“Turn-On” Fluorescent Sensor for Hg2+ via Displacement Approach

Abstract

A new Cu²⁺ compound Cu−NB, (where H₂NB is bis(2-hydroxyl-naphthalene-carboxaldehyde) benzil dihydrazone) was synthesized as a highly selective fluorescence chemosensor for the detection of Hg²⁺ in aqueous media through a displacement “turn-on” signaling strategy. Whereas the coordination of Cu²⁺ resulted in a considerable quenching of the typical luminescence of the naphthol rings in Cu−NB, the addition of Hg²⁺ ion led to a dramatic increase in the emission intensity of Cu−NB at about 530 nm (excitation at 430 nm). The competitive fluorescent experiments showed that alkali, alkaline earth metal ions, the group 12 metals Zn²⁺, Cd²⁺, the first-row transition-metal ions such as Mn²⁺, Fe²⁺, Co²⁺, and Ni²⁺, as well as Pb²⁺ could not inhibit the Hg²⁺-binding fluorescent enhancement. It is postulated that the existence of Cu²⁺ in the luminescent probe Cu−NB could turn away the interferences of other metal cations from Hg²⁺ detection. The optical responses of the free ligand upon addition of Cu²⁺ ion, and of the Hg−H₂NB compound upon the addition of Cu²⁺ were also investigated for comparisons.