In Situ Methods for Monitoring Silver Nanoparticle Sulfidation in Simulated Waters

To probe the transformation pathways of metallic nanomaterials, measurement tools capable of detecting and characterizing the broad distribution of products with limited perturbation are required. Here, we demonstrate the detection of transformation products resulting from 40 kDa PVP-coated silver nanoparticles (AgNPs) reacted in aerated, sulfide-containing water and EPA moderately hard reconstituted water standard. Using single particle inductively coupled plasma mass spectrometry, silver mass preservation in primary AgNP populations during sulfidation was observed under all reaction conditions examined. Disparate sensitivities of Ag+ and AgNPs to different media were observed, limiting confidence in the measured dissolved fraction. Examination with hyphenated asymmetric flow field-flow fractionation (A4F) methods supported similar mass preservation. Using flow-cell FTIR measurements, we provide direct evidence for the preservation of PVP-coatings in the presence of Na2S and fulvic acid, which we attributed to the observed, unprecedented Ag preservation. Using A4F and X-ray scattering, sub 10 nm AgNP populations, which have gone nearly unstudied in environmental systems, were detected and characterized in all the pristine and transformed product distributions examined. Furthermore, by distinguishing Ag+ from individual AgNPs, quantification of each population becomes tractable, which is a critical measurement need for toxicity testing and predicting NP fate in engineered and natural systems.