Monodisperse magnetic single-crystal ferrite microspheres.

It has been thought that many novel properties and potential applications would emerge from monodisperse materials with small dimensions. Therefore, the synthesis of monodisperse nanoparticles has been intensively pursued for their technological and fundamental scientific importance. The synthesis of nanostructured magnetic materials has become a particularly important area of research and is attracting a growing interest because of the potential applications such materials have in ferrofluids, advanced magnetic materials, catalysts, colored pigments, high-density magnetic recording media, and medical diagnostics. Spinel ferrites (MFe2O4; M = Fe, Mn, Zn, or Co) are among the most important magnetic materials and have been widely used in electronic devices, information storage, magnetic resonance imaging (MRI), and drug-delivery technology. 14] Magnetite (Fe3O4) has recently been considered an ideal candidate for biological applications, both as a tag for sensing and imaging, and as an activity agent for antitumor therapy. For high performance in function-specific biological applications, magnetic particles must be spherical and have smooth surfaces, narrow size distributions, large surface areas (for maximal protein or enzyme binding), high magnetic saturation (ss) to provide maximum signal, and good dispersion in liquid media. 18,19] After Sugimoto and Matijević reported the preparation of magnetite particles with a narrow size distribution in the early 1980s, monodisperse ferrite has been fabricated by various chemistry-based synthetic methods, including coprecipitation, the reverse micelle method, microwave plasma synthesis, solgel techniques, freeze drying, ultrasound irradiation, hydrothermal methods, laser pyrolysis techniques, and thermal decomposition of organometallic and coordination compounds. 9,14, 18, 20–27] However, most of these approaches were focused on the synthesis of ferrite particles limited to diameters below 30 nm. There are no reports on the synthesis of well-crystallized ferrite nanoparticles with sizes similar to protein molecules. The development of a facile and economic synthetic strategy for the synthesis of hydrophilic, biocompatible magnetite nanoparticles would benefit their technical use in biomedical fields, especially for applications in vivo. Herein we report a general approach for the fabrication of monodisperse, hydrophilic, and single-crystalline ferrite microspheres by a solvothermal reduction method. To the best of our knowledge, this is the first report on the synthesis of single-crystalline magnetic microspheres. The ferrite spheres had monodisperse diameters that were tunable in the range of 200–800 nm. This work resulted in an important method for obtaining various monodisperse, magnetic, and single-crystalline microspheres, and provided an opportunity to further apply these promising materials. Typical syntheses of Fe3O4 and ferrite microspheres were carried out in a solvothermal system by modified reduction reactions between FeCl3 and ethylene glycol. We confirmed the production of Fe3O4 by conducting controlled oxidation reactions in which aand g-Fe2O3 were produced (Supporting Information). 28–29] The crystalline structures of MFe2O4 were characterized by XRD. As shown in Figure 1, the