Ultrathin SiO₂-coated magnetic iron oxide nanoparticle clusters for magnetic hyperthermia: effect of silica layer thickness on magnetothermal performance

Magnetic nanoparticles (MNPs) for magnetically induced hyperthermia have steadily increased because they can convert alternating magnetic field (AMF) energy into localized heat. Among numerous candidates, silica-coated magnetic iron oxide nanoparticles (MIONs/SiO₂) are particularly attractive owing to their improved colloidal stability and potential biocompatibility. However, the influence of ultrathin SiO₂ layers on the magnetothermal behavior of cluster-like MION assemblies remains insufficiently understood. Here, SiO₂ layers with thicknesses of approximately 2, 4, and 6 nm were deposited onto MIONs using a sol–gel method to produce SiO₂-coated MION clusters, designated MIONs/SiO₂-1, MIONs/SiO₂-2, and MIONs/SiO₂-3, respectively. The heating performance of these SiO₂-coated MION clusters was evaluated in aqueous dispersions at different Fe concentrations under an AMF of 100 kHz and 140 Oe. Among the prepared samples, MIONs/SiO₂-1 showed the most favorable Fe-normalized heating performance within the tested sample series at an Fe concentration of 6 mg mL⁻ 1 , achieving a specific absorption rate (SAR) of 80.11 ± 3.51 W g⁻ 1 and an intrinsic loss power (ILP) of 6.454 ± 0.283 nH·m 2 ·kg⁻ 1 . These values were higher than those of uncoated MIONs and the other silica-coated samples. This study indicates that an ultrathin SiO₂ layer of approximately 2 nm was associated with a favorable balance between colloidal stabilization and magnetic heating efficiency within the tested sample series, providing useful insight into silica layer design for SiO₂-coated MION clusters in magnetic hyperthermia studies.