Multifunctional Anisotropic Rod-Shaped CoFe₂O₄ Nanoparticles for Magnetic Resonance Imaging and Magnetomechanical Therapy

One-dimensional magnetic nanomaterials, such as nanorods, are one of the most interesting but unexplored objects due to the complexity of their manufacture. In this work, we report on the template-assisted preparation of rod-shaped cobalt ferrite (CoFe 2 O 4 ) nanoparticles (NPs) using the akaganeite (β-FeOOH) matrix and their application as multifunctional agents in biomedicine. In the beginning, rod-shaped β-FeOOH NPs are prepared by a forced hydrolysis technique. Then, amorphous Co(OH) 2 is deposited onto the surface of β-FeOOH. Finally, a solid-state reaction results in mesoporous rod-shaped (130 nm × 40 nm) CoFe 2 O 4 NPs with a crystallite size of 7 nm. The proposed mechanism of synthesis involves the simultaneous transformation of β-FeOOH and amorphous Co(OH) 2 in two corresponding oxides during annealing, resulting in cation interdiffusion with the formation of a CoFe 2 O 4 phase, while the key role of the Co(OH) 2 shell in preserving the rod-shaped morphology of CoFe 2 O 4 NPs is demonstrated. NPs exhibit a high coercivity of 73 kA m –1 arising from their shape anisotropy, while their effective anisotropy constant of 1.1 × 10 5 J m –3 is compared to bulk CoFe 2 O 4 . Despite the low value of saturation magnetization (13 A m 2 kg –1 ), NPs demonstrate dual-modal relaxivity in MRI experiments with high values of r 1 = 2.3 s –1 mM –1 and r 2 = 228 s –1 mM –1 . Finally, we show that the magnetomechanical stimulus from NPs leads to the programmed death of breast cancer cells. Under the action of a low-frequency magnetic field (31 Hz), such NPs mediate mechanical vibrations, leading to cancer cell destruction even at a very low concentration of 6 μg CoFe 2 O 4 ·mL –1 .