Surface coatings of nanoparticles (NPs) play a key role in their behavior in the environment. It is well known for metal oxide nanoparticles these processes include dissolution, aggregation, and reactivity. All of these are impacted by surface coatings. This work focuses on understanding adsorption and displacement reactions of molecular and biomolecular species including humic acid (HA), citric acid (CA), ascorbic acid (AA) and bovine serum albumin (BSA). Several different techniques are employed in these studies including attenuated total reflectance–Fourier transformed infrared spectroscopy (ATR-FTIR) and quartz crystal microbalance (QCM). We observed that the small molecules, AA and CA, tend to form inner sphere complex with surface Ti ion via enediol and carboxylate, respectively. The interaction with humic acid can occur through multiple functional groups found within this large macromolecule. Furthermore, humic acid was used as a model of natural organic matter (NOM), to evaluate the displacement of coatings from the TiO2 surface. For the surface fully coated by AA or CA, humic acid tends to bind to the surface by partially displacing the pre-coated molecules, as a result of the co-adsorption. In the case of BSA, we observe that humic acid does not displace it but instead only co-adsorbs. 2D correlation spectroscopy was used to provide more details of the displacement of AA and CA by HA. The effects of displacement reactions on sedimentation and aggregation were also studied. TiO2 NPs with different coatings displayed a slow sedimentation rate in presence of humic acid due to the decrease in hydrodynamic diameter. Overall, this is the first time that displacement reactions are systematically studied. The insights this work provides are of significant relevance in the fate of nanomaterials as the ligands displacement reactions will modify the stability of nanomaterials during their transport in the environment.

Displacement reactions on coated TiO2 NPs with humic acid and the implications on their fate in the environment

256th National Meeting (2018)

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