Tetrazine-based 1D polymers for the selective chemiresistive sensing of nitrogen dioxide via the interplay between hydrogen bonding and n-heteroatom interactions

There is a great demand for the reliable and quantitative detection of NO2 to monitor and control environmental pollution and subsequently prevent harmful human health effects. Metal oxide-based chemiresistive sensors detect this notorious gas, but high operating temperatures (100–500 °C) limit their applicability. Organic polymers can perform room-temperature sensing, but such sensors are rare due to the lack of functional groups needed for effective analyte detection and discrimination. This paper describes the selective sensing of NO2 gas by incorporating well-defined N-heteroatom functionalization into organic polymers. Three structurally analogous novel one-dimensional tetrazine polymers (p-phenyl-, m-phenyl- and thienyl-substituted) were synthesized in this study. Incorporating such π units allows the polymers to be stabilized in their partially oxidized forms, resulting in various ratios of dihydrotetrazine (0–55%):tetrazine units in the polymer backbone. All of the synthesized polymers were tested as resistive sensors for NO2. The polymer pPTz, possessing the highest dihydrotetrazine (~55%) content, exhibited the best performance for detecting 25–150 ppm NO2 compared to the other two polymers (mPTz, TTz). The pPTz-based sensor was found to be fast, taking 115 s to respond to 150 ppm NO2, reversible with a recovery time of 560 s for 150 ppm NO2 and could sense NO2 at room temperature. pPTz was also found to be highly selective for NO2 over the other tested analytes, such as NH3, EtOH, MeOH, acetone, 2-nitro toluene, and humidity. Synergetic interactions arising from –NH hydrogen bonding (dihydrotetrazine) and the N-heteroatom (tetrazine) functionalities in pPTz were expected to be the reason for the superior and selective detection of NO2. The structure-property relationship studies reported in this work may pave the way for designing functional conducting polymers for excellent chemiresistive sensing. Tetrazine based one-dimensional organic polymers have been investigated as sensing materials for room-temperature sensing of NO2 gas. The well-defined N-heteroatom functionalization within the polymer’s backbone promotes strong hydrogen-bonding interactions with the analyte and detects various concentrations of NO2 (25–150 ppm). The sensing was also highly selective towards NO2 over other tested analytes such as NH3, EtOH, MeOH, Acetone, 2-nitro toluene, and humidity.