To understand plasma-induced chemical processing in liquids, we investigated the formation of free radicals in aqueous solution exposed to different types of non-contact atmospheric-pressure helium plasma using the spin-trapping technique. Both hydroxyl radical (OH·) and superoxide anion radical (O2·) adducts were observed when neutral oxygen gas was additionally supplied to the plasma. In particular, O2· can be dominantly induced in the solution via oxygen flow into the afterglow gas of helium plasma. This type of plasma treatment can potentially be used in medical applications to control infectious diseases, because the O2· is crucial for sterilization of liquids via atmospheric-pressure plasma.

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