Students

Bc. level: 2009 - 2010
Supervisor: Zdenko Machala
Title (en):
Oxidative damage of Gram positive bacteria by non-thermal plasmas
Title (sk):
OXIDAČNÉ POŠKODENIE GRAM POZITÍVNYCH BAKTÉRIÍ V NEROVNOVÁŽNEJ PLAZME
Abstract (sk)

MSc. level: 2010 - 2012
Supervisor: Zdenko Machala
Consultant: Libuša Šikurová
Title (en):
Mechanisms of the interaction of plasma with microbial cells and related chemical effects in water
Title (sk):
MECHANIZMY INTERAKCIE PLAZMY S BUNKAMI MIKROORGANIZMOV A SÚVISIACE CHEMICKÉ ZMENY VO VODE
Abstract (en)
Abstract (sk)

PhD. level: 2013 - 2019
Supervisor: Zdenko Machala
Consultant: Libuša Šikurová
Title (en):
Investigation of cold air plasma generation of aqueous reactive oxygen and nitrogen species with focus on their detection and related antibacterial effects
Title (sk):
Výskum vzniku kyslíkových a dusíkových reaktívnych častíc vo vode pôsobením studenej plazmy vo vzduchu so zameraním na ich detekciu a súvisiace antibakteriálne účinky
Abstract (en):  hide
In this PhD thesis we investigate formation of reactive oxygen and nitrogen species (RONS) in cold air plasma of transient spark activated water (PAW) and buffered solutions by electrospray and their related antibacterial effect. Aqueous reactive species are typically formed due to chemical reactions occurring at the gas plasma-liquid interface. Antibacterial properties of PAW depend on its composition and presence of RONS. Therefore, we investigate their formation in the gas phase, their transfer into the aqueous solutions and particularly we focus on their specific detection in PAW. DC transient spark discharge was generated in ambient air at atmospheric pressure in combination with water electrospray. The electrospray improved the inter-phase contact and enhanced the transfer of plasma reactive species into the droplets of electrosprayed solutions. The optical emission spectroscopy (OES) of the discharge with electrospray showed a significant number of emission lines: Fe, Fe+, Cr+, Ni+, second positive system of N2, O+, N+, O and N. We observed the cooling effect due to the water electrospray on the gas temperature of the discharge (700 K→ 400 K). Stable gaseous reactive species were identified by Fourier-transform infrared spectroscopy (FTIR), UV absorption and by electrochemical sensors. Transient spark produced mainly nitrogen oxides (NO + NO2) and in the presence of electrospray also other abundant species (H2O2, HNO2, HNO3, •OH, HO2•) were observed. Formation of O3 in other than negligible concentrations was not observed. Formation of aqueous RONS correlated with the production of gaseous reactive species. Detection of aqueous RONS in PAW showed that their formation and life-time depend on the pH. Based on the kinetic chemical analysis we proved formation of peroxynitrous acid, which was determined to be the main antibacterial agent in PAW formed by TS discharge. Highly reactive radicals •OH, HO2•, •NO and •NO2 formed in the PAW initiated the decomposition of the cell membranes via the process of lipoperoxidation and resulted in sublethal injury of the cells. We showed that antibacterial properties of PAW can be preserved by immediate deep freeze and storage at low temperatures (-70°C). Direct treatment of bacterial suspension in PAW resulted in the strong antibacterial effect (i.e. complete sterilization) and these bacteria showed no metabolic activity due to the synergetic effect of RONS induced chemistry, acidic pH and other plasma agents (e.g. UV emission or electric field). Formation of peroxynitrite/peroxynitrous acid in PAW was proved also by their fluorescent detection. We investigate the suitability and specificity of 2,7-dischlorodihydrofluorescein diacetate (H2DCFDA) fluorescent dye in plasma activated solutions. We demonstrated that in plasma activated solutions without the presence of Cl- and thus prohibiting formation of hypochlorite anions (OCl-) or hypochlorous acid (HOCl), the H2DCFDA fluorescence signal can be attributed primarily to ONOO-/ONOOH formed primary via the reaction of NO2- and H2O2 in PAW. Simple UV-spectroscopic methods for the detection of aqueous NO2- and O3 in PAW were examined for their specificity and possible interferences. We confirmed the accuracy of the widely used Griess colorimetric assay for NO2- detection and we excluded the interference of H2O2 on the assay by using the enzyme catalase. By analysis of the phenol degradation products we confirmed that transient spark did not produce aqueous O3 and thus the Indigo blue assay for dissolved O3 detection is non-specific in PAW and strongly interference with the peroxynitrite chemistry in PAW.