Influence of cobalt doping on the hyperthermic efficiency of magnetite nanoparticles

Magnetite nanoparticles (NPs) are extensively investigated for biomedical applications, particularly as contrast agents for Magnetic Resonance Imaging and as heat mediators in Magnetic Fluid Hyperthermia. For the latter, one of the goal of the research is to obtain materials with improved hyperthermic properties. A valuable strategy is the increase of the magnetic anisotropy of commonly employed magnetite through the total or partial substitution of Fe²⁺ ions with Co²⁺ ions. Here we present a study on a family of 8 nm Co-doped magnetite NPs (Co_xFe_3−xO₄), with composition ranging from pure magnetite (x=0) to stoichiometric cobalt ferrite (x=1), aimed to investigate the evolution of the hyperthermic properties with the increase of Co content. We found that the addition of a small amount of Co is enough to sharply increase the Specific Absorption Rate (SAR). The SAR further increases with x but it reaches a maximum for an intermediate value (x=0.6). Such anomalous behavior is ascribed to the intrinsic magnetic properties of the material, and, in particular, to the magnetic anisotropy, which displays the same peculiar trend. The Co-doping thus may represent an effective strategy to improve the poor hyperthermic efficiency of very small magnetite NPs (<10 nm). - Highlights: • A series of 8 nm non-stoichiometric cobalt ferrite nanoparticles was synthesized. • The Co:Fe molar ratio was varied systematically from 0 to 0.5. • The SAR was observed to have a maximum at intermediate Co content. • The hyperthermic results are explained on the basis of the magnetic anisotropy. • Co-doping is an effective strategy to improve the SAR of Fe₃O₄ NPs less than 10 nm