κ-Carrageenan-derived carbon dots for highly selective and sensitive detection of Fe³⁺ and oxytetracycline

Carbon dots (CDs) have received increasing attention as fluorescence sensors for the detection of different metal ions and organic small molecules. In this work, κ-carrageenan (κ-Carr)-derived carbon dots (C-CDs) were synthesized via a facile hydrothermal treatment with κ-Carr as a carbon source. The produced C-CDs exhibited a high fluorescence with fluorescent quantum yield up to 20.6% and excellent fluorescent stability under diverse conditions (UV radiation, pH value and ionic strength). Then, the C-CDs were utilized as dual functional sensing probe to detect both Fe³⁺ and oxytetracycline (Otc). The fluorescence intensity of C-CDs was selectively quenched by Fe³⁺, in sharp contrast to dozens of other metal ions like Al³⁺, Ba²⁺, Ca²⁺, Co²⁺, Mg²⁺, Pb²⁺, Na⁺, Hg²⁺, Ni²⁺, Zn²⁺, Cd²⁺ and Cu²⁺. Coincidentally, this fluorescence could also be selectively quenched by Otc, entirely distinct from related physiological molecules like glycine, ammonium hydroxide, glucose, urea. The prepared C-CDs could be used for label-free sensitive and selective fluorescent detector for Fe³⁺ in a wide concentration range of 0–500 μM with a detection limit of 0.21 μM and for Otc as well in the range of 0–100 µM with a detection limit of 0.05 µM. Moreover, the fluorescent C-CDs were successfully used for Fe³⁺ and Otc detection in some real samples, suggesting great application potential in fluorescence sensing of trace metal elements and antibiotics. Graphic abstract: