A first principles study of pristine and Al-doped boron nitridenanotubes interacting with platinum-based anticancer drugs

Ehsan Shakerzadeh n, Siamak NoorizadehComputational Chemistry Group, Chemistry Department, Faculty of Science, Shahid Chamran University, Golestan Street, Ahvaz, Iran

H I G H L I G H T

� Investigating cis- and neda-platininteractions with pristine andAl-doped BNNTs.

� Cis- and neda-platin preferentiallyadsorbed onto Al-doped-BNNT.

� Investigating electronic structures ofthe stable configurations.

� Introducing Al-doped-BNNT as anefficient carrier for cis- and neda-platin.

G R A P H I C A L A B S T R A C T

Interaction of cis-platin and neda-platin with pristine and Al-doped BNNTs is investigated using thedensity functional theory method.

a r t i c l e i n f o

Article history:Received 30 April 2013Received in revised form8 September 2013Accepted 11 September 2013Available online 16 October 2013

Keywords:Boron nitride nanotubeAl-doped boron nitride nanotubeCis-platinNeda-platinDFT calculations

a b s t r a c t

Interaction of cis-platin and neda-platin, two conventional platinum-based anticancer drugs, withpristine [8,8] and Al-doped [8,0] boron nitride nanotubes (BNNTs) are investigated using the densityfunctional theory (DFT) method. The obtained results indicate that cis-platin and neda-platin weaklyinteract with pristine zig zag or armchair BNNTs with a little dependency on the adsorbing positions;while both cis-platin and neda-platin are preferentially adsorbed onto the Al atom of the Al-doped BNNTwith considerable adsorption energies. Therefore the Al-doped-BNNT might be an efficient carrier fordelivery of these drugs in nanomedicine domain. The electronic structures of the stable configurationsare also investigated through both DOS and PDOS spectra. The obtained results introduce the Al-doped-BNNT as an efficient carrier for delivery of cis-platin and neda-platin in nanomedicine domain.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

The antineoplastic activity of cisplatin (cis-Pt), the first platinum-based anticancer drug, is discovered in the late 1960s [1,2].Undoubtedly this drug is one of the most common anticancerchemotherapy drugs, which is particularly effective in treatments oftestis, ovary, esophageal, bladder, non-small-cell lung cancers, headand neck malignancies [3–5]. In spite of the wide anticancerapplications of cis-Pt, its therapeutic efficacy is somewhat compro-mised by the occurrence of serious side effects such as nausea,vomiting, nephrotoxicity as well as development of resistance [6–8].For over three decades, considerable attempts have been made to

design new platinum-based antitumor agents to overcome cis-Ptresistance or enhance its antitumor activity. In addition to cis-Pt,several other platinum-based complexes have been approved fortumor therapy [9]. Neda-platin (neda-Pt) as an alternative anti-tumor agent shows lower nephrotoxicity compared to cis-Pt [10,11].Moreover, in vitro chemosensitivity test suggested that nedaplatinhas equivalent or superior antitumor activity to cis-Pt in cervicalcancer [12]. However, since the optimization of the dosing anddelivery schedule of platinum-based anticancer drugs could bepotentially minimized adverse effects while maintaining theirefficacy [13,14]. It seems that it is necessary to investigate theadsorption properties of these drugs on the selected carrier matrixin order to perform further pharmacological studies [13].

Nanoscale materials have been investigated in many biologicalapplications such as biosensing, biological separation, molecularimaging and anticancer therapy [15–19] because of their novel

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Physica E

1386-9477/$ - see front matter & 2013 Elsevier B.V. All rights reserved.http://dx.doi.org/10.1016/j.physe.2013.09.019

n Corresponding author. Tel.: þ98 611 3332156; fax: þ98 611 3331042.E-mail address: e.shakerzadeh@scu.ac.ir (E. Shakerzadeh).

Physica E 57 (2014) 47–55

properties as well as extreme different functionality comparedto their bulk counterparts. Although carbon nanotubes (CNTs)have been widely explored for these purposes, their inherent

cytotoxicity have limited their biological applications [20–23].Although surface functionlaization could be reduced the cytotoxi-city of CNTs, the possibility of in situ desorption brings consider-able risk for using them in the living organism [24–26]. Nowadays,boron nitride nanotubes (BNNTs) have attracted wide attention innanomedicine domain because of their inherent noncytotoxicitytogether with unique and significant structural, mechanical andelectronic properties [27–32]. They also possess superior chemicaland thermal stability with a high resistance to oxidation. BNNTsare electrical insulators with a wide band gap (�4.00–6.00 eV)and show high thermal conductivity [33]. In addition, they areinsoluble and hydrophobic nanomaterials, which necessarily passthrough the use of biocompatible detergent-like agents, such asamphyphilic or positively charged biomolecules [30]. These uniqueproperties distinguish BNNTs as potentially attractive candidatesfor a wide range of biological applications and innovative tools innanomedicine domain.

In spite of the inherent noncytotoxicity and the chemical inertiaof BNNTs, unfortunately their biomedical applications are almostunexplored from theoretical point of view. During the past fewyears many attempts have been made to investigate the interactionof BNNTs with different molecules [34–36]. It is noteworthy thatresearch on functionalized BNNT is a hot spot for scientists [37]. Theaim of this study is to investigate the interaction of pristine andAl-doped BNNTs with cis- and neda-platin drugs. This study isperformed through finding the changes in the electronic andstructural properties of the considered tubes due to the adsorption

Scheme 1. The structures and atom numbering of (a) cis-Pt and (b) neda-Pt.

Fig. 1. The electronic density of states of the pristine BNNT. (The Fermi level setto zero).

Fig. 2. The obtained different stable configurations for the adsorption of cis-Pt drug on [8,8] pristine BNNT.

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of cis- and neda-platin drugs. It hopes that the obtained resultsprovide new insights to the nanomedicine field.

2. Computational details

The density functional theory [38] method, implemented in SIESTAcomputational code [39,40], is applied to study cis-Pt and neda-Ptadsorptions on pristine [8,8] and [8,0] BNNTs as well as Al-doped [8,0]BNNT. Recently it is reported that SIESTA package is an excellentcomputational tool for predicting the structures and energies ofplatinated systems [41]. Describing the electron exchange-correlationterm is done within the generalized gradient approximation (GGA) inform of Perdew–Burke–Ernzerhof (PBE) correction. The interactionbetween core and valance electrons is treated by Troullier–Martins

norm-conserving pseudopotential [42]. Double-ζ basis sets pluspolarization orbitals (DZP) with an energy cutoff of 150 Ry is utilizedfor all calculations. The relaxed atomic configuration of each system isobtained using the conjugated gradient (CG) technique when the forceis smaller than 0.04 eV/Å on each atom. Since a high dense Brillouinzone sampling is necessary for a better description of physicalquantities, Monkhorst–Pack [43] block sets to 1�1�7, which leadsto convergence of energy. The supercells of [8,8] and [8,0] BNNTs areconsisted of 191 and 160 atoms, respectively, and a periodic boundarycondition is applied to the supercell. A vacuum space of 40 Å is usedwhich ensures that the z-axis of the periodic supercell is great enoughand therefore there is no interaction between BNNTs of adjacentsupercells. Also the lateral distance between drug molecules is at least12 Å to eliminate the interactions between drugs in neighboringsupercells.

Table 1The obtained deformation (Edef ), binding (Ebin), and adsorption (Eads) energies forthe stable configurations of [8,8] pristine BNNT with cis-Pt drug. (All in eV).

Configuration Ebin Eads Edef

1a �0.1600 �0.1105 0.04951b �0.2041 �0.1329 0.07121c �0.3162 �0.2292 0.08701d �0.1769 �0.1060 0.07091e �0.1676 �0.0957 0.07191f �0.2599 �0.1890 0.0709

Fig. 3. The obtained different stable configurations for the adsorption of neda-platin drug on [8,0] pristine BNNT.

Table 2The obtained deformation (Edef ), binding (Ebin), and adsorption (Eads) energies forthe stable configurations of [8,0] pristine BNNT with neda-Pt drug. (All in eV).

Configuration Ebin Eads Edef

2a �0.2979 �0.1642 0.13372b �0.8692 �0.5183 0.35092c �0.4916 �0.2132 0.27842d �0.3097 �0.1097 0.20012e �0.9082 �0.4936 0.41452f �0.5806 �0.2911 0.2895

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3. Results and discussion

In this study, the drug–carrier interaction is investigated usingthe adsorption, binding and deformation energies. The adsorptionenergy (Eads) of the cis-Pt or neda-Pt, depicted in Scheme 1, on theBNNT with or without Al-doping is defined as

Eads ¼ E½BNNTþdrug��ðEiso½BNNTghostþdrug�þEiso½BNNTþdrugghost �Þð1Þ

where E½BNNTþdrug� denotes the total energy of the adduct BNNTwith the corresponding drug molecule. The term Eiso½BNNTghostþdrug� Eiso½BNNTþdrugghost �

� �is the total energy of isolated drug

(BNNT) system plus BNNT (drug) basis wave function. This is thecounterpoise method using “ghost” atoms which ensures theelimination of basis set superposition error (BSSE). Recent studysuggests that the adsorption energy encompasses both binding(Ebin) and deformation (Edef ) energy contributions [44]. Thefollowing equations are used to separate these terms from each

other:

Ebin ¼ E½BNNTþdrug��ðEsp½BNNTghostþdrug�þEsp½BNNTþdrugghost �Þð2Þ

Edef ¼ Eads�Ebin ð3Þwhere Esp½BNNTghostþdrug� Esp½BNNTþdrugghost �� �

denotes the totalenergy of drug (BNNT) system in its relaxed geometry plus BNNT(drug) basis wave function. According to the adsorption energydefinition (Eq. (1)), the negative Eads indicates that the obtainedcomplex is stable and the positive one belongs to the local mini-mum where the adsorption of drug molecule onto the nanotube isprevented by a barrier.

Firstly, the structural optimization of pristine [8,8] and [8,0]BNNTs is performed and the calculated average B–N bond length isfound to be 1.454 Å. Moreover the obtained band gaps for theconsidered tubes are nearly 4.36 eV, which imply to the semi-conductor character of these tubes. The DOS spectrum of pristineBNNT is depicted in Fig. 1. The obtained results are in fair

Fig. 4. The obtained different stable configurations for the adsorption of cis-Pt and neda-Pt drugs on [8,0] Al-doped BNNT from two side views.

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agreements with previously reported values [33]. It is noteworthythat although armchair BNNTs are energetically less stable than zigzag BNNTs, the former have considerable roles in experiments dueto their structural importance [45]. On the other hand, since theB–N bond has great ionic character, the electronic properties ofBNNTs are almost independent of tube diameter and chirality.Therefore it seems that investigations on armchair BNNTs could besimply transferred to stable zig zag BNNTs. Such a methodology isstated by other authors [46]. The studies of cis-Pt and neda-Ptdrugs adsorptions onto BNNTs are discussed separately in the nextsections.

3.1. Adsorption of Cisplatin onto [8,8] Pristine BNNT

In this part the interaction of cis-Pt with [8,8] pristine BNNT isinvestigated. To find the most favorable adsorption structure,several initial configurations including Cl or Pt atom of cis-Ptmolecule approaches to either of the boron or nitrogen atom of thetube as well as Pt atom closes to the hexagon of the BNNT areconsidered. The obtained stable configurations (1a–1e) aredepicted in Fig. 2. Note that some other initial configurations arealso considered, but the results indicate that in all cases therelaxed geometries finally lead to one of these stable configura-tions (1a–1e). For example all of the initial configurations that Clatom closes to the hexagon, zig zag or latitudinal B–N bondschange to the 1c configuration in which Cl atom attached to boronsite of the tube. Also 1f configuration is obtained by putting thecis-Pt molecule inside the tube without imposing any constrain; i.e. the drug just sets in the middle of BNNT in the initial geometry.The obtained deformation (Edef ), binding (Ebin) and adsorption(Eads) energies for these stable configurations are calculated usingEqs. (1–3), which are summarized in Table 1. The results of thistable present that all of the obtained adsorption energies for theconsidered configurations are less than �0.23 eV imply to weakinteraction between the pristine BNNT and drug. Moreover theshortest interaction distances for these configurations (see Fig. 2)are in accordance with the corresponding weak adsorption ener-gies. The most stable configuration is 1c with Eads¼�0.2292 eVand Ebin¼�0.3162 eV in which the cis-Pt molecule adsorbed ontothe boron site of the pristine BNNT through its chlorine atom.Although the cis-Pt molecule is closer to the wall of tube in the 1fconfiguration in comparison with the other configurations (seeFig. 2, the shortest interaction distance is 2.564 Å), the obtainedadsorption energy for this configuration is not considerable(Eads¼�0.1890 eV). However, the obtained results indicate thatin all cases the cis-Pt molecule is interacted weakly with pristineBNNT. Moreover, it is obvious from Table 1 that the calculateddeformation energies for these configurations are negligible.Therefore the pristine BNNT undergoes slightly distortion whenthe cis-Pt molecule is adsorbed on the tube. Hence it seems that inall of these configurations the cis-Pt molecule is just floated on theBNNT and there is no evidence for chemisorption of cis-Pt ontopristine BNNT.

3.2. Adsorption of Nedaplatin onto [8,0] Pristine BNNT

The interaction of neda-Pt drug with pristine [8,0] BNNT isdiscussed in this section. Since neda-Pt drug is bulkier than cis-Pt,a [8,0] BNNT with enough length (20 Å) is selected for studying of itsinteraction with neda-Pt. In this case, various possible initialadsorption configurations are examined. The initial configurationsincluded those cases which O(I), O(II), O(III), H or Pt atom of neda-Ptmolecule (see Scheme 1) closes to the boron atom, the nitrogenatom, center of the hexagon ring and axial or zigzag B–N bond of the[8,0] BNNT (totally 25 configurations). Upon full structural optimiza-tion, just six stable adsorption configurations are found out for the

interaction of neda-Pt with [8,0] BNNT. The obtained configurationstogether with the shortest interaction distances are shown in Fig. 3.The calculated deformation (Edef ), binding (Ebin) and adsorption(Eads) energies for these stable configurations are also listed inTable 2. The results of this table indicate that the most stableconfiguration is 2b with Eads¼�0.5183 eV, in which O(I) atom ofneda-Pt molecule (oxygen of carbonyl group) is linked to the boronatom of the tube. The shortest interaction distance among theobtained configurations also belongs to B–O(I) bond in 2b config-uration (1.620 Å); which is in accordance with the correspondinggreat adsorption energy. Furthermore, it is clear from Table 2 thatthe evaluated deformation energies for these configurations are notsignificant, hence no significant distortion is occurred when theneda-Pt molecule approach to BNNT. According to the inconsider-able adsorption energy values for the neda-Pt adsorption onto

Fig. 5. The electronic charge difference isosurfaces for 3c configuration. (Forinterpretation of the references to color in this figure legend, the reader is referredto the web version of this article.)

Table 3The obtained deformation (Edef ), binding (Ebin), and adsorption (Eads) energies for thestable configurations of [8,0] Al-doped BNNT with cis- and neda-Pt drugs. (All in eV).

Configuration Ebin Eads Edef

Cis-platin3a �1.6517 �1.1905 0.46123b �2.1852 �1.5953 0.5899

Neda-platin3c �3.0970 �2.3687 0.72833d �2.8859 �2.0907 0.79523e �2.3944 �1.8688 0.52563f �1.9409 �1.4719 0.46903g �0.5246 �0.1254 0.3992

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pristine BNNT (in the range of –0.1642 to –0.5183 eV), it seems thatpristine BNNT does not adsorb neda-Pt molecule efficiently.

To sum up, the obtained results reveal that pristine BNNT hasno tendency to adsorb neda-Pt molecule. This may be due to theinert chemical properties of BN materials; hence it seems thatsome modifications are necessary to activate the surface of BNNTfor adsorbing the platinum-based drugs. This modification couldbe performed by replacing one of the B or N atoms of the BNNTwith a metal atom.

3.3. Adsorption of Cisplatin and Nedaplatin onto [8,0]Al-doped-BNNT

In this part it is attempted to investigate the ability ofAl-doped-BNNT for adsorbing cis-Pt and neda-Pt molecules.Therefore a boron atom of the [8,0] BNNT is substituted by onealuminum atom in supercell. Since Al atom is greater than B atom,after full optimization it is found out that the dopant Al atomprotrudes out of the tube plane and causes a drastic distortion inthe region around the doping site to relieve the stress. This causesthat the bond lengths at the doping site are elongated to lAl–N¼1.794 Å from lB–N¼1.453 Å. The diameter of the doped-tube is

6.58 Å, which is increased by 0.1 Å with respect to pristine [8,0]BNNT. It should be mentioned that these variations in bondlengths are related to the distortion of hexagonal structuresadjacent to the Al atom, which is also observed in the case ofAl-doped carbon nanostructure [47–49] as well as boron nitridenanosheet [44]. Neutral charge is present in the Al-doped-nanotube and the singlet multiplicity is taken into account foreach considered configuration.

In order to investigate the adsorption of cis-Pt and neda-Pt ontoAl-doped-BNNT, various possible configurations are considered.After full structural optimization, just seven stable configurations,in which Cl or Pt of cis-Pt as well as O(I), O(II), O(III), H or Pt atom ofneda-Pt molecule are linked to Al atom of tube are obtained (seeFig. 4). The deformation (Edef ), binding (Ebin) and adsorption (Eads)energies for these stable configurations are summarized in Table 3.The shortest interaction distance for each of the mentioned config-urations is also presented in Fig. 4. The results of Table 3 reveal thatwhen cis-Pt molecule is adsorbed through its Cl or Pt atom onto thetube (3a and 3b configurations in Fig. 4), they pull the Al atom outfrom the plane of the tube (deformation energies are 0.4612 and0.5899 eV, respectively). Also, the calculated Eads for 3a and 3bconfigurations are �1.1905 and �1.5953 eV, respectively. Therefore

Fig. 6. DOS and PDOS spectra of the isolated (I) Al-doped BNNT, (II) cis-Pt and (III) neda-Pt molecules. The red, black and blue lines represent s, p and d orbitals, respectively.The Fermi level sets to zero. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

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it seems that the interaction of cis-Pt molecule with the Al-doped-BNNT is more favorable and stronger than with the pristine BNNT.

In the 3a configuration, in which the chlorine atom attached tothe doped aluminum atom, the formed Al–Cl bond (lAl–Cl¼2.342 Å)is nearly perpendicular to the surface of tube. Also three Al–Nbond lengths are increased to 1.84 Å from 1.79 Å after the adsorp-tion of drug. The N–Al–N angle also decreases to 1041 from 1151.Furthermore, in the 3b configuration in which the Pt atom of drugis linked to the Al atom, the formed Al–Pt bond is parallel to thetube's surface and its bond length is 2.483 Å. In this configuration,three Al–N bond lengths are increased to 1.82 Å from 1.79 Å aswell as the N–Al–N angle decreases to 1071 from 1151. Theseresults indicate that the hybridization of Al site is changed tonearly sp3 from sp2 during the adsorption process. Therefore itcould be concluded that the p orbital of the Al atom is responsiblefor the formation of the bond between the drug and tube. Theobtained results imply to the chemisorption of cis-Pt moleculefrom its chlorine and platinum atoms onto Al-doped-BNNT. Thismay be due to the charge accumulation on the Cl and Pt atoms ofcis-Pt molecule and the acidic character of Al site of the tube.

The results of Table 3 present that neda-Pt molecule could bechemisorbed onto Al-doped-BNNT through 3c–3f configurationswith considerable adsorption energies. These configurationsbelong to the attach of O(I), O(II), O(III) and Pt atoms of theneda-Pt to the Al site of the tube. However, approaching the neda-Pt from its CH2 group to the tube (3g configuration) leads to weakinteraction (Eads¼�0.1254 eV). Furthermore the results of Table 3indicate that the complexes formed by chemisorbing of neda-Ptmolecule from its oxygen atoms to the tube are stronger thanthose which are formed by adsorbing the drug through its Pt atom.Moreover, the interaction of O(I) atom of neda-Pt, i.e. oxygen ofcarbonyl group, with the tube (3c configuration) is the strongestone among the other considered configurations. The evaluateddeformation energies for these configurations confirm significantlocal structural reconstruction in the adsorbing site. For instance inthe case of the most stable configuration (3c) the obtaineddeformation energy is 0.7283 eV. In this configuration, the formedAl–O(I) bond is approximately perpendicular to the tube's surfaceand its bond length is 1.831 Å. This interaction distance is the

smallest one among all of the calculated distances; which is in wellagreement with the considerable adsorption energy for thisconfiguration (Ecorrads ¼�2.3687 eV). Also three Al–N bond lengthsare increased to 1.843 Å from 1.791 Å; while the N–Al–N angledecreases to 1061 from 1171 imply to the reduction of p characterof hybridization of tube during the adsorption.

To investigate the changes of charge density in BNNT due to theadsorption of neda-Pt molecule, electron density difference (Δρ) isalso calculated for 3c configuration (the most stable configura-tion). The electron density difference is calculated as follow:

Δρ¼ ρtotal�ðρBNNT þρdrugÞ ð4Þwhere ρtotal denotes electron density of the complex, whileρBNNTand ρdrug show the charge densities of BNNT and drugmolecule in the adsorbed system, respectively. Fig. 5 shows theelectron density difference for the most stable configuration (3c).It should be mentioned that the loss of electron is depicted in blue,whereas the electron accumulation is indicated in red. It is obviousfrom this figure that there is a significant charge accumulationbetween the neda-Pt molecule and the Al-doped-BNNT. Thesecharge transfer prove that the strong binding between the neda-Ptand Al-doped-BNNT induced by adsorption. These results are inaccordance with the significant adsorption energy, which isobtained for this configuration (Eads¼�2.3687 eV). Therefore itseems that the neda-Pt molecule could be interacted considerablywith Al-doped-BNNT through its O(I) atom.

3.4. Electronic density of state (DOS)

To better understand the electronic properties of Al-doped-BNNT during the adsorption of cis-Pt or neda-Pt molecule, itselectronic properties are investigated through the electronicdensity of states (DOS) and partial density of states (PDOS) spectra.These spectra for the pure Al-doped BNNT, cis-Pt and neda-Ptmolecules are presented in parts I, II and III of Fig. 6, respectively.Moreover the PDOS spectra of the atoms which are interacted inthe considered configurations (Pt, Cl, and O atoms of drugs as wellas Al atom of tube) are presented in Fig. 6 for deeper analysis.According to the part I of this figure, replacing one of the boron

Fig. 7. DOS and PDOS spectra of (I) 3a and (II) 3b configurations. The red, black and blue lines represent s, p and d orbitals, respectively. The Fermi level sets to zero. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

E. Shakerzadeh, S. Noorizadeh / Physica E 57 (2014) 47–55 53

atoms by an aluminum atom causes slightly changes in the DOSspectrum compared with the pristine BNNT; i.e. appearing a newpeak at nearly 2 eV which belongs to 2s and 2p orbitals of theAl atom (see Fig. 1). However the band gap of the pure Al-doped-BNNT (3.67 eV) implies to its semiconductor character. Also thecalculated DOS spectra of the pure cis-Pt and neda-Pt molecules

(see parts II and III in Fig. 6) exhibit discrete peaks correspondingto separate energy levels with a wide band gap near the Fermilevel, demonstrates the insulating character of these molecules.

In the case of cis-Pt adsorption onto Al-doped BNNT, theobtained DOS and PDOS spectra for 3a and 3b configurations arepresented in part I and II of Fig. 7, respectively. According to this

Fig. 8. DOS and PDOS spectra of (I) 3c, (II) 3d, (III) 3e and (IV) 3f configurations. The red, black and blue lines represent s, p and d orbitals, respectively. (For interpretation ofthe references to color in this figure legend, the reader is referred to the web version of this article.)

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figure, cis-Pt adsorption onto Al-doped-BNNT leads to a new peakover the Fermi level at nearly 1 eV. This new peak appears near theconducting band, changes the energy gap; i.e. the band gapsreduce to 1.97 and 2.16 eV from 3.67 for 3a and 3b configurations,respectively. Therefore electrons could be transferred more easilyfrom valance level to conducting level after the adsorption of drug.By comparing Fig. 7 with the part I of Fig. 6, it is found out that inthe case of 3a configuration, this new peak is due to the interactionof 2p orbitals of Al atom of the tube with 2p orbitals of Cl atom ofthe cis-Pt molecule. However the interaction of 2p orbitals ofAl atomwith 5d orbitals of Pt atom is responsible of this new peakin the case of 3b configuration. Furthermore the PDOS of Al atomchanges near the Fermi level by the adsorption of cis-Pt moleculethrough its Cl and Pt atoms in this configuration. These observa-tions confirm the strong interaction of cis-Pt molecule with Al-doped BNNT.

For the adsorption of neda-Pt onto Al-doped BNNT, theobtained DOS and PDOS spectra for 3c, 3d, 3e and 3f configura-tions are depicted in parts I, II, III and IV of Fig. 8, respectively. PartI of this figure indicates that the adsorption of neda-Pt moleculethrough its O(I) atom (the most stable configuration among all ofthe considered configurations) leads to new peaks in the range of1–2 eV. These new peaks decrease the band gap of the systemfrom 3.67 to 2.23 eV, and the PDOS near the Fermi level for Alatom is also changed in this configuration. However the PDOS of O(I) atom does not change distinctly. Therefore it seems that the Alatom strongly participated in charge transfer process of thisadsorption system.

In the other considered configurations (See parts II, III and IV ofFig. 8), no obvious change is observed in the corresponding DOSand PDOS spectra; unless the observed 2p orbitals of Al atom inPDOS change drastically near the Fermi level. These observationsclarify the responsibility of Al atom in the charge transfer processduring the adsorption of drug in these configurations.

4. Conclusion

In summary, by performing DFT calculations it is found thatAl-doped-BNNT is an efficient carrier for cis-platin and neda-platindrugs due to the obtained appreciable adsorption energies;whereas these drugs are weakly interacted with pristine BNNT.Furthermore, it is presented that the cis-platin molecule adsorbsstrongly onto Al-doped-BNNT through its chlorine atom, while theoxygen atom of carbonyl group of neda-platin leads to strongadsorption. Moreover, the electronic structures of the stableconfigurations confirm the strong interaction between cis-platinor neda-platin and Al-doped BNNT. It seems that the obtainedresults are meaningful for the potential biological applicationsof BNNTs.

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