Translating Catalysis to Chemiresistive Sensing

Surface Defect-Induced Specific Catalysis Activates 100% Selective Sensing toward Amine Gases at Room Temperature

Achieving selective sensing toward target volatile organic compound gases is of vital importance in the fields of air quality assessment, food freshness evaluation, and diagnosis of patients via exhaled breath. However, chemiresistive sensors that exhibit specificity like biological enzymes in a complex environment are rare. Herein, we developed a strategy of optimizing oxygen vacancy structures in tin oxides to induce specific catalysis, activating 100% selective sensing toward amine gases at room temperature. In situ technologies and theoretical calculations reveal that the "donor-receptor" coordination between nitrogen atoms from amine molecules and bridging oxygen vacancies (OVBri)-induced electron-deficient center is the essence of specific catalysis and provides the bridge from the surface oxidation reaction to electrophysical characteristics evolution, which allows the sensor to exhibit amine-specific sensing behavior, even in gas mixtures. Moreover, OVBri enhances the selectivity by enabling a room-temperature sensing pathway where lattice oxygens participate in catalytic oxidation for amine molecules, resulting in record-high sensing values: 19,938.92 toward 100 ppm of triethylamine, 15,236.78 toward trimethylamine, and 123.41 toward diethylamine. Our findings illustrate the feasibility of designing specific active sites through defect engineering and can contribute to the advancement of highly selective sensors based on catalytic processes.

Trace Hydrogen Sulfide Sensing Inspired by Polyoxometalate-Mediated Aerobic Oxidation

A high-performance chemiresistive gas sensor is described for the detection of hydrogen sulfide (H₂S), an acutely toxic and corrosive gas. The chemiresistor operates at room temperature with low power requirements potentially suitable for wearable sensors or for rapid in-field detection of H₂S in settings such as pipelines and wastewater treatment plants. Specifically, we report chemiresistors based on single-walled carbon nanotubes (SWCNTs) containing highly oxidizing platinum-polyoxometalate (Pt-POM) selectors. We show that by tuning the vanadium content and thereby the oxidation reactivity of the constituent POMs, an efficient chemiresistive sensor is obtained that is proposed to operate by modulating CNT doping during aerobic H₂S oxidation. The sensor shows exceptional sensitivity to trace H₂S in air with a ppb-level detection limit, multimonth stability under ambient conditions, and high selectivity for H₂S over a wide range of interferants, including thiols, thioethers, and thiophene. Finally, we demonstrate that the robust sensing material can be used to fabricate flexible devices by covalently immobilizing the SWCNT-P4VP network onto a polyimide substrate, further extending the potentially broad utility of the chemiresistors. The strategy presented herein highlights the applicability of concepts in molecular aerobic oxidation catalysis to the development of low-cost analyte detection technologies.

Breath Acetone Sensing Based on Single-Walled Carbon Nanotube-Titanium Dioxide Hybrids Enabled by a Custom-Built Dehumidifier

Acetone is a metabolic byproduct found in the exhaled breath and can be measured to monitor the metabolic degree of ketosis. In this state, the body uses free fatty acids as its main source of fuel because there is limited access to glucose. Monitoring ketosis is important for type I diabetes patients to prevent ketoacidosis, a potentially fatal condition, and individuals adjusting to a low-carbohydrate diet. Here, we demonstrate that a chemiresistor fabricated from oxidized single-walled carbon nanotubes functionalized with titanium dioxide (SWCNT@TiO₂) can be used to detect acetone in dried breath samples. Initially, due to the high cross sensitivity of the acetone sensor to water vapor, the acetone sensor was unable to detect acetone in humid gas samples. To resolve this cross-sensitivity issue, a dehumidifier was designed and fabricated to dehydrate the breath samples. Sensor response to the acetone in dried breath samples from three volunteers was shown to be linearly correlated with the two other ketone bodies, acetoacetic acid in urine and β-hydroxybutyric acid in the blood. The breath sampling and analysis methodology had a calculated acetone detection limit of 1.6 ppm and capable of detecting up to at least 100 ppm of acetone, which is the dynamic range of breath acetone for someone with ketosis. Finally, the application of the sensor as a breath acetone detector was studied by incorporating the sensor into a handheld prototype breathalyzer.

A chemiresistive methane sensor

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.

Cascade Reaction-Based Chemiresistive Array for Ethylene Sensing

Chemiresistive sensors, which are based on semiconducting materials, offer real-time monitoring of environment. However, detection of nonpolar chemical substances is often challenging because of the weakness of the doping effect. Herein, we report a concept of combining a cascade reaction (CR) and a chemiresistive sensor array for sensitive and selective detection of a target analyte (herein, ethylene in air). Ethylene was converted to acetaldehyde through a Pd-catalyzed heterogeneous Wacker reaction at 40 °C, followed by condensation with hydroxylamine hydrochloride to emit HCl vapor. HCl works as a strong dopant for single-walled carbon nanotubes (SWCNTs), enabling the main sensor to detect ethylene with excellent sensitivity (10.9% ppm^-1) and limit of detection (0.2 ppm) in 5 min. False responses that occur in the main sensor are easily discriminated by reference sensors that partially employ CR. Moreover, though the sensor monitors the variation of normalized electric resistance (ΔR/R₀) in the SWCNT network, temporary deactivation of CR yields a sensor system that does not require analyte-free air for a baseline correction (i.e., estimation of R₀) and recovery of response. The concept presented here is generally applicable and offers a solution for several issues that are inherently present in chemiresistive sensing systems.

Trace Ethylene Sensing via Wacker Oxidation

Ethylene is a dynamic plant hormone, and its temporal monitoring can be used to glean insight into plant health and status. However, the real-time distributed detection of ethylene at trace levels under ambient conditions remains a challenge. We report a single-walled carbon nanotube-based chemiresistor catalyst combination that can detect ppb levels of ethylene in air. Cycling between Pd(II) and Pd(0) via Wacker oxidation with a nitrite cocatalyst imparts response discrimination driven by the chemoselectivity of the chemical transformation. Sensitivity is controlled by a combination of the chemical reaction efficiency and the n-doping strength of the Pd(0) species generated in situ. The covalent functionalization of the carbon nanotube sidewall with pyridyl ligands drastically improves the device sensitivity via enhanced n-doping. The utility of this ethylene sensor is demonstrated in the monitoring of senescence in red carnations and purple lisianthus flowers.