Nitrogen-doped carbon quantum dots as fluorescent sensor for doxorubicin and chlortetracycline: experimental and DFT insights

), hydroxyl (OH), and carboxyl (COOH) functionalities. The NEDA-CQDs exhibited remarkable fluorescence quenching in the presence of DOX and CTC, allowing for their quantitative detection with limits of detection (LOD) of 4.02 µM and 2.53 µM, respectively. Stern-Volmer analysis demonstrated highly linear quenching behavior, indicating a combined static quenching and inner filter effect (IFE) mechanism. Application to human serum and urine samples resulted in excellent recoveries ranging from 93.65% to 106.34%, highlighting the practical utility of the sensor. Density functional theory (DFT) calculations further elucidated the sensing mechanism, demonstrating strong binding energies, significant HOMO-LUMO gap reductions, Fermi level shifts, and enriched non-covalent interactions (NCI), including π-π stacking and hydrogen bonding. Reduced density gradient (RDG) and NCI analyses confirmed the formation of a stable drug-sensor complex, consistent with the experimentally observed fluorescence quenching. Overall, the synergy between experimental findings and theoretical insights establishes NEDA-CQDs as an efficient, low-cost, and robust fluorescent nano-sensor for monitoring DOX and CTC in biological and environmental matrices.