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Multiwalled Carbon Nanotubes Dispersion Methods Affect Their Aggregation, Deposition, and Biomarker Response Xiaojun Chang, W. Matthew Henderson, and Dermont C. Bouchard Environ. Sci. Technol., Just Accepted Manuscript • Publication Date (Web): 29 Apr 2015 Downloaded from http://pubs.acs.org on April 29, 2015

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Environmental Science & Technology

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Multiwalled Carbon Nanotubes Dispersion Methods

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Affect Their Aggregation, Deposition, and

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Biomarker Response

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Xiaojun Chang,a W. Matthew Henderson,b and Dermont C. Bouchardb,* a

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b

National Research Council Research Associate

USEPA Office of Research and Development, National Exposure Research Laboratory, 960

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College Station Road, Athens, GA 30605, USA

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*Corresponding author E-mail: [email protected]; 706-355-8333

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Total Word Length: 5267 + 6 figures*300 = 7067 words (not including references)

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1

ACS Paragon Plus Environment


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Abstract

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To systematically evaluate how dispersion methods affect multi-walled carbon nanotubes

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(MWNTs) environmental behaviors, MWNTs were dispersed in various solutions [e.g.,

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surfactants, natural organic matter (NOM), and etc.] via ultrasonication (SON) and long-term

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stirring (LT). The two tested surfactants [anionic sodium dodecyl sulfate (SDS) and nonionic

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poly(ethylene glycol)-poly(propylene glycol)-poly(ethylene glycol) (PEO-PPO-PEO) triblock

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copolymers (Pluronic)] could only disperse MWNTs via ultrasonication; while stable aqueous

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SON/MWNT and LT/MWNTs suspensions were formed in the presence of the two model

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NOMs [Suwannee river humic acid (SRHA) and fulvic acid (SRFA)]. Due to the inherent

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stochastic nature for both methods, the formed MWNTs suspensions were highly heterogeneous.

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Their physicochemical properties, including surface charge, size, and morphology, greatly

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depended upon the dispersant type and concentration but were not very sensitive to the

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preparation methods. Aggregation and deposition behaviors of the dispersed MWNTs were

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controlled by van der Waal and electrostatic forces, as well as other non-DLVO forces (e.g.,

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steric, hydrophobic forces, etc.). Unlike the preparation method-independent physicochemical

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properties, LT/NOM-MWNTs and SON/NOM-MWNTs differed in their fathead minnow

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epithelial cell metabolomics profiles.

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1. Introduction

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Since their discovery in 1991,1 carbon nanotubes (CNTs) have attracted attention due to

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exceptional properties,2,3 but their increasing production and extensive applications 4,5 may result

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in accidental and intentional releases to the environment.6,7 To investigate transport, fate, and

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impact of CNTs in the environment, a number of studies on their dispersal in various aqueous

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matrices have been conducted.8-13

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Unlike conventional chemicals, the environmental behaviors of nanoparticles are closely

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related to how they are dispersed in environmental matrices.14 Previous studies have shown that

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properties of fullerene C60 nanoparticle suspensions (nC60) are greatly affected by preparation

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methods.15-18 The transport and fate of nanomaterials are determined by their physicochemical

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properties which, in turn, are affected by dispersion methodology and dispersant species. Due to

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their strong van der Waals interaction energies of ~ 500 eV/µm of tube-tube contact,19 CNTs

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cannot be readily dispersed in water as de-bundled individual nanotubes. Other than

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functionalization via covalent bonding,20-23 in which CNTs are chemically changed, the primary

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approach for dispersing them in the aqueous phase is through ultrasonication in the presence of

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stabilizing agents such as surfactants13,24-27 and natural organic matter15,26,28,29. The appearance of

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reactive oxygen species (ROS),30 surfactant degradation,30 and nanoparticle oxidation31 usually

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occurs during ultrasonication due to the introduction of high local temperature and pressure.

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Compared to high energy ultrasonication, the milder process of extended mixing CNTs in water

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is a better mimic of processes nanoparticles undergo after release to the aquatic environment.

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Ultrasonication is often chosen over extended mixing in studies on nanoparticle toxicity,

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transport, and fate, however, since it is faster and relatively more controllable. Implicit in this

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approach is the assumption that physicochemical properties and toxicities of the ultrasonicated

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nanoparticle suspension are the same as those produced via extended mixing. However,

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ultrasonication has been reported to be able to introduce the formation of O-containing

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functional groups and the disappearance of –CHn groups in the basic CNTs structure.32

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Studies33,34 have also shown that toxicities of ultrasonicated CNTs were different from those

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dispersed via stirring. In addition, various stabilities and mobilities in porous media have been

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observed for CNTs in the presence of different dispersants through classic column studies.35,36

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To our best knowledge, no study has experimentally verified the aforementioned assumption

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regarding key stability properties and their biological effects governing MWNTs’ behavior in the

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environment.

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The purpose of this study is to systematically evaluate effects of the two dispersion methods

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(ultrasonication and long-term stirring) and various dispersants [natural water, small molecular

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weight organic acids, surfactants, and natural organic matter (NOM)] on MWNTs’

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environmental behavior. MWNTs suspension properties including mass concentration, surface

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charge, morphology, size, and size distribution were characterized with a range of instruments.

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Dispersion methods effects on MWNT aggregation and deposition, and on exposure biomarkers

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(fathead minnow cell cultures), were also evaluated.

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2. Materials and Methods

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2.1 Materials. MWNTs with purity of 95 wt% were purchased from CheapTubes Inc.

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(Brattleboro, VT); reported outside diameters and lengths are 20-30 nm and 10-30 µm,

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respectively. Analytical grade sodium dodecyl sulfate (SDS) and sodium citrate (Na3Cit) were

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purchased from Thermo-Fisher (Fremont, CA). Analytical grade sodium acetate (NaAce) was

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purchased from J. T. Baker (Center Valley, PA). Poly(ethylene glycol)-poly(propylene glycol)-

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poly(ethylene glycol) (PEO-PPO-PEO) triblock copolymers (poloxamers, or Pluronic®, PF) were

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purchased from Sigma Aldrich (St. Louis, MO). Suwanee river humic acid (SRHA, Standard II)

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and Suwannee river fulvic acid (SRFA, Standard I), purchased from the International Humic

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Substances Society (St. Paul, MN), were chosen as model NOMs. The deionized (DI) water used

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has a resistivity of 18.2 MΩ·cm and was obtained from an Aqua Solutions Type I Water

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Purification System. The natural surface water used was collected from a tributary of Calls

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Creek, a small stream near Athens, GA.26 Cell culture reagents were purchased from American

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Type Culture Collection (ATCC, Manassas, VA) and the assays for cytotoxicity were obtained

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from Promega Corporation (Madison, WI). All analytical consumables were purchased from

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Agilent Technologies (Santa Clara, CA).

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2.2 Suspension Preparation. MWNTs suspensions were produced via extended mixing (i.e.,

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long-term magnetic stirring) or ultrasonication. Here we refer to MWNTs suspensions produced

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by extended mixing as XX mg/L LT/YY-MWNTs, where XX indicates concentration of the

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dispersant and YY indicates type of dispersant. To distinguish suspensions produced by extended

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mixing, suspensions produced by ultrasonication are termed XX mg/L SON/YY-MWNTs. 4


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In the extended mixing method, purchased MWNTs were mixed with aqueous solutions of

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different dispersants (DI, Calls Creek water, SDS, PF, SRHA, SRFA, etc.) with an initial

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[MWNTs] of 100 mg/L. Mixtures were then magnetically stirred at a rate of 400 rpm under

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ambient conditions at room temperature in the dark; stirring stopped after more than 200 days.

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To prepare SON/MWNTs suspensions, a 40 mL mixture of MWNTs and dispersant solution

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(initial [MWNT] = 100 mg/L) was ultrasonicated with a probe sonicator (Sonic & Materials,

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Newton, CT) in an ice-water bath for 10 min at an average energy level of ~33 W. Mixtures

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obtained from long-term magnetic stirring and ultrasonication were gravitationally settled for 30

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days and the stable supernatants were removed and used as stock MWNTs suspensions for

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further studies. The possible microbial growth in the prepared suspensions were tested by

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culturing suspensions in the non-selective media, and no microbial growth was observed (Details

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are presented in the Supporting Information.)

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2.3 Characterization.

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2.3.1 General Physicochemical Characterization. MWNTs’ concentration in suspension was

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determined using a pre-determined calibration curve37 and UV/Vis absorbance (measured by an

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Enspire Multimode Reader 2300, PerkinElmer, MA) at 500 nm, with background correction for

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the corresponding dispersant solutions. The intensity averaged hydrodynamic diameter (Dh) and

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polydispersity index (PDI) of the MWNTs suspensions were measured by dynamic light

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scattering (DLS) using a Nano ZetaSizer (Malvern Instruments, Worcestershire, UK) with a

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helium/neon laser (λ = 633 nm). Using the same Nano ZetaSizer, the electrophoretic mobility

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(EPM) of MWNTs suspensions was determined by phase analysis light-scattering. MWNTs

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suspensions were drop cast on aluminum oxide surfaces (QSX 303, Biolin Scientific, MD), dried

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overnight, and their morphologies characterized using a Bruker MultiMode 8 Atomic Force

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Microscope (AFM) with a Nanoscope V controller and a J-Scanner. Images were taken under the

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ScanAsyst-Air mode using ScanAsyst-Air probes at the speed of 1.0 Hz and resolution of 512 ×

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512 pixels.

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2.3.2 Size Distribution Determination by Asymmetric Flow Field Flow Fractionation (AF4).

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An AF2000 Focus AF4 (Postnova, Salt Lake City, UT) was utilized to fractionate MWNTs

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based on hydrodynamic size (diffusion coefficient).38,39 The trapezoidal AF4 channel (27.5 cm

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from tip to tip) has a tapered inlet and outlet with lengths of 4 and 1 cm, respectively. A 350 µm

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spacer, 10 kDa regenerated cellulose membrane, and 500 µL injection loop accomplished the

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fractionation. A full AF4 fractionation program includes injection and focusing, elution, and

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rinsing. Injection is a 5 min period with a tip flow rate of 0.3 mL/min. In the elution phase, the

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cross flow rate was set as 1.0 mL/min, while the focusing flow [which equals the sum of cross

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flow and detector flow (0.75 mL/min) less tip flow] was set by AF4 software. In the rinsing

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phase, tip flow equaled detector flow at 1.0 mL/min, while cross flow was 0.0 mL/min.

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Absorbance at 254 nm and Dh of the fractionated effluents were sequentially measured by the

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coupled UV-Vis detector (PN 3241, Postnova) and DLS detector (Nano ZetaSizer, Malvern).

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Four polystyrene nanosphereTM (PS-NP) size standards (NIST-traceable materials, Thermo-

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Fisher, Fremont, CA), with certified diameters of 30 ± 1, 59 ± 2, 147 ± 3, and 296 ± 6 nm, were

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standards for testing the relationship between elution time t and particle hydrodynamic size Dh

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(Figure S1A) using the above-described fractionation program. As shown in Figure S1, its t

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increases with its Dh for a homogeneous standard. Using this relationship, a UV/Vis absorbance-

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weighted size distribution curve for a heterogeneous suspension can be derived from the UV-t

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profile obtained from the AF4/UV-Vis.

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2.2.3 Electrokinetic, Aggregation, and Deposition Properties. Effects of ionic strength (IS) on

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MWNTs surface charges were investigated by measuring MWNTs EPMs in NaCl solutions of

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varying IS; MWNTs aggregation rates were determined by Time-Resolved DLS. Experimental

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details can be found in our previous study.40 The deposition of MWNTs on silica dioxide

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surfaces (QSX 303, Biolin Scientific, MD) was studied using a quartz crystal microbalance with

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dissipation monitoring (QCM-D, Q-Sense E4, Västra Frölunda, Sweden). Experimental details

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are provided in Supporting Information.

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2.2.4 Exposure Biomarkers. All fathead minnow cells (ATCC® CCL-42) were incubated and

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maintained at 28ºC in T25 culture flasks and transferred to 24-well plates for exposure studies.

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MWNTs suspensions at a concentration of 300 ng/mL, dispersant-only solutions, or deionized

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water were added to the passaged cells following a 24-h acclimation period. After a 24-h

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exposure, cells were quenched and suspended in 80% methanol after being washed twice with

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phosphate buffered saline (PBS). The cell-methanol suspensions were stored at ≤ -20ºC prior to

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lyophilization. The metabolites were extracted and analyzed by gas chromatography, coupled

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with time of flight mass spectrometry (GC/ToF-MS). The acquired data were analyzed with

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partial least square discriminant analysis (PLS-DA), using commercially available software.

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Detailed description of cell culture, exposure, instrumental analysis, and data analyses can be

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found in previous studies.41,42

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3. Results and Discussion

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3.1 Physicochemical Properties

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3.1.1 Dispersant Efficacy. MWNTs were dispersed in deionized water (DI), Call’s Creek

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water, solutions of organic acids salts with short carbon chains (Na3Cit and NaAce), surfactants

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(SDS and PF), and natural organic matter (SRHA and SRFA) via long-term magnetic stirring or

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ultrasonication. For stirred samples, the suspended concentrations of MWNTs in DI, Call’s

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Creek water, Na3Cit and NaAce solutions (1-200 mg/L) were below the detection limit of UV-

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Vis measurement (0.15 mg/L) after more than 200 days of stirring. Likewise, ultrasonicated

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MWNTs samples in the same background solutions rapidly settled out from suspension, and no

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MWNTs were detected by UV-Vis after the 24-h gravitational settling period. The lack of

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stability of dispersed MWNTs in these solutions indicates that pure water, natural surface water

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with low dissolved organic carbon (DOC) concentration,26 and solutions of small molecular

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weight organic acids are unable to disperse and stabilize MWNTs in the aqueous phase.

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The two commercial surfactants tested were also ineffective in dispersing MWNTs via long-

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term stirring since no MWNTs were detected in solutions with 1-20 mg/L of SDS, or in solutions

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with 1-10 mg/L of the non-ionic surfactant PF. A very low MWNT concentration (< 0.5 mg/L)

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was observed in 20 mg/L PF solution after 200-day stirring. Stable MWNTs suspensions were

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obtained in both surfactant solutions with 5-20 mg/L dispersant via ultrasonication. Moreover,

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[MWNTs] for these SON/MWNTs were independent of surfactant concentrations ([MWNTs] ≈

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60 mg/L, (Figure 1A)).

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Natural organic matter usually carries a negative charge due to the carboxylic and phenolic

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moieties in its structure.43 Several previous studies33,44,45 investigated NOM’s capability of

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facilitating MWNTs dispersion in aqueous media, however, their time scales were relatively

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short. In the present study, SRHA and SRFA were very effective in MWNTs dispersion: up to

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~60 mg/L MWNTs were suspended in solutions with [SRHA] and [SRFA] as low as 1 mg/L

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(total organic concentration, Figure 1A) by ultrasonication. Stable LT/MWNTs suspensions were

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also obtained through extended mixing: the final [MWNTs] after 200 days of stirring increased 7



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with dispersant concentration, reaching ~30 mg/L in the presence of 20 mg/L NOM (Figure 1D).

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NOM’s greater efficacy in stabilizing MWNTs is likely due to the existence of aromatic fractions

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in their molecular structures which is absent in both aliphatic SDS and PF.45 The ability of NOM

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to disperse MWNTs at relatively high concentrations, even with low-energy mixing, is

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environmentally significant since NOM is ubiquitous in the aquatic environment.

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Figure 1 Concentration, surface charge, and size of MWNTs prepared via ultrasonication (top row) and long-term stirring (bottom row) as a function of dispersant concentration. (Note: The concentration of SRHA and SRFA were determined as total organic carbon concentration.)

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3.1.2 Electrophoretic Mobility. Dispersants also affect the physicochemical properties of

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MWNTs suspensions. The EPMs of SON/MWNTs for all four tested solutions (Figure 1B) and

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LT/MWNTs in two NOM solutions (Figure 1E) decreased with dispersant concentration up to 10

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mg/L, showing that high [dispersant]/[MWNTs] ratios facilitate formation of stable suspensions.

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The ultrasonication process likely results in some oxidation of both MWNTs and dispersant,18

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therefore MWNTs dispersed in the non-ionic surfactant Pluronic were slightly charged.

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SON/MWNTs dispersed in the anionic surfactant SDS are more negatively charged than the

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SON/PF-MWNTs. EPM values are similar for SON/MWNTs dispersed in the two tested NOMs

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and SDS solutions (SON/SDS-MWNTs ≈ SON/SRHA-MWNTs > SON/SRFA-MWNTs, Figure 8


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1B). The observation that SRFA-MWNTs are more negatively charged than SRHA-MWNTs in

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both ultrasonicated and long-term stirred samples is consistent with the fact that SRFA is more

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negatively charged than SRHA.46

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3.1.3 Morphology, Size, and Size Distribution.

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Morphology. Representative AFM images of 20 mg/L LT/SRHA-MWNTs and SON/SRHA-

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MWNTs are shown in Figure S2. As determined by AFM measurements, the diameters for both

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LT/SRHA-MWNTs and SON/SRHA-MWNTs are ~30 nm which is consistent with the

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manufacturer’s reported diameters for the pristine materials. We note that MWNTs in both

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samples had similar lengths (~100 nm to ~1 µm) which were much shorter than the

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manufacturer’s reported lengths (10 to 30 µm). Kennedy et al.33 reported that ultrasonicated

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MWNTs were more fragmented than MWNTs that were magnetically stirred for seven days. As

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observed here, similarity in lengths of MWNTs dispersed via different methodologies indicates

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that mild shear forces of long-term magnetic stirring are as effective at de-bundling pristine

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MWNTs into shorter individual nanotubes as the more intense short-term ultrasonication. .

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Hydrodynamic Diameter. In all four dispersants, average hydrodynamic diameter (Dh) of

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SON/MWNTs decreased as dispersant concentration increased from 1 mg/L to 20 mg/L. This

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decrease corresponds to the increasing [dispersant]/[MWNTs] ratio and the resultant more

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negative surface charge (Figure 1B). The extent of Dh decrease varied (Figure 1C) with

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dispersant species: slight decreases in Dh were observed in the presence of SDS (from 215 nm to

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183 nm) and the two NOMs (from ~210 to ~140 nm), while the greatest decrease (from 1460 nm

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to 167 nm) was observed in PF. The low [MWNTs], less negative EPM, and significantly large

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Dh for 1 mg/L SON/PF-MWNTs collectively suggest that this low PF concentration is unable to

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provide sufficient steric stabilization to prevent the SON/MWNTs from aggregating and settling.

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Ten and 20 mg/L SON/PF-MWNTs, however, had similar Dh values to those dispersed in other

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dispersants.

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The LT/MWNTs at the lowest NOM concentration (1 mg/L) yielded very large (> 500 nm)

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Dh values (Figure 1E), which correspond to less negative EPM measurements (Figure 1F) and

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result in low concentrations (Figure 1D). Similarly to a previous study,33 as NOM concentration

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increased (≥ 5 mg/L), the MWNTs dispersed via long-term stirring developed more negatively-

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charged surfaces (< -3 × 10-8 m2/V-s) and smaller sizes (~150 nm, size decrease of ~10 nm with 9



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[NOM] from 5 to 20 mg/L), and thus more stable LT/MWNTs suspensions with higher mass

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concentrations (Figure 1D).

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Size Distribution. Due to the inherent stochastic nature of the top-down dispersion processes,

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it is expected that highly polydisperse MWNTs suspensions would form through ultrasonication

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and extended mixing. The high polydispersity indices (PDIs) around 0.25, shown in Table S2,

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and broad size distribution (from less than< 30 nm to 1 micron (Figure S3)) confirm the high size

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heterogeneity in suspensions formed by either technique. Counter-intuitively, size (Table S2) and

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size distribution (Figures S3A, S3C, S3E, S3G, and S3I) measurements by batch DLS showed

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that LT/MWNTs have slightly smaller average size and size distribution regions than

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SON/MWNTs prepared by the much more energetic ultrasonication process. Although previous

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studies have shown that ultrasonication is more effective in reducing nanoparticle size than

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magnetic stirring,31 stirring time periods were fairly short (e.g., 300 min31). According to the

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present study’s observed decrease in size with stirring time which is likely the most important

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factor determining particle size, long-term stirring is as effective as short-term ultrasonication in

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reducing MWNT size. Also, Raman spectra for the SON/SRHA-MWNTs and LT/SRHA-

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MWNTs (Figure S4) show no difference (G/D ratios are 0.87 ± 0.04 and 0.85 ± 0.06 for

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SON/SRHA-MWNTs and LT/SRHA-MWNTs, respectively), indicating that MWNT oxidation

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by long-term stirring or 10-min of ultrasonication is comparable.

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For a highly heterogeneous suspension, the intensity-weighted hydrodynamic diameter is a

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general index of the whole particle population rather than an accurate measurement of individual

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particles. The DLS technique is not ideal for measuring particle size for non-spherical particles

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in suspensions with a range of particle sizes since DLS theory assumes the measured particles

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are monodisperse spheres.14 And since the light-scattering intensity of a particle scales to the

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sixth power of its diameter,47 DLS measurement tends to overestimate large particles in a

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heterogeneous suspension.

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To mitigate occlusion of smaller particles in batch DLS measurements, asymmetric flow

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field-flow fractionation (AF4) was employed to fractionate the highly polydisperse MWNTs

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suspensions. The separation is performed in a thin channel with a laminar flow, which has a

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parabolic flow profile. An applied cross flow, which is perpendicular to the longitudinal laminar

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flow, force particles to shift towards the permeable accumulation wall. Particles will re-distribute 10


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in the channel due to Brownian motion: smaller particles with larger diffusion coefficients

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migrate farther away from the accumulation wall,

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particle retention time is therefore a function of diffusion coefficent and the separation is thus

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achieved.38,39,48 The UV/Vis-t profiles of the 10 mg/L SRHA-MWNTs suspensions (Figure S5)

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and their scattered light intensity-t profile (Figure 2B) show that the ultrasonicated sample was

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eluted from AF4 earlier than the long-term stirred sample. Using the relationship between elution

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time and particle size established from polystyrene nanosphere standards (Figures 2A and S1),

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the UV/vis-t profile (Figure S5) was transformed to UV/vis absorbance-weighted size

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distribution profiles. The normalized UV-size profiles derived (Figure 2C) clearly show the 10

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mg/L SON/SRHA-MWNTs had a narrower and smaller size distribution than the 10 mg/L

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LT/SRHA-MWNTs which is contrary to batch DLS measurement results (Figure 2D). The AF4-

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UV/Vis-DLS fractionation-sizing experiments were conducted for all MWNT suspensions listed

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in Table S2. Differences between these measurements and batch DLS measurements were

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observed for all five groups (Figure S3). The discrepancy is likely explained by different

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mechanisms of AF4-UV/Vis and DLS measurements. Mie theory predicts a linear relationship

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between nanoparticle volume and extinction coefficient.49 Previous studies found that UV/Vis

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extinction coefficients of various spherical and non-spherical nanoparticles follow a power law

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with exponents between 2-3.50-55 This relationship indicates that size distribution curves

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determined by AF4-UV/Vis-size profiles will still overestimate larger particles, however, they

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are more sensitive to smaller particles than the batch DLS measurement in which scattered light

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intensity is proportional to the sixth power of particle size. The suspensions having similar batch

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DLS-determined average hydrodynamic diameters therefore had substantially different size

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distributions measured by AF4-UV/Vis. These results suggest that determining particle size and

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size distribution for a heterogeneous suspension is highly dependent upon the measurement

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method, and that comparing particle size and heterogeneity of different suspensions must be

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done carefully.

39

where the transport velocity is higher; the

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Figure 2 (A) The chromatographs of polystyrene standards obtained by AF4; (B) Scattered light intensity as a function of elution time; (C) normalized UV/vis absorbance weighted size distribution curves; and (D) scattered light intensity weighted size distribution curves of the AF4 fractionated 10 mg/L SRHA-MWNTs solution.

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3.2 Environmental Behaviors.

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3.2.1 EPM Changes with Ionic Strength. The EPM of MWNTs as a function of [NaCl] is

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shown in Figure 3A. Generally, all EPM values of tested MWNTs became less negative with

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increasing IS, due to the screening effect of sodium ion (Na+) commonly seen in colloidal

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suspensions.40,56 The SON/PF-MWNTs were the least negatively charged in the absence of NaCl

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[(-1.558 ± 0.231) × 10-8 m2/V-s] and their EPMs were very close to zero when [NaCl] ≥ 5 mM;

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the EPMs of SON/SDS-MWNTs and SON/SRHA-MWNTs gradually reached the range of (-1~-

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2) × 10-8 m2/V-s as [NaCl] increased from 0 to 40 mM; and the EPMs of SON/SRFA-MWNTs

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changed the least in the tested IS range. For each dispersant type, there was no substantial

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difference in EPM changes among suspensions with various concentrations (Figure S6) which

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suggests that dispersant type is the key factor dictating MWNTs surface charge in aqueous

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solutions.

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Changes in EPM of two LT/MWNTs suspensions were also measured. Interestingly, there is

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no difference in EPM between the LT/SRHA-MWNTs and SON/SRHA-MWNTs, however, the

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LT/SRHA-MWNTs is more sensitive to IS increase than SON/SRFA-MWNTs. This could be

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caused by the different structures of SRHA and SRFA or by the transformation of SRHA and

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SRFA from ultrasonication.

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Figure 3 Environmental behaviors comparison between MWNTs suspension in the presence of SDS, PF, SRHA, and SRFA dispersants (20 mg/L dispersant). (A) Ionic strength effects on surface charge; (B) aggregation attachment efficiency (α αA) as a function of [NaCl]; and (C) QCM-D deposition profile at [NaCl] = 20 mM on silica surface.

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3.2.2 Aggregation. The aggregation attachment efficiency (αA) was obtained from initial

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aggregation rates determined by time-resolved DLS.40,57 The αA and critical coagulation

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concentrations (CCCs) obtained from αA profiles were used to quantitatively describe

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aggregation in different MWNTs suspensions. No size increase was observed for all SON/PF-

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MWNTs at all [NaCl] levels (as high as 1 M), showing that MWNTs dispersed in pluronic

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solutions did not aggregate despite their almost neutral surface charges. Lack of aggregation of

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the weakly charged SON/PF-MWNTs demonstrates that the non-ionic surfactant PF is an

317

effective dispersion agent that stabilizes MWNTs mainly through steric effects. SON/SDS-

318

MWNTs started aggregating at a low IS level ([NaCl] = 10 mM, Figure S7A) which implies that

319

the steric component of the SDS electrosteric stabilization of MWNTs is not as profound as that

320

of the Pluronic co-block polymer and that electrostatic forces are the major stabilizing forces for

321

SON/SDS-MWNTs. SON/SDS-MWNTs’ αA values increased with IS in the range of 10-100

322

mM, reaching 1.0 (favorable, diffusion-controlled aggregation) as IS increased further. Like IS

323

effects on their surface charges, all SON/SDS-MWNTs exhibited very similar aggregation

324

behaviors, and had the same critical coagulation concentrations (CCC) at ~100 mM NaCl which

325

is higher than the CCC of MWNTs previously dispersed in DI.56

326

SON/SRHA-MWNTs were more stable than SON/SDS-MWNTs (Figure S7B). Although

327

their EPM values both became much less negative (≥ -1× 10-8 m2/V-s) when IS ≥ 20 mM (Figure

328

S6C), all SON/SRHA-MWNTs did not start aggregating until IS ≥ 60 mM which indicates the

329

importance of steric effects in SRHA stabilization of MWNTs suspensions. In the range of 60

330

mM ≤ IS ≤ 120 mM, 20 mg/L SON/SRHA-MWNTs were slightly more stable than those

331

dispersed in solutions with lower SRHA concentrations, however, this difference disappeared

332

with increasing [NaCl] and all CCCs were ~ 300 mM NaCl. Aggregation behaviors of

333

SON/SRFA-MWNTs showed clearer dependence upon dispersant concentration (Figure S7C).

334

MWNTs dispersed in solutions with lower SRFA concentrations started aggregating at low IS

335

levels (~20 mM NaCl), while those dispersed in solutions with higher [SRFA] levels started

336

aggregating when IS exceeded 50 mM. The CCCs of 1 and 5 mg/L SON/SRFA-MWNTs were

337

200 mM, while the CCCs of 10 and 20 mg/L SON/SRFA-MWNTs were 300 mM. 14


Page 15 of 24


338

αA as a function of [NaCl] for MWNTs dispersed in 20 mg/L dispersant solutions are

339

compared in Figure 3B. SON/SRHA-MWNTs are more stable than SON/SDS-MWNTs,

340

although IS effects on their surface charges are very similar (Figure 3A). Besides electrostatic

341

force, the high stability of SON/SRHA-MWNTs and SON/SRFA-MWNTs is attributed to non-

342

DLVO, steric effects of large NOM molecules which associate with MWNTs through π-π

343

interactions between aromatic rings on MWNTs and aromatic moieties in NOM molecules.44,45

344

The αA of two MWNTs suspensions prepared by long-term stirring are also presented in Figure

345

3B. There is no substantial difference between ultrasonicated samples and long-term stirred

346

samples.

347

3.2.3 QCM-D Deposition. In natural waters, aggregation between MWNTs is unlikely to be

348

the major factor determining their environmental fate due to the relatively low MWNTs

349

concentration relative to naturally occurring suspended particulate matter such as sediments and

350

NOM. Rather, interaction of MWNTs with environmental surfaces is expected to determine

351

MWNTs distribution in the aquatic environment. Here, MWNTs deposition on silica surfaces

352

was investigated using QCM-D. Although the mass of deposited MWNTs cannot be directly

353

calculated by the classic Sauerbrey equation,58,59 the frequency change can be used as a relative

354

index to represent the deposited mass: the decreasing third overtone (∆f3) indicates deposition

355

(i.e., larger decrease in ∆f3 suggests more deposited mass), while the increasing ∆f3 indicates

356

release of deposited mass.

357

For comparison, all tested MWNTs concentrations and ionic strengths were adjusted to 2.4

358

mg/L and 20 mM NaCl, respectively. The MWNTs suspensions deposition and release profiles

359

are presented in Figure 3C. Changes in ∆f3 during the deposition of the four MWNTs

360

suspensions decrease in the order of SON/PF-MWNTs (~20 Hz) > SON/SDS-MWNTs (~2 Hz)

361

> SON/SRHA-MWNTs = SON/SRFA-MWNTs (~0 Hz). In the presence of 20 mM NaCl,

362

SON/PF-MWNTs had almost neutral surface charge [EPM = (-0.160 ± 0.132) × 10-8 m2/V-s].

363

Under this condition, the negative DLVO interaction (i.e., double-layer interaction and Van der

364

Waals interaction) energy (Figure S9A) indicates attractive forces between SON/PF-MWNTs

365

and silica surface, resulting in a large amount of deposited SON/PF-MWNTs. In contrast, there

366

is an energy barrier (Figure S9A) between silica and the more negatively charged SON/SDS-

367

MWNTs [EPM = (-1.563 ± 0.144) × 10-8 m2/V-s] at 20 mM NaCl, resulting in less deposition.

368

Although SON/SRHA-MWNTs surface charge is similar to SON/SDS-MWNTs’ (Figure 3A), 15



Page 16 of 24

369

the former does not deposit onto the silica surface which again indicates the importance of steric

370

effects induced by large SRHA molecules. The stronger electrostatic repulsive forces between

371

the more negatively charged SON/SRFA-MWNTs [EPM = (-3.048 ± 0.088) × 10-8 m2/V-s] and

372

silica surfaces resulted in no measurable deposition. Results of aggregation and QCM-D

373

deposition collectively indicate that aggregation results alone are inadequate to predict the

374

stability of nanoparticle suspensions: for example, the most aggregation-stable SON/PF-MWNTs

375

have the greatest potential to deposit onto silica surfaces. According to these results, in a system

376

with 20 mM NaCl and available silica surfaces, the major process governing SON/PF-MWNTs

377

removal from the aqueous phase will be surface deposition rather than settlement due to

378

aggregation.

379

MWNTs release, which indicates the ability of deposited MWNTs to re-enter the water

380

column after being deposited on a surface, is an important parameter for modeling MWNTs fate

381

in the aquatic environment. After deposition, no MWNTs were released when background

382

solution conditions were held constant (i.e., background solution without MWNTs flowed across

383

crystal sensor), indicating that MWNT deposition on silica was irreversible in all of the

384

dispersants. However, the two deposited MWNTs differed greatly in release immediately after

385

DI introduction: for SON/SDS-MWNTs, ∆f3 increased from -2.56 Hz to -2.32 Hz, indicating that

386

~15% of the deposited MWNTs were released; for SON/PF-MWNTs, ∆f3 rapidly increased from

387

-20 Hz to 0 Hz, indicating that all SON/PF-MWNTs deposited on silica surfaces were released.

388

The infinite primary minimum described by DLVO interaction energy profiles under release

389

conditions (Figure S9B), however, is not consistent with the observed release. By considering the

390

Born repulsion which is short-range interactions introduced by interpenetration of electron

391

clouds surrounding the atoms on colloid and planar surfaces,60,61 a finite primary minimum can

392

be obtained. The resultant energy barrier between this primary minimum and energy maximum

393

may be responsible for release of the deposited nanoparticles. In a previous study,8 Yi and Chen

394

attributed release of highly oxidized MWNTs to this barrier. In cases of SON/PF-MWNTs and

395

SON/SDS-MWNTs, the presence of surfactants and their association with MWNTs may

396

introduce other non-DLVO interactions such as hydration effect, steric effect, hydrophobic force,

397

etc. that govern the release process. In addition, effects of salt on the hydration of PEO and PPO

398

blocks of PF62 are also likely to affect SON/PF-MWNTs release.

16


Page 17 of 24


399

3.2.4 Biomarkers Study. Given the observed effects of NOM on MWNTs properties

400

discussed above, and the limited information available on the exposure potential of carbon-based

401

nanoparticles associated with NOM,63 a metabolomics-based investigation of different

402

dispersants and dispersion methodologies was conducted on fathead minnow (FHM) epithelial

403

cells. Twenty mg/L LT/SRHA-MWNTs, SON/SRHA-MWNTs, LT/SRFA-MWNTs, and

404

SON/SRFA-MWNTs were used to dose the cell cultures; following extraction, derivatization,

405

and GC/ToF-MS analysis, a PLS-DA model was constructed to aid in visualizing class

406

separation between exposed cells and control samples. Despite the fact that no substantial

407

difference in physiochemical properties and stabilities was observed between the ultrasonicated

408

and long-term stirred MWNTs samples, a score plot derived from PLS-DA analysis of the

409

chromatograms (Figure S11) demonstrates the greatest class separation between samples

410

prepared by the two approaches in the presence of SRHA or SRFA. Due to the separation along

411

principal component 1 (x-axis), the largest variation appears to occur between ultrasonicated and

412

long-term stirred samples, independent of organic matter class.

413

To understand the biochemical changes induced by exposure to MWNTs suspensions,

414

metabolites between treatment groups were identified by Student’s t-test filtering (p ≤ 0.05). The

415

chromatogram and t-test comparisons were then performed across each nanotube suspension,

416

subtracting its respective vehicle control (Figure 4). The dispersants caused minor changes in the

417

assay (Figure S12), however, SRHA-MWNTs altered metabolites such as glucose, valine,

418

glycine and cholesterol, regardless of dispersion technique. Decreases in cellular levels of

419

glycine, serine and other amino acids potentially result from decreasing energy requirements for

420

FHM cells following exposure to MWNTs. SON/SRHA-MWNTs caused quantifiable increases

421

in the C16-C18 fatty acids, as well as other membrane-associated metabolites. Edgington et al.64

422

found that organic matter coatings resulted in increased toxicity of carbon nanoparticles, and that

423

these suspensions were more bioavailable to aquatic organisms. There is less abundant variation

424

in the statistical difference between metabolites altered in the LT/SRFA-MWNTs samples,

425

compared to the LT/SRHA-MWNTs. Fulvic acid, being of smaller molecular weight,65 has

426

greater potential to penetrate cell membranes. The SON/SRHA-MWNTs and SON/SRFA-

427

MWNTs appear to induce more biochemical changes with respect to glucose and metabolites

428

involved in cellular energetics than their respective long-term counterparts. Interestingly,

429

exposure to SON/SRFA alters more metabolites than SRFA alone or SON/SRFA-MWNTs. 17



Page 18 of 24

430 431 432 433 434

Figure 4: T-test filtered difference chromatograms of the humic or fulvic acid-dispersed MWNT solutions, compared to their respective controls. Peaks above the x-axis indicate metabolites higher in organic matter exposed cells.

4. Environmental Implications.

435

Results of the present study demonstrate that physicochemical properties, aggregation and

436

deposition, which collectively control MWNTs transport and fate in the environment, mainly

437

depend upon the dispersants and not on dispersion methodology. Interestingly, distinct

438

differences were observed between the metabolomic profiles of FHM cells in ultrasonicated and

439

long-term stirred samples. Although mild long-term magnetic stirring is a better mimic of the

440

MWNTs dispersal process in natural waters than intense ultrasonication, many previous studies

441

on MWNTs environmental transport and subsequent toxicity have chosen ultrasonication over

442

stirring. According to our results, transport-related data of SON/MWNTs can be applied to

443

model released MWNTs transport and fate in the environment, however, LT/MWNTs may be a

444

better choice for conducting MWNTs toxicity studies when NOM is present.

445

Supporting Information Available

18


Page 19 of 24


446

The fractogram and size vs. retention time for polystyrene nanosphere standards; QCM-D

447

deposition experiment protocol; Comparison of the sizes and polydispersity indices between

448

LT/MWNTs and SON/MWNTs; AFM images of representative SON/MWNTs and LT/MWNTs;

449

Size distributions of LT/MWNTs and SON/MWNTs obtained from batch DLS and AF4-UV/Vis

450

measurements; Raman spectra for SON/SRHA-MWNTs and LT/SRHA-MWNTs; AF4-UV/Vis

451

profiles for SRHA-MWNTs; Ionic strength effects on MWNTs surface charge; Aggregation

452

attachment efficiency as a function of [NaCl]; Replicate deposition profiles of 20 mg/L SON/PF-

453

MWNTs in the presence 20 mM NaCl on silica surface; DLVO profiles for SON/SDS-MWNTs

454

and SON/PF-MWNTs under deposition and release condition; QCM-D deposition profiles for

455

NOM dispersed MWNTs; PLS-DA score plot for NOM dispersed MWNTs; T-test filtered

456

difference chromatograms of the SRHA and sonicated SRHA solution exposed cells. This

457

information is available free of charge via the Internet at http://pubs.acs.org.

458

Disclaimer

459

This paper has been reviewed in accordance with the USEPA’s peer and administrative

460

review policies and approved for publication. Mention of trade names or commercial products

461

does not constitute endorsement or recommendation for use.

462

Reference

463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481

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Multiwalled Carbon Nanotube Dispersion Methods ... - ACS Publications

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