Zhang Fan, Allen Andrew J, Johnston-Peck Aaron C, Liu Jingyu, Pettibone John M
Materials Measurement Science Division, National Institute of Standards and Technology, Gaithersburg, MD 20899, USA.
Nanoscale Adv. 2019;1(1):241-53. doi: 10.1039/C8NA00103K.
Understanding the structure transformation of engineered nanomaterials (ENMs) is a grand measurement challenge, which impacts many aspects of ENMs applications, such as their efficacy, safety, and environmental consequence. To address the significant knowledge gap regarding the fundamental kinetic rate and extent of ENM transformation in the environment, we present a comprehensive and mechanistic structural investigation of the transformation, aggregation, and dissolution behavior of a polyvinylpyrrolidone-coated silver nanoparticle (AgNP) suspension upon sulfidation in moderately reduced hard water with fulvic acid and dissolved NaS. This reaction is among the most prevalent and industrially and environmentally relevant ENMs transformation. Using transmission electron microscopy (TEM) and both and synchrotron-based small angle X-ray scattering (SAXS) and X-ray diffraction (XRD), we find that sulfidation of faceted AgNPs strongly depends on the crystallographic orientation of the facets, with nanometer-scale passivation layers developed on {111} and {100} facets and continuous nucleation and growth on {110} facets. Nanobeam electron diffraction and atomic resolution imaging show Ag and AgS domains both possess a high degree of crystalline order, contradicting amorphous structures as previously reported. SAXS/XRD allowed simultaneous determination of the morphological changes and extent of sulfidation of AgNPs. SAXS/XRD results strongly indicate sulfidation follows first-order reaction kinetics without any aggregation. Aided by their size monodispersity, for the first time, using direct, morphology and atomic-structure probes whose results mutually corroborate, we unequivocally determined the sulfidation rate constant of AgNPs under an environmentally relevant condition (~0.013 min for 68 nm diameter AgNPs). A rigorous analysis of the long-term sulfidation product of the AgNPs under different S/Ag ratios using SAXS/XRD clearly demonstrates that the silver mass in the original AgNP and transformed Ag/AgS NP is preserved. This result has important environmental implications, strongly suggesting that Ag ions, a known highly effective antimicrobial agent, are not leached into the solution during sulfidation of AgNPs. The combined nondestructive methodology can be extended to unfold the structure transformation pathway and kinetics in a broad range of ENM systems.
了解工程纳米材料（ENM）的结构转变是一项重大的测量挑战，这会影响ENM应用的许多方面，例如其功效、安全性和环境影响。为了填补关于ENM在环境中基本动力学速率和转变程度的重大知识空白，我们对聚乙烯吡咯烷酮包覆的银纳米颗粒（AgNP）悬浮液在含有富里酸和溶解的NaS的中度还原硬水中硫化时的转变、聚集和溶解行为进行了全面的机理结构研究。该反应是最普遍且与工业和环境相关的ENM转变之一。使用透射电子显微镜（TEM）以及基于同步加速器的小角X射线散射（SAXS）和X射线衍射（XRD），我们发现多面体形AgNP的硫化强烈依赖于晶面的晶体取向，在{111}和{100}晶面上形成纳米级钝化层，而在{110}晶面上持续成核和生长。纳米束电子衍射和原子分辨率成像表明Ag和AgS域都具有高度的晶体有序性，这与先前报道的非晶结构相矛盾。SAXS/XRD能够同时测定AgNP的形态变化和硫化程度。SAXS/XRD结果有力地表明硫化遵循一级反应动力学且无任何聚集。借助其尺寸单分散性，首次使用直接的形态和原子结构探针，其结果相互佐证，我们明确确定了在环境相关条件下（直径68 nm的AgNP约为0.013 min）AgNP的硫化速率常数。使用SAXS/XRD对不同S/Ag比下AgNP的长期硫化产物进行严格分析，清楚地表明原始AgNP和转化后的Ag/AgS NP中的银质量得以保留。这一结果具有重要的环境意义，强烈表明已知的高效抗菌剂银离子在AgNP硫化过程中不会浸出到溶液中。这种组合的非破坏性方法可以扩展到揭示广泛的ENM系统中的结构转变途径和动力学。
