Quantifying lattice vibrational modes and optical conductivity in mixed magnetite–maghemite nanoparticles

Magnetite nanoparticles are an important nanomaterial with promising biomedical applications that depend crucially on their stoichiometry. Their tendency to oxidize and turn into isostructural maghemite means that discriminating between both oxides is a essential task that is difficult to achieve with conventional techniques. In this work, a novel methodology based on infrared spectroscopy is developed and tested with practically relevant magnetic nanoparticles. In contrast to Raman spectroscopy, which is prone to systematic errors and only offers a qualitative understanding of the vibrational properties, infrared spectroscopy is not only able to identify all the modes corresponding to magnetite and maghemite, but also capable of discriminating between two possible structural variants of maghemite. Additionally, the proposed approach also allows the acquisition of electrical conductivity, a property that is sensitive to the structure and stoichiometry of the particles. Together, vibrational mode modeling and conductivity quantification provide a detailed picture of the structure and properties of mixed iron oxide nanoparticles that is valuable towards an advanced characterization of magnetic nanomaterials in real biomedical applications.