Antifouling PVDF Membrane by Surface Covalently Anchoring Functionalized Graphene Quantum Dots

To construct antifouling polyvinylidene fluoride (PVDF) membranes, l-aspartic acid (L-asp)-modified graphene quantum dots (AGQDs) were covalently anchored on the PVDF membrane surface via a three-step modification method. The pristine PVDF membrane was first dehydrofluorinated to generate internal double bonds under the alkali solution. Then diamine (EDA) and two types of hyperbranched polyethyleneimines (HPEI) were grafted on the alkali-treated surfaces through the Michael addition reaction. Finally, the as-synthesized AGQDs were chemically immobilized on the amine-grafted surfaces via an amidation reaction. The surface morphologies and surface properties of the pristine and modified PVDF membranes were comprehensively characterized by X-ray photoelectron spectroscopy, ATR-FTIR, scanning electron microscope, atomic force microscope, and dynamic antifouling experiments. Meanwhile, the grafting efficiency of AGQDs were found to be strongly dependent on the type of amine used. Compared with EDA and HPEIL, HPEIH with a high molecular weight preferred to be grafted on membrane surfaces rather than membrane pores because of its larger steric hindrance, which facilitate the covalent anchorage of AGQDs on the HPEIH-grafted surface. Thanks to the high grafting efficiency of AGQDs, the resulting PVDF-HPEIH-AGQDs membrane possessed the excellent hydrophilicity (water contact angle as low as 58.7°). Most importantly, this membrane demonstrated superior antifouling performance over the pristine PVDF membrane in the presence of either the positively charged foulant (e.g., lysozyme) or the negatively charged foulant (e.g., bovine serum albumin). This novel membrane fabrication approach developed in this study provides a promising solution to covalently anchor hydrophilic nanoparticles on the PVDF membrane surface for antifouling enhancement.