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Strong adsorption of Al-doped carbon nanotubes toward cisplatin
Chemical Physics Letters 658 (2016) 162–167
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Strong adsorption of Al-doped carbon nanotubes toward cisplatin Wei Li, Guo-Qing Li ⇑, Xiao-Min Lu, Juan-Juan Ma, Peng-Yu Zeng, Qin-Yu He, Yin-Zhen Wang School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2016 In final form 16 June 2016 Available online 17 June 2016 Keywords: Al-doped CNTs Adsorption Cisplatin Symmetry
a b s t r a c t The adsorption of cisplatin molecule on Al-doped CNTs is investigated using density functional theory. The obtained results indicate that Al-doped carbon nanotubes can strongly absorb cisplatin. After absorbing cisplatin, the symmetry of CNTs has some changes. We innovatively defined a parameter of symmetry variation which relates to the adsorption. By analyzing the electronic structure, it can be concluded that under the circumstance that cisplatin was absorbed by Al-doped CNTs through aluminum atom of Aldoped CNTs. In conclusion, Al-doped CNTs is a kind of potential delivery carrier with high quality for anticancer drug cisplatin. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Iijima found the carbon nanotubes (CNTs) in1991 [1]. With their unique physical and chemical properties as well as with its one-dimensional structure, CNTs has been widely used in many different fields, such as physics, and chemical materials [2–6]. What deserves to be mentioned the most is that CNTs, especially single wall CNTs, has good adsorption properties with a large surface area, stable physical and chemical properties [3–5]. Therefore, CNTs has shown great prospects in applying to biomedical, environmental areas [7–9]. The antitumor properties of cisplatin were accidentally discovered by Rosenberg while examining the influence of electric current on the growth of bacteria [10–12]. Cisplatin can slow down the DNA transcription and replication by managing the cancer cells apoptosis [13]. Cisplatin has been used as an anticancer drug since it was discovered. Due to the fact that chemotherapy drugs for cancer cannot identify differences between cancer cells and normal cells, cisplatin may act on both cancerous and normal cells. And this may lead to the destruction or impairment of vital organs [14]. Apart from the wide use in anticancer, the therapeutic efficacy is somewhat compromised by the occurrence of serious side effects such as nausea, vomiting, nephrotoxicity as well as the resistance to chemotherapy drugs [15–17]. Anticancer drug molecules with the optimization of drug dosage and delivery process of cisplatin anticancer drugs could be precisely delivered to tumor sites for maximum treatment efficacy which can also minimize side effects to normal organs [18,19]. So the investigation of ⇑ Corresponding author. E-mail address: [email protected] (G.-Q. Li). http://dx.doi.org/10.1016/j.cplett.2016.06.040 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
advanced drug delivery systems indicates great promise for improving cancer therapy outcomes [20–23]. The researches showed that the CNTs was able to deliver drugs directly to cancer cells [24,25]. However, the adsorption ability of pristine CNTs is weak [26,27]. But, adsorption performance can be improved by replacing one carbon atom of pristine CNTs with another atom [26–29]. Currently, it is quite rare to adopt the first-principle to analyze the adsorption properties of cisplatin on pristine CNTs and doped CNTs. Therefore, we studied the adsorption performance of cisplatin on Al-doped (7, 7) CNTs by using the first-principle. A new insight to the nanomedicine field was considered with the obtained results. 2. Computational details The pristine (7, 7) CNTs and Al-doped (7, 7) CNTs with diameter (9.49 Å) were studied in this paper. In order to make the carbon atoms of both ends of the nanotubes reach the saturation state, both ends of nanotubes were added with hydrogen atoms. In this study, all the structures investigated were optimized using DMOL3 software package based on density functional theory (DFT) calculations in the present study [30–32]. The description the electron exchange-correlation term was done within the generalized gradient approximation (GGA) in form of Perdew–Burke–Ernzerhof (PBE) [33]correction. The electronic wave functions were expanded in double-numeric polarization basis sets (DNP) with an orbital cutoff of 4.5 Å. Along the tubes, 1 1 6 k-point mesh with Gamma centered Monkhorst–Pack scheme was used for all investigated configurations [34,35]. In order to avoid interaction between adjacent cells, a 30 30 30 Å supercell with periodic bounding conditions was constructed. Structural optimization
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was carried out on all systems with a convergence criterion for energy of 105 Ha/atom. The forces and displacements on each atom were converged to less than 2 103 Ha/Å and 5 103 Ha/Å. Here, the adsorption energy (Eads ) of a cisplatin molecule onto nanotubes was defined to be:
Eads ¼ EðAÞ EðBÞ EðCÞ
ð1Þ
where EðAÞ is the total energy of pristine CNTs or Al-doped CNTs with a cisplatin molecule per supercell, EðBÞ and EðCÞ are the total energy of optimized pristine CNTs or Al-doped-CNTs per supercell and the total energy of isolated cisplatin molecule, respectively. Additionally, a negative of value Eads indicates a favorable adsorption. That is to say, the reaction is exothermic and spontaneous without passing through any barrier. Furthermore, the more negative adsorption energy, the more stable the configuration is. On the other hand, a positive of value Eads indicates a difficult adsorption. 3. Results and discussion 3.1. Adsorption of cisplatin molecule on pristine (7, 7) CNTs In this part, the adsorption properties of cisplatin molecule on pristine (7, 7) CNTs were studied. To find out the most stable adsorption configuration, we consider several kinds of the initial structures which include the Cl, Pt and N atoms of cisplatin molecule close to the outer surface of pristine (7, 7) CNTs and the encapsulation of cisplatin molecule into the pristine (7, 7) CNTs. In the initial configurations, we have tried all kinds of atoms of cisplatin molecule to get close to the pristine (7, 7) CNTs. The selected initial configurations are representative enough. After geometry optimization, the obtained structures were showed in Fig. 1. The adsorption energy and the shortest intermolecular interaction distance were calculated (see Table 1). It is obvious that the configuration of cisplatin molecule adsorbed inside the pristine (7, 7) CNTs has the largest adsorption energy (Eads ¼ 0:24 eV) compared with others. The reason can be that when cisplatin molecule was adsorbed inside the tubes, there are more weak interaction between the molecule and approaching C atoms of the pristine (7, 7) CNTs. The adsorption energy is larger and the configuration is more stable due to the effect of these weak interactions. Even though, the adsorption energy of each model in Fig. 1 is less than 0.24 eV, and the intermolecular distance of each model in Fig. 1 is greater than 3 Å which indicate the weak electron interaction between cisplatin molecule and pristine (7, 7) CNTs. According to the previous reports [36]. We can know that the adsorption process can be regarded as physisorption when the value of adsorption energy is smaller than 0.25 eV. In 1b, 1c, 1d configurations, the bond length (C-C) still is 1.42 Å. The obtained results suggest that the adsorption of cisplatin molecule on pristine
Table 1 The obtained adsorption energies (Eads) and intermolecular distances for the configurations of cisplatin molecule with pristine (7, 7) CNTs. Configuration
Eads (eV)
D (Å)
1a 1b 1c 1d
0.24 0.12 0.23 0.20
– 3.58 3.68 3.98
(7, 7) CNTs is unstable physisorption. To sum up, it is invalid for cisplatin molecule absorbing on pristine (7, 7) CNTs. Hence, It was necessary to increase the adsorption energy by making some modifications on the surface of pristine (7, 7) CNTs. 3.2. Adsorption of cisplatin molecule on Al-doped (7, 7) CNTs In this part, we mainly studied the adsorption of cisplatin molecule on Al-doped (7, 7) CNTs. To construct the Al-doped (7, 7) CNTs model, one carbon atom of pristine (7, 7) CNTs was substituted by an aluminum atom. After optimization, there is a large deformation which lead to the fact that the bond length of lC-C = 1.42 Å turned out to be lAl-C = 1.87 Å. This is because the radius of Al atom is much larger than that of C atom. For the adsorption of cisplatin molecule on Al-doped (7, 7) CNTs, several possible initial structures were considered. In the initial configurations, we have tried all kinds of atoms of cisplatin molecule to get close to the Aldoped (7, 7) CNTs. The selected initial configurations are representative enough. After optimization, six configurations, such as N, Pt or Cl atom of cisplatin molecule was linked to Al atom of Al-doped (7, 7) CNTs as well as cisplatin molecule adsorbed into Al-doped (7, 7) CNTs, were obtained (see 2a, 2b, 2c, 2d, 2e, 2f of Fig. 2). We found the number of sensitive adsorption sites increase. The adsorption energy and the shortest interaction distances of these configurations are all presented in Table 2. Ehsan studied the adsorption of Al-doped boron nitride nanotubes toward cisplatin molecule [37]. From Table 2, we can see that the adsorption energies of 2e and 2f are greater than the value of 1.5 eV in the Ehsan’s research. In addition, after doping Al atom, the CNTs becomes more sensitive to cisplatin molecule which is not so much at first. And the adsorption energies of all most configurations are greater than those of the configurations of Fig. 1. But the results are quite different in the cases of pristine (7, 7) CNTs. In the 2a, 2b and 2c configurations, cisplatin molecule are adsorbed into Al-doped (7, 7) CNTs. The adsorption energy of 2b (0.08 eV) is small. Moreover, the adsorption energies of 2a and 2c are much larger than that of 2b. In 2d, 2e and 2f configurations, cisplatin are adsorbed on Al-doped (7, 7) CNTs. And the adsorption energy of 2d is much smaller than that of 2e and 2f. That shows adsorption energy is not relative to the situation that whether cis-
Fig. 1. The obtained different configurations for the adsorption of cisplatin on pristine (7, 7) CNTs.
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Fig. 2. The obtained different configurations for the adsorption of cisplatin on Al-doped (7, 7) CNTs.
Table 2 The obtained adsorption energies (Eads) and intermolecular distances for the configurations of cisplatin molecule with Al-doped (7, 7) CNTs. Configuration
Eads (eV)
D (Å)
2a 2b 2c 2d 2e 2f
0.74 0.08 0.45 0.08 1.93 1.92
– – – 3.33 2.54 2.55
platin adsorbed on or into Al-doped (7, 7). As far as we know, there is no exact theory to account for such a characteristic. In our opinion, the most important reason may be that the symmetry of the geometry structure is destroyed due to the adsorption. The famous Neumann’s symmetry principle in crystal physics [38–40] has stated: ‘‘The symmetry of any physical property of a crystal must include the symmetry elements of the point group of the crystal.” The symmetry is an important structure parameter of crystal which has significant influence on system stability. The symmetry variation of Al-doped (7, 7) CNTs is very complex in the process of adsorption. We mainly care about the relationship between symmetry variation near the Al atom and the adsorption energy. In order to describe the symmetry variation of Al-doped (7, 7) CNTs, the symmetry variation factor was defined to be:
d ¼ a2 a1
ð2Þ
where a1 is the average bond angle of three \C-Al-C of free Aldoped (7, 7) CNTs, and a2 is the average band angle of three \CAl-C of Al-doped (7, 7) CNTs, after adsorption. Table 3 listed the symmetry variation factor d of different configurations.
Table 3 The values of symmetry variation factor d for the configurations of cisplatin molecule with Al-doped (7, 7) CNTs. Configuration
2a
2b
2c
2d
2e
2f
d
0.99
0.63
0.76
0.58
6.81
6.62
The adsorption energy and symmetry variation factor d are showed in Tables 2 and 3. It is noted that the relationship between adsorption energy and symmetry variation factor d has a general trend which the adsorption energy increase with decreasing the value of d. And the adsorption energy is very small when d greater than zero. The adsorption energy is larger when d less than zero. The maximum adsorption energy appears in 2e (d ¼ 6:81), and the minimum adsorption energy appears in 2b (d ¼ 0:63). In the 2d configuration in which the N atom attached to the doped Al atom, the formed N-Al bond (lN-Al = 3.33 Å) is nearly perpendicular to the surface of nanotubes. According the Table 2, we can see that the adsorption energy (Eads ¼ 0:08 eV) of 2d configuration is very small, and the distance between N atom of cisplatin molecule and Al atom of Al-doped (7, 7) CNTs is far. These imply to the unreliable physisorption of cisplatin molecule from its N atom onto Al-doped (7, 7) CNTs. Furthermore, in 2e and 2f configurations which the Pt atom of drug is linked to the Al atom, the formed Al-Pt bond (lAl-Pt = 2.54 Å) of 2e and Al-Pt bond (lAl-Pt = 2.55 Å) of 2e are nearly perpendicular to the surface of nanotubes. The results of Table 2 present that the interaction between cisplatin molecule and Al-doped (7, 7) CNTs is stronger than pristine (7, 7) CNTs. In addition, cisplatin molecule can be chemisorbed strongly onto Al-doped (7, 7) CNTs through the interaction between Pt atom attach to Al atom.
W. Li et al. / Chemical Physics Letters 658 (2016) 162–167
Total electron density was calculated so as to deeply analyze the interaction between cisplatin molecule and Al-doped (7, 7) CNTs during the adsorption process. Fig. 3 demonstrated the total electron density maps of 2d, 2e and 2f configurations. The bright region showed that concentration of many electrons was an active region available for chemical reaction. Besides, the overlap of different parts of electron density indicated that chemical band would be formed between the adjacent segments. As can be seen from the part I of Fig. 3, there is no overlapped area for cisplatin molecule and Al-doped (7, 7) CNTs during the adsorption process. Indeed, adsorbed cisplatin molecule far from the nanotubes edge would have no effect on the electronic charge distribution of Al atoms of the Al-doped (7, 7) CNTs. They indicate that the unreliable physisorption of cisplatin molecule from its N atom onto Al-doped (7, 7) CNTs. In contrast, the more favorable configurations (2e, 2f) shows a large overlap area of total electron density from the part II, III of Fig. 3. The large area overlap of electronic density and high value of total electron density by 0.2 for 2e and 2f configurations reveal strong chemisorption between cisplatin molecule and Al-doped (7, 7) CNTs. The obtained electron density maps are consistent with adsorption energies. Therefore, cisplatin molecule can be strongly adsorbed onto Al-doped (7, 7) CNTs through its Pt atom. 3.3. Electronic density of state To better understand the electronic properties of Al-doped (7, 7) CNTs and cisplatin molecule during the adsorption, we investigated the electronic density of states (DOS) spectrum. The spectrum for the cisplatin molecule is presented in Fig. 4 and the Fermi level is represented by the 0 eV in the horizontal coordinates. The DOS spectrum of the pure cisplatin molecule (see Fig. 4) exhibits discrete peaks corresponding to separate energy levels with a wide band gap near the Fermi level, which demonstrates the insulation character of cisplatin. This is corresponding to the results in literature [37]. In order to further analyze the interaction between Al-doped CNTs and cisplatin molecule, the obtained electronic partial density of states (PDOS) spectra of 2d, 2e, 2f configurations and the free Al-doped CNTs, cisplatin molecule are presented in part I, II, III, IV, V of Fig. 5, respectively. According to part I of Fig. 5, there is not any overlap around the Fermi level between the PDOS spectra of Al atom of Al-doped (7, 7) CNTs and the N atom of the cisplatin molecule, indicating weak interaction between Al atom of Al-doped (7, 7) CNTs and the N atom of the cisplatin molecule. Moreover, from the part I and IV of Fig. 5, the PDOS spectra of Al atom rarely change, which indicates that cisplatin molecule has little effect on the electronic state of Al atoms of Al-doped CNTs. Therefore, cisplatin molecule can’t be adsorbed strongly on
165
Fig. 4. DOS spectrum of the isolated cisplatin molecule.
Al-doped (7, 7) CNTs through its N atom. But there is a large overlap around the 3.5 eV between the PDOS spectra of aluminum atom and the Pt atom which indicates strong hybridization between the s and p orbital of Al atom and p, d orbital of Pt atom as that can be seen from the part II of Fig. 5. Therefore, chemical bond between Al atom and Pt atom is formed. These results suggest that the cisplatin molecule can be adsorbed onto the Al atom of Al-doped (7, 7) CNTs through its Pt atom. As we can see from the part III of Fig. 5, the PDOS of s and p orbital of Pt atom and p, d orbital of Pt atom exhibit overlapped peaks near the Fermi level which proves that there is strong interaction between Al atom and Pt atom. The bond between Al atom and Pt atom is stable which means that exist strong chemical adsorption between cisplatin molecule and Al-doped (7, 7) CNTs. Furthermore, from the part II, III and V of Fig. 5, it can be seen the d orbital of Pt atom near the Fermi level split. One new peak appears at 0.5 eV while another at 4.5 eV which leads to the decreasing energy of system and better cisplatin adsorption on Al-doped (7, 7) CNTs. Furthermore, the PDOS spectra of p orbital of Al atom occur significant changes, many peaks disappear due to the adsorption. The electronic states of Al atom and Pt atom change obviously due to the adsorption which indicate that the interaction between Al atom and Pt atom is strong. The obtained results of PDOS spectra are well agreed with adsorption energies and total electron density. Therefore, the Aldoped (7, 7) CNTs have excellent adsorption properties for cisplatin. Furthermore, the Al-doped (7, 7) CNTs have a great potentiality to be a delivery carrier material for cisplatin drugs.
Fig. 3. The electron density map for (I) 2d, (II) 2e, (III) 2f configurations.
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Fig. 5. PDOS spectra of (I) 2d, (II) 2e, (III) 2f configurations and (IV) free Al-doped CNTs, (V) free cisplatin molecule.
4. Conclusion In summary, by performing DFT calculations we find that Aldoped (7, 7) CNTs has higher adsorption energy than pristine (7, 7) CNTs. There has been a certain relationship between adsorption energy and symmetry variation of Al-doped (7, 7) CNTs. In order to describe the relationship between symmetry variation of Al-doped (7, 7) CNTs and the adsorption ability, we creatively define a symmetry variation factor d. It shows that the smaller the value of the symmetry variation factor d, the
better the adsorption ability of Al-doped CNTs is. Furthermore, the electronic structures of the stable configurations confirm the strong interaction between cisplatin and Al-doped CNTs. Moreover, it presents that the cisplatin molecule can be strongly adsorbed onto Al-doped CNTs through Pt atom, while the cisplatin molecule is adsorbed weakly onto Al-doped CNTs through other atoms. Strong adsorption can guarantee the drug not to fall off in the carrier trasport. So Al-doped CNTs should be a kind of potential high quality delivery carrier for anticancer drug cisplatin.
W. Li et al. / Chemical Physics Letters 658 (2016) 162–167
Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No. 51172078) and the Young Teacher Research Program of South China Normal University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Strong adsorption of Al-doped carbon nanotubes toward cisplatin Wei Li, Guo-Qing Li ⇑, Xiao-Min Lu, Juan-Juan Ma, Peng-Yu Zeng, Qin-Yu He, Yin-Zhen Wang School of Physics and Telecommunication Engineering, South China Normal University, Guangzhou 510006, PR China
a r t i c l e
i n f o
Article history: Received 9 April 2016 In final form 16 June 2016 Available online 17 June 2016 Keywords: Al-doped CNTs Adsorption Cisplatin Symmetry
a b s t r a c t The adsorption of cisplatin molecule on Al-doped CNTs is investigated using density functional theory. The obtained results indicate that Al-doped carbon nanotubes can strongly absorb cisplatin. After absorbing cisplatin, the symmetry of CNTs has some changes. We innovatively defined a parameter of symmetry variation which relates to the adsorption. By analyzing the electronic structure, it can be concluded that under the circumstance that cisplatin was absorbed by Al-doped CNTs through aluminum atom of Aldoped CNTs. In conclusion, Al-doped CNTs is a kind of potential delivery carrier with high quality for anticancer drug cisplatin. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction Iijima found the carbon nanotubes (CNTs) in1991 [1]. With their unique physical and chemical properties as well as with its one-dimensional structure, CNTs has been widely used in many different fields, such as physics, and chemical materials [2–6]. What deserves to be mentioned the most is that CNTs, especially single wall CNTs, has good adsorption properties with a large surface area, stable physical and chemical properties [3–5]. Therefore, CNTs has shown great prospects in applying to biomedical, environmental areas [7–9]. The antitumor properties of cisplatin were accidentally discovered by Rosenberg while examining the influence of electric current on the growth of bacteria [10–12]. Cisplatin can slow down the DNA transcription and replication by managing the cancer cells apoptosis [13]. Cisplatin has been used as an anticancer drug since it was discovered. Due to the fact that chemotherapy drugs for cancer cannot identify differences between cancer cells and normal cells, cisplatin may act on both cancerous and normal cells. And this may lead to the destruction or impairment of vital organs [14]. Apart from the wide use in anticancer, the therapeutic efficacy is somewhat compromised by the occurrence of serious side effects such as nausea, vomiting, nephrotoxicity as well as the resistance to chemotherapy drugs [15–17]. Anticancer drug molecules with the optimization of drug dosage and delivery process of cisplatin anticancer drugs could be precisely delivered to tumor sites for maximum treatment efficacy which can also minimize side effects to normal organs [18,19]. So the investigation of ⇑ Corresponding author. E-mail address: [email protected] (G.-Q. Li). http://dx.doi.org/10.1016/j.cplett.2016.06.040 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
advanced drug delivery systems indicates great promise for improving cancer therapy outcomes [20–23]. The researches showed that the CNTs was able to deliver drugs directly to cancer cells [24,25]. However, the adsorption ability of pristine CNTs is weak [26,27]. But, adsorption performance can be improved by replacing one carbon atom of pristine CNTs with another atom [26–29]. Currently, it is quite rare to adopt the first-principle to analyze the adsorption properties of cisplatin on pristine CNTs and doped CNTs. Therefore, we studied the adsorption performance of cisplatin on Al-doped (7, 7) CNTs by using the first-principle. A new insight to the nanomedicine field was considered with the obtained results. 2. Computational details The pristine (7, 7) CNTs and Al-doped (7, 7) CNTs with diameter (9.49 Å) were studied in this paper. In order to make the carbon atoms of both ends of the nanotubes reach the saturation state, both ends of nanotubes were added with hydrogen atoms. In this study, all the structures investigated were optimized using DMOL3 software package based on density functional theory (DFT) calculations in the present study [30–32]. The description the electron exchange-correlation term was done within the generalized gradient approximation (GGA) in form of Perdew–Burke–Ernzerhof (PBE) [33]correction. The electronic wave functions were expanded in double-numeric polarization basis sets (DNP) with an orbital cutoff of 4.5 Å. Along the tubes, 1 1 6 k-point mesh with Gamma centered Monkhorst–Pack scheme was used for all investigated configurations [34,35]. In order to avoid interaction between adjacent cells, a 30 30 30 Å supercell with periodic bounding conditions was constructed. Structural optimization
163
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was carried out on all systems with a convergence criterion for energy of 105 Ha/atom. The forces and displacements on each atom were converged to less than 2 103 Ha/Å and 5 103 Ha/Å. Here, the adsorption energy (Eads ) of a cisplatin molecule onto nanotubes was defined to be:
Eads ¼ EðAÞ EðBÞ EðCÞ
ð1Þ
where EðAÞ is the total energy of pristine CNTs or Al-doped CNTs with a cisplatin molecule per supercell, EðBÞ and EðCÞ are the total energy of optimized pristine CNTs or Al-doped-CNTs per supercell and the total energy of isolated cisplatin molecule, respectively. Additionally, a negative of value Eads indicates a favorable adsorption. That is to say, the reaction is exothermic and spontaneous without passing through any barrier. Furthermore, the more negative adsorption energy, the more stable the configuration is. On the other hand, a positive of value Eads indicates a difficult adsorption. 3. Results and discussion 3.1. Adsorption of cisplatin molecule on pristine (7, 7) CNTs In this part, the adsorption properties of cisplatin molecule on pristine (7, 7) CNTs were studied. To find out the most stable adsorption configuration, we consider several kinds of the initial structures which include the Cl, Pt and N atoms of cisplatin molecule close to the outer surface of pristine (7, 7) CNTs and the encapsulation of cisplatin molecule into the pristine (7, 7) CNTs. In the initial configurations, we have tried all kinds of atoms of cisplatin molecule to get close to the pristine (7, 7) CNTs. The selected initial configurations are representative enough. After geometry optimization, the obtained structures were showed in Fig. 1. The adsorption energy and the shortest intermolecular interaction distance were calculated (see Table 1). It is obvious that the configuration of cisplatin molecule adsorbed inside the pristine (7, 7) CNTs has the largest adsorption energy (Eads ¼ 0:24 eV) compared with others. The reason can be that when cisplatin molecule was adsorbed inside the tubes, there are more weak interaction between the molecule and approaching C atoms of the pristine (7, 7) CNTs. The adsorption energy is larger and the configuration is more stable due to the effect of these weak interactions. Even though, the adsorption energy of each model in Fig. 1 is less than 0.24 eV, and the intermolecular distance of each model in Fig. 1 is greater than 3 Å which indicate the weak electron interaction between cisplatin molecule and pristine (7, 7) CNTs. According to the previous reports [36]. We can know that the adsorption process can be regarded as physisorption when the value of adsorption energy is smaller than 0.25 eV. In 1b, 1c, 1d configurations, the bond length (C-C) still is 1.42 Å. The obtained results suggest that the adsorption of cisplatin molecule on pristine
Table 1 The obtained adsorption energies (Eads) and intermolecular distances for the configurations of cisplatin molecule with pristine (7, 7) CNTs. Configuration
Eads (eV)
D (Å)
1a 1b 1c 1d
0.24 0.12 0.23 0.20
– 3.58 3.68 3.98
(7, 7) CNTs is unstable physisorption. To sum up, it is invalid for cisplatin molecule absorbing on pristine (7, 7) CNTs. Hence, It was necessary to increase the adsorption energy by making some modifications on the surface of pristine (7, 7) CNTs. 3.2. Adsorption of cisplatin molecule on Al-doped (7, 7) CNTs In this part, we mainly studied the adsorption of cisplatin molecule on Al-doped (7, 7) CNTs. To construct the Al-doped (7, 7) CNTs model, one carbon atom of pristine (7, 7) CNTs was substituted by an aluminum atom. After optimization, there is a large deformation which lead to the fact that the bond length of lC-C = 1.42 Å turned out to be lAl-C = 1.87 Å. This is because the radius of Al atom is much larger than that of C atom. For the adsorption of cisplatin molecule on Al-doped (7, 7) CNTs, several possible initial structures were considered. In the initial configurations, we have tried all kinds of atoms of cisplatin molecule to get close to the Aldoped (7, 7) CNTs. The selected initial configurations are representative enough. After optimization, six configurations, such as N, Pt or Cl atom of cisplatin molecule was linked to Al atom of Al-doped (7, 7) CNTs as well as cisplatin molecule adsorbed into Al-doped (7, 7) CNTs, were obtained (see 2a, 2b, 2c, 2d, 2e, 2f of Fig. 2). We found the number of sensitive adsorption sites increase. The adsorption energy and the shortest interaction distances of these configurations are all presented in Table 2. Ehsan studied the adsorption of Al-doped boron nitride nanotubes toward cisplatin molecule [37]. From Table 2, we can see that the adsorption energies of 2e and 2f are greater than the value of 1.5 eV in the Ehsan’s research. In addition, after doping Al atom, the CNTs becomes more sensitive to cisplatin molecule which is not so much at first. And the adsorption energies of all most configurations are greater than those of the configurations of Fig. 1. But the results are quite different in the cases of pristine (7, 7) CNTs. In the 2a, 2b and 2c configurations, cisplatin molecule are adsorbed into Al-doped (7, 7) CNTs. The adsorption energy of 2b (0.08 eV) is small. Moreover, the adsorption energies of 2a and 2c are much larger than that of 2b. In 2d, 2e and 2f configurations, cisplatin are adsorbed on Al-doped (7, 7) CNTs. And the adsorption energy of 2d is much smaller than that of 2e and 2f. That shows adsorption energy is not relative to the situation that whether cis-
Fig. 1. The obtained different configurations for the adsorption of cisplatin on pristine (7, 7) CNTs.
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Fig. 2. The obtained different configurations for the adsorption of cisplatin on Al-doped (7, 7) CNTs.
Table 2 The obtained adsorption energies (Eads) and intermolecular distances for the configurations of cisplatin molecule with Al-doped (7, 7) CNTs. Configuration
Eads (eV)
D (Å)
2a 2b 2c 2d 2e 2f
0.74 0.08 0.45 0.08 1.93 1.92
– – – 3.33 2.54 2.55
platin adsorbed on or into Al-doped (7, 7). As far as we know, there is no exact theory to account for such a characteristic. In our opinion, the most important reason may be that the symmetry of the geometry structure is destroyed due to the adsorption. The famous Neumann’s symmetry principle in crystal physics [38–40] has stated: ‘‘The symmetry of any physical property of a crystal must include the symmetry elements of the point group of the crystal.” The symmetry is an important structure parameter of crystal which has significant influence on system stability. The symmetry variation of Al-doped (7, 7) CNTs is very complex in the process of adsorption. We mainly care about the relationship between symmetry variation near the Al atom and the adsorption energy. In order to describe the symmetry variation of Al-doped (7, 7) CNTs, the symmetry variation factor was defined to be:
d ¼ a2 a1
ð2Þ
where a1 is the average bond angle of three \C-Al-C of free Aldoped (7, 7) CNTs, and a2 is the average band angle of three \CAl-C of Al-doped (7, 7) CNTs, after adsorption. Table 3 listed the symmetry variation factor d of different configurations.
Table 3 The values of symmetry variation factor d for the configurations of cisplatin molecule with Al-doped (7, 7) CNTs. Configuration
2a
2b
2c
2d
2e
2f
d
0.99
0.63
0.76
0.58
6.81
6.62
The adsorption energy and symmetry variation factor d are showed in Tables 2 and 3. It is noted that the relationship between adsorption energy and symmetry variation factor d has a general trend which the adsorption energy increase with decreasing the value of d. And the adsorption energy is very small when d greater than zero. The adsorption energy is larger when d less than zero. The maximum adsorption energy appears in 2e (d ¼ 6:81), and the minimum adsorption energy appears in 2b (d ¼ 0:63). In the 2d configuration in which the N atom attached to the doped Al atom, the formed N-Al bond (lN-Al = 3.33 Å) is nearly perpendicular to the surface of nanotubes. According the Table 2, we can see that the adsorption energy (Eads ¼ 0:08 eV) of 2d configuration is very small, and the distance between N atom of cisplatin molecule and Al atom of Al-doped (7, 7) CNTs is far. These imply to the unreliable physisorption of cisplatin molecule from its N atom onto Al-doped (7, 7) CNTs. Furthermore, in 2e and 2f configurations which the Pt atom of drug is linked to the Al atom, the formed Al-Pt bond (lAl-Pt = 2.54 Å) of 2e and Al-Pt bond (lAl-Pt = 2.55 Å) of 2e are nearly perpendicular to the surface of nanotubes. The results of Table 2 present that the interaction between cisplatin molecule and Al-doped (7, 7) CNTs is stronger than pristine (7, 7) CNTs. In addition, cisplatin molecule can be chemisorbed strongly onto Al-doped (7, 7) CNTs through the interaction between Pt atom attach to Al atom.
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Total electron density was calculated so as to deeply analyze the interaction between cisplatin molecule and Al-doped (7, 7) CNTs during the adsorption process. Fig. 3 demonstrated the total electron density maps of 2d, 2e and 2f configurations. The bright region showed that concentration of many electrons was an active region available for chemical reaction. Besides, the overlap of different parts of electron density indicated that chemical band would be formed between the adjacent segments. As can be seen from the part I of Fig. 3, there is no overlapped area for cisplatin molecule and Al-doped (7, 7) CNTs during the adsorption process. Indeed, adsorbed cisplatin molecule far from the nanotubes edge would have no effect on the electronic charge distribution of Al atoms of the Al-doped (7, 7) CNTs. They indicate that the unreliable physisorption of cisplatin molecule from its N atom onto Al-doped (7, 7) CNTs. In contrast, the more favorable configurations (2e, 2f) shows a large overlap area of total electron density from the part II, III of Fig. 3. The large area overlap of electronic density and high value of total electron density by 0.2 for 2e and 2f configurations reveal strong chemisorption between cisplatin molecule and Al-doped (7, 7) CNTs. The obtained electron density maps are consistent with adsorption energies. Therefore, cisplatin molecule can be strongly adsorbed onto Al-doped (7, 7) CNTs through its Pt atom. 3.3. Electronic density of state To better understand the electronic properties of Al-doped (7, 7) CNTs and cisplatin molecule during the adsorption, we investigated the electronic density of states (DOS) spectrum. The spectrum for the cisplatin molecule is presented in Fig. 4 and the Fermi level is represented by the 0 eV in the horizontal coordinates. The DOS spectrum of the pure cisplatin molecule (see Fig. 4) exhibits discrete peaks corresponding to separate energy levels with a wide band gap near the Fermi level, which demonstrates the insulation character of cisplatin. This is corresponding to the results in literature [37]. In order to further analyze the interaction between Al-doped CNTs and cisplatin molecule, the obtained electronic partial density of states (PDOS) spectra of 2d, 2e, 2f configurations and the free Al-doped CNTs, cisplatin molecule are presented in part I, II, III, IV, V of Fig. 5, respectively. According to part I of Fig. 5, there is not any overlap around the Fermi level between the PDOS spectra of Al atom of Al-doped (7, 7) CNTs and the N atom of the cisplatin molecule, indicating weak interaction between Al atom of Al-doped (7, 7) CNTs and the N atom of the cisplatin molecule. Moreover, from the part I and IV of Fig. 5, the PDOS spectra of Al atom rarely change, which indicates that cisplatin molecule has little effect on the electronic state of Al atoms of Al-doped CNTs. Therefore, cisplatin molecule can’t be adsorbed strongly on
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Fig. 4. DOS spectrum of the isolated cisplatin molecule.
Al-doped (7, 7) CNTs through its N atom. But there is a large overlap around the 3.5 eV between the PDOS spectra of aluminum atom and the Pt atom which indicates strong hybridization between the s and p orbital of Al atom and p, d orbital of Pt atom as that can be seen from the part II of Fig. 5. Therefore, chemical bond between Al atom and Pt atom is formed. These results suggest that the cisplatin molecule can be adsorbed onto the Al atom of Al-doped (7, 7) CNTs through its Pt atom. As we can see from the part III of Fig. 5, the PDOS of s and p orbital of Pt atom and p, d orbital of Pt atom exhibit overlapped peaks near the Fermi level which proves that there is strong interaction between Al atom and Pt atom. The bond between Al atom and Pt atom is stable which means that exist strong chemical adsorption between cisplatin molecule and Al-doped (7, 7) CNTs. Furthermore, from the part II, III and V of Fig. 5, it can be seen the d orbital of Pt atom near the Fermi level split. One new peak appears at 0.5 eV while another at 4.5 eV which leads to the decreasing energy of system and better cisplatin adsorption on Al-doped (7, 7) CNTs. Furthermore, the PDOS spectra of p orbital of Al atom occur significant changes, many peaks disappear due to the adsorption. The electronic states of Al atom and Pt atom change obviously due to the adsorption which indicate that the interaction between Al atom and Pt atom is strong. The obtained results of PDOS spectra are well agreed with adsorption energies and total electron density. Therefore, the Aldoped (7, 7) CNTs have excellent adsorption properties for cisplatin. Furthermore, the Al-doped (7, 7) CNTs have a great potentiality to be a delivery carrier material for cisplatin drugs.
Fig. 3. The electron density map for (I) 2d, (II) 2e, (III) 2f configurations.
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Fig. 5. PDOS spectra of (I) 2d, (II) 2e, (III) 2f configurations and (IV) free Al-doped CNTs, (V) free cisplatin molecule.
4. Conclusion In summary, by performing DFT calculations we find that Aldoped (7, 7) CNTs has higher adsorption energy than pristine (7, 7) CNTs. There has been a certain relationship between adsorption energy and symmetry variation of Al-doped (7, 7) CNTs. In order to describe the relationship between symmetry variation of Al-doped (7, 7) CNTs and the adsorption ability, we creatively define a symmetry variation factor d. It shows that the smaller the value of the symmetry variation factor d, the
better the adsorption ability of Al-doped CNTs is. Furthermore, the electronic structures of the stable configurations confirm the strong interaction between cisplatin and Al-doped CNTs. Moreover, it presents that the cisplatin molecule can be strongly adsorbed onto Al-doped CNTs through Pt atom, while the cisplatin molecule is adsorbed weakly onto Al-doped CNTs through other atoms. Strong adsorption can guarantee the drug not to fall off in the carrier trasport. So Al-doped CNTs should be a kind of potential high quality delivery carrier for anticancer drug cisplatin.
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Acknowledgements This work was supported by the National Nature Science Foundation of China (Grant No. 51172078) and the Young Teacher Research Program of South China Normal University. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19]
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