A fluorometric/colorimetric dual-channel Hg2+ sensor derived from a 4-amino-7-nitro-benzoxadiazole (ANBD) fluorophore

A new visible light excitable Hg2+ sensor, NBD-TAEE, was prepared by bridging a 4-amino-7-nitro-benzoxadiazole (ANBD) fluorophore with thiaazacrown ether via an ethylene spacer. Besides the specific Hg2+-induced absorption shift from 466 to 513 nm, this sensor exhibits the specific Hg2+-induced emission enhancement (similar to 29-fold) and fluorescent pH-independence from pH 6 to 13. Hg2+ addition even leads to the emission red shift from 532 to 570 nm, although NBD-TAEE is not a ratiometric fluorescent sensor due to its emission band being overlaid by the Hg2+-induced band. The instant and reversible turn-on'' Hg2+ response, fine cell permeability and cell imaging ability display its potential application in both environmental and Hg2+-polluted biological samples. Its analogue without the ethylene spacer, NBD-TAE, possesses the higher quantum yield and very poor Hg2+ binding ability. Their different emission behavior and sensing ability suggest that the spacer is quite essential for sensor construction.