Micronano bubble-enhanced microbial inactivation during water disinfection by underwater plasma discharges: surfactant-mediated ROS retention and subcellular mechanisms

Microbial safety remains a critical global challenge for public health and the environment, exacerbated by escalating antimicrobial resistance and increasingly stringent regulations on disinfection by-products. Plasma-activated water (PAW) emerges as a promising antimicrobial medium, yet its efficacy is constrained by the rapid decay of key reactive species. Micro-nano bubbles (MNBs) may enhance species transfer and persistence, but the interfacial mechanisms governing bubble controllability and the resulting impacts on PAW lifetime remain insufficiently understood. Here, we investigate how surfactant-mediated interfacial modulation regulates bubble characteristics and reactive species retention within an in situ plasma-bubble system. Nonionic surfactants substantially increase bubble density, narrow the microbubble size distribution while reducing zeta potential. In contrast, the anionic surfactant sodium dodecyl sulfate (SDS) is found to exert minimal influence on zeta potential, but significantly enhances O₃ retention for at least 30 min. This modulation extends the bactericidal lifetime of PAW from less than 10 min to 100 min post-discharge. Scavenger assays and intracellular ROS measurements further identified superoxide (·O₂⁻) as a critical species sustaining the bioactivity of PAW. Mechanistically, SDS appears to maintain ·O₂⁻ level through two complementary pathways: (i) physically stabilizing ROS level by improving bubble properties and (ii) altering interfacial chemistry and associated radical interactions. Overall, these findings highlight the role of interfacial chemistry in plasma–liquid interactions and provide a practical strategy for achieving sustained antimicrobial performance of PAW.