Smart pH-Responsive Covalent Organic Framework Membrane for Charge-Driven Molecular Sieving

Water purification has been identified as a far-sighted measure that benefits global sustainable development, and the purification technology based on membrane separation has gradually become the main strategy due to its virtue of low energy consumption, small footprint, convenient operation, and high separation efficiency. In this work, we have constructed 160 nm-thick robust self-standing COF membranes. For the first time, the as-synthesized imine-linked COF membrane not only exhibits intelligent optical pH stimuli-responsive performance (its color turns from bright red to black immediately when being exposed to acidic solutions and turns to light red after a few minutes of exposure to alkaline solutions) but also overcomes the ecumenical limitation of selective removal. The charge-driven molecular sieving effects induced by acid/base endow them with preeminent screening performance toward organic dyes whose dimensions are much smaller than the membrane pores and in separating oppositely charged persistent organic pollutants with analogical sizes. This work opens up a new avenue for developing smart pH-responsive 2D COF membranes, exploring the neoteric removal modes of organic compounds, and discovering more potential intelligent applications of self-standing COF membranes in various frontier domains and functional devices.