Plasma-Activated Water Generated by Surface-Wave Sustained Discharge: Physicochemical Properties and Antimicrobial Efficacy

Abstract

Electrode-less surface-wave sustained discharge (SWD) plasma jets operated with noble gases provide a chemically clean and reproducible route to plasma-activated water (PAW) generation, avoiding electrode erosion and metal contamination while enabling long, stable activations. Using an argon SWD jet (70 W), we tracked the time-resolved thermal and physicochemical evolution of PAW over 40 min and assessed antimicrobial efficacy. PAW acidified to pH ~ 3.34, with ORP ~ 230 mV, conductivity ~ 180 µS/cm, and TDS ~ 55 mg L⁻1. Spectrophotometric/colorimetric analyses showed the accumulation of RONS with H₂O₂ ~ 10–25 mg L⁻1, NO₃⁻ ~ 10–25 mg L⁻1, and NO₂⁻ ≤ 1 mg L⁻1; evaporation during long activations concentrated solutes, and volume-normalized endpoints confirmed that qualitative trends persist after correcting for mass loss. Optical emission and thermal probes indicated that water buffers the heat load yet can drift toward ~ 40 °C under the longest activations, motivating temperature control (≤ 20–25 °C) to preserve thermally labile ROS. Microbiological assays revealed strong bactericidal activity against Staphylococcus aureus and Escherichia coli but limited effect on Candida albicans, consistent with organism-specific cell-wall structure and oxidative defenses. The batch energy input (46.67 Wh for 40 mL) corresponds to EPL ≈ 1.17 kWh L⁻1, highlighting a purity–energy trade-off relative to some DBD systems; we outline straightforward optimizations (minor O₂/N₂ admixtures, reduced gap, bubbling/recirculation, and active cooling) to enhance RONS yield and energy efficiency without compromising chemical purity. Collectively, these results establish electrode-less SWD as a robust platform for sterile, contamination-free PAW and clarify operational levers that tune performance for biomedical and sanitation applications.