Synthesis and photophysical characterisation of nitrogen-doped carbon quantum dots for antibiotic sensing

The antibiotics can enter the environment through patients’ urine samples due to improper metabolism and the disposal of untreated pharmaceutical industrial waste, which can contaminate environmental samples and harm living creatures. Therefore, an economical and easy-to-use method need to be developed for their detection. Carbon dot-based optical methods are advantageous for sensing ultra-low concentrations of various antibiotics. Herein, we report microwave-assisted aqueous synthesis of Nitrogen-doped carbon quantum dots (NCQD) from tartaric acid and 3,3-diaminobenzidine as carbon and nitrogen sources, respectively, for the differential detection of antibiotic ciprofloxacin via fluorescence colour change from green to blue in pharmaceutical formulation, tap water, and milk samples. The as-synthesised NCQD was characterised using FT-IR, Raman spectroscopy, HRTEM, DLS, XRD, and EDX. Methodological investigation disclosed the distinctive properties of NCQD, including excitation-dependent emission, multi-colour fluorescence response in various solvents, and photostability. Moreover, NCQD also exhibits strong radical-scavenging activity, determined by the DPPH assay, and can be used as green and blue fluorescent ink. Aqueous solution of NCQD exhibits a selective and sensitive fluorescence response towards ciprofloxacin antibiotic in the presence of other different classes of antibiotics in an ultra-low concentration, with a detection limit of 14 nM.