Antimicrobial synergy between air DBD and UVA treatment of aqueous solution

Summary form only given. Both ambient-condition air plasmas and UVA emissions at wavelengths near 360 nm have been shown to inactivate bacteria in aqueous solution. We have recently reported that in the case of relatively high-power plasma treatment (greater than 0.25 W/cm²), important aqueous-phase chemical species include nitrate (NO₃^-), nitrite (NO₂^-), and hydrogen peroxide (H₂O₂) In this study, we demonstrate a synergistic antibacterial interaction between air plasma, using a surface micro-discharge (SMD) device, and UVA treatment, using a UVA-emitting LED. When an aqueous suspension of E. coli was treated with high-power air plasma for 5 minutes followed by UVA for 60 seconds, the antimicrobial effect was at least a 4.5 log reduction in bacterial load. Under the same conditions, the expected additive effect of plasma treatment alone plus UVA treatment alone was a 2-log reduction in load. In contrast, we show that under other conditions, such as treating with UVA first or using low-power plasma, the combined antimicrobial effect matches the expected additive effect. Of the species known to be created in high-power air plasma treatment of aqueous solution, nitrite appears to influence the observed synergy the most. Nitrite is known to absorb and photolyze in the UVA range to form NO and OH. These species are much more chemically and biologically active than nitrite. To support this hypothesis, we show that the addition of nitrite to aqueous solution increases the synergistic antimicrobial effect when treated with UVA. The interaction of UVA photons with plasma-generated chemical species has the potential to increase the speed and efficacy of both ambient-plasma disinfection and UV-based disinfection methods. Furthermore, our results suggest the possibility of a wider application of ambient-condition plasma chemistry coupled with photochemistry to produce unique chemical and biological effects.