The Distribution of Active Substances and the Bacterial Inactivation Effect Induced by a Helium Microplasma

Inactivation of bacteria by plasma is related to its active substances; the aim of this study is to investigate the impact of active substances’ spatial distribution on inactivation efficiency. In this study, a comparative analysis of the distribution and concentration of reactive oxygen and nitrogen species (RONSs) in the cross section of the plasma jet was conducted under different discharge voltages, working gas flow rates, and treatment distances. Then the impact of RONS distribution on the inactivation efficiency against Staphylococcus aureus (ATCC 25923) biofilm was analyzed. Experimental results demonstrated an influence of gas flow rates and treatment distances on RONS distribution. For instance, at the treatment distance of 3 mm, RONS distribution showed a solid circular shape at 0.4 standard liters per minute (SLM) and below, a double ring shape at 0.5 SLM, and a ring shape at 0.6 SLM. At 5 mm, the RONS distribution showed a solid circular shape at 0.8 SLM and below, a double ring shape at 0.9 SLM, and a ring shape at 0.9 SLM and above. The total redox concentration exhibited a positive correlation with the three physical parameters. Biofilm was treated for 100 s at a vertical position of approximately 3 mm. The inactivation of biofilm by the jet was slightly more efficient at 0.4 SLM (RONS was characterized by a low concentration of solid circular shape) compared to 0.8 SLM (high concentration of ring shape). Extending the treatment time to 300 s resulted in similar inactivation efficiency at 0.8 to 0.4 SLM.