodium citrate does (Henglein and Giersig, 1999; Olson et al., 2006). There can be two possible mechanisms of the NaOAc effects during the radiolytic formation of AgNPs. Firstly, acetate ion from NaOAc interacts with Ag ion prior to a irradiation to form Ag+? OAc complex which could be more easily reducible by a
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ion on the radiolytic formation of AgNPs, we demonstrate that NaOAc present in PVA as a major impurity significantly promotes the radiolytic formation of AgNPs based on an observation by utilizing the UV/vis spectral characteristics of AgNPs prepared in PVA solutions containing various amounts of NaOAc. Even though many research works have been performed without careful consideration of NaOAc present in commercial PVA, it must be given attention since the NaOAc content in a commercial PVA polymer and in the final PVA concentration for an AgNPs are various. In this work, 1H NMR spectroscopy was successfully applied to determine the NaOAc content in PVA polymer. Acknowledgments This work was supported by the nuclear R&D Program from the Ministry of Science & Technology, Korea. References
Fig. 5. UV/vis absorption spectra of the AgNPs radiotically prepared in the purified PVA solution containing various amount of AgNO3 at 3 (A), 10 (B), and 50 kGy (C).
irradiation than Ag+? NO3. Olson et al. (2006) reported that a single-crystal structure of AgOAc consists of Ag2(OAc)2 dimer and the Ag(I)–Ag(I) in this dimer may promote the formation of AgNPs during a thermally induced reduction of Ag carboxylates. The proximity of two Ag(I)–Ag(I) atoms may promote a radiolytic formation of AgNPs too. Secondly, Ag nuclei (Agn or Agn+) formed by an irradiation is interacted (or stabilized) with acetate ion to form Agn(or Agn+) OAc complex as found in the capping action of citrate (Henglein and Giersig, 1999) and this complex may facilitate in a subsequent growth process to create larger particles. 4. Conclusion In conclusion, although a more detailed mechanistic study is needed to further elucidate this effect of an acetate
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