Portable fluorescence sensing platform with 5% Eu-doped g-C₃N₄ for visual analysis of four tetracycline antibiotics

Elevated levels of tetracycline antibiotics (TCs) in animal-derived foods threaten human health, necessitating effective residue monitoring. Herein, Eu-doped g-C₃N₄ fluorescence probe was prepared via high-temperature solid-phase method. Upon interaction with four TCs, g-C₃N₄'s blue fluorescence was quenched by inner filter effect and static quenching. Simultaneously, the Eu³⁺ red fluorescence was enhanced through an antenna effect, owing to complexation between Eu³⁺ and the β-diketone/enol structures of TCs. This probe exhibited differential ratiometric responses and a sensitivity order of oxytetracycline (OTC) > tetracycline (TC) > doxycycline (DOX) > chlortetracycline (CTC), producing distinct blue to variable color changes for visual discrimination of TCs. Limits of detection were 1.00, 2.18, 3.51 and 23.66 nM for OTC, TC, DOX and CTC, respectively. A smartphone-integrated hydrogel sensor was also developed for on-site visual detection of TCs in food samples. This study offers a promising strategy for portable and real-time monitoring of antibiotic residues in food.