Oxytetracycline-derived carbon dots as a fluorescent switch in trace ferric ion sensing

The analysis of trace Fe(iii) ions in water is of vital importance in the fields of biology, environment and health. The fluorescent switch technique based on photoluminescent carbon dots (CDs) is highly favorable due to their bio-adaptability, safety and economy, but it is hindered in the highly specific recognition at the trace level. In this work, oxytetracycline-derived CDs (denoted as OCDs) with a uniform size (ca. 2.04 nm), nitrogen dopant, and rich hydroxyl-/carboxyl- groups have been successfully fabricated via a bottom-up hydrothermal carbonization approach. The as-prepared OCDs have displayed excellent photoluminescence characteristics and fluorescence switch performance for the rapid detection of Fe(iii) within 2 min with the limit of detection (LOD) as low as 0.440 mu M (24.6 ppb) and the second highest sensitivity of 3.52 x 10(-2) mu M-1 among reported CDs-based Fe(iii) fluorescent switch materials. This detection system is stable in the analysis matrix with many interfering metal ions, at least with the help of the masking agent cysteine, realizing the specific detection of Fe(iii) in real water environments. The contributions from the fluorophore concentration and the affinity between fluorophore and quencher have been revealed by the numerical model, providing a roadmap for the design of metal ion fluorescence switch materials based on a complex formation quenching mechanism. Fluorescence cell imaging in plant cells has also been performed successfully with the as-synthesized OCDs. This work presents a high-performing CDs-based Fe(iii) fluorescence switch derived from the environmentally-risky oxytetracycline, as well as a feasible strategy for recycling waste antibiotics.