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From Balls to Tubes, and Beyond

chemistry

The Door that the Buckyball Opened

From Balls to Tubes, and Beyond

By Naina S. Ahmad

The birth was accidental, stemming more from serendipity than coming after a long gestation period of scientific analysis and toil. Unexpected results from a laser blasting experiment with graphite chunks, nights of red-eyed agonizing over finding an explanation for these results, and a taped paper model later, the buckyball was born.

Buckyballs are made of carbon. Like other forms of carbon, diamond and graphite, buckyballs are large molecules of repeating units made up of carbon atoms attached to each other. They have their own distinctive arrangement of 60 carbon atoms folding over into a soccer ball. And it is this distinctive structure that imparts the exceptional tensile strength and electrical conductivity to the buckyball molecule.

Defects are key to buckyball shape

In the buckyball, each carbon atom is bonded to three other carbon atoms, forming the corners of a hexagon. Many such repeating units form a network of interconnected hexagons. At the edges of the network, however, each carbon atom can not find a full set of three partners to bond to. For a molecule, these free bonds are bad news, since they mean a lot of energy wasted in searching for a missing partner. To eliminate this energy cost, the sheet of hexagons begins to curl around, like a piece of paper curls up in the fire as it begins to burn, and forms a ball, closing up these loose ends.

Yet, if you take a piece of paper and cut out a hexagon, and tape many of these hexagons side-by-side into a large sheet, you won't be able to fold the sheet over into a smooth ball. In fact, all corners won't meet to close up into a ball at all. Then what provides the uniform curvature for such bending in the buckyball?

Apparently, the hexagonal network sheet harbors occasional defects of pentagons and heptagons, in specific parts of the sheet. These shapes in exactly those particular locations allow the sheet to bend over at the corners to form the smooth surface of the ball. Once the carbon atoms aggregate into this extremely specific conformation, the overall structure of the buckyball is extremely stable and resistant to attempts to break it open.

The big deal behind buckyballs

This structural stability and nature of the buckyball gives it its two unique physical properties.

Firstly, of its four spare electrons, carbon in the buckyball molecule uses three electrons to form bonds with its three neighbors. The lone electron per carbon atom that is not held rigid in bond formation is hence mobile. This free electron per carbon atom imparts high electrical conductivity to the molecule. If 60 copper atoms were gathered to form a single unit, then the buckyball molecule of carbon would have about 50 to 100 times higher electrical conductivity than the copper bundle.

Secondly, in a molecule of buckyball, the atoms are all interconnected with each other. This resulting energetically stable conformation gives the buckyball molecules exceptional tensile strength. Evidence shows that if buckyball molecules were used to form a rod, it would be almost 50 to 100 times as strong as steel, even at one fourth the weight.

The only barrier to mass use of the material was the difficulty with large-scale production. The problem was that the buckyballs could only be produced in miniscule amounts under extreme temperatures, which is hardly conducive to industrial scale production. However, once this extreme temperature of about 500,000 degrees Celsius was achieved by laser, carbon atoms themselves took care of the rest as they automatically self assembled into buckyballs.

In the hopes of producing greater amounts of the buckyballs under milder conditions, scientists began to experiment with different levels of temperatures for different durations. Some metal particles, like cobalt and nickel have been known to help reactions occur at milder conditions of temperature and pressure and are widely used in various industrial processes.

Buckytubes

Scientists tried adding trace amounts of these metals, separately, and then together as a mixture, to see if that made buckyball formation any easier. It was during one such adjustment that they stumbled upon a novel bucky shape - the capped buckytubes.

When a laser beam was aimed at a block of graphite with traces of cobalt and nickel, the graphite vaporized at temperatures exceeding 300,000 degrees. Now, when it came in contact with a comparatively cooler copper collector in a 1200 degree Celsius oven, the carbon atoms appeared to aggregate into molecular tubes instead of their ball shape. Almost as if the ball had been spliced in half, and a belt of another 10 carbon atoms in hexagons added as a layer in between, followed by many more belts layered on, to elongate the buckyball into a buckytube with hemispheric caps at the two opposite ends. Why did carbon suddenly opt for such variety in shape, forming these molecular capped tubes instead of the balls?

Makers of the buckytubes believe that the cobalt and nickel atoms temporarily lodge onto the dangling bonds at the sheet edges, and stall the curling up of the sheet long enough for several belts of hexagons to be layered onto the forming hemisphere. Now it is too late for the forming bucky to turn back into a ball, and it takes the path of least energy consumption, accepting many more belts and elongating into a closed hollow tube an atom thick.

What this implies may hold significant promise in the study and use of bucky molecules. Given appropriate conditions of extreme temperatures, carbon atoms can self-aggregate into bucky shapes. Tampering with the conditions can change the fate of the shape that these carbon atoms will assemble into. Once we figure out the appropriate settings, we can theoretically make buckytubes go on forever, into long fibers one atom thick. Macroscopically long nanotubes. And its exceptional strength, thermal and electrical conductivity coupled with its atomic scale size can make it a truly marvelous material for a wide range of purposes.

Tubes used in electronics, biology

So what exactly can we use these molecular tubes for? On the macroscopic scale, hundreds of thousands of such nanotubes can be laid side by side or roped into light strong wires or composites. The key to the future of lightweight vehicles, earthquake resistant buildings, light bulletproof vests; you name it and the material scientists excitedly claim that the buckytubes may hold promise for it.

On a microscopic scale, buckytubes also seem to hold great potential. Today, miniaturization of computer chips and other electronic parts are severely limited by the size of the wires that must feed the components. Scientists are hoping that the mechanical resilience and electrical conductivity of these molecular tubes may hold extensive promise for wiring up nanoscale electronic devices.

Buckytubes seem to excite scientists from a range of disciplines - from the “dry” world of electronics to the “wet” world of life sciences. Applying electrical fields to the tips of these tubes have been shown to open up the carbon connections, allowing the tips to be saddled with other chemical molecules. Such open-ended nanostraws could penetrate into a cellular structure for chemical probing in diagnostic purposes, or could be used as ultra-small pipettes to inject molecules into living cells with almost no damage to the latter. Perhaps these hollow tubes can be stuffed with drugs and closed up again, a unique drug delivery mechanism that may be coordinated to use cellular signals in the body to open up when the drugs are needed.

Yet perhaps most thrilling is what buckyballs really are. They are carbon - what every component of the living world is made up of. Previously the inanimate electronic world had monopoly over high tensile strengths and high electrical conductivities, while the living world specialized in complicated interconnected machinery on a nano scale. Now, through these buckymolecules, these two worlds may be united, and the interface between the dry and wet worlds may meet. Gadgets whose intricacy rivals that of living cells, bacteria-size sensors or drugs whose active ingredients resemble tiny, intelligent robots more than ordinary chemicals may not be as far off into the future.

Scientists have discovered the correct conditions that can instigate carbon atoms to form this extraordinary class of molecules, buckytubes. With the right set of environmental variables, can we program the carbon in our body cells to self assemble into these bucky units?

Sure, if the 300,000 degree laser pulse doesn’t fry the living cells first, claim skeptics. Remember that carbon stays comfortable in its form as graphite or charcoal until it is heated up to the extreme temperature. Only when it has the perfect condition, of high temperature and suitable traces of metals to help it along the way, only then can carbon leisurely fold into the marvelous bucky shapes.

Advocates of biological buckytube formation are careful with their replies. They claim that if carbon can self assemble into bucky shapes at a particular set of extremely stringent conditions, perhaps the same self-assembly can be instigated in a biological system under an entirely mild set of conditions. Far fetched as this may sound, it may not be entirely impossible. Biological molecules are capable of discharging a wide range of functions that their inorganic counterparts in our labs would stumble and stall at in the absence of extreme conditions. There are bodily reactions that take place even under the mild temperature and pressure within our bodies; made possible by biological molecules such as enzymes.

These same reactions may never even take place in the lab, and may proceed slowly under extreme pressure and temperatures in the industry. Scientists believe that perhaps one day it will be possible to harness this remarkable property of biomolecules to induce the carbon within living systems to self assemble into bucky tubes, without the laser.

There is a nice little trick that holds much more promise than may be immediately apparent. Even with their diverse abilities, living cells invariably take a back seat when it comes to electrical conductivity. Even what electric impulses there are in the body is generated and maintained by proteins, enzymes and a host of other complex molecules interacting in even more intricate pathways.

If we could code target parts of our cells to grow buckytubes, it could mean the key to treating nerve regeneration in paralysis cases and in neurological diseases. It could mean correction of inadequate signaling pathways, and may promise a means of treating a host of other related diseases. And laying the groundwork for such electrical circuitry at the molecular level within the living system may not be too far off in the future with these marvelous bucky shapes.