Lemon zest-derived N,S-doped carbon quantum dots as a highly sensitive and sustainable fluorescent probe for cefoxitin sodium determination

• Ultrafast green microwave-driven synthesis of luminescent N,S-CQDs. • Lemon zest N,S-CQDs have been applied as a sensitive fluorescence sensor. • A static fluorescence quenching mechanism through Stern-Volmer analysis. • ComplexMoGAPI, BAGI, CaFRI, and MoGSA as methods greenness assessment tools. • EPPI (2025) comparison demonstrates the developed N,S-CQDs method is greener and more practical than reported one. Cefoxitin (CEF), classified as a semisynthetic beta-lactam second-generation cephalosporin antibiotic, can be used against many aerobic and anaerobic bacterial infections. Elevated aspartate aminotransferase levels and a higher prevalence of eosinophilia have been linked to higher CEF dosages, which alleviates the demand for ultra-sensitive determination and monitoring of its concentration level. Sulfur and nitrogen doped carbon quantum dots (N,S-CQDs) were synthesized by microwave techniques using lemon zest as a natural biomass source for green modified CQDs synthesis. N,S-CQDs used as a highly sensitive fluorescent probe, green and economic, for the determination of CEF owing to its excellent photoluminescence (PL) characters and exhibited higher quantum yield (QY) ∼37.65 % with high aqueous stability. The quenching mechanism is meticulously demonstrated to be a static quenching process through the utilization of fluorescence spectroscopy and Stern-Volmer analysis across various temperature conditions. This work developed a new method for rapid determination of CEF residues by static quenching mechanism with a detection limit (LOD) value of 0.023 µM within a linearity of 0.1–15.0 µM. Moreover, the proposed fluorescence probe has potential practical application for CEF determination in pharmaceutical dosage forms and biological fluids. Furthermore, four green evaluation tools, the Complex Modified Green Analytical Procedure Index (ComplexMoGAPI), the blue applicability grade index (BAGI), the carbon footprint reduction index (CaFRI), and a Modified Green Star Area (MoGSA), were evaluated to assess the proposed analytical method. The Evaluation and Performance of Practicality Index (EPPI, 2025) assessment demonstrated that the developed green N,S-CQDs-based fluorometric method for CEF determination is a more sustainable, efficient, and practical analytical approach compared to the reported derivatization method.