Continuous Flow Chemistry and Bayesian Optimization for Polymer-Functionalized Carbon Nanotube-Based Chemiresistive Methane Sensors

The preparation of poly(ionic) polymer-wrapped single-walled carbon nanotube (SWCNT) dispersions was carried out for application in chemiresistive methane (CH₄) sensors with enhanced humidity tolerance. Single-walled carbon nanotubes were noncovalently functionalized using poly(4-vinylpyridine) (P4VP), which had been modified with varying amounts of a poly(ethylene glycol) (PEG) moiety bearing a bromine atom and terminal azide group (Br-R1). Quaternization of P4VP with Br-R1 was performed under continuous flow conditions, with reaction conditions selected through Bayesian optimization. The resulting polymers (PyBrR1), exhibiting different degrees of functionalization, were utilized to disperse SWCNTs and subsequently incorporated into sensors. These sensors included a platinum complex as an aerobic oxidative catalyst and a polyoxometalate (POM) redox mediator to enable CH₄ sensing at room temperature.

As the degree of quaternization in the PyBrR1–CNT composites was increased, enhanced sensor responses were observed, with a nominal 10% quaternization providing the highest response magnitude. The presence of PEG was found to improve sensor stability under relative humidity conditions ranging from 57–90%, in comparison to sensors fabricated using CNT dispersions with unfunctionalized P4VP. Devices fabricated from these dispersions demonstrated superior performance over those prepared in situ under dry conditions, exhibiting greater stability under elevated humidity. Additionally, the effect of the character of Keggin-type POMs was evaluated to identify alternatives that could further improve sensor performance in humid environments. To support further optimization of the polymer functionalization algorithm, a kinetically informed machine learning model was developed and explored to predict the reactivity of pyridine units with alkyl bromides under flow conditions.

• Dunlap, JH
• Feng, H
• Pioch, T
• Volk, AA
• Giordano, AN
• Reidell, A
• Tran, LD
• Hampton, CM
• Luo, SL
• Rao, R
• Crouse, CA
• Swager, TM
• Baldwin, LA

• Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio 45433, United States

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