Laccase-triggered one-step fabrication of positively charged phenolic acid-amine networks for nanofiltration

Molecular deposition provides a flexible and versatile platform for preparing positively charged composite nanofiltration membranes. The widely studied building units for molecular deposition are polyphenols such as catechins and tannins, which usually cause the complexity of the ultimate selective layer due to their relatively high antioxidant activity and limited positive potential originated from the large number of hydroxyl groups. In this work, a laccase-triggered one-step process has been proposed to prepare composite nanofiltration membranes. Highly positively charged phenolic acid-amine networks, which are represented by ferulic acid (FA) and polyethyleneimine (PEI), are constructed as the dense selective layer with a thickness of about 49 nm. This process is conducted in aqueous solutions under mild conditions, driven by electrostatic adsorption, catalytic oxidation, and covalent crosslinking processes. The structure of the selective layer can be finely tuned by the molecular weights of PEI, the chemical composition, and the pH values of the solutions. The prepared composite nanofiltration membranes can effectively reject MgCl₂ (94.5 %) and organic dyes such as alcian blue (AB) (>98.7 %) and methyl blue (MB) (>97.8 %). This work not only demonstrates a facile and versatile method for preparing positively charged composite nanofiltration membranes with great separation performance towards divalent cations, but also expands the membrane building blocks.