Acid-assisted ultrasonic preparation of nitrogen-doped MXene quantum dots for the efficient fluorescence "off-on-off" detection of Zn(II) in water and oxalic acid in vegetables.

A novel fluorescence "off-on-off" probe was presented to detect Zn(II) and oxalic acid (OA) based on nitrogen-doped MXene quantum dots (N-MQDs), which were synthesized by an ultrasound approach at room temperature with nitric acid and ethylenediamine. These N-MQDs displayed small size (<10 nm), water dispersibility, and good photoluminescence. Furthermore, the N-MQDs showed an selective response towards Zn(II) through fluorescence enhancement, with a limit of detection (LOD) calculated as 0.127 μM in the linear range of 0-20 μM. Then, the fluorescence of N-MQDs/Zn(II) system could be selectively quenched after adding OA, with an effective response in the range from 0 to 20 μM (LOD: 0.883 μM). The fluorescence "turn-on" and "turn-off" properties of N-MQDs were resulted from the intramolecular charge transfer (ICT) of Zn(II) and the coordination between OA and Zn(II), respectively. This sensing platform was successfully applied for Zn(II) and OA detection in actual environmental and vegetable samples.