A Reaction-based Ratiometric Fluorescent Probe with Large STOKES Shift for Cu²⁺ in Neat Aqueous Solution

A novel reaction-based ratiometric fluorescent probe 1 for Cu²⁺ using picolinate as the reaction site and hemicyanine as the fluorophore was developed. 1 displayed maximum absorption peak at 355 nm and fluorescence emission peak at 500 nm, with large Stokes shift of 145 nm. Upon reaction with Cu²⁺, the maximum absorption and fluorescence emission peaks red-shifted to 390 nm and 570 nm respectively, owing to Cu²⁺-induced hydrolysis of the picolinate moiety in 1. Meanwhile, the solution of 1 turned from green to orange under a 365 nm UV lamp. 1 not only could detect Cu²⁺ ratiometrically by the ratios of both absorbance (A_{390 nm}/A_{355 nm}) and fluorescence intensity (F_{570 nm}/F_{500 nm}), but also displayed large Stokes shift, fast response, high sensitivity and excellent selectivity over other metal ions in neat aqueous solution.