Coordination‑dominated selectivity for divalent transition‑metal ions in thiol/carboxyl‑functionalized MXene...

Biological ion channels realize exceptional selectivity for transition metal ions through the synergistic effects of angstrom-scale confinement and highly specific ligand-ion coordination. Inspired by this molecular recognition principle, we develop mercaptosuccinic acid (MSA) functionalized Ti₃C₂Tx MXene biomimetic nanochannel membranes that operate under a coordination-dominated ion transport pattern. The introduction of MSA induces chemically regulated angstrom-scale nanochannels with stabilized hydrated spacing, thereby effectively suppressing structural swelling in aqueous environments. In this system, highly selective separation of Cd²⁺ or Ni²⁺ is achieved mainly through specific coordination between thiol-carboxyl functional groups and transition metal ions, with K⁺/Cd²⁺ and K⁺/Ni²⁺ selectivity reaching ∼10³. By contrast, the K⁺/Mg²⁺ selectivity is much lower. Theoretical calculations reveal that strong coordination interactions between transition metal ions and MSA functional groups impose significantly elevated translocation energy barriers, effectively immobilizing Cd²⁺ and Ni²⁺ within the confined channels, while K⁺ and Mg²⁺ experience weaker interactions and retain facile transport pathways. The selective accumulation of divalent ions at channel entrances reflects the combined effects of strong ligand-ion interactions and high dehydration barriers. This work establishes a thiol-carboxyl coordination-dominated transition metal ion recognition pattern in angstrom-confined two-dimensional nanochannels and provides a theoretical basis for the rational design of artificial ion channels with programmable selectivity.