Tunable Ion Selectivity in Self-Standing Protein-Based Membranes via Strategic Chemical Functionalization

Protein-based biomembranes provide a sustainable and tunable platform for selective ion transport due to their structural versatility. Herein, self-standing bovine serum albumin (BSA) membranes were chemically functionalized via sulfonation and phosphorylation to enhance ion selectivity. Structural and spectroscopic analyses confirmed distinct modes of functionalization and associated morphological modifications, while electrochemical and radiotracer-based studies evaluated ion transport performance. Phosphorylation significantly increased water uptake (15-30%) and ion exchange capacity (∼30-fold), resulting in a highly hydrated and interconnected transport network with elevated ionic conductivity. Conversely, sulfonation reduced water uptake (15-30%) and moderately increased ion exchange capacity (∼8-fold), producing a compact structure with restricted ion mobility. Both functionalized membranes demonstrated superior monovalent ion selectivity compared to relevant cation ion-exchange membranes. Phosphorylated BSA exhibited a strong affinity for trivalent ions through phosphate-mediated interactions, yielding a high Cs⁺/Eu³⁺ separation factor, while sulfonated BSA showed enhanced discrimination of Cs⁺ over Ba²⁺, reflecting the influence of the introduced functional groups. These findings highlight the potential of chemically tailored BSA membranes for achieving tunable, highly selective ion transport, offering a sustainable route for advanced ion separation and water purification technologies.