Water plasma generated from in-situ adsorbent-released vapor inactivates bacterial spores and damages DNA and proteins via OH-radical-rich chemistry

Water vapor plays a central role in plasma-mediated oxidation and microbial inactivation through the generation of hydroxyl radicals (·OH) and reactive oxygen species. However, conventional water-based plasma systems require externally humidified feed gases or vapor-supply units, limiting their use. Here, we developed a low-pressure water plasma device that utilizes in situ water vapor desorbed from silica gel, enabling plasma generation without externally supplied feed gases. Plasma was produced using a 13.56-MHz radio-frequency discharge (50 W) with a discharge voltage amplitude of ~180 V and a discharge current amplitude of ~1 A. Pressure reduction from 600 to 100 Pa induced water vapor release and plasma ignition. The OH-rich non-thermal plasma gave an optical emission peak at 308 nm. Plasma generation was accompanied by continuous silica-gel mass loss, reflecting gradual vapor desorption, while the chamber temperature remained below 70 °C. Sporicidal efficacy and biomolecular damage under controlled low-pressure conditions were assessed. The plasma effectively inactivated Bacillus atrophaeus spores, a highly resistant biological indicator. Complete spore inactivation was achieved within 3 h for initial loads of 10 3 -10 4 colony forming units (CFU). An initial load of 10 5 CFU fell below the detection limit after 6 h. Quantitative polymerase chain reaction showed a time-dependent decrease in amplifiable genomic DNA. Plasma exposure also caused protein damage, including loss of luciferase activity, reduction of Lowry-reactive peptide bonds, decreased aromatic amino-acid absorbance, and attenuation of amide I/II bands in infrared spectra. Overall, we conclude the adsorbent-mediated water plasma device provides an effective means of decontamination. • Vapor desorbed from silica gel allows RF water plasma generation without feed gases • An OH radical-rich plasma operates below 70 °C under low pressure (100-600 Pa) • Bacillus atrophaeus spores (10 3 -10 5 CFU) fell below the detection limit in 3-6 h • PCR signal decreased, indicating loss of amplifiable spore genomic DNA • Proteins (luciferase, BSA) showed functional/structural changes (activity loss/FTIR changes)