Formation of reactive oxygen species by the irradiation of cold atmospheric pressure plasma to water

We examined the formation of reactive oxygen species by the irradiation of helium-based cold atmospheric pressure plasma to water using ESR and colorimetric analysis. Cold atmospheric plasma was generated with a generator PN-110TPG (NU global, Japan). The plasma was irradiated to aqueous samples changing the irradiation time and the irradiation distance between water surface and the top of the plasma-generating tip. 1) Formation of reactive oxygen species was confirmed by the decrease of the ESR signal intensity of the solution of 2,2,6,6-tetramethylpiperidine 1-oxyl (TEMPO), a stable free radical, by the irradiation. 2) Formation of hydroxyl radical was confirmed by the ESR spin-trapping technique using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin-trap reagent. 3) Formation of hydrogen peroxide was confirmed by the colorimetric method using a peroxidase reaction. In these three methods, the change (decrease or increase) in the intensity was observed as irradiation-time dependent fashion. On the other hand, complex dependency on the irradiation distance was observed. These findings suggest that the reactive oxygen species observed are largely derived from the plasma itself rather than hydrolysis or excitation of water molecules.