Sensitive sensor for mercury ion in aqueous solution by "naked-eye" and fluorescence recognition

A sensitive and selective Nile blue-based optical sensor 1-benzoyl-3-{2-[9-(ethylamino)-10-methyl-9%H%-benzo[%α%] phenoxazin-5-ylamino]ethyl}thiourea hydrochloride(NBET) in aqueous solution for Hg ²⁺ was synthesized. The maximum absorption peak of the probe molecule appeared at 640 nm, and decreased {upon} the addition of Hg ²⁺ in 0.05 mol/L Tris-HCl buffer solution at pH=7.4, a new peak of absorption at 556 nm was observed. The color of the solution changed from light blue to light purple. There was neglectable change upon the addition of other metal ions. Fluorescence monitoring revealed a fluorescence quenching of the probe's characteristic emission at 660 nm upon the addition of Hg ²⁺. The probe possessed the advantages of near-infrared excitation and emission and high fluorescence quantum yield in aqueous solution and offer an opportunity to detect Hg ²⁺ by a "naked-eye" manner. Limit of detection of fluorescence spectral for Hg ²⁺ was found to be 0.005 μmol/L in aqueous solution.