Optimal Dispersion of nanoZnO and Antimicrobial Activity against Staphylococcus aureus and Escherichia coli

Introduction: Bacterial contamination is of great concern to the food industry. Inorganic compounds in nanosize present antibacterial activity at low concentrations because of their high surface area to volume ratio. NanoZnO synthesis methods allow the production of separated nanoparticles. However, single primary nanoparticles tend to form particle collectives which may result in loss of antimicrobial activity.

Purpose: To optimize nanoZnO dispersion with response surface methodology (RSM) and test nanoZnO antibacterial activity against Staphylococcus aureus and Escherichia coli.

Methods: The effects of (1) power, (2) probe sonication time and (3) the presence of the dispersant agent (Na₄P₂O₇) on nanoZnO size were investigated using a 2³ full factorial design. To study the time and power of sonication effects over an extensive range, a central composite design (CCD) was also applied. The optimized dispersion condition was chosen to obtain lowest nanoZnO size. Concentrations of nanoZnO (0, 1, 5 and 10%) were tested by the agar well diffusion method.

Results: The lowest nanoZnO size (259 nm) was obtained at conditions of 50 min, 200 W, in the presence of Na₄P₂O₇, whereas the highest nanoZNO size (506 nm) was obtained at conditions of 20 min, 400 W, in the absence of Na₄P₂O₇. The RSM showed the lowest nanoZNO size (238 nm) with optimal conditions at 200 W, 45 min of sonication in the presence of Na₄P₂O₇. At 1% and 5%, nanoZNO presented the same inhibition zone (1.0 cm) against S. aureus, with this zone increasing to a 1.5 cm for 10% NanoZNO. NanoZnO at 1% had no effect against E. coli, whereas 5 and 10% nanoZnO created inhibition zones of 1.5 and 1.8 cm, respectively.

Significance: Optimal nanoZnO dispersion presented antimicrobial activity against foodborne pathogens. NanoZnO is a promising antimicrobial for food preservation with applications for incorporation in polymers intended as food contact surfaces.