Discharge characteristics and bactericidal mechanism of Ar plasma jet with ethanol and oxygen gas admixtures

An argon plasma jet with gas admixtures of 0.2% ethanol (EtOH) and 0.08% oxygen, which has promising prospects in sterilization and anti-infection, is studied. EtOH can convert an argon plasma jet from filamentary mode into diffuse mode, as reported previously, and O₂ can further enhance its bactericidal effect dramatically. Compared to the Ar+EtOH plasma jet, the Ar+EtOH+O₂ plasma jet is still in diffuse mode, but its bactericidal effect increases by more than five orders of magnitude when used to treat Methicillin-resistant Staphylococcus aureus (MRSA) suspensions (∼1 × 10⁸ CFU ml^-1). Such a strong bactericidal effect can also be achieved by using deionized water activated by an Ar+EtOH+O₂ plasma jet, even when the plasma-activated water has been stored for 3 min before mixing with the MRSA suspension. This implies that some new antibacterial species are produced by the Ar+EtOH+O₂ plasma jet compared to the Ar+EtOH plasma jet, which plays a crucial role in sterilization. Compared to the Ar+EtOH plasma jet, the Ar+EtOH+O₂ plasma jet decomposes more EtOH molecules, and some new gaseous species, such as CO₂, CH₃COOOH, CH₃COCH₂ ⁺, COOO^-, HCOOO^- and CH₃COOO^-, are produced. CH₃COOOH also exists in plasma-activated water with a lifetime of 3-10 min and is deduced to be the key antibacterial species. Finally, the chemical profile of EtOH decomposition in the Ar+EtOH+O₂ plasma jet is deduced and compared with that in the Ar+EtOH plasma jet.