Biocompatible Magnetic Fluids of Co-Doped Iron Oxide Nanoparticles with Tunable Magnetic Properties

Magnetite (Fe₃O₄) particles with a diameter around 10 nm have a very low coercivity (H_c) and relative remnant magnetization (M_r/M_s), which is unfavorable for magnetic fluid hyperthermia. In contrast, cobalt ferrite (CoFe₂O₄) particles of the same size have a very high H_c and M_r/M_s, which is magnetically too hard to obtain suitable specific heating power (SHP) in hyperthermia. For the optimization of the magnetic properties, the Fe²⁺ ions of magnetite were substituted by Co²⁺ step by step, which results in a Co doped iron oxide inverse spinel with an adjustable Fe²⁺ substitution degree in the full range of pure iron oxide up to pure cobalt ferrite. The obtained magnetic nanoparticles were characterized regarding their structural and magnetic properties as well as their cell toxicity. The pure iron oxide particles showed an average size of 8 nm, which increased up to 12 nm for the cobalt ferrite. For ferrofluids containing the prepared particles, only a limited dependence of H_c and M_r/M_s on the Co content in the particles was found, which confirms a stable dispersion of the particles within the ferrofluid. For dry particles, a strong correlation between the Co content and the resulting H_c and M_r/M_s was detected. For small substitution degrees, only a slight increase in H_c was found for the increasing Co content, whereas for a substitution of more than 10% of the Fe atoms by Co, a strong linear increase in H_c and M_r/M_s was obtained. Mössbauer spectroscopy revealed predominantly Fe³⁺ in all samples, while also verifying an ordered magnetic structure with a low to moderate surface spin canting. Relative spectral areas of Mössbauer subspectra indicated a mainly random distribution of Co²⁺ ions rather than the more pronounced octahedral site-preference of bulk CoFe₂O₄. Cell vitality studies confirmed no increased toxicity of the Co-doped iron oxide nanoparticles compared to the pure iron oxide ones. Magnetic heating performance was confirmed to be a function of coercivity as well. The here presented non-toxic magnetic nanoparticle system enables the tuning of the magnetic properties of the particles without a remarkable change in particles size. The found heating performance is suitable for magnetic hyperthermia application.