Upconverting nanoparticles (UCNPs) have gained significant attention in bioimaging and sensing due to their ability to convert near-infrared light into visible emissions. This unique property allows for deep tissue imaging, reduced photobleaching, and high sensitivity in detection applications[1],[2],[3]. However, their potential ecological impact remains unclear, particularly in aquatic environments, where their interactions with organisms could influence whole ecosystem dynamics.
Algae plays a key role in nutrient cycling in aquatic ecosystem and serves as a bioindicator of water quality. Their rapid growth, high surface-area-to-volume ratio, and sensitivity to environmental changes make them ideal model organisms for investigation of the potential toxicity of nanomaterials[4]. In this study, we investigated the stability of NaYF\(_{4}\):Yb\(^{3+}\),Er\(^{3+}\) UCNPs, localization of UCNPs in green microalgae Desmodesmus communis and potential toxicity. This study used two media: an algae growth media (MWC) and water from the river Neris. UCNPs were tested at concentrations ranging from 2.5 to 100 µg/mL. Confocal laser scanning microscopy (CLSM) was employed to analyze nanoparticle uptake and localization in algae. UCNPs were excited with a 980 nm laser, while algae autofluorescence was excited using a 404 nm laser. Additional optical characterization included absorbance spectroscopy, fluorescence measurements (at 480 nm excitation), upconversion emission spectra (at 980 nm excitation), and emission decay kinetics.
Our results showed that UCNPs emission signals were more intense and nanoparticles were more colloidaly stable in the river Neris water than in MWC. MWC media is a balanced algae growth media rich in nutrients and ions. The ionic strength of MWC media is approximately 5 times higher than that of the river Neris water. This might be one of the factors causing faster aggregation of UCNPs in MWC media. Microbiological investigation of Neris water and MWC media showed that bacteria content in Neris water was 10,000 times higher (although water was filtered through a 0.2 µm filter) than in MWC media. CLSM imaging also confirmed that the river Neris water had much more bacteria compared to MWC media. The results also demonstrated that UCNPs did not localize inside D. communis cells. However, we observed that UCNPs interacted with bacteria.
Algae growth (evaluated using the absorbance of chlorophyll band) in MWC media followed an exponential curve, whereas in the river Neris water, it was linear and much slower (after 4 days, chlorophyll concentration was 4 times smaller). However, there were no significant differences in samples with different UCNPs concentrations in both cases. Different algae growth rates might be caused by fewer nutrients in the river Neris water.
Our findings suggest that UCNPs do not bioaccumulate in freshwater microalgae, but further research is needed to assess long-term aquatic toxicity. This work was supported by the Research Council of Lithuania, agreement No [S-MIP-24-92] “Effects of rare earth metals-based nanoparticles on aquatic organism: correlation between nanotoxicity and physico-chemical properties”.