A chemiresistive methane sensor

Abstract

A chemiresistive sensor is described for the detection of methane (CH₄), a potent greenhouse gas that also poses an explosion hazard in air. The chemiresistor allows for the low-power, low-cost, and distributed sensing of CH₄ at room temperature in air with environmental implications for gas leak detection in homes, production facilities, and pipelines. Specifically, the chemiresistors are based on single-walled carbon nanotubes (SWCNTs) noncovalently functionalized with poly(4-vinylpyridine) (P4VP) that enables the incorporation of a platinum-polyoxometalate (Pt-POM) CH₄ oxidation precatalyst into the sensor by P4VP coordination. The resulting SWCNT-P4VP-Pt-POM composite showed ppm-level sensitivity to CH₄ and good stability to air as well as time, wherein the generation of a high-valent platinum intermediate during CH₄ oxidation is proposed as the origin of the observed chemiresistive response. The chemiresistor was found to exhibit selectivity for CH₄ over heavier hydrocarbons such as n-hexane, benzene, toluene, and o-xylene, as well as gases, including carbon dioxide and hydrogen. The utility of the sensor in detecting CH₄ using a simple handheld multimeter was also demonstrated.

Keywords: catalysis; chemiresistors; methane; selectors; sensor.

Fig. 1.

Aerobic methane oxidation with a platinum-polyoxometalate precatalyst under mild conditions and the strategy reported in the present study. SWCNT, single-walled carbon nanotube; POM, polyoxometalate [H₅PV₂Mo₁₀O₄₀].

Fig. 2.

Device fabrication schematic and chemiresistor composition. (A) Sensor fabrication and stepwise selector incorporation by spray coating of SWCNT-P4VP network (step 1), Pt coordination by soaking in [1-DMSO][SO₃OCH₃] solution (step 2), and anion exchange by soaking in [H₅PV₂Mo₁₀O₄₀] solution (step 3). (B) Proposed surface speciation of SWCNT-P4VP-Pt. (C) Proposed surface speciation of SWCNT-P4VP-Pt-POM.

Fig. 3.

Chemiresistor performance in methane sensing and control experiments. (A) Control CH₄ sensing experiments omitting selector components contrasted with the sensing response of SWCNT-P4VP-Pt-POM. (Inset) Averaged change in conductance (represented as ΔG/G₀, %) of SWCNT-P4VP-Pt-POM in response to 0.5% of CH₄ in air (RH = 10 ± 5%). (B) Averaged conductance trace of SWCNT-P4VP-Pt-POM in response to three repeated 120-s exposures of 0.5% of CH₄ each in air. (C) Chemiresistive responses of SWCNT-P4VP-Pt-POM to 120-s exposures to various CH₄ concentrations in air (maroon), dry air (gray), or N₂ (blue) carrier gas. (D) Chemiresistive responses of SWCNT-P4VP-Pt-POM to 120-s exposures to various CH₄ concentrations in air. Shaded areas and error bars represent SDs (n = 4); all data were collected at room temperature.

Fig. 4.

Synthesis of [1-Py][SO₃OCH₃] and subsequent reaction with [H₅PV₂Mo₁₀O₄₀].

Fig. 5.

Chemiresistor selectivity and stability studies. (A) Average device response (defined as change in conductance, ΔG/G₀, %) of SWCNT-P4VP-Pt-POM toward 400 ppm of various interferants in air. (B) Normalized average response of freshly prepared and aged SWCNT-P4VP-Pt-POM devices toward 0.5% of CH₄ in air. Error bars represent SDs (n = 4); all data were collected at room temperature.