Publikationsrepositorium - Helmholtz-Zentrum Dresden-Rossendorf

Colloids and nanoparticulate matter play an important role in the environment since they can act as carriers for (toxic) compounds and thereby enhance migration of substances that might normally be immobile under the given environmental conditions. The carrier properties of colloidal clay particles, humic substances and silica actinide colloids may play an important role in nuclear waste repositories. However, monitoring of these colloids in complex systems such as geological formations or groundwater is nearly impossible using conventional methods, especially at environmentally relevant concentrations and the high background load of other colloids or dissolved species of the same element. This obstacle can be overcome by the use of radiolabelling, which may be of crucial value in enabling such research.
We have developed various methods of introducing radiotracers into natural organic colloids such as humic or fulvic acids [1] and some of the most common technical nanoparticles, such as Ag⁰ [2], carbon and TiO₂ nanoparticles [3]. Current studies are dealing with radiolabelling of CeO₂ and quantum dots.
Five different approaches can be pursued in the radiolabelling of colloids or nanoparticles:
(1) Radiosynthesis – the synthesis of a compound using radioactive material
(2) Radiochemistry – the binding of a radioactive tracer to an existing compound
(3) In-diffusion – the in-diffusion of radioisotopes into existing particles
(4) Direct Activation – the activation of existing particles by proton irradiation
(5) Recoil labelling – the implantation of radionuclides into an existing particles using the recoil of a nuclear reaction

Radiosynthesis can be used to produce custom-made radiolabelled nanoparticles provided a suitable radiotracer is available. If the labelling is isotopic no difference in properties compared to non-radioactive particles are expected. We used this method to produce radiolabelled [^105/110mAg]Ag nanoparticles.
Binding a radiotracer to a compound is a way of radiolabelling existing commercial or natural materials by a suitable radiochemical protocol. Carbon nanotubes and humic acids were successfully labelled with radioactive Iodine following the one-pot Iodogen method to yield [^124/125/131I]CNTs and [¹²⁵I]humic acid. No significant change in properties was detected compared to the original compounds.
Commercial TiO₂ and Ag⁰ nanoparticles were labelled by the in-diffusion of isotopic radionuclides into surface defects and lattice structure at elevated temperatures. The resulting [^110mAg]Ag⁰ and [^44/45Ti]TiO₂ nanoparticles showed no change in properties and the radiolabel proved to be stable under various conditions.
If a cyclotron is available, nanoparticles can be radiolabelled by activation via proton irradiation. The proton irradiation causes a nuclear reaction in the nanopowder producing the radiolabel inside the particles. Commercial TiO₂ particles were labelled with ⁴⁸V via a ⁴⁸Ti(p,n)⁴⁸V reaction.
If no suitable radiotracers/nuclear reactions are available for the above described methods, nanoparticles can be labelled utilising the recoil of a nuclear reaction to implant a radiotracer. Typically a mixture of a lithium compound and the to-be-labelled particles is irradiated with protons. The nuclear reaction ⁷Li(p,n)⁷Be produces ⁷Be which is implanted in the nanoparticles due to the recoil of the nuclear reaction. [⁷Be]MWCNT were produced successfully.
Table 1 shows the results for radiolabelling of particles. The described methods are adaptable for a wide range of other nanoparticles or colloids. 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. In the research area of radioactive waste repositories the radiolabelling of humic substances, clay colloids and actinide silica colloids may be of particular interest.

Table 1: Comparison of the radiolabelling procedures and the resulting radiolabelled NP analysis.
Radiolabeling procedure Resulting NP Half-life of the radionuclide Activity concentration [MBq/mg] Detection limit [ng/L]
Radiosynthesis [^110mAg]Ag⁰ 250 d 1.5 33
[¹⁰⁵Ag]Ag⁰ 41.3 d 0.65 77

Radiochemistry [¹²⁴I]CNT 4.2 d 8.0 6
[¹²⁵I]CNT 59.4 d 19.9 2
[¹³¹I]CNT 8.0 d 3.7 14

In-Diffusion [^110mAg]Ag 250 d 1 50
[⁴⁴Ti]TiO₂ 60.4 a 0.01 5000
[⁴⁵Ti]TiO₂ 3.08 h 135 0.5

Direct Activation [⁴⁸V]TiO₂ 15.97 d 3.7 14
[⁷Be]MWCNT 53.29 d 0.041 1000

Recoil Labelling [⁷Be]TiO₂ 0.3 170
[⁷Be]SiO₂ [4] 53.29 d 1.4 36
[⁷Be]MWCNT 0.055 1000
References:
[1] Franke, K., Patt, J.T., Kupsch, H., Warwick, P.: Radioiodination of Humic Substances via Azocoupling with 3-[¹²⁵I]Iodoaniline. Environ Sci Technol 42(11) (2008) 4083 - 4087.
[2] Hildebrand, H., Franke, K.: A new radiolabeling method for commercial Ag⁰ nanopowder with ^110mAg for sensitive nanoparticle detection in complex media. J Nanopart Res 14 (2012) 1142.
[3] Hildebrand, H., Schymura, S., Holzwarth, U., Gibson, N., Dalmiglio, M., Franke, K.: Strategies for radiolabeling of commercial TiO₂ nanopowder as a tool for sensitive nanoparticle detection. J Nanopart Res, submitted.
[4] Holzwarth, U., Bellido, E., Dalmiglio, M., Kozempel, J., Cotogno, G., Gibson, N.: ⁷Be-recoil radiolabelling of industrially manufactured silica nanoparticles. J Nanopart Res 16 (2014) 2574.