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Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Switchable Single-Walled Carbon Nanotube−Polymer Composites for CO2 Sensing Bora Yoon,†,‡ Seon-Jin Choi,§ Timothy M. Swager,*,‡,§ and Gary F. Walsh*,† †
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Optical and Electromagnetic Materials Team, U.S. Army Natick Soldier Research, Development and Engineering Center (NSRDEC), Natick, Massachusetts 01760, United States ‡ Institute for Soldier Nanotechnology and §Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States S Supporting Information *
ABSTRACT: 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. KEYWORDS: carbon nanotubes, carbon dioxide sensing, chemiresistor, copolymer, amidine, surface click reaction
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depends on the type of the produce: dried products (95%).8 Single-walled carbon nanotubes (SWCNTs) are attractive materials for sensing such gaseous analytes because of their sensitive resistance changes in response to the binding of molecules.9−12 Chemical functionalization of SWCNTs with selectors or receptors via covalent/noncovalent functionalization can produce sensors selective to target analytes. Noncovalent functionalization of SWCNTs13−18 offers the advantage of minimal perturbation of the electronic properties of the nanotubes, as opposed to covalent functionalization,19−22 which disrupts the extended π-electronic states in the nanotube sidewalls. This can be accomplished by using polymer wrappers to debundle the SWCNTs through noncovalent interactions to form a stabilized dispersion in the solvent media.
INTRODUCTION
There has been a growing interest in the detection and monitoring of CO2 in agri−food industry for precise agriculture and high food quality packaging. The determination of optimal CO2 levels in greenhouse crop production, which controls the photosynthesis of plants, is important to improve the crop productivity. Increasing the CO2 level to 0.1% increases the photosynthesis of most crops by about 50% over atmospheric CO2 levels, whereas an excessive amount of CO2 may cause damage to plants.1,2 In the food packaging, CO2 generated by the microbial metabolic activity is a key foodspoilage indicator.3 Monitoring of CO2 levels in the foods packed with modified atmosphere packaging (MAP) is also required to ensure the food quality and predict the shelf life.4 In MAP, a CO2 level of 20−80% is required in the package headspace for nonrespiring food to decrease the growth rate of aerobic bacteria.5 Elevated CO2 (>1%) and reduced O2 (
