Detection of Triacetone Triperoxide (TATP) Precursors with an Array of Sensors Based on MoS₂/RGO Composites

Tunnel junction sensing of TATP explosive at the single-molecule level

Triacetone triperoxide (TATP) is a highly potent homemade explosive commonly used in terrorist attacks. Its detection poses a significant challenge due to its volatility, and the lack of portability of current sensing techniques. To address this issue, we propose a novel approach based on single-molecule TATP detection in the air using a device where tunneling current in N-terminated carbon-nanotubes nanogaps is measured. By employing the density functional theory combined with the non-equilibrium Green's function method, we show that current of tens of nanoamperes passes through TATP trapped in the nanogap, with a discrimination ratio of several orders of magnitude even against prevalent indoor volatile organic compounds (VOCs). This high tunneling current through TATP's highest occupied molecular orbital (HOMO) is facilitated by the strong electric field generated by N-C polar bonds at the electrode ends and by the hybridization between TATP and the electrodes, driven by oxygen atoms within the probed molecule. The application of the same principle is discussed for graphene nanogaps and break-junctions.

Recent development and trends in the detection of peroxide-based explosives

Peroxide-based explosives (PBEs) are increasingly common in criminal and terrorist activity due to their easy synthesis and high explosive power. The rise in terrorist attacks involving PBEs has heightened the importance of detecting trace amounts of explosive residue or vapors. This paper aims to provide a review on the developments of techniques and instruments for detecting PBEs over the past ten years, specifically discussing advancements in ion mobility spectrometry, ambient mass spectrometry, fluorescence techniques, colorimetric methods, and electrochemical methods. We provide examples to illustrate their evolution and focus on new strategies for improving detection performance, specifically in terms of sensitivity, selectivity, high-throughput, and wide explosives coverage. Finally, we discuss future prospects for PBE detection. It is hoped this treatment will serve as a guide to the novitiate and as aid memoire to the researchers.

Room Temperature Detection of Hydrogen Peroxide Vapor by Fe2O3:ZnO Nanograins

In this report, a Fe₂O₃:ZnO sputtering target and a nanograins-based sensor were developed for the room temperature (RT) detection of hydrogen peroxide vapor (HPV) using the solid-state reaction method and the radio frequency (RF) magnetron sputtering technique, respectively. The characterization of the synthesized sputtering target and the obtained nanostructured film was carried out by scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy-dispersive X-ray (EDX) analyses. The SEM and TEM images of the film revealed its homogeneous granular structure, with a grain size of 10-30 nm and an interplanar spacing of Fe₂O₃ and ZnO, respectively. EDX spectroscopy presented the real concentrations of Zn in the target material and in the film (21.2 wt.% and 19.4 wt.%, respectively), with a uniform distribution of O, Al, Zn, and Fe elements in the e-mapped images of the Fe₂O₃:ZnO film. The gas sensing behavior was investigated in the temperature range of 25-250 °C with regards to the 1.5-56 ppm HPV concentrations, with and without ultraviolet (UV) irradiation. The presence of UV light on the Fe₂O₃:ZnO surface at RT reduced a low detection limit from 3 ppm to 1.5 ppm, which corresponded to a response value of 12, with the sensor's response and recovery times of 91 s and 482 s, respectively. The obtained promising results are attributed to the improved characteristics of the Fe₂O₃:ZnO composite material, which will enable its use in multifunctional sensor systems and medical diagnostic devices.

Preparation and Gas Sensing Properties of Hair-Based Carbon Sheets

Waste human hair was carbonized into carbon sheets by a simple carbonization method, which was studied as gas sensing materials for the first time. The effect of carbonization temperature on the structure and gas sensing properties of hair-based carbon sheet was studied by scanning electron microscope, X-ray diffraction, infrared spectrum, Raman spectrum, and gas-sensitive tester. The results showed that the carbonization temperature had a significant effect on the structure and gas sensing performance of carbon sheets, which were doped with K, N, P, and S elements during carbonization. However, the sensor of the carbon sheet does not show good selectivity among six target gases. Fortunately, the carbon sheets prepared at different temperatures have different responses to the target gases. The sensor array constructed by the carbon sheets prepared at different temperatures can realize the discriminative detection of a variety of target gases. For the optimized carbon sheet, the theoretical limit of detection of hydrogen peroxide is 0.83 ppm. This work provides a reference for the resource utilization of waste protein and the development of gas sensors.

Wool-Based Carbon Fiber/MoS2 Composite Prepared by Low-Temperature Catalytic Hydrothermal Method and Its Application in the Field of Gas Sensors

Under the background of the Paris Agreement on reducing greenhouse gases, waste wools were converted into wool carbon fiber (WCF) and WCF–MoS₂ composites by low-temperature catalytic hydrothermal carbonization. Their structures and gas-sensing performances were studied for the first time. Due to the existence of heterojunctions, the responses of the WCF–MoS₂ composite to the five analytes were 3–400 times those of MoS₂ and 2–11 times those of WCF. Interestingly, because of the N, P, and S elements contained in wools, the WCF prepared by the hydrothermal method was realized the doping of N, P, and S, which caused the sensing curves of WCF to have different shapes for different analytes. This characteristic was also well demonstrated by the WCF–MoS₂ composite, which inspired us to realize the discriminative detection only by a single WCF–MoS₂ sensor and image recognition technology. What’s more, the WCF–MoS₂ composite also showed a high sensitivity, a high selectivity, and a rapid response to NH₃. The response time and the recovery time to 3 ppm NH₃ were about 16 and 5 s, respectively. The detection of limit of WCF–MoS₂ for NH₃ was 19.1 ppb. This work provides a new idea for the development of sensors and the resource utilization of wool waste.

Three-Dimensional MoS2/Reduced Graphene Oxide Nanosheets/Graphene Quantum Dots Hybrids for High-Performance Room-Temperature NO2 Gas Sensors

This study presents three-dimensional (3D) MoS₂/reduced graphene oxide (rGO)/graphene quantum dots (GQDs) hybrids with improved gas sensing performance for NO₂ sensors. GQDs were introduced to prevent the agglomeration of nanosheets during mixing of rGO and MoS₂. The resultant MoS₂/rGO/GQDs hybrids exhibit a well-defined 3D nanostructure, with a firm connection among components. The prepared MoS₂/rGO/GQDs-based sensor exhibits a response of 23.2% toward 50 ppm NO₂ at room temperature. Furthermore, when exposed to NO₂ gas with a concentration as low as 5 ppm, the prepared sensor retains a response of 15.2%. Compared with the MoS₂/rGO nanocomposites, the addition of GQDs improves the sensitivity to 21.1% and 23.2% when the sensor is exposed to 30 and 50 ppm NO₂ gas, respectively. Additionally, the MoS₂/rGO/GQDs-based sensor exhibits outstanding repeatability and gas selectivity. When exposed to certain typical interference gases, the MoS₂/rGO/GQDs-based sensor has over 10 times higher sensitivity toward NO₂ than the other gases. This study indicates that MoS₂/rGO/GQDs hybrids are potential candidates for the development of NO₂ sensors with excellent gas sensitivity.

The Effect of Surface Hydroxyls on the Humidity-Sensitive Properties of LiCl-Doped ZnSn(OH)6 Sphere-Based Sensors

Pure zinc hydroxystannate (ZnSn(OH)₆) and LiCl-doped ZnSn(OH)₆ have been synthesized through a facile wet chemical method. The LiCl-doped samples keep their original spherical morphology as pure ZnSn(OH)_6, with some LiCl particles stuck to its surface, providing more active sites for the adsorption and desorption of water molecules. The influence of LiCl doping on the humidity-sensing properties was explored by varying the dopant concentration. The 16 wt% LiCl/ZnSn(OH)₆ showed a better humidity-sensing performance than that of the pure ZnSn(OH)₆ and other doped samples, including a high resistive sensitivity, a relatively small hysteresis, and a fast response speed. Through the FTIR analysis, the number of hydroxyl groups on the surface structure after aging has been found to decline markedly. These hydroxyl groups provide a platform for the adsorption of water molecules on the surface and promote the dissociation of water molecules. The detriment of aging to sensor performance should not be underrated. The complex impedance spectrum explains the mechanism of the sensor. These results demonstrate that ZnSn(OH)₆ has potential application in fabricating humidity sensors, and the sensing performance of the sensor is enhanced by the dopant LiCl.

Nanoengineering Approaches Toward Artificial Nose

Significant scientific efforts have been made to mimic and potentially supersede the mammalian nose using artificial noses based on arrays of individual cross-sensitive gas sensors over the past couple decades. To this end, thousands of research articles have been published regarding the design of gas sensor arrays to function as artificial noses. Nanoengineered materials possessing high surface area for enhanced reaction kinetics and uniquely tunable optical, electronic, and optoelectronic properties have been extensively used as gas sensing materials in single gas sensors and sensor arrays. Therefore, nanoengineered materials address some of the shortcomings in sensitivity and selectivity inherent in microscale and macroscale materials for chemical sensors. In this article, the fundamental gas sensing mechanisms are briefly reviewed for each material class and sensing modality (electrical, optical, optoelectronic), followed by a survey and review of the various strategies for engineering or functionalizing these nanomaterials to improve their gas sensing selectivity, sensitivity and other measures of gas sensing performance. Specifically, one major focus of this review is on nanoscale materials and nanoengineering approaches for semiconducting metal oxides, transition metal dichalcogenides, carbonaceous nanomaterials, conducting polymers, and others as used in single gas sensors or sensor arrays for electrical sensing modality. Additionally, this review discusses the various nano-enabled techniques and materials of optical gas detection modality, including photonic crystals, surface plasmonic sensing, and nanoscale waveguides. Strategies for improving or tuning the sensitivity and selectivity of materials toward different gases are given priority due to the importance of having cross-sensitivity and selectivity toward various analytes in designing an effective artificial nose. Furthermore, optoelectrical sensing, which has to date not served as a common sensing modality, is also reviewed to highlight potential research directions. We close with some perspective on the future development of artificial noses which utilize optical and electrical sensing modalities, with additional focus on the less researched optoelectronic sensing modality.

Recent Developments in the Field of Explosive Trace Detection

Explosive trace detection (ETD) technologies play a vital role in maintaining national security. ETD remains an active research area with many analytical techniques in operational use. This review details the latest advances in animal olfactory, ion mobility spectrometry (IMS), and Raman and colorimetric detection methods. Developments in optical, biological, electrochemical, mass, and thermal sensors are also covered in addition to the use of nanomaterials technology. Commercially available systems are presented as examples of current detection capabilities and as benchmarks for improvement. Attention is also drawn to recent collaborative projects involving government, academia, and industry to highlight the emergence of multimodal screening approaches and applications. The objective of the review is to provide a comprehensive overview of ETD by highlighting challenges in ETD and providing an understanding of the principles, advantages, and limitations of each technology and relating this to current systems.

Qualitative Detection Toward Military and Improvised Explosive Vapors by a Facile TiO2 Nanosheet-Based Chemiresistive Sensor Array

A facile TiO₂ nanosheets-based chemiresistive gas sensor array was prepared to identify 11 kinds of military and improvised explosive vapors at room temperature. The morphology of TiO₂ nanosheets was well-controlled by adjusting the concentration of HF applied during the preparation. Owing to the morphology difference, the TiO₂ nanosheet-based sensors show different response values toward 11 kinds of explosives, which is the basis of the successful discriminative identification. This method owes lots of advantages over other detection techniques, such as the facile preparation procedure, high response value (115.6% for TNT and 830% for PNT) at room temperature, rapid identifying properties (within 30 s for 9 explosives), simple operation, high anti-interference property, and low probability of misinforming, and consequently has a huge potential application in the qualitative detection of explosives.

Enhanced gas sensing performance of perovskite YFe1−xMnxO3 by doping manganese ions

The gas sensitive performance of perovskite YFe_1−xMn_xO₃ can be tailored effectively by simple manganese ion doping.

Surface Persistence of Trace Level Deposits of Highly Energetic Materials

In the fields of Security and Defense, explosive traces must be analyzed at the sites of the terrorist events. The persistence on surfaces of these traces depends on the sublimation processes and the interactions with the surfaces. This study presents evidence that the sublimation process of these traces on stainless steel (SS) surfaces is very different than in bulk quantities. The enthalpies of sublimation of traces of four highly energetic materials: triacetone triperoxide (TATP), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and 1,3,5- trinitrohexahydro-s-triazine (RDX) deposited on SS substrates were determined by optical fiber coupled-grazing angle probe Fourier Transform Infrared (FTIR) Spectroscopy. These were compared with enthalpies of sublimation determined by thermal gravimetric analysis for bulk amounts and differences between them were found. The sublimation enthalpy of RDX was very different for traces than for bulk quantities, attributed to two main factors. First, the beta-RDX phase was present at trace levels, unlike the case of bulk amounts which consisted only of the alpha-RDX phase. Second, an interaction between the RDX and SS was found. This interaction energy was determined using grazing angle FTIR microscopy. In the case of DNT and TNT, bulk and traces enthalpies were statistically similar, but it is evidenced that at the level of traces a metastable phase was observed. Finally, for TATP the enthalpies were statistically identical, but a non-linear behavior and a change of heat capacity values different from zero was found for both trace and bulk phases.