## 4 Discussion
Decrease of pH can occur due to presence of nitrogen oxides, which can be produced from atmospheric air by the plasma and dissolve in liquids [31]. In our study, pH decreased with increasing plasma exposure time of deionised water and this result is in accordance with other reports [5; 6; 31] and remained stable for at least a week. Concentrations of some of the chemical species were altered in the same storage period, indicating that the chemical reactive species in PAW may havereacted to form other secondary products, with pH and conductivity values remaining the same. Antimicrobial effects of PAW can persist for days but artificially prepared solutions mimicking chemistry of PAW were shown to be less effective [33]. According to [PERSON] et al., post-discharge reactions between nitrites and hydrogen peroxide can occur in PAW and result in formation of peroxynitrite [34] and nitrate [35]. PAW has the potential to be formulated for use in a variety of clinical applications, such as disinfection, blood coagulation, oncotherapy, or wound healing. Formulations used in clinical settings or as carriers for therapeutics normally undergo terminal sterilisation, which may be mediated by heat and pressure. The PAW generated here has the potential to be used in a setting requiring sterilisation but might undergo chemical decomposition through a number of pathways at the elevated temperatures used for its sterilisation. A widely used method for sterilisation is moist heat in the form of saturated steam under pressure. The basic principle of steam sterilisation, as accomplished in an autoclave, is to expose each item to direct steam contact at high temperature and pressure for a specific time. Thus, there are four parameters of steam sterilisation: steam, pressure, temperature, and time. In this study different temperatures and heating times were applied to PAW, and results showed that this PAW retains antimicrobial efficacy. In some cases, autoclaving actually increased the antimicrobial efficacy by comparison with unheated PAW. From a reactive species point of view, hydrogen peroxide and peroxides of SD15 PAW were strongly reduced post autoclaving. Reduction of peroxides post autoclaving was also observed for GD15 PAW, which also retained antimicrobial effects at longer contact times. This suggests that other chemical species were primarily responsible for the bactericidal effects of PAW. The differences in the concentrations and/or structure of chemical species measured in this study could be caused by autoclave sterilisation because of the high pressure and steam. Moreover, stability of hydrogen peroxide can be altered by increase of temperature [36] and result in its decomposition to yield oxygen and water. In conclusion, these results show that PAW can be generated to provide strong disinfect efficacy and retain its stability in different conditions, including high temperatures and pressure. In addition, the possibility of post-plasma modifications of plasma activated liquids to alter their chemical composition and biological effects arises and presents an exciting avenue for further investigations. Antimicrobial stability of PAW to heat and pressure broadens the application potential of this technology. Autolaved PAW can provide a valuable decontamination tool for the removal of bacteria in many settings, including hospitals and other health care institutions. It can be used for example as a \"no-touch\" (automated) decontamination method using vaporised plasma activated water for reduction of microbial contamination of surfaces. Another example for autoclaved PAW application is thermal fogging. Thermal fogging disinfection can be used in livestock industry and medical setting, for decontamination of surfaces and aerosols in the air. In both examples, PAW could be added as a disinfectant in these devices, heated at desired temperature in order to generate fog/mist and then used in a contaminated area.