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

Abstract

The accurate and rapid detection of clinically vital but potentially toxic pharmaceutical residues, such as the anticancer agent Doxorubicin (DOX) and the broad-spectrum antibiotic Chlortetracycline (CTC), is essential for therapeutic drug monitoring and environmental safety. This work presents the development of a novel fluorescent sensor based on nitrogen-doped carbon quantum dots (NEDA-CQDs) synthesized via a simple hydrothermal method using N-(1-naphthyl) ethylenediamine (NEDA) and citric acid (CA) as precursors. The obtained CQDs exhibited excellent aqueous dispersibility, strong blue fluorescence with a quantum yield (QY) of 2.7%, and high photostability. Comprehensive characterization revealed uniform quasi-spherical nanoparticles enriched with amino (NH₂), 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.