Adsorption from aqueous solutions on opened carbon nanotubes ...

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Adsorption from aqueous solutions on opened carbon nanotubes—organic compounds speed up delivery of water from insidew Artur P. Terzyk,*a Piotr A. Gauden,a Sylwester Furmaniak,a Rados$aw P. Weso$owski,a Peter J. F. Harrisb and Piotr Kowalczykc Received 5th August 2009, Accepted 20th August 2009 First published as an Advance Article on the web 7th September 2009 DOI: 10.1039/b916067a We report the results of first systematic studies of organic adsorption from aqueous solutions onto relatively long single walled carbon nanotubes (four tubes, in initial and oxidised forms). Using molecular dynamics simulations (GROMACS package) we discuss the behaviour of tube–water as well as tube–adsorbate systems, for three different adsorbates (benzene, phenol and paracetamol). Carbon nanotubes (CNs) are fascinating materials due to their unique properties.1 Different potential applications of CNs have been proposed and, among them, one of the most promising is in drug delivery systems. However, this application is still uncertain, due to contradictory reports about the toxicity of CNs. Recent results show that single walled carbon nanotubes (SWNTs) can be successfully applied for the construction of SWNT–cisplatin systems, which reduces the growth of cancer cells.2 Also, the anti-cancer activity of oxidized carbon nanohorns (or nanohorns filled with cisplatin3) is well documented. From literature reports, it can be concluded that many proposed applications of CNs refer to solutions4 and to adsorption phenomena taking place in the liquid phase (for example, removal of dioxins5). However, there is still a lack of results explaining the mechanistic aspects of the adsorption on CNs from solutions. Gotovac et al.6 studied the adsorption of polycyclic aromatic hydrocarbons (PAHs) on CN, discussing the orientation of PAHs adsorbed on nanotubes. Some experimental results show the reduction of organic adsorption after oxidation of nanotubes;7 however, systematic theoretical studies on the mechanisms of adsorption on CN have not yet been reported. Since water is the most common solvent, there are many papers showing the behavior of water in CN but (as pointed out in a recent review by Alexiadis and Kassinos8) simulations are usually performed in relatively short tubes and there is no investigation of longer a

N. Copernicus University, Department of Chemistry, Physicochemistry of Carbon Materials Research Group, Gagarin St. 7, 87-100, Torun´, Poland. E-mail: [email protected]; Fax: +48 (0)56 654 24 77; Tel: +48 (0)56 611 43 71; Web: http://www.chem.uni.torun.pl/Baterzyk/ b Centre for Advanced Microscopy, University of Reading, Whiteknights, Reading, UK RG6 6AF c Applied Physics, RMIT University, GPO Box 2476V, Victoria, 3001, Australia w Electronic supplementary information (ESI) available: Movies showing the kinetics of water exchange between internal and external spaces of the tubes (movie_1), configurations of adsorbed molecules (movie_2) and the kinetics of the pumping effect (movie_3); detailed description of movies, simulation methodology, snapshots of equilibrated configurations and water in CN. See DOI: 10.1039/b916067a

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systems. Theoretical studies suggest two-state transitions of the nanotube–water system, between empty and filled states on a nanosecond timescale.9 The process follows simple kinetics.10 Some authors also suggest the creation of an ice structure of water inside nanotubes.11 Among the experimental studies, those reported by Naguib et al.12 are particularly interesting. These authors described the problems with the filling of defect-free tubes with water and proved the presence of gaps and bubbles inside water-filled tubes. Kolesnikov et al.13 reported that water does not freeze in CN but the molecules form a square-ice sheet, which is wrapped into a cylinder inside the carbon nanotube and interior molecules are captured in a chain-like configuration (the average number of hydrogen bonds between water molecules inside the tubes was estimated as equal to 1.86). Due to the above-mentioned reasons, in the current study, we consider the mechanisms of adsorption of water (solvent), as well as of three organic molecules (benzene, phenol and paracetamol), from aqueous solutions onto SWNTs. The applied methodology is illustrated for one type of tube in Fig. 1. Four opened zigzag SWNTs were studied: (8,0), (14,0), (20,0) and (26,0). They were investigated in the initial (left hand side in Fig. 1) and in modified forms (right hand side in Fig. 1). The latter were obtained by saturation of edge carbon atoms with carbonyl groups [16, 28, 40 and 52 oxygen atoms were attached for tubes (8,0) up to (26,0)]. Since molecular dynamics simulations (MD) play an indisputable role in the field of prediction and examination of adsorption and related phenomena in nanostructures,8–11,14 we decided to use this method in the current study. The simulated tubes are relatively long (B8.2 nm), one of the most sophisticated models of water is applied, and among the studied molecules is paracetamol—the drug that is used as the standard molecule for testing the efficiency of drug delivery systems. All simulations were performed using Gromacs Molecular Dynamics package15 (see ESIw); the parameters for organic molecules were taken from the OPLSAA force field, projected for the simulation of condensed phases;16–18 water was modeled using the TIP4P model19 and the parameters for tubes were taken from ref. 20 and 21. We also checked the influence of the box size and the initial configuration on simulation results (see ESIw). First, we performed simulations for tube–water systems. The results show that the solvent is almost absent inside the initial (8,0) tube (see Fig. 2 and S1w). Oxidation of the entrances to this tube leads to a rise in the density of water (molecules are practically not present inside), mainly at the ends of the tube. In tube (14,0), water is also not present inside Phys. Chem. Chem. Phys., 2009, 11, 9341–9345 | 9341

Fig. 1 The methodology of simulations (tube (14,0) is shown as an example). First row: initial and modified CN; next rows show equilibrated MD configurations for tubes with water only and for tubes with solutions containing the studied molecules (benzene, phenol and paracetamol).

(Fig. 1, 2 and S2w) and only the formation of a meniscus at the tube entrances is observed (see movie_1w). The maximum average number of hydrogen bonds per water molecule inside the tube (nhb), calculated using the procedure described in ref. 22, is equal to 2.7. After the appearance of carbonyls at tube entrances, we observe the filling of the internal space with water and one can observe a large density of molecules located at the vicinity of the internal wall, as well as the formation of the high density second layer at the tube centre (water is oriented with oxygen atoms to the wall; nhb = 3.0). The appearance of four peaks of high density water at the tube entrances (Fig. 2) is crucial for the mechanism of organic adsorption in this tube (discussed below). Contrary to the other studied modified tubes (where only two peaks occur), due to the dimensions of (14,0), high density water peaks also appear at the centre of the pore mouth (see Fig. 2) (nhb = 3.1). It will be shown below that these solvent molecules block the internal space of the (14,0) tube for the studied organics, by the so-called ‘‘pore blocking’’ effect (Fig. S2w). The animations (see movie_1 in the ESIw) show that the kinetics of mixing between water molecules filling the central channel and bulk water is much faster than in the hydrophobic (14,0) and one can observe the creation of gaps (Fig. 1 and S2w). Further widening of the tubes leads to the presence of two water layers inside the (20,0) SWNT, where we observe the largest density of molecules located at the vicinity of the tube wall (nhb is practically the same as in bulk water, i.e. 3.4), lower density water in the second layer and a drop in density of molecules located along the tube central axis. A similar situation (i.e. two water layers) occurs inside oxidized tubes (with a similar density and number of hydrogen bonds of water in both layers as in the initial tube) but, additionally, two peaks of high density water are observed at the entrances. As for the modified (14,0) tube here, gaps are also created (Fig. S3w). Therefore, it can be concluded that the creation of gaps inside modified tubes is induced by the presence of surface oxygen groups at the edges. This effect needs further study, and the results will be reported. In (26,0), the situation is analogous, 9342 | Phys. Chem. Chem. Phys., 2009, 11, 9341–9345

Fig. 2 Radial density profiles of water in the studied initial and modified CNs (the results for simulations of CN–water systems, i.e. without adsorbate); r/R is the ratio of the distance from tube centre to the radius of a tube. The tube central axis is located at the point (0,0).

but water additionally fills the centre of the tube and has a similar density to the bulk (Fig. S4w). In this case, gaps are not observed. To discuss the results of the simulations of the studied molecules in aqueous solutions, we start with an analysis of the histogram of angular orientations of adsorbed molecules, shown in Fig. 3. As shown in the snapshots, for some cases we observe a small number of molecules forming the second layer of adsorbate (see the ESIw), but the angular orientations of the molecules adsorbed outside (and shown in Fig. 3) are limited only to those in the monolayer. Oxidation of the tubes does not change the orientation of the adsorbed molecules for any of the studied CNs. For tube (8,0), we observe that an angle of 15 degrees is the most probable for benzene. For this molecule, one can observe a wide distribution of the angle, even up to 90 degrees (i.e. a perfectly perpendicular orientation). For phenol, the shift towards slightly smaller angles is seen (although a wide distribution of angles is still observed). However, the largest number of almost flatly oriented molecules occurs for paracetamol (the most probable angle is located at ca. 10 degrees). In the latter case, there are practically no molecules with angles larger than 60 degrees. Qualitatively, the This journal is

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same distributions are observed for larger SWNTs. However, one can see a smaller number of orientations with larger angles with a rise in the radius of the tube, because molecules interact efficiently with the external surface of the tubes via van der Waals forces (see movie_2 in the ESIw). An interesting feature is the formation of spiral structures by molecules adsorbed on the external surfaces of CNs. On the other hand, the orientation of molecules inside the nanotubes changes very slightly with the rise in tube diameter and the most probable orientations are located between 6–10 degrees for all studied adsorbates. Analysis of the animations and snapshots (ESIw), as well as the density profiles, shows that in SWNT (8,0) water and adsorbate molecules are not present inside the tube (this CN, with an effective diameter of 0.2818 nm, is too small for penetration) and molecules are adsorbed only on the external surface (water behavior is consistent with the results of Wang et al.23). In the hydrophobic (14,0) tube, one can see a progressive rise in the density of adsorbed molecules inside the tube, from benzene through to paracetamol. An interesting effect is observed for this system, i.e. the appearance of adsorbate inside and outside the tube remarkably speeds up the emptying of water from the internal space (water is ‘‘pumped up’’ by the adsorbed molecules—see movie_3 in the ESIw). Fig. 4 shows the kinetic plots of the pumping process. One can see that the tube emptying (without adsorbate), if studied for pure water, is very slow, but occurs spontaneously. However, in the case of phenol or paracetamol solutions, the appearance of solute molecules inside the tube speeds up the pumping process. The results for benzene

additionally show that this effect is also accelerated by adsorption on the external surface. As can be seen from Fig. 4 (where the kinetic curves for two regions of the tube are shown), after ca. 4000 ps, there are fluctuations in the number of water molecules at the tube mouth (we observe a filling–emptying transition), but the central part of this tube remains almost empty. Therefore, it can be seen that, if simulations are performed in very short tubes (as is often reported in literature), one can observe that the tube is filled with water, but, in fact, for long tubes, the internal space is empty for the majority of the time. Generally, the mechanism of pumping is ruled by the rise in hydrophobic interactions between water and the adsorbed molecules (movie_3w). This is confirmed by the results of the distribution of hydrogen bonds for water inside this tube in the presence of adsorbed molecules, showing a strong reduction in the number of hydrogen bonds, especially for water, at the walls of the tubes (see Fig. S5w). Making the tube entrances hydrophilic diminishes the adsorption inside (this has been recently observed experimentally for adsorption on oxidized tubes with almost the same diameters as tube (14,0) studied in this paper7), and adsorbed molecules are not present in the internal space due to the above-mentioned ‘‘pore blocking’’ effect. The appearance of high density water at the centre of pore entrances produces a barrier for solute molecules which is probably too hard to overcome. Moreover, in the case of this tube, water molecules adsorbed inside, forming the layer at the internal wall of the tube, have large densities. In tube (20,0), all three molecules penetrate the internal channel but after oxidation, only benzene and phenol (in smaller amount) can access the inside. Finally, in tube

Fig. 3 Histogram of the angular orientations of adsorbed molecules for the initial CN (note that oxidation does not change the distributions) and their corresponding snapshots.

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Fig. 4 Kinetics of molecular pumping in the (14,0) initial tube, divided into regions A and B; and selected snapshots for simulations of water–tube and water–tube–adsorbate systems.

(26,0), molecules can freely penetrate the internal tube space, and the differences between the number of molecules adsorbed inside hydrophobic and hydrophilic nanotubes are the smallest. In summary, we have performed the first systematic studies of organic adsorption onto SWNTs from aqueous solutions. Simulations of water (without solute molecules) show the absence of molecules inside (8,0) and the initial (14,0) CNs. Oxidation of the pore mouth induces the appearance of water inside (14,0), with the creation of gaps (also observed in the (20,0) modified tube). Among the studied molecules, paracetamol prefers an almost flat orientation and with the rise in tube diameter, the number of molecules oriented in parallel increases. The adsorbed molecules form spiral structures, wrapping the nanotubes. The number of molecules adsorbed inside generally decreases after the oxidation of tube entrances, which is in accordance with experimental data. The initial (14,0) tube seems to be the most promising for the construction of drug delivery systems based on drugs with a similar structure to paracetamol. However, the number of oxygen functional groups at the tube ends should be reduced to enable the loading of the drugs inside. Pore blocking by high density water disabled the penetration of the (14,0) modified tube by solute molecules. However, in the initial 9344 | Phys. Chem. Chem. Phys., 2009, 11, 9341–9345

(14,0) tube, we observe that the spontaneously-occurring emptying of water from the tube is accelerated by the adsorption of organic molecules inside and/or outside, and this is called the pumping effect. It is caused by a reduction in the number of hydrogen bonds per water molecule inside the tube channel. We have also considered the length of CN, showing that if the tube is too short, one can observe a filling–emptying mechanism which, for a long tube, is only a fluctuation in the number of molecules at the edges of pores, whereas, in fact, the central part of the long (14,0) tube remains empty.

Acknowledgements The authors thank the Information and Communication Technology Center of the Nicolas Copernicus University (Torun´, Poland). The paper was supported by grants N N204 009934 (APT and SF) and N N204 288634 (PG). SF thanks the Foundation for Polish Science.

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