Reaction of Water-Stable C60 Aggregates with Ozone

Sep 25, 2007 - significant in that they (1) demonstrate the feasibility of ..... nC60 (C0, CT ) 0) 100 mg/L. TEM scale bar 100 nm. .... 1994, 116, 451...
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Environ. Sci. Technol. 2007, 41, 7497-7502

Reaction of Water-Stable C60 Aggregates with Ozone JOHN D. FORTNER,† DOO-IL KIM,† ADINA M. BOYD,‡ JOSHUA C. FALKNER,‡ SEAN MORAN,§ VICKI L. COLVIN,‡ JOSEPH B. HUGHES,† AND J A E - H O N G K I M * ,† School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, and Departments of Chemistry and of Biochemistry and Cell Biology, Rice University, Houston, Texas 77005

While the reactivity of C60 has been described in a variety of organic solvents, little information is available regarding aqueous-based reactions due to solubility limitations. In this study, a reaction between C60, as a nanoscale suspension, and dissolved ozone in the aqueous phase was investigated. Findings indicate a facile reaction occurs, resulting in aggregate dissolution concurrent with formation of watersoluble fullerene oxide species. Product analyses, including 13C NMR, MS (LDI), FTIR, UV-Vis, and XPS, indicate highly oxidized fullerene with an average of ∼29 oxygen additions per molecule, arranged in repeating hydroxyl and hemiketal functionalities. These findings are significant in that they (1) demonstrate the feasibility of other aqueous-based fullerene chemistries, including those for alternative synthesis routes, which might otherwise be considered prohibitive on the basis of solubility limitations, and (2) imply that the aqueous reactivity of fullerenebased materials must be considered appropriately for accurate assessment of their transport, fate, and potential risk(s) in environmental systems.

Introduction The reactivity of C60 in a variety of solvents has been widely studied (1-4), yet little information is available regarding C60 reactivity in water primarily due to its extremely low solubility (estimated C60 solubility 500 nm with varying degrees of dispersivity (9-14). UVVis spectra of the suspensions not only show characteristic molecular C60 absorbance but also additional broad absorption at ca. 400-500 nm. The latter absorption has been suggested to arise from symmetry-forbidden C60-C60 transitions, similar to what has been observed in C60 thin films (9-11, 13, 15, 16). Other spectral evidence, particularly FTIR and 13C NMR, indicates that aggregation occurs without functional derivatization (13, 17). Diffraction analyses also suggest that aggregates are crystalline with a simple hexagonal unit cell assignment (10, 13). Due to negative surface potentials (ζ potential ranging from ca. -9 to -65 mV, pH 1.2-9.75), these aggregates do not readily partition back into nonpolar solvents and remain stable in water (8, 10, 13, 14, 18, 19). This stabilization phenomenon permits suspended concentrations of C60 ca. 10-11 orders of magnitude above the estimated aqueous solubility (13), opening new avenues for fullerene chemistry previously considered improbable in water (20-23). Due to olefin-like reactivity, C60 dissolved in organic solvent is readily oxidized by ozone with a relatively high oxidation potential (EH° ) 2.07 V) (1, 3). Reaction kinetics and product identities have been described under varying reaction conditions (24-27). Unlike other olefins though, steric constraints of the cage structure do not allow a typical Criegee mechanism to occur (28). Instead, it was suggested that O3 attack under typical conditions occurs primarily along the [6,6] C-C double bond (part of the hexagon ring structure), resulting in a short-lived ozonide (C60O3) which dissociates via either thermolysis (26) or photolysis (24) into epoxide ([6,6] closed epoxide) or ether ([5,6] open oxidoannulene), respectively. Multiple oxygen adducts (C60Ox) have also been observed through such mechanisms as a function of O3 availability and reaction time. The molar ratio of O added per molecule of C60 has been reported as high as 21 (25, 29), but is usually reported at lower values of 1-10 (2325, 27, 30). Specific molecular characterization of highly ozonated fullerenes is complex, as the number of possible congeners and isomers is exceedingly large. Nevertheless, a range of functional groups such as epoxide, ether, carbonyl, and hydroxyl have been previously reported (24, 25, 31, 32). Work presented here investigates the reaction of fullerene aggregates with dissolved ozone in water. It is known that, in organic nonpolar solvents, by increasing the number of oxide additions per C60 derivative, a decrease in solubility is generally observed such that products become unavailable for further reactions (25). In contrast, but for the same reason, the reaction in the aqueous phase might proceed to a greater degree than that in the organic phase. Findings in this study indicate that nC60 is reactive with dissolved ozone, resulting in highly derivatized (C:O = 2:1), water-soluble, fullerene oxide(s) with repeating hydroxyl and hemiketal functionalities. Furthermore, not only do such aqueous-phase reactions allow for more efficient production of highly polar fullerene derivatives, but they also may be of environmental significance, as unintended aqueous reactions (e.g., ozone treatment among other applicable chemistries) could occur, resulting in products of significantly different behavior(s) compared to that of the parent materials (56).

Materials and Methods nC60. Briefly, nC60 solution was prepared by rapidly adding ultrapure water (>18.2 Ω, pH-unadjusted) to an equal volume of C60-saturated (ca. 9 mg/L) THF under vigorous mixing at room temperature and subsequently evaporating THF in the mixture following a stepwise distillation procedure (10, 13). VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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Several individually prepared batches, ca. 250 mL each, were combined into one stock (11.5 L) containing 5.1 mg/L nC60, which was used throughout this study. To ensure the removal of residual organics (e.g., THF or possible THF derivatives), >99.5% of the suspension water was replaced with ultrapure water using a pressurized (N2 UHP, 20-30 psi), stirred-cell membrane unit (Amicon, molecular weight cutoff (MWCO) of 10 000). The residual THF concentration in the final stock solution was verified to be below the detection limits (subppb) of a GC-MS head space analysis (Agilent model 6890N gas chromatograph equipped with a Teledyne Tekmar HT3 headspace autosampler and a 30 m × 0.25 mm o.d. DB-5ms column connected to an Agilent model 5974 inert massselective detector). Ozone Reactions. Semibatch experiments, defined as an open system maintaining the ozone concentration constant in the liquid phase by constant bubbling of ozone gas, were performed at pH 5.4, 6.8, and 8.9 and at 19.5 ( 0.5 °C. Ozone gas was generated by a Wedeco model GSO 10 ozone generator (Herford, Germany) from pure oxygen. Prior to contact with the reaction medium, the ozone gas was washed through a solution containing 10 mM phosphate buffer solution at pH 6. Concentrations of dissolved ozone were measured by the indigo colorimetric method described by Chiou et al. (33). Selected experiments were performed using excess tert-butyl alcohol (t-BuOH) as the hydroxyl radical scavenger (k[t-BuOH + •OH] ) 5 × 108 M-1 s-1) (34, 35). Typical semibatch reactions were initiated by adding 1020 mL of nC60 concentrated solution into the ozone solution, resulting in 5-10 mg/L suspensions. Sample aliquots of 2 mL were taken at appropriate time intervals, with a total sample volume of less than 10% of the reaction volume. For consistency, reaction conditions (reaction time and ozone concentration) are normalized as CT ((mg of O3 dissolved/L) × time (min)). Additional experiments were performed in a batch mode using a custom-built, multichannel stoppedflow reactor at 20.0 ( 0.1 °C (36). Stock solutions of dissolved ozone (concentrations ranging from 2.0 to 15 mg/L) were prepared by bubbling ozone gas through ultrapure water at pH 5. For both semibatch and batch experiments, ozone in the sample was quenched using excess sodium thiosulfate (4:1 Na2S2O3/O3) (34) or stripped with N2 (UHP) (for 13C NMR, MS, XPS, IR, and total organic carbon (TOC) analyses to avoid potential background interferences). All chemicals used throughout were reagent grade or higher unless otherwise noted. Product Characterization. The size and shape of nC60 aggregates were analyzed by two primary methods as previously demonstrated (10, 13, 14, 37): dynamic light scattering (DLS) using a ZetaPALS (Brookhaven Instruments Corp.) or a Zetasizer Nano ZS (Malvern Instruments Ltd.) and transmission electron microscopy (TEM). TEM images were prepared by evaporating 40 µL of concentrated suspension on a 400 mesh carbon-coated copper grid and imaged with a JEOL FasTEM 2010 at 100 kV calibrated to an aluminum standard. Spectral analyses were performed on product samples prepared from 100 to 180 mg/L C60 (as nC60) in contact with dissolved ozone (3-6 mg/L) for 30-50 min, resulting in a complete reaction. UV-Vis absorption spectra were taken within the range of 190-800 nm at 0.5 nm intervals using a Varian Cary 50 UV-Vis spectrophotometer and corrected for the appropriate background. The 13C NMR spectrum of a nC60 suspension prepared with 13C-enriched C60 (25%) in D2O before and after ozonation was obtained using a Varian Inova 600 MHz NMR instrument equipped with a carbon-enhanced cold probe. XPS analysis was performed using a PHI Quantera SXM scanning X-ray microprobe ULVAC-PHI with an Al mono, 24.8 W X-ray source and a 100.0 µm X-ray spot size at 45.0° (26.00 eV for 1 h). Samples were prepared by first sputter coating a clean silicon 7498

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substrate with Au for 2-10 min at 100 mA and evaporating ca. 100 µL of concentrated liquid sample on the substrate overnight at room temperature in a dust-free atmosphere. Data were analyzed with PeakFit to estimate the peak position and relative peak areas. ATR-FTIR spectroscopy was performed with a Thermo Nicolet Nexus 870 FTIR instrument and a Pike HATR instrument equipped with a germanium trough. TOC was measured using a Shimadzu TOC-5050A total carbon analyzer (Shimadzu Scientific Instruments, Inc., Columbia, MD) performing catalytic combustion at 680 °C and equipped with an infrared detector. MS analyses (both nC60 and reaction products) via a laser desorption/ionization (LDI) setup were performed using a tandem time-of-flight (TOF/TOF) mass spectrometer equipped with a 200 Hz laser (Applied Biosystems 4700 proteomics analyzer) under the positive ion mode. For matrix-assisted (MALDI) analyses, samples were temporarily dried and dissolved in an organic matrix (cyano-4-hydroxycinnamic acid (CHCA)) for increased sensitivity toward less polar products.

Results and Discussion Semibatch Experimental Results. Semibatch experimental results shown in Figure 1 indicate that nC60 is susceptible to reaction with ozone. The loss of characteristic 1T1u-1Ag transition peaks at 450, 340, and 260 nm (38) suggests molecular alteration upon reaction (Figure 1A). In parallel with the UV-Vis absorbance change, the solution gradually lost its characteristic yellow-orange color and became clear without accumulation of surface residues or precipitates. Also noticeable through DLS and TEM analyses as the reaction proceeded was the decrease in aggregate sizes (Figure 1B), which suggested that the products were no longer aggregates but probably molecularly dispersed (below the detection limit of DLS, 85% (in terms of carbon), as a form of soluble product which passed through a 10 000 MWCO filter (Amicon, Millipore Corp.) and was below the DLS detection limit (ca. 5 nm). Taken together, these results illustrate direct, facile oxidation of C60 by ozone

FIGURE 1. Ozonation of nC6: (A) nC60 (5 mg/L) spectra as a function of reaction time at pH 6.8 during semibatch ozonation ([O3] ) 4.3 mg/L); (B) changes in aggregate size and shape during semibatch ozonation. nC60 (C0, CT ) 0) 100 mg/L. TEM scale bar 100 nm.

FIGURE 2. Semibatch reaction kinetics measured as C/C0 (at 340 nm) at pH 5.4, 6.8, and 8.9 with (b) and without (O) 10 mM t-BuOH. in water over a range of pH values, resulting in soluble products.

and other ozone decay processes that may be promoted in the presence of C60.

Batch Experimental Results. Ozone decay kinetics were measured in a batch system with and without nC60. The reaction was carried out without contact with the atmosphere to prevent ozone loss due to volatilization and at pH 5.4 and with 10 mM t-BuOH to minimize ozone self-decay. After approximately 30 min of reaction time 10 mg/L (270 µM) ozone was consumed when approximately 5 mg/L (10 µM) C60 was transformed (27:1 molar ratio). This ratio was close to the 26:1 mole ratio estimated by Anachkov et al. as the maximum possible ozone consumed per molecule of C60 (40). Also note that this ratio is much higher than those observed in organic solvents. These results, however, do not necessarily mean that the 27:1 stoichiometric ratio should be assumed for the reaction between an individual C60 molecule and O3. It instead gives a normalized molar ratio of ozone consumed from all processes involved including direct C60 oxidation

Product Characterization. The LDI mass spectrum of the parent compound showed a single peak at 720 m/z (Figure 3A), which is, within our detection limits, consistent with the previous finding that nC60 is underivatized in nature (13, 17). A representative product obtained after exposure of nC60 to 5 mg/L O3 for 40 min at an unadjusted pH of 3.4 exhibited not only the parent peak at 720 m/z but also multiple, less intense peaks of 720 + (16-17)x additional m/z units, indicative of addition of oxygen and or hydroxyl functionalities (Figure 3B) (41-44). An additional mass spectrum obtained with organic matrix assistance (Figure 3C) showed an increase in relative intensity for C60 molecules with less oxide addition (25), which are less polar in nature. The occurrence of 720 m/z (60 carbons) observed in the product spectrum is likely an artifact of the MS analysis, whereby functional groups are lost during the ionization process as VOL. 41, NO. 21, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. Top: ATR-FTIR product spectrum of nC60 (180 mg/L) at pH 3.4 (unadjusted) after ozonation. Bottom: ATR-FTIR spectrum of C60(OH)22 (MER Corp.) (200 mg/L). FIGURE 3. LDI-MS spectra of (A) parent nC60 (100 mg/L) analyzed in water, (B) ozonation product (100 mg/L) in water, and (C) product (100 mg/L) in the matrix-assisted mode (cyano-4-hydroxycinnamic acid).

FIGURE 4. 13C NMR spectra of (A) parent nC60 (150 mg/L, 25% 13C C60) in water and (B) ozonation product (180 mg/L) in water. reported by others (25, 26, 41, 45). The presence of the 720 m/z peak in the products is insightful, however, as it indicates that a 60-carbon cage structure remains intact throughout ozonation, thus providing a fundamental carbon architecture for the products. The 13C NMR spectrum of the 13C-labeled parent compound showed a single nC60 peak at 146 ppm, characteristic of underivatized material with Ih symmetry (Figure 4A) (13, 47-49). After the reaction with ozone, the peak at 146 ppm was no longer observed. In contrast, four new peaks were observed at 176, 168, 128, and 95 ppm (Figure 4B), indicating a high level of functional derivatization and loss of Ih symmetry (1, 3). Product peak shifts indicate the presence 7500

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of different oxygen moieties in the products. These peaks might be assigned as carbonyl carbon at 176 ppm, vinyl ether (-CdCsO-) carbon at 168 ppm, C-C cage carbon at 128 ppm (as CdC), which is shifted downfield as a result of decreased molecular strain, and C-O (or -O-C-O-) carbon at 95 ppm according to previous reports (49, 50). The relatively small number of peaks present at different chemical shifts suggests a high level of molecular symmetry indicative of repeating functional arrangements. The ATR-FTIR spectrum of the final products provides further information on the identity of oxygen moieties present in the products (Figure 5). A broad OH absorption at 3400 cm-1, C-OH in-plane bending at 1360 cm-1, and C-O stretching at 1315 cm-1 collectively indicate the existence of hydroxyl functionalities. A sharp and strong absorption at 1630 cm-1 along with a weaker shoulder at 1760-1770 cm-1 suggested other forms of oxygen moieties, as similar peaks have been observed for other fullerene oxides including fullerenes ozonated in organic solvents and hemiketal fullerols. In particular, these peaks have been attributed to carbonyl moieties such as carboxyl groups and ketones (30, 31) or to part of a hemiketal moiety (R-O-C-OH) (31, 41, 49). The ether functionality could be responsible for the broad and weak band centered at 1110 cm-1 (50). In any case, presently there are few absolute reports of IR functional identification of fullerene oxide groups, as the types of possible derivatives vary widely depending on the oxidation conditions (31, 41, 43, 49, 51). For example, an IR spectrum for a commercial fullerene oxide from MER Corp. (Tucson, AZ) prepared through substitution of brominated C60 at pH 3 (C60(O)x(OH)y, where x + y ) ∼22) (52) is provided in Figure 5 (bottom) for comparison. Parent C60 spectra have been analyzed by others with C60 as a solid and as a water-stable colloid, both demonstrating the presence of four sharp absorption peaks at 524-526, 571, 1180-1183, and 14291430 cm-1 (17, 53, 54). The XPS absorption spectrum of the product shown in Figure 6 is consistent with the above observations. In addition to C(1s) absorption with a binding energy at 285.5 eV by underivatized carbon, two additional peaks appeared at higher energy levels. Additional peaks represented carbon at different oxidation states: monooxygenated carbon (C-O) at 287.45 eV and dioxygenated carbon (O-C-O, CdO) at 298.73 eV (51). Dioxygenated carbon has been observed in fullerene oxides, particularly in hemiketal forms by Chiang et al. (2, 51). Spectrum deconvolution by curve fitting (Lorentz area curves, >0.9% goodness of fit, PeakFit second derivatives)

fullerene chemistries (e.g., electrophilic and nucleophilic additions, electron transfer, etc.) may also be possible via similar aqueous-phase route(s), expanding the applicability of C60 and its polar derivatives. Conversely, the aqueous availability and reactivity of C60 take on additional significance as inadvertent reactions may also result in soluble products. For example, an ozone disinfection process with a CT of 7.8 (mg min)/L for 2 log removal of Cryptoporidium parvum oocysts at 20 °C according to the Long Term 2 Surface Water Treatment Rule by U.S. EPA (55) might result in approximately 35-45% of the C60 in aggregate form (based on the loss of absorbance at 340 nm, Figure 2) to undergo chemical derivatization at pH 8.9. These soluble products will behave significantly differently from the parent aggregate in aqueous environments as they are molecularly dispersed (56). Consequences of such aqueous reactivity, which is certainly not limited to ozonation only, should be considered appropriately in relevant environmental fate and transport studies.

Acknowledgments FIGURE 6. C(1s) XPS spectrum and curve-fitting analysis of the ozonation product of nC60. Top: Points represent spectral data. Bottom: Curve-fitting deconvolution expressed in relative intensity. Table: Identification and integrated peak areas (relative intensities). indicates that the ratio between nonoxygenated and oxygenated carbons was ca. 31:29, suggesting an average derivative structure as C60(O)x(OH)y, where x + y ) ∼29. Among 29 oxygenated carbons, approximately 11 were monooxygenated, while the remaining 18 were dioxygenated. UV-Vis analysis of ozonated products showed a broad UV absorption increase (Figure S1, Supporting Information), similar to that of commercial fullerols (MER Corp.; 49). Compared to the highly conjugated parent C60 molecule, both ozonated products appeared to have relatively low extinction coefficients in the UV region. Interestingly, the absorbance of the ozonated products was pH dependent, with new (weak) peaks appearing at ca. 260 and 360 nm only at high pH (Figure S1A). Commercial fullerol in contrast did not exhibit a similar pH dependence (Figure S1B). Such a pH dependency is also supportive of hemiketal groups which undergo rearrangement as a function of pH (49). This is also consistent with the observation that the pH of the product solution decreased as the product concentration increased (nonbuffered solution), as the enol forms of the products were deprotonated and became anionic. Such deprotonation and consequent assumption of a negative charge by C60 derivatives at higher pH might facilitate nC60 disintegration during ozonation and accelerate the reaction kinetics as previously described. The above results collectively indicate that nC60 readily reacts with ozone in the aqueous phase over a range of pH values to form highly oxidized (average ∼29 oxygen additions) and molecularly soluble products. MS, NMR, ATR-FTIR, XPS, and UV analyses suggested that these products maintain a 60-carbon-atom cage structure and the oxygen-based functionalities in the products are most likely in repeating hemiketal and hydroxyl forms, while the presence of other minor functional groups should not be excluded. On the basis of similar reaction mechanisms proposed for organicphase reactions, the aqueous-phase reaction is likely to proceed through initial formation of a [6,6] primary ozonide (26), subsequent dissociation into a [6,6] closed epoxide (concurrent with loss of O2) (26), and further hydrolysis to form hemiketal arrangements (49). The work presented herein demonstrates the feasibility of chemically functionalizing C60 in the aqueous phase, through the formation of water-stable, nanoscale, aggregate intermediates which overcome solubility limitations of the parent molecule. Accordingly, a wide spectrum of other

We thank David Bostwick, Mass Spectroscopy Facilities, Department of Chemistry and Biochemistry at the Georgia Institute of Technology, for MS analyses; Jed Costanza, School of Civil and Environmental Engineering, Georgia Institute of Technology, for GC-MS analyses; and Brandon Lafferty, Department of Soil Chemistry, University of Delaware, for ATR-IR analyses and insightful discussions. This study was supported by the U.S. Environmental Protection Agency (STAR Grant #D832526) and the Nanoscale Science and Engineering Initiative of the U.S. National Science Foundation (EEC-0118007).

Supporting Information Available An additonal figure showing UV-Vis spectra. This material is available free of charge via the Internet at http:// pubs.acs.org.

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Received for review April 5, 2007. Revised manuscript received August 6, 2007. Accepted August 6, 2007. ES0708058