As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Synthesis and Characterization

Synthesis of nickel ferrite nanoparticles by sol-gel method

The covalently binding of yeast alcohol dehydrogenase (YADH) to magnetic nanoparticles via carbodiimide activation was studied. The magnetic nanoparticles Fe3O4 with a mean diameter of 10.6 nm were prepared by co-precipitating Fe2+ and Fe3+ ions in an ammonia solution and treating under hydrothermal conditions. Transmission electron microscopy (TEM) micrographs showed that the magnetic nanoparticles remained discrete and had no significant change in size after binding YADH. X-ray diffraction (XRD) patterns indicated both the magnetic nanoparticles before and after binding YADH were pure Fe3O4. Magnetic measurement revealed the resultant magnetic nanoparticles were superparamagnetic characteristics, and their saturation magnetization was reduced only slightly after enzyme binding. The analysis of Fourier transform infrared (FTIR) spectroscopy confirmed the binding of YADH to magnetic nanoparticles and suggested a possible binding mechanism. In addition, the measurement of protein content revealed that the maximum weight ratio of YADH bound to magnetic nanoparticles was 0.125, below which the binding efficiency of YADH was almost 100%. The kinetic measurements indicated the bound YADH retained 62% of its original activity and exhibited a 10-fold improved stability than did the free enzyme. The maximum specific activities and Michaelis constants were also determined.

Preparation and characterization of YADH-bound magnetic nanoparticles

Highly dispersed γ-Fe2O3 powders with particle sizes down to 5 nm were directly synthesized by combustion of solutions of iron pentacarbonyl or iron(III) acetylacetonate in toluene in an oxyhydrogen flame. The particle size as well as other properties of the obtained powders can be controlled simply by varying the iron concentration in the starting solutions. Phase composition, morphological and magnetic properties of the powders were studied. The reasons for the formation of γ-Fe2O3 are discussed by means of structure–chemical/kinetic considerations. The materials are interesting as recording materials, or ferrofluids, or for colour imaging and bioprocessing.

Flame pyrolysis – a preparation route for ultrafine pure γ-Fe2O3 powders and the control of their particle size and properties

The response of fine-particle magnetic oxides to applied magnetic fields is of primary importance in several areas of applied physics, such as magnetic separation, magnetic fluid devices, and magnetic recording. It is also of fundamental interest. In this article contemporary research on the important properties of magnetic oxide particles is reviewed, including both applied and basic work. The microscopic and macroscopic properties of ferrimagnetic and antiferromagnetic fine-particle systems are considered. Special attention is paid to the physics of magnetization reversal mechanisms, surface effects, and the intrinsic properties of fine-particle magnetic oxides.

http://iopscience.iop.org/article/10.1088/0953-8984/5/45/002/pdf

Fine-particle magnetic oxides

The surface effect on saturation magnetization of Co and Ti substituted Ba-ferrite platelet fine particles is reported. The saturation magnetization decreases linearly with increasing hexagonal c-plane surface area of the particles, and the effect of the side plane surface area on the saturation magnetization is negligible. This result indicates that a low saturation magnetization layer exists predominantly on the c-plane surfaces. The saturation magnetization of the c-plane surface layers increases noticeably with decreasing temperature, and approaches the saturation magnetization value for the bulk Ba-ferrite. The low saturation magnetization at the surface layer is explained by a thermal fluctuation effect on the magnetic ions at the surface layer which have a weaker exchange field than the ions inside the particles. Therefore, particles with small diameter to thickness ratio values are beneficial in obtaining higher saturation magnetization under a given particle volume.

S. Kurisu

Papers

Surface effect on saturation magnetization of Co and Ti substituted Ba-Ferrite fine particles