Surfactant-Controlled Size and Shape Evolution of Magnetic Nanoparticles

This manuscript describes a simple one-pot reaction that affords cuboid iron−manganese oxide nanoparticles with unprecedented dimensions as large as 33 ± 5 nm (average body-centered diagonal) in monodisperse form. Our unique synthetic method, which requires no multiple growth steps typical of other methods, utilizes the thermal decomposition of metal precursor complexes in the presence of specifically tailored surfactants and/or mixtures of surfactants. The size and shape of these unusually large magnetic nanoparticles (MNPs) can be manipulated at will simply by adjusting the surfactant composition, leading to enhanced control over the dimensions of the nanoparticles because of the surface-differentiating influence of the functional end groups. Our surfactant system utilizes two types of surface ligands: one strongly bound to the metal surface and the others loosely bound. This combination of ligands plays an important role in controlling particle size and morphology. With an eye toward potential biomedical applications, the magnetic properties of the MNPs were investigated through their M−H hysteresis loop behavior at 290 K. More importantly, when exposed to a magnetic field, relaxation measurements of these MNPs afforded Néel relaxation times of 3.4 s at an average body-centered diagonal size of 29 ± 4 nm.