J. Phys. Chem. C 2009, 113, 19155–19160
19155
Monte Carlo Study of the Properties of a Carbon Nanotube Functionalized by Magnetic Nanoparticles T. Panczyk*,† and T. P. Warzocha‡ Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30239 Cracow, Poland, and Department of Chemistry, Maria Curie-Sklodowska UniVersity, pl. M. Curie-Sklodowskiej 3, 20031 Lublin, Poland ReceiVed: July 2, 2009; ReVised Manuscript ReceiVed: September 4, 2009
We present a computer simulation study of the behavior of a carbon nanotube in which both ends are connected to magnetic nanoparticles by a short alkane chain. Such a composite object reveals very interesting features depending on the type of magnetic nanoparticle (ferromagnetic or superparamagnetic). In the case of ferromagnetic nanoparticles, we observed that depending on the strength of the external magnetic field the nanotube mouths can be plugged or opened. It implies that the access to the nanotube interior can be easily controlled by switching on or off the external magnetic field. Such nanodevices might be very promising candidates for drug-delivery systems or storage materials with controllable release of encapsulated molecules. This paper discusses the conditions under which the desirable properties can be attained. 1. Introduction Carbon nanotubes (CNTs)1 and magnetic nanoparticles (MNPs)2 have attracted wide interest in many areas of science, technology, and medicine. Their outstanding properties offer new ways of solving many technological and medical problems. CNTs are promising candidates as hydrogen storage materials,3 structural materials,4 electronic devices,5 and many others. MNPs are most widely known in ferrofluid technology,6 catalysis,7,8 and medicine.9 Both CNTs and MNPs reveal unique properties; however, our idea is to study the behavior and properties of a composite object, that is, a CNT covalently functionalized by MNPs as depicted in Figures 1 and 2. There are several reports in the literature concerning the study of composite objects comprising carbon nanotubes and magnetic nanoparticles. MNPs can locate in the interior of CNTs10,11 or be attached to the CNTs walls.12-14 However, to the best our knowledge, covalently immobilized MNPs to CNTs walls have not yet been synthesized and studied. Methods of functionalization of CNTs exist15,16 and seem to be advanced enough to produce such objects as depicted in Figures 1 and 2. The most likely synthesis route of such a nanodevice (ND) would encompass functionalization steps of CNTs and MNPs, both are currently possible to perform.8,15-17 One of the most important factors controlling the behavior of the ND is the magnetic state of MNPs. MNPs are usually paramagnetic or superparamagnetic, depending on their size (typically 10-20 nm) and chemical composition (typically magnetite, iron or iron oxides).8 Ferromagnetic behavior of nanoparticles has also been reported, for example CoFe2O4 with sizes greater than 9 nm18 or cobalt nanoparticles with sizes greater than 4.7 nm.19 Computer modeling is a fast and cheap approach for prediction of properties of nanosized structures. Thus, using a reasonable representation of the potentials operating between * To whom correspondence should be addressed. E-mail: panczyk@ vega.umcs.lublin.pl. † Polish Academy of Sciences. ‡ Maria Curie-Sklodowska University.
Figure 1. Schematic representation of the ND studied in this work. The CNT is connected with the MNP via short alkane chain. The terminal carbons of the chain belong to either the outermost graphene ring of the CNT or to the surface of the MNP. During MC simulation the angles θi and bond lengths rij are optimized. The whole ND can rotate around the center of mass of the CNT. The MNPs have permanent magnetic moments µ which interact with the EMF of the induction B.
the most important components of such a ND it is possible to predict its properties without the need of time-consuming and probably expensive synthesis. Therefore, the aim of this work is theoretical analysis of the structure and properties of the objects schematically shown in Figures 1 and 2. We found that such a ND reveals potentially useful properties. CNT mouths can be remotely plugged or opened by controlling the strength of the external magnetic field. Such a property creates a chance of developing novel drug delivery systems or storage materials with controllable release of encapsulated molecules. 2. Model Computational study utilized in this work is based on the Monte Carlo simulation of the ND structure under various external magnetic field (EMF) strengths. CNT of chirality (10,10), which corresponds to the diameter of 1.96 nm, is treated
10.1021/jp9062065 CCC: $40.75 2009 American Chemical Society Published on Web 10/09/2009
19156
J. Phys. Chem. C, Vol. 113, No. 44, 2009
Panczyk and Warzocha unit vectors. Contribution to the magnetic energy coming from eq 2 is small given the large separation distance between MNPs. Another contribution to the potential energy comes from dispersive interactions between the MNPs and carbon atoms creating CNT and CH2 group being the middle segment of the chain, and between CH2 group and carbon atoms. Those interactions are computed using the Lennard-Jones formula,
ULJ ) 4εx
Figure 2. States of the ND observed in simulations. (A) shows the configuration when no EMF exist, whereas configurations (B) and (C) can be observed when the EMF is switched on. State (A) is typical for superparamagnetic nanoparticles no matter what is the strength of the magnetic field. States (B) and (C) may exist only for ferromagnetic nanoparticles under nonzero magnetic field. Arrows with labels ‘B’ indicate the direction of the EMF, whereas labels ‘µ’ show the spatial orientation (illustrative) of the internal magnetic moments of the MNPs.
TABLE 1: Lennard-Jones Parameters for Nonbonded Interactions ε, K σ, nm
MNP
C21,22
CH221
180 3.00
52.86 0.375
60.00 0.393
as a rigid body. Its length is 8.4 nm and both nanotube ends are functionalized by a short alkane chain. This chain is composed of three segments, but both terminal segments belong either to the outermost nanotube rings or to the surface of the MNP. Figure 1 shows typical configuration of the ND and denotes bond angles and lengths which are being optimized during Monte Carlo simulations. The MNP is essentially treated as a structureless spherical object of radius rMNP ) 1.5 nm, and it interacts with other components of the ND via Lennard-Jones potential. It also interacts with the EMF of the induction B according to the following relation,
UB ) -µ · B
(1)
where µ is the magnetic moment of the MNP and its value is assumed to be 2 × 104 µB where µB is the Bohr magneton. That value of µ has been chosen arbitrarily in order to mimic high magnetic moments of nanoparticles and is typical for a 9 nm diameter nanoparticle made from magnetite (superparamagnetic) or a 7 nm diameter cobalt nanoparticle (ferromagnetic).19 The choice of µ is not critical since the main component of magnetic energy of the system comes from eq 1; thus, the magnetic energy can be tuned by adjusting the magnetic field B. Magnetic moments of both MNPs interact also via dipoledipole mechanism according to the Dormann-Bessais-Fiorani model,20
Udip )
µ0 µ2 [µˆ · µˆ - 3(µˆ 1 · rˆ)(µˆ 2 · rˆ)] 4π r3 1 2
(2)
where µ ) |µ1| ) |µ2|, µ0 is the magnetic permeability of free space, r is the separation vector between the MNPs and ˆ denotes
[( ) ( ) ] σx rx
12
-
σx rx
6
(3)
The parameters εx, σx, rx take suitable values depending on the interacting groups and the Lorentz-Berthelot mixing rules are used for their calculation, that is for the interaction between the MNP and carbon the parameter εMNP-C ) (εMNPεC), whereas the parameter σMNP-C ) 0.5(σMNP + σC). The separation distance rMNP-C is defined as the distance between the centers of the MNP and C. Table 1 shows the particular values of all of the LJ parameters used in the calculations. The values of the LJ parameters for CH2 group and for carbon atoms creating the CNT were taken from the literature.21,22 Interaction of the MNP with the other components of the ND is assumed to be of the LJ type as well. This assumption is a crude approximation; however, an exact treatment of that kind of interaction is impossible at the present stage of studies. Moreover, is it not a critical factor since the present model aims mainly on a rough estimation of various factors giving finally the desirable properties of the studied object. Interaction of the MNP with carbon atoms or CH2 group represents rather the case of the interaction of a big colloidal particle with a small group of atoms. Thus, a more exact treatment of that sort of interaction would involve the summation (or integration) of individual contributions from atoms creating the MNP with the probe atom (or group of interest) located at some distance from the MNP. Such a treatment would essentially lead to potential energy function similar to either Hamaker23 or Steele formulas.24 Both functions are similar in shape to the LJ potential (with slightly different curvatures), however it is still possible to adjust the LJ parameters in such way that the LJ eq 3 can mimic those more realistic potential energy curves. Thus, replacing the true but unknown potential energy function by the approximate LJ formula reduces to a proper choice of the parameter εMNP only, because σMNP will be close to the diameter of the MNP. The value of εMNP ) 180 K has been estimated from the simulations as the lower limit still giving the desired properties of the ND. This value seems to be rather low, but it still leads to stable binding of the MNP to the CNT surface at the studied temperatures. Higher values of εMNP would probably be more realistic and they would lead to stronger binding via dispersive forces. The MNP is bound to the CNT via single CH2 group; thus, the position of the MNP in reference to the position of the CNT depends on two bond lengths, r12 and r23, and three angles, θ1, θ2, θ3, as shown in Figure 1. These bonded interactions are described by the bond stretching potential,
Ustr )
Kr [(r - re)2 + (r23 - re)2] 2 12
(4)
where Kr ) 9.65 × 106 K nm-2 and re ) 0.154 nm,25 and the bond bending potential,
Carbon Nanotube Functionalized by Magnetic Nanoparticles
Ubend )
J. Phys. Chem. C, Vol. 113, No. 44, 2009 19157
Kθ [(θ1 - θπ)2 + (θ2 - θe)2 + (θ3 - θπ)2] 2
(5) where Kθ ) 6.25 × 104 K rad-2 and θe ) 114.0o.25 The equilibrium angles for θ1 and θ3 are assumed to be θπ ) 180.0o. This assumption is fully justified for the case of the angle θ1 since hybridization of the carbon atoms in CNTs is close to sp2. For the case of the angle θ3, which is defined as the angle between the r23 bond and the axis connecting the centers of the NMP and the carbon atom located on the surface of the NMP, the equilibrium angle of 180° seems to be also reasonable as the first approximation. No particular torsional potential is assumed for rotation around the r12 and r23 bonds since typical potentials of that kind, normally used for alkanes, would obviously be wrong. Instead, we assumed totally free and internally nonhindered rotation around those bonds. The last degree of freedom of our system is rotation of the whole object around the center of mass of the CNT. Contribution to the total potential energy comes from the interaction of the magnetic moments of the MNPs with the EMF due to eq 1. Optimization of the structure of the ND is done by utilizing standard Metropolis sampling scheme. A given degree of freedom is chosen at random, and a small displacement from the original state is performed. The total energy in this new configuration is computed as the sum of all contributions listed above. If the new energy is smaller than the previous one the displacement is accepted, otherwise the displacement occurs with the probability exp(-(∆E)/(kBT)), where ∆E is the difference of the total energy between the new and original configurations. The energy differences are computed as full pairwise additive sums without assuming any cutoff. No periodic boundary conditions are imposed since the structure does not perform any translations in the simulation box. Normally ca. 107 Monte Carlo steps (MCS) are performed in a single run. Averaging has been done using from 5 to 10 independent runs. 3. Results and Discussion We found that the states (configurations) of the ND may belong to three possible groups depending on the EMF strength and the magnetic nature of the MNPs. They are shown in Figure 2. In the case of superparamagnetic nanoparticles the magnetic moments of the MNPs can rotate freely (because of low value of the anisotropy constant) and thus can always align to the direction of the EMF at least at constant (non alternating) EMF. Therefore, only states A can be observed no matter what is the strength of the magnetic field. The dispersive forces tend to plug the nanotube entrances and the magnetic field cannot break those forces since the magnetic moments of the MNPs adjust to the direction of the EMF. Thus, in the case of the superparamagnetic nanoparticles no interesting features of the ND can be observed since no matter what is the EMF strength the ND always remains plugged. In the case of ferromagnetic nanoparticles the states B and C can be observed for strong enough EMFs. We found that the crucial condition for getting one of the states B or C is the initial mutual alignment of the permanent magnetic moments of the MNPs. We studied four different initial alignments of the magnetic moments leading to qualitatively different behavior of the ND in the EMF. “Parallel” corresponds to the situation when both magnetic moments are located on the axis connecting the MNPs centers of mass (which is, at the same time, approximately the nanotube axis) and their directions are the same. “Anti-parallel” means that the directions of magnetic
Figure 3. Relative occurrence of the possible ND states (presented in Figure 2) under 1 T EMF and T ) 300 K for various initial alignments of the MNPs magnetic moments.
moments are opposite but they still are located on the same axis as in the case of “parallel”. “Perpendicular 1” alignment stands for the case when first of the magnetic moments is still located on the axis connecting the MNPs centers, but the second one is perpendicular to that axis and lies in the plane created by MNPs centers of mass and the point where the alkane chain is bound to the surface of the MNP associated with the second magnetic moment. “Perpendicular 2” corresponds to the same arrangement and differs from the “perpendicular 1” by the direction of the second magnetic moment (it is opposite). Those initial alignments were set after some number of the MC steps (normally 2 × 106 MCS) carried out without the EMF. Upon completing this stage of calculations and getting the equilibrium state of the ND both MNPs stick to the CNT mouths due to dispersive forces and the vector connecting their centers of mass almost coincides with the nanotube axis. Next, we choose one of the above-described spatial alignments of the magnetic moments and treat them as permanent (they cannot rotate around the center of the MNP). Finally, we switch the EMF on and continue the simulation under magnetic field (ca. 107 MCS for single run). We found that we can control the structure of the ND by adjusting the strength of the EMF. Generally, we observe that under weak magnetic fields the nanotube entrances stay capped by the MNPs due to dispersive forces. The initial alignments of the MNP magnetic moments do not destroy that configuration since the interaction between the MNPs is weak at such distances. By performing a prolonged simulation at zero EMF (4 × 108 MCS) we found less than 0.002% of states B for any initial alignments of magnetic moments and these states quickly transformed again to the state A. Only by increasing the strength of the EMF we can remotely open (at least from one side) the CNT entrances and by switching the EMF off we can again cap the entrances because the dispersive forces become dominant. The three states A, B, and C are not static and strictly defined due to fluctuations of configuration. Therefore, we classify a given state as belonging to one of states A, B, or C by monitoring the solid angle Ω under which the MNP can be seen from the center of the CNT entrance. It is schematically displayed as the inset in Figure 3. When the MNP sticks to the CNT mouth, then Ω takes the highest value, i.e., 2π. Small values of Ω correspond to situations when the MNP is shifted away from the CNT entrance, though zero solid angle cannot be observed due to constraints imposed by the short length of the alkane chain. Because we are interested mainly in a balance between totally capped state and variety of uncapped (strongly or even slightly) we assumed the threshold value of Ω ) 5.0 for classification of a given state as capped, semiuncapped, or uncapped. Thus, state A corresponds to the case when for both entrances the
19158
J. Phys. Chem. C, Vol. 113, No. 44, 2009
Panczyk and Warzocha
TABLE 2: Mean Total Energies for Various States under 1 T EMF and the Temperature 300 K , kJ/mol antiparallel parallel perpendicular 1 perpendicular 2 random
A
B
C
B-A
71.6 ( 1.0 -6.3 ( 0.2 58.4 ( 0.9 -4.5 ( 0.5 7.0 ( 2.5
41.4 ( 1.7 42.2 ( 0.5 42.6 ( 1.3 39.7 ( 0.8 42.8 ( 2.2
60.6 ( 0.9 80.4 ( 0.6 91.1 ( 1.0 81.3 ( 1.0 82.1 ( 4.7
-30.2 48.5 -15.8 44.2 35.8
solid angles are greater than 5. State B: for one of entrances Ω1 > 5 and for the second Ω2 < 5. Finally, for state C both solid angles are less than 5. The choice of that threshold solid angle is arbitrary because the definition of an open or closed state depends on the size of a molecule trapped inside the CNT, for instance. For the assumed threshold the diameter of the slit between the edge of the CNT and surface of the MNP is 3.4 Å; thus, the ND becomes uncapped for hydrogen molecules (kinetic diameter 2.89 Å) at that threshold solid angle but still stay capped for bigger molecules. A given state consists of some spectrum of states sharing a similar configuration. However, intermediate states, for example, transition from full state A to full B must be accompanied by overcoming energy barriers associated with breaking the dispersive forces and stiffness of the alkane chain. Thus, they appear only for a short time and give small contribution to the mean energy of the system and occurrence of states. In fact, we observe mostly the states very close to full states A, B, and C. Figure 3 shows how the EMF affects the state of the ND. These results correspond to relatively strong EMF, i.e., 1 T. We can see that semiuncapped state B appears for all alignments studied but it is the most dominant for antiparallel (99%) and perpendicular 1 (89%). For parallel alignment the ND stays almost fully plugged (92% of state A) though 7% of semiuncapped states can also be observed. Similarly, the perpendicular 2 gives the most probable state A (86%) and also a nonnegligible occurrence of states B (12%). The fully uncapped state (C) appears for any alignment of magnetic moments but its population does not exceed ca. 1%. We also did analogous calculations for totally random initial alignment of magnetic moments. We got 68% of state A, 32% of state B, and 0.004% of state C. Though these results are not as impressive as in the case of antiparallel or perpendicular 1, they can still be regarded as promising. If we treat the states A, B, and C as local minima of the total energy, we can draw some conclusions about their stability. Table 2 shows the determined mean total energies for the considered states under 1 T EMF and 300 K temperature. We can see that mean energies correlate well with the associated occurrence of states. For a given alignment of magnetic moments, we observe minima of for most dominant states. The differences in energy for transitions from state to state are either negative or positive, showing the tendency of the system to attain a preferred state. Most interesting are the values of the energy difference between states A and B for antiparallel and perpendicular 1 alignments. In the case of antiparallel, it is ca. 30 kJ/mol; thus, at the considered temperature of 300 K the equilibrium between states A and B is strongly shifted toward the state B. However, for the perpendicular 1, the energy difference is much smaller and we can conclude that the transitions between states A and B occur frequently though the equilibrium state is more abounding in semiuncapped states. The existence of dynamic equilibrium
Figure 4. Changes of the mean total and structural energy of the ND with magnetic field B at T ) 300 K.
between fully capped and semiuncapped states does not challenge the promising properties of the ND since it may occur only under strong EMFs. Lack of the EMF leads to fully capped states for every alignment of magnetic moments with only a very small fraction ( 0.3 and 0.8 T is enough to reach almost the same state as for 1 T. For parallel and perpendicular 2 does not change under EMF. Thus, the structure of the ND stays almost unchanged under the EMF, i.e., remains mostly in the capped state A. Figure 5 summarizes the most important results obtained in this study. It shows how the degree of plugging of the CNT entrances changes with the EMF, temperature, and alignment of the magnetic moments. The degree of plugging has been defined in terms of the solid angle under which the MNP can be seen from the center of the CNT entrance, (Ω1 + Ω2)/4π. The degree of plugging is 1 only when both CNT entrances are totally plugged, 0.5 would correspond to the situation when one of entrances is plugged and the second fully opened, for instance. Figure 5A leads to the conclusion that the most promising behavior of the ND can be observed for antiparallel alignment of magnetic moments. Weak EMF cannot overcome the
Carbon Nanotube Functionalized by Magnetic Nanoparticles
Figure 5. Degree of plugging of the CNT entrances by the MNPs as a function of the EMF strength. (A) shows the influence of the mutual alignment of the MNP magnetic moments, µ at 300 K. (B) represents the effect of the temperature for antiparallel alignment of magnetic moments.
dispersive forces operating between the MNP and the CNT thus the ND remains plugged from both sides. Stronger EMF leads to partial opening of the CNT entrances and finally for strong enough EMF we can observe the state of the ND corresponding to low value of the degree of plugging (slightly less than 0.6). At this state one entrance of the CNT stays closed because the associated MNP magnetic moment is in agreement with the EMF. The second one gets opened because the magnetic moment of the second MNP tends to align to the direction of the EMF. As already mentioned, switching off the EMF leads to the original state with both entrances of the CNT plugged. The parallel alignment does not give such behavior since magnetic moments of both MNPs are oriented in the same direction, so they will exert a torque on the ND which leads to rotation minimizing the interaction with the EMF. Perpendicular alignments of the magnetic moments give different behaviors depending on the initial direction of the second magnetic moment. Any move of the MNP leads to change of the angle between µ2 and B and that change might be energetically favorable or not depending on the direction of µ2. For perpendicular 1 we got similar behavior of the ND as for antiparallel but the changes of the degree of plugging are slightly smaller. Perpendicular 2 gives behavior almost identical like parallel one. Figure 5B shows how the temperature affects the behavior of the ND in the case of antiparallel alignment of the magnetic moments. Generally, the desired property of the ND is preserved in wide range of temperatures. The threshold EMF is only shifted toward lower values when the temperature increases. However, the shift is not very strong what offers a possibility of taking advantage of the properties of the ND under various conditions. Obviously, the threshold EMF will depend on the strength of dispersive interactions between the MNP and the CNT. In the present case the dispersive interactions are assumed to be rather weak, so the actual system of that kind would probably require stronger EMFs in order to make the transition from fully plugged state to the open state.
J. Phys. Chem. C, Vol. 113, No. 44, 2009 19159 The discussed property of the ND, that is, the possibility of controlling its configuration by the EMF, creates unique areas of its application. Imposing a strong EMF on the ND leads to opening of the CNT interior, which becomes then accessible for other molecules. So, the guest molecules can be encapsulated in the CNT interior after removing or weakening the EMF. They can be released again in another environment by switching the EMF on. Because the open state appears under relatively strong EMF, the weaker EMFs can still be used for controlled targeting of the NDs without simultaneous opening of the CNT entrances. So, we can find many potential applications of such NDs in various areas of technology or in medicine as a novel drug delivery systems, for instance. Obviously, further experimental studies are necessary in order to confirm these theoretically predicted properties. As mentioned, the superparamagnetic nanoparticles do not give such a behavior in the EMF. However, it cannot be excluded that similar behavior can appear in alternating magnetic fields, or if one assumes the existence of magnetic communication through metallic nanotubes as already found using density functional studies.26 The present approach is based on the simplest possible computational technique for studying the behavior of such defined ND. However, the main goal of this work is to present the idea and investigate whether the ND reveals any interesting features which could stimulate further theoretical or experimental studies. We conclude that the present results are very promising and deeper investigation of such nanodevices is needed. 4. Summary The studied structure reveals unique and potentially useful properties. When nanoparticles are ferromagnetic and their magnetic moments are oriented in some specific way (antiparallelly, for instance), then the external magnetic field can induce its structural rearrangements. Under weak magnetic fields, the nanotube interiors stay plugged by the magnetic nanoparticles. Strong magnetic fields overcome the dispersive forces and the nanotube becomes opened from one or both sides. This effect is observed in wide range of temperatures; in particular, it exists at ambient temperature. Superparamagnetic nanoparticles do not reveal such property, at least under static external magnetic fields. Acknowledgment. T.P. thanks the Foundation for Polish Science (FNP) for financial support. T.P.W. thanks the British Council for funding scholarship in YSP scheme and Dr. P. Camp for fruitful discussions. Supporting Information Available: Visualizations of the structural changes of the ND occurring under the EMF (1 T and 300 K) are prepared as Windows Media Video files (wmv). “ap.wmv” shows the case of initially antiparallel alignments of magnetic moments whereas “pp.wmv” shows perpendicular 1 case. The arrow in the middle of the CNT shows the direction of the EMF; it appears and disappears depending on the EMF being on or off. Rotation of the EMF is the result of keeping the CNT static what allows for better observation of the MNPs moves. Arrows attached to the MNPs centers represent their internal magnetic moments. Numbers show the degrees of capping expressed in terms of the solid angles. Left number concerns the left entrance to the CNT, right is for the right entrance. This material is available free of charge via the Internet at http://pubs.acs.org.
19160
J. Phys. Chem. C, Vol. 113, No. 44, 2009
References and Notes (1) Iijima, S. Helical microtubules of graphitic carbon. Nature 1991, 354, 56–58. (2) Rosensweig, R. E. Ferrohydrodynamics; Cambridge University Press: Cambridge, 1985. (3) Liu, C.; et al. Hydrogen Storage in Single-Walled Carbon Nanotubes at Room Temperature. Science 1999, 286, 1127–1129. (4) Vigolo, B.; et al. Macroscopic Fibers and Ribbons of Oriented Carbon Nanotubes. Science 2000, 290, 1331–1334. (5) Bachtold, A.; Hadley, P.; Nakanishi, T.; Dekker, C. Logic Circuits with Carbon Nanotube Transistors. Science 2001, 294, 1317–1320. (6) Raj, K.; Moskowitz, B.; Casciari, R. Advances in Ferrofluid Technology. J. Magn. Magn. Mater. 1995, 149, 174–180. (7) Hu, A.; Yee, G. T.; Lin, W. Magnetically recoverable chiral catalysts immobilized on magnetite nanoparticles for asymmetric hydrogenation of aromatic ketones. J. Am. Chem. Soc. 2005, 127, 12486–12487. (8) Lu, A.-H.; Salabas, E. L.; Schuth, F. Magnetic Nanoparticles: Synthesis, Protection, Functionalization, and Application. Angew. Chem., Int. Ed. 2007, 46, 1222–1244. (9) Arruebo, M.; Fernandez-Pacheco, R.; Ibarra, M. R.; Santamaria, J. Magnetic nanoparticles for drug delivery. Nanotoday 2007, 2, 22–31. (10) Korneva, G. Carbon Nanotubes Loaded with Magnetic Particles. Nano Lett. 2005, 5, 879–884. (11) Kornev, K. G.; Halverson, D.; Korneva, G.; Gogotsi, Y.; Friedman, G. Magnetostatic interactions between carbon nanotubes filled with magnetic nanoparticles. Appl. Phys. Lett. 2008, 92, 233117. (12) Georgakilas, V.; Tzitzios, V.; Gournis, D.; Petridis, D. Attachment of magnetic nanoparticles on carbon nanotubes and their soluble derivatives. Chem. Mater. 2005, 17, 1613–1617. (13) Correa-Duarte, M. A.; et al. Aligment of carbon nanotubes under low magnetic fields through attachment of magnetic nanoparticles. J. Phys. Chem. B 2005, 109, 19060–19063. (14) Zhang, H.; Du, N.; Wu, P.; Chen, B.; Yang, D. Functionalization of carbon nanotubes with magnetic nanoparticles: general nonaqueous synthesis and magnetic properties. Nanotechnology 2008, 19, 315604.
Panczyk and Warzocha (15) Prato, M.; Kostarelos, K.; Bianco, A. Functionalized carbon nanotubes in drug design and discovery. Acc. Chem. Res. 2008, 41, 60–68. (16) Giordani, S.; et al. Multifunctional hybrid materials composed of [60]fullerene-based functionalized-single-walled carbon nanotubes. Carbon 2009, 47, 578–588. (17) Hamilton, C. E.; et al. Functionalization of SWNTs to facilitate the coordination of metal ions, compounds and clusters. Dalton. Trans. 2008, 2937–2944. (18) Ayyappan, S.; Philip, J.; Raj, B. A facile method to control the size and magnetic properties of CoFe2O4 nanoparticles. Mater. Chem. Phys. 2009, 115, 712–717. (19) Massart, R.; Rasolonjatovo, B.; Neveu, S.; Cabuil, V. Mercurybased cobalt magnetic fluids and cobalt nanoparticles. J. Magn. Magn. Mater. 2006, 308, 10–14. (20) Hansen, M. F.; Mørup, S. Models for the dynamics of interacting magnetic particles. J. Magn. Magn. Mater. 1998, 184, 262–274. (21) Jorgensen, W. L.; Madura, J. D.; Swenson, C. J. Optimized intermolecular potential functions for liquid hydrocarbons. J. Am. Chem. Soc. 1984, 106, 6638–6646. (22) Kong, Y.; Cui, D.; Ozkan, C. S.; Gao, H. Modeling Carbon Nanotube Based Bio-Nano Systems: A Molecular Dynamics Study. Mater. Res. Symp. Proc. 2003, 773, N8.5.1–6. (23) Qin, Y.; Fichthorn, K. A. Molecular-dynamics simulation of forces between nanoparticles in a Lennard-Jones liquid. J. Chem. Phys. 2003, 119, 9745–9754. (24) Steele, W. A. The physical interaction of gases with crystalline solids. I. Gas-solid energies and properties of isolated adsorbed atoms. Surf. Sci. 1973, 36, 317–352. (25) Nath, S. K.; Escobedo, F. A.; de Pablo, J. On the Simulation of Vapor-Liquid Equilibrium for Alkanes. J. Chem. Phys. 1998, 108, 9905– 9911. (26) Ruiz, E.; Nunzi, F.; Alvarez, S. Magnetic Communication through Functionalized Nanotubes: A Theoretical Study. Nano Lett. 2006, 6, 380–384.
JP9062065
