Dissolution of metal and metal oxide nanoparticles in aqueous media Niksa Odzak * , David Kistler, Renata Behra, Laura Sigg Eawag, Swiss Federal Institute of Aquatic Science & Technology, Department Environmental Toxicology (Utox), Ueberlandstrasse 133, 8600 Dübendorf, Switzerland article info Article history: Received 28 October 2013 Received in revised form 4 April 2014 Accepted 7 April 2014 Available online xxx Keywords: Nanoparticles dissolution Silver Zinc oxide Copper oxide Carbon coated copper DGT Ultrafiltration Dialysis membrane abstract The dissolution of Ag (citrate, gelatin, polyvinylpyrrolidone and chitosan coated), ZnO, CuO and carbon coated Cu nanoparticles (with two nominal sizes each) has been studied in artificial aqueous media, similar in chemistry to environmental waters, for up to 19 days. The dissolved fraction was determined using DGT (Diffusion Gradients in Thin films), dialysis membrane (DM) and ultrafiltration (UF). Relatively small fractions of Ag nanoparticles dissolved, whereas ZnO dissolved nearly completely within few hours. Cu and CuO dissolved as a function of pH. Using DGT, less dissolved Ag was measured compared to UF and DM, likely due to differences in diffusion of organic complexes. Similar dissolved metal concen- trations of ZnO, Cu and CuO nanoparticles were determined using DGT and UF, but lower using DM. The results indicate that there is a need to apply complementary techniques to precisely determine disso- lution of nanoparticles in aqueous media. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Metal and metal oxide nanoparticles are being used in industrial settings and have diverse applications (http://www. nanotechproject.org) that likely will result in release to the envi- ronment, directly, or from nanoparticle-containing products (Nowack et al., 2011). Silver nanoparticles (Ag-NPs) are used in many consumer product applications, mainly due to their antimi- crobial properties. These products range from wound dressings, socks and other textiles, paints, air filters, toothpaste, baby prod- ucts, vacuum cleaners, and washing machines. Zinc oxide nano- particles (ZnO-NPs) are used for extensive application in sunscreens, cosmetics, and bottle coatings because of their ultra- violet blocking ability and the visible transparency of nano- particulate forms. Nano sized copper particles are widely used as a bactericide, for air and liquid filtration, metallic coating, wood preservation. These NPs are very unstable in pure form, and they are used in many products as Cu stabilized (coated) with carbon, or in form of CuO. Carbon coated Cu-NPs serve as catalysts for chemical reactions (catalyst for the synthesis of methanol and glycol), electromagnetic interference (EMI) shielding and heat sinks, replacement for more expensive noble metals for printed electronics, displays and other conductive thin film applications. Once in the aqueous media these nanoparticles will change their physical and/or chemical properties and partly dissolve. The char- acterization and quantification of nanoparticles is a difficult analytical challenge since measurements are highly dependent upon (i) particle size and nature; (ii) sample concentration and solution physicochemistry (Handy et al., 2008; Klaine et al., 2008; Pourbaix, 1974; Schoonen et al., 2006). By definition, truly dissolved metals are those with a size smaller than 1 nm (w1 kDa) (Templeton et al., 2000). From the techniques used for measurements of dissolved metal fractions in metal and metal oxide NPs suspensions, ultrafiltration (Ma et al., 2012; Navarro et al., 2008) and dialysis (Vasyukova et al., 2012; Kittler et al., 2010) are well known and commonly used. Ultrafil- tration for nanoparticles is combined with centrifugation, thus in- cludes application of a centrifugal force to separate dissolved fraction from the particulate fraction. Alternatively, the efficiency of dialysis technique is dependent on diffusion of the dissolved form through a membrane. The DGT (Diffusion Gradients in Thin films) technique has been previously used almost exclusively for In situ measurements of dissolved inorganic and labile organic metal species in aqueous environments (Davison and Zhang, 2012; Odzak et al., 2002; Sigg et al., 2006). In the study on toxicity of Ag nano- particles to freshwater algae performed in this laboratory (Navarro * Corresponding author. E-mail address: odzak@eawag.ch (N. Odzak). Contents lists available at ScienceDirect Environmental Pollution journal homepage: www.elsevier.com/locate/envpol http://dx.doi.org/10.1016/j.envpol.2014.04.010 0269-7491/Ó 2014 Elsevier Ltd. All rights reserved. Environmental Pollution 191 (2014) 132e138