Dissolution of CeO2 NPs and its role in plant uptake elucidated by smart radiolabeling

Environmental transformations of nanoparticles (NPs) play a major role in determining their likely fate in the environment and the implications for toxicity, mobility and general risk assessment. A main question in this context is the dissolution of nanoparticles. If a nanoparticle dissolves quickly it may be treated as ionic species in terms of risk assessment, while particulate species can show different tendencies of transport, uptake by organisms and consequently toxicity. Slowly dissolving NPs can exhibit a so-called Trojan-horse effect, transporting and releasing ions at places where said ions would not have been transported – at least not at the same concentrations - without traveling on horseback, i.e. in particulate forms.
Using different radiolabeling techniques we have investigated the dissolution of CeO2 NPs. Through activation by proton bombardment using a cyclotron we have radiolabeled CeO2 NPs with radioactive Ce-139 via a (p,2n) nuclear reaction from Ce-140 to Pr-139 followed by the decay of Pr-139 to Ce-139. Here the radiolabel can be assumed to be uniformly distributed in the resulting [Ce-139]CeO2 NPs. In contrast to this we also have produced [Ce-139]CeO2 NP using an in-diffusion technique where ionic radioactive Ce-139 diffuses into the NPs at elevated temperatures. Here the radiolabel is located close to the surface of the NPs. This results in different leaching kinetics of Ce-139 for the two batches of [Ce-139]CeO2 NPs (Fig. 1). The comparison of the different rates allows us to calculate that about 47 % of the 139Ce introduced by in-diffusion is located in the first atomic layer of the CeO2 NPs. We can show that dissolution plays an insignificant role under environmentally relevant conditions with leaching rates well below 1 % of Ce. However, this still reflects significant changes of the surface of the CeO2, as a dissolution of only 1.5 % corresponds to a complete removal of the first atomic layer.
Furthermore, using the differently labeled [Ce-139]CeO2 NPs we can show that the uptake of Ce into plants when exposed to CeO2 NP is mainly an uptake of particulate CeO2 rather than dissolved ionic Ce.