Effect of Ambient Oxidation on Chemical Composition and Structural Properties of Iron Nanoparticles for Hyperthermia and Medical Imaging

With magnetization saturation roughly twice that of iron oxide nanoparticles, metallic iron nanoparticles (also termed zero-valent iron nanoparticles) have desirable properties for use as a magnetic resonance imagining (MRI) contrast agent as well as a medium for hyperthermia treatment of cancer. Metallic iron nanoparticles, however, are difficult to synthesize and maintain due to their high degree of reactivity and proclivity for oxidation. The main goal of this study was to investigate how ambient oxidation affects the chemical composition and structural properties of metallic iron nanoparticles initially synthesized through a facile reduction reaction of iron (III) chloride with sodium borohydride. A metallic iron nanoparticle with tunable oxidation would combine the biocompatibility of iron oxide with the magnetic strength of metallic iron.

Metallic iron nanoparticles were examined via transmission electron microscopy (TEM), scanning electron microscopy (SEM), dynamic light scattering (DLS), powder X-ray diffraction (XRD), and Mössbauer spectroscopy to determine their morphology and structure. Relaxometry experiments were conducted to investigate the potential of as-made metallic iron nanoparticles as an MRI contrast agent. Imaging data revealed nanoparticles in the range of 10-80 nm that are arranged as either spheroids or sintered aggregates. X-ray diffraction confirmed the presence of metallic iron, while Mössbauer measurements revealed core-shell nanoparticles containing a metallic iron core covered by amorphous iron and iron oxides. Oxide percentage increased as nanoparticles were left to age under ambient conditions. Oxidation rate slowed once an adequate passivation layer was formed. Polyethylene glycol (PEG) coating of nanoparticles retarded oxidation rate, thereby preserving the metallic iron content and desirable magnetic properties of the nanoparticles.