Fast Responding Amperometric CO₂ Microsensor with Ionic Liquid-Aprotic Solvent Electrolytes

Knowledge about the microscale distribution of CO₂ is essential in many environmental and technical settings, and electrochemical CO₂ sensing may be optimized to yield such information. The performance of a Clark-type CO₂ sensor was greatly improved by adding 20% dimethylformamide (DMF) to the ionic liquid 1-ethyl-3-methylimidazolium dicyanamide (EMIM-DCA) previously used as an electrolyte. The addition of DMF resulted in a much faster response to increasing (95% response of about 100 s) or decreasing CO₂ concentration, a negligible interference from low concentrations of N₂O, and a signal temperature dependence similar to that of O₂ microsensors. The use of 80% EMIM-DCA/20% DMF as an electrolyte leads to CO₂ reduction at -0.72 V (vs standard hydrogen electrode), reducing the overpotential by 0.2 V as compared to the use of 100% EMIM-DCA. The CO₂ microsensor has a calculated limit of detection of 0.5 Pa CO₂, and sensors optimized for high sensitivity exhibited a linear response within the range of 0-4.6 kPa (0-1.7 mM) CO₂. A set of four sensors exhibited no noticeable change of zero current and CO₂ sensitivity during 4 months of continuous polarization.