Received: 24 March 2016 Revised: 02 July 2016 Accepted: 06 July 2017 DOI: 10.1002/ctpp.201600021 ORIGINAL ARTICLE Antibacterial activity of silver and zinc oxide nanoparticles produced by spark discharge in deionized water Amir Jamali 1 Serajoddin Razavizadeh 1 Atousa Aliahmadi 2 Hamid Ghomi 1 * 1 Laser and Plasma Research Institute, Shahid Beheshti University, Tehran, Iran 2 Department of Biology, Medicinal Plants and Drugs Research Institute, Shahid Beheshti University, Tehran, Iran *Correspondence Hamid Ghomi, Laser and Plasma Research Institute, Shahid Beheshti University, Evin 1983963113, Tehran, Iran. Email: h-gmdashty@sbu.ac.ir Inorganic antibacterial agents such as metal nanoparticles (NPs) are very impor- tant in biomedical and pharmaceutical areas. There are many methods of syn- thesizing these NPs, but all of them have their own disadvantages. In this study, ultrasonic-assisted spark discharge is employed to produce colloidal silver (Ag) and zinc oxide (ZnO) NPs which are stable without using any stabilizers or sur- factants. Different tests such as X-ray diffraction, field emission scanning electron microscopy, and ultraviolet–visible absorption spectroscopy are used for the char- acterization of the quantity and quality of these NPs, and their antibacterial activity is evaluated by the disk diffusion method and determination of the minimum inhibitory concentrations against Escherichia coli. The results show that the overall antibacterial activity of Ag NPs is higher than that of ZnO NPs. KEYWORDS antibacterial agents, Escherichia coli, silver nanoparticles, spark discharge, zinc oxide nanoparticles 1 INTRODUCTION Over the past few years, nanotechnology has emerged as a novel technology with great and unique medical benefits and use in different aspects such as exploring the antibacterial effect of metal nanoparticles (NPs). Due to emergence of microorganisms resistant to multiple antimicrobial agents, development of new disinfection methods is of great importance. [1] In this context, efficient disinfection methods are defined as those that provide immediate and constant mortality of germs. Metal NPs are one of the widely known, efficient disinfectants. [2, 3] Metal NPs, which have a large surface area to volume ratio, are of great interest due to their unique antimicrobial properties. [4] Heavy metals such as silver (Ag), gold (Au), bismuth (Bi), and zinc (Zn) have proven antimicrobial properties and some of them has been used in medical practice for centuries. [5] Microbial experiments have established the antibacterial effect of Ag and zinc NPs against a wide variety of microorganisms. [6–10] The mechanism of the antimicrobial activity of NPs has not yet been clearly elucidated. However, it has been confirmed that Ag + and Zn 2+ ions can interact with the thiol groups of enzymes and proteins that are important for bacterial respiration, resulting in the enzymes’ inactivation and even death. Recently, other possible mechanisms of action of these NPs have been proposed. [4] The most common methods for the synthesis of metal NPs are chemical techniques, [11–13] which require the use of stabilizers in order to prevent the NPs from agglomeration. On the other hand, most physical methods for the preparation of NPs are too expensive, overcoming their advantages. Among the physical methods, electrical discharge is a suitable method for the preparation of metallic NPs and carbon nanotubes because of its cost effectiveness, its ability to be used in mass production environments, and its non-dependence on vacuum equipment. However, the NP size distribution obtained through chemical methods is narrower than that produced by electrical discharge. [14–19] In this study, we have employed an ultrasonic-assisted spark discharge method (USDM) to produce Ag and zinc oxide (ZnO) NPs. [20] The colloidal NPs produced by this method are stable without using any stabilizers or surfactants. The antibacterial activity of Ag NPs will change when they are modified with a chemical stabilizer such as sodium dodecyl sulfate (SDS). [21] The absence of stabilizers or surfactants in the colloidal solution of NPs could help us Contrib. Plasma Phys. 2017;57:316–321 www.cpp-journal.org © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 316