Fullerenes And Their Applications
A fullerene is an allotrope of carbon whose molecule consists of carbon atoms connected by single and double bonds to form a closed or partially closed mesh, with fused rings of five to seven atoms. The molecule may be a hollow sphere, ellipsoid, tube, or many other shapes and sizes. The Buckminster Fullerene alias the Bucky Ball is the most popular amongst all the fullerenes. The Bucky Ball has 60 carbon atoms arranged in a truncated icosahedron structure with twenty hexagons and twelve pentagons.
Discovery Of Fullerenes
Fullerenes were first discovered by Harrold Kroto, an organic chemist in the University of Sussex in the United Kingdom became fascinated with various “peculiar” aspects of carbon chemistry. He also was interested in astrochemistry, the makeup of space and celestial bodies in the universe. In the year of 1984, Kroto met his friend Robert Curl, an American chemist who was working with colleague Richard Smalley at Rice University in Houston, Texas, to study atom clusters using a special instrument called an AP2 (“app-two”), the machine helps scientists study clusters of any element. Kroto accompanied Curl back to his lab and examined the machine. He saw the possibility of putting carbon in it to explore his theory about carbon chain formation. But Smalley had his own research to perform and didn’t initially offer the AP2. However, a year later, Smalley agreed to let Kroto use the instrument for his experiments. Late in 1985, Kroto arrived in Houston to begin his experiment with Smalley and Curl. The three scientists, aided by graduate students Sean O’Brien, James Heath and Yuan Liu, conducted the study under the instructions of Kroto. In the first two days of the study, amazing results were obtained and one of them led to the discovery of the fullerene. The molecule, C-60, formed very readily and exhibited extraordinary stability.
Kroto remembered about Buckminster Fuller, an American architect known for designing spherical structures called geodesic domes. Buckminister had designed Montreal Biosphère at the 1967 World Exposition in Montreal which inspired Kroto to name the new molecule as “Buckminister Fullerene”.
In 1990 physicists Donald R. Huffman of the United States and Wolfgang Krätschmer of Germany announced a simple technique for producing macroscopic quantities of fullerenes, using an electric arc between two graphite rods in a helium atmosphere to vaporize carbon.
The Unique Functionalization of Fullerenes
Fullerenes are not super aromatic nature. It behaves like a polyene and has the ability to accept electrons. The fullerene molecules are hydrophobic in nature and have a limited solubility in organic solvents. . Deviation from planarity has been observed due to geometrical constraints which result in a loss of strain due to change in its hybridization from sp2 to sp3. This causes the molecule to be more reactive and therefore more versatile for several applications.
The Potential Applications of Fullerenes
Fullerenes and Hydrogen Fuel Cells
Fullerenes have an ability to retain a maximum of 6.1% of hydrogen due to their cage-like structures. The fullerene on being hydrogenated forms the C-H Bond which are weaker in bond strength as compared to that of the C-C Bond. When these hydrogenated fullerenes are heated, the C-H Bonds break and the original structure of the fullerene is preserved. This unique property of the fullerene makes it a perfectly suitable for hydrogen storage in the fuel cells. Mostly Exohedral Fullerenes are used for this application. These Exohedral Fullerenes are doped with Boron and Complexes of Transition Metals. Boron is doped with the endohedral fullerenes as it has the ability to increase the stability of the entire structure by increasing the binding energy, reduces the weight of the complex and increases the hydrogen carrying capacity of the complex. As most of the hydrogen storing hydride materials are dangerous and have low storage capabilities, fullerene hydrides can be used in fuel cells of electric cars safely.
Fullerenes and Photovoltaic Cells
C-60 is an electron-deficient system and it acts as an electrophile in various chemical reactions. It has found out that C-60 in an addition reaction with Carbene to form methanefullerene; solid-state synthesis in the presence of KCN leading to dimers and high-pressure synthesis to obtain polymers. Many of these polymer-fullerenes are used in organic photovoltaics since they have the ability to exhibit ultrafast photo-induced charge transfer, while the reversible reaction is slower. The bulk heterojunction of the solar cell uses fullerene derivate phenyl –C61 butyric acid methyl ester (PCBN) with good efficiencies (about 5%). Stanford scientists built the first all-carbon solar cells (click on the link below for more details).
Fullerenes and Pharmaceuticals
Fullerenes hold a very important place in the pharmaceutical sector. The reason behind their importance in the pharmaceutical sector can be attributed to its ability to readily react with free radicals that are harmful to the human body and act as antioxidants. The ability of the fullerenes to control the damage caused to the nervous system due to due to diseases such as Alzheimer’s and Lou Gehrig’s disease (ALS) is being studied with great interest. The sponge-like effect of fullerenes towards radicals, destroys 20 free radicals per C-60 radical. The bioactivity of the fullerene-based antioxidants is 100 times more effective than the current leading drugs in the market. Viral infection can be knocked out with fullerenes(To read more about this click the link below).
Researchers have developed a single-step method to obtain water-soluble fullerene compounds with significant biological properties, such as the ability to effectively suppress HIV(To read more about this click the link below).
Attachment of hydrophilic moieties to the hydrophobic C60 fullerene can make it soluble in water. Consequently, they become capable of transporting drugs to the cells. These functionalized fullerenes have the ability to cross the cell membrane and bind with the mitochondria.
Fullerenes and Radioactive Tracers
Fullerene cages with an encapsulated molecule have many potential applications. Such fullerene cages are called Endohedral Fullerenes. Improved synthesis of endohedral formation was introduced in the year of 1999 by replacing helium by nitrogen in the conventional Krätschmer-Huffman generator. Molecules of metals, noble gases and multimetallofullerene can be encapsulated in the fullerene cage. Rare-earth ions such as Gd inserted in the fullerene could potentially be used as diagnostic MRI contrast agents for several diseases. These materials are highly stable even at higher temperatures and more effective than fullerenes. Since the cage-like structure of the fullerene screens the magnetic ion from the
external environment, these molecules have exceptional spin properties.
Fullerenes and Superconductivity
Superconductivity is a state of matter in which the resistance of a sample becomes zero, and in which no magnetic field is allowed to penetrate the sample. Particularly important are crystalline compounds of C60 with alkali metals and alkaline earth metals; these compounds are the only molecular systems to exhibit superconductivity at relatively high temperatures above 19 K. Superconductivity is observed in the range 19 to 40 K, equivalent to −254 to −233 °C or −425 to −387 °F. In the year of 2003, the Berkeley lab started working on the electrical properties of the C-60 monolayer(To read more about this click the link below).
In the year of 2012, the NIMS team succeeded for the first time in the world in realizing superconductivity in a fullerene-based material by adding potassium to fullerene nanowhiskers, which are a nano-sized thread-like substance that can be synthesized from fullerenes, and heat-treating the resulting nanowhiskers. Even when the material manifests superconductivity, it retains its fine, fibrous structure(To read more about this click the link below).
Conclusion
There has been extensive research that has been happening in the domain of fullerenes ever since its discovery. Scientists and academia are striving hard to discover many more applications of the fullerene for the benefit of the mankind.
The future for the research can be seen moving towards implementing the fullerenes into domains like superconductivity, portable power, theranostics, purification of water, combatting biohazards and other domains.
As a concluding remark, I would like to share a video of the NASA detecting the Bucky Balls in Space.
