Rapid Inactivation of E. coli by Water Droplet-Tuned Surface Micro-Discharges

In this study, rapid inactivation of Escherichia coli ( E. coli ) by surface micro-discharge (SMD) plasma is studied by comparing the inactivation effects in air with and without water droplets (WDs) under different distances ( d ) between the samples and the grounded electrode. The discharge current, discharge power and spatial–temporal developments of SMD are also compared. The pH value, conductivity and the concentrations of reactive oxygen and nitrogen species (RONS) of condensed water are determined with the presence of WDs. In addition, the role of water vapor on the sterilization process is analyzed by evaluating the inactivation efficiency of E. coli treated with plasma effluent through the gas outlet. The measurements imply that the variation in discharge power may not be the main factor affecting E. coli inactivation in WD-tuned air. The results of sterilization display that with the presence of WDs, the log value of remaining E. coli on the surface of the sample can be rapidly reduced to 0.05 in 5 s by SMD plasma treatment as the sample surface stained with E. coli backing on the grounded electrode (GND) at d = 1.1 mm. However, a significant decrease in E. coil survival without WDs takes more than 30 s or longer. It also suggests that WD-tuned SMD can significantly contribute to the generation of reactive species associated with water, thus leading to the rapid inactivation of E. coli . These species may be transferred to the surface of the treated samples through air or water clusters.