Ever since the discovery of fullerenes, towards the end of the last century, a number of new findings about these molecules is afoot. With its unique chemical and physical properties, the applications and uses of these molecules may be infinite.
What’s in a name!
The Buckminsterfullerene was the first fullerene to be discovered and was named after the American architect, Buckminster Fuller, as it resembled the geodesic dome designed by him.
The Buckminsterfullerene is an allotrope of carbon discovered by Richard Smalley, Robert Curl, Harry Kroto et al at Rice University and Sussex University, in the year 1985, for which they were awarded the Nobel Prize in 1996. This led to the subsequent discoveries of a number of other similar compounds that were known as fullerenes.
Due its relative ease of synthesis, fullerene of C60 remains popular and a lot of research for its potential applications has been carried out. The C60 buckyball is made of 60 carbons at 60 vertices that make a spherical structure. It is composed of 12 pentagonal and 20 hexagonal rings that are adjacent to each other. These rings are conjugated with double bonds. The C-C bond length for the hexagonal rings is 1.40 A° and 1.46 A° for the pentagonal rings, with the average bond length equal to 1.44 A°.
Fullerenes have sp2 as well as sp3 hybridized carbon atoms. These molecules have extremely high affinity for electrons and can be reversibly reduced to take up 6 electrons. Although this molecule is made of conjugated carbon rings, the electrons here are not delocalized, and thus, these molecules lack the property of superaromaticity. These molecules have very high tensile strength and bounce back to their original shape after being subjected to over 3,000 atmospheric pressure! Due to the unique properties of this allotrope of carbon, it has a number of applications―a few of them have been discussed below.
Applications and Uses of Fullerene
Medical Applications of Fullerene
Extensive research on the biomedical applications of this molecule has been underway ever since its discovery. The greatest challenge faced by scientists in doing so was its insolubility in aqueous medium, and its tendency to form aggregates. This was overcome by various techniques like encapsulation of the fullerenes with hydrophilic molecules, suspending this molecule with other solvents, and conjugating it with other hydrophilic molecules.
Fullerenes can make excellent antioxidants, this property can be attributed to the large number of conjugated double bonds they possess and a very high electron affinity of these molecules (due to low energy of the unoccupied molecular orbital). Fullerenes can react with a number of radicals before being consumed. A single C60 molecule can interact with up to 34 methyl radicals before being used up. That is why, these molecules are also known as the ‘world’s most efficient radical scavengers’ or ‘radical sponge’. Perhaps, one of the major advantages of using these molecules as an antioxidant is that these can be localized within the cell.
These molecules also act as effective cytoprotectors against the ultraviolet A irradiation. These bind to the Reactive Oxygen Species (ROS) and prevent damage to cells. A water soluble derivative C60 with polyvinylpyrrolidone or Radical Sponge is usually added to cosmetics. This prevents skin damage and premature aging of the skin without any side effects. The biggest advantage of this molecule is that it is readily absorbed by the intact skin. Fullerene molecules also prevent lipid peroxidation by scavenging peroxy radicals, and thus, prevent cell cytotoxicity related with them.
Fullerenes have grabbed quite a bit of attention due to their potential as antiviral agents. Perhaps the most exciting aspect of this may be their ability to suppress the replication of the human immunodeficiency virus (HIV), and thus, delay the onset of acquired immunodeficiency syndrome (AIDS). Dendrofullerene 1 and Derivative 2, trans isomer have been seen to inhibit the HIV protease, and thus, prevent replication of HIV 1. Bivalent metal derivatives of amino acid derivatives of fullerene, like C60-1-Ala, are also seen to be active against HIV and human cytomegalovirus replication. These molecules are usually inserted in the hydrophobic domains of proteins (binding site of protease in HIV).
Another target for amino acid derivative of fullerene is the reverse transcriptase in HIV, these molecules are seen to be more active than the non nucleoside analog inhibitors usually used. Cationic fullerene derivatives are antibacterial and antiproliferative in nature. Most fullerene derivatives can inhibit hepatitis C virus.
Water insoluble derivatives of fullerene show antiviral activities against enveloped viruses, when vesicular stomatitis virus is incubated with fullerene derivatives under visible light, it loses its infectivity. This can be attributed to the generation of singlet oxygen (more details will be discussed later).
Drug Delivery and Gene Delivery
Drug delivery is the proper transportation of a pharmaceutical compound to its site of action, whereas gene delivery is the introduction of foreign DNA into cells to bring about a desired effect. It is therefore of utmost importance to deliver these molecules with safety and great efficacy. Fullerenes are a class of inorganic carriers, these molecules are preferred as they show good bio compatibility, greater selectivity, retain the biological activity, and are small enough to be diffused. DNA sequences are attached to the amino acid derivatives of fullerene. These sequences detach from their carrier with the loss or denaturation of the amino groups. Biochemical studies have shown greater protective abilities of these derivatives as compared to the traditional vector used.
Fullerenes can be used in the delivery of hydrophobic drugs. In fact, these carriers are used in the slow release of these hydrophobic drugs in the system. A significant anticancer activity has been observed for C 60-paclitaxel conjugate. An additional benefit is that they can easily diffuse through intact skin―a fullerene-based peptide has demonstrated the ability to penetrate via skin.
Photosensitizers in Photodynamic Therapy
Photodynamic Therapy (PDT) is a form of therapy of using non-toxic light sensitive compound which, when exposed to light, becomes toxic. This is used to target altered and malignant cells. Fullerenes are usually used as these compounds. Fullerenes get excited upon irradiation, when these molecules return to ground state, they give off energy that splits the oxygen present to generate singlet oxygen, which can be cytotoxic in nature.
In the presence of electron donors, fullerenes are converted to C60- radicals (fullerenes are excellent acceptors of electrons). These radical anions transfer electrons to oxygen molecules and convert them to anionic superoxide and hydroxyl radicals. These radicals damage the DNA and may bring about cell death. Sometimes, certain fullerenes form conjugates with proteins and DNA, this has a potential application in developing certain anticancer therapy as well.
The highly water-soluble C60-N vinylpyrrolidone copolymer is used as an agent for photodynamic therapy.
➡ In metalofullerenes, like Gadofullerene, metals such as gadolinium, lithium, calcium, etc., are inserted in the cage of C60. These molecules are used as contrast agents in X-ray and MRI imaging as these molecules have a long spin relaxation times and some exceptional spin properties.
Other Applications of Fullerenes
A polymer-based organic photovoltaic cell may be the answer to finding an economical and lightweight medium for the conversion of solar energy. These solar cells basically work by transfer of electrons from a material that gets excited when irradiated with light (known as the donor). This electron in its excited state is taken by an acceptor molecule, which is transferred further to the electrode. Fullerenes, due to their high electron affinity and ability to transfer these electrons, make excellent acceptors. These organic photovoltaic cells are complexes of fullerenes and polymers, and are called bulk heterojunctions.
Phenyl-C61-butyric acid methyl ester (PCBM) is a common acceptor used in organic solar cells. It usually used in conjunction with the polymer polythiophene (P3HT) as an electron donor.
In Protective Eye wear
Fullerenes have optical limiting properties. This refers to its ability to decrease the transmittance of light incident to it. These molecules can therefore be used as an optical limiter that can be used in protective eye wear and sensors. This optical limiter will only allow the light below a particular threshold to pass through as well as maintain the light being transmitted at a constant level, much below the intensity that may cause damage to the eye or the sensor.
Hydrogen Gas Storage
The one-of-a-kind molecular structure of fullerenes enables them to hydrogenate and dehydrogenate quite easily. The carbon rings in fullerene are conjugated with C=C double bonds. On hydrogenation, these bonds can be broken easily giving rise to C-C single bonds and C-H bonds. When heat is applied to these fullerene hydrides, the C-H bonds break easily to give back fullerene. This is because the bond strength of C-H is lower as compared to that of C-C. One fullerene molecule can hold up to 36 hydrogen atoms. The color of hydrogenated fullerenes changes from black to brown, red, orange, and finally, to yellow as the hydrogen content increases. These molecules hold a promise of better, safer, and more efficient hydrogen storage devices than the ones that are currently being used.
Fullerenes can be the future of developing comparatively lightweight metals with greater tensile strength, without seriously compromising the ductility of the metal. This can be attributed to the small size and high reactivity due to the sp2 hybridization of the carbon. This enables dispersion strengthening metal matrix by interaction of the fullerenes and the metals. A 30% increase in the hardness of lightweight Ti-24.5AI-17N alloy has been observed with the addition of fullerenes.
➡ Fullerenes are quite similar in structure to diamonds. Recently, Argonne National Laboratories and MER Corporation have demonstrated the conversion of fullerenes into diamonds with minor rearrangement of the atoms. These fullerenes can be used as a substitute for diamond films required in various electronic devices.
Currently, studies are underway to look into the potential use of fullerenes in sensors as well in development of molecular conductors. Perhaps, the greatest practical application of these molecules, at this time, may be in the cosmetic industries. Their use in other industries may be limited due the cost of synthesizing these molecules. However, with the aggressive research on these molecules, we can hope for a cost-effective method for synthesizing molecules to be developed shortly.