Synthesis and physical characterization of magnetite nanoparticles for biomedical applications

Iron oxide nanoparticles for biomedical applications in the size range of 15-130 nm were prepared by either oxidative hydrolysis of ferrous sulfate with KOH or precipitation from ferrous/ferric chloride solutions. The magnetite particle size is controlled by variation of pH and temperature. The synthesized magnetite nanoparticles are partially oxidized as signaled by ferrous concentrations of below 24 wt% Fe2+ and lattice parameters of a(0) <= 8.39 angstrom which are smaller compared to 8.39 angstrom for stoichiometric magnetite. The extend of oxidation increases with decreasing particle size. Heating at 150-350 degrees C topotactically transforms the magnetite nanoparticles into stoichiometric tetragonal maghemite (ferrous ion concentration CFe2+ = 0 and a(0) = 8.34 angstrom) without significant particle growth. The magnetite-maghemite transformation is studied with thermal analysis, XRD and IR spectroscopy. The saturation magnetizations of the magnetite and maghemite particles decrease with decreasing particle size. The variation of M-s with particle size is interpreted using a magnetic core-shell particle model. Magnetite particles with d <= 16 nm show superparamagnetic behavior at room temperature whereas particles with diameter >16 nm display hysteresis behavior. These particles are candidates for biomedical applications, e.g. controlled drug release or hyperthermia. (C) 2008 Elsevier B.V. All rights reserved.