Metal-Ion-Responsive Chromogenic Probe for Rapid, On-Location Detection of Foodborne Bacterial Pathogens in Contaminated Food Items

An amphiphilic chromogenic probe based on an oxidized di(indolyl)arylmethane backbone has been utilized for visual detection of both Cu2+ (detection limit = 8.5 ppb) and Hg2+ (detection limit = 10.2 ppb) ions via mutually independent sensing pathways. The Cu2+ ion binds to the carboxylate ends (donor site) and induces a color change from orange to yellow in the aqueous medium, while coordinating Hg2+ at the bisindolyl moiety (acceptor site) can result in the formation of a red-colored solution. Interestingly, by selecting the proper excitation channel, we can specifically excite either the monomer species or nanoaggregates. The addition of Hg2+ enhances the monomer fluorescence, while Cu2+ induces quenching. However, in both cases, metal-ion coordination triggers dissociation of a preformed self-assembled structure. Further, the in-situ-formed Cu(II) complex was utilized for rapid, on-location detection of food-borne pathogens, such as Escherichia coli (E. coli) in contaminated food items and water (detection limit = 52 CFU·mL–1). E. coli induces reduction of Cu2+ to Cu+ and transforms the yellow-colored solution into an orange-colored solution. Finally, low-cost, reusable paper strips were designed as an eco-friendly, sustainable strategy to detect bacterial pathogens.