Reversible and ultrafast fluorescent nanoswitch for on-site visual determination of malachite green and in vivo zebrafish imaging.

Malachite green (MG), one of the most toxic veterinary drug residues, is of great significance to establish a rapid, accurate, and highly selective on-site quantitative detection method for food safety. In this study, a new strategy for the detection of MG in aquatic products is developed, which is based on orange fluorescent carbon dots (O-CDs) as nanoswitches for rapid and reversible detection of MG content. The overlap between the strong absorption of MG at 615 nm and the fluorescence emission spectra of O-CDs at 564 nm provides effective support for the internal filter effect (IFE) leading to fluorescence quenching of the system. Under the attack of negative hydrogen ions in NaBH, MG is effectively reduced to weakly absorb leucomalachite green (LMG), blocking the IFE process and leading to the recovery of system fluorescence, thereby achieving reversible detection of MG within 30 s with up to 9 cycles. In solution, the strategy demonstrates a wide linear range of 1-120 μM for MG with a detection limit of 0.37 μM. In addition, a smartphone-assisted O-CDs agar slice sensing platform has been developed, which can accurately quantify and visually determine MG. Furthermore, it is also utilized for fluorescence imaging of exogenous MG in zebrafish. In summary, the establishment of sensing strategy provides a solution for constructing qualitative and visual semi quantitative sensors on site, which has potential application value in food safety assessment.