Potassium iodide enhances the antimicrobial activity of plasma-activated water

Plasma-activated water (PAW) is a promising disinfection strategy that generates a complex mixture of reactive oxygen and nitrogen species (ROS/RNS), including hydrogen peroxide (H₂O₂), nitrate (NO₃ ^-), and transient oxidants, in an acidic aqueous environment. These reactive species contribute to both immediate and extended antimicrobial activity. This study investigates how the addition of low concentrations (<100 μM) of potassium iodide (KI) enhances the bactericidal properties of spark-generated PAW by enabling the in-situ generation of reactive iodine species (RIS), particularly hypoiodous acid (HIO), under acidic conditions. KI addition (10-100 μM) led to a counterintuitive, dose-dependent increase in H₂O₂ concentrations, from ∼1.2 mM in PAW alone to ∼1.8 mM at 30 μM KI, possibly due to iodine-mediated catalytic effects or reduced H₂O₂ degradation. NO₃ ^- levels also increased by ∼17 % with increasing KI. Equivalent concentrations of H₂O₂ + KI failed to replicate the rapid antimicrobial activity observed in PAW + KI, which achieved complete inactivation of <i>Escherichia coli</i> and <i>Listeria monocytogenes</i> planktonic cells within 3 min, compared to over 10 min for PAW alone, indicating the involvement of additional reactive species in KI-enhanced antimicrobial activity of PAW. However, <i>Salmonella enterica planktonic cells</i> exhibited only partial inactivation even with KI, indicating species-specific tolerance under these conditions. 24h biofilms of <i>L. monocytogenes</i> and <i>E. coli</i> were eradicated with PAW + KI in 10 min, whereas <i>S. enterica</i> showed only a 2-log reduction. Scavenger assays revealed that both longer-lived species (H₂O₂) and shorter-lived oxidants such as singlet oxygen are essential for this enhanced killing, while ozone and superoxide appeared dispensable. These findings support a multi-step antimicrobial mechanism: (1) plasma treatment creates a low pH, H₂O₂-rich solution; (2) iodide is oxidised to RIS such as I₃ ^- and HIO; (3) additional PAW-derived oxidants potentiate RIS chemistry; and (4) unionised HIO diffuses across bacterial membranes to induce oxidative damage. PAW-KI remained stable for at least 14 days at 4 °C, with sustained RIS activity and minimal loss of H₂O₂ or NO₃ ^-, suggesting preserved antimicrobial capacity over time. The antimicrobial mechanism likely proceeds through a four-step pathway: plasma-mediated generation of H₂O₂ and NO₃ ^-; oxidation of I^- to I₂ and HIO; potentiation of RIS via PAW-derived ROS/RNS; and subsequent microbial inactivation via membrane damage. Together, these results demonstrate that PAW + KI forms a powerful, in situ RIS-generating system, offering a residue-minimising and environmentally sustainable disinfection platform. Its rapid action, scalability, and reliance on only air, water, electricity, and GRAS-listed KI make it an attractive intervention for food safety, clinical disinfection, and decentralised sanitation settings.