Synthesis of carbon dots from spent coffee grounds: transforming waste into potential biomedical tools

Carbon dots (CDs) are small-sized, spherical nanoparticles presenting amorphous carbon cores with nanocrystalline regions of a graphitic structure. They show unique properties such as high aqueous solubility, robust chemical inertness, and non-toxicity and can be manufactured at a relatively low cost. They are also well known for outstanding fluorescence tunability and resistance to photobleaching. Together, these properties boost their potential to act as drug delivery systems (DDSs). This work presents a low-cost synthesis of CDs by upcycling spent coffee grounds (SCGs) and transforming them into value-added products. This synthetic route eliminates the use of highly toxic heavy metals, high energy-consuming reactions and long reaction times, which can improve biocompatibility while benefiting the environment. A series of physico-chemical characterisation techniques demonstrated that these SCG-derived CDs are small-sized nanoparticles with tunable fluorescence. In vitro studies with 120 h of incubation of SCG-derived CDs demonstrated their specific antiproliferative effect on the breast cancer CAL-51 cell line, accompanied by increased reactive oxygen species (ROS) production. Importantly, no impact was observed on healthy breast, kidney, and liver cells. Confocal laser scanning microscopy confirmed the intracellular accumulation of SCG-derived CDs. Furthermore, the drug efflux pumps P-glycoprotein (P-gp) and the breast cancer resistance protein (BCRP) did not impact CD accumulation in the cancer cells.