Construction of a ratiometric fluorescence sensor using Mahonia fortunei leaf-derived N-CDs and Eu3+ for smartphone-assisted portable and visual monitoring of tetracycline residues in food

In this study, N-CDs were combined with Eu 3+ to construct a ratiometric fluorescence sensor (N-CDs@Eu 3+ ) for the determination of tetracycline (TC). The nitrogen-doped carbon dots (N-CDs) with good water solubility, excellent fluorescence performance, and high stability successfully prepared by using the leaves of Mahonia fortunei leaf as the carbon source and ethylenediamine as the nitrogen source through a one-step thermal solvent method. In the presence of TC, the fluorescence intensity of N-CDs at 510 nm was quenched due to fluorescence resonance energy transfer (FRET), while the fluorescence intensity of Eu 3+ at 616 nm was significantly enhanced through the antenna effect (AE). This ratiometric fluorescence sensor enables rapid, sensitive and highly selective detection of TC, with a detection limit as low as 7.5 nM. Moreover, a distinct color change from green to red can be observed under a 365 nm UV lamp. With the assistance of a smartphone sensing platform, visual semi-quantitative monitoring of TC was achieved. This method was successfully applied to the detection of TC in milk, pork, and honey, with recoveries ranging from 90.6% to 105.0%. This research demonstrates that this sensor holds potential application value in the field of real-time visualization detection and semi-quantitative determination of TC in biomedical and food safety sensing fields. Graphical abstract