Silver is one of the oldest materials to be used widely in a nanoparticle form. Nanosilver, or colloidal silver, has been used as an antibacterial agent for surfaces, water filters and medication since the late 19th century.
Silver nanoparticles are now used wide range of products. Whilst it is generally thought that the danger to human health is low, there has been limited research into the effects of long-term exposure. It has been shown that nanosilver in coatings and fabrics can leach out over time but the effect of silver nanoparticles on the environment has not been studied extensively.
Example Anti-Microbial Applications of Silver Nanoparticles
Silver Nanoparticles have been used as an antimicrobial agent in coatings and surfaces in a huge range of products, including:
- medical equipment such as catheters
- wound dressings
- socks and underwear
- shower curtains
- tables and worktops
- refridgerator shelves
- floor coatings
- cosmetics, sunscreen and deodorants
- paints and glues
- water filters
Human Toxicity of Silver Nanoparticles
Several studies in the last few years have tried to make the issues around the toxicity of nanosilver clearer. Elemental silver at low doses is generally considered to be non-toxic, but long-term exposure can sometime cause damage to the liver, kidneys and blood cells.
Nanosilver has been shown to be absorbed into the body by inhalation, ingestion, and through the skin. The uptake rate is much greater for damaged skin, leading to demonstrably higher levels of silver in the blood and urine after nanosilver-treated wound dressings have been used.
Although specific studies on the toxicity of silver nanoparticles are limited, it is generally accepted that their toxicity derives from the free silver ions they release - the toxicity can therefore be related to the speed of release of these ions. This is determined mostly by the particle size, and the type of environment the nanoparticles are in. It is therefore important to determine whether the ions are released before or after absorption into the body.
Environmental Toxicity of Silver Nanoparticles
As more and more nanosilver is used in consumer products, it is inevitable that it will begin to build up in the environment. In particular, the use of silver nanoparticles in waste water treatment must lead to some presence of silver in aquatic environments.
As with most toxicity studies, it seems that silver ions, rather than nanoparticles themselves, are the cause of any toxic effects. It has been shown that silver in waste water sludge mostly takes the form of silver sulfide, which is non-toxic. It is unclear whether this can naturally convert to other, more harmful forms.
Anti-Microbial Mechanism of Silver Nanoparticles
Recent research by Rice University has shown beyond doubt that even very small silver nanoparticles have no direct toxic effect on microbes - all of the anti-microbial activity comes from free silver ions, released when the nanoparticles are oxidized. If oxidization is prevented, for example by protective chemicals in the surroundings, the antimicrobial effect of silver all but disappears.
This indicates that all research into nanosilver toxicity, including human studies, and development work to produce more effective antimicrobial agents, should focus on controlling an optimizing the release of silver ions. This could be controlled using the size and shape of the nanoparticles, or embedding them in responsive polymers.
Armed with this new, deeper understanding of just how silver nanoparticles are able to kill microbes, researchers will aim to optimize this old technology, not just to improve antibacterial performance, but also to reduce the amount of this precious metal that gets flushed into the environment, and to reduce the amount of potential damage it could do to ecosystems when it is released.