Environmental Protection

New Method May Lower Toxicity of Antimicrobial Silver

Chemists at the University of Helsinki have manufactured new polymer-stabilized silver nanoparticles to reduce toxicity. The research results will soon be published in Colloid and Polymer Science.

The safety of nanoparticles (a nanometer is equal to one billionth of a meter) is a topic of debate both in research and everyday life. The antimicrobial characteristics of silver are well-known and have numerous commercial applications. In the United States, the registration of new insecticides containing silver nanoparticles has stirred more discussion of their risks. Supermarkets carry an abundance of products containing silver nanoparticles such as shower curtains, glues, and even some creams and deodorants.

Nanoparticle manufacturing methods often are based on reducing metallic salts, in this case silver nitrate, in the presence of a stabilizing compound. Polymer-stabilized silver nanoparticles have been successfully manufactured at the Laboratory of Polymer Chemistry at the University of Helsinki. The work has exploited the laboratory's prior experience with gold nanoparticles and the expertise of the School of Science and Technology of the Aalto University and its European cooperation partners.

In Helsinki, the stabilizing component is a polymer with a reactive thiol end group. Thiol groups bind effectively with silver. 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.

Many mechanisms relating to the toxicity of silver to microorganisms have been put forward. Silver ions react in cells with the thiol groups of proteins. There is 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.

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 oxidizes 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.

According to Finnish researchers, 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 as possible, enabling a reduction in the possible exposure to silver nanoparticles.

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