Gold-coated cementite nanoparticles: An oxidation-resistant alternative to α-Iron

Iron-based nanoparticles are desirable for many applications because of their magnetic properties and inherent biocompatibility. Metallic iron, or α-Fe, is the most sought after because of its high saturation magnetization (up to 220 emu/g). This magnetization in iron nanoparticles is difficult to reach or maintain because of the ease of oxidation, which greatly reduces the magnetization values (90 emu/g or less). Here, we report the synthesis of an iron-based nanoparticle comprising a magnetic cementite core (Fe₃C) that is more oxidation-resistant than α-Fe, an oxide layer, and a gold coating for passivation and easy functionalization. The nanoparticle structure was confirmed via X-ray absorption fine structure and Mössbauer experiments, and morphology was confirmed using transmission electron microscopy. Magnetic characterization yielded a saturation magnetization of 110 emu/g, thus demonstrating cementite as more stable alternative to α-Fe with higher magnetic moments than the iron oxides.