Dual-emission carbon dots for sensitive detection of sulfamethoxazole in milk samples

The detection of antibiotic pollutants such as sulfamethoxazole (SMX) remains important for both environmental and food safety monitoring. In this work, dual-emissive nitrogen and phosphorus co-doped carbon dots (N,P-CDs) were developed as a stable fluorescent and phosphorescent probe for SMX detection. The N,P-CDs were synthesized through a one-pot hydrothermal route using pentaethylenehexamine (PEHA) and phosphoric acid, producing nanoparticles that exhibited strong blue fluorescence together with visible room-temperature phosphorescence and a persistent afterglow. The prepared CDs showed excellent stability over a broad pH range, high ionic strength, and in different solvent environments. Optimization of precursor ratios indicated that a 1:1 composition provided the best sensing performance toward SMX. The probe displayed high selectivity against structurally related pharmaceuticals and biomolecules, with detection limits of 6.5 μM in fluorescence mode and 28 μM in phosphorescence mode. Mechanistic investigation suggested that the quenching process involved a combined inner filter effect and static quenching through ground-state complex formation. To further demonstrate practical applicability, the proposed sensor was successfully applied to SMX determination in spiked milk samples. Satisfactory recoveries ranging from 96.50% to 98.32% with low relative standard deviations were obtained using both emission modes, indicating minimal matrix interference after simple sample pretreatment. These findings demonstrate that the developed dual-mode carbon dot probe offers a reliable platform for SMX quantification in complex matrices and highlights its potential for real-world antibiotic monitoring.