Room-temperature operated cyano-terminated ethynylated-thiourea as a resistive-type carbon dioxide (CO2) gas sensor

Abstract Ethynylated-thiourea derivatives of 4-ethylbenzoyl-3-(4-ethynylbenzonitrile-phenyl)-thiourea ( 2a ), and 4-methylbenzoyl-3-(4-ethynylbenzonitrile-phenyl)-thiourea ( 2b ) were successfully designed, synthesised, characterised and were used as sensing layers for the detection of carbon dioxide (CO 2 ) gas. Resistive-type CO 2 gas sensor featuring 2a and 2b (6 μL with concentration of 1 × 10 −3  M) were drop-coated onto interdigitated electrodes (IDEs) surface. The surface morphology of coated IDEs with 2a and 2b were analyzed via Scanning Electron Microscopy (SEM). Upon exposure to 10–1000 ppm CO 2 at 65–70% relative humidity (RH), their synergistic effects on CO 2 sensing were subsequently examined through the variance in resistance values before and after exposure to CO 2 . The response (%) of these sensors increased significantly to a value of 25% operated at room temperature upon the increment of CO 2 concentration. Both sensors showed no obvious deterioration in the sensing response during recurrent usage which gave the good indicator of the regeneration and reproducibility technique with low relative standard deviation (RSD) values in the range 0.174%–1.698%. Furthermore, they also showed fast response (τ res ) and recovery (τ rec ) time significantly for CO 2 absorption and desorption. Their molecular response towards CO 2 and mechanism of detection were evaluated via density functional theory (DFT) at B3LYP/6-31G (d,p) level of theory. The unique properties of ethynylated-thiourea derivatives are attributed by the presence of -NH-C=O for H-bonding interaction between ethynylated-thiourea derivatives and CO 2 analyte.