Kinetics of C60 Fullerene Dispersion in Water Enhanced by Natural

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Environ. Sci. Technol. 2009, 43, 3574–3579

Kinetics of C60 Fullerene Dispersion in Water Enhanced by Natural Organic Matter and Sunlight Q I L I N L I , * ,† B I N X I E , † Y U S I K H W A N G , † AND YUANKAI XU‡ Department of Civil and Environmental Engineering, Rice University, Houston Texas 77005 and Department of Chemical and Biomolecular Engineering, Rice University, Houston Texas 77005

Received December 19, 2008. Revised manuscript received February 25, 2009. Accepted March 2, 2009.

The industrial-scale production of Buckminster fullerene C60 elicits concerns over its impact on human health and ecosystems because of the reported, albeit debatable, toxicity. Assessment of the overall environment risk requires a good estimate of the level of exposure and careful characterization of the physicochemicalpropertiesofC60 innaturalaqueousenvironments. The reported study investigates the role of various environmental factors, i.e., ionic composition, natural organic matter (NOM), and light in dispersion of C60 in the aqueous phase by simple mixing. The presence of NOM greatly enhances C60 dispersion, and the dispersion process is further accelerated by sunlight. At typical NOM concentrations found in natural waters, C60 concentrations of a few to tens of milligrams per liter can occur within 10 days of mixing, regardless of its extremely low water solubility. The rate of dispersing decreases with the increase of ionic strength. However, calcium ions significantly increase C60 concentration in the aqueous phase. Results from UV/vis absorbance characterization strongly suggest that C60 may have been chemically modified when dispersed in an NOM solution in the presence of sunlight. This reaction pathway has significant implication on the fate, transport, and environmental impact of C60 fullerene.

Introduction Carbon-based nanomaterials have received increasing attention because of their potential applications in various fields including electronics, optics, and pharmaceuticals (1-3). Although industrial-scale production of C60 fullerene has reached tons per year (4, 5), there are currently no environmental regulations on C60. Potential introduction of C60 into the environment through the use of consumer product, nonregulated discharges and incidental spill of C60 raises concern over its impact on the ecosystem as well as human health. Toxicity studies have reported contradictory results in the literature, but there has been evidence that C60 is toxic to various organisms (6-10) and human cell lines (11, 12). Therefore, careful assessment of the environmental and human health risks is imperative. Although virtually insoluble in water (13, 14), C60 can form stable aqueous suspensions of nanoparticles, usually referred * Corresponding author phone: (713)348-2046; fax: (713)348-2026; email: [email protected]. † Department of Civil and Environmental Engineering. ‡ Department of Chemical and Biomolecular Engineering. 3574

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to as nC60 (15-17). The toxicity of nC60 has been found to strongly depend on how the suspension is prepared (6-8). Reported methods for nC60 preparation fall into two general categories: direct dispersion, where dry C60 powder is directly mixed with water for an extended period of time (16, 18), and solvent exchange, where one or more intermediate solvent is used for dissolution of C60 followed by mixing with water and subsequent removal of the organic solvent (15, 17). Intuitively, the direct dispersion method simulates entry of C60 fullerene into the aqueous environment upon spill or discharge of C60 powder or deposition of C60-containing aerosols. Several studies have reported the properties of nC60 formed using the direct dispersion method (7, 19, 20). The dispersion is a slow process. It typically takes weeks to months to form a stable suspension of nanosized nC60 with detectable concentrations of C60. Very few studies reported the amount of C60 that can be dispersed in water, i.e., apparent solubility of nC60, which is very important for assessing the potential exposure in the natural aqueous environment. Dispersitivity of C60 in pure water or simple electrolyte solutions has been shown to be very low: Dhawan et al. (8) reported C60 concentrations of 0.23 and 0.26 mg/L after 2 weeks and 11 months of mixing in ultrapure water and 10 mM NaN3, respectively; Terashima and Nagao (21) reported an apparent solubility of 10-8 M (7.2 µg/L) in 0.1 M NaCl. However, real natural water is complex, and various components of natural water may interact with C60 and influence the particle dispersion process. In particular, natural organic matter (NOM), which is ubiquitous in natural water, has been found to stabilize nC60 colloidal particles (21, 22) as well as multiwalled carbon nanotubes (23). Our previous study (24) showed that NOM could also cause significant changes in particle size and morphology of nC60 formed using the solvent exchange method. In addition, both C60 and NOM are UV light sensitive (25, 26). Solar irradiation may also play an important role in the dispersion of C60 in the natural aqueous environment. In this study, we investigated the role of various environmental factors (i.e., solution condition, NOM, and sunlight) on direct dispersion of C60 in water. Measurement of C60 dispersion rate under typical natural water conditions revealed that NOM and UV light greatly enhanced dispersion of C60. The physicochemical properties of the nC60 particles formed including particle size, morphology, surface charge and UV/vis absorbance were carefully characterized to elucidate the mechanisms involved and the chemical nature of the C60-NOM interactions under dark and sunlight conditions.

Experimental Section Materials. Dry powder of sublimed C60 fullerene with purity greater than 99% was obtained from Materials Electronics Research Corporation (Tucson, AZ). Suwannee River humic acid II (SRHA) and fulvic acid I (SRFA) standards were purchased from International Humic Substances Society (IHSS, Atlanta, GA) and used as model aquatic NOM compounds. SRHA and SRFA stock solutions of 1 g/L in concentration were prepared by dissolving the as-received powder in deionized water. To ensure dissolution of SRHA, the solution pH was adjusted to alkaline condition using a 0.02 N NaOH solution. The stock solutions were then filtered through 0.45-µm-pore-size sterilized membrane filters (Millipore, Billerica, MA). Based on carbon contents provided by the IHSS, SRHA and SRFA concentrations were determined by total organic carbon (TOC) analysis using a high sensitivity 10.1021/es803603x CCC: $40.75

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TOC analyzer (Shimadzu Scientific Instruments, Columbia, MD). Reagent-grade NaCl, CaCl2 and NaN3 were obtained from Aldrich Chemicals (Milwaukee, WI). Deionized water was generated with a Barnstead Epure water clarification system (Dubuque, IA). Formation of nC60 Dispersion. Direct Dispersion. Dry C60 powder was mixed with test solutions containing different concentrations of NOM under various solution conditions in dark, with fluorescent room light or sunlight to investigate the kinetics of C60 dispersion in water. The total ionic strength of the test solutions ranged from 0.1 to 10 mM, adjusted using NaCl and CaCl2. NaN3 at 1 mM was added to test solutions mixed for more than 72 h to avoid potential microbial growth. Concentrations of SRHA or SRFA from 0 to 20 mg/L were tested. These concentrations represent those found in typical surface and ground waters. In each experiment, aliquots of 50 mg dry C60 powder were mixed with 200 mL test solutions in autoclaved clear 500-mL glass bottles. C60-free controls were run in parallel. The mixtures were vigorously stirred using a multiposition magnetic stirrer plate (ThermofFisher Scientific, Pittsburgh, PA) at 500 rpm. Experiments in sunlight were performed on 10 consecutive sunny days during the summer. The sample bottles were kept on a benchtop exposed to sunlight for ∼5 h per day. Other experiments were conducted in the laboratory with the room fluorescent light turned on. The dark controls were wrapped in aluminum foil to prevent penetration of room light. Mixing in dark or laboratory lighting was performed for up to 29 days. Duplicate samples were retrieved from each test suspension at predetermined times, filtered through 2-µm and/or 0.45-µm-pore-size membrane filters, and stored in dark at 4 °C before analysis. All samples were analyzed shortly after collection. Solvent Exchange. For comparison, an aqueous nC60 suspension was prepared following a solvent exchange procedure using tetrahydrofuran (THF). The sample was prepared in laboratory room light and stored in dark at 4 °C before use. Details of the protocol can be found elsewhere (24). Characterization of nC60 Dispersion. C60 Concentration. Concentration of C60 in all suspensions was determined by TOC measurement using a high sensitivity TOC analyzer (Shimadzu Scientific Instruments, Columbia, MD) (22, 24, 27). All samples were filtered through 2-µm and 0.45-µm- poresize membrane filters before analysis. Therefore, the TOC concentration measured represents stable nC60 particles in the suspension. When NOM was present in the suspension, the known TOC concentration of the NOM was subtracted from the TOC of the sample to yield C60 concentration. Control experiments showed that both SRHA and SRFA concentrations remained constant over time under the experimental conditions tested. Particle Size and Surface Property. Hydrodynamic diameter and electrophoretic mobility of nC60 were measured by dynamic light scattering and phase analysis light scattering, respectively, using ZetaPALS (Brookhaven Instruments, Holtsville, NY). All measurements were performed at 25 °C. Refractive index of nC60 was set at 2.20 (28). Each sample was measured consecutively at least 10 times for electrophoretic mobility and five times for hydrodynamic diameter. The measured hydrodynamic diameter was correlated using Contin’s regularized non-negatively constrained leastsquares algorithm, and the results were presented as number based particle size distribution. To obtain time-resolved information on nC60 particle size during the dispersing process, some particle size measurements were performed using samples filtered only by 2 µm-pore-size membrane filters in order to include large suspended particles at the beginning of the mixing process. The hydrodynamic diameter measured by dynamic light scattering was also corroborated

with transmission electron microscopy (TEM) analysis as described below. Particle Morphology and Structure. nC60 particle morphology and structure were analyzed by TEM imaging. Samples for TEM analysis were prepared by depositing 3 µL sample suspension on a 400-mesh Ultrafine carbon support film on copper grid (Ted Pella, Redding, CA), and drying overnight in a dust-free box. Samples were imaged using a JEOL-2010 TEM (JEOL Inc., Peabody, MA) operated at 100 kV. Representative images of the sample surface were analyzed using an image processing software SimplePCI (Compix Inc., Sewickley, PA) to obtain particle size distribution. UV/Vis Absorbance. UV/vis absorbance spectra of nC60 suspensions were obtained using a dual beam, high resolution UV/vis spectrophotometer (UV-2550, Shimadzu Scientific Instruments, Columbia, MD). The scan was performed in the wavelength range of 190 to 900 nm. The slit width and sample interval were set at 1 and 0.2 nm, respectively. The UV to visible light transition wavelength was adjusted to 390 nm to avoid interference with the characteristic absorbance peak of C60 (∼340 nm). For each sample, the corresponding background solution used to prepare the nC60 suspension was used as the reference. In addition, selected nC60 suspensions formed by direct dispersing in NOM solutions were dialyzed in deionized water using a dialysis membrane with a molecular weight cutoff of 6-8 kDa (Fisher Scientific, Pittsburgh, PA) to remove free NOM in the suspension. UV/ Vis absorbance spectra of the dialyzed nC60 samples were then obtained using deionized water as reference.

Results and Discussion NOM Enhances C60 Dispersion Kinetics. C60 dispersion in DI water is a very slow process under both dark and light conditions. The C60 concentration detected after 72 h and 3 weeks of mixing in DI water under dark conditions was only 0.31 and 0.35 mg/L, respectively. This is consistent with a previous study by Dhawan et al. (8), which reported C60 concentrations of 0.23 and 0.26 mg/L after 2 weeks and 11 months of mixing in ultrapure water and 10 mM NaN3, respectively. These concentrations, however, were significantly higher than the apparent solubility reported by Terashima and Nagao (21). In the presence of SRHA or SRFA, C60 dispersion kinetics was greatly enhanced. Figure 1 compares the nC60 suspensions formed in DI, 20 mg/L SRHA, and 20 mg/L SRFA as well as the corresponding C60-free background SRHA and SRFA solutions after 72 h of mixing and one week of storage under laboratory lighting conditions. While C60 in the DI water sample was present in the form of very large aggregates either settled on the bottom or floating on the surface of the aqueous phase, a stable dark brown suspension of nC60 was formed in both SRHA and SRFA solutions. The stable suspensions in SRHA and SRFA obtained after filtration with 0.45-µm- pore-size membranes contained 7.8 and 12.8 mg/L of C60, respectively, more than 1 order of magnitude higher than that dispersed in DI water after 3 weeks of mixing. These results indicate that C60 may occur at high concentration levels in the natural aqueous environment. The enhanced C60 dispersion in the presence of SRHA or SRFA is partially attributed to the increase in nC60 particle stability due to the steric hindrance effect exerted by NOM molecules adsorbed on nC60 surface as well as the reduced surface hydrophobicity. NOM is known to enhance colloidal stability and transport by increasing electric double layer repulsion and through the steric hindrance effect (29-31). It has been found to increase stability of nC60 formed by the solvent exchange method in several previous studies: Chen et al. (22) reported that humic acid reduces nC60 aggregation; Xie et al. (24) showed evidence of dissolution/disaggregation of preformed nC60 particles upon contact with SRHA and VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 1. Aqueous nC60 suspensions formation after 72 h of mixing under laboratory lighting conditions. (a) C60 in deionized water; (b) C60 in 20 mg/L SRHA; (c) 20 mg/L SRHA; (d) C60 in 20 mg/L SRFA; and (e) 20 mg/L SRFA.

FIGURE 3. Effect of sunlight on C60 dispersion. All solutions contain 10 mg/L SRHA, 7 mM NaCl, and 1 mM CaCl2.

FIGURE 2. C60 dispersion in SRHA solutions under laboratory lighting conditions. (a) Effect of SRHA concentration. All solutions contain 1 mM NaCl. (b) Effect of ionic composition. All solutions contain 10 mg/L SRHA. SRFA. Terashima and Nagao (21) reported an increase in apparent C60 solubility in the presence of NOM, but the concentration of NOM used in their study, 100 mg/L, is unlikely to be found in typical aqueous environment. Our study is the first to show that high concentrations of C60 can be found in typical aqueous environments. The concentrations measured in our study were also significantly higher than those reported by Terashima and Nagao (21) (0.057 and 3.89 mg/L for FA and HA, respectively) even though they used a much higher concentration of NOM and a longer mixing time (10 days). Further discussion on the potential reasons for such discrepancies is provided later. Effect of NOM Concentration. The extent of C60 dispersion was found to increase with increasing NOM concentration. Figure 2a compares the amount of C60 dispersed in solutions 3576

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containing different concentrations of SRHA and 1 mM NaCl during 72 h of mixing under laboratory lighting conditions. Concentration of stable nC60 particles increased with increasing mixing time and SRHA concentration. It is worth noting that with only 5 mg/L of SRHA (2.6 mg/L as TOC), as much as 2.4 mg/L of C60 can be dispersed in the form of stable nC60 particles after 72 h of mixing. Typical TOC concentration of surface water ranges from 1 to 20 mg/L (32). This indicates potential fast entry of C60 into natural aqueous environment if spill of C60 powder occurs. At a SRHA concentration of 20 mg/L, 12 mg/L of C60 was detected in the suspension after 10 days of mixing. Similar effect was found with SRFA (data not shown). These high concentration levels suggest potentially high exposure levels of C60 through natural water. Effect of Solution Condition. C60 dispersion strongly depended on the ionic strength of the aqueous phase. The amount of C60 dispersed in 10 mg/L SRHA increases significantly with decreasing total ionic strength (Figure 2b), as expected from the increased electrostatic repulsion at lower ionic strength. The less compact molecular conformation of NOM at lower ionic strength and therefore more effective steric hindrance may also play a role. Since Ca2+ is known to reduce colloidal stability due to more effective charge screening compared to Na+, the effect of Ca2+ on C60 dispersion was also investigated. Under both laboratory lighting and dark conditions, Ca2+ ions did not significantly affect C60 dispersion during the initial 72 h of mixing (Figure 2b). Over a longer mixing period, however, much better C60 dispersion was observed in the presence of Ca2+: After 3 weeks of mixing under dark conditions, 13.9 mg/L of C60 was dispersed as stable nanoparticles in the presence of 1 mM Ca2+ at a total ionic strength of 10 mM, whereas only 5.9 mg/L was detected in 0.1 mM NaCl. Sunlight Accelerates C60 Dispersion by NOM. Compared to that under dark and laboratory fluorescent lighting

FIGURE 5. UV/vis absorbance spectra of different C60 suspensions. (i) 3 mg/L nC60 prepared by solvent exchange using THF mixed with 10 mg/L SRHA (10 mM NaCl). Reproduced with permission from (24); (ii) dispersed in deionized water in dark (0.1 mM NaCl, 3 weeks of mixing); (iii) dispersed in 10 mg/ L SRHA under sunlight (0.1 mM NaCl, 72 h mixing); (iv) dispersed in 10 mg/L SRHA under sunlight (7 mM NaCl, 1 mM CaCl2, 72 h mixing); (v): dispersed in 10 mg/L SRHA in dark (0.1 mM NaCl, 3 weeks of mixing); and (vi) dispersed in 10 mg/L SRHA in dark (7 mM NaCl, 1 mM CaCl2, 3 weeks of mixing).

FIGURE 4. TEM images of nC60 nanoparticle dispersed under sunlight at (a) 5 h, (b) 24 h, and (c) 48 h. The solution condition is 10 mg/L SRHA at 0.1 mM IS. Particle size analysis was based on more than 2000 particles. Mean equivalent particle diameters are 32, 16, and 6 nm for 5, 24, and 48 h, respectively. conditions, dispersion of C60 was greatly accelerated when experiments were performed under sunlight (Figure 3). Dispersion under dark and fluorescent lighting conditions was not statistically different (Figure S1of the Supporting

Information). Since the visible light spectrum of the laboratory fluorescent light is similar to that of sunlight and fluorescent light bulbs emit negligible UV (33), it is hypothesized that the accelerated C60 dispersion can be attributed to the small amount of UV present in the solar spectrum. nC60 particle size decreased very quickly as mixing proceeded under sunlight, as shown in Figure S2 of the Supporting Information. At 0.1 mM total ionic strength, the mean hydrodynamic diameter of nC60 particles dispersed in the 10 mg/L SRHA solution decreased to below 5 nm within 72 h of mixing (Figure S2a of the Supporting Information). In comparison, the sizes of particles formed under dark or room light conditions stayed relatively constant over a mixing period of 29 days and were significantly larger under similar solution conditions (Figure S3 of the Supporting Information). TEM images (Figure 4) taken at different times during the 72 h mixing under sunlight reveal that the fast dispersion is mainly caused by a surface erosion or dissolution-recrystallization process instead of particle breakage. This is supported by the following evidence found in the TEM images: At the beginning of the mixing (Figure 4a), a significant number of particles smaller than 20 nm in diameter already formed; gradients of particle size and concentration were found around large aggregates, but the surrounding particles were much smaller than the primary crystals in the aggregates; Although the primary crystals are all faceted in rectangular or hexagonal shape (insert in Figure 4a), secondary crystals formed during mixing are all semispherical. This is shown by Figure 4b where very small, semispherical nC60 crystals are found surrounding the few faceted primary crystals, and Figure 4c, where all particles formed after 48 h of mixing are semispherical with a narrow particle size distribution from 4 to 11 nm. The surface erosion or dissolution-recrystallization process is likely a result of NOM interacting with C60 molecules at the surface of the primary crystals, which was catalyzed by sunlight. The decrease in particle size was accompanied by an increase in the electrophoretic mobility of the nC60 particles (Figure S2, parts b and d, of the Supporting Information), indicating that the surface charge density of the secondary crystals is greater than the primary crystals as a result of interactions with NOM. However, the particle size under different solution conditions does not seem to correlate well to the surface VOL. 43, NO. 10, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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zeta potential, suggesting that electrostatic interaction is not the only mechanism for particle stability. Potential Chemical Transformation of C60. UV/Vis absorbance characterization of nC60 suspensions formed by direct dispersion in NOM solutions suggests that C60 might have been chemically modified during the dispersion process. Figure 5 compares the UV/vis spectra of the nC60 suspensions prepared under different conditions. The absorbance spectrum of the nC60 suspension formed by the solvent exchange method using THF was essentially unchanged when the suspension was mixed with 10 mg/L SRHA (spectrum i, Figure 5) (24). It clearly shows absorbance in the 260 and 340 nm wavelength bands, characteristic absorbance bands of C60 molecules (34). This indicates that NOM adsorption on nC60 surface does not affect its UV/vis absorbance and C60 in this suspension is chemically intact. The nC60 suspension obtained by mixing in deionized water shows the same absorbance bands (spectrum ii, Figure 5) with a slight red shift on the 340 nm band due to larger aggregate sizes and overall lower absorbance due to the much lower concentration (0.35 mg/L) of C60 dispersed. However, the suspensions formed in NOM solutions under sunlight do not show any absorbance maxima (spectra iii and iv, Figure 5), even though TOC analyses suggest they contain more than 5 mg/L C60. Similar featureless UV/vis spectra were obtained after dialysis of these suspensions to remove free NOM (data not shown), further confirming that the changes in the UV/vis absorbance spectrum are not due to interference from free NOM molecules in the aqueous phase. These results indicate that C60 dispersed in the NOM solutions under sunlight has been chemically modified. Chemical transformation of C60 formed seems to have occurred also in dark. The suspension formed in dark in the absence of Ca2+ (spectrum v, Figure 5) does retain the two absorbance bands at 260 and 350 nm, but the absorbance is much lower than that of the suspension formed in deionized water even though the concentration (5.9 mg/ L) is more than 1 order of magnitude higher. When 1 mM Ca2+ is present (spectrum vi, Figure 5), no absorbance is found at 260 or 340 nm wavelength ranges; a small absorbance peak occurred at ∼310 nm, possibly due to the formation of an intermediate product. The chemical transformation may partially contribute to the discrepancies between the apparent C60 solubility reported by Terashima and Nagao (21) and the amount of dispersed C60 measured in our study. In Terashima and Nagao’s study (21), C60 concentration was determined by toluene extraction followed by measurement by HPLC with a UV detector. Any reaction products that do not absorb UV at the 335 nm band (the detection wavelength used in their study) are not accounted for, even though they may exist as part of the nanoparticles or dissolved in the aqueous phase in the molecular form. In addition, it is unclear whether the extraction protocol is effective toward C60-NOM complexes. The TOC measurement used in this study has the advantage of quantifying the total amount of C60 and C60 derivatives dispersed, but it does not provide any information on the chemical nature of the dispersed fullerene compounds. Although the mechanisms of the chemical transformation described above are unknown at this time, several scenarios are likely to result in transformation of C60 in the natural environment. Under sunlight, NOM is known to act as photosensitizers or precursors for the production of highly reactive species, e.g., singlet oxygen, hydroxyl radicals, organic peroxy radicals, and triplet state of NOM (35, 36). Reactions with these highly reactive photooxidants may lead to the chemical transformation of nC60. Fortner et al. (37) reported similar changes in the UV/vis absorbance spectra upon reaction of aqueous nC60 with ozone. It was suggested that the reaction products are highly oxidized fullerenes with repeating hydroxyl and hemiketal functionalities. Alterna3578

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tively, C60 may undergo addition reactions with amine functional groups in the NOM structure. It has been reported that C60 loses its signature absorbance peaks upon reaction with organic amines in toluene or cyclohexane under sunlight (38). Similar reaction may occur in the aqueous phase. Under dark conditions, formation of charge transfer complexes and intercalation compounds (39) with NOM may be responsible for the observed changes in the UV-vis spectrum. Clearly, these reaction pathways require a more comprehensive investigation. Implications of the Study. Direct dispersing of C60 to water most closely simulates the natural pathway of C60 into the aqueous environment. The present investigation clearly showed that the dispersitivity, physicochemical properties, and chemical nature of C60 in the aqueous phase can be significantly altered by natural organic matter (NOM), ionic strength, Ca2+, and sunlight. This strongly suggests that NOM and sunlight may play critical roles in the fate, transport (40), and toxicity (7) of C60 in the natural aqueous environment. Results from this study also indicate that high concentrations of C60 or C60 derivatives in the form of extremely small particles likely occur under typical aqueous environment. These environmental factors should therefore be carefully considered in assessing the environmental impact of C60. In particular, reactions of C60 under typical aqueous environmental conditions should be included in the fate and transport models. To our knowledge, this is the first study to report NOM-induced chemical transformation of C60. Further investigation is needed to better understand the aquatic chemistry of C60, especially the chemical nature of NOM-nC60 interactions.

Acknowledgments The reported study was funded by the NSF Center for Biological and Environmental Nanotechnology (Award EEC0647452). We thank Dr. Wenhua Guo (Department of Chemistry, Rice University) for his assistance on transmission electron microscopy.

Supporting Information Available Additional figures showing the amount of nC60 dispersed in the presence of SRHA under laboratory light and dark conditions, and particle size and surface zeta potential of nC60 dispersed under sunlight, laboratory light, and dark conditions are also provided. This material is available free of charge via the Internet at http://pubs.acs.org.

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