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Looking at silver-based nanoparticles in environmental water samples: repetitive cloud point extraction bridges gaps in electron microscopy for naturally-occurring nanoparticles.

The growing use of silver-based nanoparticles (Ag-b-NPs) in everyday products goes in hand with their release into the environment, resulting in ng L-1 traces in natural water bodies. In order to assess their fate, possible transformations and ecotoxicology - essential information to proper risk assessment - particle size, shape, and chemical composition have to be determined. Transmission electron microscopy coupled with energy dispersive X-ray spectroscopy (TEM-EDX) is a powerful tool for determining these particle characteristics, but it requires high particle concentrations in order to produce statistically reliable results. In this study, we will present the extraction of Ag-b-NPs at environmentally relevant concentrations down to 5 ng L-1 from artificial as well as environmental water samples via cloud point extraction (CPE) on a repetitive basis (rCPE). The combination with an on-grid centrifugation technique ensures an efficient concentration and deposition of the extracted particles onto the TEM grid for subsequent TEM-EDX measurements. Furthermore, electron microscopy investigations were supplemented by single particle inductively coupled plasma mass spectrometry (sp-ICP-MS) measurements. Ag-b-NPs were successfully visualized and characterized at environmentally relevant concentrations of 5 ng L-1 with TEM-EDX and sp-ICP-MS measurements. Their size, shape, and chemical composition were not affected by the sample preparation.