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Antimicrobial properties
Silver (Ag) nanoparticles (NPs) are one of the most commonly used nanomaterials in different daily life applications. The reason is the well-known antimicrobial properties of silver as well as its availability. As a disinfectant, silver has been used for several millennia, e.g. silver vessels were used in 1000 B.C to make water potable, and for centuries silver has been in use to treat burns and chronic wounds. After the discovery of penicillin in 1928, the treatment of bacterial infections using silver has gradually been reduced to a minimum. As a consequence of the emergence of antimicrobial resistant bacteria and even multi-resistant bacteria during recent decades, the use of silver as an antimicrobial surface has gained an increased interest. Silver is due to its potent and broad-spectrum antimicrobial activity, even at low doses, today one of the most interesting antibacterial materials for different applications.
Silver NPs or silver ions are preferably incorporated in coatings in an organic or inorganic (e.g. silica and phosphate glass) matrix to enable a controlled release of silver from, as an example, different touch surfaces. Silver NPs can also be embedded in the surface of metallic biomaterial implants (e.g. titanium) to avoid biomaterial-associated infections and thereby promote bio-integration with the tissue.
During the last decades, the use of silver (both as ions and NPs) as an antimicrobial agent has increased immensely and is today widely used in both traditional industries and emerging fields including coatings, paint, food, textiles (sports outfits), cosmetics, water purification, medical treatment, washing machines, packaging materials, refrigerators etc. The increased use of silver NPs in the society has resulted in significant economic benefits, but also in evident environmental and health risks and potential adverse effects on aquatic organisms and humans induced by their potential diffuse dispersion to different settings via for example waste water treatment plants or direct intake of colloidal silver products. Exposure to high concentrations of silver has for instance been reported to result in argyria, which is a condition where the skin is colored blue/grey.
Antimicrobial mechanism
The antimicrobial activity of silver is mainly attributed to the silver ion (Ag+), which can cause inhibition of cell growth, alteration of cell morphology and production of reactive oxygen species (ROS). Silver ions interact extensively with bacteria cell walls, which has been proposed to cause lysis (cell membrane breakdown). Released silver ions may also interact with sulfur-containing species, such as DNA and proteins (especially enzymes) inside the bacterial cell. This may inactivate enzymes and inhibit gene expression related to the enzymes. Silver interacting with DNA bases may cause DNA to condense and therefore loose its ability to replicate. One or several mechanisms may act in a synergistic way that may result in even more severe cell damage.
As silver is a noble metal stable toward oxidation by water, dissolution (i.e. release of ions) requires another oxidizer such as dissolved atmospheric O2. Although the most plausible mechanism for the antimicrobial activity of silver NPs is attributed to the silver ion , the silver NPs act as silver ion reservoirs that prolongs the antibacterial activity over time. The antimicrobial activity of silver NPs has shown to be dependent on the particle size and morphology and the rate of dissolution into silver ions. The dissolution rate increases typically with decreasing NP size. Small Ag NPs can further be immobilized close to a cell membrane, or even internalized within the cell, and there locally release high concentrations of silver ions (Trojan horse effect). The antimicrobial activity is moreover very sensitive to species that readily interact with the silver ion forming insoluble complexes, such as sulfide or chlorides (at high chloride concentrations), which reduces or eliminates the antibacterial effect.
References
Chernousova, S. and Epple, M. Silver as Antibacterial Agent: Ion, Nanoparticle, and Metal. Angew. Chem. Int. Ed. 2013, 52(6), 1636-1653. https://doi.org/10.1002/anie.201205923
Gliga, A.R. et al. Size-dependent cytotoxicity of silver nanoparticles in human lung cells: the role of cellular uptake, agglomeration and Ag release. Particle and Fibre Toxicology 2014, 11:11. https://doi.org/10.1186/1743-8977-11-11
Le Ouay, B. and Stellacci, F. Antibacterial activity of silver nanoparticles: A surface science insight. Review. Nano Today 2015, 10(3), 339-354. https://doi.org/10.1016/j.nantod.2015.04.002
Rai, M., Yadav, A. and Gade, A. Silver nanoparticles as a new generation of antimicrobials. Biotechnology Advances 2009, 27(1), 76-83. https://doi.org/10.1016/j.biotechadv.2008.09.002
Reidy, B. et al. Mechanisms of Silver Nanoparticle Release, Transformation and Toxicity: A Critical Review of Current Knowledge and Recommendations for Future Studies and Applications. Materials 2013, 6(6), 2295-2350. https://doi.org/10.3390/ma6062295