CO₂-Responsive Dual-Charge-Switchable Membranes for Protein Foulant Cleaning

Known CO₂-responsive membranes exhibit single-charge-switching (neutral-cationic), which limits their ability to regulate electrostatic interactions with oppositely charged foulants in membrane cleaning. Herein, CO₂-responsive poly(ether sulfone) (PES) ultrafiltration membranes with a dual charge-switchable surface are prepared by adding an amphoteric diblock copolymer additive to PES via the nonsolvent-induced phase separation (NIPS) method. By tuning the isoelectric point (IEP) of the copolymer additive to around 5, the membrane surface charge reversibly switches between positive and negative states under alternating CO₂ bubbling, lowering pH to ∼4 (below IEP), and N₂ bubbling, raising pH to ∼7 (above IEP). ATR-FTIR and water contact angle analyses indicate that increasing the copolymer content in the casting solution increases the amount of amphoteric chains on the membrane surface. Using bovine serum albumin (BSA) as a model foulant with a similar IEP, protein-fouled membranes are effectively cleaned with water when CO₂ and N₂ are alternately bubbled due to the constantly existing electrostatic repulsion between the membrane and protein. The gas-triggered cleaning efficiency is comparable to conventional acid/alkaline cleaning and remains stable over repeated fouling-cleaning cycles, underscoring the potential of this approach for environmentally friendly and chemical-free membrane cleaning.