Exceptional Monovalent Anion Selectivity in One-Dimensional Rectifying Metal-Organic Framework Subnanochannels.

Abstract

Selective anion transport is crucial for water treatment, energy harvesting, and biosensing. Inspired by biological anion channels known for their exceptional selectivity, permeability, and rectification properties, replicating these functions in artificial channels is highly desirable to enhance sensitivity in ion detection and reduce energy consumption in separation processes; however, accomplishing this remains a significant challenge. In this study, we present monovalent anion-selective channels fabricated from aluminum-based metal-organic frameworks (MOFs), MIL-53-X (X = NH₂ and N⁺(CH₃)₃), embedded in polymer substrates. These MOFs feature one-dimensional sub-1-nanometer pores and highly positive surface charges. The asymmetric configurations of the synthesized MOF channels promote unidirectional transport of the monovalent anions (Cl^- and NO₃^-), closely mimicking the function of biological anion channels. The resulting channels exhibit excellent Cl^-/SO₄^2- selectivity ranging from ∼13 to ∼80 and NO₃^-/SO₄^2- selectivity from ∼7 to ∼46, along with ion rectification ratios of up to ∼110 for Cl^- and ∼93 for NO₃^-. These results demonstrate the artificial anion channels' high monovalent anion selectivity and unidirectional transport capabilities, offering a promising approach for selective ion electrodes and energy-efficient separation technologies.