Supramolecular assembly of squaraine dye for visual detection of paraquat via an indicator displacement assay

Paraquat (PQ), a highly toxic and persistent herbicide, poses severe risks to environmental and public health. Herein, we developed a supramolecular indicator displacement assay (IDA) for PQ detection using a squaraine dye (J3-Ad) and cucurbit[7]uril (CB7). PQ displaces J3-Ad monomers from the J3-Ad&CB7 (J3&CB) complex, inducing J3-Ad self-assembly into H-aggregates. This process was characterized by Ultraviolet-visible (UV-vis) absorption and fluorescence spectroscopy, proton nuclear magnetic resonance (¹H NMR) spectroscopy, and density functional theory (DFT) calculations. The assay achieved a detection limit of 1.86 μM with a linear range of 0-30 μM, and exhibited excellent anti-interference ability. It was successfully applied to visually detect PQ in soil and fruit samples, with recovery rates of 96.21-104.66%. This method provides a simple, rapid, and reliable approach for monitoring PQ residues in environmental and food safety applications.