Salinity-dependent Reactivity of Plasma-activated Saline and Antimicrobial Response of Isotonic 0.9% NaCl Activated by Surface-wave Argon Plasma

This study investigates how clinically relevant NaCl concentrations modulate the chemistry of plasma-activated saline (PAS) produced by an electrode-less argon surface-wave discharge (SWD, 70 W, 2.5 L/min, activation up to 40 min) and evaluates the antimicrobial response of the isotonic formulation (0.9% NaCl). Using the same SWD configuration previously validated for plasma-activated water (PAW), which generated pH close to 3.3, ORP around 230 mV and clear bactericidal activity against Staphylococcus aureus and Escherichia coli, we activated saline solutions at 0.45%, 0.9%, 3% and 7.5% in order to separate the effect of the plasma from the effect of the liquid matrix. Real-time monitoring (pH, ORP, conductivity, TDS, salinity, dissolved oxygen and free chlorine), together with UV–Vis spectroscopy and semi-quantitative RONS determinations, showed that all saline matrices were acidified to about pH 3.3–3.5 and became moderately oxidizing (ORP about 200–222 mV), but the kinetics and final oxidative profile were strongly dependent on ionic strength: dilute PAS (0.45–0.9% NaCl) formed H₂O₂ and nitrogen oxyanions within the first 10 min, whereas concentrated PAS (3–7.5% NaCl) required 30–40 min to reach a nitrate-dominated and more stable oxidizing state and showed transient formation of chlorine-derived species. To avoid confounding osmotic effects and to align with clinically used irrigation solutions, microbiological assays were deliberately restricted to isotonic 0.9% PAS and performed against S. aureus, E. coli and Candida albicans. Under these conditions, only a non-significant reduction trend was observed for S. aureus and E. coli at 40 min activation and 30 min contact, and no effect was detected for C. albicans (p > 0.05). When compared with earlier SWD-PAW results obtained under identical plasma conditions, this outcome indicates that the saline matrix attenuates biocidal expression through ionic and acidic buffering, diversion of part of the oxidative budget to chloride-driven reactions, ORP values below the more aggressive redox window reported for bactericidal PAW/PAS, and removal of osmotic synergy. Based on the combined physicochemical and biological evidence, three optimization routes are outlined: activate at low salinity and readjust to isotonicity before application; drive concentrated PAS (3–7.5% NaCl) to the late, nitrate-rich regime and dilute only at the point of use; or combine isotonic PAS with co-stressors (mild acidification or sub-MIC antibiotics) to restore synergy. Thus, salinity is confirmed not as a drawback of the SWD source but as a controllable parameter for tailoring PAS to medical and dental applications under physiologically compatible conditions.