Exploring the compositional space of a metal–organic framework with ionic liquids to develop porous ionic conductors for enhanced signal and selectivity in VOC capacitive sensors

Abstract

The monitoring of atmospheric pollutants, especially non-methane-based volatile organic compounds (NMVOCs), is an important paradigm towards secure air quality surroundings. However, existing gas sensing technologies face challenges in selectively and sensitively detecting individual NMVOCs due to their low concentration in comparison to the main atmospheric components. In this research, the compositional space between a Metal–Organic Framework (MOF) and Ionic Liquids (ILs) is explored to fine tune the signal and selectivity of a capacitive gas sensing layer. Firstly, by tuning the weight ratio of the MOF : IL components, ionic conductive materials ranging from solid porous to partially porous inks are produced. Secondly, by combining the sensitivity arising from the IL's dielectric characteristics with the selectivity endowed by the MOF's porosity, varied capacitive responses are obtained. Finally, the sensing responses of thirteen sensors towards the detection of water vapor, ethanol, acetone and isopropanol demonstrate that the hybridization of the MOF/IL offers a suitable avenue to balance the porosity, magnitude of response, and partial selectivity. In addition, when the responses of multiple MOF/IL sensors are evaluated, cross-selectivity detection of individual NMVOCs is reached. This approach contributes to fine-tuning the MOF/IL sensor performance not just by expanding the MOF/IL combinations, but optimizing sensor processing by advanced printing and electronics design.