A fluorescence “turn-on” chemosensor for Hg²⁺ and Ag⁺ based on NBD (7-nitrobenzo-2-oxa-1,3-diazolyl)

A new fluorescent sensor 1 was prepared by bridging a 7-nitrobenzo-2-oxo-1,3-diazolyl (NBD) fluorophore with a dimethyl ethylene amine group via an ethylamine spacer. Distinct “turn-on” fluorescence changes of 1 were observed upon the addition of Hg²⁺ and Ag⁺ in the aqueous solution. The sensor 1 showed high sensitivity toward Hg²⁺ and Ag⁺ with detection limits of 0.05 μM and 0.12 μM, respectively. Moreover, the sensing abilities of 1 for Hg²⁺ and Ag⁺ were successfully carried out in real water samples, and 1 functioned as fluorescent test strip with silica plate. The sensing mechanisms of 1 with Hg²⁺ and Ag⁺ were studied by using photophysical experiments, NMR titration, and ESI-mass spectrometry analysis. Moreover, turn-on fluorescence of 1 toward Hg²⁺ and Ag⁺ caused by photo-induced electron transfer (PET) was explained by density functional theory (DFT) calculations.