silver in nanotechnology

Silver has been used for centuries to prevent and treat a variety of diseases, most notably infections. It has been well documented that silver coins were used in ancient Greece and Rome as a disinfectant for the storage of water and other liquids. (1,2) More recently, NASA still uses silver to maintain water purity on the space shuttle. Silver has extremely potent antimicrobial properties, as only one part per 100 million of elemental silver is an effective antimicrobial in a solution. Free silver ions, or radicals, are known to be the active antimicrobial agent. In order to achieve a bactericidal effect, silver ions must be available in solution at the bacterial surface. Efficacy depends on the aqueous concentration of these ions. Silver ions appear to kill micro-organisms instantly by blocking the respiratory enzyme system (energy production), as well as altering microbe DNA and the cell wall, while having no toxic effect on human cells in vivo.

More recent information has provided, at least a hypotheses as to the mechanism of silver’s pro-healing and anti-inflammatory effects. Initial literature reports on the use of pure silver, mainly in the electro-colloidal form, occurred prior to the 1940’s when pure silver was still being used. After 1940 a host of systemic antibiotics became prevalent, decreasing the use of silver except as a topical agent. During this transition, silver was complexed as a salt (e.g. silver nitrate and silver sulfadiazine) or other compound (e.g. silver protein) to increase the available silver ion concentration. These silver complexes remain a popular topical antimicrobial agent for the care of wounds. Silver itself is considered to be non-toxic to human cells in vivo.(4) The only reported complication is the cosmetic abnormality argyria caused by precipitation of silver salts in the skin and leading to a blue-gray color

Thus, if none of the chemical species produced include silver hydroxide complexes with the general formula Agx(OH)y (charge = x-y), then it is conceivable that uptake could occur through the orthophosphate pathway. Such a scenario would explain the rapid uptake and kill of microorganisms as well as the susceptibility of silver resistant organisms. The presence of Ag0 suggests that there must be clusters present as it is unlikely that a bare atom such as Ag0 could exist on its own. These clusters may exist as uncharged or charged entities, but it is unknown what the biological activity might be. It is known that other heavy metals such as Au and Pt have unique biological properties including anti-inflammatory and apoptosis induction (anti-tumour?) activity. Since these activities have not been observed with Ag+ in the past, but they have been observed with dissolution products of nanocrytalline silver, it is postulated that it is the other species released, including Ag0, that may be partly or wholly responsible for the unusual biological properties.