A EuII‐Containing Cryptate as a Redox Sensor in Magnetic Resonance Imaging of Living Tissue

Abstract The Eu II ion rivals Gd III in its ability to enhance contrast in magnetic resonance imaging. However, all reported Eu II ‐based complexes have been studied in vitro largely because the tendency of Eu II to oxidize to Eu III has been viewed as a major obstacle to in vivo imaging. Herein, we present solid‐ and solution‐phase characterization of a Eu II ‐containing cryptate and the first in vivo use of Eu II to provide contrast enhancement. The results indicate that between one and two water molecules are coordinated to the Eu II core upon dissolution. We also demonstrate that Eu II ‐based contrast enhancement can be observed for hours in a mouse.