On the transformation mechanism of polyethylene glycol- and citrate-coated silver nanocolloids under sunlight exposure

As the use of nanomaterials continues to increase, their release into the environment is also expected to increase. During the use phase of nano-enhanced products containing silver nanoparticles (Ag NPs) or after the release of Ag NPs in the environment, the latter are likely to be altered by the conditions they will encounter, such as sunlight irradiation. In this work, suspensions of citrate-stabilized and PEG-stabilized Ag NPs of around 20 nm in diameter were synthesized and aged for a week in a climatic chamber under controlled temperature (40 °C) and sunlight irradiation. The suspensions were analyzed by UV/visible spectroscopy, asymmetric flow field flow fractionation—inductively coupled plasma mass spectrometry (AF4-ICP-MS), dynamic light scattering (DLS), and transmission electron microscopy (TEM). We found that the electrostatic-stabilizing citrate promoted particle morphology changes with the emergence of a transient and minor secondary population of truncated 2D prisms (110–120 nm in size) but postponed the final sedimentation step. Conversely, the initial protecting role of the steric-stabilizing PEG was reduced by the photodegradation of the polymer chains and eventually favored inter-particle bridging resulting in aggregation and sedimentation. It is hypothesized that the bridging process proceeds via PEG chain interpenetration between two adjacent particles and reduction by the polymer photodegradation products of the Ag+ ions generated by oxidative dissolution.