Selective Permeability of Uranyl Peroxide Nanocages to Different Alkali Ions: Influences from Surface Pores and Hydration Shells

The precise guidance to different ions across the biological channels is essential for many biological processes. An artificial nanopore system will facilitate the study of the ion-transport mechanism through nanosized channels and offer new views for designing nanodevices. Herein we reveal that a 2.5 nm-sized, fullerene-shaped molecular cluster Li_48+mK₁₂(OH)_m[UO₂(O₂)(OH)]_60-(H₂O)_n (m≈20 and n≈310) (U₆₀) shows selective permeability to different alkali ions. The subnanometer pores on the water-ligand-rich surface of U₆₀ are able to block Rb⁺ and Cs⁺ ions from passing through, while allowing Na⁺ and K⁺ ions, which possess larger hydrated sizes, to enter the interior space of U₆₀. An interestingly high entropy gain during the binding process between U₆₀ and alkali ions suggests that the hydration shells of Na⁺/K⁺ and U₆₀ are damaged during the interaction. The ion selectivity of U₆₀ is greatly influenced by both the morphologies of the surface nanopores and the dynamics of the hydration shells. U₆₀ thing: U₆₀ clusters can selectively let Na⁺/K⁺ pass through the surface nanopores while blocking Rb⁺/Cs⁺ outside under the co-effect of hydration shell and surface pores. This ion-transport process is proved to be entropy controlled. The resulting U₆₀ surface charge density change will influence the size of their assemblies (see figure).