Trace-Level,Multi-Gas Detection for Food Quality Assessment Based on Decorated Silicon Transistor Arrays

Multiplexed gas detection at room temperature is critical for practical applications, such as for tracking the complex chemical environments associated with food decomposition and spoilage. An integrated array of multiple silicon-based, chemical-sensitive field effect transistors (CSFETs) is presented to realize selective, sensitive, and simultaneous measurement of gases typically associated with food spoilage. CSFETs decorated with sensing materials based on ruthenium, silver, and silicon oxide are used to obtain stable room-temperature responses to ammonia (NH₃), hydrogen sulfide (H₂S), and humidity, respectively. For example, one multi-CSFET sensor signal changes from its baseline by 13.34 in response to 1 ppm of NH₃, 724.45 under 1 ppm H₂S, and 23.46 under 80% relative humidity, with sensitive detection down to 10 ppb of NH₃ and H₂S. To demonstrate this sensor for practical applications, the CSFET sensor array is combined with a custom-printed circuit board into a compact, fully integrated, and portable system to conduct real-time monitoring of gases generated by decomposing food. By using existing silicon-based manufacturing methodologies, this room-temperature gas sensing array can be fabricated reproducibly and at low cost, making it an attractive platform for ambient gas measurement needed in food safety applications.     

Synthesis,characterisation and catalytic activity of gold and silver nanoparticles in the bio-sensor application

Most of the hydrogen peroxide (H₂O₂) bio-sensors developed till date are based on enzymes and proteins causing them to have a limited lifetime. Moreover, complex procedures are followed for sensor fabrication. Therefore, an inorganic material-based sensor, with a simple design and longer shelf life is highly desirable. In this work, surfactant-metal (gold and silver) nanoparticles are prepared in aqueous solutions containing cetyltrimethylammonium bromide. The particle sizes of the metal nanoparticles obtained are characterised by UV–Vis, HRTEM, X-ray diffraction and FTIR; the average sizes of gold and silver nanoparticles are 8 and 10?±?0.2?nm, respectively. The nanoparticles are tested for H₂O₂ detection. The sensor is characterised and tested using samples from M to mM H₂O₂ range and a linear response is observed. Low-detection limits and high sensitivity are some of the advantages of this work. Same principle could be extended for the detection of other substrates as well.     

Modified metal-insulator-metal (M-I-M) hydrogen gas sensors based on zinc oxide

Zinc oxide thin films (thickness ~0.6 m) were deposited on zinc substrates by the spray-CVD method and a new kind of sensor structure Pd/ZnO/Zn was fabricated for hydrogen detection. The sensor was exposed to air mixed with different concentrations of H₂ (2000–20 000 ppm) and showed high sensitivity and a short time response at room temperature. The recovery time of the sensor to the zero base level was found to be faster after exposure to higher hydrogen concentrations. The sensitivity was bias-dependent with best performance at 0.05 V. Because of the key role of sensing played by the adsorption and desorption processes at the Pd surface and the Pd/ZnO interface, the contact resistance at the Pd/ZnO interface changed reversibly. The sensor showed high selectivity to hydrogen at room temperature and was insensitive to CO, CO₂ and LPG even at higher concentrations.     

PdRh-Sensitized Iron Oxide Ultrathin Film Sensors and Mechanistic Investigation by Operando TEM and DFT Calculation

Metal oxide semiconductor (MOS) thin films are of critical importance to both fundamental research and practical applications of gas sensors. Herein, a high-performance H₂ sensor based on palladium (Pd) and rhodium (Rh) co-functionalized Fe₂O₃ films with an ultrathin thickness of 8.9 nm deposited by using atomic layer deposition is reported. The sensor delivers an exceptional response of 105.9 toward 10 ppm H₂ at 230 °C, as well as high selectivity, immunity to humidity, and low detection limit (43 ppb), which are superior to the reported MOS sensors. Importantly, the Fe₂O₃ film sensor under dynamic H₂ detection is for the first time observed by operando transmission electron microscopy, which provides deterministic evidence for structure evolution of MOS during sensing reactions. To further reveal the sensing mechanism, density functional theory calculations are performed to elucidate the sensitization effect of PdRh catalysts. Mechanistic studies suggest that Pd promotes the adsorption and dissociation of H₂ to generate PdH_x, while Rh promotes the dissociation of oxygen adsorbed on the surface, thereby jointly promoting the redox reactions on the films. A wireless H₂ detection system is also successfully demonstrated using the thin film sensors, certifying a great potential of the strategy to practical sensors.     

Sensitivity and selectivity of (Au,Pt, Pd)-loaded and (In,Fe)-doped SnO2 sensors for H2 and CO detection

(Au, Pt, Pd)-loaded and (In, Fe)-doped SnO₂ are synthesized by a sol–gel method. The composition, morphology and electrochemical property of the materials were characterized by XRD, SEM and electrochemical workstation, respectively. The results show that Au, Pd loading and In, Fe doping prefer to enhance the selectivity to CO against H₂, while Pt loading can enhance the selectivity to H₂ against CO. Furthermore, 1 mol% Pt-loaded SnO₂ sensor has preferable selectivity to H₂ against CO when Pt loading amount is changed. The response time of the Pt-loaded SnO₂ sensor to 5,000 ppm H₂ is 5 s at 400 °C, which is much shorter than that of pure SnO₂ sensor. Meanwhile the effect of operating temperature and Pt loading on n value (the slope of logarithm of response versus logarithm of gas concentration) is studied. The Pt-loaded SnO₂ sensor can detect H₂ down to 1 ppm. These results show that the Pt-loaded SnO₂ sensor is a good candidate for practical H₂ sensors.     

Selective detection of hydrogen sulfide using copper oxide-doped tin oxide based thick film sensor array

In this work, copper oxide-doped (1, 3 and 5 wt%) tin oxide powders have been synthesised by sol–gel method and thick film sensor array has been developed by screen printing technique for the detection of H₂S gas. Powder X-ray diffraction pattern shows that the tin oxide (SnO₂) doped with 3 wt% copper oxide (CuO) has smaller crystallite size in comparison to 0, 1 and 5 wt% CuO-doped SnO₂. Furthermore, field emission scanning electron microscopy manifests the formation of porous film consisting of loosely interconnected small crystallites. The effect of various amounts of CuO dopant has been studied on the sensing properties of sensor array with respect to hydrogen sulfide (H₂S) gas. It is found that the SnO₂ doped with 3 wt% CuO is extremely sensitive (82%) to H₂S gas at 150 °C, while it is almost insensitive to many other gases, i.e., hydrogen (H₂), carbon monoxide (CO), sulphur dioxide (SO₂) and liquefied petroleum gas (LPG). Moreover, at low concentration of gas, it shows fast recovery as compared to response time. Such high performance of 3 wt% CuO-doped SnO₂ thick film sensor is probably due to the diminishing of the p–n junction and the smallest crystallite size (11 nm) along with porous structure.     

Ultra-sensitive room-temperature H<Subscript>2</Subscript>S sensor using Ag–In<Subscript>2</Subscript>O<Subscript>3</Subscript> nanorod composites

Ultra-sensitive H₂S sensors operated at room temperature were fabricated using Ag–In₂O₃ nanorod composites synthesized using sol–hydrothermal method followed by NaBH₄ reduction process. TEM proved that the In₂O₃ was nanorod structures of?~?110 nm in length and?~?35 nm in diameter. Ag nanoparticles with diameters from 10 to 15 nm homogeneously decorated on the surfaces of the In₂O₃ naonorods. XRD and XPS analysis proved that the Ag elements existed as zero-valent metallic silver on the surface of the In₂O₃ nanorods. Ag nanoparticles could enhance the formation of chemisorbed oxygen species and interactions between H₂S molecules and oxygen species due to spillover effect, and the electron transfer between Ag and In₂O₃ nanorods also enhanced the sensing properties. Therefore, the H₂S sensors based on the Ag–In₂O₃ nanorod composites showed significantly improved sensing performance than those based on the pure In₂O₃ nanorods. The optimized content of Ag nanoparticles is 13.6 wt%. Operated at room temperature, the H₂S sensors made of 13.6 wt% Ag–In₂O₃ nanorod composites exhibited an ultra-high response of 93719 to 20 ppm H₂S and a superior detection limit of 0.005 ppm. The sensor also showed good reversibility, good selectivity, excellent reproducibility and stability for detection of H₂S gas.     

Transition metal oxides covered Pd film for optical H2 gas detection

We have developed a simple fiber optic Fabry-Perot interferometer (FPI) sensor that is used to detection and measure concentration of hydrogen gas in the air. The operating principle of the sensor is discussed in this paper, and it was noticed that the wavelength positions of the FPI reflectance peaks change with the concentration of hydrogen gas. The sensor has been successfully used to monitor concentration of H₂ in the air below Lower Explosion Limit (LEL). The sensor utilizes a layered sensing structure. This structure includes gasochromic titanium dioxide (TiO₂) and nickel oxide (NiO_x) sensing film. The optical H₂ gas sensor has a very short response time and a fast regeneration time at room temperature.     

Response kinetics of a fiber-optic gas sensor using Pt/WO3 thin film to hydrogen

Response kinetics of a fiber-optic hydrogen gas sensor in air- and inert-atmosphere were characterized. The sensor is mainly based on the evanescent field interaction in hydrogen sensitive cladding which is used Platinum-supported tungsten trioxide (Pt/WO₃). When the sensor was exposed to 1 vol.% H₂/air and H₂/N₂ gas, the changes in optical power propagating through the fiber were about 30% and 50%, respectively. The detection limit was about 0.1 vol.% in air-atmosphere. The humidity dependence of the response kinetics was also evaluated. While the response speed in N₂-atmosphere was accelerated, the speed in air-atmosphere was suppressed by the humidity.     

Poly(BCB)/Au-nanoparticles hybrid film modified electrode: Preparation, characterization and its application as a non-enzymatic sensor

We report electrochemical preparation and characterization of poly-brilliant cresyl blue (Poly(BCB))/gold nanoparticles (Au-NPs) modified electrode. The Poly(BCB)/Au-NPs modified electrode has been used as an electrochemical sensor for the detection of hydrogen peroxide (H₂O₂) at lower potential (− 0.2 V). The Poly(BCB)/Au-NPs film was characterized by scanning electron microscopy, Uv-visible spectroscopy (Uv-vis) and cyclic voltammetry. We have observed that, Au-NPs attached glassy carbon electrode (Au-NPs/GCE) significantly enhanced the polymerization of BCB compared to bare GCE. The Poly(BCB) film was irreversibly attached onto the Au-NPs modified electrode, the resulting hybrid film modified electrode was electrochemically active in the pH range from 2 to 11. Attachment of Poly(BCB)/Au-NPs hybrid film on the electrode surface was confirmed by Uv-vis spectra. In addition, electrocatalytic properties of the Poly(BCB)/Au-NPs/GCE towards reduction of H₂O₂ have been investigated, and it was found that the sensitivity, reduction potential as well as the corresponding detection limit were improved as compared to the voltammetric response of the Poly(BCB)/GCE and Au-NPs/GCE. Based on this study, a non-enzymatic electrochemical sensor for the determination of H₂O₂ has been reported. Moreover, analysis of commercial H₂O₂ samples was performed using the proposed method and satisfactory results were obtained.     

Synthesis of polypyrrole coated manganese nanowires and their application in hydrogen peroxide detection

This study focuses on the synthesis and application of polypyrrole coated manganese nanowires (Mn/PPy NWs) as an enzyme-less sensor for the detection of hydrogen peroxide (H₂O₂). The X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) results confirm a core–shell structure with the Mn nanowires encapsulated by the PPy. An electrochemical sensor based on the Mn/PPy NWs for amperometric determination of H₂O₂ is prepared. The electrochemical behaviour of H₂O₂ is investigated by cyclic voltammetry with the use of modified glassy carbon electrode (GCE) with Mn/PPy NWs film. The modified glassy carbon electrode (GCE) with Mn/PPy NWs shows enhanced amperometric response for the detection of H₂O₂. This is due to the high available surface area of Mn/PPy NWs which can provide a suitable area for the reaction of H₂O₂. The detection limit and limit of quantification (S/N = 3) for two linear segments (low and high concentration of H₂O₂) are estimated to be 2.12 μmol L⁻¹, 7.07 μmol L⁻¹ and 22.3 μmol L⁻¹, 74.5 μmol L⁻¹, respectively. In addition, the sensitivity for these two linear segments is 0.4762 μA mM⁻¹ and 0.0452 μA mM⁻¹ respectively.     

Kinetics-Driven Dual Hydrogen Spillover Effects for Ultrasensitive Hydrogen Sensing

Palladium (Pd)-modified metal oxide semiconductors (MOSs) gas sensors often exhibit unexpected hydrogen (H₂) sensing activity through a spillover effect. However, sluggish kinetics over a limited Pd-MOS surface seriously restrict the sensing process. Here, a hollow Pd-NiO/SnO₂ buffered nanocavity is engineered to kinetically drive the H₂ spillover over dual yolk-shell surface for the ultrasensitive H₂ sensing. This unique nanocavity is found and can induce more H₂ absorption and markedly improve kinetical H₂ ab/desorption rates. Meanwhile, the limited buffer-room allows the H₂ molecules to adequately spillover in the inside-layer surface and thus realize dual H₂ spillover effect. Ex situ XPS, in situ Raman, and density functional theory (DFT) analysis further confirm that the Pd species can effectively combine H₂ to form Pd-H bonds and then dissociate the hydrogen species to NiO/SnO₂ surface. The final Pd-NiO/SnO₂ sensors exhibit an ultrasensitive response (0.1–1000 ppm H₂) and low actual detection limit (100 ppb) at the operating temperature of 230 °C, which surpass that of most reported H₂ sensors.     

Cobalt Nanoparticles and Atomic Sites in Nitrogen‐Doped Carbon Frameworks for Highly Sensitive Sensing of Hydrogen Peroxide

In situ monitoring of hydrogen peroxide (H₂O₂) during its production process is needed. Here, an electrochemical H₂O₂ sensor with a wide linear current response range (concentration: 5 × 10^?8 to 5 × 10^?2 m ), a low detection limit (32.4 × 10^?9 m ), and a high sensitivity (568.47 µA mm ^?1 cm^?2) is developed. The electrocatalyst of the sensor consists of cobalt nanoparticles and atomic Co‐N_x moieties anchored on nitrogen doped carbon nanotube arrays (Co‐N/CNT), which is obtained through the pyrolysis of the sandwich‐like urea@ZIF‐67 complex. More cobalt nanoparticles and atomic Co‐N_x as active sites are exposed during pyrolysis, contributing to higher electrocatalytic activity. Moreover, a portable screen‐printed electrode sensor is constructed and demonstrated for rapidly detecting (cost ≈40 s) H₂O₂ produced in microbial fuel cells with only 50 µL solution. Both the synthesis strategy and sensor design can be applied to other energy and environmental fields.     

A Ratiometric Sensor Using Single Chirality Near‐Infrared Fluorescent Carbon Nanotubes: Application to In Vivo Monitoring

Advances in the separation and functionalization of single walled carbon nanotubes (SWCNT) by their electronic type have enabled the development of ratiometric fluorescent SWCNT sensors for the first time. Herein, single chirality SWCNT are independently functionalized to recognize either nitric oxide (NO), hydrogen peroxide (H₂O₂), or no analyte (remaining invariant) to create optical sensor responses from the ratio of distinct emission peaks. This ratiometric approach provides a measure of analyte concentration, invariant to the absolute intensity emitted from the sensors and hence, more stable to external noise and detection geometry. Two distinct ratiometric sensors are demonstrated: one version for H₂O₂, the other for NO, each using 7,6 emission, and each containing an invariant 6,5 emission wavelength. To functionalize these sensors from SWCNT isolated from the gel separation technique, a method for rapid and efficient coating exchange of single chirality sodium dodecyl sulfate‐SWCNT is introduced. As a proof of concept, spatial and temporal patterns of the ratio sensor response to H₂O₂ and, separately, NO, are monitored in leaves of living plants in real time. This ratiometric optical sensing platform can enable the detection of trace analytes in complex environments such as strongly scattering media and biological tissues.     

Joule Heating a Palladium Nanowire Sensor for Accelerated Response and Recovery to Hydrogen Gas

The properties of a single heated palladium (Pd) nanowire for the detection of hydrogen gas (H₂) are explored. In these experiments, a Pd nanowire, 48–98 µm in length, performs three functions in parallel: 1) Joule self‐heating is used to elevate the nanowire temperature by up to 128 K, 2) the 4‐contact wire resistance in the absence of H₂ is used to measure its temperature, and 3) the nanowire resistance in the presence of H₂ is correlated with its concentration, allowing it to function as a H₂ sensor. Compared with the room‐temperature response of a Pd nanowire, the response of the heated nanowire to hydrogen is altered in two ways: First, the resistance change (ΔR/R₀) induced by H₂ exposure at any concentration is reduced by a factor of up to 30 and second, the rate of the resistance change – observed at the beginning (“response”) and at the end (“recovery”) of a pulse of H₂ – is increased by more than a factor of 50 at some H₂ concentrations. Heating nearly eliminates the retardation of response and recovery seen from 1–2% H₂, caused by the α → β phase transition of PdH_x, a pronounced effect for nanowires at room temperature. The activation energies associated with sensor response and recovery are measured and interpreted.     

Room-temperature H2S gas sensing properties of carbonized silicon nanoporous pillar array

Through thermally treating silicon nanoporous pillar array (Si-NPA) in a graphite crucible in a vacuum furnace at 1100 °C, a continuous thin film composed of cubic SiC nanoparticles was prepared and its room-temperature resistive sensing properties were measured. The sensor was found to be with high sensitivity and an upper limit concentration of 1200 ppm for H₂S detection. Through carrying out the experiments of adsorption-desorption dynamic cycles and long-time air-ambient storage, the sensor was demonstrated to be with high repeatability and long-term stability. The response and recovery times were determined to be ~ 123 and ~ 114 s, respectively. The sensing mechanism was put forward through analyzing the possible adsorption modes of H₂S molecules on SiC/Si-NPA. The existence of the detecting limit concentration was attributed to the single-layer adsorption of H₂S molecules, whose quantity was restricted by the effective adsorption sites formed on SiC/Si-NPA. Our results show that SiC/Si-NPA might be an ideal sensing material for fabricating low-concentration H₂S gas sensors.     

Effect of rf power on the structural properties of indium tin oxide thin film prepared for application in hydrogen gas sensor

Indium tin oxide films were grown on glass substrate by rf magnetron sputtering at 648 K. Influence of rf power on structural properties of the ITO films was studied. XRD measurements showed (222) preferred orientation under the optimized deposition conditions. The surface morphology of ITO films analyzed by scanning electron microscope appears to be uniform over the entire surface area, the film exhibited dense layers with fine grains. Finally, ITO sensor device was fabricated and the sensing properties of the device towards hydrogen gas were investigated. The variation in sensitivity of the ITO sensor with operating temperature and with concentration of hydrogen gas was studied. The maximum response was found to be 1.6 at 400 K, for 1,000 ppm of hydrogen gas, and the response of the sensor was found to decrease with increase in concentration of H₂ gas.     

The study of Pt Schottky contact on porous GaN for hydrogen sensing

This article reports the studies of Pt Schottky contact on porous n-type GaN for hydrogen sensing. A simpler and improved electroless etching method has been developed to generate porous GaN in which high uniformity of the porous area could be achieved. Hydrogen sensor was subsequently fabricated by depositing Pt Schottky contacts onto the porous GaN sample. For comparative study, a standard hydrogen sensor was also prepared by depositing Pt Schottky contacts on the as-grown sample. Hydrogen detection was carried out at room temperature and 100 °C. This Pt/porous GaN sensor exhibited a significant change of current upon exposure to 2% H₂ in N₂ gas as compared to the standard Pt/GaN sensor. Morphological studies by scanning electron microscopy revealed that Pt contact deposited on porous GaN having a very rough surface morphology with pores distributed all over the contact layer. Therefore, the increase of current could be attributed to the unique microstructure at porous Pt/porous GaN interface which allowed higher accumulation of hydrogen and eventually led to stronger effect of the H-induced dipole layer.     

Ab Initio Study of Hydrogen Interaction with Defective BC2N Nanotubes

The spin polarized density functional theory is used to investigate the incorporation of hydrogen adatoms and the interaction between molecular H₂ with antisites and vacancies in both zigzag (4,0) and armchair (3,3) BC₂N nanotubes. We find that the presence of antisites and vacancies increases the binding energy of hydrogen adatoms on the tube surface. In the most stable antisites (C_B, C_N, N_CI and B_CII), the hydrogen adatoms bind preferentially on carbon atoms of the defective site (C_B, and C_N) or closer to it (N_CI and B_CII). For a single adsorbed H, the calculated binding energies show that the H adsorption on a carbon vacancy (V_CII) is the most stable site with a binding energy of ?4.23?eV. The adsorption of a second H atom near the previous one is an exothermic process compared to of a single H₂ molecule physisorbed on the nanotube surface.     

SnO<Subscript>2</Subscript>(Au<Superscript>0</Superscript>, Co<Superscript>II,III</Superscript>) nanocomposites: A synergistic effect of the modifiers in CO detection

Nanocrystalline SnO₂ has been synthesized and its surface has been modified with Au⁰ and Co(II, III). The distribution of the modifiers over the nanocomposites has been studied by X-ray diffraction, inductively coupled plasma mass spectrometry, and transmission electron microscopy. The effect of the modifiers on the hydrogen reduction of nanocrystalline SnO₂ has been assessed. The CO sensing properties of the synthesized materials (10 ppm CO in air) have been studied in situ by electrical conductance measurements. The addition of both Au⁰ and Co(II, III) allows the working temperature of the SnO₂-based semiconductor sensor to be lowered to 215°C.