Unveiling the Stability of Proper Polyethyleneimine-Functionalized Carbon Nanotube Composites-Derived Chemiresistive CO2 Sensors

Branched polyethyleneimine (PEI), consisting of numerous imine groups, is employed for CO₂ gas detection, attributed to the interaction between imine groups and CO₂. Incorporating functionalized carbon nanotubes (f-CNTs) into PEI developed in recent years has remarkably enhanced sensor performance. However, the active characteristic of imine groups makes the sensor susceptible to aging. In this research, the aging of PEIs with different molecular weights is systematically studied and compared along with their thermal stability and structure migration under various conditions, explored using thermogravimetric analysis and Fourier-transform infrared spectroscopy. Furthermore, an ink composed of high-molecular-weight PEI and f-CNTs is successfully developed, which shows outstanding printability according to its rheology results. The stability of the ink is assessed by a Turbiscan analyzer and compared with the ink composed of low-molecular-weight PEI. The ink with high-molecular-weight PEI is utilized to prepare CO₂ sensors via screen printing, which are then tested by in-house-built electronics. The sensors achieve a detection range of 300–2000 ppm CO₂, which could indicate robust sensing performance even after 40 continuous testing cycles. It should be noted that the resultant sensors have realized a wider measurement range and superior stability than other reported PEI/CNT-based CO₂ sensors, further facilitating their practical applications.