Zwitterionic-coated ultrasmall iron oxide nanoparticles for magnetic resonance imaging

Ultrasmall superparamagnetic iron oxide nanoparticles (USPIONS) have been applied in vitro and in vivo as contrast agents to improve the sensitivity of magnetic resonance imaging (MRI). Most of the clinically approved iron-containing particles are used as MRI contrast agents for the liver. This kind of particles is accumulated in the liver as a result of opsonization and scavenging by the mononuclear phagocyte system. Currently, novel coating strategies are applied to evade phagocytosis and to pave the way to evolve contrast agents with fewer off-target effects. We have developed a novel nanoparticle platform by introducing a zwitterionic polymer layer onto the magnetite particle surface (ZW-USPIONS). Firstly, magnetite (Fe3O4) crystals of about 5 nm were synthesized via thermal decomposition of iron oleate in presence of oleyl alcohol. Subsequently, the hydrophobic magnetite nanoparticles were stabilized using a zwitterionic polymeric layer to render them water-soluble and to provide an extraordinary stability over a broad pH range and different ionic strength. Purification of the polymer-coated nanoparticles via ultracentrifugation becomes a critical step for getting rid of the unbound polymer and to produce monodisperse samples with a defined hydrodynamic diameter by intensity of ca.15 nm measured by dynamic light scattering and to give a nearly neutral zeta potential in a pH range of 6.8 to 9. Thorough characterization of the ZW-USPIONS has been performed. X-ray diffraction studies show the presence of magnetite (Fe3O4). The dependence of magnetisation on temperature and magnetic field in static fields up to 7 Tesla was determined by using a commercial SQUID magnetometer. High-resolution transmission electron microscopy (HR-TEM) confirms well-defined narrow-sized particles. The presence of a tiny polymer layer around the crystals was also studied by elemental analysis, Raman spectroscopy, Fourier transform infrared spectroscopy (FT-IR) and thermogravimetric analysis (TGA). The in vitro cytotoxicity of the ZW-USPIONS was evaluated by 3-[4,5-dimethylthialzol-2-yl]-2,5-diphenyltetrazolium bromide (MTT) assay. Toxic response was not observed with nanoparticle concentrations up to 100 µg/mL in a range of human cell lines. SDS-PAGE and Gel electrophoresis studies were performed to analyse the composition of the nanoparticles-protein complex formed upon incubation with human serum. The zwitterionic-coated nanoparticles have shown anti-fouling properties and a significant decrease of non-specific bounded proteins compared to negatively and positively charged coatings. These properties render the functionalized ZW-USPIONS suitable for being used as MRI-agents.