A Highly Sensitive and Selective Nano-Fluorescent Probe for Ratiometric and Visual Detection of Oxytetracycline Benefiting from Dual Roles of Nitrogen-Doped Carbon Dots

The specific detection of oxytetracycline (OTC) residues is significant for food safety and environmental monitoring. However, rapid specific determination of OTC from various tetracyclines is still challenging due to their similar chemical structures. Here, nitrogen-doped carbon dots (NCDs) with excitation and pH-dependent optical properties and a high-fluorescence quantum yield were successfully synthesized, which were directly employed to fabricate a dual-response fluorescence probe by self-assembly with Eu3+ (NCDs/Eu3+) for the ratiometric determination of OTC. The addition of OTC into the probe greatly enhances the characteristic emission of Eu3+ due to the "antenna effect", and the incorporation of NCDs into the probe further improves the Eu3+ fluorescence by remarkably weakening the quenching effect caused by H2O molecules and efficiently shortening the distance of energy transfer from OTC to Eu3+. Meanwhile, the fluorescence of NCDs apparently decreases due to aggregation-caused quenching. The results demonstrate that a ratiometric detection of OTC (0.1-25 mu M) with a detection limit of 29 nM based on the double response signals is achieved. Additionally, visual semi-quantitative assay of OTC can be realized with the naked eye under a 365 nm UV lamp according to the fluorescence color change of the as-fabricated probe. This probe exhibits acceptable specificity and anti-interference for OTC assay, holding promise for the fast detection of OTC in real water and milk samples.