Direct upcycling of expired metformin tablets into non-cytotoxic carbon quantum dots

The accumulation of expired pharmaceuticals represents an increasing environmental challenge and highlights the need for sustainable waste valorization strategies. In this work, expired metformin tablets were directly upcycled into fluorescent carbon quantum dots (CQDs) without prior extraction of the active pharmaceutical ingredient, demonstrating a simple and environmentally responsible approach to converting pharmaceutical waste into value-added carbon nanomaterials. CQDs were synthesized using a microwave-assisted hydrothermal method with real-time temperature and pressure monitoring. A two-stage dialysis purification, using membranes with molecular weight cut-offs (MWCOs) of 14 000 Da and 100-500 Da, efficiently removed larger aggregates together with ionic and low-molecular-weight impurities, yielding purified carbon nanodots. The obtained CQDs exhibited a quantum yield of 10.02 ± 0.77%, strong fluorescence, and good aqueous dispersibility. Structural and surface analyses (HR-TEM, FTIR, XPS, and UV-vis spectroscopy) confirmed successful carbonization, nitrogen incorporation from metformin, and the presence of oxygen-rich surface functional groups. The synthesized CQDs did not exhibit cytotoxicity toward DU145 prostate cancer cells after 72 h of exposure. This study demonstrates that expired pharmaceutical products can serve as sustainable precursors for the direct synthesis of biocompatible carbon quantum dots, supporting green chemistry principles and expanding opportunities for pharmaceutical waste valorization.