A Tunable Colorimetric Carbon Dioxide Sensor Based on Ion-Exchanger- and Chromoionophore-Doped Hydrogel

We report here a colorimetric carbon dioxide (CO₂) optode sensor with a polypropylene microporous membrane on top of a thin layer (30 μm) of polyurethane hydrogel. The diffusion of CO₂ across the polypropylene membrane induced pH changes in the hydrogel containing a lipophilic indicator, a cation exchanger, and a cationic amine. The ratio of the indicator and the cation exchanger was successfully utilized to adjust the sensitivity of the CO₂ response. Increasing the relative amount of the cation exchanger made the sensor much more sensitive to CO₂ in the lower concentration range. Moreover, the carbarmic formation reaction between the primary amine and CO₂ was investigated. The results indicated a very small contribution of carbarmic formation to the overall pH change. With a detection limit of 0.014 % for the CO₂ partial pressure, the sensor was successful applied to monitor CO₂ evolution during yeast catalyzed flour fermentation.