2015 Distinguished Guest Lecture and Symposium: Nanomaterials—environmental remediants or toxicants?
Recent reports released by Research Councils UK and the Royal Society of Chemistry have stressed the importance of nanoscience and technology to the long-term prosperity of the UK. On 24 June 2015, a half-day symposium convened by the ECG considered the environmental challenges and benefits provided by the synthesis and use of nanomaterials. The symposium, attended by about 60 people, also provided the occasion to present the ECG’s DGL medal to Professor Valsami-Jones from the University of Birmingham.
David Spurgeon (Centre for Ecology and Hydrology, Wallingford, UK) examined the benefits and problems associated with the use of standard tests for assessing nanomaterial ecotoxicity. Standard tests are used to ensure inter-laboratory data comparability, and are approved inter alia by the OECD. Many commercially used nanomaterials end up in sewage sludge (see the photo below), which is spread onto soils. Thus, nanomaterial contamination is very often a soil contamination issue. Standard tests exist for examining the ecotoxicity of nanomaterials in soils. These tests rely upon the use of a limited number of test species (normally adults); a standard medium; and standard test conditions to examine a particular toxicological end-point (normally death) over a defined, short-term exposure period. The speaker showed that such standardised tests very often fail to identify critical factors that may affect toxicity in different environments and over different time scales. For example, soil pH and organic matter composition attenuate nanoparticle toxicity in soils. One study found that silver nanoparticles
Waste water treatment plant. Nanoparticles often end up in sewage sludge that is spread onto soils. Credit: SKY2015/Shutterstock
become increasingly toxic to the earthworm Eisenia fetida as they changed with time (aging). Because standard tests rapidly generate data and provide reliable information for initial risk assessments, they are the tools of choice for regulatory risk assessment. However, such tests restrict the diversity of data available for risk modelling and can fail to address key endpoints and effects. Non-standard tests can be important for detecting specific outcomes and validating hypotheses.
David Spurgeon (Centre for Ecology and Hydrology, Wallingford, UK) examined the benefits and problems associated with the use of standard tests for assessing nanomaterial ecotoxicity. Standard tests are used to ensure inter-laboratory data comparability, and are approved inter alia by the OECD. Many commercially used nanomaterials end up in sewage sludge (see the photo below), which is spread onto soils. Thus, nanomaterial contamination is very often a soil contamination issue. Standard tests exist for examining the ecotoxicity of nanomaterials in soils. These tests rely upon the use of a limited number of test species (normally adults); a standard medium; and standard test conditions to examine a particular toxicological end-point (normally death) over a defined, short-term exposure period. The speaker showed that such standardised tests very often fail to identify critical factors that may affect toxicity in different environments and over different time scales. For example, soil pH and organic matter composition attenuate nanoparticle toxicity in soils. One study found that silver nanoparticles
Waste water treatment plant. Nanoparticles often end up in sewage sludge that is spread onto soils. Credit: SKY2015/Shutterstock
become increasingly toxic to the earthworm Eisenia fetida as they changed with time (aging). Because standard tests rapidly generate data and provide reliable information for initial risk assessments, they are the tools of choice for regulatory risk assessment. However, such tests restrict the diversity of data available for risk modelling and can fail to address key endpoints and effects. Non-standard tests can be important for detecting specific outcomes and validating hypotheses.
Iseult Lynch (University of Birmingham) was a last minute replacement for Dr Tom Scott, who was unable to attend. She focussed on nanoparticle interfaces, which are key to the environmental fate and behaviour of
nanoparticles. Nanoparticle interfaces have high surface energies. Molecular adsorption lowers surface energy and thereby passivates the interface. Humans can take up nanoparticles by ingestion, inhalation or absorption through the skin. After entering the body, nanoparticles are surrounded by biological fluids. Binding of biomolecules to the nanoparticle surface can confer biological recognition. The resultant biomolecule/nanoparticle complexes can act as “Trojan Horses” for the transport of contaminants. In the environment, nanoparticles are also able to bind organic matter. Binding of biological or organic molecules may stabilise nanoparticles against aggregation or may promote it. The range of effects associated with molecular binding must be assessed to properly understand the environmental fate and toxicological properties of nanoparticles.
Debora Rodrigues (University of Houston) spoke about her group’s work on using carbon-based single-walled and multi-walled nanotubes and graphene and graphene oxide for the sustainable remediation of wastewater (see photo on the previous page). The main goal is to produce cheap nanomaterials that are readily dispersed in water and that have reduced human cytotoxicity. In toxicity measurements against human cell lines, graphene oxide functionalised with poly(N-vinyl carbazole) (PVK) showed no toxicity, and carbon nanotubes functionalised with PVK showed reduced toxicity compared to the nanomaterials without PVK.
Gold nanoparticles imaged with a transmission electron microscope. Credit: Georgy Shafeev/Shutterstock
Functionalisation of the carbon nanomaterials with PVK permits the removal of metal ions such as lead under appropriate pH conditions (pH 5). The same materials were able to inhibit microbial activity. The speaker also showed that the functionalised nanomaterials can be attached to membrane filters. Filters containing PVK-functionalised carbon nanotubes were effective in inactivating/killing bacterial cells through cell disruption, resulting in better quality water (though possibly not of drinking water quality). Unfortunately it remains difficult to regenerate the nanomaterials. These materials show promise but issues of cost and large-scale production need to be addressed.
The meeting closed with the 2015 Distinguished Guest Lecture by Eugenia (Éva) Valsami-Jones (University of Birmingham). She began by outlining the perceived economic potential of nanomaterials, which arises from their novel properties, including quantum confinement, surface plasmon resonance and superparamagnetism. However, their toxicity is difficult to predict because of major challenges of detection and characterisation. For example, nanoparticle size, shape and structure is affected by aging. Moreover, characterisation needs to be undertaken in relevant media. A particular challenge lies in assessing the uptake of metals from metal-based nanomaterials (see micrograph) at realistic exposure levels.
To address these challenges, Éva Valsami-Jones outlined the use of nanomaterials that have been either isotopically or fluorescently labelled. In one study, labelling permitted an assessment of zinc assimilation via dietary exposure. One conclusion from this work was the absence of any “off-the-scale” toxicity associated with nanoparticles. Nanoparticles may be internalised through a range of biological mechanisms, including phagocytosis for large particles, endocytosis for smaller particles, and macropinocytosis from fluids. Surface-coated nanoparticles can also be taken up by biological cells. Computer modelling shows that surface energy falls with increasing numbers of atoms, but this reduction is not smooth. Thus, apparent inconsistencies in reactivity data with size may arise from genuine changes in surface properties with particle size.
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