Wearable nitrogen oxide gas sensors based on hydrophobic polymerized ionogels for the detection of biomarkers in exhaled breath

Abstract

High-performance chemical sensors can be used for detecting biomarkers in exhaled breath. The use of chemiresistive-type gas sensors has been limited because of their ineffective feasibility and relatively poor operational stability against humidity and physical deformations. In this study, wearable chemiresistive-type gas sensors based on bar-printed ionogels are developed for the detection of nitrogen oxide (NOx) at ppb levels. The ionogels consist of polymerized ionic liquids (ILs) incorporated into a carefully chosen hydrophobic polymer matrix. The proposed sensors exhibit a sensitive response of 11% when exposed to 300 ppb of NOx target gas for 15 s of exhaled breath duration and an outstanding recovery of up to 100% for 25 s of inhaled breath duration. Furthermore, high-performance ionogel-based biosensors exhibited a theoretical detection limit of 0.11 ppb and an excellent selectivity toward different oxidizing and reducing gases. To realize practical biosensor applications, the effects of humidity in exhaled breath at different temperatures and the mechanical stability against cyclic bending stress on the sensing performance are investigated. Highly stable sensing performances were achieved under harsh conditions with a relative humidity of 90% at 45 °C for 30 days, even under a bending state measured and 20,000 bending cycles at a strain of 30%. Finally, the sensing mechanism of the chemiresistive-type gas sensors and the origin of their high sensitivity and stability are investigated by analyzing the interactions between ion pairs of the ILs and NOx molecules, as well as the incorporation of ILs into the hydrophobic polymer matrix.