Colloids and Surfaces A: Physicochem. Eng. Aspects 435 (2013) 22–27
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A preliminary assessment of the interactions between the capping agents of silver nanoparticles and environmental organics Boris L.T. Lau a,∗ , William C. Hockaday a , Kaoru Ikuma a , Olga Furman a , Alan W. Decho b a b
Department of Geology, Baylor University, One Bear Place #97354, Waco, TX,76798, United States Department of Environmental Health Sciences, Arnold School of Public Health, University of South Carolina, Columbia, SC, 29208, United States
h i g h l i g h t s
g r a p h i c a l
a b s t r a c t
T1 relaxation time is useful in detecting interactions between organics and AgNPs. NOM may have a destabilizing effect on AgNPs by displacing its capping agent. Substrates modified by NOM or model EPS have different sorptive capability for AgNPs.
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Article history: Received 22 October 2012 Accepted 27 November 2012 Available online 5 December 2012 Keywords: Nanoparticles Natural organic matter (NOM) Extracellular polymeric substances (EPS) NMR spectroscopy Raman spectroscopy Quartz crystal microgravimetry
a b s t r a c t Stability of nanoparticles (NPs) and their sorption on different environmental surfaces have important implications for their fate and transport in aquatic systems. The surfaces of both NPs and soil/sediment minerals are likely to encounter environmental organics including natural organic matter (NOM) and extracellular polymeric substances (EPS) under relevant environmental conditions. The aim of this paper was to explore the potential modes of silver NP (AgNP) interaction with NOM and a model EPS. Molecular spectroscopies were used to characterize the interactions of NOM with the AgNP-capping agents (citrate and polyvinylpyrrolidone (PVP)). NMR spectroscopy suggests that both the humic acid (HA) and fulvic acid (FA) fractions of NOM are capable of displacing citrate from the surface of AgNPs. The relaxation times of methylene (CH2 and CH) protons provide indirect evidence that carboxyl or hydroxyl groups of FA interact with the surface of AgNPs. Raman spectroscopy suggests that FA interacts with both the ring and polyvinyl domains of PVP and the oxygen atom involved in the PVP–NP complex. These spectroscopic results imply that the displacement of AgNP capping agents by NOM may have a destabilizing effect on engineered NPs that enter the aqueous environment, thus reducing their environmental mobility. Quartz crystal microgravimetry (QCM) revealed observable differences in both the extent and kinetics of AgNP adsorption on substrates coated with NOM and dextran sulfate. These exploratory QCM results are crucial in guiding future research to further investigate the role of NOM/EPS-induced adsorption in influencing environmental partitioning of NPs. Overall, our preliminary assessment highlighted the critical role of surface modifications of both the NPs and the bulk substrate by environmental organics in the stability and mobility of AgNPs. These initial findings are important in the future design of NPs to ensure successful targeted applications as well as the environmental health and safety of NPs. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
∗ Corresponding author. Tel.: +1 254 710 2534; fax: +1 254 710 2673. E-mail address: Boris
[email protected] (B.L.T. Lau). 0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.11.065
Environmental organics, including natural organic matter (NOM) [1] and extracellular polymeric substances (EPS) produced by microorganisms [2], are ubiquitous in natural waters [3,4]. It is widely recognized that NOM plays important roles in the fate and
B.L.T. Lau et al. / Colloids and Surfaces A: Physicochem. Eng. Aspects 435 (2013) 22–27
transport of organics and metals [5–9]. However, the mechanisms and implications of nanoparticle (NP) interactions with NOM and EPS are largely unknown. Though NOM is realized to be a complex mixture, containing a plethora of different organic molecules, the interactions with NPs are likely to be selective for certain organic molecules within the mixture. The interplay of the organic capping ligands on the NP surface with environmental organics must be better understood to more precisely predict the stability and mobility of NPs in aquatic systems. Bacteria are an omnipresent component of natural and many engineered aquatic systems, and often the first organisms to interact with introduced compounds. In natural environments, most bacteria occur as attached biofilms where cells are enveloped within a secreted matrix of “sticky” EPS [2]. The EPS comprise a wide range of molecules, including polysaccharides, proteins, lipids, and nucleic acids, of which polysaccharides are typically the major component [10]. These EPS are thought to be responsible for the strong adsorption of NPs to biofilms, and form an effective matrix for entrapment, sorption and binding of metals, organics and even viruses. Recent studies demonstrate that significant accumulations of NPs occur in the biofilms of both riverineand estuarine-mesocosms, and as well as in laboratory cultures of bacteria [11–13]. For example, when gold NPs (i.e., 65 × 15 nm nanorods) were added to an estuarine mesocosm ecosystem, the NPs were most strongly bioconcentrated in the biofilm components; with their bioconcentration accounting for greater than 60% of the added NPs [11]. Similar bioconcentration was found in riverine mesocosms using 20 nm TiO2 NPs [12]. These initial studies point to an important role of biofilms for influencing environmental partitioning of NPs within natural systems. However, little is known concerning interactions of biofilms with NPs. The aim of this work was to investigate the potential of NP interactions with NOM and EPS. Using silver NPs (AgNPs) as model NPs, the present study utilized molecular spectroscopy and quartz crystal microgravimetry (QCM) to investigate NP surface interactions with humic/fulvic acids and dextran sulfate (as a model polysaccharide found in EPS), and explore the potential implications for NP fate and transport. Interaction of NOM with the AgNP surface in the aqueous phase was probed using liquid-state nuclear magnetic resonance (NMR) spectroscopy and Raman spectroscopy. QCM was used to quantify the extent and kinetics of AgNP adsorption on organic-coated SiO2 (model soil/sediment mineral). 2. Materials and methods 2.1. Preparation and characterization of AgNPs Silver ions can be complexed with various inorganic and organic ligands to form AgNPs anthropogenically and naturally [14,15]. Engineered AgNPs are typically functionalized by different capping agents, depending upon the intended application. Currently, the two most common capping agents for commercially available AgNPs are citrate and polyvinylpyrrolidone (PVP). 50 nm AgNPs (citrate-capped and PVP-capped) were purchased from Nanocomposix, Inc. (San Diego, CA) for this study. A Malvern Zetasizer NS (Worcestershire, U.K.) was used to determine: 1) the hydrodynamic diameter of NPs by dynamic light scattering, and 2) the zeta potential of NPs by converting measured electrophoretic mobility using the Smoluchowski approximation. 2.2. Molecular composition and dynamics: liquid-state 1 H NMR spectroscopy We used the molecular dynamics of the citrate molecule as a probe for modification of the AgNP surface upon interaction with NOM. Citrate-capped AgNPs (10 ml, 20 mg/L) were used as
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purchased and subsequently allowed to interact with NOM (5 mg/L DOC) in Milli-Q water in the dark overnight. We used two fractions of NOM—Suwannee River humic acid (HA) and fulvic acid (FA)— purchased from the International Humic Substance Society. To remove the excess of NOM and H2 O, the NOM-coated AgNPs were centrifuged at 15,000 rpm for 30 min. The pellet was rinsed and suspended with 99.9% deuterated water (D2 O) by sonication for 60 s. The solutions were buffered with 0.4 mM Na-citrate to ensure that there was sufficient citrate in all samples for detection by 1 H nuclear magnetic resonance. Solution conditions were: 1. 0 mg/L AgNP-citrate, 0 NOM, 0.4 mM sodium citrate, 99.9% D2 O 2.
