Interactions and stability of silver nanoparticles in the aqueous phase: Influence of natural organic matter (NOM) and ionic strength

The rapid development of nanotechnology and the related production and application of nanosized materials such as engineered nanoparticles (ENP) inevitably lead to the emission of these products into environmental systems. So far, little is known about the occurrence and the behaviour of ENP in environmental aquatic systems. In this contribution, the influence of natural organic matter (NOM) and ionic strength on the stability and the interactions of silver nanoparticles (n-Ag) in aqueous suspensions was investigated using UV–vis spectroscopy and asymmetrical flow field-flow fractionation (AF⁴) coupled with UV–vis detection and mass spectrometry (ICP-MS). n-Ag particles were synthesized by chemical reduction of AgNO₃ with NaBH₄ in the liquid phase at different NOM concentrations. It could be observed that the destabilization effect of increasing ionic strength on n-Ag suspensions was significantly decreased in the presence of NOM, leading to a more stable n-Ag particle suspension. The results indicate that this behaviour is due to the adsorption of NOM molecules onto the surface of n-Ag particles (“coating”) and the resulting steric stabilization of the particle suspension. The application of AF⁴ coupled with highly sensitive detectors turned out to be a powerful method to follow the aggregation of n-Ag particle suspensions at different physical–chemical conditions and to get meaningful information on their chemical composition and particle size distributions. The method described will also open the door to obtain reliable data on the occurrence and the behaviour of other ENP in environmental aquatic systems.