Impact of imine positional isomerism on polymer/semiconducting single-walled carbon nanotube sensors for selective NO₂ detection

The accurate determination of NO 2 concentration in exhaled gases is imperative for discriminating between patients with respiratory/cardiovascular diseases and healthy individuals. Therefore, it is important to employ a streamlined and expeditious methodology for the detection of NO 2 . Semiconducting single-walled carbon nanotubes (sc-SWCNTs) exhibit immense potential as next-generation gas sensors due to their high specific surface area, distinctive atomic structure, and remarkable electrical responsiveness to the surrounding environment. In this work, two conjugated polymers (fluorene-alt-C-benzene (PFCN) and fluorene-alt-N-benzene (PFNC)) were synthesized by the Suzuki coupling, featuring positional isomerism of imine groups (-CH=N- vs. -N = CH-). The introduction of a phenyl side chain via imine bonds (-CH=N-) not only enhances the polymer but also regulates the alignment of the imine groups via steric and electronic effects, optimizing their interaction with sc-SWCNTs. The resulting polymer/sc-SWCNTs complexes were utilized for sensor fabrication. In PFCN, more electronegative N atom is positioned away from the skeleton-forming main chain, increasing its environmental exposure and enabling effective attraction of electron-deficient NO 2 . Consequently, the PFCN/sc-SWCNTs sensor showed a significantly higher response (27.6 %) to 1 ppm NO 2 , which was 1.7-fold higher than the PFNC/sc-SWCNTs sensor (16.52 %). This study presents novel strategies for designing high-performance sensors by integrating sc-SWCNTs with imine-functionalized polymers, advancing sensitive NO 2 detection for applications such as non-invasive medical diagnostics.