Size Control of Iron Oxide Nanoparticles Synthesized by Thermal Decomposition Methods

The controlled synthesis of superparamagnetic iron oxide nanoparticles is crucial for a variety of biomedical applications. Among different synthesis routes, thermal precursor decomposition methods are the most versatile, yielding monodisperse nanoparticles on the multigram scale. Recent in situ kinetic studies of the nucleation and growth processes during thermal decomposition routes revealed nonclassical nucleation and growth paths involving amorphous precursor phases and aggregative growth steps. With the knowledge of this kinetic mechanism, we systematically examined a range of different iron oxide heat-up synthesis routes to understand and conclude which methods allow good and reproducible size control over a range of relevant nanoparticle diameters. Using transmission electron microscopy (TEM) and small-angle X-ray scattering (SAXS) for the characterization of the nanoparticle size distribution, we find that a set of solvents (1-octadecene, trioctylamine, docosane) provides access to a temperature range between 300 and 370 °C, allowing us to synthesize monodisperse nanoparticles in a size range of 6–24 nm on a large scale. We confirm that a thermal pretreatment of the iron oxide precursor is essential to achieve reproducible size control. We find that each solvent provides access to a certain temperature range, within which the variation of temperature, heating rate, or precursor concentration allows us to reproducibly control the nanoparticle size.