Solid-State Ionic Liquid: Key to Efficient Detection and Discrimination in Organic Semiconductor Gas Sensors

Abstract

Pi-conjugated polymers (pi-CPs) blended with ionic liquids (ILs) have shown great potential for noninvasive diagnostics by transducing dielectric environmental changes induced by volatile organic compound (VOC) biomarkers into distinct electrical signals. However, the role of ILs in VOC sensing remains unclear, limiting further development of organic sensors for real commercialization. Herein, the key VOC detection and discrimination mechanisms in pi-CP:IL sensors are identified. Three different ILs forming either the liquid or solid (semicrystalline) state at room temperature are investigated. Superior to the liquid-state ionic liquid (LSIL), the solid-state ionic liquid (SSIL) promotes strong, stable, and reversible electrochemical interaction (electric-field-driven doping) of pi-CP yielding a significant increase in pi-CP conductivity, which is a key prerequisite for reliable and sensitive VOC sensing. These interactions are further modulated by different VOC polarities enabling highly sensitive and selective detection of various VOCs. Advanced in situ electrochemical and structural measurements reveal that polar VOC interacts directly with SSIL reducing the pi-electron density of pi-CP, while nonpolar VOC induces strong electronic coupling between pi-CP and SSIL. Our results identify the complex transducing mechanisms of organic VOC sensors and provide important insight into the materials design rule for high-performance sensors.