Interplay between longitudinal and transverse contrasts in Fe3O4 nanoplates with (111) exposed surfaces.

Iron oxide has been developed as either T1 or T2 magnetic resonance imaging (MRI) contrast agents by controlling the size and composition; however, the underlying mechanism of T1 and T2 contrasts in one iron oxide entity is still not well understood. Herein, we report that freestanding superparamagnetic magnetite nanoplates with (111) exposed facets have significant but interactional T1 and T2 contrast effects. We demonstrate that the main contribution of the T1 contrast of magnetic nanoplates is the chemical exchange on the iron-rich Fe3O4(111) surfaces, whereas the T2 relaxation is dominated by the intrinsic superparamagnetism of the nanoplates with an enhanced perturbation effect. We are able to regulate the balance of T1 and T2 contrasts by controlling structure and surface features, including morphology, exposed facets, and surface coating. This study provides an insightful understanding on the T1 and T2 contrast mechanisms, which is urgently needed to allow more sophisticated design of high-performance MRI contrast agents.