A quinoline-based fluorescence "on-off-on" probe for relay identification of Cu²⁺ and Cd²⁺ ions

A novel fluorescent probe based 8-hydroxyquinoline (8-HQ) has been designed and synthesized. The probe 1 exhibits fast relay recognition performance for Cu²⁺ and Cd²⁺ via a fluorescence "on-off-on" response signal. The probe 1 itself has a strong emission peak at 471 nm in EtOH/H₂O (v/v = 1:9) solution with a blue fluorescence. The addition of Cu²⁺ immediately results in the quenching of fluorescence and the limit of detection is 2.7 × 10^-8 M. Moreover, the formation of [1-Cu²⁺] complexes can also serve as a new efficient probe system for the relay recognition of Cd²⁺ with the emergence of a new fluorescence signal under the same conditions. The study also found that the probe 1 has high selectivity for target ions in the presence of other competing ions. The probe 1 has been successfully applied to detect and analyze the trace amount of Cu²⁺ and Cd²⁺ in environmental water samples.