Nanotechnology is unquestionably a field of the future. No country that wants to keep pace in competitive research can ignore this branch of research nowadays because the (economic) opportunities it opens up are too tempting. However, there are also sceptical voices asking to be heard over and over again. The kind of thing that is feared is that ultra-small particles could cause damage in the human body. A new study published recently by Wendelin Stark’s research group from the Institute for Chemical Engineering and Bioengineering of ETH Zurich together with researchers from the Swiss Federal Laboratories for Materials Testing and Research (EMPA), St. Gallen, now confirms that these fears cannot be entirely dismissed out of hand. (1)

The researchers studied the extent to which nanoparticles containing heavy metals can cause damage in lung cells. They used the concentration of reactive oxygen species, abbreviated ROS, as a measure. These substances are formed in various oxidation processes in cells, and nowadays it is suspected that they play a central role in a whole series of diseases. Stark and his group have now exposed human lung cells to nanoparticles that are used in industry as catalysts for various chemical reactions. Stark explains that “The purpose of our study was to answer the question of whether these particles also act as catalysts to promote the oxidation of substances in human cells.”

The results show that this really is true. Although the cells show stress responses when brought into contact with solutions containing heavy metals, they nonetheless fend off the harmful ions relatively successfully. However, the situation is different when the metals are present as nanoparticles. In this case the metal penetrates into the cells relatively easily. This leads to a more or less distinct reaction in the cells, depending on which metal is involved. With titanium oxide, for example, the researchers observed only a slight increase in ROS. The situation looks quite different when the cells are exposed to particles of cobalt oxide or manganese oxide. Stark says “I myself was amazed at the intensity of the cells’ reaction to these heavy metals. The ROS concentration was four to eight times the normal value.”

Titanium-silica nanoparticles (top left) are deposited in lung epithelial cells (bottom). The nanoparticles act as catalysts in the cells and are involved in the formation of oxygen radical species. (top right)

However, the researchers exposed the cells not only to pure metal oxide particles. In another series of experiments they bonded small amounts of metal to silica particles. In the case of cobalt and manganese, even a tiny quantity of 0.5 percent causes a massive increase in the concentration of ROS in the cells. Stark thinks that “In this case the silicon dioxide, which is itself harmless, acts as a Trojan horse – like the metal oxide particles. It carries problem-causing substances on its surface and these trigger reactions in the cells.”

By using an elegant series of experiments, Stark was able to prove that the heavy metals introduced into the cells really can also act as catalysts. Stark explains “We studied how cells respond to silicon nanoparticles doped with iron oxide. This enabled us to show that the maximum formation of ROS occurs at a concentration between 1 and 3 percent. Exactly the same relationship is observed in industrial applications: iron oxide on silica acts most efficiently in this concentration range when used as a catalyst for the selective oxidation of certain organic substances. This shows that the metal also achieves a catalytic effect in the cells.”

In Stark’s opinion, these and previous results confirm that caution is advisable when developing products containing nanoparticles. He says “Together with my group I am also researching industrial applications – which is extremely exciting. That’s why I believe it is essential that we also think about possible side-effects at an early stage. Caution is appropriate above all when nanoparticles are used in consumer goods. A careful risk assessment is absolutely essential in that case.”