Hierarchical Nanoheterostructure of HFIP-Grafted α-Fe2O3@Multiwall Carbon Nanotubes as High-Performance Chemiresistive Sensors for Nerve Agents

New and efficient sensors of nerve agents are urgently demanded to prevent them from causing mass casualties in war or terrorist attacks. So, in this work, a novel hierarchical nanoheterostructure was synthesized via the direct growth of α-Fe2O3 nanorods onto multiwall carbon nanotube (MWCNT) backbones. Then, the composites were functionalized with hexafluoroisopropanol (HFIP) and successfully applied to detect dimethyl methylphosphonate (DMMP)-sarin simulant gas. The observations show that the HFIP-α-Fe2O3@MWCNT hybrids exhibit outstanding DMMP-sensing performance, including low operating temperature (220 °C), high response (6.0 to 0.1 ppm DMMP), short response/recovery time (8.7 s/11.9 s), as well as low detection limit (63.92 ppb). The analysis of the sensing mechanism demonstrates that the perfect sensing performance is mainly due to the synergistic effect of the chemical interaction of DMMP with the heterostructure and the physical adsorption of DMMP by hydrogen bonds with HFIP that are grafted on the α-Fe2O3@MWCNTs composite. The huge specific surface area of HFIP-α-Fe2O3@MWCNTs composite is also one of the reasons for this enhanced performance. This work not only offers a promising and effective method for synthesizing sensitive materials for high-performance gas sensors but also provides insight into the sensing mechanism of DMMP.

A colorimetric analytical method based on a TCPP-CuCo2O4-like peroxidase for the detection of trichlorfon

In this work, a highly sensitive colorimetric sensing platform was designed for the detection of trichlorfon based on inhibiting thiocholine (TCh)-induced redox reaction. 5,10,15,20-Tetracarboxyphenylporphyrin (TCPP) functionalized CuCo2O4 (TCPP-CuCo2O4) was synthesized to construct a colorimetric sensing platform for trichlorfon. In the presence of H2O2, TCPP-CuCo2O4 can oxidize colorless 3,3',5,5'-tetramethylbenzidine (TMB) into blue ox-TMB, accompanied by a strong absorption peak at 652 nm, while acetylcholinesterase (AChE) can specifically hydrolyze acetylthiocholine (ATCh) into TCh, which can reduce ox-TMB back into colorless TMB, resulting in a lower absorbance at 652 nm. Trichlorfon can irreversibly inhibit the activity of AChE and thus recover the absorption peak. Under the optimized conditions, detection of trichlorfon has a wide linear range of 40-4000 ng mL-1 with a linear correlation coefficient of 0.9904. The proposed method can be applied to the detection of trichlorfon in vegetables and has good application prospects.

Detection of n-Propanol Down to the Sub-ppm Level Using p-Type Delafossite AgCrO2 Nanoparticles

As an important biomarker of lung cancer, n-propanol at the sub-ppm level is still challenging to be detected for a simple and immediate early diagnosis. In this work, a new n-propanol gas sensor with an ultralow detection limit down to 100 ppb is presented using AgCrO₂ nanoparticles synthesized by a simple hydrothermal method. Compared with the congeneric CuCrO₂ and commercial SnO₂, AgCrO₂ exhibits prevailing performances, including a higher selectivity, dynamic response, and logarithmical linearity but lower working temperature. The first-principles calculation and the energy band theoretical analysis are combined to elucidate the sensing mechanism, in which the chemical adsorption of gaseous molecules to silver followed by the dehydrogenation on chromium on the surface of AgCrO₂ is responsible for the outstanding susceptibility toward n-propanol. The proposed metal oxide semiconductor gas sensor capable of sub-ppm n-propanol detection provides a route to design and optimize the sensitive material system for the advanced trace detection of the volatile organic compounds.

Enhanced peroxidase-like activity of 2(3), 9(10), 16(17), 23(24)-octamethoxyphthalocyanine modified CoFe LDH for a sensor array for reducing substances with catechol structure

Improving the catalytic activity of artificial nanozymes to realize the real-time detection of small molecules becomes an important task. Herein, a highly active nanozyme, 2(3), 9(10), 16(17), 23(24)-octamethoxyphthalocyanine (Pc(OH)₈) modified CoFe LDH microspheres (Pc(OH)₈-CoFe LDH) have been prepared by the two-step hydrothermal method. The 3,3',5,5'-tetramylbenzidine (TMB), a chromogenic substrate, was fast oxidized into blue oxTMB by H₂O₂ in the presence of Pc(OH)₈-CoFe LDH, indicating that Pc(OH)₈-CoFe LDH possesses high peroxidase-like activity rather than pure CoFe LDH. The enhancement peroxidase-like activity of the Pc(OH)₈-CoFe LDH is ascribed to the synergistic action between Pc(OH)₈ and CoFe LDH. Experimental results of radical scavenger and fluorescence probe verify that superoxide radical (•O₂^-) plays an important role during the catalytic reaction. Interestingly, the absorption intensity of reaction system has been enhanced largely, due to adding of the reducing substances containing catechol structure. Based on this, the three reducing substances (dopamine, procyanidin B2, catechins) containing catechol structure were distinguished from other reducing substances without catechol structure. Thus, a colorimetric array has been constructed using reaction time as the sensing element to realize the sensitive and selective recognition of catechol structures at a certain concentration.

Fast-response MEMS xylene gas sensor based on CuO/WO3 hierarchical structure

CuO/WO₃ hierarchical hollow microspheres, assembled from irregular two dimensional (2D) nanosheets, were prepared by ultrasonic-wet chemical etching and pyrolysis in this study. The sensing performance of Micro-Electro-Mechanical System (MEMS) xylene gas sensor based on CuO/WO₃ hierarchical structure were evaluated. It was found that the CuO/WO₃ MEMS sensors showed an enhanced gas sensing performance compared with pristine WO₃ sensor. The CuO/WO₃-3 (the mass ratio of CuO to WO₃ is 3%) sensor exhibited faster response-recover speed and the highest response value to xylene. Moreover, the CuO/WO₃-3 sensor possessed higher selectivity and long-term stability. The good sensing properties can be attributed to the unique three dimensional (3D) hierarchical structure and p-n heterojunction of CuO-WO₃. Considering the above advantages, the CuO/WO₃-3 sensor has a great potential for the rapid detection and monitoring of xylene.

Electronic and morphological dual modulation of NiO by indium-doping for highly improved xylene sensing

Ultrathin indium-doped NiO nanosheets with simultaneously optimized nanostructures and electronic properties were developed to achieve a high response to a low concentration of xylene.

Design of p-p heterojunctions based on CuO decorated WS2nanosheets for sensitive NH3gas sensing at room temperature

Tungsten disulfide (WS₂) nanosheets (NSs) have become a promising room-temperature gas sensor candidate due to their inherent high surface-to-volume ratio, tunable electrical properties, and high on-state current density. For further practical applications of WS₂-based gas sensors, it is still necessary to overcome the insensitive response and incomplete recovery at room temperature. In this work, we controllably synthesized high-performance ammonia (NH₃) gas sensor based on CuO decorated WS₂NSs. The optimized p-p WS₂/CuO heterojunctions improve the surface catalytic effect, thereby enhancing the gas-sensing performance. The pure WS₂NSs-based gas sensors showed a low response and an incomplete recovery in the case of NH₃sensing. After the functionalization of CuO nanoparticles, the WS₂/CuO heterostructure-based gas sensor exhibits an improved response value of 40.5% to 5  ppm NH₃and full recoverability without any external assistance. Density functional theory calculations illustrate that the adsorption of CuO for NH₃is much superior to WS₂. The p-p heterojunctions strategy demonstrated in this work has great potential in the design of sensitive materials for gas sensors, and provides useful guidance for enhancing the room-temperature sensitivity and recoverability.

5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin-functionalized urchin-like CuCo2O4 as an excellent artificial nanozyme for determination of dopamine

Urchin-like peroxidase mimics 5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin-functionalized CuCo2O4 nanospheres (Por-CuCo2O4) has been fabricated as an excellent visual biosensor. X-ray diffractometry (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) have been employed to characterize the composition, morphologies, and elemental analysis of the as-synthesized Por-CuCo2O4. The catalytic activity of Por-CuCo2O4 was evaluated by the chromogenic substrate 3,3',5,5'-tetramethylbenzidine (TMB) with the aid of H2O2, which exhibited a visual blue change with an absorption maximum at 652 nm for only 10 s. The peroxidase-like behaviors of Por-CuCo2O4 conformed to the Michaelis-Menten equation. Electrochemistry, radical scavenger, and fluorescence probe experiments verified that electron transfer, •O2- radicals, and holes (h+) are the important factors during the catalytic oxidation of TMB. Based on the inhibition of dopamine (DA) on TMB oxidation, the Por-CuCo2O4-based colorimetric biosensor has been successfully constructed for sensitive determination of DA witha detection limit (LOD) of 0.94 μΜ. In addition, colorimetry was validated to detect DA in serum samples with high sensitivity and good selectivity. 5,10,15,20-tetrakis (4-carboxyl phenyl) porphyrin-functionalized urchin-like CuCo2O4 (Por-CuCo2O4) with excellent peroxidase activity, ascribed to the synergistic effect between •O2- radicals and holes (h+). A fast colorimetric sensor on the basis of Por-CuCo2O4 has been constructed to quantitatively determine dopamine concentration in human serums.

Construction of Zn/Ni Bimetallic Organic Framework Derived ZnO/NiO Heterostructure with Superior N-Propanol Sensing Performance

Bimetallic organic frameworks (Bi-MOFs) have been recognized as one of the most ideal precursors to construct metal oxide semiconductor (MOS) composites, owing to their high surface area, various chemical structures, and easy removal of the sacrificial MOF scaffolds through calcination. Herein, we synthesized Zn/Ni Bi-MOF for the first time via a facile ion exchange postsynthetic strategy, formed a three-dimensional framework consisting of infinite one-dimensional chains that is unattainable through the direct solvothermal approach, and then transformed the Zn/Ni Bi-MOF into a unique ZnO/NiO heterostructure through calcination. Notably, the obtained sensor based on a ZnO/NiO heterostructure exhibits an ultrahigh response of 280.2 toward 500 ppm n-propanol at 275 °C (17.2-fold enhancement compared with that of ZnO), remarkable selectivity, and a limit of detection of 200 ppb with a notable response (2.51), which outperforms state-of-the-art n-propanol sensors. The enhanced n-propanol sensing properties may be attributed to the synergistic effects of several points including the heterojunction at the interface between the NiO and ZnO nanoparticles, especially a one-dimensional chain MOF template structure as well as the chemical sensitization effect of NiO. This work provides a promising strategy for the development of a novel Bi-MOF-derived MOS heterostructure or homostructure with well-defined morphology and composition that can be applied to the fields of gas sensing, energy storage, and catalysis.

Inorganic-Diverse Nanostructured Materials for Volatile Organic Compound Sensing

Environmental pollution related to volatile organic compounds (VOCs) has become a global issue which attracts intensive work towards their controlling and monitoring. To this direction various regulations and research towards VOCs detection have been laid down and conducted by many countries. Distinct devices are proposed to monitor the VOCs pollution. Among them, chemiresistor devices comprised of inorganic-semiconducting materials with diverse nanostructures are most attractive because they are cost-effective and eco-friendly. These diverse nanostructured materials-based devices are usually made up of nanoparticles, nanowires/rods, nanocrystals, nanotubes, nanocages, nanocubes, nanocomposites, etc. They can be employed in monitoring the VOCs present in the reliable sources. This review outlines the device-based VOC detection using diverse semiconducting-nanostructured materials and covers more than 340 references that have been published since 2016.

Spinel-Type Materials Used for Gas Sensing: A Review

Demands for the detection of harmful gas in daily life have arisen for a period and a gas nano-sensor acting as a kind of instrument that can directly detect gas has been of wide concern. The spinel-type nanomaterial is suitable for the research of gas sensors because of its unique structure. However, the existing instability, higher detection limit, and operating temperature of the spinel materials limit the extension of the spinel material sensor. This paper reviews the research progress of spinel materials in gas sensor technology in recent years and lists the common morphological structures and material sensitization methods in combination with previous works.

Unusual Cu-Co/GO Composite with Special High Organic Content Synthesized by an in Situ Self-Assembly Approach: Pyrolysis and Catalytic Decomposition on Energetic Materials

An interesting Cu-Co/GO composite with special high organic content was accidentally fabricated for the first time via a one-pot solvothermal method in the mixed solvent of isopropanol and glycerol. The Cu-Co/GO composite was calcined separately in three different atmospheres (air, nitrogen, and argon) and further investigated by a series of characterization techniques. The results indicate that the spinel phase nano-CuCo₂O₄ composite, nanometal oxides (CuO and CoO), and nanometal mixture of Cu and Co were unexpectedly formed after calcination in air, N₂, and Ar atmospheres, respectively, and the possible reaction mechanism was discussed. The specific mass losses of the Cu-Co/GO composite calcined in air, N₂, and Ar atmospheres were 28.14 %, 21.68 %, and 23.76 %, respectively. The catalytic decomposition performances of the as-prepared samples for cyclotrimethylenetrinitramine (RDX) and the mixture of nitrocellulose (NC) and RDX (NC + RDX) were investigated and compared via DSC method, and the results demonstrate that Cu-Co/GO composites obviously decrease the thermal decomposition temperature of RDX from 242.3 to 236.5 (before calcination), 238.6 (air), 235.8 (N₂), and 228.6 °C (Ar), respectively. Cu-Co/GO(Ar) composite exhibits the best catalytic decomposition performance among all samples, which makes the decomposition temperature of RDX and NC + RDX decrease by 13.7 and 4.9 °C and the apparent activation energy of decomposition for RDX decrease by 110.1 kJ/mol. The enhanced catalytic performance of Cu-Co/GO(Ar) composite could be attributed to the smaller particle size, better crystallinity, and specific well-dispersed metal atoms, whereas the Cu-Co/GO(air) composite after air calcination presents a bad catalytic performance due to the removal of GO.

p-p Heterojunction Sensors of p-Cu3Mo2O9 Micro/Nanorods Vertically Grown on p-CuO Layers for Room-Temperature Ultrasensitive and Fast Recoverable Detection of NO2

High sensitivity, low limit of detection (LOD), and short response and recovery times at room temperature (RT) are critical for gas sensors. For NO₂, different binary metal oxide-based sensors were developed to achieve superior performance at elevated temperatures instead of RT. Herein, we report on CuO@CuO and Cu₃Mo₂O₉@CuO sensors with CuO and Cu₃Mo₂O₉ micro/nanorods vertically aligned on the CuO layers, which were directly fabricated using a facile, low-cost, and catalyst-free chemical vapor deposition (CVD) technique. Their sensing performance tests revealed that the Cu₃Mo₂O₉@CuO p-p heterojunction sensors exhibited a high response of 160% to 5 ppm NO₂, an excellent sensitivity of 50% ppm^-1, a low LOD of 2.30 ppb, a short response time of 49 s, and a rapid recovery of 241 s at RT, obviously better than those for CuO@CuO sensors. The superior performance of Cu₃Mo₂O₉@CuO sensors could be attributed to the Schottky heterojunction formed between p-Cu₃Mo₂O₉ micro/nanorods and p-CuO films, the catalytic effect, and the anisotropic nature of Cu₃Mo₂O₉ micro/nanorods. This study not only provides a simple, low-cost, and batchable fabrication method of homo/heterojunction sensors with micro/nanorods vertically aligned on films but also opens an avenue for sensor design by tuning the Schottky barrier height to enhance RT performance.

A novel rGO-decorated ZnO/BiVO4 heterojunction for the enhancement of NO2 sensing properties

A novel ZnO/BiVO₄/rGO composite exhibits excellent gas sensing performance, which is attributed to the formation of n–n heterojunctions and decoration with rGO.

Underpinning the Interaction between NO2 and CuO Nanoplatelets at Room Temperature by Tailoring Synthesis Reaction Base and Time

An approach to tailor the morphology and sensing characteristics of CuO nanoplatelets for selective detection of NO₂ gas is of great significance and an important step toward achieving the challenge of improving air quality and in assuring the safety of mining operations. As a result, in this study, we report on the NO₂ room temperature gas-sensing characteristics of CuO nanoplatelets and the underlying mechanism toward the gas-sensing performance by altering the synthesis reaction base and time. High sensitivity of ∼40 ppm^-1 to NO₂ gas at room temperature has been realized for gas sensors fabricated from CuO nanoplatelets, using NaOH as base for reaction times of 45 and 60 min, respectively at 75 °C. In both cases, the crystallite size, surface area, and hole concentration of the respective materials influenced the selectivity and sensitivity of the NO₂ gas sensors. The mechanism underpinning the superior NO₂ gas sensing are thoroughly discussed in terms of the crystallite size, hole concentration, and surface area as active sites for gas adsorption.

Multilevel Effective Heterojunctions Based on SnO2/ZnO 1D Fibrous Hierarchical Structure with Unique Interface Electronic Effects

One-step single-spinneret electrospinning synthesis of 1D fibrous hierarchical structure can not only prevent the agglomeration or restacking of fibers or particles and enlarge surface active area but also promote the directional migration of electrons in materials and achieve effective regulation of resistances. Herein, tunable SnO₂ and SnO₂/ZnO fibrous hierarchical structures with in situ growth of monodisperse spherical-like particles on surface provide a new sight for adjusting component distribution, surface absorption and chemical reaction, electronic transmission path, and electron transfer efficiency. Compared with SnO₂ porous fibers and SnO₂ hierarchical structures, the optimal SnO₂/ZnO sensors exhibit superior gas-sensing response value of 366-100 ppm ethanol at 260 °C as well as excellent gas selectivity and long-term stability, in which the enhanced gas-sensing mechanism is primarily derived from multilevel effective heterojunctions with unique interface electronic effects. Especially, these SnO₂-based sensors can achieve favorable linear relationship of the response and gas concentration for sensitive trace detection in cosmetics for the first time, providing a new strategy to design composite materials for quantitative analysis of volatiles in the cosmetics evaluation process.

HFIP-Functionalized Co3 O4 Micro-Nano-Octahedra/rGO as a Double-Layer Sensing Material for Chemical Warfare Agents

Semiconductor metal oxides (SMO)-based gas-sensing materials suffer from insufficient detection of a specific target gas. Reliable selectivity, high sensitivity, and rapid response-recovery times under various working conditions are the main requirements for optimal gas sensors. Chemical warfare agents (CWA) such as sarin are fatal inhibitors of acetylcholinesterase in the nerve system. So, sensing materials with high sensitivity and selectivity toward CWA are urgently needed. Herein, micro-nano octahedral Co₃ O₄ functionalized with hexafluoroisopropanol (HFIP) were deposited on a layer of reduced graphene oxide (rGO) as a double-layer sensing materials. The Co₃ O₄ micro-nano octahedra were synthesized by direct growth from electrospun fiber templates calcined in ambient air. The double-layer rGO/Co₃ O₄ -HFIP sensing materials presented high selectivity toward DMMP (sarin agent simulant, dimethyl methyl phosphonate) versus rGO/Co₃ O₄ and Co₃ O₄ sensors after the exposure to various gases owing to hydrogen bonding between the DMMP molecules and Co₃ O₄ -HFIP. The rGO/Co₃ O₄ -HFIP sensors showed high stability with a response signal around 11.8 toward 0.5 ppm DMMP at 125 °C, and more than 75 % of the initial response was maintained under a saturated humid environment (85 % relative humidity). These results prove that these double-layer inorganic-organic composite sensing materials are excellent candidates to serve as optimal gas-sensing materials.

Approaches to Enhancing Gas Sensing Properties: A Review

A gas nanosensor is an instrument that converts the information of an unknown gas (species, concentration, etc.) into other signals (for example, an electrical signal) according to certain principles, combining detection principles, material science, and processing technology. As an effective application for detecting a large number of dangerous gases, gas nanosensors have attracted extensive interest. However, their development and application are restricted because of issues such as a low response, poor selectivity, and high operation temperature, etc. To tackle these issues, various measures have been studied and will be introduced in this review, mainly including controlling the nanostructure, doping with 2D nanomaterials, decorating with noble metal nanoparticles, and forming the heterojunction. In every section, recent advances and typical research, as well mechanisms, will also be demonstrated.