Iron oxide nanoparticles are widely used for biological applications thanks to their outstanding balance between magnetic properties, surface-to-volume ratio suitable for efficient functionalization and proven biocompatibility. Their development for MRI or magnetic particle hyperthermia concentrates much of the attention as these nanomaterials are already used within the health system as contrast agents and heating mediators. As such, the constant improvement and development for better and more reliable materials is of key importance. On this basis, this review aims to cover the rational design of iron oxide nanoparticles to be used as MRI contrast agents or heating mediators in magnetic hyperthermia, and reviews the state of the art of their use as nanomedicine tools.

Because of the broad range of application of iron oxide nanoparticles (NPs), the control of their size and shape on demand remains a great challenge, as these parameters are of upmost importance to provide NPs with magnetic properties tailored to the targeted application. One promising synthesis process to tune their size and shape is the thermal decomposition one, for which a lot of parameters were investigated. But two crucial issues were scarcely addressed: the precursor's nature and water content. Two in house iron stearates with two or three stearate chains were synthesized, dehydrated, and then tested in standard synthesis conditions of spherical and cubic NPs. Investigations combined with modeling showed that the precursor's nature and hydration rate strongly affect the thermal decomposition kinetics and yields, which, in turn, influence the NP size. The cubic shape depends on the decomposition kinetics but also crucially on the water content. A microscopic insight was provided by first-principles simulation showing an iron reduction along the reaction pathway and a participation of water molecules to the building unit formation.

Exchange coupled core–shell nanoparticles present high potential to tune adequately the magnetic properties for specific applications such as nanomedicine or spintronics.

Iron oxide nanoparticles are widely used as a contrast agent in magnetic resonance imaging (MRI), and may be used as therapeutic agent for magnetic hyperthermia if they display in particular high magnetic anisotropy. Considering the effect of nanoparticles shape on anisotropy, a reproducible shape control of nanoparticles is a current synthesis challenge. By investigating reaction parameters, such as the iron precursor structure, its water content, but also the amount of the surfactant (sodium oleate) reported to control the shape, iron oxide nanoparticles with different shape and composition were obtained, in particular, iron oxide nanoplates. The effect of the surfactant coming from precursor was taking into account by using in house iron stearates bearing either two or three stearate chains and the negative effect of water on shape was confirmed by considering these precursors after their dehydration. Iron stearates with three chains in presence of a ratio sodium oleate/oleic acid 1:1 led mainly to nanocubes presenting a core-shell Fe1−xO@Fe3−xO4 composition. Nanocubes with straight faces were only obtained with dehydrated precursors. Meanwhile, iron stearates with two chains led preferentially to the formation of nanoplates with a ratio sodium oleate/oleic acid 4:1. The rarely reported flat shape of the plates was confirmed with 3D transmission electronic microscopy (TEM) tomography. The investigation of the synthesis mechanisms confirmed the major role of chelating ligand and of the heating rate to drive the cubic shape of nanoparticles and showed that the nanoplate formation would depend mainly on the nucleation step and possibly on the presence of a given ratio of oleic acid and chelating ligand (oleate and/or stearate).

The composition of metal oxide nanoparticles is of great importance for their applications because defects and/or deviation from stoechiometry strongly affect their physical properties. We report here on the crucial role of synthesis parameters such as solvent, ligand and iron precursors on the composition of spinel iron oxide nanoparticles synthesized by the thermal decomposition method. At first, the investigation of the thermal decomposition of iron stearates bearing either two or three stearate chains by thermogravimetric analysis, IR spectroscopy and Mössbauer spectrometry as a function of temperature and syntheses with only oleic acid and iron stearate confirmed that the composition of the first nuclei is wüstite Fe 1-x O. The synthesis of nanoparticles with high sizes requires the use of very high boiling point solvents to ensure an effective growth step. We observed that, when the grain growth and oxidation kinetics are similar, nanoparticles with a spinel composition and quite no defects are produced. An oxidation rate slower than the nuclei growth rate favours the formation of core-shell Fe 1-x O@Fe 3-x O 4 NPs. The oxidation kinetics is shown to be influenced by surfactant and solvent natures.Indeed, surfactants such as oleic acid form a dense monolayer at the nuclei surface which oxidation kinetics will depend on this monolayer permeability. Temperature, solvents with high surfactant affinity, deprotonated surfactants or decomposition products of solvents affect the monolayer stability and thus the nanoparticle composition. The solvents' nature and solvent mediated ligand-ligand interactions are thus evidenced to be important parameters to control the formation of defectsfree and stoechiometric oxide nanoparticles.

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Iron oxide nanoparticles (IONPs) are well-known contrast agents for MRI for a wide range of sizes and shapes. Their use as theranostic agents requires a better understanding of their magnetic...