Gas sensors based on TiO2 nanostructured materials for the detection of hazardous gases: A review

Synthesis of spherical and rods-like titanium oxide nanoparticles (TiO2 NPs) and evaluation of their cytotoxicity towards colon cells in vitro

Titanium oxide nanoparticles (TiO2 NPs) are currently used as ingredients in medicines and sunscreens. Unfortunately, recent information about TiO2 NPs indicates their undesirable biological effect on colon cells. Therefore, the aim of this work was to synthesize and evaluate the physicochemical characterization of spherical (TiO2 NSs) and rods-like (TiO2 NRs) NPs, followed by assessment their cytotoxicity. For this purpose, both normal colon epithelial cells (CRL-1790) and cancerous colon cells (SW480) were used. Scanning transmission electron microscopy (STEM) showed that TiO2 NSs with a diameter of ≈10 nm and TiO2 NRs with the size of the longer axis ≈25 nm and shorter axis ≈3 nm were obtained. Based on the selected area electron diffraction (SAED) patterns, it was found that crystalline phases were obtained for both TiO2 NPs. The UV-Vis spectra showed no contamination of TiO2 NPs. Zeta potential values were 9.7 mV and 3.1 mV for NSs and NRs, respectively. Cytotoxicity of TiO2 NPs was assessed using the MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxy-methoxy-phenyl)-2-(4-sulfophenyl)-2H-tetrazolium) test for various concentration of NPs. The cytotoxic effect for both TiO2 NPs was visible for concentration of 75 μg/ml (for CRL-1790) and 50 μg/ml (for SW480) and higher, and it did not depend on the shape. Moreover, both types of TiO2 NPs (in higher concentration) induce the generation of reactive oxygen species (ROS) in cells cultured with these NPs. Holotomographic microscopy studies showed increased cellular uptake of TiO2 NPs by SW480. The obtained results for the synthesized TiO2 NPs are a promising prospect for their use in biomedical application.

WO3-x/WS2 Nanocomposites for Fast-Response Room Temperature Gas Sensing

Currently, semiconductor gas sensors are being actively studied and used in various fields, including ecology, industry, and medical diagnostics. One of the major challenges is to reduce their operating temperature to room temperature. To address this issue, sensor layers based on WO3-x/WS2 nanostructures synthesized by the hydrothermal method have been proposed. In this paper, the morphology of the material's surface and its elemental composition were investigated, as well as the optical band gap. Additionally, changes in the resistance of the WO3-x/WS2 sensor layers under the influence of alcohol vapors at room temperature were analyzed. The results showed that the layers exhibited a significant response, with short response and recovery times. The achieved response value to 1000 ppm of isopropanol was 1.25, with a response time of 13 s and a recovery time of 12 s. The response to 1000 ppm of ethanol was 1.35, and the response and recovery times were 20 s. This indicates that these sensor layers have promising potential for various applications.

Inkjet-Printed Graphene-PEDOT:PSS Decorated with Sparked ZnO Nanoparticles for Application in Acetone Detection at Room Temperature

This work presents a simple process for the development of flexible acetone gas sensors based on zinc oxide/graphene/poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate). The gas sensors were prepared by inkjet printing, which was followed by a metal sparking process involving different sparking times. The successful decoration of ZnO nanoparticles (average size ~19.0 nm) on the surface of the graphene-PEDOT:PSS hybrid ink was determined by characterizations, including Raman spectroscopy, Fourier transform infrared spectroscopy, field-emission transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffractometry. The ZnO nanoparticle-decorated graphene-PEDOT:PSS with a sparking time of 2 min exhibited the highest response of 71.9% at 10 ppm of acetone, above those of samples treated with other sparking times and the undecorated control. In addition, the optimal sensor revealed high selectivity for acetone over several other kinds of gases, such as ammonia, toluene, dimethylformamide, ethanol, methanol, and benzene, at room temperature. The gas sensor also revealed a low limit of detection (0.4 ppm), high sensitivity (6.18 ppm-1), and high stability (5-week long-term) to acetone. The response and recovery times of the sensor were found to be 4.6 min and 4.2 min, respectively. The acetone-sensing mechanism was attributed to the formation of p-n heterojunctions, which were responsible for the significantly enhanced sensitivity.

An Ultrasensitive Ethanol Gas Sensor Based on a Dual-Nanoparticle In2O3/SnO2 Composite

As a VOC, ethanol can be found in human exhaled breath, and its concentration can be used as a biomarker of human liver disease. To detect trace-level concentrations of ethanol, an ultrasensitive ethanol sensor was developed based on a dual-nanoparticle In2O3/SnO2 composite that was prepared by hydrothermal synthesis, and its suspension was dipped on a flat electrode to form a gas sensor. The nanocomposite was characterized by an SEM (scanning electron microscope), XRD (X-ray diffraction), and a TEM (transmission electron microscope), and the nanoparticle structure was observed. The experimental results showed that gas sensors based on the In2O3/SnO2 nanocomposite had higher responses compared to sensors based on pure In2O3. Among the nanocomposites, the one with a In2O3-to-SnO2 mol ratio of 1:8 was used in the sensor with the highest response of 1.41 to 100 ppb ethanol at 150 °C, which also exhibited good repeatability. The ultrasensitive response to ethanol can be attributed to the faster electron migration rate and the increase in oxygen-absorbing sites caused by the n-n heterojunction in the nanocomposite. Due to its low detection limit, good repeatability, and relatively high responses in high humidity, this sensor has a potential application in exhaled breath detection.

Hydrothermally Grown Globosa-like TiO2 Nanostructures for Effective Photocatalytic Dye Degradation and LPG Sensing

The rapid expansion of industrial activities has resulted in severe environmental pollution manifested by organic dyes discharged from the food, textile, and leather industries, as well as hazardous gas emissions from various industrial processes. Titanium dioxide (TiO2)-nanostructured materials have emerged as promising candidates for effective photocatalytic dye degradation and gas sensing applications owing to their unique physicochemical properties. This study investigates the development of a photocatalyst and a liquefied petroleum gas (LPG) sensor using hydrothermally synthesized globosa-like TiO2 nanostructures (GTNs). The synthesized GTNs are then evaluated to photocatalytically degrade methylene blue dye, resulting in an outstanding photocatalytic activity of 91% degradation within 160 min under UV light irradiation. Furthermore, these nanostructures are utilized to sense liquefied petroleum gas, which attains a superior sensitivity of 7.3% with high response and recovery times and good reproducibility. This facile and cost-effective hydrothermal method of fabricating TiO2 nanostructures opens a new avenue in photocatalytic dye degradation and gas sensing applications.

Gas-Sensing Performance of Metal Oxide Heterojunction Materials for SF6 Decomposition Gases: A DFT Study

The online monitoring of GIS equipment can be realized through detecting SF6 decomposition gasses. Metal oxide heterojunctions are widely used as gas-sensing materials. In this study, the structural and electrical properties of In2O3-ZnO and TiO2-ZnO heterojunctions were analyzed based on density functional theory calculations. After heterojunction structural optimization, the electrical conductivity of these two heterojunctions was enhanced compared to each intrinsic model, and the electrical conductivity is ranked as follows: In2O3-ZnO heterojunction > TiO2-ZnO heterojunction. The gas-sensing response of these two heterojunctions to four SF6 decomposition gasses, H2S, SO2, SOF2, and SO2F2, was investigated. For gas adsorption systems, the adsorption energy, charge transfer, density of states, charge difference density, and frontier molecular orbitals were calculated to analyze the adsorption and gas-sensing performance. For gas adsorption on the In2O3-ZnO heterojunction surface, the induced conductivity changes are in the following order: H2S > SO2F2 > SOF2 > SO2. For gas adsorption on the TiO2-ZnO heterojunction surface, H2S and SOF2 increase conductivity, and SO2 and SO2F2 decrease conductivity.

TiO2 Nanoparticles with Adjustable Phase Composition Prepared by an Inverse Microemulsion Method: Physicochemical Characterization and Photocatalytic Properties

Titania nanoparticles (NPs) find wide application in photocatalysis, photovoltaics, gas sensing, lithium batteries, etc. One of the most important synthetic challenges is maintaining control over the polymorph composition of the prepared nanomaterial. In the present work, TiO2 NPs corresponding to anatase, rutile, or an anatase/rutile/brookite mixture were obtained at 80 °C by an inverse microemulsion method in a ternary system of water/cetyltrimethylammonium bromide/1-hexanol in a weight ratio of 17:28:55. The only synthesis variables were the preparation of the aqueous component and the nature of the Ti precursor (Ti(IV) ethoxide, isopropoxide, butoxide, or chloride). The materials were characterized with X-ray diffraction, scanning/transmission electron microscopy, N2 adsorption-desorption isotherms, FTIR and Raman vibrational spectroscopies, and diffuse reflectance spectroscopy. The synthesis products differed significantly not only in phase composition, but also in crystallinity, textural properties, and adsorption properties towards water. All TiO2 NPs were active in the photocatalytic decomposition of rhodamine B, a model dye pollutant of wastewater streams. The mixed-phase anatase/rutile/brookite nanopowders obtained from alkoxy precursors showed the best photocatalytic performance, comparable to or better than the P25 reference. The exceptionally high photoactivity was attributed to the advantageous electronic effects known to accompany multiphase titania composition, namely high specific surface area and strong surface hydration. Among the single-phase materials, anatase samples showed better photoactivity than rutile ones, and this effect was associated, primarily, with the much higher specific surface area of anatase photocatalysts.

Exploring affinity between organic probes and Prussian Blue Analogues via inverse gas chromatography

Prussian Blue Analogues (PBAs), which are characterized by their open structure, high stability, and non-toxic properties, have recently been the subject of research for various applications, including their use as electrode precursors for capacitive deionization, gas storage, and environmental purification. These materials can be readily tailored to enhance their affinity towards gases for integration with sensing devices. An improved understanding of PBA-gas interactions is expected to enhance material development and existing sensor deposition schemes greatly. The use of inverse gas chromatography (IGC) is a robust approach for examining the relationship between porous materials and gases. In this study, the adsorption properties of (functionalized) hydrocarbons, i.e., probe molecules, on the copper hexacyanoferrate (CuHCF) lattice were studied via IGC, demonstrating that alkylbenzenes have a higher affinity for this material than n-alkanes. This difference was rationalized by steric hindrance, π-π interactions, and vapour pressure effects. Along the same line, the five isomers of hexane showed decreasing selectivity upon increased steric hindrance. Enthalpy values for n-pentane, n-hexane and n-heptane were lower than that of toluene. The introduction of increased probe masses resulted in a surface coverage of 46% for toluene. For all n-alkane probe molecules this percentage was lower. However, the isotherms of these probes did not show saturation points and the observed linear regime proves beneficial for gas sensing. Our work demonstrates the versatility of CuHCF for gas sensing purposes and the potential of IGC to characterize the adsorption characteristics of such a porous nanomaterial.

Recent Progress on Potentiometric Sensor Applications Based on Nanoscale Metal Oxides: A Comprehensive Review

Electrochemical sensors have been the subject of much research and development as of late, with several publications detailing new designs boasting enhanced performance metrics. That is, without a doubt, because such sensors stand out from other analytical tools thanks to their excellent analytical characteristics, low cost, and ease of use. Their progress has shown a trend toward seeking out novel useful nano structure materials. A variety of nanostructure metal oxides have been utilized in the creation of potentiometric sensors, which are the subject of this article. For screen-printed pH sensors, metal oxides have been utilized as sensing layers due to their mixed ion-electron conductivity and as paste-ion-selective electrode components and in solid-contact electrodes. Further significant uses include solid-contact layers. All the metal oxide uses mentioned are within the purview of this article. Nanoscale metal oxides have several potential uses in the potentiometry method, and this paper summarizes such uses, including hybrid materials and single-component layers. Potentiometric sensors with outstanding analytical properties can be manufactured entirely from metal oxides. These novel sensors outperform the more traditional, conventional electrodes in terms of useful characteristics. In this review, we looked at the potentiometric analytical properties of different building solutions with various nanoscale metal oxides.

CuO/TiO2/MXene-Based Sensor and SMS-TENG Array Integrated Inspection Robots for Self-Powered Ethanol Detection and Alarm at Room Temperature

In this study, a high-precision CuO/TiO2/MXene ethanol sensor operating at room temperature was prepared. The sensor exhibits excellent response value (95% @1 ppm ethanol), extremely low detection limit (0.3 ppm), fast response/recovery time (16/13 s), and remarkable long-term stability for trace detection of ethanol gas at room temperature, attributed to the p-n heterojunction formed by CuO and TiO2, as well as the rich functional groups and large specific surface area of MXene. Furthermore, a high-performance triboelectric nanogenerator (SMS-TENG) was developed through the introduction of the silicone/Mxene@silicone dual dielectric layer as the triboelectric layer, which improves the charge storage capacity of the dielectric layer and greatly enhances the output performance of the TENG. At the optimal doping ratio, the open-circuit voltage of the SMS-TENG can reach 1160 V, which is sufficient to light 720 LEDs. By combining the sensor and SMS-TENG, the resistive response of ethanol sensing is converted to a voltage response, which amplifies the response value up to 15.8 times. Finally, the designed SMS-TENGs are expected to be arrayed on an inspection robot as energy supply and combined with the CuO/TiO2/Mxene ethanol sensor to build a self-powered ethanol detection alarm system, endowing the inspection robot with the capability of self-powered ethanol detection at ppb level. This work provides an effective pathway for the intelligence of ethanol detection.

Sub-PPB Detection with Gas-Phase Multiphoton Electron Extraction Spectroscopy under Ambient Conditions

Multiphoton electron extraction spectroscopy (MEES) is an advanced analytical technique that has demonstrated exceptional sensitivity and specificity for detecting molecular traces on solid and liquid surfaces. Building upon the solid-state MEES foundations, this study introduces the first application of MEES in the gas phase (gas-phase MEES), specifically designed for quantitative detection of gas traces at sub-part per billion (sub-PPB) concentrations under ambient atmospheric conditions. Our experimental setup utilizes resonant multiphoton ionization processes using ns laser pulses under a high electrical field. The generated photoelectron charges are recorded as a function of the laser's wavelength. This research showcases the high sensitivity of gas-phase MEES, achieving high spectral resolution with resonant peak widths less than 0.02 nm FWHM. We present results from quantitative analysis of benzene and aniline, two industrially and environmentally significant compounds, demonstrating linear responses in the sub-PPM and sub-PPB ranges. The enhanced sensitivity and resolution of gas-phase MEES offer a powerful approach to trace gas analysis, with potential applications in environmental monitoring, industrial safety, security screening, and medical diagnostics. This study confirms the advantages of gas-phase MEES over many traditional optical spectroscopic methods and demonstrates its potential in direct gas-trace sensing in ambient atmosphere.

Porous materials as effective chemiresistive gas sensors

Chemiresistive gas sensors (CGSs) have revolutionized the field of gas sensing by providing a low-power, low-cost, and highly sensitive means of detecting harmful gases. This technology works by measuring changes in the conductivity of materials when they interact with a testing gas. While semiconducting metal oxides and two-dimensional (2D) materials have been used for CGSs, they suffer from poor selectivity to specific analytes in the presence of interfering gases and require high operating temperatures, resulting in high signal-to-noise ratios. However, nanoporous materials have emerged as a promising alternative for CGSs due to their high specific surface area, unsaturated metal actives, and density of three-dimensional inter-connected conductive and pendant functional groups. Porous materials have demonstrated excellent response and recovery times, remarkable selectivity, and the ability to detect gases at extremely low concentrations. Herein, our central emphasis is on all aspects of CGSs, with a primary focus on the use of porous materials. Further, we discuss the basic sensing mechanisms and parameters, different types of popular sensing materials, and the critical explanations of various mechanisms involved throughout the sensing process. We have provided examples of remarkable performance demonstrated by sensors using these materials. In addition to this, we compare the performance of porous materials with traditional metal-oxide semiconductors (MOSs) and 2D materials. Finally, we discussed future aspects, shortcomings, and scope for improvement in sensing performance, including the use of metal-organic frameworks (MOFs), covalent-organic frameworks (COFs), and porous organic polymers (POPs), as well as their hybrid counterparts. Overall, CGSs using porous materials have the potential to address a wide range of applications, including monitoring water quality, detecting harmful chemicals, improving surveillance, preventing natural disasters, and improving healthcare.

Recent Advances in Nanostructured Materials for Application as Gas Sensors

Many different industries, including the pharmaceutical, medical engineering, clinical diagnostic, public safety, and food monitoring industries, use gas sensors. The inherent qualities of nanomaterials, such as their capacity to chemically or physically adsorb gas, and their great ratio of surface to volume make them excellent candidates for use in gas sensing technology. Additionally, the nanomaterial-based gas sensors have excellent selectivity, reproducibility, durability, and cost-effectiveness. This Review article offers a summary of the research on gas sensor devices based on nanomaterials of various sizes. The numerous nanomaterial-based gas sensors, their manufacturing procedures and sensing mechanisms, and most recent advancements are all covered in detail. In addition, evaluations and comparisons of the key characteristics of gas sensing systems made from various dimensional nanomaterials were done.

Ultrasensitive and reversible NO2 gas sensor based on SnS2/TiO2 heterostructures for room temperature applications

Nitrogen dioxide (NO2) is one of the toxic gases produced by chemical industries, power plants, and vehicles. In this work, we demonstrate an inexpensive sensing platform for NO2 detection at room temperature (RT-32 °C) based on a charge transfer mechanism. Three-dimensional hierarchical SnS2 and SnS2/mesoporous TiO2 nanocomposites were synthesized via the solvothermal method. SnS2/20 wt% mesoporous TiO2 nanocomposites sample showed 245.4% enhanced response compared to pristine SnS2. The fabricated device exhibits excellent selectivity among all other interfering gases with one-month stability. The rapid response and enhanced response achieved were obtained for the minimum concentration of 2 ppm NO2. The formation of heterojunction between SnS2 and mesoporous TiO2 has a synergetic effect, providing more active sites and porous structures for the detection of NO2 gas molecules.

Synthesis of TiO2-(B) Nanobelts for Acetone Sensing

Titanium dioxide nanobelts were prepared via the alkali-hydrothermal method for application in chemical gas sensing. The formation process of TiO2-(B) nanobelts and their sensing properties were investigated in detail. FE-SEM was used to study the surface of the obtained structures. The TEM and XRD analyses show that the prepared TiO2 nanobelts are in the monoclinic phase. Furthermore, TEM shows the formation of porous-like morphology due to crystal defects in the TiO2-(B) nanobelts. The gas-sensing performance of the structure toward various concentrations of hydrogen, ethanol, acetone, nitrogen dioxide, and methane gases was studied at a temperature range between 100 and 500 °C. The fabricated sensor shows a high response toward acetone at a relatively low working temperature (150 °C), which is important for the development of low-power-consumption functional devices. Moreover, the obtained results indicate that monoclinic TiO2-B is a promising material for applications in chemo-resistive gas detectors.

Low-Cost ZnO Spray-Coated Optical Fiber Sensor for Detecting VOC Biomarkers of Diabetes

A non-invasive optical fiber sensor for detecting volatile organic compounds (VOCs) as biomarkers of diabetes is proposed and experimentally demonstrated. It offers a low-cost and straightforward fabrication approach by implementing a one-step spray coating of a ZnO colloidal solution on a glass optical fiber. The structure of the optical fiber sensor is based on a single-mode fiber-coreless silica fiber-single-mode fiber (SMF-CSF-SMF) structure, where the CSF is the sensor region spliced between two SMFs. The ZnO layer of a higher refractive index coated over the sensing region improves the light interaction with the surrounding medium, leading to sensitivity enhancement. The optical properties, morphology, and elemental composition of the ZnO layer were analyzed. The sensing mechanism of the developed sensor is based on a wavelength interrogation technique showing wavelength shifts when the sensor is exposed to various VOC vapor concentration levels. Various concentrations of the three VOCs (including acetone, isopropanol, and ethanol) ranging from 20% to 100% were tested and analyzed. The sensor noticeably shows a significant response towards acetone vapor, with a better sensitivity of 0.162 nm/% vapor than for isopropanol (0.082 nm/% vapor) and ethanol (0.075 nm/% vapor) vapors. The high sensitivity and selectivity towards acetone, a common biomarker for diabetes, offers the potential for further development of this sensor as a smart healthcare system for monitoring diabetic conditions.

TiO2-modified MoS2 monolayer films enable sensitive NH3 sensing at room temperature

The research and development of high-performance NH3 sensors are of great significance for environment monitoring and disease diagnosis applications. Two-dimensional (2D) MoS2 nanomaterials have exhibited great potential for building room-temperature (RT) NH3 sensors but still suffer from relatively low sensitivity. Herein, the TiO2-modified monolayer MoS2 films with controllable TiO2 loading contents are fabricated by a facile approach. A remarkable enhancement in the RT NH3 sensing performance is achieved after the n-n hetero-compositing of the TiO2/MoS2 system. The device with 95% surface coverage of TiO2 shows enhanced sensor response, low detection limit (0.5 ppm), wide detection range (0.5-1000 ppm), good repeatability, and superior selectivity against other gases. In situ Kelvin potential force microscopy results revealed that the TiO2 modification not only improved the surface reactivity of the sensing layers but also contributed to the NH3 sensing performance by serving as the "gas-gating" layers that modulated the electron depletion layer and the conductivity of the MoS2 films. Such an n-n hetero-compositing strategy can provide a simple and cost-effective approach for developing high-performance NH3 sensors based on 2D semiconductors.

Road Map of Semiconductor Metal-Oxide-Based Sensors: A Review

Identifying disease biomarkers and detecting hazardous, explosive, flammable, and polluting gases and chemicals with extremely sensitive and selective sensor devices remains a challenging and time-consuming research challenge. Due to their exceptional characteristics, semiconducting metal oxides (SMOxs) have received a lot of attention in terms of the development of various types of sensors in recent years. The key performance indicators of SMOx-based sensors are their sensitivity, selectivity, recovery time, and steady response over time. SMOx-based sensors are discussed in this review based on their different properties. Surface properties of the functional material, such as its (nano)structure, morphology, and crystallinity, greatly influence sensor performance. A few examples of the complicated and poorly understood processes involved in SMOx sensing systems are adsorption and chemisorption, charge transfers, and oxygen migration. The future prospects of SMOx-based gas sensors, chemical sensors, and biological sensors are also discussed.

Recent Progress in Spinel Ferrite (MFe2O4) Chemiresistive Based Gas Sensors

Gas-sensing technology has gained significant attention in recent years due to the increasing concern for environmental safety and human health caused by reactive gases. In particular, spinel ferrite (MFe2O4), a metal oxide semiconductor with a spinel structure, has emerged as a promising material for gas-sensing applications. This review article aims to provide an overview of the latest developments in spinel-ferrite-based gas sensors. It begins by discussing the gas-sensing mechanism of spinel ferrite sensors, which involves the interaction between the target gas molecules and the surface of the sensor material. The unique properties of spinel ferrite, such as its high surface area, tunable bandgap, and excellent stability, contribute to its gas-sensing capabilities. The article then delves into recent advancements in gas sensors based on spinel ferrite, focusing on various aspects such as microstructures, element doping, and heterostructure materials. The microstructure of spinel ferrite can be tailored to enhance the gas-sensing performance by controlling factors such as the grain size, porosity, and surface area. Element doping, such as incorporating transition metal ions, can further enhance the gas-sensing properties by modifying the electronic structure and surface chemistry of the sensor material. Additionally, the integration of spinel ferrite with other semiconductors in heterostructure configurations has shown potential for improving the selectivity and overall sensing performance. Furthermore, the article suggests that the combination of spinel ferrite and semiconductors can enhance the selectivity, stability, and sensing performance of gas sensors at room or low temperatures. This is particularly important for practical applications where real-time and accurate gas detection is crucial. In conclusion, this review highlights the potential of spinel-ferrite-based gas sensors and provides insights into the latest advancements in this field. The combination of spinel ferrite with other materials and the optimization of sensor parameters offer opportunities for the development of highly efficient and reliable gas-sensing devices for early detection and warning systems.

Exploiting Interfacial Effects between Collapsing Bubbles and Nanocarbon/TiN Substrates for the Green Synthesis of Self-Organized Noble Metal and Nanoalloy Nanoparticles

Noble metal nanoparticles and multi-materials thereof are processed on a substrate from aqueous solutions of the metallic ions, precluding any chemical additives/catalysts. The methods reported here take advantage of interactions between collapsing bubbles and the substrate that result in the generation of reducing radicals at the substrate surface and leading to the reduction of the metal ions on those sites, followed by nucleation and growth. Two selected substrates where these phenomena take place are nanocarbon and TiN. By either using ultrasonic radiation of the substrate in ionic solution or quenching the substrate in a solution from temperatures above the Leidenfrost temperature, a high density of nanoparticles of Au, Au/Pt, Au/Pd and Au/Pd/Pt are synthesized on the substrate surface. The sites where the reducing radicals are generated determine the self-assembly of the nanoparticles. The methods yield highly adherent surface films and nanoparticles; they are materials efficient and cost effective because only the surface is modified with costly materials. The formation mechanisms of these green multi-material NPs are described. Outstanding electrocatalytic performances in acidic solutions of methanol and formic acid are demonstrated.

Recent Advancements in TiO2 Nanostructures: Sustainable Synthesis and Gas Sensing

The search for sustainable technology-driven advancements in material synthesis is a new norm, which ensures a low impact on the environment, production cost, and workers' health. In this context, non-toxic, non-hazardous, and low-cost materials and their synthesis methods are integrated to compete with existing physical and chemical methods. From this perspective, titanium oxide (TiO₂) is one of the fascinating materials because of its non-toxicity, biocompatibility, and potential of growing by sustainable methods. Accordingly, TiO₂ is extensively used in gas-sensing devices. Yet, many TiO₂ nanostructures are still synthesized with a lack of mindfulness of environmental impact and sustainable methods, which results in a serious burden on practical commercialization. This review provides a general outline of the advantages and disadvantages of conventional and sustainable methods of TiO₂ preparation. Additionally, a detailed discussion on sustainable growth methods for green synthesis is included. Furthermore, gas-sensing applications and approaches to improve the key functionality of sensors, including response time, recovery time, repeatability, and stability, are discussed in detail in the latter parts of the review. At the end, a concluding discussion is included to provide guidelines for the selection of sustainable synthesis methods and techniques to improve the gas-sensing properties of TiO₂.

The sensitivity enhancement of TiO2-based VOCs sensor decorated by gold at room temperature

Detection of hazardous toxic gases for air pollution monitoring and medical diagnosis has attracted the attention of researchers in order to realize sufficiently sensitive gas sensors. In this paper, we fabricated and characterized a Titanium dioxide (TiO₂)-based gas sensor enhanced using the gold nanoparticles. Thermal oxidation and sputter deposition methods were used to synthesize fabricated gas sensor. X-ray diffraction analysis was used to determine the anatase structure of TiO₂samples. It was found that the presence of gold nanoparticles on the surface of TiO₂enhances the sensitivity response of gas sensors by up to about 40%. The fabricated gas sensor showed a sensitivity of 1.1, 1.07 and 1.03 to 50 ppm of acetone, methanol and ethanol vapors at room temperature, respectively. Additionally, the gold nanoparticles reduce 50 s of response time (about 50% reduction) in the presence of 50 ppm ethanol vapor; and we demonstrated that the recovery time of the gold decorated TiO₂sensor is less than 40 s. Moreover, we explain that the improved performance depends on the adsorption-desorption mechanism, and the chemical sensitization and electronic sensitization of gold nanoparticles.

Evaluation of Zn: WO3 Thin Films as a Sensing Layer for Detection of NH3 Gas

Pristine WO₃ and Zn-doped WO₃ were synthesized using the spray pyrolysis technique to detect ammonia gas. The prominent orientation of the crystallites along the (200) plane was evident from X-ray diffraction (XRD) studies. Scanning Electron Microscope (SEM) morphology indicated well-defined grains upon Zn doping with a smaller grain size of 62 nm for Zn-doped WO₃ (Zn: WO₃) film. The photoluminescence (PL) emission at different wavelengths was assigned to defects such as oxygen vacancies, interstitial oxygens, localized defects, etc. X-ray Photoelectron spectroscopy (XPS) studies confirmed the formation of oxygen vacancies in the deposited films. The ammonia (NH₃) sensing analysis of the deposited films was carried out at an optimum working temperature of 250 °C. The sensor performance of Zn: WO₃ was enhanced compared to pristine WO₃ at 1 ppm NH₃ concentration, elucidating the possibility of the films in sensing applications.

Room-temperature ammonia gas sensing via Au nanoparticle-decorated TiO2 nanosheets

A high-performance gas sensor operating at room temperature is always favourable since it simplifies the device fabrication and lowers the operating power by eliminating a heater. Herein, we fabricated the ammonia (NH₃) gas sensor by using Au nanoparticle-decorated TiO₂ nanosheets, which were synthesized via two distinct processes: (1) preparation of monolayer TiO₂ nanosheets through flux growth and a subsequent chemical exfoliation and (2) decoration of Au nanoparticles on the TiO₂ nanosheets via hydrothermal method. Based on the morphological, compositional, crystallographic, and surface characteristics of this low-dimensional nano-heterostructured material, its temperature- and concentration-dependent NH₃ gas-sensing properties were investigated. A high response of ~ 2.8 was obtained at room temperature under 20 ppm NH₃ gas concentration by decorating Au nanoparticles onto the surface of TiO₂ nanosheets, which generated oxygen defects and induced spillover effect as well.

Composites of Montmorillonite and Titania Nanoparticles Prepared by Inverse Microemulsion Method: Physico-Chemical Characterization

TiO₂/montmorillonite composites were synthesized using inverse micellar route for the preparation of titania nanoparticles (4-6 nm diameter) in 1-hexanol and for the dispersion of one of the clay components. Two series of composites were obtained: one derived from cetyltrimethylammonium organomontmorillonite (CTA-Mt), exfoliated in 1-hexanol, and the other from sodium form of montmorillonite (Na-Mt) dispersed by formation of an inverse microemulsion in 1-hexanol. The TiO₂ content ranged from 16 to 64 wt.%. The composites were characterized with X-ray diffraction, scanning/transmission electron microscopy/energy dispersive X-ray spectroscopy, thermal analysis, and N₂ adsorption-desorption isotherms. The Na-Mt-derived component was shown to undergo transformation to CTA-Mt, as indicated by basal spacing of 17.5 nm, due to the interaction with the CTABr surfactant in inverse microemulsion. It was also better dispersed and intermixed with TiO₂ nanoparticles. As a result, the TiO₂/Na-Mt series displayed superior textural properties, with specific surface area up to 256 m²g^-1 and pore volume up to 0.247 cm³g^-1 compared with 208 m²g^-1 and 0.231 cm³g^-1, respectively, for the TiO₂/CTA-Mt counterpart. Members of both series were uniformly mesoporous, with the dominant pore size around 5 nm, i.e., comparable with the dimensions of titania nanoparticles. The advantage of the adopted synthesis method is discussed in the context of other preparative procedures used for manufacturing of titania-clay composites.

Metal Oxide Gas Sensors to Study Acetone Detection Considering Their Potential in the Diagnosis of Diabetes: A Review

Metal oxide (MOx) gas sensors have attracted considerable attention from both scientific and practical standpoints. Due to their promising characteristics for detecting toxic gases and volatile organic compounds (VOCs) compared with conventional techniques, these devices are expected to play a key role in home and public security, environmental monitoring, chemical quality control, and medicine in the near future. VOCs (e.g., acetone) are blood-borne and found in exhaled human breath as a result of certain diseases or metabolic disorders. Their measurement is considered a promising tool for noninvasive medical diagnosis, for example in diabetic patients. The conventional method for the detection of acetone vapors as a potential biomarker is based on spectrometry. However, the development of MOx-type sensors has made them increasingly attractive from a medical point of view. The objectives of this review are to assess the state of the art of the main MOx-type sensors in the detection of acetone vapors to propose future perspectives and directions that should be carried out to implement this type of sensor in the field of medicine.

The Impact of Side-Selective Laser Tailoring of Titania Nanotubes on Changes in Photoelectrocatalytic Activity

Over the last few decades, titanium(IV) oxide-based materials have gained particular attention due to their stability, corrosion resistance, photocatalytic activity under UV light, and possibilities for modification. Among various structures, TiO₂ nanotubes (NTs) grown on Ti foil or glass substrates and obtained through a simple anodization process are widely used as photocatalysts or photoanodes. During the anodization process, the geometry of the nanotubes (length, distribution, diameter, wall thickness, etc.) is easily controlled, though the obtained samples are amorphous. Heat treatment is required to transform the amorphous material into crystalline material. However, instead of time- and cost-consuming furnace treatment, fast and precise laser annealing is applied as a promising alternative. Nonetheless, laser treatment can result in geometry changes of TiO₂ NTs, consequently altering, their electrochemical activity. Moreover, modification of the TiO₂ NTs surfaces with transition metals and further laser treatment can result in materials with unique photoelectrochemical properties. In this regard, we gathered the latest achievements in the field of laser-treated titania for this review paper. We mainly focused on single structural and morphological changes resulting from pulsed laser annealing and their influence on the electrochemical properties of titania. Finally, the theoretical basis for and combination of laser- and metal-modifications and their impact on the resulting possibilities for electrochemical water splitting are also discussed.

Recent Advances in Sensing Materials Targeting Clinical Volatile Organic Compound (VOC) Biomarkers: A Review

In general, volatile organic compounds (VOCs) have a high vapor pressure at room temperature (RT). It has been reported that all humans generate unique VOC profiles in their exhaled breath which can be utilized as biomarkers to diagnose disease conditions. The VOCs available in exhaled human breath are the products of metabolic activity in the body and, therefore, any changes in its control level can be utilized to diagnose specific diseases. More than 1000 VOCs have been identified in exhaled human breath along with the respiratory droplets which provide rich information on overall health conditions. This provides great potential as a biomarker for a disease that can be sampled non-invasively from exhaled breath with breath biopsy. However, it is still a great challenge to develop a quick responsive, highly selective, and sensitive VOC-sensing system. The VOC sensors are usually coated with various sensing materials to achieve target-specific detection and real-time monitoring of the VOC molecules in the exhaled breath. These VOC-sensing materials have been the subject of huge interest and extensive research has been done in developing various sensing tools based on electrochemical, chemoresistive, and optical methods. The target-sensitive material with excellent sensing performance and capturing of the VOC molecules can be achieved by optimizing the materials, methods, and its thickness. This review paper extensively provides a detailed literature survey on various non-biological VOC-sensing materials including metal oxides, polymers, composites, and other novel materials. Furthermore, this review provides the associated limitations of each material and a summary table comparing the performance of various sensing materials to give a better insight to the readers.

Photocatalytic Degradation of Organic Dyes Contaminated Aqueous Solution Using Binary CdTiO2 and Ternary NiCdTiO2 Nanocomposites

The synergistic effect of binary CdTiO2 and ternary NiCdTiO2 on the photocatalytic efficiency of TiO2 nanoparticles was investigated. The SEM analysis demonstrates spherical TiO2 NPs of different sizes present in agglomerated form. The structural analysis of the nanocomposites reveals a porous structure for TiO2 with well deposited Cd and Ni NPs. TEM images show NiCdTiO2 nanocomposites as highly crystalline particles having spherical and cubical geometry with an average particle size of 20 nm. The EDX and XRD analysis confirm the purity and anatase phase of TiO2, respectively. Physical features of NiCdTiO2 nanocomposite were determined via BET analysis which shows that the surface area, pore size and pore volume are 61.2 m2/g, 10.6 nm and 0.1 cm3/g, respectively. The absorbance wavelengths of the CdTiO2 and NiCdTiO2 nanocomposites have shown red shift as compared to the neat TiO2 due to coupling with Ni and Cd that results in the enhanced photocatalytic activity. The photocatalytic activity demonstrated that TiO2, CdTiO2 and NiCdTiO2 degrade methylene blue (MB) and methyl green (MG) about 76.59, 82, 86% and 63.5, 88, 97.5%, respectively, at optimum reaction conditions.

Enhancement of H2S sensing performance of rGO decorated CuO thin films: experimental and DFT studies

We demonstrate a highly selective and sensitive Cupric oxide (CuO) thin film-based low concentration Hydrogen sulfide (H₂S) sensor. The sensitivity was improved around three times by decorating with reduced graphene oxide (rGO) nanosheets. CuO thin films were deposited by Chemical Vapor Deposition followed by inter-digital electrode fabrication by a thermal evaporations system. The crystal structure of CuO was confirmed by x-ray diffraction. The sensing response of pristine CuO was found around 54% at 100 °C to 100 ppm of H₂S. In contrast, the sensing response was enhanced to 167% by decorating with rGO of 1.5 mg ml^-1concentration solution. The sensing was improved due to the formation of heterojunctions between the rGO and CuO. The developed sensor was examined under various gas environments and found to be highly selective towards H₂S gas. The improvement in sensing response has been attributed to increased hole concentration in CuO in the presence of rGO due to the Fermi level alignment and increased absorption of H₂S molecules at the rGO/CuO heterojunction. Further, electronic structure calculations show the physisorption behavior of H₂S molecules on the different adsorption sites. Detailed insight into the gas sensing mechanism is discussed based on experimental results and electronic structure calculations.

Determination of active ingredients in antihypertensive drugs using a novel green HPLC method approach

A novel, sensitive, fast, and pratic RP-HPLC methods were presented for the quantitative amounts of Telmisartan (TEL) and Olmesartan (OLM) in the presence of Amlodipin (AML) in a binary mixture of pharmaceutical preparation. Waters Spherisorb ODS-2 C18 column was used for separation. These methods were valid over linearity ranges of 2.5-30 μμg/mlL, 2-85 μμg/mlL, and 2-35 μμg/mlL for OLM, TEL, and AML, respectively. The mobile phase system consisted of acetonitrile:methanol: phosphate buffer at pH 3.0 (65:5:30 v/v/v), and the flow rate was 1,5 mlL/min for OLM and AML. The mobile system's other mixture (TEL and AML) was acetonitrile:methanol: phosphate buffer at pH 2.5 (65:5:30 v/v/v), and the flow rate was 1,5 mlL/min. These procedures were successfully applied to bulk, laboratory synthetic mixture, and medicinal dosage forms to use active ingredients quantitatively. The studied methods were validated according to ICH guidelines. In the developed HPLC method, the limit of detection values was found to be 0.020 μμg/mlL for TEL, 0.025 μμg/mlL for OML, and 0.070 μμg/mlL for AML. The correlation coefficients for the HPLC method were found to be 0.9938 for TEL, 0.9996 for OML, and 0.9982 for AML. The calibration range is between 2.5 and -30, 5-35, and 2-85 μμg/mlL for OLM, AML, and TEL, respectively. The proposed HPLC method is a convenient, effective, sensitive, green, and time-saving method for the rapid determination of TEL and OLM in the presence of AML.

On the Morphology of Nanostructured TiO2 for Energy Applications: The Shape of the Ubiquitous Nanomaterial

Nanostructured titania is one of the most commonly encountered constituents of nanotechnology devices for use in energy-related applications, due to its intrinsic functional properties as a semiconductor and to other favorable characteristics such as ease of production, low toxicity and chemical stability, among others. Notwithstanding this diffusion, the quest for improved understanding of the physical and chemical mechanisms governing the material properties and thus its performance in devices is still active, as testified by the large number of dedicated papers that continue to be published. In this framework, we consider and analyze here the effects of the material morphology and structure in determining the energy transport phenomena as cross-cutting properties in some of the most important nanophase titania applications in the energy field, namely photovoltaic conversion, hydrogen generation by photoelectrochemical water splitting and thermal management by nanofluids. For these applications, charge transport, light transport (or propagation) and thermal transport are limiting factors for the attainable performances, whose dependence on the material structural properties is reviewed here on its own. This work aims to fill the gap existing among the many studies dealing with the separate applications in the hope of stimulating novel cross-fertilization approaches in this research field.

Investigation of the Atomic Layer Deposition of the Titanium Dioxide (TiO2) Film as pH Sensor Using a Switched Capacitor Amplifier

The electrical and chemical properties of the titanium dioxide (TiO2) coated spirals grown by the atomic layer deposition (ALD) technique in two different temperatures of 150 °C and 300 °C are studied. The thickness of the TiO2 layers studied are 20, 40, and 80 nm. A switched capacitor amplifier is used to investigate the pH response and the capacitance of the samples. It is found that the performance of the TiO2 samples depends on either the thickness or the deposition temperature due to the differences in the physical properties of the oxide layer such as surface roughness and film density. The high temperature samples are more crystalline, whereas the low temperature samples are more amorphous. Since there is a low pass filter effect in the electrolyte–sample interface, the TiO2 coated samples show the better response to the pH change for the high temperature samples as the sensor surface area for binding the hydrogen ions is larger and the charge transfer resistance is smaller. Furthermore, more roughness on the surface can be obtained by increasing the thickness, which reduces the charge transfer resistance. In this study, the 80 nm sample deposited at 300 °C gives the best pH response of 40 mV/pH.

Flexible Humidity Sensors Based on Multidimensional Titanium Dioxide/Cellulose Nanocrystals Composite Film

A humidity sensor is a crucial device in daily life; therefore, in the present study, a novel humidity sensor was designed to increase its specific surface area to improve its humid sensing capacity and conductivity. Titanium dioxide nanoparticles (TiNP) consisting of zero-dimensional nanospheres and one-dimensional nanotubes were prepared by anodic oxidation. Rod-shaped cellulose nanocrystals (CNCs) with average length and diameter of 60 nm and 800 nm, respectively, were obtained by enzymatic hydrolysis and high pressure homogenization. TiNP/CNC composite films exhibited superior hydrophilicity and large specific surface areas based on Fourier transform infrared spectroscopy and nitrogen adsorption-desorption results. The humidity sensing characteristics of sensors based on TiNP/CNC flexible composite films with varying contents of TiNP were investigated under a relative humidity range of 11-97%. The 6% TiNP/CNC-based humidity sensor exhibited high humidity response, rapid response/recovery speed, and high stability. Furthermore, the humidity sensing mechanism of TiNP/CNC composite films was analyzed based on the density functional theory. TiNP/CNC-based humidity sensors could be applied in flexible and wearable electronics.

Effect of Ag Addition on the Gas-Sensing Properties of Nanostructured Resistive-Based Gas Sensors: An Overview

Nanostructured semiconducting metal oxides (SMOs) are among the most popular sensing materials for integration into resistive-type gas sensors owing to their low costs and high sensing performances. SMOs can be decorated or doped with noble metals to further enhance their gas sensing properties. Ag is one of the cheapest noble metals, and it is extensively used in the decoration or doping of SMOs to boost the overall gas-sensing performances of SMOs. In this review, we discussed the impact of Ag addition on the gas-sensing properties of nanostructured resistive-based gas sensors. Ag-decorated or -doped SMOs often exhibit better responsivities/selectivities at low sensing temperatures and shorter response times than those of their pristine counterparts. Herein, the focus was on the detection mechanism of SMO-based gas sensors in the presence of Ag. This review can provide insights for research on SMO-based gas sensors.