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The inherent physicochemical properties of engineered nanomaterials (ENMs) are known to control the sorption of proteins, but knowledge on how the release of ENMs to the environment prior to protein exposure affects this reaction is limited. In this study, time-resolved, in situ infrared spectroscopy was used to investigate the sorption of a model protein, bovine serum albumin (BSA), onto two different types of titanium dioxide (TiO 2 ) ENMs (catalytic-grade P90 and food-grade E171) in the presence and absence of a simple dissolved organic carbon molecule, oxalate. Infrared spectroscopy results showed that oxalate adsorbed to P90 through chemisorption interactions, but it adsorbed to E171 through physisorption interactions due to the presence of inherent surface-bound phosphates. Secondary structure and two-dimensional correlation spectroscopy analyses showed that BSA interacted with and unfolded on the surface of P90, but not E171, presumably due to the repulsive forces from the negatively charged phosphates on E171. When oxalate was pre-adsorbed to either P90 or E171, the unfolding of BSA occurred, but along different pathways. This suggests both the “outer” surface chemistry ( e.g. , oxalate layers) and the mechanism by which this layer is bound to the ENM play a significant role in the adsorption of proteins. Collectively, the results indicate the exposure of ENMs to natural and engineered environments prior to biological uptake affects the resulting protein corona formation, and thus the transport and bioactivity of ENMs. 

With the increased use of nanoparticles (NPs) in consumer, food, and pharmaceutical products, their eventual release into streams is inevitable. Critical factors affecting the transport of NPs in streams are the hyporheic exchange between the water column and porous streambed substrate and the interaction with biofilms. In this study, the transport behavior of two titanium dioxide NPs – catalytic- (P90) and food-grade (E171) – was evaluated in four field streams lined with different streambed substrate sizes for varying seasonal biofilm conditions. When biofilm growth was minimal, NP retention in the streams increased with increasing substrate size due to increased hyporheic exchange and subsequent physical and chemical interactions between the NPs and substrate. For all streams, the average mass recovery at the 40 m sampling point for E171 and P90 was 44 ± 8.7% and 16 ± 8.0%, respectively. The greater mobility of E171 was due to the inherent presence of negatively charged surface phosphates that reduced aggregation and decreased its interaction with the substrate. When biofilms were thriving in the streams the average mass recovery at 40 m for both NPs decreased significantly (E171 = 5.8 ± 7.3%, P = 0.0017; P90 = 2.4 ± 0.7%, P = 0.041), and the mass recovery difference between the two NPs became insignificant ( P = 0.38).