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Theoretical investigation of the solubilization of COOH-functionalized single wall carbon nanotubes in water
Journal of Molecular Liquids 215 (2016) 780–786
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Theoretical investigation of the solubilization of COOH-functionalized single wall carbon nanotubes in water Michael Mananghaya, Michael Angelo Promentilla, Kathleen Aviso, Raymond Tan De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines DLSU STC Laguna Boulevard, LTI Spine Road Barangays Biñan and Malamig, Biñan City, Laguna, Philippines National Research Council of the Philippines, General Santos Ave., Taguig, 1631 Metro Manila, Philippines
a r t i c l e
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Article history: Received 7 June 2015 Received in revised form 14 December 2015 Accepted 12 January 2016 Available online 8 February 2016 Keywords: Binding energy Density functional theory Gibbs free energy of solvation Single-walled carbon nanotubes
a b s t r a c t The paper discusses the impact of functionalization in understanding the solubility of (n, 0) single walled carbon nanotube (SWCNT) with varying diameters (n = 8–16) to increase its dispersion in water with the aide of spinunrestricted density functional theory (DFT). A finite nanotube model saturated with hydrogen at both ends was functionalized with a carboxylic acid at the sidewall. Functionalization resulted in an enhancement in the solubility of the nanotubes in water which can be explained by the increase in dipole moment as visualized in the HOMO–LUMO surface plots. This behavior depends on the tube diameter marked with saw tooth like periodic features which originated from their different π bonding structures manifested in the electronic band gaps. Furthermore, as the degree of sidewall functionalization increases, the SWCNT sample becomes more soluble as assessed by the calculated Gibbs free energies of solvation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The biocompatibility and cytotoxicity of carbon nanotubes (CNTs) are attributed to size, dose, duration, testing systems, surface functionalization chirality as well as the strong van der Waals (vdW) forces of the as-produced samples. This makes it difficult to obtain individualized CNTs that retain their intrinsic properties in applications specifically intended for nanomedicine [1–11]. Single-wall carbon nanotubes (SWCNTs) with very high specific surface areas can be derived easily with biomolecules through chemical attachment, adsorption, or encapsulation. Such bioconjugates on SWCNTs have the ability to deliver bioactive molecules across cell membranes and even into the cell nuclei [9–11]. Scientists have shown that nanotubes can release drugs in cells without damaging the healthy cells of the tissue. The advantages of using a carbon nanotube-assisted drug delivery system include improved efficiency and amplification of tumor targeting due to an enhanced permeability and retention effect of the carbon nanotube which can be efficiently loaded with the drug. For example, the use of a nontoxic drug, which is activated to its cytotoxic form in the tumor cells, helps preserve the non-targeted normal tissue of the patient, thereby potentially reducing the side effects resulting from the therapy [11–15]. Surface modifications or functionalization of nanoparticles could play a crucial role in improving their physicochemical and surface properties. The overall objective of functionalizing CNTs (fCNT) for biomedical applications is to increase their solubility or dispersion in
E-mail address: [email protected] (M. Mananghaya).
http://dx.doi.org/10.1016/j.molliq.2016.01.041 0167-7322/© 2016 Elsevier B.V. All rights reserved.
biocompatible aqueous media. It is well recognized that the transport, bioavailability and solubility are significant factors in improving distribution, therapeutics, selectivity, and amelioration of toxic effects. It has been reported that after modifications, fCNT solubility increased significantly [7–10] and alters their cellular interaction pathways, resulting in much-reduced cytotoxic effects. Because of this, it is imperative to examine the dispersion of SWCNTs as individuals in a liquid and hopefully address the toxicity of these carbon-based nanostructures [12–17]. fCNTs are promising novel materials for chemical manipulation and cutting which can potentially lead to improved compatibility with a variety of biological components [1–15]. Essentially, the addition of a layer of biocompatible material can be used to remove the toxicity of pristine CNT aggregates by making them more dispersible in aqueous solutions. The most efficient way to transform the surface of CNTs from hydrophobic to hydrophilic is by attaching different water-soluble and functional moieties. fCNTs can be achieved by using carboxylic terminal group (–COOH) commonly present in oligonucleotides, biomolecules, surfactants, and polymers. Furthermore, SWCNTs are made up of the sp2-bonding network of carbon atoms and its chemical reactivity in aqueous media is dependent on local curvature of the independent C–C bonds. Since unmodified CNTs often cause adverse reactions to living cells and tissues, whereas the aforementioned fCNTs are expected to be less toxic due to more biocompatible carboxylic functional groups. The motivation of this study is to understand how the properties of water-dispersible SWCNT functionalized with –COOH change. The improvement of the solubility of the corresponding nanotubes can be addressed not only on the degree of –COOH functionalization but also to curvature. The possible diameter
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selectivity of the chemical reactions of –COOH with nanotubes and the dependence of solubility on the nanotubes diameter for the efficient control of the amount of dispersed species are crucial in the biocompatibility of CNT. Specifically, the objective of this work is to determine the extent of solubility as a function of variations in tube diameter and degree of sidewall –COOH functionalization. Improved solubility for fCNTs depends not only with the quantity of functional group attached but also with the tube diameter that may permit miscibility in cell culture with satisfactory distribution, low aggregation and reduced cytotoxicity for future medical applications. 2. Methodology The (n, 0) nanotubes with n ranging from 8 to 16 each having a length of 7.38 Å was constructed wherein the optimized structural geometry of (10, 0) SWCNT is presented in Fig. 1(a) as an example; following is the assembly of acid-functionalized SWCNT with formic acid, CO2H2, as a model of carboxylic acid. All calculations were performed using DFT with a B3LYP functional [19] and computational cost will be lowered by the use of highly localized grid representations wherein each electronic wave function is expanded in a localized atomcentered basis set with each basis function defined numerically. For cluster geometries, spin-unrestricted calculations were carried out with a double numeric polarized (DNP) basis set available. Charge densities were analyzed by the Mulliken method [20]. For open-shell molecular radicals, the unrestricted formalism was used. The present level of calculation is known to produce reasonable results [21] for bond lengths, bond angles, and bond energies for a wide range of molecules. The computations were carried out using the ab initio quantum chemistry package, Density functional theory for MOLecules (DMOL) code, available from Accelerys [22]. Electronic structure descriptors have been computed to analyze the geometrical and electronic changes that may lead to better solubility of the functionalized nanotubes. Among them are the dipole moment (μdip) and the Gibbs free energy of solvation (ΔGsolv). Measuring its Gibbs free energy of solvation can assess the solubility of a given molecule in a solvent. A number of quantum mechanical continuum solvation models were developed for this purpose [23,24]. The COnductorlike Screening MOdel (COSMO) as implemented in DMOL [22] was chosen. These originated from the Onsager continuum model [24], and were formulated by Tomasi et al. [23,25–27] as the polarizable continuum model (PCM) which is a commonly used method in computational chemistry for modelling solvation effects. If it were necessary to consider each solvent molecule as a separate molecule, the computational cost of modeling a solvent-mediated chemical reaction would grow prohibitively high. Modeling the solvent as a polarizable continuum, and not as individual molecules, makes ab initio computation feasible. Two types of PCMs have been popularly used: dielectric PCM (D-PCM) which
Fig. 1. Optimized geometry of the (a) finite (10, 0) zigzag SWCNT; (b) (10, 0) zigzag SWCNT with the formic acid radical at the tube sidewall. Gray color depicts carbon atoms; white is hydrogen and red is oxygen.
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models the continuum as a polarizable dielectrics and conductor-like PCM (C-PCM) which models the continuum as a conductor-like picture similar to COSMO. COSMO approximates the solvent by a dielectric continuum, surrounding the solute molecules outside of a molecular cavity. The details of the cavity construction implementations is constructed as an assembly of atom-centered spheres with radii approximately 20% larger than the vdW radius and the actual calculation the cavity surface is approximated by segments, e.g., hexagons, pentagons, or triangles [28]. Functionalization of CNTs with –COOH results in highly soluble materials that are further derived with active molecules, making them compatible with biological systems. In addition, functionalized CNTs have wider biological applications compared to non functionalized CNTs. [15,17]. Since one of the objectives of the calculation is to investigate how the solubility of SWCNTs change as a function of the degree of sidewall functionalization, the resulting optimized structures are presented in Fig. 2. Figure 2 serves as a model for determining the
Fig. 2. The optimized geometry of the SWCNT functionalized with: (a) one, (b) two, (c) three, (d) four formic acid radical at the tube sidewall. Gray color depicts carbon atoms; white spheres — hydrogen atoms, and red spheres — oxygen atoms.
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dependence of solubility with respect to the added formic acid radical. The reported density of functionalization on the sidewalls of the SWCNT is a function of the number of attached COOH groups. 3. Results and discussions 3.1. Electronic effects with varying tube diameter The optimized structural geometry of (10, 0) SWCNT with COOH attached is presented in Fig. 1(b) [9,10] as an illustrative example. The finite (10, 0) nanotube has an optimized length of 7.38 Å, and a computed diameter of 7.83 Å agrees well in literature [29,30] wherein a periodic supercell model was employed. The agreement between the cluster and periodic model with respect to diameter is well exhibited. The model presented in Fig. 1(b) is a representative of biomaterials due to their superior polar characteristics that constitutes a major improvement of unmodified CNTs, since fCNTs have emerged as a new alternative and efficient tool for transporting and translocating therapeutic molecules with improved dispersability and decreased cytotoxicity due to more biocompatible COOH groups [17]. The selectivity of certain reactions has been reported to be diameter dependent as well [2,17] specifically for –COOH groups, the major effects seem to be tube species and diameter. The influence of tube diameter for carbon nanotubes for – COOH adsorption was investigated in order to relate the possible diameter selectivity of the chemical reactions of –COOH with nanotubes. The bond lengths and adsorption energies of –COOH adsorbed on nanotubes of different diameters is analyzed. Adsorption energy, Ea, is the energy required to disassemble a whole system into separate parts. A bound system typically has a lower potential energy than the sum of its constituent parts of the radicals to the nanotube, defined for the reaction –COOH þ SWCNT→COOH–SWCNT;
ð1Þ
where –COOH is the carboxylic radical and COOH-SWCNT is the corresponding functionalized nanotube. Therefore. Ea ¼ ESWCNT þ E–COOH −ECOOH–SWCNT :
ð2Þ
where ESWCNT, E–COOH and ECOOH–SWCNT denote the total energy of the optimized nanotube, the –COOH radical and the sidewallfunctionalized nanotube with –COOH radical, respectively. A stable optimized configuration and bonding is dictated by a thermodynamically favorable functionalization — this is what keeps the system together. As shown in Fig. 3(a) the adsorption energies decrease with increasing diameter having a saw tooth like periodicity with n as an even integer at a higher adsorption energy compared to the odd case. This trend can be explained by examining the band gap as a function of diameter as will be explained later. The structures of both SWCNTs and graphite are made up of the sp2-bonding network of carbon atoms, and the difference of the reactivity in –COOH is therefore deeply related to the local curvature. Our results strongly suggest that it induces the structure deformation from sp2 to sp3-like bonding, depending on the diameter of SWCNTs. As the reactant carbon atoms shift from three to four coordinate, the local preferred geometry correspondingly shifts from planar to tetrahedral. Since nanotubes are obtained by rolling up a graphene sheet, the resultant distortions generate structures with the bond lengths deviating from the ideal value. Smaller diameter gives higher curvature and larger deviations. CNTs possess strain in their highly curved lattice which causes them to have high chemical reactivity; this strain is released through the formation of sp3 covalent sites. The source of graphene's chemical stability comes, in part, from the reduced strain present in its sp2 network as the strain releases from the highly curved CNT with relatively small diameter to bigger ones, the stability increases to the value for graphene due to it resembling closely to an ideal planar structure [31–38]. Furthermore, any local conjugation is broken and the carbon atoms rehybridize from sp2 to sp3. The energy
required inducing such curvature scales inversely with diameter, therefore it is expected that small diameter nanotubes with greater strain can be functionalized more easily than the larger ones. The adsorption energy data plotted in Fig. 3(a) supports this; the most stable adsorption structure is that with the least diameter. Thus reaction can take place with carbon atoms in the nanotube transformed from sp2 to sp3 hybridization. The reactivity is correlated with the extent of this hybridization transformation. Careful analysis of Mulliken charges (Table 1) reveals that, whether for very small or for very large diameter nanotubes, there is charge transfer from the –COOH to the surface of the nanotubes in the range of 0.052 (for the (8, 0) tube) to 0.059 (for the (16, 0) nanotube). The other nanotubes show in-between values. The charge transfer data reveals that maximum charge is transferred from the – COOH moiety to the carbon nanotube as the diameter increases which correlates well with adsorption energy plot. The results presented in Fig. 3(a) also indicate a higher reactivity of zigzag versus armchair SWCNT toward carboxylation. This is in agreement with earlier observations [24]. The product of magnitude of charges and the distance of separation between the charges can be expressed in terms of dipole moments (μdip) as displayed in Table 1 wherein there is a substantial increase in the value of the magnitude of the dipole moments of the functionalized nanotubes both zigzag and armchair configuration compared to the unfunctionalized nanotube which has practically no dipole moment as can be seen quantitatively in the plots of the HOMO–LUMO mapped into the total electron density as shown in Fig. 4, the pure nanotube has localization with respect to the terminal ends whereas the nanotube with a sidewall functional group has localization in the opposite end as far as possible from the organic acid attached. In a nucleophile, we should think about the localization of the HOMO orbital because electrons from this orbital are mostly free to participate in the reaction. Similarly, the frontier orbital theory predicts that a site where the lowest unoccupied orbital is localized serves as a good electrophilic site. The radical has a preference to attach itself at the wall near the terminal end of the tube saturated with hydrogens. After which the succeeding radical attaches as far as possible, typically at the opposite end of the tube again near the hydrogens. Polarizability correlates well with the HOMO–LUMO separations in atoms and molecules. The electron distribution can be distorted readily if the LUMO lies close to the HOMO in energy, compared to the pure nanotube the red spots of the functionalized tube is intense in color, so the polarizability is then larger. The large electric dipole moment of a molecule of the functionalized SWCNT versus the finite pure nanotube model suggests a modification of the functionalized nanotubes interaction with a polar solvent such as water, potentially increasing the dispersion. The dipole moment of the functionalized nanotube decreases monotonically as the diameter increases as shown in Fig. 3(b). This is in agreement with the claim that small diameter SWCNT disperse at higher concentration than do large-diameter SWCNT. Scanning tunneling spectroscopy (STS) experiments confirmed that nanotubes could be either metallic or semi conducting. Observations show that, for narrow carbon nanotubes functionalized with –COOH, the effect of curvature can convert nanotubes, which are expected to be semiconductors into metals. As the band gap decreases with decreasing diameter again with saw tooth like periodicity as displayed in Fig. 3(c) with n as an even integer at a smaller band gap compared to the odd case which makes the (n, 0) tubes with n = 2 k (k is integer) more metallic compared to n = 2 k + 1 upon functionalization of – COOH. This peculiar behavior is reflected implicitly in the adsorption energy plot as shown earlier, the (n, 0) tubes with n = 2 k possess more metallic character has delocalized electron that can intensify the binding of the –COOH compared to n = 2 k + 1 which resulted in a higher adsorption energy for (n, 0) tubes with n = 2 k vs. n = 2 k + 1. The zigzag pattern can be traced in the plot of the HOMO of the pure nanotube as advertised in Fig. 3(d). This was carried over to the functionalized case and was accompanied by the charge transferred
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Fig. 3. The plot of the calculated adsorption energies, band gaps and solvation free energies in electron volts (eV) versus nanotube diameter in angstrom (Å). The graph of the dipole moments in Debye (D) with respect to nantoube diameter is also expressed in angstrom unit. Red curve denotes that Tkatchenko–Scheffler (TS) scheme was adapted in the functional calculation with basis set superposition error (BSSE) correction approach and yellow curve denotes that the armchair (n,n) nanotube with n = 4 to 9 was taken into account as basis for the calculation with a different nanotube chirality.
from the –COOH to the CNT. The inclusion of long range correlation facilitates the accurate determination of vdW correction method wherein Tkatchenko–Scheffler (TS) scheme was adapted in the functional calculation with basis set superposition error (BSSE) correction approach employed by means of counterpoise method (CP) computations that were carried out using DMOL [22]. The adsorption energy values were
updated in Fig. 3(a) and the saw tooth character was also observed prominent. According to rigorous calculations, vdW interactions produce an increase in the average adsorption energy as presented in Fig. 3(a) and consequently reduces the equilibrium distance. The rest of the parameters such as dipole moment in Fig. 3(b) and band gap in Fig. 3(c) have no significant change with respect to the uncorrected
Table 1 Data on functionalization of –COOH and SWCNTs. n
8 9 10 11 12 13 14 15 16
Ea
μdip
ΔGsolv
CCNT
Cc
CO
C–O
CH
HOMO
LUMO
Gap
(eV)
(D)
(eV)
(e)
(e)
(e)
(e)
(e)
(eV)
(eV)
(eV)
0.992 0.827 0.919 0.759 0.843 0.690 0.808 0.672 0.781
1.368 1.337 1.215 1.156 1.123 1.065 1.044 1.006 0.923
−0.298 −0.303 −0.293 −0.299 −0.289 −0.294 −0.287 −0.290 −0.283
−0.334 −0.339 −0.343 −0.347 −0.350 −0.353 −0.353 −0.356 −0.357
0.537 0.537 0.538 0.537 0.539 0.533 0.538 0.540 0.540
−0.382 −0.381 −0.381 −0.380 −0.381 −0.379 −0.380 −0.380 −0.379
−0.392 −0.392 −0.393 −0.392 −0.392 −0.393 −0.392 −0.392 −0.392
0.289 0.289 0.290 0.289 0.289 0.289 0.289 0.290 0.290
−4.027 −4.030 −4.047 −4.124 −4.077 −4.168 −4.095 −4.086 −4.105
−4.003 −3.970 −4.039 −4.034 −4.047 −4.070 −4.049 −3.978 −4.063
0.024 0.059 0.008 0.090 0.030 0.098 0.045 0.107 0.041
CCNT = carbon of the nanotube attached to the COOH, CC = carbon of the COOH attached to the nanotube, CO = oxygen attached to the COOH in a double bond fashion, C–O = oxygen attached to the COOH in a single bond fashion, CH = hydrogen of the COOH radical.
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Fig. 4. HOMO–LUMO of the pure and –COOH functionalized nanotube.
values, as vdW do not influence the spatial configuration of the adsorbed COOH radical. The increase in the adsorption energy can be rooted to the dispersion forces, the nonlocal correlation between electrons. A balance between the long-range attractive vdW forces and the short-range Pauli repulsion mainly determines the equilibrium distance. Apparently, the nonlocal correlations reduce the repulsion between the electrons. Its effect when a proper description of nonlocal interactions is considered allows the functionalized COOH to reach a shorter equilibrium distance, where adsorption energy increases significantly. Interestingly, as the chirality of the nanotube is changed the saw tooth character vanishes completely as in the case of armchair (n, n) nanotubes as shown in Fig. 3(a) and (c). This observation can be attributed to the metallic nature of the armchair nanotubes in which the electrons are already delocalized. The molar Gibbs energy (ΔG) is the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. Measuring the Gibbs free energy of solvation can assess solubility as such; it is a convenient criterion for the spontaneity of the solvation processes with respect to
constant pressure and temperature. The Gibbs free energy of solvation, ΔGsolv, is defined as ΔGsolv ¼ ðEDPCM þ ΔGnonelectrostatic Þ−Egas :
ð3Þ
where E denotes the computed optimized DMOL3 total energy of the species in the vacuum (gas phase), ED-PCM is the total DMOL3/COSMO energy of the species in solvent (D-PCM) as mentioned in the methodology section. The non-electrostatic contribution ΔGnonelectrostatic due to dispersion and cavity effects are estimated from a linear interpolation of the free energies of hydration for linear-chain alkanes as a function of surface area [22]. The magnitude of the ΔGsolv of nanotubes decreases with increasing tube diameter with local maximum occurring at n with multiples of 3 that results from their different π bonding structures for the pure nanotube as shown in Fig. 3(e). Specifically, (9, 0), (12, 0) and (15, 0) exhibits these kind of behavior due to their metallic nature as compared to the other nanotubes as expected. On the other hand, a large negative value of ΔGsolv for all the –COOH functionalized nanotubes as given in Table 1. This implies that the technique of functionalization with –COOH produces soluble species substantially.
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Strong variations of Gibbs free energy of solvation as a function of diameter marked with periodic features was observed wherein the traditional profile in the pure case is now transformed to that of a saw tooth like character as a consequence of the saw tooth shaped band gap plot of the corresponding COOH–SWCNT from n = 8 to 16. Interestingly, for the functionalized case the even integer nanotubes are at a lower solubility compared to the odd numbered case. The zigzag band gap profile of the functionalized tube can also modify its behavior in aqueous media such as water as displayed. It can be seen that aside from the fact that small diameter SWCNT disperse at higher concentration than do largediameter SWCNT as supported in the plot of the Gibbs free energy vs. diameter a saw tooth behavior persists in its profile. However, the armchair configuration does not exhibit any of the periodicity found in the zigzag case. 3.2. Electronic effects of adsorbed –COOH groups The density of functionalization on the sidewalls of the SWCNT can be depicted as a function of the number of adsorbed COOH groups. As an illustrative example, up to 18 –COOH groups are attached to the (10, 0) CNT. However, only 16 –COOH groups attached uniformly to the CNT. For higher concentrations, owing to steric attraction between –COOH groups, the system was observed to have unbounded –COOH groups. However, this process does not destroy the structure of the CNT. Functionalization of CNTs not only modifies the morphology of the functionalized system locally but it also influences the optimized length of the functionalized CNTs based on B3LYP/DNP calculations. The calculated equilibrium length along the symmetry axis (z) of the functionalized (9, 0) CNT as a function of the number of sidewall attached groups generally causes elongation of the tube. The elongation further increases with the increase in concentration of functionalizing molecules. The results clearly show that the functionalization of CNTs with – COOH molecules leads to strong chemisorption of the adsorbants. The value of adsorption energy for one –COOH group bound to (10, 0) CNT containing 80 carbon atoms is 0.919 eV which nicely agrees with other theoretical prediction [35]. For these systems, the adsorption energy per molecule decreases as a function of adsorbants concentration, indicating that CNTs can be saturated with a high density of 16 adsorbants. The destabilization of CNTs is not significant within the critical density of attached –COOH groups. However, −COOH groups induce small local distortions along the radial direction on the tube sidewall. These geometry changes are usually described as the local sp3 rehybridization as mentioned before. The calculations show that the C-C bond length are close to the C–C typical distance in the sp3hybridized diamond (1.54 Å) and significantly larger than the C–C bond length in the perfect graphene sheet constituting the CNT surface with sp2 hybridization (1.42 Å). The strong chemisorption of –COOH species to CNT surface can be also seen in Fig. 4, where the distribution of the HOMO–LUMO is depicted. These HOMO–LUMO rearrangements have mostly local character and the picture does not change essentially with the growing density of the adsorbed molecules. CNT functionalized by –COOH groups can be easily synthesized [8, 22] and exchanged by other groups using standard chemical reactions allowing one to attach to CNTs more complex molecules. The molar Gibbs energy of the pure nanotube that is practically insoluble in water has an average value of about −0.08 eV. Pristine CNTs are not soluble in water or in organic solvents and have tendencies to create bundles, mostly because of dangling π-bonds of the C atoms on CNT's surface, which could build strong covalent bonds among themselves. Using the technique of –COOH sidewall functionalization, the calculated ΔGsolv of the functionalized nanotubes with low to high concentrations of –COOH groups in Table 2 suggests that as the degree of –COOH sidewall functionalization increases, the SWCNT sample becomes more and more soluble. This is in agreement with experimental [8,9,17] and theoretical calculations [16–18] performed, wherein the sidewall
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Table 2 The functionalization density dependence of SWCNTs Gibbs free energy of solvation (ΔGsolv). na
Ea (eV)
μdip (D)
ΔGsolv (eV)
HOMO (eV)
LUMO (eV)
HOMO–LUMO gap (eV)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
– 0.919 0.897 0.806 0.875 0.651 0.583 0.521 0.353 0.415 0.372 0.334 0.324 0.278 0.260 0.257 0.251
0.034 1.215 1.147 1.095 1.059 1.039 1.036 1.049 1.078 1.123 1.184 1.262 1.356 1.451 1.597 1.737 1.879
−0.08 −0.29 −0.54 −0.78 −1.06 −1.164 −1.242 −1.282 −1.294 −1.291 −1.281 −1.276 −1.168 −1.324 −1.397 −1.519 −1.425
−4.034 −4.047 −3.920 −3.938 −3.957 −4.077 −4.197 −4.167 −4.136 −4.189 −4.242 −4.047 −4.047 −4.059 −4.012 −4.042 −4.073
−3.619 −4.039 −3.911 −3.924 −3.937 −4.054 −4.172 −4.140 −4.109 −4.161 −4.212 −4.039 −4.039 −4.033 −3.990 −4.028 −4.066
0.281 0.008 0.009 0.014 0.020 0.023 0.025 0.026 0.027 0.028 0.030 0.027 0.027 0.027 0.022 0.014 0.006
a
Number of COOH attached per (10, 0) CNT with 80 C.
functionalized SWCNT are found out to be substantially less cytotoxic than unfunctionalized SWCNT in water. The functionalized (10, 0) CNTs band gap decreases, originating from the approach of the HOMO and LUMO. The near metallic character for higher degree of functionalization is well exhibited at least for the concentrations studied. These results clearly show which type of molecules should be used in order to make semiconducting nanotubes approach its metallic counterpart and give important practical hint for technological applications. The results presented in this paper correspond to the equilibrium situation at 0 K. They will provide the optimized geometry of the functionalized CNTs that is basis for calculations of the electronic structure. However, for many cases the performed molecular dynamics studies (to be published elsewhere) suggest that the functionalized structures remain stable for simulation time of 2 ps and temperatures reaching 1400 K. 4. Conclusions The solubility in water of SWCNT covalently functionalized with formic acid radical is improved under the aide of the density functional theory formalism wherein electronic exchange and correlation effects have been treated with the gradient-corrected (B3LYP) functional/ DNP as basis set implemented within the DMOL code, which uses cluster models and localized atom-centered basis set. The functionalization enhanced the dipole moment of SWCNT expected to modify its interaction with a polar solvent such as water that depends on the tube diameter marked with saw tooth like periodicity which originated from their different π bonding structures manifested in the electronic band gaps. Furthermore as the degree of sidewall functionalization increases the solubility also increases. The cytotoxicity of CNTs has been well addressed through various methods of surface functionalization of CNTs employing C-PCM, thereby improving their interaction within biological systems. Given CNTs' relatively lower toxicity, their surface functionalization is a promising strategy for delivering different biological molecules. Functionalization of their surface can result in highly soluble materials, which can be further derivatized with active molecules, making them compatible with biological systems. Therefore, many biomedical applications can be envisaged. References [1] N. Sinha, J.T.W. Yeow, Carbon nanotubes for biomedical applications, IEEE Trans. Nanobiosci. 4 (2) (2005) 180–195. [2] N. Sinha, J. Ma, J.T.W. Yeow, Carbon nanotube-based sensors, J. Nanosci. Nanotechnol. 6 (3) (2006) 573–590.
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Theoretical investigation of the solubilization of COOH-functionalized single wall carbon nanotubes in water Michael Mananghaya, Michael Angelo Promentilla, Kathleen Aviso, Raymond Tan De La Salle University, 2401 Taft Avenue, 0922 Manila, Philippines DLSU STC Laguna Boulevard, LTI Spine Road Barangays Biñan and Malamig, Biñan City, Laguna, Philippines National Research Council of the Philippines, General Santos Ave., Taguig, 1631 Metro Manila, Philippines
a r t i c l e
i n f o
Article history: Received 7 June 2015 Received in revised form 14 December 2015 Accepted 12 January 2016 Available online 8 February 2016 Keywords: Binding energy Density functional theory Gibbs free energy of solvation Single-walled carbon nanotubes
a b s t r a c t The paper discusses the impact of functionalization in understanding the solubility of (n, 0) single walled carbon nanotube (SWCNT) with varying diameters (n = 8–16) to increase its dispersion in water with the aide of spinunrestricted density functional theory (DFT). A finite nanotube model saturated with hydrogen at both ends was functionalized with a carboxylic acid at the sidewall. Functionalization resulted in an enhancement in the solubility of the nanotubes in water which can be explained by the increase in dipole moment as visualized in the HOMO–LUMO surface plots. This behavior depends on the tube diameter marked with saw tooth like periodic features which originated from their different π bonding structures manifested in the electronic band gaps. Furthermore, as the degree of sidewall functionalization increases, the SWCNT sample becomes more soluble as assessed by the calculated Gibbs free energies of solvation. © 2016 Elsevier B.V. All rights reserved.
1. Introduction The biocompatibility and cytotoxicity of carbon nanotubes (CNTs) are attributed to size, dose, duration, testing systems, surface functionalization chirality as well as the strong van der Waals (vdW) forces of the as-produced samples. This makes it difficult to obtain individualized CNTs that retain their intrinsic properties in applications specifically intended for nanomedicine [1–11]. Single-wall carbon nanotubes (SWCNTs) with very high specific surface areas can be derived easily with biomolecules through chemical attachment, adsorption, or encapsulation. Such bioconjugates on SWCNTs have the ability to deliver bioactive molecules across cell membranes and even into the cell nuclei [9–11]. Scientists have shown that nanotubes can release drugs in cells without damaging the healthy cells of the tissue. The advantages of using a carbon nanotube-assisted drug delivery system include improved efficiency and amplification of tumor targeting due to an enhanced permeability and retention effect of the carbon nanotube which can be efficiently loaded with the drug. For example, the use of a nontoxic drug, which is activated to its cytotoxic form in the tumor cells, helps preserve the non-targeted normal tissue of the patient, thereby potentially reducing the side effects resulting from the therapy [11–15]. Surface modifications or functionalization of nanoparticles could play a crucial role in improving their physicochemical and surface properties. The overall objective of functionalizing CNTs (fCNT) for biomedical applications is to increase their solubility or dispersion in
E-mail address: [email protected] (M. Mananghaya).
http://dx.doi.org/10.1016/j.molliq.2016.01.041 0167-7322/© 2016 Elsevier B.V. All rights reserved.
biocompatible aqueous media. It is well recognized that the transport, bioavailability and solubility are significant factors in improving distribution, therapeutics, selectivity, and amelioration of toxic effects. It has been reported that after modifications, fCNT solubility increased significantly [7–10] and alters their cellular interaction pathways, resulting in much-reduced cytotoxic effects. Because of this, it is imperative to examine the dispersion of SWCNTs as individuals in a liquid and hopefully address the toxicity of these carbon-based nanostructures [12–17]. fCNTs are promising novel materials for chemical manipulation and cutting which can potentially lead to improved compatibility with a variety of biological components [1–15]. Essentially, the addition of a layer of biocompatible material can be used to remove the toxicity of pristine CNT aggregates by making them more dispersible in aqueous solutions. The most efficient way to transform the surface of CNTs from hydrophobic to hydrophilic is by attaching different water-soluble and functional moieties. fCNTs can be achieved by using carboxylic terminal group (–COOH) commonly present in oligonucleotides, biomolecules, surfactants, and polymers. Furthermore, SWCNTs are made up of the sp2-bonding network of carbon atoms and its chemical reactivity in aqueous media is dependent on local curvature of the independent C–C bonds. Since unmodified CNTs often cause adverse reactions to living cells and tissues, whereas the aforementioned fCNTs are expected to be less toxic due to more biocompatible carboxylic functional groups. The motivation of this study is to understand how the properties of water-dispersible SWCNT functionalized with –COOH change. The improvement of the solubility of the corresponding nanotubes can be addressed not only on the degree of –COOH functionalization but also to curvature. The possible diameter
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selectivity of the chemical reactions of –COOH with nanotubes and the dependence of solubility on the nanotubes diameter for the efficient control of the amount of dispersed species are crucial in the biocompatibility of CNT. Specifically, the objective of this work is to determine the extent of solubility as a function of variations in tube diameter and degree of sidewall –COOH functionalization. Improved solubility for fCNTs depends not only with the quantity of functional group attached but also with the tube diameter that may permit miscibility in cell culture with satisfactory distribution, low aggregation and reduced cytotoxicity for future medical applications. 2. Methodology The (n, 0) nanotubes with n ranging from 8 to 16 each having a length of 7.38 Å was constructed wherein the optimized structural geometry of (10, 0) SWCNT is presented in Fig. 1(a) as an example; following is the assembly of acid-functionalized SWCNT with formic acid, CO2H2, as a model of carboxylic acid. All calculations were performed using DFT with a B3LYP functional [19] and computational cost will be lowered by the use of highly localized grid representations wherein each electronic wave function is expanded in a localized atomcentered basis set with each basis function defined numerically. For cluster geometries, spin-unrestricted calculations were carried out with a double numeric polarized (DNP) basis set available. Charge densities were analyzed by the Mulliken method [20]. For open-shell molecular radicals, the unrestricted formalism was used. The present level of calculation is known to produce reasonable results [21] for bond lengths, bond angles, and bond energies for a wide range of molecules. The computations were carried out using the ab initio quantum chemistry package, Density functional theory for MOLecules (DMOL) code, available from Accelerys [22]. Electronic structure descriptors have been computed to analyze the geometrical and electronic changes that may lead to better solubility of the functionalized nanotubes. Among them are the dipole moment (μdip) and the Gibbs free energy of solvation (ΔGsolv). Measuring its Gibbs free energy of solvation can assess the solubility of a given molecule in a solvent. A number of quantum mechanical continuum solvation models were developed for this purpose [23,24]. The COnductorlike Screening MOdel (COSMO) as implemented in DMOL [22] was chosen. These originated from the Onsager continuum model [24], and were formulated by Tomasi et al. [23,25–27] as the polarizable continuum model (PCM) which is a commonly used method in computational chemistry for modelling solvation effects. If it were necessary to consider each solvent molecule as a separate molecule, the computational cost of modeling a solvent-mediated chemical reaction would grow prohibitively high. Modeling the solvent as a polarizable continuum, and not as individual molecules, makes ab initio computation feasible. Two types of PCMs have been popularly used: dielectric PCM (D-PCM) which
Fig. 1. Optimized geometry of the (a) finite (10, 0) zigzag SWCNT; (b) (10, 0) zigzag SWCNT with the formic acid radical at the tube sidewall. Gray color depicts carbon atoms; white is hydrogen and red is oxygen.
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models the continuum as a polarizable dielectrics and conductor-like PCM (C-PCM) which models the continuum as a conductor-like picture similar to COSMO. COSMO approximates the solvent by a dielectric continuum, surrounding the solute molecules outside of a molecular cavity. The details of the cavity construction implementations is constructed as an assembly of atom-centered spheres with radii approximately 20% larger than the vdW radius and the actual calculation the cavity surface is approximated by segments, e.g., hexagons, pentagons, or triangles [28]. Functionalization of CNTs with –COOH results in highly soluble materials that are further derived with active molecules, making them compatible with biological systems. In addition, functionalized CNTs have wider biological applications compared to non functionalized CNTs. [15,17]. Since one of the objectives of the calculation is to investigate how the solubility of SWCNTs change as a function of the degree of sidewall functionalization, the resulting optimized structures are presented in Fig. 2. Figure 2 serves as a model for determining the
Fig. 2. The optimized geometry of the SWCNT functionalized with: (a) one, (b) two, (c) three, (d) four formic acid radical at the tube sidewall. Gray color depicts carbon atoms; white spheres — hydrogen atoms, and red spheres — oxygen atoms.
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dependence of solubility with respect to the added formic acid radical. The reported density of functionalization on the sidewalls of the SWCNT is a function of the number of attached COOH groups. 3. Results and discussions 3.1. Electronic effects with varying tube diameter The optimized structural geometry of (10, 0) SWCNT with COOH attached is presented in Fig. 1(b) [9,10] as an illustrative example. The finite (10, 0) nanotube has an optimized length of 7.38 Å, and a computed diameter of 7.83 Å agrees well in literature [29,30] wherein a periodic supercell model was employed. The agreement between the cluster and periodic model with respect to diameter is well exhibited. The model presented in Fig. 1(b) is a representative of biomaterials due to their superior polar characteristics that constitutes a major improvement of unmodified CNTs, since fCNTs have emerged as a new alternative and efficient tool for transporting and translocating therapeutic molecules with improved dispersability and decreased cytotoxicity due to more biocompatible COOH groups [17]. The selectivity of certain reactions has been reported to be diameter dependent as well [2,17] specifically for –COOH groups, the major effects seem to be tube species and diameter. The influence of tube diameter for carbon nanotubes for – COOH adsorption was investigated in order to relate the possible diameter selectivity of the chemical reactions of –COOH with nanotubes. The bond lengths and adsorption energies of –COOH adsorbed on nanotubes of different diameters is analyzed. Adsorption energy, Ea, is the energy required to disassemble a whole system into separate parts. A bound system typically has a lower potential energy than the sum of its constituent parts of the radicals to the nanotube, defined for the reaction –COOH þ SWCNT→COOH–SWCNT;
ð1Þ
where –COOH is the carboxylic radical and COOH-SWCNT is the corresponding functionalized nanotube. Therefore. Ea ¼ ESWCNT þ E–COOH −ECOOH–SWCNT :
ð2Þ
where ESWCNT, E–COOH and ECOOH–SWCNT denote the total energy of the optimized nanotube, the –COOH radical and the sidewallfunctionalized nanotube with –COOH radical, respectively. A stable optimized configuration and bonding is dictated by a thermodynamically favorable functionalization — this is what keeps the system together. As shown in Fig. 3(a) the adsorption energies decrease with increasing diameter having a saw tooth like periodicity with n as an even integer at a higher adsorption energy compared to the odd case. This trend can be explained by examining the band gap as a function of diameter as will be explained later. The structures of both SWCNTs and graphite are made up of the sp2-bonding network of carbon atoms, and the difference of the reactivity in –COOH is therefore deeply related to the local curvature. Our results strongly suggest that it induces the structure deformation from sp2 to sp3-like bonding, depending on the diameter of SWCNTs. As the reactant carbon atoms shift from three to four coordinate, the local preferred geometry correspondingly shifts from planar to tetrahedral. Since nanotubes are obtained by rolling up a graphene sheet, the resultant distortions generate structures with the bond lengths deviating from the ideal value. Smaller diameter gives higher curvature and larger deviations. CNTs possess strain in their highly curved lattice which causes them to have high chemical reactivity; this strain is released through the formation of sp3 covalent sites. The source of graphene's chemical stability comes, in part, from the reduced strain present in its sp2 network as the strain releases from the highly curved CNT with relatively small diameter to bigger ones, the stability increases to the value for graphene due to it resembling closely to an ideal planar structure [31–38]. Furthermore, any local conjugation is broken and the carbon atoms rehybridize from sp2 to sp3. The energy
required inducing such curvature scales inversely with diameter, therefore it is expected that small diameter nanotubes with greater strain can be functionalized more easily than the larger ones. The adsorption energy data plotted in Fig. 3(a) supports this; the most stable adsorption structure is that with the least diameter. Thus reaction can take place with carbon atoms in the nanotube transformed from sp2 to sp3 hybridization. The reactivity is correlated with the extent of this hybridization transformation. Careful analysis of Mulliken charges (Table 1) reveals that, whether for very small or for very large diameter nanotubes, there is charge transfer from the –COOH to the surface of the nanotubes in the range of 0.052 (for the (8, 0) tube) to 0.059 (for the (16, 0) nanotube). The other nanotubes show in-between values. The charge transfer data reveals that maximum charge is transferred from the – COOH moiety to the carbon nanotube as the diameter increases which correlates well with adsorption energy plot. The results presented in Fig. 3(a) also indicate a higher reactivity of zigzag versus armchair SWCNT toward carboxylation. This is in agreement with earlier observations [24]. The product of magnitude of charges and the distance of separation between the charges can be expressed in terms of dipole moments (μdip) as displayed in Table 1 wherein there is a substantial increase in the value of the magnitude of the dipole moments of the functionalized nanotubes both zigzag and armchair configuration compared to the unfunctionalized nanotube which has practically no dipole moment as can be seen quantitatively in the plots of the HOMO–LUMO mapped into the total electron density as shown in Fig. 4, the pure nanotube has localization with respect to the terminal ends whereas the nanotube with a sidewall functional group has localization in the opposite end as far as possible from the organic acid attached. In a nucleophile, we should think about the localization of the HOMO orbital because electrons from this orbital are mostly free to participate in the reaction. Similarly, the frontier orbital theory predicts that a site where the lowest unoccupied orbital is localized serves as a good electrophilic site. The radical has a preference to attach itself at the wall near the terminal end of the tube saturated with hydrogens. After which the succeeding radical attaches as far as possible, typically at the opposite end of the tube again near the hydrogens. Polarizability correlates well with the HOMO–LUMO separations in atoms and molecules. The electron distribution can be distorted readily if the LUMO lies close to the HOMO in energy, compared to the pure nanotube the red spots of the functionalized tube is intense in color, so the polarizability is then larger. The large electric dipole moment of a molecule of the functionalized SWCNT versus the finite pure nanotube model suggests a modification of the functionalized nanotubes interaction with a polar solvent such as water, potentially increasing the dispersion. The dipole moment of the functionalized nanotube decreases monotonically as the diameter increases as shown in Fig. 3(b). This is in agreement with the claim that small diameter SWCNT disperse at higher concentration than do large-diameter SWCNT. Scanning tunneling spectroscopy (STS) experiments confirmed that nanotubes could be either metallic or semi conducting. Observations show that, for narrow carbon nanotubes functionalized with –COOH, the effect of curvature can convert nanotubes, which are expected to be semiconductors into metals. As the band gap decreases with decreasing diameter again with saw tooth like periodicity as displayed in Fig. 3(c) with n as an even integer at a smaller band gap compared to the odd case which makes the (n, 0) tubes with n = 2 k (k is integer) more metallic compared to n = 2 k + 1 upon functionalization of – COOH. This peculiar behavior is reflected implicitly in the adsorption energy plot as shown earlier, the (n, 0) tubes with n = 2 k possess more metallic character has delocalized electron that can intensify the binding of the –COOH compared to n = 2 k + 1 which resulted in a higher adsorption energy for (n, 0) tubes with n = 2 k vs. n = 2 k + 1. The zigzag pattern can be traced in the plot of the HOMO of the pure nanotube as advertised in Fig. 3(d). This was carried over to the functionalized case and was accompanied by the charge transferred
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Fig. 3. The plot of the calculated adsorption energies, band gaps and solvation free energies in electron volts (eV) versus nanotube diameter in angstrom (Å). The graph of the dipole moments in Debye (D) with respect to nantoube diameter is also expressed in angstrom unit. Red curve denotes that Tkatchenko–Scheffler (TS) scheme was adapted in the functional calculation with basis set superposition error (BSSE) correction approach and yellow curve denotes that the armchair (n,n) nanotube with n = 4 to 9 was taken into account as basis for the calculation with a different nanotube chirality.
from the –COOH to the CNT. The inclusion of long range correlation facilitates the accurate determination of vdW correction method wherein Tkatchenko–Scheffler (TS) scheme was adapted in the functional calculation with basis set superposition error (BSSE) correction approach employed by means of counterpoise method (CP) computations that were carried out using DMOL [22]. The adsorption energy values were
updated in Fig. 3(a) and the saw tooth character was also observed prominent. According to rigorous calculations, vdW interactions produce an increase in the average adsorption energy as presented in Fig. 3(a) and consequently reduces the equilibrium distance. The rest of the parameters such as dipole moment in Fig. 3(b) and band gap in Fig. 3(c) have no significant change with respect to the uncorrected
Table 1 Data on functionalization of –COOH and SWCNTs. n
8 9 10 11 12 13 14 15 16
Ea
μdip
ΔGsolv
CCNT
Cc
CO
C–O
CH
HOMO
LUMO
Gap
(eV)
(D)
(eV)
(e)
(e)
(e)
(e)
(e)
(eV)
(eV)
(eV)
0.992 0.827 0.919 0.759 0.843 0.690 0.808 0.672 0.781
1.368 1.337 1.215 1.156 1.123 1.065 1.044 1.006 0.923
−0.298 −0.303 −0.293 −0.299 −0.289 −0.294 −0.287 −0.290 −0.283
−0.334 −0.339 −0.343 −0.347 −0.350 −0.353 −0.353 −0.356 −0.357
0.537 0.537 0.538 0.537 0.539 0.533 0.538 0.540 0.540
−0.382 −0.381 −0.381 −0.380 −0.381 −0.379 −0.380 −0.380 −0.379
−0.392 −0.392 −0.393 −0.392 −0.392 −0.393 −0.392 −0.392 −0.392
0.289 0.289 0.290 0.289 0.289 0.289 0.289 0.290 0.290
−4.027 −4.030 −4.047 −4.124 −4.077 −4.168 −4.095 −4.086 −4.105
−4.003 −3.970 −4.039 −4.034 −4.047 −4.070 −4.049 −3.978 −4.063
0.024 0.059 0.008 0.090 0.030 0.098 0.045 0.107 0.041
CCNT = carbon of the nanotube attached to the COOH, CC = carbon of the COOH attached to the nanotube, CO = oxygen attached to the COOH in a double bond fashion, C–O = oxygen attached to the COOH in a single bond fashion, CH = hydrogen of the COOH radical.
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Fig. 4. HOMO–LUMO of the pure and –COOH functionalized nanotube.
values, as vdW do not influence the spatial configuration of the adsorbed COOH radical. The increase in the adsorption energy can be rooted to the dispersion forces, the nonlocal correlation between electrons. A balance between the long-range attractive vdW forces and the short-range Pauli repulsion mainly determines the equilibrium distance. Apparently, the nonlocal correlations reduce the repulsion between the electrons. Its effect when a proper description of nonlocal interactions is considered allows the functionalized COOH to reach a shorter equilibrium distance, where adsorption energy increases significantly. Interestingly, as the chirality of the nanotube is changed the saw tooth character vanishes completely as in the case of armchair (n, n) nanotubes as shown in Fig. 3(a) and (c). This observation can be attributed to the metallic nature of the armchair nanotubes in which the electrons are already delocalized. The molar Gibbs energy (ΔG) is the chemical potential that is minimized when a system reaches equilibrium at constant pressure and temperature. Its derivative with respect to the reaction coordinate of the system vanishes at the equilibrium point. Measuring the Gibbs free energy of solvation can assess solubility as such; it is a convenient criterion for the spontaneity of the solvation processes with respect to
constant pressure and temperature. The Gibbs free energy of solvation, ΔGsolv, is defined as ΔGsolv ¼ ðEDPCM þ ΔGnonelectrostatic Þ−Egas :
ð3Þ
where E denotes the computed optimized DMOL3 total energy of the species in the vacuum (gas phase), ED-PCM is the total DMOL3/COSMO energy of the species in solvent (D-PCM) as mentioned in the methodology section. The non-electrostatic contribution ΔGnonelectrostatic due to dispersion and cavity effects are estimated from a linear interpolation of the free energies of hydration for linear-chain alkanes as a function of surface area [22]. The magnitude of the ΔGsolv of nanotubes decreases with increasing tube diameter with local maximum occurring at n with multiples of 3 that results from their different π bonding structures for the pure nanotube as shown in Fig. 3(e). Specifically, (9, 0), (12, 0) and (15, 0) exhibits these kind of behavior due to their metallic nature as compared to the other nanotubes as expected. On the other hand, a large negative value of ΔGsolv for all the –COOH functionalized nanotubes as given in Table 1. This implies that the technique of functionalization with –COOH produces soluble species substantially.
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Strong variations of Gibbs free energy of solvation as a function of diameter marked with periodic features was observed wherein the traditional profile in the pure case is now transformed to that of a saw tooth like character as a consequence of the saw tooth shaped band gap plot of the corresponding COOH–SWCNT from n = 8 to 16. Interestingly, for the functionalized case the even integer nanotubes are at a lower solubility compared to the odd numbered case. The zigzag band gap profile of the functionalized tube can also modify its behavior in aqueous media such as water as displayed. It can be seen that aside from the fact that small diameter SWCNT disperse at higher concentration than do largediameter SWCNT as supported in the plot of the Gibbs free energy vs. diameter a saw tooth behavior persists in its profile. However, the armchair configuration does not exhibit any of the periodicity found in the zigzag case. 3.2. Electronic effects of adsorbed –COOH groups The density of functionalization on the sidewalls of the SWCNT can be depicted as a function of the number of adsorbed COOH groups. As an illustrative example, up to 18 –COOH groups are attached to the (10, 0) CNT. However, only 16 –COOH groups attached uniformly to the CNT. For higher concentrations, owing to steric attraction between –COOH groups, the system was observed to have unbounded –COOH groups. However, this process does not destroy the structure of the CNT. Functionalization of CNTs not only modifies the morphology of the functionalized system locally but it also influences the optimized length of the functionalized CNTs based on B3LYP/DNP calculations. The calculated equilibrium length along the symmetry axis (z) of the functionalized (9, 0) CNT as a function of the number of sidewall attached groups generally causes elongation of the tube. The elongation further increases with the increase in concentration of functionalizing molecules. The results clearly show that the functionalization of CNTs with – COOH molecules leads to strong chemisorption of the adsorbants. The value of adsorption energy for one –COOH group bound to (10, 0) CNT containing 80 carbon atoms is 0.919 eV which nicely agrees with other theoretical prediction [35]. For these systems, the adsorption energy per molecule decreases as a function of adsorbants concentration, indicating that CNTs can be saturated with a high density of 16 adsorbants. The destabilization of CNTs is not significant within the critical density of attached –COOH groups. However, −COOH groups induce small local distortions along the radial direction on the tube sidewall. These geometry changes are usually described as the local sp3 rehybridization as mentioned before. The calculations show that the C-C bond length are close to the C–C typical distance in the sp3hybridized diamond (1.54 Å) and significantly larger than the C–C bond length in the perfect graphene sheet constituting the CNT surface with sp2 hybridization (1.42 Å). The strong chemisorption of –COOH species to CNT surface can be also seen in Fig. 4, where the distribution of the HOMO–LUMO is depicted. These HOMO–LUMO rearrangements have mostly local character and the picture does not change essentially with the growing density of the adsorbed molecules. CNT functionalized by –COOH groups can be easily synthesized [8, 22] and exchanged by other groups using standard chemical reactions allowing one to attach to CNTs more complex molecules. The molar Gibbs energy of the pure nanotube that is practically insoluble in water has an average value of about −0.08 eV. Pristine CNTs are not soluble in water or in organic solvents and have tendencies to create bundles, mostly because of dangling π-bonds of the C atoms on CNT's surface, which could build strong covalent bonds among themselves. Using the technique of –COOH sidewall functionalization, the calculated ΔGsolv of the functionalized nanotubes with low to high concentrations of –COOH groups in Table 2 suggests that as the degree of –COOH sidewall functionalization increases, the SWCNT sample becomes more and more soluble. This is in agreement with experimental [8,9,17] and theoretical calculations [16–18] performed, wherein the sidewall
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Table 2 The functionalization density dependence of SWCNTs Gibbs free energy of solvation (ΔGsolv). na
Ea (eV)
μdip (D)
ΔGsolv (eV)
HOMO (eV)
LUMO (eV)
HOMO–LUMO gap (eV)
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16
– 0.919 0.897 0.806 0.875 0.651 0.583 0.521 0.353 0.415 0.372 0.334 0.324 0.278 0.260 0.257 0.251
0.034 1.215 1.147 1.095 1.059 1.039 1.036 1.049 1.078 1.123 1.184 1.262 1.356 1.451 1.597 1.737 1.879
−0.08 −0.29 −0.54 −0.78 −1.06 −1.164 −1.242 −1.282 −1.294 −1.291 −1.281 −1.276 −1.168 −1.324 −1.397 −1.519 −1.425
−4.034 −4.047 −3.920 −3.938 −3.957 −4.077 −4.197 −4.167 −4.136 −4.189 −4.242 −4.047 −4.047 −4.059 −4.012 −4.042 −4.073
−3.619 −4.039 −3.911 −3.924 −3.937 −4.054 −4.172 −4.140 −4.109 −4.161 −4.212 −4.039 −4.039 −4.033 −3.990 −4.028 −4.066
0.281 0.008 0.009 0.014 0.020 0.023 0.025 0.026 0.027 0.028 0.030 0.027 0.027 0.027 0.022 0.014 0.006
a
Number of COOH attached per (10, 0) CNT with 80 C.
functionalized SWCNT are found out to be substantially less cytotoxic than unfunctionalized SWCNT in water. The functionalized (10, 0) CNTs band gap decreases, originating from the approach of the HOMO and LUMO. The near metallic character for higher degree of functionalization is well exhibited at least for the concentrations studied. These results clearly show which type of molecules should be used in order to make semiconducting nanotubes approach its metallic counterpart and give important practical hint for technological applications. The results presented in this paper correspond to the equilibrium situation at 0 K. They will provide the optimized geometry of the functionalized CNTs that is basis for calculations of the electronic structure. However, for many cases the performed molecular dynamics studies (to be published elsewhere) suggest that the functionalized structures remain stable for simulation time of 2 ps and temperatures reaching 1400 K. 4. Conclusions The solubility in water of SWCNT covalently functionalized with formic acid radical is improved under the aide of the density functional theory formalism wherein electronic exchange and correlation effects have been treated with the gradient-corrected (B3LYP) functional/ DNP as basis set implemented within the DMOL code, which uses cluster models and localized atom-centered basis set. The functionalization enhanced the dipole moment of SWCNT expected to modify its interaction with a polar solvent such as water that depends on the tube diameter marked with saw tooth like periodicity which originated from their different π bonding structures manifested in the electronic band gaps. Furthermore as the degree of sidewall functionalization increases the solubility also increases. The cytotoxicity of CNTs has been well addressed through various methods of surface functionalization of CNTs employing C-PCM, thereby improving their interaction within biological systems. Given CNTs' relatively lower toxicity, their surface functionalization is a promising strategy for delivering different biological molecules. Functionalization of their surface can result in highly soluble materials, which can be further derivatized with active molecules, making them compatible with biological systems. Therefore, many biomedical applications can be envisaged. References [1] N. Sinha, J.T.W. Yeow, Carbon nanotubes for biomedical applications, IEEE Trans. Nanobiosci. 4 (2) (2005) 180–195. [2] N. Sinha, J. Ma, J.T.W. Yeow, Carbon nanotube-based sensors, J. Nanosci. Nanotechnol. 6 (3) (2006) 573–590.
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