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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. |
