Quantifying the Origin of Released Ag+ Ions from Nanosilver

Nanosilver is most attractive for its bactericidal properties in modern textiles, food packaging, and biomedical applications. Concerns, however, about released Ag+ ions during dispersion of nanosilver in liquids have limited its broad use. Here, nanosilver supported on nanostructured silica is made with closely controlled Ag size both by dry (flame aerosol) and by wet chemistry (impregnation) processes without any surface functionalization that could interfere with its ion release. It is characterized by electron microscopy, atomic absorption spectroscopy, and X-ray diffraction, and its Ag+ ion release in deionized water is monitored electrochemically. The dispersion method of nanosilver in solutions affects its dissolution rate but not the final Ag+ ion concentration. By systematically comparing nanosilver size distributions to their equilibrium Ag+ ion concentrations, it is revealed that the latter correspond precisely to dissolution of one to two surface silver oxide monolayers, depending on particle diameter. When, however, the nanosilver is selectively conditioned by either washing or H2 reduction, the oxide layers are removed, drastically minimizing Ag+ ion leaching and its antibacterial activity against E. coli. That way the bactericidal activity of nanosilver is confined to contact with its surface rather than to rampant ions. This leads to silver nanoparticles with antibacterial properties that are essential for medical tools and hospital applications.