Amino acid-mediated synthesis of zinc oxide nanostructures and evaluation of their facet-dependent antimicrobial activity

• ZnO nanostructures (ZnO-NSs) were synthesized with amino acids as capping agents.
• The concentration of surface defects increases with the number of polar facets.
• ZnO-NSs show a facet-dependent antibacterial activity.
• Reactive oxygen species contribute mostly to antimicrobial activity.
• Cellular internalization of NSs has a minor effect on antimicrobial activity.

ZnO nanostructures (ZnO-NSs) of different morphologies are synthesized with the amino acids l-alanine, l-threonine, and l-glutamine as capping agents. X-ray diffraction (XRD) shows the formation of a crystalline wurtzite phase of ZnO-NSs. The surface modification of ZnO-NSs due to the capping agents is confirmed using Fourier transform infrared (FTIR) spectroscopy. Photoluminescence spectroscopy reveals that the concentration of surface defects correlates positively with the number of polar facets in ZnO-NSs. The antimicrobial activity of the ZnO-NSs has been tested against Escherichia coli and the common pathogens Staphylococcus aureus, Klebsiella pneumoniae, and Bacillus subtilis. Culture-based methods in rich medium show up to 90% growth inhibition, depending on the ZnO-NSs. Flow cytometry analyses indicate that the reactive oxygen species (ROS) generated by ZnO-NSs contribute mostly to the antibacterial activity. Control experiments in minimal medium show that amino acids and other reducing agents in Luria-Bertani (LB) medium quench ROS, thereby decreasing the antimicrobial activity of the ZnO-NSs.

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