Sarin (isopropyl methyl fluorophosphonate) is an organophosphorus nerve agent regulated by the Convention on the Banning of Chemical Weapons. It can enter the body through the respiratory system, skin, or eyes, paralyzing the central nervous system by inhibiting acetylcholinesterase, which can lead to death. Therefore, rapid and sensitive detection of trace Sarin is vital for safety and environmental protection.

Due to its high toxicity, Sarin's use is strictly controlled, leading researchers to use diethyl chlorophosphate (DCP) as a safer simulant. The common fluorescence detection method takes advantage of DCP's strong electrophilicity, using recognition sites like hydroxyl oxime and imine for fluorescence quenching to identify the target.

However, this method is affected by photobleaching, acid, and other environmental factors, limiting its application. Most studies focus on DCP in liquid solutions rather than its gaseous form, which is more relevant in practice. Thus, developing a new sensing material that combines high sensitivity, anti-interference, and rapid detection for both liquid and gaseous DCP remains a significant challenge.

To tackle 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 to regulate the density of recognition sites for ultrasensitive and specific fluorescence sensing of gaseous DCP. Their findings, published in the journal Analytical Chemistry, highlight the importance of adjusting the recognition site density and the specific surface area of Schiff base materials. This approach enhances both the adsorption capacity and the collision efficiency with gaseous DCP.

In this study, the researchers designed and synthesized a series of zero-background fluorescence Schiff base materials—namely, FDBA, DFDBA, and DFDBA-POP—with varying densities of C=N bonds as recognition sites. This was achieved by modulating the chain length. The findings indicate that increasing the density of C=N bonds and the specific surface area enhances the collision efficiency with DCP, leading to faster response times.

Specifically, when the density of C=N bonds reaches 3.86 × 10²¹/cm³ and the specific surface area is 128.5 m²/g, DFDBA-POP exhibits excellent sensing performance for the target analyte. It is capable of detecting gaseous DCP with a rapid response time of just one second and demonstrates superior selectivity, even in the presence of 15 different interferents, including hydrochloric acid, which closely resembles DCP.

Furthermore, the practicality of DFDBA-POP was validated through the development of a DFDBA-POP solid-state sensor, which can specifically identify gaseous DCP. The researchers anticipate that this design strategy for DFDBA-POP will provide new insights into the customization of organic porous polymers with specific sensing functions, offering an advanced model for the development of solid-state sensors aimed at detecting and distinguishing trace hazardous substances with similar structures and properties.

This work was supported by the National Key Research and Development Program of China, and the National Natural Science Foundation of China, among other sources.

Schematic illustration of the designed strategy of recognition site density regulation. (Image by Prof. DOU Xincun's team)