High magnetization superparamagnetic core-shell nanoparticles

Abstract

Magnetic nanoparticles and nanocomposites have been special focus of high research interest due to their high magnetic moment and bioaffinity surface properties and promising applications in nanoscience, nanotechnology and bio-applications. These properties can be obtained by synthesizing various magnetic nanoparticles and core-shell type magnetic nanocomposites. In this research, we synthesized high magnetization superparamagnetic nanoparticles and core-shell nanocomposites by chemical and sonochemical method for using as carriers/labels in bio-sensing purpose. Iron oxide magnetic nanoparticles (NPs) have been synthesized by sonochemical method using inexpensive and non-toxic metal salts as reactants. Transmission electron microscopy (TEM) data demonstrated that the particles were narrow range in size distribution with 11 nm average particle size and spherical in shape. The magnetization curve from vibrating sample magnetometer (VSM) measurement shows that assynthesized NPs were nearly superparamagnetic in magnetic properties with very low coercivity, and magnetization values were 80 emu/g. Monodisperse magnetite nanocubes with uniform particle size of about 80 nm have been synthesized in aqueous medium by sonochemical method. The magnetic characterization of the NPs reveals saturation magnetization of 91 emu/g at 5 K for as-synthesized sample and 94.8 emu/g for the sample which annealed at the temperature of 600 °C in a vacuum chamber. However, the saturation magnetization has been observed to decrease with further increase in annealing temperature and this has been attributed to the presence of a thin magnetic dead layer at the surface caused by shape anisotropy distortion and broken exchange bonds, and spin canting on the surface of the particles in addition to formation of a small amount of maghemite phase. Highly crystalline and monodisperse cobalt ferrite (CoFe2O4) nanoparticles have been synthesized via rapid one-pot sonochemical techniques and without subsequent calcination. The size of CoFe2O4 nanoparticle was controlled in the range from 20 to 110 nm based on the solvent medium used in the synthesis process. Furthermore, the evolution from spherical to cubic morphology of cobalt ferrite is achieved by simply changing the solvent medium from aqueous to alcoholic medium. High saturation magnetization (Ms) and high coercivity (Hc) values of 87 emu/g and 1610 Oe, respectively were obtained for the CoFe2O4 NPs. For synthesis of core-shell type of silica coated iron oxide magnetic (Fe3O4@SiO2 core-shell) NPs, sono-chemical approach was applied using inexpensive and non-toxic chemicals. TEM data demonstrated that the thickness of silica coating on iron oxide magnetic NPs 10-15 nm in average. The magnetization curve from VSM measurement shows that the magnetization has also been decreased of as synthesized silica coated iron oxide NPs compared to freshly prepared bare iron oxide magnetic NPs, which is also a evidence of synthesizing of Fe3O4@SiO2 core-shell NPs. Besides the synthesis of metal oxide core-shell NPs, high magnetization FeCo nanoparticles with different Fe/Co ratios have been successfully synthesized by surfactant free simple modified polyol method. In this process, polyethylene glycol was used as a solvent media and it has been found to play a key role to act as a reducing agent as well as a stabilizer simultaneously. TEM data suggest that the annealed FeCo nanoparticles are of 50-90 nm in size. The physical Property Measurement System (PPMS) reveals that the Fe60Co40 composition among all the samples exhibit highest saturation magnetization of 230.14 emu/g at 5K. In another study, high magnetic monodisperse NiFe NPs with different compositions have been successfully synthesized polyol method. TEM images displayed formation of a thin oxide layer around the nanoparticles, and confirmed by detection of some oxygen element using EDS measurement. The magnetic properties of the synthesized NiFe NPs samples were mea