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Correlation of the Physicochemical Properties of Natural Organic Matter Samples from Different Sources to Their Effects on Gold Nanoparticle Aggregation in Monovalent Electrolyte Stacey M. Louie,†,‡ Eleanor R. Spielman-Sun,⊥,¶ Mitchell J. Small,‡ Robert D. Tilton,†,§,∥ and Gregory V. Lowry*,†,‡,§ †

Center for the Environmental Implications of NanoTechnology (CEINT), ‡Department of Civil and Environmental Engineering, Department of Chemical Engineering, ∥Department of Biomedical Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213, United States ⊥ Department of Chemistry, Oberlin College, Oberlin, Ohio 44074, United States §

S Supporting Information *

ABSTRACT: Engineered nanoparticles (NPs) released into natural environments will interact with natural organic matter (NOM) or humic substances, which will change their fate and transport behavior. Quantitative predictions of the effects of NOM are difficult because of its heterogeneity and variability. Here, the effects of six types of NOM and molecular weight fractions of each on the aggregation of citrate-stabilized gold NPs are investigated. Correlations of NP aggregation rates with electrophoretic mobility and the molecular weight distribution and chemical attributes of NOM (including UV absorptivity or aromaticity, functional group content, and fluorescence) are assessed. In general, the >100 kg/ mol components provide better stability than lower molecular weight components for each type of NOM, and they contribute to the stabilizing effect of the unfractionated NOM even in small proportions. In many cases, unfractionated NOM provided better stability than its separated components, indicating a synergistic effect between the high and low molecular weight fractions for NP stabilization. Weight-averaged molecular weight was the best single explanatory variable for NP aggregation rates across all NOM types and molecular weight fractions. NP aggregation showed poorer correlation with UV absorptivity, but the exponential slope of the UV−vis absorbance spectrum was a better surrogate for molecular weight. Functional group data (including reduced sulfur and total nitrogen content) were explored as possible secondary parameters to explain the strong stabilizing effect of a low molecular weight Pony Lake fulvic acid sample to the gold NPs. These results can inform future correlations and measurement requirements to predict NP attachment in the presence of NOM.

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INTRODUCTION The environmental risk of engineered nanoparticles (NPs) will be determined by their physicochemical characteristics. In particular, the NP surface properties will determine the forces acting between NPs and other surfaces, and hence the NP’s attachment behavior (aggregation and deposition) and interactions with biological components. Upon release into natural environments, NPs interact with organic macromolecules, including natural organic matter (NOM), which is ubiquitous and tends to adsorb to solid surfaces. Prediction of the fate and toxicity of NPs will require a thorough understanding of their interactions with NOM and the resulting effects on the colloidal behavior of the NPs. The effect of NOM on the attachment behavior of NPs has been investigated in many studies. Typically, higher NOM concentrations result in better NP stability:1−8 increasing adsorbed mass of NOM on the NPs produces electrostatic repulsion (attributable to negatively charged carboxylate © XXXX American Chemical Society

groups) and electrosteric repulsion for high molecular weight components. Alternatively, NOM can destabilize NPs by charge neutralization2,9,10 (for positively charged NPs) or bridging in conjunction with divalent cations.11−17 Further studies are needed to attain a more quantitative and mechanistic understanding of the effects of NOM on NP attachment behavior. One significant problem is that NOM represents a “supermixture” of decay products;18 i.e., it is polydisperse in molecular weight (possibly forming aggregates19 with sizes up to hundreds of nanometers20) and contains a multiplicity of chemical components.18,21 Buffle et al. suggested that humic and fulvic substances stabilize colloids, whereas polysaccharides could enhance flocculation by Received: October 25, 2014 Revised: January 20, 2015 Accepted: January 22, 2015

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bridging.22 Higher molecular weight and more aromatic components often adsorb preferentially23−26 (with some exceptions27,28) and provide better stability against NP aggregation.29,30 Our group previously fractionated Suwannee River NOM (SRNOM) by molecular weight and investigated the effects on the homoaggregation of citrate-stabilized gold NPs.31 At high ionic strength, the >100 kg/mol components provided better NP stability than the 100 kg/ mol components) are similar, whereas Mw decreases significantly after the 100 kg/mol components are removed (SI Table S4). Even for Mg, the molecular weights determined by MALS here and elsewhere41 are significantly higher than those obtained in many38,42−50 (but not all51−53) studies. Regardless, the trends observed here among NOM sources follow expectations (higher molecular weights for humic acids and soil-derived NOM). Artifacts in the MALS results here include coelution of high and low molecular weight NOM (due to adsorption to the SEC column). Other methods are also subject to artifacts, e.g., imperfect representation by polymer standards,50,54 variation of SEC elution times with eluent composition,53 nonuniversal detection, and fragmentation/ multiple charging in mass spectrometry.55 Chemical Characterization of NOM. Fluorescence EEMs for all NOM fractions are shown in SI Figure S5. Within each sample, the fluorescence intensity at the “humic-like” peak for the retentate fraction is consistently lower than that of the filtrate or whole NOM. However, no trend of peak intensities with Mw is apparent across the six types of NOM. A secondary peak in the protein-like region is observed in some retentate fractions (SRNOM, ESFA, and PLFA); while we cannot definitively state the mechanism, it is noted that labile proteins and amino acids can be stabilized by association with humic aggregates,56 consistent with these results. The UV−vis absorbance spectra and slope coefficients, S, are presented in Figure 2. Comparisons within each NOM type and exponential fits are presented in SI Figure S6. SUVA at 254 nm (SUVA254) and 280 nm (SUVA280) and the estimated aromaticity (using various correlations42,57,58) are provided in SI Table S6. Comparing across the six unfractionated NOM isolates (Figure 2a), SUVA shows a positive correlation with Mg. However, comparing fractions within each NOM type (SI Figure S6), SUVA for the retentate fractions are similar to, or lower than in ESFA and PLFA, the whole or filtrate fractions. Therefore, SUVA and molecular weight are not always correlated and are not intrinsically related. The lower SUVA of the retentate fraction of ESFA and PLFA may indicate higher polysaccharide or aliphatic content as opposed to aromatic content.51

are not artifacts of possible modifications during the ultrafiltration process. Initial aggregation rates (kagg) were estimated and used for quantitative correlations of NP aggregation behavior to NOM properties. Ideally, kagg is taken as the linear slope (over several data points) of size versus time during doublet formation (until the hydrodynamic radius, Rh, reaches ∼1.3 times the initial size, Rh,0).17,39 However, particle aggregation here was often too rapid to capture this period, so kagg was computed between Rh,0 and the first measurement after NaCl addition. Uncertainties in measurement time and standard deviation in Rh over replicate measurements were propagated to estimate uncertainty in kagg. Characterization of NOM-Coated NPs. Electrophoretic mobility (EPM) was measured on 20 ppm cit-Au NPs coated in 10 ppm NOM (Zetasizer Nano ZSP, Malvern Instruments, Westborough, MA) at 40 V, averaging five replicate measurements. The background electrolyte was 20 mM NaCl with 1 mM NaHCO3 (pH 8.3) and was chosen (in contrast to 100 mM NaCl in the aggregation experiments) to reduce NP aggregation and electrode corrosion.40 Various treatments of the NP suspensions were tested (details in the SI, “Electrophoretic mobility...”): washed or unwashed of excess NOM, and with adsorption of NOM in 100 mM NaCl followed by washing and resuspension in 20 mM NaCl. All treatments yielded similar conclusions.

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RESULTS AND DISCUSSION Molecular Weight Distributions of NOM. SEC-MALS chromatograms and cumulative weight distributions for the whole NOM and the 100 kg/mol filtrate and retentate fractions are provided in SI Figures S1 and S2. Insufficient quantities of retentate were recovered for PLFA or POFA for SEC-MALS (however, sufficient PLFA retentate was recovered for other analyses and aggregation experiments). Molecular weight distributions and weight-averaged molecular weights (Mw) for the unfractionated NOM are summarized in Figure 1. The humic acids (PPHA and SRHA) have the highest molecular weights and a higher percentage of components with apparent Mw > 100 kg/mol. The marine fulvic acids (PLFA and POFA) have the lowest molecular weights, but contain a small amount of >100 kg/mol material. For all samples, SEC-MALS confirmed good separation at the 100 kg/mol cutoff by ultrafiltration. The fitted log-normal distributions accounting for the unanalyzed portion of the NOM are shown in SI Figures S2 D

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Figure 3. Intensity-averaged radius measured by time-resolved DLS for 20 ppm cit-Au NPs in 100 mM NaCl, in 1 mM NaHCO3 at pH 8.3 with 10 ppm PPHA (a), SRHA (b), ESFA (c), SRNOM (d), PLFA (e), and POFA (f), ordered from highest to lowest Mg. Error bars represent the standard deviation in the radius measured at each time point for duplicate or more runs. The retentate consistently produces the best NP stability against aggregation, whereas the filtrate produces the poorest NP stability. An exception is observed for PPHA (similar NP stability for all fractions). Insufficient retentate was collected for POFA to conduct an aggregation experiment, so it is not included. SRNOM (d) plot adapted with permission from Louie et al.31 Copyright 2013, American Chemical Society.

In contrast to SUVA, the slope coefficient, S, better correlates with the molecular weight difference between fractions: the retentate fraction consistently exhibits a lower S than the corresponding filtrate and whole NOM (SI Figure S6). S may be representative of the NOM composition (e.g., the ratio of unsubstituted aromatic groups and those with polar substitutions)59 or extended aromatic structures in higher molecular weight NOM.60 Implications for distinguishing SUVA, S, and molecular weight (which typically covary for bulk NOM) are further discussed later in this article. Effect of Molecular Weight Fractions on Nanoparticle Aggregation. Aggregation of the cit-Au NPs in the presence of 10 ppm NOM and 100 mM NaCl is shown in Figure 3. Similarly to results for SRNOM,31 the >100 kg/mol retentate

fractions of SRHA, ESFA, PLFA, and POFA provide better NP stability against aggregation than the filtrate fractions, and the whole NOM provides intermediate stability. An exception is observed for PPHA, for which all fractions stabilize the NPs against aggregation similarly well. No enhancement of NP aggregation by NOM was observed, although divalent or multivalent cations (e.g., Ca2+) that may induce bridging were not assessed. The observed behavior is attributable to enhanced steric repulsion imparted by the larger retentate components, which may form thicker layers or have a higher adsorption affinity.31 For PPHA, the similarity among fractions may be attributable to the relatively high molecular weight of all fractions, whereas E

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Figure 4. Intensity-averaged radius measured by time-resolved DLS for 20 ppm citrate-stabilized gold NPs in the presence of 1 ppm PPHA (a), SRHA (b), ESFA (c), PLFA (d), and POFA (e) in 100 mM NaCl, in 1 mM NaHCO3 at pH 8.3. NOM types are ordered from highest to lowest Mg (a−e) of the unfractionated NOM. Error bars represent the standard deviation for two or more runs. SRNOM was assessed in our previous study only at 10 ppm;31 additional data were not taken at 1 ppm in this study.

the other five samples showed greater differences in molecular weight between the filtrate and retentate fractions (Figure 1). The conclusion that adsorbed NOM inhibits aggregation by steric, rather than electrostatic, stabilization is supported by analysis of the EPMs of the NOM-coated NPs (SI Figure S7). Comparing across different NOM isolates (e.g., each unfractionated NOM), no correlation between EPM and aggregation behavior is observed (similar results have previously been found).4,7,11,61−63 The EPM data are also contradictory to an electrostatic repulsion mechanism. For example, the EPMs for all NOM-coated NPs are less negative than for the cit-Au NPs, but all NOM types reduced NP aggregation. Moreover, NPs coated with the retentate NOM have less negative EPMs than those coated with the whole or filtrate NOM. This result may be attributable to greater citrate displacement, lower charge density in the adsorbed layer due to

greater layer extension or lesser total charge, or a thicker or less permeable adsorbed layer64,65 (all consistent with a steric repulsion mechanism). Although we could not measure EPM in higher than 20 mM NaCl, simple electrostatic models (e.g., the Gouy−Chapman model, assuming constant surface charge density) suggest that trends should hold at higher ionic strength, and, furthermore, EPM will approach zero (making electrostatic effects even less important at 100 than 20 mM NaCl), as shown previously.12,62,63,66 A synergistic effect of the mixture of filtrate and retentate fractions (i.e., the unfractionated NOM), shown previously for SRNOM,31 is observed for all samples here except PPHA. Neither the filtrate nor retentate fraction alone (at their estimated concentrations in the unfractionated NOM) provides the full NP stabilization observed for the whole NOM, suggesting an interaction between the fractions (SI Figure F

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Figure 5. Correlation of gold NP initial aggregation rate with Mw directly measured by MALS (a); the geometric mean, Mg, of a fitted log-normal distribution of weight-averaged molecular weights (b); SUVA280 (c); the exponential slope coefficient of the UV−vis absorbance spectrum (d); reduced sulfur content (e), and a model including both Mw and reduced sulfur content (f). Correlations excluding PLFA data (black line) and including PLFA data (orange line) are assessed. Whole, retentate, and filtrate fractions of NOM are considered for all correlations except reduced sulfur content (only whole NOM). The dotted line in (f) represents a 1:1 correlation.

number concentrations of retentate NOM and NPs. The quantity of retentate present in 1 ppm unfractionated NOM (100 kg/mol NOM. Mw provided better correlation than Mg with the aggregation rates. In particular, it better captured differences between filtrate and whole fractions, e.g. in ESFA (blue triangles) and SRHA (purple squares), consistent with the qualitative observation that the retentate fraction contributed significantly in the unfractionated NOM to stabilize NPs against aggregation. This result suggests that number-averaged molecular weights or measurements from SEC without MALS (where the high molecular weight void peak can not be analyzed) may not fully represent the effects of a polydisperse NOM mixture on NP aggregation; weighting toward the high molecular weight NOM components may be necessary. Correlations between aggregation rates and UV−vis absorbance properties (SUVA280 and the exponential slope coefficient) were then assessed (Figure 5c and d). SUVA is more easily measured than molecular weight and would therefore be advantageous in practice, if the two properties predicted NP aggregation behavior equally well (e.g., due to covariance), as was found for unfractionated NOM.29 However, in this study, SUVA280 did not always correlate with molecular weight among the fractions: the retentate for ESFA and PLFA had lower SUVA280 than the corresponding filtrate or unfractionated NOM (SI Figure S6). Aggregation rates showed poor correlation with SUVA280 (R2 = 0.39), indicating that molecular weight was a better explanatory variable. This result suggests that the mechanism for NP stabilization was driven by molecular weight effects, rather than interactions with aromatic groups on NOM (represented by SUVA). Although SUVA280 yielded poorer correlation than Mw or Mg, the slope coefficient of the absorbance spectrum correlated better with molecular weight and thus yielded better correlation with aggregation rate (R2 = 0.74) than SUVA280. The slope coefficient requires no additional experimental effort to acquire compared to SUVA and may be of interest for future studies to parameterize correlations of NP attachment behavior with NOM properties. Ratios of absorbances at two wavelengths were also considered (SI Figure S13), but the slope coefficient incorporates more data and showed better correlation with aggregation rates. Correlations that include elemental or functional group composition (which may contribute to the effect of PLFA) were assessed, as shown in Figure 5 for reduced sulfur and SI Figure S14 for other moieties. Poor correlations (R2 < 0.5) were observed considering any functional group alone (Figures 5e and SI S14). Models including both molecular weight and chemical composition were then assessed (SI, “Comparison of two- and three-parameter correlations”). Models including reduced sulfur, total nitrogen, or total oxygen content produced the strongest improvements in the correlations (R2 = 0.87, 0.86, and 0.81, respectively, including PLFA data; Figures 5f and SI S14). However, Akaike Information Criteria indicate that these improvements are significant only when PLFA is considered, but not for the other five NOM types. In summary, chemical composition appears to be important for PLFA, but the importance of any specific moiety is inconclusive. Data on additional NOM isolates covering a broader range of chemistries are needed to better assess the role of functional group composition. It is also noted that the importance of each functional group may differ for other types of NPs depending on their affinity for the NP material, whereas H

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polydispersity71 or protein mixtures with varying numbers of components.72 Knowledge of the comprehensiveness of a NOM sample needed to adequately represent its effects on NP or colloidal behavior would inform improved design of future studies.

enhanced adsorption of higher molecular weight macromolecules is a universal phenomenon.67 Environmental Implications. The strong influence of high molecular weight NOM components observed previously for SRNOM31 was consistent across the other NOM samples (except PPHA): these components provide better NP stability than lower molecular weight components and can contribute significantly to the stabilizing effect of the unfractionated NOM. These results emphasize that molecular weight distributions (or weighting toward higher molecular weights) may be needed to predict NP attachment behavior in the presence of NOM. Weight-averaged molecular weight was a better predictor of aggregation rate than SUVA (or aromaticity) or EPM. The distinction between molecular weight and SUVA was not made previously29,30 because these properties covary for bulk samples. Here, the use of fractionated materials allowed for these properties to be distinguished. The UV−vis slope coefficient was also identified as a useful parameter to consider for future correlations. Finally, the PLFA results suggest that functional group composition can be important for some NOM samples. Additional studies are needed to assess the effects of pH and ionic strength and composition. The pH used here (8.3) is high but environmentally relevant.5 pH is expected to be relatively unimportant down to pH ∼ 6, considering the pKa of the primary charge-determining group (carboxylates). At lower ionic strength, electrostatic forces may dominate over steric forces, and molecular weight may have lesser importance; however, trends in attachment efficiency at high ionic strength typically hold at lower ionic strengths.11,12,62,68,69 High concentrations of divalent cations such as Ca 2+ may significantly change the trends observed here: bridging of NOM (and enhanced aggregation) has been observed,11−17 but not consistently6,63,66,68,69 (likely depending on Ca2+ concentration11,12 and NOM composition).62 For fractionated SRNOM, Yin et al. reported that high molecular weight fractions provided the best stability of Ag NPs in both monoand divalent electrolytes (up to 30 mM CaCl2).33 Shen et al. also showed that the critical coagulation concentration (CCC) of fullerene in NaCl, CaCl2, and MgCl2 increased monotonically with the molecular weight of SRNOM fractions; however, the >30 kg/mol fractions enhanced aggregation in Ca2+ concentrations higher than the CCC.34 Further work is needed to assess the role of Ca2+ with respect to the composition and heterogeneity of NOM. Overall, the results of this study suggest that NOM collection and sample preparation processes (e.g., filtration) can significantly affect its interactions with NPs and effects on NP behavior. In the environmental nanotechnology community, the importance of characterization and sample preparation for the NPs has often been discussed. We suggest that more thorough characterization and reporting for NOM are also needed. More broadly, the results imply that extrapolation of the results of laboratory studies to real systems may not be accurate. Laboratory NOM samples are often prefractionated (e.g., via extraction from the source and further fractionation into humic or fulvic acids). However, real systems contain a “supermixture” of humic and fulvic acids and other components,18 which may not be adequately represented by prefractionated NOM. This point was emphasized in a review of general environmental studies using NOM70 and is supported by the results of this study, as well as studies on NP aggregation when coated with synthetic polymers of varying

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ASSOCIATED CONTENT

S Supporting Information *

Detailed methods and additional NOM characterization, EPMs, aggregation data, and quantitative correlations. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (412) 268-2948. Fax: (412) 268-7813. E-mail: [email protected]. Address: 5000 Forbes Ave., 119 Porter Hall, Pittsburgh, PA 15213. Present Address ¶

Civil & Environmental Engineering and Center for Environmental Implications of Nanotechnology, Carnegie Mellon University.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. George Aiken for providing the POFA sample, Chuanjia Jiang and Prof. Heileen Hsu-Kim for preparation of the stock POFA, Dr. Stella Marinakos for synthesis and TEM characterization of the cit-Au NPs, and three anonymous reviewers for helpful comments. This research was funded by the National Science Foundation Center for Environmental Implications of Nanotechnology (CEINT) (EF-0830093 and EF-1266252) and U.S. EPA STAR Graduate Fellowship Assistance Agreement FP-91714101. Additional funding was provided by the Prem Narain Srivastava Legacy Fellowship, Carnegie Institute of Technology Dean’s Fellowship, and the Steinbrenner Institute for Environmental Education and Research through the Jared and Maureen Cohon Graduate Fellowship. Funding for E.S−S. was through an REU with CEINT (EEC-1004985).

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