Targeted ion sieving via single-chain hydrogels with sub-nanometer meshes

Hydrogels containing 3D nano-meshes formed by polymer chains provide pathways for mass transport. The typical hydrogel mesh sizes range from 5 to 100 nanometers, one or more orders of magnitude higher than ionic diameters, making them ineffective for precise ion sieving. Achieving sub-nanometer meshes in an upscaled hydrogel membrane remains a major challenge. Herein, we demonstrate single-chain hydrogels (SCHs) with sub-nanometer meshes, fabricated via a solid-phase synthesis process, which function as precise ion sieves. The SCHs are prepared through intrachain crosslinking of polyethyleneimine (h-PEI), which is precisely pre-grafted at the entrance of the nanopores in an isolated feature. The sub-nanoscale confinement in SCHs significantly influences three coexisting mesh-ion interactions: ionic dehydration, electrostatic repulsion, and coordination affinity, leading to varied energy barriers for ion transport. As a result, SCHs exhibit graded cation transport, serving as a versatile platform for targeted ion sieving with both exceptional ion selectivity and high target ion flux. In key applications, including seawater desalination, lithium extraction, and heavy metal ion removal, our SCHs demonstrate state-of-the-art performance. The SCH concept provides a promising template for future applications in fields such as energy, environmental science, and bionics.