A coumarin-derived Cu²⁺-fluorescent chemosensor and its direct application in aqueous media

A novel coumarin-based receptor bearing a benzohydrazide (FCBH) was developed as a fluorescent chemosensor with high selectivity toward Cu²⁺. The sensor was successfully applied to the monitoring of Cu²⁺ in aqueous solution. After the addition of Cu²⁺ to FCBH, the color of the solution changed from greenish-yellow to red, and the absorption band at 457nm red-shifted to 517nm. The fluorescent green color of FCBH disappeared and the fluorescence emission was completely quenched in the presence of Cu²⁺. Upon the addition of Cu²⁺, deprotonation of FCBH occurred, and a 1:1 metal-ligand complex formed. DFT theoretical investigation was carried out to understand the behavior of the sensing probe toward Cu²⁺. Additionally, the quenched fluorescence of the FCBH-Cu²⁺ complex was restored upon the addition of CN^- ions. The possible sensing mechanism of FCBH toward Cu²⁺ was derived from experimental and theoretical examinations.