Oxidation of iron in iron/gold core/shell nanoparticles

Abstract

Recently, reverse micelles were used to produce core/shell nanoparticles of iron and gold of highly uniform size distribution and reported to contain metallic iron. A composite structure of this sort is attractive in biosensor and bioactuator applications; the metallic iron core provides high magnetic sensitivity and the gold coating is easily functionalized. With iron/gold nanoparticles prepared by this method, we directly measure the chemical state of the iron component using x-ray absorption spectroscopy (XAS) and interpret our results along with the measured magnetization of the particles. In earlier reports, metallicity of the iron core was inferred from magnetization measurements rather than directly measured. In contrast, XAS directly measures the structural environment of the iron atoms. Because XAS interpretation is independent of symmetry or periodicity, it is an ideal tool for characterizing nanoparticles. Using XAS, we demonstrate that the iron component of the iron/gold nanoparticles is fully oxidized. We suggest that oxidation is a persistent problem in core/shell nanoparticles of this variety and recommend that characterization by XAS be a standard part of nanoparticle preparation.