Two-Luminescent-Site Synergistic Responses in a Lanthanide-Functionalized Covalent Organic Framework for Ratiometric Fluorescence and Visual Detection of Cinoxacin Antibiotics

Abstract There is an urgent need to design a sensitive and rapid sensor for visual sensing of cinoxacin (CIN) antibiotics. Herein, a two-luminescent-site synergistic responsive ratiometric fluorescent sensing platform was rationally constructed by postsynthetically anchoring lanthanide Eu3+ onto a luminescent covalent organic framework (COF@Eu3+). The resulting COF@Eu3+ features two luminescent sites specifically tailored for CIN. Upon exposure to CIN, the green emission of the COF is efficiently quenched by the inner filter effect, while the characteristic red luminescence of Eu3+ is remarkably enhanced via the antenna effect. This two-luminescent-site response strategy generates a self-calibrated and synergistic ratiometric signal for CIN detection. Consequently, the two-luminescent-site-responsive COF@Eu3+ ratiometric fluorescent probe exhibits high sensitivity, a low detection limit (0.065 μM), a broad linear range (0.01–60 μM), and a rapid response (within 30 s) for CIN detection. Furthermore, the sensor exhibits a distinct fluorescence color evolution from green to orange and ultimately to red as the CIN concentration increases. Given its prominent advantages, a portable and user-friendly smartphone-assisted visual sensor was also exploited for CIN detection, offering a promising strategy for on-site qualitative identification and semi-quantitative monitoring of CIN. This work not only offers a reliable strategy for the on-site visual sensing of CIN but also expands the application of lanthanide-functionalized COFs.