Atmospheric pressure plasma jet utilizing Ar and Ar/H₂O mixtures and its applications to bacteria inactivation

An atmospheric pressure plasma jet generated with Ar with H₂O vapor is characterized and applied to inactivation of Bacillus subtilis spores. The emission spectra obtained from Ar/H₂O plasma shows a higher intensity of OH radicals compared to pure argon at a specified H₂O concentration. The gas temperature is estimated by comparing the simulated spectra of the OH band with experimental spectra. The excitation electron temperature is determined from the Boltzmann's plots and Stark broadening of the hydrogen Balmer H_β line is applied to measure the electron density. The gas temperature, excitation electron temperature, and electron density of the plasma jet decrease with the increase of water vapor concentration at a fixed input voltage. The bacteria inactivation rate increases with the increase of OH generation reaching a maximum reduction at 2.6% (v/v) water vapor. Our results also show that the OH radicals generated by the Ar/H₂O plasma jet only makes a limited contribution to spore inactivation and the shape change of the spores before and after plasma irradiation is discussed.