Aging of silver nanocolloids in sunlight: particle size has a major influence

Abstract

Environmental contextTransformation of silver nanoparticles in the environment is an important issue because the form they take directly influences what effect they have. We show that the size of the primary particles of silver nanosuspensions is a major factor determining their evolution under sunlight irradiation (dissolution, formation of nanoseeds and nanoprisms, agglomeration). The persistence of nano-sized silver particles after exposure to sunlight irradiation implies that their ecotoxicological impact will likely last well beyond their introduction in the environment. AbstractThe environmental fate of silver nanoparticles (Ag NPs) is a serious cause for concern with regard to their ecotoxicity. In this study, an aging scenario intended to evaluate the effect of sunlight on three Ag NP suspensions of various particle size was assessed. Suspensions of citrate-stabilised Ag NPs of 20, 60, and 100 nm diameter were aged for a week in a climatic chamber under controlled temperature (40 °C) and irradiation (1.44 W m−2 at 420 nm). The suspensions were analysed by asymmetric flow field flow fractionation – inductively coupled plasma mass spectrometry (AF4-ICP-MS), dynamic light scattering (DLS) and transmission electron microscopy (TEM). The AF4-ICP-MS monitoring showed that only a small fraction (10 % at most) of the primary 20 nm particles are converted into multi-faceted particles. Larger particles undergo shape modifications correlated with dissolution (60 nm Ag NPs) and aggregation (100 nm Ag NPs) processes. Silicate structures – supposedly originating from the glassware degradation – stabilise the primary particles. The occurrence of smaller Ag seeds, also associated with silicates, was revealed and quantified by AF4-ICP-MS and confirmed by TEM. The physical fractionation of the particles according to their size provided by AF4, together with the quantitative analysis provided by ICP-MS, helped to determine the role of size in the fate of silver nanoparticles under sunlight exposure.