Abstract
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 Na2S. This reaction is among the most prevalent and industrially and environmentally relevant ENMs transformation. Using ex situ transmission electron microscopy (TEM) and both in situ and ex situ 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 Ag2S domains both possess a high degree of crystalline order, contradicting amorphous structures as previously reported. In situ 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, in situ 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−1 for 68 nm diameter AgNPs). A rigorous analysis of the long-term sulfidation product of the AgNPs under different S/Ag ratios using ex situ SAXS/XRD clearly demonstrates that the silver mass in the original AgNP and transformed Ag/Ag2S 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.
Highlights
Due to their novel physical and chemical properties, engineered nanomaterials (ENMs) have found increasing applications in medicine,[1] energy,[2] sensor technologies,[3] and consumer industries.[4]
The vertices of the particle were nominally {110} terminated, and the particle edges o en had a rounded appearance resulting from surface steps and higher-order surface terminations, a feature identi ed earlier in a silver cube nanoparticle system.[55]
Taking advantage of the high quality of the colloidal AgNPs, we applied the synchrotronbased in situ ultra-small angle X-ray scattering (USAXS), small angle X-ray scattering (SAXS), and X-ray diffraction (XRD) techniques, which are sensitive to the NP size, morphology, electron density, and phases, to precisely track the sul dation process of the colloidal AgNPs–Ag/Ag2S NPs in real time
Summary
Due to their novel physical and chemical properties, engineered nanomaterials (ENMs) have found increasing applications in medicine,[1] energy,[2] sensor technologies,[3] and consumer industries.[4]. To predict ENM performance and environmental impacts, knowledge of the extent and rate of speci c transformations must be acquired. Due to their antimicrobial capabilities, silver nanoparticles (AgNPs) are among the most widely used ENMs.[13] For ENMs, one central concern is their environmental impact and consequent social cost.[6] For AgNPs in particular, while they can effectively release silver ions in targeted applications, they can pose signi cant risks to the ecosystem and overall environment.[14,15,16] it is important to characterize and understand AgNP transformation and its kinetics in realistic environmental settings to elucidate their toxicological behavior. Once discharged into the environment, AgNPs are subject to physiochemical interactions with natural organic matter (NOM), especially humic substances, which act to modify the stability and mobility of AgNPs through electrosteric interactions or hydrophobic effects.[17,18,19] humic substances in uence the stability, dissolution, and aggregation behaviors of AgNPs and affect their transport properties and environmental persistence.[20,21,22]
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