Fluorescence 'on-off-on' chemosensor for the sequential recognition of Hg2+ and cysteine in water

A simple fluorescent chemosensor 1 for the sequential detection of Hg2+ and cysteine was developed by combination of benzene-1,2-diamine and 6-bromopyridine-2-carboxaldehyde. The sensor 1 exhibited an 'on-off' fluorescent quenching response in the presence of Hg2+, and could be applied for detection of Hg2+ with a good recovery in water samples. The sensing mechanism of 1 for Hg2+ was supported by theoretical calculations. Moreover, the resulting Hg2+-2.1 complex acted as an efficient 'off-on' sensor for cysteine, showing recovery of 1 from Hg2+-2. 1 complex. Therefore, the sensor 1 can be employed as a practical fluorescent chemosensor for recognition of Hg2+ and cysteine in aqueous solution.