Buckminster fullerenes with three metal atoms caged inside of them could give designers new materials for medicine, tribology, and molecular computing.
Buckminster fullerenes such as C60, C70, and C80 are hollow molecules made of interlocking pentagons and hexagons of carbon atoms. They were discovered in 1985 after being produced in a lab and got their name for their resemblance to R. Buckminster Fuller's geodesic domes. But they do exist in nature and are produced in small quantities by fires and lightning.
Scientists and engineers have been excited about them ever since. The molecules represent a form of carbon (after graphite and diamond) and seem to have a spectrum of properties that includes superconductivity, insulation, tolerance to heat, and incredible strength. And the fact they are made of carbon means there are many sites on the molecule to attach other atoms and chemically customize them.
But so far, buckyballs have been somewhat disappointing, generating more curiosity than cash. However, researchers and engineers at Luna nanoWorks, Danville, Va., are developing a new type of fullerene called Trimetaspheres that show promise in several industries, especially medicine.
Trimetaspheres consist of three metallic atoms surrounding a nitrogen atom, and all four are enclosed in a C80 buckyball. They were first synthesized at the Virginia Polytechnic Institute and State University. Luna Innovations, Blacksburg,Va., the parent company of nanoWorks, has licensed this composition-of-matter patent, which gives them a lock on selling them for the next decade or longer.
Luna makes Trimetaspheres in a sealed reactor or generator by running an arc through two electrodes. The electrodes are packed with graphite, a form of carbon, and other ingredients Luna researchers are reluctant to talk about. Luna staff is also tight-lipped about how tightly they pack the graphite electrodes and other parameters such as current, pressure (or vacuum), and what type of atmospheres that optimize yield, purity, and composition. The arc vaporizes the carbon and other compounds in the electrodes and leaves a sooty deposit. But if you run the soot through a mass spectrometer, results show peaks at abnormally high mass readings. These correspond to buckyballs, which are relatively massive, as well as even larger Trimetaspheres.
Relying on chemical engineering, the technicians eliminate amorphous carbon and metal oxides from the soot, the main by-products of the process. Then they use solvent extraction to isolate nanomaterials. High-pressure liquid chromatography purifies the mixture of nanomaterial and solvent. The solvent/nonmaterial mixture is pumped through a special silica-bed column. Smaller molecules and lower-weight materials make it through the column first, with the heavier ones following. Based on specific retention times, technicians know when solvent coming out of the tower will contain Trimetaspheres.
"For example, if I know a C60 takes 10 min to get through the column and stops coming through after 15 min, and I wanted C60, I'd collect what came out between those two times," says Charles Gause, vice president of operations at Luna. "And, since Trimetaspheres are larger than C60, they are pumped out after them at a specific time."
This purification method (called "time of flight" because it is based on how long it takes for specific molecules to exit the extraction tower) is also used to isolate different buckyballs.
After getting the Trimetaspheres separated, Luna "functionalizes" them, which means they chemically alter the molecule's surface, adding compounds and other molecules to give them specific properties. One common functionalization adds hydroxylated (OH) molecules to dangle off several carbon atoms making up the Trimetasphere cage. This chemistry is soluble in water, an important property for medical applications.
"There are many variables that affect output, including the recipe used to pack rods or electrodes, arc conditions, and pressure inside the reactor," says Gause. "But we've had years of experience and learned about these materials' behaviors and can make educated guesses as how to improve the process and yields."
An easy way to boost throughput, for example, is to use larger graphite rods and extraction towers, physically scaling up the current process. The company has already done some of this and seen yields increase by two orders of magnitude over the last three or four years. "But we want to go from a batch process, where we are now, to a more continuous and automated process," notes Gause. "We plan on building new reactors and separation stations, but one of our biggest bottlenecks is liquid chromatography. So we'll be looking at using higher solvent pressures as well as different methods of sorting and purifying materials."
One of the first steps on the road to optimizing yields and throughputs is to tightly monitor their processes. "We want to know exactly what is happening," says Gause, "We use statisticalprocess controls to understand when processes are going out of control and learn how to bring them back in line."
Luna is also taking proactive steps to ensure the safety of their staff and customers. Little is known on the effects of inhaling nanomaterials, getting them in cuts, or other potential health risks, so there are no government regulations yet. And even HEPA filters are useless against them. "But we have everyone wear safety gear, enclose the reactors in hoods with glovebox access, and we use custom filters built for nanomaterials," says Gause.
Currently, Luna is still building the labs, developing a cost structure, and working with other companies. "Our initial goal is to be selling kilos of nanomaterials per year, and in the long term, kilos per month and perhaps hundreds of kilos per year," says Gause. But even he readily admits, how fast the company grows depends on finding the right applications for Trimetaspheres and lessexpensive manufacturing methods.
That Killer APP
The most promising use for Trimetaspheres, so far, is as a contrast agent for magnetic-resonance imaging (MRI). Trimetaspheres containing gadolinium, a toxic rare-earth element, are altered to be water soluble. They are injected into a patient, usually near the site of interest, just before being scanned. The molecules react to the magnetic field, exciting the water molecules around them and altering the magnetic field. Meanwhile, the cage protects the body from harmful affects of the metal atoms. The metal changes the reflexivity of the water molecules and seems to increase the dynamic range of the image, giving better contrast and definition to small details.
"Suppose you're getting an MRI on your knee and you've had knee problems before. Scar tissue from previous injuries or surgery might block or obscure underlying details," explains Gause. "The Trimetasphere contrast agent would let doctors see the difference between normal tissue and scar tissue."
Ideally, Luna wants to craft spheres with affinities to specific organs and types of cells, then insert radioactive metals or other therapeutic agents inside the cages. Put inside the body, these compounds would travel to the thyroid or liver, or perhaps to cancer cells or tumors, and deliver treatment.
Tests show that less of Luna's contrast agent is needed than other MRI imaging agents, which should cut costs and potential side effects. (Other agents on the market cause allergy-type reactions, including nausea, headaches, dizziness, chills, and occasionally convulsions.) The company says its agent is a vast improvement over currently used agents and estimates the market for MRI agents at $1 billion annually. Researchers are looking into how the body eliminates Trimetaspheres to ensure there are no long-term biological problems such as liver or kidney damage.
Other possible applications include molecular electronics, coatings, and a different type of solar cell.
"If we can put a large and small atom inside the cage, they could create a dipole moment," says Gause. "Apply a magnetic field and it would swing one way, remove the field and it would keep spinning. This could be used as a memory device or a switch."
The structure of Trimetaspheres make them stable, so you can deposit them on various substrates using several different methods. "For example, they survive temperatures to 400°C without the cages burning off," says Gause. The material might be used as an anticorrosion coating, either as a barrier, blocking the substrate from oxygen and water, or acting as a charge receptor and taking the electrochemical approach.
They could also play an important role in anti-EMI coatings, paints, and films. Another early idea was to use them as lubricants or antifriction coatings. Being only 0.8 nm in diameter, they are already much smaller than the graphite used as a lubricant. There are few particles or debris smaller than Trimetaspheres, so they might be too small to make a difference. Of course, they might find use lubricating MEMS devices.
And while the material can be used to build photovoltaics, silicon is hard to beat in terms of efficiency. "Silicon in photovoltaics is always crystalline or polycrystalline, and extremely stiff," says Gause. "But we can fashion Trimetaspheres into extremely thin and flexible films or coatings."
|Buckyballs||1985||Large spherical molecules made of carbon||Lubricants, superconducting wires, antifriction coatings, photovoltaics, copier toner, diamond films, drugs and drug-delivery systems|| |
|Trimetaspheres||1999||Buckyballs containing three metal atoms and a nitrogen atom||Same as buckyballs||MRI imaging agent|
|Nanotubes, double-walled||1991||Concentric tubes of carbon atoms||Nano-sized test tubes, molecular electronics||Tips for atomic-force microscopes, reinforcing fibers in plastics and composites.|
|Nanotubes, single-walled||1993||Tubes made of interlocked carbon atoms, much like an elongated buckyball with truncated ends||Cables, fabrics, EMI shielding, superconductive wires||Field emitters for display panels|
|Peas in a pod, or fullerenes inside nanotubes||1998|| |
Nanotubes with fullerenes insides
|Molecular electronics, sensors|| |
The Ultimate in Recycling
Luna nanoWorks is investing nearly $6.5 million to retrofit a 24,000-ft 2 tobacco warehouse that dates from the 19th century into a high-tech nanomaterial production facility. It is located in Danville, Va., an area that has seen better days but seems to be on the road to recovery. "We looked at other sites in southwest and northern Virginia, including some near D.C. and Maryland, as well as the Tidewater area around Norfolk," says Charles Gause, vice president of operations at Luna. "But we ended up in this great old building because it just made sense. The quality of life is outstanding, the workforce has a manufacturing heritage, and there have been many major investments and initiatives by the State and local agencies to turn things around.
"It's also easy to get the people we need to move here," he says. He points to Dr. Stephen Wilson, a leading researcher on fullerenes and nanomaterials, as well as a successful entrepreneur, as exhibit number one. Luna lured him out of his tenured position at NYU partially on the basis of their Danville location.
Prior to the mid-1980s, a few scientists looking at returns from radiotelescopes suspected that large molecules made of long, strange chains of carbon existed in outer space near red giant stars. A team of researchers tried to replicate conditions found near these stars and ended up creating buckminster fullerenes in a lab at Rice University in 1985. The oversized molecules hadn't been seen before, but since then have been found in nature.
In 1990, German and American scientists independently stumbled on a more effective method of making large quantities of buckyballs. Still, prices are in the $10,000/lb range, depending on purity. This keeps them in the labs and out of consumer goods, at least for now. Engineers are still looking at new ways to use these carbon-based materials, as well as new forms and less-expensive production methods.