Is one of the best materials known to mankind toxic?

Graphene is a two-dimensional carbon layer and in many aspects the best material known to mankind. It is highly robust, has very good conductive properties, and due to its stucture an immense surface to mass ratio.

Graphene exists as nanoplatelets. Like all nanoparticles it is subject to environmental and toxicological investigations. Nanoparticles are suspected to enter cells and cause damage, harm cell membranes, and act as transport vehicles for contaminats due to their sorptive capacity for unpolar molecules.

http://polpix.sueddeutsche.com/bild/1.1008406.1355787097/640x360/physiknobelpreis.jpg

It is this possible sorption of putative pollutants that makes graphene also very interesting in terms of soil and sediment remediation. The vast surface of only a little amount of graphene platelets promises a high efficiency in capturing toxic organic molecules and thus making them unavailable for uptake by organisms.

However, prior to application in remediation measures two crucial questions need to be answered:

1. How strong is the sorption? Will the particles be a reliable sink?

2. Are graphene platelets toxic, and if yes, at which concentration? Could they be still successfully applied in concentrations where they do no harm?

We investigated these questions within two bachelor's theses in collaboration with colleagues from the Polytechnical University of Bucharest (UPB), Romania, and the RWTH Aachen University Institute of Plant Physiology. Objects of study were not only pure graphene platelets but also differently oxidised types. Since we knew from previous studies that nanoparticles cause mainly oxidative stress, we investigated induction of reactive oxygen species (ROS) in different test systems.

Graphene platelet suspensions were subjected to transgenic yeasts that can reveal different modes of action for ROS induction. They were further used to expose algae and also zebrafish embryos. The assays made use of dyes that react with ROS after metabolic activation to fluorescent molecules. Fluorescence is hence proportional to the strength of ROS induction and can be determined by means of microscopy or photometric measurement. For zebrafish investigations we used a new protocol from colleagues in Spain on semi-quantitative evaluation of ROS induction.

Our study showed that graphene nanoplatelets do not induce ROS induction in yeasts and zebrafish up to a nominal concentration of 2 mg/L. However, higher concentrations were not yet tested, since this concentration would be likely the one for environmental application according to the sorption studies conducted by the UPB. Algae gave weak ROS induction, but with high variability and hence not significant.

While our results give a first good indication that graphene platelets are suitable for soil and sediment remediation, these were only initial investigations. Further studies will test higher concentrations, as graphene platelets are likely to accumulate. Thus, the relevant environmental concentrations could clearly exceed the applied. Also, more sensitive assays using yeasts and zebrafish will be carried out. But most importantly, we will measure the exposure concentration, since chances exist that the real concentrations of platelets were lower than the one of the initial suspension.