Antibacterial activity of ZnO nanoparticles: dependence on particle size, dispersion media and storage time

Zinc oxide nanoparticles have a high potential as novel antimicrobial products. In this study ZnO nanoparticles of various sizes: 20-100 nm (ZnO-1) and 50-300 nm (ZnO-2) have been examined. This paper presents the results of an integrated study of the influence of physicochemical properties such as particle size and concentration of the freshly prepared and 24-hour suspensions of ZnO nanoparticles (0. 001...1000 mg/L) in distilled water and in normal saline upon their toxicity against bacteria E. coli. Ionic solutions of ZnCl2 were used for comparison study. Fresh aqueous solutions of nanoparticles displayed the highest antibacterial effect (E. coli survival under 5-25%). Storage for 24 hours reduced the toxicity of all the samples. In normal saline solutions the toxicity of the studied nanoparticles decreased at concentrations below 10 g/L. In all cases ZnO-2 suspensions displayed lower toxicity levels, especially after ageing for 24 hours. Stability analysis showed that the highest antibacterial effect was produced by less stable solutions containing the largest aggregates. Thus, it has been shown that antibacterial effects of zinc oxide nanoparticles are affected by the size of particles and their aggregates in colloidal solutions, chemical composition of the media and the suspensions storage time. The obtained results can be used for creating new types of antibacterial products including phytoprotectors based on nano-sized zinc oxide particles as well as for development of methods to assess and predict toxicity of zinc-containing suspensions.