Cation-Dependent Hierarchical Assembly of U60 Nanoclusters into Macro-Ion Assemblies Imaged via Cryogenic Transmission Electron Microscopy

Self-assembly of ([UO₂(O₂)OH]₆₀)^60- (U60), an actinide polyoxometalate with fullerene topology, can be induced by the addition of mono- and divalent cations to aqueous U60 solutions. Dynamic light scattering and small-angle X-ray scattering lend important insights into assembly in this system, but direct imaging of U60 and its assemblies via transmission electron microscopy (TEM) has remained an elusive goal. In this work, we used cryogenic TEM to image U60 and secondary and tertiary assemblies of U60 to characterize the size, morphology, and rate of formation of the secondary and tertiary structures. The kinetics and final morphologies of the secondary and tertiary structures strongly depend on the cation employed, with monovalent cations (Na⁺ and K⁺) leading to the highest rates and largest secondary and tertiary structures.