A simple turn-off fluorescent chemosensor based on a Schiff base structure for ultrafast and highly selective trace detection of Cu2+ ions in aqueous solutions.

A simple turn-off fluorescent probe, 5-(diethylamino)-2-(hydrazonomethyl)phenol (DHP), is designed and synthesized for the sensitive and selective detection of Cu²⁺. The bright green fluorescence of DHP is quenched after the addition of Cu²⁺. The probe DHP exhibits good anti-interference performance against Cu²⁺ in the presence of multiple metal ions. The fluorescence intensity of DHP (10 μM) at 522 nm is well linearized with the Cu²⁺ concentration at 0-5 μM, and it has a detection limit of 0.29 nM (R² = 0.9949). The complexation ratio of the probe DHP to Cu²⁺ is 2 : 1 and the complexation constant is 3.44 × 10⁴ M^-1 (R² = 0.9974). In addition, the probe DHP can be recovered using EDTA and Cu²⁺ can be effectively monitored at pH 5-11, with good results in dipstick experiments and actual water samples. HepG-2 cells remained viable in excess of 90% after being exposed to DHP (50 μM) for 24 h, which demonstrates the extremely low toxicity of DHP, and it can be used for in vivo cell imaging.