Ultrathin 2D NbWO6 Perovskite Semiconductor Based Gas Sensors with Ultrahigh Selectivity under Low Working Temperature

Recent Advances in Structure Design and Application of Metal Halide Perovskite-Based Gas Sensor

Metal halide perovskites (MHPs) are emerging gas-sensing materials and have attracted considerable attention in gas sensors due to their unique bandgap structure and tunable optoelectronic properties. The past decade has witnessed significant developments in the gas-sensing field; however, their intrinsic structural instability and ambiguous gas-sensing mechanisms hamper their practical applications. Herein, we summarize the recent advances in MHP-based gas sensors. The physicochemical properties of MHPs are discussed at first. The structure design, including dimension design and engineering design, is overviewed as well as their fabrication methods, and we put forward our insights into the gas-sensing mechanism of MHPs. It is believed that enhanced understanding of gas-sensing mechanisms of MHPs are helpful for their application as gas-sensing materials, and structure design can enhance their stability, sensing sensitivity, and selectivity to target gases as gas sensors. Subsequently, the latest developments in MHP-based gas sensors are summarized according to their different application scenarios. Finally, we conclude with the current status and challenges in this field and propose future perspectives.

Conformal sulfidation of HKUST-1 for constructing porous Cu2S/CuO octahedrons realizing highly sensitive non-enzymatic glucose detection

Metal-organic frameworks (MOFs) are believed to be promising precursors for constructing novel and efficient catalysts for glucose sensing. Herein, HKUST-1 precursors are first fabricated using a one-pot hydrothermal approach, and then HKUST-1 is converted into porous Cu2S/CuO octahedrons through conformal sulfidation with the help of OH-ions. The as-obtained Cu2S/CuO composite can provide rich electrochemical active sites and promoted electric transfer kinetics. Benefiting from these combined merits, the as-fabricated Cu2S/CuO composite is confirmed to be a high-performance catalyst, with high sensitivities of 8269.45 and 4140.82μA mM-1cm-2in the corresponding ranges of 0.05 ∼ 0.6 mM and 0.6 ∼ 1.2 mM, respectively. Moreover, the as-prepared electrode materials possess good anti-interference ability, reproducibility and long-term stability. This work opens up new avenues for the design and preparation of transition metal sulfide composites.

Venus Flytrap-Inspired Data-Center-Free Fast-Responsive Soft Robots Enabled by 2D Ni3(HITP)2 MOF and Graphite

The rapid and responsive capabilities of soft robots in perceiving, assessing, and reacting to environmental stimuli are highly valuable. However, many existing soft robots, designed to mimic humans and other higher animals, often rely on data centers for the modulation of mechanoelectrical transduction and electromechanical actuation. This reliance significantly increases system complexity and time delays. Herein, drawing inspiration from Venus flytraps, a soft robot employing a power modulation strategy is presented for active stimulus reaction, eliminating the need for a data center. This robot achieves mechanoelectrical transduction through Ni3(2,3,6,7,10,11-hexaiminotriphenylene)2 (Ni3(HITP)2) metal-organic framework (MOF) with an ultralow time delay (256 ns) and electromechanical actuation via graphite. The Joule heating effect in graphite is effectively modulated by Ni3(HITP)2 before and after the presence of pressure, thus enabling the stimulus reaction of soft robots. As demonstrated, three soft robots are created: low-level edge tongue robots, Venus flytrap robots, and high-level nerve-center-controlled dragonfly robots. This power modulation strategy inspires designs of edge soft robots and high-level robots with a human-like effective fusion of conditioned and unconditioned reflexes.

Sabatier phenomenon in chemoselective gas-sensing reactions induced by Ag cluster coordination

The Sabatier principle in heterogeneous catalysis provides guidance for designing optimal catalysts with the highest activity. We report a new Sabatier phenomenon induced by nanoclusters on different atomic scales in gas-sensitive reactions. We prepared a series of Ag nanocluster catalysts with coordination structures ranging from Ag0 to Ag13 through a surface coordination strategy. When used as catalysts for gas-sensitive reactions, a volcano-type relationship between the coordination number of Ag nanoclusters and gas-sensitive activity emerges, with a summit at a moderate coordination of Ag5. Mechanistic studies show that the efficient adsorption of activated *C2H6O on electron-rich Ag5 clusters is a key factor for the Sabatier phenomenon (adsorption energy from -0.322 eV to -0.663 eV), which leads to highly selective sensing. We found that the catalyst electron-rich surface layer induced by Ag5 clusters serves as a descriptor to explain the structure-activity relationship. Furthermore, due to the well-defined geometric and electronic structures in the Ag5 clusters, the optimized catalyst achieves both maximum activity and selectivity in chemoselective sensing reactions. This study reveals the Sabatier principle and provides insightful guidance for the rational design of more efficient and practical nanocluster catalysts for heterogeneous catalysis.

Room-Temperature Sensing Mechanism of GQDs/BiSbO4 Nanorod Clusters: Experimental and Density Functional Theory Study

Creating high-performance gas sensors for heptanal detection at room temperature demands the development of sensing materials that incorporate distinct spatial configurations, functional components, and active surfaces. In this study, we employed a straightforward method combining hydrothermal strategy with ultrasonic processing to produce mesoporous graphene quantum dots/bismuth antimonate (GQDs/BiSbO4) with nanorod cluster forms. The BiSbO4 was incorporated with appropriate contents of GQDs resulting in significantly improved attributes such as heightened sensitivity (59.6@30 ppm), a lower threshold for detection (356 ppb), and quicker period for response (40 s). A synergistic mechanism that leverages the inherent advantages of BiSbO4 was proposed, while its distinctive mesoporous hollow cubic structure, the presence of oxygen vacancies, and the catalytic enhancement provided by GQDs lead to a marked improvement in heptanal detection. This work introduces a straightforward and effective method for crafting sophisticated micro-nanostructures that optimize spatial design, functionality, and active mesoporous surfaces, showing great promise for heptanal sensing applications.

Nanowire Array Breath Acetone Sensor for Diabetes Monitoring

Diabetic ketoacidosis (DKA) is a life-threatening acute complication of diabetes characterized by the accumulation of ketone bodies in the blood. Breath acetone, a ketone, directly correlates with blood ketones. Therefore, monitoring breath acetone can significantly enhance the safety and efficacy of diabetes care. In this work, the design and fabrication of an InP/Pt/chitosan nanowire array-based chemiresistive acetone sensor is reported. By incorporation of chitosan as a surface-functional layer and a Pt Schottky contact for efficient charge transfer processes and photovoltaic effect, self-powered, highly selective acetone sensing is achieved. The sensor has exhibited an ultra-wide acetone detection range from sub-ppb to >100 000 ppm level at room temperature, covering those in the exhaled breath from healthy individuals (300-800 ppb) to people at high risk of DKA (>75 ppm). The nanowire sensor has also been successfully integrated into a handheld breath testing prototype, the Ketowhistle, which can successfully detect different ranges of acetone concentrations in simulated breath samples. The Ketowhistle demonstrates the immediate potential for non-invasive ketone monitoring for people living with diabetes, in particular for DKA prevention.

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.

Utilizing Data Mining for the Synthesis of Functionalized Tungsten Oxide with Enhanced Oxygen Vacancies for Highly Sensitive Detection of Triethylamine

The optimal combination of metal ions and ligands for sensing materials was estimated by using a data-driven model developed in this research. This model utilized advanced computational algorithms and a data set of 100,000 literature pieces. The semiconductor metal oxide (SMO) that is most suitable for detecting triethylamine (TEA) with the highest probability was identified by using the Word2vec model, which employed the maximum likelihood method. The loss function of the probability distribution was minimized in this process. Based on the analysis, a novel hierarchical nanostructured tungsten-based coordination with 2,5-dihydroxyterephthalic acid (W-DHTA) was synthesized. This synthesis involved a postsynthetic hydrothermal treatment (psHT) and the self-assembly of tungsten oxide nanorods. The tungsten oxide nanorods had a significant number of oxygen vacancies. Various techniques were used to characterize the synthesized material, and its sensing performance toward volatile organic compound (VOC) gases was evaluated. The results showed that the functionalized tungsten oxide exhibited an exceptionally high sensitivity and selectivity toward TEA gas. Even in a highly disturbed environment, the detection limit for TEA gas was as low as 40 parts per billion (ppb). Furthermore, our findings suggest that the control of oxygen vacancies in sensing materials plays a crucial role in enhancing the sensitivity and selectivity of gas sensors. This approach was supported by the utilization of density functional theory (DFT) computation and machine learning algorithms to assess and analyze the performance of sensor devices, providing a highly efficient and universally applicable research methodology for the development and design of next-generation functional materials.

2D Materials in Flexible Electronics: Recent Advances and Future Prospectives

Flexible electronics have recently gained considerable attention due to their potential to provide new and innovative solutions to a wide range of challenges in various electronic fields. These electronics require specific material properties and performance because they need to be integrated into a variety of surfaces or folded and rolled for newly formatted electronics. Two-dimensional (2D) materials have emerged as promising candidates for flexible electronics due to their unique mechanical, electrical, and optical properties, as well as their compatibility with other materials, enabling the creation of various flexible electronic devices. This article provides a comprehensive review of the progress made in developing flexible electronic devices using 2D materials. In addition, it highlights the key aspects of materials, scalable material production, and device fabrication processes for flexible applications, along with important examples of demonstrations that achieved breakthroughs in various flexible and wearable electronic applications. Finally, we discuss the opportunities, current challenges, potential solutions, and future investigative directions about this field.

Gas nanosensors for health and safety applications in mining

The ever-increasing demand for accurate, miniaturized, and cost-effective gas sensing systems has eclipsed basic research across many disciplines. Along with the rapid progress in nanotechnology, the latest development in gas sensing technology is dominated by the incorporation of nanomaterials with different properties and structures. Such nanomaterials provide a variety of sensing interfaces operating on different principles ranging from chemiresistive and electrochemical to optical modules. Compared to thick film and bulk structures currently used for gas sensing, nanomaterials are advantageous in terms of surface-to-volume ratio, response time, and power consumption. However, designing nanostructured gas sensors for the marketplace requires understanding of key mechanisms in detecting certain gaseous analytes. Herein, we provide an overview of different sensing modules and nanomaterials under development for sensing critical gases in the mining industry, specifically for health and safety monitoring of mining workers. The interactions between target gas molecules and the sensing interface and strategies to tailor the gas sensing interfacial properties are highlighted throughout the review. Finally, challenges of existing nanomaterial-based sensing systems, directions for future studies, and conclusions are discussed.

Synergistic Effect of Electron Scattering and Space Charge Transfer Enabled Unprecedented Room Temperature NO2 Sensing Response of SnO2

Metal oxide gas sensors have long faced the challenge of low response and poor selectivity, especially at room temperature (RT). Herein, a synergistic effect of electron scattering and space charge transfer is proposed to comprehensively improve gas sensing performance of n-type metal oxides toward oxidizing NO2 (electron acceptor) at RT. To this end, the porous SnO2 nanoparticles (NPs) assembled from grains of about 4 nm with rich oxygen vacancies are developed through an acetylacetone-assisted solvent evaporation approach combined with precise N2 and air calcinations. The results show that the as-fabricated porous SnO2 NPs sensor exhibits an unprecedented NO2 -sensing performance, including outstanding response (Rg /Ra  = 772.33 @ 5 ppm), fast recovery (<2 s), an extremely low detection limit (10 ppb), and exceptional selectivity (response ratio >30) at RT. Theoretical calculation and experimental tests confirm that the excellent NO2 sensing performance is mainly attributed to the unique synergistic effect of electron scattering and space charge transfer. This work proposes a useful strategy for developing high-performance RT NO2 sensors using metal oxides, and provides an in-depth understanding for the basic characteristics of the synergistic effect on gas sensing, paving the way for efficient and low power consumption gas detection at RT.

Wearable Nano-Based Gas Sensors for Environmental Monitoring and Encountered Challenges in Optimization

With a rising emphasis on public safety and quality of life, there is an urgent need to ensure optimal air quality, both indoors and outdoors. Detecting toxic gaseous compounds plays a pivotal role in shaping our sustainable future. This review aims to elucidate the advancements in smart wearable (nano)sensors for monitoring harmful gaseous pollutants, such as ammonia (NH3), nitric oxide (NO), nitrous oxide (N2O), nitrogen dioxide (NO2), carbon monoxide (CO), carbon dioxide (CO2), hydrogen sulfide (H2S), sulfur dioxide (SO2), ozone (O3), hydrocarbons (CxHy), and hydrogen fluoride (HF). Differentiating this review from its predecessors, we shed light on the challenges faced in enhancing sensor performance and offer a deep dive into the evolution of sensing materials, wearable substrates, electrodes, and types of sensors. Noteworthy materials for robust detection systems encompass 2D nanostructures, carbon nanomaterials, conducting polymers, nanohybrids, and metal oxide semiconductors. A dedicated section dissects the significance of circuit integration, miniaturization, real-time sensing, repeatability, reusability, power efficiency, gas-sensitive material deposition, selectivity, sensitivity, stability, and response/recovery time, pinpointing gaps in the current knowledge and offering avenues for further research. To conclude, we provide insights and suggestions for the prospective trajectory of smart wearable nanosensors in addressing the extant challenges.

Design of stretchable and self-powered sensing device for portable and remote trace biomarkers detection

Timely and remote biomarker detection is highly desired in personalized medicine and health protection but presents great challenges in the devices reported so far. Here, we present a cost-effective, flexible and self-powered sensing device for H2S biomarker analysis in various application scenarios based on the structure of galvanic cells. The sensing mechanism is attributed to the change in electrode potential resulting from the chemical adsorption of gas molecules on the electrode surfaces. Intrinsically stretchable organohydrogels are used as solid-state electrolytes to enable stable and long-term operation of devices under stretching deformation or in various environments. The resulting open-circuit sensing device exhibits high sensitivity, low detection limit, and excellent selectivity for H2S. Its application in the non-invasive halitosis diagnosis and identification of meat spoilage is demonstrated, emerging great commercial value in portable medical electronics and food security. A wireless sensory system has also been developed for remote H2S monitoring with the participation of Bluetooth and cloud technologies. This work breaks through the shortcomings in the traditional chemiresistive sensors, offering a direction and theoretical foundation for designing wearable sensors catering to other stimulus detection requirements.

Intelligent Multifunctional Sensing Systems based on Ordered Macro-Microporous Metal Organic Framework and Its Derivatives

Compared with nanomaterials-based sensors with single function, the development of multifunctional sensors shows high potential in comprehensive monitoring of personal health and environment, intelligent human-machine interfaces, and realistic imitation of human skin in prosthetics. Ordered macro-microporous metal-organic frameworks (MOFs)-enabled flexible and stretchable electronics are promising candidates for integrated multifunctional sensing systems. Herein, a three-dimensional ordered macro-microporous zeolite imidazolate framework-8 (3DOM ZIF-8) for humidity sensing and the derived ZnO within a hierarchically ordered macroporous-mesoporous-microporous carbon matrix (ZnO@HOMC) for gas sensor is constructed. Benefit from hierarchically ordered macroporous-mesoporous-microporous structure, the active site is fully exposed, and the charge transfer is accelerated. As a result, the multifunctional sensing systems show ultrafast response and recovery speed (10 s and 34 s), high sensitivity (R_air /R_gas = 38.6@50 ppm) to acetone, rapid humidity response speed (0.23 s) within changing humidity (RH 21%-99%), excellent stability and repeatability. Furthermore, in order to realize real-time monitoring of gas concentrations and humidity on mobile devices, an intelligent and portable sensor module is fabricated and wirelessly connected to a smartphone to effectively detect acetone concentration and humidity. This sensing technology shows fascinating applications in personal health, fitness tracking, electronic skins, artificial nervous systems, and human-machine interactions.

Biocompatible Material-Based Flexible Biosensors: From Materials Design to Wearable/Implantable Devices and Integrated Sensing Systems

Human beings have a greater need to pursue life and manage personal or family health in the context of the rapid growth of artificial intelligence, big data, the Internet of Things, and 5G/6G technologies. The application of micro biosensing devices is crucial in connecting technology and personalized medicine. Here, the progress and current status from biocompatible inorganic materials to organic materials and composites are reviewed and the material-to-device processing is described. Next, the operating principles of pressure, chemical, optical, and temperature sensors are dissected and the application of these flexible biosensors in wearable/implantable devices is discussed. Different biosensing systems acting in vivo and in vitro, including signal communication and energy supply are then illustrated. The potential of in-sensor computing for applications in sensing systems is also discussed. Finally, some essential needs for commercial translation are highlighted and future opportunities for flexible biosensors are considered.

Exploitation of Schottky-Junction-based Sensors for Specifically Detecting ppt-Concentration Gases

Gas species and concentrations of human-exhaled breath correlate with health, wherein disease markers contain volatile organic compounds (VOCs) of concentrations in parts per billion. It is expected that a gas-sensing strategy possesses a gas specificity and detection limit in the parts per trillion (ppt) range; however, it is still a challenge. This investigation has exploited the Schottky junction of gas sensors for detecting the reactance signal of ppt VOC, aiming for a specific and rapid detection toward disease marker acetone. In this new sensing paradigm, formed by the engineered energy band between metal-semiconductor contact, the Schottky junction is accessed to specific modulation of different adsorbate dopings and the corresponding reactance signal is measured. Regarding the detection toward ppt concentration of acetone, this sensing paradigm possesses rapid (∼100 s) and room-temperature response, molecular specificity, and 34 ppt of detection limit. The proposed detection paradigm is demonstrated to show a high feasibility toward detection of disease marker acetone.

Recent Advances in Photo-Activated Chemical Sensors

Gas detectors have attracted considerable attention for monitoring harmful gases and air pollution because of industry development and the ongoing interest in human health. On the other hand, conventional high-temperature gas detectors are unsuitable for safely detecting harmful gases at high activation temperatures. Photo-activated gas detectors improve gas sensing performance at room temperature and enable low-power operation. This review presents a timely overview of photo-activated gas detectors that use illuminated light instead of thermal energy. Illuminated light assists in gas detection and is classified as visible or ultraviolet light. The research on photo-activated gas detectors is organized according to the type of gas that can be intensively detected. In addition, a development strategy for advancing photo-activated gas detectors is discussed.

Efficient and Stable 3D/2D Perovskite Solar Cells through Vertical Heterostructures with (BA)4 AgBiBr8 Nanosheets

Perovskite solar cells (PVSCs) have drawn great attention due to their high processability and superior photovoltaic properties. However, their further development is often hindered by severe nonradiative recombination at interfaces that decreases power conversion efficiency (PCE). To this end, a facile strategy to construct a 3D/2D vertical heterostructure to reduce the energy loss in PVSCs is developed. The heterostructure is contrived through the van der Waals integration of 2D perovskite ((BA)₄ AgBiBr₈ ) nanosheets onto the surface of 3D-FAPbI₃ -based perovskites. The large bandgap of (BA)₄ AgBiBr₈ enables the formation of type-I heterojunction with 3D-FAPbI₃ -based perovskites, which serves as a barrier to suppress the trap-assisted recombination at the interface. As a result, a satisfying PCE of 24.48% is achieved with an improved open-circuit voltage (V_OC ) from 1.13 to 1.17 V. Moreover, the 2D perovskite nanosheets can effectively mitigate the iodide ion diffusion from perovskite to the metal electrode, hence enhancing the device stability. 3D/2D architectured devices retain ≈90% of their initial PCE under continuous illumination or heating after 1000 h, which are superior to 3D-based devices. This work provides an effective and controllable strategy to construct 3D/2D vertical heterostructure to simultaneously boost the efficiency and stability of PVSCs.

Quasi-aligned Cu2S/Cu(OH)2nanorod arrays anchored on Cu foam as self-supported electrode for non-enzymatic glucose detection

Self-supported Cu₂S/Cu(OH)₂composite nanorods for highly sensitive non-enzymatic glucose sensing werein situgrown on Cu foam by simple hydrothermal treatment of aligned Cu(OH)₂nanorods. The physicochemical and electrochemical properties of the as-fabricated Cu₂S/Cu(OH)₂composite nanorods were characterized by scanning electron microscopy, transmission electron microscopy, x-ray diffraction, Raman spectroscope, x-ray photoelectron spectroscope, cyclic voltammetry, electrochemical impedance spectroscopy, amperometrici-tmeasurements. The mechanism of the composite nanorods produced on conductive substrates was also explored. The electrode exhibits a sensitivity of 9626.88μA mM^-1cm^-2towards glucose with good anti-interference ability, indicating it a promising electrode material for the enhanced non-enzymatic glucose detection.

Ag-Modified Porous Perovskite-Type LaFeO3 for Efficient Ethanol Detection

Perovskite (ABO₃) nanosheets with a high carrier mobility have been regarded as the best candidates for gas-sensitive materials arising from their exceptional crystal structure and physical–chemical properties that often exhibit good gas reactivity and stability. Herein, Ag in situ modified porous LaFeO₃ nanosheets were synthesized by the simple and efficient graphene oxide (GO)-assisted co-precipitation method which was used for sensitive and selective ethanol detection. The Ag modification ratio was studied, and the best performance was obtained with 5% Ag modification. The Ag/LaFeO₃ nanomaterials with high surface areas achieved a sensing response value (R_g/R_a) of 20.9 to 20 ppm ethanol at 180 °C with relatively fast response/recovery times (26/27 s). In addition, they showed significantly high selectivity for ethanol but only a slight response to other interfering gases. The enhanced gas-sensing performance was attributed to the combination of well-designed porous nanomaterials with noble metal sensitization. The new approach is provided for this strategy for the potential application of more P-type ABO₃ perovskite-based gas-sensitive devices.

Effect of Stability of Two-Dimensional (2D) Aminoethyl Methacrylate Perovskite Using Lead-Based Materials for Ammonia Gas Sensor Application

Changes in physical properties of (H2C=C(CH3)CO2CH2CH2NH3)2PbI2Cl2 and (H2C=C(CH3)CO2CH2CH2NH3)2Pb(NO3)2Cl2 (2D) perovskite materials from iodide-based (I-AMP) and nitrate-based (N-AMP) leads were investigated at different durations (days) for various storage conditions. UV-Vis spectra of both samples showed an absorption band of around λmax 420 nm due to the transition of n to π* of ethylene (C=C) and amine (NH2). XRD perovskite peaks could be observed at approximately 25.35° (I-AMP) and 23.1° (N-AMP). However, a major shift in I-AMP and dramatic changes in the crystallite size, FHWM and crystallinity percentage highlighted the instability of the iodide-based material. In contrast, N-AMP showed superior stability with 96.76% crystallinity even at D20 under the S condition. Both materials were exposed to ammonia (NH3) gas, and a new XRD peak of ammonium lead iodide (NH4PbI3) with a red-shifted perovskite peak (101) was observed for the case of I-AMP. Based on the FWHM, crystallite size, crystallinity and lattice strain analysis, it can be concluded N-AMP's stability was maintained even after a few days of exposure to the said gases. These novel nitrate-based lead perovskite materials exhibited great potential for stable perovskite 2D materials and recorded less toxicity compared to famous lead iodide (PbI2) material.