Molecular engineered optical probes for chemical warfare agents and their mimics: Advances, challenges and perspectives

Smart Sensing of Nerve Agents

The recent international scenario highlights the importance to protect human health and environmental quality from toxic compounds. In this context, organophosphorous (OP) Nerve Agents (NAs) have received particular attention, due to their use in terrorist attacks. Classical instrumental detection techniques are sensitive and selective, but they cannot be used in real field due to the high cost, specialized personnel requested and huge size. For these reasons, the development of practical, easy and fast detection methods (smart methods) is the future of this field. Indeed, starting from initial sensing research, based on optical and/or electrical sensors, today the development and use of smart strategies to detect NAs is the current state of the art. This review summarizes the smart strategies to detect NAs, highlighting some important parameters, such as linearity, limit of detection and selectivity. Furthermore, some critical comments of the future on this field, and in particular, the problems to be solved before a real application of these methods, are provided.

A ratiometric fluorescence platform for on-site screening meat freshness

Meat freshness is related to food safety and human health. Developing a simple and effective method for on-site detection of meat freshness is essential to ensure food safety. This study aimed to explore a ratiometric fluorescence platform for on-site screening of meat freshness. We synthesized a series of benzothiazole-based fluorescent compounds (BM, BHM and BTH), each with different recognition groups for detecting meat freshness biomarkers cadaverine (Cad) and putrescine (Pte). The optimized 2-(2'-hydroxyphenyl-3-aldehyde-5-1,3-indanedione) benzothiazole (BTH) demonstrated a noticeable color and fluorescence change, a fast response (<15 min), and high selectivity and sensitivity (LOD = 70 nM) to Cad. Portable test strips based on BTH were prepared for rapid visual detection of meat freshness, which exhibited visible color and fluorescen color changes to Cad and Pte. Furthermore, a portable smartphone-based fluorescence device integrated with a self-programmed Python program was fabricated and used on-site to monitor Cad and Pte within 5 min. The BTH-loaded portable test strips were successfully employed as low-cost, high-contrast, fast-response, and smartphone-adaptable fluorescent labels for detecting Cad and Pte in meat samples under different temperatures (25 °C, 4 °C, and -20 °C). This enabled consumers and food supply chain stakeholders to quickly and visually monitor the meat freshness in real beef, chicken, and pork products.

Highly selective and sensitive chromogenic recognition of sarin gas mimicking diethylchlorophosphate

The high-level toxic effects of organophosphate (OP) nerve agents severely threaten national security and public health. Generating trustworthy, accurate methods for quickly identifying these poisonous chemicals is urgently necessary. In this study, we have presented an azine-based colorimetric sensor (HBD) for the highly sensitive and selective identification of poisonous sarin gas surrogate diethylchlorophosphate (DCP). Our introduced sensor shows a purple color in contact with DCP, which is fully reversible upon the addition of triethylamine (TEA). The detection limit of our sensor for the toxic nerve agent mimic DCP is in the μM range. We have fabricated a test kit to verify the capability of HBD for on-the-spot identification of DCP to execute its practical use. To prove that HBD is an effective chemosensor, dip-stick investigation was conducted to detect DCP in the vaporous stage in the presence of different OPs, inorganic phosphates (IPs), and many other deadly analytes. A cellphone-based display method was also undertaken for on-the-spot recognition and measurement of DCP in isolated regions.

A benzoxazole-triphenylamine conjugated fluorogenic probe for specific detection of sarin gas mimic diethylchlorophosphate

In this study, an excellent chromo-fluorogenic PMPA probe, (E)-4-(((4-(benzoxazole-2-yl)phenyl)imino)methyl)-N,N-diphenylamine, is introduced for the detection of sarin gas mimic diethyl chlorophosphate (DCP) in solution and gaseous phases. On adding DCP into PMPA solution in a pure DMSO and water-DMSO (4 : 1) medium, it exhibits a hypsochromic shift from yellow to colorless and from no fluorescence to highly intense blue-violet photoluminescence via the formation of a phosphorylated PMPA-DCP product due to the inhibition of intramolecular charge transfer (ICT) and photoinduced electron transfer (PET) mechanism. The sensor could detect DCP in the presence of several other notorious guest analytes with a detection limit in the μM range. Moreover, to accomplish the on-spot detection of DCP and explore the practical applicability of the probe, a paper strip-based test kit, "dip-stick" method, and, more interestingly, a real sample analysis was demonstrated in spiked soil samples.

A highly sensitive fluorescence probe for on-site detection of nerve agent mimic diethylchlorophosphonate DCP

Nerve agents are the most toxic chemical warfare agents that pose severe threat to human health and public security. In this work, we developed a novel fluorescent probe NZNN based on naphthylimide and o-phenylenediamine to detect nerve agent mimic diethylchlorophosphonate (DCP). DCP underwent a specific nucleophilic reaction with the o-phenylenediamine group of NZNN to produce a significant fluorescence turn-on response with high selectivity, exceptional linearity, bright fluorescence, rapid response (<6 s) and a low detection limit (30.1 nM). Furthermore, a portable sensing device was fabricated for real-time detection of DCP vapor with excellent performance. This portable and sensitive device is favorable for monitoring environmental pollution and defense against chemical warfare agents.

A fluorescent probe based on the ESIPT (excited state intramolecular proton transfer) mechanism for rapid detection of endogenous and exogenous H2O2 (hydrogen peroxide) in cells

Hydrogen peroxide (H2O2) is one of the important reactive oxygen species in the body and can be used as a marker of some diseases such as cancer and neurodegenerative diseases. Therefore, it is of great significance to develop fluorescent probes that can detect H2O2 in living organisms for early diagnosis of diseases. However, slow response time and low fluorescence quantum yield limit the application of many probes. In this study, using 2-(2-hydroxyphenyl) benzothiazole (HBT) as the fluorophore, the introduction of weakly absorbing bromine atoms can accelerate the speed of electron transfer during the recognition process. Three ESIPT (excited state intramolecular proton transfer) fluorescent probes JLO/JLM/JLP were designed and synthesized. The detection of H2O2 can be achieved with all three probes, and we screened a probe JLO with the fastest response time (30 min) and highest fluorescence quantum yield (Ф = 0.731). The probe also has a large Stokes shift, which can reduce fluorescence self-absorption and background interference, and also has a high sensitivity, which is designed to accurately detect endogenous and exogenous H2O2 in living cells, which has great potential for biological applications.

Ratiometric Fluorescent Probe for the Detection of Nanomolar Phosgene in Solution and Gaseous Phase: Advancing Crime Detection Applications

Phosgene, an exceptionally hazardous gas, poses a grave concern for the health and safety of the general public. The present study describes a fluorescent ratiometric probe for phosgene employing 2-(naphthalen-2-yl) benzo[d]oxazol-5-amine (NOA) with an amino group as the recognition site. NOA detects phosgene through the intramolecular charge transfer mechanism. The electron-rich amine group of NOA attacks the electrophilic carbonyl group of phosgene, resulting in a quick response within 20 s. NOA demonstrates a low detection limit of 60 nM while maintaining high selectivity and sensitivity toward phosgene. The final product was isolated and verified by nuclear magnetic resonance spectroscopy. The probe can detect phosgene not just quickly in a solution environment but also in its solid state. The probe's applications in fingerprint imaging and bioimaging are also demonstrated.

A near-infrared fluorescent probe for tracking endogenous and exogenous H2O2 in cells and zebrafish

H2O2 is an important signaling molecule in the body, and its levels fluctuate in many pathological sites, therefore, it can be used as a biomarker for early diagnosis of disease. Since the environment in vivo is extremely complex, it is of great significance to develop a probe that can accurately monitor the fluctuation of H2O2 level without interference from other physiological processes. Based on this, we designed and synthesized two new near-infrared H2O2 fluorescent probes, LTA and LTQ, based on the ICT mechanism. Both of them have good responses to H2O2, but LTA has a faster response speed. In addition, the probe LTA has good biocompatibility, good water solubility, and a large Stokes shift (95 nm). The detection limit is 4.525 μM. The probe was successfully used to visually detect H2O2 in living cells and zebrafish and was successfully used to monitor the changes in H2O2 levels in zebrafish due to APAP-induced liver injury.

Differentiating Between V- and G-Series Nerve Agent and Simulant Vapours Using Fluorescent Film Responses

In-field rapid and reliable identification of nerve agents is critical for the protection of Defence and National Security personnel as well as communities. Fluorescence-based detectors can be portable and provide rapid detection of chemical threats. However, most current approaches cannot differentiate between dilute vapors of nerve agent classes and are susceptible to false positives due to the presence of common acids. Here a fluorescence-based method is shown for rapid differentiation between the V-series and phosphonofluoridate G-series nerve agents and avoids false positives due to common acids. Differentiation is achieved through harnessing two different mechanisms. Detection of the V-series is achieved using photoinduced hole transfer whereby the fluorescence of the sensing material is quenched in the presence of the V-series agent. The G-series is detected using a turn-on mechanism in which a silylated excited state intramolecular proton transfer sensing molecule is selectively deprotected by hydrogen fluoride, which is typically found as a contaminant and/or breakdown product in G-series agents such as sarin. The strategy provided discrimination between classes, as the sensor for the G-series agent class is insensitive to the V-series agent, and vice versa, and neither responded to common acids.

Highly sensitive and selective detection of triphosgene with a 2-(2'-hydroxyphenyl)benzimidazole derived fluorescent probe

In this work, a 2-(2'-hydroxyphenyl)benzimidazole derived fluorescent probe, 2-(2'-hydroxy-4'-aminophenyl)benzimidazole (4-AHBI), was synthesized and its fluorescent behavior toward triphosgene were evaluated. The results showed that 4-AHBI exhibited high sensitivity (limit of detection, 0.08 nM) and excellent selectivity for triphosgene over other acyl chlorides including phosgene in CH2Cl2 solution. Moreover, 4-AHBI loaded test strips were prepared for the practical sensing of triphosgene.

Fluorescent dyes based on rhodamine derivatives for bioimaging and therapeutics: recent progress, challenges, and prospects

Since their inception, rhodamine dyes have been extensively applied in biotechnology as fluorescent markers or for the detection of biomolecules owing to their good optical physical properties. Accordingly, they have emerged as a powerful tool for the visualization of living systems. In addition to fluorescence bioimaging, the molecular design of rhodamine derivatives with disease therapeutic functions (e.g., cancer and bacterial infection) has recently attracted increased research attention, which is significantly important for the construction of molecular libraries for diagnostic and therapeutic integration. However, reviews focusing on integrated design strategies for rhodamine dye-based diagnosis and treatment and their wide application in disease treatment are extremely rare. In this review, first, a brief history of the development of rhodamine fluorescent dyes, the transformation of rhodamine fluorescent dyes from bioimaging to disease therapy, and the concept of optics-based diagnosis and treatment integration and its significance to human development are presented. Next, a systematic review of several excellent rhodamine-based derivatives for bioimaging, as well as for disease diagnosis and treatment, is presented. Finally, the challenges in practical integration of rhodamine-based diagnostic and treatment dyes and the future outlook of clinical translation are also discussed.

Detection of exposed phosgene in household bleach: development of a selective and cost-effective sensing tool

Sensing of gaseous environment pollutants and health hazards is in demand these days and in this regard, lethal phosgene has emerged as a leading entrant. In this contribution, we have successfully developed a facile chemodosimeter (ANO) based on an anthracene fluorophore and oxime recognition site with an interesting mechanism to sense lethal phosgene evolved from bleaching powder, a very popular disinfectant and sanitizer. The ANO probe is highly competent in recognizing deadly phosgene in solution and in the gaseous phase with a detection limit in the nanomolar range (1.52 nM). The sensing mechanism is confirmed by UV-vis, emission spectroscopy, mass spectrometry, and computational studies.

A small-molecule fluorogenic probe for the detection of hypochlorite and its application in the bio-imaging of human breast cancer cells

A certain amount of hypochlorite can help to regulate the body's defense system while excessive hypochlorite has some complex influence on health. Herein, a thiophene-derived biocompatible turn-on fluorescent probe (TPHZ) was synthesized and characterized for the detection of hypochlorite (ClO^-). The fluorescence and colorimetric sensing of the probe followed an ICT OFF strategy. The experimental results showed a remarkable turn on fluorescence enhancement from colorless to bright blue after the addition of ClO^- within 130 s in a solvent system having 80% water with high selectivity and a low detection limit of 53.8 nM. The sensing mechanism was attributed to ClO^- mediated electrophilic addition to the imine bond which was justified by DFT calculations, and ESI-MS and ¹H-NMR titration experiments. The probe was used in an application to visualize ClO^- in human breast cancer cells which can be helpful for investigating the functions of hypochlorite in living cells. Finally, by virtue of fine photophysical properties, good sensing performance, good water solubility and low limit of detection, the probe TPHZ was successfully applied to TLC test strips, and commercial bleach and water samples.

Dual-State Fluorescent Probe for Ultrafast and Sensitive Detection of Nerve Agent Simulants in Solution and Vapor

The development of fluorescent probes for detecting nerve agents has been the main concern focus of research because of their lethal toxicity for humans. Herein, a probe (PQSP) based on the quinoxalinone unit and the styrene pyridine group was synthesized and could visually detect a sarin simulant diethyl chlorophosphate (DCP) with excellent sensing properties in solution and solid states. Interestingly, PQSP showed an apparent intramolecular charge-transfer process by catalytic protonation after reacting with DCP in methanol, accompanied with the aggregation recombination effect. The sensing process was also verified by nuclear magnetic resonance spectra, scanning electron microscopy, and theoretical calculations. In addition, the papered test strips of loading probe PQSP exhibited an ultrafast response time within 3 s and high sensitivity with a limit of detection of 3 ppb for the detection of DCP vapor. Therefore, this research provides a designed strategy for developing the probes with dual-state emission fluorescence in solution and solid states for detecting DCP sensitively and rapidly, which can be fabricated as chemosensors to visually detect nerve agents in practice.

Visual and Rapid Detection of Nerve Agent Mimics in Gas and Solution Phase by a Simple Fluorescent Probe

Chemical nerve agents are highly toxic organophosphorus compounds that are easy to obtain and can be utilized by terrorists to threaten homeland security and human safety. Those organophosphorus nerve agents contain nucleophilic ability that can react with acetylcholinesterase leading to muscular paralysis and human death. Therefore, there is great importance to explore a reliable and simple method to detect chemical nerve agents. Herein, the o-phenylenediamine-linked dansyl chloride as a colorimetric and fluorescent probe has been prepared to detect specific chemical nerve agent stimulants in the solution and vapor phase. The o-phenylenediamine unit serves as a detection site that can react with diethyl chlorophosphate (DCP) in a rapid response within 2 min. A satisfied relationship line was obtained between fluorescent intensity and the concentration of DCP in the range of 0-90 μM. In the optimized conditions, we conducted the fluorescent titration to measure the limits of detection (0.082 μM) with the fluorescent enhancement up to 18-fold. Fluorescence titration and NMR studies were also conducted to explore the detection mechanism, indicating that the formation of phosphate ester causes the intensity of fluorescent change during the PET process. Finally, probe 1 coated with the paper test is utilized to detect DCP vapor and solution by the naked eye. We expect that this probe may give some admiration to design the small molecule organic probe and applied in the selectivity detection of chemical nerve agents.

Molecular engineering of fluorescence probe for real-time non-destructive visual screening of meat freshness

Spoiled meat poses a great challenge to food security and human health, which should be addressed by the early monitoring and warning of the meat freshness. We herein exploited a molecular engineering strategy to construct a set of fluorescence probes (PTPY, PTAC, and PTCN) with phenothiazine as fluorophore and cyanovinyl as recognition site for the facile and efficient monitoring of meat freshness. These probes produce an obvious fluorescence color transition from dark red to bright cyan in response to cadaverine (Cad) through the nucleophilic addition/elimination reaction. The sensing performances were elaborately improved to achieve quick response (16 s), low detection limit (LOD = 3.9 nM), and high contrast fluorescence color change by enhancing the electron-withdrawing strength of cyanovinyl moiety. Furthermore, PTCN test strips were fabricated for portable and naked-eye detection of Cad vapor with fluorescence color change from crimson to cyan, and accurate determination of Cad vapor level with RGB color (red, green, blue) mode analysis. The test strips were employed to detect the freshness of real beef samples, and demonstrated a good capability of non-destructive, non-contact and visual screening meat freshness on site.

Bifunctional Fluorescent Probes for the Detection of Mustard Gas and Phosgene

Mustard gas [sulfur mustard (SM)] and phosgene are the most frequently used chemical warfare agents (CWAs), which pose a serious threat to human health and national security, and their rapid and accurate detection is essential to respond to terrorist attacks and industrial accidents. Herein, we developed a fluorescent probe with o-hydroxythioketone as two sensing sites, AQso, which can detect and distinguish mustard gas and phosgene. The dual-sensing-site probe AQso reacts with mustard gas to form a cyclic product with high sensitivity [limit of detection (LOD) = 70 nM] and is highly selective to SM over phosgene, SM analogues, active alkylhalides, acylhalides, and nerve agent mimics, in ethanol solutions. When encountering phosgene, AQso rapidly converts to cyclic carbonate, which is sensitive (LOD = 14 nM) and highly selective. Their sensing mechanisms of AQso to mustard gas and phosgene were well demonstrated by separation and characterization of the sensing products. Furthermore, a facile test strip with the probe was prepared to distinguish 2-chloroethyl ethyl sulfide (CEES) and phosgene in the gas phase by different fluorescence colors and response rates. Not using the complicated instrument, the qualitative and quantitative detection of CEES or phosgene can be achieved only by measuring the red-green-blue (RGB) channel intensity of the test strip after being exposed to CEES or phosgene gas by the smartphone with an RGB color application.

Advances in Noble-Metal Nanoparticle-Based Fluorescence Detection of Organophosphorus Chemical Warfare Agents

Efficient and simple detection of chemical warfare agents (CWAs) is an essential step in minimizing the potentially lethal consequences of chemical weapons. CWAs are a family of organic chemicals that are used as chemical weapons because of their enormous severity and lethal effects when faced with unforeseen challenges. To stop the spread of CWAs, it is critical to develop a platform that detects them in a sensitive, timely, selective, and minimally invasive manner. Rapid advances in the demand for on-site sensors, metal nanoparticles, and biomarker identification for CWAs have made it possible to use fluorescence as a precise real-time and point-of-care (POCT) testing technique. For POCT-based applications, the new capabilities of micro- and nanomotors offer enormous prospects. In recent decades, significant progress has been made in the design of fluorescent sensors and the further development of noble metal nanoparticles for the detection of organophosphorus CWAs, as described in this review. Through this work, recent attempts to fabricate sensors that can detect organophosphorus CWAs through changes in their fluorescence properties have been summarized. Finally, an integrated outlook on how noble metal nanoparticles could be used to develop smart sensors for organophosphorus CWAs that communicate with and control electronic devices to monitor and improve the health of individuals.