Sulfur- and nitrogen-co-doped carbon dots derived from expired ceftriaxone powder for sensitive fluorometric and electrochemical detection of vandetanib

a one-step hydrothermal carbonization of expired ceftriaxone powder (180 °C, 8 h) with a product yield of 18.6%. The as-prepared N,S-CDs exhibited bright blue fluorescence (quantum yield 34.9%), excellent water dispersibility, and remarkable electrochemical activity. In the fluorometric mode, VADB quenched the N,S-CD fluorescence through an inner filter effect (IFE), providing a linear response from 0 to 300 µM with a detection limit of 0.0045 µM. In the electrochemical mode, N,S-CDs modified on a glassy carbon electrode enabled sensitive voltammetric detection of VADB oxidation over 0.1-500 nM with an ultralow detection limit of 0.025 nM. Both modes showed excellent selectivity against common interfering species and good reproducibility (RSD < 4%). The dual-mode platform was successfully applied to quantify VADB in pharmaceutical tablets and spiked human serum, with recoveries of 96.8-105.3% and results in good agreement with a reference HPLC-UV method. Beyond analytical performance, this work demonstrates a green, low-cost strategy that converts pharmaceutical waste into a high-value nanomaterial for reliable drug monitoring.