A novel ion-selective POSS-based pore from amphiphilic diblock copolymers

Abstract

Biological ion-selective channels possess an unparalleled combination of high ion conductance and selectivity. Pores with comparable characteristics are highly required in industrial applications, e.g., for ion separation from brines. Yet, bio-channels' application is limited due to their instability, complexity in production, and low surface density. Artificial channels with a satisfactory trade-off between selectivity and conductance may represent an alternative. We propose taking advantage of the amphiphilic properties and high chemical variability of polyhedral oligomeric silsesquioxane (POSS), thus far used as a filler in nano-composite membranes. We designed and synthesized diblock copolymers where the hydrophobic block contained POSS cages decorated with isobutyl groups. Polymers containing four (SP5B3-5) or eight (Au22) POSS cages appeared capable of self-assembling into alkali ion-selective pores in planar lipid bilayers. Based on polymer structure and dose effect experiments, we propose that SP5B3-5 forms a tetrameric pore, with two monomers from each membrane leaflet, and Au22 forms an antiparallel dimeric pore. Both polymers prefer cations over anions. The unitary conductance decreases in the order Cs+ > K+ > Na+ of the Eisenman sequence I, indicating that the selectivity primarily stems from the penalty for ion dehydration. The permeabilities for Cs+, K+ and Na+ relate as 2.5 : 1.9 : 1 for SP5B3-5, and as 2.5 : 1.5 : 1 for Au22. The observation is compatible with a conductivity pathway where six oxygen atoms of the surrounding four POSS cages coordinate an ion within the pore. The artificial POSS-based pores pave the way for producing ion-selective membranes with high throughput.