Colloidal Quantum Dots for Explosive Detection: Trends and Perspectives

Sensitive, accurate, and reliable detection of explosives has become one of the major needs for international security and environmental protection. Colloidal quantum dots, because of their unique chemical, optical, and electrical properties, as well as easy synthesis route and functionalization, have demonstrated high potential to meet the requirements for the development of suitable sensors, boosting the research in the field of explosive detection. Here, we critically review the most relevant research works, highlighting three different mechanisms for explosive detection based on colloidal quantum dots, namely photoluminescence, electrochemical, and chemoresistive sensing. We provide a comprehensive overview and an extensive discussion and comparison in terms of the most relevant sensor parameters. We highlight advantages, limitations, and challenges of quantum dot-based explosive sensors and outline future research directions for the advancement of knowledge in this surging research field.

Green synthesis of Ag nanoparticle supported on graphene oxide for efficient nitrite sensing in a water sample

Water is essential for conserving biodiversity, ecology, and human health, but because of population growth and declining clean water supplies, wastewater must be treated to meet demand. Nitrite is one of the contaminants in wastewater that is well-known. It is crucial to identify nitrite since it can be fatal to humans in excessive doses. Utilizing a straightforward and effective electrochemical sensor, nitrite in actual water samples may be determined electrochemically. The sensor is created by coating the surface of a GC electrode with a thin layer of graphene oxide (GO), followed by a coating of silver nanoparticles. The modified electrode reached a linear detection range of 1-400 µM. thus, the activity of the electrode was investigated at different pH values ranging from 4 to 10 to cover acidic to highly basic environments. However, the electrode recorded limit of detection (LOD) is equal to 0.084, 0.090, and 0.055 µM for pH 4, 7, and 10, respectively. Additionally, the electrode activity was utilized in tap water and wastewater that the LOD reported as 0.16 and 0.157 µM for tape water and wastewater, respectively.

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.

Impedimetric detection of 2,4,6-trinitrotoluene using surface-functionalized halloysite nanotubes

Herein, we report the application of amine-surface-functionalized halloysite nanotubes (HAs) as active materials for the quantitative detection of 2,4,6-trinitrotoluene (TNT). The findings indicated that HA could selectively capture TNT via a strong reaction between the amine groups on its surface and the TNT molecules. Plate electrodes were fabricated from HA to evaluate its TNT-sensing capacity by electrochemical impedance spectroscopy. Upon binding with TNT, the proton conductivity on the HA plate electrodes increased linearly with the TNT concentration from 1.0 × 10⁻¹¹ M to 1.0 × 10⁻⁴ M. The HA plate electrodes exhibited good sensitivity with a detection limit of 1.05 × 10⁻¹² M. Subsequently, the cycling measurements of the TNT binding/removal were performed on the HA plate electrode, and the material exhibited high stability, good regenerative ability, and good reversibility without a significant decrease in efficiency. The present work highlights the significant application potential of HAs for the electrochemical detection of TNT.

Amine-surface-functionalized halloysite nanotubes are used for electrochemical sensing TNT.

All-printed soft human-machine interface for robotic physicochemical sensing

Ultrasensitive multimodal physicochemical sensing for autonomous robotic decision-making has numerous applications in agriculture, security, environmental protection, and public health. Previously reported robotic sensing technologies have primarily focused on monitoring physical parameters such as pressure and temperature. Integrating chemical sensors for autonomous dry-phase analyte detection on a robotic platform is rather extremely challenging and substantially underdeveloped. Here, we introduce an artificial intelligence-powered multimodal robotic sensing system (M-Bot) with an all-printed mass-producible soft electronic skin-based human-machine interface. A scalable inkjet printing technology with custom-developed nanomaterial inks was used to manufacture flexible physicochemical sensor arrays for electrophysiology recording, tactile perception, and robotic sensing of a wide range of hazardous materials including nitroaromatic explosives, pesticides, nerve agents, and infectious pathogens such as SARS-CoV-2. The M-Bot decodes the surface electromyography signals collected from the human body through machine learning algorithms for remote robotic control and can perform in situ threat compound detection in extreme or contaminated environments with user-interactive tactile and threat alarm feedback. The printed electronic skin-based robotic sensing technology can be further generalized and applied to other remote sensing platforms. Such diversity was validated on an intelligent multimodal robotic boat platform that can efficiently track the source of trace amounts of hazardous compounds through autonomous and intelligent decision-making algorithms. This fully printed human-machine interactive multimodal sensing technology could play a crucial role in designing future intelligent robotic systems and can be easily reconfigured toward numerous practical wearable and robotic applications.

Dummy Molecularly Imprinted Polymers Using DNP as a Template Molecule for Explosive Sensing and Nitroaromatic Compound Discrimination

This work reports a rapid, simple and low-cost voltammetric sensor based on a dummy molecularly imprinted polymer (MIP) that uses 2,4-dinitrophenol (DNP) as a template for the quantification of 2,4,6-trinitrotoluene (TNT) and DNP, and the identification of related substances. Once the polymer was synthesised by thermal precipitation polymerisation, it was integrated onto a graphite epoxy composite (GEC) electrode via sol–gel immobilisation. Scanning electron microscopy (SEM) was performed in order to characterise the polymer and the sensor surface. Responses towards DNP and TNT were evaluated, displaying a linear response range of 1.5 to 8.0 µmol L−1 for DNP and 1.3 to 6.5 µmol L−1 for TNT; the estimated limits of detection were 0.59 µmol L−1 and 0.29 µmol L−1, for DNP and TNT, respectively. Chemometric tools, in particular principal component analysis (PCA), demonstrated the possibilities of the MIP-modified electrodes in nitroaromatic and potential interfering species discrimination with multiple potential applications in the environmental field.

Electroanalysis and Spectroelectrochemistry of Nonaromatic Explosives in Acetonitrile Containing Dissolved Oxygen

In search of a rapid, low-cost, and solution-phase detection technique for explosives, the (spectro-)electrochemistry of compounds from two major nonaromatic classes, namely nitramines (RDX and HMX) and nitrate esters (pentaerythritol tetranitrate (PETN) and the plastic explosive composite Semtex 1A) in acetonitrile (AN) is reported. In electrochemical screening, 5 μg of explosive material was detectable in 10 s by multicomponent cyclic voltammetric (CV) analysis on unmodified glassy carbon under ubiquitous environmental influences (i.e., trace water and dissolved oxygen). The explosives were identified with high recoveries under a battery of proof-of-concept testing scenarios in various matrices. In AN containing naturally dissolved oxygen (approx. 2 mM), the superoxide radical is co-electrogenerated during analyte reduction. Free superoxide yields prominent signals that the explosives attenuate quantitatively. To gain further insight into the electrochemical transformation mechanism, spectroelectrochemistry was employed to monitor changes in ultraviolet (UV) absorbance during CV and identify transient intermediates and product species, which could be targeted by future chemical sensors. Overlapping UV spectra of multiple species are deconvoluted using a new strategy, spectral regional baselining, for time- and potential-resolved spectroelectrochemical (SEC) analysis. This study shows that dissolved oxygen, hitherto an interferent purposefully removed from the solution, can be exploited advantageously in electrochemical sensing. The work expands our understanding of high-explosive solution-phase chemistry and offers a novel route to signal transduction for the sensing of energetic materials.

p-Pyridine BODIPY-based fluorescence probe for highly sensitive and selective detection of picric acid

A novel fluorescence probe p-PBP for PA was synthesized based on a basic N atom as the electronic donor. The probe could detect PA over TNT, CE, PETN, RDX, HMX, NB, NT, DNT, NP, DNP, and common inorganic explosive ions (K⁺, Ba²⁺, NH₄⁺, NO₃^-, ClO₃^-, and ClO₄^-), and common ions (Na⁺, Ca²⁺, and Mg²⁺) with high selectivity. The fluorescence quenching was attributed to the photo-induced electron transfer (PET) processes from the excited state of p-PBP to the ground state PA. The detection limit of probe p-PBP for PA was as low as 13.06 nmol/L, which is far lower than the concentration stipulated by the Environmental quality standards for surface water. The response time was less than 30 s. Hence, the fluorescence probe p-PBP was successfully developed to detect the concentration level of PA in real samples, which would provide a novel quantitative analysis method of PA in forensic science.

A 3D-Printed Hybrid Nasal Cartilage with Functional Electronic Olfaction

Advances in biomanufacturing techniques have opened the doors to recapitulate human sensory organs such as the nose and ear in vitro with adequate levels of functionality. Such advancements have enabled simultaneous targeting of two challenges in engineered sensory organs, especially the nose: i) mechanically robust reconstruction of the nasal cartilage with high precision and ii) replication of the nose functionality: odor perception. Hybrid nasal organs can be equipped with remarkable capabilities such as augmented olfactory perception. Herein, a proof-of-concept for an odor-perceptive nose-like hybrid, which is composed of a mechanically robust cartilage-like construct and a biocompatible biosensing platform, is proposed. Specifically, 3D cartilage-like tissue constructs are created by multi-material 3D bioprinting using mechanically tunable chondrocyte-laden bioinks. In addition, by optimizing the composition of stiff and soft bioinks in macro-scale printed constructs, the competence of this system in providing improved viability and recapitulation of chondrocyte cell behavior in mechanically robust 3D constructs is demonstrated. Furthermore, the engineered cartilage-like tissue construct is integrated with an electrochemical biosensing system to bring functional olfactory sensations toward multiple specific airway disease biomarkers, explosives, and toxins under biocompatible conditions. Proposed hybrid constructs can lay the groundwork for functional bionic interfaces and humanoid cyborgs.

Direct and Selective Electrochemical Vapor Trace Detection of Organic Peroxide Explosives via Surface Decoration

The ability to detect traces of highly energetic explosive materials sensitively, selectively, accurately, and rapidly could be of enormous benefit to civilian national security, military applications, and environmental monitoring. Unfortunately, the detection of explosives still poses a largely unmet arduous analytical problem, making their detection an issue of burning immediacy and a massive current challenge in terms of research and development. Although numerous explosive detection approaches have been developed, these methods are usually time-consuming, require bulky equipment, tedious sample preparation, a trained operator, cannot be miniaturized, and lack the ability to perform automated real-time high-throughput analysis, strongly handicapping their mass deployment. Here, we present the first demonstration of the "direct" electrochemical approach for the sensitive, selective, and rapid vapor trace detection of TATP and HMTD, under ambient conditions, unaffected by the presence of oxygen and hydrogen peroxide species, down to concentrations lower than 10 ppb. The method is based on the use of Ag-nanoparticles-decorated carbon microfibers air-collecting electrodes (μCF), which allow for the selective direct detection of the organic peroxide explosives, through opening multiple redox routes, not existent in the undecorated carbon electrodes. Finally, we demonstrate the direct and rapid detection of TATP and HMTD explosive species from real-world air samples.

Electrochemical Detection of Explosive Compounds in an Ionic Liquid in Mixed Environments: Influence of Oxygen, Moisture, and Other Nitroaromatics on the Sensing Response

From a security point of view, detecting and quantifying explosives in mixed environments is required to identify potentially concealed explosives. Electrochemistry offers a viable method to detect nitroaromatic explosive compounds owing to the presence of easily reducible nitro groups that give rise to a current signal. However, their reduction potentials can overlap with interfering species, making it difficult to distinguish particular compounds. We have therefore examined the effect of oxygen, moisture, and other nitroaromatic species on the cyclic voltammetry and square wave voltammetry of nitroaromatic compounds of a range of mixed environments, focussing on 2,4,6-trinitrotoluene (TNT) and 2,4-dinitrotoluene (DNT) as model analytes, and using the hydrophobic room-temperature ionic liquid (RTIL) [P14,6,6,6][NTf2] as the solvent. Oxygen (0–20% vol.) minimally affected the current of the first reduction peak of TNT in [P14,6,6,6][NTf2], but significantly affects the current for DNT. The impact of water (0 to 86% relative humidity), however, was much more dramatic – even in the hydrophobic RTIL, water significantly affected the currents of the analyte peaks for TNT and DNT, and gave rise to additional reduction features, further contributing to the current. Additionally, the voltammetry of other related di- and tri-nitro compounds (2,6-dinitrotoluene, 1,3-dinitrobenzene, 2,4,6-trinitrotoluene, 1,3,5-trinitrobenzene, and musk xylene) was also studied to understand how different substituents on the aromatic ring may affect the reduction potentials. A 50:50 mixture of TNT and DNT revealed that both analytes could be separately identified and quantified using square wave voltammetry. Overall, this information is useful in determining the effect of other species on the current signals of electrochemical explosive sensors, and reveals that it may be necessary to dry the aprotic RTIL electrolyte when used in humid environments.

Two-Dimensional Electrochemiluminescence on Porous Silicon Platform for Explosive Detection and Discrimination

This work established a rapid and sensitive explosive detection and recognition technique. We report a two-dimensional electrochemiluminescence (2-D ECL) method based on porous silicon (pSi) by monitoring the dynamic change in peak position and peak intensity of pSi-ECL. Gold nanoparticles (AuNPs) were deposited on the pSi surface to promote the electrochemical reaction and electron transfer efficiency at the pSi-electrolyte interface. The 2-D ECL can effectively detect and discriminate different classes of explosives including nitro compounds, peroxides with nitrogen atoms, and peroxides without nitrogen atoms due to their different oxidation and electron transfer ability.

A Comparison of ACQ, AIE and AEE-Based Polymers Loaded on Polyurethane Foams as Sensors for Explosives Detection

An aggregation-caused quenching (ACQ)-active polymer (PF), an aggregation-induced emission (AIE)-active polymer (PFTPE) and an aggregation-enhanced emission (AEE)-active polymer (PTTPE) were synthesized by tetraphenylethane (TPE), fluorene and thiophene moieties. Polyurethane (PU) foams modified by PF, PFTPE and PTTPE, namely PU-PF, PU-PFTPE and PU-PTTPE, using ultrasonication-assisted method have been prepared. A comparative study of PU-PF, PU-PFTPE and PU-PTTPE for detection explosives had been performed, and significant fluorescence quenching was observed with the introduction of PA solutions. The as-prepared PU-PF, PU-PFTPE and PU-PTTPE sensors exhibited a superior sensitivity for PA solutions with different concentrations. Remarkably, PU-PF gave a quenching efficiency of 96.2%, higher than 93.5% for PU-PFTPE and 86.7% for PU-PTTPE at a PA concentration of 180 µg·mL⁻¹ in methanol, which was attributed to the effective energy transfer from the fluorophore (PF) to the nitro explosive (PA). This suggested that some ACQ polymers, applied to detect explosives, could afford better performances than AIE or AEE polymers through modification of structures and selection of adequate carriers. At the same time, these chemical sensors can be recycled many times.

Electrochemical Enzyme Biosensors Revisited: Old Solutions for New Problems

Worldwide legislation is driving the development of novel and highly efficient analytical tools for assessing the composition of every material that interacts with Consumers or Nature. The biosensor technology is one of the most active R&D domains of Analytical Sciences focused on the challenge of taking analytical chemistry to the field. Electrochemical biosensors based on redox enzymes, in particular, are highly appealing due to their usual quick response, high selectivity and sensitivity, low cost and portable dimensions. This review paper aims to provide an overview of the most important advances made in the field since the proposal of the first biosensor, the well-known hand-held glucose meter. The first section addresses the current needs and challenges for novel analytical tools, followed by a brief description of the different components and configurations of biosensing devices, and the fundamentals of enzyme kinetics and amperometry. The following sections emphasize on enzyme-based amperometric biosensors and the different stages of their development.

Towards the fluorogenic detection of peroxide explosives through host-guest chemistry

Two dansyl-modified β-cyclodextrin derivatives (1 and 2) have been synthesized as host-guest sensory systems for the direct fluorescent detection of the peroxide explosives diacetone diperoxide (DADP) and triacetone triperoxide (TATP) in aqueous media. The sensing is based on the displacement of the dansyl moiety from the cavity of the cyclodextrin by the peroxide guest resulting in a decrease of the intensity of the fluorescence of the dye. Both systems showed similar fluorescent responses and were more sensitive towards TATP than DADP.

Paper-Based Electrochemical Detection of Chlorate

We describe the use of a paper-based probe impregnated with a vanadium-containing polyoxometalate anion, [PMo₁₁VO₄₀]⁵⁻, on screen-printed carbon electrodes for the electrochemical determination of chlorate. Cyclic voltammetry (CV) and chronocoulometry were used to characterize the ClO₃⁻ response in a pH = 2.5 solution of 100 mM sodium acetate. A linear CV current response was observed between 0.156 and 1.25 mg/mL with a detection limit of 0.083 mg/mL (S/N > 3). This performance was reproducible using [PMo₁₁VO₄₀]⁵⁻-impregnated filter paper stored under ambient conditions for as long as 8 months prior to use. At high concentration of chlorate, an additional catalytic cathodic peak was seen in the reverse scan of the CVs, which was digitally simulated using a simple model. For chronocoulometry, the charge measured after 5 min gave a linear response from 0.625 to 2.5 mg/mL with a detection limit of 0.31 mg/mL (S/N > 3). In addition, the slope of charge vs. time also gave a linear response. In this case the linear range was from 0.312 to 2.5 mg/mL with a detection limit of 0.15 mg/mL (S/N > 3). Simple assays were conducted using three types of soil, and recovery measurements reported.

Electrochemical Reduction of 2,4-Dinitrotoluene in Room Temperature Ionic Liquids: A Mechanistic Investigation

The reduction mechanism of 2,4-dinitrotoluene (DNT) has been studied in eight room temperature ionic liquids (RTILs) using cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry, and digital simulation. Two distinctive peaks are observed in the voltammetry, corresponding to the stepwise reduction of the two nitro groups on the aromatic ring. Diffusion coefficients (D) and electron counts (n) were calculated from chronoamperometric transients, revealing an electron count of one in most RTILs, and a linear relationship between D and the inverse of viscosity. Focusing on the first reduction only, the peak appears to be chemically reversible at low concentrations. However, as the concentration increases, the current of the reverse peak diminishes, suggesting that one or more chemical steps occur after the electrochemical step. The results from digital simulation of the CVs in one of the RTILs reveal that the most likely mechanism involves a deprotonation of the methyl group of a parent DNT molecule by the electrogenerated radical anion and/or a dimerisation of two electrogenerated radical anions. Elucidation of the reduction mechanism of DNT (and other explosives) is vital if electrochemical techniques are to be employed to detect these types of compounds in the field.

Electronic Tongue-A Tool for All Tastes?

Electronic tongue systems are traditionally used to analyse: food products, water samples and taste masking technologies for pharmaceuticals. In principle, their applications are almost limitless, as they are able to almost completely reduce the impact of interferents and can be applied to distinguish samples of extreme complexity as for example broths from different stages of fermentation. Nevertheless, their applications outside the three principal sample types are, in comparison, rather scarce. In this review, we would like to take a closer look on what are real capabilities of electronic tongue systems, what can be achieved using mixed sensor arrays and by introduction of biosensors or molecularly imprinted polymers in the matrix. We will discuss future directions both in the sense of applications as well as system development in the ever-growing trend of low cost analysis.

Design, synthesis and properties of a reactive chromophoric/fluorometric probe for hydrogen peroxide detection

A succinct chromophoric/fluorometric probe, AVPM, for sensitive and selective H₂O₂detection.

Dual-opposite injection capillary electrophoresis: Principles and misconceptions

Dual-opposite injection capillary electrophoresis (DOI-CE) is a separation technique that utilizes both ends of the capillary for sample introduction. The electroosmotic flow (EOF) is suppressed to allow all ions to reach the detector quickly. Depending on the individual electrophoretic mobilities of the analytes of interest and the effective length that each analyte travels to the detection window, the elution order of analytes in a DOI-CE separation can vary widely. This review discusses the principles, applications, and limitations of dual-opposite injection capillary electrophoresis. Common misconceptions regarding DOI-CE are clarified.

Chemical Sniffing Instrumentation for Security Applications

Border control for homeland security faces major challenges worldwide due to chemical threats from national and/or international terrorism as well as organized crime. A wide range of technologies and systems with threat detection and monitoring capabilities has emerged to identify the chemical footprint associated with these illegal activities. This review paper investigates artificial sniffing technologies used as chemical sensors for point-of-use chemical analysis, especially during border security applications. This article presents an overview of (a) the existing available technologies reported in the scientific literature for threat screening, (b) commercially available, portable (hand-held and stand-off) chemical detection systems, and (c) their underlying functional and operational principles. Emphasis is given to technologies that have been developed for in-field security operations, but laboratory developed techniques are also summarized as emerging technologies. The chemical analytes of interest in this review are (a) volatile organic compounds (VOCs) associated with security applications (e.g., illegal, hazardous, and terrorist events), (b) chemical "signatures" associated with human presence, and

Electronic tongue for nitro and peroxide explosive sensing

This work reports the application of a voltammetric electronic tongue (ET) towards the simultaneous determination of both nitro-containing and peroxide-based explosive compounds, two families that represent the vast majority of compounds employed either in commercial mixtures or in improvised explosive devices. The multielectrode array was formed by graphite, gold and platinum electrodes, which exhibited marked mix-responses towards the compounds examined; namely, 1,3,5-trinitroperhydro-1,3,5-triazine (RDX), octahydro-1,3,5,7-tetranitro-1,3,5,7-tetrazocine (HMX), pentaerythritol tetranitrate (PETN), 2,4,6-trinitrotoluene (TNT), N-methyl-N,2,4,6-tetranitroaniline (Tetryl) and triacetone triperoxide (TATP). Departure information was the set of voltammograms, which were first analyzed by means of principal component analysis (PCA) allowing the discrimination of the different individual compounds, while artificial neural networks (ANNs) were used for the resolution and individual quantification of some of their mixtures (total normalized root mean square error for the external test set of 0.108 and correlation of the obtained vs. expected concentrations comparison graphs r>0.929).

A Microfluidic-Based Fabry-Pérot Gas Sensor

We developed a micro-gas detector based on a Fabry-Pérot (FP) cavity embedded in a microfluidic channel. The detector was fabricated in two steps: a silicon substrate was bonded to a glass slide curved with a micro-groove, forming a microfluidic FP cavity; then an optical fiber was inserted through a hole drilled at the center of the groove into the microfluidic FP cavity, forming an FP cavity. The light is partially reflected at the optical fiber endface and the silicon surface, respectively, generating an interference spectrum. The detection is implemented by monitoring the interference spectrum shift caused by the refractive index change of the FP cavity when a gas analyte passes through. This detection mechanism (1) enables detecting a wide range of analytes, including both organic and inorganic (inertia) gases, significantly enhancing its versatility; (2) does not disturb any gas flow so that it can collaborate with other detectors to improve sensing performances; and (3) ensures a fast sensing response for potential applications in gas chromatography systems. In the experiments, we used various gases to demonstrate the sensing capability of the detector and observed drastically different sensor responses. The estimated sensitivity of the detector is 812.5 nm/refractive index unit (RIU) with a detection limit of 1.2 × 10⁻⁶ RIU assuming a 1 pm minimum resolvable wavelength shift.

Integrating Paper Chromatography with Electrochemical Detection for the Trace Analysis of TNT in Soil

We report on the development of an electrochemical probe for the trace analysis of 2,4,6-trinitrotoluene (TNT) in soil samples. The probe is a combination of graphite electrodes, filter paper, with ethylene glycol and choline chloride as the solvent/electrolyte. Square wave chromatovoltammograms show the probes have a sensitivity for TNT of 0.75 nA/ng and a limit of detection of 100 ng. In addition, by taking advantage of the inherent paper chromatography step, TNT can be separated in both time and cathodic peak potential from 4-amino-dinitrotolene co-spotted on the probe or in soil samples with the presence of methyl parathion as a possible interferent.