A Colorful Detector For Phosgene

Chemists have identified six molecules that can react with a highly toxic gas to produce a blue fluorescent signal (Anal. Chem., DOI: 10.1021/ac300737g). The compounds could find use in a color-based phosgene sensor for the plastics industry, where the gas is commonly used to make building blocks for the materials, the researchers say.

Commercial phosgene sensors detect the notoriously toxic gas based on its characteristic absorption of infrared light. Researchers have also developed sensors that detect the gas using a color-changing material. But these materials change color mainly in the presence of the gas’s chemical cousin triphosgene, not phosgene itself.

Pradip Kundu and Kuo Chu Hwang, both at National Tsing Hua University, in Taiwan, wanted to design a simple color-changing sensor for phosgene and thought the chemical’s reactivity could help.

The carbonyl of a phosgene molecule can react with two electron-rich groups, such as amines, alcohols, or carboxylic acids. The researchers wanted to find compounds with two such groups. They envisioned phosgene grabbing the two reactive groups and connecting them to form an additional aromatic ring, which could make the molecule fluoresce.

The two chemists identified six molecules of that type in chemical catalogs. They dissolved each molecule in chloroform and bubbled phosgene into the liquid.

Each molecule fluoresced weakly before it reacted with phosgene. When the ring formed, the resulting molecules shined bright blue. The molecule most sensitive to the presence of phosgene, (2E)-3-(2-aminophenyl)-2-propenoic acid, started producing visible fluorescence at a phosgene concentration of 1 nM, about 160 to 400 times more sensitive than commercial gas-phase phosgene detectors.

“There is still a lot of room for improvement,” Hwang says, for real-world applications of the method. For example, to test air samples for low concentrations of phosgene, companies would need to bubble large volumes of air through the sensing solution; Hwang notes that the researchers need to devise a way to prevent the sensing solution from evaporating under those conditions.