Langmuir 2005, 21, 5251-5255
5251
Molecular Dynamics Simulation Study of the Role of Evenly Spaced Poly(ethylene oxide) Tethers on the Aggregation of C60 Fullerenes in Water Dmitry Bedrov,* Grant D. Smith, and Liwei Li Department of Materials Science & Engineering, Department of Chemical Engineering, University of Utah, 122 South Central Campus Drive, Room 304, Salt Lake City, Utah 84112 Received February 23, 2005. In Final Form: April 27, 2005 The aggregation behavior of C60 fullerenes and C60 fullerenes with six symmetrically tethered poly(ethylene oxide) oligomers [(PEO)-6-C60] in aqueous solutions has been studied using implicit solvent molecular dynamics simulations. Our simulations reveal that while the attraction between two (PEO)6-C60 fullerenes in aqueous solution is stronger and longer range than that between two bare C60 fullerenes, the (PEO)-6-C60 fullerenes do not phase-separate in water but rather aggregate in chainlike clusters at concentrations where unmodified fullerenes completely phase-separate.
Modification of nanoparticles by attachment of polymer chains can in principle allow altering and control of the geometry and interaction of fullerenes on the nanometer scale that can in turn result in unique binding properties or self-assembly. For example, molecular dynamics (MD) simulations using simplified coarse-grained models have shown that model nanoparticles tethered by polymer chains with various degrees of asymmetry, polymer chain length, and polymer/particle interaction exhibit a rich spectrum of nanostructured morphologies.1 These simulations have shown that polymer-tethered nanoparticles can aggregate to form spherical, cylindrical, lamellar, sheetlike, and bicontinuous morphologies. Self-assembly of nanoparticles tethered with oligomers into spherical vesicles2 and nanorods3 has also been observed experimentally. Chemical modification of fullerene nanoparticles with various polymeric oligomers has recently attracted particular attention as a potential pathway to design and control nanoscale structures and interactions.4 Applications of chemically modified fullerenes include formation of complex nanoscale structures, drug delivery, imaging contrast agent, specific binding to biological molecules/ structures, and polymer nanocomposites with superior properties. In this letter we investigate the aggregation behavior of chemically realistic fullerene nanoparticles in aqueous solutions using MD simulations. Specifically, we focus on C60 fullerenes and C60 fullerenes tethered with six poly(ethylene oxide) (PEO) chains of molecular weight Mw ) 266 evenly distributed on the surface of the sphere (placed at the vertexes of a hexahedron), referred to as (PEO)6-C60. Bare C60 fullerenes are thermodynamically insoluble in water. However, it has been shown that suspensions with fullerene aggregates ranging in size from 3 to 72 nm can be stable at a relatively high fullerene concentration for days.5 PEO on the other hand is water soluble for all molecular weights at all concentrations for temperatures below 400 K and has been extensively used in biomedical (1) Zhang, Z.; Horsch, M. A.; Lamm, M. H.; Glotzer, S. C. Nano Lett. 2003, 3, 134. (2) Zhou, S.; Burger, C.; Chu, B.; Sawamura, M.; Nagahama, N.; Toganoh, M.; Hackler, U. E.; Isobe, H.; Nakamura, E. Science 2001, 291, 1944. (3) Sawamura, M.; Kawai, K.; Matsuo, Y.; Kanie, K.; Kato, T.; Nakamura, E. Nature 2002, 419, 702. (4) Nakamura, E.; Isobe, H. Acc. Chem. Res. 2003, 36, 807. (5) Andrievsky, G. V.; Klochkov, V. K.; Karyakina, E. L.; MchedlovPetrosyan, N. O. Chem. Phys. Lett. 1999, 300, 392.
and bioengineering applications to improve the biocompatibility of various synthetic materials.6 Therefore, it is reasonable to anticipate that tethering of C60 fullerenes with PEO chains should reduce the strong tendency of fullerenes to aggregate in water and perhaps result in complete solubility of PEO-tethered fullerenes in water. However, as we discuss below, symmetric modification of C60 fullerenes with PEO oligomers results in complex interactions between the modified fullerenes in water and causes unexpected chainlike aggregation. We have utilized two types of models in our MD simulations of C60 and (PEO)-6-C60 aqueous solutions, namely, a fully atomistic model with explicit representation of water and a parametrized coarse-grained model where the influence of water on fullerene-fullerene, fullerene-PEO, and PEO-PEO interactions is included implicitly. In the fully atomistic simulations two C60 fullerenes each tethered with six -[O-CH2-CH2]6O-CH3 oligomers are solvated by 1500 TIP4P7 water molecules. PEO-PEO and PEO-water inter- and PEOPEO intramolecular interactions have been described using our well-validated quantum-chemistry-based force field.8 C60-C60 interactions were described using all-atom potential,9 simulations with which predicted fullerene properties in good agreement with experiment. C60-water interactions were adopted from ref 10 where the potential was parametrized to accurately recover the macroscopic contact angle of a water droplet on a graphite sheet. Nonbonded interactions between C60 and PEO were determined using standard combining rules. Ewald summation was used to treat long-range electrostatic interaction, and a cutoff radius of 10 Å was used for all nonbonded interactions. Following our previous study of interactions between bare C60 fullerenes in water,11 we utilized umbrella sampling simulations using a harmonic biasing potential that allowed us to sample phase space as a function of (6) Poly(Ethylene Glycol): Chemistry and Biological Applications; ACS Symposium Series; Harris, J. M., Zalipsky, S., Eds.; American Chemical Society: Washington, DC, 1997; Vol. 680. (7) Jorgensen, W. L.; Chandrasekhar, J.; Madura, J.D.; Impey, R. W.; Klein, M. J. Chem. Phys. 1983, 79, 926. (8) Smith, G. D.; Borodin, O.; Bedrov, D. J. Comput. Chem. 2002, 23, 1480. (9) Girifalco, L. A. J. Phys. Chem. 1992, 96, 858. (10) Werder, T.; Walther, J. H.; Jaffe, R. L.; Halicioglu, T.; Koumoutsakos, P. J. Phys. Chem. B 2003, 107, 1345. (11) Li, L.; Bedrov, D.; Smith, G. D. Phys. Rev. E. 2005, 71, 011502 (1-4).
10.1021/la0504816 CCC: $30.25 © 2005 American Chemical Society Published on Web 05/12/2005
5252
Langmuir, Vol. 21, No. 12, 2005
Figure 1. PMF between two bare and tethered fullerenes as a function of their separation in aqueous solution obtained from atomistic, explicit solvent and coarse-grained, implicit solvent MD simulations at 298 K.
fullerene separation. The self-consistent multiple histogram approach12 has been used to calculate a potential of mean force (PMF) between two bare11 or two (PEO)-6-C60 fullerenes in water as a function of their separation13 at 298 K and atmospheric pressure. More details about atomistic MD simulations of these systems can be found in refs 8, 11, and 13. Figure 1 shows the PMF as a function of center of mass separation (r) between C60 fullerenes calculated for two bare and two (PEO)-6-C60 fullerenes in water. At short distances the bare fullerenes show strong attraction which is primarily due to direct C60-C60 van der Waals interactions.11 We showed in a recent study that the waterinduced part of the PMF (total C60-C60 PMF in water minus C60-C60 PMF in a vacuum) is actually repulsive11 despite the well-accepted belief of fullerene hydrophobicity. Tethering of six PEO oligomers to the C60 fullerenes resulted in an even more surprising result. The PMF for two (PEO)-6-C60 fullerenes exhibits a weak long-range attraction (12 Å < r
