Fast and efficient “on-off-on” fluorescent sensor from N-doped carbon dots for detection of mercury and iodine ions in environmental water

A kind of nitrogen doped carbon dots (N-CDs) was facilely fabricated from polyethyleneimine and anhydrous citric acid, and which was adopted to develop a neoteric "on-off" and "off-on" fluorescent sensor for rapidly and efficiently sensing Hg2+ and I-. The fluorescence of N-CDs was notably quenched (off) in the existence of Hg2+ derived from strong interaction and the electron transfer between N-CDs and Hg2+, while the quenched fluorescence of the N-CDs and Hg2+ system was strikingly regained by addition of I- (on) resulted from the separation of N-CDs and Hg2+ due to the higher binding preference between Hg2+ and I-. Under optimal conditions, the linear detection ranges were 0.01-20 μM for Hg2+ and 0.025-7 μM for I-, respectively. Meanwhile, the detection limits could be down to 3.3 nM for Hg2+ and 8.5 nM for I-, respectively. Satisfied recoveries had also been gained for measuring Hg2+ and I- in practical water samples. The constructed "on-off-on" fluorescent sensor provided a simple, rapid, robust and reliable platform for detecting Hg2+ and I- in environmental applications.