Dissolution Kinetics and Solubility of ZnO Nanoparticles Followed by AGNES Calin A. David, † Josep Galceran,* ,† Carlos Rey-Castro, † Jaume Puy, † Encarnació Companys, † Jose ́ Salvador, † Josep Monne ́ , † Rachel Wallace, ‡ and Alex Vakourov ‡ † Departament de Química, Universitat de Lleida, Rovira Roure 191, 25198 Lleida, Catalonia, Spain ‡ Centre for Molecular Nanoscience (CMNS), School of Chemistry, University of Leeds, Leeds LS2 9JT, United Kingdom * S Supporting Information ABSTRACT: 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 Zn 2+ 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 Zn 2+ 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/m 2 . AGNES also allows the study of the kinetics of Zn 2+ 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 Zn 2+ 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. 1. INTRODUCTION Increased use of engineered nanoparticles has given rise to heightening concern for their biological activity. Nanoparticles (NPs) in aqueous dispersion have dimensions in between those of micrometer-sized particles and molecular-sized dissolved compounds. 1 Their thermodynamics, transport, mechanical, and chemical properties are dependent on their small dimension and their large surface area that generates a considerable surface activity. 2 As a result, the nanoparticles become highly active, even if they are made of inert material. In addition, the biological impact of nanoparticles depends on their size and structure in the solution environment as well as on their functionality, since a small cluster of metal atoms can have a different chemical potential than the bulk solid and be more easily dissolved. 3 In fact, some nanoparticles can dissolve and release species which may itself be toxic. This, coupled with the putative ability of nanoparticles to cross cell membranes and enter cells (because of their small size 3 ), will imply the presence of toxic species in the cell interior. Taking into account the full complexity of a nanoparticle’s toxicity, an initial step in a study of their biological activity will involve an investigation into the factors controlling the dissolution of specific nanoparticles. A known biologically active nanoparticle species which dissolves in aqueous media is ZnO, and it remains equivocal whether the toxicity of ZnO stems from the particle or from the dissolved Zn 2+ . Deriving from the biological concern, an important consideration in nanotechnology is the impact of manufactured nanoparticles on the environment, which has further promoted the study of their characteristics and behavior. 3-6 Dissolution of the NP in water has been pointed out as an important physicochemical property to be taken into account 7,8 to assess the environmental impact. Similar to their behavior in biological media, ZnO NPs can dissolve in aqueous environ- ments significantly, so that some studies attribute the toxicity mainly to the dissolved Zn 2+ , 9-11 others to the nanoscale properties of ZnO, 12,13 and still others to a combination of both factors (with a different weight). 14-22 The dissolution process of nanoparticulate ZnO has been tackled in several studies. 9-11,15,17,18,20-26 A particularly important thermodynamic question is the equilibrium concen- tration of Zn 2+ , while the main kinetic question addresses the Received: February 20, 2012 Revised: April 14, 2012 Published: April 26, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 11758 dx.doi.org/10.1021/jp301671b | J. Phys. Chem. C 2012, 116, 11758-11767