Outstanding Heat Loss via Nano-Octahedra above 20 nm in Size: From Wustite-rich Nanoparticles to Magnetite Single-Crystals
Most studies of magnetic nanoparticle-based hyperthermia utilize iron oxide nanoparticles smaller than 20 nm, which are intended to have a superparamagnetic behavior (SP-MNPs). However, the heating power of larger magnetic nanoparticles with non-fluctuating or fixed magnetic dipoles (F-MNPs) can be significantly greater than the one of SP-MNPs if high enough fields (H>15mT) are used. But the synthesis of larger single nanocrystals of magnetite (Fe3O4) with regular shape, narrow size distribution and devoid of secondary phases remains a challenge. Iron oxide nanoparticles, grown over 25 nm, often present large shape and size polydispersities, twinning defects and a significant fraction of wüstite-type (FeO) paramagnetic phase, resulting in degraded magnetic properties. Herein, we introduce an improved procedure to synthesize monodisperse F-MNPs in a range of 25 to 50 nm with distinct octahedral morphology and very crystalline magnetite phase. We unravel the subtle phase transformation that takes place during the synthesis by a thorough study in several non-optimized nanoparticles presenting core-shell structure or composed of magnetite-type clusters embedded in a wüstite lattice. Optimized magnetite samples present a slight decrease in the saturation magnetization compare to bulk magnetite, which is successfully explained by the presence of Fe+2 vacancies. AC magnetometry measurements have shown one of the largest specific absorption rates reported to date for magnetite NPs (>1000 W/g Fe3O4) at moderate magnetic fields (<40 mT) and at hundreds of kilohertz (300 kHz). Furthermore, the magnetic properties and hyperthermia performance of polymer-coated F-MNPs remain unaltered after modifying the ionic strength and the viscosity of the medium. Using a physical model based on single magnetic domain approaches, we derive a novel connection between the octahedral shape and the high hyperthermia performance.