Surface Structure of Silver Nanoparticles as a Model for Understanding the Oxidative Dissolution of Silver Ions

The toxicity of silver nanoparticles (AgNPs) has been related to the release of ionic silver. This process is influenced by a large variety of factors and is poorly understood. The key to understanding Ag+ release by AgNPs is its subvalency. This is a fundamental property of Ag that can be elucidated by analyzing the crystal structures of a specific class of Ag materials as well as MO/DFT (molecular orbital/ density functional theory)-optimized Ag-13(OH)(4) clusters, being precursors of AgNPs. Semimetallic silver at the (111) faces of AgNPs has a subvalency of +1/3, v.u., forming Ag3OH0 surface groups with a maximum site density of 4.7 sites/nm(2). Oxidative dissolution may remove these groups with the simultaneous formation of oxygen radicals that may further interact with the surface via different pathways. Reactive oxygen species (ROS) can create a circular process with the dissolution of Ag3OH0, exposure of new metallic sites at the underlying lattice, and subsequent oxidation to Ag3OH0. This regeneration process is interrupted by the penetration of O-center dot radicals into the lattice, forming highly stable Ag6O octahedra with subvalent silver that protects the AgNP from further oxidation. A thermodynamic model has been developed that quantitatively describes the equilibrium condition between Ag3OH0 and Ag6O0 and explains a large variety of collectively observed phenomena.