Sarin (isopropyl methyl fluorophosphonate) is an organophosphorus nerve agent in the list of the Convention on the Banning of Chemical Weapons, which can enter the human body through the respiratory system, skin, eyes, and other parts and paralyze the central nervous system by inhibiting the enzyme of acetyl cholinesterase, which in turn causes death. Thus, the highly sensitive, rapid and on-site identification of trace Sarin is of great significance for environmental and safety requirements. The use of Sarin is strictly controlled by the relevant authorities because of the high toxicity and irreversible damage to human beings. Therefore, researchers generally use diethyl chlorophosphate (DCP), which has similar reactivity to Sarin and low toxicity, as the simulant of Sarin. At present, the commonly used fluorescence method is based on the strong electrophilicity of DCP, employing hydroxyl oxime, aliphatic hydroxyl, imine (-C=N-), and pyridine with good nucleophilicity as recognition sites to realize the fluorescence quenching detection of the target analyte. However, this method is susceptible to the interference of photobleaching, acid, and conventional fluorescent quenchers such as halogen ions, nitro compounds, and the surrounding environment, which limits its application in the actual environment. In addition, many reports focus only on the detection of DCP in solutions, less on gaseous DCP, which is more valuable in practical applications. Therefore, it is a challenging issue to propose a new sensing material design strategy with high sensitivity, anti-interference, and rapid detection performance toward solution and gaseous DCP based on the effective employment of the recognition sites.

To address this challenge, a research team led by Prof. DOU Xincun from the Xinjiang Technical Institute of Physics and Chemistry of the Chinese Academy of Sciences (CAS) has developed a design strategy of recognition site density regulation for ultrasensitive and specific fluorescence sensing toward gaseous DCP. Their findings, published in Analytical Chemistry, emphasize tuning the recognition site density and the specific surface area of the Schiff base materials to increase the adsorption capacity and collision efficiency with gaseous DCP.

In this work, researchers based on the electrophilicity of DCP, a series of zero-background fluorescence Schiff base materials (FDBA, DFDBA, DFDBA-POP) with different densities of -C=N- bonds as recognition sites were designed and synthesized by modulating the chain length. It is found that the increase of the -C=N- bond density and the specific surface area could improve the collision efficiency with DCP, thereby improving the response speed. When the density of -C=N- bonds is 3.86 × 10²¹/cm³ and the specific surface area is 128.5 m²/g, DFDBA-POP demonstrated a more superior sensing performance toward the target analyte, including the ability to detect gaseous DCP, a rapid response (1 s), and superior selectivity even in the presence of 15 kinds of interferents including the very similar hydrochloric acid. Moreover, the practicality of DFDBA-POP was further verified by a DFDBA-POP solid-state sensor, which is capable of specifically identifying gaseous DCP. We except that the present DFDBA-POP design strategy could provide new ideas for the customization of organic porous polymers with specific sensing functions, and provide an advanced scientific paradigm for the construction of solid-state sensors to detect and discriminate trace hazardous substances with similar structures and properties.

This work was supported by the National Key Research and Development Program of China, the National Natural Science Foundation of China, the Tianshan Talents Plan, the Youth Innovation Promotion Association of the Chinese Academy of Sciences, and the Tianshan Innovation Team Plan.

Figure: Schematic illustration of the designed strategy of recognition site density regulation. (Image by the research team)