Repurposing disposable medical syringes into valuable fluorescent carbon dots: application to the fluorometric determination of nintedanib.

Monitoring nintedanib (NTB) using reliable analytical methods is essential for ensuring safe dosing, minimizing toxicity, assessing drug-drug interactions, and supporting quality control in personalized cancer therapy. In this work, we present a cost-effective and energy-efficient hydrothermal strategy for synthesizing nitrogen-doped carbon dots (CDs) from disposable plastic syringes. The process, carried out at 200 °C following a calcination pretreatment, not only provides a sustainable route for valorizing biomedical waste but also addresses pressing environmental challenges. The as-prepared CDs exhibited intense green fluorescence, outstanding photostability, and a high quantum yield of 46.42%, reflecting their superior optical performance. Upon exposure to NTB, a concentration-dependent quenching response at 470 nm was observed, primarily mediated by the inner filter effect (IFE). This mechanism enabled highly sensitive NTB detection, with an ultralow detection limit of 2.5 nM (S/N = 3). The probe demonstrated remarkable selectivity, showing negligible interference from common coexisting ions, biomolecules, and anticancer drugs. Analytical accuracy was validated by recovery studies in spiked serum and urine samples, which ranged from 97.6% to 103.2%, while RSD values below 3.48% confirmed excellent precision and reproducibility. These findings establish the proposed CD-based probe as a robust, reproducible, and clinically relevant tool for NTB quantification. By demonstrating the conversion of discarded medical plastics into high-value nanomaterials, this work presents a strategy that aligns with the goals of green nanotechnology and delivers a practical platform for bioanalytical sensing, therapeutic drug monitoring, and pharmacokinetic studies. The dual focus on waste repurposing and clinical utility underscores the potential the potential of syringe plastic-derived CDs for translation into next-generation biomedical diagnostics.