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.