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Indirectly monitoring halitosis via the detection of hydrogen sulfide (H2S) biomarkers using gas sensors is a newly emerging technique. However, such H2S sensors are required with critically high selectivity and sensitivity, as well as a ppb-level detection limit, which remains technologically challenging. To address such issues, here, we have developed highly sensitive and selective H2S sensors with NiO/WO3 nanoparticles (NPs), which have been synthesized by firstly hydrolyzing WO3 NPs and subsequently decorating with NiO NPs in a hydrothermal process. Theoretically, the NiO/WO3 NPs assist in forming a thicker electron depletion layer, adsorbing more oxygen species O2- to oxidize H2S and finally release more electrons. Beneficially, 2.1 wt % NiO/WO3 NPs show high sensitivity to H2S (Ra/Rg = 15031 ± 1370 @ 10 ppm, 100 °C), which is 42.6-fold higher than that of the pristine WO3 NPs (Ra/Rg = 353 ± 5.6 @ 10 ppm, 100 °C). Further, the H2S sensor shows ppb-level detection limit (Ra/Rg = 4.95 ± 2.9 @ 0.05 ppm, 100 °C) and high selectivity. Practically, NiO/WO3 NP sensor prototype has been employed to detect the simulated exhaled halitosis compared with that of gas chromatography, revealing a close concentration of H2S. Our investigation offers an experimental base in future intelligent medical applications.

Hydrogen sensing simultaneously endowed with fast response, high sensitivity and selectivity is highly desired in detecting hydrogen leakages such as in hydrogen-driven vehicles and space rockets. Here, hydrogen sensing reined via a hydrogen spillover effect has been developed using palladium nanoparticles photochemically decorated on WO3 nanoparticles (Pd-NPs@WO3-NPs). Theoretically, the Pd-NP catalysts and WO3-NP support are used to construct the hydrogen spillover system, in which Pd NPs possess high catalytic activity, promoting the electron transfer and therefore the reaction kinetics. Beneficially, the Pd-NPs@WO3-NP sensor prototypes toward 500 ppm hydrogen simultaneously exhibit fast response time (∼1.2 s), high response (Ra/Rg = 22 867) and selectivity at a working temperature of 50 °C. Such advanced hydrogen sensing provides an experimental basis for the smart detection of hydrogen leakage in the future hydrogen economy.

The newly-emerged breath sensing detection of disease biomarkers (e.g., H2S and acetone) offers rapid and noninvasive early diagnosis of diseases (e.g., halitosis and diabetes). Utilizing a single sensor to monitor dual biomarkers may contribute to both miniaturized size and multi-detection if implemented, but remains challenging. Here, interconnected BiFeO3/Bi25FeO40 nanoparticles (NPs, ∼22.5 nm in diameter) synthesized via combined microwave hydrothermal and annealing methods have been developed to selectively detect dual biomarkers of halitosis and diabetes. The selective detection has been modulated by an n/p semiconducting transformation, and has been experimentally observed and theoretically interpreted, in which electron–hole pairs are modulated due to the synergistic effect of temperature-dependent adsorbed oxygen molecules and semiconducting band bending. Remarkably, the sensor prototypes enable the selective detection of both H2S (n-type mode) and acetone (p-type mode) biomarkers with superior stability and a ppb-level detection limit. Furthermore, practical human breath has been experimentally simulated. Critically, the sensors with a waterproof membrane have been tested by immersing them into water. Our strategy of the detection of dual exhaled biomarkers by a single gas sensor may contribute to the integration and miniaturization of sensors, for upcoming intelligent medical treatment.

The stability for a hydrogen sensor is of crucial importance under a low-temperature range (e.g., 200-400 K), especially in critical environments (e.g., aerospace). However, the "reverse sensing behavior" of Pd-based sensing materials at low temperatures limits their wide application. Herein, a three-dimensional (3D) hydrogen-sensing material of interconnected Pd nanoparticles supported on zeolite-AFI (zeolite-AFI@Pd NPs) is designed for the hydrogen sensor at low temperature. The interconnected Pd NPs of ∼15 nm in diameter are achieved onto the zeolite-AFI framework by reduction-controlled self-assembly growth, followed by partially etching-off zeolite. The 3D structure provides a larger surface ratio for improving hydrogen adsorption onto Pd, and more space for PdHx intermediate expansion, which effectively facilitates response to hydrogen and suppresses the α-β phase transition. Remarkably, there is no "reverse sensing behavior" observed in zeolite-AFI@Pd NPs, though temperature is as low as to 200 K compared with that of pristine Pd nanowires at 287 K. Furthermore, the zeolite-AFI@Pd NPs sensors yield excellent sensing response and high stability to hydrogen at temperature from 200 to 400 K. Such Zeolite-AFI@Pd NPs sensors are expected to detect hydrogen leakage, especially in critical environments of low temperature.

Listeria monocytogenes (LMs) that release 3-hydroxy-2-butanone (3H-2B) biomarker, can cause serious human illness and even death; in this regard, conventional techniques for the smart detection of LMs are of vital importance; however, they still suffer from the requirement of either sophisticated techniques or complicated instrumentation. Herein, we report a metal-oxide semiconductor (MOS) sensor, built with 3D mesoporous nickel oxide nanocuboids (M-NiO NCs), for the detection of LMs. The M-NiO NCs were obtained by directly sacrificing nickel foam in oxalic acid, followed by subsequent annealing. The M-NiO NCs have a rough surface and many multichannel pathways. Impressively, gas sensors built with M-NiO NCs display excellent sensing performance towards the 3H-2B biomarker, exhibiting an ultrahigh sensitivity (Rg/Ra = 302 at 50 ppm) and excellent selectivity. Moreover, the 3H-2B sensor presents excellent long-term stability and, particularly, a low limit of detection (LOD = 0.5 ppm) at 120 °C. The multichannel pathways of the M-NiO NCs contribute to gas adsorption and diffusion, thus enhancing the sensing behavior. Our study provides an ingenious and low-cost strategy to produce mesoporous NiO nanostructures as well as new insights for the detection of pathogenic microbes in food.

The metallic dopants in palladium (Pd) sensing materials enable modification of the d-band center of Pd, which is expected to tune the α–β phase transitions of the PdHx intermediate, thus improve the sensing stability to hydrogen. Here, the boosted hydrogen-sensing stability at ultra-low temperatures has been achieved with palladium/cobalt nanowires (PdCo NWs) as the sensing material. The various Co contents in PdCo NWs were modulated via AAO-template-confined electrodeposition. The temperature-dependent sensing evaluations were performed in 0.1–3 v/v% hydrogen. Such sensors integrated with PdCo NWs are able to stably detect hydrogen as low as 0.1 v/v%, even when the temperature is lowered to 273 K. In addition, the critical temperatures of “reverse sensing behavior” of the PdCo NWs (Pd82Co18: Tc = 194 K; Pd63Co37: Tc = 180 K; Pd33Co67: Tc = 184 K) are observed much lower than that of pristine Pd NWs (Tc = 287 K). Specifically, the Pd63Co37 NWs (∼37 at% Co content) sensor shows outstanding stability of sensing hydrogen against α–β phase transitions within the wide temperature range of 180–388 K, which is attributed to both the electronic interactions between Pd and Co and the lattice compression strain caused by Co dopants. Moreover, the “reverse sensing behavior” of the PdCo NWs is explicitly interpreted using the α–β phase transition model.

We report zinc oxide (core) coated with alumina (shell) single-shelled nanocables (ZnO@Al2O3 NCBs) for detection of 3-hydroxy-2-butanone (3H-2B) biomarker, which was prepared via aluminum-anodic-oxide (AAO) template confined electrodeposition, chemical etching and leaving thin AAO-channel wall with broken surface. In the core/shell nanostructure, the outer Al2O3 layer protects ZnO from interference gases, while the pores allow the target gas to react with the oxygen specie on the surface of ZnO. For as-built ZnO@Al2O3 NCBs sensors, the detection limit is as low as 1 ppm, and the response value to 50 ppm 3H-2B is 37.2 at 300 °C. Moreover, the ZnO@Al2O3 NCBs show excellent selectivity and cyclic stability. Our ZnO@Al2O3 NCBs sensors in future monitoring Listeria monocytogenes.

As a high-energy-density, renewable and green energy carrier, hydrogen has recently received increasing attention. However, the reliable detection for hydrogen leakage, especially in harsh environmental conditions, and the efficient high-purity hydrogen production are still challenging issues. Here, we report that uniform palladium-nickel nanowires (PdNi NWs) arrays, which have been synthesized via anode-aluminum-oxide (AAO) template-confined electrodeposition as bi-functional material for hydrogen leakage detection and hydrogen evolution reaction (HER). PdNi NWs arrays exhibit rough surface with the diameter of ~60 nm and present alloyed crystallization. Benefited from the synergic effect of the rough surface of PdNi NWs etched during synthesis and the electronic structure of Pd modified by Ni, PdNi NWs enable to stably work within the wide temperature range of 153–388 K, in which the Pd-Ni alloying decreases the critical temperature of “reverse sensing behavior” to 153 K. Meanwhile, the PdNi NWs show the overpotential of 91 mV at 10 mA/cm2, and the Tafel slope of 96 mV/dec in 0.5 M H2SO4. The PdNi NWs show great potential in both hydrogen leakage detection and hydrogen evolution reaction.

The newly emerged gas sensing detection of 3-hydroxy-2-butanone (3H-2B) biomarker is deemed as an effective avenue to indirectly monitor Listeria monocytogenes (LM). However, 3H-2B sensing materials requiring critically high sensitivity and selectivity, and ppb-level detection limit, remain challenging. Here, we report the advanced gas sensors built with bismuth vanadate microdecahedron (BiVO4 MDCD) {010} facets selectively decorated with Pd nanoparticles (Pd NPs, Pd-{010}BiVO4 MDCDs) for boosted detection of the 3H-2B biomarker. Meanwhile, BiVO4 MDCDs with overall facets are randomly deposited with Pd NPs (Pd-BiVO4 MDCDs). Comparatively, Pd-{010}BiVO4 MDCD sensors show 1 order of magnitude higher response toward the 3H-2B biomarker at 200 °C. Further, Pd-{010}BiVO4 MDCD sensors enable to detect as low as 0.2 ppm 3H-2B and show best selectivity and stability, and fastest response and recovery. Density functional theory calculations reveal a lower adsorption energy of 3H-2B onto Pd-{010}BiVO4 MDCDs than those of pristine and Pd-BiVO4 MDCDs. The extraordinary Pd-{010}BiVO4 sensing performance is ascribed to the Pd NP-assisted synergetic effect of the preferential adsorption of 3H-2B target molecules, accumulated sensing agent of ionic oxygen species, and concentrated catalysts on the {010} facets. This strategy offers rapid and noninvasive detection of LMs and is thus of great potential in the upcoming Internet of Things.

Detection of 3-hydroxy-2-butanone (3H-2B) biomarker is a newly emerging and feasible way to monitor Listeria monocytogenes (LMs). Although progress has been made in gas-sensing methodology, the long-term adsorption of the reactive products onto the sensing materials leads to a decrease in stability and selectivity. Herein, we report highly selective and stable 3H-2B sensors, which are developed from a core / shell structure of tin oxide nanowire coated with alumina (SnO2@Al2O3 NCBs), via combining AAO template-confined electrodeposition and wet-chemical etching. As-built sensors exhibit high sensitivity of Ra/Rg = 43.3 to 5 ppm 3H-2B at 120 °C, which is around 6–40 times against other interfering gases. Also, our sensors exhibit excellent stability after 42 days′ evaluation. Furthermore, the sensors can detect as low as 0.1 ppm 3H-2B (Ra/Rg = 2.4; 120 °C), and the sensitivity reach Ra/Rg = 27.2 at 50 °C. Our SnO2@Al2O3 NCBs sensors are endowed with ppb-level detection limit, excellent selectivity and stability at low temperature, which is of great potential in future detection of LMs.