Block Copolymer-Stabilized Complex Emulsions for Flow-Injection Detection of Perfluorooctanoic Acid in Multiple-Well Flow Chips

Rapid, field-deployable screening methods are needed for per- and polyfluoroalkyl substances (PFAS), particularly in high-load and non-potable water streams. Here, we report a responsive complex-emulsion platform in which acid-reactive block copolymers (BCPs) stabilize hydrocarbon/fluorocarbon/water droplets and convert perfluorooctanoic acid (PFOA) binding into an optically readable change in morphology. Polystyrene-block-poly(4-vinylpyridine) (PS-b-P4VP) and polystyrene-block-poly(glycidyl methacrylate) (PS-b-PGMA) were synthesized by atom transfer radical polymerization and screened for interfacial activity and emulsion stability. The selected copolymers, PS2.4k-b-P4VP2.7k and PS2.5k-b-PGMA1.8k, reduced the hydrocarbon/water interfacial tension to 25.77 and 13.23 mN m–1, respectively, and stabilized fluorocarbon-in-hydrocarbon-in-water droplets. Exposure to PFOA transformed the droplets from engulfed to Janus morphologies. The response is consistent with pyridine protonation and pyridinium–perfluorooctanoate ion-pair formation in the PS-b-P4VP system and primarily hydrogen bonding, with a minor contribution from epoxide ring opening, in the PS-b-PGMA system. The morphology transition was retained in PFOA-spiked river water and was distinct from the responses to nonfluorinated carboxylic acids, humic acid, and sodium dodecyl sulfate. Integration with a multiple-well flow chip enabled concentration-dependent response times over 10–500 ppb PFOA, with PS-b-P4VP droplets reaching the Janus state in 1133 s at 10 ppb. A ResNet18 model classified droplet transitions with 92.8% accuracy, a weighted F1 score of 92.8%, and an area under the receiver operating characteristic curve of 0.98. This work establishes chemically programmed complex emulsions as dynamic optical transducers for rapid, semiquantitative PFOA screening.