Stable Graphene-Based Membrane with pH-Responsive Gates for Advanced Molecular Separation

Graphene-based stable pH-responsive membranes (GPMs) were developed by alternative deposition of graphene oxide (GO) with polyethylenimine (PEI) in a layer-by-layer manner. Different from the conventional pore-blocking pH-responsive membranes, the size of the gaps among the GO sheets were first designed to respond to the surrounding pH. Atomic force microscopy was used to dynamically explore the internal structure alteration of GPM in the pH range from 3 to 11. It was found that the PEI molecules not only cross-linked the GO sheets through amide bonds to ensure the membrane stability but also reversibly altered the gate size of GPM in a certain extent according to the surrounding pH. In filtration, the gates of GPM were widened with the decreasing pH of the feed and vice versa. As a result, the permeate flux of GPM increased with the decreasing feed pH. More importantly, the molecular weight cutoff of GPM could be continuously regulated by the feed pH in a certain range; during the filtration of the polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO) mixed solution, only PVP (58 kDa) could penetrate GPM at pH 11, while the left PEO (600 kDa) would penetrate GPM at pH 3. The controlled penetration through GPM led to a complete separation and recovery of the molecules in different sizes, which is highly desirable for advanced molecular separation in environmental applications.