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.