An MRI contrast composition includes a liposome and a europium metal complex disposed within the liposome. The europium metal complex includes a europium metal ion and a multi-dentate ligand selected from the group consisting of cryptands and thiacryptands and one or more counter-ions that balances a charge of the europium metal ion and the multi-dentate ligand, the europium metal ion being switchable between a 2+ and 3+ oxidation state. The contrast composition advantageously provides an oxidation-responsive dual-mode contrast agent because it would enhance either T1-weighted images or CEST images depending on the oxidation state of Eu.
