Ion transport in carbon nanotube porins with a pH-switchable entrance gate

Molecular transport in nanofluidic channels whose sizes approach molecular dimensions often differs drastically from conventional bulk transport. Strong confinement forces molecules in those channels into close proximity with channel walls, amplifying the roles of surface transport, surface defects, and molecular gates. In this work, machine-learning-accelerated first-principles molecular dynamics (ML-FPMD) simulations combined with well-tempered metadynamics simulations are used to probe these effects at the atomistic level and to elucidate the gating behavior observed experimentally in pH-responsive carbon nanotube porins. The simulations validate the experimental findings, demonstrating that defects introduced through defect-induced chemical etching (DICE) lead to rim functionalization that forms the structural basis of the pH-responsive molecular gate. Under neutral conditions, the rim groups remain dissociated, maintaining an open conformation that permits unhindered ion transport. At acidic pH, protonation of these groups drives a cooperative rearrangement that closes the pore entrance, blocking ionic flow. Beyond reproducing experimental observations, the MD trajectories reveal the molecular-level hydrogen-bond rearrangements and hydration structures providing a detailed picture of transport regulation in sub-nanometer functional nanopores.