Xenon capsule: A smart 129Xe NMR‐based sensor of Zn2+ ions for magnetic resonance imaging (MRI) is proposed. The resonance frequency of xenon encapsulated in a cryptophane that bears a nitrilotriacetic ligand moiety varies when Zn2+ ions are present in solution (see picture). With hyper‐polarized gas, such a construct enables detection of 100 nM zinc in one xenon batch, a threshold 300 times lower than achieved with gadolinium chelates.

Cryptophanes, cage molecules for which xenon exhibits a high affinity, are good candidates for xenon biosensing. Cryptophane-1.1.1 (1) exhibits the highest binding constant for xenon encapsulation in organic solution determined to date. This property suggests that the cryptophane-1.1.1 core (1) is optimal for sensing applications. A high-yielding scalable synthesis of compound 1 is reported as well as an easy way to functionalize it.