Interplay between Longitudinal and Transverse Contrasts in Fe3O4 Nanoplates with (111) Exposed Surfaces

Iron oxide has been developed as either T-1 or T-2 magnetic resonance imaging (MRI) contrast agents by controlling the size and composition; however, the underlying mechanism of T-1 and T-2 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 T-1 and T-2 contrast effects. We demonstrate that the main contribution of the T-1 contrast of magnetic nanoplates is the chemical exchange on the iron-rich Fe3O4(111) surfaces, whereas the T-2 relaxation is dominated by the intrinsic superparamagnetism of the nanoplates with an enhanced perturbation effect We are able to regulate the balance of T-1 and T-2 contrasts by controlling structure and surface features, including morphology, exposed facets, and surface coating. This study provides an insightful understanding on the T-1 and T-2 contrast mechanisms, which is urgently needed to allow more sophisticated design of high-performance MRI contrast agents.