Fabrication of Novel Magnetic Janus Microparticles Amro K. F. Dyab and Vesselin N. Paunov*, Department of Chemistry, University of Hull, Hull, HU6 7RX, United Kingdom.
ABSTRACT We have designed a novel technique for fabrication of magnetic Janus microparticles based on “trapping” the alignment of magnetite nanoparticles dispersed within the oil drops of polymerizable oil-in-water emulsion. We polymerized the oil drops after gelling the continuous aqueous phase in the presence of an external magnetic filed. This allowed us to produce magnetic Janus particles with optical and magnetic anisotropy which form unusual zigzag chains and structures when an external magnetic field is applied to a suspension of such particles. These novel microparticles retain high remanence magnetization and coercivity values indicative of ferromagnetic behavior, which indicates that the composite polymeric Janus microparticles posses a net magnetic dipole and behave like micro-magnets due to the “trapped” orientation of the magnetite nanoparticles in their polymeric matrix. INTRODUCTION Anisotropic microparticles have attracted substantial interest over the past few years.1 Anisotropy from non-spherical particle shape or non-uniform surface properties makes their physical properties different from those of isotropic microparticles, which puts them as potentially promising building blocks for assembling photonic crystals with novel symmetries, colloidal substitutes for liquid crystals and electrorheological fluids.1,2 In addition, anisotropic colloids can be used to control suspension rheology and optical properties,3 for stabilization of emulsions4 and foams,5 engineering of biomaterials6 and complex colloidal composites.7 A range of techniques for fabrication of anisotropic particles have been developed, including micro-contact printing,8 lithography-based micro-stamping,9 clusterization of microspheres10 or partial coating of particle monolayers,11 micro-fluidics,12 electrohydrodynamic jetting,13 and controlled nucleation and precipitation.14 “In bulk” preparation of anisotropic polymeric particles from emulsions was performed by deforming droplets in a stretched gel matrix during polymerization,15 by solvent attrition of elongated polymer solution droplets in shear flow,16 and by using liquid crystal droplets in surfactant director fields.17 Recently, we published a simple method for fabricating anisotropic non-spherical polymeric magnetic microparticles of various morphologies by a surface formation technique.18 In this paper, we report the fabrication of novel magnetic Janus microparticles which behave like micro-magnets and can be assembled and manipulated in solution by external stimuli such as magnetic field. The basics of the method are illustrated in Fig.1 and involve the following stages: (i) a magnetic o/w emulsion is prepared from a polymerizable oil, loaded with a known amount of hydrophobized magnetite nanoparticles, and stabilised by a polymerizable anionic surfactant; (ii) the resultant stable o/w emulsion is then mixed with a hot agarose solution *
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and the system is allowed to cool to gel the aqueous phase; (iii) the polymerisation of the oil drops is started by heating the gelled magnetic emulsion while applying an external magnetic field on the sample; (iv) after completing the polymerisation the microparticles are released from the gel and washed and re-dispersed in an aqueous phase.
Figure 1: Schematic representation of our method for the preparation of magnetic Janus microparticles from a polymerizable oil-in-water magnetic emulsion template. MATERIALS AND METHODS We present results with magnetite (Fe3O4) nanoparticles which were produced by coprecipitation19 of Fe3+ and Fe2+ (2:1 molar ratio) with NH4OH in the presence of oleic acid at 80 o C. The precipitated nanoparticle powder was washed five times with water and ethanol, separated by magnetic decantation and dried in an oven at 80 °C. 2-4% wt magnetite nanoparticles were dispersed in the oil phase (1,6 hexanediol diacrylate, HDDA or styrene) by using Branson digital sonifier 450W, 20KHz at 20% of the full power for 10 minutes. After the sonication process, 2 wt.% of the thermal initiator (Vazo) was dissolved into the oil phase. We prepared oil-in-water emulsions using typically 20% of the oil phase in water in the presence of polymerizable surfactant (Hitenol BC-10/20), typically at 2-3 wt.% with respect to the oil phase. The resulting emulsions, produced from either methods mentioned above, were rapidly added to 2 wt.% hot Agarose solution (melting temperature 88 °C) with the ratio of (1:1) and the mixture was cooled under tap water to set the gel. A neodymium magnet (from E-magnets, UK) was applied to the gelled magnetic o/w emulsion which was heated to 72 °C for 3.5 hours to complete the oil polymerization. Microparticles were then recovered by melting the agarose gel at 88°C, followed by 50 fold dilution in Milli-Q water at the same temperature. Microparticles were collected by magnetic decantation and then washed with miliQ water and IPA several times to remove the traces of un-reacted oil and excess of surfactant.
RESULTS AND DISCUSSION Fig. 2A shows the X-ray diffraction analysis for the oleic acid-coated magnetite nanoparticles powder. The XRD patterns revealed diffraction peaks which are characteristic of the Fe3O4 crystal with a cubic spinal structure. The diffraction peaks at 2θ=35.62°, 62.9°, 30.24°, 57.26°, and 43.21° can be assigned to (3 1 1), (4 4 0), (2 2 0), (5 1 1) and (4 0 0) planes of Fe3O4 (JCPDS 88-0866), respectively. The similarity of maghemite and magnetite XRD patterns makes it difficult to exclude the formation of trace amount of maghemite. However, the value of the saturation magnetization and the distinctive black color of the sample suggest that magnetite comprises a significant fraction of the sample. The crystallite size of the OCMNs was estimated form the XRD patterns by measuring the half-height width of the strongest reflection (311) plane of Fe3O4 using Scherrer formula20 and was found to be 10.8 nm. The magnetization curve of the magnetic nanoparticles is presented on Fig. 2B and shows a typical superparamagnetic behavior.
Figure 2: (A) XRD patterns of oleic acid-coated magnetite nanoparticles powder; (B) Magnetization curve of oleic acid-coated magnetic nanoparticles at 298 K. Fig. 3A shows that the magnetite nanoparticles form chains aligned in parallel to the applied magnetic field (one side of the magnet) within the oil drops and have concentrated on one side of the drops. The position of the permanent magnet with respect to the gelled magnetic oil-inwater emulsion sample during its polymerization is an important factor that controls the morphology of the produced microparticles. This was demonstrated by the microparticles shown in Figs. 3B where the gelled HDDA-in-water magnetic emulsion sample was placed in the middle between the north (N) and south (S) poles of a single magnet during the polymerization. The entrapped magnetite nanoparticles within the HDDA drops were aligned towards the N and S poles forming chains and flakes with a void depleted of magnetite particles in the middle of the spherical drops. Moreover, the produced magnetic Janus microparticles formed straight chains when exposed to an external magnetic field, as seen in Fig 3B.
A B Figure 3: Optical micrographs of HDDA oil drops and polymeric microparticles containing magnetite nanoparticles and stabilized with 5 wt.% Hitenol BC-10 polymerizable surfactant. (A) An emulsion oil drop contains 2 wt.% magnetite nanoparticles before thermally initiated polymerization and before applying the magnet; (B) Chaining of the polymerized microparticles near a magnet.
Figure 4 (A) Optical micrographs of HDDA based polymeric microparticles containing 3 wt.% magnetite nanoparticles and stabilized by 3 wt.% Hitenol BC-10. (B) Magnetisation curve of the magnetic Janus particles o/w emulsion of HDDA was stabilised by 5 wt.% Hitenol BC-10 and contain 2 wt.% magnetite nanoparticles in the HDDA.at 298K We found that the emulsion drops exhibited chain structure where the magnetic parts of neighboring microparticles align together forming magnetic “lanes”. An optical image of the aqueous suspension of these particles is shown in Fig.4A. The produced microparticles exhibit shape, magnetic and optical anisotropies. They formed zigzag chains in an external magnetic field as their magnetic dipoles interact with each other. This structuring behavior resembles recently published results by Velev et al. 21 where 50% gold capped polystyrene particles have been shown to exhibit very similar chain behavior in AC external field. Fig. 4B shows the magnetization curve of anisotropic magnetic Janus microparticles loaded with 2 wt.%
hydrophobized magnetite nanoparticles at 298 K. The hysteresis loop indicates ferromagnetic behavior, i.e. the microparticles behave like micro-magnets.
Figure 5. Optical micrographs of styrene oil drops and polymeric microparticles containing 2 wt.% magnetite nanoparticles and stabilised by 3 wt.% Hitenol BC-10. (A) a magnetic o/w emulsion before polymerisation. (B) Microparticles after polymerisation (initiated with 2 wt.% Vaso), dispersed in water. Fig.5A shows a magnetic styrene-in-water emulsion drops, loaded with 2 wt.% magnetite nanoparticles, before polymerization and under an external magnetic field. The emulsion was prepared using the homogenizer and was stabilized by 3 wt% Hitenol BC-10. The magnetite nanoparticles aligned in at least two flakes that repel each other as can be seen in Fig.5A. The resulting anisotropic microparticles after the polymerization of the styrene-in-water emulsion retain this “cat eye”-like morphology, as seen in Fig. 5B. The formed microparticles were smaller in sizes (average diameters of about 10 µm) compared to those prepared with HDDA oil monomer at the same conditions.
CONCLUSIONS We have developed a novel method for fabricating magnetic Janus microparticles with permanent magnetic dipole moments and various morphologies, which can be manipulated by an external magnetic field. The fabrication method is based on "arresting" the alignment and the magnetic polarisation of hydrophobized magnetite nanoparticles dispersed into oil-in-water emulsion drops. The later is achieved by polymerising the oil phase while applying an external magnetic field to the emulsion sample. To prevent the interdrop coalescence in the magnetic emulsion the aqueous phase was gelled with agarose prior to the application of external magnetic field. These Janus microparticles showed ferromagnetic behaviour with clear hysteresis loops in the magnetisation curve. Such particles exhibit interesting zigzag chain formation in aqueous dispersions under external magnetic filed where the magnetic parts of neighbouring microparticles are aligned together forming magnetic “lanes”. These anisotropic Janus particles can find many potential applications for magnetically controlled optical switches and valves, fabrication of micro engines and assembling of colloid crystal structures of novel symmetries.
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