Cu²⁺ sensing via noncovalent complexes of fluorescent whitening agents and imidazole-based polymeric dye transfer inhibitors

Fluorescent whitening agents (FWAs) and polymeric dye transfer inhibitors (DTIs) are widely used in detergent formulations to combat garment yellowing, fading, and discoloration. We report a new application for these inexpensive, mass-produced, laundry additives as sensitive fluorescence “turn-off” sensors for Cu²⁺. We show that Cu²⁺-sensitive FWA–DTI complexes form spontaneously when aqueous solutions of FWA and imidazole-based DTI are mixed. We also show that the imidazole groups present in the resulting fluorescent FWA–DTI complex selectively bind Cu²⁺, a potent fluorescence quencher, to form nonfluorescent FWA–DTI-Cu²⁺ complexes. Our Cu²⁺-sensing FWA–DTI complexes are completely water-soluble and have a Cu²⁺ detection limit of ~ 0.14 μM in water. Our simple approach not only converts the Cu²⁺-insensitive FWAs into sensitive fluorescent probes for the metal ion but also significantly enhances the brightness of triazinylaminostilbene FWAs. The present study provides a facile, synthesis-free strategy for producing inexpensive fluorescent sensors for Cu²⁺.