Real-Space Tracking of Redox Cluster Transport within Nanochannels

Real-space tracking of molecular transport in confined environments is crucial to understanding nanofluidics. Herein, we elucidated the diffusion mechanism of atomically precise polyoxometalates (POMs) within single-walled carbon nanotubes (SWCNTs) by leveraging their intrinsic structural and electronic tunability, serving as an ideal model for molecular transport. Aqueous-phase assembly enables the dense packing of both soluble and insoluble POMs into SWCNTs. Electron microscopy and spectroscopy characterizations revealed distinct cation-dependent POM transport behaviors, where protons permeated SWCNTs while Na⁺/K⁺/Cs⁺/NH₄⁺ cations were removed, dictated by the redox strength of POM-SWCNT interactions. Time-dependent thermogravimetric analysis showed that Coulombic forces from POM-SWCNT redox reactions drove rapid cluster diffusion into nanotubes, with filling efficiency correlating with the POM oxidation potential and solvent polarity. A selective sieving strategy revealed the POM anion charge density, governed by diffusion coefficients, as the key selection criterion. These findings will facilitate the efficient and controlled assembly of diverse substances (e.g., ionic crystals) within confined cavities.