Comparing sulfidation kinetics of silver nanoparticles in simulated media using direct and indirect measurement methods
Jingyu Liu, Fan Zhang, Andrew J. Allen, Aaron C. Johnston-Peck, John M. Pettibone
The processing of nanomaterials in natural and engineered systems, which controls their fate and performance, is determined by the relative rates of the predominant reaction pathways present. Commonly used techniques for determining the reaction kinetics of metal nanoparticles use proxy measurements to infer chemical transformations, but extrapolation of these methods into increasing complex media has proven difficult. Here, we compare the sulfidation rate of AgNPs using two ion selective electrode (ISE)-based methods, which rely on either i) direct measurement of free sulfide, or ii) monitor the free Ag+ available in solution over time in the presence of sulfide species. Most experiments were carried out in moderately hard reconstituted water at pH 7 containing fulvic acid or humic acid, which represented a broad set of known interferences in ISE. Distinct differences in the measured rates were observed between the two proxy-based methods and details of the divergent results are discussed. The two ISE based methods were then compared to direct monitoring of AgNP chemical conversion to Ag2S using in situ X-ray diffraction. Using the direct method, the fitting of the Ag0 decay resulted in different observed rates than from either ISE-based technique. Additionally, the X-ray diffraction method exhibited distinct transformation regimes, which were identified by distinct changes in the Ag0 decay rates, that could not be observed with other commonly employed techniques. The implications of distinct transformation regimes in the well-characterized AgNP systems are discussed and demonstrate the need for more sensitive (direct monitoring) methods to determine reaction kinetics of nanomaterials in increasingly complex media.