Switchable Single-Walled Carbon Nanotube-Polymer Composites for CO₂ Sensing

We report a chemiresistive CO₂ sensor based on single-walled carbon nanotubes (SWCNTs) noncovalently functionalized with a CO₂ switchable copolymer containing amidine pendant groups that transform into amidinium bicarbonates in response to CO₂. To fabricate a robust surface-anchored polymer-SWCNT dispersion via spray coating, we first designed and synthesized a precursor copolymer, P(4VP-VBAz), bearing both 4-vinylpyridine (4VP) groups and azide groups. The SWCNT dispersant group, 4VP, is capable of debundling and stabilizing nanotubes to improve their solubility in organic solvents for solution processing. Well-dispersed P(4VP-VBAz)-SWCNT composites are covalently immobilized onto a glass substrate functionalized with alkyl bromides, and then the amidine moieties are subsequently attached to form the resulting CO₂-switchable P(Q4VP-VBAm)-SWCNT composites via a copper(I)-catalyzed azide-alkyne cycloaddition click reaction at the film surface. The amidine groups are strong donors that compensate or pin carriers in the SWCNTs. In the presence of CO₂ under humid conditions, the generated amidinium bicarbonates from the polymer wrapping increase the concentration and/or liberate the hole carriers in the nanotubes, thereby increasing the net conductance of the composites. The amidinium moieties revert back to the amidines when purged with a CO₂-free carrier gas with a reversible decrease in conductance. We also demonstrate high selectivity to CO₂ over the other atmospheric gases such as O₂ and Ar.