pH-gated ion transport in covalent organic framework membranes functionalized with oxygen-containing groups

Covalent organic framework (COF) membranes with pH-gated ion transport hold promise for smart nanofluidic devices, yet the regulatory mechanisms of different oxygen-containing functional groups remain poorly understood. Herein, we systematically investigate the pH-dependent ionic conductance of TpPa-R COF membranes (R = -SO3H, -COOH, -OH) across pH 3 to 11.5 using unmodified TpPa as a reference. All membranes exhibit nanofluidic transport with a U-shaped conductance profile as a function of pH. The neutral-pH conductance is governed by the effective surface charge density that drives counterion enrichment. Among the three modifications, the TpPa-SO3H membrane displays the broadest pH response and the highest conductance at both acidic and alkaline extremes. Notably, the TpPa-COOH membrane shows unexpectedly lower conductance than TpPa-OH at pH ≥ 5, which we attribute to hydration screening that reduces the effective charge of the carboxylate groups, whereas deprotonation of the phenol groups in TpPa-OH enhances effective surface charge density and conductance at high pH. All membranes demonstrate reversible pH-switching over the tested range. This work provides a molecular-level design rationale for tailoring pH-gating performance in COF membranes and a foundation for developing tunable ion-transport devices with predictable pH responsiveness.