Redoxable heteronanocrystals functioning magnetic relaxation switch for activatable T1 and T2 dual-mode magnetic resonance imaging.

T 1 /T 2 dual-mode magnetic resonance (MR) contrast agents (DMCAs) have gained much attention because of their ability to improve accuracy by providing two pieces of complementary information with one instrument. However, most of these agents are “always ON” systems that emit MR contrast regardless of their interaction with target cells or biomarkers, which may result in poor target-to-background ratios. Herein, we introduce a rationally designed magnetic relaxation switch (MGRS) for an activatable T 1 /T 2 dual MR imaging system. Redox-responsive heteronanocrystals, consisting of a superparamagnetic Fe 3 O 4 core and a paramagnetic Mn 3 O 4 shell, are synthesized through seed-mediated growth and subsequently surface-modified with polysorbate 80. The Mn 3 O 4 shell acts as both a protector of Fe 3 O 4 in aqueous environments to attenuate T 2 relaxation and as a redoxable switch that can be activated in intracellular reducing environments by glutathione. This simultaneously generates large amounts of magnetically decoupled Mn 2+ ions and allows Fe 3 O 4 to interact with the water protons. This smart nanoplatform shows an appropriate hydrodynamic size for the EPR effect (10–100 nm) and demonstrates biocompatibility. Efficient transitions of OFF/ON dual contrast effects are observed by in vitro imaging and MR relaxivity measurements. The ability to use these materials as DMCAs is demonstrated via effective passive tumor targeting for T 1 - and T 2 -weighted MR imaging in tumor-bearing mice. [ABSTRACT FROM AUTHOR]