Crown-Ether Crystal Channel Membranes with Subnanometer Pores for Selective Na⁺ Transport

Emulating biological sodium ion channels to achieve high selectivity and rapid Na⁺ transport is important for water desalination, energy conversion, and separation processes. However, the development of artificial ion channels, especially multichannels, to achieve high ion selectivity, remains a challenge. In this work, we demonstrate the fabrication of ion channel membranes utilizing crown-ether crystals (DA18C6-nitrate crystals), which feature extremely consistent subnanometer pores. The polyethylene terephthalate (PET) membranes were initially subjected to amination, followed by the in situ growth of DA18C6-nitrate crystals to establish ordered multichannels aimed at facilitating selective Na⁺ conductance. These channels allow rapid Na⁺ transport while inhibiting the migration of other ions (K⁺ and Ca²⁺). The Na⁺ transport rate was 2.15 mol m^-2 h^-1, resulting in the Na⁺/K⁺ and Na⁺/Ca²⁺ selectivity ratios of 6.53 and 12.56, respectively. Due to the immobilization of the crown-ether ring, when the size of the transmembrane ion exceeded that of the crown-ether ring's cavity, the ions had to undergo a dehydration process to pass through the channel. This resulted in the ions encountering a higher energy barrier upon entering the channel, making it more difficult for them to permeate. However, the size of Na⁺ was compatible with the cavity of the crown-ether ring and was able to displace the hydrated layer effectively, facilitating selective Na⁺ translocation. In summary, this research offers a promising approach for the future development of functionalized ion channels and efficient membrane materials tailored for high-performance Na⁺ separation.