The effect of gas additives on reactive species and bacterial inactivation by a helium plasma jet

In this paper, the influences of gas doping (O₂, N₂, Air) on the concentrations of reactive species and bactericidal effects induced by a He plasma jet are studied. Firstly, results show that gas doping causes an increase in voltage and a decrease in current compared with the pure He discharge under the same discharge power, which might be attributed to the different chemical characteristics of O₂ and N₂ and verified by the changes in the gaseous reactive species shown in the optical emission spectroscopy (OES) and Fourier transform infrared (FTIR) spectroscopy. Secondly, the concentrations of aqueous reactive oxygen species (ROS) and reactive nitrogen species (RNS) are tightly related to the addition of O₂ and N₂ into the working gas. The concentrations of aqueous NO₂- and NO3- significantly increase while the concentrations of aqueous ROS decrease with the admixture of N₂. The addition of O₂ has little effect on the concentrations of NO₂- and NO₃- and pH values; however, the addition of O₂ increases the concentration of O₂- and deceases the concentrations of H₂O₂ and OH. Finally, the results of bactericidal experiments demonstrate that the inactivation efficiency of the four types of plasma jets is He+O₂>He+Air>He>He+N₂, which is in accordance with the changing trend of the concentration of aqueous O₂-. Simultaneously to the better understanding of the formation and removal mechanisms of reactive species in the plasma–liquid interaction, these results also prove the effectiveness of regulating the concentrations of aqueous reactive species and the bacteria inactivation effects by gas doping.