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Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles
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CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles Milan P. Nikolića, Konstantinos P. Giannakopoulosb, Vladimir V. Srdićc,n b
a Department of Chemical Engineering, Faculty of Agronomy, University of Kragujevac, Čačak, Serbia Institiute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Athens, Greece c Department of Materials Engineering, Faculty of Technology, University of Novi Sad, Serbia
Received 29 May 2015; received in revised form 23 July 2015; accepted 23 July 2015
Abstract Multifunctional bilayered shell around monodispersed silica core particles was prepared by wet-chemical synthesis process. The first outer layer was composed of mesoporous silica and the second internal layer was composed of superparamagnetic Fe3O4 nanoparticles. Ferrite nanoshell was synthesized by assembling of oppositely charged ferrite (Fe3O4) nanoparticles on the surface of the monodispersed silica core particles (having average size of 0.5 mm and prepared by hydrolysis and condensation of tetraethylortosilicate). The obtained Fe3O4 layer was superparamagnetic and had very weakly developed pore structure. The second layer on the previously obtained particles with silica core and Fe3O4 shell was formed by deposition of silica nanoparticles from a silicate solution. To allow electrostatic assembling of the silica nanoparticles and formation of external silica layer around the silica/Fe3O4 particles, the latter were functionalized with poly(diallyldimethylammonium chloride) (PDDA). Obtained silica layer had well developed pore structure with average pore size of 10 nm. The obtained composite particles with two functional layers can be used in bioengineering for immobilization of enzymes inside external mesoporous silica layer while internal superparamagnetic ferrite layer would allow magnetic separation from reaction mixture. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Core/shell particles; Bilayered silica/ferrite shell; Synthesis; Characterization
1. Introduction Mesoporous silica particles have been widely used due to their chemical stability, biocompatibility, non-toxicity and relative easy synthesis, enabling adjustable size, morphology and porosity. Thus, mesoporous silica particles found many applications, such as “nanocarriers” for delivery of drugs and other cargos to cells [1], the templates for synthesis of various materials [2], enzyme immobilization and biocatalysis [3], bioseparation [4], biosensors [3], etc. Furthermore, mesoporous silica nanoparticle with high porosity induced the reduction of in vitro cytotoxicity and inflammation compared with non-porous silica nanoparticle (colloidal silica) [5].
n
Corresponding author. Tel.: þ381 214853665; fax: þ 381 21450413. E-mail address: [email protected] (V.V. Srdić).
On the other side, magnetic nanoparticles have demonstrated a wide range of applications including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage and environmental remediation [6]. In most applications listed above, the magnetic particles have shown the best features when their size is below the critical value and then each nanoparticle becomes a single magnetic domain and shows superparamagnetic behavior when the temperature is above the so-called blocking temperature [6]. However, successful application of magnetic nanoparticles is highly dependent on the stability of the particles under a range of different conditions. Such small particles tend to form agglomerates to reduce the energy associated with the high surface area to volume ratio of the nanosized particles. Moreover, naked metallic nanoparticles are chemically highly active, and are easily oxidized in air, resulting generally in loss of magnetism and dispersibility [6]. To avoid
http://dx.doi.org/10.1016/j.ceramint.2015.07.139 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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M.P. Nikolić et al. / Ceramics International ] (]]]]) ]]]–]]]
these problems it is necessary to develop protection strategies to chemically stabilize the naked magnetic nanoparticles against degradation during or after the synthesis. Around iron-group magnetic nanoparticles can be fabricated protective layers preventing their oxidation in air or moisture and fast dissolution under acidic conditions [7,8]. Furthermore, core/shell structure not only provides a stabilized magnetic property and oxidative resistance, but also broadens their potential applications since the shell can be functionalized with organic [9], biomolecular [10] or inorganic [11] materials. The combination of mesoporous silica with magnetic functional groups to form separable core/shell structured composite found its application in controlled drug release [12–14], removal of bacterial toxins [15] or organic and inorganic pollutants [16,17], bioseparations [18] and catalytic degradation of pollutants [19]. Formation of mesoporous silica layers around dense cores is mainly based on the use of surfactants that are aggregated on the surface of the cores and then condensation of TEOS is carried out on the surface of modified cores [14–17,20]. In these cases calcination or extraction had to be performed to remove template from the structure. However, mesoporous silica shell can be synthesized without templates by simple procedure where silica nanoparticles, prepared from highly
basic sodium silicate solution, were electrostaticaly deposited on the surface of functionalized cores [21]. Multifunctional core/shell particles with bilayered shells have also been utilized for drug delivery and catalytic application [12,22,23]. Thus, magnetic Fe3O4 were successfully encapsulated within dense SiO2 layer in the first step and then covered with uniform layer of mesoporous silica [22] and found application in catalysis. Yung et al. [12] demonstrated that hydrothermal synthesized Fe3O4 microspheres encapsulated with nonporous silica and a further layer of ordered mesoporous silica through a simple sol–gel process could be efficient for drug delivery. In some cases even magnetic ZnFe2O4 hollow microsphere were used as the core particles and mesoporous silica with folic acid molecules were deposited as the outer shell [23]. In this work, uniform bilayered shells composed of internal superparamagnetic ferrite layer and external mesoporous layer were synthesized around silica cores. Obtained particles can find potential application in heterogenous catalysis or bioseparations. 2. Experimental The core/shell structures were prepared by three-step process with starting reagents which were of analytical purity
Fig. 1. SEM image of SiO2 core particles (a), SEM image of core/shell particles (b), TEM micrographs of core/shell particles – bright (c) and dark field (d). Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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Germany) was dissolved in deionized water, stirred and heated for 15 min at about 80 1C. The warm ferric nitrate solution was slowly added into the stirred vessel containing silica core particles dispersed in aqueous solution having the constant pH value (pH ¼ 5.4) and temperature of 75 1C. The quantity of added iron nitrate solution was adjusted so that the weight ratio between the SiO2 core and the ferrite shell was 3. After addition of the last amount of nitrate ions, reaction continued for 10 min. The precipitated powder (sample CF3) was centrifuged and washed with distilled water several times and finally dried at 120 1C for 1 day. Fig. 2. Isoelectric titration graphs of silica core, Fe3O4 and silica/ferrite particles.
2.3. Functionalization of silica/ferrite particles In order to achieve the electrostatic assembly of silica nanoparticles on the surface of the previously obtained particles with silica core and ferrite (Fe3O4) shell, the latter were functionalized with PDDA (poly(diallyldimethylammonium chloride)). The PDDA-functionalized silica/ferrite particles (sample CF3p) were prepared by dispersing silica/ferrite particles (0.6 g) in 60 ml PDDA solution (concentration of 6 mg/ml) which also contained 0.1 M NaCl. The suspension was stirred for 20 min at 45 1C. Modified particles were centrifuged, successively washed with distilled water and finally dried at 80 1C. 2.4. Formation of silica shell on silica/ferrite particles
Fig. 3. Isoelectric titration graphs of PDDA-functionalized silica/Fe3O4 particles (CF3p) and bilayered-shell particles (CF3p-S).
grade and used without further purification. In the first step, monodispersed silica core particles were synthesized via the hydrolysis and condensation of tetraethylortosilicate, TEOS (Si (OC4H9)4, Fluka) [24]. In the second step the ferrite (Fe3O4) shell was prepared by assembling ferrite nanoparticles on the surface of the silica core particles [25]. In the last step, the external mesoporous silica shell was formed by assembling of silica nanoparticles on the surface of the previously obtained particles with silica core and Fe3O4 shell.
The PDDA-functionalized particles with silica core and ferrite (Fe3O4) shell (sample CF3p) were used as templates for assembling the second layer, which was composed of silica nanoparticles. The functionalized silica/ferrite particles were dispersed in highly basic sodium silicate solution (Water glass, Alumina Factory-Birac, Zvornik) having SiO2/Na2O molar ratio 2.8 mol/mol and SiO2 concentration of 55 g/l. Sulfuric acid (H2SO4 ¼ 1 mol/l) was slowly added into this well stirred dispersion at 70 1C to decrease pH value and enable generation of silica nanoparticles. The reaction time was 6 min. When reaction was finished, the obtained particles (sample CF3p-S) were separated from the liquid phase by centrifugation, washed with distilled water and finally dried at 120 1C for one day.
2.1. Preparation of silica core particles 2.5. Characterization of particles TEOS was dissolved in anhydrous ethanol and hydrolyzed with distilled water under basic condition (25% NH3, Merck). Silica core particles (sample C) were synthesized using a molar ratio TEOS:H2O:NH4OH¼ 1:40:4 and TEOS concentration of 0.25 mol/l. After feeding, the produced suspension was continuously stirred at room temperature for one hour. The white precipitated powder was centrifuged and washed with distilled water until the effluent was free of NH4 groups, and finally dried at 120 1C for 1 day. 2.2. Formation of ferrite shell on silica core particles The ferrite (Fe3O4) shell was synthesized by the method used in our previous work [25]. Fe(NO3)3 9H2O (Merck,
Particle size distribution was measured by dynamic light scattering (DLS) and the zeta potential of particles was determined by phase analysis light scattering and mixed mode measurement (Zetasizer Nano ZS with MPT-2 Autotitrator Malvern Instruments). Size and morphology of particles were examined using a scanning electron microscope (SEM JEOL, 6460 LV) operating at 20 kV and a transmission electron microscope (Philips CM 20), operating at 200 kV. The specific surface area (according to the BET method), pore size distribution (according to the BJH method) and pore volume of the as-synthesized core/shell particles were measured by low temperature nitrogen adsorption using a Quantachrom Autosorb-3B instrument.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
M.P. Nikolić et al. / Ceramics International ] (]]]]) ]]]–]]]
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Fig. 4. SEM micrographs of: (a) PDDA-functionalized silica/Fe3O4 particles (CF3p) and (b) silica particles with SiO2/Fe3O4 bilayered-shell (CF3p-S).
Fig. 5. Particle size distribution of PDDA-functionalized silica/Fe3O4 particles (CF3p) and silica particles with SiO2/Fe3O4 bilayered-shell (CF3p-S).
3. Results and discussion The silica core particles (Fig. 1a), synthesized by hydrolysis and condensation of TEOS, were used as templates for in situ assembling of ferrite nanoparticles generated by coprecipitation from iron nitrate precursor. The smooth surface of the SiO2 core particles (Fig. 1a) was roughened due to deposition of the ferrite nanoparticles on the surface of core particles (Fig. 1b). The thickness of the obtained ferrite shell was about 50 nm (Fig. 1c) while dark field TEM micrograph (Fig. 1d) shows that the ferrite shell is composed of nanocrystallites structure. Fig. 2 shows that the silica core and the ferrite particles had oppositely charged surfaces in the pH range between 3.0 and 6.2. Thus, the surface electric charge of silica core particles is negative while the newly synthesized ferrite nanoparticles are positive at pH ¼ 5.4. This resulted in assembling of ferrite nanoparticles on the surface of core particles by attractive electrostatic forces. The continuity of the obtained ferrite shell was confirmed with the electrophoretic measurement presented in Fig. 2. The isoelectric point of Fe3O4 particles and core/shell particles (sample CF3) was 6.4 and 5.6, respectively. This indicates that the surface of silica core particles is almost completely covered with a ferrite shell. The electrophoretic measurement of the PDDA-functionalized particles with silica core and ferrite shell (sample CF3p)
shows positively charged particles for pH between 2 and 10 and this should allow electrostatic assembly of the negatively charged silica nanoparticles on the surface of functionalized ferrite shell. On the other side, the sample obtained by deposition of silica nanoparticles on the surface of PDDAfunctionalized silica/Fe3O4 particles (sample CF3p-S) has isoelectric point at 3.6 similar to the silica (Fig. 3) which indicates that the PDDA-functionalized ferrite shell is almost completely covered with silica nanoparticles. SEM micrographs of the PDDA-functionalized of silica/Fe3O4 particles (sample CF3p) and the bilayered-shell particles (particles CF3p-S with silica core and two Fe3O4/silica shells) are shown in Fig. 4. The surface of the sample CF3p is smooth in comparison to the rough surface of CF3p-S particles confirming the formation of external silica layer. According to DLS measurements (Fig. 5) the average particles size of the samples CF3p and CF3p-S is 660 nm and 880 nm, respectively, indicating that the silica shell thickness is about 100 nm. The adsorption–desorption isotherms of silica core (C), silica core with ferrite shell (CF3) and silica core with SiO2/Fe3O4 bilayered shell (CF3p-S) particles, measured by low-temperature nitrogen adsorption, are shown in Fig. 6a and Table 1. The adsorption–desorption isotherms of the samples C and CF3 belong to the type I, indicating that these samples have microporous structure. On the other side, the adsorption– desorption isotherm of the sample CF3p-S belongs to the type IV, indicating that external silica shell has mesoporous structure. Pore structure of the investigated samples can also be clearly seen in Fig. 6b where it can be observed that the average pore size of the bilayered core/shell particles (sample CF3p-S) is 10 nm. The bilayered core/shell particles also have relatively high specific surface area and total pore volume, Table 1. It is important to underline that the results which could be found in the literature [12–15,22] mostly considered surfactant-templated mesoporous silica monolayer prepared around superparamagnetic Fe3O4 core particles. These multifunctional nanostructures might have better magnetic properties, but the silica layers obtained in this work have a greater total pore volume which would allow a greater amount of protein or other material to be absorbed inside the silica layer.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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Fig. 6. Adsorption–desorption isotherms (a) and pore size distribution (b) of synthesized particles: silica core (C), silica with Fe3O4 shell (CF3) and silica with SiO2/Fe3O4 bilayered-shell (CF3p-S).
Table 1 The specific surface area (Sv) and total pore volume (Vp) of synthesized particles: silica core (C) silica with Fe3O4 shell (CF3) and silica with bilayeredshell (CF3p-S). Sample
Sv [m2/g]
Vp [cm3/g]
C CF3 CF3p-S
324 97 294
0.186 0.075 0.638
Magnetization measurements [25] confirmed the superparamagnetic behavior of the ferrite (Fe3O4) shell. Thus, the obtained composite particles with two functional layers might be used in bioengineering for the immobilization of enzymes inside the external mesoporous silica layer, while the internal superparamagnetic ferrite layer would allow the magnetic separation from a reaction mixture. 4. Conclusions Bilayered-shells composed of internal ferrite layer and external silica layer were synthesized by assembling oppositely charged ferrite (Fe3O4) nanoparticles on the surface of monodispersed silica core particles (having an average size of 0.5 μm) that was followed by the deposition of silica nanoparticles on the surface of the functionalized ferrite layer. Both shells completely cover the core particles, while the corresponding thickness of the ferrite and the silica layers is 20 nm and 100 nm, respectively. The obtained ferrite layer is superparamagnetic and the external silica layer is mesoporous with an average pore size of 10 nm and a large total pore volume. Due to the multifunctionality of the bilayered-shell obtained, the core/shell particles may find potential applications in enzyme immobilization and bioseparation. Acknowledgments The research was supported by the Serbian Ministry of Science under the Project no. III45021. The cooperation under the COST IC1208 Project is also highly acknowledged. We are
also thankful to Milos Bokorov, Faculty of Natural Sciences, University of Novi Sad for performing SEM measurements.
References [1] D. Tarn, C.E. Ashley, M. Xue, E.C. Carnes, J.I. Zink, C.J. Brinker, Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility, Acc. Chem. Res. 46 (2013) 792–801. [2] R. Ryoo, S.H. Joo, S. Jun, Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation, J. Phys. Chem. B 103 (1999) 7743–7746. [3] C. Thörn, H. Gustafsson, L. Olsson, QCM-D as a method for monitoring enzyme immobilization in mesoporous silica particles, Microporous Mesoporous Mater. 176 (2013) 71–77. [4] J. Liu, B. Wang, S.B. Hartono, T. Liu, P. Kantharidis, A.P.J. Middelberg, G.Q. (Max), Lu, L. He, S.Z. Qiao, Magnetic silica spheres with large nanopores for nucleic acid adsorption and cellular uptake, Biomaterials 33 (2012) 970–978. [5] S. Lee, H.-S. Yun, S.-H. Kim, The comparative effects of mesoporous silica nanoparticles and colloidal silica on inflammation and apoptosis, Biomaterials 32 (2011) 9434–9443. [6] A.-H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. [7] S. Wei, Q. Wang, J. Zhu, L. Sun, H. Line, Z. Guo, Multifunctional composite core–shell nanoparticles, Nanoscale 3 (2011) 4474–4502. [8] J. Cheng, X. Ni, H. Zheng, B. Li, X. Zhang, D. Zhang, Preparation of Fe (core)/SiO2 (shell) composite particles with improved oxidationresistance, Mater. Res. Bull. 41 (2006) 1424–1429. [9] F.H. Chen, Q. Gao, J.Z. Ni, The grafting and release behavior of doxorubincin from Fe3O4@SiO2 core–shell structure nanoparticles via an acid cleaving amide bond: the potential for magnetic targeting drug delivery, Nanotechnology 19 (2008) 165103. [10] R. He, X. You, J. Shao, F. Gao, B. Pan, D. Cui, Core/shell fluorescent magnetic silica-coated composite nanoparticles for bioconjugation, Nanotechnology 18 (2007) 315601. [11] L. Qu, S. Tie, Mesoporous silica-coated superparamagnetic magnetite functionalized with CuO and its application as a desulfurizer, Microporous Mesoporous Mater. 117 (2009) 402–405. [12] P. Yang, Z. Quan, Z. Hou, C. Li, X. Kang, Z. Cheng, J. Lin, A magnetic, luminescent and mesoporous core–shell structured composite material as drug carrier, Biomaterials 30 (2009) 4786–4795. [13] S. Gai, P. Yang, J. Hao, W. Wang, N. Niu, F. He, D. Wang, J. Lin, Fabrication of luminescent and mesoporous core–shell structured nanocomposites and their application as drug carrier, Microporous Mesoporous Mater. 131 (2010) 128–135.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
6
M.P. Nikolić et al. / Ceramics International ] (]]]]) ]]]–]]]
[14] J.M. Rosenholm, J. Zhang, W. Sun, H. Gu, Large-pore mesoporous silicacoated magnetite core–shell nanocomposites and their relevance for biomedical application, Microporous Mesoporous Mater. 145 (2011) 14–20. [15] Y. Deng, D. Qi, C. Deng, X.M. Zhang, D. Zhao, Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28–29. [16] X. Fu, X. Chen, J. Wang, J. Liu, Fabrication of carboxylic functionalized superparamagnetic mesoporous silica microspheres and their application for removal basic dye pollutants from water, Microporous Mesoporous Mater. 139 (2011) 8–15. [17] S. Tao, C. Wang, W. Ma, S. Wu, C. Meng, Designed multifunctionalized magnetic mesoporous microsphere for sequential sorption of organic and inorganic pollutants, Microporous Mesoporous Mater. 147 (2012) 295–301. [18] T. Sen, I.J. Bruce, Mesoporous silica-magnetite nanocomposites: fabrication, characterisation and applications in biosciences, Microporous Mesoporous Mater. 120 (2009) 246–251. [19] M. Xia, C. Chen, M. Long, C. Chen, W. Cai, B. Zhou, Magnetically separable mesoporous silica nanocomposite and its application in Fenton catalysis, Microporous Mesoporous Mater. 145 (2011) 217–223.
[20] M.P. Nikolić, K.P. Giannakopoulos, M. Bokorov, V.V. Srdić, Effect of surface functionalization on synthesis of mesoporous silica core/shell particles, Microporous Mesoporous Mater. 155 (2012) 8–13. [21] N.Ž. Knežević, Core/shell magnetic mesoporous silica nanoparticles with radially oriented wide mesopores, Process. Appl. Ceram. 8 (2) (2014) 109–112. [22] F. Wang, G. Wang, W. Sun, T. Wang, X. Chen, Metallophthalocyanine functionalized magnetic mesoporous silica nanoparticles and its application in ultrasound-assisted oxidation of benzothiophene, Microporous Mesoporous Mater. 217 (2015) 203–209. [23] D. Yang, K. Wei, Q. Liu, Y. Yang, X. Guo, H. Rong, M.-L. Cheng, G. Wang, Folic acid-functionalized magnetic ZnFe2O4 hollow microsphere core/mesoporous silica shell composite particles: synthesis and application in drug release, Mater. Sci. Eng. C 33 (2013) 2879–2884. [24] W. Stöber, A. Fink, E. Bohn, Controlled growth of mesoporous silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62. [25] M.P. Nikolić, K.P. Giannakopoulos, D. Stamppoulos, E.G. Moshopoulou, V.V. Srdić, Synthesis and characterization of silica core/nano-ferrite shell particles, Mater. Res. Bull. 47 (2012) 1513–1519.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
CERAMICS INTERNATIONAL
Ceramics International ] (]]]]) ]]]–]]] www.elsevier.com/locate/ceramint
Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles Milan P. Nikolića, Konstantinos P. Giannakopoulosb, Vladimir V. Srdićc,n b
a Department of Chemical Engineering, Faculty of Agronomy, University of Kragujevac, Čačak, Serbia Institiute of Nanoscience and Nanotechnology, National Center for Scientific Research “Demokritos”, Athens, Greece c Department of Materials Engineering, Faculty of Technology, University of Novi Sad, Serbia
Received 29 May 2015; received in revised form 23 July 2015; accepted 23 July 2015
Abstract Multifunctional bilayered shell around monodispersed silica core particles was prepared by wet-chemical synthesis process. The first outer layer was composed of mesoporous silica and the second internal layer was composed of superparamagnetic Fe3O4 nanoparticles. Ferrite nanoshell was synthesized by assembling of oppositely charged ferrite (Fe3O4) nanoparticles on the surface of the monodispersed silica core particles (having average size of 0.5 mm and prepared by hydrolysis and condensation of tetraethylortosilicate). The obtained Fe3O4 layer was superparamagnetic and had very weakly developed pore structure. The second layer on the previously obtained particles with silica core and Fe3O4 shell was formed by deposition of silica nanoparticles from a silicate solution. To allow electrostatic assembling of the silica nanoparticles and formation of external silica layer around the silica/Fe3O4 particles, the latter were functionalized with poly(diallyldimethylammonium chloride) (PDDA). Obtained silica layer had well developed pore structure with average pore size of 10 nm. The obtained composite particles with two functional layers can be used in bioengineering for immobilization of enzymes inside external mesoporous silica layer while internal superparamagnetic ferrite layer would allow magnetic separation from reaction mixture. & 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Keywords: Core/shell particles; Bilayered silica/ferrite shell; Synthesis; Characterization
1. Introduction Mesoporous silica particles have been widely used due to their chemical stability, biocompatibility, non-toxicity and relative easy synthesis, enabling adjustable size, morphology and porosity. Thus, mesoporous silica particles found many applications, such as “nanocarriers” for delivery of drugs and other cargos to cells [1], the templates for synthesis of various materials [2], enzyme immobilization and biocatalysis [3], bioseparation [4], biosensors [3], etc. Furthermore, mesoporous silica nanoparticle with high porosity induced the reduction of in vitro cytotoxicity and inflammation compared with non-porous silica nanoparticle (colloidal silica) [5].
n
Corresponding author. Tel.: þ381 214853665; fax: þ 381 21450413. E-mail address: [email protected] (V.V. Srdić).
On the other side, magnetic nanoparticles have demonstrated a wide range of applications including magnetic fluids, catalysis, biotechnology/biomedicine, magnetic resonance imaging, data storage and environmental remediation [6]. In most applications listed above, the magnetic particles have shown the best features when their size is below the critical value and then each nanoparticle becomes a single magnetic domain and shows superparamagnetic behavior when the temperature is above the so-called blocking temperature [6]. However, successful application of magnetic nanoparticles is highly dependent on the stability of the particles under a range of different conditions. Such small particles tend to form agglomerates to reduce the energy associated with the high surface area to volume ratio of the nanosized particles. Moreover, naked metallic nanoparticles are chemically highly active, and are easily oxidized in air, resulting generally in loss of magnetism and dispersibility [6]. To avoid
http://dx.doi.org/10.1016/j.ceramint.2015.07.139 0272-8842/& 2015 Published by Elsevier Ltd and Techna Group S.r.l.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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M.P. Nikolić et al. / Ceramics International ] (]]]]) ]]]–]]]
these problems it is necessary to develop protection strategies to chemically stabilize the naked magnetic nanoparticles against degradation during or after the synthesis. Around iron-group magnetic nanoparticles can be fabricated protective layers preventing their oxidation in air or moisture and fast dissolution under acidic conditions [7,8]. Furthermore, core/shell structure not only provides a stabilized magnetic property and oxidative resistance, but also broadens their potential applications since the shell can be functionalized with organic [9], biomolecular [10] or inorganic [11] materials. The combination of mesoporous silica with magnetic functional groups to form separable core/shell structured composite found its application in controlled drug release [12–14], removal of bacterial toxins [15] or organic and inorganic pollutants [16,17], bioseparations [18] and catalytic degradation of pollutants [19]. Formation of mesoporous silica layers around dense cores is mainly based on the use of surfactants that are aggregated on the surface of the cores and then condensation of TEOS is carried out on the surface of modified cores [14–17,20]. In these cases calcination or extraction had to be performed to remove template from the structure. However, mesoporous silica shell can be synthesized without templates by simple procedure where silica nanoparticles, prepared from highly
basic sodium silicate solution, were electrostaticaly deposited on the surface of functionalized cores [21]. Multifunctional core/shell particles with bilayered shells have also been utilized for drug delivery and catalytic application [12,22,23]. Thus, magnetic Fe3O4 were successfully encapsulated within dense SiO2 layer in the first step and then covered with uniform layer of mesoporous silica [22] and found application in catalysis. Yung et al. [12] demonstrated that hydrothermal synthesized Fe3O4 microspheres encapsulated with nonporous silica and a further layer of ordered mesoporous silica through a simple sol–gel process could be efficient for drug delivery. In some cases even magnetic ZnFe2O4 hollow microsphere were used as the core particles and mesoporous silica with folic acid molecules were deposited as the outer shell [23]. In this work, uniform bilayered shells composed of internal superparamagnetic ferrite layer and external mesoporous layer were synthesized around silica cores. Obtained particles can find potential application in heterogenous catalysis or bioseparations. 2. Experimental The core/shell structures were prepared by three-step process with starting reagents which were of analytical purity
Fig. 1. SEM image of SiO2 core particles (a), SEM image of core/shell particles (b), TEM micrographs of core/shell particles – bright (c) and dark field (d). Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
M.P. Nikolić et al. / Ceramics International ] (]]]]) ]]]–]]]
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Germany) was dissolved in deionized water, stirred and heated for 15 min at about 80 1C. The warm ferric nitrate solution was slowly added into the stirred vessel containing silica core particles dispersed in aqueous solution having the constant pH value (pH ¼ 5.4) and temperature of 75 1C. The quantity of added iron nitrate solution was adjusted so that the weight ratio between the SiO2 core and the ferrite shell was 3. After addition of the last amount of nitrate ions, reaction continued for 10 min. The precipitated powder (sample CF3) was centrifuged and washed with distilled water several times and finally dried at 120 1C for 1 day. Fig. 2. Isoelectric titration graphs of silica core, Fe3O4 and silica/ferrite particles.
2.3. Functionalization of silica/ferrite particles In order to achieve the electrostatic assembly of silica nanoparticles on the surface of the previously obtained particles with silica core and ferrite (Fe3O4) shell, the latter were functionalized with PDDA (poly(diallyldimethylammonium chloride)). The PDDA-functionalized silica/ferrite particles (sample CF3p) were prepared by dispersing silica/ferrite particles (0.6 g) in 60 ml PDDA solution (concentration of 6 mg/ml) which also contained 0.1 M NaCl. The suspension was stirred for 20 min at 45 1C. Modified particles were centrifuged, successively washed with distilled water and finally dried at 80 1C. 2.4. Formation of silica shell on silica/ferrite particles
Fig. 3. Isoelectric titration graphs of PDDA-functionalized silica/Fe3O4 particles (CF3p) and bilayered-shell particles (CF3p-S).
grade and used without further purification. In the first step, monodispersed silica core particles were synthesized via the hydrolysis and condensation of tetraethylortosilicate, TEOS (Si (OC4H9)4, Fluka) [24]. In the second step the ferrite (Fe3O4) shell was prepared by assembling ferrite nanoparticles on the surface of the silica core particles [25]. In the last step, the external mesoporous silica shell was formed by assembling of silica nanoparticles on the surface of the previously obtained particles with silica core and Fe3O4 shell.
The PDDA-functionalized particles with silica core and ferrite (Fe3O4) shell (sample CF3p) were used as templates for assembling the second layer, which was composed of silica nanoparticles. The functionalized silica/ferrite particles were dispersed in highly basic sodium silicate solution (Water glass, Alumina Factory-Birac, Zvornik) having SiO2/Na2O molar ratio 2.8 mol/mol and SiO2 concentration of 55 g/l. Sulfuric acid (H2SO4 ¼ 1 mol/l) was slowly added into this well stirred dispersion at 70 1C to decrease pH value and enable generation of silica nanoparticles. The reaction time was 6 min. When reaction was finished, the obtained particles (sample CF3p-S) were separated from the liquid phase by centrifugation, washed with distilled water and finally dried at 120 1C for one day.
2.1. Preparation of silica core particles 2.5. Characterization of particles TEOS was dissolved in anhydrous ethanol and hydrolyzed with distilled water under basic condition (25% NH3, Merck). Silica core particles (sample C) were synthesized using a molar ratio TEOS:H2O:NH4OH¼ 1:40:4 and TEOS concentration of 0.25 mol/l. After feeding, the produced suspension was continuously stirred at room temperature for one hour. The white precipitated powder was centrifuged and washed with distilled water until the effluent was free of NH4 groups, and finally dried at 120 1C for 1 day. 2.2. Formation of ferrite shell on silica core particles The ferrite (Fe3O4) shell was synthesized by the method used in our previous work [25]. Fe(NO3)3 9H2O (Merck,
Particle size distribution was measured by dynamic light scattering (DLS) and the zeta potential of particles was determined by phase analysis light scattering and mixed mode measurement (Zetasizer Nano ZS with MPT-2 Autotitrator Malvern Instruments). Size and morphology of particles were examined using a scanning electron microscope (SEM JEOL, 6460 LV) operating at 20 kV and a transmission electron microscope (Philips CM 20), operating at 200 kV. The specific surface area (according to the BET method), pore size distribution (according to the BJH method) and pore volume of the as-synthesized core/shell particles were measured by low temperature nitrogen adsorption using a Quantachrom Autosorb-3B instrument.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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Fig. 4. SEM micrographs of: (a) PDDA-functionalized silica/Fe3O4 particles (CF3p) and (b) silica particles with SiO2/Fe3O4 bilayered-shell (CF3p-S).
Fig. 5. Particle size distribution of PDDA-functionalized silica/Fe3O4 particles (CF3p) and silica particles with SiO2/Fe3O4 bilayered-shell (CF3p-S).
3. Results and discussion The silica core particles (Fig. 1a), synthesized by hydrolysis and condensation of TEOS, were used as templates for in situ assembling of ferrite nanoparticles generated by coprecipitation from iron nitrate precursor. The smooth surface of the SiO2 core particles (Fig. 1a) was roughened due to deposition of the ferrite nanoparticles on the surface of core particles (Fig. 1b). The thickness of the obtained ferrite shell was about 50 nm (Fig. 1c) while dark field TEM micrograph (Fig. 1d) shows that the ferrite shell is composed of nanocrystallites structure. Fig. 2 shows that the silica core and the ferrite particles had oppositely charged surfaces in the pH range between 3.0 and 6.2. Thus, the surface electric charge of silica core particles is negative while the newly synthesized ferrite nanoparticles are positive at pH ¼ 5.4. This resulted in assembling of ferrite nanoparticles on the surface of core particles by attractive electrostatic forces. The continuity of the obtained ferrite shell was confirmed with the electrophoretic measurement presented in Fig. 2. The isoelectric point of Fe3O4 particles and core/shell particles (sample CF3) was 6.4 and 5.6, respectively. This indicates that the surface of silica core particles is almost completely covered with a ferrite shell. The electrophoretic measurement of the PDDA-functionalized particles with silica core and ferrite shell (sample CF3p)
shows positively charged particles for pH between 2 and 10 and this should allow electrostatic assembly of the negatively charged silica nanoparticles on the surface of functionalized ferrite shell. On the other side, the sample obtained by deposition of silica nanoparticles on the surface of PDDAfunctionalized silica/Fe3O4 particles (sample CF3p-S) has isoelectric point at 3.6 similar to the silica (Fig. 3) which indicates that the PDDA-functionalized ferrite shell is almost completely covered with silica nanoparticles. SEM micrographs of the PDDA-functionalized of silica/Fe3O4 particles (sample CF3p) and the bilayered-shell particles (particles CF3p-S with silica core and two Fe3O4/silica shells) are shown in Fig. 4. The surface of the sample CF3p is smooth in comparison to the rough surface of CF3p-S particles confirming the formation of external silica layer. According to DLS measurements (Fig. 5) the average particles size of the samples CF3p and CF3p-S is 660 nm and 880 nm, respectively, indicating that the silica shell thickness is about 100 nm. The adsorption–desorption isotherms of silica core (C), silica core with ferrite shell (CF3) and silica core with SiO2/Fe3O4 bilayered shell (CF3p-S) particles, measured by low-temperature nitrogen adsorption, are shown in Fig. 6a and Table 1. The adsorption–desorption isotherms of the samples C and CF3 belong to the type I, indicating that these samples have microporous structure. On the other side, the adsorption– desorption isotherm of the sample CF3p-S belongs to the type IV, indicating that external silica shell has mesoporous structure. Pore structure of the investigated samples can also be clearly seen in Fig. 6b where it can be observed that the average pore size of the bilayered core/shell particles (sample CF3p-S) is 10 nm. The bilayered core/shell particles also have relatively high specific surface area and total pore volume, Table 1. It is important to underline that the results which could be found in the literature [12–15,22] mostly considered surfactant-templated mesoporous silica monolayer prepared around superparamagnetic Fe3O4 core particles. These multifunctional nanostructures might have better magnetic properties, but the silica layers obtained in this work have a greater total pore volume which would allow a greater amount of protein or other material to be absorbed inside the silica layer.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
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Fig. 6. Adsorption–desorption isotherms (a) and pore size distribution (b) of synthesized particles: silica core (C), silica with Fe3O4 shell (CF3) and silica with SiO2/Fe3O4 bilayered-shell (CF3p-S).
Table 1 The specific surface area (Sv) and total pore volume (Vp) of synthesized particles: silica core (C) silica with Fe3O4 shell (CF3) and silica with bilayeredshell (CF3p-S). Sample
Sv [m2/g]
Vp [cm3/g]
C CF3 CF3p-S
324 97 294
0.186 0.075 0.638
Magnetization measurements [25] confirmed the superparamagnetic behavior of the ferrite (Fe3O4) shell. Thus, the obtained composite particles with two functional layers might be used in bioengineering for the immobilization of enzymes inside the external mesoporous silica layer, while the internal superparamagnetic ferrite layer would allow the magnetic separation from a reaction mixture. 4. Conclusions Bilayered-shells composed of internal ferrite layer and external silica layer were synthesized by assembling oppositely charged ferrite (Fe3O4) nanoparticles on the surface of monodispersed silica core particles (having an average size of 0.5 μm) that was followed by the deposition of silica nanoparticles on the surface of the functionalized ferrite layer. Both shells completely cover the core particles, while the corresponding thickness of the ferrite and the silica layers is 20 nm and 100 nm, respectively. The obtained ferrite layer is superparamagnetic and the external silica layer is mesoporous with an average pore size of 10 nm and a large total pore volume. Due to the multifunctionality of the bilayered-shell obtained, the core/shell particles may find potential applications in enzyme immobilization and bioseparation. Acknowledgments The research was supported by the Serbian Ministry of Science under the Project no. III45021. The cooperation under the COST IC1208 Project is also highly acknowledged. We are
also thankful to Milos Bokorov, Faculty of Natural Sciences, University of Novi Sad for performing SEM measurements.
References [1] D. Tarn, C.E. Ashley, M. Xue, E.C. Carnes, J.I. Zink, C.J. Brinker, Mesoporous silica nanoparticle nanocarriers: biofunctionality and biocompatibility, Acc. Chem. Res. 46 (2013) 792–801. [2] R. Ryoo, S.H. Joo, S. Jun, Synthesis of highly ordered carbon molecular sieves via template-mediated structural transformation, J. Phys. Chem. B 103 (1999) 7743–7746. [3] C. Thörn, H. Gustafsson, L. Olsson, QCM-D as a method for monitoring enzyme immobilization in mesoporous silica particles, Microporous Mesoporous Mater. 176 (2013) 71–77. [4] J. Liu, B. Wang, S.B. Hartono, T. Liu, P. Kantharidis, A.P.J. Middelberg, G.Q. (Max), Lu, L. He, S.Z. Qiao, Magnetic silica spheres with large nanopores for nucleic acid adsorption and cellular uptake, Biomaterials 33 (2012) 970–978. [5] S. Lee, H.-S. Yun, S.-H. Kim, The comparative effects of mesoporous silica nanoparticles and colloidal silica on inflammation and apoptosis, Biomaterials 32 (2011) 9434–9443. [6] A.-H. Lu, E.L. Salabas, F. Schüth, Magnetic nanoparticles: synthesis, protection, functionalization, and application, Angew. Chem. Int. Ed. 46 (2007) 1222–1244. [7] S. Wei, Q. Wang, J. Zhu, L. Sun, H. Line, Z. Guo, Multifunctional composite core–shell nanoparticles, Nanoscale 3 (2011) 4474–4502. [8] J. Cheng, X. Ni, H. Zheng, B. Li, X. Zhang, D. Zhang, Preparation of Fe (core)/SiO2 (shell) composite particles with improved oxidationresistance, Mater. Res. Bull. 41 (2006) 1424–1429. [9] F.H. Chen, Q. Gao, J.Z. Ni, The grafting and release behavior of doxorubincin from Fe3O4@SiO2 core–shell structure nanoparticles via an acid cleaving amide bond: the potential for magnetic targeting drug delivery, Nanotechnology 19 (2008) 165103. [10] R. He, X. You, J. Shao, F. Gao, B. Pan, D. Cui, Core/shell fluorescent magnetic silica-coated composite nanoparticles for bioconjugation, Nanotechnology 18 (2007) 315601. [11] L. Qu, S. Tie, Mesoporous silica-coated superparamagnetic magnetite functionalized with CuO and its application as a desulfurizer, Microporous Mesoporous Mater. 117 (2009) 402–405. [12] P. Yang, Z. Quan, Z. Hou, C. Li, X. Kang, Z. Cheng, J. Lin, A magnetic, luminescent and mesoporous core–shell structured composite material as drug carrier, Biomaterials 30 (2009) 4786–4795. [13] S. Gai, P. Yang, J. Hao, W. Wang, N. Niu, F. He, D. Wang, J. Lin, Fabrication of luminescent and mesoporous core–shell structured nanocomposites and their application as drug carrier, Microporous Mesoporous Mater. 131 (2010) 128–135.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139
6
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[14] J.M. Rosenholm, J. Zhang, W. Sun, H. Gu, Large-pore mesoporous silicacoated magnetite core–shell nanocomposites and their relevance for biomedical application, Microporous Mesoporous Mater. 145 (2011) 14–20. [15] Y. Deng, D. Qi, C. Deng, X.M. Zhang, D. Zhao, Superparamagnetic high-magnetization microspheres with an Fe3O4@SiO2 core and perpendicularly aligned mesoporous SiO2 shell for removal of microcystins, J. Am. Chem. Soc. 130 (2008) 28–29. [16] X. Fu, X. Chen, J. Wang, J. Liu, Fabrication of carboxylic functionalized superparamagnetic mesoporous silica microspheres and their application for removal basic dye pollutants from water, Microporous Mesoporous Mater. 139 (2011) 8–15. [17] S. Tao, C. Wang, W. Ma, S. Wu, C. Meng, Designed multifunctionalized magnetic mesoporous microsphere for sequential sorption of organic and inorganic pollutants, Microporous Mesoporous Mater. 147 (2012) 295–301. [18] T. Sen, I.J. Bruce, Mesoporous silica-magnetite nanocomposites: fabrication, characterisation and applications in biosciences, Microporous Mesoporous Mater. 120 (2009) 246–251. [19] M. Xia, C. Chen, M. Long, C. Chen, W. Cai, B. Zhou, Magnetically separable mesoporous silica nanocomposite and its application in Fenton catalysis, Microporous Mesoporous Mater. 145 (2011) 217–223.
[20] M.P. Nikolić, K.P. Giannakopoulos, M. Bokorov, V.V. Srdić, Effect of surface functionalization on synthesis of mesoporous silica core/shell particles, Microporous Mesoporous Mater. 155 (2012) 8–13. [21] N.Ž. Knežević, Core/shell magnetic mesoporous silica nanoparticles with radially oriented wide mesopores, Process. Appl. Ceram. 8 (2) (2014) 109–112. [22] F. Wang, G. Wang, W. Sun, T. Wang, X. Chen, Metallophthalocyanine functionalized magnetic mesoporous silica nanoparticles and its application in ultrasound-assisted oxidation of benzothiophene, Microporous Mesoporous Mater. 217 (2015) 203–209. [23] D. Yang, K. Wei, Q. Liu, Y. Yang, X. Guo, H. Rong, M.-L. Cheng, G. Wang, Folic acid-functionalized magnetic ZnFe2O4 hollow microsphere core/mesoporous silica shell composite particles: synthesis and application in drug release, Mater. Sci. Eng. C 33 (2013) 2879–2884. [24] W. Stöber, A. Fink, E. Bohn, Controlled growth of mesoporous silica spheres in the micron size range, J. Colloid Interface Sci. 26 (1968) 62. [25] M.P. Nikolić, K.P. Giannakopoulos, D. Stamppoulos, E.G. Moshopoulou, V.V. Srdić, Synthesis and characterization of silica core/nano-ferrite shell particles, Mater. Res. Bull. 47 (2012) 1513–1519.
Please cite this article as: M.P. Nikolić, et al., Synthesis and characterization of mesoporous and superparamagnetic bilayered-shell around silica core particles, Ceramics International (2015), http://dx.doi.org/10.1016/j.ceramint.2015.07.139