First Principles Calculations of Alanine Radicals Adsorbed on Pristine

14812

J. Phys. Chem. C 2008, 112, 14812–14815

First Principles Calculations of Alanine Radicals Adsorbed on Pristine and Functionalized Carbon Nanotubes M. A. Carneiro and P. Venezuela Instituto de Fı´sica, UniVersidade Federal Fluminense, 24210-346, Nitero´i, RJ, Brazil

Solange B. Fagan* A´rea de Cieˆncias Naturais e Tecnolo´gicas, Centro UniVersita´rio Franciscano, 97010-032, Santa Maria, RS, Brazil ReceiVed: April 22, 2008; ReVised Manuscript ReceiVed: June 20, 2008

The structural and electronic properties of alanine radicals adsorbed on pristine and carboxyl-functionalized (5,5) single-walled carbon nanotubes (SWCN) are investigated through ab initio calculations based on density functional theory. We find that the bonds between the alanine radicals and pristine SWCNs are stronger when the interaction occurs via the amine group. In particular, the strongest bond occurs for a structure where all C atoms belonging to the SWCN keep sp2-like hybridization. Interestingly, this is the only case where the system remains metallic. In all other cases, the systems result in a semiconductor or semiconductor with a half-filled level in the gap, depending on how the alanine radical is adsorbed. On the other hand, for carboxyl-functionalized SWCNs the properties of the systems do not significantly depend on how the alanine radical is absorbed. Introduction Functionalization is a crucial step to increase the applicability of carbon nanotubes (CNTs) and fullerenes.1-3 Several chemical and mechanical processes have been used in order to functionalize CNTs by means of physical or chemical adsorption of specific molecules.4 Usually, functionalization leads to very interesting modifications of CNT chemical and physical properties1-6 Recently, potential applications of functionalized CNTs have been proposed in the biomedical field. It has been done, for instance, by connecting amino acids1,7,8 and drugs9,10 to the nanotube surface. Functionalized CNTs are promising as nanovectors for delivery of different types of therapeutic molecules.11 Amino acids interacting with CNTs are expected to be used as biological or chemical sensors1,7 due to conductance and capacitance modifications by adsorption of a very small amount of target molecules.7 The adsorption of glycine, the simplest amino acid, on singlewalled carbon nanotubes (SWCNs) has been studied recently by means of ab initio calculations.12 It is shown that the glycine-SWCN interaction depends on the structural configuration. However, they do not focus on the important electronic properties of CNTs functionalized by amino acids. Systematical theoretical investigations about the energetic, structural, and electronic properties of SWCNs functionalized by amino acids are still missing. Alanine, in particular, is interesting because it is a simple molecule and is a common part of many proteins. In this paper the interaction of the alanine radicals with pristine and functionalized metallic (5,5) SWCNs are analyzed through ab initio simulations. The electronic and structural properties of these systems are investigated to elucidate the * Corresponding author. E-mail: [email protected]; phone: 55-5532201200; fax: 55-55-3222-6484.

Figure 1. Schematic view for the different configurations of alanine radicals adsorbed on pristine (schemes I-V) and carboxyl-functionalized (schemes VI to VII) (5,5) SWCNs.

different ways that this amino acid can be adsorbed on the SWCN surface and the significant modifications observed from the original properties.

10.1021/jp803512b CCC: $40.75  2008 American Chemical Society Published on Web 09/03/2008

Alanine Radicals Adsorbed on Carbon Nanotubes

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14813 space grid integration is defined as being equivalent to a plane wave cutoff of 150 Ry.15 Periodic boundary conditions are used so that the lateral separation between tube centers are 4 nm and the total length of the tubes are 0.984 nm. All systems studied here are structurally relaxed until the residual forces were smaller than 0.05 eV/Å. Binding energies19 are calculated considering the basis set superposition error (BSSE) scheme.20 Finally, concerning the optimization problem, several structural relaxation calculations with different conformational starting points have been made. Here, we present only the stable systems with smaller final energies.

Figure 2. Alanine, without two H atoms of the amine group interacting with a SWCN (structures II and III from Figure 1). The system shown in (b) is the most strongly bonded of all systems studied here.

TABLE 1: Bond Lengths (dAla-SWCN) and Binding Energies (EB), Calculated within the LDA, for the Different Structures (see Figure 1) of Alanine Radicals Interacting with Pristine and Carboxylated SWCNsa alanine@SWCN structures

dAla-SWCN (Å)

EB (eV)

I II III IV V VI VII

1.48 1.47 1.41 1.50 1.59 1.44 1.37

-2.78 (-2.21) -3.14 (-3.05) -6.80 (-5.18) -2.19 (-1.58) -2.16 (-1.63) -2.49 -2.55

a We show the GGA binding energies in parentheses (single point energy calculations done with the LDA relaxed structures).

Methodology Ab initio simulations based on density functional theory13,14 are used to evaluate the structural and electronic properties of pristine and carboxylated (5,5) SWCNs interacting with alanine radicals. The simulations are done using the SIESTA program,15 which performs self-consistent calculations solving the KohnSham equations by means of numerical atomic orbital basis sets. We use double-ζ basis sets plus polarization functions (DZP), the local density (LDA)16 or generalized gradient approximation (GGA)17 for the exchange-correlation potential, and norm conserving Troullier-Martins pseudopotentials.18 Although, the calculations are done by means of localized orbitals, the real

Results and Discussion In this section, initially we will present and discuss the results concerning pristine SWCNs interacting with alanine radicals (structures I-V in Figure 1). In the second part of the section we will deal with carboxyl-functionalized SWCNs (c-SWNCs) interacting with alanine radicals (structures VI and VII in Figure 1). In Figure 1, structures I-III are formed by approaching the alanine toward the SWCN via the amine group. When this is done by taking off one H atom from the amine group there is only one stable final configuration. However, when two H atoms are taken from the amine group we find two stable configurations. The relaxed configurations of structures II and III are displayed in Figure 2, panels a and b, respectively. For both cases, the distances between the C atoms belonging to the SWCN increase. However, in structure III this distance increases almost 50%, implying that the bond between the C atoms is broken, see Figure 2b. In this configuration these C atoms can bind to the N atoms while still keeping sp2-like hybridization. On the other hand, in structure II (Figure 2a), the distance between the C atoms increases only 8%; thus, the bond is stretched but does not break. As a consequence, these two C atoms become four-coordinated. Structures IV and V are formed by approaching the alanine toward the SWCN via the carboxyl group, where an H atom and an OH radical have been taken off from the alanine molecule, respectively. Comparing the binding energies, calculated within the LDA, for pristine SWCNs interacting with alanine radicals (Table 1,

Figure 3. Electronic band structure for the pristine SWCN compared with SWCNs interacting with alanine radicals in several configurations as indicated. The Fermi energies correspond to the horizontal dashed lines at 0 eV.

14814 J. Phys. Chem. C, Vol. 112, No. 38, 2008 structures I-V),21 we see that, in general, the bonds are stronger when the interaction occurs via the amine group. In particular, the strongest bond occurs for structure III. As a matter of fact, in this case the bond between the alanine radical and the SWCN is much stronger than any other case investigated here. Interestingly, this is the only case where all C atoms belonging to the SWCN keep sp2-like hybridization, as discussed above. The binding energies shown in parentheses in Table 1 refer to GGA single point energy calculations done with the LDA relaxed structures. As expected, the absolute values of GGA binding energies are smaller (it is known that LDA overestimates binding and GGA underestimates it). However, our main conclusions do not change because LDA and GGA binding energies are qualitatively similar. In Table 1, we also display the distance between the atoms from the alanine and the SWCN, which are bound. For structures I-III, these distances refer to N-C bonds, and for structures IV and V they refer to O-C and C-C bonds, respectively. It is also interesting to compare the total energies of structures II and III. This comparison can be done directly because these structures have the same number and type of atoms. The total energy of structure III is 1.23 eV smaller than the total energy of structure II. It means that structure III is the most stable, and it partially explains its strong binding to the SWCN surface. Considering the interaction of pristine SWCNs with alanine via the carboxyl group (Table 1, structures IV and V), we see that the binding energies are almost the same. The distances between alanine and the SWCNs are different because they refer to different bonds, O-C and C-C bonds for structure IV and V, respectively. Another comparison between total energies can be done for structures I and IV. The total energy of structure I is 0.52 eV smaller than the total energy of structure IV. It corroborates our finding that the alanine prefers to interact with SWCN via the amine group. The same behavior is observed for the binding energies of the glycine radical adsorbed on the (8,0) and (4,4) SWNTs,12 when the interaction through the glycine N-approach is more stable than for the C radical. The results for binding energies of the COOH and NH2 radicals22 also show the preference for the interaction through the N atom (energy differences larger than 0.3 eV between the considered systems). Figure 3 presents the electronic band structures for the pristine (5,5) SWCN (for comparison) and the alanine radicals adsorbed on the (5,5) SWCN in the configurations I-V. For configurations I, IV, and V, the metallic SWCN becomes a semiconductor with a localized half-filled level in the gap region. In Figure 4a, the electronic charge density plot for the half-filled level is shown for structure IV. This level is basically a defect level, and the charges are mainly localized around the adsorbed radicals. Previous calculations for -COOH, -NH2, and -CONH2 adsorbed on SWCNs22 show similar defect levels localized around the adsorbed radicals. Charge densities for the configurations I and V half-filled levels are not shown here because they are similar to the one presented in Figure 4a. It is interesting to notice that, considering the cases where the SWCN is directly functionalized with alanine radicals (configurations I-V), the structures with smaller binding energies (configurations I, IV, and V) have similar electronic properties, as described above. On the other hand, configurations II and III, which present the largest binding energies, are quite different, as far as electronic properties are concerned. In these configurations the same alanine radical is adsorbed in different geometries on the SWCN, resulting in different electronic properties. As can be seen in Figure 3, configuration II presents

Carneiro et al.

Figure 4. Electronic charge densities (10-4 electrons/bohr3 isosurfaces) for: (a) structure IV half-filled level, (b) structure II HOMO and LUMO levels and (c) structure III HOMO and LUMO levels.

a semiconductor character and configuration III preserves the original metallic nature of the pristine SWCN. The metallic character of structure III is a consequence of the fact that in this configuration all C atoms belonging to the SWCN present sp2-like hybridization. The electronic charge densities plots for the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) for configurations II and III are presented in Figure 4, panels b and c, respectively. In both levels, for configuration II, the charge is more concentrated near the adsorption site and on the N atom of the alanine radical. On the other hand, for configuration III, the charge of the HOMO and LUMO levels are homogeneously distributed throughout the tube. The difference in charge distribution of configurations II and III is consistent with distinct (semiconductor vs metallic) behavior shown previously in Figure 3. One can say that configuration III (II) preserves (does not preserve) the metallic nanotube character because the original charge distribution does not change (changes) significantly with functionalization.

Alanine Radicals Adsorbed on Carbon Nanotubes

J. Phys. Chem. C, Vol. 112, No. 38, 2008 14815 In particular, the strongest bond occurs in a configuration where all C atoms belonging to the SWCN keep sp2-like hybridization (Figure 2b). Interestingly, the electronic properties of this configuration, for energies close to the Fermi energy, are almost identical to the pristine SWCN electronic properties. Thus, it is remarkable that in the configuration depicted in Figure 2b, alanine is strongly covalently bound to the SWCN and the electronic properties of the tube remain unaltered. For the structure shown in Figure 2a, two C atoms belonging to the SWCN make bonds with the alanine radical and become 4-fold coordinated. Thus, sp2-like hybridization is no longer present for these atoms and, as a consequence, an energy gap between the HOMO and LUMO levels appear. For all other cases studied here the covalent bonding of the functionalized radical with the tube leads to a distortion of the SWCN carbon atom position. Because of this distortion, the hybridization of the C atom becomes sp3-like. This hybridization leads to a localized half-filled level near the Fermi energy. When the previously carboxylated SWCN is functionalized by alanine, we see that binding energies and electronic properties do not depend on how the alanine radical is attached to the carboxyl group.

Figure 5. Electronic band structure for c-SWCN and c-SWCN interacting with alanine radicals in the configurations VI and VII, as indicated. The Fermi energies correspond to the horizontal dashed lines at 0 eV.

Acknowledgment. The authors acknowledge the financial support from Brazilian agencies CNPq, FAPERJ, and FAPERGS. Part of the calculations was performed at CENAPAD. S.B.F. acknowledges Lóreal/Paris for the Grant for the Brazilian Woman in Science, 2006. References and Notes

Now we will discuss our results for the cases where the SWCN is initially functionalized by COOH (here denoted as c-SWCN) and then interacts with the alanine radicals. The stable configurations that we have found in our calculations are schematically represented as VI and VII in Figure 1. In both cases the SWCN was previously functionalized by a carboxyl group without the H atom. After that, the alanine radical (also without one of the H atoms) is bounded to the O atom of the carboxyl group in two different ways via the amine and acid groups, respectively. In general we can say that, after the SWCN is carboxylated, the properties of the system do not significantly depend on how the alanine radical is attached. For instance, the binding energies for configurations VI and VII are very similar, as can be seen in Table 1. Also in Figure 5, we compare the electronic band structure of configurations VI and VII with the c-SWCN.17 For the c-SWCN, a half-filled defect level appears near the Fermi energy. The attachment of the alanine radial to the c-SWCN almost does not change the band structure. The electronic charge distribution (not shown here) of configurations VI and VII are also very similar to the c-SWCN. The charge densities for all three cases concentrate near the defect created by the bound of the carboxyl group with the SWCN consistently with the localized levels shown in Figure 5. Conclusion We have made a systematical theoretical study about the structural and electronic properties of alanine-functionalized SWCNs. We investigated several configurations for the direct alanine functionalization of the tubes and also for the alanine functionalization of previously carboxylated tubes. The covalent bonds between alanine and pristine SWCN are stronger when the interaction occurs via the amine group, compared to the interaction of the SWCN with the carboxyl group.

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