Nanosilver, the active component of more than 20% of the nanoproducts currently available on the market, is the most commonly used nanomaterial for commercial applications. Approximately 88% of these products have some form of antibacterial or antimicrobial activity. Silver nanoparticles exhibit physical properties that are different from both the ion and the bulk material. Because of their strong antibacterial properties, several studies have shown the potential use of silver nanoparticles in biomedical and environmental applications, such as the treatment of wounds and burns and water disinfection. While the antibacterial properties of silver nanoparticles have been extensively demonstrated, their disinfectant mechanism(s) and kinetics in the inactivation of bacteria, viruses, and protozoa have not yet been elucidated. Indeed, there have been no studies testing the ability of silver nanoparticles to deactivate protozoan pathogens like Cryptosporidium parvum and Giardia lamblia. Previous studies have not addressed the effect of environmental conditions on the antimicrobial properties of silver nanoparticles.
The researchers hypothesize that there are three possible antimicrobial processes for silver nanoparticles: (1) direct interaction of the silver particle with the cell membrane and subsequent damage to the membrane and complexation with intracellular components, (2) release of Ag+ ions and subsequent disinfection, and (3) formation of reactive oxygen species (ROS). None of these mechanisms have been conclusively confirmed, nor has the relative importance of each mechanism in the inactivation of different types of pathogenic microorganisms been elucidated. This work will synthesize silver nanoparticles with different mean particle sizes and specific surface areas and quantify their effects on pathogen disinfection mechanism and rate. Pathogens proposed for study include a virus (MS2), a bacteria (E. coli), and a protozoa (Cryptosporidium parvum). Experimental design will allow us to identify each potential mechanism individually and determine its relative contribution to the overall disinfection rate for each pathogen.
It is anticipated that this work will have broader impacts in several ways, including a summer program for high-school students, adding an environmental nanotechnology course module to an existing course, and working with the University of Rhode Island chapter of Engineering Students without Borders (ESWB) to develop a service project involving the manufacture and distribution of nanosilver particles specifically designed for use in ceramic water filters manufactured by Potters for Peace. They envision this latter effort to be a sustainable service project for the student chapter that will teach them about developing-world water problems, introduce them to silver nanotechnology, and provide funds to support their student chapter service efforts.
In this study, we have investigated the disinfection effects of siver and copper nanoparticles. Silver nanoparticles capped with citrate and proteinate show effective disinfection capabilities for coliform bacteria and Escherichia coli (E. coli). The primary mechanism of disinfection is the oxidation of metallic silver to ionic silver and subsequent disinfection. Copper nanoparticles also exhibit significant disinfection capabilites, but these nanoparticles are less stable in solution. At even moderate ionic strengths, they form aggregates that are less effective at releasing ionic copper. Silver and copper nanoparticles can be impregnated into ceramic porous media to form effective water filters. However, we have also observed that silver nanoparticles are relatively mobile in ceramic filters. Nanoparticles that are painted onto the filters as part of an aqueous suspension can be released back into solution when the filter is in use. An alternative method is to "fire-in" the silver nanoparticles. Using this approach, silver nanoparticles suspended in water are mixed with clay and a fine-grained burnout material (sawdust, flour, rice husk) prior to firing. This approach results in a more homogeneous distribution of silver nanoparticles in the ceramic filter medium and approximately 10 times better retention in the porous media. By retaining more metallic silver in the filter media, the filter performance lifetime will likely be extended. Finally, the effects of silver nanoparticles on Cryptosporidium parvum have been examined using a murine model and dielectrophoresis. Results suggest that ionic silver released from the silver nanoparticles has a modest but measureable effect on the infectivity of C. parvum in mice. Given the resistance of this pathogen to other common disinfectants, including hypochlorous acid, the identification of a new metallic disinfectant may have important implications for future C. parvum treatment technologies. C. parvum was also shown to be removed (1.8 mean log reduction) from water during physical filtration using ceramic water filters.
