Electrochemical Anion Sensing in Conductive Porous Manifolds

Anion sensing technology is motivated by the widespread and critical roles played by anions in biological systems and the environment. Electrochemical approaches comprise a major portion of this field, but so far have relied on redox-active molecules appended to electrodes that often lack the ability to produce distinct signatures for different anions. Here, we demonstrate the ability to differentiate anions based on size using metal-organic framework (MOF) nanocrystal thin films with sub-nanometer pores. The reversible oxidation of Cr(1,2,3-triazolate)2 couples to the intercalation of charge-balancing anions such that larger anions (ClO4–, PF6–, and OTf–) give rise to redox potentials shifted anodically by hundreds of mV due to the additional work of solvent reorganization and anion desolvation, whereas smaller anions (BF4–) may enter partially solvated. As a proof-of-concept, we leverage this “nanoconfinement” approach to report an electrochemical ClO4– sensor in aqueous media that is recyclable, reusable, and sensitive to sub-100-nM concentrations. Taken together, these results highlight the unusual combination of distinct external versus internal surface chemistry in MOF nanocrystals and the interfacial chemistry they enable as a novel supramolecular approach for redox voltammetric anion sensing.