Electrochemical Anion Sensing Using Conductive Metal–Organic Framework Nanocrystals with Confined Pores

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 mixtures of distinct signatures from mixtures of different anions. Here, nanocrystalline films of the conductive metal–organic framework (MOF) Cr(1,2,3-triazolate)2 are used to differentiate anions based on size, which consequently affect the reversible oxidation of the MOF. During framework oxidation, the intercalation of larger charge-balancing anions (e.g., ClO4–, PF6–, and OTf–) gives rise to redox potentials shifted anodically by hundreds of mV due to the additional work of solvent reorganization and anion desolvation. Smaller anions (e.g., BF4–) may enter partially solvated, while larger ansions (e.g., OTf–) intercalate with complete desolvation. 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 exemplify an 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.