Diols as a novel signal enhancer for electrochemical ammonia gas sensing in ionic liquid electrolytes

Electrochemical gas sensors have been widely utilized for ammonia (NH₃) gas detection due to their high sensitivity and selectivity, easy miniaturization and operation, and low power consumption. Room temperature ionic liquids (RTILs) have gained a lot of attention as an electrolyte for electrochemical gas sensors owing to their high ionic conductivity, wide electrochemical stability window, non-volatility, and non-flammability. However, low NH₃ solubility in RTILs and their high viscosity impede mass transfer of NH₃ gas to electrodes for electrochemical oxidation, thereby limiting the sensitivity. To overcome these limitations, for the first time we apply diols to RTIL electrolytes. Ethylene glycol (EG), 1,2-propanediol (1,2-PD), and 1,3-propanediol (1,3-PD) were simply added to 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C₂mim][NTf₂]) electrolyte. Their addition not only increased the NH₃ solubility, but also decreased the viscosity and density of the electrolyte, thereby enhancing sensitivity and selectivity of amperometric NH₃ gas sensors. Simple addition of 1,2-PD to the electrolyte increased a sensitivity from 5.40 × 10⁻⁹ A/ppm to 2.58 × 10⁻⁸ A/ppm and decreased a limit of detection (LOD) from 20.1 ppm to 5.53 ppm. The signal enhancement by diol addition was also shown even when other electrodes and RTIL electrolytes were used. It is highly expected that diols will be widely used as an effective signal enhancer for electrochemical ammonia gas sensing in RTIL electrolytes.