Q. Zhang, Y. Hu, C. Masterson,V.L. Colvin
Keywords: silver nanoparticles, iron oxide nanoparticles, anti-microbial materials, size-dependent dissolution
Summary:The release of metal ions from a crystallite is central to understanding the biological and environmental impact of nanoparticles. This dissolution process can be described in relatively simple thermodynamic terms, allowing the equilbrium amounts of dissolved material to be found from knowledge of the diameter, shape and surface coating of the material. We illustrate these using two different nanoparticle systems. Silver nanoparticles are of great commercial interest for their antimicrobial activity, a feature that after their disposal can result in unintended impacts on natural microbial systems. Their biological properties arise from oxidative dissolution: silver nanoparticles dissolve to produce silver ions in water and these ions are largely responsible for their toxicity to microbes. Using uniform silver nanoparticle libraries we can create an empirical model for predicting the silver ion concentration from a variety of different types of materials. Shape has the most profound influence on the extent of nanoparticle dissolution: silver triangles, for example, produced the highest equilibrium concentration of silver ions at the fastest rate. Surface coatings also can alter both the kinetics and extent of dissolution in a manner that depends sensitively on the coating surface density as opposed to its chemical composition. Finally, smaller nanoparticles both dissolve faster and yield higher equilibrium concentrations of silver ions. While this trend is expected because of the greater free energy of smaller nanocrystals, quantitatively the size dependence does not follow the anticipated Gibbs-Thompson relationship. This may reflect the complex surface composition of the nanocrystals whose oxidized surface structure is thought to depend on nanocrystal size. The antibacterial toxicity of these nanoparticles, produced by different methods, depend solely on the equilibrium free silver concentration they produce. It is notable that over the wide range of material characteristics represented in these nanoparticle libraries the differences in dissolution rate and extent between the least active and most active particles is only an order of magnitude. These results illustrate how the material properties of silver nanocrystals may influence their dissolution properties, and suggests strategies for optimizing antimicrobial properties as well as designing safer silver nanoparticles. We also use a similar framework to consider the dissolution of iron oxide nanocrystals in acidic media, such as that encountered in lysosomes; this dissolution process can be slowed and even halted through the deposition of protective carbon coatings.