A novel, green, facile and dual turn-off/on sensor for detection of Hg2+ and I- was developed based on carbon dots. Carbon dots were synthesized from citric acid, urea, and thiourea by microwave-assisted method. The size of the carbon dots (CDs) was about 10 nm and the synthesized CDs showed a strong emission at 523 nm upon excitation at 416 nm. The fluorescence quantum yield was 19.2%. Mercury (II) quenched the fluorescence of carbon dots. This turn off sensor had linear response for Hg2+ over a concentration range from 0.1 to 20 µM with detection limit as low as 62 nM. The carbon dots/Hg2+ system was also used as a turn on sensor for detection of iodide. Linear concentration range for I- was 0.1–10 µM with detection limit as low as 72 nM. The proposed method showed good sensitivity and selectivity with respect to interference ions. Finally, this system was successfully used for the detection of Hg2+ and I- in tap, river and mineral waters and fish samples.
