Conductive polymers for carbon dioxide sensing

Augmented levels of carbon dioxide (CO2) in greenhouses stimulate plant growth through photosynthesis. Wireless sensor networks monitoring CO2 levels in greenhouses covering large areas require preferably low power sensors to minimize energy consumption. Therefore, the main objective of this research is to develop CO2 sensors using conductive polymer/polyelectrolyte blends as low power sensing layers operating at room temperature. The transduction principle is based on a relative change in conductivity of the polymer/blend film with regard to variation in CO2 concentration. Conductive polymers including emeraldine base polyaniline (EB-PANI), sodium salt of sulfonated polyaniline(SPAN-Na) and their blends with poly(vinyl alcohol) (PVA) were investigated for CO2 sensing. Conductivity of EB-PANI did not vary in the required pH range for CO2 sensing (pH4 - pH7), however a sulfonated derivative (SPAN-Na) showed an appropriate conductivity change in this pH range. Frequency-dependent impedance of the polymer films casted on interdigitated platinum electrodes were measured. A significant decrease in impedance of the SPAN-Na:PVA blend films was observed at high CO2 concentrations (above 20,000 ppm) under high humidity. The effect of humidity on intrinsic and ionic conductivity of the polymerswas investigated by electrochemical impedance spectroscopy. In addition, polyethyleneimine (PEI) and its blends with other polyelectrolytes including SPAN-Na, poly(sodium 4-styrenesulfonate) (PSS-Na) and Nafion sodium salt (Nafion-Na) exhibited a better sensitivity over a wide range of CO2 concentrations (from 400 ppm to 10,000 ppm). Both dc resistance and ac impedance increased when the films were exposed to CO2 at high humidity. The relative change in impedance of the PEI films was about 6-12%. The response time was 4-5 min but recovery time was quite long from 20 to 60 min. A novel solution to reduce the recovery time was achieved with PEI blends. The blend of PEI:SPAN-Na exhibited a fast response (1.5-4 min) and a short recovery time (1.5-10 min) but a reduced sensitivity in comparison with pure PEI. Furthermore, blends of PEI with PSS-Na, Nafion-Na gave a good sensitivity (up to 2-3 order improvement) and relatively short recovery time (10-20 min). The interactions between sulfonate groups with amine groups of PEI might explain the higher CO2 sensitivity of this PEI blend. Some perspectives are sketched for polymer sensors to be applied in wireless sensor network for greenhouses and other potential applications.