Chemical Sensor Applications Research Articles

NOx gas sensors based on layer-transferred n-MoS2/p-GaN heterojunction at room temperature: Study of UV light illuminations and humidity

Transition metal dichalcogenides (TMDs) have gained extensive interest due to their phenomenal optoelectrical and electrochemical properties. Recently, TMDs are used as one of the auspicious materials for the chemical gas sensor application owing to their low operating temperatures. In this study, we developed room-temperature nitrogen oxide (NOx) gas sensors using n-MoS2/p-GaN hybrid-structure made by layer-transfer technique. The device response to NO2, NO, NH3, H2, H2S, and CO target gases under UV illumination at 27 °C has been evaluated. The n-MoS2/p-GaN heterostructure was noticed to have significant selectivity to NO2 gas compared to pristine p-GaN film with particularly high response of 98.42 % to 50 ppm of NO2 under UV illumination. Under UV light, MoS2/p-GaN heterostructure works effectively at a lower temperature than in dark. Further, by using photonic energy we also boosted NO2 response and made recovery signal of device faster. Our results divulged that temperature significantly influenced the response of the heterostructure and it escalated from 47.48 % to 148.81 % to 50 ppm of NO2 in the temperature range of 27–150 °C. The heterostructure sensor showed slight degradation (less than 15 %) to NO2 at 60 % of relative humidity. The proposed device seems to be a good candidate for room temperature gas sensing applications.

ZnO decorated flexible and strong graphene fibers for sensing NO2 and H2S at room temperature

Here we present toxic gas fiber-based sensor, operating at room temperature, which is important in wearable electronics for monitoring the environment and human health. Continuous strong and flexible graphene oxide (GO) fibers, with aligned GO sheets are synthesized by transforming a prealigned liquid crystalline GO solution using a wet spinning assembly. GO fibers are chemically reduced (rGO) for imparting resistance-based gas sensing. ZnO decorated rGO fibers (ZrGO) are synthesized by hydrothermal-assisted chemical reduction of GO fibers. ZnO nanoparticles and flowers, grown on ZrGO fibers, provided more surface area for adsorption of gas molecules compared to rGO fibers. Furthermore, ZrGO fibers provided more catalytic sites for gas interactions to enhance the sensing performance. The fiber-sensor can detect minimum levels of 1.5 and 8 ppm of NO2 and H2S gas, respectively. Interestingly, the sensing performance of ZrGO fiber-sensor is increased by 8 and 24-fold for NO2 and H2S, respectively, compared to rGO. The fiber-sensor shows fast recovery for H2S compared to NO2 gas. This work opens a new way for the application of robust, lightweight, cheap, and room temperature chemical sensors, that can detect hazardous gases.

Carbon nanotubes: functionalisation and their application in chemical sensors.

Carbon nanotubes (CNTs) have been recognised as a promising material in a wide range of applications, from safety to energy-related devices. However, poor solubility in aqueous and organic solvents has hindered the utilisation and applications of carbon nanotubes. As studies progressed, the methodology for CNTs dispersion was established. The current state of research in CNTs either single wall or multiwall/polymer nanocomposites has been reviewed in context with the various types of functionalisation presently employed. Functionalised CNTs have been playing an increasingly central role in the research, development, and application of carbon nanotube-based nanomaterials and systems. The extremely high surface-to-volume ratio, geometry, and hollow structure of nanomaterials are ideal for the adsorption of gas molecules. This offers great potential applications, such as in gas sensor devices working at room temperature. Particularly, the advent of CNTs has fuelled the invention of CNT-based gas sensors which are very sensitive to the surrounding environment. The presence of O2, NH3, NO2 gases and many other chemicals and molecules can either donate or accept electrons, resulting in an alteration of the overall conductivity. Such properties make CNTs ideal for nano-scale gas-sensing materials. Conductive-based devices have already been demonstrated as gas sensors. However, CNTs still have certain limitations for gas sensor application, such as a long recovery time, limited gas detection, and weakness to humidity and other gases. Therefore, the nanocomposites of interest consisting of polymer and CNTs have received a great deal of attention for gas-sensing application due to higher sensitivity over a wide range of gas concentrations at room temperature compared to only using CNTs and the polymer of interest separately.

Temperature dependent characteristics of graphene/silicon Schottky junction

Graphene/silicon Schottky junction has been reported as a promising device for chemical sensor, biological sensor and photodetector applications. However, abnormal characteristics are often reported and these ambiguities are explained inconsistently in the literature. This work aims at characterising the temperature effects on the current-voltage characteristics so as to explore the physical mechanisms leading to the abnormal behaviours of graphene/silicon Schottky junction. Particular attention is on the effect of silicon surface states which have been studied quite comprehensively in various silicon devices but have not been given enough attention for the graphene/Si structure. Results show that a graphene/Si Schottky junction could have quite different temperature dependences on the barrier height, ideality factor and reverse characteristics as compared with metal/semiconductor contacts. The dangling bonds on silicon surface, isolated by the ultrathin graphene layer, are still electrically active and play an important role in the carrier transport, photonic and chemical sensing capabilities of the graphene/Si junction. Graphene/Si contact prepared by transfer method cannot be a good Schottky junction from the electronic property and stability points of view.

Contactless and Simultaneous Measurement of Water and Acid Contaminations in Oil Using a Flexible Microstrip Sensor.

The assessment of the petroleum product quality often involves multiple indicators, among which water content and acid value are two major parameters. The complexity of an oil sample and the narrow space in pipeline transport make it difficult to monitor the oil quality in real-time. Considering the practical requirements, a new type of flexible microstrip sensor is proposed in this work. The shape and line width of the microstrip sensor are studied and optimized by theory and experiments. The proposed square spiral-based microstrip sensor has good water content detection resolution at high frequencies with less acid interference, and it can determine the acid value in the low-frequency band. The sensor surface is further passivated, protecting it from direct contact with the oil sample to enhance the electrochemical robustness, and still achieves good detection linearity and high sensitivity. After encapsulation on a flexible substrate, the proposed microstrip sensor realized the non-contact determination of the water content and acid value of oil at the same time, which is only a few millimeters in size and can conform to various tubing wall shapes. Due to the fact that the manufacture of the sensor is CMOS-compatible, we expect it to be readily applied to many other miniaturized chemical-sensing applications.

Silver Metallization Characterization and Resistivity Performance Potential toward Chemical Sensor Application

Performance of limiting resistive and capacitive signal delays increases gradually depends on the complexity degree of multilayer metallization. Electrodes sensor for electrical applications requires low surface roughness and low resistivity metal layers. Amongst conductive metal series, silver (Ag) has the lowest resistivity. On the other hand, compared to aluminum and copper, this metal also has higher oxidation resistance. This study aims to characterize Ag thin film on the glass substrate and the resistivity performance by using a physical deposition technique for chemical sensor application. A series of Ag thin film with different thickness were prepared from thermal vacuum evaporator at 3.45 x 10-5 Pa with applied current at 28 Ampere in 21 minutes. Four-point resistivity probing instrument was used for resistivity testing of the thin films with different thickness. The prepared Ag thin film shows a low average roughness at 1.89 nm. A smooth and homogeneous of Ag thin film is an advantage to provide a sensitive surface for element recognition in the development of chemical sensor and an adsorbate can be justified whereas it schematically assembled with the arrangement onto smooth and perfectly flat thin film surface. Ag thin film has shown a crystallite size with respect to 50.84 nm. The low rough surfaces have fewer nucleation sites, therefore fewer grains (crystallites) will appear. The optimum thickness was determined at 107 nm and the resistivity of Ag thin film was an average at 1.988 x10-8 Ohm m.  Â

Electrocatalytic activity and chemical sensor application of Mn-MOF: synthesis, crystal structure and photo luminescent properties

A solvothermal method was used to synthesize a Manganese metal-organic framework (Mn-MOF), [Mn(Tpa)(Mi)(DMF)] (Tpa = Terephthalic acid, Mi = Methylimidazole, DMF = Dimethylformamide). The Mn-MOF molecule has a three dimensional network in which the θ ranges from 5.942–50 with monoclinic I2/a space group. The final full matrix least-squares refinement over F2 is converged on R1 = 0.0558, wR2 = 0.1696 through GOF = 1.095. Mn-MOF shows a strong luminescence along with red-shift, which indicates that these are the potential candidates for various luminescent applications. Electro catalytic activities of Mn-MOF electrode have also been tested for in 5 mM K4FeCN6 and K3FeCN6 in 0.5 M KCl solution. The electrochemical sensor detection of ascorbic acid (AA) in Mn-MOF was used as a buffer solution (pH-7.0). The ascorbic acid peak potential is a great deal nearer to the Mn-MOF electrode peak potential is ΔEp = 0.062 V. These results indicate that Mn-MOF electrode exhibits a very good electro catalytic activity and good chemical sensor by a quick reply in favor of ascorbic acid detection.

Textile-based non-invasive lithium drug monitoring: A proof-of-concept study for wearable sensing

Flexible wearable chemical sensors are emerging tools which target diagnosis and monitoring of medical conditions. One of the potential applications of wearable chemical sensors is therapeutic drug monitoring for drugs that have a narrow therapeutic range such as lithium. We have investigated the possibility of developing a fibre-based device for non-invasive lithium drug monitoring in interstitial fluid. A flexible cotton-based lithium sensor was coupled with a carbon fibre-based reference electrode to obtain a potentiometric device. In vitro reverse iontophoresis experiments were performed to extract Li+ from under porcine skin by applying a current density of 0.4 mA cm−2 via two electrodes. Carbon fibre-based reverse iontophoresis electrodes were fabricated and used instead of a conventional silver wire-based version and comparable results were obtained. The fibre-based Li+ sensor and reference electrodes were capable of determining the Li+ concentration in samples collected via reverse iontophoresis and the results compared well to those obtained by ion chromatography. Additionally, biocompatibility of the materials used have been tested. Promising results were obtained which confirm the possibility of monitoring lithium in interstitial fluid using a wearable sensor.

Recent Developments of High-Resolution Chemical Imaging Systems Based on Light-Addressable Potentiometric Sensors (LAPSs).

A light-addressable potentiometric sensor (LAPS) is a semiconductor electrochemical sensor based on the field-effect which detects the variation of the Nernst potential on the sensor surface, and the measurement area is defined by illumination. Thanks to its light-addressability feature, an LAPS-based chemical imaging sensor system can be developed, which can visualize the two-dimensional distribution of chemical species on the sensor surface. This sensor system has been used for the analysis of reactions and diffusions in various biochemical samples. In this review, the LAPS system set-up, including the sensor construction, sensing and substrate materials, modulated light and various measurement modes of the sensor systems are described. The recently developed technologies and the affecting factors, especially regarding the spatial resolution and temporal resolution are discussed and summarized, and the advantages and limitations of these technologies are illustrated. Finally, the further applications of LAPS-based chemical imaging sensors are discussed, where the combination with microfluidic devices is promising.

Microwave-Initiated Synthesis of Metal Chalcogenides/Graphene-Catalyst for Enhanced Hydrogen Evolution Reaction

In present days, new types of renewable energy resources are being developed to resolve the world energy crisis. Nanomaterials, such as carbon nanotubes, metal oxides, metal chalcogenides (MCs) and conducting polymers with superior mechanical, thermal and electrical properties, lead to broad applications in composite materials, smart structures, chemical sensors, energy storage and nano-electronic devices. However, the high cost and difficulty in producing large scale, high quality nanomaterials remain challenges. The present work successfully demonstrates the production of MCs on graphene substrate through ultra-fast (60 seconds), facile and energy-efficient microwave-initiated approach. Graphene plays a vital role during the synthesis process by absorbing microwave energy and converting it to heat energy, which facilitates the precursors to react vigorously. Moreover, high specific surface area and conductivity of graphene delivers a favorable conductive network for the uniform growth of MCs, along with rapid charge transfer kinetics. The electrochemical characterizations reveal that the as-produced nanocomposites can be used for hydrogen evolution reaction (HER) to generate useful hydrogen fuel. As an example, the molybdenum disulfide on graphene (MoS2/graphene) composite exhibits superior electrocatalytic activity for the HER in acidic medium, with a low onset potential of only 100 mV and a Tafel slope of 43.3 mV per decade, suggesting the Volmer-Heyrovsky mechanism of hydrogen evolution. Beyond excellent catalytic activity, MoS2/graphene reveals an unusual ability to enhance the accessible active sites during the HER proceeds. This leads to a long cycling stability with a very high cathodic current density, providing the opportunity of practical application for scalable processing. Therefore, this single-step microwave approach can be universally employed to produce other useful MCs (e.g., MoSe2, WS2, WSe2 etc.), that will catalyze substantial development in more widespread uses of MC-based nanocomposites for successful energy applications.

Small gas adsorption on Co–N4 porphyrin-like CNT for sensor exploitation: a first-principles study

Using first-principles theory, we investigated the adsorption performance of CoN4-CNT towards six small gases including NO, O2, H2, H2S, NH3, and CH4, for exploiting its potential application for chemical gas sensors. The frontier molecular orbital theory was conducted to help understand the conductivity change of the proposed material at the presence of gas molecules. The desorption behavior of gas molecules from CoN4-CNT surface at ambient temperature was analyzed as well to determine its suitability for sensing application. Results show that CoN4-CNT is a promising material for O2 and NH3 sensing due to their desirable adsorption and desorption behaviors while not appropriate for sensing NO due to the poor desorption ability and for sensing CH4 and H2 given the poor adsorption behavior. Our calculation would provide a first insight into the CoN4-embedded effect on the structural and electronic properties of single-walled CNT, and shed light on the application of CoN4-CNT towards sensing of small gases.

A Perspective on Recent Advances in Piezoelectric Chemical Sensors for Environmental Monitoring and Foodstuffs Analysis

This paper provides a selection of the last two decades publications on the development and application of chemical sensors based on piezoelectric quartz resonators for a wide range of analytical tasks. Most of the attention is devoted to an analysis of gas and liquid media and to industrial processes controls utilizing single quartz crystal microbalance (QCM) sensors, bulk acoustic wave (BAW) sensors, and their arrays in e-nose systems. The unique opportunity to estimate several heavy metals in natural and wastewater samples from the output of a QCM sensor array highly sensitive to changes in metal ion activity in water vapor is shown. The high potential of QCM multisensor systems for fast and cost-effective water contamination assessments “in situ” without sample pretreatment is demonstrated.

WS2 and MoS2 thin film gas sensors with high response to NH3 in air at low temperature

Transition metal dichalcogenides (TMDs) have received immense research interest in particular for their outstanding electrochemical and optoelectrical properties. Lately, chemical gas sensor applications of TMDs have been recognized as well owing to the low operating temperatures of devices, which is a great advantage over conventional metal oxide based sensors. In this work, we elaborate on the gas sensing properties of WS2 and MoS2 thin films made by simple and straightforward thermal sulfurization of sputter deposited metal films on silicon chips. The sensor response to H2, H2S, CO and NH3 analytes in air at 30 °C has been assessed and both MoS2 and WS2 were found to have an excellent selectivity to NH3 with a particularly high sensitivity of 0.10 ± 0.02 ppm−1 at sub-ppm concentrations in the case of WS2. The sensing behavior is explained on the bases of gas adsorption energies as well as carrier (hole) localization induced by the surface adsorbed moieties having reductive nature.

Hierarchical Structure Including Nanoscale Passages By Using Wet-Etching of Nickel Thin Films in Lamellae Layers for Bio-Sensors Applications

In recent years, research for surface hierarchical structures for chemical sensor and bio-sensor devices applications has been highlighted owing to their relatively large ratio of surface-area compared to the volume. Especially, the hierarchical micro-nanostructures (organization of the nanostructure within the microstructure) including nanoscale passages have been intensively studied, because nanoscale passages are necessary for single-molecule DNA sequencing. In order to realize a hierarchical micro-nanostructure, the fabrication process for micro- and nano-scale structures should be applied together, controlling each pattern size precisely. Therefore, the properties of lateral Ni film etching by TFB etchant at room temperature were investigated by using samples with patterned lamellae layers consist of Ni, Al2O3, and SiO2 for fabricating hierarchical structures for the fabrication of hierarchical structure including nanoscale passages. Lateral etching length was increased with increasing etching time, despite of the difficult passage through nanoscale passage in lamellae layers. However, higher etching rate (2.1 nm/s) was observed in lower Ni film, which has contact with SiO2 and Al2O3 layers, compared to that (1.6 nm/s) of upper Ni film, which has contact with only Al2O3, due to stronger wetting property of SiO2, inducing easier penetration of etchant into nanoscale passage between surrounding layers. Moreover, the influence of Ni film thickness on lateral etching characteristics was also investigated. In this presentation, the fabrication process based on lateral Ni film etching in lamellae layers by liquid etchant at room temperature will be presented systematically for bio-sensor applications.

Construction of (3,8)-connected three-dimensional cobalt(II) and copper(II) coordination polymers with 1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene and benzene-1,3,5-tricarboxylate ligands.

Coordination polymers (CPs) have been widely studied because of their diverse and adjustable topologies and wide-ranging applications in luminescence, chemical sensors, magnetism, photocatalysis, gas adsorption and separation. In the present work, two coordination polymers, namely poly[(μ5-benzene-1,3,5-tricarboxylato-κ6O1:O1':O3:O3:O5,O5'){μ3-1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene-κ3N:N':N''}di-μ3-hydroxido-dicobalt(II)], [Co2(C9H3O6)(OH)(C12H12N6)]n or [Co2(btc)(OH)(mtrb)]n, (1), and poly[[diaquabis(μ3-benzene-1,3,5-tricarboxylato-κ3O1:O3:O5)bis{μ3-1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene-κ3N:N':N''}tetra-μ3-hydroxido-tetracopper(II)] monohydrate], {[Cu4(C9H3O6)2(OH)2(C12H12N6)2(H2O)2]·H2O}n or {[Cu4(btc)2(OH)2(mtrb)2(H2O)2]·H2O}n, (2), were synthesized by the hydrothermal method using 1,3-bis[(1,2,4-triazol-4-yl)methyl]benzene (mtrb) and benzene-1,3,5-tricarboxylate (btc3-). CP (1) exhibits a (3,8)-coordinated three-dimensional (3D) network of the 3,8T38 topological type, with a point symbol of {4,5,6}2{42·56·616·72·82}, based on the tetranuclear hydroxide cobalt(II) cluster [Co4(μ3-OH)2]. CP (2) shows a (3,8)-coordinated tfz-d topology, with a point symbol of {43}2{46·618·84}, based on the tetranuclear hydroxide copper(II) cluster [Cu4(μ3-OH)2]. The different (3,8)-coordinated 3D networks based on tetranuclear hydroxide-metal clusters of (1) and (2) are controlled by the different central metal ions [CoII for (1) and CuII for (2)]. The thermal stabilities and solid-state optical diffuse-reflection spectra were measured. The energy band gaps (Eg) obtained for (1) and (2) were 2.72 and 2.29 eV, respectively. CPs (1) and (2) exhibit good photocatalytic degradation of the organic dyes methylene blue (MB) and rhodamine B (RhB) under visible-light irradiation.

Nanocrystalline LaCoO3 modified by Ag nanoparticles with improved sensitivity to H2S

Semiconductor mixed-metal oxides with perovskite structure are of increasing interest for the application in chemical sensors. LaCoO3 has been extensively studied for its peculiar electronic and magnetic properties, but less is known of the gas sensing mechanisms of this material and its composites. In this work, nanocrystalline LaCoO3 was synthesized by sol-gel method and functionalized by Ag nanoparticles. An improved sensitivity to H2S was observed for Ag/LaCoO3 sensors, in contrast to pure LaCoO3 sensor. The Ag/LaCoO3 sensors had minimum cross-sensitivity to CO and NH3 interfering gases, which were present at higher concentrations than H2S. The role of Ag nanoparticles in the selective sensitivity of Ag/LaCoO3 nanocomposites to H2S gas were unveiled using in situ infrared (DRIFT) spectroscopy.

In-Fiber Cascaded FPI Fabricated by Chemical-Assisted Femtosecond Laser Micromachining for Micro-Fluidic Sensing Applications

An ultra-compact cascaded Fabry Perot (F-P) interferometer in fiber, which exhibits a high-visibility and enhanced detection limit, has been developed by chemical-assisted femtosecond laser micromachining. The reflection spectrum of the cascaded F-P interferometer shows stronger and narrower resonance than single F-P cavity. The liquid refractive index (RI) measurement demonstrates that the wavelength sensitivity and the dip intensity sensitivity are ∼888 nm/RIU and −882 dB/nm/RIU, respectively, and the obtained detection limit is 4.22 × 10−5 RIU in RI range 1.325–1.330, which is higher than that of the conventional single F-P cavity in optical fibers based on wavelength modulation (10−4 RIU). Moreover, the gas pressure measurement demonstrates that the wavelength sensitivity is 2.86 nm/MPa and the detection limit reaches 1.311 × 10−2 MPa in pressure range 1.5–2.0 MPa. The proposed in-fiber cascaded F-P interferometer will find potential applications in chemical and biological sensors, biomedical probes, and low-cost care products.

Highly sensitive and selective ethanol detection at room temperature utilizing holey SWCNT-Sn/SnO2 nanocomposites synthesized by microwave irradiation

Carbon-based nanocomposites that can operate at room temperature (25 °C) are potential sensing materials for application in chemical sensors owing to their unique physical properties such as extremely large surface-to-volume ratios and very high carrier mobility. However, most studies on sensing materials consisting of carbon-based nanocomposites have focused on synthesizing various material systems that contain metals, oxides, and chalcogenides, which are of limited use in practical gas sensors owing to their low response, poor selectivity, and irreversibility. Therefore, a simple approach to synthesizing carbon-based nanocomposites with high response and good gas selectivity utilizing microwave (MW) irradiation is proposed, and the sensing mechanisms underlying the enhanced sensing performance are discussed in detail. First, the perforation of single-walled carbon nanotubes (SWCNTs) during MW irradiation is investigated systematically, and then nanocomposite gas sensors based on the opened SWCNTs with large surface areas and without additional functionalization are fabricated. The nanocomposite sensors exhibit an extremely high response (>6 at 10 ppm) and selectivity for ethanol (C2H5OH) at room temperature. In addition, the C2H5OH response of the holey SWCNT-Sn/SnO2 nanocomposites is much higher than those of carbon-based nanocomposites. These results show that the simple synthesis method utilizing MW irradiation would be useful for various material systems requiring surface engineering or functionalization.

Approaching the Intrinsic Lifetime and Modulating a Graphene Plasmonic Resonance at a Few Hundred GHz

AbstractGraphene‐based metamaterials exhibit large tunability, low insertion loss, and high field confinement compared with metal‐based structures. Graphene‐based metamaterials may find numerous applications in optical modulators, optoelectronic devices, and chemical sensors in the few‐hundred‐GHz and low‐THz regime, constituting a new frontier as electronics and wireless communication continuously push further to higher frequencies. However, the main challenge is overcoming loss in graphene‐based resonant applications. Here, a low‐loss graphene plasmonic resonant modulator is demonstrated that operates at a few hundred GHz and 1 THz. The modulation depth is in excess of 35% with two layers of graphene. Numerical simulations show good agreement with the experimental results and reveal 100–200 fs lifetimes, which are approaching the intrinsic lifetime of the graphene plasmon due to the suppression of loss from phonon and edge scattering. The above values are the closest to electronic frequencies and represent the longest plasmon lifetime and largest modulation depth in graphene plasmon resonance devices reported so far.

Colorimetric Determination Of Hydrogen Peroxide Based on Localized Surface Plasmon Resonance of Silver Nanoprisms Using a Microchannel Chip

Silver nanoprisms (AgNPrs) have unique optical phenomena due to their localized surface plasmon resonance that results in the extinction of light from the visible to the near-infrared spectral region. In this study, we propose the colorimetric determination of silver nanoprisms in microchannels using a smartphone camera. Image acquisition was performed by capturing an image of the colloidal solution of the silver nanoprisms in the microchannel using the transmitted light. Red, green, and blue chromaticity levels were extracted from the recorded images for further quantification of the silver nanoprisms. This technique was employed for the detection and colorimetric determination of hydrogen peroxide (H2O2). Good linearity between the change in the green chromaticity level and concentration of hydrogen peroxide was observed for values from 10 to 300 μM with an R2 value of 0.9670. We anticipate that the developed methodology for the quantification of silver nanoprisms and hydrogen peroxide by monitoring the change in color in the images of transmitted light will enhance the development of simple, rapid, and reliable detection systems for quality control in the production of silver nanoprisms as well as in chemical sensor applications.