Strategies for the radiolabelling of carbon nanoparticles

The employment of radiotracers is a versatile tool for the detection of nano-particulate materials in complex systems such as environmental samples or organisms. With the increasing usage of nanoparticles in applications outside of research laboratories a careful risk assessment of their release into the environment becomes mandatory. However the monitoring of nanoparticles in such complex natural systems as geological formations or ground water is nearly impossible using conventional methods, especially at environmentally relevant concentrations. This obstacle can be overcome by radiolabelling, which may be of crucial value in enabling such research. We present here different radiolabelling strategies for carbon nanoparticles, in particular carbon nanotubes (CNTs) whose intriguing physical properties predestine them for widespread application, so that future release into the environment is to be expected.
We have developed three different approaches for the radiolabelling of CNTs. The first is the iodination of carbon nanotubes using radioactive iodine, e.g. I-125 or I-131. Using the Iodogen method known from protein labelling strategies it is possible to radiolabel single- and multi-wall CNTs by binding radioactive iodine on the CNT side wall.
The other strategies involve proton irradiation using a cyclotron and cause the incorporation of radioactive beryllium in between the layers of multi-wall CNTs. The first option is to directly activate carbon by high-energy proton irradiation (> 34 MeV), which causes a (p,3d) nuclear reaction creating Be-7, which intercalates between the graphitic layers of the CNT. The second option is to mix the CNTs with a lithium containing compound like LiH and irradiate at a much lower proton energy to create Be-7 via the (p,n) reaction on Li. The recoil of the Be-7 from the nuclear reaction causes the incorporation of the radiotracer into the structure of multi-wall CNTs.
The methods were tested for labeling yield, achievable activity concentration, pH-dependent stability of the labeling and the influence on NP-properties. Data thus obtained enables the selection of a radiolabeling method appropriate for different experimental conditions.