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A preliminary assessment of the interactions between the capping agents of silver nanoparticles and environmental organics
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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.
a r t i c l e
i n f o
Article history: Received 22 October 2012 Accepted 27 November 2012 Available online xxx 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).
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
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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
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. <200 mg/L AgNP-citrate, 0 NOM, 0.4 mM sodium citrate, 99.9% D2 O 3. <200 mg/L AgNP-FA, 0.4 mM sodium citrate, 99.9% D2 O 4. <200 mg/L AgNP-HA, 0.4 mM sodium citrate, 99.9% D2 O The NMR experiments were conducted with a 360MHz Bruker DSX spectrometer. Standard one-dimensional 1 H NMR spectra were acquired without sample spinning by applying a 90o1 H excitation pulse and a water signal saturation pulse. Chemical shift values were externally referenced to trimethylsilane. Spin-lattice relaxation times were measured using a standard inversion-recovery pulse program and a recycle delay of 20 s, without the use of water suppression. Eight 1 H spectra were acquired with the following recovery delay times (): 0, 0.2, 0.5, 0.75, 1, 2, 6 and 8 s. Phase correction of spectra 1–7 was performed relative to spectrum #8 ( = 8 s), baseline was set manually. Delay times () were plotted against peak integrals (It ); and exponential fitting was conducted following Eq. (1) and using the SigmaPlot 11.0 software. It = A + Be−/T1
(1)
where is a variable delay time, It = integral of the peak of interest recorded for eight spectra at variable delay time, A and B = constants, T1 = relaxation time (derived from exponential fitting). 2.3. Surface chemistry: Raman spectroscopy Samples prepared for Raman spectroscopy 1. 2.2 mg Suwanee River Fulvic Acid in 0.5 mL Milli-Q water 2. 4.1 mg/mL PVP-capped AgNPs in 0.5 mL Milli-Q water 3. 4.1 mg/mL PVP-capped AgNPs, 2.2 mg Suwanee River Fulvic Acid in 0.5 mL Milli-Q water Samples were placed in a 5 mm (o.d.) quartz tube and Raman spectra were acquired with a Nicolet Fourier transform Raman spectrometer with Nd:YAG laser (1064 cm−1 excitation wavelength) operating at a power level of 0.636 W. Raman scattering was recorded by an InGaAs detector (4000–100 cm−1 ) operated at a spectral resolution of 4 cm−1 . Spectra were summed for 500 scans and Fourier transformed without smoothing functions. 2.4. Adsorption dynamics: Quartz crystal microgravimetry (QCM) An E1 quartz crystal microbalance (Q-Sense, Gothenburg, Sweden) was used to characterize AgNP adsorption. Before and after each experiment, a standard cleaning procedure developed by Q-Sense was used to remove trace organics and to render the surface of sensor substrate sufficiently clean. The cleaning protocols involves soaking and rinsing the sensors with 2% sodium dodecyl sulfate solution and Milli-Q water, drying under nitrogen gas, and an UV–ozone treatment. After obtaining a stable baseline with background electrolyte solution, a 10 mg/L working suspension of
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AgNPs was pumped through the surface of a quartz crystal sensor (silica sensors were coated with NOM and dextran sulfate respectively). Changes in resonance frequency (f) and in resonance dissipation (D) were monitored over time as AgNPs adsorbed onto the surface, while constant and intermittent alternating voltages were applied to the quartz crystal. Seven harmonics were detected, leading to 14 simultaneous measurements of parameters (seven frequencies and seven dissipation values). With the best signalto-noise ratio, f and D obtained from the third overtone are presented in this paper. The extent of adsorption was determined after the sensor electrode was washed with background electrolyte solution and a new stable frequency reading was reached. Measured frequency shift is proportional to the mass deposited on the QCM sensor and the initial adsorption rate is determined by calculating the slope of the measured frequency shifts over the first 10 min of adsorption. The temperature of the solutions in the flow module was maintained at 25 ± 0.02 ◦ C for all experiments. The flow rate was kept at 100 L/min. The same procedure was repeated for all experimental conditions. 2.4.1. Substrate preparation Dextran is a branched polysaccharide made of glucose chains of varying lengths (from 3 to 2000 kilodaltons). The straight chain consists of ␣-1,6 glycosidic linkages between glucose molecules, while branches begin from ␣-1,3 linkages. Dextran sulfates (CAS NUMBER: 9011-18-1), which have hydroxyl groups of dextran substituted with sulfonate functional groups, were used in experiments and consisted of the sodium salt forms (Sigma Chem. Co., St. Louis, MO) and were derived from the bacterium Leuconostoc mesenteroides. Dextran sulfate contains approx.17% S, (i.e., approx. 2.3 sulfates per glucose). Silica (SiO2 ) quartz crystals (14 mm diameter) with a fundamental resonant frequency of 5 MHz (QSX 303, Q-Sense AB, Gothenburg, Sweden) were used as substrate for the immobilization of organics (NOM and dextran sulfate). The silica sensor was first coated with 0.1 mg/mL of poly-L-lysine (PLL) to establish a positively charged surface as an electrostatically favorable condition for later adsorption of organics. After deposition of the PLL layer, the sensor was rinsed with background electrolyte (10 mM NaNO3 ) and then exposed to one of the following: 10 mg/L of SRHA/SRFA or 5 mg/mL of dextran sulfate. After the adsorption of organics onto PLL-coated silica reached saturation, the substrate was again rinsed with the background electrolyte to remove any unbound organics prior to the introduction of AgNPs. 2.5. Substrate characterization The zeta potential of the substrate surface was determined by measuring the streaming potential using a SurPASS electrokinetic analyzer (Anton Paar GmbH, Graz, Austria). An adjustable gap cell customized to hold the QCM sensor substrates was used for sample mounting. The gap between the two sensors was adjusted to ∼120 m. Duplicate measurements were performed by pumping 10 mM NaNO3 (pH 6) electrolyte through the micro-channel with syringe pumps. A pressure ramp from 50 to 300 mbar in 15 mbar steps was employed to force the electrolyte solution through the channel with a flow rate between 150 and 200 mL/min. Measured streaming potentials were converted to zeta potentials using the Fairbrother–Mastin equation [16]. 3. Results and discussion 3.1. Spectroscopic evidences of AgNP– NOM interactions NMR is sensitive to both covalent and non-covalent interactions, making it feasible to determine the identity of the interacting
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functional groups as well as the nature of the interactions. Regarding the latter, liquid-state 1 H NMR with proton spin-lattice relaxation measurements were used to identify changes in molecular dynamics (rotational and segmental motion) of NOM components upon interaction with AgNPs. Similarly, the localized surface enhancement of Raman scattering by the Ag metal surface provides information about which organic functional groups are closely associated with the AgNP surface. 3.1.1. Liquid-state 1 H NMR spectroscopy Liquid-state 1 H NMR provides structural information on NOM in aqueous solution. We used 1 H NMR to study the composition of NOM in D2 O before and during interaction with AgNPs. The interaction of AgNPs with NOM could occur through covalent or non-covalent bonding. The chemical shift of NMR frequencies provides information about the electron distribution surrounding the nucleus, and is most influenced by covalent bond formation, whereas spin-lattice relaxation times (T1 ) provide information about non-covalent interactions. The T1 value is characteristic of overall molecular motion, including translation, rotation, and vibration, and has been used to examine non-covalent interactions of contaminants with NOM [17]. The spin-lattice relaxation time (T1 ) of proton magnetic moments are strongly modulated when the timescale of molecular motions ( c ) correspond to the 1 H nuclear magnetic resonance frequency (ωo ). Efficient relaxation (short T1 ) occurs when 1/ c ≈ ωo . On the contrary, molecular reorientations that occur more rapidly (1/ c » ωo ) or more slowly (1/ c « ωo ) than the NMR frequency do not provide efficient interactions between proton magnetic moments, therefore, relaxation rates are slowed and T1 relaxation times are lengthened. Citrate T1 relaxation time. To first establish the viability of spin-lattice relaxation times as a sensitive means of detecting interactions between organic molecules and AgNPs, we compared the T1 of citrate in solution (without AgNPs) to the T1 of citrate when a fraction is bound as a “capping agent” on the AgNP surface. The citrate molecule, shown in Fig. 1, contains 3 carboxylate groups (a tridentate ligand) and 4 methylene 1 H atoms giving rise to 2 NMR signals (doublets) centered at 2.55 and 2.65 ppm (HA and HB , respectively). We used the combined magnetization of these four protons as a measure of the 1 H T1 relaxation time of citrate. Table 1 shows an increase in the average T1 relaxation time of the citrate protons in the presence of AgNPs. In aqueous solution, the relaxation of protons (A and B) in sodium citrate is relatively rapid (T1 = 739 ms). This may be attributed to several causes; molecular motion at rates approximating the NMR frequency (1/tc ≈ ωo ), or the paramagnetic effects of sodium ions. Polyacids like citrate are known to form oligomeric structures by self-association through extensive hydrogen bonding and ion bridging interactions. These self-associations and bridging with the paramagnetic ions, sodium, are likely to cause efficient dipolar interaction and rapid spin-lattice relaxation (short T1 ). Upon interaction with the surface of AgNPs, the relaxation of citrate protons is substantially slower (T1 = 964 ms).
B
H OH
+
Na
H
-
A
H
A
-
O
B
O
H
O
-
O
O O Na+
+
Na
Fig. 1. Na-citrate with protons used for T1 relaxation study shown in red.
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Table 1 Relaxation times of aliphatic proton of citrate. #
Sodium citrate (mM)
50 nm AgNP-citrate (ppm, Ag)
Additional coating
Relaxation time T1 a (s)
R2
1 2 3 4
0.4 0.4 0.4 0.4
0 200 200 200
0 0 FA HA
0.739 0.964 1.030 1.135
0.9997 0.9973 0.9942 0.9919
a
determined by fitting of inversion recovery data to Eq. (1).
Fig. 2. A schematic representation of the competition of HA and FA for the surface of the AgNPs, and the subsequent displacement of citrate ligands from the surface.
We attribute this exclusively to changes in the citrate molecular motion (1/ c » ωo or 1/ c « ωo ) because the concentration of sodium remained unchanged. Relaxation rates were progressively slowed upon addition of FA (T1 = 1.030 s) and HA (T1 = 1.135 s), respectively. One possible interpretation is that the increased T1 with added FA and HA is due to the formation of an organic layer on top of the citrate-capped AgNP’s resulting in a reduced rates of molecular reorientation and dipolar interaction of citrate (i.e., 1/ c « ωo ). An alternative interpretation of the increase in citrate T1 with the addition of FA and HA invokes an increase in molecular mobility of citrate due to displacement of citrate from the surface of AgNPs. The schematic (Fig. 2) depicts the competition of HA and FA for the surface of the AgNPs, and the subsequent displacement of citrate ligands from the surface. This interpretation reconciles the T1 data because the displacement of citrate from the NP surface and into the bulk solution would increase the rate of molecular motion (1/ c » ω) to the extent that relaxation rate is slower. Fulvic Acid T1 relaxation times. To further elucidate the nature of interactions between citrate and NOM at the AgNP surface, we measured T1 relaxation times of aliphatic protons in FA in the presence and absence of citrate-capped AgNPs. Our inversion recovery experiments indicated a substantial decrease (∼20 ms) in the T1 relaxation time of methylene (CH2 ) protons in FA after 24 h of interaction with citrate-capped AgNPs (Table S1). Meanwhile, AgNPs caused an increase (∼70 ms) in the methyl (CH3 ) protons T1 relaxation times. Taken together, these data may suggest that motions of methylene (CH2 ) groups near the carboxyl termini are slowed due to interaction of the carboxylate group with the AgNP, while the methyl termini of aliphatic FA are more mobile, perhaps because the methyl termini are exposed to the solution rather than interacting directly with the AgNP surface. A schematic interpretation of these data is shown in Fig. 3. Taken together, the 1 H relaxation time data suggest that the organic capping ligands on the surface of engineered AgNPs interact or react with and/or may be replaced by aqueous NOM.
3.1.2. Raman spectroscopy Raman spectroscopy is a powerful complement to NMR due to a highly selective enhancement of scattering by AgNPs [18]. We compared Raman spectra of pure PVP with that of PVP-capped AgNPs to determine how the PVP polymer is linked to the AgNP surface (Figures S2 and S3). In the PVP-capped AgNPs, we observed the disappearance of the PVP carbonyl (C O) stretches at 1311 cm−1 and 1667 cm−1 and the emergence of signals corresponding to N C and N C O vibrations (1597 and 2074 cm−1 , respectively). These results may indicate that PVP is attached to the surface of AgNPs through charge complexation or ester linkage to silver oxide. The association of PVP with the AgNP surface caused substantial changes in the relative intensity of signals in the Raman spectrum of PVP. We observed a relative enhancement of signals from less abundant C O and C N, likely due to electromagnetic plasmon resonance enhancement that occurs on the surface of AgNPs [19].
Fig. 3. Idealized fulvic acid molecule on the surface of a spherical nanoparticle (5 nm diameter, drawn to scale). Longer T1 values suggest greater molecular motion of shielded methylene (CH2 ) and terminal methyl (CH3 ) protons in fulvic acid, and increased motion of the terminal methyl (CH3 ) groups.
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Fig. 4. Raman spectroscopy of Suwanee River fulvic acid and silver nanoparticles in aqueous solution.
Fig. 5. Zeta potentials of nanoparticles and substrates measured in 10 mM NaNO3 (pH 6). Error bars represent the standard deviation of at least three measurements.
Raman spectroscopy can also provide molecular information about NP-NOM interactions. Fig. 4 shows the spectra of FA (gray curve), and PVP-capped NPs (dashed line). The FA generates a relatively featureless Raman spectrum dominated by broad signals tentatively assigned to C C, C H, and C C stretching vibrations, whereas the NP spectrum contains several sharp signals that can be assigned to the Raman-active vibrational modes of the PVP capping agent. The introduction of FA solution to the PVP-capped AgNPs (bold line in Fig. 4) generates notable changes in the PVP signals. The decreased intensity of signals at 690 cm−1 , 1465 cm−1 and 2074 cm−1 in Fig. 4 (which we attribute to the C C ring vibration, CH2, and N C O stretching, respectively) suggest that FA interacts with both the ring and polyvinyl domains of PVP and the oxygen atom involved in the PVP–NP complex. This result implies that FA may have displaced the PVP from the direct contact with the AgNPs where the signal is enhanced by the surface plasmon resonance [19]. Displacement of NP capping agents by FA may have a destabilizing effect on engineered nanoparticles that enter the aqueous environment, thus reducing their environmental mobility. Indeed, some precipitation of AgNPs was observed during our preliminary experiment. We also noted the appearance or enhancement of a small signal at 898 cm−1 , from C O C stretching vibrations in Fig. 4. This result implies that the AgNP surface interacts with O-alkyl groups in NOM, such as those found in sugars, polysaccharides, and ether lipids. To understand, more precisely, the exact cause of changes in Raman peak intensities will require further investigation with carefully controlled conditions in the solution. Nevertheless, these Raman spectroscopy conspire with the NMR data to suggest that FA can alter the surface composition, and perhaps the reactivity, of the NP surface. To quantify the effects of NOM on AgNP reactivity, Section 3.2 discusses the initial QCM measurements of AgNP adsorption at the solid–water interface.
QCM results demonstrated that AgNP adsorption is influenced by i) the composition of organics that were immobilized on the silica substrate and ii) its capping agents. Despite the presence of electrostatic repulsion between PVP-capped AgNPs and the HA/FA coated silica, substantial degree of adsorption (328–440 ng/cm2 ) occurred (Fig. 6). This underscored the importance of nonelectrostatic forces in controlling the sorption dynamics. There were observable differences in both the extent and kinetics of PVP-capped AgNP adsorption on substrates coated with HA/FA (Fig. 6) and dextran sulfate (as a model polysaccharide found in EPS) (Fig. 7). HA and FA behaved similarly in terms of their relative rate and capacity to adsorb PVP-capped AgNPs. However, the amounts of PVP-capped AgNPs deposited on HA/FA-coated substrate were about 4 times greater than dextran sulfate (Figs. 6 and 7). This difference in adsorption extent seems to suggest that NOM may serve as a bigger sink than polysaccharides in immobilizing AgNPs on environmental surfaces. PVP-capped AgNPs were depositing to dextran sulfate about 16 times faster and 43 times more than citrate-capped AgNPs (Fig. 7). This is probably due to the difference in properties of citrate and PVP. The adsorption behavior of citrate-capped AgNPs is expected to be primarily governed by the degree of protonation on citrate due to pH. The minimal adsorption of citrate-capped AgNPs seems to
3.2. NP adsorption on substrate coated with organics Prior to the adsorption experiments, zeta potentials of the AgNPs and the organic-coated silica substrates were determined (Fig. 5). The results from Figs. 6 and 7 were interpreted in the light of the charge characteristics of the AgNPs and the substrate surfaces. All surfaces were negatively charged in the study conditions. The coated silica substrates were more negatively charged than the AgNPs (Fig. 5). Size of AgNPs was monitored and no aggregation was observed throughout the adsorption experiments. The hydrodynamic diameters of citrate AgNPs and PVP-capped AgNPs were 54.8 ± 0.9 nm and 60.1 ± 0.5 nm, respectively.
Fig. 6. The rate (gray column) and extent (black column) of PVP-capped AgNPs adsorption on NOM-modified silica substrates. Error bars represent the upper range of duplicate measurements.
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.colsurfa.2012.11.065. References
Fig. 7. The adsorption rate (gray column) and extent (black column) of citrateand PVP-capped AgNPs onto dextran sulfate-modified silica substrates. Error bars represent the upper range of duplicate measurements.
indicate that electrostatically flavorable condition is essential. On the other hand, hydrophobic/steric forces potentially play a more important role in yielding the observed adsorption of PVP-capped AgNPs.
4. Conclusions This paper illustrated how molecular spectroscopy and QCM could provide crucial fundamental information for investigating the relative importance of NOM/EPS in influencing environmental partitioning of AgNPs. The spectroscopy (NMR and Raman) data each suggest several modes of AgNP interaction with NOM. Each provides evidence for: i) displacement of the capping ligands, ii) alteration of the AgNP–NOM interface, and iii) alteration of the NOM–solution interface. QCM revealed that AgNP adsorption dynamics is a function of i) the composition of organics that were coated on silica and ii) its capping ligands. A more complete picture is needed to confirm our observations. Either spectroscopy or QCM alone is insufficient to reveal the complexities involved. To gain a better mechanistic understanding of the interactions, future research should couple surface-sensitive tools with bulk techniques to provide complementary lines of evidence. Our preliminary evaluation highlighted the critical role of surface modification of both the NPs and the bulk substrate by environmental organics in the stability and mobility of AgNPs. These initial findings will help improve future design of NPs to ensure successful targeted applications while keeping the NPs safe upon environmental release.
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Contents lists available at SciVerse ScienceDirect
Colloids and Surfaces A: Physicochemical and Engineering Aspects journal homepage: www.elsevier.com/locate/colsurfa
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.
a r t i c l e
i n f o
Article history: Received 22 October 2012 Accepted 27 November 2012 Available online xxx 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).
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
0927-7757/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.colsurfa.2012.11.065
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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
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. <200 mg/L AgNP-citrate, 0 NOM, 0.4 mM sodium citrate, 99.9% D2 O 3. <200 mg/L AgNP-FA, 0.4 mM sodium citrate, 99.9% D2 O 4. <200 mg/L AgNP-HA, 0.4 mM sodium citrate, 99.9% D2 O The NMR experiments were conducted with a 360MHz Bruker DSX spectrometer. Standard one-dimensional 1 H NMR spectra were acquired without sample spinning by applying a 90o1 H excitation pulse and a water signal saturation pulse. Chemical shift values were externally referenced to trimethylsilane. Spin-lattice relaxation times were measured using a standard inversion-recovery pulse program and a recycle delay of 20 s, without the use of water suppression. Eight 1 H spectra were acquired with the following recovery delay times (): 0, 0.2, 0.5, 0.75, 1, 2, 6 and 8 s. Phase correction of spectra 1–7 was performed relative to spectrum #8 ( = 8 s), baseline was set manually. Delay times () were plotted against peak integrals (It ); and exponential fitting was conducted following Eq. (1) and using the SigmaPlot 11.0 software. It = A + Be−/T1
(1)
where is a variable delay time, It = integral of the peak of interest recorded for eight spectra at variable delay time, A and B = constants, T1 = relaxation time (derived from exponential fitting). 2.3. Surface chemistry: Raman spectroscopy Samples prepared for Raman spectroscopy 1. 2.2 mg Suwanee River Fulvic Acid in 0.5 mL Milli-Q water 2. 4.1 mg/mL PVP-capped AgNPs in 0.5 mL Milli-Q water 3. 4.1 mg/mL PVP-capped AgNPs, 2.2 mg Suwanee River Fulvic Acid in 0.5 mL Milli-Q water Samples were placed in a 5 mm (o.d.) quartz tube and Raman spectra were acquired with a Nicolet Fourier transform Raman spectrometer with Nd:YAG laser (1064 cm−1 excitation wavelength) operating at a power level of 0.636 W. Raman scattering was recorded by an InGaAs detector (4000–100 cm−1 ) operated at a spectral resolution of 4 cm−1 . Spectra were summed for 500 scans and Fourier transformed without smoothing functions. 2.4. Adsorption dynamics: Quartz crystal microgravimetry (QCM) An E1 quartz crystal microbalance (Q-Sense, Gothenburg, Sweden) was used to characterize AgNP adsorption. Before and after each experiment, a standard cleaning procedure developed by Q-Sense was used to remove trace organics and to render the surface of sensor substrate sufficiently clean. The cleaning protocols involves soaking and rinsing the sensors with 2% sodium dodecyl sulfate solution and Milli-Q water, drying under nitrogen gas, and an UV–ozone treatment. After obtaining a stable baseline with background electrolyte solution, a 10 mg/L working suspension of
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AgNPs was pumped through the surface of a quartz crystal sensor (silica sensors were coated with NOM and dextran sulfate respectively). Changes in resonance frequency (f) and in resonance dissipation (D) were monitored over time as AgNPs adsorbed onto the surface, while constant and intermittent alternating voltages were applied to the quartz crystal. Seven harmonics were detected, leading to 14 simultaneous measurements of parameters (seven frequencies and seven dissipation values). With the best signalto-noise ratio, f and D obtained from the third overtone are presented in this paper. The extent of adsorption was determined after the sensor electrode was washed with background electrolyte solution and a new stable frequency reading was reached. Measured frequency shift is proportional to the mass deposited on the QCM sensor and the initial adsorption rate is determined by calculating the slope of the measured frequency shifts over the first 10 min of adsorption. The temperature of the solutions in the flow module was maintained at 25 ± 0.02 ◦ C for all experiments. The flow rate was kept at 100 L/min. The same procedure was repeated for all experimental conditions. 2.4.1. Substrate preparation Dextran is a branched polysaccharide made of glucose chains of varying lengths (from 3 to 2000 kilodaltons). The straight chain consists of ␣-1,6 glycosidic linkages between glucose molecules, while branches begin from ␣-1,3 linkages. Dextran sulfates (CAS NUMBER: 9011-18-1), which have hydroxyl groups of dextran substituted with sulfonate functional groups, were used in experiments and consisted of the sodium salt forms (Sigma Chem. Co., St. Louis, MO) and were derived from the bacterium Leuconostoc mesenteroides. Dextran sulfate contains approx.17% S, (i.e., approx. 2.3 sulfates per glucose). Silica (SiO2 ) quartz crystals (14 mm diameter) with a fundamental resonant frequency of 5 MHz (QSX 303, Q-Sense AB, Gothenburg, Sweden) were used as substrate for the immobilization of organics (NOM and dextran sulfate). The silica sensor was first coated with 0.1 mg/mL of poly-L-lysine (PLL) to establish a positively charged surface as an electrostatically favorable condition for later adsorption of organics. After deposition of the PLL layer, the sensor was rinsed with background electrolyte (10 mM NaNO3 ) and then exposed to one of the following: 10 mg/L of SRHA/SRFA or 5 mg/mL of dextran sulfate. After the adsorption of organics onto PLL-coated silica reached saturation, the substrate was again rinsed with the background electrolyte to remove any unbound organics prior to the introduction of AgNPs. 2.5. Substrate characterization The zeta potential of the substrate surface was determined by measuring the streaming potential using a SurPASS electrokinetic analyzer (Anton Paar GmbH, Graz, Austria). An adjustable gap cell customized to hold the QCM sensor substrates was used for sample mounting. The gap between the two sensors was adjusted to ∼120 m. Duplicate measurements were performed by pumping 10 mM NaNO3 (pH 6) electrolyte through the micro-channel with syringe pumps. A pressure ramp from 50 to 300 mbar in 15 mbar steps was employed to force the electrolyte solution through the channel with a flow rate between 150 and 200 mL/min. Measured streaming potentials were converted to zeta potentials using the Fairbrother–Mastin equation [16]. 3. Results and discussion 3.1. Spectroscopic evidences of AgNP– NOM interactions NMR is sensitive to both covalent and non-covalent interactions, making it feasible to determine the identity of the interacting
3
functional groups as well as the nature of the interactions. Regarding the latter, liquid-state 1 H NMR with proton spin-lattice relaxation measurements were used to identify changes in molecular dynamics (rotational and segmental motion) of NOM components upon interaction with AgNPs. Similarly, the localized surface enhancement of Raman scattering by the Ag metal surface provides information about which organic functional groups are closely associated with the AgNP surface. 3.1.1. Liquid-state 1 H NMR spectroscopy Liquid-state 1 H NMR provides structural information on NOM in aqueous solution. We used 1 H NMR to study the composition of NOM in D2 O before and during interaction with AgNPs. The interaction of AgNPs with NOM could occur through covalent or non-covalent bonding. The chemical shift of NMR frequencies provides information about the electron distribution surrounding the nucleus, and is most influenced by covalent bond formation, whereas spin-lattice relaxation times (T1 ) provide information about non-covalent interactions. The T1 value is characteristic of overall molecular motion, including translation, rotation, and vibration, and has been used to examine non-covalent interactions of contaminants with NOM [17]. The spin-lattice relaxation time (T1 ) of proton magnetic moments are strongly modulated when the timescale of molecular motions ( c ) correspond to the 1 H nuclear magnetic resonance frequency (ωo ). Efficient relaxation (short T1 ) occurs when 1/ c ≈ ωo . On the contrary, molecular reorientations that occur more rapidly (1/ c » ωo ) or more slowly (1/ c « ωo ) than the NMR frequency do not provide efficient interactions between proton magnetic moments, therefore, relaxation rates are slowed and T1 relaxation times are lengthened. Citrate T1 relaxation time. To first establish the viability of spin-lattice relaxation times as a sensitive means of detecting interactions between organic molecules and AgNPs, we compared the T1 of citrate in solution (without AgNPs) to the T1 of citrate when a fraction is bound as a “capping agent” on the AgNP surface. The citrate molecule, shown in Fig. 1, contains 3 carboxylate groups (a tridentate ligand) and 4 methylene 1 H atoms giving rise to 2 NMR signals (doublets) centered at 2.55 and 2.65 ppm (HA and HB , respectively). We used the combined magnetization of these four protons as a measure of the 1 H T1 relaxation time of citrate. Table 1 shows an increase in the average T1 relaxation time of the citrate protons in the presence of AgNPs. In aqueous solution, the relaxation of protons (A and B) in sodium citrate is relatively rapid (T1 = 739 ms). This may be attributed to several causes; molecular motion at rates approximating the NMR frequency (1/tc ≈ ωo ), or the paramagnetic effects of sodium ions. Polyacids like citrate are known to form oligomeric structures by self-association through extensive hydrogen bonding and ion bridging interactions. These self-associations and bridging with the paramagnetic ions, sodium, are likely to cause efficient dipolar interaction and rapid spin-lattice relaxation (short T1 ). Upon interaction with the surface of AgNPs, the relaxation of citrate protons is substantially slower (T1 = 964 ms).
B
H OH
+
Na
H
-
A
H
A
-
O
B
O
H
O
-
O
O O Na+
+
Na
Fig. 1. Na-citrate with protons used for T1 relaxation study shown in red.
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Table 1 Relaxation times of aliphatic proton of citrate. #
Sodium citrate (mM)
50 nm AgNP-citrate (ppm, Ag)
Additional coating
Relaxation time T1 a (s)
R2
1 2 3 4
0.4 0.4 0.4 0.4
0 200 200 200
0 0 FA HA
0.739 0.964 1.030 1.135
0.9997 0.9973 0.9942 0.9919
a
determined by fitting of inversion recovery data to Eq. (1).
Fig. 2. A schematic representation of the competition of HA and FA for the surface of the AgNPs, and the subsequent displacement of citrate ligands from the surface.
We attribute this exclusively to changes in the citrate molecular motion (1/ c » ωo or 1/ c « ωo ) because the concentration of sodium remained unchanged. Relaxation rates were progressively slowed upon addition of FA (T1 = 1.030 s) and HA (T1 = 1.135 s), respectively. One possible interpretation is that the increased T1 with added FA and HA is due to the formation of an organic layer on top of the citrate-capped AgNP’s resulting in a reduced rates of molecular reorientation and dipolar interaction of citrate (i.e., 1/ c « ωo ). An alternative interpretation of the increase in citrate T1 with the addition of FA and HA invokes an increase in molecular mobility of citrate due to displacement of citrate from the surface of AgNPs. The schematic (Fig. 2) depicts the competition of HA and FA for the surface of the AgNPs, and the subsequent displacement of citrate ligands from the surface. This interpretation reconciles the T1 data because the displacement of citrate from the NP surface and into the bulk solution would increase the rate of molecular motion (1/ c » ω) to the extent that relaxation rate is slower. Fulvic Acid T1 relaxation times. To further elucidate the nature of interactions between citrate and NOM at the AgNP surface, we measured T1 relaxation times of aliphatic protons in FA in the presence and absence of citrate-capped AgNPs. Our inversion recovery experiments indicated a substantial decrease (∼20 ms) in the T1 relaxation time of methylene (CH2 ) protons in FA after 24 h of interaction with citrate-capped AgNPs (Table S1). Meanwhile, AgNPs caused an increase (∼70 ms) in the methyl (CH3 ) protons T1 relaxation times. Taken together, these data may suggest that motions of methylene (CH2 ) groups near the carboxyl termini are slowed due to interaction of the carboxylate group with the AgNP, while the methyl termini of aliphatic FA are more mobile, perhaps because the methyl termini are exposed to the solution rather than interacting directly with the AgNP surface. A schematic interpretation of these data is shown in Fig. 3. Taken together, the 1 H relaxation time data suggest that the organic capping ligands on the surface of engineered AgNPs interact or react with and/or may be replaced by aqueous NOM.
3.1.2. Raman spectroscopy Raman spectroscopy is a powerful complement to NMR due to a highly selective enhancement of scattering by AgNPs [18]. We compared Raman spectra of pure PVP with that of PVP-capped AgNPs to determine how the PVP polymer is linked to the AgNP surface (Figures S2 and S3). In the PVP-capped AgNPs, we observed the disappearance of the PVP carbonyl (C O) stretches at 1311 cm−1 and 1667 cm−1 and the emergence of signals corresponding to N C and N C O vibrations (1597 and 2074 cm−1 , respectively). These results may indicate that PVP is attached to the surface of AgNPs through charge complexation or ester linkage to silver oxide. The association of PVP with the AgNP surface caused substantial changes in the relative intensity of signals in the Raman spectrum of PVP. We observed a relative enhancement of signals from less abundant C O and C N, likely due to electromagnetic plasmon resonance enhancement that occurs on the surface of AgNPs [19].
Fig. 3. Idealized fulvic acid molecule on the surface of a spherical nanoparticle (5 nm diameter, drawn to scale). Longer T1 values suggest greater molecular motion of shielded methylene (CH2 ) and terminal methyl (CH3 ) protons in fulvic acid, and increased motion of the terminal methyl (CH3 ) groups.
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Fig. 4. Raman spectroscopy of Suwanee River fulvic acid and silver nanoparticles in aqueous solution.
Fig. 5. Zeta potentials of nanoparticles and substrates measured in 10 mM NaNO3 (pH 6). Error bars represent the standard deviation of at least three measurements.
Raman spectroscopy can also provide molecular information about NP-NOM interactions. Fig. 4 shows the spectra of FA (gray curve), and PVP-capped NPs (dashed line). The FA generates a relatively featureless Raman spectrum dominated by broad signals tentatively assigned to C C, C H, and C C stretching vibrations, whereas the NP spectrum contains several sharp signals that can be assigned to the Raman-active vibrational modes of the PVP capping agent. The introduction of FA solution to the PVP-capped AgNPs (bold line in Fig. 4) generates notable changes in the PVP signals. The decreased intensity of signals at 690 cm−1 , 1465 cm−1 and 2074 cm−1 in Fig. 4 (which we attribute to the C C ring vibration, CH2, and N C O stretching, respectively) suggest that FA interacts with both the ring and polyvinyl domains of PVP and the oxygen atom involved in the PVP–NP complex. This result implies that FA may have displaced the PVP from the direct contact with the AgNPs where the signal is enhanced by the surface plasmon resonance [19]. Displacement of NP capping agents by FA may have a destabilizing effect on engineered nanoparticles that enter the aqueous environment, thus reducing their environmental mobility. Indeed, some precipitation of AgNPs was observed during our preliminary experiment. We also noted the appearance or enhancement of a small signal at 898 cm−1 , from C O C stretching vibrations in Fig. 4. This result implies that the AgNP surface interacts with O-alkyl groups in NOM, such as those found in sugars, polysaccharides, and ether lipids. To understand, more precisely, the exact cause of changes in Raman peak intensities will require further investigation with carefully controlled conditions in the solution. Nevertheless, these Raman spectroscopy conspire with the NMR data to suggest that FA can alter the surface composition, and perhaps the reactivity, of the NP surface. To quantify the effects of NOM on AgNP reactivity, Section 3.2 discusses the initial QCM measurements of AgNP adsorption at the solid–water interface.
QCM results demonstrated that AgNP adsorption is influenced by i) the composition of organics that were immobilized on the silica substrate and ii) its capping agents. Despite the presence of electrostatic repulsion between PVP-capped AgNPs and the HA/FA coated silica, substantial degree of adsorption (328–440 ng/cm2 ) occurred (Fig. 6). This underscored the importance of nonelectrostatic forces in controlling the sorption dynamics. There were observable differences in both the extent and kinetics of PVP-capped AgNP adsorption on substrates coated with HA/FA (Fig. 6) and dextran sulfate (as a model polysaccharide found in EPS) (Fig. 7). HA and FA behaved similarly in terms of their relative rate and capacity to adsorb PVP-capped AgNPs. However, the amounts of PVP-capped AgNPs deposited on HA/FA-coated substrate were about 4 times greater than dextran sulfate (Figs. 6 and 7). This difference in adsorption extent seems to suggest that NOM may serve as a bigger sink than polysaccharides in immobilizing AgNPs on environmental surfaces. PVP-capped AgNPs were depositing to dextran sulfate about 16 times faster and 43 times more than citrate-capped AgNPs (Fig. 7). This is probably due to the difference in properties of citrate and PVP. The adsorption behavior of citrate-capped AgNPs is expected to be primarily governed by the degree of protonation on citrate due to pH. The minimal adsorption of citrate-capped AgNPs seems to
3.2. NP adsorption on substrate coated with organics Prior to the adsorption experiments, zeta potentials of the AgNPs and the organic-coated silica substrates were determined (Fig. 5). The results from Figs. 6 and 7 were interpreted in the light of the charge characteristics of the AgNPs and the substrate surfaces. All surfaces were negatively charged in the study conditions. The coated silica substrates were more negatively charged than the AgNPs (Fig. 5). Size of AgNPs was monitored and no aggregation was observed throughout the adsorption experiments. The hydrodynamic diameters of citrate AgNPs and PVP-capped AgNPs were 54.8 ± 0.9 nm and 60.1 ± 0.5 nm, respectively.
Fig. 6. The rate (gray column) and extent (black column) of PVP-capped AgNPs adsorption on NOM-modified silica substrates. Error bars represent the upper range of duplicate measurements.
Please cite this article in press as: B.L.T. Lau, et al., A preliminary assessment of the interactions between the capping agents of silver nanoparticles and environmental organics, Colloids Surf. A: Physicochem. Eng. Aspects (2012), http://dx.doi.org/10.1016/j.colsurfa.2012.11.065
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Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/ j.colsurfa.2012.11.065. References
Fig. 7. The adsorption rate (gray column) and extent (black column) of citrateand PVP-capped AgNPs onto dextran sulfate-modified silica substrates. Error bars represent the upper range of duplicate measurements.
indicate that electrostatically flavorable condition is essential. On the other hand, hydrophobic/steric forces potentially play a more important role in yielding the observed adsorption of PVP-capped AgNPs.
4. Conclusions This paper illustrated how molecular spectroscopy and QCM could provide crucial fundamental information for investigating the relative importance of NOM/EPS in influencing environmental partitioning of AgNPs. The spectroscopy (NMR and Raman) data each suggest several modes of AgNP interaction with NOM. Each provides evidence for: i) displacement of the capping ligands, ii) alteration of the AgNP–NOM interface, and iii) alteration of the NOM–solution interface. QCM revealed that AgNP adsorption dynamics is a function of i) the composition of organics that were coated on silica and ii) its capping ligands. A more complete picture is needed to confirm our observations. Either spectroscopy or QCM alone is insufficient to reveal the complexities involved. To gain a better mechanistic understanding of the interactions, future research should couple surface-sensitive tools with bulk techniques to provide complementary lines of evidence. Our preliminary evaluation highlighted the critical role of surface modification of both the NPs and the bulk substrate by environmental organics in the stability and mobility of AgNPs. These initial findings will help improve future design of NPs to ensure successful targeted applications while keeping the NPs safe upon environmental release.
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Please cite this article in press as: B.L.T. Lau, et al., A preliminary assessment of the interactions between the capping agents of silver nanoparticles and environmental organics, Colloids Surf. A: Physicochem. Eng. Aspects (2012), http://dx.doi.org/10.1016/j.colsurfa.2012.11.065