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Food contamination has long plagued agriculture, posing significant health risks to consumers. The use of volatile gases for food safety detection has proven highly effective, with composite gas sensors that leverage the two-dimensional material MXene exhibiting notable advancements in detecting various target gases. This paper reviews the progress of MXene-based composite gas sensors in the detection of food safety-related gases. The review begins by examining MXene material synthesis methods and then presents an overview of techniques aimed at enhancing MXene-based sensor detection capabilities. Recently, advancements in MXene composite gas sensors tailored for food safety gases have been highlighted. Finally, challenges encountered in gas-sensing applications of MXene-based composites are outlined, alongside predictions for their future development, aiming to offer insights for the application and advancement of intelligent gas sensors for target gases in food safety.

Asunto(s)

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The detection of toxic, harmful, explosive, and volatile gases cannot be separated from gas sensors, and gas sensors are also used to monitor the greenhouse effect and air pollution. However, existing gas sensors remain with many drawbacks, such as lower sensitivity, lower selectivity, and unstable room temperature detection. Thus, there is an imperative need to find more suitable sensing materials. The emergence of a new 2D layered material MXenes has brought dawn to solve this problem. The multiple advantages of MXenes, namely high specific surface area, enriched terminal functionality groups, hydrophilicity, and good electrical conductivity, make them among the most prolific gas-sensing materials. Therefore, this review paper describes the current main synthesis methods of MXenes materials, and focuses on summarizing and organizing the latest research results of MXenes in gas sensing applications. It also introduces the possible gas sensing mechanisms of MXenes materials on NH3 , NO2 , CH3 , and volatile organic compounds (VOCs). In conclusion, it provides insight into the problems and upcoming challenges of MXenes materials for gas sensing.

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A new technique of polarization doping was adopted to improve NO2 gas sensing properties of the polypyrrole (PPy) sensor. PPy nanosheets polarization doped with sodium dodecyl benzenesulfonate (SDBS) were synthesized by low-temperature polymerization. The semiagglomerated PPy nanosheets were well-dispersed and a large specific surface areas due to the introduction of dodecyl benzenesulfonate (DBS). The DBS doped PPy sensor shows excellent NO2 sensing performance. Polarization of sulfonate ions to PPy enhanced the adsorption ability of NO2 with the synergistic effect of NO2. The adsorption energy (-0.676 eV) and electron transfer (0.521 |e|) of PPy to NO2 increased greatly after doping. An unoccupied electron state is observed in the valence band electron structure of PPy/DBS after the adsorption of NO2 by calculations of Density Functional Theory (DFT). The intermolecular synergy between NO2 and PPy/DBS causes a strong polarization, resulting in a high polarization potential, which enhances the NO2 sensing performance of PPy sensor. It is of great significance to develop NO2 detection device based on PPy that works at room temperature.

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Liquefied petroleum gas (LPG), which is mainly composed of hydrocarbons, such as propane and butane, is a flammable gas that is considered a clean source of energy. Currently, the overwhelming use of LPG as fuel in vehicles, domestic settings, and industry has led to several incidents and deaths globally due to leakage. As a result, the appropriate detection of LPG is vital; thus, gas-sensing devices that can monitor this gas rapidly and accurately at room temperature, are required. This work reviews the current advances in LPG gas sensors, which operate at room temperature. The influences of the synthesis methods and parameters, doping, and use of catalysts on the sensing performance are discussed. The formation of heterostructures made from semiconducting metal oxides, polymers, and graphene-based materials, which enhance the sensor selectivity and sensitivity, is also discussed. The future trends and challenges confronted in the advancement of LPG room temperature operational gas sensors, and critical ideas concerning the future evolution of LPG gas sensors, are deliberated. Additionally, the advancements in the next-generation gas sensors, such as the wireless detection of LPG leakage, self-powered sensors driven by triboelectric/piezoelectric mechanisms, and artificial intelligent systems are also reviewed. This review further focuses on the use of smartphones to circumvent the use of costly instruments and paves the way for cost-efficient and portable monitoring of LPG. Finally, the approach of utilizing the Internet of Things (IoT) to detect/monitor the leakage of LPG has also been discussed, which will provide better alerts to the users and thus minimize the effects of leakages.

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Different hierarchical porous In2O3 nanostructures were synthesized by regulating the hydrothermal time and combining it with a self-pore-forming method. The gas-sensing test results show that the response of the sensor based on In2O3 obtained after hydrothermal reaction for 48 h is about 10.4 to 500 ppm methane. Meanwhile, it possesses good reproducibility, stability, selectivity and moisture resistance as well as a good exponential linear relationship between the response to methane and its concentration. In particular, the sensor based on In2O3 can detect a wide range of methane (10~2000 ppm) at near-room temperature (30 °C). The excellent methane sensitivity of the In2O3 sensor is mainly due to its unique nanostructure, which has the advantages of both porous and hierarchical structures. Combined with the DFT calculation, it is considered that the sensitive mechanism is mainly controlled by the surface adsorbed oxygen model. This work provides a feasible strategy for enhancing the gas sensitivity of In2O3 toward methane at low temperatures.

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SnO2is widely used for ethanol-sensing applications due to its excellent physicochemical properties, low toxicity and high sensitivity. However it is a challenge to construct 3D-hierarchical structures with sub 5 nm primary grain particle, which is the optimized size for ethanol sensor. Herein, genetic tri-level hierarchical SnO2microstructures are synthesised by the genetic conversion of 3D hierarchical SnS2flowers assembled by ultrathin nanosheets. The SnS2nanosheets are morphology genetic converted to porous nanosheets with sub 5 nm SnO2nanoparticles during the calcination process. When used for the detection of ethanol, the sensor exhibits a high sensitivity of 0.5 ppm (Ra/Rg = 6.8) and excellent gas-sensing response (Ra/Rg= 183 to 100 ppm) with short response/recovery time (12 s/11 s). The excellent gas sensing performance is much better than that of the previous reported SnO2-based sensors. The highly sensitivity is attributed to the large surface area derived from the recrystallization and volume changes, which offers more active sites during the morphology genetic conversion from SnS2to SnO2. Furthermore, the flower-like 3D structure enhances the stability of the materials and is beneficial for the mass diffusion dynamics of ethanol.

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High surface area nickel oxide nanowires (NiO NWs), Fe-doped NiO NWs andα-Fe2O3/Fe-doped NiO NWs were synthesized with nanocasting pathway, and then the morphology, microstructure and components of all samples were characterized with XRD, TEM, EDS, UV-vis spectra and nitrogen adsorption-desorption isotherms. Owing to the uniform mesoporous template, all samples with the same diameter exhibit the similar mesoporous-structures. The loadedα-Fe2O3nanoparticles should exist in mesoporous channels between Fe-doped NiO NWs to form heterogeneous contact at the interface of n-typeα-Fe2O3nanoparticles and p-type NiO NWs. The gas-sensing results indicate that Fe-dopant andα-Fe2O3-loading both improve the gas-sensing performance of NiO NWs sensors.α-Fe2O3/Fe-doped NiO NWs sensors presented the highest response to 100 ppm ethanol gas (55.264) compared with Fe-doped NiO NWs (24.617) and NiO NWs sensors (3.189). The donor Fe-dopant increases the ground state resistance and the absorbed oxygen content in air.α-Fe2O3nanoparticles in electron depletion region result in the increasing resistance in ethanol gas and decreasing resistance in air. In this way,α-Fe2O3/Fe-doped NiO NWs sensor presents the excellent gas-sensing performance due to the formation of heterogeneous contact at the interface.

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Two-dimensional transition metal dichalcogenides (2D TMDs), such as WS2, are considered to have the potential for high-performance gas sensors. It is a pity that the interaction between gases and pristine 2D WS2 as the sensitive element is too weak so that the sensor response is difficult to detect. Herein, the sensing capabilities of Al- and P-doped WS2 to NO, NO2, and SO2 were evaluated. Especially, we considered selectivity to target gases and dopant concentration. Molecular models of the adsorption systems were constructed, and density functional theory (DFT) was used to explore the adsorption behaviors of these gases from the perspective of binding energy, band structure, and density of states (DOS). The results suggested that doping atoms could increase the adsorption strength between gas molecules and substrate. Besides, the sensitivity of P-doped WS2 to NO and NO2 was hardly affected by CO2 or H2O. The sensitivity of Al-doped WS2 to NO2 and SO2 was also hard to be affected by CO2 or H2O. For NO detection, the WS2 with 7.4% dopant concentration had better sensitive properties than that with a 3.7% dopant concentration. While for SO2, the result was just the opposite. This work provided a comprehensive reference for choosing appropriate dopants (concentration) into 2D materials for sensing noxious gases.

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Gas sensors have been wildly used in various fields related to people's lives. Gas sensor materials were the core factors that affected the performances of various gas sensors, and these have attracted much attention from scientific researchers due their high sensitivity, high selectivity, adjustable reliability, low cost, and other advantages. The preparation of nanostructures with a highly specific surface area was a useful method to improve the gas-sensing performance of a metal oxide semiconductor. Meanwhile, lots of research has focused on preparing nanostructures with a highly specific surface area. This paper has explored some fabricated sensors with high sensitivity, good selectivity, and long-term stability, which has also made them promising candidates for toxic gas detection. Besides, this paper has reviewed the development status of metal oxides used as gas sensors.

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SnO2/graphitic carbon nitride (g-C3N4) composites were synthesized via a facile solid-state method by using SnCl4·5H2O and urea as the precursor. The structure and morphology of the as-synthesized composites were characterized by the techniques of X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), energy dispersive spectrometer (EDS), thermogravimetry-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS), and N2 sorption. The results indicated that the composites possessed a two-dimensional (2-D) structure, and the SnO2 nanoparticles were highly dispersed on the surface of the g-C3N4 nanosheets. The gas-sensing performance of the samples to ethanol was tested, and the SnO2/g-C3N4 nanocomposite-based sensor exhibited admirable properties. The response value (Ra/Rg) of the SnO2/g-C3N4 nanocomposite with 10 wt % 2-D g-C3N4 content-based sensor to 500 ppm of ethanol was 550 at 300 °C. However, the response value of pure SnO2 was only 320. The high surface area of SnO2/g-C3N4-10 (140 m²·g-1) and the interaction between 2-D g-C3N4 and SnO2 could strongly affect the gas-sensing property.

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Hierarchical morphology-dependent gas-sensing performances have been demonstrated for three-dimensional SnO2 nanostructures. First, hierarchical SnO2 nanostructures assembled with ultrathin shuttle-shaped nanosheets have been synthesized via a facile and one-step hydrothermal approach. Due to thermal instability of hierarchical nanosheets, they are gradually shrunk into cone-shaped nanostructures and finally deduced into rod-shaped ones under a thermal treatment. Given the intrinsic advantages of three-dimensional hierarchical nanostructures, their gas-sensing properties have been further explored. The results indicate that their sensing behaviors are greatly related with their hierarchical morphologies. Among the achieved hierarchical morphologies, three-dimensional cone-shaped hierarchical SnO2 nanostructures display the highest relative response up to about 175 toward 100 ppm of acetone as an example. Furthermore, they also exhibit good sensing responses toward other typical volatile organic compounds (VOCs). Microstructured analyses suggest that these results are mainly ascribed to the formation of more active surface defects and mismatches for the cone-shaped hierarchical nanostructures during the process of thermal recrystallization. Promisingly, this surface-engineering strategy can be extended to prepare other three-dimensional metal oxide hierarchical nanostructures with good gas-sensing performances.

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In the present study, the nanotubes of 5-(4-hydroxyphenyl)-10, 15, 20-tri(4-chlorophenyl) porphyrin (p-HTClPP) (1) and 5-(4-hydroxyphenyl)-10, 15, 20-tri(4-chlorophenyl) porphyrin cobalt (p-HTClPPCo) (2) were successfully prepared by using anodize alumina oxide (AAO) template method. The p-HTClPP and p-HTClPPCo nanotubes have been confirmed by scanning electron microscopy (SEM), transmission electron microscopy (TEM), electronic absorption spectra, fluorescence spectroscopy, fourier transform infrared spectroscopy (FT-IR), low-angle X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS) techniques. Both p-HTClPPCo and p-HTClPP nanotubes showed excellent sensitivity, reproducibility and selectivity toward NO2. Especially the prepared sensor of p-HTClPPCo nanotubes exhibited faster response/recovery characteristics and lower detection limit of NO2 (up to 500ppb) than that of p-HTClPP nanotubes, which pave a new avenue in the gas sensitive field.

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NiO nanocones decorated with ZnO nanothorns on NiO foil substrates are shown to be an ammonia sensor with excellent comprehensive performance, which could, in real-time, detect and monitor NH3 in the surrounding environment. Gas-sensing measurements indicate that assembling nanocones decorated with nanothorns on NiO foil substrate is an effective strategy for simultaneously promoting the stability, reproducibility, and sensitivity of the sensor, because the NiO foil substrate as a whole can quickly and stably transfer electrons between the gas molecules and the sensing materials and the large specific surface area of both nanocones and nanothorns provide good accessibility of the gas molecules to the sensing materials. Moreover, p-type NiO, with majority charge carriers of holes, has higher binding affinity for the electron-donating ammonia, resulting in a significant increase in selectivity toward NH3 over other organic gases. Compared with the NiO nanowires and pure NiO nanocones, the heterogeneous NiO nanocones/ZnO nanothorns exhibit less dependence on the temperature and humidity in response/recovery speed and sensitivity of sensing NH3. Our investigation indicates that two factors are responsible for reducing the dependence on the gas sensing characteristics under various environmental conditions. One is that the n-type ZnO nanothorns growing on the surface of nanocones, with majority charge carriers of electrons, speed up adsorption and desorption of gas molecules. The other is that the abundant cone-shaped and thornlike superstructures on the substrate are favorable for constructing a hydrophobic surface, which prevents the gas sensing material from being wetted.