Colloidal maghemite nanoparticles with oxyhydroxide-like interface and chiroptical properties

Magnetic nanoparticles, mainly constituted of magnetite and maghemite, are widely used for environmental, biomedical and biotechnological applications. Herein, the synthesis and properties of maghemite nanoparticles characterized by chiroptical activity, representing an unprecedented feature for iron oxide nanoparticles, is described in detail. Among nanosized iron oxides, these maghemite nanoparticles stand out for the excellent colloidal stability in water and the witnessed ability to specifically bind selected molecules. The surface properties of nanoparticles were probed with AsIII and AsV oxyacids, studied by x-ray photoelectron spectroscopy and correlated to circular dichroism. The dichroic signal of nanoparticles was differently influenced by the coordination of ligands. Crystalline vacancies on the nanomaterial surface were identified as the chiral centers responsible of the dichroic behavior and the selectivity toward ligands. The latter was correlated with the ability of restructuring the nanomaterial at the crystal truncation. Furthermore, surface binding sites emerged for bearing labile coordination water in analogy with iron oxyhydroxides. The present report, besides enriching the colloidal chemistry of iron oxide based nanomaterials, can stimulate further research on inorganic systems expressing intrinsic chiral properties.