Abstract
Among the most studied processes of self-organization1, 2 are the coiling and ring formation of biopolymers such as DNA and proteins. These processes are complex, involving several different types of interaction. We have found that single-walled carbon nanotubes (SWNTs), which are renowned for their extremely high flexural rigidity3, 4, can also be induced to organize themselves into rings or coils, with high yields of up to 50%. But unlike coils of biopolymers, in which hydrogen bonding and ionic interactions are usually involved, coils of nanotubes can be stabilized by van der Waals forces alone.
Main
Scanning electron micrographs (Fig. 1a) of SWNTs with an average diameter of 1.4 nm were prepared using laser ablation5. The nanotubes were shortened and induced to coil by using an acid treatment with ultrasound. Transmission electron microscope (TEM) images (Fig. 1b) confirm that the rings consist of aligned ropes of SWNT. The size of the rings formed is shown in Fig. 1c.
The structure of rings (tori or coils) can be deduced from several observations. First, long SWNTs are shortened by oxidation, leaving the tube ends functional with carboxylic acid groups6, 7, arguing against the formation of a torus involving covalent bonds between carbon atoms. Images obtained by TEM and atomic force microscopy show that the rings do not have a constant thickness and height around their circumference (Fig. 1b), indicating that they may be formed by separate ropes being curled together. The rings can be taken apart and the ends of the ropes exposed (not shown), so we conclude that they are indeed produced by a coiling process.
Trace quantities (0.01 to 0.04%) of rings have previously been observed8 and larger yields have also been claimed9. The rings were assumed to be perfect tori8, stabilized by covalent bonds between carbon atoms, but our analysis suggests that they were actually coiled SWNTs.
The simplest model of the ring formation process has a SWNT coiling over itself to form a loop. Coiling involves significant strain energy because of the increased curvature, but van der Waals interactions stabilize the tubes.
The critical ring radius, R, for forming thermodynamically stable rings a few micro-metres long is small, about 0.03 μm for single tubes or ropes of SWNTs 1.4 nm in diameter. According to our calculations, much lower values of R are energetically allowed than are actually observed, indicating that ring formation may be kinetically controlled. The activation energy, EA, should be of the order of the strain energy, and ΔEA∝ R-2. In our experiments, the activation energy is provided by ultrasonic irradiation. The most likely mechanism involves the hydrophobic nanotubes acting as nuclei for bubble formation and being bent mechanically at the bubble-liquid interface as a result of the bubbles collapsing during cavitation10.
References
Lehn, J.-M. Supramolecular Chemistry: Concepts and Perspectives (VCH, Weinheim, 1995).
Whitesides, G. M., Mathias, J. P. & Seto, C. T. Science 254, 1312–1319 (1991).
Treacy, M. M. J., Ebbesen, T. W. & Gibson, J. M. Nature 381, 678–680 (1996).
Wong, E. W., Sheehan, P. E. & Lieber, C. M. Science 277, 1971–1975 (1997).
Thess, A. et al. Science 273, 483–487 (1996).
Liu, J. et al. Science 280, 1253–1256 (1998).
Wong, S. S., Joselevich, E., Woolley, A. T., Cheung, C. L. & Lieber, C. M. Nature 394, 52–55 (1998).
Liu, J. et al. Nature 385, 780–781 (1997).
McEuen, P. reported in Vossmeyer, T. et al. Adv. Mater. 10, 351–353 (1998).
.Suslick, K. S. (ed.) Ultrasound: Its Chemical, Physical and Biological Effects(VCH, Weinheim, 1988).
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Martel, R., Shea, H. & Avouris, P. Rings of single-walled carbon nanotubes. Nature 398, 299 (1999). https://doi.org/10.1038/18589
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DOI: https://doi.org/10.1038/18589
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