Colorimetric “off–on” fluorescent probe for selective detection of toxic Hg2+ based on rhodamine and its application for in-vivo bioimaging

Abstract

A novel “turn on” fluorimetric sensor 1-benzyl-N-(3',6'-bis(ethylamino)-2',7'-dimethyl-3-oxospiro[isoindoline-1,9'-xanthen]-2-yl)-7-hydroxy-5-oxo-1,2,3,5-tetrahydroimidazo[1,2-a]pyridine-8-carboxamide (RhB) based on rhodamine moiety has been designed and synthesized. The cation sensing ability of RhB was examined and validated using UV–visible, fluorescence, and NMR spectral studies. The probe RhB exhibits selective and sensitive sensing towards Hg2+ over the other competitive metal cations like Ag+, Al3+, Ba2+, Cu2+, Cd2+, Fe3+, Fe2+, K+, Li+, Mg2+, Mn2+, Pb2+, Ni2+, Na+, Zn2+ in EtOH:HEPES buffer solution (1:1, v/v) medium and a rapid 1:1 stoichiometric irreversible reaction were obtained between RhB and Hg2+. The UV–vis and fluorescence spectral changes suggest the effectiveness of the probe and the rapid response of RhB towards Hg2+ could be due to the formation of the coordinate bond between the ligand and metal. The spirolactam ring opening followed by the formation of suitable coordinate geometry in the RhB is the reason for the selective sensing of Hg2+ ion. NMR titration experiments further confirmed this. The binding constant was calculated by the B-H plot and found to be 4.0 × 109 M−1. The LOD for UV–vis and fluorescencebased detection was found to be 2.57 µM (0.516 mg L−1). Likewise, an economically feasible, easy-to-use test strips of RhB was developed for the naked-eye detection of Hg2+ in 100% aqueous solution. However, it was found to be much less sensitive than the spectrophotometric method. Further, in vivo experiments were performed using Drosophila melanogaster that led to demonstrate its efficacy at identifying mercury toxicity within an organism.