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Particle Coating-dependent Interaction of Molecular Weight Fractionated Natural Organic Matter: Impacts on the Aggregation of Silver Nanoparticles Yongguang Yin, Mohai Shen, Zhiqiang Tan, Sujuan Yu, Jing-fu Liu, and Guibin Jiang Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/es5061287 • Publication Date (Web): 05 May 2015 Downloaded from http://pubs.acs.org on May 11, 2015
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Particle Coating-dependent Interaction of Molecular Weight Fractionated
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Natural Organic Matter: Impacts on the Aggregation of Silver
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Nanoparticles
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Yongguang Yin,† Mohai Shen,† Zhiqiang Tan,† Sujuan Yu,† Jingfu Liu,*,†
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and Guibin Jiang†
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†
State Key Laboratory of Environmental Chemistry and Ecotoxicology,
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Research Center for Eco-Environmental Sciences, Chinese Academy of
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Sciences, P.O. Box 2871, Beijing 100085, China
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* Corresponding author.
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Tel.: +86-10-62849192; Fax: +86-10-62849192;
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E-mail:
[email protected] 18
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ABSTRACT
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Ubiquitous natural organic matter (NOM) plays an important role in the aggregation
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state of engineered silver nanoparticles (AgNPs) in aquatic environment, which
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determines the transport, transformation and toxicity of AgNPs. As various capping
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agents are used as coatings for nanoparticles and NOM are natural polymer mixture
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with
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coating-dependent interaction of MW fractionated natural organic matter (Mf-NOM)
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with various coatings is helpful for understanding the differential aggregation and
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transport behavior of engineered AgNPs as well as other metal nanoparticles. In this
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study, we investigated the role of pristine and Mf-NOM on the aggregation of AgNPs
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with bare, citrate and PVP coating (Bare-, Cit- and PVP-AgNP) in mono- and
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di-valent electrolyte solutions. We observed that the enhanced aggregation or
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dispersion of AgNPs in NOM solution highly depends on the coating of AgNPs.
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Pristine NOM inhibited the aggregation of Bare-AgNPs but enhanced the aggregation
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of PVP-AgNPs. In addition, Mf-NOM fractions have distinguishing roles on the
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aggregation and dispersion of AgNPs, which also highly depend on the AgNPs
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coating as well as the MW of Mf-NOM. Higher MW Mf-NOM (>100 kDa and 30-100
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kDa) enhanced the aggregation of PVP-AgNPs in mono- and di-valent electrolyte
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solutions, while lower MW Mf-NOM (10-30 kDa, 3-10 kDa and 99.5%) and hydroxylammonium chloride (>98.5%) were
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purchased from Sinopharm Chemicals (Shanghai, China). Polyvinylpyrrolidone (PVP,
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MW=58000) was from Aladdin Chemistry Co. Ltd. (Shanghai, China). Suwannee
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River natural organic matter (SRNOM) (1R101N) was obtained from the International
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Humic Substance Society (St. Paul, MN). Amicon Ultra-15 centrifugal filter (with 3
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kDa, 10 kDa, 30 kDa and 100 kDa nominal MW cut-off, respectively) from Millipore
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(Darmstadt, Germany) were used for fractionation of SRNOM. The other reagents
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were purchased from Beijing Chemicals (Beijing, China). All the reagents were used
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as obtained without further purification. Ultrapure water (18.3 MΩ) produced with a
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Milli-Q Gradient system (Millipore, Bedford) was used throughout the experiments.
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Fractionation and Characterization of NOM. Mf-NOM was prepared using
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Amicon Ultra-15 centrifugal filters with different MW cut-off. The detailed procedure
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of ultra-filtration and the characterization of pristine and Mf-NOM were given in our
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previous study.18, 20 Briefly, the stock solution of SRNOM was firstly loaded and
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centrifuged (30 min at 3743 g) using Amicon Ultra-15 centrifugal filters (100 kDa).
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The filtrate was then collected and filtered sequentially by 30 kDa, 10 kDa and 3 kDa
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filters at centrifuging rates 5095, 6654, and 8422 g, respectively. The retentate or
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filtrate was collected as Mf-NOM (>100 kDa, 30-100 kDa, 10-30 kDa, 3-10 kDa and
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100 kDa and 30-100 kDa NOM) significantly
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enhanced the aggregation of PVP-AgNPs in NaClO4 solution (Figure 3A), while in
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the presence of low MW NOM fraction (10-30 kDa, 3-10 kDa and 100 kDa Mf-NOM. However, the aggregation of PVP-AgNPs was enhanced in
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the presence of >100 kDa Mf-NOM, which suggested that the presence of >100 kDa
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Mf-NOM decreased the steric repulsion between PVP-AgNPs. Thus, considering both
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the electrostatic and steric repulsion, high MW Mf-NOM accelerated the aggregation
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of PVP-AgNPs. However, low MW Mf-NOM could adsorb on PVP-AgNPs surface
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but not screen PVP significantly, which result in enhanced electrostatic repulsion but
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minor change of steric repulsion. Therefore, low MW Mf-NOM inhibited the
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aggregation of PVP-AgNPs. The detailed mechanism should be studied in the future
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via advanced characterization technique (such as microscale thermogravimetric
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analysis)41 to further elucidate the role of Mf-NOM on nanoparticle aggregation.
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Cit-AgNP
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The aggregation of Cit-AgNPs in both mono- and di-valent electrolytes was not
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significantly influenced by pristine and Mf-NOM, compared with PVP-AgNPs
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(Figure 4). Specially, the aggregation of Cit-AgNPs was inhibited in the presence
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of >100 kDa NOM and low concentration of Ca2+, and an enhanced aggregation of
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Cit-AgNPs was observed in the presence of high MW Mf-NOM (>100 kDa and
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30-100 kDa NOM) and high concentration of Ca2+ (Figure 4C). The zeta potentials of
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Cit-AgNPs in electrolyte solution (Figure S5) showed that the zeta potential was also
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not significantly varied in the presence of pristine and Mf-NOM, with or without
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mono/di-valent electrolytes. This should be ascribed to the inhibition of NOM
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adsorption on AgNPs by the electrostatic repulsion between NOM and coated citrate.
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The inhibition of aggregation by >100 kDa NOM should be ascribed to the steric
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repulsion provided by adsorbed NOM, while the enhancement of aggregation by high
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MW Mf-NOM (>100 kDa and 30-100 kDa NOM) in the presence of high
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concentration of Ca2+ was ascribed to the interparticle bridging of NOM.21
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Bare-AgNP
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The aggregation of Bare-AgNPs in both mono- and di-valent electrolytes was
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significantly inhibited by pristine NOM (Figure 5). The CCCs were shifted from 8.10
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and 1.06 mmol L-1 to 81.55 and 2.11 mmol L-1 in NaClO4 and Ca(ClO4)2,
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respectively (Figure 5A and C). Accordingly, the α value in the presence of pristine
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NOM decreased to 0.70 and 0.64 in NaClO4 and Ca(ClO4)2, respectively.
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In the presence of Mf-NOM, the aggregation of Bare-AgNPs in both mono- and
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di-valent electrolytes was also inhibited significantly (Figure 5). Generally, this
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inhibition effect was correlated with the MW of Mf-NOM. High MW Mf-NOM could
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disperse Bare-AgNPs much better than low MW counterpart. This MW-dependent
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dispersion of nanoparticle by Mf-NOM was also observed previously for fullerene18
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and citrate-coated gold nanopartiles.17 High adsorption and strong steric repulsion of
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high MW Mf-NOM possibly account for this MW-dependent dispersion.17 Compared
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with Cit-AgNPs, Mf-NOM with the same MW enhanced the stabilization of
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Bare-AgNPs more effectively, possibly owning to different adsorption amount of
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Mf-NOM on AgNPs. This result also supports our previous hypothesis that the
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electrostatic repulsion between NOM and coated citrate decreases the adsorption of
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Mf-NOM on Cit-AgNPs. Additionally, when the concentration of Ca2+ were higher
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than 10 mmol L-1, enhanced aggregation of Bare-AgNPs was observed in pristine and
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high MW Mf-NOM (>100 kDa and 30-100 kDa NOM) (Figure 5C). However, this
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enhanced aggregation was not significant for low MW Mf-NOM (10-30 kDa, 3-10
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kDa and 100 kDa NOM enhanced
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aggregation more significantly than other fractions, followed by fraction of 30-100
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kDa Mf-NOM. Previous studies also observed enhanced aggregation of fullerene,42
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gold,43 silver,21 nano zero-valent iron,44 CeO2,45 and silicon nanoparticles,46 in the
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presence of high concentration of Ca2+, which were possibly induced by the
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Ca2+-induced inter-particle bridging of NOM adsorbed on nanoparticles.42 However,
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how specific compositions in NOM behave in this enhanced aggregation is still not
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well clarified. The findings of MW-dependent aggregation in the presence of high
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concentration of Ca2+ in our study, suggested that previous observation of this
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enhanced aggregation should be mainly ascribed to the high MW fraction of NOM.
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The zeta potentials of Bare-AgNPs in the absence and presence of NOM in electrolyte
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solution were given in Figure S6. The results clearly showed that the zeta potential of
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Bare-AgNPs was more negative in the presence of pristine and Mf-NOM, with or
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without mono/di-valent electrolytes. The zeta potential of Bare-AgNPs in the
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presence of >100 kDa NOM was even more negative than that in the presence of 100 kDa Mf-NOM in the case
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of PVP-AgNP), the over-coating or displacing of the coating agent by Mf-NOM could
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generally accelerate the aggregation. However, if the adsorbed Mf-NOM could
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provide additional repulsion (e.g. electrostatic repulsion from 100 kDa Mf-NOM is higher than that of
