These iron core–gold shell nanoparticles (Fe@Au) have been proposed for use as MRI contrast agents,13 CT contrast agents,14 molecular diagnostic tools,15 magnetic guidable drug-carriers,14 and thermotherapeutic probes.16 Intriguingly, our previous research demonstrated that Fe@Au showed cancer-preferential cytotoxicity toward oral cancer in vitro and in vivo, which was caused by the nonoxidized iron in the nanoparticles.