Dissolution Kinetics and Solubility of ZnO Nanoparticles Followed by AGNES

There is a current debate on whether the toxicity of engineered ZnO nanoparticles (NPs) can be traced back to their nanoscale properties or rather to the simple fact of their relatively high solubility and consequent release of Zn2+ ions. In this work, the emerging electroanalytical technique AGNES (Absence of Gradients and Nernstian Equilibrium Stripping), which is specially designed to determine free metal ion concentration, is shown to be able to measure the Zn2+ concentration resulting from dissolution of ZnO nanoparticles dispersed in aqueous salt solutions. Three NP samples from different sources (having average primary particle diameters of 6, 20, and 71 nm) were tested and compared with bulk ZnO material. The enhanced solubility of the nanoparticles with decreasing primary radius allows for an estimation of the surface energy of 0.32 J/m2. AGNES also allows the study of the kinetics of Zn2+ release as a response to a change in the solution parameters (e.g., pH, ZnO concentration). A physicochemical model has been developed to account for the observed kinetic behavior. With this model, only one kinetic parameter is required to describe the time dependence of the free Zn2+ concentration in solution. Good agreement with this prediction is obtained when, starting from an equilibrated NP dispersion, the pH of the medium is lowered. Also, the independence of this parameter from pH, as expected from the model, is obtained at least in the pH range 7–9. When dissolution is studied by dispersing ZnO nanoparticles in the medium, the kinetic parameter initially decreases with time. This decrease can be interpreted as resulting from the increase of the radius of the clusters due to the agglomeration/aggregation phenomena (independently confirmed). For the larger assayed NPs (i.e., 20 and 71 nm), a sufficiently large pH increase leads to a metastable solubility state, suggesting formation of a hydroxide interfacial layer.