pubs.acs.org/Langmuir © 2009 American Chemical Society
Hydration and Dispersion of C60 in Aqueous Systems: The Nature of Water-Fullerene Interactions )
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Jer^ome Labille,*,† Armand Masion,† Fabio Ziarelli,‡ Jer^ome Rose,† Jonathan Brant,§,^ Frederic Villieras, Manuel Pelletier, Daniel Borschneck,† Mark R. Wiesner,§ and Jean-Yves Bottero† †
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CEREGE, CNRS and Aix-Marseille University, Europole M editerran een de l’Arbois, BP80, 13545 Aix en Provence Cedex 04, France, ‡F ed eration des Sciences Chimiques de Marseille CNRS FR1739, Spectropole, Service 511, av. Escadrille Normandie Ni emen, 13397 Marseille Cedex 20, France, §Department of Civil and Environmental Engineering, Duke University, Durham, North Carolina 27708-0287, and Laboratoire Environnement et Min eralurgie, Nancy-Universit e-CNRS, Ecole Nationale Sup erieure de G eologie, BP 40, 54501 Vandoeuvre-l es-Nancy, France. ^ Current address: University of Wyoming, Department of Civil & Architectural Engineering, 1000 E. University Avenue, Department 3295, Laramie, Wyoming 82071. Received June 25, 2009. Revised Manuscript Received August 24, 2009 The nature of fullerene-water interactions and the role that they play in the fate of C60 in aqueous systems is poorly understood. This work provides spectroscopic evidence for the surface hydroxylation of the initially hydrophobic C60 molecule when immersed in water. This mechanism appears to be the basis for stabilizing the hydrophilic nC60 aggregates in suspension. It is remarkable that such a chemical transformation and dispersion are achieved under mild conditions that are readily produced in an aquatic environment. This acquired affinity for water is likely to play a subsequent role in the reactivity, mobility, and bioavailability of fullerenes in aqueous media.
Introduction Fullerene (C60) molecules, discovered recently,1 have a very stable polyhedral nanosized cage structure involving a conjugated π system, conferring on them unique electronic properties at this scale. Commercial applications range from consumer electronics to medical therapeutics.2,3 Many of these applications require the dispersal of C60 in a solvent, with aqueous dispersions being preferred because of biocompatibility, safety, or environmental concerns. Although typically considered to be hydrophobic in nature,4-6 C60 has been successfully dispersed in water using functionalization,7-9 solvent exchange,10-13 or even more recently *Corresponding author. E-mail:
[email protected]. (1) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley, R. E. Nature 1985, 318, 162–163. (2) Jensen, A. W.; Wilson, S. R.; Schuster, D. I. Bioorg. Med. Chem. 1996, 4, 767–779. (3) Bosi, S.; Da Ros, T.; Spalluto, G.; Prato, M. Eur. J. Med. Chem. 2003, 38, 913–923. (4) Ruoff, R. S.; Tse, D. S.; Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97, 3379–3383. (5) Marcus, Y.; Smith, A. L.; Korobov, M. V.; Mirakyan, A. L.; Avramenko, N. V.; Stukalin, E. B. J. Phys. Chem. B 2001, 105, 2499–2506. (6) Alargova, R. G.; Deguchi, S.; Tsujii, K. J. Am. Chem. Soc. 2001, 123, 10460– 10467. (7) Guldi, D. M.; Asmus, K.-D. Radiat. Phys. Chem. 1999, 56, 449–456. (8) Gao, Y.; Tang, Z. X.; Watkins, E.; Majewski, J.; Wang, H. L. Langmuir 2005, 21, 1416–1423. (9) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807–815. (10) Andrievsky, G. V.; Kosevich, M. V.; Vovk, O. M.; Shelkovsky, V. S.; Vashchenko, L. A. Electrochem. Soc. Proc. 1995, 12, 1591–1601. (11) Brant, J. A.; Labille, J.; Bottero, J. Y.; Wiesner, M. R. Langmuir 2006, 22, 3878–3885. (12) Deguchi, S.; Alargova, R. G.; Tsujii, K. Langmuir 2001, 17, 6013–6017. (13) Scrivens, W. A.; Tour, J. M.; Creek, K. E.; Pirisi, L. J. Am. Chem. Soc. 1994, 116, 4517–4518. (14) Labille, J.; Brant, J.; Villieras, F.; Pelletier, M.; Thill, A.; Masion, A.; Wiesner, M.; Rose, J.; Bottero, J. Y. Fullerenes, Nanotubes, Carbon Nanostruct. 2006, 14, 307–314.
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after prolonged mixing of C60 in pure water.11 The underlying nature of fullerene-water interactions that may explain an evolving affinity of fullerenes with water under certain conditions14 is the topic of the current communication. The experimental approach followed here was based on the water vapor adsorption isotherm and physical-chemical characterizations of C60 in different stages of hydration. It enabled us to propose a reaction that transforms the C60 characteristics from hydrophobic to hydrophilic when C60 is brought into contact with water.
Materials and Methods Pristine C60 fullerite (C60 99.9%, MER Corp., Tucson, AZ) was used in all experiments. The affinity of C60 for water was quantified by water vapor adsorption/desorption isotherms. Gravimetric measurements were performed on a 100 mg fullerite sample at 30 C in a quasi-equilibrium state during continuous water supply/removal. According to previous observations,11,14 stable aqueous dispersions of C60 clusters (nC60) in water can be obtained after prolonged mixing of fullerite and water (1 g/L). In this work, the mixture was first sonicated (155 kJ/h, 20 h) under a nitrogen atmosphere and then centrifuged (34 000g, 30 min) to remove excess solid material. The nC60 obtained was characterized in terms of size by photon correlation spectroscopy (PCS), in terms of internal lattice structure by X-ray diffraction (XRD), and in terms of shape by transmission electron microscopy (TEM). The spectroscopic fingerprints of freeze-dried samples of nC60 were compared to initial pristine C60 using Fourier transformed infrared spectroscopy (FTIR) and 1H solid state nuclear magnetic resonance (NMR). A commercial fullerenol C60(OH)24 sample was also studied by 1H NMR as a polyhydroxylated C60 reference.
Results Fullerite Affinity for Water. The water vapor adsorption/ desorption isotherm has been performed to study the hydration of
Published on Web 09/02/2009
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Figure 1. Water vapor adsorption/desorption isotherm. The number of H2O monolayers adsorbed is estimated from the BET method. At the first monolayer adsorbed, P(H2O)/P0 = 0.24 and Vm = 0.44 cm3/g.
the C60 surface (Figure 1). At low relative pressure, the adsorption branch is typical of a nonmicroporous surface (type II isotherm), and multilayer adsorption occurs at medium and high relative pressure with a condensation phenomenon around 0.7. The steeper slope of the isotherm above P/P0 ≈ 0.7 indicates that adsorption occurs on the hydrophilic surface at higher relative pressures. This is interpreted as being due to water molecule clustering at the fullerite surface, favoring multilayer adsorption and 3D condensation. The number of water monolayers adsorbed on fullerite, Nm, was estimated on the basis of the BET approximation,15,16 which revealed that the first monolayer was statistically completed at (P/ P0) ≈ 0.25. The obtained specific surface area is around 1.8 m2/g, in agreement with nitrogen adsorption data that show that the BET specific surface area is lower than 2 m2/g (not shown because nitrogen adsorption experiments are not accurate for low specific surface area, as is the case for fullerite). The hysteresis between adsorption and desorption branches reveals an important affinity of water molecules for fullerite and their irreversible bonding to the C60 surface. This is typical of systems sensitive to water hydration, such as cements, because hydration phenomena occur at high relative pressure (i.e., when bulk water is present on the surface or on interparticle mesopores). For medium to low relative pressure, the desorption isotherm remains parallel to the adsorption isotherm, showing that the surface area of C60 particles remains unchanged. In addition, the more concave shape of the desorption curve for P/P0 < 0.1 suggests that the surface is more hydrophilic, which is in agreement with irreversible reaction at the fullerite/water interface during the adsorption step that modifies the C60 to yield a more hydrophilic surface. About one monolayer of water remains adsorbed at P/P0 =0. Physical Characterization of nC60. Both TEM observation of the nC60 dried on a copper grid (Figure 2a) and PCS measurement of the nC60 aqueous suspension (Figure 2b) reveal an nC60 size distribution ranging from 10 to 200 nm. nC60 particles also appear to be dense and differentiated in shape according to their size. Small clusters (tens of nanometers) are smooth and round, whereas large ones (hundreds of nanometers) are more random, blunt, and angular. This strongly suggests a mechanical erosion (15) Adamson, A. W.; Gast, A. P., Physical Chemistry of Surfaces, 6th ed.; John Wiley & Sons: New York, 1997; p 783. (16) Brunauer, S.; Emmett, P. H.; Teller, E. J. Am. Chem. Soc. 1938, 60, 309– 319.
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Figure 2. (a) TEM photograph of nC60 dried on a copper grid (scale bar = 20 nm). (b) Size distribution in an nC60 aqueous suspension after sonication (120 W/s, 20 h, 6 106 J) and centrifugation (340 000g, 2 h). (c) X-ray diffractograms of dried nC60 and fullerite. Peak labels indicate the corresponding distances (A˚) and lattice planes (hkl references).
phenomenon due to the energy dissipated by physical stirring, separating small fragments from larger particles. This is also confirmed by the X-ray diffraction pattern (Figure 2c), which revealed that the nC60 core lattice structure is very similar to that of the original pure fullerite (i.e., organized in a face-centeredcubic (fcc) arrangement of the C60 molecules). The nC60 clusters thus result from an erosion of the larger C60 crystallites, preserving the internal lattice structure. Moreover, lattice planes (111) and (220) appear underexpressed and overexpressed, respectively, in the nC60 diffraction pattern compared to that of the original fullerite. This indicates that the erosion process occurs preferentially via (111) lattice plane exfoliation. This implies a relative integrity of the C60 molecules at the surface of nC60 crystallites during this step because (111) planes correspond to a cleavage plane not crossing any C60 molecule, whereas (220) planes run exactly through the center of the C60 buckyballs (Supporting Information). This is consistent with the binding energies characterizing C60-C60 attractions inside the crystallite and C-C bonds inside the C60 molecule (EC60 - C60 = 1.6 eV < Esp2C - C=7.71 eV).17,18 Molecular Scale Characterization of nC60. The FTIR spectra of pristine fullerite and dried nC60 (Figure 3) both display four intense peaks at 1428, 1182, 576, and 526 cm-1 attributed to the C-C vibrational modes of the C60 molecules.19 For the nC60 sample, however, additional peaks are observed. Bands at 3435 and 1625 cm-1, correspond to O-H stretching and bending, (17) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Crystalline Structure of Fullerene Solids. In Science of Fullerenes and Carbon Nanotubes; Academic Press: San Diego, 1995; pp 171-223. (18) Hua, X.; Cagin, T.; Che, J.; Goddard, W. A. Nanotechnology 2000, 11, 85– 88. (19) Kratschmer, W.; Lamb, L. D.; Fostiropoulos, K.; Huffman, D. R. Nature 1990, 347, 354–358.
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Figure 3. Comparison in the FTIR spectra of the nC60 and the original pristine C60.
respectively, and indicate the presence of remaining free water despite heating the sample to 100 C prior to analysis. This agrees with the irreversible water physisorption observed on the water adsorption isotherm (Figure 1). However, the most interesting feature on the IR spectrum of nC60 is the large, intense band near 1100 cm-1 attributed to C-O stretching.20 Given the composition of the sample (i.e., a freeze-dried aqueous suspension of C60), these C-O vibrations strongly suggest the presence of hydroxyls forming alcohol functional groups in the nC60 structure. The width of the band (900-1250 cm-1) indicates that these C-O(H) groups are located in various chemical environments. Solid-state 1H NMR provided additional evidence for the chemical transformations of the nC60. The spectrum of pristine C60 displays a single resonance at 0.97 ppm attributed to mobile adsorbed water. For the hydrated nC60, the spectrum shows two sharp resonances around 1 ppm that are probably due to the water molecules in two slightly different magnetic environments21 and a broad signal at 4 ppm attributed to the proton of the hydroxyl in C-OH groups. This confirms that the C-O vibrations observed by FTIR correspond to alcohol functional groups. The width of this resonance points to a nonunique molecular environment for these newly formed hydroxyls, as already hypothesized from the IR data, and possibly interactions such as fast exchange between hydroxylated sites. There are marked similarities in the overall shape between the spectra of nC60 and commercial fullerenol C60(OH)24 (Figure 4). The observed resonances for fullerenol centered at ca. 1.2 and 6 ppm (C-OH) are consistent with previous findings.22,23 For the hydrated nC60, the relatively low chemical shift of the hydroxyl compared to that of the fullerenol may originate from local magnetic field inhomogeneities created by neighboring π-electron systems, resulting in an upfield shift of the resonance.24 In the commercial fullerenol sample, where statistically nearly half of the C atoms in C60 carry a hydroxyl group, this phenomenon is expected to be marginal because of the reduced number of π systems. Indeed, only a (20) Lu, C. Y.; Yao, S. D.; Lin, W. Z.; Wang, W. F.; Lin, N. Y.; Tong, Y. P.; Rong, T. W. Radiat. Phys. Chem. 1998, 53, 137–143. (21) Collins, C.; Kolodziejski, W.; Foulkes, J.; Klinowski, J. Chem. Phys. Lett. 1998, 289, 338–340. (22) Chiang, L. Y.; Upasani, R. B.; Swirczewski, J. W. J. Am. Chem. Soc. 1992, 112, 10154–10157. (23) Chiang, L. Y.; Wang, L. Y.; Swirczewski, J. W.; Soled, S.; Cameron, S. J. Org. Chem. 1994, 59, 3960–3968. (24) Thomas, F.; Masion, A.; Bottero, J.-Y.; Rouiller, J.; Montigny, F.; Genevrier, F. Environ. Sci. Technol. 1993, 27, 12.
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Figure 4. 1H ssNMR spectra at 400 MHz recorded at magic angle spinning (sr 20 kHz) on original C60, hydrated nC60, and commercial fullerenol C60(OH)24.
shoulder at 4 ppm/peak dissymmetry is barely discernible on the fullerenol spectrum. Solid-state 13C NMR could not discern the C-OH groups in nC60 with respect to the overall C speciation (data not shown), implying a small fraction of the hydroxylated binding sites below the
