Dynamic analysis of absorbance behavior and peak shift of RONS in plasma-activated water by UV absorption spectroscopy: dependency on gas impurity, pulse polarity, and solution pH

In this paper, we employ UV absorption spectroscopy to monitor the generation and permeation of reactive oxygen and nitrogen species (RONS) in plasma-activated water (PAW) to revealthe dynamic variation mechanism of RONS chemistry. Parameters including gas impurity, pulse polarity and solution pH value are varied to explore their effects on the absorbance behavior and peak shift of absorption spectra as well as the permeation distribution of RONS. Regarding the absorbance behavior, experimental results show that introducing air and N-2 into He working gas would effectively improve RONS absorbance, proportions of about 0.2% air and 0.5% N-2 would result in the maximum absorbance, while the addition of O-2 would result in a significant decrease in RONS absorbance. Under positive polarity, the RONS absorbance is about 20% higher than that under negative polarity. Changing the solution pH from acidic to alkaline is beneficial in increasing RONS absorbance, indicating that alkaline solution could effectively promote RONS formation. Regarding the characteristic peak shift, different parameter conditions seriously affect the shift of the absorption peak toward low wavelength or high wavelength due to the change in the ratio of the concentration of each component of RONS in PAW. Furthermore, with respect to the permeation distribution of H2O2 and NO2-, the results show that the addition of O-2 would result in the fastest production rate of H2O2 and introducing air and N-2 would generate the fastest rate of NO2- production. Interestingly, the NO2- permeation distribution displays a 'columnar mode' and a 'filamentous mode' under positive and negative polarity, respectively. An alkaline solution promotes the formation of NO2- while having an obvious inhibiting effect on the NO2- permeation; conversely, an acidic solution has a promotional effect on NO2-. This study provides a new in-depth understanding of the dynamic evolutionary behavior of RONS in PAW, helping to reveal the network relationship between RONS, and assisting in the development of applications of PAW.