Optimizing superparamagnetic ferrite nanoparticles: microwave-assisted <i>vs.</i> thermal decomposition synthesis methods

Superparamagnetic iron oxide nanoparticles are of crucial importance for various applications in medicine and biology as well as in materials science, where properties such as magnetism and inductive heating are advantageous. In this study, we systematically compare the synthesis methods for ferrite nanoparticles with those of pure iron oxide, focusing on their final properties. We synthesized superparamagnetic substituted ferrite nanoparticles with an average diameter of 5 to 8 nm with the general formula of M _<i>x</i> Fe_3-<i>x</i> O₄ (M = Fe²⁺, Mn²⁺, Co²⁺) using both conventional thermal decomposition (TD) method and microwave-assisted (MW) methods. Although the manganese-substituted particles obtained through both methods exhibited a narrow size distribution and high surface coverage with oleic acid, they demonstrated lower heating efficiency in an induction field compared to the cobalt-substituted particles. In particular, the replacement of Fe²⁺ ions with Co²⁺ ions significantly improved the self-heating ability and increased the specific absorption rate (SAR) from 22.7 for Fe₃O₄ to 106.3 W g_NP ^-1 for Co_0.88Fe_2.12O₄ nanoparticles. In addition, the concentration of 1,2-dodecanediol in the reaction mixture significantly influenced the shape and size distribution of the particles. Microwave-assisted synthesis resulted in higher incorporation of M²⁺ ions, as confirmed by ICP-MS and EDX spectroscopy, and more uniform particle sizes due to homogeneous nucleation. By optimizing the microwave method, we were able to produce small size superparamagnetic particles with high saturation magnetization (89.2 emu g^-1 at 300 K), capable of generating more heat in the magnetic field, making these particles suitable candidates for induction heating in materials.