The environmental aging of functional iron oxide nanomaterials, especially those used in biomedical applications, may have significant consequences for both their mechanism of action and their safety profile. However, to date, there has been no way to systematically measure this property, and the problem has been largely overlooked. We show here that a recently proposed “center of gravity” method for determining the composition of magnetite (Fe3O4) and maghemite (γ-Fe2O3) mixtures via 57Fe Mössbauer spectroscopy can be applied to the matter. We show that a long-established diffusion model can be used to characterize the oxidative aging process, yielding a parametric (and therefore predictive) description of the magnetite-to-maghemite oxidation processes occurring within the nanoparticles.
Doublet lines should be of equal area; in sextets the area ratios of the outermost:middle:innermost pairs should be 3:2:1.
Maghemite is less dense than magnetite but has the same underlying structure; hence the “concentration gradient” here can be thought of as the concentration of iron ions in the oxygen matrix, which is higher in the more magnetite-like particle core than in the more maghemite-like surface shell.
The summation was performed with n = 50 terms.