Tuning the Enzymatic Activity of Aqueous Phase Separation-Based PEI-PSS Membranes

Aqueous phase separation (APS), a highly sustainable method of membrane production, has already been successfully applied to prepare biocatalytic membranes. In this work, membranes were prepared by APS through the complexation of polyethyleneimine (PEI) and poly(sodium 4-styrene sulfonate) (PSS). Lysozyme and laccase were successfully introduced to give enzymatic function to the membranes at room temperature. Changing the PEI:PSS mixing ratio affected the membrane structure and thereby also the resulting enzymatic activity. The optimum PEI:PSS mixing ratio, in terms of enzymatic activity and mechanical properties, was found to be 1.9:1, and this ratio was subsequently used to study the influence of enzyme concentration. PEI-PSS membranes showed a linear increase in activity with enzyme concentration, with an optimum activity of 3.0 ± 0.2 U·cm⁻² for a casting solution with 0.25 wt.% lysozyme. Above this concentration, activity decreased, likely due to enzyme aggregation. For laccase functionalized membranes, it was demonstrated to be possible to oxidize syringaldazine during active filtration, while a higher enzyme loading was found compared to lysozyme. This work demonstrates the possibility of tuning the enzymatic activities of APS-produced membranes by changing the polyelectrolyte mixing ratio and enzyme concentration. Moreover, the incorporation of laccase demonstrates the versatility of the APS approach and its potential to degrade organic pollutants in wastewater treatment.