Designing oxide chemiresistors for detecting volatile aromatic compounds: recent progresses and future perspectives

Pd-W18O49 Nanowire MEMS Gas Sensor for Ultraselective Dual Detection of Hydrogen and Ammonia

Demand for real-time detection of hydrogen and ammonia, clean energy carriers, in a sensitive and selective manner, is growing rapidly for energy, industrial, and medical applications. Nevertheless, their selective detection still remains a challenge and requires the utilization of diverse sensors, hampering the miniaturization of sensor modules. Herein, a practical approach via material design and facile temperature modulation for dual selectivity is proposed. A Pd nanoparticles-decorated W18O49 nanowire gas sensor is prepared for dual detection of hydrogen and ammonia. The sensor exhibits distinct operating temperatures for ultraselective detection of hydrogen (125 °C) and ammonia (225 °C), with high responses of 35.3 and 133.8, respectively. This dual selectivity with high sensitivity is attributed to enhanced oxygen adsorption, the chemical affinity of sensing materials for target gases, and distinct reactivity profiles of gases. The proposed sensor is further integrated into a microelectromechanical system, enabling its small size, low power consumption, and rapid temperature modulation. Moreover, the practical feasibility of this sensor platform for smart energy monitoring systems is demonstrated by assessing its sensing properties in electrochemical ammonia oxidation reaction systems. This work can provide a practical approach for developing a single gas sensor with multiple functionalities for application in electronic nose systems.

Multiple-Yolk-Shell NiO Microspheres for Selective Detection of m-Xylene

m-Xylene is a volatile organic compound that is extensively used in various industrial processes. It is toxic, posing significant risks to human health and the environment. Therefore, developing gas sensors with high sensitivity and selectivity for m-xylene detection is critical. In this work, we demonstrated the synthesis of NiO-yolk double-shell (NiO-YDS) and NiO-yolk triple-shell (NiO-YTS) derived from NiO/Ni-BTC and NiO/Ni-PTA composites, respectively, using the microwave-assisted solvothermal method from Ni-BTC-derived NiO spheres. The NiO/Ni-BTC composite has trimesic acid (H3BTC) as an organic linker, while NiO/Ni-PTA has p-terephthalic acid (PTA). We investigated the sensing properties of these materials for 2-butanone, 2-nonanone, 3-methyl-1-butanol, acetone, benzene, ethanol, methanol, and m-xylene. These composites exhibited excellent sensitivity and selectivity for detecting m-xylene under dry conditions. Specifically, the NiO-YTS sensor showed a sensitivity of 217.5% to m-xylene, while the NiO-YDS sensor demonstrated a sensitivity of 179.8% at 350 °C in dry air. We emphasize the NiO-YTS composite due to its superior sensitivity and selectivity in detecting m-xylene compared with the NiO-YDS composite. The NiO-YTS sensor exhibited stable and reproducible sensing performance for 100 ppm of m-xylene under optimum working conditions, with a theoretical detection limit of 5.43 ppb and relatively fast response time (89 s) and recovery time (191 s). This work describes an easy method for synthesizing NiO-YDS and NiO-YTS derived from NiO/Ni-BTC and NiO/Ni-PTA composites. It demonstrates that these composites represent a new class of materials that can potentially enhance the sensitivity and selectivity of m-xylene gas sensors.

Highly Selective and Reversible Detection of Simulated Breath Hydrogen Sulfide Using Fe-Doped CuO Hollow Spheres: Enhanced Surface Redox Reaction by Multi-Valent Catalysts

The precise and reversible detection of hydrogen sulfide (H2S) at high humidity condition, a malodorous and harmful volatile sulfur compound, is essential for the self-assessment of oral diseases, halitosis, and asthma. However, the selective and reversible detection of trace concentrations of H2S (≈0.1 ppm) in high humidity conditions (exhaled breath) is challenging because of irreversible H2S adsorption/desorption at the surface of chemiresistors. The study reports the synthesis of Fe-doped CuO hollow spheres as H2S gas-sensing materials via spray pyrolysis. 4 at.% of Fe-doped CuO hollow spheres exhibit high selectivity (response ratio ≥ 34.4) over interference gas (ethanol, 1 ppm) and reversible sensing characteristics (100% recovery) to 0.1 ppm of H2S under high humidity (relative humidity 80%) at 175 °C. The effect of multi-valent transition metal ion doping into CuO on sensor reversibility is confirmed through the enhancement of recovery kinetics by doping 4 at.% of Ti- or Nb ions into CuO sensors. Mechanistic details of these excellent H2S sensing characteristics are also investigated by analyzing the redox reactions and the catalytic activity change of the Fe-doped CuO sensing materials. The selective and reversible detection of H2S using the Fe-doped CuO sensor suggested in this work opens a new possibility for halitosis self-monitoring.

Imparting Chemiresistor with Humidity-Independent Sensitivity toward Trace-Level Formaldehyde via Substitutional Doping Platinum Single Atom

The modification of metal oxides with noble metals is one of the most effective means of improving gas-sensing performance of chemiresistors, but it is often accompanied by unintended side effects such as sensor resistance increases up to unmeasurable levels. Herein, a carbonization-oxidation method is demonstrated using ultrasonic spray pyrolysis technique to realize platinum (Pt) single atom (SA) substitutional doping into SnO2 (named PtSA-SnO2). The substitutional doping strategy can obviously enhance gas-sensing properties, and meanwhile decrease sensor resistance by two orders of magnitude (decreased from ≈850 to ≈2 MΩ), which are attributed to the tuning of band gap and fermi-level position, efficient single atom catalysis, and the raising of adsorption capability of formaldehyde, as validated by the state-of-the-art characterizations, such as spherical aberration-corrected scanning transmission electron microscopy (Cs-corrected STEM), in situ diffuse reflectance infrared Fourier transformed spectra (in situ DRIFT), CO temperature-programmed reduction (CO-TPR), and theoretical calculations. As a proof of concept, the developed PtSA-SnO2 sensor shows humidity-independent (30-70% relative humidity) gas-sensing performance in the selective detection of formaldehyde with high response, distinguishable selectivity (8< Sformaldehyde/Sinterferant <14), and ultra-low detection limit (10 ppb). This work presents a generalized and facile method to design high-performance metal oxides for chemical sensing of volatile organic compounds (VOCs).

2D nanomaterials for realization of flexible and wearable gas sensors: A review

Gas sensors are extensively employed for monitoring and detection of hazardous gases and vapors. Many of them are produced on rigid substrates, but flexible and wearable gas sensors are needed for intriguing usage including the internet of things (IoT) and medical devices. The materials with the greatest potential for the fabrication of flexible and wearable gas sensing devices are two-dimensional (2D) semiconducting nanomaterials, which consist of graphene and its substitutes, transition metal dichalcogenides, and MXenes. These types of materials have good mechanical flexibility, high charge carrier mobility, a large area of surface, an abundance of defects and dangling bonds, and, in certain instances adequate transparency and ease of synthesis. In this review, we have addressed the different 2D nonmaterial properties for gas sensing in the context of fabrication of flexible/wearable gas sensors. We have discussed the sensing performance of flexible/wearable gas sensors in various forms such as pristine, composite and noble metal decorated. We believe that content of this review paper is greatly useful for the researchers working in the research area of fabrication of flexible/wearable gas sensors.

Data-centric artificial olfactory system based on the eigengraph

Recent studies of electronic nose system tend to waste significant amount of important data in odor identification. Until now, the sensitivity-oriented data composition has made it difficult to discover meaningful data to apply artificial intelligence in terms of in-depth analysis for odor attributes specifying the identities of gas molecules, ultimately resulting in hindering the advancement of the artificial olfactory technology. Here, we realize a data-centric approach to implement standardized artificial olfactory systems inspired by human olfactory mechanisms by formally defining and utilizing the concept of Eigengraph in electrochemisty. The implicit odor attributes of the eigengraphs were mathematically substantialized as the Fourier transform-based Mel-Frequency Cepstral Coefficient feature vectors. Their effectiveness and applicability in deep learning processes for gas classification have been clearly demonstrated through experiments on complex mixed gases and automobile exhaust gases. We suggest that our findings can be widely applied as source technologies to develop standardized artificial olfactory systems.

Multi-Functional Applications of H-Glass Embedded with Stable Plasmonic Gold Nanoislands

Metal nanoparticles (MNPs) are synthesized using various techniques on diverse substrates that significantly impact their properties. However, among the substrate materials investigated, the major challenge is the stability of MNPs due to their poor adhesion to the substrate. Herein, it is demonstrated how a newly developed H-glass can concurrently stabilize plasmonic gold nanoislands (GNIs) and offer multifunctional applications. The GNIs on the H-glass are synthesized using a simple yet, robust thermal dewetting process. The H-glass embedded with GNIs demonstrates versatility in its applications, such as i) acting as a room temperature chemiresistive gas sensor (70% response for NO2 gas); ii) serving as substrates for surface-enhanced Raman spectroscopy for the identifications of Nile blue (dye) and picric acid (explosive) analytes down to nanomolar concentrations with enhancement factors of 4.8 × 106 and 6.1 × 105 , respectively; and iii) functioning as a nonlinear optical saturable absorber with a saturation intensity of 18.36 × 1015 W m-2 at 600 nm, and the performance characteristics are on par with those of materials reported in the existing literature. This work establishes a facile strategy to develop advanced materials by depositing metal nanoislands on glass for various functional applications.

Integrated portable food safety testing pipette based on a color-switchable fluorescence probe for rapid visual discrimination of mild food deterioration

A sensitive, portable, easy-to-operate, directly-readable food freshness monitoring device has been developed for rapid visual identification of mild food spoilage.

Exclusive detection of volatile aromatic hydrocarbons using bilayer oxide chemiresistors with catalytic overlayers

The accurate detection and identification of volatile aromatic hydrocarbons, which are highly toxic pollutants, are essential for assessing indoor and outdoor air qualities and protecting humans from their sources. However, real-time and on-site monitoring of aromatic hydrocarbons has been limited by insufficient sensor selectivity. Addressing the issue, bilayer oxide chemiresistors are developed using Rh-SnO₂ gas-sensing films and catalytic CeO₂ overlayers for rapidly and cost-effectively detecting traces of aromatic hydrocarbons in a highly discriminative and quantitative manner, even in gas mixtures. The sensing mechanism underlying the exceptional performance of bilayer sensor is systematically elucidated in relation to oxidative filtering of interferants by the CeO₂ overlayer. Moreover, CeO₂-induced selective detection is validated using SnO₂, Pt-SnO₂, Au-SnO₂, In₂O₃, Rh-In₂O₃, Au-In₂O₃, WO₃, and ZnO sensors. Furthermore, sensor arrays are employed to enable pattern recognition capable of discriminating between aromatic gases and non-aromatic interferants and quantifying volatile aromatic hydrocarbon classifications.

Acetone and Toluene Gas Sensing by WO3: Focusing on the Selectivity from First Principle Calculations

Sensitivity and selectivity are the two major parameters that should be optimized in chemiresistive devices with boosted performances towards Volatile Organic Compounds (VOCs). Notwithstanding a plethora of metal oxides/VOCs combinations that have been investigated so far, a close inspection based on theoretical models to provide guidelines to enhance sensors features has been scarcely explored. In this work, we measured experimentally the sensor response of a WO3 chemiresistor towards gaseous acetone and toluene, observing a two orders of magnitude higher signal for the former. In order to gain insight on the observed selectivity, Density Functional Theory was then adopted to elucidate how acetone and toluene molecules adsorption may perturb the electronic structure of WO3 due to electrostatic interactions with the surface and hybridization with its electronic structure. The results of acetone adsorption suggest the activation of the carbonyl group for reactions, while an overall lower charge redistribution on the surface and the molecule was observed for toluene. This, combined with acetone's higher binding energy, justifies the difference in the final responses. Notably, the presence of surface oxygen vacancies, characterizing the nanostructure of the oxide, influences the sensing performances.