Ultrathin Silica-Coated Iron Oxide Nanoparticles: Size-Property Correlation

Magnetic nanostructures (MNS) in their superparamagnetic form are promising materials systems for contrast enhancement in magnetic resonance imaging (MRI) and thermal-activation therapeutics via RF heating. These biomedical applications and associated properties are very much dependent on the structure, composition and size/shape of MNS. Although there are many speculations on the effect of various parameters on MNS, the size dependent properties are not well understood, largely due to challenges in size-selective large-scale synthesis for precise measurements and interference from typical large surface coating. Herein we report the size effect of iron oxide MNS on hydrodynamic radius, zeta potential, magnetic property and relaxivity. For this study, we have synthesized a wide range of monodispersed iron oxide MNS, from 7 nm to 20 nm nominal size and phase transferred to aqueous media with very thin layer of silica coating using Polyhedral Oligomeric Selesquioxanes (POSS). The dynamic light scattering (DLS) study and zeta potential measurement were carried out to determine the hydrodynamic radius and surface charges. The hydrodynamic radius follow the linear correlation but the zeta potential does not maintain it due to nonlinear mass/charge ratio relations with varying size. We have also conducted correlative studies of magnetic saturation (M_s) and (Formula presented.) relaxivities. This work will highlight the size dependent correlation and tailoring of MNS properties for desired biomedical applications.