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Influence of Diameter, Length, and Chirality of Single-Walled Carbon Nanotubes on Their Free Radical Scavenging Capability Annia Galano* Departamento de Quı´mica, UniVersidad Auto´noma Metropolitana-Iztapalapa, San Rafael Atlixco 186, Col. Vicentina, Iztapalapa, C. P. 09340, Mexico ReceiVed: May 18, 2009; ReVised Manuscript ReceiVed: August 18, 2009
Density functional theory calculations have been used to model the influence of diameter, length, and chirality of single-walled carbon nanotubes (SWCNT) on their free radical scavenging activity. SWCNTs with wide distributions of different diameter, length, and chirality are proposed to have good free radical scavenging activity in the gas phase and in nonpolar environments. Therefore, they can be used as free radical traps with potential application in environmental and biological systems. In general, thinner tubes are expected to have better antiradical activities. However, the curvature of the tubes seems to modify the antiradical activity of armchair nanotubes. Therefore, for wide distributions of tube diameter, the zigzag SWCNTs are expected to be more efficient for free radical scavenging purposes than the armchair ones. The length of the tubes only has a minor influence on the free radical trapping efficiency of SWCNTs. From the results reported in this work, thin and zigzag nanotubes are recommended as those with the best antiradical activity, regardless of their length. Introduction Carbon nanotubes (CNTs) are fascinating molecules that have impacted broad areas of science and technology.1–7 Their wide applicability and peculiar properties arise from their unique structure. CNTs constitute large arrays of conjugated double bonds, and therefore they are expected to show great electron donor and acceptor capabilities. This particular feature makes them particularly reactive toward free radicals, which are highly dangerous to human health and environment. Therefore, the capability of CNTs to act as efficient free radical scavengers is a promising attribute that can be applied to fight these damaging species. Despite the importance of such application, very little research has been devoted to it. In fact, there are only four reports on this subject so far.8–11 Chronologically, Watts et al.8 were the first to report that multiwalled carbon nanotubes (MWCNT) can act as antioxidants as well as halogen absorbers. These authors found that the oxidation of polystyrene, polyethylene, polypropylene, and poly(vinylidene fluoride) is retarded by the presence of carbon nanotubes. Shortly after, Fenoglio et al.9 observed that when in contact with hydroxyl or superoxide radicals, MWCNTs exhibit a remarkable radical scavenging capacity. After these two experimental results, the first and only previous theoretical investigation on this subject was performed.10 The reactions of a (5,5) singled-walled carbon nanotube (SWCNT) fragment with six different free radicals were modeled, and it was concluded that SWCNTs can act as free-radical sponges based on thermodynamic and kinetic considerations. Moreover, it was found that once a first radical is attached to the tube, further additions are increasingly feasible. The most recent work on this topic experimentally confirmed that SWCNTs are potent antioxidants.11 In the same work, cytotoxicity assays also showed that SWCNTs have little or no toxic effect on cell viability, which is very important for biological applications. * Corresponding author. E-mail:
[email protected].
When CNTs are synthesized, a mixture of tubes with different diameter, length, and chirality is obtained.12 Actually producing SWNTs of defined structures is a major technological challenge.13,14 Therefore, now that we know that CNTs can efficiently act as free radical scavengers, the next questions are if their diameter, length, and chirality affect this valuable property and how. Accordingly, it is the main aim of the present work to address these questions. Computational Details Electronic structure calculations have been performed with the Gaussian 0315 package of programs. Full geometry optimizations and frequency calculations were carried out for all of the stationary points using the B3LYP hybrid HF-density functional and the 3-21G basis set. No symmetry constraints have been imposed in the geometry optimizations. The energies of all of the stationary points were improved by single point calculations at B3LYP/6-311+G(d) level of theory. Thermodynamic corrections at 298 K were included in the calculation of relative energies. Spin-restricted calculations were used for closed shell systems and unrestricted ones for open shell systems. Local minima were identified by the number of imaginary frequencies (NIMAG ) 0). It seems worthwhile to emphasize the fact that any theoretical model aiming to make predictions concerning practical applications must be analyzed in terms of Gibbs free energies, which implies the necessity of performing frequency calculations that are particularly expensive. Accordingly, it seems a better compromise to perform frequency calculations at a low level of theory than increase the level and analyze the results only in terms of electronic energy. The stationary points were first modeled in the gas phase (vacuum), and solvent effects were included a posteriori by single point calculations using a polarizable continuum model, specifically the integral-equation-formalism (IEF-PCM)16 at B3LYP/6-311+G(d) level of theory, with benzene as solvent
10.1021/jp904646q CCC: $40.75 2009 American Chemical Society Published on Web 10/07/2009
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Figure 1. Definition of roll-up vector as linear combinations of base vectors a1 and a2. Zigzag: θ ) 0 (n,0). Armchair: θ ) 30 (n,n). Chiral: 0 < θ < 30 (n,m).
TABLE 1: Diameters (nm) of the Studied (n,0) and (n,n) Nanotubes tube
diameter
tube
diameter
(5,0) (7, 0) (9,0) (10,0) (12,0) (14,0)
0.392 0.548 0.705 0.783 0.940 1.096
(3,3) (4,4) (5,5) (6,6) (7,7) (8,8)
0.407 0.542 0.678 0.814 0.949 1.085
Figure 2. Selected addition products. (A) Frontal view, (B) lateral view.
for mimicking nonpolar environments. Polar environments were not included because pure carbon nanotubes are not expected to be soluble in such media. Results and Discussion Single-walled carbon nanotubes (SWCNTs) are cylindrical molecules composed of carbon atoms that can be thought of as rolled-up graphene sheets. Their structures can be unambiguously defined by a chiral vector that represents the roll-up direction:
Ch ) (n, m) ) na1 + ma2
(1)
where a1 and a2 denote equivalent lattice vectors of the graphene sheet, and n and m are integers (0 e |m| e n) (Figure 1). Finite SWNTs fragments of extreme chirality (armchair and zigzag) with diameters ranging from 0.4 to 1.1 nm have been selected for the present study. The thinnest tubes were selected with diameters ≈ 0.4 nm because it is the smallest experimentally achievable diameter.17 Different lengths ranging from 0.7 to 2.0 nm (from 3 to 8 hexagons long) have also been tested for the thinnest tubes: (3,3) and (5,0). The dangling bonds at the ends of the nanotubes have been saturated by hydrogen atoms to avoid unwanted distortions. The SWNTs free radical scavenging activity has been modeled through the reaction of these fragments with the methoxyl radical (OCH3). These reactions have been computed in the gas phase as well as in benzene solution, aiming for environmental and biological applications, respectively. The tubes have been selected in such a way that in every case there is an armchair and a zigzag fragment of similar diameter (Table 1). This selection has been made for fair comparisons accounting for the effect of chirality. The diameters (D) reported through this Article have been estimated (in nm) according to:
D ) 0.0783√n2 + m2 + nm
(2)
Each addition product has been modeled with the free radical attached to a central hexagon of the tube (Figure 2) to prevent
unwanted interactions with H atoms at the end of the tubes. The most relevant geometrical parameter associated with formation of the adducts is the C-O distance corresponding to the newly formed bond (r1). Another geometrical parameter that might be interested to analyze is the O-C distance in the methoxy moiety (r2), particularly relative to its value in the reacting O-CH3 radical (1.430 Å, at the used level of theory). The influence of the SWCNTs diameter and length on this bond distance is shown in Figure 3. The shortest d(C-O) was found for the thinnest tubes, for both armchair and zigzag nanotubes. In addition, a regular trend was observed, indicating that as the diameter of the tube increases so does the d(C-O) distance. It seems logical to assume that an upper limit value must exist. Even though it was not reached with the studied fragments for the wider tubes, the d(C-O) seems to be near such limit. For SWCNTs with diameters up to ∼0.7 nm, the new formed bond was found to be larger for zigzag nanotubes than for the corresponding armchair ones. This difference practically vanishes for tubes with diameters g 0.96 nm. The length of the tubes influences the magnitude of d(C-O) to a much less extent. In fact, the maximum variation of d(C-O) with diameter was found to be 0.04 and 0.03 Å for armchair and zigzag fragments, respectively, while the maximum variation of d(C-O) with length is only 0.004 and 0.003 Å. A slight zigzag pattern arises as the length of the tubes increases. However, the variations are so small that it can be considered that the d(C-O) distance remains unchanged, at least for the lengths considered in the present study. There is no reason to expect a sudden change in this behavior for longer tubes. The energy barriers have not been computed in the present work, because it was already proposed that the reactions of pristine carbon nanotubes with the OCH3 radical are diffusion controlled.10 This is because such reactions are barrierless in terms of enthalpy and have a very low barrier in terms of Gibbs free energies of reaction (∆Gq ≈ 4 kcal/mol); therefore, this work only focuses on Gibbs free energies of reaction, which controlled the products formation under equilibrium conditions. The Gibbs free energies of reaction (∆G), at 298.15 K, for • OCH3 additions to the studied (n,0) and (n,n) fragments (three hexagons long), are reported in Table 2. For the gas phase, they were performed using a standard state of 1 atm, as calculated from the Gaussian program outputs. However, for reactions in solution, the reference state has been changed from 1 atm to 1
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Figure 3. Distance of the formed C-O bond (A) as a function of the tube diameter, and (B) as a function of the tube length.
TABLE 2: Enthalpies (∆H, kcal/mol), Gibbs Free Energies of Reaction (∆G, kcal/mol) Corresponding to OCH3 Additions to the Studied (n,0) and (n,n) Fragments (Three Hexagons Long), and HOMO Energies (eV) ∆H (5,0) (7,0) (9,0) (10,0) (12,0) (14,0) (3,3) (4,4) (5,5) (6,6) (7,7) (8,8)
∆G
HOMO
gas
benzene
gas
benzene
gas
benzene
-41.39 -53.27 -44.69 -43.89 -50.26 -48.12 -29.38 -14.61 -10.26 -6.42 -4.58 -3.11
-40.31 -52.64 -43.81 -43.19 -49.34 -51.28 -28.75 -13.95 -9.70 -5.82 -3.97 -2.58
-29.60 -40.94 -32.45 -32.08 -39.55 -36.01 -17.30 -2.42 1.90 6.25 8.02 9.71
-32.95 -44.74 -35.99 -35.81 -43.06 -43.59 -21.10 -6.19 -1.97 2.42 4.20 5.82
-3.891 -4.078 -4.168 -4.184 -4.212 -4.194 -4.748 -4.954 -4.727 -4.612 -4.527 -4.464
-3.839 -4.004 -4.102 -4.118 -4.152 -4.078 -4.738 -4.918 -4.695 -4.583 -4.503 -4.440
M, and the solvent cage effects have been included according to the corrections proposed by Okuno,18 taking into account the free volume theory.19 These corrections are in good agreement with those independently obtained by Ardura et al.20 and have been successfully used by other authors.21–24 The expression used to correct the Gibbs free energy is: VM ∆Gsol = ∆Ggas - RT{ln[n10(2n-2)] - (n - 1)}
(3)
where n represents the total number of reactant moles. According to expression 3, the cage effects in solution cause ∆G to decrease by 2.54 kcal/mol for bimolecular reactions, at 298 K. This lowering is expected because the cage effects of the solvent reduce the entropy loss associated with any adduct formation, in reactions with molecularity equal to or larger than two. Therefore, if the translational degrees of freedom in solution are treated as in the gas phase, the cost associated with their loss when two or more molecules form a complex system in solution is overestimated, and consequently these processes are overpenalized in solution. All of the studied reactions were found to be exothermic, and most of them were found to be exergonic, regardless of the tube diameter (Table 2). The exceptions, with ∆G > 0, are the additions to (6,6), (7,7) and (8,8) fragments in benzene solutions, and also to (5,5) in the gas phase. The feasibility of the studied reactions was found to be increased by the presence of nonpolar solvents, benzene in the present study; that is, the Gibbs free
energies of reaction are systematically more negative in benzene solution than in gas phase. The dependence of the Gibbs free energies of reaction with the tube diameter is shown in Figure 4A. It should be noticed that the present study is aiming to predict practical applications. In addition, addition reactions are involved, which have known entropy losses. Therefore, the magnitude that would determine the viability of the studied processes is the Gibbs free energy. From the results in Table 2, it stands out that zigzag nanotubes are systematically more reactive toward the OCH3 free radical, and probably toward free radicals in general, than their corresponding armchair partners of similar diameter. This can be explained by the fact that the highest occupied molecular orbitals (HOMO) of zigzag tubes are systematically higher in energy than the HOMOs of the armchair tubes with equivalent diameter. Therefore, the reactivity of the zigzag tubes toward electrophilic radicals is higher than that of the corresponding armchair tubes. Armchair, (n,n), nanotubes show a regular trend of decreasing their free radical scavenging activity as the tubes become wider. Zigzag tubes, on the other hand, show no indication of such trend. Apparently their reactivity toward free radicals is less sensitive to the tube diameter than that of armchair SWCNTs. According to the results shown in Figure 4A, for wide distributions of tube diameter, the zigzag structure is more efficient for free radical scavenging purposes. The influence of the length of the tubes on their reactivity toward free radicals has also been analyzed in terms of ∆H and ∆G (Table 3). In this case, all of the reactions were found to be exothermic and exergonic (but they all correspond to the thinnest fragments). The presence of the solvent also promotes the studied reactions; that is, the exothermicity and exergonicity of the reactions are larger in benzene solutions than in gas phase. To help in visualizing any possible trend of the free radical scavenging activity of the tubes with their lengths, a plot of ∆G values versus the length in hexagons is shown in Figure 4B. For the shortest tubes, the zigzag fragments show better antiradical activity than do the armchair fragments. However, this tendency is inverted for tubes six, or more, hexagons long. However, actual SWCNTs are longer than those modeled in the present study. Because for the (3,3) tubes the ∆G values did not converge up to 8 hexagons, additional calculations have been performed for this particular case for tubes up and to 10 hexagons long. For such lengths, it seems that the convergence is achieved. In any case, what really matters is that all of the addition processes were found to be energetically viable.
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Figure 4. Gibbs free energy of reaction in benzene solution (A) as a function of the tube diameter, and (B) as a function of the tube length. Continuous lines represent benzene solutions, and dotted lines represent gas phase.
TABLE 3: Enthalpies (∆H, kcal/mol) and Gibbs Free Energies of Reaction (∆G, kcal/mol) Corresponding to OCH3 Additions to (5,0) and (3,3) Fragments from Three to Seven Hexagons (h) Long ∆H (5,0) (5,0) (5,0) (5,0) (5,0) (5,0) (3,3) (3,3) (3,3) (3,3) (3,3) (3,3) (3,3) (3,3)
3h 4h 5h 6h 7h 8h 3h 4h 5h 6h 7h 8h 9h 10 h
∆G
gas
benzene
gas
benzene
-41.39 -44.53 -39.67 -30.55 -23.19 -22.89 -29.38 -28.10 -32.27 -36.87 -29.01 -35.61 -31.46 -31.39
-40.31 -44.48 -38.71 -29.52 -23.44 -21.25 -28.75 -27.56 -31.59 -36.34 -28.39 -34.82 -31.01 -30.68
-29.60 -32.93 -27.69 -17.96 -9.49 -10.00 -17.30 -15.91 -20.49 -25.72 -16.52 -23.89 -19.03 -18.99
-32.95 -37.31 -31.15 -21.36 -14.17 -12.79 -21.10 -19.80 -24.24 -29.62 -20.34 -27.53 -23.02 -22.71
Therefore, thin nanotubes can be proposed as compounds with excellent free radical scavenging activity, regardless of their length and chirality. They seem to be viable choices for both biological and environmental applications because the reactions are exergonic in both the gas phase and benzene solutions. Conclusions Analyzing all of the previously discussed results together, it seems that some generalizations can be made. SWCNTs with wide distributions of different diameter, length, and chirality are expected to have good free radical scavenging activity in the gas phase and in nonpolar environments. Therefore, they can be used as free radical traps with potential application in environmental and biological systems. In general, thinner tubes are expected to have better antiradical activities. However, the curvature of the tubes seems to play an important role in the antiradical activity only for armchair nanotubes. Therefore, for wide distributions of tube diameter, the zigzag SWCNTs are expected to be more efficient for free radical scavenging purposes than the armchair ones. It was also found that the length of the tubes only has a minor influence in the free radical trapping efficiency of SWCNTs. Taking into account all of these
findings together, thin and zigzag nanotubes are recommended as those with the best antiradical activity, regardless of their length. Acknowledgment. I thank Laboratorio de Visualizacio´n y Co´mputo Paralelo at UAM-Iztapalapa for the access to its computer facilities. References and Notes (1) Dresselhaus, M. S.; Dresselhaus, G.; Eklund, P. C. Science of Fullerenes and Carbon Nanotubes; Academic: San Diego, CA, 1996. (2) Dalton, A. B.; Stephan, C.; Coleman, J. N.; McCarthy, B.; Ajayan, P. M.; Lefrant, S.; Bernier, P.; Blau, W.; Byme, H. J. J. Phys. Chem. B 2000, 104, 10012. (3) Rinzler, A. G.; Hafner, J. H.; Nikolaev, P.; Lou, L.; Kim, S. G.; Toman´ek, D.; Nordlander, P.; Colbert, D. T.; Smalley, R. E. Science 1995, 269, 1550. (4) de Heer, W. A.; Chatˆelain, A.; Ugarte, D. Science 1995, 270, 1179. (5) Treacy, M. M. J.; Ebbesen, T. W.; Gibson, J. M. Nature 1996, 381, 678. (6) Yakobson, B. I.; Smalley, R. E. Am. Sci. 1997, 85, 324. (7) Dekker, C. Phys. Today 1999, 52, 22. (8) Watts, P. C. P.; Fearon, P. K.; Hsu, W. K.; Billingham, N. C.; Kroto, H. W.; Walton, D. R. M. J. Mater. Chem. 2003, 13, 491. (9) Fenoglio, I.; Tomatis, M.; Lison, D.; Muller, J.; Fonseca, A.; Nagy, J. B.; Fubini, B. Free Radical Biol. Med. 2006, 40, 1227. (10) Galano, A. J. Phys. Chem. C 2008, 112, 8922. (11) Lucente-Schultz, R. M.; Moore, V. C.; Leonard, A. D.; Price, B. K.; Kosynkin, D. V.; Lu, M.; Partha, R.; Conyers, J. L.; Tour, J. M. J. Am. Chem. Soc., Article ASAP, DOI: 10.1021/ja805721p. (12) Liu, Q.; Ren, W.; Chen, Z.-G.; Wang, D.-W.; Liu, B.; Yu, B.; Li, F.; Cong, H.; Cheng, H.-M. ACS Nano 2008, 2, 1722, and references therein. (13) Zheng, M.; Semke, E. D. J. Am. Chem. Soc. 2007, 129, 6084. (14) Joselevich, E.; Dai, H.; Liu, J.; Hata, K.; Windle, A. H. Top. Appl. Phys. 2008, 111, 101. (15) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision D.01; Gaussian, Inc.: Wallingford, CT, 2004.
Free Radical Scavenging Capability of SWCNTs (16) (a) Cances, M. T.; Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 107, 3032. (b) Mennucci, B.; Tomasi, J. J. Chem. Phys. 1997, 106, 5151. (c) Mennucci, B.; Cances, E.; Tomasi, J. J. Phys. Chem. B 1997, 101, 10506. (d) Tomasi, J.; Mennucci, B.; Cances, E. THEOCHEM 1999, 464, 211. (17) Dresselhaus, M. S.; Dresselhaus, G.; Jorio, A. Annu. ReV. Mater. Res. 2004, 34, 247. (18) Okuno, Y. Chem.-Eur. J. 1997, 3, 212. (19) Benson, S. W. The Foundations of Chemical Kinetics; Krieger: Florida, 1982.
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