# Multimodal Comparison of Cold Atmospheric Plasma Sources for Disinfection
## 1 Introduction  
Following the rapid development of plasma medicine in recent years, a plethora of different cold atmospheric plasma sources have been developed in laboratories across the world [1, 2, 3, 4, 5]. Most of these sources have been designed for a specific purpose and tested within their framework of interest, achieving, in some cases, outstanding results up to reaching the clinical trial phase [6]. Among the most studied applications, many experiments have been carried out on disinfection and decontamination. Although the bactericidal properties of plasma treatments are commonly recognised, there is still open debate about the actual processes involved. Plasma is a complex system, involving free charges, excited atoms, reactive species, and radiation. Moreover, biological systems are even more complex. A complete description of all the biochemical processes involved, with the understanding of the triggered biochemical pathways, is still far from being available. Among the other components, a fundamental role in the plasma-cells interaction is expected to be played by reactive oxygen and nitrogen species (RONS) [7; 8; 9; 10]. They are typically the results of multi-step chemical chains. Primary species like hydroxyl radical, atomic oxygen, or atomic nitrogen are directly obtained from operating gases by electron-induced dissociation and then react with each other to produce secondary RONS [11]. In the presence of a liquid surface, interfacial phenomena and solubility drive the absorption of species into water, with the consequent development of higher-order chemistry [12]. ROS, which include hydrogen peroxide (H\({}_{2}\)O\({}_{2}\)) and ozone (O\({}_{3}\)), are characterised by a strong redox potential. Maintaining a redox equilibrium between the cytoplasm and the environment is fundamental for biological systems, and most of them possess defence mechanisms to counteract imbalances [13]. However, when these mechanisms are not sufficient, the so-called oxidative stress can lead to protein damage and compromise the organism homeostasis. Similarly, RNS, such as nitrous acid (HNO\({}_{2}\)), nitric acid (HNO\({}_{3}\)) and their unprotonated forms (NO\({}_{2}^{-}\) and NO\({}_{3}^{-}\)), can lead to cell damage via nitrosative stress. The cell membrane is known to have a key role in maintaining equilibrium, but its integrity can be harmed by the plasma treatments [14]. The whole picture of the different biochemical pathways is still mostly obscure, with thousands of proteins involved whose exact function in still unknown [13]. This poses important obstacles to a complete theoretical understanding of the chemistry, suggesting a more suitable phenomenological modelling. A full characterisation of a plasma source in action requires investigation of the plasma state, the gas phase, the liquid phase and the biological effects. Moreover, all these points of view should be combined in a single setup. In fact, it has been observed that the same plasma source can behave differently when operated under different environmental conditions [15], meaning that each setup is unique and diagnostic data cannot be easily compared. On the other side, for each diagnostic (or set of diagnostics) multiple operating parameters and sources should be characterised under a fixed data collection protocol. The gathering of all the collected data, for the different measured quantities and the different sources, constitute the behavioural map which can suggest a phenomenological model. There are a few studies in the literature that perform a systematic comparison of plasma sources for biological and medical applications, as an example comparing the RONS in water [16; 17; 18]. From the point of view of bacteria inactivation, [PERSON] et al. [19] published a study on the comparison of DBD and nanosecond pulsed plasma for the inactivation of multidrug resistance bacteria, and [PERSON] et al. [20] compared a plasma jet and a surface DBD, proposing also a standardized technique for source comparison.