Discharge mode transition in a He/Ar atmospheric pressure plasma jet and its inactivation effect against tumor cells in vitro

Discharge characteristic comparisons between He and Ar plasma jets have been extensively reported, but is rarely reported for the comprehensive study of discharge mode transition from He jet to Ar jet, especially its induced liquid chemistry and biological effect. In this paper, we investigate the plasma jet mode transformation by varying the Ar contents in the He/Ar mixing working gas, particularly focusing on the effect of liquid chemistry of plasma activated water (PAW) and the corresponding inactivation effect against tumor cells in vitro. The mode transition process from He jet to Ar jet is characterized by the discharge images, the spatial temporal evolution, and the electrical and spectra measurements. It is found that the plasma jet mode displays a transition from diffuse to filamentous mode, which significantly affects the gaseous reactive species production and the ability to deliver into liquid, resulting in a huge difference in physicochemical properties and the concentration of reactive oxygen and nitrogen species in PAW. Furthermore, the A549 lung cancer cell is utilized to reveal the inactivation effect against tumor cells induced by PAW during discharge mode transition, and it is found that the PAW induced by Ar plasma jet can lead to the higher apoptosis efficiency of cancer cells due to the high production of key species (NO₂⁻, H₂O₂, and ONOO⁻/ONOOH) under filamentous mode. This study would provide deep insights into the nature of liquid physicochemistry and its anticancer effect during plasma jet mode transformation.