Q. Zhang, Y. Hu, C. Masterson, V. Colvin
Keywords: Silver nanoparticles; Antimicrobial; Dissolution.
Summary:Silver nanoparticles are of great commercial interest for their antimicrobial activity. Their antimicrobial 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. The antimicrobial properties and oxidative dissolution of silver nanoparticles depends on the size, shape and surface coatings of the particles. Establishment of the relationship between the structure of silver nanoparticles and their antimicrobial properties will enable us to customize the attributes of silver nanoparticles for their applications to achieve optimal performance and to reduce unintended environmental impact. We developed several new synthesis methodologies to produce a large library of uniform silver nanoparticles with various shape, size and surface coatings. This nanoparticle library enabled us to unambiguously identify the influence of the shape, size, and surface chemistry on their dissolution properties and antimicrobial activities. We established a semi-empirical modal to predict the dissolution and antimicrobial properties of silver nanoparticles based on the measured data. We found that both the dissolution thermodynamics and kinetics highly depend on the particle parameters. 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. These results illustrate how the material properties of silver nanocrystals may influence their dissolution properties, and suggest strategies for optimizing antimicrobial properties as well as designing safer silver nanoparticles.