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Synthesis and characterization of monodisperse silica colloids loaded with superparamagnetic iron oxide nanoparticles
Chemical Physics Letters 401 (2005) 19–23 www.elsevier.com/locate/cplett
Synthesis and characterization of monodisperse silica colloids loaded with superparamagnetic iron oxide nanoparticles Sang Hyuk Im, Thurston Herricks, Yun Tack Lee, Younan Xia
*
Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA Received 12 October 2004; in final form 4 November 2004 Available online 23 November 2004
Abstract Silica colloids embedded with superparamagnetic iron oxide nanoparticles were synthesized by combining commercial ferrofluids with the well-known Sto¨ber process. In a typical procedure, organophilic iron oxide nanoparticles were extracted from a ferrofluid and redispersed in toluene. The suspension was then added to an alcoholic medium to produce emulsion drops consisting of aggregates of iron oxide nanoparticles and toluene. When tetraethylorthosilicate was introduced into the system, it hydrolyzed and formed silica coating around each emulsion drop. The final size of silica colloids depended on the concentration of iron oxide nanoparticles and the type of solvent used for the Sto¨ber synthesis. Larger colloids were obtained at lower concentrations of iron oxide nanoparticles and in alcohols with higher molecular weights. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Design and synthesis of uniform, nanometer-sized particles has been intensively studied in recent years because of their technological and fundamental scientific importance [1–3]. Due to their extremely small dimensions, these particles often exhibit novel electronic, optical, magnetic, and chemical properties [4–6]. Especially, magnetic nanoparticles have been of interest as driven by their unique applications in areas, such as information storage, biomedical imaging, magnetic refrigeration, ferrofluids, and drug delivery [7–9]. In general, suitable coatings (made of either inorganic ceramics or organic polymers) are required to endow the magnetic nanoparticles with great stability against agglomeration and harsh chemical environments [10–15]. Silica has been exploited as a coating material for magnetic nanoparticles. As a major advantage, the surface of silica is often terminated by silanol groups that *
Corresponding author. Fax: +1 206 685 8665. E-mail address: [email protected] (Y. Xia).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.028
can react with various coupling agents to covalently attach specific ligands to the surfaces of these magnetic particles [16]. Such a capability opens the door to the design and synthesis of magnetic carriers that can be used to deliver drugs to targeted organs via highly specific recognition (e.g. antibody–antigen interaction). In our previous Letter, we reported a procedure based on the Sto¨ber method to coat the surface of iron oxide nanoparticles (extracted from a commercial ferrofluid) with amorphous silica [17]. Since each of these colloids (50 nm in diameter) only contained a few iron oxide nanoparticles, their response to external magnetic field was very weak as compared with the commercial polymer beads containing multiple magnetic nanoparticles [18]. In the present Letter, we modified the previous method to produce larger silica colloids (100–700 nm in diameter) embedded with multiple magnetic nanoparticles. The modification was executed by replacing the hydrophilic iron oxide nanoparticles (ferrofluid EMG304) with organophilic ones (ferrofluid EMG911) and forming emulsion drops in an alcohol in the presence of toluene. Introduction of a sol–gel precursor into
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S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
the system led to the formation of a silica shell around each emulsion drop. The size of resultant particles could be conveniently controlled by varying the ratio between the concentrations of iron oxide nanoparticles and sol– gel precursor and the type of alcohol used as solvent for the Sto¨ber process [19].
2. Experimental 2.1. Extraction of iron oxide nanoparticles from a commercial ferrofluid In a typical process, 20 ml of acetone was added to 1 ml of ferrofluid EMG 911 (Ferrotec, Nashua, NH) and shaken vigorously to extract iron oxide nanoparticles from the dispersion medium (light mineral oil). The nanoparticles were then collected with a permanent magnet. This extraction procedure was repeated 5 times. These iron oxide nanoparticles were dried in oven at 60 °C for 1 h and finally redispersed in 20 ml of toluene by sonication for 1 h. 2.2. Synthesis of silica colloids containing iron oxide nanoparticles Magnetic silica colloids of 360 and 700 nm in diameter were synthesized by using toluene solutions containing 9.3 and 4.65 mg/ml of iron oxide nanoparticles, respectively. In a typical procedure, 1 ml of the iron oxide suspension in toluene was added to a solution containing 20 ml isopropanol, 2 ml deionized water, and 1 ml ammonium hydroxide (30 wt%). After stirring at 300 rpm for 10 min, 2 ml of tetraethylorthosilicate (TEOS) was introduced into the solution. The reaction was then allowed to proceed at room temperature under continuous magnetic stirring for one day. The final product was collected via magnetic extraction. To alter the size of the silica colloids, the isopropanol was also replaced with an ethanol–isopropanol mixture (50:50 w/w) and pure isobutanol, with other parameters unchanged. 2.3. Characterization Magnetization curves were measured on a superconducting susceptometer (SQUID, MPMS-5S, Quantum
Design, San Diego, CA). Two different procedures were used for measuring the magnetic susceptibilities: zerofield cooling (ZFC) and field cooling (FC) at a field strength of 0.01 T. Hysteresis curves were measured at 5 and 300 K with a maximum field strength of 5 T. All results were normalized against the sample weight. Samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were prepared by dropping suspensions of the colloids on a piece of silicon wafer and carbon-coated copper grid (Ted Pella, Redding, CA), respectively, and drying in a fume hood. SEM images were taken using a field emission scanning electron microscopy (FEI-SEM, Sirion XL) operated at an accelerating voltage of 5 kV. The TEM images were taken with a JEOL microscope (1200EX II) operated at 80 kV.
3. Results and discussion Fig. 1 schematically illustrates the mechanism for generating silica colloids loaded with iron oxide nanoparticles. Since the surfaces of extracted iron oxide nanoparticles are covered by oleic acid, they can be readily redispersed in toluene. When the iron oxide–toluene mixture is added to isopropanol, emulsion drops will be formed because toluene is more miscible with hydrophobic iron oxide particles than with isopropanol even though some of the toluene may be dissolved in isopropanol. In the next step, a sol–gel precursor is introduced to coat the emulsion drops with silica shells. Most of the iron oxide nanoparticles will be located in the center of each silica colloid. Only a small portion of iron oxide nanoparticles that exist as individual ones in the solution will be incorporated into the silica shells as the coating proceeds. Fig. 2a shows a TEM image of the extracted hydrophobic iron oxide nanoparticles. These particles had a mean diameter around 10 nm and they formed large agglomerates upon drying because of strong hydrophobic interactions between their surfaces. Fig. 2b shows two typical TEM images of silica colloids sampled in the early stage of the reaction (1 h after the addition of TEOS). These images clearly indicate that the iron oxide nanoparticles were mainly located at the center of each silica colloid. Because the iron oxide nanoparti-
Fig. 1. A schematic illustration of the procedure for synthesizing silica colloids embedded with iron oxide nanoparticles.
S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
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Fig. 2. (a) TEM image of iron oxide nanoparticles extracted from the commercial ferrofluid. (b) TEM images of silica colloids embedded with iron oxide nanoparticles sampled in the early stage (t 1 h) of sol–gel coating. The inset is a magnified image showing the typical structure of these colloids. (c) SEM image of the final product obtained at t 24 h. The inset shows a TEM image depicting the core-shell structure of these colloids, with the iron oxide nanoparticles being concentrated in the cores. (d) SEM image of another sample that was synthesized under the same conditions as for (c) except that the concentration of iron oxide nanoparticles was reduced by two times. In this case, the size of silica colloids was increased from 360 to 700 nm.
cles were hydrophobic, they tended to be trapped at the surface of each emulsion drop. When the toluene was evaporated, a void could be produced in the center of each silica colloid. Of course, the individual iron oxide nanoparticles in the dispersion medium could be incorporated into the particles at any point as the silica shell grew. For this particular synthesis, the diameters of silica colloids and magnetic shells are 200 and 60 nm, respectively. If we assume that the surface of each emulsion drop is closely packed with iron oxide particles of 10 nm in diameter to form a monolayer, the number of iron oxide nanoparticles contained in each silica colloid can be estimated as the following: 2
N ¼ 4ðRdrop =Rnanoparticle Þ ;
ð1Þ
where N is the number of iron oxide particles embedded in each silica particle, and Rdrop and Rnanoparticle represent the radii of emulsion drops and iron oxide nanoparticles, respectively. If an emulsion drop had a radius of 30 nm and the iron oxide particles had a radius of 5 nm, then there should be about 144 iron oxide nanoparticles encapsulated in each silica colloid. Fig. 2c shows an SEM image of the final product (after reaction for 24 h): silica colloids loaded with iron
oxide nanoparticles. This image clearly indicates that the final colloids are fairly uniform in size and shape, with a mean diameter around 360 nm. The inset shows a TEM image of these particles confirming that the iron oxide nanoparticles were mainly confined to the center of each silica colloid. Since no residual iron oxide nanoparticles are observed in the TEM image, we believe that all of them have been incorporated into the silica colloids. The surfaces of these final particles are also smooth and free of iron oxide nanoparticles. By varying the ratio between the concentrations of TEOS and iron oxide nanoparticles, we could control the size of the iron oxide–silica colloids. Based on the heterogeneous nucleation model, the size of silica colloids should be mainly determined by the number of seeds (i.e., the emulsion drops). Under the synthetic conditions used here, homogeneous nucleation of silica colloids should not occur when there are seeds present in the reaction mixture. Decreasing the number of seeds is expected to increase the diameter of magnetic silica colloids as long as other conditions are kept the same. Fig. 2d shows an SEM image of larger silica colloids encapsulated with iron oxide nanoparticles that were synthesized by reducing the concentration of iron oxide
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S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
while keeping all other experimental parameters unchanged. More specifically, 4.65 mg/ml of iron oxide particles (which corresponds to half of the concentration used in Fig. 2c) was employed and the size of resultant silica colloids became 700 nm in diameter. These results clearly establish that one can tune the size of magnetic silica colloids by varying the concentration of iron oxide nanoparticles. The size of silica colloids resulting from the Sto¨ber process have been found to be less sensitive to the concentrations of water and ammonia [20,21]. The alcohol solvent, however, has a strong influence on the final size of silica colloids. In general, change of solvent is expected to affect the size of emulsion drops, as well as to alter the hydrolysis of TEOS. When solvents containing secondary and tertiary alcohols were employed, larger silica particles were obtained. Figs. 3a and b show SEM images of the final products that were prepared
with a mixture of ethanol and isopropanol (50:50 by weight) and pure isobutanol was used as the solvent, respectively. The corresponding diameters of these magnetic colloids were 140 nm and 1.5 lm, and there was a tendency towards broader size distribution with the use of heavier alcohols due to difference in solvent properties. Silica colloids synthesized using the current method still maintain the superparamagnetic feature associated with the original iron oxide nanoparticles [22]. The blocking temperature of these silica colloids was observed at 86 K from the ZFC measurement shown in Fig. 4a. The steady decline of magnetic moment as temperature increases indicates that the iron oxide nanoparticles are physically isolated from each another due to the oleic acid coatings on their surfaces. From hysteresis measurements, the coercivity of the silica colloids is 139 Oe at 5 K and 1 Oe at 300 K, while the
Fig. 3. SEM images of the silica colloids containing iron oxide nanoparticles that were prepared using two different solvents: (a) a mixture of ethanol and isopropanol (50:50, w/w); (b) pure isobutanol.
Fig. 4. (a) Magnetization vs. temperature and (b) magnetic hysteresis loops for the sample shown in Fig. 2d.
S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
coercivity of the commercial ferrofluid is 0 Oe at 300 K [23]. This difference can be attributed to the fact that the iron oxide nanoparticles suspended in a liquid medium have the ability to rotate freely while the iron oxide nanoparticles embedded in a silica matrix do not have this freedom. In addition, the iron oxide nanoparticles embedded in silica are grouped together so that their magnetic domain sizes might be slightly over the limit for superparamagnetism. It is wellknown that large aggregates of iron oxide nanoparticles require a slightly higher magnetic field to change their direction of magnetization when compared with nanoparticles isolated from each other. For these reasons, it is not difficulty to understand why the silica colloids exhibit a very small coercivity at room temperature. Similar to the commercial ferrofluid, the remanence of our silica colloids is 0 emu/g at 300 K indicating that they do not possess a net magnetic moment at room temperature and are superparamagnetic. The saturation magnetization is about 0.371 emu/g and from this value we can estimate that iron oxide nanoparticles occupy about 0.5% of the magnetic silica colloids by mass. This measured value is close to the number of 0.86% calculated from the feeding concentration of iron oxide nanoparticles. Although this number is relatively small, the amount of iron oxide per colloidal particle is significantly improvement over the previous system.
4. Conclusion Silica colloids loaded with superparamagnetic iron oxide nanoparticles have been prepared using commercial ferrofluids and the well-known Sto¨ber process. The organophilic behavior of iron oxide nanoparticles plays an important role in forming emulsion drops in an alcohol on account of the function of the iron oxide particles as surfactants and difference of miscibility of toluene toward the iron oxide particles and the medium. The final particle size is dependent on the concentration of iron oxide nanoparticles and the type of alcohol used as solvent in the Sto¨ber process because the size of silica particles is closely related to the number of seeds (emulsion drops). Accordingly, larger silica colloids can be obtained at lower concentrations of iron oxide nanoparticles and in alcohols of higher molecular weights.
23
Acknowledgments This work has been supported in part by a DARPADURINT subcontract from Harvard University and a Fellowship from the David and Lucile Packard Foundation. Y.X. is a Camille Dreyfus Teacher Scholar and an Alfred P. Sloan Research Fellow. S.H.I. and Y.T.L. have also been supported in part by the Post-doctoral Fellowship Program of the Korean Science and Engineering Foundation (KOSEF). T.H. thanks the Center for Nanotechnology at the UW for an IGERT fellowship that is funded by the National Science Foundation (DGE-9987620).
References [1] C. Murray, D. Norris, M. Bawendi, J. Am. Chem. Soc. 115 (1993) 8706. [2] T. Hyeon, S.S. Lee, J. Park, Y. Chung, H. Na, J. Am. Chem. Soc. 123 (2001) 12798. [3] Z.L. Wang, Adv. Mater. 15 (2004) 432. [4] A. Alivisatos, Science 271 (1996) 933. [5] Z.L. Wang, J. Phys. Chem. B 104 (2000) 1153. [6] A. Schroedter, H. Weller, Angew. Chem. Int. Ed. 41 (2002) 3218. [7] R. Ziolo, E. Giannelis, B. Weinstin, M. OÕHoro, B. Ganguly, V. Mehrotra, M. Russell, D. Huffman, Science 257 (1992) 219. [8] C.B. Murray, S.H. Sun, H. Doyle, T. Betley, Mater. Res. Soc. Bull. 26 (2001) 985. [9] T. Hyeon, Chem. Commun. 8 (2003) 927. [10] A. Philipse, M. van Vruggen, C. Pathmamanoharan, Langmuir 10 (1994) 92. [11] Y. Wang, X.W. Teng, J.S. Wang, H. Yang, Nano Lett. 3 (2003) 789. [12] C.J. Xu, K.M. Xu, H.W. Gu, R.K. Zheng, H. Liu, X.X. Zhang, Z.H. Guo, B. Xu, J. Am. Chem. Soc. 126 (2004) 9938. [13] N. Yanase, H. Noguchi, H. Asakura, T. Suzuta, J. Appl. Polym. Sci. 50 (1993) 765. [14] X. Xu, G. Friedman, K.D. Humfeld, S.A. Majetich, S.A. Asher, Adv. Mater. 13 (2001) 1681. [15] X. Xu, S.A. Majetich, S.A. Asher, J. Am. Chem. Soc. 124 (2002) 13864. [16] A. Ulman, Chem. Rev. 96 (1996) 1533. [17] Y. Lu, Y. Yin, B.T. Mayers, Y. Xia, Nano Lett. 2 (2002) 183. [18] See, for example, http://www.dynalbiotech.com. [19] W. Sto¨ber, A. Fink, E. Bohn, Colloid Interf. Sci. 26 (1968) 62. [20] D. Green, J. Lin, Y.-F. Lam, M.Z.-C. Hu, D. Schaefer, M. Harris, J. Colloid Interf. Sci. 266 (2003) 346. [21] A. Lyberatos, R.W. Chantrell, IEEE Trans. Magn. 26 (1990) 2119. [22] J.L. Dormann, L. Bessias, D. Fiorani, J. Phys. C: Solid State Phys. 21 (1988) 2015. [23] See the data sheet at http://www.ferrotec-europe.de/pdf/ emgdatentabelle.en.pdf.
Synthesis and characterization of monodisperse silica colloids loaded with superparamagnetic iron oxide nanoparticles Sang Hyuk Im, Thurston Herricks, Yun Tack Lee, Younan Xia
*
Department of Chemistry, University of Washington, Seattle, WA 98195-1700, USA Received 12 October 2004; in final form 4 November 2004 Available online 23 November 2004
Abstract Silica colloids embedded with superparamagnetic iron oxide nanoparticles were synthesized by combining commercial ferrofluids with the well-known Sto¨ber process. In a typical procedure, organophilic iron oxide nanoparticles were extracted from a ferrofluid and redispersed in toluene. The suspension was then added to an alcoholic medium to produce emulsion drops consisting of aggregates of iron oxide nanoparticles and toluene. When tetraethylorthosilicate was introduced into the system, it hydrolyzed and formed silica coating around each emulsion drop. The final size of silica colloids depended on the concentration of iron oxide nanoparticles and the type of solvent used for the Sto¨ber synthesis. Larger colloids were obtained at lower concentrations of iron oxide nanoparticles and in alcohols with higher molecular weights. Ó 2004 Elsevier B.V. All rights reserved.
1. Introduction Design and synthesis of uniform, nanometer-sized particles has been intensively studied in recent years because of their technological and fundamental scientific importance [1–3]. Due to their extremely small dimensions, these particles often exhibit novel electronic, optical, magnetic, and chemical properties [4–6]. Especially, magnetic nanoparticles have been of interest as driven by their unique applications in areas, such as information storage, biomedical imaging, magnetic refrigeration, ferrofluids, and drug delivery [7–9]. In general, suitable coatings (made of either inorganic ceramics or organic polymers) are required to endow the magnetic nanoparticles with great stability against agglomeration and harsh chemical environments [10–15]. Silica has been exploited as a coating material for magnetic nanoparticles. As a major advantage, the surface of silica is often terminated by silanol groups that *
Corresponding author. Fax: +1 206 685 8665. E-mail address: [email protected] (Y. Xia).
0009-2614/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2004.11.028
can react with various coupling agents to covalently attach specific ligands to the surfaces of these magnetic particles [16]. Such a capability opens the door to the design and synthesis of magnetic carriers that can be used to deliver drugs to targeted organs via highly specific recognition (e.g. antibody–antigen interaction). In our previous Letter, we reported a procedure based on the Sto¨ber method to coat the surface of iron oxide nanoparticles (extracted from a commercial ferrofluid) with amorphous silica [17]. Since each of these colloids (50 nm in diameter) only contained a few iron oxide nanoparticles, their response to external magnetic field was very weak as compared with the commercial polymer beads containing multiple magnetic nanoparticles [18]. In the present Letter, we modified the previous method to produce larger silica colloids (100–700 nm in diameter) embedded with multiple magnetic nanoparticles. The modification was executed by replacing the hydrophilic iron oxide nanoparticles (ferrofluid EMG304) with organophilic ones (ferrofluid EMG911) and forming emulsion drops in an alcohol in the presence of toluene. Introduction of a sol–gel precursor into
20
S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
the system led to the formation of a silica shell around each emulsion drop. The size of resultant particles could be conveniently controlled by varying the ratio between the concentrations of iron oxide nanoparticles and sol– gel precursor and the type of alcohol used as solvent for the Sto¨ber process [19].
2. Experimental 2.1. Extraction of iron oxide nanoparticles from a commercial ferrofluid In a typical process, 20 ml of acetone was added to 1 ml of ferrofluid EMG 911 (Ferrotec, Nashua, NH) and shaken vigorously to extract iron oxide nanoparticles from the dispersion medium (light mineral oil). The nanoparticles were then collected with a permanent magnet. This extraction procedure was repeated 5 times. These iron oxide nanoparticles were dried in oven at 60 °C for 1 h and finally redispersed in 20 ml of toluene by sonication for 1 h. 2.2. Synthesis of silica colloids containing iron oxide nanoparticles Magnetic silica colloids of 360 and 700 nm in diameter were synthesized by using toluene solutions containing 9.3 and 4.65 mg/ml of iron oxide nanoparticles, respectively. In a typical procedure, 1 ml of the iron oxide suspension in toluene was added to a solution containing 20 ml isopropanol, 2 ml deionized water, and 1 ml ammonium hydroxide (30 wt%). After stirring at 300 rpm for 10 min, 2 ml of tetraethylorthosilicate (TEOS) was introduced into the solution. The reaction was then allowed to proceed at room temperature under continuous magnetic stirring for one day. The final product was collected via magnetic extraction. To alter the size of the silica colloids, the isopropanol was also replaced with an ethanol–isopropanol mixture (50:50 w/w) and pure isobutanol, with other parameters unchanged. 2.3. Characterization Magnetization curves were measured on a superconducting susceptometer (SQUID, MPMS-5S, Quantum
Design, San Diego, CA). Two different procedures were used for measuring the magnetic susceptibilities: zerofield cooling (ZFC) and field cooling (FC) at a field strength of 0.01 T. Hysteresis curves were measured at 5 and 300 K with a maximum field strength of 5 T. All results were normalized against the sample weight. Samples for scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies were prepared by dropping suspensions of the colloids on a piece of silicon wafer and carbon-coated copper grid (Ted Pella, Redding, CA), respectively, and drying in a fume hood. SEM images were taken using a field emission scanning electron microscopy (FEI-SEM, Sirion XL) operated at an accelerating voltage of 5 kV. The TEM images were taken with a JEOL microscope (1200EX II) operated at 80 kV.
3. Results and discussion Fig. 1 schematically illustrates the mechanism for generating silica colloids loaded with iron oxide nanoparticles. Since the surfaces of extracted iron oxide nanoparticles are covered by oleic acid, they can be readily redispersed in toluene. When the iron oxide–toluene mixture is added to isopropanol, emulsion drops will be formed because toluene is more miscible with hydrophobic iron oxide particles than with isopropanol even though some of the toluene may be dissolved in isopropanol. In the next step, a sol–gel precursor is introduced to coat the emulsion drops with silica shells. Most of the iron oxide nanoparticles will be located in the center of each silica colloid. Only a small portion of iron oxide nanoparticles that exist as individual ones in the solution will be incorporated into the silica shells as the coating proceeds. Fig. 2a shows a TEM image of the extracted hydrophobic iron oxide nanoparticles. These particles had a mean diameter around 10 nm and they formed large agglomerates upon drying because of strong hydrophobic interactions between their surfaces. Fig. 2b shows two typical TEM images of silica colloids sampled in the early stage of the reaction (1 h after the addition of TEOS). These images clearly indicate that the iron oxide nanoparticles were mainly located at the center of each silica colloid. Because the iron oxide nanoparti-
Fig. 1. A schematic illustration of the procedure for synthesizing silica colloids embedded with iron oxide nanoparticles.
S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
21
Fig. 2. (a) TEM image of iron oxide nanoparticles extracted from the commercial ferrofluid. (b) TEM images of silica colloids embedded with iron oxide nanoparticles sampled in the early stage (t 1 h) of sol–gel coating. The inset is a magnified image showing the typical structure of these colloids. (c) SEM image of the final product obtained at t 24 h. The inset shows a TEM image depicting the core-shell structure of these colloids, with the iron oxide nanoparticles being concentrated in the cores. (d) SEM image of another sample that was synthesized under the same conditions as for (c) except that the concentration of iron oxide nanoparticles was reduced by two times. In this case, the size of silica colloids was increased from 360 to 700 nm.
cles were hydrophobic, they tended to be trapped at the surface of each emulsion drop. When the toluene was evaporated, a void could be produced in the center of each silica colloid. Of course, the individual iron oxide nanoparticles in the dispersion medium could be incorporated into the particles at any point as the silica shell grew. For this particular synthesis, the diameters of silica colloids and magnetic shells are 200 and 60 nm, respectively. If we assume that the surface of each emulsion drop is closely packed with iron oxide particles of 10 nm in diameter to form a monolayer, the number of iron oxide nanoparticles contained in each silica colloid can be estimated as the following: 2
N ¼ 4ðRdrop =Rnanoparticle Þ ;
ð1Þ
where N is the number of iron oxide particles embedded in each silica particle, and Rdrop and Rnanoparticle represent the radii of emulsion drops and iron oxide nanoparticles, respectively. If an emulsion drop had a radius of 30 nm and the iron oxide particles had a radius of 5 nm, then there should be about 144 iron oxide nanoparticles encapsulated in each silica colloid. Fig. 2c shows an SEM image of the final product (after reaction for 24 h): silica colloids loaded with iron
oxide nanoparticles. This image clearly indicates that the final colloids are fairly uniform in size and shape, with a mean diameter around 360 nm. The inset shows a TEM image of these particles confirming that the iron oxide nanoparticles were mainly confined to the center of each silica colloid. Since no residual iron oxide nanoparticles are observed in the TEM image, we believe that all of them have been incorporated into the silica colloids. The surfaces of these final particles are also smooth and free of iron oxide nanoparticles. By varying the ratio between the concentrations of TEOS and iron oxide nanoparticles, we could control the size of the iron oxide–silica colloids. Based on the heterogeneous nucleation model, the size of silica colloids should be mainly determined by the number of seeds (i.e., the emulsion drops). Under the synthetic conditions used here, homogeneous nucleation of silica colloids should not occur when there are seeds present in the reaction mixture. Decreasing the number of seeds is expected to increase the diameter of magnetic silica colloids as long as other conditions are kept the same. Fig. 2d shows an SEM image of larger silica colloids encapsulated with iron oxide nanoparticles that were synthesized by reducing the concentration of iron oxide
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S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
while keeping all other experimental parameters unchanged. More specifically, 4.65 mg/ml of iron oxide particles (which corresponds to half of the concentration used in Fig. 2c) was employed and the size of resultant silica colloids became 700 nm in diameter. These results clearly establish that one can tune the size of magnetic silica colloids by varying the concentration of iron oxide nanoparticles. The size of silica colloids resulting from the Sto¨ber process have been found to be less sensitive to the concentrations of water and ammonia [20,21]. The alcohol solvent, however, has a strong influence on the final size of silica colloids. In general, change of solvent is expected to affect the size of emulsion drops, as well as to alter the hydrolysis of TEOS. When solvents containing secondary and tertiary alcohols were employed, larger silica particles were obtained. Figs. 3a and b show SEM images of the final products that were prepared
with a mixture of ethanol and isopropanol (50:50 by weight) and pure isobutanol was used as the solvent, respectively. The corresponding diameters of these magnetic colloids were 140 nm and 1.5 lm, and there was a tendency towards broader size distribution with the use of heavier alcohols due to difference in solvent properties. Silica colloids synthesized using the current method still maintain the superparamagnetic feature associated with the original iron oxide nanoparticles [22]. The blocking temperature of these silica colloids was observed at 86 K from the ZFC measurement shown in Fig. 4a. The steady decline of magnetic moment as temperature increases indicates that the iron oxide nanoparticles are physically isolated from each another due to the oleic acid coatings on their surfaces. From hysteresis measurements, the coercivity of the silica colloids is 139 Oe at 5 K and 1 Oe at 300 K, while the
Fig. 3. SEM images of the silica colloids containing iron oxide nanoparticles that were prepared using two different solvents: (a) a mixture of ethanol and isopropanol (50:50, w/w); (b) pure isobutanol.
Fig. 4. (a) Magnetization vs. temperature and (b) magnetic hysteresis loops for the sample shown in Fig. 2d.
S.H. Im et al. / Chemical Physics Letters 401 (2005) 19–23
coercivity of the commercial ferrofluid is 0 Oe at 300 K [23]. This difference can be attributed to the fact that the iron oxide nanoparticles suspended in a liquid medium have the ability to rotate freely while the iron oxide nanoparticles embedded in a silica matrix do not have this freedom. In addition, the iron oxide nanoparticles embedded in silica are grouped together so that their magnetic domain sizes might be slightly over the limit for superparamagnetism. It is wellknown that large aggregates of iron oxide nanoparticles require a slightly higher magnetic field to change their direction of magnetization when compared with nanoparticles isolated from each other. For these reasons, it is not difficulty to understand why the silica colloids exhibit a very small coercivity at room temperature. Similar to the commercial ferrofluid, the remanence of our silica colloids is 0 emu/g at 300 K indicating that they do not possess a net magnetic moment at room temperature and are superparamagnetic. The saturation magnetization is about 0.371 emu/g and from this value we can estimate that iron oxide nanoparticles occupy about 0.5% of the magnetic silica colloids by mass. This measured value is close to the number of 0.86% calculated from the feeding concentration of iron oxide nanoparticles. Although this number is relatively small, the amount of iron oxide per colloidal particle is significantly improvement over the previous system.
4. Conclusion Silica colloids loaded with superparamagnetic iron oxide nanoparticles have been prepared using commercial ferrofluids and the well-known Sto¨ber process. The organophilic behavior of iron oxide nanoparticles plays an important role in forming emulsion drops in an alcohol on account of the function of the iron oxide particles as surfactants and difference of miscibility of toluene toward the iron oxide particles and the medium. The final particle size is dependent on the concentration of iron oxide nanoparticles and the type of alcohol used as solvent in the Sto¨ber process because the size of silica particles is closely related to the number of seeds (emulsion drops). Accordingly, larger silica colloids can be obtained at lower concentrations of iron oxide nanoparticles and in alcohols of higher molecular weights.
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Acknowledgments This work has been supported in part by a DARPADURINT subcontract from Harvard University and a Fellowship from the David and Lucile Packard Foundation. Y.X. is a Camille Dreyfus Teacher Scholar and an Alfred P. Sloan Research Fellow. S.H.I. and Y.T.L. have also been supported in part by the Post-doctoral Fellowship Program of the Korean Science and Engineering Foundation (KOSEF). T.H. thanks the Center for Nanotechnology at the UW for an IGERT fellowship that is funded by the National Science Foundation (DGE-9987620).
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