Influence of Retrogradation on KI–Starch Visualization of Reactive Oxygen Species Emitted by Plasma Jets

Plasma jets have been widely researched in the fields of medicine, biology, and agriculture and their positive effects were reported. To understand the working mechanism of plasma-target interaction, the spatial distribution of reactive oxygen species (ROS) at the target's surface should be examined as they are considered to be the most affectable factor to treat the target. The authors have used KI-starch method in the form of gel or liquid, which uses reactions among potassium iodide, starch, water, and oxidative species, to visualize the distribution of ROS. This method gives information regarding the distribution of ROS by coloration; however, the retrogradation of starch possibly influences on the visualization especially in a liquid KI-starch reagent. In this article, the visualization on the distribution of ROS was done via the liquid KI-starch reagent during plasma jets' irradiation with different helium gas flow rates from 1 to 4 L/min. The retrogradation of starch was confirmed when the gas flow rate was higher than 3 L/min, because the reagent's surface was cooled down, which incapacitated the ability of coloration from the reagent. It was revealed that a ring-like coloration pattern could be observed at the gas flow rate of 3 L/min, because the coloration occurred at the outside of an uncolored circular region where the retrogradation of starch was formed by the gas flow and insensitive to ROS.