The dual effect of natural organic matter on the two-step internalization process of Au@Sio₂ in freshwater

Dissolved organic matter (DOM) in aquatic ecosystems reshapes the surface of nanoparticles (NPs) greatly. Understanding how these changes influence the bioavailability of NPs is critical for accurately predicting the ecological risks of NPs. A quantitative model based on the two-step internalization process enabled the differentiation between the adhesion ability of NPs to membranes and the internalization capacity. Using protozoa Tetrahymena thermophila as the test organism, fluorescein isothiocyanate-modified silica NPs (FITC-SiO₂) and silica-coated gold NPs (Au@SiO₂) were prepared to validate the model and study the influence of DOM on uptake. DOM reduced the ability of Au@SiO₂ to adhere onto cell membranes and the inhibitory effect of bovine serum albumin (BSA) and β-lactoglobulin was higher due to their higher molecular weights and the weaker interaction. Moreover, DOM increased the internalization capacity. 80% Au@SiO₂ was internalized in the presence of humic acid (HA), over 90% Au@SiO₂ was internalized in the presence of the two proteins, whereas only 60% were internalized by the control group. Next, the specific recognition of the cell internalization in the presence of DOM was confirmed. We concluded that the traditional “accumulation” may misestimate the true biological effect caused by NPs coated with DOM. NPs coated with highly bioavailable DOM pose a greater risk to aquatic ecosystems because they are more likely to be internalized by living organisms.