Surface Superoxide Complex Defects-Boosted Ultrasensitive ppb-Level NO2 Gas Sensors

Electronic Noses: From Gas-Sensitive Components and Practical Applications to Data Processing

Artificial olfaction, also known as an electronic nose, is a gas identification device that replicates the human olfactory organ. This system integrates sensor arrays to detect gases, data acquisition for signal processing, and data analysis for precise identification, enabling it to assess gases both qualitatively and quantitatively in complex settings. This article provides a brief overview of the research progress in electronic nose technology, which is divided into three main elements, focusing on gas-sensitive materials, electronic nose applications, and data analysis methods. Furthermore, the review explores both traditional MOS materials and the newer porous materials like MOFs for gas sensors, summarizing the applications of electronic noses across diverse fields including disease diagnosis, environmental monitoring, food safety, and agricultural production. Additionally, it covers electronic nose pattern recognition and signal drift suppression algorithms. Ultimately, the summary identifies challenges faced by current systems and offers innovative solutions for future advancements. Overall, this endeavor forges a solid foundation and establishes a conceptual framework for ongoing research in the field.

The Challenges and Opportunities for TiO2 Nanostructures in Gas Sensing

Chemiresistive gas sensors based on metal oxides have been widely applied in industrial monitoring, medical diagnosis, environmental pollutant detection, and food safety. To further enhance the gas sensing performance, researchers have worked to modify the structure and function of the material so that it can adapt to different gas types and environmental conditions. Among the numerous gas-sensitive materials, n-type TiO2 semiconductors are a focus of attention for their high stability, excellent biosafety, controllable carrier concentration, and low manufacturing cost. This Perspective first introduces the sensing mechanism of TiO2 nanostructures and composite TiO2-based nanomaterials and then analyzes the relationship between their gas-sensitive properties and their structure and composition, focusing also on technical issues such as doping, heterojunctions, and functional applications. The applications and challenges of TiO2-based nanostructured gas sensors in food safety, medical diagnosis, environmental detection, and other fields are also summarized in detail. Finally, in the context of their practical application challenges, future development technologies and new sensing concepts are explored, providing new ideas and directions for the development of multifunctional intelligent gas sensors in various application fields.

A Fully Integrated Flexible Tunable Chemical Sensor Based on Gold-Modified Indium Selenide Nanosheets

In this work, a novel light-modulated bifunctional gas sensor based on Au nanoparticles-modified 2D InSe nanosheets was demonstrated. The prepared sensor displayed a reversible and extremely high response for recognition of nitrogen dioxide (NO₂) under visible-light illumination. The sensitivity (1192%) was about 10 times higher than that under dark condition, and the limit of detection (LOD) was 0.17 ppb. In contrast, when sensing ammonia (NH₃), higher sensitivity and selectivity were obtained in darkness rather than in light, with sensitivity and LOD of 11% and 0.2 ppm. Furthermore, the sensor possesses decent stability, repeatability, and anti-interference ability. The tunable sensing behavior with light modulation has been clearly studied with the help of density functional theory. A new principle called "carrier storage box" of Au nanoparticles was proposed to explain the change in surface state of InSe under light modulation. Finally, the prepared sensor has been successfully applied to construct a fully integrated wearable device to measure NH₃ and NO₂ in ambient environment. In all, this work provides a highly competitive gas detection method and paves the way for designing 2D materials-based optoelectronic devices with tunable and multifunctional features.

Boosting room-temperature ppb-level NO2 sensing over reduced graphene oxide by co-decoration of α-Fe2O3 and SnO2 nanocrystals

As promising sensing materials, reduced graphene oxide (RGO)-based nanomaterials have drawn considerable attention in the fields of gas monitoring owing to their low operating temperature. However, constructing RGO-based room-temperature gas sensors possessing ppb-level limit of detection with high sensitivity remains challenging. In this work, a series of highly sensitive NO₂ sensors were fabricated using α-Fe₂O₃ and SnO₂ co-decorated RGO hybrids (designated as α-Fe₂O₃/SnO₂-RGO) as sensing materials. They were rationally synthesized by a one-pot hydrothermal method. Compared to SnO₂ modified RGO hybrids (SnO₂-RGO with bandgap of 3.88 eV), the bandgap energy of α-Fe₂O₃/SnO₂-RGO hybrids (3.53 eV) was reduced by adding α-Fe₂O_3; the narrower bandgap facilitated the sensing materials to release more electrons and form more oxygen ions at room temperature. Besides, the high carrier migration of RGO, which served as continuous phase, identical structure with ultrasmall particle size of α-Fe₂O₃ and SnO₂ (about 3-6 nm), and abundant chemisorbed oxygen species on the surface (20.8%) of the sensing materials, as well as their suitable bandgap (3.53 eV) in the sensing materials, significantly improved NO₂ response at room temperature. Among the sensors fabricated, α-Fe₂O₃/SnO₂-RGO-15-based NO₂ sensor had the highest response of 7.4 with a short response time of 59 s towards 1 ppm NO₂; it could even reach a response of 2.6 towards 100 ppb NO₂. Notably, α-Fe₂O₃/SnO₂-RGO-15 sample has excellent capability to recognize NO₂, where the response value (7.4) towards 1 ppm NO₂ is about 7 times higher than that of 100 ppm ammonia and common volatile organic compounds (formaldehyde, toluene, ethanol and acetone). Such NO₂ sensor has superior repeatability with negligible response deviation towards 1 ppm NO₂ for four reversible cycles. This makes it to have a great potential application in the field of NO₂ detection.

Plasma-induced oxygen vacancies enabled ultrathin ZnO films for highly sensitive detection of triethylamine

Metal oxide semiconductor (MOS) thin films hold great promise for electronic devices such as gas sensors. However, the low surface activity of pristine MOS often leads to inferior sensitivity and the sensitization mechanism of ultrathin MOS films has received rare attention. Herein, we report a high performance gas sensor based on plasma-etched ZnO thin films. The ultrathin ZnO films (20 nm) were deposited on SiO₂ wafers by atomic layer deposition (ALD), which enables high-throughput production of sensor devices. The ZnO sensor shows typical n-type conductivity, which is highly variable to the exposure of triethylamine (TEA). Annealing temperature of the films is found to impact the sensor response, revealing calcination at a moderate temperature, i.e. 700 °C, leads to the best response. Further treatment by Ar plasma results in a remarkable decrease of sensor working temperature from 300 °C of untreated films to 250 °C and nearly 4-fold enhancement in the sensor response to 10 ppm TEA. Notably, the plasma-treated ZnO sensor also shows decent response even at room temperature (RT), which has been seldom reported for ZnO-based sensors. Structure and mechanism investigations reveal that the superior sensor properties are derived from the abundant oxygen vacancies generated by Ar plasma etching.

Heterostructural (Sr0.6Bi0.305)2Bi2O7/ZnO for novel high-performance H2S sensor operating at low temperature

Exploring novel sensing materials enabling selective discrimination of trace ambient H₂S at lower temperature is of utmost importance for diverse practical applications. Herein, heterostructural (Sr_0.6Bi_0.305)₂Bi₂O₇/ZnO (SBO/ZnO) nanomaterials were proposed. Synergetic effect of promoting analyte adsorption (via multiplying oxygen vacancy defects) and reversible sulfuration-desulfuration reaction induced unique band alignment among SBO/ZnO/ZnS, contributes to the sensitive and selective response toward H₂S molecules. Novel SBO/ZnO (10%) sensor possesses excellent sensing H₂S performances, including a high response (107.6 for 10 ppm), low limit of detection of 20 ppb, good selectivity and long-term stability. Together with the merits of low operation temperature of 75 °C and weak humidity dependence (endowed by the hydrophobic SBO), present heterostructural SBO/ZnO sensor paves the way for the practical monitoring of trace H₂S pollutants in diverse workplaces including petroleum and natural gas industries.

Adjustment of oxygen vacancy states in ZnO and its application in ppb-level NO2 gas sensor

Oxygen vacancy (V_O) is long believed as a key factor influencing the gas sensing properties. However, the concentration of V_O is generally focused while the V_O state is neglected, which masks the inherent mechanism of gas sensor. Using a post annealing process, the influence of V_O states on the response of ZnO nanofilm to NO₂ gas is investigated in this study. The systematical analysis of the results obtained by different methods indicates a transformation of V_O from the neutral to the doubly ionized state during post annealing treatment. The results also imply that the gas sensing properties is not directly correlated with the V_O concentration. And due to the competitive adsorption of ambient O₂, the neutral V_O is majorly occupied by the adsorbed O₂ while the V_O in doubly ionized state can promote the adsorption of NO₂. Consequently, the transition of V_O from the neutral to the doubly ionized state can lead to a dramatic increase of the response to NO₂, from 733 to 3.34 × 10⁴ for 100 ppm NO₂. Guided by this mechanism, NO₂ gas sensing in ppb-level is also achieved: the response reaches 165% to 25 ppb (0.025 ppm) NO₂ with a good repeatability.

Treatment-dependent surface chemistry and gas sensing behavior of the thinnest member of titanium carbide MXenes

MXenes, a rapidly developing family of two-dimensional materials possessing tunable electronic properties and abundant surface functional groups, are promising gas-sensing materials. Ti₂CT_x, with a thinner unit cell thickness compared to its compositional analogue Ti₃C₂T_x and thus more profound surface-dependent properties, has been less explored over the past years. Herein, by etching the precursor Ti₂AlC with a concentrated HF or LiF/HCl mixture, semiconducting Ti₂CT_x (HF) nanosheets and metallic Ti₂CT_x (LiF/HCl) nanosheets were obtained, respectively, arising from their treatment-dependent surface functionalization. In addition, the resulting metallic nanosheets were partially oxidized into TiO₂/Ti₂CT_x (LiF/HCl) hybrid, which exhibited superior sensitivity toward NH₃ gas as compared with Ti₂CT_x (HF) and Ti₂CT_x (LiF/HCl). Detailed analysis suggests that a high concentration of surface oxygen containing species, such as -O_x, -(OH)_x and Ti-O-Ti, is generally beneficial for NH₃ sensing, and a relatively higher -O_x concentration allows rapid gas desorption and sensor recovery.

Gas Sensor Detecting 3-Hydroxy-2-butanone Biomarkers: Boosted Response via Decorating Pd Nanoparticles onto the {010} Facets of BiVO4 Decahedrons

The newly emerged gas sensing detection of 3-hydroxy-2-butanone (3H-2B) biomarker is deemed as an effective avenue to indirectly monitor Listeria monocytogenes (LM). However, 3H-2B sensing materials requiring critically high sensitivity and selectivity, and ppb-level detection limit, remain challenging. Here, we report the advanced gas sensors built with bismuth vanadate microdecahedron (BiVO₄ MDCD) {010} facets selectively decorated with Pd nanoparticles (Pd NPs, Pd-{010}BiVO₄ MDCDs) for boosted detection of the 3H-2B biomarker. Meanwhile, BiVO₄ MDCDs with overall facets are randomly deposited with Pd NPs (Pd-BiVO₄ MDCDs). Comparatively, Pd-{010}BiVO₄ MDCD sensors show 1 order of magnitude higher response toward the 3H-2B biomarker at 200 °C. Further, Pd-{010}BiVO₄ MDCD sensors enable to detect as low as 0.2 ppm 3H-2B and show best selectivity and stability, and fastest response and recovery. Density functional theory calculations reveal a lower adsorption energy of 3H-2B onto Pd-{010}BiVO₄ MDCDs than those of pristine and Pd-BiVO₄ MDCDs. The extraordinary Pd-{010}BiVO₄ sensing performance is ascribed to the Pd NP-assisted synergetic effect of the preferential adsorption of 3H-2B target molecules, accumulated sensing agent of ionic oxygen species, and concentrated catalysts on the {010} facets. This strategy offers rapid and noninvasive detection of LMs and is thus of great potential in the upcoming Internet of Things.

Oxygen Vacancies Enabled Porous SnO2 Thin Films for Highly Sensitive Detection of Triethylamine at Room Temperature

Detection of volatile organic compounds (VOCs) at room temperature (RT) currently remains a challenge for metal oxide semiconductor (MOS) gas sensors. Herein, for the first time, we report on the utilization of porous SnO₂ thin films for RT detection of VOCs by defect engineering of oxygen vacancies. The oxygen vacancies in the three-dimensional-ordered SnO₂ thin films, prepared by a colloidal template method, can be readily manipulated by thermal annealing at different temperatures. It is found that oxygen vacancies play an important role in the RT sensing performances, which successfully enables the sensor to respond to triethylamine (TEA) with an ultrahigh response, for example, 150.5-10 ppm TEA in a highly selective manner. In addition, the sensor based on oxygen vacancy-rich SnO₂ thin films delivers a fast response and recovery speed (53 and 120 s), which can be further shortened to 10 and 36 s by elevating the working temperature to 120 °C. Notably, a low detection limit of 110 ppb has been obtained at RT. The overall performances surpass most previous reports on TEA detection at RT. The outstanding sensing properties can be attributed to the porous structure with abundant oxygen vacancies, which can improve the adsorption of molecules. The oxygen vacancy engineering strategy and the on-chip fabrication of porous MOS thin film sensing layers deliver great potential for creating high-performance RT sensors.

Rheotaxially Grown and Vacuum Oxidized SnOx Nanolayers for NO2 Sensing Characteristics at Ppb Level and Room Temperature

* Correspondence: Barbara [...].

Sn13-Oxo Clusters with an Open Hollow Structural Motif and Decorated by Different Functional Ligands

The first open hollow Sn₁₃-oxo cluster family has been successfully prepared and characterized. These Sn₁₃ clusters contain a {Sn₇} moiety that is similar to the basic structure unit of rutile SnO₂. Interestingly, the Sn₁₃ clusters show labile coordination sites on the edge, which could be functionalized by different ligands. With the different decorated types of functionalized ligands, the open hollow Sn₁₃ clusters present different structural details and framework diameters. The presented results provide a new open hollow structural motif of tin-oxo clusters and also a good platform for their ligand functionalization.

Designing SnO2 Nanostructure-Based Sensors with Tailored Selectivity toward Propanol and Ethanol Vapors

The application of metal oxide-based sensors for the detection of volatile organic compounds is restricted because of their high operating temperatures and poor gas sensing selectivity. Driven by this fact, we report the low operating temperature and high performance of C₃H₇OH and C₂H₅OH sensors. The sensors comprising SnO₂ hollow spheres, nanoparticles, nanorods, and fishbones with tunable morphologies were synthesized with a simple hydrothermal one-pot method. The SnO₂ hollow spheres demonstrated the highest sensing response (resistance ratio of 20) toward C₃H₇OH at low operating temperatures (75 °C) compared to other tested interference vapors and gases, such as C₃H₅O, C₂H₅OH, CO, NH₃, CH₄, and NO₂. This improved response can be associated with the higher surface area and intrinsic point defects. At a higher operating temperature of 150 °C, a response of 28 was witnessed for SnO₂ nanorods. A response of 59 was observed for SnO₂ nanoparticle-based sensor toward C₂H₅OH at 150 °C. This variation in the optimal temperature with respect to variations in the sensor morphology implies that the vapor selectivity and sensitivity are morphology-dependent. The relation between the intrinsic sensing performance and vapor selectivity originated from the nonstoichiometry of SnO₂, which resulted in excess oxygen vacancies (V_O) and higher surface areas. This characteristic played a vital role in the enhancement of the target gas absorptivity and the charge transfer capability of SnO₂ hollow sphere-based sensor.

Facile Synthesis of Hierarchical Tin Oxide Nanoflowers with Ultra-High Methanol Gas Sensing at Low Working Temperature

In this work, the hierarchical tin oxide nanoflowers have been successfully synthesized via a simple hydrothermal method followed by calcination. The as-obtained samples were investigated as a kind of gas sensing material candidate for methanol. A series of examinations has been performed to explore the structure, morphology, element composition, and gas sensing performance of as-synthesized product. The hierarchical tin oxide nanoflowers exhibit sensitivity to 100 ppm methanol and the response is 58, which is ascribed to the hierarchical structure. The response and recovery time are 4 s and 8 s, respectively. Moreover, the as-prepared sensor has a low working temperature of 200 °C which is lower than that for other gas sensors of such type has been reported elsewhere. The excellent sensitivity of the sensor is caused by its complex phase mixture of SnO, SnO2, Sn2O3, and Sn6O4 revealed by XRD analysis. The proposed hierarchical tin oxide nanoflowers gas sensing material is promising for development of methanol gas sensor. The as-obtained hierarchical tin oxide nanoflower (HTONF) gas sensor shows excellent gas-sensing performance at low working temperature (200 °C) and high annealing temperature (400 °C).

Surface oxygen vacancy defect engineering of p-CuAlO2via Ar&H2 plasma treatment for enhancing VOCs sensing performances

Ar&H₂ plasma treatment offers a facile approach to engineer surface V_O defects, which substantially enhance the VOCs responses of p-type delafossite CuAlO₂ sensor.

Detection of NO2 Down to One ppb Using Ion-in-Conjugation-Inspired Polymer

Nitrogen dioxide (NO₂ ) emission has severe impact on human health and the ecological environment and effective monitoring of NO₂ requires the detection limit (limit of detection) of several parts-per-billion (ppb). All organic semiconductor-based NO₂ sensors fail to reach such a level. In this work, using an ion-in-conjugation inspired-polymer (poly(3,3'-diaminobenzidine-squarine, noted as PDBS) as the sensory material, NO₂ can be detected as low as 1 ppb, which is the lowest among all reported organic NO₂ sensors. In addition, the sensor has high sensitivity, good reversibility, and long-time stability with a period longer than 120 d. Theoretical calculations reveal that PDBS offers unreacted amine and zwitterionic groups, which can offer both the H-bonding and ion-dipole interaction to NO₂ . The moderate binding energies (≈0.6 eV) offer high sensitivity, selectivity as well as good reversibility. The results demonstrate that the ion-in-conjugation can be employed to greatly improve sensitivity and selectivity in organic gas sensors by inducing both H-bonding and ion-dipole attraction.

Oxygen Vacancy Defects Boosted High Performance p-Type Delafossite CuCrO2 Gas Sensors

p-type ternary oxides can be extensively explored as alternative sensing channels to binary oxides with diverse structural and compositional versatilities. Seeking a novel approach to magnify their sensitivities toward gas molecules, e.g., volatile organic compounds (VOCs), will definitely expand their applications in the frontier area of healthcare and air-quality monitoring. In this work, delafossite CuCrO₂ (CCO) nanoparticles with different grain sizes have been utilized as p-type ternary oxide sensors. It was found that singly ionized oxygen vacancies (V_o^•) defects, compared with the grain size of CCO nanoparticles, play an important role in enhancing the charge exchange at the VOCs molecules/CCO interface. In addition to suppressing the hole concentration of the sensor channel, the unpaired electron trapped in V_o^• provides an active site for chemisorptions of environmental oxygen and VOCs molecules. The synergetic effect is responsible for the observed increase of sensitivity. Furthermore, the sensitive (V_o^• defect-rich) CCO sensor exhibits good reproducibility and stability under a moderate operation temperature (<325 °C). Our work highlights that V_o^• defects, created via either in situ synthesis or postannealing treatment, could be explored to rationally boost the performance of p-type ternary oxide sensors.

Self-Assembled Hierarchical Interfaces of ZnO Nanotubes/Graphene Heterostructures for Efficient Room Temperature Hydrogen Sensors

Herein, we report the novel nanostructural interfaces of self-assembled hierarchical ZnO nanotubes/graphene (ZNT/G) with three different growing times of ZNTs on graphene substrates (namely, SH₁, SH₂, and SH₃). Each sample was fabricated with interdigitated electrodes to form hydrogen sensors, and their hydrogen sensing properties were comprehensively studied. The systematic investigation revealed that SH₁ sensor exhibits an ultrahigh sensor response even at a low detection level of 10 ppm (14.3%) to 100 ppm (28.1%) compared to those of the SH₂ and SH₃ sensors. The SH₁ sensor was also found to be well-retained with repeatability, reliability, and long-term stability of 90 days under hydrogenation/dehydrogenation processes. This outstanding enhancement in sensing properties of SH₁ is attributed to the formation of a strong metalized region in the ZNT/G interface due to the inner/outer surfaces of ZNTs, establishing a multiple depletion layer. Furthermore, the respective band models of each nanostructure were also purposed to describe their heterostructure, which illustrates the hydrogen sensing properties. Moreover, the long-term stability can be ascribed by the heterostructured combination of ZNTs and graphene via a spillover effect. The salient features of this self-assembled nanostructure are its reliability, simple synthesis method, and long-term stability, which makes it a promising candidate for new generation hydrogen sensors and hydrogen storage materials.

“Close network” effect of a ZnO micro/nanoporous array allows high UV-irradiated NO2 sensing performance

The “close network” effect of a ZnO micro/nanoporous array allows high UV-irradiated NO₂ sensing performance at room temperature.

Two-dimensional NiO nanosheets with enhanced room temperature NO2sensing performance via Al doping

Defects caused by Al³⁺doping significantly affect the gas-sensing properties of NiO nanosheets.