Reducing the toxicity of silver
Silver is known as one of the oldest antimicrobial agents (i.e. substances that kill or inhibit the growth of microorganisms such as bacteria, fungi, or protozoans).
Many mechanisms relating to the antimicrobial property of silver have been put forward. It has been demonstrated that silver ions react in micro-organisms with the thiol groups of proteins. There is also evidence to show that silver ions damage DNA by inhibiting its replication. Silver’s ability to form extremely sparingly soluble salts is also considered one of its impact mechanisms. When the chloride ions precipitate as silver chloride from the cytoplasm of cells, cell respiration is inhibited. The antibacterial efficiency of silver nanoparticles is also well-known, especially against Gram-negative bacteria such as E.coli. The silver nanoparticles work by releasing silver ions and by penetrating cells.
Harmful effects of silver nanoparticles
Silver, silver ions and silver nanoparticles have generally been considered to be quite harmless to people. However, the most recent research has demonstrated that nanoparticles also penetrate mammalian cells and damage the genotype. There is even evidence to suggest that silver nanoparticles may actively find their way into cells through endocytosis. Inside the cell, hydrogen peroxide formed in cell respiration oxidises silver nanoparticles and releases silver ions from them, consequently increasing the toxicity. Thus, it can even be assumed that silver nanoparticles are cyto- or genotoxic. Moreover, it has been demonstrated that silver nanoparticles penetrate the skin via pores and glands. If the skin is damaged, this facilitates the penetration of silver particles through the skin.
Reducing the toxicity of silver
According to chemists at the University of Helsinki it is important that coatings containing silver nanoparticles do not release nanoparticles. The effect of the coating should only be based on silver ions dissolving from them. Consequently, nanoparticles should be as well bound to the coating (i.e. products with added silver or silver nanoparticles) as possible, enabling a reduction in the possible exposure to silver nanoparticles.
Keeping this in mind the Finnish researchers have managed to manufacture new polymer-stabilised silver nanoparticles. By binding the silver nanoparticles to polymers these researchers are confident that silver nanoparticles will be prevented from penetrating mammalian cells thus reducing the toxicity of silver.
In order to manufacture polymer-stabilised silver nanoparticles the researchers used a reactive thiol end group. It is known that thiol groups bind effectively with silver, which enables the effective colloidal stabilisation of silver nanoparticles and binding to polymers. The polymer is in itself a soft, rubber-like acrylate, which contains a water-soluble block that enables silver ions to be released from the otherwise hydrophobic coating. The idea is that these silver nanoparticles could be used as a coating or its component.
Source: University of Helsinki
February 24, 2010