Onion-Peel Carbon Quantum Dots: Antimicrobial Effect and Biofilm Control on Food Contact Surfaces

As by-products rich in flavonoids and phenolic compounds, onion peels are globally undervalued and often discarded. This study reports the synthesis of carbon quantum dots (CQDs) from onion peels and evaluates their antimicrobial effectiveness against key foodborne pathogens and biofilms on common food contact surfaces, including plastic, stainless steel, and rubber. The CQDs exhibited a quasi-spherical shape with particle sizes ranging from 1.7 to 9.0 nm and contained abundant oxygen- and nitrogen-functional groups, as confirmed by FT-IR and XPS analyses. The CQDs showed significant antimicrobial activity, with minimum inhibitory concentrations (MICs) and minimum bactericidal concentrations (MBCs) against <i>Salmonella typhimurium</i>, <i>Escherichia coli</i> O157: H7, <i>Listeria monocytogenes</i>, and <i>Staphylococcus aureus</i> of 2200/2800 µg/mL, 1400/2000 µg/mL, 1200/1800 µg/mL, and 400/600 µg/mL, respectively. Time-kill assays confirmed these results. In biofilm tests, <i>S. typhimurium</i> formed biofilms more easily than <i>L. monocytogenes</i>. Washing with CQD solution for 5 min reduced biofilm presence by 81.6-91.5% for <i>S. typhimurium</i> and over 74% for <i>L. monocytogenes</i>, with more than 94% reduction after 10 min of treatment (over 94% for <i>S. typhimurium</i>; 95.8-98.8% for <i>L. monocytogenes</i>) across all surfaces, especially on plastic and stainless steel. These findings indicate that onion peel-derived CQDs are promising, eco-friendly agents for disrupting biofilms and turning undervalued waste into valuable products.