Although metal organic frameworks (MOFs) have proven to be effective for electronic ion sensing, they lack key desirable characteristics such as solution processability due to their insolubility in most common solvents. Metal-organic supercontainers (MOSCs) have a significant advantage over MOFs, as they can be readily dispersed in solution while retaining their unique endo- and exocavities. Thus, MOSCs are especially suitable for incorporating into ion-selective electrodes (ISEs). Incorporating ionophores with ion-exchange sites into a polymer matrix to form a mixed-matrix membrane (MMM) not only improved mechanical stability, but also increased ion-selectivity. Currently, there are commercially available ionophores and protocols for fabricating potentiometric ISEs with specific binding affinities to about thirty cations and anions. It is important to note that most of the ionophores available today are for the detection of elemental and other small ions despite a great demand for technologies that allow for rapid and precise monitoring of larger organic ions for both biomedical and environmental applications.
In collaboration with a team from Uppsala University, USD researchers have developed an application of MOSCs as a new type of ionophore by direct incorporation of MOSC molecules into a polyvinyl chloride (PVC) based MMM or solid sensing surface (SSS) for ISEs. With this approach, they take advantage of MOSCs’ solubility and exploit their unique multi-cavities as adsorption sites for molecular ion sensing. In this regard, MOSCs are particularly attractive as designer molecules: the MOSC structure, its constituent elements, and the number of binding sites can be easily tuned via a bottom-up approach, thus enabling a straightforward design of novel ionophores for highly specific ion detection.