The Challenge of Studying TiO₂ Nanoparticle Bioaccumulation at Environmental Concentrations: Crucial Use of a Stable Isotope Tracer

The ecotoxicity of nanoparticles (NPs) is a growing area of research with many challenges ahead. To be relevant, laboratory experiments must be performed with well-controlled and environmentally realistic (i.e., low) exposure doses. Moreover, when focusing on the intensively manufactured titanium dioxide (TiO₂) NPs, sample preparations and chemical analysis are critical steps to meaningfully assay NP’s bioaccumulation. To deal with these imperatives, we synthesized for the first time TiO₂ NPs labeled with the stable isotope ⁴⁷Ti. Thanks to the ⁴⁷Ti labeling, we could detect the bioaccumulation of NPs in zebra mussels (D<i>reissena polymorpha</i>) exposed for 1 h at environmental concentrations via water (7–120 μg/L of ⁴⁷TiO₂ NPs) and via their food (4–830 μg/L of ⁴⁷TiO₂ NPs mixed with 1 × 10⁶ cells/mL of cyanobacteria) despite the high natural Ti background, which varied in individual mussels. The assimilation efficiency (AE) of TiO₂ NPs by mussels from their diet was very low (AE = 3.0 ± 2.7%) suggesting that NPs are mainly captured in mussel gut, with little penetration in their internal organs. Thus, our methodology is particularly relevant in predicting NP’s bioaccumulation and investigating the factors influencing their toxicokinetics in conditions mimicking real environments