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Adsorption of natural organic matter (NOM) on nanomaterials (NMs) results in the formation of interfacial area between NMs and the surrounding environment (referred to as NOM-corona), giving rise to NMs' unique surface identity. This unique surface identity is determined by the ligands and their interactions with NM surfaces. Since the chemical structure and functionality is heterogeneous and polydisperse, the molecular composition of NOM-corona is the result of competitive adsorption of NOM molecules on the NM surface. Here, we investigate the molecular composition of NOM-corona formed from two different NOM samples (isolated from the Yukon River and Milwaukee River) on the surface of AgNMs using electrospray ionization-Fourier-transform ion cyclotron resonance mass spectrometry (ESI-FT-ICR-MS). The composition of AgNM-NOM corona varied with the composition of the original NOM. In general, AgNM-NOM corona is rich with N- and S-containing compounds. Furthermore, AgNM-NOM corona is rich with compounds with high molecular weight, high unsaturation, and high number of oxygenated groups. However, CHOS (carbon, hydrogen, oxygen and sulfur) compounds adsorbed on AgNMs from the Yukon River NOM have low molecular weight (LMW) and low saturation index, which might be due to selective adsorption via chemical complexation (Ag–S). On the other hand, NOM compounds with LMW and low unsaturation or compounds containing few oxygenated groups (mainly alcohols and ethers) are preferentially maintained in solution phase. The results here provide evidence of molecular interactions between NOM and NMs, which are critical to understanding NM behavior and toxicity in natural environments.