Atmospheric pressure plasma treatment of lipopolysaccharide in a controlled environment

The atmospheric pressure plasma jet (APPJ) has been widely investigated for sterilization of surfaces, but studies on surface chemical changes of model compounds in controlled environments have been lacking. We present measurements on lipopolysaccharide (LPS) using x-ray photoelectron spectroscopy after 1% O-2 in Ar APPJ treatments in controlled ambients composed of N-2/Ar mixtures. By varying the N-2 concentration from 20% to 100%, we find that the interaction of the jet with the environment plays a major role in modifying surface reactions. This is due to the plasma exciting N-2, which quenches reactive oxygen species (ROS) that would otherwise modify the film surface. By minimizing the interaction of the APPJ with the environment, e.g. by changing the APPJ geometry, we show that surface modifications increase even when the plasma itself is removed farther from the LPS surface. Measurements on the biological activity, optical emission, and ozone production of the jet using O-2, N-2 and O-2/N-2 admixtures all demonstrate that ROS are readily quenched by N-2 species excited by the plasma. These results clearly reveal the importance of considering plasma-environment interactions for APPJ treatments of surfaces.