Engineering of brewery waste-derived graphene quantum dots with ZnO nanoparticles for treating multi-drug resistant bacterial infections

In this study, the antibacterial efficiency of graphene quantum dots (GQDs) derived from brewery spent grain (BSG) in combination with zinc oxide (ZnO) nanoparticles against the methicillin-resistant Staphylococcus aureus (MRSA) isolated from clinical wound specimens is investigated for the first time. The crystallinity, morphology, and structural defects of the nanomaterials were analysed in detail by the X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and Raman spectroscopy techniques. The layered morphology of BSG-derived reduced graphene oxide (BrGO), d-spacing, and the average particle size of GQDs were further examined by a high-resolution transmission electron microscopy (HR-TEM). Compared to individual GQDs and ZnO, ZnO/GQDs composites showed a better antibacterial activity against Escherichia coli, and Staphylococcus aureus. ZnO/GQDs also demonstrated significant efficiency in disinfecting the MRSA (ATCC and clinical isolates from wound specimens). Density functional theory (DFT) calculations confirmed the substantial clustering of functional groups at the edges of nanomaterial. The disinfection mechanism of MRSA is elucidated for the first time with DFT calculations. Cytotoxicity experiments with the 3T3 mouse embryo fibroblasts testified that ZnO/GQDs are not toxic to mammalian cells.