Investigation into the extraction speciation of rare-earth radioisotopes from aqueous solution using polyoxometalates

Lanthanides are attractive elements for many applications due to their unique magnetic and spectroscopic properties and are particularly appealing as luminescence and magnetic resonance imaging agents. The radiolanthanides possess a range of β− energies, penetration depths and half-lives that can be matched to tumor size, tumor density and the residence time of peptides and antibodies. This allows for tailor-made radiotherapeutic therapy depending on the patient. Lutetium-177 (177Lu) is one such isotope. ‘Carrier-free’ productions, resulting in >99.9% of radioactive atoms, via the enrichment of ytterbium-176 (176Yb) requires that the 177Lu be separated from the Yb parent. While separation strategies do exist, they are often time consuming and require large, cumbersome solvent volumes. In this study we investigate the use of the Wells–Dawson lacunary [α2-P2W17O62]10− polyoxometalate (α2) to bind and sequester lanthanide ions. Specifically we explore the speciation of the radiolanthanide–α2 complexes in order to tailor the separation strategies to separate neighboring lanthanides through extraction into organic solvents. Using species-specific analytical techniques (31P NMR, luminescence spectroscopy and reverse phase radio-TLC) under extraction conditions, we find that solution speciation is controlled by pH and stoichiometry of Ln:α2. Specifically at a pH of 0–1, the 1:1 Ln:α2 complexes are formed exclusively with 177Lu and 153Sm.