Introduction to fullerenes. Misnomers and misconception of nanotechnology

4

Introduction to fullerenes. Misnomers and misconception of nanotechnology

A fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and they resemble the balls used in association football. Cylindrical ones are called carbon nanotubes or buckytubes. Fullerenes are similar in structure to graphite, which is composed of stacked graphene sheets of linked hexagonal rings; but they may also contain pentagonal (or sometimes heptagonal) rings.

 The first fullerene to be discovered, and the family's namesake, buckminsterfullerene (C₆₀), was prepared in 1985 by Richard Smalley, Robert Curl, James Heath, Sean O'Brien, and Harold Kroto at Rice University. The name was an homage to Buckminster Fuller, whose geodesic domes it resembles. The structure was also identified some five years earlier by Sumio Iijima, from an electron microscope image, where it formed the core of a "bucky onion." Fullerenes have since been found to occur in nature. More recently, fullerenes have been detected in outer space. According to astronomer Letizia Stanghellini, "It’s possible that buckyballs from outer space provided seeds for life on Earth.”

The discovery of fullerenes greatly expanded the number of known carbon allotropes, which until recently were limited to graphite, diamond, and amorphous carbon such as soot and charcoal. Buckyballs and buckytubes have been the subject of intense research, both for their unique chemistry and for their technological applications, especially in materials science, electronics, and nanotechnology.

Properties

For the past decade, the chemical and physical properties of fullerenes have been a hot topic in the field of research and development, and are likely to continue to be for a long time. Popular Science has published articles about the possible uses of fullerenes in armor. In April 2003, fullerenes were under study for potential medicinal use: binding specific antibiotics to the structure to target resistant bacteria and even target certain cancer cells such as melanoma. The October 2005 issue of Chemistry & Biology contains an article describing the use of fullerenes as light-activated antimicrobial agents.

In the field of nanotechnology, heat resistance and superconductivity are some of the more heavily studied properties.

A common method used to produce fullerenes is to send a large current between two nearby graphite electrodes in an inert atmosphere. The resulting carbon plasma arc between the electrodes cools into sooty residue from which many fullerenes can be isolated.

There are many calculations that have been done using ab-initio quantum methods applied to fullerenes. By DFT and TD-DFT methods one can obtain IR, Raman and UV spectra. Results of such calculations can be compared with experimental results.

Aromaticity

Researchers have been able to increase the reactivity of fullerenes by attaching active groups to their surfaces. Buckminsterfullerene does not exhibit "superaromaticity": that is, the electrons in the hexagonal rings do not delocalize over the whole molecule.

A spherical fullerene of n carbon atoms has n pi-bonding electrons, free to delocalize. These should try to delocalize over the whole molecule. The quantum mechanics of such an arrangement should be like one shell only of the well-known quantum mechanical structure of a single atom, with a stable filled shell for n = 2, 8, 18, 32, 50, 72, 98, 128, etc.; i.e. twice a perfect square number; but this series does not include 60. This 2(N + 1)² rule (with N integer) for spherical aromaticity is the three-dimensional analogue of Hückel's rule. The 10+ cation would satisfy this rule, and should be aromatic. This has been shown to be the case using quantum chemical modelling, which showed the existence of strong diamagnetic sphere currents in the cation.

As a result, C₆₀ in water tends to pick up two more electrons and become an anion. The nC₆₀ described below may be the result of C₆₀ trying to form a loose metallic bond.

 Fullerene chemistry

Fullerenes are stable, but not totally unreactive. The sp²-hybridized carbon atoms, which are at their energy minimum in planar graphite, must be bent to form the closed sphere or tube, which produces angle strain. The characteristic reaction of fullerenes is electrophilic addition at 6,6-double bonds, which reduces angle strain by changing sp²-hybridized carbons into sp³-hybridized ones. The change in hybridized orbitals causes the bond angles to decrease from about 120° in the sp² orbitals to about 109.5° in the sp³ orbitals. This decrease in bond angles allows for the bonds to bend less when closing the sphere or tube, and thus, the molecule becomes more stable.

Other atoms can be trapped inside fullerenes to form inclusion compounds known as endohedral fullerenes. An unusual example is the egg shaped fullerene Tb₃N@C₈₄, which violates the isolated pentagon rule. Recent evidence for a meteor impact at the end of the Permian period was found by analyzing noble gases so preserved. Metallofullerene-based inoculates using the rhonditic steel process are beginning production as one of the first commercially-viable uses of buckyballs.

Solubility

C₆₀ in solution

Fullerenes are sparingly soluble in many solvents. Common solvents for the fullerenes include aromatics, such as toluene, and others like carbon disulfide. Solutions of pure buckminsterfullerene have a deep purple color. Solutions of C₇₀ are a reddish brown. The higher fullerenes C₇₆ to C₈₄ have a variety of colors. C₇₆ has two optical forms, while other higher fullerenes have several structural isomers. Fullerenes are the only known allotrope of carbon that can be dissolved in common solvents at room temperature.

Some fullerene structures are not soluble because they have a small band gap between the ground and excited states. These include the small fullerenes C₂₈, C₃₆ and C₅₀. The C₇₂ structure is also in this class, but the endohedral version with a trapped lanthanide-group atom is soluble due to the interaction of the metal atom and the electronic states of the fullerene. Researchers had originally been puzzled by C₇₂ being absent in fullerene plasma-generated soot extract, but found in endohedral samples. Small band gap fullerenes are highly reactive and bind to other fullerenes or to soot particles.

Solvents that are able to dissolve buckminsterfullerene (C₆₀ and C₇₀) are listed at left in order from highest solubility. The solubility value given is the approximate saturated concentration.

Solubility of C₆₀ in some solvents shows unusual behaviour due to existence of solvate phases (analogues of crystallohydrates). For example, solubility of C₆₀ in benzene solution shows maximum at about 313 K. Crystallization from benzene solution at temperatures below maximum results in formation of triclinic solid solvate with four benzene molecules C₆₀·4C₆H₆ which is rather unstable in air. Out of solution, this structure decomposes into usual fcc C₆₀ in few minutes' time. At temperatures above solubility maximum the solvate is not stable even when immersed in saturated solution and melts with formation of fcc C₆₀. Crystallization at temperatures above the solubility maximum results in formation of pure fcc C₆₀. Millimeter-sized crystals of C₆₀ and C₇₀ can be grown from solution both for solvates and for pure fullerenes.

Hydrated Fullerene (HyFn)

C₆₀HyFn water solution with a C₆₀ concentration of 0.22 g/L.

Hydrated fullerene C₆₀HyFn is a stable, highly hydrophilic, supra-molecular complex consisting of С₆₀ fullerene molecule enclosed into the first hydrated shell that contains 24 water molecules: C₆₀@(H₂O)₂₄. This hydrated shell is formed as a result of donor-acceptor interaction between lone-electron pairs of oxygen, water molecules and electron-acceptor centers on the fullerene surface. Meanwhile, the water molecules which are oriented close to the fullerene surface are interconnected by a three-dimensional network of hydrogen bonds. The size of C₆₀HyFn is 1.6–1.8 nm. The maximal concentration of С₆₀ in the form of C₆₀HyFn achieved by 2010 is 4 mg/mL.

Quantum mechanics

In 1999, researchers from the University of Vienna demonstrated that wave-particle duality applied to molecules such as fullerene. One of the co-authors of this research, Julian Voss-Andreae, has since created several sculptures symbolizing wave-particle duality in fullerenes.

Science writer Marcus Chown stated on the CBC radio show Quirks and Quarks in May 2006 that scientists are trying to make buckyballs exhibit the quantum behavior of existing in two places at once (quantum superposition).

Safety and toxicity

Moussa et al. (1996-7) studied the in vivo toxicity of C₆₀ after intra-peritoneal administration of large doses. No evidence of toxicity was found and the mice tolerated a dose of 5 000 mg/kg of body weight (BW). Mori et al. (2006)  could not find toxicity in rodents for C₆₀ and C₇₀ mixtures after oral administration of a dose of 2 000 mg/kg BW and did not observe evidence of genotoxic or mutagenic potential in vitro. Other studies could not establish the toxicity of fullerenes: on the contrary, the work of Gharbi et al. (2005) suggested that aqueous C₆₀ suspensions failing to produce acute or subacute toxicity in rodents could also protect their livers in a dose-dependent manner against free-radical damage.

A comprehensive and recent review on fullerene toxicity is given by Kolosnjaj et al. (2007a,b, c). These authors review the works on fullerene toxicity beginning in the early 1990s to present, and conclude that very little evidence gathered since the discovery of fullerenes indicate that C₆₀ is toxic.

With reference to nanotubes, a recent study by Poland et al. (2008) on carbon nanotubes introduced into the abdominal cavity of mice led the authors to suggest comparisons to "asbestos-like pathogenicity". It should be noted that this was not an inhalation study, though there have been several performed in the past, therefore it is premature to conclude that nanotubes should be considered to have a toxicological profile similar to asbestos. Conversely, and perhaps illustrative of how the various classes of molecules which fall under the general term fullerene cover a wide range of properties, Sayes et al. found that in vivo inhalation of C₆₀(OH)₂₄ and nano-C₆₀ in rats gave no effect, whereas in comparison quartz particles produced an inflammatory response under the same conditions. As stated above, nanotubes are quite different in chemical and physical properties to C₆₀, i.e., molecular weight, shape, size, physical properties (such as solubility) all are very different, so from a toxicological standpoint, different results for C₆₀ and nanotubes are not suggestive of any discrepancy in the findings.

When considering toxicological data, care must be taken to distinguish as necessary between what are normally referred to as fullerenes: (C₆₀, C₇₀, ...); fullerene derivatives: C₆₀ or other fullerenes with covalently bonded chemical groups; fullerene complexes (e.g., water-solubilized with surfactants, such as C₆₀-PVP; host-guest complexes, such as with cyclodextrin), where the fullerene is physically bound to another molecule; C₆₀ nanoparticles, which are extended solid-phase aggregates of C₆₀ crystallites; and nanotubes, which are generally much larger (in terms of molecular weight and size) molecules, and are different in shape to the spheroidal fullerenes C₆₀ and C₇₀, as well as having different chemical and physical properties.

The above different molecules span the range from insoluble materials in either hydrophilic or lipophilic media, to hydrophilic, lipophilic, or even amphiphilic molecules, and with other varying physical and chemical properties. Therefore any broad generalization extrapolating for example results from C₆₀ to nanotubes or vice versa is not possible, though technically all are fullerenes, as the term is defined as a close-caged all-carbon molecule. Any extrapolation of results from one molecule to other molecules must take into account considerations based on a quantitative structural analysis relationship study (QSARS), which mostly depends on how close the molecules under consideration are in physical and chemical properties.

Superconductivity

After the synthesis of macroscopic amounts of fullerenes, their physical properties could be investigated. Very recently, Haddon et al. found that intercalation of alkali-metal atoms in solid C₆₀ leads to metallic behavior. In 1991, it was revealed that potassium-doped C₆₀ becomes superconducting at 18 K. This was the highest transition temperature for a molecular superconductor. Since then, superconductivity has been reported in fullerene doped with various other alkali metals. It has been shown that the superconducting transition temperature in alkaline-metal-doped fullerene increases with the unit-cell volume V. As caesium forms the largest alkali ion, caesium-doped fullerene is an important material in this family. Recently, superconductivity at 38 K has been reported in bulk Cs₃C₆₀, but only under applied pressure. The highest superconducting transition temperature of 33 K at ambient pressure is reported for Cs₂RbC₆₀.

The increase of transition temperature with the unit-cell volume had been believed to be evidence for the BCS mechanism of C₆₀ solid superconductivity, because inter C₆₀ separation can be related to an increase in the density of states on the Fermi level, N(ε_F). Therefore, there have been many efforts to increase the interfullerene separation, in particular, intercalating neutral molecules into the A₃C₆₀ lattice to increase the interfullerene spacing while the valence of C₆₀ is kept unchanged. However, this ammoniation technique has revealed a new aspect of fullerene intercalation compounds: the Mott-Hubbard transition and the correlation between the orientation/orbital order of C₆₀ molecules and the magnetic structure.

The C₆₀ molecules compose a solid of weakly bound molecules. The fullerites are therefore molecular solids, in which the molecular properties still survive. The discrete levels of a free C₆₀ molecule are only weakly broadened in the solid, which leads to a set of essentially nonoverlapping bands with a narrow width of about 0.5 eV. For an undoped C₆₀ solid, the 5-fold h_u band is the HOMO level, and the 3-fold t_1u band is the empty LUMO level, and this system is a band insulator. But when the C₆₀ solid is doped with metal atoms, the metal atoms give electrons to the t_1u band or the upper 3-fold t_1g band. This partial electron occupation of the band leads to sometimes metallic behavior. However, A₄C₆₀ is an insulator, although the t_1u band is only partially filled and it should be a metal according to band theory. This unpredicted behavior may be explained by the Jahn-Teller effect, where spontaneous deformations of high-symmetry molecules induce the splitting of degenerate levels to gain the electronic energy. The Jahn-Teller type electron-phonon interaction is strong enough in C₆₀ solids to destroy the band picture for particular valence states.

A narrow band or strongly correlated electronic system and degenerated ground states are important points to understand in explaining superconductivity in fullerene solids. When the inter-electron repulsion U is greater than the bandwidth, an insulating localized electron ground state is produced in the simple Mott-Hubbard model. This explains the absence of superconductivity at ambient pressure in caesium-doped C₆₀ solids. Electron-correlation-driven localization of the t_1u electrons exceeds the critical value, leading to the Mott insulator. The application of high pressure decreases the interfullerene spacing, therefore caesium-doped C₆₀ solids turn to metallic and superconducting.

A fully developed theory of C₆₀ solids superconductivity is still lacking, but it has been widely accepted that strong electronic correlations and the Jahn-Teller electron-phonon coupling produce local electron-pairings that show a high transition temperature close to the insulator-metal transition.

Chirality

Some fullerenes (e.g. C₇₆, C₇₈, C₈₀, and C₈₄) are inherently chiral because they are D₂-symmetric, and have been successfully resolved. Research efforts are ongoing to develop specific sensors for their enantiomers.

Popular culture

Examples of fullerenes in popular culture are numerous. Fullerenes appeared in fiction well before scientists took serious interest in them. In New Scientist there used to be a weekly column called "Daedalus" written by David Jones, which contained humorous descriptions of unlikely technologies. In 1966  Jones suggested that it may be possible to create giant hollow carbon molecules by distorting a plane hexagonal net by the addition of impurity atoms.

On 4 September 2010, Google used an interactively rotatable fullerene C₆₀ as the second 'o' in their logo to celebrate the 25th anniversary of the discovery of the fullerenes.

Misnomers

Nanoscience

"Nanoscience" is probably a misnomer, since it refers to the "meso-scale" of microscopic physics -- from many Angstroms to about a micron -- in which quantum effects are usually minor.

Mike Adams is a natural health author and award-winning journalist with a passion for sharing empowering information to help improve personal and planetary health . His viewa about nanotechnology as follows

“You probably noticed that nanotechnology isn't on the top 10 list. This is no oversight. Nanotech isn't on the list because nanotechnology has been so distorted by the popular press and researchers who add "nano" to their projects in order to get funding that, today, it essentially means "anything that's really tiny." That's not a technology. That's a scale.

By this measure, everything is nanotechnology. I mean that literally: every "thing" is nanotech because it's made of a collection of very tiny molecules. Your computer has a nanotech CPU, your radio has nanotech transistors, your brain is made of nanotech neurons, and the carrots in your refrigerator were built from an impressive nanotechnology infrastructure that fed them nutrients from tiny molecules diffused into the plants' nanotech roots through soil.

The whole universe is, in fact, nanotech. In fact, every item of matter in the known universe is made up of tiny particles and, ultimately, waves of energy and probability. Under this vast umbrella, nearly anything can be called "nanotechnology." And it's not a misnomer: everything really is nanotech!

Since everything is, technically, nanotech, product makers and science researchers can legitimately claim to be using nanotechnology on practically any project. As a result, the term has lost any real meaning. "Nanotech" now belongs on the scrap heap of catchy buzzwords that sound cool but are devoid of any real meaning.

The preferred term for the "classic" definition of nanotechnology is molecular assembly technologies. This phrase remains specific: it means the assembly of objects or machines at the molecular scale. And that's what classic "nanotech" was really all about.

So why haven't I covered molecular assembly technologies in this report? While the field does look potentially promising, it's still a bit early to say what the real-world applications are going to look like. I plan to cover this subject in more detail in a future report, however.

MisconceptionsIn 2008, Advances in Colloid and Interface Science published a study in which it was concluded that every self-assembly process in reality presents a co-assembly, which makes the former term a misnomer of a kind. The thesis is built on the concept of mutual ordering of the self-assembling system and its environment.

Nanotechnology and the products spawned from this exciting field continue to grow at an explosive rate. From electronics to medical devices to automotive and even cosmetic products, the potential applications of nanotechnology seem endless.

Yet with the promise of new breakthroughs comes concern over the safety and environmental issues that nanotechnology raises.

Because the nanoparticles that comprise stronger and more efficient materials are slower to degrade, it is thought they can pass more easily from our organs into our bloodstream.

Given that a human hair is about 75,000 nanometers wide, it's easy to see how nanoparticles can be inhaled, ingested or absorbed through the skin.

Take, for example, carbon nanotubes, which show great potential for strengthening building materials, improving cancer therapy and generating energy. In 2009, researchers with the National Institute for Occupational Safety and Health released preliminary findings showing inflammation and fibrosis in the lungs of lab mice, after the mice had “inhaled” small droplets of water containing carbon nanotubes.

This is just one example of how scientists and regulators are exploring the potential risks associated with nanotechnology, to determine what steps should be taken to reduce and even eliminate environmental, health and safety risks.

Robert M. Dunn, Ph.D., managing director of the Midwest Institute for Nanoelectronics Discovery, based at the University of Notre Dame, points out that nanoscience itself ultimately will help us understand the specific problems — and solutions — associated with nanotechnology.

“Nanoparticles are all around us in our environment,” Dunn says. “Nanotechnology allows us to observe them for the first time, and nanoscience is going to be the mechanism that will identify dangerous nanoparticles, as well as discover ways to reduce health concerns.”

Dispelling misconceptions about nanoelectronics research

When it comes to one specific area of nanotechnology — nanoelectronics — the more pressing health and safety concerns do not realistically apply, says Wolfgang Porod, Ph.D., director of Notre Dame's Center for Nano Science and Technology. Nanoelectronics involve electronic components on the nanoscale level.

“Most of the health concerns related to nanotechnology are about nanoparticles, which may enter the environment via the air or water and be inhaled or ingested,” says Porod. “Yet our work in nanoelectronics is very different because these electronic components do not become airborne. Instead, they are fabricated on a semiconductor chip.”

Patrick Fay, Ph.D., professor of electrical engineering and director of the clean room at Notre Dame's Stinson-Remick Hall of Engineering, understands firsthand the concerns the public may have about the safety of nanoparticles.

“While people are fearful about the possibility of inhaling particulates, within the nanoelectronics sphere, concerns about particulates are a non-issue,” says Fay. “A completed chip is still macroscopic in size, so it is not any different than the microelectronics that existed before. What is different is that the individual devices on the chip are much smaller, and as a consequence, the chip can offer greatly enhanced functionality.”

Solid safety measures at Stinson-Remick

So, the good news about nanoelectronics research is that it does not really involve the toxicity problems associated with nanoparticles.

And even better news is that Stinson-Remick has excellent safety measures in place to protect researchers who occasionally use certain toxic substances in their research.

“For example, some of the gases we work with are toxic, and so we have comprehensive monitoring systems to detect any kind of leak,” Fay says. “This monitoring system tests the air in the lab and in the lab exhaust for toxic chemicals. The sensors are placed in various locations around the laboratories and run continuously.”

Fay says this system is similar to what is in place for many semiconductor and processing facilities around the world. If any problem is detected, the system is designed to automatically shut off the gas supply to ensure the situation does not escalate.

“The sensitivity of the system is excellent,” he says. “In addition, all of the effluent from the lab — the leftover liquid chemicals and gases from processing — is neutralized either on-site or in conjunction with Notre Dame's Risk Management and Safety Department.”

Equally critical in the overall safety plan at Stinson-Remick is the thorough training of all users at the facility.

“We train everyone involved so that they have an appreciation for the safety requirements of the processes they use, and the proper procedures and precautions to take,” Fay says. “They also wear specialized clothing, which they put on in separate gowning rooms. Not only does this practice serve to protect the environment in which they work, but it also protects our researchers.”

The Reilly Center leads important safety discussions

Along with its reputation for conducting cutting-edge nanotechnology research, Notre Dame is a leader in exploring the issues that surround such valuable research. Critical discussions and policy-making take place at Notre Dame's John J. Reilly Center for Science, Technology and Values.

Kathleen Kolakovich Eggleson, Ph.D., who formerly served as the Reilly Center's associate director, believes that the most pressing concern related to nanotechnology is “the vast unknown” about its nature.

“It is true that both naturally occurring and engineered nanoparticles can persist in the environment as aerosols and enter the body by inhalation,” Eggleson says. “The threat to our health, however, would depend on exposure — the amount, the duration and the frequency. In general, we do not yet have enough data to form a complete picture of which specific nanomaterials pose a significant threat to human health or the environment, and which are relatively benign.”

But the Reilly Center is actively taking steps to reduce the information gap surrounding nanomaterials and safety.

The mission of the Reilly Center is to promote the development of science and technology to the benefit of the common good in an ethical manner that upholds the dignity and rights of each person. With this in mind, the Reilly Center is studying the health and safety concerns related to nanotechnology. Its leaders took a significant step in that direction by hosting a conference on that subject last spring.

“We collaborated with the Center for Nano Science and Technology in bringing together experts, decision-makers and stakeholders who seldom gather around the same conversational table,” Eggleson says.

The conference, entitled “Toward Regulation of Nanomaterials: a conversation between industry, academia, law and government,” was such a success that plans to continue this collaboration are under way.

Reilly's scholars and staff are optimistic about the various measures now in place to better understand and address safety and health issues surrounding nanotechnology research. A portion of federal expenditures dedicated to the investigation of such issues is proof that federal agencies are paying attention.

“The National Institute for Occupational Safety and Health had previously established guidelines for the personal protective equipment that is effective in protecting nanotechnology workers who handle fine and ultrafine particles in the workplace,” Eggleson says. “NIOSH is also conducting research specific to the health and safety of these workers and is adapting guidelines as new data become available.”

Eggleson very recently assumed a new and elevated role at Notre Dame when it comes to exploring these important issues in the field of nanotechnology. As of this month, she joined the Center for Nano Science and Technology as a research scientist, and she will focus specifically on the environmental, health, safety, ethical, legal and social implications of nanotechnology.

The increasing emphasis specifically on the ethical, legal and social implications of nanotechnology (commonly called ELSI in the field) is part of a national trend. Case in point: The 10-year strategic direction of the National Nanotechnology Initiative, a coordinating agency for 25 federal agencies, reflects an increased focus on ELSI. Toward that end, the National Nanotechnology Initiative will develop special research, educational and communication programs pertaining to the ethical, legal and social implications of nanotechnology development.

The impressive advances stemming from today's nanotech world already have yielded numerous innovative materials, devices and structures. However, as Porod points out, it is important to realize that nanoparticles and their potential health effects are not new, and have in fact existed since the beginning of time.

But, thanks to ongoing efforts at Notre Dame and elsewhere to understand the nature of nanoparticles, we're increasingly arming ourselves with the knowledge needed to counter potential health and safety risks associated with them.

Put simply, by becoming smarter about nanotechnology, we're becoming safer in the process.

“Nanoparticles naturally occur in the environment, such as in dust and smoke,” Porod says. “When cavemen sat around a fire, they inhaled nanoparticles. When digging in the dust, one inhales nanoparticles. What is new, however, is our capability to ‘see' these nanoparticles and study their properties, thanks to nanotechnology research.

“We have already learned that the interactions between nanoparticles and living cells are very complex, and they depend not only on the chemical composition of these particles, but also on their size, and even shape. Based on our new technological capabilities, our equally new understanding of the interactions between nanoparticles and life processes likely will lead to better preventative measures, cures and the regulation of health issues that have been around for a long time.”