Debbie S. SILVESTER-DEAN1
1School of Molecular and Life Sciences, Curtin University, , Australia
In 2004, Compton’s group first proposed the use of room temperature ionic liquids (RTILs) as replacements for traditional electrochemical solvents in amperometric gas sensors (AGSs).1 The favourable properties of RTILs – high intrinsic conductivity, wide electrochemical windows, high chemical and thermal stability – combined with their low volatility and good solvation ability, meant that the porous membrane in an AGS could be removed. This allows the whole sensor device to be miniaturized, using a planar electrode in a ‘membrane free’ design with just a tiny droplet of RTIL.2
In this presentation, I will discuss some of my group’s recent developments in this area, and how our fundamental studies over the last decade on toxic gas and explosive behaviour in RTILs have progressed towards sensor commercialization. Using electrochemical techniques such as cyclic voltammetry and chronoamperometry, we have explored the physical properties of these systems, enabling us to extract new information regarding electrode reaction mechanisms, as well as solubilities, diffusion coefficients, and gas–liquid partition coefficients. Since ionic liquids are well-known to be hygroscopic, the impact of humidity and dissolved water in ionic liquids is explored, showing how its presence significantly affects the outcome of electrochemical reactions such as the oxygen reduction reaction. It was found that using ionic liquids with hydrophobic cations and anions is extremely important in high humidity environments for oxygen sensing.3 This was supported by our atomic force microscopy (AFM) studies,3 uncovering the strong structuring of the ions in the electrical double layer (EDL) at the interface that becomes weakened in the presence of water.
Since RTILs exist in the liquid state, which is not ideal for sensors that need to be moved, shaken, or agitated, one way to mechanically stabilise them on planar electrode devices is to combine them with polymers. I will discuss some of our developments using poly(ionic liquid)/ionic liquid mixtures (collaboration with Prof David Mecerreyes) as favourable and mechanically robust electrolytes. Different mixing ratios of the poly(IL) with the IL were investigated to find the optimum mixture that gives adequate robustness, conductivity and sensitivity.4 The voltametric behaviour of oxygen, ammonia and sulfur dioxide at different concentrations show linear calibration graphs and excellent limits of detection, despite the more viscous gel-type electrolytes having increased viscosities.4 Finally, I will highlight the promising possibility to use the poly(IL)/IL mixtures as peelable membranes to collect and detect solid explosive contaminants, such as 2,4,6-trinitrotoluene (TNT), for forensic and environmental applications.
1_MC Buzzeo, C Hardacre, RG Compton Use of Room Temperature Ionic Liquids in Gas Sensor Design Anal. Chem. 2004, 76, 4583-4588.
2_DS Silvester New innovations in ionic liquid–based miniaturized amperometric gas sensors Curr. Opin. Electrochem. 2019, 15, 7-17.
3_S Doblinger, J Lee, DS Silvester Effect of Ionic Liquid Structure on the Oxygen Reduction Reaction Under Humidified Conditions J. Phys. Chem. C 2019, 123, 10727−10737.
4_S Doblinger, CE Hay, LC Tomé, D Mecerreyes, DS Silvester Ionic liquid/poly(ionic liquid) membranes as non-flowing, conductive materials for electrochemical gas sensing Anal. Chim. Acta 2022, 1193, 339414.