Impedance Spectroscopy-Based Reduced Graphene Oxide-Incorporated ZnO Composite Sensor for H2S Investigations

Graphene-based chemiresistive gas sensors

Gas sensors allow the monitoring of the chemical environment of humans, which is often crucial for their wellbeing or even survival. Miniaturization, reversibility, and selectivity are some of the key challenges for serial use of chemical sensors. This tutorial review describes critical aspects when using nanomaterials as sensing substrates for the application in chemiresistive gas sensors. Graphene has been shown to be a promising candidate, as it allows gas sensors to be operated at room temperature, possibly saving large amounts of energy. In this review, an overview is given on the general mechanisms for gas-sensitive semiconducting materials and the implications of doping and functionalization on the sensing parameters of chemiresistive devices. It shows in detail how different challenges, like sensitivity, response time, reversibility and selectivity have been approached by material development and operation modes. In addition, perspectives from the area of data analysis and intelligent algorithms are presented, which can further enhance these sensors' usability in the field.

Graphene Inks Printed by Aerosol Jet for Sensing Applications: The Role of Dispersant on the Inks' Formulation and Performance

This study presents graphene inks produced through the liquid-phase exfoliation of graphene flakes in water using optimized concentrations of dispersants (gelatin, triton X-100, and tween-20). The study explores and compares the effectiveness of the three different dispersants in creating stable and conductive inks. These inks can be printed onto polyethylene terephthalate (PET) substrates using an aerosol jet printer. The investigation aims to identify the most suitable dispersant to formulate a high-quality graphene ink for potential applications in printed electronics, particularly in developing chemiresistive sensors for IoT applications. Our findings indicate that triton X-100 is the most effective dispersant for formulating graphene ink (GTr), which demonstrated electrical conductivity (4.5 S·cm-1), a high nanofiller concentration of graphene flakes (12.2%) with a size smaller than 200 nm (<200 nm), a low dispersant-to-graphene ratio (5%), good quality as measured by Raman spectroscopy (ID/IG ≈ 0.27), and good wettability (θ ≈ 42°) over PET. The GTr's ecological benefits, combined with its excellent printability and good conductivity, make it an ideal candidate for manufacturing chemiresistive sensors that can be used for Internet of Things (IoT) healthcare and environmental applications.

Biosynthesis and Optimization of ZnO Nanoparticles Using Ocimum lamifolium Leaf Extract for Electrochemical Sensor and Antibacterial Activity

In this study, zinc oxide nanoparticles (ZnO NPs) were synthesized using an aqueous extract of the Ocimum lamifolium (O. lamifolium) plant. The I-optimal coordinate exchange randomized response surface methodology (RSM) was used to optimize the effect of the zinc acetate precursor, temperature, and time on ZnO NPs by designing nine runs. From ANOVA analysis, the significance and validity of the designed model showed that the optimal values of the zinc acetate precursor, temperature, and time during ZnO NPs synthesis were found to be ∼0.06 M, ∼30 °C, and ∼1.35 h, respectively. The obtained ZnO NPs under these optimized conditions were characterized and explored by UV-vis, TGA/DTA, FTIR, XRD, SEM-EDX, TEM, HRTEM, and SAED. Furthermore, the electrocatalytic performance of ZnO NPs was performed for sulfamethoxazole (SMZ) sensing activity with a 0.3528 μM (S/N = 3) limit of detection (LOD). In addition, an antibacterial study revealed that ZnO NPs confirmed an excellent zone of inhibition against E. coli, S. aureus, P. aeruginosa, and S. pyogen pathogenic drug resistance bacterial strains at concentrations of 50, 75, and 100 μg/mL. Thus, ZnO NPs synthesized using the O. lamifolium leaf have a potential electrocatalytic activity for diverse organic pollutant detection as well as a desirable material for such drug resistance antimicrobial strains.

Engineering of ZnO/rGO towards NO2 Gas Detection: Ratio Modulated Sensing Type and Heterojunction Determined Response

Nanoscale heterostructured zinc oxide/reduced graphene oxide (ZnO/rGO) materials with p-n heterojunctions exhibit excellent low temperature NO2 gas sensing performance, but their doping ratio modulated sensing properties remain poorly understood. Herein, ZnO nanoparticles were loaded with 0.1~4% rGO by a facile hydrothermal method and evaluated as NO2 gas chemiresistor. We have the following key findings. First, ZnO/rGO manifests doping ratio-dependent sensing type switching. Increasing the rGO concentration changes the type of ZnO/rGO conductivity from n-type (<0.6% rGO) to mixed n/p -type (0.6~1.4% rGO) and finally to p-type (>1.4% rGO). Second, interestingly, different sensing regions exhibit different sensing characteristics. In the n-type NO2 gas sensing region, all the sensors exhibit the maximum gas response at the optimum working temperature. Among them, the sensor that shows the maximum gas response exhibits a minimum optimum working temperature. In the mixed n/p-type region, the material displays abnormal reversal from n- to p-type sensing transitions as a function of the doping ratio, NO2 concentration and working temperature. In the p-type gas sensing region, the response decreases with increasing rGO ratio and working temperature. Third, we derive a conduction path model that shows how the sensing type switches in ZnO/rGO. We also find that p-n heterojunction ratio (np-n/nrGO) plays a key role in the optimal response condition. The model is supported by UV-vis experimental data. The approach presented in this work can be extended to other p-n heterostructures and the insights will benefit the design of more efficient chemiresistive gas sensors.

Silica sand-supported nano zinc oxide-graphene oxide composite induced rapid photocatalytic decolorization of azo dyes under sunlight and improved antimicrobial activity

Here, silica sand-supported heterojunction composite of nano zinc oxide (nZnO) and graphene oxide nanosheet (nZnO-GO@SS) was prepared, and its potential as an efficient photocatalyst for the degradation of methylene blue (MB) and Rhodamine-B (Rh-B) under sunlight was demonstrated. Transmission electron microscopy confirmed the uniform distribution of spherically shaped nZnO of average size of approximately 8 nm over graphene oxide nanosheet (GO) in the composites. Photodegradation yields of 95.3% and 97.5% for 100 ppm of MB and Rh-B dye within 150 and 220 min, respectively, were achieved under sunlight by the prepared nanocatalyst (nZnO-GO), while sand microparticle-supported nanocatalyst (nZnO-GO@SS) demonstrated faster degradation of MB and Rh-B, i.e., within 120 and 160 min, respectively. Furthermore, when the recyclability of the photocatalyst was studied, the nZnO-GO exhibited more than 80% degradation efficiency after five cycles for both the dyes and nZnO-GO@SS demonstrated 10% higher (~90%) removal capability after five cycles of reuse. Furthermore, the antibacterial assay showed complete inactivation of Escherichia coli and Staphylococcus aureus bacterial strain by nZnO-GO@SS. Hence, our proposed strategy for the removal of toxic dyes from the aquatic environment under sunlight proved that sand microparticle-supported nanocatalyst (nZnO-GO@SS) might be a superior, cost-effective, and suitable photocatalytic system for industrial applications toward toxic dye removal and decontamination from industrial wastewater.

UV-Enhanced Formaldehyde Sensor Using Hollow In2O3@TiO2 Double-Layer Nanospheres at Room Temperature

Hollow In₂O₃@TiO₂ double-layer nanospheres were prepared via a facile water bath method using the sacrifice template of carbon nanospheres. It is shown that the size of the In₂O₃/TiO₂ nanocomposites is 150-250 nm, the thickness of the In₂O₃ shell is about 10 nm, and the thickness of the TiO₂ shell is about 15 nm. The sensing performances of the synthesized In₂O₃/TiO₂ nanocomposites-based chemiresistive-type sensor to formaldehyde (HCHO) gas under UV light activation at room temperature have been studied. Compared to the pure In₂O₃- and pure TiO₂-based sensors, the In₂O₃/TiO₂ nanocomposite sensor exhibits much better sensing performances to formaldehyde. The response of the In₂O₃/TiO₂ nanocomposite-based sensor to 1 ppm formaldehyde is about 3.8, and the response time and recovery time are 28 and 50 s, respectively. The detectable formaldehyde concentration can reach as low as 0.06 ppm. The role of the formed In₂O₃/TiO₂ heterojunctions and the involved chemical reactions activated by UV light have been investigated by AC impedance spectroscopy and the in situ diffuse reflectance Fourier transform infrared spectroscopy. The improvement of the sensing properties of In₂O₃/TiO₂ nanocomposites could be attributed to the nanoheterojunctions between the two components and the "combined photocatalytic effects" of UV-light-emitting diode irradiation. Density functional theory calculations demonstrated that introducing heterojunctions could improve the adsorption energy and charge transfer between formaldehyde and sensing materials.

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.

Cadmium sulfide in-situ derived heterostructure hybrids with tunable component ratio for highly sensitive and selective detection of ppb-level H2S

Herein, we reported cadmium sulfide derivatives pine needles-like CdS/CdO heterostructure hybrids synthesized by hydrothermal treatment and subsequent self-template oxidation approach. The component ratio of the CdS/CdO hybrids can be controlled specifically via tuning the annealing treatment protocol, and thereby giving rise to the optimization of morphology, electrical characteristics, and gas sensing properties of derived hybrids. As proof of concept, the pine needles-like CdS/CdO, which obtained after different annealing temperatures and durations, as sensitive material was employed to manufacture H₂S gas sensors. The sensor based on CdS/CdO hybrids (400 °C & 1 h) exhibited high sensitivity (73.5 to 5 ppm), ppb-level limit of detection (10 ppb), and excellent selectivity regardless of the interference of other gases at optimal working temperature of 200 °C. Due to the abnormal resistance variation of n-type cadmium sulfide derived hybrids while contacting with H₂S, the sensing mechanism mainly depends on the surface chemical conversion from oxide to sulfide. The pine needles-like hierarchical morphology provided an excellent scaffold for the carriers transportation and the growth of the CdO, which played a key role in resistance modulation both in air and target gas, resulting in the enhanced H₂S sensing performance ultimately.

Semiconductor-Type Gas Sensors Based on γ-Fe2O3 Nanoparticles and Its Derivatives in Conjunction with SnO2 and Graphene

The gas sensitivity of semiconductor metal oxides, such as γ-Fe2O3 and SnO2, is investigated together with the synergistic effects in conjunction with grapheme. Nanoparticles of γ-Fe2O3, γ-Fe2O3/SnO2, and γ-Fe2O3/SnO2/RGO, prepared by two-step fabrication, were assembled in gas-sensing devices to assess their sensitivities; response and recovery times for the detection of ethanol, methanol, isopropanol, formaldehyde, H2S, CO, and NO gases at different temperatures but constant concentrations of 100 particles per million (ppm); and H2S, which underwent the dynamic gas sensitivity test in different concentrations. Each sample’s crystallinity and microscopic morphology was investigated with X-ray diffraction and a scanning electron microscope. In comparative gas sensitivity measurements, the ternary composite of γ-Fe2O3/SnO2/RGO was identified as an ideal candidate, as it responds to all four tested liquids in the gas phase as well as H2S with a response value equal to 162.6. Further, only the ternary composite γ-Fe2O3/SnO2/RGO hybrid nanoparticles responded to NO gas with a sensor response value equal to 4.09 in 12 s. However, only the binary composite γ-Fe2O3/SnO2 responded to CO with a corresponding sensitivity of 1.59 units in 7 s.

Impedance Spectroscopy of Hierarchical Porous Nanomaterials Based on por-Si, por-Si Incorporated by Ni and Metal Oxides for Gas Sensors

Approaches are being developed to create composite materials with a fractal-percolation structure based on intercalated porous matrices to increase the sensitivity of adsorption gas sensors. Porous silicon, nickel-containing porous silicon, and zinc oxide have been synthesized as materials for such structures. Using the impedance spectroscopy method, it has been shown that the obtained materials demonstrate high sensitivity to organic solvent vapors and can be used in gas sensors. A model is proposed that explains the high sensitivity and inductive nature of the impedance at low frequencies, considering the structural features and fractal-percolation properties of the obtained oxide materials.

Simultaneous Detection of NH3 and NO2 by Modified Impedance Spectroscopy in Sensors Based on Carbon Nanotubes

There are many gaseous substances that need to be monitored for possible damage to health or the environment. This requires many sensors. The solution to reducing the number of sensors is to use one sensor to detect several gaseous substances simultaneously. Efforts to simplify sensor systems thus lead to the use of a sensor with a suitable sensitive layer and to finding a suitable method of detecting individual gaseous substances within one sensor. The aim is to find a suitable method to detect various gaseous substances acting on the sensor. For this purpose, modified impedance spectroscopy in the high-frequency range is applied, where the scattering parameters of the sensor based on carbon nanotubes are measured under the action of NO2 and NH3 gases. For this method of detection of gaseous substances, a suitable sensor platform structure was designed to enable the measurement of the electrical properties of the sensor in the GHz range. Based on the obtained results, it is possible to use one sensor to detect different types of gaseous substances.

Microheater: material, design, fabrication, temperature control, and applications-a role in COVID-19

Heating plays a vital role in science, engineering, mining, and space, where heating can be achieved via electrical, induction, infrared, or microwave radiation. For fast switching and continuous applications, hotplate or Peltier elements can be employed. However, due to bulkiness, they are ineffective for portable applications or operation at remote locations. Miniaturization of heaters reduces power consumption and bulkiness, enhances the thermal response, and integrates with several sensors or microfluidic chips. The microheater has a thickness of ~ 100 nm to ~ 100 μm and offers a temperature range up to 1900℃ with precise control. In recent years, due to the escalating demand for flexible electronics, thin-film microheaters have emerged as an imperative research area. This review provides an overview of recent advancements in microheater as well as analyses different microheater designs, materials, fabrication, and temperature control. In addition, the applications of microheaters in gas sensing, biological, and electrical and mechanical sectors are emphasized. Moreover, the maximum temperature, voltage, power consumption, response time, and heating rate of each microheater are tabulated. Finally, we addressed the specific key considerations for designing and fabricating a microheater as well as the importance of microheater integration in COVID-19 diagnostic kits. This review thereby provides general guidelines to researchers to integrate microheater in micro-electromechanical systems (MEMS), which may pave the way for developing rapid and large-scale SARS-CoV-2 diagnostic kits in resource-constrained clinical or home-based environments.

Graphene and Perovskite-Based Nanocomposite for Both Electrochemical and Gas Sensor Applications: An Overview

Perovskite and graphene-based nanocomposites have attracted much attention and been proven as promising candidates for both gas (H₂S and NH₃) and electrochemical (H₂O₂, CH₃OH and glucose) sensor applications. In this review, the development of portable sensor devices on the sensitivity, selectivity, cost effectiveness, and electrode stability of chemical and electrochemical applications is summarized. The authors are mainly focused on the common analytes in gas sensors such as hydrogen sulfide, ammonia, and electrochemical sensors including non-enzymatic glucose, hydrazine, dopamine, and hydrogen peroxide. Finally, the article also addressed the stability of composite performance and outlined recent strategies for future sensor perspectives.

Structural refinement and electrochemical properties of one dimensional (ZnO NRs)1-x(CNs)x functional hybrids for serotonin sensing studies

Herein, the efficient serotonin (5-HT) sensing studies have been conducted using the (ZnO NRs)1-x(CNs)x nanocomposites (NCs) having appropriate structural and electrochemical properties. Initially, the different compositions of ZnO nanorods (NRs), with varying content of carbon nanostructures (CNs=MWCNTs and RGO), are prepared using simple in-situ wet chemical method and thereafter these NCs have been characterized for physico-chemical properties in correlation to the 5-HT sensing activity. XRD Rietveld refinement studies reveal the hexagonal Wurtzite ZnO NRs oriented in (101) direction with space group 'P63mc' and both orientation as well as phase of ZnO NRs are also retained in the NCs due to the small content of CNs. The interconnectivity between the ZnO NRs with CNs through different functional moieties is also studied using FTIR analysis; while phases of the constituents are confirmed through Raman analysis. FESEM images of the bare/NCs show hexagonal shaped rods with higher aspect ratio (4.87) to that of others. BET analysis and EIS measurements reveal the higher surface area (97.895 m2/g), lower charge transfer resistance (16.2 kΩ) for the ZCNT 0.1 NCs to that of other NCs or bare material. Thereafter, the prepared NCs are deposited on the screen printed carbon electrode (SPCE) using chitosan as cross-linked agent for 5-HT sensing studies; conducted through cyclic voltammetry (CV) and square wave voltammetry (SWV) measurements. Among the various composites, ZCNT0.1 NCs based electrodes exhibit higher sensing activity towards 5-HT in accordance to its higher surface area, lower particle size and lower charge transfer resistance. SWV measurements provide a wide linear response range (7.5-300 μM); lower limit of detection (0.66 μM), excellent limit of quantification (2.19 μM) and good reproducibility to ZCNT 0.1 NCs as compared to others for 5-HT sensing studies.

Assessment of GO/ZnO nanocomposite for solar-assisted photocatalytic degradation of industrial dye and textile effluent

An ecofriendly and solar light-responsive graphene oxide wrapped zinc oxide nanohybrid has been synthesized hydrothermally using lemon and honey respectively as chelating and complexing agents. By tuning the reaction conditions, a heterostructure between GO and ZnO has been formed during synthesis. The photocatalytic activity of the synthesized nanohybrid was investigated by degradation of hazardous organic textile dye (methylene blue) as well as wastewater under natural solar light. The nanohybrid exhibited excellent photocatalytic activity towards degradation (~ 89%) of methylene blue (MeB). Furthermore, along with decolorization, 71% of mineralization was also achieved. Interestingly, the nanohybrid has been found to be reusable up to 4 cycles without significant loss of photocatalytic activity. Along with this, the physicochemical parameters of the wastewater generated from textile industry have been also monitored before and after exposure to nanohybrid. The results revealed significant reduction in chemical oxygen demand (COD) (96.33%), biochemical oxygen demand (BOD) (96.23%), and total dissolved solids (TDS) (20.85%), suggesting its potential applicability in textile wastewater treatment.

A covalent organic polymer as an efficient chemosensor for highly selective H2S detection through proton conduction

An interdigitated electrode fabricated with a covalent organic polymer (COP) acts as an efficient H₂S gas sensor at room temperature.