Mechanistically unique 19F-EuII/III complex reports redox in vivo using both 1H- and 19F-MRI and displays temperature-dependent contrast enhancement.

Considerable research effort has focused on the in vivo use of responsive imaging probes that change imaging properties upon reacting with oxygen because hypoxia is relevant to diagnosing, treating, and monitoring diseases. One promising class of compounds for oxygen-responsive imaging is EuII-containing complexes because the EuII/III redox couple enables imaging with multiple modalities including magnetic resonance and photoacoustic imaging. The use of EuII requires care in handling to avoid unintended oxidation during synthesis and characterization. This review describes recent advances in the field of imaging agents based on discrete EuII-containing complexes with specific focus on the synthesis, characterization, and handling of aqueous EuII-containing complexes.

We report, for the first time, a multimodal, oxidation-responsive contrast agent for magnetic resonance imaging and photoacoustic imaging that uses the differences in the properties between Eu in the +2 and +3 oxidation states. The enhancement of contrast in T1-weighted magnetic resonance and photoacoustic imaging was observed in the +2 but not in the +3 oxidation state, and the complex is a known chemical exchange saturation transfer agent for magnetic resonance imaging in the +3 oxidation state.