Smartphone-assisted ratiometric fluorescence sensing platform based on Eu-DES@hydrogel for visual quantitative detection of levofloxacin in milk and honey

The presence of fluoroquinolone antibiotic residues brings significant threats to food safety and human health. This situation has necessitated the development of rapid visual detection methods for identifying these residues in food products. This study constructed a self-calibrating ratiometric fluorescent sensor based on europium and deep eutectic solvent complex (Eu-DES) for the sensitive visual detection of levofloxacin (LEV). By mixing Eu 3+ with the DES (oxalic acid:choline chloride = 1:1) ligand at room temperature, a stable red fluorescence at 623 nm was successfully achieved in the Eu-DES system. Upon the addition of LEV, the Eu-DES induced a red-shift fluorescence of LEV, enabling a self-calibrating ratiometric fluorescent response to LEV with a wide linear range (0.5–100 μM) and a low detection limit (86.50 nM, S/N = 3). Furthermore, integrating Eu-DES into a gel matrix and combining it with smartphone-assisted RGB color analysis enabled quantitative LEV detection through multicolor shifts from red to green. This approach was successfully applied to detect LEV in honey and milk with high sensitivity and good recovery (92.7%–106.4%). The Eu-DES-based detection platform developed in this study provides a powerful tool for on-site LEV detection in actual food samples, promising to advance rapid food safety testing technologies.