## 1 Introduction
Compared to conventional methods (hydrothermal, solvothermal), this method has several advantages, including rapid heating to the crystallization temperature, acceleration of chemical reactions, and increased crystallinity [19]. Moreover, it is a simple and efficient method both energetically and financially. Compared to conventional conduction-based heating, when using a microwave, heat is generated in the whole volume of the sample [20]. In this case, the entire system is heated simultaneously by the vibration at an extremely rapid frequency, at the gigahertz level, of each water molecule (or any other polar molecule). This fact allows a higher reaction rate, and, in some cases, the reaction yield is much higher than in the case of traditional heating (standard hydrothermal method) [21]. Previous experimental studies have shown that microwave-assisted synthesis can be easily controlled by precisely setting the temperature, pressure, and reaction time and can enhance reaction rates by 10-100 times compared to conventional heating methods [22; 23]. Particularly for ZnO nanoparticles, our group focused on the synthesis with microwave-assisted hydrothermal versus polyol methods, using different synthesis parameters and Zn\({}^{2+}\) precursors. It was observed that the synthesis approach impacts the subsequent biological effects of ZnO nanoparticles due to the different morphologies and sizes of the powders. Hence, spherical morphology was observed using the polyol method, while flower-type morphologies were formed via the microwave-assisted hydrothermal method [24]. Over time, numerous studies have reported the influence of precursor concentration, the molar ratio of Zn\({}^{2+}\) precursors and reducing agents and microwave-assisted hydrothermal parameters (e.g., irradiation power, time and temperature) on the nanostructural, optical properties and morphology of ZnO. Among the most common chemical precursors used were zinc acetate, zinc nitrate, zinc sulfate, sodium hydroxide, ammonia water, and hexamethylenetetramine (HMTA) [21]. Even though zinc chloride is convenient in this case because the Cl\({}^{-}\) ions resulting from the reaction do not exhibit significant toxicity at low concentrations, it should be emphasized that ZnCl\({}_{2}\) stands out as a troublesome reactant, primarily due to the potential occurrence of a stable by-product or impurity known as simonkolleite (Zn\({}_{5}\)(OH)\({}_{8}\)Cl\({}_{2}\cdot\)H\({}_{2}\)O) during the synthesis of ZnO, hence the reduced number of related publications [25; 26]. The main objective of this study is to investigate the intrinsic factors influencing the antibacterial activity of ZnO nanoparticles, with a specific focus on particle size, and to provide insights into how these factors can be controlled through varying synthesis conditions. The findings aim to contribute to the development of ZnO nanoparticles with optimized properties for antimicrobial applications. Zinc oxide nanoparticles were synthesized with a microwave-assisted hydrothermal method, with the controlled variation of two parameters, the concentration of the Zn\({}^{2+}\) precursor and the reaction time, using zinc chloride and sodium hydroxide as starting materials. Various characterization techniques were employed to determine the mineral composition, crystallinity, particle size, morphology, and specific surface area of the obtained powders while correlating the findings with their antimicrobial properties.