Transformation of Aggregated C60 in the Aqueous Phase by UV

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Environ. Sci. Technol. 2009, 43, 4878–4883

Transformation of Aggregated C60 in the Aqueous Phase by UV Irradiation JAESANG LEE,† MIN CHO,‡ JOHN D. FORTNER,§ JOSEPH B. HUGHES,‡ AND J A E - H O N G K I M * ,‡ School of Civil and Environmental Engineering, Rice University, Houston, Texas 77005, School of Civil and Environmental Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, and Department of Chemistry, Rice University, Houston, Texas 77005

Received December 19, 2008. Revised manuscript received April 29, 2009. Accepted May 1, 2009.

This study demonstrates that water-stable C60 cluster (nC60) undergoes a photochemical transformation(s) when irradiated with monochromatic UV light at 254 nm. Upon UV exposure, characteristic absorption of nC60 in the visible (ca. 450-550 nm, indicative of a cluster structure) and UV regions (indicative of underivatized molecular C60) gradually disappeared. Concurrently, a new product with absorption centered at 210 nm formed. Dynamic light scattering (DLS) and transmission electron microscopy (TEM) analyses confirmed a complete reduction in aggregation and formation of a soluble product. Negligible loss of total organic carbon (TOC) and drastic retardation in degradation kinetics in the absence of oxygen collectively implied that photochemical transformation was accomplished via oxidative pathway without carbon mineralization. MS (LDI), FTIR, and XPS analyses indicated a 60 carbon cage product, containing various oxygen functional groups such as epoxides and ethers. In addition, this product demonstrated significantly less antibacterial effects on Escherichia coli when compared to the parent nC60. The results of this study suggest that accurate assessment of C60 in environmental life cycles and impact should consider the light-mediated transformation of C60 in the aqueous phase and resulting water-soluble products.

Introduction Fullerenes including C60 are known to be susceptible to various oxidation and reduction reactions as well as photochemical reactions, forming diverse derivatized fullerene products (1-10). Electrophilic addition of radicals and oxidants such as singlet oxygen and ozone is the result of the C60’s electron-rich cage structure with localized, conjugated π-systems (1, 7, 8, 11). These reactions are facilitated by UV irradiation, which excites ground-state C60 to the more reactive triplet-state (9, 12). For example, C60 dissolved in organic solvents such as benzene readily acquires functional groups such as carbonyl and carboxylic during UV irradiation in the presence of oxygen (4, 10). A photochemical trans* Corresponding author. E-mail: [email protected]. Phone: (404) 894-2216. Fax: (404) 385-7087. † School of Civil and Environmental Engineering, Rice University. ‡ School of Civil and Environmental Engineering, Georgia Institute of Technology. § Department of Chemistry, Rice University. 4878

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formation of C60 in solid-state thin film to C60 oxide has been also reported (13). Most of these reactions have been previously studied in nonaqueous medium, because molecular water solubility of C60 is extremely low. Recent findings that C60 forms water-stable aggregates, often referred to as nC60, via various preparation methods have raised both interest and concern on C60’s reactivity and toxicity in aqueous environments (14). Toxicological effects of nC60 on microorganisms and higher organisms have been reported (15-19), although results vary depending on the preparation method (20). In order to get an accurate assessment of environmental impact of nC60, it is necessary to understand its fate in the environment, including potential chemical transformations. If transformation occurs, products may have very different fate(s) in the environment and may also have different toxicological effects. Compared to abundant reports of reactivity of C60 in organic solvents (1, 4-10, 13, 21, 22), little is known regarding reactions of C60 (as nC60) in the aqueous phase. A recent study by Fortner et al. (11) demonstrated that nC60 can be transformed into highly oxygenated, molecularly soluble products upon reaction with dissolved ozone. The study presented herein examines the reaction of the water-stable colloidal form of C60 (nC60) by irradiation of monochromatic UV at 254 nm. While photons in the UVC wavelength regions of sunlight (λ < 290 nm) do not penetrate into the lower troposphere and reach the ground surface (23), C60 transported to the upper atmosphere, for example, that generated from coal-burning power plants (24) or from lightning and volcanic action (25), might form similar nC60 in water droplets with direct exposure to short-wavelength UV irradiation. The nC60 released to wastewater and source water for drinking water could be exposed to germicidal UV light during disinfection processes. Considering that nC60 could transform into unknown products by prolonged exposure to sunlight (26), photochemical transformation by higher-energy photon irradiation might be also possible. To test the above hypothesis, reaction kinetics were examined and the products were characterized by a suite of analytical techniques including UV-vis, total organic carbon (TOC) analysis, dynamic light scattering (DLS), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), attenuated total reflectance-Fourier transform infrared (ATR-FTIR) spectroscopy, and laser desorption ionization-mass spectroscopy (LDI-MS). Experiments were also conducted to assess the toxicological properties of the product compared to parent nC60 using Escherichia coli (E. coli) growth bioassays. Results demonstrating facile transformation of nC60 by short-wavelength UV irradiation are significant when considering the environmental half-life and the ubiquitous presence of UV light. Furthermore, the corresponding change in biological response upon UV irradiation highlights the dynamic nature of the material’s life cycle as it relates risk(s).

Experimental Section Chemicals. C60 (Sublimed, 99.9%) was purchased from the Materials Electronics Research Corporation (Tucson, AZ). All the solutions were prepared using ultrapure water (>18 MΩ) produced by a Milli-Q water purification system (Millipore, Billerica, MA). Preparation of Aqueous Stable C60. To avoid possible solvent interference(s), nC60 was prepared without using organic solvent. Ultrasound (20 W) was applied to 500 mL of ultrapure water containing 100 mg of C60 in a sealed bottle for 24 h (continuously) at an ambient temperature using an 10.1021/es8035972 CCC: $40.75

 2009 American Chemical Society

Published on Web 06/01/2009

FIGURE 1. UV-vis spectra of nC60 as a function of UV irradiation time at pH (a) 3, (b) 5.5, and (c) 10 ([C60]0 ) 5 mg/L). ultrasonicator (S-4000, Misonix Co., Farmingdale, NY). The solution color changed to light orange, indicating that C60 formed water-stable clusters (27). The resultant solution was filtered through a 0.45 µm cellulose filter (Millipore) and then stored under dark. The filtrate containing 2-3 mg/L of nC60 was further concentrated to a stock solution of 20 mg/L using a rotary evaporator. The concentration of C60 in the aqueous phase was measured based on absorbance at 350 nm. Past studies have suggested that C60 concentration up to 5 mg/L was linearly proportional to the concentration measured using HPLC after extraction to toluene (11, 28). Photochemical Experiments. Photochemical experiments were carried out using a magnetically stirred 60 mL cylindrical quartz reactor surrounded by four U-type 9-W germicidal lamps (Philips PL-S 9W) at an ambient temperature (25 °C). The incident light intensity of each lamp at the location of the reactor was measured at 11 mW/cm2 using a UVX-25 radiometer (UVP Co., U.S.A.). The solution contained 5 mg/L of nC60, and the pH was adjusted using 1 M HCl or NaOH standard solution. For experiments under anoxic condition, the reactor was purged with ultrapure N2 gas for 30 min and sealed. The reactor headspace was continuously purged with nitrogen using a needle during photochemical experiments. As the photochemical reaction proceeded, sample aliquots of 2 mL were withdrawn from the reactor using a syringe and immediately injected to a UV quartz cell for UV-vis spectrophotometric analysis (Agilent 8453). All kinetic experiments were performed in triplicate. Characterization. In order to obtain a sufficient amount of sample for product characterization, a solution containing 20 mg/L of nC60 was irradiated for 110 h and further concentrated using a reverse osmosis membrane (GE Osmonic Sepa CF Polymaide RO AG membrane). The XPS analysis on the product was carried out using a PHI Quanteria SXM scanning X-ray microprobe ULVAC-PHI with an Al mono, 24.8 WX- ray source and a 100.0 µm X-ray spot size at 45.0° (26.00 eV for 1 h). Samples were prepared by sputter coating a clean silicon substrate with Au and then evaporating ca. 100 µL of product solution on the substrate overnight at room temperature in dust-free atmosphere. Data were analyzed with PeakFit to estimate peak position and relative peak areas. ATR-FTIR analysis was conducted using a Thermo Nicolet Nexus 870 FTIR spectrometer equipped with a Pike Technologies horizontal attenuated total reflectance (HATR) germanium trough. MS analyses on both nC60 and reaction product via an LDI setup were performed using a time-offlight (TOF) mass spectrometer (Bruker Corporation, MS Reflux IV) under the positive ion mode. The size of nC60 and product were analyzed by DLS in a suspension using a Zetasizer Nano ZS90 (Malvern Instruments, Worcestershire, OK). Electron microscope images were taken by a Philips 120 TEM. TEM specimens were prepared by first placing

several droplets of sample on a copper carbon grid (Electron Microscopy Science, Hatfield, PA) and drying overnight at ambient temperature. UV-vis absorption spectra were analyzed using a Spectropro-500 spectrophotometer (Acton Research Co., U.S.A.). TOC was measured using a Shimadzu TOC-5050A analyzer (Shimadzu Scientific Instruments, Inc., Columbia, MD). Antibacterial Activity Test. Antibacterial activity of nC60 was evaluated as a function of UV irradiation time using the minimal inhibitory concentration (MIC) assay with E. coli (ATCC strain DH5R) as described elsewhere (29). E. coli was cultured on Luria-Bertani (LB, Difco Co., U.S.A.) broth at 37 °C for 18 h, and the stock concentration of cultured E. coli was determined by a spreading plate method. A 40 mL solution containing 20 mg/L of nC60 was irradiated for varying time periods and diluted to various concentrations. These samples were added to Minimal Davis media (i.e., Davis media with potassium phosphate reduced by 90%) after purging with nitrogen under heat. After inoculation and incubation overnight at 37 °C, E. coli in the media was enumerated by the spread plate method and the optical density method. The concentration of nC60 resulting in no growth of E. coli after 18 h of incubation was denoted as the MIC.

Results and Discussion Photochemical Degradation of nC60. UV-vis spectra of nC60 are shown as a function of UV irradiation time and pH in Figure 1. Characteristic molecular C60 absorption peaks at 260 and 350 nm decreased with exposure to UV light and ultimately disappeared, suggesting alteration of the electronic structure of the C60 cage. Reaction kinetics, examined by plotting the absorption at 350 nm (14, 30) versus time (results not shown), were not significantly dependent on pH, although enhanced at pH 10. Increased surface negativity at higher pH (31) might be related to increased repulsion between parent nC60 and derivatized products, leading to enhanced nC60 disaggregation during the course of the reaction (discussed below), while the exact mechanism is currently unknown. Broadband absorption in the wavelength region from 450 to 550 nm (14, 32, 33), which is indicative of aggregated C60-C60 interactions, steadily decreased during the course of UV irradiation, which eventually disappeared after 30 h. This observation suggests that the C60 cluster disaggregates by UV irradiation as more hydrophilic product forms, consistent with the findings by Fortner et al. (11). Figure 2 compares the nC60 and the product after prolonged exposure (110 h) to UV irradiation. The product exhibited a strong absorption centered at 210 nm and with no absorption at other wavelengths including broadband absorption in the visible range from 400 to 500 nm. While the original nC60 exhibited highly aggregated structure in VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. (a) UV-vis spectrum of the product after 110 h of UV irradiation; (b) TEM images of nC60 (left) and the product after 110 h UV irradiation (right) ([C60]0 ) 20 mg/L; initial pH ) 5.5). TEM images, most of these particles disappeared in the product sample (Figure 2b). The average size of the product by the DLS analysis gradually decreased (124.6 nm at 0 h, 107.9 nm at 25 h, and 90.3 nm at 50 h) until the particle concentration became too low for reliable analysis. Although extensive analysis of entire TEM sample grids indicated sporadic occurrence of particulate matter in the product, the number concentration was also too low to be statistically analyzed via TEM. These results also suggest that nC60 transformed by prolonged UV irradiation into very small aggregates or even molecular species that are close to the detection limits of the employed DLS and TEM techniques. Product Characterization. It has been reported that dry C60 could be oxidized to CO and CO2 as well as partially mineralized carbonyl intermediates when subjected to relatively harsh oxidation (6, 21). However, for the reaction observed in this study, the total organic carbon (TOC) of the solution (5 mg/L nC60) did not decrease even after 30 h of UV irradiation (4.3, 4.5, and 4.2 mg/L for 10, 20, and 30 h, respectively), indicating that nC60 transformation did not involve any measurable level of carbon mineralization. The XPS absorption spectrum of the product shown in Figure 3, after spectral deconvolution by curve fitting (Lorentz area curves, >0.9% goodness of fit; PeakFit second derivatives), indicates that oxidized C60 carbons formed upon UV irradiation. The two additional peaks correspond to the higher carbon oxidation states absorption peaks (as C(1s) binding energies) with the main peak at 283.87 eV indicative of underivatized carbon (34). The peak at 285 eV represents mono-oxygenated carbon (C-O) and accounts for approximately 23% of carbon in C60. The peak at 288.7 eV represents dioxygenated carbon (CdO and O-C-O) and accounts for approximately 13% of carbon in C60. The ratio between underivatized carbon and oxygenated carbon was 4880

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64:36, i.e., approximately one-third of C60 carbon was oxidized by UV irradiation for 110 h. The ATR-FTIR spectrum of the product shown in Figure 4 provides further information on the identity of oxygen moieties. A broad OH absorption centered at 3400 cm-1 and C-OH in-plane bending or C-O stretch absorption at 1375 cm-1 are strong indications that hydroxyl functionalities exist in the product. An absorption at 1630 cm-1 along with weaker shoulder at 1700 cm-1 suggested the presence of carboxyl groups or ketone (1, 2). Strong peaks appearing at 1125 and 1250 cm-1 can be attributed to ether and epoxide, respectively (1, 11). Considering that the IR spectra of oxidized C60 vary sensitively depending on oxidation conditions (1, 3, 11, 35), it is noteworthy that this spectrum has several similarities with that of C60 oxygenated by ozone in the aqueous phase (1, 11) as well as a commercial fullerene oxide from MER Corp. (Tucson, AZ) prepared through substitution of brominated C60 (C60(O)x(OH)y, where x + y ) ∼22) (Figure 4), although relative peak intensities varied rather largely depending on the compound. The LDI-MS spectrum of the product was characterized by the appearance of parent C60 peak at 720 m/z as well as multiple peaks indicating fragmentation and coupling (Figure 5). This spectrum was in marked contrast to that of parent nC60, which contained a single peak at 720 m/z (not shown). In particular, multiple peaks with 24 m/z intervals below the parent compound peak represent fullerenes with stepwise loss of C2 fragments during LDI. Such a stepwise fragmentation during LDI is typical for C60 oxide with ether or epoxide functional groups (36). Multiple peaks at 1050 to 1350 m/z and 1600 to 1800 m/z might represent the dimers and trimers that form during UV photolysis or during MS analysis (i.e., recombination of fullerene fragments). Furthermore, the peak at 720 m/z and accompanying peaks for 720 m/z based fragments strongly suggest that the resulting derivatives maintain the 60-carbon cage structure. The above analytical results collectively indicate that nC60 in the aqueous phase is transformed by UV irradiation into products that contain various mono- and dioxygenated functionalities. The product seems to retain a stable 60carbon atom cage structure, since no organic carbon was lost and the parent compound peak was observed in MS analysis. Although no further quantitative information on the relative abundance of each functional group on the oxidized C60 is currently available, it is postulated that epoxide or ether functionality are likely the dominant functional groups based on XPS and FTIR peak intensities and fragmentation pattern during MS analysis. Other functional groups detected may result from hydrolysis of epoxide and ethers, resulting in hemiketal functionality (11, 37). This result is partly in accordance with previous findings that C60 in benzene could be photochemically oxidized to form monoxide C60O having two isomers, i.e., ether and epoxide (38). It is also noteworthy that fullerene oxide is a major product of singlet oxygen addition to photoexcited triplet C60 (9), consistent with the hypothesized reaction mechanism below. Reaction Mechanism. The rate of nC60 transformation was much slower in the absence of oxygen (Figure 6). This suggests that oxygen participates in nC60 photochemical transformation, possibly via oxidative pathway(s). Although UV photolytic oxidation of molecularly dissolved C60 in organic solvents (4, 9, 10) and C60 in multilayer solid-state film (13) has been reported, little is known regarding the mechanism of photochemical oxidation of nC60 in the aqueous phase. On the basis of the current level of understanding on C60 reactivity upon UV irradiation and the very recent finding (26) that C60 clusters could be also phototransformed by sunlight irradiation, the following mechanism is hypothesized. First, C60 in the cluster may be excited to its triplet state by UV irradiation. At the same time, singlet

FIGURE 3. C(1s) XPS spectrum and curve-fitting analysis of the 110 h UV irradiation product. (Top) Points represent spectral data; (Bottom) curve-fitting deconvolution expressed in relative intensity (Gaussian fits); (Table) identification and integrated peak areas (relative intensities).

FIGURE 4. (Top) ATR-FTIR spectrum of 110 h UV irradiation product; (Bottom) ATR-FTIR spectrum of commercial fullerol (C60(OH)22-24). oxygen (1O2) might be generated via energy transfer from excited triplet-state C60 to ground triplet-state oxygen. It has been reported that triplet-state C60 photochemically transforms to C60O through the reaction with 1O2 (9, 13), while ground-state C60 was inert toward 1O2 attack (39). However, the past study (40) also suggested that the lifetime of triplet-state C60 within the aggregate structure is extremely short (i.e., pico-second scale). It was also reported that the 1O2 production by energy transfer was drastically reduced when C60 formed clusters, below the detection limit of commonly used surrogate method (e.g., employing furfuryl alcohol as a 1O2 indicator) or electron spin resonance analysis (40, 41). As a result, the above oxidative reaction pathway should be very slow, consistent with the earlier reports that solid-state C60 would react with oxygen only when it was heated to 200 °C (21) or irradiated with high-energy photon such as X-ray under vacuum (6). For the reaction in the

FIGURE 5. LDI-MS spectrum of 110 h UV irradiation product. aqueous phase, however, as the reaction proceeds and the oxidized C60 is released from the cluster structure (as products become more hydrophilic due to oxygen addition), it is possible that the product becomes more photoactive and 1 O2 production rate is enhanced (42). The above mechanism could not be verified since the concentrations of key reactants, triplet-state C60 and 1O2, are below the detection limit of state-of-the-art instrumental techniques. However, it partially explains why the photochemical reaction of nC60 was relatively slow even under a very harsh UV irradiation condition as shown in Figure 1. For example, 45% transformation (i.e., concentration estimated based on absorption at 350 nm) of 5 mg/L (7 µM) of nC60 requires as much as 1.6 kJ/cm2 of UV irradiation. In comparison, 50% of phenol at an initial concentration of 2.2 mM can be photochemically degraded by UV dose of ca. 78 J/cm2 (43). As another comparison, the required UV doses for 4 log inactivation of virus and Cryptosporidium parvum oocysts are 186 and 22 mJ/cm2, respectively (44). VOL. 43, NO. 13, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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effect on E. coli compared to parent nC60. In addition, this study demonstrates that UV application may be suitable for aqueous-based green fullerene chemistries and even material detoxification.

Acknowledgments This study was supported by the U.S. Environmental Protection Agency (STAR Grant #D832526). The authors thank David Bostwick at the Mass Spectroscopy Facilities at Department of Chemistry and Biochemistry, Georgia Institute of Technology, for MS analyses and Brandon Lafferty at Department of Soil Chemistry, University of Delaware, for ATR-FTIR analyses and insightful discussions.

Literature Cited

FIGURE 6. Normalized absorbance of nC60 at 350 nm as a function of UV irradiation time in the absence and presence of oxygen ([C60]0 ) 5 mg/L; initial pH ) 5.5).

TABLE 1. Minimal Inhibitory Concentration (MIC) of nC60 before and after UV Irradiation for E. coli (+ and - Indicate Positive Microbial Growth and No Growth, Respectively); The Concentration Refers to the Initial Concentration of nC60 Prior to UV Irradiation and after Dilution UV irradiation time (h)

concentration of nC60 (or UV-treated products) (mg/L)

0

25

50

70

90

110

0 1 2 4 6 8 10

+ + -

+ + -

+ + + -

+ + + + + -

+ + + + + + -

+ + + + + + +

Antibacterial Activity. As the concentration of nC60 (no UV treatment) was increased beyond 2 mg/L, the growth of E. coli was inhibited (Table 1). This result was consistent with the earlier findings that MIC value for nC60, prepared without a transfer solvent, ranges from 0.5 to 2.0 mg/L (29). In contrast, as nC60 was exposed to UV and transformed, MIC values continuously increased as high as 10 mg/L for nC60 upon 110 h irradiation. This result suggests that photochemical transformation of nC60 leads to a decrease in toxicological effect (as measured by this assay), similar to earlier findings that reported decreased toxicity corresponding with increased C60 functionalization (18). Environmental Implication. This study demonstrates that nC60 in the aqueous phase undergoes photochemical transformation by short-wavelength UV irradiation in the presence of oxygen. As a result, colloidal C60 disaggregates into much smaller clusters (i.e., below the detection limit of DLS) or molecularly soluble species that contain various oxygen functional groups. These findings are critical for accurate life cycle assessment of C60 in environmental systems as key physiochemical material properties are significantly altered, leading to different dispersion status in the aqueous phase (i.e., particle versus molecule), surface interaction and transport behaviours, potential for further chemical and biochemical transformation, and interaction with living cells. Highlighting this dynamic are the results demonstrating the C60 photolysis product having significantly less antibacterial 4882

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