Silver nanoparticles represent a class of manmade nanomaterials that are the most widely used in commercial medical and consumer products including household antiseptic sprays and antimicrobial bandages.  Although silver ions released from the silver nanoparticles exhibit toxicity with no doubt, there is still no confirmative evidence to show whether the metallic silver nanoparticles themselves exert the particle-specific toxicity.    

The research group led by Prof. Alvarez and Prof. Colvin at Rice University recently reported their interesting results in Nano Letters (DOI: 10.1021/nl301934w).  They designed specific experiments by incubating the silver nanoparticles with an E. coli strain K12 (ATCC 25404) that exhibits equal susceptibility to silver ions under aerobic and anaerobic conditions.  Although metallic silver could be easily oxidized to release toxic silver ions in acidic aqueous solutions exposing to air, the oxidation reaction could be essentially prevented under strict anaerobic conditions.  As a result, the influence of the metallic silver nanoparticles on the E. coli bacteria could be precisely evaluated.  The viability assays showed the nonlethal concentrations were 6224 and 7665 times higher than the minimum lethal concentration (MLC) of silver ions for the 5-nm and 11-nm silver nanoparticles, respectively.  Moreover, regardless of the parameters (e.g., shape, size, surface coating, etc.) of the silver nanoparticles, their toxicity only depended on the concentration of the silver ions released under aerobic conditions.  These observations suggested that the antibacterial activity could be controlled by modulating the release of silver ions from silver nanoparticles (similar to a drug release process) through controlling the oxygen availability and the parameters of the silver nanoparticles.  

The London 2012 Olympic Games started last Friday and attracted attentions from all over the world.  I read an article introducing how the medals were manufactured.  The gold medals are not made of pure gold and they are primarily made of silver coated with very thin layers of gold.  The gold coating is not only for the golden looking but also can help to prevent the inside silver being oxidized.  Similarly, the silver nanoparticle can be coated with a seamless gold layer to avoid oxidization, leading to no release of silver ions, the toxic species.  At the end, the gold-coated silver nanoparticles can behave as biologically friendly materials as gold nanoparticles.  

Z.-M. Xiu, Q.-B. Zhang, H. L. Puppala, V. L. Colvin, P. J. J. Alvarez, “Negligible particle-specific antibacterial activity of silver nanoparticles”, Nano Lett. DOI: 10.1021/nl301934w