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.

Giant Redox Entropy in the Intercalation vs Surface Chemistry of Nanocrystal Frameworks with Confined Pores.

Redox intercalation involves coupled ion-electron motion within host materials, finding extensive application in energy storage, electrocatalysis, sensing, and optoelectronics. Monodisperse MOF nanocrystals, compared to their bulk phases, exhibit accelerated mass transport kinetics that promote redox intercalation inside nanoconfined pores. However, nanosizing MOFs significantly increases their external surface-to-volume ratios, making the intercalation redox chemistry into MOF nanocrystals difficult to understand due to the challenge of differentiating redox sites at the exterior of MOF particles from the internal nanoconfined pores. Here, we report that Fe(1,2,3-triazolate)2 possesses an intercalation-based redox process shifted ca. 1.2 V from redox at the particle surface. Such distinct chemical environments do not appear in idealized MOF crystal structures but become magnified in MOF nanoparticles. Quartz crystal microbalance and time-of-flight secondary ion mass spectrometry combined with electrochemical studies identify the existence of a distinct and highly reversible Fe2+/Fe3+ redox event occurring within the MOF interior. Systematic manipulation of experimental parameters (e.g., film thickness, electrolyte species, solvent, and reaction temperature) reveals that this feature arises from the nanoconfined (4.54 Å) pores gating the entry of charge-compensating anions. Due to the requirement for full desolvation and reorganization of electrolyte outside the MOF particle, the anion-coupled oxidation of internal Fe2+ sites involves a giant redox entropy change (i.e., 164 J K-1 mol-1). Taken together, this study establishes a microscopic picture of ion-intercalation redox chemistry in nanoconfined environments and demonstrates the synthetic possibility of tuning electrode potentials by over a volt, with profound implications for energy capture and storage technologies.

Solvent-controlled ion-coupled charge transport in microporous metal chalcogenides.

Interactions between ions and itinerant charges govern electronic processes ranging from the redox chemistry of molecules to the conductivity of organic semiconductors, but remain an open frontier in the study of microporous materials. These interactions may strongly influence the electronic behavior of microporous materials that confine ions and charges to length scales comparable to proton-coupled electron transfer. Yet despite mounting evidence that both solvent and electrolyte influence charge transport through ion-charge interactions in metal-organic frameworks, fundamental microscopic insights are only just beginning to emerge. Here, through electrochemical analysis of two open-framework chalcogenides TMA2FeGe4S10 and TMA2ZnGe4S10, we outline the key signatures of ion-coupled charge transport in band-type and hopping-type microporous conductors. Pressed-pellet direct-current and impedance techniques reveal that solvent enhances the conductivity of both materials, but for distinct mechanistic reasons. This analysis required the development of a fitting method that provides a novel quantitative metric of concerted ion-charge motion. Taken together, these results provide chemical parameters for a general understanding of electrochemistry in nanoconfined spaces and for designing microporous conductors and electrochemical methods used to evaluate them.