This module was developed as part of a Rice University Class called "Nanotechnology: Content and Context" initially funded by the National Science Foundation under Grant No. EEC-0407237. It was conceived, researched, written and edited by students in the Fall 2005 version of the class, and reviewed by participating professors.
““This year's Nobel Prize in Chemistry has implications for all the natural sciences. The seeds of the discovery were sowed by a desire to understand the behavior of carbon in red giant stars and interstellar gas clouds. The discovery of fullerenes has expanded our knowledge and changed our thinking in chemistry and physics. It has given us new hypotheses on the occurrence of carbon in the universe. It has also led us to discover small quantities of fullerenes in geological formations. Fullerenes are probably present in much larger amounts on earth than previously believed. It has been shown that most sooty flames contain small quantities of fullerenes. Think of this the next time you light a candle!””
-From the presentation speech for the Nobel Prize in Chemistry, 1996
In 1996, the Royal Swedish Academy of Sciences awarded the Nobel Prize in Chemistry, the most prestigious award in the world for chemists, to Richard Smalley, Robert Curl, and Harold Kroto for their discovery of fullerenes. They discovered fullerenes (also called buckyballs) in 1985, but the special properties of the buckyballs took a few years to prove and categorize. Although by 1996 no practical applications of buckyballs had been produced, scientists appreciated the direction this discovery based in organic chemistry had led scientific research, as well as its specific contributions to various other fields. The accidental discovery of fullerenes also emphasizes the benefits and unexpected results which can arise when scientists with different backgrounds and research aims collaborate in the laboratory.
Before going into detail about the actual buckyball, we should discuss the element that makes its structure possible, carbon. Carbon is the sixth element on the periodic table, and has been found to be at least a partial constituent in over 90 per cent of all chemicals known to man. Indeed, its electron-bonding properties grant it a versatility specific to carbon, allowing it to be so widely functionalized, and more importantly, the reason for life on Earth. Anything that is living is necessarily chemically based on Carbon atoms, and for this reason, substances containing carbon are called organic compounds, and the study of them is called organic chemistry.
Though carbon is involved in chemistry with all sorts of other elements and compounds, it can also exist in pure carbon states such as graphite and diamond. Graphite and diamond are two different allotropes of carbon. An allotrope is a specific physical arrangement of atoms of an element. So although diamond and graphite are both pure carbon, because the crystalline structure of each is significantly different, their chemical and physical properties (as well as value) are very different.
Above: diamond Below: carbon. Notice how the structure of the two allotropes vary, even though they are both made of the same carbon atoms (black)
Diamond and graphite are not the only known allotropes pf carbon, chaoit and carbon(VI), discovered in 1968 and 1972, respectively, have also been found. Even more recently, the Buckminsterfullerenes, the subject of this module, were discovered at Rice by Smalley, Kroto,and Curl. Buckminsterfullerenes is actually a class of allotropes
Above: C540 Below: C60 Both of these are different allotropes of carbon. C60 is the most common and the most popularized of the Buckminsterfullerenes. Not shown is the second most common Buckyball, C70.
In fact, scientists have now discovered hundreds of buckyballs of different sizes, all with the trademark spherical-like shape. To differentiate them, each allotrope is denoted as C (for carbon) with the number of carbon atoms in the subscript (i.e. C80). Technically, the geometrical shapes that these buckyballs share are actually known as geodesics, or rather, polyhedrons that approximate spheres. Specifically, the commonly depicted C60 buckyball is a truncated icosahedron. A more satisfactory representation of it can be had in a soccer ball, with which it shares the exact same shape. It is made up of 12 pentagons, each surrounded by 5 hexagons (20 in all).
British chemist Harold W. Kroto at the University of Sussex was studying strange chains of carbon atoms found in space through microwave spectroscopy, a science that studies the absorption spectra of stellar particles billions of kilometers away to identify what compounds are found in space. This is possible because every element radiates a specific frequency of light that is unique to that element, which can observed using radiotelescopes. The elements can then be identified because a fundamental rule of matter stating that the intrinsic properties of elements apply throughout the universe, which means that the elements will emit the same frequency regardless of where they are found in the universe. Kroto took spectroscopic readings near carbon-rich red giants, or old stars with very large radii and relatively low surface temperatures, and compared them to spectrum lines of well-characterized substances. He identified the dust to be made of long alternating chains of carbon and nitrogen atoms known as cynopolyynes, which are also found in interstellar clouds. However Kroto believed that the chains were formed in the stellar atmospheres of red giants and not in interstellar clouds, but he had to study the particles more closely.
At the same time, Richard Smalley was doing research on cluster chemistry, at Rice University in Houston, Texas. “Clusters” are aggregates of atoms or molecules, between microscopic and macroscopic sizes, that exist briefly. Smalley had been studying clusters of metal atoms with the help of Robert Curl, using an apparatus Smalley had in his laboratory. This laser-supersonic cluster beam apparatus had the ability to vaporize nearly any known material into plasma using a laser, which is a highly concentrated beam of light with extremely high energy.
Through an acquaintance with Curl, Kroto contacted Smalley and discussed the possibility of using his apparatus to recreate the high-heat conditions of a red giant’s atmosphere in order to study the clusters of carbon produced, which might give Kroto insight as to the formation of the carbon chains. Smalley conceded and Kroto arrived in Smalley’s laboratory in Rice University on September 1, 1985 whom began working on the experiment along with graduate students J.R. Heath and S.C. O’Brien.
Smalley’s apparatus, shown above, fires a high energy laser beam at a rotating disk of graphite in a helium-filled vacuum chamber. Helium is used because it is an inert gas and therefore does not react with the gaseous carbon. The intense heating of the surface of the graphite breaks the C—C bonds because of the intense energy. Once vaporized, the carbon atoms cool and condense in the high-pressure helium gas, colliding and forming new bond arrangements. Immediately upon cooling several degrees above absolute zero in a chamber, the carbon leads to a mass spectrometer for further analysis.
A mass spectrometer uses an atom or molecule’s weight and electric charge to separate it from other molecules. This is done by ionizing the molecules, which is done by bombarding the molecules with high energy electrons which then knocks off electrons. If an electron is removed from an otherwise neutral molecule, then the molecule becomes a positively charged ion or cation. The charged particles are then accelerated by passing through electric plates and then filtered through a slit. A stream of charged particles exits the slit and is then deflected by a magnetic field into a curved path. Because all the particles have a charge of +1, the magnetic field exerts the same amount of force on them, however, the more massive ions are deflected less, and thus a separation occurs. By adjusting the strength of the accelerating electric plates or the deflecting magnetic field, a specific mass can be selected to enter the receptor on the end. After adjusting the experiment, it became greatly evident that the most dominant molecule measured was 720 amu (atomic mass units). By dividing this number by the mass of a single carbon atom (12 amu), it was deduced that the molecule was comprised of 60 carbon atoms (720 / 12 = 60).
The next task was to develop a model for the structure of C60, this new allotrope of carbon. Because it was overwhelmingly dominant, Smalley reasoned the molecule had to be the very stable. The preferred geometry for stable molecule would reasonably be spherical, because this would mean that all bonding capabilities for carbon would be satisfied. If it were a chain or sheet like graphite, the carbon atoms could still bond at the ends, but if it were circular all ends would meet. Another hint as to the arrangement of the molecule was that there must be a high degree of symmetry for a molecule as stable as C60. Constructing a model that satisfied these requirements was fairly difficult and the group of scientists experimented with several models before coming to a conclusion. As a last resort, Smalley made a paper model by cutting out paper pentagons and hexagons in which he tried to stick them together so that the figure had 60 vertices. Smalley found that he create a sphere made out of 12 pentagons interlocking 20 hexagons to make a ball. The ball even bounced. To ensure that the shape fulfilled the bonding capabilities of carbon, Kroto and Curl added sticky labels to represent double bonds. The resulting shape is that of a truncated icosahedron, the same as that of a soccer ball. Smalley, Curl, and Kroto named the molecule buckminsterfullerene after the American architect and engineer Richard Buckminster Fuller who used hexagons and pentagons for the basic design of his geodesic domes.
Eleven days after they had begun, the scientist submitted their discovery to the prestigious journal Nature in a manuscript titled “C60 Buckminsterfullerene.” The journal received it on the 13th of September and published it on the 14th of November 1985. The controversial discovery sparked approval and criticism for a molecule that was remarkably symmetrical and stable.
Experimentally, Smalley, Kroto, and Curl, first created the buckyballs using Smalley’s laser-supersonic cluster beam apparatus to knock carbons off of a plate and into a high pressure stream of helium atoms. They would be carried off and immediately be cooled to only a few degrees above absolute zero, where they would aggregate and form these buckyballs. This method however, resulted in low yields of buckyballs, and it took nearly five years until in 1990 newer methods developed by American and German scientists could manufacture buckyballs in large quantities.
The common method today involves transmitting a large current between two graphite electrodes in an inert atmosphere, such as Helium. This gives rise to a carbon plasma arc bridging the two electors, which cools instantaneously and leaves behind a sooty residue from which the buckyballs can be extracted.
These methods of producing buckyballs do deserve a great deal of applaud. However, humans cannot take all, or even most of, the credit for the production of fullerenes. As a matter of fact, buckyballs occur in nature, naturally, and in greater amounts than expected. Buckyballs are known to exist in interstellar dust and in geological formations on Earth. Even closer to home are the buckyballs that naturally form in the wax and soot from a burning candle, as the flame on the wick provides the sufficient conditions for such processes to occur. Buckyballs are the new sensation for us, but to Nature, they are old news.
Since buckyballs are still relatively new, there properties are still being heavily studied. Buckyballs’ unique shape and electron bonding give them interesting properties on the physical level, and on the chemical level.
Since spheres in nature are known to be the most stable configurations, one could expect the same from fullerenes. Indeed this is one of the reasons why Smalley, Curl, and Kroto initially considered its shape. Their tests showed that it was extremely stable, and thus, they reasoned, it could be a spherical-like geodesic. Also, fullerenes are resilient to impact and deformation. This means, that squeezing a buckyball and then releasing it would result in its popping back in shape. Or perhaps, if it was thrown against an object it would bounce back; ironically just like the very soccer ball it resembles.
Buckyballs are also extremely stable in the chemical sense. Since all the carbon-carbon bonds are optimized in their configuration, they become very inert, and are not as prone to reactions as other carbon molecules. What makes these bonds special is a property called aromaticity. Normally, electrons are fixed in whatever bond they constitute. Whereas in aromatic molecules, of which hexagonal carbon rings are a prime example, electrons are free to move (“delocalize”) among other bonds. Since all the fullerenes have the cyclo-hexanes in abundance, they are very aromatic, and thus have very stable, inert, carbon bonds. Buckyballs, though sparingly soluble in many solvents, are in fact the only known carbon allotropes to be soluble.
An interesting feature of Fullerenes is that their hollow structure allows them to hold other atoms inside them. The applications of this are abound, and are being studied to great extent.
Important to note about any new material is its health concerns. Although believed to be relatively inert, experiments by Eva Oberdörster at Southern Methodist University, presented some possible dangers of fullerenes. She introduced buckyballs into water at concentrations of 0.5 parts per million, and found that largemouth bass suffered a 17-fold increase in cellular damage in the brain tissue after 48 hours. The damage was of the type lipid peroxidation, which is known to impair the functioning of cell membranes. Their livers were also inflamed and genes responsible for producing repair enzymes were activated. As of 10/20/05, the SMU work had not been peer reviewed.
After the astrophysicists D.R. Huffmann and W. Kratschmer managed to produce larger quantities of fullerenes in 1990, scientists further investigated the structure and characteristics of buckyballs. Research on buckyballs has led to the synthesis of over 1000 new compounds with exciting properties, and over 100 patents related to buckyballs have been filed in the US. In addition, an important new material, nanotubes, has exploded onto the scientific scene in recent years. The discovery and manufacture of nanotubes resulted directly from research on buckyballs. Finally, although buckyballs have not yet been used in any practical applications, partly due to the high cost of material, researchers are using buckyballs to learn more about the history of our world, and companies are devising some interesting uses for buckyballs even today.
The discovery of nanotubes in 1991 by S. Iijima has been by far the buckyball’s most significant contribution to current research. Nanotubes, both single- and multi-walled, can be thought of as sheets of graphite rolled into cylinders and sometimes capped with half-fullerenes. Nanotubes, like fullerenes, possess some very unique properties, such as high electrical and thermal conductivity, high mechanical strength, and high surface area. In fact, carbon nanotubes provide a clear example of the special properties inherent at the quantum level because they can act as either semi-conductors or metals, unlike macroscopic quantities of carbon molecules. These properties make nanotubes extremely interesting to researchers and companies, who are already developing many potentially revolutionary uses for them.
A paper published on March 28, 2000 in the Proceedings of the National Academy of Sciences (PNAS) by Becker, Poreda, and Bunch uses buckyballs to provide new evidence for early periods in earth’s geological and biological history. By exploiting the unique properties of buckyballs, these three scientists were able to study geology in a new way. First of all, the unique ability to extract fullerenes (unlike graphite and diamond) from organic solvents allowed them to isolate carbon material in the meteorites, then the unique cage-like structure of fullerenes allowed them to investigate the noble gases enclosed within the ancient fullerenes. In their study, the researches found helium of extraterrestrial origin trapped inside buckyballs extracted from two meteorites and sedimentary clay layers from 2 billion and 65 million years ago respectively. The helium inside these buckyballs bears unusual ratios of 3He/4He coupled with non-atmospheric ratios of 40Ar/36Ar, which according to their research indicates extraterrestrial origin. In addition, they have shown that these fullerenes could not have been formed upon impact of the meteorite or during subsequent forest fires.iBecker, Poreda, and Bunch. 2982.
The discovery of the extraterrestrial origin of the enclosed helium has far-reaching implications for the history of the earth. For example, the existence of the carrier phase of fullerenes suggests that “fullerenes, volatiles, and perhaps other organic compounds were being exogenously delivered to the early Earth and other planets throughout time.”iiBecker, Poreda, and Bunch, 2982. With more research, it might even be possible to determine whether meteorite impacts on earth could have triggered global changes or even brought carbon and gases to earth that allowed for the development of life!
Why does it matter? Why should anyone care? These buckyballs are giving scientists information about allotropes of carbon never before conceived. More importantly, these buckyballs might allow engineers and doctors do what was never before possible. These are some of the applications for buckyballs currently in research.
Medical uses for buckyballs
Buckyballs are now being considered for uses in the field of medicine, both as diagnostic tools and drug candidates. Simon Friedman, a researcher at the University of Kansas, began experimenting with buckyballs as possible drug treatments in 1991. Because buckyballs have a rigid structure (unlike benzene rings, often used for similar purposes), researchers are able to attach other molecules to it in specific configurations to create precise interactions with a target molecule. For example, Friedman has created a protease inhibitor that attaches to the active site of HIV 50 times better than other molecules. C Sixty, a Toronto based company that specializes in medical uses of fullerenes, plans to test on humans two new fullerene-based drugs for Lou Gehrig’s disease and HIV in the near future.
Another medical use for buckyballs is taking place in the field of diagnostics. Buckyballs unique cage-like structure might allow it to take the place of other molecules in shuttling toxic metal substances through the human body during MRI scans. Usually, the metal gadolinium is attached to another molecule and sent into the body to provide contrast on the MRI scans, but unfortunately these molecules are excreted from the system quickly to reduce the chance of toxic poisoning in the subject. Lon Wilson of Rice University and researchers at TDA Research have encased gadolinium inside buckyballs, where they cannot do harm to the patient, allowing them to remain inside the body longer, but still appear in MRI’s. So far this application has been successfully tested in one rat. Wilson and others have begun to develop even more applications for the tiny little cages that could one day help revolutionize medicine.
Engineering Uses
The Scanning Tunneling Microscope (STM) is one of the foremost tools in microscopy today; boasting the ability to to map out the topology of material surfaces at atomic resolution (i.e. on the order of 0.2 nanometers). The STM achieves this feat by bringing a needle point, functioning as a probe, within just several nanometers of a sample's surface. At these minute scales, even small disturbances can cause the tip to crach into the sample and deform itself. A possible solution to this problem would be the replacement of the standard needle point with a buckyball. As discussed previously, fullerenes bear amazing resilience due to their spherical geometry, and would resist distortions from such collisions.
European scientists are aiming to use buckyballs in circuit. So far, they have been able to attach a single fullerene to a copper surface, and then, through a process called shrink wrapping, fitted its center with a metal ion and made it smaller to increases electric conductivity by a hundred times.
Because of their shapes, they could be used equivalently to ball bearings, and thus allow surfaces to roll over each other, making the fullerenes equivalently lubricants
It has been shown that fitting a potassium ion in the buckyball causes it to become superconductive. Ways to exploit this are in the research stages.
Attaching metals onto the surface of fullerenes offers the possibility for buckyballs to become catalysts.
As we can see, we have come along way since that fateful year of 1985. Strides have been made. We have seen the rise of nanotubes and the new science of Nanotechnology. We are still studying the chemical and physical properties of buckyballs and continue to be amazed. They have already proved to us why they are important; their possible uses in medicine and in engineering are broad and profound, while the health risks they posed have yet to be fully analyzed. Only time will tell whether they will meet, or exceed our expectations as we unfold this brave new world.
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Gorman, Jessica. Buckymedicine: Coming soon to a pharmacy near you?. Science News Online: July 13, 2002, vol. 162, no. 2. http://www.sciencenews.org/articles/2002071
