Interplay between Longitudinal and Transverse Contrasts in Fe_3O_4 Nanoplates with (111) Exposed Surfaces

摘要

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 Fe_3O_4(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.