Supported fluorine-free ionic liquids with highly sensitive gas-sensing performance

Abstract

Monitoring volatile organic compounds (VOCs) is of increasing significance in the fields of environmental protection, health and safety, economic issues in industries, etc. Ionic liquids (ILs) arouse a great interest as potential sensing materials for VOCs. Here, the gas-sensing ability of fluorine-free ILs such as [N4,4,4,4][MEEA], [P6,6,6,14][MEEA], and [P6,6,6,14][TpA] have been thoroughly investigated. These ILs with phosphonium and ammonium cations as well as fluorine-free anions were successfully supported on carboxyl and amino groups terminated surfaces (denoted as COOH and NH2) as electrical gas sensors to tailor the response to the electronic resistance after sensing of VOCs. The [P6,6,6,14][MEEA] IL supported by either COOH or NH2 exhibits the strongest electrical sensing response within five cycles at a 150 ppm concentration of acetone or toluene, followed by [P6,6,6,14][TpA], and [N4,4,4,4][MEEA] with the shorter ammonium cation exhibiting the weakest response. The achieved higher gas sensing capability of the supported [P6,6,6,14][MEEA] is strongly associated with the accelerated ion mobility reflected from the smaller friction coefficient (μ), while [N4,4,4,4][MEEA] possesses the highest μ and [P6,6,6,14][TpA] in between. The reduced friction coefficient of the supported [P6,6,6,14][MEEA] IL originates from the observed more ordered ion layering, speeding up the kinetics of gaseous molecules in the supported IL. The monitoring of VOCs in these supported ILs offers an excellent opportunity in e-nose sensing devices to easily and cost-effectively detect VOCs at ppm concentrations.