Oligomer-coated carbon nanotube chemiresistive sensors for selective detection of nitroaromatic explosives

Flexible Phenanthracene Nanotubes for Explosive Detection

Phenanthracene nanotubes with arylene-ethynylene-butadiynylene rims and phenanthracene walls are synthesized in a modular bottom-up approach. One of the rims carries hexadecyloxy side chains, mediating the affinity to highly oriented pyrolytic graphite. Molecular dynamics simulations show that the nanotubes are much more flexible than their structural formulas suggest: In 12, the phenanthracene units act as hinges that flip the two macrocycles relative to each other to one of two possible sites, as quantum mechanical models suggest and scanning tunneling microscopy investigations prove. Unexpectedly, both theory and experiment show for 13 that the three phenanthracene hinges are deflected from the upright position, accompanied by a deformation of both macrocycles from their idealized sturdy macroporous geometry. This flexibility together with their affinity to carbon-rich substrates allows for an efficient host-guest chemistry at the solid/gas interface opening the potential for applications in single-walled carbon nanotube-based sensing, and the applicability to build new sensors for the detection of 2,4,6-trinitrotoluene via nitroaromatic markers is shown.

A luminescent Cu4 cluster film grown by electrospray deposition: a nitroaromatic vapour sensor

We present the fabrication and use of a film of a carborane-thiol-protected tetranuclear copper cluster with characteristic orange luminescence using ambient electrospray deposition (ESD). Charged microdroplets of the clusters produced by an electrospray tip deposit the clusters at an air-water interface to form a film. Different microscopic and spectroscopic techniques characterized the porous surface structure of the film. Visible and rapid quenching of the emission of the film upon exposure to 2-nitrotoluene (2-NT) vapours under ambient conditions was observed. Density functional theory (DFT) calculations established the favourable binding sites of 2-NT with the cluster. Desorption of 2-NT upon heating recovered the original luminescence, demonstrating the reusability of the sensor. Stable emission upon exposure to different organic solvents and its quenching upon exposure to 2,4-dinitrotoluene and picric acid showed selectivity of the film to nitroaromatic species.

Electrical properties and chemiresistive response to 2,4,6 trinitrotoluene vapours of large area arrays of Ge nanowires

We study the electrical and morphological properties of random arrays of Ge nanowires (NW) deposited on sapphire substrates. NW-based devices were fabricated with the aim of developing chemiresistive-type sensors for the detection of explosive vapours. We present the results obtained on pristine and annealed NWs and, focusing on the different phenomenology observed, we discuss the critical role played by NW-NW junctions on the electrical conduction and sensing performances. A mechanism is proposed to explain the high efficiency of the annealed arrays of NWs in detecting 2,4,6 trinitrotoluene vapours. This study shows the promising potential of Ge NW-based sensors in the field of civil security.

Detection of Nitroaromatic Explosives in Air by Amino-Functionalized Carbon Nanotubes

Nitroaromatic explosives are the most common explosives, and their detection is important to public security, human health, and environmental protection. In particular, the detection of solid explosives through directly revealing the presence of their vapors in air would be desirable for compact and portable devices. In this study, amino-functionalized carbon nanotubes were used to produce resistive sensors to detect nitroaromatic explosives by interaction with their vapors. Devices formed by carbon nanotube networks working at room temperature revealed trinitrotoluene, one of the most common nitroaromatic explosives, and di-nitrotoluene-saturated vapors, with reaction and recovery times of a few and tens of seconds, respectively. This type of resistive device is particularly simple and may be easily combined with low-power electronics for preparing portable devices.

Eu-Doped MOF-based high-efficiency fluorescent sensor for detecting 2,4-dinitrophenol and 2,4,6-trinitrophenol simultaneously

Nitroaromatic explosives pose a great threat to the environment and human safety. It is very important to design simple, highly efficient and multifunctional sensors for detecting nitroaromatic explosives. However, a few sensors can determine multicomponent nitroaromatic explosives simultaneously. Eu functionalized MOF-253 (Eu@MOF-253) hybrid material was synthesized using the post-synthetic modification method. The introduction of Eu³⁺ in MOF-253 caused the fluorescence peak of the ligand to show a distinct red-shift due to its polarization enhancement effect in the presence of 2,4-DNP. The emission and excitation spectra of the Eu@MOF-253 sensor showed overlap with the ultraviolet-visible (UV-vis) absorption spectra of the representative nitroaromatic explosives 2,4-dinitrophenol (2,4-DNP) and 2,4,6-trinitrophenol (TNP). Therefore, it is feasible to discriminate and quantify TNP and 2,4-DNP simultaneously. As proposed, the Eu@MOF-253 luminescent sensor was highly sensitive and selective towards TNP and 2,4-DNP. The other coexisting nitroaromatic explosives did not interfere with the determination. Upon addition of TNP, the fluorescence of the Eu@MOF-253 sensor decreased dramatically and showed an excellent quenching constant (K_sv) of 1.58 × 10⁶. The fluorescence intensities of the Eu@MOF-253 sensor presented good linear relationships with concentrations of TNP and 2,4-DNP ranging from 0.01-100 μM and 0.01-25 μM, respectively. Low limits of detection (LOD) for both 2,4-DNP and TNP were approximately 10 nM. The determination mechanism is mainly ascribed to the internal filtration effect (IFE) and electron transfer. This work provides a practical method for the highly efficient determination of nitroaromatic explosives.

Exploring the performance of a functionalized CNT-based sensor array for breathomics through clustering and classification algorithms: from gas sensing of selective biomarkers to discrimination of chronic obstructive pulmonary disease

An array of carbon nanotube (CNT)-based sensors was produced for sensing selective biomarkers and evaluating breathomics applications with the aid of clustering and classification algorithms. We assessed the sensor array performance in identifying target volatiles and we explored the combination of various classification algorithms to analyse the results obtained from a limited dataset of exhaled breath samples. The sensor array was exposed to ammonia (NH₃), nitrogen dioxide (NO₂), hydrogen sulphide (H₂S), and benzene (C₆H₆). Among them, ammonia (NH₃) and nitrogen dioxide (NO₂) are known biomarkers of chronic obstructive pulmonary disease (COPD). Calibration curves for individual sensors in the array were obtained following exposure to the four target molecules. A remarkable response to ammonia (NH₃) and nitrogen dioxide (NO₂), according to benchmarking with available data in the literature, was observed. Sensor array responses were analyzed through principal component analysis (PCA), thus assessing the array selectivity and its capability to discriminate the four different target volatile molecules. The sensor array was then exposed to exhaled breath samples from patients affected by COPD and healthy control volunteers. A combination of PCA, supported vector machine (SVM), and linear discrimination analysis (LDA) shows that the sensor array can be trained to accurately discriminate healthy from COPD subjects, in spite of the limited dataset.

Extensive application of clustering and classification algorithms shows the potential of a CNT-based sensor array in breathomics.

Empirical determination of explosive vapor transport efficiencies

The transport efficiency of 2,4-dinitrotoluene (2,4-DNT), 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitro-1,3,5-triazinane (RDX) trace vapors through tubing materials that commonly constitute vapor handling infrastructures have been determined for a variety of tubing dimensions and sampling conditions. Using a programmable temperature vaporization inlet coupled with a gas chromatography mass spectrometer (PTV-GC-MS), the explosive vapors were quantified both with and without a length of tubing of a specific material in the sampling flow path. At vapor temperatures of 30 °C and 66 °C, minimal attenuations were observed for 2,4-DNT and TNT vapor concentrations when the tubing material was in-line with the sampling flow path, indicating that the transport is largely unaffected by interactions with the surface of the tubing materials. In contrast, RDX vapors showed large attenuations as a function of both sampling conditions and tubing materials/dimensions. For those experiments where attenuated RDX vapor transport was observed, the mass sequestered by interactions between the flowing vapor and the internal tubing surface was determined to be in the range of tens to hundreds of picograms. Of all the materials examined for RDX transport, fluorinated ethylene propylene (FEP) tubing resulted in the least amount of mass loss to surface interactions, with vapor transport efficiencies (VTEs) between 95-100%. However, for some materials, the combination of tubing dimensions and sampling conditions resulted in no RDX transport, even after sampling more than 250.0 L of vapor through the tubing.

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.

Development of a Sensing Array for Human Breath Analysis Based on SWCNT Layers Functionalized with Semiconductor Organic Molecules

A sensor array based on heterojunctions between semiconducting organic layers and single walled carbon nanotube (SWCNT) films is produced to explore applications in breathomics, the molecular analysis of exhaled breath. The array is exposed to gas/volatiles relevant to specific diseases (ammonia, ethanol, acetone, 2-propanol, sodium hypochlorite, benzene, hydrogen sulfide, and nitrogen dioxide). Then, to evaluate its capability to operate with real relevant biological samples the array is exposed to human breath exhaled from healthy subjects. Finally, to provide a proof of concept of its diagnostic potential, the array is exposed to exhaled breath samples collected from subjects with chronic obstructive pulmonary disease (COPD), an airway chronic inflammatory disease not yet investigated with CNT-based sensor arrays, and breathprints are compared with those obtained from of healthy subjects. Principal component analysis shows that the sensor array is able to detect various target gas/volatiles with a clear fingerprint on a 2D subspace, is suitable for breath profiling in exhaled human breath, and is able to distinguish subjects with COPD from healthy subjects based on their breathprints. This classification ability is further improved by selecting the most responsive sensors to nitrogen dioxide, a potential biomarker of COPD.

Electrochemical determination of nitroaromatic explosives at boron-doped diamond/graphene nanowall electrodes: 2,4,6-trinitrotoluene and 2,4,6-trinitroanisole in liquid effluents

The study is devoted to the electrochemical detection of trace explosives on boron-doped diamond/graphene nanowall electrodes (B:DGNW). The electrodes were fabricated in a one-step growth process using chemical vapour deposition without any additional modifications. The electrochemical investigations were focused on the determination of the important nitroaromatic explosive compounds, 2,4,6-trinitrotoluene (TNT) and 2,4,6-trinitroanisole (TNA). The distinct reduction peaks of both studied compounds were observed regardless of the pH value of the solution. The reduction peak currents were linearly related to the concentration of TNT and TNA in the range from 0.05-15 ppm. Nevertheless, two various linear trends were observed, attributed respectively to the adsorption processes at low concentrations up to the diffusional character of detection for larger contamination levels. The limit of detection of TNT and TNA is equal to 73 ppb and 270 ppb, respectively. Moreover, the proposed detection strategy has been applied under real conditions with a significant concentration of interfering compounds - in landfill leachates. The proposed bare B:DGNW electrodes were revealed to have a high electroactive area towards the voltammetric determination of various nitroaromatic compounds with a high rate of repeatability, thus appearing to be an attractive nanocarbon surface for further applications.

Solid State Electronic Sensors for Detection of Carbon Dioxide

Detection of carbon dioxide (CO₂) is very important for environmental, health, safety and space applications. We have studied novel multiwall carbon nanotubes (MWCNTs) and an iron oxide (Fe₂O₃) nanocomposite based chemiresistive sensor for detection of CO₂ at room temperature. The sensor has been miniaturized to a chip size (1 cm × 2 cm). Good sensing performance was observed with a wide detection range of CO₂ concentrations (100–6000 ppm). Structural properties of the sensing materials were characterized using Field-Emission Scanning Electron Microscopy, Fourier-Transform Infrared and Raman spectroscopies. The greatly improved sensitivity of the composite materials to CO₂ can be attributed to the formation of a depletion layer at the p-n junction in an MWCNT/iron oxide heterostructure, and new CO₂ gas molecules adhere to the high surface area of MWCNTs due to the concentration gradient. The test results showed that the CO₂ sensor possesses fast response, compact size, ultra-low power consumption, high sensitivity and wide dynamic detection range.

Carbon Nanotube Chemical Sensors

Carbon nanotubes (CNTs) promise to advance a number of real-world technologies. Of these applications, they are particularly attractive for uses in chemical sensors for environmental and health monitoring. However, chemical sensors based on CNTs are often lacking in selectivity, and the elucidation of their sensing mechanisms remains challenging. This review is a comprehensive description of the parameters that give rise to the sensing capabilities of CNT-based sensors and the application of CNT-based devices in chemical sensing. This review begins with the discussion of the sensing mechanisms in CNT-based devices, the chemical methods of CNT functionalization, architectures of sensors, performance parameters, and theoretical models used to describe CNT sensors. It then discusses the expansive applications of CNT-based sensors to multiple areas including environmental monitoring, food and agriculture applications, biological sensors, and national security. The discussion of each analyte focuses on the strategies used to impart selectivity and the molecular interactions between the selector and the analyte. Finally, the review concludes with a brief outlook over future developments in the field of chemical sensors and their prospects for commercialization.

Bias- and Gate-Tunable Gas Sensor Response Originating from Modulation in the Schottky Barrier Height of a Graphene/MoS2 van der Waals Heterojunction

We report on the gas-sensing characteristics of a van der Waals heterojunction consisting of graphene and a MoS₂ flake. To extract the response actually originating from the heterojunction area, the other gas-sensitive parts were passivated by gas barrier layers. The graphene/MoS₂ heterojunction device demonstrated a significant change in resistance, by a factor of greater than 10³, upon exposure to 1 ppm NO₂ under a reverse-bias condition, which was revealed to be a direct reflection of the modulation of the Schottky barrier height at the graphene/MoS₂ interface. The magnitude of the response demonstrated strong dependences on the bias and back-gate voltages. The response further increased with increasing reverse bias. Conversely, it dramatically decreased when measured at a large forward bias or a large positive back-gate voltage. These behaviors were analyzed using a metal-semiconductor-metal diode model consisting of graphene/MoS₂ and counter Ti/MoS₂ Schottky diodes.

Orthogonal Ambipolar Semiconductors with Inherently Multi-Dimensional Responses for the Discriminative Sensing of Chemical Vapors

Numerous examples of field-effect transistor (FET) biosensors and chemical sensors with good sensitivity and selectivity have now been developed. However, effectively discriminating between analytes has required either the use of receptors that selectively bind specific analytes or the fabrication of an array of sensors with varying but nonspecific responses. Both approaches exhibit significant limitations. In the first case, it can be difficult to design sufficiently specific receptors for many compounds, whereas the number of receptors required scales with the number of analytes to be detected, making it impractical to recognize many different compounds. In the second case, existing approaches to FET sensor arrays are generally material-inefficient and provide modest sensitivity. Here, we demonstrate that orthogonal ambipolar semiconductors consisting of semiconducting p-type polymers and n-type small-molecule nanowires with perpendicular in-plane orientations provide a platform with high sensitivity and inherently multi-dimensional response. This allows for discrimination between even closely related derivatives such as aromatic isomers and n-alkyl alcohols varying in length by a single carbon atom resolution using only a single sensor element.

Carbon Nanotube-Based Chemiresistive Sensors

The development of simple and low-cost chemical sensors is critically important for improving human life. Many types of chemical sensors have been developed. Among them, the chemiresistive sensors receive particular attention because of their simple structure, the ease of high precise measurement and the low cost. This review mainly focuses on carbon nanotube (CNT)-based chemiresistive sensors. We first describe the properties of CNTs and the structure of CNT chemiresistive sensors. Next, the sensing mechanism and the performance parameters of the sensors are discussed. Then, we detail the status of the CNT chemiresistive sensors for detection of different analytes. Lastly, we put forward the remaining challenges for CNT chemiresistive sensors and outlook the possible opportunity for CNT chemiresistive sensors in the future.

Graphene based nanosensor for aqueous phase detection of nitroaromatics

Nitroaromatics sensor composed of monolayer graphene and molecularly imprinted chitosan thin film was fabricated and responded selectively against imprinted nitrotriazolone.

Factors affecting the intramolecular decomposition of hexamethylene triperoxide diamine and implications for detection

Hexamethylene triperoxide diamine (HMTD) is an easily synthesized and highly sensitive organic peroxide frequently used as a primary explosive. The vapor pressure of HMTD is very low, impeding vapor detection, especially when compared to other peroxide explosives, such as triacetone triperoxide (TATP) or diacetone diperoxide (DADP). Despite this fact, HMTD has a perceptible odor that could be utilized in the indirect detection of HMTD vapor. Headspace measurements above solid HMTD samples confirm that HMTD readily decomposes under ambient conditions to form highly volatile products that include formic acid, ammonia, trimethylamine and formamides. The presence and quantity of these compounds are affected by storage condition, time, and synthetic method, with synthetic method having the most significant effect on the content of the headspace. A kinetic study of HMTD decomposition in solution indicated a correlation between degradation rate and the presence of decomposition species identified in the headspace, and provided further insight into the mechanism of decomposition. The study provided evidence for a proton assisted decomposition reaction with water, as well as an intramolecular decomposition process facilitated by the presence of water.

Carbon Nanotube-Based Chemical Sensors

The need to sense gases and vapors arises in numerous scenarios in industrial, environmental, security and medical applications. Traditionally, this activity has utilized bulky instruments to obtain both qualitative and quantitative information on the constituents of the gas mixture. It is ideal to use sensors for this purpose since they are smaller in size and less expensive; however, their performance in the field must match that of established analytical instruments in order to gain acceptance. In this regard, nanomaterials as sensing media offer advantages in sensitivity, preparation of chip-based sensors and construction of electronic nose for selective detection of analytes of interest. This article provides a review of the use of carbon nanotubes in gas and vapor sensing.

Highly Photoluminescent Molybdenum Oxide Quantum Dots: One-Pot Synthesis and Application in 2,4,6-Trinitrotoluene Determination

As a well-studied transition-metal semiconductor material, MoOx has a wider band gap than molybdenum disulfide (MoS2), and its property varies dramatically for the existence of several different allotropes and suboxide phases of molybdenum oxides (MoOx, x < 3). In this manuscript, a one-pot method possessing the advantages of one pot, easily prepared, rapid, and environmentally friendly, has been developed for facile synthesis of highly photoluminescent MoOx quantum dots (MoOx QDs), in which commercial molybdenum disulfide (MoS2) powder and hydrogen peroxide (H2O2) are employed as the precursor and oxidant, respectively. The obtained MoOx QDs can be further utilized as an efficient photoluminescent probe, and a new turn-off sensor is developed for 2,4,6-trinitrotoluene (TNT) determination based on the fact that the photoluminescence of MoOx QDs can be quenched by the Meisenheimer complexes formed in the strong alkali solution through the inner filter effect (IFE). Under the optimal conditions, the decreased photoluminescence of MoOx QDs shows a good linear relationship to the concentration of TNT ranging from 0.5 to 240.0 μM, and the limit of detection was 0.12 μM (3σ/k). With the present turn-off sensor, TNT in river water samples can be rapidly and selectively detected without tedious sample pretreatment processes.