Aging Influences on the Biokinetics of Functional TiO₂ Nanoparticles with Different Surface Chemistries in Daphnia magna

Nanoparticles functionalized with various surface capping moieties are now widely used in different fields, thus there is a major need to understand the behavior and fate of these nanoparticles in the environment. The present study investigated the biokinetics of fresh titanium dioxide nanoparticles (TiO₂ NPs) or TiO₂ NPs aged under artificial sunlight (16 h light: 8 h dark) for 1, 3, and 5 days, respectively. Two commercial functionalized TiO₂ NPs (with SiO₂ coating or SiO₂ and polydimethylsiloxane coating) were employed in this study. Dynamic light scattering (DLS), Fourier transform infrared spectroscopy (FT-IR), and contact angle (CA) measurements demonstrated that the surface properties had changed due to the degradation during aging. The biokinetic parameters including dissolved uptake and depuration rate constant as well as bioconcentration factors were calculated by a biokinetic model. All the biokinetic parameters were significantly dependent on the aging process. Further data analysis showed that the CA of the TiO₂ NPs affected the uptake rate constant and the fast compartmental efflux, and both CA and hydrodynamic diameter affected the fast compartmental efflux. These results were due to the changes of corresponding indexes during the aging process. Our work highlighted the necessity of monitoring the physicochemical indexes of functionalized NPs during aging in evaluation of their environmental risks.