A Host–Guest Switched Fluorescent Detection Method for Oxytetracycline via Quenching of N,Mg-CDs and Target-Induced Emission

The issue of antibiotic pollution in the environment continues to attract attention. Herein, a host-guest switched ratiometric fluorescent detection method for oxytetracycline (OTC) was developed by quenching the fluorescence of nitrogen- and magnesium-codoped carbon dots (N,Mg-CDs) and inducing fluorescence enhancement of OTC. N,Mg-CDs were synthesized by dissolving waste distillers' grains in a type IV deep eutectic solvent, yielding a structure rich in graphitic nitrogen, diverse functional groups, and active magnesium sites. During OTC detection, the quenching of blue fluorescent N,Mg-CDs and concurrent enhancement of green OTC fluorescence produce a distinct color transition with a 0.126 μM detection limit, enabling rapid monitoring via a smartphone-assisted chromaticity analysis platform. Through characterization analysis, DFT/TDDFT, and AIM theory, we propose that the electron-rich oxygen atoms in the β-keto-enol structures of OTC interact with the magnesium active sites on N,Mg-CDs, increasing the rigidity of the OTC molecules and thereby enhancing OTC's green fluorescence, while simultaneously quenching the fluorescence of N,Mg-CDs through the internal filter effect (IFE). This method was successfully validated in real sample detection experiments, demonstrating promising application prospects.