A preliminary assessment of the interactions between the capping agents of silver nanoparticles and environmental organics

Stability of nanoparticles (NPs) and their sorption on different environmental surfaces have important implications for their fate and transport in aquatic systems. The surfaces of both NPs and soil/sediment minerals are likely to encounter environmental organics including natural organic matter (NOM) and extracellular polymeric substances (EPS) under relevant environmental conditions. The aim of this paper was to explore the potential modes of silver NP (AgNP) interaction with NOM and a model EPS. Molecular spectroscopies were used to characterize the interactions of NOM with the AgNP-capping agents (citrate and polyvinylpyrrolidone (PVP)). NMR spectroscopy suggests that both the humic acid (HA) and fulvic acid (FA) fractions of NOM are capable of displacing citrate from the surface of AgNPs. The relaxation times of methylene (CH2 and CH) protons provide indirect evidence that carboxyl or hydroxyl groups of FA interact with the surface of AgNPs. Raman spectroscopy suggests that FA interacts with both the ring and polyvinyl domains of PVP and the oxygen atom involved in the PVP–NP complex. These spectroscopic results imply that the displacement of AgNP capping agents by NOM may have a destabilizing effect on engineered NPs that enter the aqueous environment, thus reducing their environmental mobility. Quartz crystal microgravimetry (QCM) revealed observable differences in both the extent and kinetics of AgNP adsorption on substrates coated with NOM and dextran sulfate. These exploratory QCM results are crucial in guiding future research to further investigate the role of NOM/EPS-induced adsorption in influencing environmental partitioning of NPs. Overall, our preliminary assessment highlighted the critical role of surface modifications of both the NPs and the bulk substrate by environmental organics in the stability and mobility of AgNPs. These initial findings are important in the future design of NPs to ensure successful targeted applications as well as the environmental health and safety of NPs.