Reaction Mechanisms for Low Temperature Plasma Interactions with Complex Surfaces and Molecules

Abstract

Low temperature plasmas (LTPs) can be used as sources of reactive chemistry for various existing and emerging commercial applications. These applications include but are not limited to synthesis of nanoparticles for optoelectronic devices, improvement to adhesive properties of bulk commodity polymers, produce disinfection, medical instrument sterilization, and therapeutic treatments for wounds and various cancers. In each of these applications, reactive plasma species interact with a target molecule or surface to add commercial value or induce modifications that promote a desired effect, such as cell death. In many of these applications, the target molecule or surface for plasma treatment consists of complex metallic or organic molecules. Improving and optimizing these systems relies on understanding the mechanisms through which reactive species interact with these complex molecules, however these mechanisms are often unclear. In this dissertation, reaction mechanisms for several commercially relevant systems are developed and used in a 0-dimensional plasma chemistry model to better understand how LTPs interact with complex surfaces and molecules. In some instances, algorithms were added to the 0D model to provide new capability. An algorithm for describing nanoparticle nucleation and growth was developed by the author to predict the average mass density and size of silicon nanoparticles formed in a low-pressure flowing plasma. The model was used together with experiments to provide insights into how changing plasma operating conditions such as inlet gas composition, pressure, and reactor diameter effect the growth regime (onset of nucleation or growth by coagulation) of the silicon nanoparticles. Nucleation of silicon nanoparticles was shown to be sensitive to the reactor radius and flow rate. Recommendations for operating conditions that can promote or suppress particle growth are suggested. A reaction mechanism was developed to predict the addition of O-functionality to the surface of polystyrene (PS) by an atmospheric pressure plasma jet (APPJ). The addition of O functionality to polystyrene increases the wettability of the polystyrene and is desired in the production of biocompatible well-plates and petri dishes. The reaction mechanism was validated by comparison to experimental data. Results indicated that O-occupancy on the PS surface is highly sensitive to the flux of O-atoms delivered to the surface by the plasma. Operating conditions that achieve optimum O-occupancy were identified. Plasma interactions with organic molecules in liquid are inherent to plasma medical applications. A reaction mechanism for the APPJ treatment of cysteine in solution was developed and validated against experimental data. The model elucidated reaction pathways responsible for the addition of O or NO functionality to the cysteine molecule as a result of plasma exposure. Results showed that cysteine oxidation production formation can be adjusted by changing plasma operating conditions such as distance from the substrate and inlet gas composition. The cysteine reaction mechanism was extended to produce a hierarchal model for bacterial inactivation by APPJ treatment. The model enabled comparison of time to cell death between dissimilar plasma devices which is difficult to achieve experimentally. Results indicated that plasma systems that can produce reactive nitrogen species are most efficient at bacterial inactivation. Modelling approaches and mechanisms in this dissertation can be extended to studies of plasma interactions with similar complex targets. Increased understanding of plasma interactions that add value or induce modifications to a target molecule were demonstrated and recommendations to improve or optimize the efficiency of existing or emerging plasma systems were identified.