Harold Kroto Contemplates Applications of Nobel-Winning Fullerenes

Editor's Note: Last month, Sir Harold Kroto, the Royal Society Research Professor at the University of Sussex in Brighton, U.K., along with Richard E. Smalley, the Hackerman Professor of Chemistry at Rice University and Robert F. Curl, Jr., also a professor of chemistry at Rice, received the Nobel Prize in chemistry in Stockholm. They were honored for their discovery of buckyballs, the now-famous soccer-ball-shaped molecules named for architect R. Buckminster Fuller and his geodesic domes. The 60-carbon-atom molecules are also called fullerenes. Since the discovery of C60 almost 12 years ago, "buckyballs" have captured the imagination of scientists and the public alike. At first the existence of buckyballs drew much skepticism, but over the last decade they have grown into a flourishing area of research. Few stories in the annals of contemporary science provide a stronger argument for pursuing fundamental research than the discovery of C60. In the early 1970s, Kroto hatched a research program at the University of Sussex to seek long chains of carbon in interstellar space. During 1975-78 this effort eventually led to the detection of several different carbon chains, including HC5N, HC7N, and HC9N. Kroto collaborated with David Walton at Sussex and Takeshi Oka and astronomers at the Canadian National Research Council in Ottawa in the discovery of these extraterrestrial molecules. Kroto surmised that these chains were the product of carbon-rich, red-giant stars. "But how did the chains form?" he wondered. A few years later, in 1984, Kroto encountered a unique instrument-a laser vaporization cluster beam apparatus-in Smalley's laboratory at Rice University. Smalley and his group had developed this instrument and were using it for semiconductor research on silicon and germanium clusters, but Kroto immediately realized that it could be used to simulate the high-temperature conditions under which the carbon chains might form in stars. In September 1985, Kroto, together with Smalley's group and Curl, tried to create such conditions and found much more than they had bargained for: a stable molecule consisting of exactly 60 carbon atoms. They suggested that C60 took the form of a closed cage resembling a soccer ball and called the molecule buckminsterfullerene (H.W. Kroto et al., Nature, 318:162, 1985). Subsequently, this short communication has been cited 1,719 times through October 1996, according to the Philadelphia-based Institute for Scientific Information (ISI). The suggestion of this new and third form of carbon (besides graphite and diamond) was both intriguing and controversial. But it was not a subject of widespread investigation until late 1990, when a team of scientists at the University of Arizona and the Max Planck Institute for Nuclear Physics in Heidelberg discovered a method for making C60 in bulk quantities (W. Kratschmer et al., Nature, 347:354-8, 1990; cited 2,117 times through October 1996). Close on the heels of the Arizona/Heidelberg group was Kroto's team at the University of Sussex, who chromatographically purified C60 for the first time and confirmed its structure (R. Taylor et al., Journal of the Chemical Society, 20:1423-5, 1990; cited 411 times through October 1996). As soon as the chemistry community could obtain enough C60 to explore its properties, fullerene fever spread like wildfire. In short order, physicists found high-temperature superconductivity in alkali-doped buckyballs. In the last six years, buckyball research has taken off. Now scientists make fullerenes by the pound instead of by the nanogram. And some predict that one day buckyballs will be used for all sorts of applications-including drug-delivery systems and superconductors. In the following interview, reprinted from the January 1992 issue of ISI's newsletter Science Watch (3[1]:3-4, 1992) Kroto speculates on applications and research horizons for C60. One of his aspirations-to find extraterrestrial buckyballs-was realized last summer when Jeffrey Bada, a geochemist at the Scripps Institution of Oceanography in La Jolla, Calif., described buckyballs that may have come from outside our solar system. By analyzing the amount and isotope ratio of helium atoms trapped inside the buckyballs found in rocks taken from a 1.8 billion-year-old meteor crash site in Canada, Bada surmised that the molecules were formed near a red-giant star. This is the very place Kroto had suggested almost 20 years ago that unusual interstellar carbon chains might be found. The Science Watch interview with Kroto is reprinted with the permission of the newsletter and ISI. For more information on the papers and information discussed in the article, contact Christopher King, editor of Science Watch, ISI, 3501 Market St., Philadelphia, Pa. 19104; (800) 523-1850, Ext. 1341. Fax: (215) 387-1266. E-mail: cking@isinet.com. See also a subsequent series of interviews with the "Discoverers of Buckminsterfullerene" conducted by Istvan Hargittai, the editor of the Chemical Intelligencer, 1:6-26, July 1995. The journal is available from Springer-Verlag, New York. KICK-STARTING A FIELD: Buckyballs, codiscovered by Harold Kroto in 1985, started to become a hot research area in late 1990. Science Watch (SW): It's an inevitable question, but what do you see as the most promising uses for fullerenes? Kroto: Well, it's true. Most people do ask, "What can C60 be used for?" In fact, there was a question in the House of Lords about its potential applications. The answer has to be, at the moment, that it may have many uses, and then again it may turn out to have none at all. I suspect that the latter is highly unlikely, however. It seems to me that the C60 field will find and develop its own applications. The situation appears to be analogous to the discovery of lasers; it was at least a decade before applications of lasers came along. I think that C60 is or will be similar. The chemistry itself is quite novel, and it presents major challenges. For example, with benzene there are six possible positions, but with buckminsterfullerene there are 60. So, it's difficult to foresee all the potential of the fullerenes. I guess I should also say that fundamental scientists are not necessarily the best people to ask about applications. People like myself have, in a sense, spent a lifetime avoiding applications. We're puzzled about interesting things for their own sake, and we follow up on them. However, having said that, I do see some possibilities. SW: Not so long ago, Science Watch spoke to Donald Cram [a professor of chemistry] at the University of California, Los Angeles [Science Watch, 1:1 (3-4), Jan. 1990] about his work on carceplexes and their potential application in drug-delivery systems. Many have talked about using fullerenes for the same purpose. Kroto: Yes. The work of Cram and others in producing these cages or carcerands is one route. Perhaps C60 and the other fullerenes represent the ultimate way to encage an atom. I think it will take some time before we have the requisite chemical technology to incarcerate anything we want. Whether we can do it efficiently in the near future remains to be seen. Certainly we know it can be done with certain metal species. As for drug delivery, the idea is to encage an atom or a molecule, move it to a specific site in the body, and then open up the cage to release the agent. As I mentioned, at this stage we don't yet know exactly how to encage atoms efficiently. We can certainly do it by brute force, vigorous methods. C60 itself is formed in a very crude manner; rational synthesis is a non-trivial matter. But since we have learned to encage certain metal atoms, one can see a way to trap a radioactive species and take it somewhere in the body. For a radioactive atom, cage opening is not necessary. As long as the cage surface is non-toxic and stable, it can be left there or, if not, excreted. SW: What are some other potential applications? Kroto: Well, C60 is a three-dimensional compound on which one can add many different groups. Therefore, we can consider these as nucleation centers. Linking up C60 into polymeric matrices with encapsulated species is, in principle, a possibility. One would hope that this might be a way of designing computer memories at the molecular level. Applications involving the superconducting and ferromagnetic properties of certain types of fullerenes represent another area which people are looking at very carefully. With endohedral complexes, one can choose not only the physical properties but also the electrical or magnetic properties of the cage. We know the behavior of C60 crystals depends on how close atoms are together, and other aspects, and if one can put atoms inside them, then it may be possible to tune the electrical properties to produce very-high-temperature superconductivity. SW: You mentioned that the fullerenes present chemists with many challenges. Kroto: I think, in fact, fullerenes present massive problems for chemists. Fullerene research is expanding chemists' horizons. It's a demanding field analytically and technically, and it's taking chemists and others into areas where they've never gone before. Physicists have already put a lot of effort into understanding the properties of fullerenes. High-temperature superconductivity was one drive, but there will be many others. For example, C60 exhibits strange phases. That's an area for exploration, too. SW: Do you anticipate biologists will get in the game as well? Kroto: I do. One can think of taking these molecules and modifying their overall hydrophobic or hydrophilic properties in order to make a palatable biochemical system. In particular, if one thinks of the larger fullerenes, one imagines that it should be possible to manipulate their shape, and we know, of course, that shape is rather important in biological systems. SW: Has the discovery of the fullerenes pointed to other areas worth pursuing? Kroto: There have been many, many exciting and interesting molecules made in small quantities over the years. One thinks of dodecahedrane, C20H20, which is effectively a hydrogenated fullerene. Its exploitation hasn't gone very far because it's so difficult to make. That was the case with C60 before we could make it in reasonable amounts, or enough so that everyone could start working on some aspect of it. So, I think that there may be many other compounds, ones that we already know about, whose exploitation is inhibited because we don't have a good way of making sufficient material. Probably more effort should be directed at compounds that we already have. SW: What are you and your colleagues at Sussex focusing on now? Kroto: We-which is to say myself, Roger Taylor, David Walton, and others-are developing a Sussex Fullerene Program, which will include topics in transition-metal, organic, and polymer chemistry. One of my own aims at present is to show that C60 is, in fact, in space, but it's still early days. First, we have to decide how to detect it in space. As I've said before, it's a celestial sphere that fell to earth, and now I'm wondering whether it will bounce back into space. I do believe it is there. It was discovered by simulating certain astrophysical conditions, after all. The question is whether it is stable in space. I think it is. I believe that it would be very surprising if C60, in some form, is not responsible for the diffuse interstellar bands that have puzzled astronomers for so long. That's my gut reaction, the feeling in my heart. But if it's not there, then there's something out there that may be even more exciting. My other main interest right now concerns how C60 forms. It forms under the same sort of conditions that graphite forms. In fact, C60 is really a small round form of graphite. On a large scale, such as in large slabs, it is flat. But that's not the most stable form at the microscopic level. The most stable form would be a giant fullerene of some sort. What this means is that, unbelievably, we have overlooked something fundamental from the very beginnings of organic chemistry. The discovery of the fullerenes has changed our picture of how graphite forms and what are the factors controlling its structure. We're starting to recognize that graphite doesn't just happen, and it doesn't want to be flat unless it's in a very large piece. And it doesn't get to be a large piece unless it was in a smaller form beforehand. We now need to focus on why carbon networks are flat on a large scale and why they are round on a small scale. So, my aim is to rewrite the textbook picture of what the most stable form of carbon is and why it takes the different forms it does.

Harold Kroto Contemplates Applications of Nobel-Winning Fullerenes

The Scientist Staff

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