From Core-Shell FeO/Fe₃O₄ to Magnetite Nanocubes: Enhancing Magnetic Hyperthermia and Imaging Performance by Thermal Annealing

Wereport the synthesis of mostly magnetite nanocubes (NCs) inthe size range of 16-23 nm starting from FeO/Fe3O4 core-shell NCs and employing a thermal decompositionmethod, followed by thermal treatment. To induce the phase transformation,the as-prepared NCs were thermally annealed either directly in anorganic solvent before their transfer into water or by a thermal processin an aqueous solution after coating the NCs with an amphiphilic polymershell to render them soluble in water. The effects of NCs'size and type of post-synthesis annealing on improving their magneticproperties were investigated. The specific absorption rate (SAR) values,which measure the magnetic hyperthermia heating efficiency, were compareddirectly in water or in a viscous medium of glycerol/water mixturesto mimic the intratumoral dense environment. Among all samples, the20 nm NCs annealed in organic solvent show the best heating capabilities.The 24 nm NCs exposed to the water annealing process are also ableto achieve very high SAR values. Remarkably, all NC samples subjectedto annealing exhibit heating efficiency values which are negligiblyaffected by the viscous media, thus demonstrating their unique SARperformance in a viscous-independent manner. For 20 nm NCs, a threefoldhigher magnetic particle imaging signal compared to the commercialtracer VivoTrax, and, at the same time, good magnetic resonance imagingcontrast were measured.