Chemical analysis of reactive species and antimicrobial activity of water treated by nanosecond pulsed DBD air plasma

Deionized water, phosphate buffer and dilute electrolyte solution from sodium dihydrogenphosphate were treated by nanosecond pulsed DBD air plasma. Hydrogen peroxide, nitrates and nitrites produced by plasma treatment were measured by means of both quantitative and semi-quantitative methods. Phenol was used as chemical probe for ROS and RNS. Post-discharge kinetic of phenol degradation gave evidence of the formation of peroxynitrite by H+-catalyzed reaction of H2O2 and NO2- in plasma activated water (PAW). Antimicrobial activity of two types of PAWs prepared by exposing deionized water with DBD air plasma for different times (5 and 10 min, named PAW-5MIN and PAW-10MIN, respectively) was investigated on Candida albicans and Staphylococcus aureus, among the most common causes of nosocomial infections. The activity of PAWs decreased strongly with post-discharge time. For PAW-5MIN the activity held for about 30 min while for PAW-10MIN just less than 15 min. This correlated with the time evolution of peroxynitrite chemistry observed on phenol in both PAWs. OH, NO2 radicals with acidified NO2- and acidic pH itself were most likely the main biocidal agents in PAWs acting on tested microorganisms. Since the initial concentrations of H2O2 and NO2- (as precursors of peroxynitrite formation in PAW) were comparable in both PAWs, the main reason for difference in their antimicrobial activity was most likely in their pH (pH 2.3 for PAW-10MIN vs pH 2.7 for PAW-5MIN), which accelerated the rate of peroxynitrite formation in PAW-10MIN but lowered time of its antimicrobial activity. These relatively short latency times of antimicrobial activity should be taken into the consideration while using PAW in biological applications, as time delays the between preparation and application of PAW should be minimized.