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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - MolNANOtox (Nanomaterial surface interactions at the molecular level and their impact on ecotoxicity)

Teaser

Summary

Nanotechnology has revolutionised many industries. Engineered nanomaterials (ENMs), the small, nanoscale materials at the heart of this technology, have novel and unique properties that drive the nanotechnology industry. However, increasing ENM abundance and availability has led to concerns regarding the risks they may pose to the environment. Significant advances have been made towards understanding ENM core chemistry, behaviour and transport, but a knowledge gap exists regarding their surface chemistry and its evolution in the environment, and how this may impact interactions with living organisms. In particular, the dynamic, environmentally-acquired surface coating called the â€œeco-coronaâ€ is a new concept that has seen little exploration until recently. The coating formed may be composed of entities right from small ions or molecules to large macromolecular material such as natural organic matter, and may have different attachment modes and strengths. Since ENMs have a high specific surface area and surfaces play a major role in ENM interactions and reactivity, understanding their surface chemistry is important in a comprehensive assessment of ENM fate and impact.
The overall objective of the project was to characterise the fundamental interactions occurring at the surface of ENM and its impact on environmental ENM chemistry and bio-nano interactions with a range of analytical and imaging techniques. The project focused on the exploration of Raman spectroscopy and surface-enhanced Raman spectroscopy (SERS) as a novel application to probe the surface chemistry of ENMs in the context of eco-corona formation, as well as the application of a multimodal approach to imaging the bio-nano interactions.

Work performed

The first Task Set examined the utility of Raman spectroscopy and SERS to probe the surface chemistry of the ENMs in ecotoxicogical test conditions. Silver-based ENMs were selected for use in this project because they are a major ENM available in the consumer market and are characterised by high reactivity and toxicity (especially of its dissolved, ionic form). Most importantly, silver ENMs have a Raman signal enhancing property that underpin the SERS technique for examining ENM surface interactions. Here, we were interested in exploiting the Raman signal enhancement arising from the surface of silver nanoparticles (Ag-NPs) in SERS to gain an understanding of the environmentally relevant ligands that may form an eco-corona.
Raman spectroscopy and SERS were found to be useful for certain classes of ligands. However, it was more powerful combined with other techniques in a protocol. A method based on three techniques: 1) nanoparticle tracking analysis, 2) UV-Visible spectroscopy and 3) Raman spectroscopy (SERS) was developed and used to probe the eco-corona and its impact on the ENM chemistry. It was found that small, strongly binding ligands (chloride, cytosine) played a role in dissolving the Ag-NPs, while larger macromolecules (e.g. natural organic matter, algal exudates) stabilised Ag-NPs during incubation in freshwater media (Figure 1). Moreover, Raman spectroscopy and SERS clearly showed the attachment of chloride from the media on to Ag-NPs, and that this signal was reduced when Ag-NPs were incubated with macromolecules. The results were presented at several major international conferences in vibrational spectroscopy, biogeochemistry and environmental nanoscience. A manuscript is also in preparation for publication.
The second Task Set focused on understanding the effect of the surface chemistry on ENM interactions with a micro-green algae, Raphidocelis subcapitata. One of the challenges in capturing the delicate bio-nano interactions is that there is no single imaging technique with sufficiently high spatial resolution that does not require intrusive sample preparation steps. Therefore, a multimodal approach utilising three complementary methods that maximised sample fidelity (i.e. no sample preparation), high spatial resolution and nanoscale chemistry was developed using dark-field light microscopy (DF-LM), secondary electron microscopy (SEM) and nanoscale secondary ion mass spectrometry (NanoSIMS). This was able to identify that the interaction of Ag-NPs with algal cells occurred primarily at the cell surface, and that it was preserved in the samples that had undergone preparation for electron microscopy and mass spectrometry. First set of results were presented at a major environmental chemistry and toxicology conference (SETAC), and have been published as an Open Access publication, while the second set of results are currently being analysed for further dissemination (Figure 2: full article can be viewed at https://pubs.acs.org/doi/abs/10.1021/acsnano.7b04556, and further permissions are required for reuse)

Final results

Our understanding of the environmentally acquired coatings on ENMs is still in its infant stages. The data in the first Task Set enables us to recognise that Raman spectroscopy and SERS can be utilised to probe ENM surface interactions that may affect their stability in environmental systems. It also confirmed the role of chloride in Ag-NP dissolution and the protective role humic acids can play. Therefore, while there are limitations, this is one of the few techniques that are able to gain chemical data selectively from the surface and is likely to be important for investigating Ag-NPs and other ENMs in an ecotoxicological context.
The impact of these coatings, or the â€œeco-coronaâ€, on the ENM interactions with biota is also an emerging research area. The advantage of Raman spectroscopy where one can analyse ENMs in water makes it a highly promising method to explore in environmental nano(eco)toxicology. Combined with the multimodal imaging approach demonstrated in the second Task Set, we will be able to examine the potential effect of the surface chemistry on environmental bio-nano interactions. These capabilities will contribute directly to understanding the overall impact of ENMs in the environment and, in the future, towards engineering their properties for safe and beneficial use for the environment and society. During the project collaborations with larger EU H2020 projects on environmental fate (NanoFASE) and advanced characterisation (ACEnano) have let the findings of this fellowship flow directly into initiatives that will develop robust and more affordable analytical techniques and equipment to inform us on the fate of ENMs in the environment to help deliver safe industrial progress and new products benefitting society economically and functionally.