Ammonia gas sensors: A comprehensive review

High-performance PANI sensor on silicon nanowire arrays for sub-ppb NH3 detection

For industrial production and disease diagnosis, real-time detection of low concentrations of NH3 is crucial, necessitating a gas-sensitive sensor compatible with integrated processes and exhibiting excellent performance. Herein, we employed wet etching and rapid in-situ polymerization on silicon nanowire substrates to grow polyaniline fibers, thereby fabricating NH3 gas sensors with p-p heterojunction and three-dimensional network structures. Characterization and gas sensing performance testing were conducted. The results demonstrate the outstanding NH3 detection capabilities of the sensor, providing stable responses down to concentrations as low as 1 ppb, which indicates its LOD is one to two orders of magnitude lower than current similar products. It also exhibits verified selectivity and long-term reliability. The excellent sensing performance is attributed to the high surface area from the silicon nanowire structure and efficient synergy of p-p heterojunction. Additionally, the influence of doping types of the substrates and annealing process were explored. This work serves as a reference for the design of silicon-based gas sensors with high sensitivity, low detection limits, and extended operational lifetimes, suitable for deployment in commercial integrated monitoring systems.

Wool powder assisted colorimetric sensing yarn with high sensitivity for NH3 monitoring

Colorimetric sensors have applications in gas monitoring due to their simple and quick detection through visible color changes. However, it remains challenging to prepare colorimetric sensors with high sensitivity. Herein, this work fabricated a biomass-based colorimetric sensing yarn with high sensitivity using anthocyanins as the colorimetric dye and wool powder as an effective ammonia (NH3) adsorbent. The sensitivity of the prepared yarns was evaluated for detection limit and response time. Surprisingly, the addition of 3% wool powder greatly improved the sensitivity of the prepared yarns, with a reduction of both detection limit and responsive time from 100 ppm to 20 ppm, and 2 min to 20 s, respectively when exposed in 150 ppm NH3. The prepared yarns also showed good selectivity and reusability. An example of the practical use of colorimetric yarns was presented. This work provides a facile strategy for fabricating wearable devices for toxic gas monitoring with visual output.

Unraveling the ammonia sensing behavior and degradation pathways of a novel lead-free MA2CuBr4 based ammonia sensor

We report a lead-free copper-based halide perovskite gas sensor to detect ammonia gas at ambient temperature. The sensor uses methylammonium copper bromide as the active material and can trace ammonia through both a visual color change method and electrical readout. The maximum calibrated sensitivity based on the optical response of the sensor is ∼95% upon exposure to 10 ppm ammonia gas, which is the best among the colorimetric sensors using halide perovskites. The sensor can be operated at 0.5 V bias with an output current of ∼12 μA at 2 ppm ammonia gas exposure, making our device compatible with low-power gas sensors. Furthermore, we studied the degradation mechanism by subjecting the MA2CuBr4 film to ammonia-exposure cycles. We found that there were two factors responsible for the degradation of the sensor: (i) loss of methylamine gas due to formation of NH4Br, and (ii) reduction of Cu2+ to Cu+. Increasing the proportion of MABr in the system increased the material's tolerance to ammonia exposure by solving the methylamine gas escape problem. Further, we showed that the stability of the device could be enhanced by depositing porous polymethylmethacrylate over the copper perovskite.

Investigation of a flexible, room-temperature fiber-shaped NH3 sensor based on PANI-Au-SnO2

A sensitive compound was successfully obtained by coating polyaniline (PANI) on the surface of composite nanoparticles consisting of Au-loaded tin dioxide, named as PANI-Au-SnO2, using an in situ polymerization method. NH3 sensors in thin-film and fiber-shaped forms were prepared by inkjet printing and impregnation methods, respectively, based on PANI-Au-SnO2. The response characteristics of these NH3 sensors developed from composite sensitive materials were investigated in detail. Results indicate an effective response of the sensors to NH3 at room temperature. The thin-film sensor demonstrated a good linear relationship between the resistance change and NH3 concentration within the range of 5-40 ppm, indicating its excellent repeatability and long-term stability. In comparison to the thin film sensor, the fiber-shaped sensor showed a consistently stable response to NH3 even after 1000 cycles of repeated bending deformation. To demonstrate the practical application of the flexible fiber-shaped NH3 sensor, a cap designed for NH3 detection was fabricated by integrating the as-prepared sensor with a circuit board and an LED digital display. This assembly was incorporated into a commercially available ducktail cap, resulting in a wearable device capable of dynamically monitoring environmental NH3 levels and displaying real-time values. This innovative application underscores the potential of these sensors in real-world scenarios, particularly in occupational safety, where workers might be exposed to harmful levels of NH3. The cap could serve as a personal safety device, alerting the wearer to hazardous concentrations of NH3, which is particularly relevant in industrial or agricultural settings.

Janus nanocellulose membrane by nitrogen plasma: Hydrophilicity to hydrophobicity selective switch

Cellulose nanofibrils are one of the keystone materials for sustainable future, yet their poor water repellency hinders their push into industrial applications. Due to complexity and poor economical outcome and/or processing toxicity of the existing hydrophobization methods, nanocellulose loses against its antagonist plastic in medical and food industries. Herein, we demonstrate for the first time the "one-side selective water-repellency activation" in nanocellulose membranes by the means of mild N2-plasma treatment, exhibiting lowest wettability after 20 s of treatment. Hydrophobicity and accompanying Janus character were justified by the topological, chemical and structural reorganizations in cellulose nanofibrils. The findings suggest that the mechanism behind the hydrophilic/hydrophobic change primarily relies on the interplay between OH removal and appearance of SiCH3, originating from the polysiloxanes-based substrate, as well as complementary CNH2 groups formation. First-principles calculations show that NH2 groups moderately increase hydrophobicity, while various SiCH3 substitutions wholly change the character of the surface to repel water. Using nitrogen is shown to be crucial, as N(H)Si(CH3)3 groups induce greater hydrophobicity than simple OSi(CH3)3. Finally, the obtained materials absorb water on the hydrophilic side, while remaining hydrophobic on the other, exhibit high tensile strength, and protection against UV light, demonstrating applicability over wide range of applications.

Graphene-Based Fiber Materials for Gas Sensing Applications: State of the Art Review

The importance of gas sensors is apparent as the detection of gases and pollutants is crucial for environmental monitoring and human safety. Gas sensing devices also hold the potential for medical applications as health monitoring and disease diagnostic tools. Gas sensors fabricated from graphene-based fibers present a promising advancement in the field of sensing technology due to their enhanced sensitivity and selectivity. The diverse chemical and mechanical properties of graphene-based fibers-such as high surface area, flexibility, and structural stability-establish them as ideal gas-sensing materials. Most significantly, graphene fibers can be readily tuned to detect a wide range of gases, making them highly versatile in gas-sensing technologies. This review focuses on graphene-based composite fibers for gas sensors, with an emphasis on the preparation processes used to achieve these fibers and the gas sensing mechanisms involved in their sensors. Graphene fiber gas sensors are presented based on the chemical composition of their target gases, with detailed discussions on their sensitivity and performance. This review reveals that graphene-based fibers can be prepared through various methods and can be effectively integrated into gas-sensing devices for a diverse range of applications. By presenting an overview of developments in this field over the past decade, this review highlights the potential of graphene-based fiber sensors and their prospective integration into future technologies.

High-Performance Field-Effect Sensing of Ammonia Based on High-Density and Ultrathin Silicon Nanowire Channels

Ultrathin silicon nanowires (SiNWs), grown via a high-yield and low-cost catalytic approach, are ideal building blocks for the construction of highly sensitive field-effect transistor (FET) sensors. In this work, we demonstrate a high-density growth integration of an ultrathin SiNW array, with diameter down to DNW = 24 ± 3 nm and narrow NW-to-NW spacing of only 120 nm, fabricated via an in-plane solid-liquid-solid (IPSLS) approach. Junctionless bottom-gated SiNW FETs are successfully constructed, exhibiting a high on/off current ratio of >107 and a sharp subthreshold swing of 156 mV/dec These provide an excellent platform for realizing high-performance NH3 sensing at room temperature, with a high response of 96.9% at 25 ppm and 38.6% at 2.5 ppm, rapid response time of 7.9 s for 5% response (or 85.8 s for 50% response), and superior selectivity against common volatile organic compound gases in ambient environments. Finally, the field-effect sensing mechanism is attributed to the Schottky barrier modulation by the adsorbed NH3 molecules at the metal/SiNW interface, as confirmed through an epoxy-masked selective region comparative analysis. These results provide a solid basis for the ultrathin catalytic IPSLS-SiNWs to serve as advantageous one-dimensional (1D) channels for the scalable integration of various high-performance and flexible gas sensing applications.

Ultrathin Copper Monosulfide Films for an Optically Semitransparent, Highly Selective Ammonia Chemosensor

Transition-metal sulfides are emerging as promising materials for chemiresistive gas sensors─a field still dominated by semiconducting metal oxides. Despite the availability of materials with tunable electronic, optical, physical, and chemical properties, few studies have moved beyond synthesis to provide strategies for enhancing gas sensing performance through material modification. Here, we present a simple, scalable synthetic strategy for developing an optically semitransparent, flexible NH3 gas sensor with a highly uniform, ultrathin CuS (covellite) active sensing layer. The optical and chemical properties of the CuS were precisely controlled near the percolation threshold of thin-film formation by varying key experimental parameters such as the Cu film thickness (<10 nm) and the sulfurization time (∼90 s) under ambient conditions. Experimental and computational studies of CuS and its NH3 sensing characteristics identify key physicochemical properties. The controlled surface chemistry and morphology of the ultrathin CuS layer demonstrate its effectiveness in functional NH3 sensing devices, which achieve a calculated detection limit of 1.38 ppm for NH3 gas at 150 °C, along with exceptional mechanical robustness and optical semitransparency in the visible-light spectrum.

A multiple biomolecules-based rapid life detection protocol embedded in a rover scientific subsystem for soil sample analysis

The study of whether life exists, is extinct, or not depends on various sophisticated experimental studies, as many different signatures of life can be used. The experimental procedures that can be performed to identify life can be further restricted by time, resources, and mobility constraints. Therefore, any research analyzing the presence of extraterrestrial life must be precise and unambiguous. This research focuses on the objective of the extraterrestrial life detection domain and seeks to provide an efficient protocol that can produce life detection decisions based on empirical data obtained through chemical analysis under time and resource-constrained conditions. While the majority of existing frameworks in this field are designed to identify biomolecules, our goal is to accomplish the same with minimal operational expense and mission complexity. We argue that the thoughtful integration of multiple biomolecular detections with lesser complexity and a robust framework can improve overall mission performance by satisfying the necessary time and resource constraints. In this study, a rapid multiple biomolecules-based life detection protocol (MBLDP-R) from soil samples is developed from scratch and embedded in an operational scientific rover subsystem targeted for planetary analysis missions. The study uses artificial biomolecule samples and simulated extraterrestrial environments to illustrate the suggested protocol's end-to-end process. First, we list a few significant biomolecules, including lipids, proteins, carbohydrates, nucleic acids, ammonia, and pigments. Then, a weighted qualitative test scoring is applied to sort out the best test method for the finally selected biomolecules which are used as operational analogue to showcase the protocol's in-situ analysis and decision-making capabilities. Based on the suitable biomolecules, a scientific exploration subsystem is developed, and the implemented protocol is built to perform onboard sample analysis. Evaluation results show that: (1) the proposed MBLDP-R protocol could effectively predict the classes with an average f1-score of 98.65% (macro) and 90.00% (micro), (2) the area under the Receiver Operating Characteristics (AUC-ROC) curve shows the sample categories to be correctly predicted 92% of the time (97% for Extant, 88% for Extinct, and 92% in the case of NPL), and (3) the protocol is time-efficient with an average completion time of 17.60 min, demonstrating the protocol's rapid nature in detecting biosignatures in soil samples. The research outcome yields useful additional data for related future studies, particularly in the design of scientific frameworks for mission-specific requirements with limited resources while also serving as a reference point for constraint evaluation methods for similar systems.

Electrochemical Impedance Spectroscopy Study of Ceria- and Zirconia-Based Solid Electrolytes for Application Purposes in Fuel Cells and Gas Sensors

In this study, the structural and electrochemical properties of commercial powders of the nominal compositions Ce0.8Gd0.2O1.9, Sc0.1Ce0.01Zr0.89O1.95, and Sc0.09Yb0.01Zr0.9O1.95 were investigated. The materials are prospective candidates to be used in electrochemical devices, i.e., gas sensors and fuel cells. Based on a comparison of the EIS spectra in different atmospheres (synthetic air, 3000 ppm NH3 in argon, 10% H2 in argon), the reactions on the three-phase boundaries were proposed, as well as the conduction mechanisms of the electrolytes were described. The Ce0.8Gd0.2O1.9 material is a mixed ionic-electronic conductor, which makes it suitable for anode material in fuel cells. Moreover, it exhibits an apparent and reversible response for ammonia, indicating the possibility of usage as an NH3 gas-sensing element. In zirconia-based materials, electrical conduction is realized by oxygen ion carriers. Among them, the most promising from an applicative point of view seems to be Sc0.09Yb0.01Zr0.9O1.95, showing a high, reversible reaction with hydrogen.

An ultra-sensitive ammonia sensor based on a quartz crystal microbalance using nanofibers overlaid with carboxylic group-functionalized MWCNTs

Detecting ammonia at low concentrations is crucial in various fields, including environmental monitoring, industrial processes, and healthcare. This study explores the development and performance of an ultra-sensitive ammonia sensor using carboxylic group-functionalized multi-walled carbon nanotubes (f-MWCNTs) overlaid on polyvinyl acetate nanofibers coated on a quartz crystal microbalance (QCM). The sensor demonstrates high responsiveness, with a frequency shift response of over 120 Hz when exposed to 1.5 ppm ammonia, a sensitivity of 23.3 Hz ppm-1 over a concentration range of 1.5-7.5 ppm, and a detection limit of 50 ppb. Additionally, the sensor exhibits a rapid response time of only 14 s, excellent selectivity against other gases, such as acetic acid, formaldehyde, methanol, ethanol, propanol, benzene, toluene, and xylene, and good stability in daily use. These characteristics make the sensor a promising tool for real-time ammonia detection in diverse applications.

Single stranded 1D-helical Cu coordination polymer for ultra-sensitive ammonia sensing at room temperature

With the increasing demand for ammonia applications, there is a significant focus on improving NH3 detection performance at room temperature. In this study, we introduce a groundbreaking NH3 gas sensor based on Cu(I)-based coordination polymers, featuring semiconducting, single stranded 1D-helical nanowires constructed from Cu-Cl and N-methylthiourea (MTCP). The MTCP demonstrates an exceptional response to NH3 gas (>900% at 100 ppm) and superior selectivity at room temperature compared to current materials. The interaction mechanism between NH3 and the MTCP sensor is elucidated through a combination of empirical results and computational calculations, leveraging a crystal-determined structure. This reveals the formation of NH3-Cu and NH3-H3C complexes, indicative of a thermodynamically favorable reaction. Additionally, Ag-doped MTCP exhibits higher selectivity and a response over two times greater than the original MTCP, establishing it as a prominent NH3 detection system at room temperature.

Vapochromic wool fibers for hazardous ammonia detection using xanthohumol biomolecule from natural extract of common hop (Humulus lupulus L.)

Ammonia is a colorless gas, yet it can be fatal if inhaled or ingested in high enough concentrations. Herein, a solid-state colorimetric smart wool (WL) sensor for ammonia was developed. Common hop (Humulus lupulus L.) is a natural resource of spectroscopical dyestuff known as xanthohumol (XN). Wool fabrics were dyed with different concentrations of xanthohumol extract using the high-temperature high-pressure method in the presence of a mordant. The coloration parameters and absorption spectra were employed to explore the yellow-to-white colorimetric shift of the wool fabric after it was exposed to aqueous ammonia. The wool fabric showed an excellent detection limit of 5 to 125 ppm. When the ammonia concentration was increased, the absorbance spectra demonstrated a hypsochromic shift from 498 nm to 367 nm. This could be attributed to changes in the molecular structure of xanthohumol that happen owing to intramolecular charge delocalization. Using transmission electron microscopy (TEM), the mordant/xanthohumol nanoparticles were measured to have diameters of 15-40 nm. The xanthohumol-finished wool fabrics showed good colorfastness properties. The incorporation of mordant/xanthohumol nanoparticles into wool fabrics showed no negative effects on their stiffness or air-permeability.

In-Sensor Organic Electrochemical Transistor for the Multimode Neuromorphic Olfactory System

The olfactory system is one of the six basic sensory nervous systems. Developing artificial olfactory systems is challenging due to the complexity of chemical information decoding and memory. Conventional chemical sensors can convert chemical signals into electric signals to decode gas information but they lack memory functions. Additional storage and processing units would significantly increase the complexity and power consumption of the devices, especially for portable and wearable devices. Here, an olfactory-inspired in-sensor organic electrochemical transistor (OI-OECT) is proposed, with the integrated functions of chemical information decoding, tunable memory level, and selectivity of vapor sensing. The ion-gel electrolyte endows the OI-OECT with the function of tunable memory levels and a low operating voltage. Typical synaptic behaviors, including inhibitory postsynaptic current and paired-pulse facilitations, are successfully achieved. Importantly, the gas memory level can be effectively modulated by the gate voltages (0 and -1 V), which realized the transformation of volatile and nonvolatile memory. Furthermore, benefiting from the recognition of multiple gases and ability to detect cumulative damage caused by gases, the OI-OECT is demonstrated for early warning system targeting leakage detection of two gases (NH3 and H2S). This work achieves the integrated functions of chemical gas information decode, tunable gas memory level, and selectivity of gas in a single device, which provides a promising pathway for the development of future artificial olfactory systems.

Resistive Gas Sensors Based on 2D TMDs and MXenes

ConspectusGas sensors are used in various applications to sense toxic gases, mainly for enhanced safety. Resistive sensors are particularly popular owing to their ability to detect trace amounts of gases, high stability, fast response times, and affordability. Semiconducting metal oxides are commonly employed in the fabrication of resistive gas sensors. However, these sensors often require high working temperatures, bringing about increased energy consumption and reduced selectivity. Furthermore, they do not have enough flexibility, and their performance is significantly decreased under bending, stretching, or twisting. To address these challenges, alternative materials capable of operating at lower temperatures with high flexibility are needed. Two-dimensional (2D) materials such as MXenes and transition-metal dichalcogenides (TMDs) offer high surface area and conductivity owing to their unique 2D structure, making them promising candidates for realization of resistive gas sensors. Nevertheless, their sensing performance in pristine form is typically weak and unacceptable, particularly in terms of response, selectivity, and recovery time (trec). To overcome these drawbacks, several strategies can be employed to enhance their sensing properties. Noble-metal decoration such as (Au, Pt, Pd, Rh, Ag) is a highly promising method, in which the catalytic effects of noble metals as well as formation of potential barriers with MXenes or TMDs eventually contribute to boosted response. Additionally, bimetallic noble metals such as Pt-Pd and Au/Pd with their synergistic properties can further improve sensor performance. Ion implantation is another feasible approach, involving doping of sensing materials with the desired concentration of dopants through control over the energy and dosage of the irradiation ions as well as creation of structural defects such as oxygen vacancies through high-energy ion-beam irradiation, contributing to enhanced sensing capabilities. The formation of core-shell structures is also effective, creating numerous interfaces between core and shell materials that optimize the sensing characteristics. However, the shell thickness needs to be carefully optimized to achieve the best sensing output. To reduce energy consumption, sensors can operate in a self-heating condition where an external voltage is applied to the electrodes, significantly lowering the power requirements. This enables sensors to function in energy-constrained environments, such as remote or low-energy areas. An important advantage of 2D MXenes and TMDs is their high mechanical flexibility. Unlike semiconducting metal oxides that lack mechanical flexibility, MXenes and TMDs can maintain their sensing performance even when integrated onto flexible substrates and subjected to bending, tilting, or stretching. This flexibility makes them ideal for fabricating flexible and portable gas sensors that rigid sensors cannot achieve.

Construction of "ant-like tentacle" structure for ultra-sensitive detection of low-concentration ammonia through colorimetric fluorescent dual-signal gas-sensitive cotton fabric

Detection and monitoring of ammonia (NH3) are crucial in various industries, including plant safety management, food freshness testing, and water pollution control. Nevertheless, creating portable, low-cost, highly sensitive, and easily regenerated ppm-level NH3 sensors poses a significant challenge. In this investigation, an innovative "ant-like tentacle" fabrication strategy was proposed, and a colorimetric fluorescent dual-signal gas-sensitive cotton fabric (PAH-fabric) for NH3 detection was successfully prepared by conventional dyeing using suitable molecular-level photoacid (PAH) sensitive units. The visual recognition lower detection limit of the ultra-low is 1.09 ppm-level. PAH-fabric is not only straightforward, convenient, and cost-effective to prepare, but it can also be efficiently regenerated and recycled multiple times (maintaining excellent gas-sensitive performance even after 100 cycles) by strategically leveraging volatile acid fumigation. Detailed molecular reaction mechanisms involved in the NH3 response and PAH-fabric regeneration are elucidated. PAH-fabric, available either as a portable kit or an alarm system, offers a promising approach for ultra-low NH3 detection. The demonstrated "ant-like tentacle" fabrication strategy introduces numerous possibilities for designing and developing sensors with adjustable response thresholds, particularly those requiring high sensitivity.

MXene Supported Surface Plasmon Polaritons for Optical Microfiber Ammonia Sensing

The properties of surface plasmons are notoriously dependent on the supporting materials system. However, new capabilities cannot be obtained until the technique of surface plasmon enabled by advanced two-dimensional materials is well understood. Herein, we present the experimental demonstration of surface plasmon polaritons (SPPs) supported by single-layered MXene flakes (Ti3C2Tx) coating on an optical microfiber and its application as an ammonia gas sensor. Enabled by its high controllability of chemical composition, unique atomistically thin layered structure, and metallic-level conductivity, MXene is capable of supporting not only plasmon resonances across a wide range of wavelengths but also a selective sensing mechanism through frequency modulation. Theoretical modeling and optics experiments reveal that, upon adsorbing ammonia molecules, the free electron motion at the interface between the SiO2 microfiber and the MXene coating is modulated (i.e., the modulation of the SPPs under applied light), thus inducing a variation in the evanescent field. Consequently, a wavelength shift is produced, effectively realizing a selective and highly sensitive ammonia sensor with a 100 ppm detection limit. The MXene supported SPPs open a promising path for the application of advanced optical techniques toward gas and chemical analysis.

Simultaneous measurement of ammonia concentration and gas pressure based on fiber multi-mode and Fabry-Pérot interference

A design of an integrated fiber optic sensor for simultaneously measuring ammonia (NH3) concentration and gas pressure is proposed, which is based on a single mode fiber (SMF)-capillary-no core fiber (NCF) single-mode fiber (SCNS) structure. The measurement process is achieved by simultaneously monitoring the wavelength drift appearing in the transmission and reflection spectrum based on multi-mode and Fabry-Pérot interference. In achieving the desired sensor response, the NCF is coated with the zinc oxide (ZnO) film serving as the sensitive material for NH3, and a gas micro-channel is created on the sidewall of the capillary for gas pressure measurement. The experimental results indicate that the sensor with structural parameters optimization presents sensitivities of -35.52 pm/ppm for NH3 concentration and 4158.57 pm/MPa for gas pressure. The proposed sensor based on SCNS structure is capable of achieving excellent stability and consistency in the presence of mutual interference induced by simultaneous measurement of dual-parameter, demonstrating significant potential in gas measurement applications.

Advances in 2D Materials Based Gas Sensors for Industrial Machine Olfactory Applications

The escalating development and improvement of gas sensing ability in industrial equipment, or "machine olfactory", propels the evolution of gas sensors toward enhanced sensitivity, selectivity, stability, power efficiency, cost-effectiveness, and longevity. Two-dimensional (2D) materials, distinguished by their atomic-thin profile, expansive specific surface area, remarkable mechanical strength, and surface tunability, hold significant potential for addressing the intricate challenges in gas sensing. However, a comprehensive review of 2D materials-based gas sensors for specific industrial applications is absent. This review delves into the recent advances in this field and highlights the potential applications in industrial machine olfaction. The main content encompasses industrial scenario characteristics, fundamental classification, enhancement methods, underlying mechanisms, and diverse gas sensing applications. Additionally, the challenges associated with transitioning 2D material gas sensors from laboratory development to industrialization and commercialization are addressed, and future-looking viewpoints on the evolution of next-generation intelligent gas sensory systems in the industrial sector are prospected.

Resistive gas sensors for the detection of NH3gas based on 2D WS2, WSe2, MoS2, and MoSe2: a review

Transition metal dichalcogenides (TMDs) with a two-dimensional (2D) structure and semiconducting features are highly favorable for the production of NH3gas sensors. Among the TMD family, WS2, WSe2, MoS2, and MoSe2exhibit high conductivity and a high surface area, along with high availability, reasons for which they are favored in gas-sensing studies. In this review, we have discussed the structure, synthesis, and NH3sensing characteristics of pristine, decorated, doped, and composite-based WS2, WSe2, MoS2, and MoSe2gas sensors. Both experimental and theoretical studies are considered. Furthermore, both room temperature and higher temperature gas sensors are discussed. We also emphasized the gas-sensing mechanism. Thus, this review provides a reference for researchers working in the field of 2D TMD gas sensors.

Ammonia Detection by Electronic Noses for a Safer Work Environment

Providing employees with proper work conditions should be one of the main concerns of any employer. Even so, in many cases, work shifts chronically expose the workers to a wide range of potentially harmful compounds, such as ammonia. Ammonia has been present in the composition of products commonly used in a wide range of industries, namely production in lines, and also laboratories, schools, hospitals, and others. Chronic exposure to ammonia can yield several diseases, such as irritation and pruritus, as well as inflammation of ocular, cutaneous, and respiratory tissues. In more extreme cases, exposure to ammonia is also related to dyspnea, progressive cyanosis, and pulmonary edema. As such, the use of ammonia needs to be properly regulated and monitored to ensure safer work environments. The Occupational Safety and Health Administration and the European Agency for Safety and Health at Work have already commissioned regulations on the acceptable limits of exposure to ammonia. Nevertheless, the monitoring of ammonia gas is still not normalized because appropriate sensors can be difficult to find as commercially available products. To help promote promising methods of developing ammonia sensors, this work will compile and compare the results published so far.

Unraveling Ammonia and Trimethylamine Uptake on Conductive Doped Polyaniline

Polyaniline (PAni)-based sensors are a promising solution for ammonia (NH3) detection at the ppb level. However, the nature of the NH3-PAni interaction and underlying drivers remain unclear. This paper proposes to characterize the interaction between doped PAni (dPAni) sensing material and NH3 by using a Knudsen cell. First, to characterize the dPAni interface, the probe-gas method, i.e., titration of surface sites with a gas of specific properties, is deployed. The dPAni interface is found to be homogeneous with more than 96% of surface sites of acid nature or with hydroxyl functional groups. This result highlights that basic gases such as amines might act as interfering gases for NH3 detection by polyaniline-based sensors. Second, the adsorption isotherms of NH3 and trimethylamine (TMA) on dPAni are reported at ambient temperature conditions, 293 K. The uptake of NH3 and TMA on dPAni follows a Langmuir-type behavior. This approach allows for the first time to quantify the uptake of NH3 and TMA on gas-sensor materials and determine typical Langmuir adsorption parameters, i.e., the partitioning coefficient, KLang, and the maximum surface coverage, Nmax. The corresponding values obtained for NH3 and TMA are Klang (NH3) = 19.7 × 10-15 cm3 molecules-1 Nmax (NH3) = 11.6 × 1014 molecules cm-2, KLang (TMA) = 7.0 × 10-15 cm3 molecules-1 Nmax (TMA) = 5.0 × 1014 molecules cm-2. KLang and Nmax values of NH3 are higher than those of TMA, suggesting that NH3 is more efficiently taken up than TMA on dPAni. The results of this work suggest that strong hydrogen bonding drives the performance of a polyaniline-based gas sensor for NH3 and amines. In conclusion, the Knudsen cell approach allows reconsidering the fundamentals of NH3 interactions with dPAni and provides new insights on drivers to enhance sensing properties.

Enhancing Sensitivity in Gas Detection: Porous Structures in Organic Field-Effect Transistor-Based Sensors

Gas detection is crucial for detecting environmentally harmful gases. Organic field-effect transistor (OFET)-based gas sensors have attracted attention due to their promising performance and potential for integration into flexible and wearable devices. This review examines the operating mechanisms of OFET-based gas sensors and explores methods for improving sensitivity, with a focus on porous structures. Researchers have achieved significant enhancements in sensor performance by controlling the thickness and free volume of the organic semiconductor layer. Additionally, innovative fabrication techniques like self-assembly and etching have been used to create porous structures, facilitating the diffusion of target gas molecules, and improving sensor response and recovery. These advancements in porous structure fabrication suggest a promising future for OFET-based gas sensors, offering increased sensitivity and selectivity across various applications.

Preparation and NH3 gas-sensing properties of Ag/β-AgVO3 nanorods

NH3 gas sensors operating at room temperature, consisting of Ag nanoparticles decorated β-AgVO3 nanorods (Ag/β-AgVO3 NRs), were fabricated via a facile hydrothermal method without the need for a template. The surface characteristics and compositions of Ag/β-AgVO3 NRs were analyzed using X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Ag nanoparticles, ranging in diameter from approximately 20 to 40 nm, were dispersed on the surface of monoclinic β-AgVO3 NRs with diameters ranging from 50 to 105 nm and lengths from 0.3 to 1.3 μm. The NH3 gas sensing properties of Ag/β-AgVO3 NRs were studied under both dry air and humid conditions at room temperature. Comparative analysis demonstrated that the Ag/β-AgVO3 NRs exhibited a strong response to NH3 gas under 70% relative humidity (RH) at room temperature compared to α-AgVO3 NRs. Specifically, the response of the Ag/β-AgVO3 NRs to 5 ppm NH3 increased by 2.25 times as the RH varied from 20% to 80% at room temperature. This enhanced response was attributed to the effects of formation of nanoheterojunctions, nano-metallic Ag activity and the conductivity of NH4+ and OH- ions induced by the presence of humidity. The room temperature NH3 gas sensors based on Ag/β-AgVO3 NRs demonstrated strong responses to low NH3 concentrations, high selectivity, good reproducibility, and long-term stability, and show promise for the development of low-power and cost-effective NH3 gas sensors for practical applications even under high humidity.

Surface Acoustic Wave Resonator Chip Setup for the Elimination of Interfering Conductivity Responses

A surface acoustic wave (SAW) resonator chip setup is presented that eliminates interfering signal responses caused by changes in the electrical environment of the surrounding media. When using a two-port resonator, applying electrically shielding layers between the interdigital transducers (IDTs) can be challenging due to the limited dimensions. Therefore, a layered setup consisting of an insulating polymer layer and a conductive gold layer was preferred. The SAW resonators were provided with polycarbonate housings, resulting in SAW resonator chips. This setup enables easy application of a wide range of coatings to the active part of the resonator surface, while ensuring subsequent electrical and fluidic integration of the resonator chips into a microfluidic array for measurements. The signal responses of uncoated SAW resonators and those with polymer coatings with and without a gold layer were tested with aqueous potassium chloride (KCl) solutions up to 3 mol/L, corresponding to conductivities up to 308 mS/cm. The use of a polymer coating at the thickness of the first Love mode resonance and a conductive gold layer completely reduced the electrical impact on the SAW resonator signal response, making small signals resulting from changes in viscosity and density of the KCl solutions visible.

Ammonia gas sensors based on undoped and Ca-doped ZnO nanoparticles

Due to its large use in different industrial sectors, high toxicity, and corrosion, the demand for sensing techniques towards ammonia gas has become urgent. In this study we report on the sensing performances of a conductometric sensor for NH3 gas based on Ca-doped ZnO nanoparticles with different calcium concentrations (0, 1, and 3 at%) synthesized using the sol-gel process under supercritical dry conditions of ethanol. All samples were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and Fourier-transform infrared (FTIR) spectroscopy. Pure and Ca-doped ZnO are polycrystalline and well crystallized in the hexagonal wurtzite structure. TEM images revealed that pure ZnO is composed of spherical particles with dimensions in the nanometer range. Larger particles were observed after the incorporation of Ca ions. The average crystallite size, estimated by the Williamson-Hall method, was 43, 80, and 96 nm for pure, Ca-1 at% and Ca-3 at%, respectively. Furthermore, FTIR spectroscopy was used to prove the formation of ZnO and the incorporation of calcium ions in the Ca-doped ZnO samples. The gas sensing performances towards ammonia gas clearly ameliorated after the addition of Ca ions in the ZnO structure. The gas response to NH3, R0/Rg, of the 1% Ca-doped ZnO sensor reached a value of 33 for 4000 ppm of ammonia at T = 300 °C with good selectivity compared to other gases such as CO, CO2, and NO2. The response and recovery times were 5 s and 221 s, respectively. The reported good sensing performances indicate the potential application of Ca-doped ZnO as a sensor material for ammonia detection.

Highly sensitive ammonia sensor based on a PMMA/PANI microwire structure

In this paper, a highly sensitive ammonia (N H 3) sensor based on a polymethyl methacrylate/polyaniline (PMMA/PANI) microwire structure is designed and implemented. First, a micron-sized PMMA microwire was fabricated and connected with two tapered single-mode fibers to form a coupling structure; thus, the Mach-Zehnder (MZ) interference was successfully excited due to the good light conductivity of the PMMA. It was demonstrated that the coupling structure behaved with a high refractive index detection sensitivity of 3044 nm/RIU. To make it sensitive to N H 3, the PANI was selected to mix with PMMA and then formed a micron-level PMMA/PANI fiber. The experimental results showed that the PMMA/PANI fiber can selectively sense N H 3 with a high sensitivity of 65.3 pm/ppm. This proposed N H 3 sensor not only solves the problem of sensitive film shedding, but also possesses the advantages of good integration, high sensitivity, good selectivity, and short response time.

Photoacoustic Spectroscopy Gas Detection Technology Research Progress

Photoacoustic spectroscopy (PAS) can be utilized as an ultrasensitive gas detection method. The basic principles of gas detection using PAS are discussed in this paper. First, the basic instrumentation for a PAS gas detection system is introduced focusing on the photoacoustic cell. The discussion includes non-resonant photoacoustic cells and the different types of resonant photoacoustic cells, including the longitudinal photoacoustic cell, the Helmholtz photoacoustic cell, the T-type photoacoustic cell, and the high-frequency resonant photoacoustic cell. The basic working principles of each of these, cells as well as the advantages and disadvantages of photoacoustic cells are discussed, and the development of newer types of photoacoustic cells in recent years is outlined in detail. This review provides detailed reference information and guidance for interested researchers who would like to design and build advanced photoacoustic cells for gas detection.

Mixed-Potential Ammonia Sensor Based on a Dense Yttria-Stabilized Zirconia Film Manufactured at Room Temperature by Powder Aerosol Deposition

Powder aerosol deposition (often abbreviated as PAD, PADM, or ADM) is a coating method used to obtain dense ceramic films at room temperature. The suitability of this method to obtain ammonia mixed-potential sensors based on an yttria-stabilized zirconia (YSZ) electrolyte that is manufactured using PAD and a V2O5-WO3-TiO2 (VWT)-covered electrode is investigated in this study. The sensor characteristics are compared with data from sensors with screen-printed YSZ solid electrolytes. The PAD sensors outperform those in terms of sensitivity with 117 mV/decade NH3 compared to 88 mV/decade. A variation in the sensor temperature shows that the NH3 sensitivity strongly depends on the sensor temperature and decreases with higher sensor temperature. Above 560 °C, the characteristic curve shifts from exponential to linear dependency. Variations in the water and the oxygen content in the base gas (usually 10% oxygen, 2% water vapor in nitrogen) reveal a strong dependence of the characteristic curve on the oxygen content. Water vapor concentration variations barely affect the sensor signal.

One-pot synthesis of novel chitosan-salicylaldehyde polymer composites for ammonia sensing

Chitosan (Chs)-salicylaldehyde (Sal) polymer derivatives were formed via the reaction of Chs-Sal with zinc oxide nanoparticles (ZnO NPs) and beta-cyclodextrin (β-CD). These polymers were synthesized through inclusion with β-CD and doping with ZnO NPs to give pseudopolyrotaxane and Chs-Sal/ZnO NPs composite, respectively, for low-temperature detection and sensing of NH3 vapors as great significance in environmental control and human health. Additionally, the polymer (Chs-Sal/β-CD/ZnO NPs) was prepared via the insertion of generated composite (Chs-Sal/ZnO NPs) through β-cyclodextrin ring. The structural and morphological characterizations of the synthesized derivatives were confirmed by utilizing FTIR, XRD and, SEM, respectively. Also, the optical properties and thermal gravimetric analysis (TGA) of the synthesized polymers were explored. The obtained results confirmed that using β-CD or ZnO NPs for modification of polymer (Chs-Sal) dramatically enhanced thermal stability and optical features of the synthesized polymers. Investigations on the NH3-sensing properties of Chs-Sal/β-CD/ZnO NPs composite were carried out at concentrations down to 10 ppm and good response and recovery times (650 s and 350 s, respectively) at room temperature (RT) and indicated that modification by β-CD and doping with ZnO NPs effectively improves the NH3-sensing response of Chs-Sal from 712 to 6192 using Chs-Sal/β-CD/ZnO NPs, respectively, with low LOD and LOQ of 0.12 and 0.4 ppb, respectively.

Wearable Gas Sensor Based on Reticular Antimony-Doped SnO2/PANI Nanocomposite Realizing Intelligent Detection of Ammonia within a Wide Range of Humidity

Wearable gas sensors demonstrate broad potential for environmental monitoring and breath analysis applications. Typically, they require a highly stable and high-performance flexible gas sensing unit that can work with a small, flexible circuit to enable real-time accurate concentration analysis and prediction. This work proposes a flexible gas sensor using antimony-doped tin dioxide composite polyaniline as the sensing material for room-temperature ammonia detection over a wide humidity range. The sensor exhibits high sensitivity (response value at 33.1 toward 100 ppm ammonia at 70% relative humidity), excellent selectivity, and good long-term and mechanical stability. The increased sensitivity is due to a reduction in the hole concentration of polyaniline in air, achieved through compositing and doping. Subsequently, regression analysis equations are developed to establish the relationship between the gas concentration and sensor response under varying environmental humidity conditions. The sensor was integrated with a small, low-power circuit module to form a wearable smart bracelet with signal acquisition, processing, and wireless transmission functions, which could achieve early and remote warning of gas leakage in different humidity environments. This research demonstrates a promising approach to designing high-performance, high-stability, and flexible gas sensors and their corresponding wireless sensing systems.

Advantages of Slow Sensing for Ambient Monitoring: A Practical Perspective

Air pollution is a ubiquitous threat, affecting 99% of the global populace and causing millions of premature deaths annually. Monitoring ambient air quality is essential, aiding policymakers and environmental agencies in timely interventions. This study delves into the advantages of slower gas sensors over their ultrafast counterparts, with a keen focus on their practicality in real-world scenarios. Slow sensors offer accurate time-averaged exposure assessments, harmonizing with established regulatory benchmarks. Their heightened precision and reliability, complemented by their cost-effectiveness, render them eminently suitable for large-scale deployment. The slow sensing ensures compatibility with regulations, fostering robust risk management practices. In contrast, ultrafast sensors, while claiming rapid detection, despite touting swift detection capabilities, grapple with formidable challenges. The sensitivity of ultrafast sensors to uncontrolled atmospheric effects, fluctuations in pressure, rapid response times, and uniform gas dispersion poses significant hurdles to their reliability. Addressing these issues assumes paramount significance in upholding the integrity of air quality assessments.

Chitosan Film Sensor for Ammonia Detection in Microdiffusion Analytical Devices

Chitosan films have attracted increased attention in the field of sensors because of chitosan's unique chemico-physical properties, including high adsorption capacity, filmability and transparency. A chitosan film sensor was developed through the dispersion of an ammonia specific reagent (Nessler's reagent) into a chitosan film matrix. The chitosan film sensor was characterized to assess the film's properties by Fourier transform infrared spectroscopy (FTIR), thermogravimetric analysis (TGA), scanning electron microscopy (SEM) and differential scanning calorimetry (DSC). A gas diffusion device was prepared with the chitosan film sensor, enabling the collection and detection of ammonia vapor from biological samples. The chitosan film sensor color change was correlated with the ammonia concentration in samples of human serum and artificial urine. This method enabled facile ammonia detection and concentration measurement, making the sensor useful not only in clinical laboratories, but also for point-of-care devices and wherever there is limited access to modern laboratory facilities.

Polycaprolactone/Anthocyanin-Based Electrospun Volatile Amines Gas Indicator with Improved Visibility by Varying Bi-Solvent Ratio: A Case of Intelligent Packaging of Mackerel

Electrospun nanofibers have been applied as a new technology for gas indicators in food intelligent packaging. A poly(ε-caprolactone) (PCL)/red cabbage anthocyanin (RCA)-based nanofiber volatile amines gas indicator was developed by applying a bi-solvent of acetic acid (AA) and formic acid (FA) in electrospinning. The visibility of color change was improved from pink to blue, compared to blue to yellow-green, when using a single solvent of AA. The solutes of PCL (12.5, 15, 17.5, and 20%) and RCA (10, 20, 30, and 40%) and the solvents of AA/FA (9:1, 7:3, 5:5, 3:7, and 1:9) were applied in electrospinning under the condition of 12.5 cm, 1.0 mL/h, and 20 kV. The optimal microstructure with the thinnest fiber diameter and constant arrangement without forming NF beads appeared under the 7:3 FA/AA, 15% PCL, and 20% RCA condition. The indicator changed from pink to blue with the values of total color change (ΔE) of 10, 14, and 18 when exposed to the saturated gas of ammonia solutions of 8, 80, and 800 mM, respectively. The indicator was stable and unchanged in color for 28 days when exposed to light at room temperature. In the application to mackerel packaging, the built-in indicator changed from pink to purple regardless of storage temperature when the spoilage point was reached.

Nanotechnology and E-Sensing for Food Chain Quality and Safety

Nowadays, it is well known that sensors have an enormous impact on our life, using streams of data to make life-changing decisions. Every single aspect of our day is monitored via thousands of sensors, and the benefits we can obtain are enormous. With the increasing demand for food quality, food safety has become one of the main focuses of our society. However, fresh foods are subject to spoilage due to the action of microorganisms, enzymes, and oxidation during storage. Nanotechnology can be applied in the food industry to support packaged products and extend their shelf life. Chemical composition and sensory attributes are quality markers which require innovative assessment methods, as existing ones are rather difficult to implement, labour-intensive, and expensive. E-sensing devices, such as vision systems, electronic noses, and electronic tongues, overcome many of these drawbacks. Nanotechnology holds great promise to provide benefits not just within food products but also around food products. In fact, nanotechnology introduces new chances for innovation in the food industry at immense speed. This review describes the food application fields of nanotechnologies; in particular, metal oxide sensors (MOS) will be presented.

Room-temperature chemiresistive ammonia sensors based on 2D MXenes and their hybrids: recent developments and future prospects

Detection of ammonia (NH3) gas at room temperature is essential in a variety of sectors, including pollution monitoring, commercial safety and medical services, etc. Two-dimensional (2D) materials have emerged as fascinating candidates for gas-sensing applications due to their distinct properties. MXenes, a type of 2D transition metal carbides/nitrides/carbonotrides, have drawn the interest of researchers due to their high conductivity, large surface area, and changing surface chemistry. The review begins by describing the NH3 gas-detecting methods of 2D materials and then concentrates on MXene-based sensors, emphasising the benefits that MXenes provide in this context. The study also explains the prime factors involved in evaluating sensor performance, which include sensor response, sensitivity, selectivity, stability, charge transfer values, adsorption energy and response/recovery times. Subsequently, the review covers two main categories: pristine/intercalated MXenes and MXene-based hybrid materials. The review investigates the approaches for improving the sensing characteristics of pristine and intercalated MXenes by introducing MXene hybrids like MXene-metal oxide hybrids, MXene-transition metal dichalcogenides hybrid, MXene-other 2D materials hybrid, MXene-polymers and other hybrids and other MXene-derived materials. In summary, this review offers a thorough overview of current advancements and potential applications for room-temperature ammonia sensors based on 2D MXenes and their hybrids. In order to pave the way for future improvements in MXene-based gas-sensing technology for room temperature ammonia detection, the study concludes by outlining potential future scope and conclusions.

Thin Films of Chlorinated Vanadyl Phthalocyanines as Active Layers of Chemiresistive Sensors for the Detection of Ammonia

Halogenated metal phthalocyanines are promising materials for the manufacture of active layers of chemiresistive sensors for the detection of various gases. Despite the high interest in such sensors, there are few systematic studies of the position of halogen substituents in phthalocyanine macroring on the chemiresistive response of their films to gases. In this work, we prepared and studied films of novel tetrachlorosubstituted vanadyl phthalocyanine derivatives with Cl substituents in the peripheral (VOPcCl4-p) and nonperipheral (VOPcCl4-np) positions of the phthalocyanine ring as active layers of chemiresistive sensors to reveal the effect of the position of substituents on their structure and sensor response to low concentrations of NH3. It was shown that the films of VOPcCl4-p exhibited a noticeably higher sensor response to NH3 than the VOPcCl4-np ones. The limit of detection of NH3 was 0.7 ppm. The sensing layers demonstrated a reversible sensor response at room temperature with fairly low response/recovery times. It was also demonstrated that NH3 can be detected in the presence of various interfering gases (CO2 and H2) and some volatile organic vapors, as well as in a mixture of gases with a composition close to exhaled air.

Long-term performance validation of NH3 concentration prediction model for virtual sensor application in livestock facility

Livestock facilities commonly generate NH3, a hazardous substance that may also harm livestock. Therefore, monitoring of NH3 concentrations in livestock facilities is necessary to ensure proper control. However, NH3 is alkaline and toxic, causing corrosion inside detection sensors and making monitoring difficult. This study proposes a virtual sensor concept to complement the durability of NH3 physical sensors. The study also conducts a long-term performance validation of a data-driven NH3 concentration prediction model. Results indicate that the model's prediction performance declines sharply when the data generation pattern inside the livestock facility changes due to changes in outdoor conditions and facility operation. Furthermore, the prediction performance of the model differed depending on the training data period settings when updating the model. Hence, the model needs versioning and update management to respond to the data generation pattern in the livestock facility when operating the NH3 concentration virtual sensor. The virtual sensor is expected to enhance monitoring and reduce sensor management costs in livestock facilities.

Tailoring the microstructure of BiVO4 sensing electrode by nanoparticle decoration and its effect on hazardous NH3 sensing

Real-time monitoring and quantification of exhaust pollutants is crucial but is troublesome because of extremely harsh thermochemical conditions, and in this regard mixed-potential sensing technology can be a realistic solution. In this study, BiVO₄ nanoparticles are decorated onto the preformed porous sensing electrode (SE) backbone by homogeneous infiltration process to improve the sensing performance in mixed-potential sensor. The influence of nanoparticle decoration on phase composition, microstructure and sensing performance are analyzed by physical and electrochemical techniques. Corresponding results indicate that the microstructure tailoring enhances the sensor performance, by extending the triple phase boundary (TPB) and surface area of SE itself. The sensitivity (-119.47 mV/decade) and response time (20 s) of i-BVO SE-based sensor at 600 ℃ are 20 % higher and 8 s faster than bare BiVO₄ SE-based sensor (99.24 mV/decade and 28 s). Additionally, the i-BVOǀYSZǀPt cell exhibits good selectivity and cross-sensitivity toward NH₃ without any dependency on oxygen partial pressure (pO₂). The fabricated sensor is also found stable towards cyclic and long-term operations. Electrochemical Impendence Spectroscopy (EIS) and DC polarization studies were performed to confirm the mixed-potential behavior. Conclusively, the superior sensing performance of i-BVO SE compared to various oxide based SEs highlights its suitability for mixed-potential NH₃ sensing.

Quantum Annealing Boosts Prediction of Multimolecular Adsorption on Solid Surfaces Avoiding Combinatorial Explosion

Quantum annealing has been used to predict molecular adsorption on solid surfaces. Evaluation of adsorption, which takes place in all solid surface reactions, is a crucially important subject for study in various fields. However, predicting the most stable coordination by theoretical calculations is challenging for multimolecular adsorption because there are numerous candidates. This report presents a novel method for quick adsorption coordination searches using the quantum annealing principle without combinatorial explosion. This method exhibited much faster search and more stable molecular arrangement findings than conventional methods did, particularly in a high coverage region. We were able to complete a configurational prediction of the adsorption of 16 molecules in 2286 s (including 2154 s for preparation, only required once), whereas previously it has taken 38 601 s. This approach accelerates the tuning of adsorption behavior, especially in composite materials and large-scale modeling, which possess more combinations of molecular configurations.

Accelerating the Gas-Solid Interactions for Conductometric Gas Sensors: Impacting Factors and Improvement Strategies

Metal oxide-based conductometric gas sensors (CGS) have showcased a vast application potential in the fields of environmental protection and medical diagnosis due to their unique advantages of high cost-effectiveness, expedient miniaturization, and noninvasive and convenient operation. Of multiple parameters to assess the sensor performance, the reaction speeds, including response and recovery times during the gas-solid interactions, are directly correlated to a timely recognition of the target molecule prior to scheduling the relevant processing solutions and an instant restoration aimed for subsequent repeated exposure tests. In this review, we first take metal oxide semiconductors (MOSs) as the case study and conclude the impact of the semiconducting type as well as the grain size and morphology of MOSs on the reaction speeds of related gas sensors. Second, various improvement strategies, primarily including external stimulus (heat and photons), morphological and structural regulation, element doping, and composite engineering, are successively introduced in detail. Finally, challenges and perspectives are proposed so as to provide the design references for future high-performance CGS featuring swift detection and regeneration.

Fiber Optic Sensor of Ammonia Gas Using Plasmonic Extraordinary Optical Transmission

While standard surface plasmon resonance (bio) sensing, relaying on propagating surface plasmon polariton sensitivity on homogeneous metal/dielectric boundaries, represents nowadays a routine sensing technique, other alternatives, such as inverse designs with nanostructured plasmonic periodic hole arrays, have been far less studied, especially in the context of gas sensing applications. Here, we present a specific application of such a plasmonic nanostructured array for ammonia gas sensing, based on a combination of fiber optics, extraordinary optical transmission (EOT) effect, and chemo-optical transducer selectively sensitive to ammonia gas. The nanostructured array of holes is drilled in a thin plasmonic gold layer by means of focused ion beam technique. The structure is covered by chemo-optical transducer layer showing selective spectral sensitivity towards gaseous ammonia. Metallic complex of 5-(4'-dialkylamino-phenylimino)-quinoline-8-one dye soaked in polydimethylsiloxane (PDMS) matrix is used in place of the transducer. Spectral transmission of the resulting structure and its changes under exposition to ammonia gas of various concentrations is then interrogated by fiber optics tools. The observed VIS-NIR EOT spectra are juxtaposed to the predictions performed by the rigorous Fourier modal method (FMM), providing useful theoretical feedback to the experimental data, and ammonia gas sensing mechanism of the whole EOT system and its parameters are discussed.

Quantifying the dynamic characteristics of indoor air pollution using real-time sensors: Current status and future implication

People generally spend most of their time indoors, making indoor air quality be of great significance to human health. Large spatiotemporal heterogeneity of indoor air pollution can be hardly captured by conventional filter-based monitoring but real-time monitoring. Real-time monitoring is conducive to change air assessment mode from static and sparse analysis to dynamic and massive analysis, and has made remarkable strides in indoor air evaluation. In this review, the state of art, strengths, challenges, and further development of real-time sensors used in indoor air evaluation are focused on. Researches using real-time sensors for indoor air evaluation have increased rapidly since 2018, and are mainly conducted in China and the USA, with the most frequently investigated air pollutants of PM_2.5. In addition to high spatiotemporal resolution, real-time sensors for indoor air evaluation have prominent advantages in 3-dimensional monitoring, pollution peak and source identification, and short-term health effect evaluation. Huge amounts of data from real-time sensors also facilitate the modeling and prediction of indoor air pollution. However, challenges still remain in extensive deployment of real-time sensors indoors, including the selection, performance, stability, as well as calibration of sensors. In future, sensors with high performance, long-term stability, low price, and low energy consumption are welcomed. Furthermore, more target air pollutants are also expected to be detected simultaneously by real-time sensors in indoor air monitoring.

Electronic sensitization enhanced p-type ammonia gas sensing of zinc doped MoS2/RGO composites

Zinc (Zn) doping induced synergetic effects of defects engineering and heterojunction in Molybdenum disulphide/Reduced graphene oxide (MoS₂/RGO) effectively enhances the p-type Volatile organic compounds (VOC) gas sensing traits and helps in tailoring the over dependence on noble metals for surface sensitization. Through this work, we have successfully prepared Zn doped MoS₂ grafted on RGO employing an in-situ hydrothermal method. Optimal doping concentration of Zn dopants in the MoS₂ lattice triggered more active sites on the basal plane of MoS₂ with the aid of defects promoted by the zinc dopants. Effective intercalation of RGO further boost up the exposed surface area of Zn doped MoS₂ for further interaction of ammonia gas molecules. Besides, smaller crystallite size brought out by 5% Zn dopants aids in efficient charge transfer across the heterojunctions that further amplifies the ammonia sensing traits with a peak response of 32.40% along with a response time of 21.3 s and recovery time of 44.90 s. The as prepared ammonia gas sensor exhibited excellent selectivity and repeatability. The obtained results reveal that transition metal doping into the host lattice proves to be a promising approach for VOC sensing characteristics of p-type gas sensors and gives insight about the importance of dopants and defects for the development of highly efficient gas sensors in the future.

Threshold-Responsive Colorimetric Sensing System for the Continuous Monitoring of Gases

Colorimetric sensors are widely used because of their inherent advantages including accuracy, rapid response, ease-of-use, and low costs; however, they usually lack reusability, which precludes the continuous use of a single sensor. We have developed a threshold-responsive colorimetric system that enables repeated analyte measurements by a single colorimetric sensor. The threshold responsive algorithm automatically adjusts the sensor exposure time to the analyte and measurement frequency according to the sensor response. The system registers the colorimetric sensor signal change rate, prevents the colorimetric sensor from reaching saturation, and allows the sensor to fully regenerate before the next measurement is started. The system also addresses issues common to colorimetric sensors, including the response time and range of detection. We demonstrate the benefits and feasibility of this novel system, using colorimetric sensors for ammonia and carbon dioxide gases for continuous monitoring of up to (at least) 60 detection cycles without signs of analytical performance degradation of the sensors.

Room-temperature ammonia gas sensing via Au nanoparticle-decorated TiO2 nanosheets

A high-performance gas sensor operating at room temperature is always favourable since it simplifies the device fabrication and lowers the operating power by eliminating a heater. Herein, we fabricated the ammonia (NH₃) gas sensor by using Au nanoparticle-decorated TiO₂ nanosheets, which were synthesized via two distinct processes: (1) preparation of monolayer TiO₂ nanosheets through flux growth and a subsequent chemical exfoliation and (2) decoration of Au nanoparticles on the TiO₂ nanosheets via hydrothermal method. Based on the morphological, compositional, crystallographic, and surface characteristics of this low-dimensional nano-heterostructured material, its temperature- and concentration-dependent NH₃ gas-sensing properties were investigated. A high response of ~ 2.8 was obtained at room temperature under 20 ppm NH₃ gas concentration by decorating Au nanoparticles onto the surface of TiO₂ nanosheets, which generated oxygen defects and induced spillover effect as well.

High-sensitivity NH3 gas sensor using pristine graphene doped with CuO nanoparticles

A highly sensitive and selective NH₃ gas sensor was developed based on single-layer pristine graphene doped with copper(II) oxide (CuO) nanoparticles of a specific size. High-quality single-layer graphene was grown using chemical vapor deposition. Approximately 15 nm-sized CuO colloidal nanoparticles were fabricated by a microwave-assisted thermal method using copper acetate as the precursor, and dimethylformamide as the reducing and stabilizing agent. Pristine graphene was doped with an aqueous suspension of CuO nanoparticles at a coating speed of 1500 rpm using a simple spin coater. CuO nanoparticle doping induces changes in the electronic properties of graphene; in particular, p-type doping significantly altered graphene resistivity in the presence of NH₃ gas. Upon exposure of the pristine graphene surface to NH₃ gas, NH₃ reacted with O₂^-/ O^-/ O^2- species on the graphene surface and released electrons into graphene. This caused a change in the concentration of charge carriers in the valence channel of graphene and an increase in graphene resistivity, facilitating real-time NH₃ monitoring with quick response and rapid recovery at 25 ℃ and ~ 55% relative humidity. Our results indicated that graphene doped with ~ 15 nm-sized CuO nanoparticles can sense NH₃ gas selectively with a resistivity response of ~ 83%. Moreover, the sensor exhibited good reusability, fast response (~ 19 s), and rapid recovery (~ 277 s) with a detection limit of 0.041 ppm and a relative standard deviation of 0.76%.

Highly sensitive Cu-ethylenediamine/PANI composite sensor for NH3 detection at room temperature

Ammonia detection is needed in several sectors including environmental monitoring, automobile industry, and in medical diagnosis. Conducting polymers, such as polyaniline (PANI), have been utilized to develop NH₃ sensors operating at room temperature. However, the performance of these sensors in terms of sensitivity and selectivity need improvement. Functionalization of conducting PANI with metal nanocomposites have shown improved sensor performance. In this work, we report a highly sensitive copper-based nanocomposite for NH₃ detection. The novelty lies in utilization of copper-ethylenediamine (Cu-en) nanocomposite functionalized over PANI for gas sensing. Resistance of the 20 wt% Cu-en with PANI increased 3.8 times upon exposure to 100 ppm of NH₃. The nanocomposite sensor detected NH₃ concentrations as low as 2 ppm. Further, the sensing mechanism was studied by in-situ IV characteristics and impedance spectroscopy during NH₃ exposure. NH₃ showed ionic interaction with PANI, and Cu²⁺. The strong affinity of Cu²⁺ for the lone pair of NH₃ enhanced the sensor response from 0.78 to 3.8 for 100 ppm of NH₃ at 20 °C. The sensor response was completely recovered after heating at 75 °C, which indicates reusability of the sensor. The sensor showed selectivity for NH₃ over ethanol and H₂S. The response was reasonably stable after bending the flexible sensor for 1000 times at a radius of 5 mm.