Ionic Liquids as Green Solvents and Electrolytes for Robust Chemical Sensor Development

Ionic liquids (ILs) exhibit complex behavior. Their simultaneous dual nature as solvents and electrolytes supports the existence of structurally tunable cations and anions, which could provide the basis of a novel sensing technology. However, the elucidation of the physiochemical properties of ILs and their connections with the interaction and redox mechanisms of the target analytes requires concerted data acquired from techniques including spectroscopic investigations, thermodynamic and solvation models, and molecular simulations. Our laboratory is using these techniques for the rational design and selection of ILs and their composites that could serve as the recognition elements in various sensing platforms. ILs show equal utility in both piezoelectric and electrochemical formats through functionalized ionics that provide orthogonal chemo- and regioselectivity. In this Account, we summarize recent developments in and applications of task-specific ILs and their surface immobilization on solid supports. Such materials can serve as a replacement for conventional recognition elements and electrolytic media in piezoelectric and electrochemical sensing approaches, and we place a special focus on our contributions to these fields. ILs take advantage of both the physical and chemical forces of interaction and can incorporate various gas analytes. Exploiting these features, we have designed piezoelectric sensors and sensor arrays for high-temperature applications. Vibrational spectroscopy of these ILs reveals that hydrogen bonding and dipole-dipole interactions are typically responsible for the observed sensing profiles, but the polarization and cavity formation effect as an analyte approaches the recognition matrix can also cause selective discrimination. IL piezoelectric sensors can have low sensitivity and reproducibility. To address these issues, we designed IL/conducting polymer host systems that tune existing molecular templates with highly selective structure specific interactions. We can also modulate the IL microenvironment so that ILs act as filler molecules to optimize host template cavity size, shape, and functionality. When used as non-volatile and tunable electrolytes, ILs show great potential for the development of both amperometric and electrochemical double layer capacitance sensors for the detection of oxygen and explosives. We also designed and tested a two dimensional electrode chip that enabled simultaneous monitoring of both piezoelectric and electrochemical signals. This device imparted additional selectivity and overcame the limitations of the typical sensing protocol. The integrated piezoelectric and electrochemical sensing approach allows the measure of the charge to mass ratio under a dynamic regime. The electrogravimetric dynamic relationship allows for further discrimination between and accurate quantification of the interfacial transfer of different species. In summary, although new systematic and mechanistic studies of ILs are needed, we show that the self-organized phases of the aggregated non-polar and charged domains of ILs are useful sensing materials for electrochemical and quartz crystal microbalance transducers.