- Email: [email protected]
Facile synthesis of monodispersed silica-coated magnetic nanoparticles
G Model
JIEC-1744; No. of Pages 4 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Facile synthesis of monodispersed silica-coated magnetic nanoparticles Won-Yeop Rho a, Hyung-Mo Kim a, San Kyeong b, Yoo-Lee Kang a, Dong-Hyuk Kim a, Homan Kang c, Cheolhwan Jeong b, Dong-Eun Kim a, Yoon-Sik Lee b,c, Bong-Hyun Jun a,* a b c
Department of Bioscience and Biotechnology, Konkuk University, Seoul 143-701, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Republic of Korea Nano Systems Institute and Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul 151-742, Republic of Korea
A R T I C L E I N F O
Article history: Received 14 November 2013 Accepted 4 December 2013 Available online xxx Keywords: Silica-coated magnetic nanoparticles Sto¨ber method Oleate-MNPs Ligand exchange
A B S T R A C T
Silica-coated magnetic nanoparticles (MNPs) have great potential for use in field of biotechnology owing to their unique properties, which can be manipulated by an external magnetic field gradient. Herein, we describe a method for facile synthesis of monodispersed silica-coated MNPs (MNP@SiO2 NPs). Commercially available oleate-MNPs were successfully converted to polyvinylpyrrolidone-MNPs (PVPMNPs), and then coated with silica by the modified Sto¨ber method. More than 95% of MNPs were individually coated with a silica shell; non-magnetic core silica nanoparticles (NPs) were not detected. Notably, the MNP@SiO2 NPs are highly monodispersed in size (size distribution < 2.5%) and synthesis at the scale of grams was easily obtained by a simple scale up process. Moreover, aggregation was not detected upon storage of over three months. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Because the magnetic nanoparticles (MNPs) have unique properties, which can be manipulated by an external magnetic field gradient, they have great potential for applicability in various applications such as micro fluid chips, for catalysis, and biosensing [1,2]. Synthesis of large quantities of monodispersed particles is very important, especially for biomedical applications [3,4]. One of the most promising methods for preparing monodispersed MNPs on a large scale is Hyeon’s method, which is based on thermal decomposition of an iron source in the presence of surfactants and a mild oxidant [5–7]. However, the as-prepared MNPs exhibit hydrophobicity due to the surfactants, and therefore, proper surface modification of the MNPs is required for biomedical applications. Silica has been widely incorporated in various nanoparticles (NPs), because it is nontoxic, biocompatible, optically transparent, chemically inert, thermally stable, and has a well-known surface chemistry [8–16]. Thus, coating MNPs with a silica layer can be a good strategy for bio-applications. Silica coating of oleatestabilized MNPs can be performed by simple microemulsionbased methods that allowed for monodispersed ones [14,16,17]. However, there are some disadvantages of these methods, such as aggregation of the produced MNPs upon long-term storage, non-
* Corresponding author. Tel.: +82 2 450 0521. E-mail address: [email protected] (B.-H. Jun).
reproducibility, and formation of particles without a core (noncore particle) formation. The Sto¨ber method is a widely used method for coating silica because of its advantages such as relatively mild reaction conditions, low cost, and a broad range of achievable particle size (tens to hundreds of nanometers) [18]. Thus far, hydrophilic MNPs (mostly non-uniform in size) have been coated with silica using this method [19,20]. However, the Sto¨ber method cannot be directly applied for the synthesis of hydrophobic oleate-MNPs which can be prepared in uniform size because of their hydrophobicity [21,22]. In this paper, we report large-scale synthesis of monodispersed silica coated MNPs (MNP@SiO2 NPs) generated by applying the Sto¨ber method. They were prepared from commercially available oleate-MNPs after ligand exchange with polyvinylpyrrolidone (PVP) followed by silica coating. To the best of our knowledge, our method can provide the most highly monodispersed (104.2 2.4 nm) silicacoated MNPs on the scale of grams. This was possible by applying the ligand exchange method and a well-ordered preparation step. 2. Experimental 2.1. Materials A dispersion of MNPs (18 nm in average diameter, oleatestabilized in toluene) was purchased from Ocean Nanotech, Korea. Tetraethyl orthosilicate (TEOS), polyvinylpyrrolidone (PVP-10) and
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.014
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
2
Fig. 1. (a) Illustration of the synthesis of silica-coated MNPs and (b) TEM images of MNPs at each step: (i) oleate-MNPs, (ii) PVP-MNPs, (iii) MNP@SiO2 NPs.
dichloromethane (DCM) were purchased from Sigma–Aldrich. Diethyl ether (anhydrous), dimethylformamide (DMF), ethanol (EtOH), toluene, iron oxide, cyclohexane, and ammonium hydroxide (30 wt% in water) were sourced from Daejung Parm. All chemicals were used as received without any further purification. 2.2. Ligand exchange of oleate-stabilized MNP with PVP A dispersion of oleate-stabilized MNPs in toluene (0.2 mL) was transferred into a 15 mL vial and diluted with 5 mL of DMF–DCM (1:1, v/v). To this, 60 mg of PVP was added and refluxed at 100 8C for 12 h or overnight. The reaction mixture was then added dropwise into diethyl ether (10 mL) to precipitate the polymerstabilized nanoparticles. The precipitate was washed once with diethyl ether and centrifuged at 4500 rpm for 5 min. The precipitate was transferred to 6.5 mL of EtOH to yield a stable dispersion of the PVP-stabilized MNPs. The precipitate could also be dispersed in several other solvents such as water, chloroform, dichloromethane, DMF, and DMSO to yield optically transparent dispersions. 2.3. Silica coating on PVP-stabilized MNP EtOH dispersion of phase-transferred NPs (6.5 mL) was transferred into a 15 mL vial, and 0.28 mL of ammonium hydroxide (30 wt% in water) was added, followed by addition of 0.065 mL of TEOS in EtOH solution (10 vol%). The vial was stirred for 15 h at room temperature. The silica-coated nanoparticles were then isolated by centrifugation at 9000 rpm for 1 h and washed with EtOH. The collected silica-coated NPs were dispersed in distilled water. Then, 4 mL of TEOS in EtOH solution (3 vol%) was added by means of a syringe pump at a rate of 0.4 mL/h. After stirring for one day at room temperature, the resulting silica-coated MNPs were then centrifuged at 8500 rpm for 5 min and the silica-coated MNPs were dispersed in EtOH.
a sol–gel process based on the modified Sto¨ber method. Each preparation step of MNP@SiO2 NPs was visualized as in Fig. 1a and analyzed using transmission electron microscopy (TEM) images shown in Fig. 1b. The preparation steps are described in detail as follows. We used commercially available MNP NPs (18 nm). As the preparation of MNP NPs is based on the thermal decomposition of iron–oleate complexes, MNP NPs were stabilized by oleates and thus exhibit hydrophobicity [23–25]. The hydrophobic surfaces of MNPs were modified to exhibit hydrophilicity by adding an excess amount of amphiphilic PVP to the oleate-MNPs, so that the oleate ligand on MNPs could be exchanged with PVP, and the surface of the MNPs could be silicated via the Sto¨ber method. Oleate-stabilized MNPs in chloroform were mixed with PVP dissolved in a mixture of DCM and DMF to form a clear dispersion. This mixture was kept at 100 8C overnight to enable the exchange of the oleates with excess PVP [26]. The ligand exchange process did not affect the size and shape of the MNPs, as is shown in Fig. 1b(ii). Moreover, any aggregation of the MNPs was not detected after ligand exchange. Additionally, the MNPs were able to be redispersed in hydrophilic solvents such as EtOH. This property change indicates that the oleate ligands on MNPs were successfully replaced by PVP. This ligand exchange method has been applied to other types of NPs, but, to the best of our knowledge, we are the first to apply this to MNPs.
3. Results and discussion Fig. 1a shows the fabrication process flow for the preparation of monodispersed silica-coated magnetic NPs (MNP@SiO2 NPs). The MNP@SiO2 NPs have a core–shell structure, with a magnetic core (18 nm in diameter) and a silica shell with a thickness of ca 43 nm. The MNP@SiO2 NPs were prepared through a ligand exchange and
Fig. 2. Field-dependent magnetization of silica-coated MNP NPs at 300 K.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
3
Fig. 3. (a) TEM image of synthesized MNP@SiO2 NPs and (b) photograph of MNP@SiO2 NP dispersion in EtOH.
To completely exclude non-core-type silica NPs from core–shell type MNPs, residual PVP, which might have acted as a nucleation site, was completely removed from the MNP dispersion before the silica growth step by washing with EtOH. The PVP-coated MNPs were then redispersed in EtOH overnight to avoid the formation of multiple magnetic cores. Ammonium hydroxide and TEOS was then added to the PVP-coated MNPs in EtOH to make MNP@SiO2 NPs. The silica coating step was performed by two-step condensation reactions, which consist of the formation of a silica shell on PVP-coated MNPs, and growth of a silica shell on the silica-coated PVP-coated MNPs. This two-step condensation method allowed for highly monodispersed MNPs. Because PVP has an affinity to silica, the silica layer grew around the PVP-coated MNPs in the first step. During the second step a silica source (TEOS) was slowly added to the silica-coated PVP-coated MNP dispersion to avoid nucleation in solution so that the silica-coated PVP-coated MNPs could act as the sole seeds for deposition of the silica shell. The TEM images in Fig. 1b(iii) clearly shows the following: obtained MNP@SiO2 NPs with magnetic cores, silica shells, and an average MNP@SiO2 NPs size of 88.7 3.0 nm. The color of the resulting NPs changed from black to dark yellow. More than 95% of the NPs had one magnetic core as shown in Fig. 1a(iii). Thus, not only did we successfully obtain silica NPs free of non-magnetic cores, but also highly monodispersed ones (size distribution < 5%) even of a size of ca 90 nm, which cannot be easily obtained through the Sto¨ber method. There are several strong advantages of Sto¨ber method compared with the microemulsion method; for one, the thickness of the silica shell can be easily controlled by tuning the concentration of TEOS in the reaction mixture, under relatively milder conditions, and the products are not easily aggregated during storage. The field-dependent magnetism of the silica-coated MNPs at 300 K exhibited superparamagnetic properties that reached saturation at 0.07 emu/g, as shown in Fig. 2. Our process can be easily scaled up. When ten times the source of PVP-MNP, NH4OH, and TEOS were utilized, MNP@SiO2 NPs could be measured in grams (1.08 g). The prepared MNP@SiO2 NPs were also highly monodispersed in size (104.2 2.4 nm), as shown in Fig. 3a. Each of the NPs had a single magnetic core (multi-magnetic core NPs < 5%), and no empty core silica NPs were produced. We also confirmed the stability of NPs during storage. Being different from the microemulsion method, the silica NPs prepared using the Sto¨ber method did not contain any surfactant on their surface and thus less aggregation occurred during storage. The
prepared MNP@SiO2 NPs remained well dispersed in EtOH for at least three months, as shown in Fig. 3b. Even if some precipitations were detected, they can be redispersed by simple shaking, and thus aggregation did not occur even after long-term storage. 4. Conclusion We have described a facile method for the preparation of MNP@SiO2 NPs by modifying the Sto¨ber method. Commercially available oleate-MNPs were successfully converted to PVP-MNPs. These PVP-stabilized NPs were individually coated with a silica shell (>95%) and non-core silica NPs were not detected. Especially, the NPs were highly monodispersed in size (<2.5%) and aggregation did not happen during the ligand exchange and silica coating steps. The MNP@SiO2 NPs have superparamagnetic properties. The MNPs can be prepared in gram scale and no aggregation occurred upon storage for at least three months. Because our method can produce highly monodispersed MNPs without particle aggregation, it has great potential in various applications, such as a labeling tool in on-chip immunoassay, in magnetic manipulation of cells, and for controlled cell sorting. Our synthesis strategy is currently being expanded in scope to include metallic and semiconducting hybrid materials, the results from which will be reported elsewhere in the near future. Acknowledgment This study was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (HI13C-1299-010013). W.-Y. Rho and H.-M. Kim contributed equally to this work. References [1] J.M. Nam, C.S. Thaxton, C.A. Mirkin, Science 301 (2003) 1884. [2] Z.L. Cheng, A. Al Zaki, J.Z. Hui, V.R. Muzykantov, A. Tsourkas, Science 338 (2012) 903. [3] P. Majewski, B. Thierry, Critical Reviews in Solid State and Materials Sciences 32 (2007) 203. [4] L.L. Vatta, R.D. Sanderson, K.R. Koch, Journal of Magnetism and Magnetic Materials 311 (2007) 114. [5] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H. Bin Na, Journal of the American Chemical Society 123 (2001) 12798. [6] J. Park, K.J. An, Y.S. Hwang, J.G. Park, H.J. Noh, J.Y. Kim, J.H. Park, N.M. Hwang, T. Hyeon, Nature Materials 3 (2004) 891. [7] J. Park, Y.H. Kim, H.J. Yoon, B.H. Jun, Y.S. Lee, Journal of Industrial and Engineering Chemistry 17 (2011) 794.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 4
W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
[8] M.S. Noh, B.H. Jun, S. Kim, H. Kang, M.A. Woo, A. Minai-Tehrani, J.E. Kim, J. Kim, J. Park, H.T. Lim, S.C. Park, T. Hyeon, Y.K. Kim, D.H. Jeong, Y.S. Lee, M.H. Cho, Biomaterials 30 (2009) 3915. [9] W.R. Zhao, J.L. Gu, L.X. Zhang, H.R. Chen, J.L. Shi, Journal of the American Chemical Society 127 (2005) 8916. [10] R.I. Nooney, T. Dhanasekaran, Y.M. Chen, R. Josephs, A.E. Ostafin, Advanced Materials 14 (2002) 529. [11] K. Qian, Z.Q. Jiang, W.X. Huang, Journal of Molecular Catalysis A: Chemical 264 (2007) 26. [12] B.H. Jun, D.W. Hwang, H.S. Jung, J. Jang, H. Kim, H. Kang, T. Kang, S. Kyeong, H. Lee, D.H. Jeong, K.W. Kang, H. Youn, D.S. Lee, Y.S. Lee, Advanced Functional Materials 22 (2012) 1843. [13] J.H. Kim, J.S. Kim, H. Choi, S.M. Lee, B.H. Jun, K.N. Yu, E. Kuk, Y.K. Kim, D.H. Jeong, M.H. Cho, Y.S. Lee, Analytical Chemistry 78 (2006) 6967. [14] B.H. Jun, M.S. Noh, J. Kim, G. Kim, H. Kang, M.S. Kim, Y.T. Seo, J. Baek, J.H. Kim, J. Park, S. Kim, Y.K. Kim, T. Hyeon, M.H. Cho, D.H. Jeong, Y.S. Lee, Small 6 (2010) 119. [15] C.Z. Yu, J. Fan, B.Z. Tian, D.Y. Zhao, Chemistry of Materials 16 (2004) 889.
[16] B.H. Jun, G. Kim, M.S. Noh, H. Kang, Y.K. Kim, M.H. Cho, D.H. Jeong, Y.S. Lee, Nanomedicine 6 (2011) 1463. [17] D.C. Lee, F.V. Mikulec, J.M. Pelaez, B. Koo, B.A. Korgel, Journal of Physical Chemistry B 110 (2006) 11160. [18] R. Khatiri, A. Reyhani, S.Z. Mortazavi, M. Hossainalipour, Journal of Industrial and Engineering Chemistry 19 (2013) 1642. [19] Y. Kobayashi, M. Horie, M. Konno, B. Rodriguez-Gonzalez, L.M. Liz-Marzan, Journal of Physical Chemistry B 107 (2003) 7420. [20] Y.A. Barnakov, M.H. Yu, Z. Rosenzweig, Langmuir 21 (2005) 7524. [21] C.L. Fang, K. Qian, J.H. Zhu, S.B. Wang, X.X. Lv, S.H. Yu, Nanotechnology 19 (2008) 125601. [22] C. Graf, D.L.J. Vossen, A. Imhof, A. van Blaaderen, Langmuir 19 (2003) 6693. [23] D. Le Garrec, S. Gori, L. Luo, D. Lessard, D.C. Smith, M.A. Yessine, M. Ranger, J.C. Leroux, Journal of Controlled Release 99 (2004) 83. [24] O. Rabin, J.M. Perez, J. Grimm, G. Wojtkiewicz, R. Weissleder, Nature Materials 5 (2006) 118. [25] L.D. Pachon, G. Rothenberg, Applied Organometallic Chemistry 22 (2008) 288. [26] N.J.J. Johnson, N.M. Sangeetha, J.C. Boyer, F. van Veggel, Nanoscale 2 (2010) 771.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
JIEC-1744; No. of Pages 4 Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
Contents lists available at ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Short communication
Facile synthesis of monodispersed silica-coated magnetic nanoparticles Won-Yeop Rho a, Hyung-Mo Kim a, San Kyeong b, Yoo-Lee Kang a, Dong-Hyuk Kim a, Homan Kang c, Cheolhwan Jeong b, Dong-Eun Kim a, Yoon-Sik Lee b,c, Bong-Hyun Jun a,* a b c
Department of Bioscience and Biotechnology, Konkuk University, Seoul 143-701, Republic of Korea School of Chemical and Biological Engineering, Seoul National University, Seoul 151-742, Republic of Korea Nano Systems Institute and Interdisciplinary Program in Nano-Science and Technology, Seoul National University, Seoul 151-742, Republic of Korea
A R T I C L E I N F O
Article history: Received 14 November 2013 Accepted 4 December 2013 Available online xxx Keywords: Silica-coated magnetic nanoparticles Sto¨ber method Oleate-MNPs Ligand exchange
A B S T R A C T
Silica-coated magnetic nanoparticles (MNPs) have great potential for use in field of biotechnology owing to their unique properties, which can be manipulated by an external magnetic field gradient. Herein, we describe a method for facile synthesis of monodispersed silica-coated MNPs (MNP@SiO2 NPs). Commercially available oleate-MNPs were successfully converted to polyvinylpyrrolidone-MNPs (PVPMNPs), and then coated with silica by the modified Sto¨ber method. More than 95% of MNPs were individually coated with a silica shell; non-magnetic core silica nanoparticles (NPs) were not detected. Notably, the MNP@SiO2 NPs are highly monodispersed in size (size distribution < 2.5%) and synthesis at the scale of grams was easily obtained by a simple scale up process. Moreover, aggregation was not detected upon storage of over three months. ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction Because the magnetic nanoparticles (MNPs) have unique properties, which can be manipulated by an external magnetic field gradient, they have great potential for applicability in various applications such as micro fluid chips, for catalysis, and biosensing [1,2]. Synthesis of large quantities of monodispersed particles is very important, especially for biomedical applications [3,4]. One of the most promising methods for preparing monodispersed MNPs on a large scale is Hyeon’s method, which is based on thermal decomposition of an iron source in the presence of surfactants and a mild oxidant [5–7]. However, the as-prepared MNPs exhibit hydrophobicity due to the surfactants, and therefore, proper surface modification of the MNPs is required for biomedical applications. Silica has been widely incorporated in various nanoparticles (NPs), because it is nontoxic, biocompatible, optically transparent, chemically inert, thermally stable, and has a well-known surface chemistry [8–16]. Thus, coating MNPs with a silica layer can be a good strategy for bio-applications. Silica coating of oleatestabilized MNPs can be performed by simple microemulsionbased methods that allowed for monodispersed ones [14,16,17]. However, there are some disadvantages of these methods, such as aggregation of the produced MNPs upon long-term storage, non-
* Corresponding author. Tel.: +82 2 450 0521. E-mail address: [email protected] (B.-H. Jun).
reproducibility, and formation of particles without a core (noncore particle) formation. The Sto¨ber method is a widely used method for coating silica because of its advantages such as relatively mild reaction conditions, low cost, and a broad range of achievable particle size (tens to hundreds of nanometers) [18]. Thus far, hydrophilic MNPs (mostly non-uniform in size) have been coated with silica using this method [19,20]. However, the Sto¨ber method cannot be directly applied for the synthesis of hydrophobic oleate-MNPs which can be prepared in uniform size because of their hydrophobicity [21,22]. In this paper, we report large-scale synthesis of monodispersed silica coated MNPs (MNP@SiO2 NPs) generated by applying the Sto¨ber method. They were prepared from commercially available oleate-MNPs after ligand exchange with polyvinylpyrrolidone (PVP) followed by silica coating. To the best of our knowledge, our method can provide the most highly monodispersed (104.2 2.4 nm) silicacoated MNPs on the scale of grams. This was possible by applying the ligand exchange method and a well-ordered preparation step. 2. Experimental 2.1. Materials A dispersion of MNPs (18 nm in average diameter, oleatestabilized in toluene) was purchased from Ocean Nanotech, Korea. Tetraethyl orthosilicate (TEOS), polyvinylpyrrolidone (PVP-10) and
1226-086X/$ – see front matter ß 2013 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jiec.2013.12.014
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
2
Fig. 1. (a) Illustration of the synthesis of silica-coated MNPs and (b) TEM images of MNPs at each step: (i) oleate-MNPs, (ii) PVP-MNPs, (iii) MNP@SiO2 NPs.
dichloromethane (DCM) were purchased from Sigma–Aldrich. Diethyl ether (anhydrous), dimethylformamide (DMF), ethanol (EtOH), toluene, iron oxide, cyclohexane, and ammonium hydroxide (30 wt% in water) were sourced from Daejung Parm. All chemicals were used as received without any further purification. 2.2. Ligand exchange of oleate-stabilized MNP with PVP A dispersion of oleate-stabilized MNPs in toluene (0.2 mL) was transferred into a 15 mL vial and diluted with 5 mL of DMF–DCM (1:1, v/v). To this, 60 mg of PVP was added and refluxed at 100 8C for 12 h or overnight. The reaction mixture was then added dropwise into diethyl ether (10 mL) to precipitate the polymerstabilized nanoparticles. The precipitate was washed once with diethyl ether and centrifuged at 4500 rpm for 5 min. The precipitate was transferred to 6.5 mL of EtOH to yield a stable dispersion of the PVP-stabilized MNPs. The precipitate could also be dispersed in several other solvents such as water, chloroform, dichloromethane, DMF, and DMSO to yield optically transparent dispersions. 2.3. Silica coating on PVP-stabilized MNP EtOH dispersion of phase-transferred NPs (6.5 mL) was transferred into a 15 mL vial, and 0.28 mL of ammonium hydroxide (30 wt% in water) was added, followed by addition of 0.065 mL of TEOS in EtOH solution (10 vol%). The vial was stirred for 15 h at room temperature. The silica-coated nanoparticles were then isolated by centrifugation at 9000 rpm for 1 h and washed with EtOH. The collected silica-coated NPs were dispersed in distilled water. Then, 4 mL of TEOS in EtOH solution (3 vol%) was added by means of a syringe pump at a rate of 0.4 mL/h. After stirring for one day at room temperature, the resulting silica-coated MNPs were then centrifuged at 8500 rpm for 5 min and the silica-coated MNPs were dispersed in EtOH.
a sol–gel process based on the modified Sto¨ber method. Each preparation step of MNP@SiO2 NPs was visualized as in Fig. 1a and analyzed using transmission electron microscopy (TEM) images shown in Fig. 1b. The preparation steps are described in detail as follows. We used commercially available MNP NPs (18 nm). As the preparation of MNP NPs is based on the thermal decomposition of iron–oleate complexes, MNP NPs were stabilized by oleates and thus exhibit hydrophobicity [23–25]. The hydrophobic surfaces of MNPs were modified to exhibit hydrophilicity by adding an excess amount of amphiphilic PVP to the oleate-MNPs, so that the oleate ligand on MNPs could be exchanged with PVP, and the surface of the MNPs could be silicated via the Sto¨ber method. Oleate-stabilized MNPs in chloroform were mixed with PVP dissolved in a mixture of DCM and DMF to form a clear dispersion. This mixture was kept at 100 8C overnight to enable the exchange of the oleates with excess PVP [26]. The ligand exchange process did not affect the size and shape of the MNPs, as is shown in Fig. 1b(ii). Moreover, any aggregation of the MNPs was not detected after ligand exchange. Additionally, the MNPs were able to be redispersed in hydrophilic solvents such as EtOH. This property change indicates that the oleate ligands on MNPs were successfully replaced by PVP. This ligand exchange method has been applied to other types of NPs, but, to the best of our knowledge, we are the first to apply this to MNPs.
3. Results and discussion Fig. 1a shows the fabrication process flow for the preparation of monodispersed silica-coated magnetic NPs (MNP@SiO2 NPs). The MNP@SiO2 NPs have a core–shell structure, with a magnetic core (18 nm in diameter) and a silica shell with a thickness of ca 43 nm. The MNP@SiO2 NPs were prepared through a ligand exchange and
Fig. 2. Field-dependent magnetization of silica-coated MNP NPs at 300 K.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
3
Fig. 3. (a) TEM image of synthesized MNP@SiO2 NPs and (b) photograph of MNP@SiO2 NP dispersion in EtOH.
To completely exclude non-core-type silica NPs from core–shell type MNPs, residual PVP, which might have acted as a nucleation site, was completely removed from the MNP dispersion before the silica growth step by washing with EtOH. The PVP-coated MNPs were then redispersed in EtOH overnight to avoid the formation of multiple magnetic cores. Ammonium hydroxide and TEOS was then added to the PVP-coated MNPs in EtOH to make MNP@SiO2 NPs. The silica coating step was performed by two-step condensation reactions, which consist of the formation of a silica shell on PVP-coated MNPs, and growth of a silica shell on the silica-coated PVP-coated MNPs. This two-step condensation method allowed for highly monodispersed MNPs. Because PVP has an affinity to silica, the silica layer grew around the PVP-coated MNPs in the first step. During the second step a silica source (TEOS) was slowly added to the silica-coated PVP-coated MNP dispersion to avoid nucleation in solution so that the silica-coated PVP-coated MNPs could act as the sole seeds for deposition of the silica shell. The TEM images in Fig. 1b(iii) clearly shows the following: obtained MNP@SiO2 NPs with magnetic cores, silica shells, and an average MNP@SiO2 NPs size of 88.7 3.0 nm. The color of the resulting NPs changed from black to dark yellow. More than 95% of the NPs had one magnetic core as shown in Fig. 1a(iii). Thus, not only did we successfully obtain silica NPs free of non-magnetic cores, but also highly monodispersed ones (size distribution < 5%) even of a size of ca 90 nm, which cannot be easily obtained through the Sto¨ber method. There are several strong advantages of Sto¨ber method compared with the microemulsion method; for one, the thickness of the silica shell can be easily controlled by tuning the concentration of TEOS in the reaction mixture, under relatively milder conditions, and the products are not easily aggregated during storage. The field-dependent magnetism of the silica-coated MNPs at 300 K exhibited superparamagnetic properties that reached saturation at 0.07 emu/g, as shown in Fig. 2. Our process can be easily scaled up. When ten times the source of PVP-MNP, NH4OH, and TEOS were utilized, MNP@SiO2 NPs could be measured in grams (1.08 g). The prepared MNP@SiO2 NPs were also highly monodispersed in size (104.2 2.4 nm), as shown in Fig. 3a. Each of the NPs had a single magnetic core (multi-magnetic core NPs < 5%), and no empty core silica NPs were produced. We also confirmed the stability of NPs during storage. Being different from the microemulsion method, the silica NPs prepared using the Sto¨ber method did not contain any surfactant on their surface and thus less aggregation occurred during storage. The
prepared MNP@SiO2 NPs remained well dispersed in EtOH for at least three months, as shown in Fig. 3b. Even if some precipitations were detected, they can be redispersed by simple shaking, and thus aggregation did not occur even after long-term storage. 4. Conclusion We have described a facile method for the preparation of MNP@SiO2 NPs by modifying the Sto¨ber method. Commercially available oleate-MNPs were successfully converted to PVP-MNPs. These PVP-stabilized NPs were individually coated with a silica shell (>95%) and non-core silica NPs were not detected. Especially, the NPs were highly monodispersed in size (<2.5%) and aggregation did not happen during the ligand exchange and silica coating steps. The MNP@SiO2 NPs have superparamagnetic properties. The MNPs can be prepared in gram scale and no aggregation occurred upon storage for at least three months. Because our method can produce highly monodispersed MNPs without particle aggregation, it has great potential in various applications, such as a labeling tool in on-chip immunoassay, in magnetic manipulation of cells, and for controlled cell sorting. Our synthesis strategy is currently being expanded in scope to include metallic and semiconducting hybrid materials, the results from which will be reported elsewhere in the near future. Acknowledgment This study was supported by a grant from the Korean Health Technology R&D Project, Ministry of Health & Welfare, Republic of Korea (HI13C-1299-010013). W.-Y. Rho and H.-M. Kim contributed equally to this work. References [1] J.M. Nam, C.S. Thaxton, C.A. Mirkin, Science 301 (2003) 1884. [2] Z.L. Cheng, A. Al Zaki, J.Z. Hui, V.R. Muzykantov, A. Tsourkas, Science 338 (2012) 903. [3] P. Majewski, B. Thierry, Critical Reviews in Solid State and Materials Sciences 32 (2007) 203. [4] L.L. Vatta, R.D. Sanderson, K.R. Koch, Journal of Magnetism and Magnetic Materials 311 (2007) 114. [5] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H. Bin Na, Journal of the American Chemical Society 123 (2001) 12798. [6] J. Park, K.J. An, Y.S. Hwang, J.G. Park, H.J. Noh, J.Y. Kim, J.H. Park, N.M. Hwang, T. Hyeon, Nature Materials 3 (2004) 891. [7] J. Park, Y.H. Kim, H.J. Yoon, B.H. Jun, Y.S. Lee, Journal of Industrial and Engineering Chemistry 17 (2011) 794.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014
G Model
JIEC-1744; No. of Pages 4 4
W.-Y. Rho et al. / Journal of Industrial and Engineering Chemistry xxx (2013) xxx–xxx
[8] M.S. Noh, B.H. Jun, S. Kim, H. Kang, M.A. Woo, A. Minai-Tehrani, J.E. Kim, J. Kim, J. Park, H.T. Lim, S.C. Park, T. Hyeon, Y.K. Kim, D.H. Jeong, Y.S. Lee, M.H. Cho, Biomaterials 30 (2009) 3915. [9] W.R. Zhao, J.L. Gu, L.X. Zhang, H.R. Chen, J.L. Shi, Journal of the American Chemical Society 127 (2005) 8916. [10] R.I. Nooney, T. Dhanasekaran, Y.M. Chen, R. Josephs, A.E. Ostafin, Advanced Materials 14 (2002) 529. [11] K. Qian, Z.Q. Jiang, W.X. Huang, Journal of Molecular Catalysis A: Chemical 264 (2007) 26. [12] B.H. Jun, D.W. Hwang, H.S. Jung, J. Jang, H. Kim, H. Kang, T. Kang, S. Kyeong, H. Lee, D.H. Jeong, K.W. Kang, H. Youn, D.S. Lee, Y.S. Lee, Advanced Functional Materials 22 (2012) 1843. [13] J.H. Kim, J.S. Kim, H. Choi, S.M. Lee, B.H. Jun, K.N. Yu, E. Kuk, Y.K. Kim, D.H. Jeong, M.H. Cho, Y.S. Lee, Analytical Chemistry 78 (2006) 6967. [14] B.H. Jun, M.S. Noh, J. Kim, G. Kim, H. Kang, M.S. Kim, Y.T. Seo, J. Baek, J.H. Kim, J. Park, S. Kim, Y.K. Kim, T. Hyeon, M.H. Cho, D.H. Jeong, Y.S. Lee, Small 6 (2010) 119. [15] C.Z. Yu, J. Fan, B.Z. Tian, D.Y. Zhao, Chemistry of Materials 16 (2004) 889.
[16] B.H. Jun, G. Kim, M.S. Noh, H. Kang, Y.K. Kim, M.H. Cho, D.H. Jeong, Y.S. Lee, Nanomedicine 6 (2011) 1463. [17] D.C. Lee, F.V. Mikulec, J.M. Pelaez, B. Koo, B.A. Korgel, Journal of Physical Chemistry B 110 (2006) 11160. [18] R. Khatiri, A. Reyhani, S.Z. Mortazavi, M. Hossainalipour, Journal of Industrial and Engineering Chemistry 19 (2013) 1642. [19] Y. Kobayashi, M. Horie, M. Konno, B. Rodriguez-Gonzalez, L.M. Liz-Marzan, Journal of Physical Chemistry B 107 (2003) 7420. [20] Y.A. Barnakov, M.H. Yu, Z. Rosenzweig, Langmuir 21 (2005) 7524. [21] C.L. Fang, K. Qian, J.H. Zhu, S.B. Wang, X.X. Lv, S.H. Yu, Nanotechnology 19 (2008) 125601. [22] C. Graf, D.L.J. Vossen, A. Imhof, A. van Blaaderen, Langmuir 19 (2003) 6693. [23] D. Le Garrec, S. Gori, L. Luo, D. Lessard, D.C. Smith, M.A. Yessine, M. Ranger, J.C. Leroux, Journal of Controlled Release 99 (2004) 83. [24] O. Rabin, J.M. Perez, J. Grimm, G. Wojtkiewicz, R. Weissleder, Nature Materials 5 (2006) 118. [25] L.D. Pachon, G. Rothenberg, Applied Organometallic Chemistry 22 (2008) 288. [26] N.J.J. Johnson, N.M. Sangeetha, J.C. Boyer, F. van Veggel, Nanoscale 2 (2010) 771.
Please cite this article in press as: W.-Y. Rho, et al., J. Ind. Eng. Chem. (2013), http://dx.doi.org/10.1016/j.jiec.2013.12.014