# Water Modification by Cold Plasma Jet with Respect to Physical and Chemical Properties
## 1 Introduction  
Interest continues to grow worldwide in weakly ionized, low temperature, atmospheric pressure plasmas [1]. Plasmas generated and maintained at atmospheric pressure enjoyed a renaissance in the last decades mostly driven by the employment of dielectric-barrier discharges (DBDs) and plasma jets (DBD-based or not) as sources of high-density, reactive species. Such plasmas avoid the restrictions imposed by costly, cumbersome vacuum chambers and by destructively high temperatures, extending their applicability in the fields of defense [2], electronics [3], material science [4], environmental health [5], aviation [6], biomedicine [7], agriculture [8], food industry [9], etc. However, despite their apparent engineering simplicity, these systems vastly complicate the plasma physics and chemistry involved, particularly during their incidence on liquid-phase targets. The prominent interest in the latter case is prompted by attempts to interpret fundamental physicochemical phenomena along the plasma-liquid interface [10, 11, 12, 13] and to properly exploit the properties of the plasma activated liquids for tailored applications [14, 15, 16].  
Among the various liquids, a special focus has been given on the water treatment by atmospheric pressure cold plasmas with respect to remediation [17], sterilization [18],fertilizer alternatives [19], dentistry [20], food preservation [21], nanoscience [22], etc., and the concept has already been commercialized (e.g., VitalFluid BV). Although it is widely accepted that, in most applications, the plasma induced effects are mainly mediated via reactive oxygen and nitrogen species (ROS and RNS, respectively; cumulatively designated RONS), influencing redox regulated processes [23], detailed consideration of the physical and chemical impacts of the plasma species on the liquids is a prerequisite for any targeted application. This need is yet more pressing when living specimens are involved. Such dedicated, worthy representative studies, unveiling the potential plasma-induced modifications of water (and other liquids too), are laid out in Appendix A in chronological order over the last ten years. The works cited therein denote the wide interest in the topic of plasma-based modification of water, as well as the divergence of setups and diagnostic methods used in various case studies.  
However, despite the intensive research investments during the recent decennia, the field is plagued by controversies and gaps in knowledge, which might restrict further progress [24]. Within this context, the present work is an attempt to enrich the available data by communicating experimental results acquired with one of the most popular plasma types employed in liquid treatment. Accordingly, an atmospheric pressure plasma jet, consuming mean power less than 400 mW, is optimized in terms of reactive species formation when the carrier gas is argon (Ar), and this optimal operational window is applied to the activation of demineralized, doubled-distilled water (DDW) and phosphate-buffered saline (PBS). The dynamic evolution of the evaporation rate, temperature, acidity and basicity, resistivity, and oxidation-reduction potential are recorded. Finally, the formation of principal ROS, i.e., \({}^{\bullet}\)OH, H\({}_{2}\)O\({}_{2}\) and O\({}_{2}^{\bullet}\), is demonstrated and their concentration is measured by means of strict protocols that have not previously tried on plasma-treated liquids.