A review of recent developments in tin dioxide composites for gas sensing application

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.

CA19-9 and CEA biosensors in pancreatic cancer

Cancer is a complex pathophysiological condition causing millions of deaths each year. Early diagnosis is essential especially for pancreatic cancer. Existing diagnostic tools rely on circulating biomarkers such as Carbohydrate Antigen 19-9 (CA19-9) and Carcinoembryonic Antigen (CEA). Unfortunately, these markers are nonspecific and may be increased in a variety of disorders. Accordingly, diagnosis of pancreatic cancer generally involves more invasive approaches such as biopsy as well as imaging studies. Recent advances in biosensor technology have allowed the development of precise diagnostic tools having enhanced analytical sensitivity and specificity. Herein we examine these advances in the detection of cancer in general and in pancreatic cancer specifically. Furthermore, we highlight novel technologies in the measurement of CA19-9 and CEA and explore their future application in the early detection of pancreatic cancer.

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.

Gate-controlled gas sensor utilizing 1D-2D hybrid nanowires network

Novel gas sensors that work at room temperature are attracting attention due to their low energy consumption and stability in the presence of toxic gases. However, the development of sensing characteristics at room temperature is still a primary challenge. Diverse reaction pathways and low adsorption energy for gas molecules are required to fabricate a gas sensor that works at room temperature with high sensitivity, selectivity, and efficiency. Therefore, we enhanced the gas sensing performance at room temperature by constructing hybridized nanostructure of 1D-2D hybrid of SnSe2 layers and SnO2 nanowire networks and by controlling the back-gate bias (Vg = 1.5 V). The response time was dramatically reduced by lowering the energy barrier for the adsorption on the reactive sites, which are controlled by the back gate. Consequently, we believe that this research could contribute to improving the performance of gas sensors that work at room temperature.

Engineering SnO2 nanorods/ethylenediamine-modified graphene heterojunctions with selective adsorption and electronic structure modulation for ultrasensitive room-temperature NO2 detection

Ever-increasing concerns over air quality and the newly emerged internet of things (IoT) for future environmental monitoring are stimulating the development of ultrasensitive room-temperature gas sensors, especially for nitrogen dioxide (NO₂), one of the most harmful air pollution species released round-the-clock from power plants and vehicle exhausts. Herein, tin dioxide nanorods/ethylenediamine-modified reduced graphene oxide (SnO₂/EDA-rGO) heterojunctions with selective adsorption and electronic structure modulation were engineered for highly sensitive and selective detection of NO₂ at room temperature. The modified EDA groups not only enable selective adsorption to significantly enrich NO₂ molecules around the interface but also realize a favorable modulation of SnO₂/EDA-rGO electronic structure by increasing the Fermi level of rGO, through which the sensing performance of NO₂ is synergistically enhanced. The response of the SnO₂/EDA-rGO sensor toward 1 ppm NO₂ reaches 282%, which exceeds the corresponding SnO₂/rGO sensor by a factor of 2.8. It also exhibits a low detection limit down to 100 ppb, enhanced selectivity, and rapid response/recovery kinetics. This approach to designing a novel heterojunction with significantly enhanced chemical and electric effects may shed light on the future engineering of gas-sensing materials.

The Key Role of Active Sites in the Development of Selective Metal Oxide Sensor Materials

Development of sensor materials based on metal oxide semiconductors (MOS) for selective gas sensors is challenging for the tasks of air quality monitoring, early fire detection, gas leaks search, breath analysis, etc. An extensive range of sensor materials has been elaborated, but no consistent guidelines can be found for choosing a material composition targeting the selective detection of specific gases. Fundamental relations between material composition and sensing behavior have not been unambiguously established. In the present review, we summarize our recent works on the research of active sites and gas sensing behavior of n-type semiconductor metal oxides with different composition (simple oxides ZnO, In₂O₃, SnO₂, WO₃; mixed-metal oxides BaSnO₃, Bi₂WO₆), and functionalized by catalytic noble metals (Ru, Pd, Au). The materials were variously characterized. The composition, metal-oxygen bonding, microstructure, active sites, sensing behavior, and interaction routes with gases (CO, NH₃, SO₂, VOC, NO₂) were examined. The key role of active sites in determining the selectivity of sensor materials is substantiated. It was shown that the metal-oxygen bond energy of the MOS correlates with the surface acidity and the concentration of surface oxygen species and oxygen vacancies, which control the adsorption and redox conversion of analyte gas molecules. The effects of cations in mixed-metal oxides on the sensitivity and selectivity of BaSnO₃ and Bi₂WO₆ to SO₂ and VOCs, respectively, are rationalized. The determining role of catalytic noble metals in oxidation of reducing analyte gases and the impact of acid sites of MOS to gas adsorption are demonstrated.

Room-Temperature Hydrogen-Sensing Capabilities of Pt-SnO2 and Pt-ZnO Composite Nanoceramics Occur via Two Different Mechanisms

Impressive room-temperature gas-sensing capabilities have been reported for nanomaterials of many metal oxides, including SnO₂, ZnO, TiO₂, WO₃, and Fe₂O₃, while little attention has been paid to the intrinsic difference among them. Pt-SnO₂ and Pt-ZnO composite nanoceramics have been prepared through convenient pressing and sintering. The former shows strong and stable responses to hydrogen in 20% O₂-N₂ (synthetic air) at room temperature, while the responses to hydrogen in N₂ cannot be stabilized in limited times; the latter shows strong and stable responses to hydrogen in N₂, while the responses to hydrogen in synthetic air are greatly depressed. Further analyses reveal that for Pt-ZnO, the responses result from the reaction between hydrogen and oxygen chemisorbed on ZnO; while for Pt-SnO₂, the responses result from two reactions of hydrogen, one is that with oxygen chemisorbed on SnO₂ and the other is hydrogen chemisorption on SnO₂. These results reveal two different room-temperature hydrogen-sensing mechanisms among MOXs, which results in highly contrasting room-temperature hydrogen-sensing capabilities attractive for sensing hydrogen in oxygen-contained and oxygen-free environments, separately.

Palladium-Doped Tin Oxide Nanosensor for the Detection of the Air Pollutant Carbon Monoxide Gas

The exhaust gases from various sources cause air pollution, which is a leading contributor to the global disease burden. Hence, it has become vital to monitor and control the increasing pollutants coming out of the various sources into the environment. This paper has designed and developed a sensor material to determine the amount of carbon monoxide (CO), which is one of the major primary air pollutants produced by human activity. Nanoparticle-based sensors have several benefits in sensitivity and specificity over sensors made from traditional materials. In this study, tin oxide (SnO2), which has greater sensitivity to the target gas, is selected as the sensing material which selectively senses only CO. Tin oxide nanoparticles have been synthesized from stannous chloride dihydrate chemical compound by chemical precipitation method. Palladium, at the concentration of 0.1%, 0.2%, and 0.3% by weight, was added to tin oxide and the results were compared. Synthesized samples were characterized by X-ray diffraction (XRD) and field emission scanning electron microscope (FESEM) techniques. XRD revealed the tetragonal structure of the SnO2 nanoparticles and FESEM analysis showed the size of the nanoparticles to be about 7-20 nm. Further, the real-time sensor testing was performed and the results proved that the tin oxide sensor, doped with 0.2% palladium, senses the CO gas more efficiently with greater sensitivity.

Electrospun (Nickel and palladium) tin(IV) oxide/polyaniline/polyhydroxy-3-butyrate biodegradable nanocomposite fibers for low temperature ethanol gas sensing

Tin (IV) oxide (SnO2) nanostructures are regarded as one of the most popular materials for conventional gas sensors, due to their high surface area and fast response in regard to most reducing and oxidizing gases. However, their high operating temperature (>200 °C) leads to high power consumption and limits their applications. Here, a new nanocomposite fiber materials, consisting of undoped and doped (nickel and palladium) SnO2 nanorods, polyaniline (PANI), and polyhydroxy-3-butyrate (P3HB) are synthesized via the hydrothermal method,followed by an in situ polymerization and electrospinning technique. The as-synthesized nanocomposites are tested using ethanol gas at different operating temperatures: 25 °C (room temperature), 60 °C, and 80 °C. The results reveal that all samples began to show a response at 80 °C. Pd:SnO2/PANI/P3HB nanocomposite fiber sensors demonstrate a relatively higher response than that of SnO2/PANI/P3HB and Ni:SnO2/PANI/P3HB nanocomposite sensors. At 80 °C , the Pd:SnO2/PANI/P3HB nanocomposite sensor records a response (R0/Rg ) of 1610, with a response time (Tres) of 90 s and a recovery time (Trec ) of 9 min in relation to 1000 ppm ethanol gas in N2. The sensor also displays a good level of response (R0/Rg = 200) at a low concentration level (50 ppm) of ethanol gas. Structural and chemical characterizations indicate that the ethanol gas sensing performance of Pd:SnO2/PANI/P3HB nanocomposite fibers can mainly be attributed to the p-n heterojunction, fiber geometry, and one-dimensional structure of SnO2 and to the presence of the Pd catalyst. This bio-nanocomposite fiber has the potential to be a breakthrough material in biodegradable low temperature ethanol sensing applications.

Laser-generated BiVO4 colloidal particles with tailoring size and native oxygen defect for highly efficient gas sensing

To alleviate the poor sensing performance of BiVO₄, developing new strategies for the fabrication of unique device with improved sensing properties is very necessary and has great practical significance. In this work, size-tailored and uniform black BiVO₄ colloids with abundant oxygen vacancy were synthesized by a unique method of pulsed laser irradiation of colloidal nanoparticles (PLICN). The corresponding laser irradiation effects on the sensing properties are comparatively investigated. The results indicate that the BiVO₄ nanospheres with average size of 50 nm shows best sensing properties with high sensitivity, superior selectivity, low detection limit (44 ppb) to H₂S at low working temperature (75 °C). Its sensing response is over 4 times higher when comparing with that of the raw material. Further investigation manifests that laser irradiation could induce quantity of the oxygen vacancy and decrease the resistance of the sensing device, which is mainly responsible for the enhanced sensing performance. Moreover, the density functional theories (DFT) calculations suggest that the oxygen vacancies can greatly decrease the surface absorption energy with enhanced H₂S absorption capability on BiVO₄ surface and lower the bader charger transfer from the absorbed H₂S molecules to the BiVO₄, thus enabling the implementation for the enhanced gas-sensing properties.

A highly responsive NH3 sensor based on Pd-loaded ZnO nanoparticles prepared via a chemical precipitation approach

The gas-detecting ability of nanostructured ZnO has led to significant attention being paid to the development of a unique and effective approach to its synthesis. However, its poor sensitivity, cross-sensitivity to humidity, long response/recovery times and poor selectivity hinder its practical use in environmental and health monitoring. In this context, the addition of noble metals, as dopants or catalysts to modify the ZnO surface has been examined to enhance its sensing performance. Herein, we report preparation of Pd-loaded ZnO nanoparticles via a chemical precipitation approach. Various Pd loadings were employed to produce surface-modified ZnO nanostructure sensors, and their resulting NH₃ sensing capabilities both in dry and humid environments were investigated. Through a comparative gas sensing study between the pure and Pd-loaded ZnO sensors upon exposure to NH₃ at an optimal operating temperature of 350 °C, the Pd-loaded ZnO sensors were found to exhibit enhanced sensor responses and fast response/recovery times. The influence of Pd loading and its successful incorporation into ZnO nanostructure was examined by X-ray diffraction, high resolution-transmission electron microscopy, and X-ray photoelectron spectroscopy. XPS studies demonstrated that in all samples, Pd existed in two chemical states, namely Pd° and Pd²⁺. The possible sensing mechanism related to NH₃ gas is also discussed in detail.

The effects of amino substituents on the enhanced ammonia sensing performance of PcCo/rGO hybrids

Three reversible ammonia (NH3) gas sensors were fabricated using tetra-α-(p-aminobenzyloxy)phthalocyanine cobalt (ABOPcCo), tetra-α-aminophthalocyanine cobalt (APcCo) and substituent-free phthalocyanine cobalt (FPcCo) functionalized reduced graphene oxide (rGO), with cost-efficient, highly sensitive and stable sensing performance. These hybrid materials were prepared via a facile physical solution mixing self-assembly reaction with rGO and PcCo solutions. The obtained PcCo/rGO hybrid sensors exhibit excellent sensing performance; especially the ABOPcCo/rGO sensor, whose response is about 23.3% (50 ppm), with a limit of detection as low as 78 ppb, and response and recovery times about as fast as 225 s and 250 s. The performance of the PcCo/rGO hybrid sensors can be optimized by adjusting the concentrations of the PcCo/rGO aqueous dispersions. More importantly, the NH3-sensing performance of the PcCo/rGO sensors was tuned by adjusting the substituent structure of PcCo. The enhanced NH3-sensing performance may be attributed to synergistic effects between PcCo and rGO, e.g., stronger adsorption interactions between PcCo with an aminophenoxy substituent and NH3, the high electrical conductivity of rGO, and fast charge transfer between PcCo and rGO. These are further confirmed via first-principle density functional theory (DFT) calculations and electrochemical impedance spectra (EIS) measurements.

Effect of Surfactants on the Microstructures of Hierarchical SnO2 Blooming Nanoflowers and their Gas-Sensing Properties

Hierarchical SnO₂ blooming nanoflowers were successfully fabricated via a simple yet facile hydrothermal method with the help of different surfactants. Here we focus on exploring the promotion effects of surfactants on the self-assembly of 2D SnO₂ nanosheets into 3D SnO₂ flower-like structures as well as their gas-sensing performances. The polyporous flower-like SnO₂ sensor exhibits excellent gas-sensing performances to ethanol and H₂S gas due to high porosity when polyvinyl pyrrolidone is added into the precursor solution as a surfactant. The response/recovery times were about 5 s/8 s for 100 ppm ethanol and 4 s/20 s for 100 ppm H₂S, respectively. Especially, the maximum response value of H₂S is estimated to be 368 at 180 °C, which is one or two orders of magnitude higher than that of other test gases in this study. That indicates that the sensor fabricated with the help of polyvinyl pyrrolidone has good selectivity to H₂S.

Comparison between Different Structures of Suspended-Core Microstructured Optical Fibers for Volatiles Sensing

In this paper, different core structures of microstructured optical fibers (MOFs) for low-finesse Fabry⁻Pérot (FP) sensors are experimentally compared to get the highest sensitivity. These devices are designed for volatile organic compounds (VOCs) measurements. Indium tin oxide (ITO) thin films were deposited by sputtering on the MOFs and different optical fast Fourier transform (FFT) phase responses from the FP were measured for saturated atmospheres of ethanol. It has been demonstrated that the sensitivities of the developed sensors depend strongly on the geometry and the dimensions of the MOF-cores. The sensors show recovery times shorter than 100 s and the baselines are fully recovered after every exposure to ethanol vapors.

Using a Second Order Sigma-Delta Control to Improve the Performance of Metal-Oxide Gas Sensors

Controls of surface potential have been proposed to accelerate the time response of MOX gas sensors. These controls use temperature modulations and a feedback loop based on first-order sigma-delta modulators to keep constant the surface potential. Changes in the surrounding gases, therefore, must be compensated by average temperature produced by the control loop, which is the new output signal. The purpose of this paper is to present a second order sigma-delta control of the surface potential for gas sensors. With this new control strategy, it is possible to obtain a second order zero of the quantization noise in the output signal. This provides a less noisy control of the surface potential, while at the same time some undesired effects of first order modulators, such as the presence of plateaus, are avoided. Experiments proving these performance improvements are presented using a gas sensor made of tungsten oxide nanowires. Plateau avoidance and second order noise shaping is shown with ethanol measurements.

Tin chemical shift anisotropy in tin dioxide: On ambiguity of CSA asymmetry derived from MAS spectra

Two different axial symmetries of the ¹¹⁹Sn chemical shift anisotropy (CSA) in tin dioxide with the asymmetry parameter (η) of 0 and 0.27 were reported previously based on the analysis of MAS NMR spectra. By analyzing the static powder pattern, we show that the ¹¹⁹Sn CSA is axially symmetric. A nearly axial symmetry and the principal axis system of the ¹¹⁹Sn chemical shift tensor in SnO₂ were deduced from periodic scalar-relativistic density functional theory (DFT) calculations of NMR parameters. The implications of fast small-angle motions on CSA parameters were also considered, which could potentially lead to a CSA symmetry in disagreement with a crystal symmetry. Our analysis of experimental spectra using spectral simulations and iterative fittings showed that MAS spectra recorded at relatively high frequencies do not show sufficiently distinct features in order to distinguish CSAs with η ≈ 0 and η ≈ 0.4. The example of SnO₂ shows that both the MAS lineshape and spinning sideband analyses may overestimate the η value by as much as ∼0.3 and ∼0.4, respectively. The results confirm that a static powder pattern must be analysed in order to improve the accuracy of the CSA asymmetry measurements. The measurements on SnO₂ nanoparticles showed that the asymmetry parameter of the ¹¹⁹Sn CSA increases for nm-sized particles with a larger surface area compared to μm-sized particles. The increase of the η value for tin atoms near the surface in SnO₂ was also confirmed by DFT calculations.

Three-Dimensional Porous Nitrogen-Doped NiO Nanostructures as Highly Sensitive NO₂ Sensors

Nickel oxide has been widely used in chemical sensing applications, because it has an excellent p-type semiconducting property with high chemical stability. Here, we present a novel technique of fabricating three-dimensional porous nitrogen-doped nickel oxide nanosheets as a highly sensitive NO₂ sensor. The elaborate nanostructure was prepared by a simple and effective hydrothermal synthesis method. Subsequently, nitrogen doping was achieved by thermal treatment with ammonia gas. When the p-type dopant, i.e., nitrogen atoms, was introduced in the three-dimensional nanostructures, the nickel-oxide-nanosheet-based sensor showed considerable NO₂ sensing ability with two-fold higher responsivity and sensitivity compared to non-doped nickel-oxide-based sensors.

Enhanced catalytic activity and near room temperature gas sensing properties of SnO2 nanoclusters@mesoporous Sn(iv) organophosphonate composite

A simple route for preparing SnO₂ nanoclusters embedded on mesoporous Sn(iv) organophosphonate framework is described.