Turning bubbles into sensors: foam-based detection of paraquat in water

Intensive global agricultural growth has increased pesticide use, raising concerns about water contamination. Paraquat (PQ), a highly water-soluble, nonselective herbicide used in over 130 countries, poses serious risks to human health because its high solubility and long environmental half-life make it a persistent pollutant. Although conventional analytical methods provide excellent sensitivity, they require expensive instruments and complex sample preparation, limiting their practicality for on-site applications. Here, we present a fast, visual, on-site paraquat detection strategy based on smart liquid foam systems. The method exploits a highly selective host-guest complex between water soluble pillar[5]arene (WS-P[5]A) and PQ. First, WS-P[5]A hosts are loaded with the bio-based surfactant sucrose laurate (SL), which interacts less with WS-P[5]A than PQ. Secondly, PQ is added which replaces SL due to the higher binding constant between WS-P[5]A and PQ. Upon replacement of SL by PQ, surfactant is released which enables foam generation by hand-shaking. Changes in foam height correlate with PQ concentration, providing an intuitive visual readout. We demonstrate a high selective method even in the presence of competing contaminants, and we validate its performance across multiple water types. This supramolecular foam sensor could offer a simple, selective, rapid, and low-cost approach for the preliminary detection of paraquat in contaminated water. The proposed foam-based method is designed as an on-site screening tool that can be used by virtually anyone without specialized equipment, technical expertise, or laboratory infrastructure. Foam formation provides a straightforward visual indication of the presence of paraquat, enabling fast field-based assessment. If paraquat is detected qualitatively, quantification can subsequently be performed using well-established analytical techniques.