Side-Chain Engineering Enabling Ion-In-Conjugation Polymers Built-In Moisture Resistivity Detection of NO₂ at Room...

Organic semiconductors emerge as promising chemiresistive gas-sensing materials, benefiting from their high selectivity and sensitivity toward various gases. However, these materials usually work at room temperature; thus, the adsorption of water vapor is usually unavoidable and produces interfering signals, which significantly hinders the detection of nitric dioxide (NO₂) at the parts per billion (ppb) level and their practical application. In this work, side-chain engineering was applied to ion-in-conjugation (IIC) polymers for addressing moisture interference in NO₂ sensing. By controlling the condensing precursors, different numbers of methyl groups (n = 0/1/2) were introduced on the phenyl ring. The side-chain engineering could enable the polymers to stably detect NO₂ even at relative humidity levels ranging from 0 to 75% at the parts per billion level. Specifically, the p-PTS (n = 1)-based sensor shows only a 10.3% decrease in response to NO₂ at room temperature without requiring heating or hydrophobic layer coverage. Moreover, these polymers demonstrate the highest charge transfer after adsorbing NO₂ compared to other interfering gases, which endow the sensors with high selectivity for NO₂. Theoretical calculations reveal that the introduction of methyl groups improves the planarity of the polymer backbone upon water adsorption, which enhances the charge transfer following NO₂ adsorption. At the same time, the presence of methyl groups prevents water molecules from accessing the adsorption sites and provides an inherent moisture resistance. These results offer an approach for creating high-performance NO₂ sensors with built-in moisture resistance and establish a foundation for future use in more challenging environmental conditions.