*3.3.1 ZnO nanoparticles*

Zinc oxide is a compound with excellent antimicrobial properties. It is an n-type semiconductor with a band gap of 3.3 eV. ZnO NPs can adopt a wide variety of morphologies such as; rings, propellers, belts, wires, among others [41, 42]. The antimicrobial activity of ZnO NPs happens by different mechanisms, one of these is the ROS generation [43] inside the cell. It has been proposed that ZnO NPs can act to generate cell death, or the release of Zn2+ ions, whose excess generates an alteration of cellular metabolism. Some species reported as susceptible to ZnO nanoparticles are; *S aureus*, *S. epidermidis*, *Streptococcus pyogenes*, *Enterococcus faecalis* [44], *Bacillus subtilis*, *Escherichia coli* and *Klebsiella pneumonia* [41]. These bacteria can generate intra-hospital infections causing serious infectious diseases and some strains are found in water or food, so ZnO NPs can have a possible application in these areas.
