Non-Stoichiometric Cobalt Ferrite Nanoparticles by Green Hydrothermal Synthesis and their Potential for Hyperthermia Applications

Magnetic Co_xFe_3-xO₄ nanoparticles (NPs, 0.2 < x < 0.5) were prepared by a two-step synthesis of cigar-shaped akaganeite precursors and subsequently conversion through hydrothermal reaction with various molar ratios of Co²⁺/Fe²⁺/Fe³⁺ chloride salts. Both steps were successfully conducted in aqueous media without using toxic surfactants and solvents. To explore the effect of cobalt ion amounts, magnetic and hyperthermia measurements were investigated in dependency on the size, shape, and composition of the samples. Cubic and spherical Co_xFe_3-xO₄ NPs showed an increasing saturation magnetization of up to 68 Am²/kg with the cobalt ion amount and pressure of the synthesis. The temperature induced by hyperthermia rises up to 9.9 K within 10 min (10 kHz, 24.4 mT), ensuring medical applications such as medical imaging and cancer treatment. The temperature change was increased to 43 K/10 min (150.5 kHz, 19.0 mT) without exceeding the biological limit for nonselective heating. The specific absorption rate of up to 183 W/g_Co+Fe of the NPs with x = 0.38 is 30% higher than that of near-stoichiometric cobalt ferrite NPs with x = 0.80 synthesized by coprecipitation. The reduction of the cobalt content by up to 52% did not affect the hyperthermia values negatively and enables a greener synthesis method.