Rapid and facile synthesis of nitrogen-doped carbon dots for fluorescence-smartphone dual-mode detection of doxycycline and anti-counterfeiting applications

The extensive use of antibiotics in livestock has raised concerns regarding the potential presence of harmful residue levels in animal feed and food products. Herein, nitrogen-doped carbon dots (N-CDs) are rapidly synthesized via pyrolysis with citric acid (CA) and tris(hydroxymethyl)aminomethane (Tris) as precursors. The N-CDs exhibit optimal excitation/emission at 359/429 nm and a quantum yield of 56 %. A novel fluorescence sensing system based on smartphones has been created for the dual-mode detection of doxycycline (DC) by utilizing the inner filter effect (IFE) to quench fluorescence. The platform shows two linear ranges of 1.5–16.7 μM and 16.7–66.8 μM, with corresponding limits of detection (LOD) of 0.45 and 0.81 μM, respectively. The method quantifies DC in milk samples with recoveries of 98.80 %–103.90 %, validating its accuracy and practicality. Furthermore, the N-CDs enable DC monitoring in human hepatocarcinoma cells (SMMC-7721) and zebrafish, owing to their minimal toxicity and outstanding biocompatibility. Interestingly, the N-CDs embedded in the filter paper have remarkable thermal stability in the range of 20–100 °C, even after being heated at 100 °C for 60 min. Their outstanding optical properties enable their use in invisible ink for encryption and anti-counterfeiting. This dual-mode platform has shown significant potential in environmental monitoring and advanced security technologies.