Toxicity of silver nanoparticles increases during storage

(Nanowerk Spotlight) Silver had already been recognized in ancient Greece and Rome for its infection-fighting properties but in modern times pharmaceutical companies made more money developing antibiotics. However, thanks to emerging nanotechnology applications, silver has made a comeback in the form of antimicrobial nanoparticle coatings for textiles, surgical instruments, lab equipment, floors or wall paints (see for instance: "Antibacterial nanotechnology multi-action materials that work day and night").
The flip side of silver's desired toxicity towards microbes is that it might have toxic effects for humans as well ("As nanotechnology goes mainstream, 'toxic socks' raise concerns") and this has raised debate about the safety of nanosilver products. Although scientists have worked to reduce the toxicity of antimicrobial nanosilver in products, concerns remain.
Not helping to put these concerns to rest is a new report from a group of researchers in Germany that shows that toxicity of silver nanoparticles increases during storage because of slow dissolution under release of silver ions.
The group, led by Matthias Epple, a professor for inorganic chemistry at the University of Duisburg-Essen, prepared and characterized silver nanoparticles with different surface functionalization and studied their dissolution in ultrapure water at three different temperatures.
SEM image of a large area of HCP monolayer of silica microspheres forming on top of photoresist
Influence of added silver nanoparticles (Ag-NP) on the viability of human mesenchymal stem cells. Individual wells of a 24-well cell culture plate are shown. The cells were treated with 50 µg/mL silver nanoparticles of different ages (i.e., immersed in water at 5 °C) for 24 hours under cell culture conditions. In the control, no silver was added. Viable cells are indicated by green fluorescence (calcein-AM staining). Scale bar: 2 mm. (Reprinted with permission from American Chemical Society)
According to Epple, there is a general agreement that dissolved silver ions are responsible for the biological action that is especially pronounced against microorganisms. The lethal silver concentration of silver nanoparticles for human mesenchymal stem cells is about three times higher than that of silver ions (in terms of the absolute concentration of silver in a given solution).
The researchers note that only very little is known about the rate of dissolution of silver nanoparticles. "As this rate directly determines the concentration of silver ions in the vicinity of a nanoparticle, it is highly important for any antimicrobial application of silver nanoparticles, and also for assessment of the toxicity of silver nanoparticles in humans," they say. "In addition, the final fate of silver nanoparticles that are released into the environment (e.g., from silver-containing clothes into sewage plants) depends on these data."
It is likely that the rate of dissolution depends not only on the chemical species (i.e., 'metallic silver in nanoparticulate form') but also on the particle size, the surface functionalization, and the particle crystallinity. In addition, the temperature and the nature of the immersion medium (e.g., the presence of salts or biomolecules) will be major factors.
Epple's team found that the rate and degree of the dissolution of silver nanoparticles in water depend on their surface functionalization, their concentration, and the temperature.
"In a given system under given conditions, a steady state was reached after several hours, i.e., the nanoparticles do not fully dissolve. This will change in a dynamic environment, e.g., during a perfusion experiment."
The researchers point out that such changes in the nanoparticle dispersions may escape the attention of the experimentalist because the classical analytical methods – e.g., dynamic light scattering, electron microscopy, or ultracentrifugation – are insensitive to released ions and because the particle diameter undergoes only a minor change.
"A dynamic light scattering experiment of aged particles would typically be accepted as quality control that the particles did not change during storage, but this experiment would not reveal such dissolution phenomena" they write.
With regard to the toxicity of nanoparticles in the body and in the environment, the biological action of freshly prepared and aged nanoparticles is strongly different due to the different amounts of released ions. Unfortunately, the dissolution in a biological medium is much more complicated to measure and describe because of the presence of various compounds in the medium, and the fate of the released silver ions is also unclear. Therefore, the dissolution in pure water, as in the experiments of the team, gives first indications on the fate of immersed silver nanoparticles in biological environments.
"Nevertheless, nanoparticles are typically not stored in biological media but in water before they are tested for their biological action, and therefore the reported results represent typical laboratory investigations of nanoparticle toxicity studies," writes Epple. "Some published discrepancies in reported toxicological levels may be explained by this fact. Of course, if nanoparticles are stored in the dry state, they will not dissolve, but this is not typical for surface functionalized, dispersible nanoparticles because of redispersion problems. 'Dry' silver nanoparticles that could be embedded in a matrix will also partially dissolve under release of silver ions when they come into contact with water, e.g., with washing water or with rain if applied outdoors."
Michael Berger By – Michael is author of three books by the Royal Society of Chemistry:
Nano-Society: Pushing the Boundaries of Technology,
Nanotechnology: The Future is Tiny, and
Nanoengineering: The Skills and Tools Making Technology Invisible
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