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ZnO and polymers

Zinc oxide, ZnO

Antimicrobial properties

The antimicrobial properties of Zn2+ have been known for a long time, both against bacterial (both Gram-positive and Gram-negative) and fungal strains. It is recognised that part of the antimicrobial activity of ZnO particles originates from their ability to dissolve in aqueous media, partially generating Zn2+. ZnO particles have been investigated as antibacterial agents in both micro- and nanoscale formulations. However, it has been shown that ZnO exhibits enhanced antimicrobial activities when the particle size is reduced to the nanometer range since nano-sized ZnO can interact with the bacteria surface and via the cell membrane subsequently enter the cell.

ZnO as an antimicrobial agent is popularly employed in numerous fields due to its unique physical and chemical properties. ZnO NPs (nanoparticles) are increasingly used in personal care products, e.g. cosmetics and sunscreen, and textile fabrics because of the excellent UV blocking properties. For the medical and textile industry, the antimicrobial activity of ZnO is also an important property, especially for protective clothing in medical care and as surface modification of surgical meshes. ZnO (bulk, > 100 nm) is graded as a “GRAS” (generally recognised as safe) substance by the US Food and Drug Administration (FDA). Therefore, ZnO NPs have received more attention in biomedical applications, such as anticancer, drug delivery, anti-inflammation and wound healing. In the food industry, ZnO NPs are utilised in coatings and food packaging materials as antimicrobial agents to prevent contamination of foods with harmful bacteria or as UV light absorbers to protect foods that are sensitive to UV light exposure.

Antimicrobial mechanism

The antimicrobial activity of ZnO NPs is partly attributed to their ability to penetrate the cell membrane of the bacterial cells. Inside the cells, zinc ions (Zn2+) may be released that can alter the cellular structure or function. The ZnO NPs can generate reactive oxygen species (ROS) which give rise to oxidative stress that damages cellular components and will eventually lead to cytotoxicity. The size of the NPs has been shown to influence the uptake and release of Zn2+ from ZnO NPs in the cells, smaller particles leading to higher uptake and release of Zn2+. The increased specific surface area with reduced particle size leads to enhanced particle surface reactivity and, hence, to larger ROS formation.

Accumulation and precipitation of ZnO NPs on the bacterial exterior, i.e. direct contact with the cell walls, cause destabilisation of the cell membrane. ZnO NPs attached on the cell wall may also induce ROS formation as well as release Zn2+, conditions that would cause cell membrane disintegration, membrane protein damage and genomic instability.

References

Jiang, J., Pi, J. and Cai, J. The Advancing of Zinc Oxide Nanoparticles for Biomedical Applications. Bioinorganic Chemistry and Applications, Volume 2018, Article ID 1062562, 18 pages. https://doi.org/10.1155/2018/1062562

Pasquet, J. et al. The contribution of zinc ions to the antimicrobial activity of zinc oxide. Colloids and Surfaces A: Physicochemical and Engineering Aspects 2014, 457, 263-274. https://doi.org/10.1016/j.colsurfa.2014.05.057

Sirelkhatim, A. et al. Review on Zinc Oxide Nanoparticles: Antibacterial Activity and Toxicity Mechanism. Nano-Micro Letters 2015, 7, 219. https://doi.org/10.1007/s40820-015-0040-x

Villapún, V.M. et al.  Antibacterial Metallic Touch Surfaces. Materials 2016, 9, 736. https://doi.org/10.3390/ma9090736

Antimicrobial polymers

Antimicrobial polymers, also known as polymeric biocides, is a class of polymers with antimicrobial activity, or the ability to inhibit the growth of microorganisms such as bacteria, fungi or protozoans. These polymers have been engineered to mimic antimicrobial peptides which are used by the immune systems of living things to kill bacteria. Typically, antimicrobial polymers are produced by attaching or inserting an active antimicrobial agent onto a polymer backbone via an alkyl or acetyl linker. Antimicrobial polymers may enhance the efficiency and selectivity of currently used antimicrobial agents, while decreasing associated environmental hazards because antimicrobial polymers are generally nonvolatile and chemically stable. This makes this material a prime candidate for use in areas of medicine as a means to fight infection, in the food industry to prevent bacterial contamination, and in water sanitation to inhibit the growth of microorganisms in drinking water.

Antimicrobial agents kill bacteria through different methods depending on the type of bacteria. Most antiseptics and disinfectants kill bacteria immediately on contact by causing the bacterial cell to burst, or by depleting the bacteria’s source of food preventing bacterial reproduction, also known as bacterial conjugation. Antimicrobial polymers commonly kill bacteria through this first method, which is accomplished through a series of steps. First, the polymer must adsorb onto the bacterial cell wall. Most bacterial surfaces are negatively charged, therefore the adsorption of polymeric cations has proved to be more effective than adsorption of polymeric anions. The antimicrobial agent must then diffuse through the cell wall and adsorb onto the cytoplasmic membrane. Small molecule antimicrobial agents excel at the diffusion step due to their low molecular weight, while adsorption is better achieved by antimicrobial polymers. The disruption of the cytoplasmic membrane and subsequent leakage of cytoplasmic constituents leads to the death of the cell.

References

Cloutier, M., Mantovani, D. and Rosei, F. Antibacterial Coatings: Challenges, Perspectives, and Opportunities. Trends Biotechnol. 2015, 33, 637–652.