Helical ice-sheets inside carbon nanotubes in the physiological condition

Molecular dynamics simulations were performed, in the physiological condition (300 K and 1 atm), on nanotube segments of various diameters submerged in water. The results show that water molecules can exist inside the nanotube segments, and,most importantly, the water molecules inside the tubes tend to organize themselves into a highly hydrogenbonded network, i.e., solid-like wrapped-around ice sheets. The disorder-to-order transition of these ice-sheets can be achieved purely by tuning the size of the tubes. The results also suggest that the nanotubes have the potential to be used as proton-conducting pores for a variety of biological applications. 2002 Elsevier Science B.V. All rights reserved. Single-walled carbon nanotubes (SWNT) have elicited great research interest in recent years due to their unique anisotropic mechanical and electrical properties [1]. A promising area of study involves introducing nanotubes, with unusual new properties of both organic and inorganic materials, into biological systems. However, in order to successfully incorporate foreign materials into living cells, issues such as water solubility, biocompatibility, and biodegradability must be addressed. This can only be possible with a comprehensive understanding of the interactions of nanoparticles with basic biological molecules such as proteins, nucleic acids, and membrane lipids in the aqueous environment [2]. Therefore, the first fundamental step is to understand the interactions of nanoparticles with water molecules. Carbon nanotubes are generally hydrophobic, therefore, the dynamical and structural properties of surrounding water molecules are expected to differ greatly from those of bulk water. In this work, we have carried out molecular dynamics (MD) simulations [3] of segments of pristine SWNT of different sizes submerged in a periodic hexagonal prism of water in the physiological condition (300 K and 1 atm). Segments of SWNT with a length of 20 A and indices of (n; n) were studied (n 1⁄4 5, 6, 7, 8, 9, 10, 15). We are particularly interested in the behavior of water molecules inside the nanotube segments. 8 April 2002 Chemical Physics Letters 355 (2002) 445–448 www.elsevier.com/locate/cplett * Corresponding author. Fax: +713-796-9438. E-mail address: [email protected] (J. Ma). 0009-2614/02/$ see front matter 2002 Elsevier Science B.V. All rights reserved. PII: S0009-2614 (02 )00209-9 We found the size of the ð5; 5Þ tube (6.75 A in diameter) is too small to accommodate any water molecules inside, and the behavior of the water molecules inside the ð15; 15Þ tube (20.26 A in diameter) is the same as that of bulk water. However, the behavior of the water molecules inside tubes of intermediate sizes is significantly different. The oxygen–oxygen radial distribution functions of water molecules, gOOðrÞ, inside the ð7; 7Þ; ð8; 8Þ, and ð9; 9Þ tubes are shown in Fig. 1. The origin is set at the center of the tube. From the positions of the peaks, it is evident that water molecules tend to stay about 3 A away from the wall, consistent with the results of a recent high-level quantum mechanical calculation of smaller model systems [4]. Strikingly, along the central axis, there is a much lower water density. In addition, we also observed unconventional behavior of the water trajectory along the inner wall of the tube. The water molecules inside the tube appear to be arranged in a very ordered fashion. For example, there are a total of six columns of water molecules inside the ð9; 9Þ tube (12.16 A in diameter) forming a perfectly hydrogen-bonded network (water cylinder), reminiscent of the wrapped-around graphene sheet of a nanotube (Fig. 2). Due to the obtuse triangular shape of a water molecule, the cylinder formed by the six water columns present some degree of spiral nature in the axial direction of the tube. Each water molecule in the water sheet is hydrogen-bonded with four nearest neighbors in a nearly two-dimensional net. This is clearly different from the conventional tetrahedral arrangement of water molecules in ice. The formation of such a water sheet is due to the unique shape and size of the tube and also due to the iceberg effect [5,6], which states that, on a hydrophobic surface, the water molecules in the first shell tend to have their O–H bonds tangential to the surface. The water sheet appears to be very stable in the simulations. Only occasionally, individual water molecules fall off from the sheet and result in a ‘defect’, but it is quickly re-sealed. Similarly, the simulations of tubes of various sizes show that the ð6; 6Þ tube (8.11 A in diameter) can accommodate a hydrogen-bonded single-file water network [7], the ð7; 7Þ tube (9.46 A in diameter) with a three-column water network, the ð8; 8Þ tube (10.81 A in diameter) with a four-column water network, and the ð10; 10Þ tube (13.51 A in diameter) with a sevencolumn water network. An earlier model study on simulation of water molecules in confined spatial regions [8] suggested qualitatively similar features of the radial distribution function to what is shown in Fig. 1. However, it did not demonstrate the ordered spiral network that we have observed. More recently, Koga et al. [9] presented the possibility of forming n-gonal ring structures of water within carbon nanotubes. However, it was reported that such ring structures could only be formed at much higher pressures (500–5000 atm) that are far away from physiological conditions. This clearly contradicts what we have shown in this report that the ordered water network can be formed inside the nanotubes in the much milder physiological condition. It is also clear from our study that the disorder-to-order transition of the water network can be achieved by varying the tube size, rather than the pressure [9]. The discrepancy may also come from the fact that the tube indices discussed in the article [9] do not seem to be in accord with the stated diameter values of the ðn; nÞ armchair Fig. 1. Oxygen–oxygen radial distribution functions, gOO(r), of water molecules inside the tubes. The x-axis is the radial distance from the center of the tube. The results of the ð7; 7Þ tube (9.46 A in diameter), the ð8; 8Þ tube (10.81 A in diameter), and the ð9; 9Þ tube (12.16 A in diameter) are shown. The positions of the walls for the tubes are indicated by the arrows. 446 W.H. Noon et al. / Chemical Physics Letters 355 (2002) 445–448