Fast Response and High Sensitivity of ZnO Nanowires-Cobalt Phthalocyanine Heterojunction Based H2S Sensor

Combinatorial selectivity with an array of phthalocyanines functionalized TiO2/ZnO heterojunction thin film sensors

The development of electronic noses requires the control of the selectivity pattern of each sensor of the array. Organic chemistry offers a manifold of possibilities to this regard but in many cases the chemical sensitivity is not matched with the response of electronic sensor. The combination of organic and inorganic materials is an approach to transfer the chemical sensitivities of the sensor to the measurable electronic signals. In this paper, this approach is demonstrated with a hybrid material made of phthalocyanines and a bilayer structure of ZnO and TiO₂. Results show that the whole spectrum of sensitivity of phthalocyanines results in changes of the resistance of the sensor, and even the adsorption of compounds, such as hexane, which cannot change the resistance of pure phthalocyanine layers, elicits changes of the sensor resistance. Furthermore, since phthalocyanines are optically active, the sensitivity in dark and visible light are different. Thus, operating the sensor in dark and light two different signals per sensors can be extracted. As a consequence, an array of 3 sensors made of different phthalocyanines results in a virtual array of six sensors. The sensor array shows a remarkable selectivity respect to a set of test compounds. Principal component analysis scores plot illustrates that hydrogen bond basicity and dispersion interaction are the dominant mechanisms of interaction.

High-Voltage, High-Current Electrical Switching Discharge Synthesis of ZnO Nanorods: A New Method toward Rapid and Highly Tunable Synthesis of Oxide Semiconductors in Open Air and Water for Optoelectronic Applications

A novel method of oxide semiconductor nanoparticle synthesis is proposed based on high-voltage, high-current electrical switching discharge (HVHC-ESD). Through a subsecond discharge in the HVHC-ESD method, we successfully synthesized zinc oxide (ZnO) nanorods. Crystallography and optical and electrical analyses approve the high crystal-quality and outstanding optoelectronic characteristics of our synthesized ZnO. The HVHC-ESD method enables the synthesis of ZnO nanorods with ultraviolet (UV) and visible emissions. To demonstrate the effectiveness of our prepared materials, we also fabricated two UV photodetectors based on the ZnO nanorods synthesized using the subsecond HVHC-ESD method. The UV-photodetector test under dark and UV light irradiation also had a promising result with a linear ohmic current-voltage output. In addition to the HVHC-ESD method's excellent tunability for ZnO properties, this method enables the rapid synthesis of ZnO nanorods in open air and water. The results demonstrate the preparation, highlight the synthesis of fine hexagonal-shaped nanorods under a second with controlled oxygen vacancies, and point defects for a wide range of applications in less than a second.

Unveiling the Electronic Interaction in ZnO/PtO/Pt Nanoarrays for Catalytic Detection of Triethylamine with Ultrahigh Sensitivity

The detection of harmful volatile organic compounds is of great significance to environmental quality and human health. However, it still remains a challenge to achieve high detection sensitivity at a relatively low temperature. Herein, an ultrasensitive catalytic sensor for the detection of triethylamine (TEA) based on ZnO/PtO/Pt nanoarray thin films was realized. Sensor measurements reveal that the PtO/Pt sensitizer dramatically reduces the working temperature from 195 °C of a pristine ZnO sensor to 125 °C of ZnO/PtO/Pt sensors. The ZnO/PtO/Pt sensors exhibit an extremely high response of 3513 to 50 ppm TEA, which is three orders of magnitude higher than that of pristine ZnO. Meanwhile, an ultralow limit of detection of 8.3 ppb is achieved. The outstanding performances are superior to those in most previous reports on TEA detection. Mechanistic investigations reveal that the outstanding performances are ascribed to the strong electronic interaction between PtO and ZnO and the catalytic spillover effect of Pt.

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

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

Gas Sensors Based on Chemi-Resistive Hybrid Functional Nanomaterials

Chemi-resistive sensors based on hybrid functional materials are promising candidates for gas sensing with high responsivity, good selectivity, fast response/recovery, great stability/repeatability, room-working temperature, low cost, and easy-to-fabricate, for versatile applications. This progress report reviews the advantages and advances of these sensing structures compared with the single constituent, according to five main sensing forms: manipulating/constructing heterojunctions, catalytic reaction, charge transfer, charge carrier transport, molecular binding/sieving, and their combinations. Promises and challenges of the advances of each form are presented and discussed. Critical thinking and ideas regarding the orientation of the development of hybrid material-based gas sensor in the future are discussed.

Year 2020: A Snapshot of the Last Progress in Flexible Printed Gas Sensors

A review of recent advances in flexible printed gas sensors is presented. During the last years, flexible electronics has started to offer new opportunities in terms of sensors features and their possible application fields. The advent of this technology has made sensors low-cost, thin, with a large sensing area, lightweight, wearable, flexible, and transparent. Such new characteristics have led to the development of new gas sensor devices. The paper makes some statistical remarks about the research and market of the sensors and makes a shot of the printing technologies, the flexible organic substrates, the functional materials, and the target gases related to the specific application areas. The conclusion is a short notice on perspectives in the field.

IoT-Enabled Gas Sensors: Technologies, Applications, and Opportunities

Ambient gas detection and measurement had become essential in diverse fields and applications, from preventing accidents, avoiding equipment malfunction, to air pollution warnings and granting the correct gas mixture to patients in hospitals. Gas leakage can reach large proportions, affecting entire neighborhoods or even cities, causing enormous environmental impacts. This paper elaborates on a deep review of the state of the art on gas-sensing technologies, analyzing the opportunities and main characteristics of the transducers, as well as towards their integration through the Internet of Things (IoT) paradigm. This should ease the information collecting and sharing processes, granting better experiences to users, and avoiding major losses and expenses. The most promising wireless-based solutions for ambient gas monitoring are analyzed and discussed, open research topics are identified, and lessons learned are shared to conclude the study.

SnS2/SnS p-n heterojunctions with an accumulation layer for ultrasensitive room-temperature NO2 detection

The unique features of SnS₂ make it a sensitive material ideal for preparing high-performance nitrogen dioxide (NO₂) gas sensors. However, sensors based on pristine tin disulfide (SnS₂) fail to work at room temperature (RT) owing to their poor intrinsic conductivity and weak adsorptivity toward the target gas, thereby impeding their wide application. Herein, an ultrasensitive and fully recoverable room-temperature NO₂ gas sensor based on SnS₂/SnS p-n heterojunctions with an accumulation layer was fabricated. The amounts of SnS₂/SnS heterojunctions can be effectively controlled by tuning the ratios of tin and sulfur precursors in the easy one-step solvothermal synthesis. Compared with pristine SnS₂, the conductivity of SnS₂/SnS heterostructures improved considerably. Such improvement was caused by the electron transfer from p-type SnS to n-type SnS₂ because the Fermi level of SnS was higher than that of SnS₂. The sensing response of optimized SnS₂/SnS toward 4 ppm NO₂ was 660% at room temperature, which was higher than most reported sensitivity values of other two-dimensional (2D) materials at room temperature. The superior sensing response of SnS₂/SnS heterostructures was attributed to the enhanced electron transport and the increased adsorption sites caused by the SnS₂/SnS p-n heterojunctions. Moreover, the SnS₂/SnS sensor showed good selectivity and long-term stability. These achievements of SnS₂/SnS heterostructured sensors make them highly desirable for practical applications.

Recent progress and perspectives of gas sensors based on vertically oriented ZnO nanomaterials

Vertically oriented zinc oxide (ZnO) nanomaterials, such as nanorods (NRs), nanowires (NWs), nanotubes (NTs), nanoneedles (NNs), and nanosheets (NSs), are highly ordered architectures that provide remarkable properties for sensors. Furthermore, these nanostructures have fascinating features, including high surface-area-to-volume ratios, high charge carrier concentrations, and many surface-active sites. These features make vertically oriented ZnO nanomaterials exciting candidates for gas sensor fabrication. The development of efficient methods for the production of vertically oriented nanomaterial electrode surfaces has resulted in improved stability, high reproducibility, and gas sensing performance. Moving beyond conventional fabrication processes that include binders and nanomaterial deposition steps has been crucial, as the materials from these processes suffer from poor stability, low reproducibility, and marginal sensing performance. In this feature article, we comprehensively describe vertically oriented ZnO nanomaterials for gas sensing applications. The uses of such nanomaterials for gas sensor fabrication are discussed in the context of ease of growth, stability on an electrode surface, growth reproducibility, and enhancements in device efficiency as a result of their unique and advantageous features. In addition, we summarize applications of gas sensors for a variety of toxic and volatile organic compound (VOC) gases, and we discuss future directions of the vertically oriented ZnO nanomaterials.

Self-Assembly of Cu2O Monolayer Colloidal Particle Film Allows the Fabrication of CuO Sensor with Superselectivity for Hydrogen Sulfide

CuO monolayer colloidal particle films with controllable thickness and homogeneous microstructure were prepared by self-assembly and subsequent calcination based on Cu₂O colloidal particles. Large-scale CuO monolayer colloidal particle films have the particle size of 300-500 nm, and CuO colloidal particles are hollow. It was found that such a structure exhibits excellent room-temperature H₂S-gas-sensing properties. It not only has high sensing response and excellent selectivity, but also has a low limit of detection of 100 ppb. The sensors exhibit different sensitive characteristics at low and high concentrations of H₂S. At low concentration (100-500 ppb), the sensor can be recovered with the increase of gas response, although it takes a longer recovery time at room temperature. At medium concentration (1-100 ppm), although the gas response still increases, the sensor is irreversible at room temperature. When the concentration continues to increase (>100 ppm), the sensor is irreversible at room temperature, and the gas response first increases and then decreases. Two reaction mechanisms are proposed to explain the above-mentioned sensing behavior. More importantly, quasi in situ X-ray photoelectron spectra confirm the existence of CuS. The CuO sensor with room-temperature response and superselectivity will find potential applications in industry, environment, or intelligent electronics.

Improved luminescence properties by the self-assembly of lanthanide compounds with a 1-D chain structure for the sensing of CH3COOH and toxic HS− anions

A family of lanthanide compounds has been explored and enhanced luminescence has been accomplished by doping method. A brand new and multifunctional fluorescence probe was developed, which was able to detect CH₃COOH and HS⁻ with different mechanisms.

Facile on-chip electrospinning of ZnFe2O4 nanofiber sensors with excellent sensing performance to H2S down ppb level

ZnFe₂O₄ nanofiber gas sensors are cost-effectively fabricated by direct electrospinning on microelectrode chip with Pt interdigitated electrodes and subsequent calcination under different conditions to maximize their response to H₂S gas. The synthesized nanofibers of approximately 30-100 nm in diameter show typical spider-net-like morphology of the electrospun nanofibers. The ZnFe₂O₄ nanofibers comprise many 10-25 nm nanograins, which results in multi-porous structures. Moreover, the nanofibers exhibit the single phase of cubic-spinel-structure ZnFe₂O₄. The density, crystallinity and grain size of ZnFe₂O₄ nanofiber that strongly affect gas-sensing properties can be optimized by controlling electrospun time, annealing temperature, annealing time and heating rate. Under optimal conditions, the ZnFe₂O₄ nanofiber sensors exhibit high sensitivity and selectivity to H₂S at sub-ppm levels. Excellent gas-sensing performances are attributed to effects of multi-porous structure, nanograin size and crystallinity, which is explained by the sensing mechanisms of ZnFe₂O₄ nanofiber sensors to H₂S gas.

ZnO Nanowire Application in Chemoresistive Sensing: A Review

This article provides an overview of the recent development of ZnO nanowires (NWs) for chemoresistive sensing. Working mechanisms of chemoresistive sensors are unified for gas, ultraviolet (UV) and bio sensor types: single nanowire and nanowire junction sensors are described, giving the overview for a simple sensor manufacture by multiple nanowire junctions. ZnO NW surface functionalization is discussed, and how this effects the sensing is explained. Further, novel approaches for sensing, using ZnO NW functionalization with other materials such as metal nanoparticles or heterojunctions, are explained, and limiting factors and possible improvements are discussed. The review concludes with the insights and recommendations for the future improvement of the ZnO NW chemoresistive sensing.

Gold–tin co-sensitized ZnO layered porous nanocrystals: enhanced responses and anti-humidity

Gold–tin co-sensitized ZnO layered porous nanocrystals were synthesized and performed enhanced responses and significantly reduced negative effects of RH on responses to both reducing and oxidizing gases (good anti-humidity).

Fast and real-time acetone gas sensor using hybrid ZnFe2O4/ZnO hollow spheres

Special hybrid ZnFe₂O₄/ZnO hollow spheres show excellent acetone sensing properties.

Improved H2S gas sensing properties of ZnO nanorods via decoration of nano-porous SiO2 thin layers

The decoration of nano-porous SiO₂ thin film prevents the poisoning phenomenon of ZnO nanorod in H₂S detection.