Magnetic Janus Particles Prepared by a Flame Synthetic Approach: Synthesis, Characterizations and Properties

Abstract

2009 WILEY-VCH Verlag Gm Magnetic iron oxide nanoparticles have shown application potentials in position sensing, ultrahigh-density magnetic storage media, clinical diagnosis and treatment, and biological labeling, tracking, imaging, detection, and separation. Silica has been demonstrated to be a very useful material in many fields due to its nontoxicity and excellent chemical stability. For example, the mature silica chemistry has offered versatile choices for covalently attaching biomolecules to the surface of magnetic nanoparticles coated with silica, to enable their biological applications. Until now, magnetic nanoparticles have been commonly combined with silica by forming a core/shell structure, with the nanoparticles acting as a core, or a composite structure, with a number of magnetic nanoparticles simultaneously incorporated in single silica beads. Nevertheless, no heterostructured spherical particles consisting of both magnetic iron oxide and silica have ever been investigated. In fact, such ‘‘Janus’’-type particles, herein referring to spherical particles consisting of bicompartments with different chemical compositions, are very challenging to synthesize. Most successful examples are found in polymeric systems, such as Janus dendrimers and Janus polymeric microbeads. Inorganic heterostructured particles have also been widely investigated. Nonetheless, the synthetic methods in most cases rely on the seed-mediated growth of the second material, and only quasi-spherical Janus particles have been reported. In principle, Janus nanoparticles with biphasic geometry and distinct composition and properties should be superior to symmetrical nanoparticles as they hold both physical and chemical properties of each phase. Therefore, opposite properties can be simply combined into single particles, offering Janus particles great potentials for numerous applications. Herein, we report a new synthetic route for preparing spherical Janus particles composed of magnetic iron oxide and silica, denoted as g -Fe2O3jjSiO2. Systematic experiments were performed to characterize these particles. In addition, a number of experiments were carried out to demonstrate the effects related to their anisotropic properties. The magnetic g -Fe2O3jjSiO2 Janus particles were prepared by flame synthesis. A cartoon image of the setup for Janus particle preparation is shown in Figure 1. This method is similar to the liquid-feed flame spray synthesis, with the main difference being that no gas is introduced to aerosolize the organic solution of the precursors while they are combusting. The reaction is quenched by a thin water film on a rapidly rotating glass tube. The iron oxide particles carried by the water were eventually collected by a plate placed near the rotating tube, on the opposite side of the fire. Figure 2a shows a typical transmission electron microscopy (TEM) image of the magnetic particles collected from the flame envelope. A clear interface, appearing across most particles, depicts a perfect Janus structure for the resultant spherical particles, whose average particle size is of (71 33) nm. Highresolution TEM results (Fig. 2b) suggest that the dark hemisphere of the Janus particles is made up of polycrystalline maghemite. In contrast, the gray region exhibits a typically amorphous nature. Electron dispersive spectroscopy (EDS) results (Fig. 2c) further demonstrate that the dark region is an Fe-rich area with a small percentage of Si present, whereas the gray region is a Si-rich area with no Fe present. As O is detected in both inspected regions, it can be concluded that the Janus particle is made up of maghemite and amorphous silica. Powder X-ray diffraction measurements (Fig. 2d) also confirm the maghemite phase in the Janus particles, which present ferromagnetic properties with a relatively small magnetic coercivity of 78 Oe. The room-temperature saturation magnetization of the Janus particles is determined to be about 42 emu g 1 (Fig. 2e). A number of investigations on inorganic particles, consisting of single and mixed oxides prepared by flame-involved approaches, have been reported. However, to the best of our knowledge, metal oxide particles with such a perfect Janus structure as shown in Figure 1a were not reported before. Therefore, two additional samples were collected using glass