Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism

Abstract

Antibacterial activity of zinc oxide nanoparticles (ZnO-NPs) has received significant interest worldwide particularly by the implementation of nanotechnology to synthesize particles in the nanometer region. Many microorganisms exist in the range from hundreds of nanometers to tens of micrometers. ZnO-NPs exhibit attractive antibacterial properties due to increased specific surface area as the reduced particle size leading to enhanced particle surface reactivity. ZnO is a bio-safe material that possesses photo-oxidizing and photocatalysis impacts on chemical and biological species. This review covered ZnO-NPs antibacterial activity including testing methods, impact of UV illumination, ZnO particle properties (size, concentration, morphology, and defects), particle surface modification, and minimum inhibitory concentration. Particular emphasize was given to bactericidal and bacteriostatic mechanisms with focus on generation of reactive oxygen species (ROS) including hydrogen peroxide (H₂O₂), OH^- (hydroxyl radicals), and O₂ ^-2 (peroxide). ROS has been a major factor for several mechanisms including cell wall damage due to ZnO-localized interaction, enhanced membrane permeability, internalization of NPs due to loss of proton motive force and uptake of toxic dissolved zinc ions. These have led to mitochondria weakness, intracellular outflow, and release in gene expression of oxidative stress which caused eventual cell growth inhibition and cell death. In some cases, enhanced antibacterial activity can be attributed to surface defects on ZnO abrasive surface texture. One functional application of the ZnO antibacterial bioactivity was discussed in food packaging industry where ZnO-NPs are used as an antibacterial agent toward foodborne diseases. Proper incorporation of ZnO-NPs into packaging materials can cause interaction with foodborne pathogens, thereby releasing NPs onto food surface where they come in contact with bad bacteria and cause the bacterial death and/or inhibition.

Keywords: Antibacterial activity; Food antimicrobial; Reactive oxygen species; Toxicity mechanism; Zinc ions release; ZnO-NPs.

Fig. 1

a ZnO crystal structures. Adapted from Ozgur et al. [41]. b Bacterial cell structures, reused from Earth Doctor, Inc., formerly Alken-Murray [45]. c S. aureus plating for colony count [13]

Fig. 2

Correlation between the a influence of essential ZnO-NPs parameters on the antibacterial response and the b different possible mechanisms of ZnO-NPs antibacterial activity, including: ROS formation, Zn²⁺ release, internalization of ZnO-NPs into bacteria, and electrostatic interactions

Fig. 3

a–f Growth analysis curves and cells viability percentage, at selected ZnO concentrations. g Growth curves through optical density (OD_600 nm) measurements. h Percentage of viable cells after overnight incubation. Adapted with permissions from Raghupathi et al. [13]

Fig. 4

A Bactericidal efficacies of ZnO suspensions, for tested samples namely sample 1, sample 2, and bulk with three different particle sizes after 24 h incubation. Reproduced by permission from Padmavathy and Vijayaraghavan [12]. B Antibacterial activity of ZnO-NPs towards: Enteritidis and E. coli O157:H7, adapted from Xie et al. [91]

Fig. 5

a NPs internalization into the cell and translocation. NPs penetrate through holes, pits or protrusions in the cell wall. b Schematic representation of collapsed cell showing disruption of cell wall and extrusion of cytoplasmic contents. c Bacterial cell showing important variations in envelope composition (slight invaginations and thickening of cell wall) and extrusion of cytoplasm. d Probable mechanisms, involves the following: metal ions uptake into cells, intracellular depletion, and disruption of DNA replication, releasing metallic ions and ROS generation and accumulation and dissolution of NPs in the bacterial membrane. Reused from Díaz-Visurraga et al. [128]

Fig. 6

a EDS spectrum of E. coli in ZnO mixture, b FESEM micrographs of E. coli exposed to ZnO, arrows show ZnO particles on the bacteria surface, c untreated bacteria cells, d E. coli treated with ZnO, and e, f percentage inhibition of E. coli treated with ZnO-AP and ZnO–O₂ at different concentrations with and without UVA illumination, respectively (experiment was done by authors of this manuscript, triplicated)