Low and Atmospheric Pressure Plasma Interactions with Biomolecules and Polymers

Abstract

Cold atmospheric plasma (CAP) sources have emerged as economical and environmentally friendly sources of reactive species with promising industrial and biomedical applications. Many different sources are studied in the literature for advanced applications including surface disinfection, wound healing, and cancer treatment, but the underlying mechanisms for these applications are not well-understood. The overall goals of this dissertation are to 1) identify how plasma treatments induce surface modifications and which plasma species are responsible for those modifications; 2) identify how changes in surface and plasma chemistry contribute to changes in biological activity of biomolecules; and 3) investigate how fluxes of reactive species produced by atmospheric pressure plasma devices can be controlled. As a first step, a well-studied low pressure plasma system was used to isolate the effects of ions, high energy photons, and radicals using Ar and H2 plasma. The finding that plasma-generated radicals can biodeactivate and modify films with negligible etching motivated further study at atmospheric pressure. Two very different CAP sources were used under mild, remote conditions to study the biological deactivation of two immune-stimulating biomolecules: lipopolysaccharide (LPS), found in bacteria such as Escherichia coli, and peptidoglycan, found in bacteria such as Staphylococcus aureus. The surface chemistry was measured to understand which plasma- generated species and surface modifications are important for biological deactivation. To simplify the complex molecular structure of the biomolecules and study specific moieties, model polymer films were studied including polystyrene, poly(methyl methacrylate), polyvinyl alcohol, and polypropylene. The interaction of the plasma plume with the environment was studied as a parameter to tune surface modifications. It was found that increasing ambient N2 concentrations in an N2/Ar ambient decreased surface modifications of LPS, similarly to how adding N2 to the O2/Ar feed gas decreased the plasma-generated O3 density and O atom optical emission. In this work, we first observed the formation of surface-bound NO3 after plasma treatment, which had not been reported in the literature. The plasma-ambient interaction was further studied using polystyrene as a model system. This detailed study demonstrated a competition between surface oxidation and nitridation, the latter of which occurs under very specific conditions. It was found that NO3 formed on all the materials studied in this dissertation after plasma treatment. This NO3 formed after treatment by both sources, but in different concentrations. The surface-bound NO3 correlated better with changes in biological activity than general oxidation, demonstrating its importance. Studying model polymers revealed that this surface moiety preferentially forms on – OH containing surfaces. Since the atmospheric pressure plasma jet (APPJ) operates with low N2/O2 admixtures to Ar and the surface microdischarge (SMD) operates with N2/O2 mixtures, the mechanisms that cause biological deactivation must be different, and are discussed.