Microwave Heating of Superparamagnetic Iron-Oxide Nanoparticles Toward Environmental Hyperthermia-Based Applications

This article reports the effect of spherical particle size (4–30 nm) on magnetic properties and microwave (MW) reactivity of superparamagnetic iron oxide nanoparticles (SPIONs) toward environmental hyperthermia-based applications. For this, silica-coated, single domain iron oxide nanoparticles (IONPs@silica) were precisely synthesized via thermal decomposition and subsequently coated by a reverse microemulsion. Transmission electron microscopy and X-ray diffraction confirmed the formation of spherical, monodisperse, single continuous layer silica-coated magnetite nanoparticles. Magnetic measurements revealed size-dependent superparamagnetism with negligible coercivity for particles smaller than 30 nm at 300 and 350 K. Saturation magnetization increased with particle size, reaching its highest value at 30 nm due to reduced surface spin disorder. MW reactivity was evaluated by irradiating IONPs@silica in quartz sand beds (1 wt %) at 2.45 GHz for 120 s. Core sizes of 4, 17, and 30 nm IONPs@silica at 1 wt % produced statistically significant temperature increases in sand compared to the control where 17 nm particles showed the highest heating response For single-domain particles (4–21 nm), heat generationwas attributed toNéel relaxation induced by the alternating magnetic field component and electronic excitation driven by the electric field component of the MW. For larger, multidomain particles (30 nm), magnetic heating dominated, primarily through hysteresis, eddy currents, and residual losses. Among the materials evaluated, 17 nm IONPs@silica were optimal with regard to both superparamagnetism and superior MW reactivity, with their single-domain magnetic structure being retained, and relaxation mechanisms were not compromised by thermal exposure for up to five MW cycles.