Functionalization of Zinc Oxide Nanoflowers with Palladium Nanoparticles via Microwave Absorption for Room Temperature-Operating Hydrogen Gas Sensors in the ppb Level

Ultrasensitive Chemiresistive Gas Sensors Based on Dual-Mesoporous Zinc Stannate Composites for Room Temperature Rice Quality Monitoring

The integration of dual-mesoporous structures, the construction of heterojunctions, and the incorporation of highly concentrated oxygen vacancies are pivotal for advancing metal oxide-based gas sensors. Nonetheless, achieving an optimal design that simultaneously combines mesoporous structures, precise heterojunction modulation, and controlled oxygen vacancies through a one-step process remains challenging. This study proposes an innovative method for fabricating zinc stannate semiconductors featuring dual-mesoporous structures and tunable oxygen vacancies via a direct solution precursor plasma spray technique. As a proof of concept, the resulting zinc stannate-based coatings are applied to detect 2-undecanone, a key biomarker for rice aging. Remarkably, the zinc oxide/zinc stannate heterojunctions with a well-defined secondary pore structure exhibit exceptional gas-sensing performance for 2-undecanone at room temperature. Furthermore, practical experiments indicate that the developed sensor effectively identifies adulteration in various rice varieties. These results underscore the potential of this method for designing metal oxides with tailored properties for high-performance gas sensors. The enhanced adsorption capacity and dual-mesoporous features of this semiconductor make it a promising candidate for sensing applications in agricultural food safety inspections.

High Response and ppb-Level Detection toward Hydrogen Sensing by Palladium-Doped α-Fe2O3 Nanotubes

Developing hydrogen sensors with parts per billion-level detection limits, high response, and high stability is crucial for ensuring safety across various industries (e.g., hydrogen fuel cells, chemical manufacturing, and aerospace). Despite extensive research on parts per billion-level detection, it still struggles to meet stringent requirements. Here, high performance and ppb-level H2 sensing have been developed with palladium-doped iron oxide nanotubes (Pd@Fe2O3 NTs), which have been prepared by FeCl3·6H2O, PdCl2, and PVP electrospinning and air calcination techniques. Various characterization techniques (FESEM, TEM, XRD, and so forth) were used to prove that the nanotube structure was successfully prepared, and the doping of Pd nanoparticles was realized. The experiments show that palladium doping can significantly improve the gas response of iron oxide nanotubes. Specifically, 0.59 wt % Pd@Fe2O3 NTs have high response (Ra/Rg = 41,000), high selectivity, and excellent repeatability for 200 ppm hydrogen at 300 °C. Notably, there was still a significant response at a low detection limit (LOD) of 50 ppb (Ra/Rg = 16.8). This excellent hydrogen sensing performance may be attributed to the high surface area of the nanotubes, the p-n heterojunction of PdO/Fe2O3, which allows more oxygen to be adsorbed on the surface, and the catalytic action of Pd nanoparticles, which promotes the reaction of hydrogen with surface-adsorbed oxygen.

Nano-Schottky-junction-engineered Pd/SnO2 nanotube array for ultrasensitive hydrogen sensing at room temperature

Detecting H2 at low concentrations is important due to it being a major safety concern in practical applications. However, semiconductor chemiresistive gas sensors always suffer from high operating temperatures and power consumption, as well as a limited concentration detection range, which restricts their widespread use. Herein, we developed a 3D nanostructured gas sensor employing a Pd-nanocluster-decorated SnO2 nanotube array as the sensing layer. The sensor showed sensitive and selective properties for detecting low concentrations of H2 at room temperature, with a low limit of detection of 1.6 ppb. It also showed good long-term stability, as long as 100 days. Moreover, systematical characterizations were performed in conjunction with density functional theory (DFT) calculations to determine the ability of Pd/SnO2 junctions to improve the gas-sensing properties. The engineering of the nano-Schottky junction allows us to expand the library for designing low-power-consumption H2 sensors for widespread applications.

Recent advances in biosensors based on metal-oxide semiconductors system-integrated into bioelectronics

Metal-oxide semiconductors (MOSs) have emerged as pivotal components in technology related to biosensors and bioelectronics. Detecting biomarkers in sweat provides a glimpse into an individual's metabolism without the need for sample preparation or collection steps. The distinctive attributes of this biosensing technology position it as an appealing option for biomedical applications beyond the scope of diagnosis and healthcare monitoring. This review encapsulates ongoing developments of cutting-edge biosensors based on MOSs. Recent advances in MOS-based biosensors for human sweat analyses are reviewed. Also discussed is the progress in sweat-based biosensing technologies to detect and monitor diseases. Next, system integration of biosensors is demonstrated ultimately to ensure the accurate and reliable detection and analysis of target biomarkers beyond individual devices. Finally, the challenges and opportunities related to advanced biosensors and bioelectronics for biomedical applications are discussed.

Decorating Pd-Au Nanodots Around Porous In2O3 Nanocubes for Tolerant H2 Sensing Against Switching Response and H2S Poisoning

With the recently-booming hydrogen (H2) economy by green H2 as the energy carriers and the newly-emerged exhaled diagnosis by human organ-metabolized H2 as a biomarker, H2 sensing is simultaneously required with fast response, low detection limit, and tolerant stability against humidity, switching, and poisoning. Here, reliable H2 sensing has been developed by utilizing indium oxide nanocubes decorated with palladium and gold nanodots (Pd-Au NDs/In2O3 NCBs), which have been synthesized by combined hydrothermal reaction, annealing, and chemical bath deposition. As-prepared Pd-Au NDs/In2O3 NCBs are observed with surface-enriched NDs and nanopores. Beneficially, Pd-Au NDs/In2O3 NCBs show 300 ppb-low detection limit, 5 s-fast response to 500 ppm H2, 75%RH-high humidity tolerance, and 56 days-long stability at 280 °C. Further, Pd-Au NDs/In2O3 NCBs show excellent stability against switching sensing response, and are tolerant to H2S poisoning even being exposed to 10 ppm H2S at 280 °C. Such excellent H2 sensing may be attributed to the synergistic effect of the boosted Pd-Au NDs' spillover effect and interfacial electron transfer, increased adsorption sites over the porous NCBs' surface, and utilized Pd NDs' affinity with H2 and H2S. Practically, Pd-Au NDs/In2O3 NCBs are integrated into the H2 sensing device, which can reliably communicate with a smartphone.

Simultaneously Engineering Oxygen Defects and Heterojunction into Ho-Doped ZnO Nanoflowers for Enhancing n-Propanol Gas Detection

Lung cancer poses a serious threat to people's lives and health due to its high incidence rate and high mortality rate, making it necessary to effectively conduct early screening. As an important biomarker for lung cancer, the detection of n-propanol gas suffers from a low response value and a high detection limit. In this paper, flower-like Ho-doped ZnO was fabricated by the coprecipitation method for n-propanol detection at subppm concentrations. The gas sensor based on the 3% Ho-doped ZnO showed selectivity to n-propanol gas. Its response value to 100 ppm n-propanol was 341 at 140 °C, and its limit of detection (LOD) was about 25.6 ppb, which is lower than that of n-propanol in the breath of a healthy person (150 ppb). The calculation results show that the adsorption of n-propanol on a Ho-doped ZnO surface releases more energy than isopropanol, ethanol, formaldehyde, acetone, and ammonia. The enhanced gas-sensing properties of the Ho-doped ZnO material can be attributed to the fact that the Ho-doping distorts the crystal lattice of the ZnO, increases the specific surface area, and generates a large amount of oxygen defects. In addition, the doped Ho partially forms a Ho2O3/ZnO heterojunction in the material and improves the gas-sensing properties. The 3% Ho-doped ZnO material is expected to be a promising candidate for the trace detection of n-propanol gas.

Critical Sensing Modalities for Hydrogen: Technical Needs and Status of the Field to Support a Changing Energy Landscape

Decarbonization of the energy system is a key aspect of the energy transition. Energy storage in the form of chemical bonds has long been viewed as an optimal scheme for energy conversion. With advances in systems engineering, hydrogen has the potential to become a low cost, low emission, energy carrier. However, hydrogen is difficult to contain, it exhibits a low flammability limit (>40000 ppm or 4%), low ignition energy (0.02 mJ), and it is a short-lived climate forcer. Beyond commercially available sensors to ensure safety through spot checks in enclosed environments, new sensors are necessary to support the development of low emission infrastructure for production, transmission, storage, and end use. Efficient scalable broad area hydrogen monitoring motivates lowering the detection limit below that (10 ppm) of best in class commercial technologies. In this perspective, we evaluate recent advances in hydrogen gas sensing to highlight technologies that may find broad utility in the hydrogen sector. It is clear in the near term that a sensor technology suite is required to meet the variable constraints (e.g., power, size/weight, connectivity, cost) that characterize the breadth of the application space, ranging from industrial complexes to remote pipelines. This perspective is not intended to be another standard hydrogen sensor review, but rather provide a critical evaluation of technologies with detection limits preferably below 1 ppm and low power requirements. Given projections for rapid market growth, promising techniques will also be amenable to rapid development in technical readiness for commercial deployment. As such, methods that do not meet these requirements will not be considered in depth.

Advanced NH3 Detection by 1D Nanostructured La:ZnO Sensors with Novel Intrinsic p-n Shifting and Ultrahigh Baseline Stability

Due to its stability, transportability, and ability to be produced using renewable energy sources, NH3 has become an attractive option for hydrogen production and storage. Detecting NH3 is then essential, being a toxic and flammable gas that can pose dangers if not properly monitored. ZnO chemiresistive sensors have shown great potential in real NH3 monitoring applications; yet, research and development in this area are ongoing due to reported limitations, like baseline instabilities and sensitivity to environmental factors, including temperature, humidity, and interferent gases. Herein, we suggest an approach to obtain sensors with competitive performance based on ZnO semiconducting metal oxides. For this purpose, one-dimensional nanostructured pure and La-doped ZnO films were synthesized hydrothermally. Incorporating large rare earth ions, like La, into the bulk lattice of ZnO is challenging and can lead to surface defects that are influential in gas-sensing reactions. The sensors experienced a temperature-induced p-n shifting at about 100 °C, verified by the Hall effect and AC impedance measurements. The doped sensor showed exceptional stepwise baseline stability and outstanding performance at a relatively low operating temperature (150 °C) with a sensing response of 91 at best (@ 50 ppm NH3) and recorded a tolerance to water vapor up to 70% RH. Alongside p-n shifting, the enhanced performance was discussed in correlation with La doping-triggered changes in the nanostructural and surfacial properties of the films. We validated the proposed technique by producing similar sensors and performing multiple replicates to ensure consistency and reproducibility. We also introduced the fill factor concept into the gas sensor field as a new trustworthy parameter that could improve sensor performance assessment and help rate sensors based on deviation from ideality.

Bioinspired nanoplatforms for human-machine interfaces: Recent progress in materials and device applications

The panoramic characteristics of human-machine interfaces (HMIs) have prompted the needs to update the biotechnology community with the recent trends, developments, and future research direction toward next-generation bioelectronics. Bioinspired materials are promising for integrating various bioelectronic devices to realize HMIs. With the advancement of scientific biotechnology, state-of-the-art bioelectronic applications have been extensively investigated to improve the quality of life by developing and integrating bioinspired nanoplatforms in HMIs. This review highlights recent trends and developments in the field of biotechnology based on bioinspired nanoplatforms by demonstrating recently explored materials and cutting-edge device applications. Section 1 introduces the recent trends and developments of bioinspired nanomaterials for HMIs. Section 2 reviews various flexible, wearable, biocompatible, and biodegradable nanoplatforms for bioinspired applications. Section 3 furnishes recently explored substrates as carriers for advanced nanomaterials in developing HMIs. Section 4 addresses recently invented biomimetic neuroelectronic, nanointerfaces, biointerfaces, and nano/microfluidic wearable bioelectronic devices for various HMI applications, such as healthcare, biopotential monitoring, and body fluid monitoring. Section 5 outlines designing and engineering of bioinspired sensors for HMIs. Finally, the challenges and opportunities for next-generation bioinspired nanoplatforms in extending the potential on HMIs are discussed for a near-future scenario. We believe this review can stimulate the integration of bioinspired nanoplatforms into the HMIs in addition to wearable electronic skin and health-monitoring devices while addressing prevailing and future healthcare and material-related problems in biotechnologies.

Self-Powered Thermoelectric Hydrogen Sensors Based on Low-Cost Bismuth Sulfide Thin Films: Quick Response at Room Temperature

Thermoelectric (TE)-based gas sensors have attracted significant attention due to their high selectivity, low power consumption, and minimum maintenance requirements. However, it is challenging to find low-cost, environmentally friendly materials and simple device fabrication processes for large-scale applications. Herein, we report self-powered thermoelectric hydrogen (TEH) sensors based on bismuth sulfide (Bi₂S₃) fabricated from a low-cost Bi₂S₃ TE layer and platinum (Pt) catalyst. When working at room temperature, the monomorphic-type TEH sensor obtained an output response signal of 42.2 μV with a response time of 17 s at a 3% hydrogen atmosphere. To further improve device performance, we connected the patterned Bi₂S₃ films in series to increase the Seebeck coefficient to -897 μV K^-1. For comparison, the resulting N tandem-type TEH sensor yielded a distinguished output voltage of 101.4 μV, which was greater than the monomorphic type by a factor of 2.4. Significantly, the response and recovery time of the N-tandem-type TEH sensor to 3% hydrogen were shortened to 14 and 15 s, respectively. This work provides a simple, environmentally friendly, and low-cost strategy for fabricating high-performance TEH sensors by applying low-cost Bi₂S₃ TE materials.

Electrospun ZnO/Pd Nanofibers as Extremely Sensitive Material for Hydrogen Detection in Oxygen Free Gas Phase

The development of safety sensors is an urgent necessity for the successful use of hydrogen in real conditions, which may differ, in particular, by the oxygen content in the surrounding atmosphere. Palladium-modified zinc oxide shows the high sensitivity when detecting hydrogen in air; however, studies of the sensor properties and the operation mechanism of the ZnO/Pd sensor when reducing gases are detected in an oxygen deficient or inert atmosphere have not been effectuated. In this work, we synthesized the ZnO and ZnO/Pd nanofibers by electrospinning and for the first time determined their sensor properties in the detection of CO, NH3 and H2 in different oxygen backgrounds. The microstructure and composition of nanofibers were characterized by electron microscopy, X-ray diffraction, X-ray fluorescent spectroscopy, and X-ray photoelectron spectroscopy. The interaction with the gas phase was investigated in situ by diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). The sensor properties of ZnO and ZnO/Pd nanofibers were studied at 100–450 °C towards CO, NH3 and H2 in the N2/O2 gas mixtures containing 0.0005–20% O2. When detecting CO, a decrease in the oxygen concentration from 20 to 0.0005% in the gas phase does not lead to a significant change in the sensor response. At the same time, when detecting NH3 and especially H2, a decrease in oxygen concentration down to 0.0005% results in the dramatic increase in the sensor response of ZnO/Pd nanofibers. This result is discussed in terms of palladium hydride formation, modulation of the potential barrier at the ZnO/Pd interface, as well as changes in the concentration of donor defects and charge carriers in the ZnO matrix. Synthesized electrospun ZnO/Pd nanofibers are extremely promising materials for sensors for detecting hydrogen in an oxygen free atmosphere.

Low-Temperature Highly Robust Hydrogen Sensor Using Pristine ZnO Nanorods with Enhanced Response and Selectivity

We report the hydrogen-sensing response on low-cost-solution-derived ZnO nanorods (NRs) on a glass substrate, integrated with aluminum as interdigitated electrodes (IDEs). The hydrothermally grown ZnO NRs on ZnO seed-layer-glass substrates are vertically aligned and highly textured along the c-axis (002 plane) with texture coefficient ∼2.3. An optimal hydrogen-sensing response of about 21.46% is observed for 150 ppm at 150 °C, which is higher than the responses at 100 and 50 °C, which are ∼12.98 and ∼10.36%, respectively. This can be attributed to the large surface area of ∼14.51 m²/g and pore volume of ∼0.013 cm³/g, associated with NRs and related defects, especially oxygen vacancies in pristine ZnO nanorods. The selective nature is investigated with different oxidizing and reducing gases like NO₂, CO, H₂S, and NH₃, showing relatively much lower ∼4.28, 3.42, 6.43, and 3.51% responses, respectively, at 50 °C for 50 ppm gas concentration. The impedance measurements also substantiate the same as the observed surface resistance is initially more than bulk, which reduces after introducing the hydrogen gas during sensing measurements. The humidity does not show any significant change in the hydrogen response, which is ∼20.5 ± 1.5% for a large humidity range (from 10 to 65%). More interestingly, the devices are robust against sensing response, showing no significant change after 10 months or even more.

Porous Pd-Sn Alloy Nanotube-Based Chemiresistor for Highly Stable and Sensitive H2 Detection

While H₂ is indispensable as a green fuel source, it is highly flammable and explosive. Because it is difficult to detect due to its lack of odor and color, a solution for proper monitoring of H₂ leakage is essential to ensure safe handling. To this end, we have successfully fabricated hollow Pd-Sn alloy nanotubes (NTs) with a Brunauer-Emmett-Teller surface area of 223.0 m²/g through electrospinning and a subsequent etching method, which is the first demonstration of synthesizing Pd-based hollow alloy nanofibers with ultrafine grain sizes. We found that the alloying of Pd with Sn could effectively prevent degradation of the sensing performance upon the α-β phase transition during hydrogen detection. Besides, the highly porous structure with smaller nanograins offered more exposed active sites and higher gas accessibility to bulk materials. The resultant Pd-Sn NTs exhibited excellent sensitivity toward H₂ (0.00005-3%). Notably, the limit of detection of 0.0001% is an outstanding achievement on H₂ sensing among state-of-the-art H₂ sensors. Moreover, when exposed to a high concentration of H₂ (3%), Pd-Sn NTs showed excellent cycling stability with a standard deviation of 0.07% and a sensitivity of 9.27%. These obtained sensing results indicate that Pd-Sn NTs can be used as a highly sensitive and stable H₂ gas sensor at room temperature (25 °C).

Metal Oxide Chemiresistors: A Structural and Functional Comparison between Nanowires and Nanoparticles

Metal oxide nanowires have become popular materials in gas sensing, and more generally in the field of electronic and optoelectronic devices. This is thanks to their unique structural and morphological features, namely their single-crystalline structure, their nano-sized diameter and their highly anisotropic shape, i.e., a large length-to-diameter aspect ratio. About twenty years have passed since the first publication proposing their suitability for gas sensors, and a rapidly increasing number of papers addressing the understanding and the exploitation of these materials in chemosensing have been published. Considering the remarkable progress achieved so far, the present paper aims at reviewing these results, emphasizing the comparison with state-of-the-art nanoparticle-based materials. The goal is to highlight, wherever possible, how results may be related to the particular features of one or the other morphology, what is effectively unique to nanowires and what can be obtained by both. Transduction, receptor and utility-factor functions, doping, and the addition of inorganic and organic coatings will be discussed on the basis of the structural and morphological features that have stimulated this field of research since its early stage.

Preparation and Antibacterial Performances of Electrocatalytic Zinc Oxide Nanoparticles with Diverse Morphologies

This study exploits the potential of zinc oxide nanoparticles (ZnO-NPs) with diverse morphologies as catalysts and antibacterial agent. Spherical ZnO-NPs, rod-shaped ZnO-NPs and flower-shaped ZnO-NPs were prepared by microemulsion method, solvent heat method and hydrothermal method, respectively. The structural characterizations of samples were investigated by X-ray diffraction (XRD) and scanning electron microscopy (SEM) techniques. XRD results revealed the formation of spherical ZnO-NPs, rod-shaped ZnO-NPs and flower-shaped ZnO-NPs were all wurtzite crystal structure. SEM results showed that spherical ZnO-NPs had an average particle size of 30-40 nm, rod-shaped ZnO-NPs were about 500 nm long and 100 nm wide with obvious hexagonal crystals. Flower-shaped ZnO-NPs had a three-dimensional appearance with obvious petals. Results of electrochemical HER (Hydrogen evolution reaction) experiments revealed that spherical ZnO-NPs exhibited the highest electrocatalytic activity at the lowest potential voltage due to their largest specific surface area. The antibacterial property of ZnO-NPs samples were studied by the optical density method and disc diffusion method. All samples had antibacterial effects against E. coli. and flower-shaped ZnO-NPs showed the best antibacterial activity due to the largest surface area in comparison with spherical ZnO-NPs and rod-shaped ZnO-NPs, which promised the maximum Zn²⁺ release as bactericide mechanism that registered in the case of different ZnO-NPs morphologies.