A dual-mode ratiometric probe using europium-doped cyclen-functional carbon dots for fluorescent and point-of-care detection of tetracycline.

The overuse of tetracycline (TC) has led to the accumulation of antibiotic residues in drinking water and animal products, which can consequently lead to bacteria resistance and chronic disease in humans. Urgently addressing the need for a rapid, user-friendly, and point-of-care test for TC detection. In this work, we use cyclen and citric acid to synthesise carbon dots (CDs) with a unique ring-shaped structure on their surface and combine them with europium (Eu) to form an Eu-CDs fluorescent probe. In the presence of TC in aqueous systems, the Eu-CDs probe emits two distinctive fluorescent signals: the stable blue emission from cyclen-modified CDs and the red emission from Eu,showing a proportional increase with TC concentration. The developed Eu-CDs probe demonstrates accurate and selective detection capabilities for TC class antibiotics among various interfering factors. The Eu-CDs probe exhibits excellent linearity within the concentration range of 0.04-2.4 µM and achieves an impressive detection limit of 2.7 nM. Moreover, point-of-care Eu-CDs test strips are designed, allowing convenient on-site TC analysis through the detection of a colour change from blue to red under a portable UV light. The results highlight the effectiveness of the proposed dual-mode ratiometric fluorescent Eu-CDs probe and test strips, offering a practical point-of-care testing strategy for real-world TC detection applications.