Measuring at relevant concentrations - Radiolabelling as a versatile tool for sensitive nanoparticle detection in complex systems

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, ground water or organisms is nearly impossible or at least extremely laborious in terms of sample preparation 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 under laboratory conditions. Radiolabelling allows to localize, to trace and to detect nanoparticles in complex media without intensive sample pre-treatment for the measurements.

We have developed various methods of introducing radiotracers into some of the most common nanoparticles, such as Ag, carbon, CeO₂, Silica and TiO₂ nanoparticles. The labelling techniques are the synthesis of the nanoparticles using radioactive starting materials, the binding of the radiotracer to the nanoparticles, the activation of the nanoparticles using proton irradiation, the recoil labelling utilizing the recoil of a nuclear reaction to introduce a radiotracer into the nanoparticle, and the in-diffusion of radiotracers into the nanoparticles at elevated temperatures. Using these methods we have produced [^105/110mAg]Ag⁰ [1], [^124/125/131I]CNTs, [¹³⁹Ce]CeO₂, [⁴⁸V]TiO₂ [2], [⁷Be]MWCNT, [⁷Be]SiO₂ [3], [^44/45Ti]TiO₂ [4], etc.. The radiolabelled nanoparticles were intensively tested concerning radiolabel stability and nanoparticle properties (size, zeta potential) were compared with the original (non-labelled) material. It could be shown, that radiolabels are suitable for application in scientific studies under the individual experimental conditions and nanoparticle properties remained largely unalterd by the radiolabelling procedure.

The developed radiolabelling methods are adaptable for a wide range of other nanoparticles. The so-labelled nanoparticles can be detected at minimal concentrations well in the ng/L range even with a background of the same element and without complicated sample preparations necessary. This means that nanoparticles can be localized also in e.g. intact geological samples or during transport studies using appropriate detector arrangement.

Using our methods one can radiolabel commercial nanoparticle samples for sensitive detection in environmentally relevant trace concentrations.

[1] H. Hildebrand and K. Franke: A new radiolabeling method for commercial Ag⁰ nanopowder with ^110mAg for sensitive nanoparticle detection in complex media, J Nanopart Res (2012) 14:1142.
[2] K. Abbas, I. Cydzik, R. Del Torchio, M. Farina, E. Forti, N. Gibson, U. Holzwarth, F. Simonelli, W. Kreyling: Radiolabelling of TiO₂ nanoparticles for radiotracer studies, J Nanopart Res (2010) 12:2435–2443.
[3] U. Holzwarth, E. Bellido, M. Dalmiglio, J. Kozempel, G. Cotogno, N. Gibson: ⁷Be-recoil radiolabelling of industrially manufactured silica nanoparticles, J Nanopart Res (2014) 16:2574.
[4] H. Hildebrand, S. Schymura, U. Holzwarth, N. Gibson, M. Dalmiglio, K. Franke: Strategies for radiolabeling of commercial TiO₂ nanopowder as a tool for sensitive nanoparticle detection in complex matrices, J Nanopart Res (2015) 17:278.