Investigating Electron And Radical Interactions With Biomolecules And Cells Using A Droplet In Plasma Laboratory

Abstract

Plasmas-liquid interactions are a valuable source of radical species for plasma medicine including cancer treatment and antimicrobial applications. However untangling the complex interplay between species and biology over very short timescales and distances is proving extremely challenging. Radiolysis induced damage to DNA is considered to be significantly affected by low energy secondary species such as OH radicals but also via direct interaction with low energy solvated electrons (LEE). Pt adducts and the presence of Au nanoparticles are also known to enhance the LEE effectiveness. However direct investigation of LEE and radical interactions with biomolecules e.g. DNA in a liquid environment at atmospheric pressure or in a living cell present very significant experimental challenges. LEE sources have been proposed such as 2D radioactive layers and UV-induced emission from metals. In this work we look at the potential of a living laboratory based on small liquid droplets containing biomolecules or cells passed through a plasma and exposed to a high flux of electrons and selectable radicals. We also consider the potential of using droplets to deliver plasma-activated media near instantaneously downstream.

Transport of micron-sized liquid droplets through a low temperature non-equilibrium RF plasma [1] at atmospheric pressure has demonstrated a number of remarkable and unexpected effects. The microdroplet system allows for a controlled (air-free) gas ambient environment, a large surface area to volume ratio, very small reaction volume and low droplet temperature. In addition, flow induced convection in the liquid can be minimised. After a very short plasma exposure time, ~120°μ s, there is evidence that chemical reactions induced by the plasma and gas flux proceed at a rate that is significantly faster that observed in plasma – bulk liquid studies and many orders of magnitude faster than in standard bulk chemistry [2]. The high chemical reactivity is thought to depend not only on the picolitre droplet volume, as in microreaction chemistry, but also on the high level of surface charge due to net electron bombardment in the plasma and the high flux of low energy electrons which arrive at the charged droplet with almost zero net energy. We have observed very rapid electron reduction of metal (Au) salts to form Au nanoparticles at rates that are much greater than observed with gamma radiolysis or high energy electron beams. This points to the possible effectiveness of plasma-generated Ultra-Low Energy Electrons (ULEE) in reduction reactions that may prove valuable for electron – biomolecule studies. We have also observed H₂O₂ and OH in the liquid most likely due to generation of these species in the plasma phase, which contains only He and H₂O vapour. We have used droplets as carriers for single bacteria cells, which are then exposed to plasma species (electrons, OH and H₂O₂) for a very short time (~0.1 ms) and the effects on cell viability and properties determined. It is clear that this system is complex but plasma-droplet system may yet offer unique access for plasma – liquid chemistry and biology studies. We will present plasma-DNA and cell studies in conjunction with liquid chemistry and electric field modelling.