Ion-tracked membranes have been widely used in the fields of water treatment, medicine sieving, molecular detection and preparation of nanomaterials due to their features of uniformity of channel diameter distribution, high paralleled channels, good controllability of channel size and density. However, the ion-tracked membranes with smaller channel diameter (< 4 nm) are difficult to obtain with typical chemical etching method, which limits their further applications. 

In a recent study published in ACS Applied Materials & Interfaces, researchers at the Institute of Modern Physics (IMP) of Chinese Academy of Sciences (CAS) reported a convenient approach to prepare sub-nanoporous polycarbonate (PC) membranes. 

Through swift heavy ion irradiation accompanied by UV sensitization and pulsed-electrical etching, researchers successfully prepared the sub-nanoporous PC membrane with the mean channel diameter ranging from < 2.4 to 9.7 ？.  

It is proved that the sub-nanometer channel not only exhibits good cation selectivity, but also performs the voltage-activated non-linear ionic transport behavior, which is different from the Ohmoic ionic transport observed in the ion-tracked membrane with the typical chemical etching method. In the ion sieving experiment, the sub-nanoporous membrane possesses excellent ion separation property, for example, the separation ratio of Li⁺: Mg²⁺: La³⁺ can reach up to 3230: 85: 1. 

The new approach of preparing sub-nanoporous membrane is simple and controllable, and the prepared membrane can exhibit excellent ionic sieving property. This work not only provides a new insight into research on the membranes of confined mass transfer, but also will promote further applications of polymer membranes in the fields of resources extracting from ocean and salty lake.  

This work was financed by the National Nature Science Foundation of China, the National Key Research and Development Program of China and Outstanding Young Scientist Project, Key research Program of Frontier Sciences, CAS. 

(a) The schematic approach of sub-nanochannel preparation. (b) The voltage-activated I-V curves of sub-nanochannel. (c) The relationship between ion transport rate, selectivity and channel diameter.  (Image by IMP)