Abstract.The acute ecotoxicity of different diameters of silica and polyethyleneimine polystyrene (PS-PEI) nanoparticles (NPs) was assessed on a test battery of aquatic organisms representing different trophic levels. Daphnia magna, Thamnocephalus platyurus, Pseudokirchneriella subcapitata and Vibrio fischeri, were employed in a series of standard acute ecotoxicity tests and work was complemented with two cytotoxicological end points on a rainbow trout gonadal cell line (RTG-2). Physico-chemical characterization of the NPs was performed in the different test media employed, using dynamic light scattering (DLS) and zeta potentiometry. In contrast to silica NPs exposure, for which no effect was observed for concentrations up to 1000 µg ml -1 for all in vivo aquatic organisms tested, significant toxicity was detected after exposure to PS-PEI NPs at concentrations from 0.40 µg ml -1 to 416.5 µg ml -1.Differing sensitivities for each NP diameter for the different organisms was observed as: P. subcapitata
> D. magna > T. platyurus > V. fischeri.The effects observed were dependent in some cases on the NP size, a higher effect being observed for the larger NPs. Finally, cytotoxicity studies showed an effect at the highest concentrations for both sets of NPs which was greater in the case of the PS-PEI NPs.However, as agglomeration and sedimentation of the nanoparticles was observed at these concentrations, the cytotoxicity studies were found not to be a reliable ecotoxicity test model.Keywords: Nanopolystyrene; Silica Nanoparticles; Ecotoxicity; Cytotoxicity.3 1 Introduction.The development of materials and products at the nanoscale has become a major investment area on a global level. Nanotechnology is largely based on these materials, generally defined as nanomaterials (NMs), which, for regulatory purposes, have been recently defined by the European Commission (EC) as any natural, incidental, or manufactured particulate material which is in the unbounded, aggregated or agglomerated form and with at least a 50 % of the particles in the number size distribution that has at least one dimension in the size between 1 and 100 nanometers (nm) (EC 2011). NPs fall within this definition, but to be more specific, NPs are defined as particulate materials with three dimensions of the order of 100 nm or less (Loevestam et al. 2010).NMs often exhibit enhanced or different properties when compared with the bulk material due to their extremely small size, consequent high specific surface area, surface energy and other factors such as larger proportions of under co-ordinated bonds and spatially constrained electronic wavefunctions (Lead and Wilkinson 2006). Certain NMs can offer, amongst others, distinct optical, electrical and magnetic properties, rendering them of great potential in a very wide range of fields and applications (Rao and Cheetham 2001). These, and other properties, make NMs very useful in technology and their use is rapidly increasing due to their applications in areas such as textiles, electronics, pharmaceutics, co...