Ionic Liquids in Gas Sensors

Ionic liquids possess some unique properties (Seddon 1997 Demus et al. 1998 Welton 1999 Earle and Seddon 2000 Pinkert et al. 2009 Silvester 2011 Singh et al. 2012) and can be classified as a special category of nonaqueous electrolytes as well as serve as a gas-permeable membrane or a solvent. It was found that a diverse range of organie, inorganic, and organometallic compounds are 

Korotcenkov, Handbook of Gas Sensor Materials, Integrated Analytical Systems, 

Examples of simple room-temperature ionic liquids 

Source Data from Singh et al. (2012). Published by Hindawi PubUshing Corporation 

ILs have negligible vapor pressures, so there is no drying out of the electrolytes, thus reducing hazards associated with flash points and flammability (Baker et al. 2005 Anastas 2007). The low volatility of ILs has been d onstrated in gas-separation membranes for separation of SO and CO (Jiang et al. 2007). The SO selectivity of separations using IL membranes has been shown to be 9-19 times that of CO.  

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Buzzeo, M.G., Hardacre, C., and Compton, R.G., Use of room temperature ionic liquids in gas sensor design, Anal. Chem., 76,4583-4588, 2004. 

After reviewing the properties and structure of ionic liquids, leading specialists explore the role of these materials in optical, electrochemical, and biochemical sensor technology. The book then examines ionic liquids in gas, liquid, and countercurrent chromatography, along with their use as electrolyte additives in capillary electrophoresis. It also discusses gas solubilities and measurement techniques, liquid-liquid extraction, and the separation of metal ions. The final chapters cover molecular, Raman, nuclear magnetic resonance, and mass spectroscopies. 

Wang Z, Guo M, Baker GA, Stetter J, Lin L, Mason AJ, Zeng X (2014) Methane-oxygen electrochemical coupling in an ionic liquid a robust sensor for simultaneous quantification. Analyst, 139, 5140-5147... 

The magnitude of the dissociation constant A plays an important role in the response characteristics of the sensor. For a weakly dissociated gas (e.g., CO2, K = 4.4 x 10-7), the sensor can reach its equilibrium value in less than 100 s and no accumulation of CO2 takes place in the interior layer. On the other hand, SO2, which is a much stronger acid (K = 1.3 x 10-2), accumulates inside the sensor and its rep-sonse time is in minutes. The detection limit and sensitivity of the conductometric gas sensors also depend on the value of the dissociation constant, on the solubility of the gas in the internal filling solution, and, to some extent, on the equivalent ionic conductances of the ions involved. Although an aqueous filling solution has been used in all conductometric gas sensors described to date, it is possible, in principle, to use any liquid for that purpose. The choice of the dielectric constant and solubility would then provide additional experimental parameters that could be optimized in order to obtain higher selectivity and/or a lower detection limit. 

Wang R, Okajima T, Kitamura F, Ohsaka T (2004) A novel amperometric Oj gas sensor based on supported room-temperature ionic liquid porous polyethylene membrane-coated electrodes. Electroanalysis 16 66-72 Wang Z, Lin P, Baker GA, Stetter J, Zeng X (2011) Ionic liquids as electrolytes for the development of a robust amperometric oxygen sensor. Anal Chem 83 7066-7073 Wasserschied P, Welton T (2003) Ionic Uquids in synthesis. Wiley, Weinheim... 

The advantages of using room temperature ionic liquid (RTIL) electrolyte in membrane-coated electrode as an O2 gas sensor compared to solid electrolyte gas sensors and classic Clark-type gas sensors included easy constraction and the ability to operate at ambient temperature. This would be the direction for future ambient temperature oxygen sensor development. 

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