Fluorescent chemosensor for mercury(II) cations in an aqueous solution based on 4-acetylamino-1, 8-naphthalimide derivative containing the N-phenylazadithia-15-crown-5-ether receptor

The 4-acetylamino-1,8-naphthalimide derivative containing the N-phenylazadithia-15-crown-5-ether fragment in the N-aryl substituent at the imide nitrogen atom of the naphthalimide core was synthesized, and its cation-dependent spectral properties were studied. The resulting compound in the photoexcited state exhibits low-intensity fluorescence due to the process of electron transfer from the N-aryl group to the naphthalimide residue, which is confirmed by the data of quantum chemical calculations performed using the PM6 method. The binding of Hg²⁺ in an aqueous acetate buffer solution at pH 6.0 is accompanied by the formation of a 1: 1 metal—ligand complex in which the electron transfer is suppressed leading to fluorescence enhancement. The observed spectral changes were used for the determination of the stability constant K of the complex (logK = 6.51±0.03). The found limit of Hg²⁺ detection using the synthesized sensor (28 nmol L^?1) is fairly close the maximum permissible concentration for mercury in drinking water. The study of the selectivity of complexation showed that the presence of Cu²⁺, Zn²⁺, Ni²⁺, Pb²⁺ Cd²⁺, Ca²⁺, Mg²⁺, and Fe²⁺ cations did not impede the determination of Hg²⁺. The presented results indicate that the synthesized chemosensor is promising as a selective and highly sensitive fluorescent reagent for Hg²⁺ ions in an aqueous solution.