No abstract

Cubic-like shaped Zn x Fe 3−x O 4 particles with crystallite mean sizes D between 15 and 117 nm were obtained by co-precipitation. Particle size effects and preferential occupation of spinel tetrahedral site by Zn 2+ ions led to noticeable changes of physical properties. D 30 nm particles displayed nearly bulk properties, which were dominated by Zn concentration. For D 30 nm, dominant magnetic relaxation effects were observed by Mössbauer spectroscopy, with the mean blocking size D B ∼ 13 to 15 nm. Saturation magnetization increased with x up to x ∼ 0.1-0.3 and decreased for larger x. Power absorbed by water and chitosan-based ferrofluids from a 260 kHz radio frequency field was measured as a function of x, field amplitude H 0 and ferrofluid concentration. For H 0 = 41 kA m −1 the maximum specific absorption rate was 367 W g −1 for D = 16 nm and x = 0.1. Absorption results are interpreted within the framework of the linear response theory for H 0 41 kA m −1 . A departure towards a saturation regime was observed for higher fields. Simulations based on a two-level description of nanoparticle magnetic moment relaxation qualitatively agree with these observations. The frequency factor of the susceptibility dissipative component, derived from experimental results, showed a sharp maximum at D ∼ 16 nm. This behaviour was satisfactorily described by simulations based on moment relaxation processes, which furthermore indicated a crossover from Néel to Brown mechanisms at D ∼ 18 nm. Hints for further improvement of magnetite particles as nanocalefactors for magnetic hyperthermia are discussed.

Radiolabeled albumin microspheres with encapsulated citric acid-coated magnetite nanoparticles were developed as a targeting approach to localize both radioactivity and magnetic energy at the tumor site. We present in vitro and in vivo studies of yttrium-90 ( 90 Y)-labeled human serum albumin magnetic microspheres (HSAMMS) as a multifunctional agent for possible applications in a bimodal radionuclide-hyperthermia cancer therapy. The HSAMMS were produced using a modified emulsification-heat stabilization technique and contained 11 nm magnetite nanoparticles coated with citric acid, distributed as inhomogeneous clusters within the albumin microspheres. The size, size distribution and the morphology of magnetite nanoparticles and HSAMMS were determined by FESEM, HRTEM and SEM/FIB dual beam. The average particle size of the complete HSAMMS was 20 mm, and they exhibited superparamagnetic behavior at room temperature. The in vitro experiments (in saline and human serum) revealed the high stability of the labeled HSAMMS in saline and human serum after 72 h. Following the intravenous administration of the 90 Y-HSAMMS in rats, 88.81% of the activity localizes in the lungs after 1 h, with 82.67% remaining after 72 h. These data on 90 Y-HSAMMS provide good evidence for their potential use in bimodal radionuclide-hyperthermia cancer therapy.