Grantee Research Project Results
2007 Progress Report: Fate and Transformation of C60 Nanoparticles in Water Treatment Processes
EPA Grant Number: R832526Title: Fate and Transformation of C60 Nanoparticles in Water Treatment Processes
Investigators: Kim, Jae Hyung , Hughes, Joseph
Institution: Georgia Institute of Technology
EPA Project Officer: Packard, Benjamin H
Project Period: September 1, 2005 through December 31, 2008
Project Period Covered by this Report: September 1, 2006 through December 31, 2007
Project Amount: $375,000
RFA: Exploratory Research: Nanotechnology Research Grants Investigating Environmental and Human Health Effects of Manufactured Nanomaterials: A Joint Research Solicitation - EPA, NSF, NIOSH (2005) RFA Text | Recipients Lists
Research Category: Human Health , Safer Chemicals , Nanotechnology
Objective:
B. WORK STATUS/PROGRESS
B.1. Overview
In Year 2 of the project, we continued to evaluate the interaction of natural organic matter (NOM) with multi-walled carbon nanotubes (MWNTs) and the mechanism of C60 photochemical reactivity in the aqueous phase. These studies were based on our findings made during the first year of the project. Three manuscripts were submitted (currently under review) during this project year, all to ACS Environmental Science & Technology. The findings from these studies are presented in Chapters B.2, B.3, and B.4 (each chapter representing each manuscript submitted).
First, we investigated the effect of NOM characteristics and water quality parameters on NOM adsorption to MWNT (Chapter B.2). The isotherm experimental results fitted well with a modified Freundlich model that took into account of the heterogeneous nature of NOM. Accordingly, the preferential adsorption of the higher molecular weight fraction of NOM was observed by a size exclusion chromatographic analysis. Experiments performed with various NOM samples suggested that the degree of NOM adsorption varied greatly depending on the type of NOM and was proportional to the aromatic carbon content of NOM. The adsorption of NOM to MWNT was also dependent on water quality parameters: adsorption increased as pH decreased and ionic strength increased. As a result of NOM adsorption to MWNT, a fraction of MWNTs formed a stable suspension in water, the concentration of which depended on the amount of NOM adsorbed per unit mass of MWNT. The amount of MWNT suspended in water was also affected by ionic strength and pH. The findings in this study suggested that the fate and transport of MWNT in the natural system would be largely influenced by NOM characteristics and water quality parameters.
We also continue to explore the mechanism of photochemical reactivity of C60 in the aqueous phase based on our findings made in the first year of the project. First, in Chapter B.3., we demonstrate that the degree of C60 clustering in the aqueous phase is strongly dependent on the type and concentration of encapsulating agents such as surfactant, polymer and natural organic matter that interact with C60. Degree of C60 clustering was quantitatively analyzed using UV-Vis spectral characteristics. The dispersion status played a critical role in determining C60’s photochemical reactivity, in particular, its ability to mediate energy transfer and produce singlet oxygen in the presence of oxygen. Consistent with findings in the organic phase, C60 in the aqueous phase lost its intrinsic photochemical reactivity when they formed aggregates. Experiments performed using a laser flash photolysis suggested that the loss of reactivity resulted from drastic decrease in lifetime of key reaction intermediate, i.e. triplet state C60. This study suggests that the photochemical reactivity of C60 in the aqueous phase, which has been linked to oxidative damage in biological systems in earlier studies, is strongly dependent on the media environment surrounding C60.
In work presented in Chapter B.4., The mechanism involved with energy and electron transfer by C60 in the aqueous phase during UV irradiation and subsequent production of reactive oxygen species (ROS) such as singlet oxygen and superoxide radical anion was investigated. Electron paramagnetic resonance (EPR) study showed that C60 embedded in micelles of nonionic surfactant (Triton X 100) or anionic surfactant (sodium dodecylbenzenesulfonate) produced ROS, but aggregated C60 did not, consistent with our earlier findings made using indicator chemicals. Nanosecond and femtosecond laser flash photolysis showed that the aggregation of C60 significantly accelerates the decay of excited triplet state C60, which is a key intermediate for energy and electron transfer, thus blocking the pathway for ROS production. This finding suggests that C60 clusters will not contribute to oxidative damage or redox reactions in natural environment and biological systems in the same way molecular C60 in organic phase reportedly does. In contrast, C60 embedded in surfactant micelles produces ROS and the evidence is presented for the formation of C60 radical anion as an intermediate.
Progress Summary:
B.2. Natural Organic Matter (NOM) Adsorption to Multi-Walled Carbon Nanotubes: Effect of NOM Characteristics and Water Quality Parameters
B.2.1. Acknowledgements
This work was submitted to ACS Environmental Science & Technology and currently under review. The authors thank Dr. Jim Millette and Whitney Hill at MVA Scientific Consultants (Duluth, Georgia) for their assistance on TEM analysis. Dr. Vernon Snoeyink at University of Illinois at Urbana-Champaign is also thanked for reviewing the manuscript and providing valuable comments.
B.2.2. Introduction
As the evidence for toxicological effects of carbon nanotubes (CNT) is rapidly accumulating [1-3], understanding the fate and transport characteristics of CNT in the natural environment during unintended discharge is becoming an important issue. In particular, an exposure route involving natural waterways, which has traditionally not been considered as these molecules are extremely hydrophobic, has been receiving a widespread interest. Our recent study [4] showed that pristine multi-walled carbon nanotubes (MWNT) could be stabilized (suspended) in the aqueous phase by natural organic matter (NOM) which might provide sterically and electrostatically stable surfaces to MWNT after adsorption to the MWNT surface. This finding suggested that the dispersal of carbon based nanomaterials, CNT in particular, in the natural aquatic environment might occur to a higher extent than predicted based only on the hydrophobicity of these materials. In order to accurately predict the behaviors of MWNT in the environment, the mechanism of interaction between NOM and CNT and the effect of water quality on this interaction need to be elucidated.
NOM is a mixture of chemically complex polyelectrolytes with varying molecular weights, produced mainly from the decomposition of plant and animal residues [5]. Due to the carboxylic and phenolic moieties distributed throughout the entire molecule, NOM generally carries a negative charge in the natural environment [6]. These physical and chemical characteristics of NOM are likely to be closely related to the mechanism of NOM interaction with CNT. Compared to NOM adsorption onto CNT, the mechanism of NOM adsorption onto activated carbon is relatively well known due to the rich history of application to water treatment. A few characteristics of NOM interaction with activated carbon are noteworthy and might be helpful for interpretation of CNT-NOM interaction.
First, the adsorption capacity and strength strongly depend on the type of NOM and the type of activated carbon. Factors affecting adsorption have been reported to include size and chemical characteristics of NOM as well as pore structure and surface chemistry of activated carbon [5-8, 11]. Second, due to the polydisperse nature of NOM, different fractions of NOM tend to have a different degrees of adsorptive interactions with the adsorbent [7]. This preferential adsorption is reflected by the occurrence of dose dependent isotherm relationship. For example, the strongly adsorbable fraction of NOM exhibits a more favorable adsorption at lower activated carbon dose. Finally, NOM adsorption is affected by water quality parameters such as ionic strength and pH which influence the charge and configuration of NOM [6]. Specifically, the adsorption of negatively charged NOM to the activated carbon surface generally increases as ionic strength increases and pH decreases [5, 8-11].
Differences between activated carbon and CNTs need to be also recognized for the proper interpretation of CNT-NOM adsorption phenomena. First, the activated carbon consists of pores of different sizes which provide sites for NOM adsorption. CNTs in contrast provide adsorption sites only along the surface of a cylindrical structure [12]. Second, the chemical structure of activated carbon, which contains carbons of varying degree of saturation and oxidation state as well as functional groups formed during activation process [7], is fundamentally different from that of CNT, which consists only of globally conjugated unsaturated carbons in two dimensional arrays.
In this study, the effect of NOM characteristics on its adsorption to MWNT and the stability of MWNT in water was studied using NOMs obtained from different sources and extracted by different methods. The effect of water quality parameters such as ionic strength and pH was also assessed. Batch isotherm experiments were performed and the results were analyzed using the Freundlich adsorption isotherm equation which has been frequently used to describe aqueous phase adsorption. Preferential adsorption of different molecular size fractions of NOM was examined using size exclusion chromatography (SEC). Finally, the amount of stable MWNT suspension formed in water as a result of NOM adsorption under varying conditions was quantitatively analyzed.
B.2.3. Experimental
Materials. MWNT (10-20 nm diameter × 10-30 μm length) with over 95% purity was obtained from the Cheap Tubes Inc. (Brattleboro, VT). Suwannee River natural organic matter (SRNOM), Suwannee River humic acid standard II (SRHA), Suwannee River fulvic acid standard II (SRFA), Leonardite humic acid standard (LHA), Elliott soil humic acid standard (ESHA), Nordic lake humic acid reference (NLHA), Nordic lake fulvic acid reference (NLFA), Waskish peat humic acid standard (WPHA), and Waskish peat fulvic acid standard (WPFA) were purchased from the International Humic Substances Society (IHSS) (St. Paul, MN). Elemental and carbon compositions of the NOMs are provided in Table B.2.1. NOM stock solution was prepared by mixing a known amount of NOM with ultrapure water for 24 hours. Dissolution of NOM was facilitated by adding NaOH to increase the solution pH to 7. After measuring the total organic carbon (TOC) content of the stock solution by a TOC-Vw analyzer (Shimadzu, Columbia, MD), the target NOM concentration was prepared by diluting the stock solution. Ultrapure water produced by a Milli-Q water filtration system (Millipore, Billerica, MA) was used for the preparation of all the solutions.
Isotherm test. Isotherm relationships for NOM adsorption to MWNT were evaluated by a modified bottle-point technique [5], where each data point of the isotherm was determined by an individual batch experiment. Both constant adsorbent method and constant adsorbate method were adopted. For all the isotherm experiments, NOM solution was buffered with 1 mM phosphate (NaH2PO4) and ionic strength was adjusted with NaCl. The solution pH was adjusted using NaOH and HCl. The mixture of MWNT powder and NOM solution in a 40 mL vial was agitated using a magnetic stirrer for 6 days. Independent kinetic study suggested that the adsorption of NOM reached saturation within 2 days of mixing. After 2 days of settling, each suspension was filtered using a Whatman 541 filter (20-25 μm nominal pore size, Florham Park, NJ).
Analysis. The concentration of MWNT suspended in the filtrate was determined by visible light absorbance at 800 nm (VIS800) (8453 UV-Vis Spectroscopy System, Agilent, Palo Alto, CA). Our past research verified that the absorbance at this wavelength (i.e., transmittance decrease due to light scattering by suspended MWNT) was linearly correlated with MWNT concentrations measured by a Thermal Optical Transmittance (TOT) analyzer [4]. That study also verified that the TOT method provided an accurate measure of the concentration of MWNT in a solution containing both nanotubes and NOM [4]. In this study, VIS800 were also calibrated with those obtained using a TOT analyzer (Sunset Laboratory, Tigard, OR). The equilibrium concentration of NOM in the liquid phase, Ce (mg C/L), was measured by UV absorbance at 254 nm (UV254) (8453 UV-Vis Spectroscopy System, Agilent, Palo Alto, CA) after removing MWNT with a 0.2 μm Acrodisc nylon membrane syringe filter (Pall Corporation, Ann Arbor, MI). UV254 measurement was calibrated with TOC measurement (TOC-Vw, Shimadzu, Columbia, MD) for each NOM type. Once Ce was obtained, the equilibrium concentration of NOM adsorbed on the unit mass of MWNT, qe (mg C/g MWNT), was obtained by the following equation:
(1)
where C0 (mg C/L) = the initial concentration of NOM and D (g MWNT/L) = dosage of MWNT.
The molecular weight distribution of NOM was analyzed by a Hewlett-Packard 1100 high performance liquid chromatography (HPLC) system (Wilmington, DE) equipped with a Waters Protein-Pak™ 125 SEC column (Milford, MA), a commonly used column for NOM fractionation [13-15] (mobile phase = 0.1 M NaCl solution buffered with 1 mM phosphate at pH 6.8, flow rate = 1 mL/min at 40°C). UV absorbance at 254 nm (UV254) was monitored by a diode-array detector (DAD). Electron microscopic images were analyzed by a Philips 120 transmission electron microscope (TEM) (New York, NY). A TEM specimen was prepared by placing a droplet of fullerene suspension on a copper carbon grid (Electron Microscopy Science, Hatfield, PA) and drying overnight at room temperature.
B.2.4. Results and Discussion
Adsorption Isotherm. The results obtained from isotherm experiments performed under varying initial concentrations of SRNOM and MWNT are shown in Figure B.2.1a. The isotherms were adsorbent-dose dependent, i.e., different linear isotherms were obtained at different MWNT doses. Similar adsorbent-dose dependent phenomenon has been observed for adsorption of SRNOM onto activated carbons in past studies [5, 10]. Each isotherm at the same MWNT dose fitted well with the following Freundlich adsorption model which has been commonly used to represent aqueous phase adsorption phenomena:
(2)
where KF (L/mg) and n (dimensionless) represent Freundlich constant and Freundlich exponent, respectively. Generally, KF increases as the adsorption capacity of the adsorbent increases and n increases as the adsorption strength increases. As the initial MWNT dose decreased, more SRNOM adsorbed per unit mass of MWNT and consequently qe increased. This increase was less pronounced when equilibrium SRNOM concentration (Ce) increased, and the isotherms merged at the highest Ce.
Deviation from a single solute isotherm (i.e. a unique isotherm is obtained regardless of the initial adsorbate concentration or adsorbent dose) is attributed to the heterogeneity of NOM and consequential occurrence of preferential adsorption [16, 17]. In such a case, it is known that a unique isotherm is obtained by normalizing the equilibrium adsorbate concentration (Ce) by adsorbent dose (D) as follows [5, 13, 18, 19]:
(3)
A normalized Freundlich adsorption model fitted well with experimental data (Figure B.2.1b) regardless of MWNT dose. The experimental results obtained with the other NOMs used in this study also matched well with the normalized model (Figure B.2.2). The fitted model parameters for all the NOMs are summarized in Table B.2.2.
Effect of NOM Type. Results summarized in Table B.2.2. suggest that the adsorptive interaction between NOM and MWNT was strongly dependent on the type of NOM. For example, less soluble, higher molecular weight humic acids had generally higher adsorption capacity than fulvic acids. Various functionalities of the NOMs identified in the previous study [20] using 13C NMR were compared with adsorption characteristics. Among various carbon functionalities present in NOM (e.g., carbonyl, carboxyl, aromatic, acetal, heteroaliphatic, and aliphatic carbons), aromatic carbon content showed the most strong linear relationship with KF (Figure B.2.3) regardless of source (i.e., lake, soil, or river) and type (i.e., bulk, fulvic or humic) of NOMs. Some deviation from the linearity, which might originate from the existence of different elemental composition and functional groups in NOMs, was also observed. Nevertheless, the finding that the adsorption capacity is closely related to the aromatic functional group content in NOM is consistent with past studies which reported that the attractive interaction between chemical compounds containing the aromatic moiety and CNT was largely driven by π-π interactions [21, 22]. Specifically, a previous study [21] suggested that a benzene ring present in a surfactant that shielded the CNT surface would stack upon benzene ring present in the CNT. Gotovac et al. [23] reported that the adsorption of the tetracene (4 benzene rings) was 6 times greater than that with phenanthrene (3 benzene rings). The strong correlation between adsorption capacity and aromatic content of NOM implies that aromatic fractions of NOM, which can range from ca. 10-40 % (C/C) depending on the source and age [20] could be a useful measure to evaluate the level of NOM adsorption onto MWNT and consequently the dispersion of MWNT in natural waters.
From the isotherm parameters calculated for each NOM (Table B.2.2.), it is also notable that KF was inversely proportional to 1/n for all the NOMs tested in this study (Figure B.2.4). This suggests that the capacity (KF) and adsorption strength (n) of NOM adsorption are proportional to each other, which is not always the case for NOM adsorption to activated carbons. In case of activated carbon, adsorption capacity is mainly determined by surface area of carbon’s porous structure available for adsorption, and many of the smaller pores are not accessible to the larger, more strongly adsorbing NOM molecules. Even for the same activated carbon, the area available for adsorption varies with molecular size distribution and characteristics of NOM. Such variation is not necessarily related to chemical composition of NOM that governs adsorption strength between the NOM and the surface. In contrast, MWNT do not have pores available for adsorption and, therefore, NOM adsorption is less influenced by the physical structure of the adsorbent. Consequently, NOMs with greater adsorption strength are likely to have greater adsorption capacity to MWNT as experimental results suggested.
Preferential Adsorption. Preferential adsorption of the higher molecular weight faction of SRNOM to MWNT was evident when size exclusion chromatograms of the non-adsorbed portion of NOM at different MWNT doses were examined (Figure B.2.5a). As MWNT dose was increased, higher molecular weight faction (i.e., fractions appearing at lower retention time) was removed to a greater extent. Similar phenomena were observed from the SEC analysis performed with other types of NOM such as SRHA and SRFA (Figures B.2.5b and B.2.5c). This observation is in accordance with previous studies performed with the polydispersed polymers and non-porous adsorbents where preferential adsorption of high molecular weight fractions was observed [16, 17]. However, the opposite molecular weight dependence was reported when adsorbent was porous (e.g., activated carbon). From the SEC study on the Laurentian humic acid adsorbed to activated carbon, Kilduff et al. [10] suggested that lower molecular weight fraction of the humic acid more favorably adsorbed to activated carbon than the higher molecular weight fraction. Summers and Roberts [6] examined activated carbon adsorption of Aldrich humic acid fractionated by molecular weight and observed greater adsorption of the low molecular weight fraction. The apparent preferential adsorption of the low molecular weight fraction in the case of activated carbon would result from the size exclusion effect of pores within the activated carbon structure (i.e., small adsorbates can access both small and large pores, but large adsorbates can not access small pores) [6]. However, when the adsorption was evaluated on the basis of the accessible surface area, the larger molecules were found to have greater adsorption capacity to activated carbon [5], which is consistent with findings from this study.
Effect of Water Quality Parameters. The adsorption of NOM to MWNT was greatly influenced by water quality parameters such as ionic strength and pH. At the same equilibrium SRNOM concentration in the liquid phase (Ce), the solid phase SRNOM concentration (qe) was the highest in the solution containing 0.1 M NaCl and the lowest without NaCl (Figure B.2.6a). The experimental data fitted well with the Freundlich isotherm model at each ionic strength. Adsorption capacity, expressed in terms of KF, increased as ionic strength increased (KF = 6.72 at NaCl = 0 M, 7.05 at NaCl = 0.01 M and 7.55 at I = 0.1 M) presumably due to change in molecular configuration of NOM. Gosh and Schnitzer [24] suggested that humic substances would become increasingly coiled and form more compact structures as ionic strength increased. Adsorption capacity would consequently increase as more molecules could occupy the same surface area [10]. As the NOM molecule becomes more compact, the area for NOM-MWNT interaction would be reduced and the attractive force per individual NOM molecule to MWNT surface would decrease. Consequently, the adsorption strength expressed in terms of 1/n, decreased as ionic strength was increased (1/n = 0.39 at NaCl = 0 M, 0.48 at NaCl = 0.01 M and 0.60 at NaCl = 0.1 M). This is also consistent with the observation made in Figure B.2.5 that NOM with larger molecular size adsorbed more effectively to MWNT due to greater level of adsorptive interaction possible per NOM molecule with MWNT surface. At higher ionic strength, enhanced double layer compression in SRNOM-MWNT agglomerates would also enhance SRNOM adsorption onto MWNT.
Figure B.2.6b shows the effect of pH on SRNOM adsorption to MWNT. The adsorption capacity decreased as pH increased (i.e., KF = 8.31 at pH = 5.0, 5.10 at pH = 7.0 and 2.96 at pH = 9.0). As pH increases, the NOM molecules will become less coiled and less compact due to greater charge repulsion, and the adsorption capacity might consequently decrease as discussed above. In addition, as pH increases, weakly acidic SRNOM with carboxylic and phenolic moieties becomes more negatively charged [25]. Thus, in higher pH, repulsion between SRNOM and MWNT surface coated with SRNOM would increase, hindering further adsorption of SRNOM. However, the strength of adsorption did not show appreciable change (i.e. 1/n = 0.39 at pH = 5.0, 0.40 at pH = 7.0 and 0.47 at pH = 9.0).
Stability of MWNT in the Aqueous Phase. As a result of NOM adsorption to MWNT, a fraction of MWNTs are dispersed in the aqueous phase and form a stable suspension. TEM images of MWNT in SRNOM solutions (Figure B.2.7) indicate that most of the MWNT were individually dispersed. The amount of stable MWNT suspension in aqueous phase (CMWNT) was mainly determined by the amount of SRNOM adsorbed per MWNT (qe) (Figure B.2.8). The MWNT suspension is facilitated by the shielding of extremely hydrophobic MWNT surface with NOM which provides thermodynamically more favorable surface. Adsorbed NOM is also expected to contribute to steric and electrostatic stabilization. Therefore, the amount of the NOM adsorbed onto MWNT would determine the extent of MWNT stability in water, i.e., CMWNT increased as qe increased. CMWNT seemed to be also influenced by the type of NOM. For example, at the same qe, CMWNT with SRHA was higher than that with SRNOM, implying that SRHA might have greater MWNT stabilization capacity than SRNOM (Figure B.2.8). However, it was difficult to find a obvious relationship between CMWNT and properties of NOM. For the same qe, more MWNTs were suspended when more MWNTs were initially added to the solution (i.e., higher D).
The amount of MWNT suspended in water also strongly depended on solution ionic strength. The amount of MWNT suspended in water (CMWNT) was plotted versus qe at different ionic strengths in Figure B.2.9. Experimental results scattered at lower CMWNT due to analytical limitations. At higher ionic strength, much less MWNT was suspended despite the similar amount of SRNOM was adsorbed. For instance, even at high qe (> 10 mg C/g MWNT), MWNTs were only slightly dispersed (i.e., less than 1 mg/L) in the SRNOM solution with 0.1 M NaCl. This might be due to greater double layer compression in the high ionic strength solution. However, as MWNT used in this study has a very high aspect ratio (ca. diameter ranges from 10 to 20 nm and length ranges from 10 to 30 μm) it is difficult to quantitatively analyze the zeta potential of MWNT. The effect of pH was less obvious than that of ionic strength, although electrostatic stabilization of NOM-MWNT agglomerates would be more efficient due to the deprotonation of NOM at higher pH and experimental data showed slight increase in CMWNT at higher pH for the same qe.
Environmental Significance. The results of this study suggested that the environmental fate of MWNT will be largely influenced by the amount and the type of NOM as well as solution chemistry such as ionic strength and pH. Other water quality parameters such as divalent ions and inorganic composition, which were not examined in this study, might play a critical role in determining the degree of NOM-MWNT interaction. For natural waters from different sources with different characteristics, the aromatic content and molecular weight distribution of NOM might be useful parameters to predict the extent of NOM adsorption and level of MWNT dispersion. Even though a wide range of NOM concentrations (5 to 50 mg C/L) was investigated, it is noted that NOM concentrations in some natural surface and ground waters are lower than concentration levels used in this study. Therefore, further study with actual surface and ground waters with relatively low NOM contents might be necessary for a comprehensive understanding of interaction between NOM and MWNT.
In some aspects, NOM adsorption to MWNT was similar to that to activated carbon, i.e., occurrence of preferential adsorption and fitting to the Freundlich adsorption isotherm model. However, difference in physical structure of MWNT and activated carbon led to a few key differences in adsorption behavior such as preferential adsorption of higher molecular weight NOM onto MWNT. Understanding both similarities and differences between well-characterized activated carbon adsorption and MWNT adsorption should be helpful to understand not only the fate of MWNT in natural waters but also that of other carbon based nanomaterials such as single-walled carbon nanotubes and C60.
B.2.5. Literature Cited
- Kang, S.; Pinault, M.; Pfefferle, L. D.; Elimelech, M. Single-walled carbon nanotubes exhibit strong antimicrobial activity. Langmuir 2007, 23, 8670-8673.
- Lam, C. W.; James, J. T.; McCluskey, R.; Hunter, R. L. Pulmonary toxicity of single-wall carbon nanotubes in mice 7 and 90 days after intratracheal instillation. Toxicol. Sci. 2004, 77, 126-134.
- Warheit, D. B.; Laurence, B. R.; Reed, K. L.; Roach, D. H.; Reynolds, G. A.; Webb, T. R. Comparative pulmonary toxicity assessment of single-wall carbon nanotubes in rats. Toxicol. Sci. 2004, 77, 117-25.
- Hyung, H.; Fortner, J. D.; Hughes, J. B.; Kim, J. H. Natural organic matter stabilizes carbon nanotubes in the aqueous phase. Environ. Sci. Technol. 2007, 41, 179-184.
- Summers, R. S.; Roberts, P. V. Activated carbon adsorption of humic substances.1. Heterodisperse mixtures and desorption. J. Colloid Interface Sci. 1988, 122, 367-381.
- Summers, R. S.; Roberts, P. V. Activated carbon adsorption of humic substances. 2. Size exclusion and electrostatic interactions. J. Colloid Interface Sci. 1988, 122, 382-397.
- Sontheimer, H.; Crittenden, J. C.; Summers, R. S., Activated Carbon for Water Treatment. DVGW-Forschungsstelle: Karlsruhe, Germany, 1988.
- McCreary, J. J.; Snoeyink, V. L. Characterization and activated carbon adsorption of several humic substances. Wat. Res. 1980, 14, 151-160.
- Randtke, S. J.; Jepsen, C. P. Effects of salts on activated carbon adsorption of fulvic acids. J. Am. Water Works Assoc. 1982, 74, 84-93.
- Kilduff, J. E.; Karanfil, T.; Weber, W. J. Competitive interactions among components of humic acids in granular activated carbon adsorption systems: Effects of solution chemistry. Environ. Sci. Technol. 1996, 30, 1344-1351.
- Letterman, L. D. Water Quality and Treatment. McGraw Hill: New York, 1999.
- Yang, K.; Xing, B. S. Desorption of polycyclic aromatic hydrocarbons from carbon nanomaterials in water. Environ. Pollut. 2007, 145, 529-537.
- Kilduff, J. E.; Karanfil, T.; Chin, Y. P.; Weber, W. J. Adsorption of natural organic polyelectrolytes by activated carbon: A size-exclusion chromatography study. Environ. Sci. Technol. 1996, 30, 1336-1343.
- Li, Q. L.; Snoeyink, V. L.; Mariaas, B. J.; Campos, C. Elucidating competitive adsorption mechanisms of atrazine and NOM using model compounds. Wat. Res. 2003, 37, 773-784.
- Her, N.; Amy, G.; Foss, D.; Cho, J.; Yoon, Y.; Kosenka, P. Optimization of method for detecting and characterizing NOM by HPLC-size exclusion chromatography with UV and on-line DOC detection. Environ. Sci. Technol.2002, 36, 1069-1076.
- Stuart, M. A. C.; Scheutjens, J.; Fleer, G. J. Polydispersity effects and the interpretation of polymer adsorption-isotherms. J. Polym. Sci. B 1980, 18, 559-573.
- Koopal, L. K. The effect of polymer polydispersity on the adsorption-isotherm. J. of Colloid Interface Sci. 1981, 83, 116-129.
- Karanfil, T.; Kitis, M.; Kilduff, J. E.; Wigton, A. Role of granular activated carbon surface chemistry on the adsorption of organic compounds. 2. Natural organic matter. Environ. Sci. & Technol. 1999, 33, 3225-3233.
- Harrington, G. W.; Digiano, F. A. Adsorption equilibria of natural organic-matter after ozonation. J. Am. Water Works Assoc. 1989, 81, 93-101.
- Thorn, T. A.; Folan, D. W.; MacCarthy, P. Characterization of the International Humic Substances Society Standard and Reference Fulvic Acids by Solid State Carbon-13 and Hydrogen-1 Nuclear Magnetic Resonance Spectrometry; Water-Resources Investigation Report 89-4196; U.S. Geological Survey: Denver, CO, 1989.
- Islam, M. F.; Rojas, E.; Bergey, D. M.; Johnson, A. T.; Yodh, A. G. High weight fraction surfactant solubilization of single-wall carbon nanotubes in water. Nano Lett. 2003, 3, 269-273.
- Tan, Y.; Resasco, D. E. Dispersion of Single-Walled Carbon Nanotubes of Narrow Diameter Distribution. J. Phys. Chem. B 2005, 109, 14454-14460.
- Gotovac, S.; Honda, H.; Hattori, Y.; Takahashi, K.; Kanoh, H.; Kaneko, K. Effect of nanoscale curvature of single-walled carbon nanotubes on adsorption of polycyclic aromatic hydrocarbons. Nano Lett. 2007, 7, 583-587.
- Ghosh, K.; Schnitzer, M. Macromolecular structures of humic substances. Soil Sci. 1980, 129, 266-276.
- Ritchie, J. D.; Perdue, E. M. Proton-binding study of standard and reference fulvic acids, humic acids, and natural organic matter. Geochim. Cosmochim.Acta 2003, 67, 85-96.
Figure B.2.1. Adsorption of SRNOM to MWNT. (a) Isotherm experimental data and Freundlich adsorption isotherm model fit. (b) Normalized data and normalized Freundlich adsorption isotherms model fit. (both at 22 °C, pH=7.0, [NaCl] = 5 mM, [NaH2PO4] = 1 mM)
Figure B.2.2. Isotherm experimental result and Freundlich adsorption model fit for (a) SRHA, (b) SRFA, (c) ESHA, (d) LHA, (e) NLHA, (f) NLFA, (g) WPHA, and (h) WPFA. (Isotherm experimental condition: pH 7, 5 mM NaCl, and 1 mM NaH2PO4)
Figure B.2.3. Relationship between the aromatic group content of NOM and NOM-MWNT adsorption capacity (KF).
Figure B.2.4. Correlation between KF and 1/n for various NOMs.
Figure B.2.5. Size exclusion chromatograms of (a) SRNOM (b) SRHA and (c) SRFA that were not adsorbed after adding MWNT at varying doses.
Figure B.2.6. Effect of (a) ionic strength (at 22 °C, pH 7.0, [NaH2PO4] = 1 mM) and (b) pH (at 22 °C, [NaCl] = 5 mM, [NaH2PO4] = 1 mM) on SRNOM adsorption to MWNT. Lines denote Freundlich adsorption isotherm model fit.
(a) (b)
Figure B.2.7. TEM images of MWNT stabilized in SR NOM shown at different magnifications. The bars represent 2 μm and 25 nm for (a) and (b), respectively. Evolution of bubble-like structures adjacent to MWNT surfaces, which formed as organic matter sublimed by high energy electron beam irradiation, was visible during the first a few seconds of TEM analysis (Figure 5b).
Figure B.2.8. Dependency of the amount of MWNT suspended in water (CMWNT) on the amount of various NOMs adsorbed onto MWNT (qe).
Figure B.2.9. Effect of (a) ionic strength (at 22 °C, pH 7.0, [NaH2PO4] = 1 mM) and (b) pH (at 22 °C, [NaCl] = 5 mM, [NaH2PO4] = 1 mM) on the amount of MWNT suspended in water.
Table B.2.1. Carbon Distribution and Elemental Composition of NOMs Investigated in This Study. Data Are Excerpted from International Humic Substances Society (IHSS) webpage (http://www.ihss.gatech.edu Exit ).
|
Carbon Distribution |
|
Elemental Composition |
|||||||||||
|
Carbonyl |
Carboxyl |
Aromatic |
Acetal |
Heteroaliphatic |
Aliphatic |
|
H2O |
Ash |
C |
H |
O |
N |
S |
SRHA |
6 |
15 |
31 |
7 |
13 |
29 |
|
20.4 |
1.04 |
52.63 |
4.28 |
42.04 |
1.17 |
0.54 |
ESHA |
6 |
18 |
50 |
4 |
6 |
16 |
|
8.2 |
0.88 |
58.13 |
3.68 |
34.08 |
4.14 |
0.44 |
LHA |
8 |
15 |
58 |
4 |
1 |
14 |
|
7.2 |
2.58 |
63.81 |
3.7 |
31.27 |
1.23 |
0.76 |
SRFA |
5 |
17 |
22 |
6 |
16 |
35 |
|
16.9 |
0.58 |
52.34 |
4.36 |
42.98 |
0.67 |
0.46 |
NLHA |
10 |
19 |
38 |
7 |
11 |
15 |
|
9.1 |
0.31 |
53.33 |
3.97 |
43.09 |
1.16 |
0.58 |
WPHA |
8 |
18 |
42 |
6 |
8 |
18 |
|
6.93 |
1.6 |
54.72 |
4.04 |
38.54 |
1.47 |
0.36 |
NLFA |
10 |
24 |
31 |
7 |
12 |
18 |
|
9.2 |
0.45 |
52.31 |
3.98 |
45.12 |
0.68 |
0.46 |
WPFA |
7 |
19 |
36 |
6 |
12 |
20 |
|
8.29 |
0.16 |
53.63 |
4.24 |
41.81 |
1.07 |
0.29 |
SRNOM |
8 |
20 |
23 |
7 |
15 |
27 |
|
8.15 |
7 |
52.47 |
4.19 |
42.69 |
1.1 |
0.65 |
Table B.2.2. Freundlich Adsorption Model Parameters for Various NOMs.
|
SRNOM |
SRHA |
SRFA |
ESHA |
LHA |
NLHA |
NLFA |
WPHA |
WPFA |
1/n |
0.3840 |
0.1998 |
0.3476 |
0.2247 |
0.2122 |
0.2223 |
0.3237 |
0.2247 |
0.2777 |
KF |
5.47 |
9.82 |
5.45 |
11.34 |
13.08 |
11.63 |
6.94 |
12.42 |
8.44 |
r2 |
0.9702 |
0.8395 |
0.9692 |
0.9085 |
0.9004 |
0.9026 |
0.9652 |
0.9341 |
0.9029 |
B.3. Effect of Encapsulating Agents on Dispersion Status and Photochemical Reactivity of C60 in the Aqueous Phase
B.3.1. Acknowledgements
This work was submitted to ACS Environmental Science & Technology and currently under review. The authors would like to thank Dr. Prashant Kamat, Dr. Anusorn Kongkanand, and Dr. Yoichiro Matsunaga for their assistance during laser flash photolysis, which was performed at Notre Dame Radiation Laboratory, South Bend, IN.
B.3.2. Introduction
With unique physical and chemical properties, C60 has been widely recognized as a promising nanomaterial with various potential applications (1). One of notable characteristics is its strong photochemical reactivity: i.e. provided with photon energy above 2.3 eV (2), C60 is readily excited with very high quantum yield (nearly 1.0) to a triplet state (3C60*) which subsequently produces reactive oxygen species (ROS) such as singlet oxygen (1O2) and superoxide radical anion (O2·-) in the presence of oxygen (1, 3-9). C60’s capability to generate ROS has been instrumental in biomedical applications such as enzyme inhibition, antiviral activity, and photodynamic therapy (9-12). Accordingly, various methods have been developed to effectively disperse extremely hydrophobic C60 in the aqueous media and biological systems. These include: 1) generation of water-stable fullerene colloidal aggregates (often termed as nano-C60 or nC60) via ultrasonication (13) or stepwise solvent exchange (14, 15), 2) incorporation of fullerene into micelle/polymer/vesicle structures (4, 6, 11, 16-18), and 3) surface modification by addition of hydrophilic functional moieties (10, 19-21).
Strong photosensitizing activity and aqueous availability combined have been at the center of concerns in environmental and ecological impacts of this material. The exactly same property mentioned above (i.e. production of ROS) has been claimed responsible for oxidative damages and cell death in biological receptors (15, 22-24). In addition, following the similar mechanisms discussed above, dispersion of C60 in the natural aqueous environment might be facilitated by: 1) formation of colloidal aggregates, which has been demonstrated possible after prolonged exposure to water (15, 25), 2) interaction with natural encapsulant analogues such as natural organic matter (NOM) and other anthropogenic contaminants such as surfactant that strongly interact with C60, and 3) functionalization of C60 by chemical reactions in natural environment or during water and wastewater treatment processes (e.g., chemical treatment). Therefore, it is possible that C60 can be widely dispersed in the natural waterways, potentially interacting with biological systems, due to unintended events of mass discharge to the environment.
However, accurate assessment of environmental impact of this material should consider how these two properties are related to each other. Our recent study suggested that C60’s photochemical reactivity (i.e. related to toxicological effect) would be dependent on its dispersion status in the aqueous phase (i.e. related to exposure route). Although C60 associated with encapsulating agents such as surfactants or polymers retained the above intrinsic photochemical property (size of C60 colloids < 5 nm), this property was lost when C60 formed aggregates of size ca. 100 nm (7). Alternatively, varying levels of photochemical reactivity and ROS production might be possible with C60’s that are released to the aqueous phase under different disposal scenarios, since dispersion status (i.e. degree of aggregation/clustering) of C60 varies as it interacts with different encapsulating molecules present in the aqueous system (14, 15, 30). This study therefore examines the effect of a wide spectrum of both anthropogenic and natural encapsulants on the C60’s dispersion status and photochemical reactivity at the same time. Degree of aggregation (clustering) was quantified based on UV-Vis spectral characteristics and compared to 1O2 production rate. A laser flash photolysis was performed to trace the decay kinetics of 3C60* which is a critical transient intermediate for energy transfer and subsequent 1O2 production. Our study shows that the pathway for 1O2 production is prohibited when C60 is present as an aggregate in the aqueous phase, since the lifetime of 3C60* is drastically reduced, consiste nt with observations made in organic phase (1).
B.3.3. Experimental
Materials. C60 (99.9%, sublimed) was purchased from MER Corp. Other chemicals used were of the highest purity available and used without the further treatment. Chemicals used as received in this study include: furfuryl alcohol (FFA, Aldrich), Na2HPO4·7H2O (Aldrich), NaH2PO4·H2O (Aldrich), sodium dodecyl sulfate (SDS, Sigma-Aldrich), sodium decyl sulfate (SDES, Fluka), sodium octyl sulfate (SOS, Fluka), sodium dodecylbezene sulfonate (SDBS, Aldirch), cetyltrimethylammonium bromide (CTAB, Sigma), dodecyltrimethylammonium bromide (DTAB, Sigma), octyltrimethylammonium bromide (OTAB, Sigma), Triton X100 (TX 100, Sigma-Aldrich), Triton X100 reduced (TX 100-R, Sigma-Aldrich), Triton X405 (TX 405, Sigma-Aldrich), Brij 35 (Sigma), Brij 78 (Sigma), Tween 65 (Sigma), sodium polyacrylate (PA, Fluka), polyvinylpyrrolidone (PVP, Sigma-Aldrich), polyethylene glycol (PEG, Fluka), γ-cyclodextrin (Sigma), tetrahydrofuran (THF, where), and toluene (Fisher Scientific). Suwannee River humic acid (SRHA) and fulvic acid (SRFA) isolated by RO were purchased from International Humic Substances Society (IHSS, St. Paul, MN). Ultrapure water (>18.2 MΩ) produced by a Milli-Q water purification system (Millipore, Billerica, MA) was used for the preparation of all solutions.
Preparation of aqueous stable C60. Ultrasound (50/60 Hz, 125 W) was applied to a heterogeneous mixture of 10 mL toluene containing 5 mg C60 and 90 mL ultrapure water in a sealed bottle for 24 hrs using an ultrasonicator (Model 8845-40, Cole-Parmer, US). The water phase gradually assumed brownish-orange hue as C60 was dispersed. Ultrasound was further applied to the mixture open to atmosphere for additional 24 hrs at 60 °C in order to evaporate toluene and the solution was further filtered through a 0.45-μm PTFE filter (Millipore Corp.). Aqueous suspension of aggregate form of C60 prepared according to this specific method is herein referred to as son/C60. An alternative method based on solvent exchange (15) was also used to prepare aqueous suspension of C60 aggregate (termed separately as nC60) for comparison purpose. C60 associated with target encapsulating agent, referred to as C60/encapsulant, was prepared following the same method, except that varying concentration of encapsulating agent was dissolved in the aqueous phase prior to sonication. Note that the previously established preparation methods (4, 18) were not applicable in our study, since some target compounds (e.g., ionic surfactants) were negligibly soluble in organic solvents such as toluene and chloroform. However, C60/TX 100 (above c.m.c.) and C60/ γ-cyclodextrin prepared by our method was comparable to one prepared according to the methods described in (4, 7, 30) based on UV spectral analysis. Preliminary experiments also suggested that UV spectra of C60 suspensions prepared by this method (C60/SDS, C60/SDBS, C60/TX 100, and C60/CTAB) were reproducible (see Table B.3.1. for abbreviations of chemical names).
Photochemical Experiments. Photochemical experiments were performed following the method previously described by Lee et al. (2006). Under air-equilibrated condition at ambient temperature (22 °C) in a 60 mL cylindrical quartz reactor which was surrounded by six 4-W black light blue (BLB, Philips TL4W) lamps. The incident light intensity in the active wavelength region (350 – 400 nm) was measured at 3.33 x 10-4 Einstein×min-1L-1 by ferrioxalate actinometry. Production of 1O2 by aqueous colloidal C60 during UV irradiation was monitored using FFA as a probe compound [k(FFA + 1O2) = 1.2 x 108 M-1s-1 (31)]. An aliquot of 10 mM FFA stock solution was added to a reaction mixture to achieve target concentration of 1 mM. Initial concentration of aqueous C60 suspension was fixed at 5 mg/L and reaction solutions were buffered at pH 7 using 10 mM phosphate. As the photochemical reaction proceeded, sample aliquots of 1 mL were withdrawn from the reactor using a syringe, filtered through a 0.45-μm PTFE filter (Millipore), and transferred to a 2-mL amber glass vial for further analyses. The FFA concentration was measured using an Agilent 1100 HPLC equipped with a C-18 column (Agilent Zorbax RX-C18) and a diode-array detector. Each photochemical experiment was duplicated.
Laser Flash Photolysis. Prior to experiment, a solution (3 mL) containing 5 mg/L C60 was placed in a rectangular quartz reactor, purged with argon gas for 30 min and sealed to inhibit the transfer of absorbed energy from 3C60* to oxygen. A laser pulse at 355 nm (5 mJ, pulse width = 6 ns) generated from a Quanta Ray Nd:YAG laser system was used as an excitation source for C60. A monochromatic laser at 740 nm, produced by filtering the light from a xenon lamp using a monochromator and aligned perpendicular to the excitation laser, was used to monitor the concentration of 3C60* at 740 nm. Each laser flash photolytic experiment was duplicated.
B.3.4. Results and Discussion
UV-Vis Spectral Characteristics of C60 associated with Surfactants. UV-Vis analysis of C60 associated with selected surfactant suggests that spectral characteristics that reflect dispersion status of C60 are strongly affected by the presence of surfactants (30) (Figure B.3.1). Spectra of son/C60 and nC60 both showed a specific absorption peak at around 350 nm and broad band absorption in the wavelength region from 400 to 500 nm, consistent with the previous literature (14, 15). However, C60 associated with TX 100, a non-ionic surfactant widely used as an artificial cell membrane (4, 6, 30), showed a sharp absorption peak at 330 nm (i.e. blue-shifted compared to that in son/C60 and nC60) and negligible absorption in the wavelength ranges from 400 to 500 nm. This spectrum is fairly similar with that of C60 molecularly dissolved in toluene or hexane (30). The spectrum of C60 further depended on the type and amount of surfactants applied. When anionic SDBS and non-ionic Brij 35 were applied, relatively weak broad band absorption in 400-500 nm along with moderate level of peak shifting at UV range were observed. In contrast, C60 associated cationic CTAB exhibited higher absorption at 400-500 nm region and the position of the characteristic UV peak was comparable to son/C60.
Quantitative analyses on UV-Vis spectra of all the C60 suspensions are summarized in Table B.3.1. Characteristic UV absorption peaks of C60 associated with surfactant applied below c.m.c. were red-shifted compared to C60/surfactant above c.m.c. This red-shifting has been reported to occur when C60 forms aggregates in binary organic solvent mixture or in an artificial model lipid membrane (30, 33). In the aqueous phase, this bathochromic shift indicates that the surfactant applied below c.m.c. does not significantly suppress C60 aggregation. Consequently, the maximum bathochromic shift was observed with son/C60 (λmax = 218, 269 and 347 nm) which was prepared in the absence of surfactant. In contrast, all C60 associated with surfactant applied above c.m.c. showed significant hypsochromic shifts relative to son/C60 indicating that aggregation is inhibited and C60 is exposed to a media environment closer to organic phase (i.e. inclusion within the hydrophobic micelle cores). As surfactant concentration was further increased beyond c.m.c. and more micelles were available for C60 encapsulation, λmax became closer to that of individually dispersed C60. For example, when the concentration of SDBS was gradually increased as 0.25, 10, 20, and 50 g/L, λmax for the third specific peak gradually moved to shorter wavelength as 344, 336, 331, and 330 nm, respectively. λmax at the highest surfactant concentration is comparable to that of C60 associated with γ-cyclodextrin (λmax = 214, 260, and 332 nm) where single C60 molecule resides within the relatively hydrophobic (i.e. compared to water) cavity of each γ-cyclodextrin molecule (16). These values are also comparable to those of C60 dispersed in hexane (i.e. λmax = 211, 257, and 328 nm in the absence of solvent interference).
A450/A330 (Table B.3.1) represents the absorbance at 450 nm normalized by that of specific peak at 330-350 nm region (i.e. λmax varies depending on the sample). This broad band absorption in visible range (400-500 nm) results from solid state C60-C60 interactions (14, 15, 34). A450/A330 was as high as 0.41 in the absence of surfactant (i.e. nC60 and son/C60) and slightly lower when surfactants were applied below c.m.c. When applied above c.m.c., substantial decrease in A450/A330 was observed. This observation is consistent with the peak shifting which suggested that that C60 aggregation would be limited by applying surfactant above c.m.c. C60 clustering was more effectively prevented as more micelles were available in the solution. As surfactant concentration was gradually increased beyond c.m.c., A450/A330 also further decreased (Figure B.3.2a).
Although the presence of micelle structure is critical for determining dispersion status of C60 as discussed above, it might not be essential for promoting C60 transfer to and stabilization in the aqueous phase. The peak absorbance of the sample at 330-350 nm region was compared to absorbance son/C60 at 347 nm (As) and designated as A330/As in Table B.3.1. This analysis suggests that the presence of surfactants accelerated the dispersion of C60 (A330/As > 1.0) in most cases, even when concentration was below c.m.c. This enhanced transfer might be related to surfactants arranged at the interface between organic and aqueous phases, acting as phase transfer agent, while no definite answer was available at current stage.
UV-Vis spectra provide critical information regarding dispersion status (i.e. aggregate vs. molecule) especially when a large amount of surfactants are present. Dynamic light scattering (DLS) of C60 associated with surfactant above c.m.c. suggests that sizes were below detection limit of DLS (ca. 5 nm) in our past study (7), but no further insights as to exact number of C60 inside the micelle cores could be gained. However, based on the above UV analysis performed with varying surfactant concentration, it is very plausible that C60 is molecularly dispersed inside surfactant micelles. Microscopic imaging using transmission electron microscope (TEM) was not possible as excessive surfactant deposited on carbon grid prevented clear images. In following discussions, spectral shifting and A450/A330 were used as criteria to conjuncture dispersion status of C60 in the aqueous phase.
Effect of Surfactant Types. Effect of surfactant on the dispersion status of C60 depended not only on the surfactant concentration but its type. Among non-ionic surfactants (i.e. in the absence of any potential coulombic interaction between C60 and surfactant) with the same hydrophilic moieties (Brij and TX series), TX 100 was the most effective in suppressing aggregate formation. This resulted since aromatic moiety in hydrophobic tail of TX 100 would interact with C60’s conjugated p system more favorably compared to n-alkyl (Brij 78 and 35) or cycloalkyl (TX 100-R) groups (6). Functional groups in hydrophilic head also seemed to affect the dispersion characteristics. Even though TX 405 has the same hydrophobic moiety as TX 100, it was much less effective in suppressing aggregate formation. This difference should have resulted from the difference in hydrophilic chain length (-(OCH2CH2)n, n = 40 on average for TX 405 and n = 9~10 for TX 100). An attractive interaction has been reported between polyethylene glycol ((OCH2CH2)n) of TX series surfactant and π system of C60 (17). It is also consistent with the observation that polyethylene glycol moderately facilitated the dispersion of C60 in water (i.e. greater A330/As value), while other water-soluble polymers did not (Table B.3.2). Therefore, the presence of longer ethylene glycol chain in TX 405 might have hindered C60 from being incorporated into the hydrophobic inner phase.
Aliphatic anionic surfactants (e.g., SDS, SDES and SOS) were less effective in inhibiting the clustering of C60 than aliphatic non-ionic counterparts (e.g., Brij 78 and 35), suggesting that coulombic interaction also played a role in aggregate formation. During ultrasonication, it is unlikely that C60 as individual molecule directly transfers from organic phase into the micelles dispersed in the aqueous phase, as molecular C60 is extremely hydrophobic. A transient formation of water stable aggregates with negatively charged surface (15, 25) and subsequent translocation into the micelle are more likely during phase transition. Therefore, a repulsive interaction between C60 aggregates with negatively charged hydrophilic group might have limited the encapsulation of C60 by anionic surfactants, which caused higher A450/A350 values, compared to C60 associated with non-ionic surfactants. SDBS was more effective than other anionic surfactants (with the same hydrophilic functionality) in suppressing aggregate formation, following the similar mechanism that TX 100 was more efficient that other non-ionic surfactants. That is, SDBS contains a benzene ring in hydrophobic tail which has the more favorable solvent-solute interaction, while other surfactants (SOS, SDES, and SDS) contained n-alkyl groups (6).
Accordingly, attractive electrostatic interaction is possible between negatively charged C60 aggregates and quaternary ammonium group of cationic surfactants (CTAB, DTAB and OTAB). In this case, therefore, it would be feasible that this attractive interaction between cationic surfactant and aqueous C60 colloid might allow C60 particles to favorably merge into larger clusters by neutralizing negative charge on colloidal surface. When OTAB and DTAB were applied at concentrations below c.m.c., yellowish aqueous C60 suspension resulted, but quickly settled down and completely filtered through a 0.45-μm PTFE filter. C60 colloid prepared in the presence of CTAB below c.m.c. did not settle down. However, it exhibited the greatest level of red shift in characteristic peaks and largest A450/A330 value, indicating formation of larger particles. These should result from agglomeration and floc formation of C60 aggregates due to charge neutralization by positively charged surfactants. In a separate test, we observed the similar spectral change when 50 μM of ferric ion was added to son/C60 (results not shown). When CTAB and DTAB were applied above c.m.c., charge neutralization effect becomes less important and provides similar environment to C60 as compared to their anionic counterpart (i.e. anionic aliphatic surfactants such as SDS). C60/OTAB, even over c.m.c., did not produce stable suspensions. Overall, longer aliphatic moiety facilitated encapsulation of C60 in the aqueous phase and preventing aggregation.
Photochemical Activities of Aqueous C60 Associated with Surfactants. Photochemical activity of C60 in the aqueous phase, measured herein as an ability to produce 1O2 during UV irradiation, was strongly affected by the dispersion status of C60 and consistent with UV-Vis spectral characteristics. When surfactants were applied at concentrations below c.m.c. and C60 forms aggregates similar to son/C60 and nC60, no photoactivity was observed. Only when surfactants were applied above c.m.c. forming micelles that can encapsulate C60, 1O2 production was observed. Rate of 1O2 generation was inversely proportional to A450/A330, which is indicative of the degree of C60 aggregation (i.e., 1O2 generation rate: TX 100 > Brij 78 > Brij 35 » SDBS > TX 405 > TX 100-R; A450/A330: TX 100-R > Brij 35 = SDBS = TX 405 > Brij 78 > TX 100).
When surfactant concentration was further increased above c.m.c., 1O2 production rate also increased (Figure B.3.2b). C60 associated with surfactants containing aromatic moieties (SDBS and TX 100) applied at the highest concentration (50 g/L) produced 1O2 at extremely high rates, which were comparable to 16.3 μM/min measured with molecularly dispersed C60 associated with γ-cyclodextrin (16) (Table B.3.2). At such a high concentration, even relatively ineffective encapsulant, i.e. aliphatic SDS, also rendered C60 photochemically active. For cationic surfactant CTAB, no 1O2 production was observed even when concentration was increased as high as 50 g/L due to floc formation and settling of colloids.
Decay Kinetics of Triplet-State C60. In our past study (7), we have hypothesized that “photoexcited triplet C60 (3C60*) at the aggregate surface may be effectively quenched by surrounding ground state C60 (self-quenching) and another triplet C60 (triplet-triplet annihilation) within the aggregate structure and dissipated as heat”. As a result, the pathway for energy transfer to oxygen and subsequent production of 1O2 might be prohibited when C60 aggregates in the aqueous phase. This hypothesis was verified in this study by tracing the fate of 3C60*, which is the key transient intermediate for energy transfer process, using a nanosecond transient spectroscopy under anoxic, Ar-saturated condition.
First, the lifetime of 3C60* in toluene/acetonitrile co-solvent (i.e. aggregates of sizes approximately 100-200 nm, (36, 37)) was measured to be approximately three orders of magnitude shorter than that in toluene (i.e. molecularly dispersed) (Figure B.3.3). This result was consistent with the previous report that the lifetime of 3C60* in aggregated C60 derivatives was within 0.1 μs, while that in surfactant-encapsulated or molecular C60 in organic phase was typically measured as several tens to a hundred of microseconds (21). Figure B.3.4 shows the transient absorption of 3C60* in the aqueous phase: son/C60, nC60 and C60 encapsulated with TX 100 and SDBS (i.e. applied above c.m.c. to prevent C60 clustering). Similar to the observations made for organic solvents obtained in this study and reported in the literature, 3C60* in son/C60 and nC60 decayed within 100 ns, with the kinetics that was too fast to be accurately characterized using the nanosecond transient spectroscopy, whereas the lifetime of 3C60* in C60 associated with TX 100 and SDBS extended to several hundred μs scale.
This result demonstrates that C60 clustering in the aqueous phase prevents the formation of 1O2 by limiting the availability of key intermediate for energy transfer process, i.e. 3C60*. In contrast, when C60 is dispersed in the aqueous phase associated with TX 100 and SDBS, 3C60* lifetime was comparable to that in toluene, indicating that C60 should probably be dispersed as individual molecule within hydrophobic micelle cores. However, not all surfactants function in the same way: when C60 was associated with other surfactants such as SDS (aliphatic anionic) and DTAB (aliphatic cationic), 3C60* lifetime was also too short to be measured in nano-second scale (Supporting Information Figure B.3.5). This was consistent with the fact that they exhibited UV-Vis spectral features indicating aggregate formation (i.e. red-shifted characteristic peak and appearance of broad band absorption in the visible regions) and 1O2 production was negligible.
C60 Associated with NOM. C60 associated with SRH and SRF exhibited red-shifted characteristic UV peaks and lower A450/A330 values compared to son/C60. A330/As values for C60/SRH and C60/SRF significantly increased, although they were slightly overestimated by the absorbance of SRH and SRF at this wavelength. These spectral features suggest that the presence of SRH and SRF contributed to inhibiting C60 aggregation to some degree and enhancing dispersion in water. Portions of heterogeneous mixture of SRH and SRF might interact closely with C60 providing sites for C60 to be present closer to molecular state. Based on above finings, organic matter with greater aromatic moieties might be more efficient in providing sites for C60 encapsulation. Production of 1O2 production was observed, albeit at an extremely slow rate after considering 1O2 production by SRH and SRF only, suggesting that ROS production in this case should have negligible toxicological effects.
As C60 is released to natural aqueous environment, C60 will interact with various natural and anthropogenic macromolecules. This study suggests that association with these encapsulating agents determines the dispersion status of C60, i.e. degree of clustering, which is critical for accurate estimation of C60 transport in natural waterways and exposure route to human. Surfactants used in this study represent anthropogenic pollutants that strongly interact with C60. But more importantly, the findings obtained with surfactant as surrogate for lipid (31) and NOM (32) suggest that functionalities and ionic charge of these macromolecule would play a critical role in interaction with C60 in the aqueous phase. Some of these encapsulants prohibit clustering of C60 in water, leading to production of ROS and consequently toxicological effect. Diverse scenarios seem possible, for example, C60 encapsulation by surfactant-like molecules which will facilitate transport not only in the subsurface media but across the cell membrane and induce ROS production. Future study should further focus on evaluation of C60 interaction with diverse organic molecules both in natural waters and biological systems.
B.3.5. Literature Cited
- Kadish, K. M.; Ruoff, R. S., Fullerenes : chemistry, physics, and technology. Wiley-Interscience: New York, 2000; p 1-916.
- Lof. R. W.; van Veenendaal, M. A. K., B.; Jonkman, H. T.; Sawatzky, G. A., Band gap, excitons, and Coulomb interaction in solid C60. Phys. Rev. Lett, 1992, 68, 3924-3927.
- Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L., Photophysical Properties of C60. J. Phys. Chem. 1991, 95, 11-12.
- Beeby, A.; Eastoe, J.; Heenan, R. K., Solubilization of C60 in aqueous micellar solution. J. Chem. Soc. Chem. Communs. 1994, 173-175.
- Belousov, V. P.; Belousova, I. M.; Krisko, A. V.; Krisko, T. K.; Muraveva, T. D.; Sirotkin, A. K., Aqueous micellar solution C60: Preparation, properties, and capability for generation of singlet oxygen. Russ. J. Gen. Chem. 2006, 76, 251-257.
- Eastoe, J.; Crooks, E. R.; Beeby, A.; Heenan, R. K., Structure and photophysics in C60 micellar solutions. Chem. Phys. Letts. 1995, 245, 571-577.
- Lee, J.; Fortner, J. D.; Hughes, J. B.; Kim, J.-H., Photochemical production of reactive oxygen species by C60 in the aqueous phase during UV irradiation. Environ. Sci. Technol. 2007, 41, 2529-2535.
- Yamakoshi, Y.; Sueyoshi, S.; Fukuhara, K.; Miyata, N., OH and O2- generation in aqueous C60 and C70 solutions by photoirradiation: An EPR study. J. Am. Chem. Soc. 1998, 120, 12363-12364.
- Yamakoshi, Y.; Umezawa, N.; Ryu, A.; Arakane, K.; Miyata, N.; Goda, Y.; Masumizu, T.; Nagano, T., Active oxygen species generated from photoexcited fullerene (C60) as potential medicines: O2- versus 1O2. J. Am. Chem. Soc. 2003, 125, 12803-12809.
- Bosi, S.; Da Ros, T.; Spalluto, G.; Prato, M., Fullerene derivatives: an attractive tool for biological applications. European Journal of Medicinal Chemistry 2003, 38, 913-923.
- Iwamoto, Y.; Yamakoshi, Y., A highly water-soluble C60-NVP copolymer: a potential material for photodynamic therapy. Chem. Commun. 2006, 4805-4807.
- Kasermann, F.; Kempf, C., Buckminsterfullerene and photodynamic inactivation of viruses. Rev. Med. Virol. 1998, 8, 143-151.
- Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A., On the production of an aqueous colloidal solution of fullerenes. J. Chem. Soc. Chem. Communs. 1995, 1281-1282.
- Deguchi, S.; Alargova, R. G.; Tsujii, K., Stable dispersions of fullerenes, C60 and C70, in water. Preparation and characterization. Langmuir 2001, 17, 6013-6017.
- Fortner, J. D.; Lyon, D. Y.; Sayes, C. M.; Boyd, A. M.; Falkner, J. C.; Hotze, E. M.; Alemany, L. B.; Tao, Y. J.; Guo, W.; Ausman, K. D.; Colvin, V. L.; Hughes, J. B., C60 in water: nanocrystal formation and microbial response. Environ. Sci. Technol. 2005, 39, 4307-4316.
- Andersson, T.; Nilsson, K.; Sundahl, M.; Westman, G.; Wennerstrom, O., C60 embedded in r-cyclodextrin - a water-soluble fullerene. J. Chem. Soc. Chem. Communs 1992, 604-606.
- Hungerbuhler, H.; Guldi, D. M.; Asmus, K. D., Incorporation of C60 into artificial lipid membranes. J. Am. Chem. Soc. 1993, 115, 3386-3387.
- Yamakoshi, Y. N.; Yagami, T.; Fukuhara, K.; Sueyoshi, S.; Miyata, N., Solubilization of fullerenes into water with polyvinylpyrrolidone applicable to biological tests. J. Chem. Soc. Chem. Communs. 1994, 517-518.
- Chiang, L. Y.; Swirczewski, J. W.; Hsu, C. S.; Chowdhury, S. K.; Cameron, S.; Creegan, K., Multihydroxy additions onto C60 fullerene molecules. J. Chem. Soc. Chem. Communs. 1992, 1791-1793.
- Tokuyama, H.; Yamago, S.; Nakamura, E.; Shiraki, T.; Sugiura, Y., Photoinduced biochemical activity of fullerene carboxylic acid. J. Am. Chem. Soc. 1993, 115, 7918-7919.
- Guldi, D. M.; Prato, M., Excited-state properties of C60 fullerene derivatives. Acc. Chem. Res. 2000, 33, 695-703.
- Irie, K.; Nakamura, Y.; Ohigashi, H.; Tokuyama, H.; Yamago, S.; Nakamura, E., Photocytotoxicity of water-soluble fullerene derivatives. Biosci. Biotechnol. Biochem. 1996, 60, 1359-1361.
- Kamat, J. P.; Devasagayam, T. P. A.; Priyadarsini, K. I.; Mohan, H.; Mittal, J. P., Oxidative Damage Induced by the Fullerene C60 on Photosensitization in Rat Liver Microsomes. Chem. Biol. Interact. 1998, 114, 145-159.
- Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman, B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L., The differential cytotoxicity of water-soluble fullerenes. Nano Lett. 2004, 4, 1881-1887.
- Brant, J.; Lecoanet, H.; Hotze, M.; Wiesner, M., Comparison of electrokinetic properties of colloidal fullerenes (nC60) formed using two procedures. Environ. Sci. Technol. 2005, 39, 6343-6351.
- Dhawan, A.; Taurozzi, J. S.; Pandey, A. K.; Shan, W. Q.; Miller, S. M.; Hashsham, S. A.; Tarabara, V. V., Stable colloidal dispersions of C60 fullerenes in water: Evidence for genotoxicity. Environ. Sci. Technol. 2006, 40, 7394-7401.
- Dimitrijevic, N. M.; Kamat, P. V., Triplet excited state behavior of fullerenes - pulse radiolysis and laser flash photolysis of C60 and C70 in benzene. J. Phys. Chem. 1992, 96, 4811-4814.
- Ebbesen, T. W.; Tanigaki, K.; Kuroshima, S., Excited-state properties of C60. Chem. Phys. Lett. 1991, 181, 501-504.
- Fraelich, M. R.; Weisman, R. B., Triplet-states of C60 and C70 in solution - long intrinsic lifetimes and energy pooling. J. Phys. Chem. 1993, 97, 11145-11147.
- Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Leach, S.; Seta, P., C60 in model biological systems - a Visible-Uv absorption study of solvent-dependent parameters and solute aggregation. J. Phys. Chem. 1994, 98, 3492-3500.
- Qiao, R.; Ke, P. C., Lipid-carbon nanotube self-assembly in aqueous solution. J. Am. Chem. Soc. 2006, 128, 13656-13657.
- Conte, P.; Agretto, A.; Spaccini, R.; Piccolo, A., Soil remediation: humic acids as natural surfactants in the washings of highly contaminated soils. Environ. Pollut. 2005, 135, 515-522.
- Gayathri, S. S.; Agarwal, A. K.; Suresh, K. A.; Patnaik, A., Structure and dynamics in solvent-polarity-induced aggregates from a C60 fullerene-based dyad. Langmuir 2005, 21, 12139-12145.
- Larsson, S.; Volosov, A.; Rosen, A., Optical-Spectrum of the Icosahedral C60 - Follene-60. Chem. Phys. Lett. 1987, 137, 501-504.
- Mukerjee, P.; Mysels, K. J., Critical Micelle Concentrations of Aqueous Surfactant Systems. In National Bureau of Standards: Washington DC, 1971; pp 51-185.
- Barazzouk, S.; Hotchandani, S.; Kamat, P. V., Nanostructured fullerene films. Adv. Mater. 2001, 13, 1614-+.
- Beck, M. T., Solubility and molecular state of C60 and C70 in solvents and solvent mixtures. Pure Appl. Chem. 1998, 70, 1881-1887.
Figure B.3.1. UV-Visible spectra of 5 mg/L C60 in the aqueous phase: son/C60, nC60 and C60 associated with CTAB, SDBS, Brij35, and TX 100 ([CTAB]0 = 0.01 g/L (below c.m.c.); [SDBS]0 = 20 g/L (above c.m.c.); [Brij35]0 = 10 g/L (above c.m.c.); [TX 100]0 = 50 g/L (above c.m.c.); [phosphate]0 = 10 mM; pHi = 7)
Figure B.3.2. Effect of surfactant concentration on (a) A450/A330 and (b) 1O2 production rate ([C60]0 = 5 mg/L; [phosphate]0 = 10 mM; pHi = 7). Average of duplicate experiments plotted.
Figure B.3.3. Absorption time profile of 3C60* in toluene and binary mixture of toluene and acetonitrile. ([C60]0 = 5 mg/L; Ar-saturated condition)
Figure B.3.4. Absorption time profile of 3C60* recorded at 740 nm in aqueous (a) son/C60, (b) nC60, (c) C60/TX 100, and (d) C60/SDBS suspensions ([C60]0 = 5 mg/L; [SDBS]0 = 50 g/L; [TX 100]0 = 50 g/L; [phosphate]0 = 10 mM; pHi = 7; Ar-saturated condition)
Figure B.3.5. Absorption time profile of 3C60* in aqueous dispersions of C60 associated with SDS and CTAB. ([C60]0 = 5 mg/L; [SDS]0 = 5 g/L; [DTAB]0 = 10 g/L; [phosphate]0 = 10 mM; pHi = 7; Ar-saturated condition)
Table B.3.1. UV-Vis spectral features and photochemical reactivity of C60 associated with surfactants
Surfactant Type |
Surfactanta |
Surfactant |
λmax (nm) |
A450/A330 |
A330/As |
Photoactivityb (μM/min) |
||
above c.m.c. |
|
|
|
|
|
|
|
|
anionic |
SDS |
5 |
218 |
262 |
340 |
0.27 |
1.88 |
N.D |
|
SDES |
10 |
220 |
265 |
343 |
0.31 |
1.96 |
N.D |
|
SOS |
50 |
218 |
263 |
340 |
0.27 |
2.11 |
N.D |
|
SDBS |
10 |
- |
261 |
336 |
0.23 |
1.80 |
2.04 |
cationic |
CTAB |
1 |
- |
263 |
340 |
0.28 |
2.10 |
N.D |
|
DTAB |
10 |
- |
261 |
338 |
0.27 |
1.75 |
N.D |
|
OTAB |
50 |
- |
- |
- |
- |
- |
- |
non-ionic |
Brij 78 |
10 |
- |
259 |
337 |
0.17 |
1.01 |
8.81 |
|
Brij 35 |
10 |
214 |
259 |
335 |
0.23 |
1.46 |
2.10 |
|
TX 100 |
50 |
- |
- |
330 |
0.10 |
0.85 |
15.9 |
|
TX 100-R |
10 |
218 |
269 |
350 |
0.26 |
1.10 |
0.86 |
|
TX 405 |
50 |
- |
- |
337 |
0.23 |
1.51 |
1.59 |
|
Tween 65 |
0.05 |
- |
- |
337 |
0.38 |
1.05 |
N.D |
|
|
|
|
|
|
|
|
|
below c.m.c. |
|
|
|
|
|
|
|
|
anionic |
SDS |
1 |
219 |
265 |
345 |
0.36 |
1.84 |
N.D |
|
SDES |
5 |
220 |
264 |
343 |
0.32 |
2.02 |
N.D |
|
SOS |
10 |
220 |
266 |
345 |
0.35 |
1.77 |
N.D |
|
SDBS |
0.25 |
223 |
263 |
344 |
0.36 |
2.01 |
N.D |
cationic |
CTAB |
0.01 |
- |
272 |
352 |
0.50 |
1.62 |
N.D |
|
DTAB |
0.2 |
- |
- |
- |
- |
- |
- |
|
OTAB |
10 |
- |
- |
- |
- |
- |
- |
non-ionic |
Brij 78 |
0.003 |
221 |
266 |
347 |
0.37 |
1.15 |
N.D |
|
Brij 35 |
0.06 |
221 |
267 |
347 |
0.37 |
1.28 |
N.D |
|
TX 100 |
0.1 |
223 |
274 |
345 |
0.34 |
1.73 |
N.D |
|
TX 100-R |
0.1 |
222 |
268 |
349 |
0.39 |
1.04 |
N.D |
|
TX 405 |
1 |
222 |
264 |
340 |
0.28 |
1.40 |
N.D |
|
Tween 65 |
0.00017 |
221 |
266 |
345 |
0.40 |
0.95 |
N.D |
a: SDS = sodium dodecyl sulfate, SDES = sodium decyl sulfate, SOS = sodium octyl sulfate, SDBS = sodium dodecylbezene sulfonate, CTAB = cetyltrimethylammonium bromide, DTAB = dodecyltrimethylammonium bromide, OTAB = octyltrimethylammonium bromide, TX 100 = Triton X100, TX 100-R = Triton X100 reduced, TX 405 = Triton X405.
b: initial (20 min) degradation rate of FFA as 1O2 indicator (duplicate)
Table B.3.2. UV-Vis spectral features and photo-activities of aqueous C60 colloids associated with other encapsulants
Type |
Encapsulating Agenta |
Conc’n (g/L) |
λmax (nm) |
A450/A330 |
A330/As |
Photoactivity (mM/min) |
||
carbohydrate |
γ-cyclodextrin |
3 |
214 |
260 |
332 |
0.12 |
1.28 |
16.3 |
polymer |
PA |
10 |
- |
264 |
344 |
0.34 |
0.37 |
N.D |
|
PVP |
10 |
- |
- |
345 |
0.44 |
0.22 |
N.D |
|
PEG |
10 |
218 |
269 |
347 |
0.36 |
1.47 |
N.D |
natural organic matter |
SRH |
0.05b |
214 |
262 |
338 |
0.30 |
1.94 |
1.22 |
|
SRF |
0.05b |
215 |
262 |
339 |
0.28 |
2.17 |
1.32 |
a: PA = polyacrylate, PVP = polyvinylpyrrolidone, PEG = polyethylene glycol, SRH = Suwannee River humic, SRF = Suwannee River fulvic
b: carbon g/L measured by TOC (total organic carbon) analyzer
c: absorbance ratio
B.4. Mechanism of C60 Photochemistry in the Aqueous Phase: Fate of Triplet State and Radical Anion and Production of Reactive Oxygen Species
B.4.1. Acknowledgements
This work was submitted to ACS Environmental Science & Technology and currently under review. The authors thank Mark Lingwood at Department of Chemistry and Biochemistry, University of California, Santa Barbara for assistance with EPR measurements, Dr. Prashant Kamat, Dr. Matsunaga Yoichiro and Kevin Tvrdy at Radiation Laboratory, University of Notre Dame, for fruitful discussions and assistance with laser flash photolysis experiments.
B.4.2. Introduction
Understanding photochemical properties of C60 is critical to accurately assess C60’s ecotoxicological impact upon unwanted release to natural environment (1-3). When irradiated with UV light (4), C60 is photochemically excited to singlet state (1C60*), which subsequently converts to triplet state (3C60*) through intersystem crossing (5-7). The excess energy in 3C60* is efficiently transferred to oxygen present in the same media as C60 returns to ground state (6, 7), producing singlet oxygen, 1O2 (5, 8-11). In the presence of electron donor with an adequate redox potential, 3C60* can be reductively converted to C60 radical anion (C60·-) (9-12), which further transfers an electron to oxygen, producing superoxide radial anion, O2·- (9-11). Such electron transfer mechanisms are thermodynamically much more favorable with 3C60* than ground state C60 (E0(3C60*/3C60·-) = +1.1 VNHE vs. E0(1C60/1C60·-) = -0.2 VNHE (13)). Therefore, it is possible that photoexcited C60 can involve in various redox reactions as either an electron acceptor or electron shuttle. These properties, particularly C60’s ability to produce reactive oxygen species (ROS) such as 1O2 and O2·-, have been proposed as the mechanism of observed toxicological effects of C60 (2, 14, 15).
However, C60 does not always exhibit the above intrinsic molecular properties. Our past study (9) suggested that when C60 formed stable aggregates in water, 1O2 and O2·- production was not measurable by wet chemical methods. Note that the aggregate forms of C60 can be produced via different preparation methods including 1) solvent exchange using polar organic solvent such as tetrahydrofuran (16-18), 2) contacting organic solvent containing C60 with water and applying ultrasound (9, 19, 20), 3) mixing dry C60 with water for an extended period (16), among others. These aggregate forms of C60 have raised an environmental issue as the new form of C60 that can potentially contaminate natural waterways (1-3, 18, 21, 22). In contrast, when C60 was associated with encapsulating agents such as surfactant or polymer, C60 retained its intrinsic photochemical reactivity (9-11, 23). These findings are significant as they suggest that C60 released to the aqueous environment as aggregates may not be directly involved with ROS-induced toxicological effects reported in the past, as long as they are not physically or chemically modified as they enter a biological system.
The present study provides key experimental evidences to elucidate the mechanisms of how energy and electron transfer by C60 is affected by the dispersion status of C60 in the aqueous phase. We hypothesized in our past study (9) that, when C60 forms aggregate, the reaction intermediate, i.e. photoexcited triplet C60 (3C60*), is quenched by surrounding ground state C60 (self-quenching) and another triplet C60 (triplet-triplet annihilation), resulting in loss of intrinsic photochemical reactivity. This hypothesis is tested herein by measuring the lifetime of key intermediate species for energy transfer (3C60*) and electron transfer (C60·-) using nanosecond and femtosecond laser flash photolysis. In particular, detection of C60·- in the presence of strong electron donor (i.e. triethylamine (TEA)) would indicate that a direct involvement of C60 as an electron acceptor or mediator in various redox reactions might be possible in the natural environment and biological systems. We examine several representative water stable C60 samples, including C60 aggregates prepared according to previously established methods (9, 19) and C60 associated with two different types of surfactants, Triton X 100 (TX 100, nonionic) and sodium dodecylbezenesulfonate (SDBS, anionic), that were applied either below or above critical micelle concentration (c.m.c.). In our past study (9), ROS productions were indirectly measured using furfuryl alcohol and nitro blue tetrazolium as an indicator for 1O2 and O2·- respectively. In this study, we apply an electron paramagnetic resonance (EPR) spin trapping technique which is more commonly used to detect radical species.
B.4.3. Experimental
Materials. Chemicals used as received in this study include: C60 (99.9%, sublimed, MER. Corp.), SDBS (Aldrich), TX 100 (Sigma-Aldrich), TEA (Aldrich), toluene (Fisher Scientific), 2,2,6,6-tetramethyl-4-piperidone (4-oxo-TEMP, Tokyo Chemical Industries), 5,5-dimethyl-1-pyrroline-N-oxide (DMPO, DOJINDO Laboratories), FeSO4 (Sigma-Aldrich), and diethylenetriaminepentaacetic acid (DETAPAC, Sigma-Aldrich). Ultrapure water (>18.2 MΩ) produced by a Milli-Q water purification system (Millipore, Billerica, MA) was used for the preparation of all solutions.
Preparation of aqueous stable C60. Ultrasound (50/60 Hz, 125 W) was applied to a heterogeneous mixture of 10 mL toluene containing 5 mg C60 and 90 mL ultrapure water in a sealed bottle for 24 hrs using an ultrasonicator (Model 8845-40, Cole-Parmer, US). The ultrasound was further applied to the mixture open to atmosphere for additional 24 hrs at 60 °C in order to evaporate toluene. The solution was then filtered through a 0.45-μm PTFE filter (Millipore Corp.). The aqueous suspension of aggregate form of C60 prepared according to this specific method is herein referred to as son/C60. C60 associated with surfactants were prepared following the same procedure except that the ultrapure water contained surfactant. These are referred to in this study as C60/TX 100 or C60/SDBS.
EPR Analysis. EPR analyses were performed using a Bruker EMX spectrometer under the following conditions: temperature = 298 K, microwave frequency = 9.8 GHz, microwave power = 16 mW, field modulation = 0.1 mT at 100 kHz, and scan time = 89.3 s. (1) 1O2 Detection. Production of 1O2 was monitored using 4-oxo-TEMP as a spin-trapping reagent (11). Experimental solution with the target C60 concentration of 5 mg/L was prepared by mixing 1 M 4-oxo-TEMP (20 μL), 12.5 mg/L C60 stock suspension (100 μL), 250 mM phosphate buffer at pH 7.0 (50 μL) and ultrapure water (80 μL). The solution was placed in a cylindrical quartz cell and UV light (350-400 nm) was irradiated using six 4-W black light bulbs (BLB, Philips TL4W). After the light irradiation for 3-20 min (i.e. not beyond 30 min to avoid the excessive degradation of 4-oxo-TEMP), the test cell was quickly subjected to EPR measurement. When 1O2 was produced, 4-oxo-TEMP was oxidized by 1O2 to produce 4-oxo-2,2,6,6-tetramethyl-1-piperdinyloxy radical (4-oxo-TEMPO). The EPR signals for 4-oxo-TEMPO therefore indicated the formation of 1O2. (2) O2·- Detection. The generation of O2·- was indirectly measured by converting O2·- to ·OH in-situ and monitoring ·OH, since ·OH can be more sensitively detected by the EPR spin-trapping technique (11). Fe(II)-DETAPAC complex was used to reduce O2·- to ·OH [k(Fe(II)-DETAPAC + O2·-) = (2 ± 0.5) x 107 M-1s-1 (24)]. DMPO was utilized as a spin-trapping agent for ·OH and TEA as an electron donor. The reaction mixture was prepared by mixing DMPO (20 μL), 5 mM Fe(II)-DETAPAC in 250 mM phosphate buffer (50 μL), 12.5 mg/L C60 stock suspension (100 μL), 100 mM TEA (25 μL), and distilled water (55 μL). The photochemical reaction was performed following the same procedure described above for 1O2 and EPR signals of DMPO-OH adduct were monitored.
Laser Flash Photolysis. (1) Decay Kinetics of 3C60*. Both nanosecond and femtosecond laser flash photolysis experiments were carried out to trace the decay kinetics of 3C60* in different aqueous C60 samples. The experimental solution contained 5 mg/L of C60 and was buffered at pH 7. To inhibit the pathway for energy transfer from 3C60* to oxygen, solution (3 mL) was placed in a rectangular cell, bubbled with argon gas for 30 min, and sealed from the atmosphere. For nanosecond experiments, a 355 nm laser pulse (10 mJ, pulse width = 6 ns) from a Quanta Ray Nd:YAG laser system was used as an excitation source and a xenon lamp as a monitoring source. Instantaneous formation of 3C60* after laser pulse and subsequent decay were monitored at 740 nm (6). For the C60 samples with 3C60* having short life times (less than a nanosecond), femtosecond laser flash photolysis experiments were performed using a Clark-MXR 2010 laser. A 387 nm laser pulse (attenuated to 5 μJ, pulse width = 150 fs) obtained by introducing a second harmonic generator to the path of the laser beam from a Clark laser system was used as an excitation source. Time-resolved spectra in the wavelength region from 425 to 800 nm were obtained using a Helios transient absorption spectrometer (Ultrafast System). Decay kinetics for 3C60* was obtained from the time-resolved data using absorbance at 740 nm. (2) Decay Kinetics of C60·-. Transient formation of C60·- through one-electron transfer from TEA under UV irradiation and subsequent decay were monitored using a nanosecond laser flash photolysis was conducted with 308 nm laser pulses from Lambda Physik excimer laser system (10 mJ, pulse width 10 ns). The experiments were carried out under the anoxic condition (by Argon purging) so that energy transfer from 3C60* to O2 and electron transfer from C60·- to O2 are both blocked. Concentration of C60·- was monitored by recording the transient absorption at 1050 nm in Near-IR region (25).
B.4.4. Results and Discussion
Photochemical Production of Singlet Oxygen. UV-Vis spectra of son/C60 and C60 associated with TX 100 or SDBS (Figure B.4.1), suggested that son/C60 and C60 associated with TX 100 and SDBS applied below c.m.c. formed aggregates, as evidenced by a rather strong broad band absorption in the wavelength region from 400 to 500 nm (17, 18). C60 aggregates in these samples are ca. 50 to 200 nm in size according to dynamic light scattering (DLS) measurements by our past study as well as others (9, 19, 20). In contrast, when surfactants were applied above c.m.c., aggregation was considerably reduced, as evidenced by lower absorbance in the visible regions and less red-shifted specific peaks compared to UV-Vis spectrum of son/C60 (17, 18, 26). It is suspected that C60 associated with SDBS applied above c.m.c. might have a minor degree of aggregation, while no definite evidence was not possible. Note that DLS measurements of these samples approached the instrument detection limit. Microscopic imaging using transmission electron microscope (TEM) was not possible as excessive surfactant deposited on carbon grid prevented clear images.
Figures B.4.2a and B.4.2b show that the specific EPR signals for 4-oxo-TEMPO (i.e. 1O2 adduct with 4-oxo-TEMP) were produced when the UV light was irradiated to the C60 associated with TX 100 and SDBS (above c.m.c.). The peak height increased as UV irradiation time was increased. In marked contrast, C60 associated with TX 100 and SDBS (below c.m.c.) and son/C60 did not produce any specific EPR signal for 4-oxo-TEMPO (Figures B.4.2c to B.4.2e). These C60 present as aggregate form and exhibited strong broadband visible light absorption. These EPR observations are consistent with our past conclusion driven from wet chemical analyses that C60 with greater degree of aggregation is less photochemically active with respect to 1O2 production (i.e. inefficient transfer of energy from 3C60* to oxygen) (9). It is also noteworthy that C60 encapsulated in the micelles of TX 100 produced stronger EPR signals than that with SDBS. TX 100 prohibited C60 aggregation more effectively than SDBS, resulting in lower visible light absorption, more 1O2 production, and stronger 4-oxo-TEMPO signals.
Photochemical Production of Superoxide Radical Anion. The EPR signals of DMPO-OOH adducts that are produced from a slow reaction between DMPO and O2·- [k(DMPO + O2·-) = 10 M-1s-1 (24)] were short lived and exhibited complicated patterns (data not shown). Although the appearance of signals indicated the generation of O2·- in some samples (i.e. C60 associated with TX 100 and SDBS applied above c.m.c.), it was difficult to draw a definite conclusion based on these signals. As an alternative approach, O2·- was converted to ·OH in situ by ferrous complex (Fe(II)-DETAPAC) and DMPO-OH adduct that results from the reaction between DMPO and ·OH was examined. A control test was performed with Fe(II)-DETAPAC in the absence of C60, and no DMPO-OH signals were detected during UV irradiation, suggesting that the iron catalyst would not contribute to formation of both OH radical and O2·-.
As shown in Figures B.4.3, C60 associated with TX 100 and SDBS applied above c.m.c. produced specific signals for DMPO-OH adducts of which strengths were dependent on UV irradiation time, while son/C60 and C60 associated with TX 100 and SDBS applied below c.m.c. did not. This result suggests that electron transfer from TEA to oxygen is effectively mediated by C60 embedded in micelle assembly which inhibits C60 aggregation. Accordingly, C60 encapsulated with TX 100 showed greater signal strength than SDBS, as TX 100 was more efficient for inhibiting C60 clustering in the aqueous phase. These EPR results are consistent with our past conclusion, similar to 1O2, that production of O2·- is also dependent on the degree of C60 clustering, i.e. the more aggregation, the less O2·- production (9).
Decay Kinetics of 3C60*. The above phenomena might result as 3C60*, a critical transient intermediate for both energy transfer (i.e. 1O2 generation) and electron transfer (i.e. O2·- production), is rapidly lost when C60 forms a cluster. When 3C60* is in direct contact with other C60 molecules, energy can be rapidly dissipated through a self-quenching mechanism (i.e. energy transfer to surrounding ground state C60) and triplet-triplet annihilation (i.e. energy transfer to another 3C60*). A time-resolved laser flash photolysis was used to trace 3C60* under anoxic, argon saturated condition (where 3C60* decay pathway due to energy transfer to oxygen can be excluded) to verify the above hypothesis (6). Specifically, the absorption of 3C60* at 740 was measured.
Figure B.4.4 shows that 3C60* decayed over the time scale of a few hundred microseconds when C60 was embedded in TX 100 and SDBS micelles. The rate of anoxic depletion of 3C60* observed herein was comparable to that of 3C60* molecularly dissolved in organic phase or monomeric analogue of C60 derivative in water (27). C60 embedded in TX 100 micelles is most likely to be molecularly dispersed within the micelle core (i.e., no specific visible range absorption, Figure B.4.1), and therefore the decay of triplet state by the self-quenching and triplet-triplet annihilation proceeds most slowly, among all C60 samples. C60 in SDBS may not be monomeric but have some level of aggregation (i.e., some visible absorption, Figure B.4.1), while no direct experimental evidence is currently available as mentioned above. Consequently, the anoxic decay kinetics of triplet state of this C60 is, albeit considerably retarded as well, faster than triplet depletion rate of C60/TX 100 (above c.m.c.). The laser flash photolysis results suggest that the lifetime of 3C60* in these C60 colloids is sufficiently long such that energy transfer from 3C60* to O2 competes other anoxic quenching pathways. As a result, 1O2 production was observed in EPR analysis. This spectral difference is also well reflected by the laser flash photolysis and EPR measurements. The initial absorption difference, DA, obtained immediately after laser-excitation was much higher for C60 in TX 100 than SDBS (Figure B.4.4), which, combined with much longer lifetime of 3C60* of C60/TX 100 (above c.m.c.), might imply more efficient formation of 3C60* when C60 is molecularly dispersed. Accordingly, the EPR signals were greater with C60 in TX 100 indicating greater level of both 1O2 and O2·- production (Figures B.4.2 and B.4.3).
However, no distinguishable peaks were observed for C60 clusters (i.e. son/C60 and C60 associated with TX 100 and SDBS applied below c.m.c.) using the nanosecond laser flash photolysis. Lack of peaks could have resulted if the time scale of decay is smaller than the time scale of measurement. Therefore, the measurement time scale was reduced and additional experiments were performed using femtosecond laser flash photolysis. Figures B.4.5a to B.4.5c show the absorption spectra recorded during initial 4 ps after the excitation by pulse laser irradiation. The peak centered around 740 nm, which indicated 3C60* increased over the first 2 ps, and rapidly disappeared within next 2 ps.
Compared to C60 in TX 100 and SDBS micelles, the decay kinetics of triplet state was significantly accelerated when C60 formed aggregates. The half lives of 3C60* in TX 100 and SDBS micelles were ca. 70 and 30 μs, respectively, while those of 3C60* in clusters were less than 1 ps. This result confirms the hypothesis that energy transfer process and 1O2 production pathway are fundamentally blocked due to C60 aggregation and consequent rapid loss of critical intermediate during energy transfer process. As 3C60* becomes short lived and less available, electron transfer from electron donor to 3C60* is also limited and consequently O2·- production pathway is also prohibited.
Decay Kinetics of Radical Anion of C60 Colloid. C60 radical anion (C60·-) is a critical intermediate for production of O2·- and for any other redox reactions involving electron abstraction or electron transfer mediation by C60. Using a laser flash photolysis, transient formation and decay of C60·- was monitored. An absorption at 1050 nm was recorded as C60·- exhibits strong absorption in IR region (25).
Figure B.4.6 shows that C60·- formed when a solution containing C60 in TX 100 micelle and TEA was irradiated with a pulse laser at 355 nm and decayed over approximately 0.3 ms. The kinetics of formation and decay were comparable to 3C60* (Figure B.4.4) suggesting that electron transfer from the electron donor to 3C60* occurred at a faster rate than the time scale of experiments. In contrast, no measurable peak for C60·- was detected for clustered C60 (son/C60 and C60 with surfactants below c.m.c.) (results not shown). This result was consistent with the observations made for 3C60* as well as O2·- production recorded by EPR and spectral properties.
The results presented in this study help understand the environmental impact of C60 aggregate, a form of C60 that has received a widespread concern as a possible pollutant form of C60 in natural waterways. The EPR measurement of ROS presented in this study confirms that C60 clustering results in loss of C60’s intrinsic photoreactivity. Laser flash photolysis suggests that this loss results as the lifetime of key intermediate species is drastically reduced when C60 forms aggregate. Therefore, aggregate C60 themselves will not be responsible for oxidative damage by photochemical ROS production in natural environment and biological systems, contrary to some suggestions made in the past (28-30). In contrast, C60 might participate in ROS production as it is embedded in surfactant micelles or other natural encapsulating agents which prevent C60 aggregation.
It is also noteworthy that C60·- did not form when C60 is aggregated even under relatively favorable conditions for reduction (i.e. the presence of a strong electron donor and under UV irradiation). C60 reduction is thermodynamically favorable when the reduction potential of triplet state, molecular C60 is considered (E0(3C60*/βC60·-) = +1.1). Therefore, the electron transfer process is kinetically limited when C60 forms aggregates and the reaction intermediate is quenched. This suggests that aggregate C60 in most natural and biological environment will not directly participate in redox reactions (i.e. as an electron acceptor or electron mediator), while a definite conclusion cannot be made at current stage. Since changes in dispersion status as C60 enters the biological system is suspected, further study is necessary to trace the photochemical properties of C60 that interact with biomolecules.
B.4.5. Literature Cited
- Colvin, V. L., The potential environmental impact of engineered nanomaterials. Nature Biotechnol. 2003, 21, 1166-1170.
- Sayes, C. M.; Fortner, J. D.; Guo, W.; Lyon, D.; Boyd, A. M.; Ausman, K. D.; Tao, Y. J.; Sitharaman, B.; Wilson, L. J.; Hughes, J. B.; West, J. L.; Colvin, V. L., The differential cytotoxicity of water-soluble fullerenes. Nano Letts. 2004, 4, 1881-1887.
- Wiesner, M. R.; Lowry, G. V.; Alvarez, P.; Dionysiou, D.; Biswas, P., Assessing the risks of manufactured nanomaterials. Environ. Sci. Technol. 2006, 40, 4336-4345.
- Lof. R. W.; van Veenendaal, M. A. K., B.; Jonkman, H. T.; Sawatzky, G. A., Band gap, excitons, and Coulomb interaction in solid C60. Phys. Rev. Lett. 1992, 68, 3924-3927.
- Arbogast, J. W.; Darmanyan, A. P.; Foote, C. S.; Rubin, Y.; Diederich, F. N.; Alvarez, M. M.; Anz, S. J.; Whetten, R. L., Photophysical properties of C60. J. Phys. Chem. 1991, 95, 11-12.
- Dimitrijevic, N. M.; Kamat, P. V., Triplet excited-state behavior of fullerenes - pulse-radiolysis and laser flash photolysis of C60 and C70 in benzene. J. Phys. Chem. 1992, 96, 4811-4814.
- Fraelich, M. R.; Weisman, R. B., Triplet-states of C60 and C70 in solution - Long intrinsic lifetimes and energy pooling. J. Phys. Chem. 1993, 97, 11145-11147.
- Jensen, A. W.; Daniels, C., Fullerene-coated beads as reusable catalysts. J. Org. Chem. 2003, 68, 207-210.
- Lee, J.; Fortner, J. D.; Hughes, J. B.; Kim, J. H., Photochemical production of reactive oxygen species by C60 in the aqueous phase during UV irradiation. Environ. Sci. Technol. 2007, 41, 2529-2535.
- Yamakoshi, Y.; Sueyoshi, S.; Fukuhara, K.; Miyata, N., OH and O2- generation in aqueous C60 and C70 solutions by photoirradiation: An EPR study. J. Am. Chem. Soc. 1998, 120, 12363-12364.
- Yamakoshi, Y.; Umezawa, N.; Ryu, A.; Arakane, K.; Miyata, N.; Goda, Y.; Masumizu, T.; Nagano, T., Active oxygen species generated from photoexcited fullerene (C60) as potential medicines: O2- versus 1O2. J. Am. Chem. Soc. 2003, 125, 12803-12809.
- Arbogast, J. W.; Foote, C. S.; Kao, M., Electron transfer to triplet C60. J. Am. Chem. Soc. 1992, 114, 2277-2279.
- Kamat, P. V.; Haria, M.; Hotchandani, S., C60 cluster as an electron shuttle in a Ru(II)-polypyridyl sensitizer-based photochemical solar cell. J. Phys. Chem. B 2004, 108, 5166-5170.
- Dhawan, A.; Taurozzi, J. S.; Pandey, A. K.; Shan, W. Q.; Miller, S. M.; Hashsham, S. A.; Tarabara, V. V., Stable colloidal dispersions of C60 fullerenes in water: Evidence for genotoxicity. Environ. Sci. Technol. 2006, 40, 7394-7401.
- Isakovic, A.; Markovic, Z.; Todorovic-Markovic, B.; Nikolic, N.; Vranjes-Djuric, S.; Mirkovic, M.; Dramicanin, M.; Harhaji, L.; Raicevic, N.; Nikolic, Z.; Trajkovic, V., Distinct cytotoxic mechanisms of pristine versus hydroxylated fullerene. Toxicol. Sci. 2006, 91, 173-183.
- Brant, J.; Lecoanet, H.; Hotze, M.; Wiesner, M., Comparison of electrokinetic properties of colloidal fullerenes (nC60) formed using two procedures. Environ. Sci. Technol. 2005, 39, 6343-6351.
- Deguchi, S.; Alargova, R. G.; Tsujii, K., Stable dispersions of fullerenes, C60 and C70, in water. Preparation and characterization. Langmuir 2001, 17, 6013-6017.
- Fortner, J. D.; Lyon, D. Y.; Sayes, C. M.; Boyd, A. M.; Falkner, J. C.; Hotze, E. M.; Alemany, L. B.; Tao, Y. J.; Guo, W.; Ausman, K. D.; Colvin, V. L.; Hughes, J. B., C60 in water: Nanocrystal formation and microbial response. Environ. Sci. Technol. 2005, 39, 4307-4316.
- Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A., On the production of an aqueous colloidal solution of fullerenes. J. Chem. Soc., Chem. Commun. 1995, 1281-1282.
- Chen, K. L.; Elimelech, M., Aggregation and deposition kinetics of fullerene (C60) nanoparticles. Langmuir 2006, 22, 10994-11001.
- Fang, J. S.; Lyon, D. Y.; Wiesner, M. R.; Dong, J. P.; Alvarez, P. J. J., Effect of a fullerene water suspension on bacterial phospholipids and membrane phase behavior. Environ. Sci. Technol. 2007, 41, 2636-2642.
- Lyon, D. Y.; Adams, L. K.; Falkner, J. C.; Alvarez, P. J. J., Antibacterial activity of fullerene water suspensions: Effects of preparation method and particle size. Environ. Sci. Technol. 2006, 40, 4360-4366.
- Beeby, A.; Eastoe, J.; Heenan, R. K., Solubilization of C60 in aqueous micellar solution. J. Chem. Soc., Chem. Commun. 1994, 173-175.
- Bielski, B. H. J.; Cabelli, D. E.; Arudi, R. L.; Ross, A. B., Reactivity of HO2/O2- radicals in aqueous solution. J. Phys. Chem. Ref. Data 1985, 14, 1041-1100.
- Nakanishi, I.; Fukuzumi, S.; Konishi, T.; Ohkubo, K.; Fujitsuka, M.; Ito, O.; Miyata, N., DNA cleavage via superoxide anion formed in photoinduced electron transfer from NADH to r-cyclodextrin-bicapped C60 in an oxygen-saturated aqueous solution. J. Phys. Chem. B 2002, 106, 2372-2380.
- Bensasson, R. V.; Bienvenue, E.; Dellinger, M.; Leach, S.; Seta, P., C60 in model biological systems - a Visible-Uv absorption study of solvent-dependent parameters and solute aggregation. J. Phys. Chem. 1994, 98, 3492-3500.
- Guldi, D. M.; Prato, M., Excited-state properties of C60 fullerene derivatives. Acc. Chem. Res. 2000, 33, 695-703.
- Nakajima, N.; Nishi, C.; Li, F. M.; Ikada, Y., Photo-induced cytotoxicity of water-soluble fullerene. Fullerene Sci. Technol. 1996, 4, 1-19.
- Tsuchiya, T.; Oguri, I.; Yamakoshi, Y. N.; Miyata, N., Novel harmful effects of [60]fullerene on mouse embryos in vitro and in vivo. FEBS Letts. 1996, 393, 139-145.
- Yang, X. L.; Fan, C. H.; Zhu, H. S., Photo-induced cytotoxicity of malonic acid [C60]fullerene derivatives and its mechanism. Toxicol. in Vitro 2002, 16, 41-46.
Figure B.4.1. UV-Visible absorption spectra of son/C60 and C60 colloids associated with TX 100 and SDBS applied below and above c.m.c.. ([TX 100 (below c.m.c.)]0 = 0.1 g/L; [TX 100 (above c.m.c.)]0 = 50 g/L; [SDBS (below c.m.c.)]0 = 0.2 g/L; [SDBS (above c.m.c.)]0 = 20 g/L (above c.m.c.); [phosphate]0 = 50 mM; pHi = 7)
Figure B.4.2. EPR spectra of 4-oxo-TEMP adduct with 1O2 produced in the UV-Vis illuminated suspension of (a) C60/TX 100 (above c.m.c.), (b) C60/SDBS (above c.m.c.), (c) son/C60, (d) C60/TX 100 (below c.m.c.), and (e) C60/SDBS (below c.m.c.) ([son/C60]0 = [C60/TX 100]0 = [C60/SDBS]0 = 5 mg/L; [4-oxo-TEMP]0 = 80 mM; [phosphate]0 = 50 mM; pHi = 7).
Figure B.4.3. EPR spectra of DMPO adduct with OH radical produced in the UV-Vis illuminated suspension of (a) C60/TX 100 (above c.m.c.), (b) C60/SDBS (above c.m.c.), (c) son/C60, (d) C60/TX 100 (below c.m.c.), and (e) C60/SDBS (below c.m.c.) with Fe(II)-DETAPAC and TEA as an electron donor ([son/C60]0 = [C60/TX 100]0 = [C60/SDBS]0 = 5 mg/L; [DMPO]0 = 0.72 M; [Fe(II)-DETAPAC]0 = 5 mM; [TEA]0 = 10 mM; [phosphate]0 = 50 mM; pHi = 7).
Figure B.4.4. Absorption time profile of 3C60* recorded at 740 nm in aqueous suspension of (a) C60/TX 100 (above c.m.c.) and (b) C60/SDBS (above c.m.c.) ([C60/TX 100]0 = [C60/SDBS]0 = 5 mg/L; [phosphate]0 = 50 mM; pHi = 7; Ar-saturated condition).
Figure B.4.5. Time-resolved differences absorption spectra of triplet state of (a) aqueous son/C60, (b) C60/TX 100 (below c.m.c.) and (c) C60/SDBS (below c.m.c.) colloid ([son/C60]0 = [C60/TX 100]0 = [C60/SDBS]0 = 5 mg/L; [phosphate]0 = 50 mM; pHi = 7).
Figure B.4.6. Absorption time profile of C60·- recorded at 1050 nm in aqueous suspension of C60/TX 100 (above c.m.c.) ([C60/TX 100]0 = 5 mg/L; [TEA]0 = 10 mM; [phosphate]0 = 50 mM; pHi = 7; Ar-saturated condition).
C. PROGRESS EVALUATION
C.1. Comparison of Actual Accomplishments with Stated Goals
The following table summarizes the schedule of the project tasks outlined in Year 1 Progress report. Based on the progress of the research and experience gained so far, we believe that tasks involved in metal hydroxide destabilization and filtration would require much less time and efforts compared to oxidative transformation by ozone and chlorine/chloramines and UV irradiation. We have done extensive adsorption study involving NOM and MWNT and we plan to reduce the work related to activated carbon adsorption. The progress of the work is indicated as a percent completed as of November 2007.
C.2. Expenditure to Date
The following table summarizes the expenditure to date (as of November 27, 2007). Based on Georgia Tech’s budgeting system, some salaries for the term Spring 2008 have been included as the past expense.
Table 1. Project Expenditure to Date
Category | Budget |
Expense |
Balance |
Personal Services |
$161,081.00 |
$121,948.86 |
$39,132.14 |
Fringe Benefits |
$22,735.00 |
$17,476.03 |
$5,258.97 |
Materials & Supplies |
$45,456.00 |
$41,298.35 |
$4,225.32 |
Tuition |
$16,631.00 |
$17,114.59 |
-$483.59 |
Travel |
$10,600.00 |
$10,510.67 |
$89.33 |
|
|
|
|
Total Direct |
$256,503.00 |
$208,348.50 |
$48,222.17 |
|
|
|
|
Total Indirect |
$118,497.00 |
$96,060.08 |
$22,466.93 |
|
|
|
|
Total |
$375,000.00 |
$304,408.58 |
$70,689.10 |
C.3. Project Participants
C.3.1. Key Personnel
Name | Jaehong Kim, Ph.D. |
Title | Assistant Professor |
Participation | September 2006 to present |
Role | Principal Investigator |
Name | Joseph Hughes, Ph.D. |
Title | Professor and School Chair |
Participation | September 2006 to present |
Role | Co-Principal Investigator |
Name | Guangxuan Zhu, Ph.D. |
Title | Senior Research Scientist |
Participation | September 2006 to present |
Role | QA/QC Manager |
C.3.2. Graduate Students and Post-docs
Name | Dooil Kim |
Title | Ph.D. Candidate, Graduate Research Assistant |
Participation | January 2006 to May 2006 |
Task | Studying nano-C60 reaction with ozone in the aqueous phase |
Name | Hoon Hyung |
Title | Ph.D. Candidate, Graduate Research Assistant |
Participation | November 2005 to present |
Task | Aqueous stability of C60 and other carbon nanomaterials |
Name | Jaesang Lee |
Title | Postdoctoral Researcher (half of salary supported by Korean Research Foundation) |
Participation | March 2006 to present |
Task | Photochemical reactivity of C60 |
C.3.3. Undergraduate Student
Name | Varun Ghandi |
Title | Undergraduate Research Assistant, PURA awardee |
Participation | September 2006 to December 2006 |
Task | Reaction of nano-C60 with hydroxyl radical |
Note that John D. Fortner, a graduate research assistant of the co-PI, funded by NSF Center for Biological and Environmental Nanotechnology, has been collaborating very closely with the project team |
Future Activities:
C.5. Next Phase Plan
In year 3, we will continue to study the response of aqueous stable C60 to UV irradiation, especially at lower wavelength to verify the potential for direct photolysis of C60. Experiments are currently performed to evaluate the fate of C60 colloids in conventional water treatment processes.
Journal Articles on this Report : 3 Displayed | Download in RIS Format
Other project views: | All 20 publications | 10 publications in selected types | All 10 journal articles |
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Type | Citation | ||
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Fortner JD, Kim DI, Boyd AM, Falkner JC, Moran S, Colvin VL, Hughes JB, Kim J-H. Reaction of water stable C60 aggregates with ozone. Environmental Science & Technology 2007;41(21):7497-7502. |
R832526 (2007) R832526 (Final) |
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Hyung H, Fortner JD, Hughes JB, Kim J-H. Natural organic matter stabilizes carbon nanotubes in the aqueous phase. Environmental Science & Technology 2007;41(1):179-184. |
R832526 (2006) R832526 (2007) R832526 (Final) |
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Lee J, Fortner JD, Hughes JB, Kim J-H. Photochemical production of reactive oxygen species by C60 in the aqueous phase during UV irradiation. Environmental Science & Technology 2007;41(7):2529-2535. |
R832526 (2007) R832526 (Final) |
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Supplemental Keywords:
RFA, Health, Scientific Discipline, PHYSICAL ASPECTS, Water, Environmental Chemistry, Health Risk Assessment, Risk Assessments, Biochemistry, Physical Processes, Drinking Water, Engineering, Chemistry, & Physics, fate and transport, health effects, human health effects, carbon fullerene, epidemelogy, exposure, nanotechnology, other - risk assessment, particle exposure, community water system, ambient particle health effects, human exposure, engineered nanomaterials, toxicity, nanomaterials, water quality, cellular responses, drinking water contaminants, biochemical research, human health risk, drinking water systemProgress and Final Reports:
Original AbstractThe perspectives, information and conclusions conveyed in research project abstracts, progress reports, final reports, journal abstracts and journal publications convey the viewpoints of the principal investigator and may not represent the views and policies of ORD and EPA. Conclusions drawn by the principal investigators have not been reviewed by the Agency.