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 Ag13(OH)4 clusters, being precursors of AgNPs. Semimetallic silver at the (111) faces of AgNPs has a subvalency of +(1)/3 v.u., forming ≡Ag3OH(0) 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 ≡Ag3OH(0), exposure of new metallic sites at the underlying lattice, and subsequent oxidation to ≡Ag3OH(0). This regeneration process is interrupted by the penetration of O(•) 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 ≡Ag3OH(0) and ≡Ag6O(0) and explains a large variety of collectively observed phenomena.