Color-change strips detect trace benzene

A quick and simple fluorescent test can detect trace quantities of benzene in both liquid and vapor form. The new test uses paper strips impregnated with a supramolecular host-guest assembly system developed by Chuan-Feng Chen and his team at the Chinese Academy of Sciences (Angew. Chem. 2025, DOI: 10.1002/ange.202508987).

Benzene is a carcinogen, and strict workplace exposure limits protect people in many parts of the world. Air quality monitoring in high-risk environments is essential—for example, in industrial chemical facilities, where the organic chemical is often an unavoidable by-product of other processes. But benzene’s lack of functionality, coupled with the greasy, apolar character of the molecule, means sensitive detection typically requires laboratory-grade analytical equipment.

The new test developed by Chen and his team uses bowl-shaped acridan units that act as electron-rich hosts to cyanopyrimidine guest motifs, which are tethered by a polymeric chain. The resulting supramolecular assembly produces a baseline yellow fluorescent emission because of charge-transfer interactions between these host and guest components. But preferential binding of benzene in the acridan unit displaces the pyrimidine guest, suppressing the charge-transfer interactions and the yellow fluorescence. The naked pyrimidine fluoresces blue once liberated from the acridan pocket, thereby signaling the presence of benzene with a yellow-to-blue transition.

The researchers coated this host-guest complex onto samples of filter paper, which they then exposed to differing concentrations of hydrocarbon vapors. Tests demonstrated that the paper has a remarkable sensitivity and specificity toward benzene, with a calculated detection limit of just 3.5 mg/L and a 10-fold selectivity ratio for the target molecule over toluene, xylenes, and cyclohexane. In a final proof-of-principle test, Chen and his colleagues tried the paper sensor in liquid samples of cyclohexane feedstocks (which often contain trace benzene impurities) and successfully produced a visual signal at concentrations as low as 1%.

“It’s a very nice, creative use of a host-guest interaction, and they demonstrate impressive selectivity of benzene over other aromatics,” says optoelectronics researcher Eli Zysman-Colman from the University of St. Andrews.

But, Zysman-Colman adds, it’s too soon to say if this test has a commercial future. “We don’t know the cost-effectiveness of this versus other benzene sensors or how it would behave in real-world-type conditions—i.e., whether there’d be other contaminants that would quench the emission and render the sensor ineffective,” he says. “This is nonetheless an example of an exploitation of a supramolecular interaction to make a material that shows utility. But whether it’s useful for society as a whole? That is hard to predict.”