Abstracts Biological Sciences

On the uses of atmospheric and laser plasmas for biomedical applications

by James Connolly

This thesis describes work with different types of low temperature plasmas and their use in microbiological applications. Herein is described the development and results from a novel system for decontamination of packaged produce using a dielectric barrier discharge. It also describes the results of growth by pulsed laser deposition of nominally undoped and doped zinc oxide thin films at low temperature on plastic substrates. This work was carried out to develop transparent conducting electrodes for the growth of electroactive biofilms for microbial fuel cells. The system described herein utilises a floating high voltage (±15 kV) that sustains an atmospheric plasma in a plastic bag between parallel plate electrodes. This is characterised using current, voltage and charge measurement and also emission spectroscopy over electrode separations from 30 to 100 mm. Results are analysed to unravel physical processes present in this plasma for optimisation as a method of food decontamination. Plasma parameters are measured by the interpretation of electrical measurements by the “capacitor-in-series” model. The power consumed by the discharge and the reduced field strength decrease quadratically from 12.0W to 4.5W and linearly from 140 Td to 50 Td, respectively, in this range. Spectroscopy reveals the presence of molecular nitrogen, oxygen, NO, OH and helium. Temperature estimates are made by comparing intensity calibrated spectra with plots generated by the SPECAIR program. At 40 mm, this comparison yielded electron, vibrational and translational (gas) temperatures of (4980 ± 100) K, (2700 ± 200) K and (300 ± 100) K, respectively. In conjunction with colleagues from the DIT School of Food Science and Environmental Health, the bacterial inactivation properties of the system were examined. A 1.5 log reduction in bacterial populations was achieved through indirect exposure to the plasma over a 5 minute treatment. This work also presents the results of characterisation of zinc oxide and ZnO:Al (2%) thin films deposited at pressures of 1 to 300 mTorr in oxygen using several investigative techniques (AFM, XRD, contact profilometry, Van der Pauw and water contact angle). These investigate the effects of growth parameters on physical and electrical properties of thin films grown on amorphous substrates at low temperatures as a function of growth pressure, thickness and target doping. Films grown at 5,000 to 20,000 shots vary in thickness from 125 nm to 500 nm. Resistivity ranges from 10^-2 to 10^7 Ω cm, with low pressure films exhibiting the lowest resistivities. Crystal structure becomes apparent above 1 μm, the majority of films, which have thicknesses of 500 nm or less, exhibit no crystallinity. Water contact angle decreases with increasing deposition pressure. In conjunction with colleagues from the DCU School of Biotechnology, electroactive biofilms were grown on the thin films and their electrical behaviour measured with chronoamperometry and cyclic voltammetry. The results outlined herein show the viability of non-equilibrium…