Synthesis of carbon quantum dot from spent coffee ground exhibiting visible-light-driven antimicrobial activity against Escherichia coli O157:H7

Abstract

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. CQD was synthesized from SCG through microwave treatment. It was confirmed that typical CQD was synthesized through analysis of morphology (high-resolution transmission electron microscopy), size distribution (dynamic light scattering), and chemical structure (X-ray photoelectron spectroscopy). VLD antimicrobial activities of CQD synthesized from SCG against Escherichia coli O157:H7 were found to be increased as pH decreased. Fluorescence quantum yield (FLQY), band gap energy (Eg), and fluorescence lifetime as photodynamic properties of CQD related to VLD antimicrobial activity did not show any significant changes according to pH change. On the other hand, 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 E. coli O157:H7 was gradually increased due to a decrease in repulsive force between them. It was confirmed that major reactive oxygen species (ROS) that contributed to the inactivation of E. coli O157:H7 was singlet oxygen. In addition, it was found that as pH decreased, activities of enzymes (superoxide dismutase and catalase) known to scavenge ROS in cells decreased. In the presence of CQD, when visible light was applied, the amount of ROS generated inside the cell increased as the pH decreased. In addition, VLD antimicrobial activities of CQD were applied to the apple slices washing and significant (P < 0.05) reduction of E. coli O157:H7 occurred in both apple and liquid, and cross-contamination was also prevented. The activity of CQD was maintained after reuse for three consecutive treatment cycles, and there was no difference in activity by the date of manufacturing of CQD for four weeks. Results of this study suggest that it is advantageous to use CQD synthesized from SCG under acidic conditions and VLD antimicrobial activities can be applied to the fresh produce washing process. Further research is needed on a method for synthesizing CQD that is not affected by pH conditions.