Optimal sizes of iron oxide nanoflowers for magnetic hyperthermia depend on the alternating magnetic field conditions

Iron oxide nanoflowers can be synthesized through a polyol route firstly introduced almost 2 decades ago by Caruntu et al, presenting multi-core morphology, several grains (around 10 nm) being attached together (and obviously sintered). These IONFs present outstanding properties for magnetic field hyperthermia, which is considered as promising therapy against cancer. Although they have a significantly smaller diameter, the specific adsorption rate (SAR) of IONFs can reach values of the order of 1 kWg-1, as large as for magnetosomes that are natural magnetic nanoparticles typically ~40 nm found in certain bacteria, which can be grown artificially but with lower yield compared to chemical synthesis. This work aims at better understanding the structure-property relationships between the internal nanostructure of IONFs as observed by HR-TEM and their properties, in particular magnetic ones. A library of mono and multicore IONFs is presented, with diameters ranging from 11 to 30 nm and narrow size dispersity. By relating their structural features (diameter, morphology, defects) to their magnetic properties investigated in particular by AC magnetometry over a wide range of alternating magnetic field (AMF) conditions, the SAR values of all synthesized batches vary with overall diameter and number of constituting cores in qualitative agreement with theoretical predictions by the Linear Response Theory (LRT) at low fields or with the Stoner-Wohlfarth (SW) model for larger amplitudes, and with numerical simulations reported previously, in particular by showing a pronounced maximum at an IONF diameter of 22 nm.