1 Department of Physiology, McGill University, Montreal, Quebec H3G 1Y6, Canada.
2 Department of Chemistry & Biochemistry and Centre for NanoScience Research, Concordia University, Montreal, Quebec H4B 1R6, Canada.
3 Department of Biology, McGill University, Montreal, Quebec H3A 1B1, Canada.
4 McConnell Brain Imaging Centre, Montreal Neurological Institute and Hospital, McGill University, Montreal, Quebec H3A 2B4, Canada.
5 Department of Neurology and Neurosurgery, McGill University, Montreal, Quebec H3A 0G4, Canada.
6 Department of Biomedical Engineering, McGill University, Montreal, Quebec H3A 0G4, Canada.

Because of their unique physicochemical properties, lanthanide-doped upconverting nanoparticles (Ln-UCNPs) have exceptional potential for biological applications. However, the use in biological systems is hampered by the limited understanding of their bionano interactions. Our multidisciplinary study has generated these insights through in-depth and quantitative analyses. The Ln-UCNPs examined here are spherical, monodisperse, and stable in aqueous environments. We show that Ln-UCNPs were associated with HeLa (cervical cancer) and LLC-PK1 (renal proximal tubule) cells and were nontoxic over a wide concentration range. Multiple biomarkers were assessed to monitor the cellular homeostasis in Ln-UCNP-treated cells. To this end, we evaluated the nuclear lamina, nucleoli, and nuclear transport factors. Single-cell analyses quantified the impact on Nrf2 and NF-?B, two transcription factors that control stress and immune responses. Moreover, we measured Ln-UCNP-induced changes in the abundance of molecular chaperones. Collectively, in vitro studies confirmed that Ln-UCNPs are nontoxic and trigger minor cellular stress responses. This lack of toxicity was verified in vivo, using the model organism Caenorhabditis elegans. The compatibility with biological systems prompted us to assess Ln-UCNPs as potential contrast agents for magnetic resonance imaging. We demonstrated that the Ln-UCNPs examined here were especially suitable as T₂ contrast agents; they clearly outperformed the clinically used Gadovist. Taken together, our interdisciplinary work provides robust evidence for the nontoxicity of Ln-UCNPs. This sets the stage for the translation of Ln-UCNP for use in complex biological systems.