Environmental behavior and biological effects of rare-earth upconverting nanoparticles on freshwater green algae

Lanthanide-doped upconverting nanoparticles (UCNPs) have demonstrated promising applications in biomedicine and environmental sensing; however, their behavior and biological effects in freshwater environments remain poorly understood. This study involved the synthesis of hydrophilic, monodisperse NaYF4:18%Yb3+,2%Er3+ UCNPs via thermal co-precipitation and their evaluation in the freshwater green alga Desmodesmus communis under environmentally relevant conditions. The UCNPs were made water-dispersible by removing oleate ligands, and their effects were investigated in two different exposure media: a nutrient-rich algal growth medium (MWC) and natural Neris River water. Optical analysis revealed progressive destabilization of the UCNPs in both media over a 96 hour period; however, dispersions in river water exhibited greater homogeneity, likely due to stabilization by natural organic matter. Biological responses were more strongly influenced by the composition of the medium than by the concentration of the UCNPs. The content of pigments, such as chlorophyll a and carotenoids, was not notably affected by UCNP exposure (p > 0.05), whereas oxidative and antioxidant responses displayed clear patterns dependent on the medium. Confocal imaging revealed that the UCNPs were primarily associated with the algal cell surface, with no evidence of intracellular internalization. Multivariate analysis further differentiated response patterns, indicating that oxidative and antioxidant variables were more prominent in MWC medium, while growth-related variables were more significant in river water. Overall, these findings demonstrate that freshwater chemistry profoundly affects UCNP stability and bioavailability, underscoring that assessments based solely on artificial media may underestimate the effects of nanomaterials in natural aquatic environments.