Stability of 51Cr-radiolabeled SPIOs with a polymer shell. (A) Size-exclusion chromatography (SEC) results of the polymer-coated 51Cr-SPIOs. The radioactivity measured in fractions corresponds to the elution of the NPs measured by UV absorption at 280 nm. (B) Release of 51Cr3+ from radiolabeled nanoparticles when incubated with 0.1 M HCl at room temperature. A centrifuge was used for the separation of released chromium ions (filtrate) from nanoparticles (filter) at distinct times during the degradation of the particles. (C) Release of Fe3+ and 51Cr3+ from degraded iron oxide cores was measured in the filtrate via atomic absorption spectrometry and 51Cr-γ-counting. The results indicate the constant release of iron and chromium over time during the degradation of the particles.