Synthesis of carbon quantum dot synthesized using spent coffee ground as a biomass exhibiting visible-light-driven antimicrobial activity against foodborne pathogens

The aim of this study was to synthesize carbon quantum dot (CQD) exhibiting visible-light-driven (VLD) antimicrobial activity using spent coffee ground (SCG) as biomass. SCG was used to synthesize CQD through microwave treatment. It was confirmed that typical CQD was synthesized well based on morphology (high resolution transmission electron microscopy), size distribution (dynamic light scattering), and chemical structure (X-ray photoelectron spectroscopy) analyses. VLD antimicrobial activities of CQD synthesized from SCG against Staphylococcus aureus and Escherichia coli O157:H7 were found to be increased as pH decreased. Photodynamic properties of CQDs such as fluorescence quantum yield (FLQY), band gap energy (Eg), and fluorescence lifetime as photodynamic properties of CQDs related to VLD antimicrobial activity did not show any significant changes according to pH change. However, as pH decreased, negative zeta potential (mV) values of CQD and pathogenic bacteria gradually decreased. As a result, it was confirmed that CQD uptake into pathogens was gradually increased due to a decrease in repulsive force between them. In addition, it was found that as pH decreased, activities of enzymes (superoxide dismutase and catalase) known to scavenge reactive oxygen species (ROS) in cells decreased. In the presence of CQD, when visible light was applied, the amount of ROS generated inside cells increased as pH decreased. Based on these results, CQD synthesized from SCG was combined with organic acid (malic acid) for practical application in fresh produce (apple surface) washing. It was confirmed that such combination had an excellent control effect against pathogenic bacteria, suggesting that this strategy could be effectively utilized in the food industry.