Europium-containing carbon dots (Eu-CDs) for ratiometric fluorescence determination of oxytetracycline and tetracycline antibiotics differentiation

Herein, a novel ratiometric fluorescent nanosensor was prepared by in-situ incorporation of Eu 3+ into nitrogen-rich carbon dots (Eu-CDs) via a one-step hydrothermal route. The obtained Eu-CDs display dual emission: a strong blue band at 440   nm (carbon core) and a weak red band at 615   nm ( 5 D 0 → 7 F 2 transition of Eu 3+ ). Upon titrating oxytetracycline (OTC) into the system, the blue signal remained invariant as an internal reference, whereas the red emission dramatically enhanced by the OTC-mediated antenna effect. The intensity ratio ( I 615 / I 440 ) increases linearly with OTC concentration over 1.0 – 50   µM, affording a limit of detection of 35   nM. Concomitantly, the dispersion exhibits a naked-eye color switch from blue to red under 365   nm UV illumination. Mechanistic investigations reveal that the specific recognition originates from (i) coordination between Eu 3+ and the β-diketone moiety of OTC and (ii) hydrogen-bonding interactions between OTC and the CDs surface. Although the probe is highly selective toward OTC over common ions and amino acids, it suffers from cross-reactivity among tetracycline antibiotics. This limitation was overcome by coupling the ratiometric read-out with linear discriminant analysis (LDA) of dual-mode (fluorescence and absorption) data, enabling 100% correct classification of OTC, tetracycline (TC), chlortetracycline (CTC), and doxycycline (DC) in both individual and mixed samples. The proposed strategy furnishes a simple, green and low-cost route to Eu-CDs and establishes a robust, portable platform for ultrasensitive quantification and discriminative sensing of tetracycline antibiotics in complex matrices.