Impact of Natural Organic Matter on the Physicochemical Properties of

Mar 12, 2008 - Evaluating the environmental impact of C60 requires careful characterization of its physicochemical properties in the natural aqueous ...
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Environ. Sci. Technol. 2008, 42, 2853–2859

Impact of Natural Organic Matter on the Physicochemical Properties of Aqueous C60 Nanoparticles BIN XIE,† ZHIHUA XU,† W E N H U A G U O , ‡ A N D Q I L I N L I * ,† Department of Civil and Environmental Engineering and Department of Chemistry, Rice University, Houston Texas 77005

Received September 5, 2007. Revised manuscript received January 29, 2008. Accepted January 31, 2008.

Existing toxicity data indicate that industrial-scale production of C60 fullerene poses a potential threat to the environment. Evaluating the environmental impact of C60 requires careful characterization of its physicochemical properties in the natural aqueous environment. Our study aims to determine the effects of aquatic natural organic matter (NOM) on the physicochemical properties of aqueous C60 nanoparticles, nC60. Stable nC60 suspensions were formed using three different solvent exchange protocols. They were thoroughly characterized for particle size, morphology, and electrophoretic mobility in the absence or presence of two model NOM components, Suwannee River humic acid and fulvic acid. NOM caused disaggregation of nC60 crystals and aggregates under typical solution conditions of natural water, leading to significant changes in particle size and morphology. Such effect increased with increasing NOM concentration. The changes in nC60 size and morphology strongly depended on the nC60 formation pathway. Results from this study indicate that NOM may play a critical role in the transport and toxicity of C60 in the natural aqueous environment.

Introduction C60 fullerene is receiving increasing attention for its existing and potential applications in various fields including electronics, optics, cosmetics, and pharmaceutics (1–9). Although the industrial-scale fullerene production reaches tons per year (10), the environmental impact of this material is still largely unknown. C60 has very low solubility in water (estimated at 1.3 × 10- µg/L) (11, 12). However, stable aqueous suspensions of nanosized C60 colloids (nC60) can be formed through several relatively simple methods (13–16), indicating that C60 can enter the aqueous environment in the form of nC60 nanoparticles. A wide range of nC60 particle sizes, from several nanometers to around 200 nm have been reported in the literature (13–18). The particle size, morphology and surface zeta potential have been shown to be sensitive to different sample formation pathways as well as a function of solution chemistry such as alkalinity and ionic strength (16, 18–20). Although the toxicity of C60 is still a controversial topic, recent studies have reported nC60 toxicity to microorganisms (15–17, 21), aquatic species (22, 23), and human cell lines * Corresponding author phone: (713)-348-2046; fax: (713)348-2026; e-mail: [email protected]. † Department of Civil and Environmental Engineering. ‡ Department of Chemistry. 10.1021/es702231g CCC: $40.75

Published on Web 03/12/2008

 2008 American Chemical Society

(24, 25). These findings raise concerns on the potential impact of C60 on humans and the ecosystem. Moreover, nC60 can potentially serve as a “carrier” for hydrophobic organic pollutants, increasing their mobility through colloid-facilitated transport (8). These concerns on the potential environmental impact of C60 call for better understanding of the transport behavior of nC60 in the aqueous environment. Physicochemical properties of nanoparticles govern their transport as well as uptake by, and hence toxicity to, living organisms. In the natural aqueous environment, various components of natural water may interact with nC60 and interfere with its transport. In particular, natural organic matter (NOM), ubiquitous in natural water, is known to readily adsorb onto colloidal particles, altering their surface properties and, consequently, the transport pattern. For example, NOM was found to facilitate the transport of natural particles (26–28), model colloids (29–31), and bacteria (32) in saturated porous media by increasing electric double layer repulsion and through the steric hindrance effect (33–35). Recently, NOM was reported to stabilize nC60 as well as multiwall carbon nanotubes (36, 37) in the aqueous phase and to reduce nC60 particle aggregation rate, indicating its capability of increasing nC60 particle mobility in the environment (37). In this study, the effects of NOM on the physicochemical properties of nC60, including size, structure, and surface properties, were investigated under solution conditions — pH, ionic strength, cation concentration, and NOM concentration — typically found in natural water. The results obtained have important implication on the toxicity and transport in the natural aqueous environment.

Experimental Section Materials. Dry powder of sublimed C60 fullerene (purity g99%) was obtained from MER Corp. (Tucson, Arizona). Suwannee River humic acid (SRHA II) and fulvic acid (SRFA I) standards (International Humic Substances Society, Atlanta, Georigia) were used as model NOM compounds. SRHA and SRFA stock solutions of 1 g/L in concentration were prepared by dissolving the SRHA or SRFA powder in deionized water. The pH was adjusted to 8.2 with NaOH to ensure complete dissolution of SRHA. The concentrations of the stock solutions were confirmed by total organic carbon (TOC) analysis based on the carbon content reported by the International Humic Substances Society: 52.6% for SRHA and 52.4% for SRFA, respectively. Reagent-grade toluene, tetrahydrofuran (THF), and NaCl were obtained from Aldrich Chemicals (Milwaukee, Wisconsin). Analytical-grade sodium dodecyl sulfate (SDS) was purchased from Fisher Scientific (Hampton, New Hampshire). Deionized water used in all experiments was generated using an Epure water purification system (Barnstead, Dubuque, Iowa). Preparation of nC60 Suspensions. Three different sample preparation protocols were used to form aqueous nC60 suspensions based on a solvent exchange method modified from previously published protocols (13–15). The suspensions obtained were referred to as type I, type II, and type III. In the first preparation protocol, 20 mg of dry C60 powder was dissolved in 10 mL of toluene. The solution was passed through a 2 µm pore-size membrane filter (Millipore, Billerica, Massachusetts) to remove undissolved C60 powder. The fullerene solution in toluene was then added to 1000 mL of deionized water (toluene-to-water ratio of 1:100). Toluene was subsequently removed by sonication at 80–100 W for 3 h in 25 min cycles with 5 min interruption between each cycle using a sonicating probe (Heat Systems-Ultrasonic Inc., Plainview, New York). The aqueous nC60 suspension formed VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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is referred to as the type I suspension. The protocol used to prepare the type II nC60 suspension was similar to that for type I except that 100 mL of deionized water was used (toluene-to-water ratio of 1:10) and that sonication was performed for 3 h continuously. To prepare the type III nC60, 10 mg of dry C60 powder was added to 500 mL of THF. The mixture was purged with N2, and continuous stirring was provided for 12 h. Undissolved fullerene was filtered out using a 2 µm pore-size membrane. A 500 mL portion of water was then added to the THF solution of C60 (THF-to-water ratio of 1:1) at a rate of approximately 1.5 mL/sec using a pipet. Finally, THF was removed by evaporation at 80 °C using a rotary evaporator. For all three types of nC60, the prepared aqueous suspension was passed through 2 µm pore-size sterilized membrane filters and stored in the dark at 4 °C. The particle sizes of the stock suspensions were routinely measured, and the shift of the mean particle size was within ( 2 nm in the period of the study with a slight increase in polydispersity. Analysis of C60 Concentration. C60 concentration in the stock suspension was determined by UV absorbance (15) and TOC measurements (38). UV absorbance of aqueous nC60 suspensions were measured at 342 nm using a dualbeam high resolution UV/vis spectrophotometer (UV-2550, Shimadzu Scientific Instruments, Columbia, Maryland). The calibration standards of aqueous nC60 were prepared by serial dilution of one reference nC60 sample with known concentration. The concentration of the nC60 reference sample was determined following an oxidation-extraction protocol (15). Briefly, the sample was oxidized by potassium perchlorate followed by extraction with toluene. C60 concentration was then determined by measuring UV absorbance of the extract based on a pre-established calibration curve for C60 in toluene. The concentration was also verified by TOC analysis using a combustion-based high-sensitivity TOC analyzer (Shimadzu TOC-VCSH, Shimadzu Scientific Instruments, Columbia, Maryland). It was confirmed that the recovery of the oxidation-extraction process was greater than 98%. UV absorbance instead of TOC measurement was used for all other nC60 samples due to the large sample volume required for TOC analysis. Characterization of nC60 Particles. The nC60 suspensions prepared were characterized for particle size, morphology, structure, and surface charge in the absence or presence of NOM or SDS at various pH. For all analyses, aliquots of the stock suspensions were diluted with background electrolyte solutions to achieve the desired solution conditions and a C60 concentration of 3 mg/L. The ionic strength in all samples tested was kept constant at 10 mM using NaCl, representative of typically fresh water. NOM compounds or SDS was then introduced into the samples to achieve the desired concentrations. Next, the samples were gently hand shaken for 5 min and equilibrated for 20 min before characterization. All experiments for each nC60 type were performed using the same stock suspension within one week after the preparation of the stock solution to avoid discrepancies due to changes in the stock suspension. Control experiments were performed using corresponding NOM solutions containing no nC60. Particle Size and Electrophoretic Mobility Measurement. Particle size distribution was determined by dynamic light scattering (DLS) measurement using a Nano Zeta-sizer (Malvern Instruments, Worcestershire, United Kingdom) with a 633 nm laser source and a detection angle of 173°. The instrument measures particle hydrodynamic diameter based on time-dependent fluctuation in light scattering using Contin’s regularized non-negatively constrained leastsquares algorithm. The particle size range of detection is 5 nm to 10 µm, according to the manufacturer. Because the concentration of NOM or SDS in all sample suspensions was very low (e20 mg/L), the refractive index (1.33 at 25 °C) and 2854

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viscosity (0.0887 cps at 25 °C) of water were used for the dispersant. A refractive index of 2.20 was used for nC60 (18). Particle surface charge was characterized by electrophoretic mobility (EPM) measurement based on phase analysis light scattering using a ZetaPALS particle analyzer (Brookhaven Instruments, Holtsville, New York). All samples for particle size and EPM analysis were prepared in triplicates. Measurements were performed at pH values ranging from 3 to 10, adjusted using 0.005 N HCl or NaOH. Temperature was maintained at 25 °C for all measurements. Each sample was measured at least 5 times for particle size and 10 times for electrophoretic mobility. Characterization of Particle Morphology. Transmission electron microscopy (TEM) was performed to characterize particle structure and morphology as well as to confirm the particle size measurement by DLS. Samples were prepared by depositing 3 µL sample suspension on a 400-mesh removable ultrathin carbon support film (Ted Pella, Redding, California) on copper grids. Samples were air-dried overnight in a sterilized, dust-free box. Imaging was performed using a JEOL-1230 TEM (JEOL Inc., Peabody, Massachusetts) operated at 120 kV. An image analysis software, SimplePCI (Compix Inc., Sewickley, Pennsylvania), was used to obtain particle size distribution from TEM images.

Result and Discussion Characteristics of nC60 in NOM-free Suspensions. Stable aqueous nC60 suspensions were formed with all three sample preparation protocols. Figure 1 presents typical size distributions of the three suspension types. The suspensions showed a relatively narrow monomodal particle size distribution over the pH range tested. The nC60 particles formed are highly negatively charged with the particle EPM ranging from -2.5 × 10-8 to -3.5 × 10-8 m2/Vs when measured in 10 mM NaCl at pH 7, similar to previous reports (18, 19). The stock suspensions stayed very stable in DI water, and the changes in mean particle size were within ( 2 nm during the period of this study. However, the three types of nC60 particles differ greatly in size, morphology, and structure, as shown by the TEM images in Figure 2. Type I nC60 suspension (Figure 2a) contains porous aggregates of primary particles less than 10 nm in diameter. The primary particles are crystalline, as indicated by the electron diffraction pattern shown in Figure 2a. The morphology of the nC60 aggregates was similar to that previously reported by Andrievsky et al. (39). At pH 8, the mean particle size of type I nC60 suspension measured by DLS was 56 ( 8 nm. This is in reasonable agreement with the size of the aggregates found in the TEM image, indicating that the aggregates in the TEM images were not formed during the drying process in TEM sample preparation. In contrast, the type II nC60 suspension exhibited completely different particle structure and morphology (Figure 2c). It contains individual particles of crystalline structure. Most particles are semispherical, hexagonal, or rectangular. However, particles in the shape of rods and triangles were also found. This is the first time rod and triangle shaped nC60 particles are reported in samples prepared with toluene. Analysis of TEM images gave an average particle size of 60 ( 3 nm. This is in good agreement with the mean particle size measured by DLS, 65 ( 5 nm, confirming that the crystalline particles found in the TEM images exist in the suspension as individual particles. Similar to type II, type III nC60 suspension also consists of large crystalline particles. However, the mean particle sizes measured by DSL and TEM, 100 ( 3 and 89 ( 5 nm, respectively, were much larger than those of type I and type II suspensions. Particles in triangular, rectangular, and hexagonal shapes were found in type III suspension, and the larger particles tend to be more angular. The observed particle morphology was similar to that

FIGURE 1. Particle size distribution of the three types of nC60 suspensions measured in 10 mM NaCl at pH 8 without NOM and with 20 mg/L SRHA. reported in previous studies using the same sample preparation method (15, 16, 18). The results described above demonstrate that the size, structure, and morphology of nC60 particles are highly dependent on the pathway of nC60 formation. When comparing type I and type II with type III nC60, the effect of organic solvent type and subsequent removal method was obvious. The nC60 particles prepared by the toluene/sonication method were overall smaller than those prepared by the THF/ thermal evaporation method. The pronounced difference in particle structure and morphology observed between type I and type II nC60 particles, both prepared by the toluene/ sonication method, is attributed to the different water-totoluene ratio and sonication pattern. In the type II suspension, it appears that particles are formed by continuous crystal growth when transferred from the toluene phase to the water phase during sonication. In the type I nC60 suspension, however, both crystal growth and particle aggregation are important particle formation mechanisms, as demonstrated by the crystalline structure of the primary particles and the presence of large porous aggregates. It is hypothesized that the high water-to-toluene ratio yielded small nC60 crystal sizes and that the intermission between the sonication cycles interrupted crystal growth and allowed time for particle aggregation. In preparation of the type II nC60, aggregation was inhibited when continuous sonication was applied. The higher C60 concentration in the aqueous phase led to large crystals. The difference in particle structure caused by the different sonication patterns also indicates that aqueous nC60 suspensions formed with the aid of sonication may not represent what forms in the natural environment, where sonication is absent. Impact of NOM on nC60 Physicochemical Properties. The addition of NOM into nC60 suspensions induced distinctive sample type-dependent responses in nC60 particle properties. Examples of particle size distributions of the three suspensions after addition of 20 mg/L SRHA are presented in Figure 1. Representative examples of TEM images of nC60 nanoparticles in all three types of suspensions before and after addition of 20 mg/L NOM were shown in Figure 2. The mean particle size in the absence and presence of NOM or

a typical surfactant SDS are compared in Figures 3 and 4. It is noted that DLS analyses of control samples, that is, C60free NOM and SDS solutions at 20 mg/L, returned no results due to insufficient light scattering caused by the low concentration of NOM and SDS. Similarly, the TEM images of SRHA and SRFA control samples did not show distinguishable features (data not shown) due to the extremely low contrast as a result of the low electron density of these molecules. Therefore, observed changes in particle size and morphology after NOM or SDS addition are attributed to changes in nC60 particle properties due to interactions with NOM or SDS. Type I nC60. The addition of NOM to the type I nC60 suspension was found to significantly reduce the size of nC60 particles (Figure 1). Comparisons clearly show that the particle size distribution shifted toward smaller sizes after 20 mg/L SRHA was added. Similar effects were also found with SRFA. In addition, the mean particle size decreased with increasing concentration of SRHA or SRFA, as shown in Figure 3. For example, the mean particle size at pH 9 decreased from 56 to 45 and then to 20 nm, respectively, when the concentration of SRFA was increased from 0 to 5 and then 20 mg/L. When compared to a commonly used surfactant, SDS, at the same concentration of 20 mg/L, SRFA and SRHA at alkaline pHs were more effective in reducing nC60 particle size. These results indicate that nC60 may be much more stable and mobile in the natural aqueous environment than expected. Comparison of the TEM images in Figures 2a and 2b reveals that the large nC60 aggregates disaggregated into smaller aggregates containing similar crystalline primary particles. As shown in Figure 2b, very small nC60 particles are present on TEM grid with equivalent diameter around 5 nm, and they could be primary particles whose presence was theoretically calculated (40) and experimentally verified in polar organic solvents (41). The disaggregation observed is attributed to changes in surface properties of nC60 upon adsorption of NOM or SDS. Because the particle size measurement was performed at an ionic strength much lower than the critical coagulation concentration (38), particle–particle interactions control the rate of aggregation VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. TEM imaging of type I nC60 without NOM (a) and with 20 mg/L SRFA (b), type II nC60 without NOM (scale bars for all images are the same) (c) and with 20 mg/L SRHA (d), and type III nC60 without NOM (e) and with 20 mg/L SRHA (f). All suspensions contain 10 mM NaCl, and the pH is 8. (reaction limited regime). Measurement of particle EPM showed that nC60 particle surface became more negatively charged when SDS was added (see Figure S1a in the Supporting Information), indicating that the disaggregation observed with SDS was caused by increased electrostatic repulsion. Note that the SDS concentration used was far below the critical micelle formation concentration. Therefore, micelle formation is not considered as a potential mechanism. No significant changes in particle EPM were observed when SRHA was added. The disaggregation in the presence of SRHA can be explained by the steric hindrance effect and the reduced surface hydrophobicity. NOM molecules consist of a hydrophobic backbone and hydrophilic side chains (carboxyl and amine groups). They can readily adsorb onto the nC60 particle surface via hydrophobic interaction. The exposed carboxyl and amine groups make the particle surface more hydrophilic, leading to increased particle stability and, hence, smaller aggregate size. Changes in electrostatic interactions seem to also play a role in particle disaggregation 2856

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in the presence of SRFA. As shown in Figure S1b in the Supporting Information, particle surface negative charge increased with increasing SRFA concentration. However, significant disaggregation occurred under conditions when changes in surface charge was negligible, for example, at a SRFA concentration of 5 mg/L and SRFA concentrations of 10 and 20 mg/L at pH 9.8, indicating that steric hindrance and reduced surface hydrophobicity are dominant mechanisms. Type II nC60. Despite the crystalline structure of type II nC60, noticeable decrease in particle size was observed when 20 mg/L SRHA or SRFA was added to the suspension (see Figures 1 and 4a). The reduction in particle size is more pronounced at higher pH. Although EPM measurements (Figure S2a in Supporting Information) found increased negative potential when NOM was added, the changes in particle surface charge do not correlate to the changes in particle size. For example, the particle surface negative charge was the highest in the presence of SRFA, but the mean particle size of the suspension was the smallest when SRHA was

FIGURE 3. Mean particle size measured by DLS of the type I nC60: (a) at different SRHA concentrations; (b) at different SRFA concentrations. All suspensions contain 10 mM NaCl. The error bars indicate 95% confidence interval. present. Furthermore, the nC60 particles in the NOM-free suspension were not in aggregates as the mean particle size measured by DSL agrees well with the individual crystal size measured by TEM. These findings point out that electrostatic interaction is not the main mechanism for the changes in particle size. TEM images reveal an astounding phenomenon: the large, multiple-faceted crystal particles (Figure 2c) were converted to smaller, semispherical particles (Figure 2d) upon addition of NOM. Although semispherical particles were also found in the NOM-free suspension, they were a much smaller population and were with larger particle sizes. After addition of NOM, all particles found in the TEM images were semispherical. The electron diffraction pattern in Figure 2d shows that these particles are still crystalline. It appears that the edges of the large, multifaceted crystal particles were eroded away after NOM addition, although pieces resembling the broken edges were not found upon careful inspection of the TEM images. These observations indicate that the association between C60 molecules in a type II nC60 crystal particle is rather weak and can be readily disrupted by the presence of NOM to form small, semispherical particles through a dissolution and recrystallization process. Type III nC60. Contrary to type I and type II suspensions, only small changes in particle size were found with type III nC60 when 20 mg/L SRHA or SRFA was added, as shown in Figure 4b. The mean particle size was slightly smaller in the presence of NOM at pH values greater than 6, but the effect was opposite at lower pH values. However, these changes

FIGURE 4. Mean particle size measured by DLS. (a) Particle size of type II nC60 without and with NOM. (b) Particle size of type III nC60 without and with NOM. All suspensions contain 10 mM NaCl. Error bars indicate 95% confidence interval. are not conclusive when considering the large variation between measurements as indicated by the error bars. The particle surface charge (Figure S2b in Supporting Information) was higher in the presence of NOM at pH greater than 6, probably due to the deprotonation of the carboxyl groups on NOM molecules. TEM images (Figure 2, panels e and f) show an intriguing phenomenon: in the presence of NOM, type III nC60 crystal particles seem to have partially disintegrated upon interaction with NOM, as indicated by the “shadow” area found attached to each particle. These “shadow” areas were found in all TEM images acquired with type III nC60 in the presence of NOM. Control experiments with C60-free SRHA or SRFA at the same concentration, 20 mg/L, following the same TEM sample preparation method did not obtain any discernible image, thereby verifying that the “shadow” areas were not NOM aggregates. Careful examination of the TEM images found nC60 particles with layered attachment, as shown in the insert in Figure 2f. It is speculated that NOM causes partial dissolution of nC60, similar to that observed in type II suspension. The dissolved C60 molecules then form the layered structure on the nC60 crystal particle surface. The dramatic changes in nC60 particle structure observed after addition of NOM into type II and type III nC60 suspensions indicate that the nC60 crystalline structures are formed through weak association between C60 molecules. Such association can be readily broken by NOM in natural water. As a result, very small nC60 particles can exist when C60 is released into the natural aqueous environment. VOL. 42, NO. 8, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. UV/vis spectra of type II and type III nC60 in the presence and absence of NOM. The samples containing 20 mg/ L SRHA were measured using 20 mg/L SRHA as the reference sample. Potential of Chemical Transformation. In addition to changes in physicochemical properties of nC60 particles due to NOM adsorption, another possible mechanism for the observed changes in nC60 particle size and morphology is chemical reaction with NOM. Comparison of UV/vis spectra of nC60 suspensions measured in the presence and absence of NOM (Figure 5) found no sign of chemical transformation. However, the possibility of chemical reaction only at the nC60 particle surface cannot be excluded. Implications on the Environmental Behavior and Toxicity of C60. Accurate characterization of physicochemical properties of nanoparticles is necessary for assessing the environmental impact of nanomaterials. Results from this study show that aqueous nC60 nanoparticles of very different structures, sizes, and surface properties can form depending on factors including organic solvent type, mixing condition, and pH. More importantly, these properties can change dramatically within a short time upon interaction with NOM. These changes indicate that a wide spectrum of nC60 particle properties may be found in the natural aqueous environment, corresponding to the local solution chemistry. As a result, the partition and deposition behaviors of nC60 in the natural environment will be a complex function of many factors, including NOM type and concentration. Therefore, fate and transport studies should be conducted under solution conditions representative of those in natural waters. Moreover, it is important to understand how the interactions of nC60 with NOM affect its toxicity. As indicated by the differences in the physicochemical properties of nC60 in the presence and absence of NOM, toxicity studies performed in NOM-free media may not represent the actual exposure that occurs in the natural aqueous environment.

Acknowledgments We thank the Center for Biological and Environmental Nanotechnology (NSF Award EEC-0647452) for funding this study.

Supporting Information Available The electrophoretic mobility of type I, II, and III nC60 before and after NOM addition is presented. This material is available free of charge via the Internet at http://pubs.acs.org.

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