In situ characterisation of physicochemical state and concentration of nanoparticles in soil ecotoxicity studies using environmental scanning electron microscopy

Abstract

Environmental context Characterisation of nanoparticles in terms of number concentration and aggregation state is essential for interpreting data from toxicological tests. These parameters have never been measured in situ in tests carried out in soil matrices. Here, environmental scanning electron microscopy imaging is evaluated for particles in soil, and a method for determining the number concentrations by counting the particles in the images is developed. Abstract The interpretation of nanoparticle toxicity data in soils is currently impeded by the lack of methods capable of characterising particles in situ. To draw relevant and accurate conclusions it would be desirable to characterise particle sizes, agglomeration state and number concentrations. In this article, methodologies for imaging nanoparticles in soils are evaluated for conventional scanning electron microscopy (SEM) and environmental or variable pressure scanning electron microscopy (ESEM). A protocol for dispersing Au particles (~25 to ~450nm) into soil without causing aggregation was developed. The number of particles observed per imaged area of soil correlated linearly with concentration. To determine the number of particles per volume of soil it was also necessary to know how deep in the sample the particles can be visualised. The depth was estimated by both using the Kanaya Okayama model, and spiking the soil with dispersions of known number concentration. These concentrations were determined with a range of methods to ensure their accuracy. Because larger particles can be detected deeper in the matrix, such a calibration should be performed over a range of particle sizes.