Room-Temperature High-Performance H2S Sensor Based on Porous CuO Nanosheets Prepared by Hydrothermal Method

Cu/TiO2 adsorbents modified by air plasma for adsorption-oxidation of H2S

In this study, non-thermal plasma (NTP) was employed to modify the Cu/TiO2 adsorbent to efficiently purify H2S in low-temperature and micro-oxygen environments. The effects of Cu loading amounts and atmospheres of NTP treatment on the adsorption-oxidation performance of the adsorbents were investigated. The NTP modification successfully boosted the H2S removal capacity to varying degrees, and the optimized adsorbent treated by air plasma (Cu/TiO2-Air) attained the best H2S breakthrough capacity of 113.29 mg H2S/gadsorbent, which was almost 5 times higher than that of the adsorbent without NTP modification. Further studies demonstrated that the superior performance of Cu/TiO2-Air was attributed to increased mesoporous volume, more exposure of active sites (CuO) and functional groups (amino groups and hydroxyl groups), enhanced Ti-O-Cu interaction, and the favorable ratio of active oxygen species. Additionally, the X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) results indicated the main reason for the deactivation was the consumption of the active components (CuO) and the agglomeration of reaction products (CuS and SO42-) occupying the active sites on the surface and the inner pores of the adsorbents.

Efficient fluoride removal using nano MgO: mechanisms and performance evaluation

In this study, highly efficient fluoride removal of nano MgO was successfully synthesized using a simple hydrothermal precipitation method. Hexadecyl trimethyl ammonium bromide (CTMAB) was utilized as a surfactant. Its long-chain structure tightly wrapped around the precursor crystal of basic magnesium chloride, inhibiting the growth of precursor crystals, reducing their size, and improving crystal dispersion. This process enhanced the adsorption capacity of nano MgO for fluoride. The adsorption performance of nano MgO on fluoride was investigated. The results indicate that pseudo-second-order kinetics and the Langmuir isotherm model can describe the adsorption behavior for fluoride, with a maximum adsorption capacity of 122.47 mg/g. Methods such as XRD, SEM, XPS, and FTIR were employed to study the adsorption mechanisms of the adsorbent. Additionally, factors potentially affecting adsorption performance in practical applications, such as pH and competing ions, were examined. This study enhances our profound understanding of the defluorination effectiveness and mechanisms of nano MgO.

Recent Advances on Metal Oxide Based Sensors for Environmental Gas Pollutants Detection

Optimizing materials and associated structures for detecting various environmental gas pollutant concentrations has been a major challenge in environmental sensing technology. Semiconducting metal oxides (SMOs) fabricated at the nanoscale are a class of sensor technology in which metallic species are functionalized with various dopants to modify their chemiresistivity and crystalline scaffolding properties. Studies focused on recent advances of gas sensors utilizing metal oxide nanostructures with a special emphasis on the structure-surface property relationships of some typical n-type and p-type SMOs for efficient gas detection are presented. Strategies to enhance the gas sensor performances are also discussed. These oxide material sensors have several advantages such as ease of handling, portability, and doped-based SMO sensing detection ability of environmental gas pollutants at low temperatures. SMO sensors have displayed excellent sensitivity, selectivity, and robustness. In addition, the hybrid SMO sensors showed exceptional selectivity to some CWAs when irradiated with visible light while also displaying high reversibility and humidity independence. Results showed that TiO2 surfaces can sense 50 ppm SO2 in the presence of UV light and under operating temperatures of 298-473 K. Hybrid SMO displayed excellent gas sensing response. For example, a CuO-ZnO nanoparticle network of a 4:1 vol.% CuO/ZnO ratio exhibited responses three times greater than pure CuO sensors and six times greater than pure ZnO sensors toward H2S. This review provides a critical discussion of modified gas pollutant sensing capabilities of metal oxide nanoparticles under ambient conditions, focusing on reported results during the past two decades on gas pollutants sensing.

Detection of PPB-Level H2S Concentrations in Exhaled Breath Using Au Nanosheet Sensors with Small Variability, High Selectivity, and Long-Term Stability

The continuous monitoring of hydrogen sulfide (H2S) in exhaled breath enables the detection of health issues such as halitosis and gastrointestinal problems. However, H2S sensors with high selectivity and parts per billion-level detection capability, which are essential for breath analysis, and facile fabrication processes for their integration with other devices are lacking. In this study, we demonstrated Au nanosheet H2S sensors with high selectivity, ppb-level detection capability, and high uniformity by optimizing their fabrication processes: (1) insertion of titanium nitride (TiN) as an adhesion layer to prevent Au agglomeration on the oxide substrate and (2) N2 annealing to improve nanosheet crystallinity. The fabricated Au nanosheets successfully detected H2S at concentrations as low as 5.6 ppb, and the estimated limit of detection was 0.5 ppb, which is superior to that of the human nose (8-13 ppb). In addition, the sensors detected H2S in the exhaled breath of simulated patients at concentrations as low as 175 ppb while showing high selectivity against interfering molecules, such as H2, alcohols, and humidity. Since Au nanosheets with uniform sensor characteristics enable easy device integration, the proposed sensor will be useful for facile health checkups based on breath analysis upon its integration into mobile devices.

A first principles study of RbSnCl3 perovskite toward NH3, SO2, and NO gas sensing

The sensitivity of a RbSnCl3 perovskite 2D layer toward NH3, SO2, and NO toxic gases has been studied via DFT analysis. The tri-atomic layer of RbSnCl3 possessed a tetragonal symmetry with a band gap of 1.433 eV. The adsorption energies of RbSnCl3 for NH3, SO2 and NO are -0.09, -0.43, and -0.56 eV respectively with a recovery time ranging from 3.4 × 10-8 to 3.5 ms. RbSnCl3 is highly sensitive toward SO2 and NO compared to NH3. The adsorption of SO2 and NO results in a significant structural deformation and a semiconductor-to-metal transition of RbSnCl3 perovskite. A high absorption coefficient (>103 cm-1), excessive optical conductivity (>1014 s-1), and a very low reflectivity (<3%) make RbSnCl3 a potential candidate for numerous optoelectronic applications. A significant shift in optical responses is observed through SO2 and NO adsorption, which can enable identification of the adsorbed gases. The studied characteristics signify that RbSnCl3 can be a potential candidate for SO2 and NO detection.

Graphene-based chemiresistive gas sensors

Gas sensors allow the monitoring of the chemical environment of humans, which is often crucial for their wellbeing or even survival. Miniaturization, reversibility, and selectivity are some of the key challenges for serial use of chemical sensors. This tutorial review describes critical aspects when using nanomaterials as sensing substrates for the application in chemiresistive gas sensors. Graphene has been shown to be a promising candidate, as it allows gas sensors to be operated at room temperature, possibly saving large amounts of energy. In this review, an overview is given on the general mechanisms for gas-sensitive semiconducting materials and the implications of doping and functionalization on the sensing parameters of chemiresistive devices. It shows in detail how different challenges, like sensitivity, response time, reversibility and selectivity have been approached by material development and operation modes. In addition, perspectives from the area of data analysis and intelligent algorithms are presented, which can further enhance these sensors' usability in the field.

Ultrafast Response and High Selectivity of Diethylamine Gas Sensors at Room Temperature Using MOF-Derived 1D CuO Nano-Ellipsoids

Environmental and health monitoring requires low-cost, high-performance diethylamine (DEA) sensors. Materials based on metal-organic frameworks (MOFs) can detect hazardous gases due to their large specific surface area, many metal sites, unsaturated sites, functional connectivity, and easy calcination to remove the scaffold. However, developing facile materials with high sensitivity and selectivity in harsh environments for accurate DEA detection at a low detection limit (LOD) at room temperature (RT) is challenging. In this study, p-type semiconducting porous CuOx sensing materials were synthesized using a simple solvothermal process and annealed in an argon atmosphere at three different temperatures (x = 400, 600, and 800 °C). Significant variations in particle size, specific area, crystallite size, and shape were noticed when the annealing temperature was elevated. Cu-MIL-53 annealed at 400 °C (CuO-400) has a typical nanoellipsoid (NEs) shape with a length of 61.5 nm and a diameter of 33.2 nm. Surprisingly, CuO-400 NEs showed an excellent response to DEA with an ultra-LOD (Rg/Ra = 7.3 @ 100 ppb, 55% relative humidity), excellent selectivity and sensitivity (Rg/Ra = 236 @ 15 ppm), exceptional long-term stability and repeatability, and a fast response/recovery period at RT, outperforming most previously reported materials. CuO-400 NEs have outstanding gas-sensing characteristics due to their high porosity, 1D nanostructure, unsaturated Cu sites (Cu+ and Cu2+), large specific surface area, and numerous oxygen vacancies. This study presents a generic approach to produce future CuO derived from Cu-MOFs-sensitive materials, revealing new insights into the design of effective sensors for environmental monitoring at RT.

Highly selective molecularly imprinted polymer nanoparticles (MIP NPs)-based microfluidic gas sensor for tetrahydrocannabinol (THC) detection

A highly selective microfluidic integrated metal oxide gas sensor for THC detection is reported based on MIP nanoparticles (MIP NPs). We synthesized MIP NPs with THC recognition sites and coated them on a 3D-printed microfluidic channel surface. The sensitivity and selectivity of coated microfluidic integrated gas sensors were evaluated by exposure to THC, cannabidiol (CBD), methanol, and ethanol analytes in 300-700 ppm at 300 °C. For comparison, reference signals were obtained from a microfluidic channel coated with nonimprinted polymers (NIP NPs). The MIP and NIP NPs were characterized using scanning electron microscopy (SEM) and Raman spectroscopy. MIP and NIP NPs channels response data were combined and classified with 96.3% accuracy using the Fine KNN classification model in MATLAB R2021b Classification Learner App. Compared to the MIP NPs coated channel, the NIP NPs channel had poor selectivity towards THC, demonstrating that the THC recognition sites in the MIP structure enabled selective detection of THC. The findings demonstrated that the recognition sites of MIP NPs properly captured THC molecules, enabling the selective detection of THC compared to CBD, methanol, and ethanol.

Metal-organic frameworks derived inverse/normal bimetallic spinel oxides toward the selective VOCs and H2S sensing

As the typical toxic and hazardous gases, volatile organic compounds (VOCs) and hydrogen sulfide (H₂S) pose a threat to the environment and human health. The demand for real-time detection of VOCs and H₂S gases is growing in many application to protect human health and air quality. Therefore, it is essential to develop advance sensing materials for the construction of effective and reliable gas sensors. Herein, bimetallic spinel ferrites with different metal ions (MFe₂O₄, M = Co, Ni, Cu and Zn) were designed by using metal-organic frameworks as templates. The evaluation of cation substitution on crystal structures (inverse/normal spinel structure) and electrical properties (n/p type and band gap) is systematically discussed. The results indicate that p-type NiFe₂O₄ and n-type CuFe₂O₄ nanocubes with inverse spinel structure exhibit high response and great selectivity towards acetone (C₃H₆O) and H₂S, respectively. Moreover, the two sensors also display the detection limits as low as 1 ppm (C₃H₆O) and 0.5 ppm (H₂S), which are far below the threshold values of 750 ppm to acetone and 10 ppm to H₂S for 8 h exposure set by American Conference of Governmental Industrial Hygienists (ACGIH). The finding provides new possibilities for the design of high-performance chemical sensors, which display tremendous potential for practical applications.

Review of Chemical Sensors for Hydrogen Sulfide Detection in Organisms and Living Cells

As the third gasotransmitter, hydrogen sulfide (H₂S) is involved in a variety of physiological and pathological processes wherein abnormal levels of H₂S indicate various diseases. Therefore, an efficient and reliable monitoring of H₂S concentration in organisms and living cells is of great significance. Of diverse detection technologies, electrochemical sensors possess the unique advantages of miniaturization, fast detection, and high sensitivity, while the fluorescent and colorimetric ones exhibit exclusive visualization. All these chemical sensors are expected to be leveraged for H₂S detection in organisms and living cells, thus offering promising options for wearable devices. In this paper, the chemical sensors used to detect H₂S in the last 10 years are reviewed based on the different properties (metal affinity, reducibility, and nucleophilicity) of H₂S, simultaneously summarizing the detection materials, methods, linear range, detection limits, selectivity, etc. Meanwhile, the existing problems of such sensors and possible solutions are put forward. This review indicates that these types of chemical sensors competently serve as specific, accurate, highly selective, and sensitive sensor platforms for H₂S detection in organisms and living cells.

A review of piezoelectric MEMS sensors and actuators for gas detection application

Piezoelectric microelectromechanical system (piezo-MEMS)-based mass sensors including the piezoelectric microcantilevers, surface acoustic waves (SAW), quartz crystal microbalance (QCM), piezoelectric micromachined ultrasonic transducer (PMUT), and film bulk acoustic wave resonators (FBAR) are highlighted as suitable candidates for highly sensitive gas detection application. This paper presents the piezo-MEMS gas sensors' characteristics such as their miniaturized structure, the capability of integration with readout circuit, and fabrication feasibility using multiuser technologies. The development of the piezoelectric MEMS gas sensors is investigated for the application of low-level concentration gas molecules detection. In this work, the various types of gas sensors based on piezoelectricity are investigated extensively including their operating principle, besides their material parameters as well as the critical design parameters, the device structures, and their sensing materials including the polymers, carbon, metal-organic framework, and graphene.

Selective detection of trace carbon monoxide at room temperature based on CuO nanosheets exposed to (111) crystal facets

In recent years, carbon monoxide (CO) intoxication incidents occur frequently, and the sensitive detection of CO is particularly significant. At present, most reported carbon monoxide (CO) sensors meet the disadvantage of high working temperature. It is always a challenge to realize the sensitive detection of carbon monoxide at room temperature. In this study, CuO nanosheets exposed more (111) active crystal facets and oxygen vacancy defects were synthesized by a simple and environmentally friendly one-step hydrothermal method. The sensor has good comprehensive gas sensing performance, compared with other sensors that can detect CO at room temperature. The response value to 100 ppm CO at room temperature is as high as 39.6. In addition, it also has excellent selectivity, low detection limit (100 ppb), good reproducibility, moisture resistance and long-term stability (60 days). This excellent gas sensing performance is attributed to the special structural characteristics of 2D materials and the synergistic effect of more active crystal facets exposed on the crystal surface and oxygen vacancy defects. Therefore, it is expected to become a promising sensitive material for rapid and accurate detection of trace CO gas under low energy consumption, reduce the risk of poisoning, and then effectively protect human life safety.

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations-Part 2: Porous 2D Nanomaterials

This article discusses the features of the synthesis and application of porous two-dimensional nanomaterials in developing conductometric gas sensors based on metal oxides. It is concluded that using porous 2D nanomaterials and 3D structures based on them is a promising approach to improving the parameters of gas sensors, such as sensitivity and the rate of response. The limitations that may arise when using 2D structures in gas sensors intended for the sensor market are considered.

Highly sensitive work function type room temperature gas sensor based on Ti doped hBN monolayer for sensing CO2, CO, H2S, HF and NO. A DFT study

The adsorptions of toxic gas molecules (CO2, CO, H2S, HF and NO) on pristine and Ti atom doped hexagonal boron nitride (hBN) monolayer are investigated by density functional theory. Weak physisorption of gas molecules on pristine hBN results in micro seconds recovery time, limiting the gas sensing ability of pristine hBN. However Ti atom doping significantly enhances the adsorption ability. Ti atom best fits to be doped at B vacancy in hBN with lowest formation energy (-3.241 eV). Structural analysis reveals that structures of gas molecules change after being chemisorbed to Ti doped hBN monolayer. Partial density of states analysis illustrates strong hybridization among Ti-3d, gas-2p and BN-2p orbitals, moreover Bader charge transfer indicates that gas molecules act as charge acceptors. Ti doped hBN monolayer undergoes transition from semiconductor to narrow band semiconductor with adsorption of CO2, H2S and NO, while with CO and HF adsorption it transforms into metal. The change of conductance of Ti doped hBN monolayer in response to adsorption of gas molecules reveals its high sensitivity, however it is not selective to HF and NO gases. The recovery times of gas molecules desorption from monolayer are too long at ambient condition however it can significantly be shortened by annealing at elevated temperature with UV exposure. Since recovery time for NO removal from monolayer is still very long at 500 K with UV exposure, Ti doped hBN monolayer is more suitable as a scavenger of NO gas rather than as a gas sensor. It is thus predicted that Ti doped hBN monolayer can be a work-function type CO2, CO, H2S and HF sensor and NO gas scavenger.

Density functional theory study of Mobius boron-carbon-nitride as potential CH4, H2S, NH3, COCl2 and CH3OH gas sensor

The interesting properties of Mobius structure and boron-carbon-nitride (BCN) inspired this research to study different characteristics of Mobius BCN (MBCN) nanoribbon. The structural stability and vibrational, electrical and optical properties are analysed using the density functional theory. The gas-sensing ability of the modelled MBCN structure was also studied for methane, hydrogen sulfide, ammonia, phosgene and methanol gases. The negative adsorption energy and alteration of electronic bandgap verified that MBCN is very sensitive toward the selected gases. The complex structures showed a high absorption coefficient with strong chemical potential and 7 ps-0.3 ms recovery time. The negative change in entropy signifies that all the complex structures were thermodynamically stable. Among the selected gases, the MBCN showed the strongest interaction with methanol gas.

A Room Temperature ZnO-NPs/MEMS Ammonia Gas Sensor

This study uses ultrasonic grinding to grind ZnO powder to 10−20-nanometer nanoparticles (NPs), and these are integrated with a MEMS structure to form a ZnO-NPs/MEMS gas sensor. Measuring 1 ppm NH3 gas and operating at room temperature, the sensor response for the ZnO-NPs/MEMS gas sensor is around 39.7%, but the origin-ZnO powder/MEMS gas sensor is fairly unresponsive. For seven consecutive cycles, the ZnO-NPs/MEMS gas sensor has an average sensor response of about 40% and an inaccuracy of <±2%. In the selectivity of the gas, the ZnO-NPs/MEMS gas sensor has a higher response to NH3 than to CO, CO2, H2, or SO2 gases because ZnO nanoparticles have a greater surface area and more surface defects, so they adsorb more oxygen molecules and water molecules. These react with NH3 gas to increase the sensor response.

High-Performance Room-Temperature Conductometric Gas Sensors: Materials and Strategies

Chemiresistive sensors have gained increasing interest in recent years due to the necessity of low-cost, effective, high-performance gas sensors to detect volatile organic compounds (VOC) and other harmful pollutants. While most of the gas sensing technologies rely on the use of high operation temperatures, which increase usage cost and decrease efficiency due to high power consumption, a particular subset of gas sensors can operate at room temperature (RT). Current approaches are aimed at the development of high-sensitivity and multiple-selectivity room-temperature sensors, where substantial research efforts have been conducted. However, fewer studies presents the specific mechanism of action on why those particular materials can work at room temperature and how to both enhance and optimize their RT performance. Herein, we present strategies to achieve RT gas sensing for various materials, such as metals and metal oxides (MOs), as well as some of the most promising candidates, such as polymers and hybrid composites. Finally, the future promising outlook on this technology is discussed.

The Combination of Two-Dimensional Nanomaterials with Metal Oxide Nanoparticles for Gas Sensors: A Review

Metal oxide nanoparticles have been widely utilized for the fabrication of functional gas sensors to determine various flammable, explosive, toxic, and harmful gases due to their advantages of low cost, fast response, and high sensitivity. However, metal oxide-based gas sensors reveal the shortcomings of high operating temperature, high power requirement, and low selectivity, which limited their rapid development in the fabrication of high-performance gas sensors. The combination of metal oxides with two-dimensional (2D) nanomaterials to construct a heterostructure can hybridize the advantages of each other and overcome their respective shortcomings, thereby improving the sensing performance of the fabricated gas sensors. In this review, we present recent advances in the fabrication of metal oxide-, 2D nanomaterials-, as well as 2D material/metal oxide composite-based gas sensors with highly sensitive and selective functions. To achieve this aim, we firstly introduce the working principles of various gas sensors, and then discuss the factors that could affect the sensitivity of gas sensors. After that, a lot of cases on the fabrication of gas sensors by using metal oxides, 2D materials, and 2D material/metal oxide composites are demonstrated. Finally, we summarize the current development and discuss potential research directions in this promising topic. We believe in this work is helpful for the readers in multidiscipline research fields like materials science, nanotechnology, chemical engineering, environmental science, and other related aspects.

Insights into the mechanism of low-temperature H2S oxidation over Zn-Cu/Al2O3 catalyst

Odor pollution caused by toxic chemicals with low human olfactory thresholds, such as hydrogen sulfide (H₂S), has become a major cause of environmental grievance world-wide. Although the low-temperature (<180 °C) catalytic oxidation of H₂S using metal oxides has received widespread attention, desulfurization performance is not ideal. Herein, a series of Zn-Cu/Al₂O₃ catalysts were developed using an impregnation method based on the Al₂O₃ hydrophilicity and the effects of zinc loading on the catalyst physicochemical properties and performance were systematically studied. The catalysts were characterized using inductively coupled plasma-optical emission spectrometry (ICP-OES), N₂ adsorption-desorption isotherms, scanning electron microscopy with energy dispersive spectrometry (SEM-EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier-transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). It was found that optimization of zinc doping could improve the hydrophilicity of the catalyst, and hence its activity. Catalytic activity was also dependent on operational parameters such as temperature, humidity and space velocity. The Zn₃Cu₃ catalyst exhibited the highest breakthrough capacity of 353.91 mg/g at 50 °C and at a relative humidity of 50%. The excellent desulfurization performance was attributed to oxygen vacancies contributed by CuO, Cu₂O and ZnO, which facilitated the conversion of H₂O into hydroxyl radicals. Consequently, a hydroxyl radical-induced desulfurization mechanism over Zn-Cu/Al₂O₃ is proposed. This work provides a potential green and efficient catalyst for the selective oxidation of H₂S.

Large-Area Synthesis of Ultrathin, Flexible, and Transparent Conductive Metal-Organic Framework Thin Films via a Microfluidic-Based Solution Shearing Process

Iminosemiquinone-linker-based conductive metal-organic frameworks (c-MOFs) have attracted much attention as next-generation electronic materials due to their high electrical conductivity combined with high porosity. However, the utility of such c-MOFs in high-performance devices has been limited to date by the lack of high-quality MOF thin-film processing. Herein, a technique known as the microfluidic-assisted solution shearing combined with post-synthetic rapid crystallization (MASS-PRC) process is introduced to generate a high-quality, flexible, and transparent thin-film of Ni₃ (hexaiminotriphenylene)₂ (Ni₃ (HITP)₂ ) uniformly over a large-area in a high-throughput manner with thickness controllability down to tens of nanometers. The MASS-PRC process utilizes: 1) a micromixer-embedded blade to simultaneously mix and continuously supply the metal-ligand solution toward the drying front during solution shearing to generate an amorphous thin-film, followed by: 2) immersion in amine solution for rapid directional crystal growth. The as-synthesized c-MOF film has transparency of up to 88.8% and conductivity as high as 37.1 S cm^-1 . The high uniformity in conductivity is confirmed over a 3500 mm² area with an arithmetic mean roughness (R_a ) of 4.78 nm. The flexible thin-film demonstrates the highest level of transparency for Ni₃ (HITP)₂ and the highest hydrogen sulfide (H₂ S) sensing performance (2,085% at 5 ppm) among c-MOFs-based H₂ S sensors, enabling wearable gas-sensing applications.

H2S sensing under various humidity conditions with Ag nanoparticle functionalized Ti3C2Tx MXene field-effect transistors

Despite the critical need to monitor H₂S, a hazardous gas, in environmental and medical settings, there are currently no reliable methods for rapid and sufficiently discriminative H₂S detection in real-world humid environments. Herein, targeted hybridizing of Ti₃C₂T_x MXene with Ag nanoparticles on a field-effect transistor (FET) platform has led to a step change in MXene sensing performance down to ppb levels, and enabled the very high selectivity and fast response/recovery time under room temperature for H₂S detection in humid conditions. For the first time, we present a novel relative humidity (RH) self-calibration strategy for the accurate detection of H₂S. This strategy can eliminate the influence of humidity and enables the accurate quantitative detection of gas in the total RH range. We further elucidate that the superior H₂S sensing performance is attributed to the electron and chemical sensitization effects. This study opens new avenues for the development of high-performance MXene-based sensors and offers a viable approach for addressing real-world humidity effect for gas sensors generally.

Hydrogen Sulfide: An Emerging Precision Strategy for Gas Therapy

Advances in nanotechnology have enabled the rapid development of stimuli-responsive therapeutic nanomaterials for precision gas therapy. Hydrogen sulfide (H₂ S) is a significant gaseous signaling molecule with intrinsic biochemical properties, which exerts its various physiological effects under both normal and pathological conditions. Various nanomaterials with H₂ S-responsive properties, as new-generation therapeutic agents, are explored to guide therapeutic behaviors in biological milieu. The cross disciplinary of H₂ S is an emerging scientific hotspot that studies the chemical properties, biological mechanisms, and therapeutic effects of H₂ S. This review summarizes the state-of-art research on H₂ S-related nanomedicines. In particular, recent advances in H₂ S therapeutics for cancer, such as H₂ S-mediated gas therapy and H₂ S-related synergistic therapies (combined with chemotherapy, photodynamic therapy, photothermal therapy, and chemodynamic therapy) are highlighted. Versatile imaging techniques for real-time monitoring H₂ S during biological diagnosis are reviewed. Finally, the biosafety issues, current challenges, and potential possibilities in the evolution of H₂ S-based therapy that facilitate clinical translation to patients are discussed.

MXene Heterostructures as Perspective Materials for Gas Sensing Applications

This paper provides a summary of the recent developments with promising 2D MXene-related materials and gives an outlook for further research on gas sensor applications. The current synthesis routes that are provided in the literature are summarized, and the main properties of MXene compounds have been highlighted. Particular attention has been paid to safe and non-hazardous synthesis approaches for MXene production as 2D materials. The work so far on sensing properties of pure MXenes and MXene-based heterostructures has been considered. Significant improvement of the MXenes sensing performances not only relies on 2D production but also on the formation of MXene heterostructures with other 2D materials, such as graphene, and with metal oxides layers. Despite the limited number of research papers published in this area, recommendations on new strategies to advance MXene heterostructures and composites for gas sensing applications can be driven.

State-of-the-Art Research on Chemiresistive Gas Sensors in Korea: Emphasis on the Achievements of the Research Labs of Professors Hyoun Woo Kim and Sang Sub Kim

This review presents the results of cutting-edge research on chemiresistive gas sensors in Korea with a focus on the research activities of the laboratories of Professors Sang Sub Kim and Hyoun Woo Kim. The advances in the synthesis techniques and various strategies to enhance the gas-sensing performances of metal-oxide-, sulfide-, and polymer-based nanomaterials are described. In particular, the gas-sensing characteristics of different types of sensors reported in recent years, including core-shell, self-heated, irradiated, flexible, Si-based, glass, and metal-organic framework sensors, have been reviewed. The most crucial achievements include the optimization of shell thickness in core-shell gas sensors, decrease in applied voltage in self-heated gas sensors to less than 5 V, optimization of irradiation dose to achieve the highest response to gases, and the design of selective and highly flexible gas sensors-based WS2 nanosheets. The underlying sensing mechanisms are discussed in detail. In summary, this review provides an overview of the chemiresistive gas-sensing research activities led by the corresponding authors of this manuscript.

Morphology-driven gas sensing by fabricated fractals: A review

Fractals are intriguing structures that repeat themselves at various length scales. Interestingly, fractals can also be fabricated artificially in labs under controlled growth environments and be explored for various applications. Such fractals have a repeating unit that spans in length from nano- to millimeter range. Fractals thus can be regarded as connectors that structurally bridge the gap between the nano- and the macroscopic worlds and have a hybrid structure of pores and repeating units. This article presents a comprehensive review on inorganic fabricated fractals (fab-fracs) synthesized in labs and employed as gas sensors across materials, morphologies, and gas analytes. The focus is to investigate the morphology-driven gas response of these fab-fracs and identify key parameters of fractal geometry in influencing gas response. Fab-fracs with roughened microstructure, pore-network connectivity, and fractal dimension (D) less than 2 are projected to be possessing better gas sensing capabilities. Fab-fracs with these salient features will help in designing the commercial gas sensors with better performance.

Ultrathin Leaf-Shaped CuO Nanosheets Based Sensor Device for Enhanced Hydrogen Sulfide Gas Sensing Application

Herein, a simple, economical and low temperature synthesis of leaf-shaped CuO nanosheets is reported. As-synthesized CuO was examined through different techniques including field emission scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), X-ray diffraction (XRD), fourier transform infrared spectroscopic (FTIR) and Raman spectroscopy to ascertain the purity, crystal phase, morphology, vibrational, optical and diffraction features. FESEM and TEM images revealed a thin leaf-like morphology for CuO nanosheets. An interplanar distance of ~0.25 nm corresponding to the (110) diffraction plane of the monoclinic phase of the CuO was revealed from the HRTEM images XRD analysis indicated a monoclinic tenorite crystalline phase of the synthesized CuO nanosheets. The average crystallite size for leaf-shaped CuO nanosheets was found to be 14.28 nm. Furthermore, a chemo-resistive-type gas sensor based on leaf-shaped CuO nanosheets was fabricated to effectively and selectively detect H2S gas. The fabricated sensor showed maximum gas response at an optimized temperature of 300 °C towards 200 ppm H2S gas. The corresponding response and recovery times were 97 s and 100 s, respectively. The leaf-shaped CuO nanosheets-based gas sensor also exhibited excellent selectivity towards H2S gas as compared to other analyte gases including NH3, CH3OH, CH3CH2OH, CO and H2. Finally, we have proposed a gas sensing mechanism based upon the formation of chemo-resistive CuO nanosheets.

Heterostructural (Sr0.6Bi0.305)2Bi2O7/ZnO for novel high-performance H2S sensor operating at low temperature

Exploring novel sensing materials enabling selective discrimination of trace ambient H₂S at lower temperature is of utmost importance for diverse practical applications. Herein, heterostructural (Sr_0.6Bi_0.305)₂Bi₂O₇/ZnO (SBO/ZnO) nanomaterials were proposed. Synergetic effect of promoting analyte adsorption (via multiplying oxygen vacancy defects) and reversible sulfuration-desulfuration reaction induced unique band alignment among SBO/ZnO/ZnS, contributes to the sensitive and selective response toward H₂S molecules. Novel SBO/ZnO (10%) sensor possesses excellent sensing H₂S performances, including a high response (107.6 for 10 ppm), low limit of detection of 20 ppb, good selectivity and long-term stability. Together with the merits of low operation temperature of 75 °C and weak humidity dependence (endowed by the hydrophobic SBO), present heterostructural SBO/ZnO sensor paves the way for the practical monitoring of trace H₂S pollutants in diverse workplaces including petroleum and natural gas industries.

In Situ Assembly of Ordered Hierarchical CuO Microhemisphere Nanowire Arrays for High-Performance Bifunctional Sensing Applications

Seeking a facile approach to directly assemble bridged metal oxide nanowires on substrates with predefined electrodes without the need for complex postsynthesis alignment and/or device procedures will bridge the gap between fundamental research and practical applications for diverse biochemical sensing, electronic, optoelectronic, and energy storage devices. Herein, regularly bridged CuO microhemisphere nanowire arrays (RB-MNAs) are rationally designed on indium tin oxide electrodes via thermal oxidation of ordered Cu microhemisphere arrays obtained by solid-state dewetting of patterned Ag/Cu/Ag films. Both the position and spacing of CuO microhemisphere nanowires can be well controlled by as-used shadow mask and the thickness of Cu film, which allows homogeneous manipulation of the bridging of adjacent nanowires grown from neighboring CuO hemispheres, and thus benefits highly sensitive trimethylamine (TMA) sensors and broad band (UV-visible to infrared) photodetectors. The electrical response of 3.62 toward 100 ppm TMA is comparable to that of state-of-the-art CuO-based sensors. Together with the feasibility of in situ assembly of RB-MNAs device arrays via common lithographic technologies, this work promises commercial device applications of CuO nanowires.

CH3SH and H2S Sensing Properties of V2O5/WO3/TiO2 Gas Sensor

Resistive-type semiconductor-based gas sensors were fabricated for the detection of methyl mercaptan and hydrogen sulfide. To fabricate these sensors, V2O5/WO3/TiO2 (VWT) particles were deposited on interdigitated Pt electrodes. The vanadium oxide content of the utilized VWT was 1.5, 3, or 10 wt.%. The structural properties of the VWT particles were investigated by X-ray diffraction and scanning electron microscopy analyses. The resistance of the VWT gas sensor decreased with increasing methyl mercaptan and hydrogen sulfide gas concentrations in the range of 50 to 500 ppb. The VWT gas sensor with 3 wt.% vanadium oxide showed high methyl mercaptan and hydrogen sulfide responses and good gas selectivity against hydrogen at 300 °C.

Study on the adsorption selection of CH$$_4$$ on CuO (110) versus (111) surfaces: a density functional theory study

Finding the active sites of suitable metal oxides is a key prerequisite for detecting CH$$_4$$ 4 . The purpose of the paper is to investigate the adsorption of CH$$_4$$ 4 on intrinsic and oxygen-vacancies CuO (111) and (110) surfaces using density functional theory calculations. The results show that CH$$_4$$ 4 has a strong adsorption energy of −0.370 to 0.391 eV at all site on the CuO (110) surface. The adsorption capacity of CH$$_4$$ 4 on CuO (111) surface is weak, ranging from −0.156 to −0.325 eV. In the surface containing oxygen vacancies, the adsorption capacity of CuO surface to CH$$_4$$ 4 is significantly stronger than that of intrinsic CuO surface. The results indicate that CuO (110) has strong adsorption and charge transfer capacity for CH$$_4$$ 4 , which may provide experimental guidance.

Electroactive Cu2O nanocubes engineered electrochemical sensor for H2S detection

An electrochemical sensor was proposed for the detection of hydrogen sulfide (H₂S) at room temperature, by using electroactive Cu₂O nanocubes (NCs) as an electrochemical beacon. Electroactive Cu₂O NCs were synthesized on the surface of reduced graphene oxide (rGO)/Fe₃O₄ nanosheets (NSs) due to the good electronic conductivity and well-responded magnetic responses. The fabricated rGO/Fe₃O₄/Cu₂O NSs not only showed electrochemical oxidization peak at -0.1 V from Cu₂O NCs, and could be served as sensitive electrochemical beacon for the simple modification on magnetic electrodes in the applications. The unique redox reaction between Cu₂O NCs and H₂S enabled the transformation of Cu₂O NCs to Cu₉S₈ NCs, resulting in decreased electroxidation responses at -0.1 V. The constructed electrochemical platform had a limit of detection (LOD) of 230 pM and a detection range of 500 pM-100 μM. The simple and cheap electrochemical sensor developed in this paper showed potential application for H₂S detection.

Gas Sensors Based on Copper Oxide Nanomaterials: A Review

Metal oxide semiconductors have found widespread applications in chemical sensors based on electrical transduction principles, in particular for the detection of a large variety of gaseous analytes, including environmental pollutants and hazardous gases. This review recapitulates the progress in copper oxide nanomaterial-based devices, while discussing decisive factors influencing gas sensing properties and performance. Literature reports on the highly sensitive detection of several target molecules, including volatile organic compounds, hydrogen sulfide, carbon monoxide, carbon dioxide, hydrogen and nitrogen oxide from parts-per-million down to parts-per-billion concentrations are compared. Physico-chemical mechanisms for sensing and transduction are summarized and prospects for future developments are outlined.

Highly selective gas sensing enabled by filters

Portable and inexpensive gas sensors are essential for the next generation of non-invasive medical diagnostics, smart air quality monitoring & control, human search & rescue and food quality assessment to name a few of their immediate applications. Therein, analyte selectivity in complex gas mixtures like breath or indoor air remains the major challenge. Filters are an effective and versatile, though often unrecognized, route to overcome selectivity issues by exploiting additional properties of target analytes (e.g., molecular size and surface affinity) besides reactivity with the sensing material. This review provides a tutorial for the material engineering of sorption, size-selective and catalytic filters. Of specific interest are high surface area sorbents (e.g., activated carbon, silica gels and porous polymers) with tunable properties, microporous materials (e.g., zeolites and metal-organic frameworks) and heterogeneous catalysts, respectively. Emphasis is placed on material design for targeted gas separation, portable device integration and performance. Finally, research frontiers and opportunities for low-cost gas sensing systems in emerging applications are highlighted.

A highly selective electron affinity facilitated H2S sensor: the marriage of tris(keto-hydrazone) and an organic field-effect transistor

Conjugated polymers (CPs) are emerging as part of a promising future for gas-sensing applications. However, some of their limitations, such as poor specificity, humidity sensitivity and poor ambient stability, remain persistent. Herein, a novel combination of a polymer-monomer heterostructure, derived from a CP (PDVT-10) and a newly reported monomer [tris(keto-hydrazone)] has been integrated in an organic field-effect transistor (OFET) platform to sense H₂S selectively. The hybrid heterostructure shows an unprecedented sensitivity (525% ppm^-1) and high selectivity toward H₂S gas. In addition, we demonstrated that the PDVT-10/tris(keto-hydrazone) OFET sensor has the lowest limit of detection (1 ppb), excellent ambient stability (∼5% current degradation after 150 days), good response-recovery behavior, and exceptional electrical behavior and gas response reproducibility. This work can help pave the way to incorporate futuristic gas sensors in a multitude of applications.

CuWO4 with CuO and Cu(OH)2 Native Surface Layers for H2S Detection under in-Field Conditions

The paper presents the possibility of detecting low H₂S concentrations using CuWO₄. The applicative challenge was to obtain sensitivity, selectivity, short response time, and full recovery at a low operating temperature under in-field atmosphere, which means variable relative humidity (%RH). Three different chemical synthesis routes were used for obtaining the samples labeled as: CuW1, CuW2, and CuW3. The materials have been fully characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). While CuWO₄ is the common main phase with triclinic symmetry, different native layers of CuO and Cu(OH)₂ have been identified on top of the surfaces. The differences induced into their structural, morphological, and surface chemistry revealed different degrees of surface hydroxylation. Knowing the poisonous effect of H₂S, the sensing properties evaluation allowed the CuW2 selection based on its specific surface recovery upon gas exposure. Simultaneous electrical resistance and work function measurements confirmed the weak influence of moisture over the sensing properties of CuW2, due to the pronounced Cu(OH)₂ native surface layer, as shown by XPS investigations. Moreover, the experimental results obtained at 150 °C highlight the linear sensor signal for CuW2 in the range of 1 to 10 ppm H₂S concentrations and a pronounced selectivity towards CO, CH₄, NH₃, SO₂, and NO₂. Therefore, the applicative potential deserves to be noted. The study has been completed by a theoretical approach aiming to link the experimental findings with the CuW2 intrinsic properties.

Recent advances in energy-saving chemiresistive gas sensors: A review

With the tremendous advances in technology, gas-sensing devices are being popularly used in many distinct areas, including indoor environments, industries, aviation, and detectors for various toxic domestic gases and vapors. Even though the most popular type of gas sensor, namely, resistive-based gas sensors, have many advantages over other types of gas sensors, their high working temperatures lead to high energy consumption, thereby limiting their practical applications, especially in mobile and portable devices. As possible ways to deal with the high-power consumption of resistance-based sensors, different strategies such as self-heating, MEMS technology, and room-temperature operation using especial morphologies, have been introduced in recent years. In this review, we discuss different types of energy-saving chemisresitive gas sensors including self-heated gas sensors, MEMS based gas sensors, room temperature operated flexible/wearable sensor and their application in the fields of environmental monitoring. At the end, the review will be concluded by providing a summary, challenges, recent trends, and future perspectives.

A review of low-temperature H2S gas sensors: fabrication and mechanism

Reduced detection temperature of hazardous gases such as H2S can lower power consumption and increase the long-term stability. The decreased operating temperature can be achieved via physical and chemical modification of the sensing layer.

Enhancement of Low-Temperature Gas-Sensing Performance Using Substoichiometric WO3-x Modified with CuO

To verify the effect of oxygen vacancy on gas sensitivity, we have systematically investigated the gas-sensing performance of copper oxide/substoichiometric tungsten oxide (CuO/WO_3-x) nanocomposite sensors. Oxygen deficiency in WO_3-x facilitates the reaction of hydrogen sulfide (H₂S) gas with chemisorbed oxygen species (i.e., O₂^-, O^-, and O^2-) at low temperature. The oxygen/sulphur exchange reaction between CuO and H₂S in the sensing process can achieve room temperature operation of gas sensors. After the WO_3-x nanorods were modified by a low content of CuO nanoparticles (Cu:W = 1:20), the sensors present an n-type sensing behavior. Their best working temperatures drop from 289 °C (or 386 °C) to 99 °C (or 70 °C) at which the responses are improved by 14 to 163 times for different x values. Among them, CuO(L)/W₅O₁₄ shows the highest sensitivity of 1575.7 to 10 ppm H₂S at 99 °C and 171.5 to 10 ppm H₂S at room temperature. Once WO_3-x were loaded with a high concentration of CuO nanoparticles (Cu:W = 1:2), they exhibit a p-type behavior, and the optimal working temperatures reduce suddenly to room temperature at which CuO(H)/W₁₈O₄₉ displays the most sensitive response of 7.2 even toward trace amounts of H₂S as low as 100 ppb. In addition, p-type CuO weakens the metal-like characteristics of W₁₈O₄₉ and such weakening effect enhances with an increase in the CuO content. Therefore, the sensing performance of the CuO/W₁₈O₄₉ composite is the best among the four CuO/WO_3-x sensors. The two designs for low and high Cu/W molar ratios all achieve enhanced room-temperature H₂S gas response, with a fast recovery time of ∼60 s under heating pulse, as well as an excellent selectivity, which makes the sensors a promising candidate for practical applications. Moreover, the micro-Raman spectra confirmed CuS formation and the thermal effect on the decomposition of CuS in the sensing process was studied.

Hierarchically porous CuO spindle-like nanosheets grown on a carbon cloth for sensitive non-enzymatic glucose sensoring

Herein, porous CuO spindle-like nanosheets were fabricated on a carbon cloth using a facile hydrothermal method, and surface morphology, microstructure, and glucose sensing performance were studied. The porous spindle-like nanosheets are constructed by nanoparticles and slit-like pores, exhibiting a hierarchical structure. When used for non-enzymatic glucose sensoring, the obtained CuO nanosheet electrode exhibits a wide linear range from 0.05 to 3.30 mM, a high sensitivity of 785.2 μA mM^-1 cm^-2 and a low detection limit of 0.22 μM (S/N = 3). Besides, good selectivity, stability, and reproducibility for glucose detection indicate a promising application of CuO nanosheet electrodes as non-enzymatic glucose sensors.

Nanomolar Detection of H2S in an Aqueous Medium: Application in Endogenous and Exogenous Imaging of HeLa Cells and Zebrafish

The homeostasis of short-lived reactive species such as hydrogen sulfide/hypochlorous acid (H2S/HOCl) in biological systems is essential for maintaining intercellular balance. An unchecked increase in biological H2S concentrations impedes homeostasis. In this report, we present a molecular probe pyrene-based sulfonyl hydrazone derived from pyrene for the selective detection of H2S endogenously as well as exogenously through a "turn-off" response in water. The structure of the receptor is confirmed by Fourier-transform infrared spectroscopy, 1H and 13C nuclear magnetic resonance spectroscopy, electrospray ionization mass spectrometry, and single-crystal X-ray diffraction studies. The receptor shows excellent green emission in both the aqueous phase and solid state. Quenching of green emission of the receptor is observed only when H2S is present in water with a detection limit of 18 nM. Other competing anions and cations do not have any influence on the receptor's optical properties. The efficiency of H2S detection is not negatively impacted by other reactive sulfur species too. The sensing mechanism of H2S follows a chemodosimetric reductive elimination of sulfur dioxide, which is supported by product isolation. The receptor is found to be biocompatible, as evident by the 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide assay, and its utility is extended to endogenous and exogenous fluorescence imaging of HeLa cells and zebrafish.

Current Trends in Nanomaterials for Metal Oxide-Based Conductometric Gas Sensors: Advantages and Limitations. Part 1: 1D and 2D Nanostructures

This article discusses the main uses of 1D and 2D nanomaterials in the development of conductometric gas sensors based on metal oxides. It is shown that, along with the advantages of these materials, which can improve the parameters of gas sensors, there are a number of disadvantages that significantly limit their use in the development of devices designed for the sensor market.

Near-Infrared Laser-Annealed IZO Flexible Device as a Sensitive H2S Sensor at Room Temperature

A metal-oxide material (indium zinc oxide [IZO]) device with near-infrared (NIR) laser annealing was demonstrated on both glass and bendable plastic substrates (polycarbonate, polyethylene, and polyethylene terephthalate). After only 60 s, the sheet resistance of IZO films annealed with a laser was comparable to that of thermal-annealed devices at temperatures in the range of 200-300 °C (1 h). XPS, ATR, and AFM were used to investigate the changes in the sheet resistance and correlate them to the composition and morphology of the thin film. Finally, the NIR-laser-annealed IZO films were demonstrated to be capable of detecting changes in humidity and serving as a highly sensitive gas sensor of hydrogen sulfide (in ppb concentration), with room-temperature operation on a bendable substrate.

In situ deposited hierarchical CuO/NiO nanowall arrays film sensor with enhanced gas sensing performance to H2S

Hierarchical and heterogeneous CuO/NiO nanowall arrays were in situ grown on ceramic tubes via a facile template-free hydrothermal route, and then were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and nitrogen adsorption-desorption techniques. The resultant composites exhibit network-like CuO/NiO array structures constructed by interconnected porous nanosheets, in which the decoration of CuO nanoparticles in NiO nanowall arrays was confirmed by XRD, XPS and TEM analyses. The 2.84 at % CuO decorated NiO sensor exhibits excellent sensing properties at 133 °C. The response to 5 ppm H₂S attains 36.9, which increases as high as 5.6 times compared to the NiO one. The detection limit to H₂S is further decreased from 1 ppb for the pure NiO sensor to 0.5 ppb. The CuO/NiO sensor shows a wide linear range from 50 to 1000 ppb, good repeatability, selectivity and long-term stability, which is expected to be a candidate for ppb-level H₂S detection in real and complex environment of industrial production. Furthermore, the dominant H₂S sensing mechanism is discussed from the view of the homo- and hierarchical architecture of the CuO/NiO arrays as well as the chemical and electronic sensitization effects of CuO decoration.