Size controlled, antimicrobial ZnO nanostructures produced by the microwave assisted route

Zinc oxide nanostructures (ZnO-NS) have shown to be of great value for several biological and biomedical applications. In particular, they have been used in bioimaging and delivery applications as well as inhibitors of microbial growth. In this work a new methodology for producing highly crystalline, size controlled ZnO-NS using a chemical microwave assisted synthetic route is described. A wide range of sizes and shapes of ZnO-NS could be controlled by varying the molar ratio of zinc nitrate to hexamethylenetetramine (HMT) from 3:20 to 30:20. The produced ZnO-NS systematically changed from 25 nm spherical nanoparticles to well-shaped micro sized hexagonal nanorods. Pronounced oxygen defects were also noticed, particularly at higher molar ratios. However, this is not the case with the lattice constant c, whose value is found to decrease by increasing this ratio. The produced ZnO-NS were tested as antimicrobial agent against Gram-negative (E. coli), Gram-positive (B. subtilis) bacteria and yeast (S. cerevisiae). Significant inhibition of these microbial strains was noticed even at low concentrations of ZnO-NS. The ZnO-NS with the molar ratio 3:20 was the most effective against the microbes tested. The results showed 80, 71 and 50% inhibition of E. coli, B. subtilis and S. cerevisiae, respectively. Using the "surfactant stress model" we describe the nanostructure formation of ZnO-NS. The antimicrobial activity of ZnO-NS correlated well with lattice constant c and particle size, where smaller particles with higher value of c displayed increase inhibitory activity. No clear correlation between the oxygen defects and bacterial inhibitions was observed. This highly crystalline, size tunable ZnO-NS could prove to be effective antimicrobial agents at low concentrations (e.g. 20 μg per 10 mL) and might be tested against other microorganisms.