Multimodal, pH Sensitive, and Magnetically Assisted Carrier of

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Multimodal, pH sensitive and magnetically assisted carrier of doxorubicin designed and analyzed by means of computer simulations Pawel Wolski, Krzysztof Nieszporek, and Tomasz Panczyk Langmuir, Just Accepted Manuscript • Publication Date (Web): 27 Jan 2018 Downloaded from http://pubs.acs.org on January 27, 2018

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Multimodal, pH sensitive and magnetically assisted carrier of doxorubicin designed and analyzed by means of computer simulations Pawel Wolski1, Krzysztof Nieszporek,2 Tomasz Panczyk1,* 1

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences ul. Niezapominajek 8, 30239 Cracow, Poland e-mail: [email protected] phone: +48 815375620; fax:+48 815375685 2

Department of Chemistry, Maria Curie-Sklodowska University pl. M. Curie-Sklodowskiej 3, 20031 Lublin, Poland

Abstract This work deals with an analysis of drugs carriers based on the structure of a carbon nanotube using large-scale atomistic molecular-dynamics simulations. The analyzed systems link several functions in a single architecture. They are: (i) the sidewalls and tips of carbon nanotubes are covalently functionalized by polyethylene glycol – folic acid conjugates and this approach allows for creation of hydrophyllic and biocompatible systems; (ii) doxorubicin is kept in the internal space of carbon nanotube as a mixture with dyes (p-phenylenediamine or neutral red) – it allows for pH controlled release or alteration of the interactions topology; (iii) the mixture of doxorubicin and dyes in the nanotube interior is additionally sealed by fullerenes nanoparticles which act as pistons at acidic pH and loosen the tangle of polyethylene glycol chains at the nanotube tips. This enhances the release of doxorubicin from the nanotube when compared to the analogous system but without the fullerene caps; (iv) another function of the carrier can be activated by filling of the fullerenes by magnetic material – then the carrier can be visualized by means of magnetic resonance imaging, it can realize magnetic hyperthermia of tumor cells and intense rotation of the nanoparticles can be induced by the application of an external magnetic field. That rotation enhances the release of doxorubicin from the nanotube and leads to the increase of the rotational temperature. The studies show that the proposed design of the drugs/doxorubicin carrier reveals very promising properties. It fabrication is absolutely feasible as all individual stages necessary for its construction have been confirmed in the literature.

Keywords: carbon nanotube; doxorubicin; folic acid; drug delivery; magnetic nanoparticle, tumor microenvironment *

Corresponding Author, e-mail: [email protected] phone: +48 815375620, fax: +48 815375685 ACS Paragon Plus Environment

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Introduction

The reduction of extracellular pH at tumor sites is one of natural chemical signals of cancer.1,2 It can be utilized as a triggering factor for drugs unloading from their carriers and in this way realize the active delivery mechanism of drugs. pH responsive liposomes, micelles or various polymeric nanoparticles are considered as carriers of drugs taking advantage of pH change from the neutral to the acidic one in the extracellular tumor mass. 3-6 Carbon nanotubes (CNT) are considered as promising carriers of drugs due to their unique physical and chemical properties.7,8 They reveal igh mechanical strength, needle-like shape, large external surface area, large inner volume and ability of attachment of various functional groups to the tips or to the sidewalls. The drawbacks are insolubility in aqueous media and possible toxicity. However, both these problems can be removed by chemical treatments leading to shortening, purification, and development of hydrophilic functional groups located at the nanotube tips and sidewalls.9 CNTs have been studied as carriers of various small molecule anticancer drugs including doxorubicin, DOX.7 Interestingly, enhanced release of DOX from the conjugates with CNTs often occurs at acidic pH.10-13 Because the protonation pKa of DOX amino group is above 8 14,15 the observed in the literature enhanced release of DOX when pH drops from 7.4 to ca. 6 – 5.5 cannot be attributed to the mentioned protonation process only. CNTs toxicity should be taken into account when their application as drugs carriers is considered. This is because it has been demonstrated that the biodistribution, long-term fate and toxicity of CNTs are closely associated with their surface chemistry, size, doses, and administration routes. Generally, it is recognized that larger and pristine CNTs (which are hardly soluble in aqueous media) would form bundles and aggregates that induce inflammation.16,17 On the other hand, smaller and individualized CNTs do not reveal such property.18-21 In addition, highly-functionalized CNTs with well-known biocompatible moieties (such as polyethylene glycol) have demonstrated reduced in vivo toxicity after being intravenously injected into animals as compared to their raw, non functionalized counterparts.22-24 In a series of our recent publications25,26 we proposed a design of the CNT based carrier of DOX which by definition should release the drug in response to the pH change from 7.4 to 6. The crux of the proposed approach was application of the mixture of DOX with some modifier molecule which reveals the protonation pKa close to 7. At the neutral pH that mixture should be kept in the internal space of the CNT and the main role of the modifier molecule is the stabilization of the fluid in the CNT against the release to the bulk. At acidic pH the modifier should undergo protonation and due to the

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change of the molecular charge it should escape from the nanotube. The correct choice of that smart modifier molecule was a matter of research and finally we identified only two (among dozens of tested cases) molecules which worked in the prescribed manner.25 These were: p-phenylene diamine (PPD) and the neutral red (NR). Both molecules are dyes and are used in some health care applications,27,28 thus they are not harmful. It should be underlined that these dyes are not fluorescent and their unique and useful property is in this case just the protonation reaction occurring at pH close to 7. Incorporation of DOX to the inner cavity of CNTs has been studied theoretically using molecular dynamics and quantum chemical methods

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and the results suggest stronger binding of

DOX inside the CNT than on its sidewall. Encapsulation of DOX inside carbon nanotubes has not been strictly addressed in experimental studies. Nevertheless, the amount of DOX which can be encapsulated in the nanotube is probably high and is dictated mainly by the geometrical conditions, as follows from the theoretical analysis.25,29 In a more recent work 26 we studied the effect of polyethylene glycol (PEG) functionalization of the CNT on the free energy barrier of DOX and dyes release from the nanotube. We found that the presence of folic acid terminated PEG chains (FA-PEG) on the CNT tips enhanced that energy and the spontaneous release of DOX have been blocked. At the same time, we observed the escape of PPD and NR molecules from the nanotube at acidic conditions. This spontaneous and intense transfer of dyes molecules from the CNT interior to the bulk is still interesting and can be utilized as a driving force for drugs release. This contribution, based on the large-scale atomistic molecular-dynamics simulations, discusses two approaches which reduce the blocking effect of DOX by the FA-PEG clusters localized on the CNT tips. The first approach utilizes light fullerenes C320 (FU) particles which act either as additional blockers of DOX-dye mixture at neutral pH or they work as pistons or form nanovalves, weakening the tightness of the FA-PEG clusters at acidic pH. The second approach tries to equip the standard architecture of the carrier in another property, namely, the sensitivity to the external magnetic field. To that purpose the empty inner space of the FU particles was filled in magnetic material. Thus, such magnetic nanoparticles (MNP) play multiple roles: they can act as pistons and additionally their rotation can be driven by the external magnetic field.

2 Methods The starting point of the considerations is the molecular architecture of the drug carrier described in very detailed way in our recent work.26 Therefore, we skip the description of the rationales for studying that kind of constructs, step by step description of the topology and the force field type and its ACS Paragon Plus Environment

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parameters. However, the clarity of presentation needs a short description of the computational model. Thus, the system is composed of (30,0) carbon nanotube (diameter 23.4 Å, length 80 Å) which is wide enough to collect DOX and dyes molecules. The nanotube is filled in mixture of DOX (12 molecules) and PPD (60 molecules) or NR (24 molecules). DOX exists in its protonated form while dyes switch the state from the deprotonated (at neutral pH) to the protonated one (at acidic pH).26 The CNT sidewalls and tips are covalently functionalized by PEG molecules which are additionally linked with folic acid segments. The above summary concerns the already studied systems architectures. 26 This contribution extends that list by the incorporation to the CNT interior of two fullerene C320 nanoparticles at the CNT ends. Fig. 1 shows the overall view of that extended computational model at neutral pH. As can be seen the FU nanoparticles cap the CNT ends quite tightly but they are not immobilized. These architectures are similar in concept to the carbon nano-bottle studied and characterized by Ren et.al.30 They applied the nanoextraction method to fill the CNT interior in the hexametylmelamine anticancer drug and to seal the CNT ends by the fullerene molecules. Thus, our model represents formally realistic construction because the most critical part of its structure (encapsulation of FU molecules in the CNT interior) has been experimentally confirmed.30,31 The FU molecules were described by the aiREBO force field 32 that is the same force field like in the case of carbon nanotube, thus, the force field structure in this case is identical as in. 26 However, a major part of this study focuses on the magnetic nanoparticles obtained by filling the empty inner space of FU molecules by magnetic material. In such a case the molecular dynamics code had to be equipped in functions working on magnetic torques. The theoretical foundation for coupling the stochastic dynamics of magnetization reversal with the newtonian dynamics of atomic systems was outlined in refs. 33,34 In practice, it was implemented as a few extra classes in lammps 35 working on the Brown rotations of the nanoparticles within their body frames, the Neel rotations of their magnetic moments induced by the magnetic anisotropy of the nanoparticles and their interaction with the external magnetic field and the integration of the stochastic equation for inertialess magnetization reversal. Thus, upon incorporation of magnetic material to the FU particles we have to deal with three extra parameters, these are: the magnetization of the nanoparticle, the magnetic anisotropy constant of the nanoparticle and the strength of the external magnetic field. We assume that FU molecules filled in magnetic material are single domain superparamagnetic nanoparticles. Thus, their magnetization is a product of the saturation magnetization Ms and volume. Similarly, the magnetic anisotropy energy is a product of the anisotropy constant Ka and volume.36 Both, Ms and Ka are material properties and we assumed their values to be representative of metallic cobalt (Ms = 1070 kA m-1, Ka = 5×106 J m-3), i.e. ACS Paragon Plus Environment

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one of the hardest magnetic materials. The magnetic core is represented as structureless solid sphere with the density ρ, thus its dispersion interaction with the rest of the system is represented by the Hamaker potential33 with the Hamaker constant A = 50 × 10-20 J.37 It is coupled with the magnetic dipole and the magnetic anisotropy energy comes from the displacement of the magnetic dipole from its easy axis direction. That solid sphere (magnetic core) is covered by the fullerene layer and both species form together a single rigid body. The fullerene plays in this case the role of a protective layer for the metallic core. All calculations were carried out using an explicit TIP3P water model. The number of water molecules was ca. 30 000 and the dimension of the box size was ca. 88x88x126 Å. 3 Results and Discussion 3.1 Fullerenes This section discusses the role of light fullerenes C320 as modifiers of the pH responsive drugs carriers discussed in 26. Fig. 1 shows the architectures of two systems differing by the use of the dye molecule, i.e. the top part shows the case of PPD while the bottom part is for NR. The FU molecules are in both cases located at the CNT ends and form a kind of corks. However, the structures presented in Fig. 1 were obtained without any constraints imposed on dyes, DOX or just FU molecules. And, in view of a high stability of the systems without FU,

26

the FU molecules are not necessary for the tightness of

these systems at the neutral pH. However, they play an important role after lowering the pH, as will be discussed soon.


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Figure 1. The final structures of the constructs at neutral pH obtained after 1.5 ns of simulations. In both cases the yellow sticks represent the FA-PEG molecules, green: DOX molecules, cyan: fullerenes C320. In part (A) the PPD molecules are in red while in part (B) the NR molecules are in blue. The structures were obtained in unbiased simulations without any constraints imposed on the FU molecules. As can be seen, we do not assume the presence of DOX or dyes in the bulk or just adsorbed on the surface of the carrier. This is because, their presence cannot affect the controlled release processes and thus it would make the analysis confusing. We simply assume that we start from an already purified system which has passed the impregnation and sealing steps. Therefore, such a carrier should be free of some residual DOX or dyes molecules adsorbed on its surface. Figure 2 shows the distributions of the numbers of molecules of all compounds as functions of the positions within the CNT. The distributions were determined by dividing the CNT along its axis into slices and counting and averaging the numbers of atoms found in each slice. In that way we can analyze the positions of all molecules during the whole MD trajectory.


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Figure 2. Distributions of the number of molecules of a given compound in the systems determined at neutral pH. The distributions are defined as the number of molecules found in 2Å-wide slice of the nanotube located at a given z (z=0 corresponds to the nanotube center and z = ± 41Å corresponds to the CNT ends). (top) DOX with PPD; (bottom) DOX with NR. The mean numbers of water molecules residing within the nanotubes were: (top) 123; (bottom) 174. Please note that the distribution of the FAPEG molecules accounts only the molecules bound to the CNT tips. The FA-PEG molecules bound to the CNT sidewall are not accounted for due to clarity of presentation. The same concerns Figs. 4 and 6. As can be seen in Fig. 2 the FU molecules stay close to CNT ends during the whole simulation time and they are surrounded by the FA-PEG chains. Doxorubicin is tightly kept in the CNT interior and its distribution within the CNT is almost uniform. Similarly, the dyes molecules reside in the CNT interior and no escape of these molecules has been detected during the simulations. The differences between these two systems concern only the distributions of DOX and dyes but their qualitative behaviors are identical. Clearly, the NR molecules spread across the CNT more effectively than PPD and also DOX molecules are distributed more uniformly in this case. Generally, we can conclude that FU molecules positions are stable during the simulations (without any external constraints applied) and this is the result of the attractive interactions between dyes and DOX and also of both these compounds with the CNT, as found in

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. Moreover, we can expect that the decompositions of these constructs

would require huge external energy supply since the free energies associated with the dragging of PPD, NR or DOX from the CNT interior at neutral pH were very high. 26


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Figure 3. The structures of the constructs at acidic pH obtained after 1.5 ns simulations. The starting configurations were taken from the final states of computations at neutral pH, i.e. directly from Fig.1. The yellow sticks represent the FA-PEG molecules, green: DOX molecules, cyan: fullerenes C320. In part (A) the PPD molecules are in red while in part (B) the NR molecules are in blue. The structures were obtained in unbiased simulations without any extra forces imposed on the FU molecules.

The next stage of the studies concerns the process of drug unloading at acidic pH. To that purpose the equilibrated at the neutral pH structures from Fig. 1 were used as the starting configurations in the acidic pH. In order to mimic the acidic pH, the deprotonated forms of dyes from Fig.1 have been replaced by their protonated counterparts which exist at the pH lower than their pKa. The replacement of dyes is accompanied by the change of charge distribution of dyes molecules and some slight change of their molecular topologies associated with the addition of hydrogens. The pKa values of the considered dyes are: 6.8 for NR, and 6.31 and 2.97 for the first and the second step of PPD.38 As found in our previous works the switch of the deprotonated forms of dyes into their protonated counterparts leads to strong electrostatic repulsion between dyes and also between dyes and DOX.

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That repulsion appears in the confined space of the CNT interior and it leads to local

increase of pressure. In previous studies that pressure was found to be enough for the unloading of dyes from the CNT, however, in the case of DOX the conclusions were not very strict. We observed some shift of DOX towards the CNT ends but the absolute release has not been observed in the unbiased simulations. On the other hand, the analysis of the free energy barrier of DOX release, obtained in biased simulations, suggest that the spontaneous release of DOX is rather unlikely.

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That conclusion

applies especially to the FA-PEG functionalized CNT model where FA-PEG chains, attached to the CNT tips, formed densely packed clusters capping the CNT ends. ACS Paragon Plus Environment

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Fig. 3 shows the results concerning the acidic pH cases. As can be seen the release of dyes occurs also in the case of the FU filled CNT, and similarly like in the previous studies, the PPD release is more intense than the NR. The most interesting is, however, the behavior of FU molecules. We can see that they have been pushed out the CNT and they broke the networks of FA-PEG chains, at least in the case of PPD. In the case of NR dye the piston effect of the FU molecules is weaker but still we can observe the uncapping of the CNT and loosening of the FA-PEG chains structure. Fig. 4 confirms these qualitative conclusions. We can notice that the positions of the FU molecules are shifted beyond the limits of the CNT in the case of PPD. In the case of NR only one of the FU molecules has left the CNT. The densities of dyes are smeared beyond the limits of the CNT and, what is the most important, the density profiles of DOX suggest its release from the CNT. Of course, the release of DOX has not completed within the computation time and the same concern the FU molecules. Of course, the networks of FA-PEG molecules have been loosen significantly but it seems that the FU molecules need another energy impulse in order to complete the detachment from the nanotube. It can be a collision with another nanotube, for instance, or application of some external energy supply from e.g. magnetic field.


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Figure 4. Distributions of the number of molecules of a given compound in the systems from Fig. 3 determined at acidic pH. The meaning of all symbols is the same as in Fig. 2. The mean numbers of water molecules residing within the nanotubes were: (top) 383; (bottom) 294. 3.2 Magnetic nanoparticles The equilibrated systems from Fig.1 were taken as starting points for construction of the magnetically sensitive versions of the drugs carriers. To that purpose the empty internal spaces of fullerenes were filled in magnetic material. Technically, it was done by the incorporation of the solid spheres with diameters 18 Å inside the fullerenes. That diameter was the largest possible to use without the risk of overlapping with the C320 carbon atoms, which act as a protective layer for the magnetic cores. The magnetic material was assumed to be metallic cobalt, however it was represented as structureless continuous phase with a given density ρ, saturation magnetization Ms and magnetic anisotropy constant Ka. Thus, the FU filled in magnetic material are a kind of core-shell nanoparticles with graphere/fullerene external protective layers and metallic cores.39 The behavior of those modified constructs at neutral pH and in the absence of the external magnetic field (EMF) turned out to be actually identical to those without magnetic cores. Thus, the pictures from Fig. 1 do not change qualitatively after switching to the magnetic versions of the constructs at the neutral pH.


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Similarly, the case of the acidic pH does not differ much from the states in Fig. 3. But in the present case of heavy magnetic cores the weight of the FU nanoparticles is larger and reaches 20200 g mol-1 versus 3840 g mol-1 for the empty FU molecules. This leads to slightly smaller mobility of the nanoparticles when compared to Fig.3 but the qualitative picture is the same. Thus, as shown in Fig. 5 the FU nanoparticles have been pushed out of the nanotube and located beyond its limits and this process was driven by the pressure generated by the repulsion between the protonated form of dyes and DOX. However, similarly like in Fig. 3 the uncapping of the CNT is not complete and is less advanced in the case of NR than the PPD dye. The above conclusions are also supported by the distribution functions of the molecules encapsulated in the CNT. As seen in Fig. 6, the intense escape of PPD from the nanotube strongly affected the distribution of DOX. It is flat and exceeds the limits of the CNT length, thus the release of DOX is confirmed in this case. Similarly, the NR distribution has been shifted to the left and the distribution of DOX indicates its release from the lhs of the nanotube. It seems that the NR molecule is in all the considered case studies, less active/fast than the PPD.

Figure 5. The structures of the constructs with the FU filled in magnetic material at acidic pH and in the absence of the external magnetic field obtained after 1.5 ns simulations. Part (A) the co-adsorption of DOX with PPD molecules (red); part (B) the co-adsorption of DOX with the NR molecules (blue). The meaning of other colors is the same as in Fig. 3. The arrows indicate the directions of the two vectors


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associated with the magnetic FU nanoparticles – the black arrows show the directions of the easy axes of the nanoparticles, while the red arrows show the directions of the magnetic moments. The arrows attached to the nanoparticles (Fig. 5) indicate the direction of the magnetizations (red) and the easy axes (black) of the nanoparticles. The situation in Fig. 5 corresponds to the absence of the external magnetic field, therefore the magnetizations are aligned with the directions of the associated easy axes. Some small displacements are due to thermal fluctuations of the magnetizations directions which obey the stochastic motion dynamics.

Figure 6. Distributions of the number of molecules of a given compound in the systems from Fig. 5 determined at acidic pH and in the absence of the external magnetic field. The meaning of all symbols is the same as in Fig. 2. The mean numbers of water molecules residing within the nanotubes were: (top) 375; (bottom) 286.

The pictures in Fig. 5 correspond basically to the acidic pH triggered detachments of the nanoparticles as these processes appear first after endocytosis of the carriers by cancer cells. And, both Fig. 3 and Fig. 5 suggest that the corresponding carriers are able to unload their cargoes at the tumor site. However, due to the presence of the magnetic cores in the FU frameworks the considered constructs can be visualized in the magnetic resonance imaging since the superparamagnetic nanoparticles enhance the protons transverse relaxations times and act as contrast agents.22 Moreover,


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interaction of the constructs with the external magnetic field may lead to new phenomena like the magnetically triggered uncapping of the nanotube. 36 The calculations devoted to the analysis of the interaction of the constructs with the external magnetic field led to the conclusion that the applied sizes of the magnetic cores are actually too small. The magnetostatic energy of the magnetization vector scales as r3, where r is the radius of the nanoparticle. Similarly, the magnetic anisotropy energy scales with the same power of the nanoparticle radius.36 However, given the size of the magnetic cores, i.e. 18 Å, both these energies are very small for such tiny nanoparticles. It can be easily calculated that the anisotropy energy is less than 10 kJ mol-1 while the energy of the magnetic dipole depends on the magnetic field strength and for very strong field 12 Tesla it reaches not more than 25 kJ mol-1. These energy values mean that the effect of the application of the external magnetic field will be actually undetectable.

Figure 7. The structures of the constructs with the FU filled in magnetic material at acidic pH and after the exposition to 12 Tesla external magnetic field. Part (A) the co-adsorption of DOX with PPD molecules (red); part (B) the co-adsorption of DOX with the NR molecules (blue), (the l.h.s. magnetization vector is in the CNT interior). The meaning of other colors is the same as in Fig. 5. Larger nanoparticles with radii ca. 30-50Å cannot be directly studied due to the computational limitations. But obviously such sizes are most likely to be used in experimental studies.39 Again, simple calculations allow us to predict that the magnetic anisotropy energy for the radius of 35Å will be 540 kJ mol-1 while the magnetostatic energy of the magnetization at 12 Tesla will reach 1380 kJ mol-1.


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Thus, these are high values and we can expect visible effects associated with the interaction with the external magnetic field. Because, the magnetizations of the NPs and the magnetic anisotropy barriers are intrinsic parameters and they do not affect the spatial structure and interatomic interactions, their values can be formally rescaled in order to reach the experimentally reasonable sizes of the nanoparticles. In that way we are able to track what could happen in systems having bigger nanoparticles. Fig. 7 shows the structures of the analyzed constructs obtained after rescaling the values of magnetic parameters to the radii 35Å. The external magnetic field was applied along the nanotube axis to the structures from Fig. 5. As can be seen in Fig.7 the atomic/molecular distributions did not change much upon exposition to the external magnetic field. Particularly, the detachment of the nanoparticles from the CNT has not proceeded to a large extent. Some configurational changes however appeared, mainly related to the migration and rotation of the systems as a whole. Fig. 7 does not display these changes as it shows reoriented pictures prepared for the visualization purposes. The most important effect of the EMF is observed in the strongly altered directions of the easy axes and magnetizations. The latter are oriented roughly around the CNT axis because the EMF was applied in this direction and the magnetizations passed the reversal processes. The alignment of the magnetization around the EMF direction drives the Brown rotation of the nanoparticles which can be visualized as the rotations of the easy axes. Indeed the easy axes have been strongly displaced from their initial positions shown in Fig.5. This means that the EMF induced very intense turns of the nanoparticles around their body frames. These phenomena can be observed in the animations provided as Supplementary Information. These movies show the whole processes starting from the initial equilibration periods and finalize when the magnetically induced rotations of the nanoparticles vanish. Looking at these movies we can see that further calculations will not bring new observations provided that a change in the EMF will not occur.


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Figure 8. Combined effect of the pH reduction and the exposition to the external magnetic field for systems from Figs. 5 and 7. The starting points/times correspond to systems states just switched from the neutral to acidic pH. Top part shows the rotations of the FU particles filled in magnetic material, while the bottom part corresponds to their translations/displacements from the starting points. The external magnetic field was switched on at 0.3 ns as indicated by the arrows. Fig. 8 shows a kind of history of the analyzed systems starting from the moment when the pH has been switched from the neutral to acidic one. Two important parameters have been selected as indicators of the systems states during their evolution. The first is the angle of rotation of a given FU particle calculated in reference to the initial state at t=0, the second one is the displacement of the FU particles centers of mass from the reference states at t=0. Analysis of these factors leads to interesting conclusions. Namely, after the switch of the deprotonated forms of dyes into their protonated counterparts (reduction of pH below a given pKa) rapid moves of the FU particles commence. The moves last for short period of time, less than 0.1 ns but relatively long distances are passed during this time. Particularly, in the case of PPD system the FU molecules were pushed by ca. 25Å towards the CNT ends, as seen in Fig. 5. In the case of NR system, only one FU molecules performed the move, the second one stays in the same place. In both cases no rotation or just only small rotational fluctuations of the FU particles occur as a result of the pH reduction. At t = 0.3ns, as indicated by the arrows in Fig. 8, the magnetic field is switched on. Then, another phenomenon commence. These are very intense turns of the FU particles around their body ACS Paragon Plus Environment

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frames, however, their translations are not significantly enhanced and actually the uncapping of the nanotube is little improved by the presence of the magnetic field. Let us, however, focus on the rotation of the FU particles; it is clear that the rotations start quickly after the switch of the magnetic field, but the nanoparticles do only part of the full turn around their body frames and afterwards the rotation angle goes towards the initial value and next the another and weaker turn occurs. This is due to the fact that magnetizations of the FU particles align with the field direction and some time after that alignment the nanoparticles recover the orientations of the easy axes with the magnetizations. However, due to significant inertia a series of turns with vanishing intensities is performed. When that process finishes the magnetic field do not play any further role, however application of the magnetic field at another angle will lead to another turn of the nanoparticles. Therefore, we can expect that an alternating magnetic field will lead to intense turns of the nanoparticles and the energy absorbed from the field will dissipate around the magnetic nanoparticles. This must lead to local increase of the temperature which, in turn, can enhance the displacements of the nanoparticle or just directly lead to thermal ablation of cells. The studied model systems represent molecular structures which fabrications are possible but they have not been attempted yet. Therefore, the obtained results are useful because they represent a theoretical prediction of properties of the systems which can be fabricated when it will be found beneficial. 4

Summary

In this work we studied the role of fullerene and magnetic nanoparticles as nanovalves enhancing the performance of the carbon nanotube in the release of the anticancer drug – doxorubicin. Our previous studies led to the conclusion that doxorubicin is likely to be kept strongly in the nanotube interior due to the presence of some extra functional groups at the nanotube tips. These groups (polyethylene glycol and folic acid) are important as they provide biocompatibility and targeting property of the construct. We therefore studied the role of some nanoparticles present in the interior of the nanotube together with doxorubicin and some dyes molecules. The dyes molecules are adjusted in such a way that they undergo protonation at pH values typical to tumor microenvironment. We found that FU nanoparticles facilitate the release of doxorubicin at acidic pH, however, at the available simulation times we were not able to observe a full unloading of DOX from the CNT. Therefore, in the next stage of the studies we equipped the system in magnetic property by filling the internal spaces of FU molecules in magnetic material. Analysis of the effects associated with the exposition of the systems to the external magnetic field led to the conclusion that the uncapping of the CNT is mainly produced by the pH ACS Paragon Plus Environment

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reduction. The magnetic field leads, however, to intense turns of the nanoparticles around their body frames. The energy necessary to drive the rotation of the nanoparticles comes from the external magnetic field and it must dissipate in the vicinity of the constructs. Thus, we conclude that an alternating magnetic fields must produce local temperature increase which either enhance the release of the drug from the CNT or just directly do the thermal ablation of the cells. Therefore, the proposed design of the drugs carrier is very promising as it provides multimodal function in a single and relatively simple system.

5

Supporting Information

Animations showing the temporal evolution of the systems with the NR and the PPD while changing pH and switching on the external magnetic field (AVI). 6

Acknowledgments This work was supported by Polish National Science Centre grant UMO-2012/07/E/ST4/00763.

7

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fig.1 172x141mm (300 x 300 DPI)


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fig.3 317x186mm (300 x 300 DPI)


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fig.4 172x230mm (300 x 300 DPI)


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fig.5 211x191mm (300 x 300 DPI)


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fig.7 217x155mm (300 x 300 DPI)


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table of contents image 400x80mm (300 x 300 DPI)


Multimodal, pH Sensitive, and Magnetically Assisted Carrier of

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