The researchers evaluated the experimental data published in the scientific literature using a network analysis. This analysis reveals which types of nanoparticles (blue) have been studied under which environmental conditions (red). (Visualisations: Thomas Kast)
Nanoparticles behave in an extremely complex manner in the environment. In a large overview study, ETH environmental scientists have proved that currently there is a lack of systematic experimental data that could help in understanding the nanoparticles in a comprehensive way.
The nanotech industry is flourishing. Several thousands of tons of man-made nanoparticles are globally produced every year; sooner or later, a particular part of them will end up in bodies of soil or water. However, even experts have difficulty in explaining what exactly happens to the nanoparticles there. The question is indeed complex, because there are a variety of man-made (engineered) nanoparticles, and also because the particles behave differently in the environment based on the prevailing conditions.
Researchers headed by Martin Scheringer, Senior Scientist at the Department of Chemistry and Applied Biosciences, aimed at bringing about some clarity to this issue. These researchers reviewed 270 scientific studies, including almost 1,000 laboratory experiments described in these studies, in order to search for patterns in the behavior of engineered nanoparticles. The team aimed at making universal predictions about the behavior of the particles.
Particles attach themselves to everything
However, an extremely mixed picture was discovered by the researchers when they looked at the data.
“The situation is more complex than many scientists would previously have predicted,” says Scheringer. “We need to recognise that we can’t draw a uniform picture with the data available to us today.”
Nicole Sani-Kast, a doctoral student in Scheringer’s group and first author of the analysis featured in the journal PNAS, adds:
“Engineered nanoparticles behave very dynamically and are highly reactive. They attach themselves to everything they find: to other nanoparticles in order to form agglomerates, or to other molecules present in the environment.”
The speed at which the particles react and to what exactly they react depends on a wide range of factors such as the acidity of the soil or water, the concentration of the existing salts and minerals, and most of all, the composition of the organic substances present in the soil or dissolved in the water. What makes things a lot more complicated refers to the fact that the engineered nanoparticles frequently have a surface coating. Based on the environmental conditions, the particles lose or retain their coating, thus influencing their reaction behavior.
Sani-Kast evaluated the results available in the literature by using a network analysis for the very first time in this research. This technique, mostly used in social research for measuring networks of social relations, allowed Sani-Kast to demonstrate that the data available on engineered nanoparticles is poorly structured, insufficiently diverse and inconsistent.
More method for machine learning
“If more structured, consistent and sufficiently diverse data were available, it may be possible to discover universal patterns using machine learning methods,” says Scheringer, “but we’re not there yet.” Adequately structured experimental data must first be available.
“In order for the scientific community to carry out such experiments in a systematic and standardised manner, some kind of coordination is necessary,” adds Sani-Kast, but she is aware that coordinating such work is difficult. Generally, scientists are well known for preferring to explore new conditions and methods instead of routinely performing standardized experiments.
Distinguishing man-made and natural nanoparticles
Besides the lack of systematic research, a second tangible problem is also present while researching the behavior of engineered nanoparticles: a number of engineered nanoparticles comprise of chemical compounds that naturally occur in the soil. Till date, there has been difficulty in measuring the engineered particles in the environment as it is hard to distinguish them from particles that occur naturally with the same chemical composition.
However, under the supervision of ETH Professor Detlef Günther, researchers at ETH Zurich’s Department of Chemistry and Applied Biosciences recently established an efficient technique that makes way for this type of a distinction in routine investigations. The research used spICP-TOF mass spectrometry, which is a state-of-the-art and highly sensitive mass spectrometry technique, to determine which chemical elements make up separate nanoparticles in a sample.
The ETH researchers worded together with scientists from the University of Vienna to apply the technique to soil samples with natural cerium-containing particles, into which they added engineered cerium dioxide nanoparticles. They used machine learning methods, ideally suited to this particular issue, to trace variations in the chemical fingerprints of the two particle classes.
“While artificially produced nanoparticles often consist of a single compound, natural nanoparticles usually still contain a number of additional chemical elements,” explains Alexander Gundlach-Graham, a postdoc in Günther’s group.
The new measuring technique is extremely sensitive: the researchers were able to measure engineered particles present in samples with up to one hundred times more natural particles.