Nanoparticle cluster gas sensor: Pt activated SnO2 nanoparticles for NH3 detection with ultrahigh sensitivity

Adsorption of Cu Nanoparticles on Polystyrene-Based Microspheres

Nanoparticle composite microspheres are a versatile material with unique features and wide-ranging applications, including catalysis, biological medicine, and electronic devices. The adsorption behavior of nanoparticles on the surface of microspheres plays a crucial role in determining the further application potentials. The understanding of nanoparticle adsorption behavior on microsphere surfaces is essential for guiding future applications in nanoparticle composite microspheres. In this work, the adsorption behavior of unstable copper nanoparticles (Cu NPs) on polystyrene-based (PS-based) microspheres was investigated. The influence of PS-based microspheres' surface properties and the oxidation degree of Cu NPs were determined. The adsorption mechanism of Cu NPs on PS-based microspheres was analyzed. Furthermore, the amounts and rates of adsorption were examined. It was found that the Cu NPs can be rapidly and firmly adsorbed on the surface of carboxyl-modified polystyrene microspheres. Additionally, precise control over the distribution of Cu NPs on the surface of PS-based microspheres can be achieved by manipulating the solvent's polarity.

Sensing nature's alarm: SnO2/MXene gas sensor unveils methyl jasmonate signatures of plant insect stress

The incorporation of artificial intelligence into agriculture presents challenges, particularly due to hardware limitations, especially in sensors. Currently, pest detection relies heavily on manual scouting by humans. Therefore, the objective of this study is to create a chemoresistive sensor that enables early identification of the characteristic volatile compound, viz., methyl jasmonate, released during pest infestations. Given the lower reactivity of esters, we have fine-tuned a composite consisting of SnO2 nanoparticles and 2D-MXene sheets to enhance adsorption and selective oxidation, resulting in heightened sensitivity. The optimized composite demonstrated a notable response even at concentrations as low as 120 ppb, successfully confirming pest infestations in tomato crops.

Fabrication of Yolk-Shell Structure with Multifarious Nanoparticles via Double-Layered Encapsulation Strategy

The post-encapsulation method (such as single-layered encapsulation) is a promising strategy to synthesize yolk-shell structures that protect functional nanoparticles by the molecular sieving effect. However, this method exhibited limited loading capacity and nonuniform encapsulation during the co-encapsulation of various nanoparticles owing to the insufficient surface area for nanoparticle attachment. To address these limitations, we proposed a double-layered encapsulation method comprising an increased number of silica template layers and separate attachment of multifarious nanoparticles to different layers. Compared with conventional methods, this strategy can precisely adjust the ratio of encapsulated nanoparticles and increase the loading amount, which improves the functionality of yolk-shell structures, such as the photothermal properties of gold nanoparticle-encapsulated yolk-shell structures (∼69%). We describe, for the first time, the precise control of the ratio of encapsulated nanoparticles and the loading of numerous nanoparticles. Consequently, this strategy has significant potential for various applications of yolk-shell structures.

Controllable Synthesis of Sheet-Flower ZnO for Low Temperature NO2 Sensor

ZnO is a wide band gap semiconductor metal oxide that not only has excellent electrical properties but also shows excellent gas-sensitive properties and is a promising material for the development of NO₂ sensors. However, the current ZnO-based gas sensors usually operate at high temperatures, which greatly increases the energy consumption of the sensors and is not conducive to practical applications. Therefore, there is a need to improve the gas sensitivity and practicality of ZnO-based gas sensors. In this study, three-dimensional sheet-flower ZnO was successfully synthesized at 60 °C by a simple water bath method and modulated by different malic acid concentrations. The phase formation, surface morphology, and elemental composition of the prepared samples were studied by various characterization techniques. The gas sensor based on sheet-flower ZnO has a high response value to NO₂ without any modification. The optimal operating temperature is 125 °C, and the response value to 1 ppm NO₂ is 125. At the same time, the sensor also has a lower detection limit (100 ppb), good selectivity, and good stability, showing excellent sensing performance. In the future, water bath-based methods are expected to prepare other metal oxide materials with unique structures.

Nanomaterial Gas Sensors for Biosensing Applications: A Review

BACKGROUND

Nanomaterial is one of the most used materials for various gas sensing applications to detect toxic gases, human breath, and other specific gas sensing. One of the most important applications of nanomaterial based gas sensors is biosensing applications. In this review article, the gas sensors for biosensing are discussed on the basis of crystalline structure and different categories of nanomaterial.

METHODS

In this paper, firstly, rigorous efforts have been made to find out research questions by going through a structured and systematic survey of available peer reviewed high quality articles in this field. The papers related to nanomaterial based biosensors are then reviewed qualitatively to provide substantive findings from the recent developments in this field.

RESULTS

In this mini-review article, firstly, classifications of nanomaterial gas sensors have been presented on the basis of the crystalline structure of nanomaterial and different types of nanomaterial available for biosensing applications. Further, the gas sensors based on nanomaterial for biosensing applications are collected and reviewed in terms of their performance parameters such as sensing material used, target gas component, detection ranges (ppm-ppb), response time, operating temperature and method of detection, etc. The different nanomaterials possess slightly different sensing and morphological properties due to their structure; therefore, it can be said that a nanomaterial must be selected carefully for a particular application. The 1D nanomaterials show the best selectivity and sensitivity for gases available in low concentration ranges due to their miniaturised structure compared to 2D and 3D nanomaterials. However, these 2D and 3D nanomaterials also so good sensing properties compared to bulk semiconductor materials. The polymer and nanocomposites which are also discussed in this patent article have opened the door for future research and have great potential for new generation gas sensors for detecting biomolecules.

CONCLUSION

These nanomaterials extend great properties towards sensing the application of different gases for a lower concentration of particular gas particles. Nano polymer and nanocomposites have great potential to be used as gas sensors for the detection of biomolecules.

Comparative Study on Gas-Sensing Properties of 2D (MoS2, WS2)/PANI Nanocomposites-Based Sensor

NH₃ is a highly harmful gas; when inhaled at levels that are too high for comfort, it is very dangerous to human health. One of the challenging tasks in research is developing ammonia sensors that operate at room temperature. In this study, we proposed a new design of an NH₃ gas sensor that was comprised of two-dimensional (TMDs, mainly WS₂ and MoS₂) and PANI. The 2D-TMDs metal was successfully incorporated into the PANI lattice based on the results of XRD and SEM. The elemental EDX analysis results indicated that C, N, O, W, S and Mo were found in the composite samples. The bandgap of the materials decreased due to the addition of MoS₂ and WS₂. We also analyzed its structural, optical and morphological properties. When compared to MoS₂ and PANI, the proposed NH₃ sensor with the WS₂ composite was found to have high sensitivity. The composite films also exhibited response and recovery times of 10/16 and 14/16 s. Therefore, the composite PANI/2D-TMDs is a suitable material for NH₃ gas detection applications.

A Review on Polyaniline: Synthesis, Properties, Nanocomposites, and Electrochemical Applications

The development in the use of polyaniline (PANI) in advanced studies makes us draw attention to the presented research and combine it into one study like this one. The unique composition of PANI qualifies it for use in electrochemical applications in addition to many other applications whose use depends on its mechanical properties. Based on this, it is necessary to limit the reactions that produce PANI and the cheapest cost, and then limit the current uses in the formation of nanocomposites with metals, their oxides, and/or carbon nanocomposites in order to determine what is missing from them and work on it again to expand its chemistry. The development in the use of PANI in advanced studies makes us draw attention to the research presented on PANI and combine it into one study. One of the very important things that made PANI possess a very huge research revolution are preparation in a variety of ways, easy and inexpensive, from which a daily product can be obtained with very high purity, as well as its distinctive properties that made it the focus of researchers in various scientific departments. The unique structure of PANI, which is easy to prepare in its pure form or with various chemical compounds including metals, metal oxides, and carbon nanomaterials (such as carbon nanotubes, graphene, graphene oxide, and reduced graphene oxide), qualifies it for use in electrochemical applications. The various studies reviewed showed that PANI gave good results in the applications of super capacitors. In some of the studies mentioned later, it gave a specific capacitance of 503 F/g, cycle stability 85% at 10,000 cycles, energy density 8.88 kW/kg, and power density 96 W h/kg. It was also noted that these values improved significantly when using PANI with its nanocomposites. Because of its good electrical conductivity and the possibility of preparing it with a high surface area with nanostructures in the form of nanowires, nanofibers, and nanotubes, PANI was used as a gas sensor. We have noticed, through the studies conducted in this field, that the properties of PANI as a basic material in gas sensors are greatly improved when it is prepared in the form of PANI nanocomposites, as explained in detail later. From this review, we tried with great effort to shed light on this attractive polymer in terms of its different preparation methods, its distinctive properties, its nanocomposites, and the type of polymerization used for each nanocomposites, as well as its applications in its pure form or with its nanocomposites in the supercapacitor and gas sensor applications.

High Sensing Performance Toward Acetone Vapor Using TiO2 Flower-Like Nanomaterials

For real-application gas sensors, high performances (response, selectivity, response/recovery time and stability) are demanded. An effective strategy is applying nanomaterials in gas sensors. In this study, the anatase TiO₂ flower-like nanomaterials (FLNMs) are prepared through a one-step hydrothermal method which exhibit high-performance toward acetone vapor. TiO₂ FLNMs sensors property are characterized at optimal working temperature of 330 °C with selectivity (acetone), response (S = 33.72 toward 250 ppm acetone), linear dependence (R² = 0.9913), response/recovery time (46/24 s toward 250 ppm acetone) and long-term stability (30 days). These demonstrate that TiO₂ FLNMs get a high performance for acetone sensor. Moreover, the limit of detection of acetone is 0.65 ppm which is lower than that of exhaled air for diabetes (0.8 ppm), indicating that TiO₂ FLNMs gas sensor gets potential application in medical diagnosis.

Ultrasensitive Formaldehyde Sensor Based on SnO2 with Rich Adsorbed Oxygen Derived from a Metal Organic Framework

SnO₂ has been a commonly researched gas-sensing material due to its low cost, good performance, and good stability. However, gas sensors based on pure SnO₂ usually show a low response or high working temperature. In this work, laminar SnO₂ was obtained by using a Sn-based metal organic framework(Sn-MOF)@SnO₂ as a precursor. Sn-MOF@SnO₂ is prepared at low temperatures using water and dimethylformamide as a solvent, which is simple, low cost, and easily reproducible. After sintering, Sn-MOF@SnO₂ is derived to SnO₂ with rich adsorbed oxygen, large specific surface area, and unique nanoparticle piled pores, thus showing excellent gas-sensing properties. The prepared SnO₂ has an ultrahigh response value of 10,000 to 10 ppm formaldehyde at an optimal working temperature of 120 °C, a fast response/recovery time of 33 s/142 s, and an actual detection limit of lower than 10 ppb as well as high selectivity and high stability. Density functional theory calculations show that the exposed (110) plane of oxygen-rich vacancies in laminar SnO₂ can effectively increase the coadsorption capacity of O₂ and formaldehyde molecules, thereby improving the formaldehyde gas-sensing performance of the material. The present original approach paves the way to design advanced materials with excellent gas-sensing properties as well as broad application prospects in formaldehyde monitoring.

Heteronanostructural metal oxide-based gas microsensors

The development of high-performance, portable and miniaturized gas sensors has aroused increasing interest in the fields of environmental monitoring, security, medical diagnosis, and agriculture. Among different detection tools, metal oxide semiconductor (MOS)-based chemiresistive gas sensors are the most popular choice in commercial applications and have the advantages of high stability, low cost, and high sensitivity. One of the most important ways to further enhance the sensor performance is to construct MOS-based nanoscale heterojunctions (heteronanostructural MOSs) from MOS nanomaterials. However, the sensing mechanism of heteronanostructural MOS-based sensors is different from that of single MOS-based gas sensors in that it is fairly complex. The performance of the sensors is influenced by various parameters, including the physical and chemical properties of the sensing materials (e.g., grain size, density of defects, and oxygen vacancies of materials), working temperatures, and device structures. This review introduces several concepts in the design of high-performance gas sensors by analyzing the sensing mechanism of heteronanostructural MOS-based sensors. In addition, the influence of the geometric device structure determined by the interconnection between the sensing materials and the working electrodes is discussed. To systematically investigate the sensing behavior of the sensor, the general sensing mechanism of three typical types of geometric device structures based on different heteronanostructural materials are introduced and discussed in this review. This review will provide guidelines for readers studying the sensing mechanism of gas sensors and designing high-performance gas sensors in the future.

Emerging single atom catalysts in gas sensors

Single atom catalysts (SACs) offer unprecedented opportunities for high-efficiency reactions taking place in many important fields of catalytic processes, electrochemistry, and photoreactions. Due to their maximized atomic utilization and unique electronic and chemical properties, SACs can provide high activity and excellent selectivity for gas adsorption and electron transport, leveraging SACs that enhance the detection sensitivity and selectivity to target gases. In the past few years, SACs including both noble (Pt, Pd, Au, etc.) and non-noble (Mn, Ni, Zn etc.) metals have been demonstrated to be very useful in optimizing sensing performances. However, a comprehensive review on this topic is still missing. Herein, we summarize the synthesis technologies of SACs that are applicable to gas sensors. The electronic and chemical interactions between SACs and host sensing materials, which are crucial to sensor functions, are discussed. Then, we highlight the application progress of various SACs in gas sensors. Prospects in the creation of new sensing materials with emerging SACs and versatile supports are also present. Finally, the challenges and prospects of SACs in the future development of sensors are analyzed.

Understanding the Increasing Trend of Sensor Signal with Decreasing Oxygen Partial Pressure by a Sensing-Reaction Model Based on O2- Species

Although the increasing trend of sensor signal with decreasing oxygen partial pressure was observed quite early, the underlying mechanism is still elusive, which is a hindrance to accurate gas detection under varying oxygen partial pressure. In this work, a sensing model based on previous experimental and theoretical results is proposed, in which the O^2- species is determined to be the main oxygen species because O^- species has not been observed by direct spectroscopic studies. On this basis, combined with the band bending of SnO₂ at different oxygen partial pressures, the functional relationship between the surface electron concentration, oxygen partial pressure, and reducing gas concentration is established, which includes three forms corresponding to the depletion layer, accumulation layer, and flat band. In the depletion layer case, the variation of the sensor resistance to different concentrations of CO and oxygen can be well fitted with our function model. Besides, this model predicts that the response of sensors will no longer maintain the increasing trend in an extremely hypoxic atmosphere but will decrease and approach 1 with the background oxygen content further going down to 0.

Superior acetone sensor based on hetero-interface of SnSe2/SnO2 quasi core shell nanoparticles for previewing diabetes

To improve gas sensing performance of SnO₂ sensor, a heterostructure constructed by SnO₂ and SnSe₂ is designed and synthesized via hydrothermal method and post thermal oxidation treatment. The obtained SnSe₂/SnO₂ composite nanoparticles demonstrate a special core-shell structure with SnO₂ nanograins distributed in the shell and mixed SnSe₂ and SnO₂ nanograins in the core. Owning to the promoted charge transfer effect invited by SnSe₂, the sensor based on SnSe₂/SnO₂ composite nanoparticles exhibit expressively enhanced acetone sensing performance compared to the pristine SnO₂ sensor. At the working temperature of 300 °C, the SnSe₂/SnO₂ composite sensor with optimized composition exhibits superior sensing property towards acetone, including high response (10.77-100 ppm), low theoretical limit of detection (0.354 ppm), high selectivity and good reproducibility. Moreover, the sensor shows a satisfactory sensing performance in trace acetone gas detection under high humidity condition (relative humidity: 70-90%), making it a promising candidate to constructing exhaled breath sensors for acetone detection.

Recent Progress in Metal-Organic Framework Based Fluorescent Sensors for Hazardous Materials Detection

Population growth and industrial development have exacerbated environmental pollution of both land and aquatic environments with toxic and harmful materials. Luminescence-based chemical sensors crafted for specific hazardous substances operate on host-guest interactions, leading to the detection of target molecules down to the nanomolar range. Particularly, the luminescence-based sensors constructed on the basis of metal-organic frameworks (MOFs) are of increasing interest, as they can not only compensate for the shortcomings of traditional detection techniques, but also can provide more sensitive detection for analytes. Recent years have seen MOFs-based fluorescent sensors show outstanding advantages in the field of hazardous substance identification and detection. Here, we critically discuss the application of MOFs for the detection of a broad scope of hazardous substances, including hazardous gases, heavy metal ions, radioactive ions, antibiotics, pesticides, nitro-explosives, and some harmful solvents as well as luminous and sensing mechanisms of MOF-based fluorescent sensors. The outlook and several crucial issues of this area are also discussed, with the expectation that it may help arouse widespread attention on exploring fluorescent MOFs (LMOFs) in potential sensing applications.

Highly sensitive gas sensor based on a parity-time-symmetric system

Achieving extremely high sensitivity is an important indicator in the development of novel and stable gas concentration sensors. In this paper, we present a gas concentration sensor with parity-time symmetry for high sensitivity at low concentrations. The proposed sensor can detect toxic gases, such as benzene, bromine, and acetone, by probing the faint changing of the permittivity. Furthermore, the level of the sensitivity can be adjusted by the resistance segment, which is realized by various metallic formations. Our proposed structure provides a novel idea for the development of future gas concentration sensors, showing an exciting prospect for gas sensing technologies.

Formaldehyde gas sensor with extremely high response employing cobalt-doped SnO2 ultrafine nanoparticles

Formaldehyde is a common carcinogen in daily life and harmful to health. The detection of formaldehyde by a metal oxide semiconductor gas sensor is an important research direction. In this work, cobalt-doped SnO₂ nanoparticles (Co-SnO₂ NPs) with typical zero-dimensional structure were synthesized by a simple hydrothermal method. At the optimal temperature, the selectivity and response of 0.5% Co-doped SnO₂ to formaldehyde are excellent (for 30 ppm formaldehyde, R _a/R _g = 163 437). Furthermore, the actual minimum detectable concentration of 0.5%Co-SnO₂ NPs is as low as 40 ppb, which exceeds the requirements for formaldehyde detection in the World Health Organization (WHO) guidelines. The significant improvement of 0.5%Co-SnO₂ NPs gas performance can be attributed to the following aspects: firstly, cobalt doping effectively improves the resistance of SnO₂ NPs in the air; moreover, doping creates more defects and oxygen vacancies, which is conducive to the adsorption and desorption of gases. In addition, the crystal size of SnO₂ NPs is vastly small and has unique physical and chemical properties of zero-dimensional materials. At the same time, compared with other gases tested, formaldehyde has a strong reducibility, so that it can be selectively detected at a lower temperature.

Noninvasive Detection of Ammonia in the Breath of Hemodialysis Patients Using a Highly Sensitive Ammonia Sensor Based on a Polypyrrole/Sulfonated Graphene Nanocomposite

In this work, we fabricated fast-responsive and highly sensitive chemiresistive sensors based on nanocomposites of polypyrrole and graphitic materials such as graphene oxide (GO), reduced graphene oxide (RGO), and sulfonated graphene (SRGO) by an in situ chemical oxidative polymerization method. The synthesized nanocomposites were characterized using field emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), Fourier transform infrared (FT-IR) spectroscopy, and X-ray diffraction (XRD). The effects of the operating temperature of different nanocomposites were investigated at four temperatures (28, 40, 50, and 60 °C), and the results were compared with that of the polypyrrole-based sensor. The experimental results for sensors indicate that the proposed PPy/SRGO sensor could be an appropriate choice for NH₃ detection at 28 °C in the range of 0.50 parts per billion (ppb) to 12 parts per million (ppm). The PPy/SRGO nanocomposite gas sensor exhibited fast responsivity, good repeatability, and high chemical selectivity to low-concentration ammonia against humidity, methanol, ethanol, acetone, formaldehyde, dibutylamine, dimethylamine, methylamine, carbon monoxide, and nitrogen oxide at 28 °C. We utilized the PPy/SRGO sensor for studying the variation of the ammonia concentration in hemodialysis (HD) patients' breath before and after dialysis and correlated it with the blood urea nitrogen (BUN) levels. The results of the PPy/SRGO sensor indicated that the breath ammonia concentration significantly decreased after dialysis in agreement with BUN. The results demonstrate the potential application of the PPy/SRGO sensor for noninvasive detection of ammonia in breath and make this type of sensor a promising tool for the diagnosis of renal and liver diseases.

CsPbI3NC-Sensitized SnO2/Multiple-Walled Carbon Nanotube Self-Assembled Nanomaterials with Highly Selective and Sensitive NH3 Sensing Performance at Room Temperature

It is an effective strategy to enhance the sensitivity of semiconductor metal oxides (SMOs) being sensitized with CsPbI₃ nanocrystals (NCs) by adjusting the heterostructure between CsPbI₃NC and SMO nanomaterials. In this work, for the first time, a porous 3D multiple-walled carbon nanotube (MWCNT) network uniformly coated with SnO₂ quantum nanoparticles (QNPs) and CsPbI₃ nanocrystals were prepared via a simple solvent vapor-induced self-assembly method. The fabricated CsPbI₃NC-SnO₂QNP/MWCNT nanocomposite with vapor-induced self-assembly exhibits superior stability against the moisture as well as an excellent sensing response. The results imply that the rational design of the metal halide perovskite NC/SMO heterostructure can not only improve the stability but also meet the requirements of sensing application. The self-assembled SnO₂QNP/MWCNT can facilitate the dispersion of small-sized nanoparticles and efficaciously prevent the detachment of CsPbI₃NC. Compared with pristine SnO₂QNP and SnO₂/MWCNT sensors, the CsPbI₃NC-modified SnO₂QNP/MWCNT nanostructure exhibited a remarkable sensitivity of 39.2 for 0.2 ppm NH₃, rapid response/recovery time of 17/18 s, and excellent selectivity towards NH₃. In particular, we applied machine learning methods, including principal component analysis (PCA) and support vector machines (SVMs), to analyze the sensing performance of the CsPbI₃NC-SnO₂QNP/MWCNT sensor and found that the combined effects of CsPbI₃NC-SnO₂QNP/MWCNT heterointerfaces contributed to the improvement of selectivity of sensors. The excellent NH₃ for sub-ppm level concentration is ascribed to the high sensing activity of the CsPbI₃ NC-based heterojunction. This work may not only enrich the family of high-performance breath detection materials but also provide a good example for designing reasonable composite materials with specific properties in the field of metal halide perovskite/SMO heterojunctions.

Mesoporous WO3 modified by Au nanoparticles for enhanced trimethylamine gas sensing properties

Au-Doped mesoporous WO₃ is prepared and it exhibited higher response to TMA gas.

Construction of PdO-decorated double-shell ZnSnO3 hollow microspheres for n-propanol detection at low temperature

The sensor based on 4 wt% PdO-loaded double-shell ZnSnO3 hollow microspheres shows rapid response/recovery speed to n-propanol at low working temperature.

Room-temperature NH3 gas sensor based on atomically dispersed Co with a simple structure

Atomically dispersed Co was first used as an NH₃ gas sensor in this work, which realized stable and repeatable gas sensing performance at room temperature. A mechanism was proposed to explain the sensing behavior based on atomically dispersed Co.

Construction of hierarchical trimetallic organic framework leaf-like nanostructures derived from carbon nanotubes for gas-sensing applications

Unique trimetallic organic material (TMOM)-based nanostructures combined with the new architectures of metal-organic frameworks (MOFs) are promising candidates for gas-sensing applications. This work is the first to successfully convert MOF nanomaterials into nano-porous carbon through carbon nanotubes (CNT) catalytic reaction via a simple and facile hydrothermal method. The leaf-like nanostructures exhibit a high surface-to-volume ratio of 363 m² g^-1. The TMOM nanostructures were subsequently exposed to different types of target gases for a wide range of gas concentrations at different operating temperatures. The carbon nanotubes (TMOM-CNT) hybrid nanocomposites were characterized using X-ray powder diffraction, X-ray photoelectron spectroscopy, Brunauer-Emmett-Teller, scanning electron microscopy, energy dispersion spectrum analysis, thermo-gravimetric analysis, and transmission electron microscopy. The fabricated Zn-Co-Ni MOF@CNT sensors exhibit high selectivity and gas-sensing response toward H₂S gas at an optimal temperature of 325 °C for 100 ppm. These superior gas-sensing performances reveal that the Zn-Co-Ni MOF@CNT sensors with a unique leaf shape exhibit potential applications for the environment applications in gas sensor industry.

Rational Design of Semiconductor-Based Chemiresistors and their Libraries for Next-Generation Artificial Olfaction

Artificial olfaction based on gas sensor arrays aims to substitute for, support, and surpass human olfaction. Like mammalian olfaction, a larger number of sensors and more signal processing are crucial for strengthening artificial olfaction. Due to rapid progress in computing capabilities and machine-learning algorithms, on-demand high-performance artificial olfaction that can eclipse human olfaction becomes inevitable once diverse and versatile gas sensing materials are provided. Here, rational strategies to design a myriad of different semiconductor-based chemiresistors and to grow gas sensing libraries enough to identify a wide range of odors and gases are reviewed, discussed, and suggested. Key approaches include the use of p-type oxide semiconductors, multinary perovskite and spinel oxides, carbon-based materials, metal chalcogenides, their heterostructures, as well as heterocomposites as distinctive sensing materials, the utilization of bilayer sensor design, the design of robust sensing materials, and the high-throughput screening of sensing materials. In addition, the state-of-the-art and key issues in the implementation of electronic noses are discussed. Finally, a perspective on chemiresistive sensing materials for next-generation artificial olfaction is provided.

NH3 Sensor Based on 2D Wormlike Polypyrrole/Graphene Heterostructures for a Self-Powered Integrated System

The rapid development of a NH₃ sensor puts forward a great challenge for active materials and integrated sensing systems. In this work, an ultrasensitive NH₃ sensor based on two-dimensional (2D) wormlike mesoporous polypyrrole/reduced graphene oxide (w-mPPy@rGO) heterostructures, synthesized by a universal soft template method is reported, revealing the structure-property coupling effect of the w-mPPy/rGO heterostructure for sensing performance improvement, and demonstrates great potential in the integration of a self-powered sensor system. Remarkably, the 2D w-mPPy@rGO heterostructrure exhibits preferable response toward NH₃ (ΔR/R₀ = 45% for 10 ppm NH₃ with a detection limit of 41 ppb) than those of the spherical mesoporous hybrid (s-mPPy@rGO) and the nonporous hybrid (n-PPy@rGO) due to its large specific surface area (193 m²/g), which guarantees fast gas diffusion and transport of carriers. Moreover, the w-mPPy@rGO heterostructures display outstanding selectivity to common volatile organic compounds (VOCs), H₂S, and CO, prominent antihumidity inteference superior to most existing chemosensors, superior reversibility and favorable repeatability, providing high potential for practicability. Thus, a self-powered sensor system composed of a nanogenerator, a lithium-ion battery, and a w-mPPy@rGO-based sensor was fabricated to realize wireless, portable, cost-effective, and light-weight NH₃ monitoring. Impressively, our self-powered sensor system exhibits high response toward 5-40 mg NH₄NO₃, which is a common explosive to generate NH₃ via alkaline hydrolysis, rendering it a highly prospective technique in a NH₃-based sensing field.

Efficient NH3 Detection Based on MOS Sensors Coupled with Catalytic Conversion

A solution of NH₃ detection based on catalytic conversion of NH₃ into NO_x was proposed by using MOS gas detectors and Pt-supported catalysts. The catalysts convert NH₃ into NO_x, which is a very sensitive analyte for MOS detectors. Catalysts based on Pt-loaded HZSM-5 and Al₂O₃ were prepared by wet impregnation. MOS detectors were fabricated from nanosized In₂O₃ and WO₃ using screen-printing techniques. As expected, MOS sensors based on In₂O₃ and WO₃ have an extremely high sensitivity to NO₂; nevertheless, they have a relatively low response to NH₃ and a large cross-sensitivity to typical interfering gases such as CO and ethanol. By the present solution, MOS sensors could very sensitively respond to NH₃, even down to 0.25 ppm. In addition, it was also found that the catalysis also combusts the reducing gases into CO₂ and water and consequently significantly improves the selectivity of NH₃. Lastly, we would to like to stress that the proposed concept of the catalytic conversion method suggests the potential utility for broader measurements by using different catalysts and gas detectors and that only a part of the usage for NH₃ was presented here.

Controlling Pt co-catalyst loading in a WO3 quantum dot and MoS2 nanosheet composite Z-scheme system for enhanced photocatalytic H2 evolution

A solid-state Z-scheme system, with the synergistic integration of the advantages of various narrow-band semiconductors, is considered to be a potential strategy to develop efficient photocatalysts for operation under visible light illumination. However, the charge separation efficiency of these systems has always been reduced by disordered electron transfer between coupling semiconductors. In this work, we constructed a direct Z-scheme system WO₃-MoS₂-Pt through the loading of WO₃ quantum dots onto MoS₂ nanosheets and the selective depositing of a Pt co-catalyst onto MoS₂. X-ray diffraction, transmission electron microscopy, atomic electron microscopy and x-ray photoelectron spectroscopy, etc were used to confirm the successful preparation of the targeted photocatalyst. This photocatalytic system showed high visible-light-driven H₂ evolution activity (802.2 μmol · h^-1 · g^-1) and good photostability. Control experiment and mechanism analysis suggested that the remarkable performance can be attributed to the heterojunction formed WO₃ and MoS₂ and the vectorial electron transfer (WO₃ → MoS₂ → Pt) achieved by selectively loading the Pt co-catalyst.

Hydrogel-Based Gas Sensors for NO2 and NH3

In this study, an innovative gas sensing mechanism, self-responsive sensing mechanism, has been detected in the supramolecular hydrogel-based sensors. The self-responsive ability of as-fabricated hydrogel-based sensors to the target gas (e.g., NO₂, NH₃, etc.) is determined by three synergetic supramolecular interactions, namely, hydrogen bonding, molecule crystallization, and electrostatic interactions existing in hydroxyls, poly(vinyl alcohol) (PVA) crystallization, and poly(ionic liquids) of the intrinsic hydrogel networks, respectively. On account of unique synergetic supramolecular interactions, the sensors not only exhibit a rapid, reversible, and reproducible response but also show good tensile and compressive properties and excellent recovery property. The results demonstrate the potential of the self-responsive sensing mechanism as a pathway to realize a new generation of highly responsive hydrogel-based gas sensors.

Surface Modification and Charge Injection in a Nanocomposite Of Metal Nanoparticles and Semiconductor Oxide Nanostructures

Combining two materials in a nanoscale level can create a composite with new functionalities and improvements in their physical and chemical properties. Here we present a high-throughput approach to produce a nanocomposite consisting of metal nanoparticles and semiconductor oxide nanostructures. Volmer-Weber growth, though unfavorable for thin films, promotes nucleation of dense and isolated metal nanoparticles on crystalline oxide nanostructures, resulting in new material properties. We demonstrate such a growth of Au nanoparticles on SnO₂ nanostructures and a remarkable sensitivity of the nanocomposite for detecting traces of analytes in surface enhanced Raman spectroscopy. Au nanoparticles with tunable size enable us to modify surface wettability and convert hydrophilic oxide surfaces into super-hydrophobic with contact angles over 150°. We also find that charge injection through electron beam exposure shows the same effect as photo-induced charge separation, providing an extra Raman enhancement up to an order of magnitude.

A Highly Sensitive and Room Temperature CNTs/SnO2/CuO Sensor for H2S Gas Sensing Applications

Gas sensors based on tin dioxide-carbon nanotube composite films were fabricated by a simple inexpensive sol-gel spin-coating method using PEG400 as a solvent. Nanostructured copper was coated on CNTs/SnO₂ film, and then copper was transformed into copper oxide at 250 °C. Resistivity of the final composite films is highly sensitive to the presence of H₂S, which became easily attached or detached at room temperature. The response and recovery time of the sensor are 4 min and 10 min, and the value of sensitivity is 4.41, respectively. Meanwhile, the CNTs/SnO₂/CuO sensor also has low detection limit, high selectivity toward H₂S, and stable performance with different concentrations of H₂S.

ZnO twin-rods decorated with Pt nanoparticles for butanone detection

ZnO twin-rods were synthesized using a combination of the ultrasonic spray nozzle and microwave-assisted hydrothermal methods. The VOC detection test revealed that the decoration with 2% of Pt provides a more sensitive and selective butanone sensor.

Facile fabrication of shell crosslinked microcapsule by visible light induced graft polymerization for enzyme encapsulation

A strategy to encapsulate enzymes into microcapsule fabricated by visible light-induced graft polymerization using CaCO₃microparticles as template was developed.

Controlled Synthesis of Pt Doped SnO2 Mesoporous Hollow Nanospheres for Highly Selective and Rapidly Detection of 3-Hydroxy-2-Butanone Biomarker

Listeria monocytogenes (L. monocytogenes) has been recognized as one of the extremely hazardous and potentially life-threatening food-borne pathogens, its real-time monitoring is of great importance to human health. Herein, a simple and effective method based on platinum sensitized tin dioxide semiconductor gas sensors has been proposed for selective and rapid detection of L. monocytogenes. Pt doped SnO2 nanospheres with particular mesoporous hollow structure have been synthesized successfully through a robust and template-free approach and used for the detection of 3-hydroxy-2-butanone biomarker of L. monocytogenes. The steady crystal structure, unique micromorphology, good monodispersit, and large specific surface area of the obtained materials have been confirmed by X-ray diffraction (XRD), Raman spectroscopy, Scanning Electron Microscopy (SEM), Transmission Electron Microscopy (TEM), X-ray Photoelectron Spectroscopy (XPS), Brunauer-Emmett-Teller (BET), and Photoluminescence spectra (PL). Pt doped SnO2 mesoporous hollow nanosphere sensors reach the maximum response of 3-hydroxy-2-butanone at 250°C. Remarkably, sensors based on SnO2 mesoporous hollow nanospheres with 0.16 wt% Pt dopant exhibit excellent sensitivity (Rair/Rgas = 48.69) and short response/recovery time (11/20 s, respectively) to 10 ppm 3-hydroxy-2-butanone at the optimum working temperature. Moreover, 0.16 wt% Pt doped SnO2 gas sensors also present particularly low limit of detection (LOD = 0.5 ppm), superb long-term stability and prominent selectivity to 3-hydroxy-2-butanone. Such a gas sensor with high sensing performance foresees its tremendous application prospects for accurate and efficient detection of foodborne pathogens for the food security and public health.

Application of gas cyclone-liquid jet absorption separator for purification of tail gas containing ammonia

In this experiment, with stainless steel gas cyclone-liquid jet absorption separator as carrier, NH₃ as experimental gas, and water and H₃PO₄ solution as absorbents, corresponding NH₃ absorption rate change is obtained through the adjustment of experimental parameters, such as NH₃ inlet concentration, inlet velocity of mixed gas, injection flow rate of absorbent, temperature of absorbent, and H₃PO₄ absorbent concentration. The NH₃ absorption rate decreases with the increase in NH₃ inlet concentration and inlet gas velocity. The NH₃ absorption rate will increase first and then tends to remain unchanged after reaching a certain degree with the increase in liquid injection flow rate and absorbent concentration. The NH₃ absorption rate will increase first and then decrease with the increase in the absorbent temperature. The maximum NH₃ removal efficiencies of water and H₃PO₄ were 96% and 99%, respectively.

Ultrahigh Selective Room-Temperature Ammonia Gas Sensor Based on Tin-Titanium Dioxide/reduced Graphene/Carbon Nanotube Nanocomposites by the Solvothermal Method

Resistive-based gas sensors have been considered as the most favorable gas sensors for detection of toxic gases and volatile organic compounds (VOCs) because of their simple structure, low cost, high sensitivity, ease of use, and high stability. Unfortunately, wide application of resistive-based gas sensors is limited by their low selectivity. In this article, we present the fabrication of ultrahigh selective NH₃ gas sensor based on tin-titanium dioxide/reduced graphene/carbon nanotube (Sn-TiO₂@rGO/CNT) nanocomposites. The Sn-TiO₂@rGO/CNT nanocomposites with different molar ratios of Sn/Ti (1:10, 3:10, and 5:10) were synthesized via the solvothermal method. Characterizations by scanning electron microscopy, transmission electron microscopy, and X-ray photoelectron spectroscopy confirmed the decoration of Sn-TiO₂ nanoparticles on rGO/CNT nanocomposite surfaces. The Sn-TiO₂@rGO/CNT nanocomposite gas sensor exhibited high response and ultrahigh selectivity to NH₃ against toluene, dimethylformamide, acetone, ethanol, methanol, isopropanol, formaldehyde, hydrogen, carbon dioxide, acetylene, and VOCs in paint thinners at room temperature. The Sn-TiO₂@rGO/CNT nanocomposite gas sensor with molar ratio of Sn/Ti = 1:10 showed the highest response to NH₃ over other molar ratios of Sn/Ti as well as pure rGO/CNT and Sn-TiO₂ gas sensors. The ammonia-sensing mechanisms of the Sn-TiO₂@rGO/CNT gas sensor were proposed based on the formation of p-n heterojunctions of p-type rGO/CNT and n-type Sn-TiO₂ nanoparticles via a low-temperature oxidizing reaction process.