As providers of nanomaterials, both industrial ones for real world relevance and bespoke synthesized NMs to allow more controlled experiments suitable for parameterization of the models, WP3 leaders Iseult Lynch and Marianne Matzke have an excellent overview of the project activities. As we approach the mid-point of the NanoFASE project, it seems appropriate to reflect on which materials are being utilized in the different aspects of the project, and where the major synthesis challenges have arisen.
Silver nanoparticles are widely used in a range of products, and are thus expected to be one of the largest NM fractions in the environment, however, not as Ag0 NMs given their large propensity for transformation, including dissolution and reprecipitation, and sulfidation leading to insoluble Ag2S NMs. Partner AppNano have been busy scaling up synthesis of spherical 20nm Ag2S NMs to the gram-scale as a first proxy for transformed Ag0 NMs, with the caveat that the Ag2S particles resulting from waste water treatment plants (for example) are often hollow in the centre (as a result of dissolution) and typically horn shaped due to re-precipitation of Ag2S in a non-symmetric manner. All partners in WP9 (bioaccumulation across a wide variety of organisms and plants) are utilizing these NMs.
Another significant challenge faced in the project has been the development of Titanium dioxide NMs that could be experimentally distinguished from the background TiO2. Since TiO2 is one of the most ubiquitous oxides in the environment, UoB proposed the development of core-shell particles of a rare earth element core (for tracking and distinction from background particles) surrounded by a TiO2 (or CeO2) shell. Initial proposals for the rare earth element included holmium or hafnium (with europium also attempted by a commercial producer of particles), but in all cases the particles were not successful as core-shell particles. The justification for the core-shell approach rather than just doping was to keep the shell identical to the undoped particles thereby allowing for direct comparison. After significant trial-and-error, an optimized protocol was developed to produce NaHoF 4@TiO2@TiO2 NMs – a double shell approach was utilized as the initial TiO2 shell was just a couple of nm thick, which was considered too thin to ensure environmental behavior similar to pure TiO2 NMs. These NMs will be utilized in experiments in WP5 to track NMs ageing and transformation during waste water treatment.
A range of other NMs have been produced by partners Promethean particles for the consortium, including bare and PVP capped CeO2, Fe2O3 and Fe3O4, TiO2, and CuO NMs, which are being utilized in the experiments on fate and behavior of NMs in aquatic and soil compartments (WPs 7 and 8). The role of capping agent as stabilizer, and its fate during particle ageing in the different environmental compartments (including during waste water treatment) is a key topic of investigation within NanoFASE. Additionally, gold NMs of 20 and 80 nm have been utilized in a number of WPs (WP5, WP8 and WP9) as an insoluble, and easily detectable (e.g. by ICP-MS) material, and is being used, for example, to confirm that no uptake of NMs by isopods occurs (WP9, partner UniLj with UoB). Dyspium-doped copper hydroxide nanorods were successfully synthesized by UoB for use in the WP mesocosms (see figure opposite), as these are widely used as a nano-pesticide material, and thus understanding their uptake by soil organisms and plants under realistic exposures (mesocosms) is a critical aspect for NanoFASE case study development.
Figure 1: Left: TEM imaging showing a Dy-doped CuOH nanowire; Right: EDX imaging of the CuOH wires indicating the overlap of the Dy signal and the Cu signal, confirming that the Dy is distributed throughout the CuOH wires.