Biocatalytic PEI–PSS membranes through aqueous phase separation: influence of casting solution pH and operational temperature

Abstract

Biocatalytic membranes combine the separation properties of membranes and the catalytic abilities of enzymes, holding great promise for industries where both purification and conversion are required. In this work, polyelectrolyte complex membranes incorporated with lysozyme were prepared using polyethyleneimine (PEI) and poly(sodium 4-styrenesulfonate) (PSS) through a one-step and mild pH shift aqueous phase separation (APS) approach. The effects of lysozyme addition and casting solution pH on the membrane properties were studied. All the membranes, both with and without added lysozyme, exhibited asymmetric structures with relatively dense top surfaces and porous cross-sections with finger-like macrovoids. The incorporation of lysozyme did not significantly influence the structure and permeability of the formed membranes. The PEI–PSS biocatalytic membranes exhibited temperature dependent enzymatic activity. The activity strongly increased with increased operational temperature, with the highest activity of 4.30 ± 0.15 U cm⁻² at 45 °C. This indicates a responsive effect, where a higher temperature leads to some swelling of the polyelectrolyte complex membrane, making the enzyme more accessible to the used substrate. Moreover, the biocatalytic membranes demonstrate desirable enzymatic stability, maintaining 60% activity even after 60 days of storage. This study validates the potential of the water-based APS process as a straightforward approach for integrating enzymes into responsive biocatalytic membranes.