Fluorescein-N-Methylimidazole Conjugate as Cu²⁺ Sensor in Mixed Aqueous Media Through Electron Transfer

A new highly selective, chromogenic, and fluorogenic Cu²⁺ chemosensor, fluorescein-N-methylimidazole conjugate 1, and another fluorescein-N-imidazole conjugate 2 were synthesized and investigated by UV-visible and fluorescence spectroscopy. The sensing of Cu²⁺ quenches the emission band of 1 at λ_max = 525 nm, with an association constant (K _a = 1.0 x 10⁷ M^-1) and a stoichiometry of 1:1 in a buffered H₂O: MeOH solution (4:1, pH = 7.4). The Cu²⁺ detection limit for chemosensor 1 is 37 nM. The presence of the N-methyl group in 1 increased the Cu²⁺ binding selectivity, resulting in a stronger binding constant and a broader pH working range (pH 5-10) in comparison to 2. The fluorescence in 1 and 2 is caused by electron transfer phenomenon from the imidazole nitrogen to fluorescein, which is readily inhibited by Cu²⁺ binding.