Fullerene Uses – The Comprehensive Overview
Despite their unusual name and their relatively short time on the research scene, there are many possible fullerene uses. These so called carbon allotropes (as well as graphite, graphene and diamond known to date) consist of carbon molecules in the form of various shapes as hollow sphere, tube or ellipsoid. This fact made them very interesting for science and industries, as the other known allotropes have many various commercial uses. Based on the molecular shape, these molecules could be divided into spherical (also called buckminsterfullerenes or buckyballs) and cylindrical (termed as nanotubes or buckytubes). Due to their features, buckyballs and buckytubes have been intensively examined especially for their possible technical applications in nanotechnology, material science and electronics but also in pharmaceutical and cosmetic industry and last but not least in medicine.
Fullerene uses could be infinite
The most abundant and the first studied representative of these novel family of molecules is buckminsterfullerene (C60) which was discovered in 1985 by Richard Smalley, Robert Curl, James Heath, Sean O’Brian and Harold Kroto at Rise University.
It was named after Richard Buckminster Fuller, an architect known for popularization of geodesic dome with similar shape as C60.
Scientists who discovered C60 were awarded by Nobel Prize in 1996.
This led to extensive interest of many research groups all over the world and subsequently to the discovery of a large number of structural variants derived from basic molecule such as megatubes, fullerene rings or polymers.
After that, the shorter name “fullerene” started to be commonly used for the whole family of these carbon allotropes. The suffix “-ene” refers to the fact, that each carbon atom is bonded to three others by covalent binding.
The main properties that made fullerenes so interesting for many different research groups but also for nanomaterial companies include:
- Simple structure – contain 60 atoms of carbon and has soccer-ball shape
- Hollowness – suggesting their potential uses as they can be filled
- Uniformity – contain only one molecule typ
These novel molecules have high potential to be better in many aspects compared to carbon allotropes known so far. Based on their unique and uniform structure, they are predicted to be synthesized by cheaper method and even have more possible commercial uses than other carbon compounds which are world-widely used in many different fields including medicine, nanotechnology or industry.
The common process of fullerene production
- Fullerene or fullerene-containing soot synthesis which is mediated by sending a large current between two graphite electrodes. It has to be done in an inert atmosphere. The carbon plasma arc between electrodes subsequently cools and gives rise to sooty residues from which, C60 could be isolated in following steps.
- Synthesis of derivatives
- Post-processing (for example dispersion into a matrix)
The diversity in buckyballs possible usage
Due to their many described properties, fullerenes and their derivatives seem to have an infinite number of uses across various disciplines including the following ones:
- Few years ago utilization of C60 in armor was discussed in science community
- They were also studied for their potential to be very nice tool in medicine for (among others) targeting resistant bacteria by antibiotics conjugated with carbon molecule or for targeting cancer cells with special benefits for patients undergoing cancer treatment which is described in more detail bellow
- In 2005, buckyballs utilization as light-activated antimicrobial agents was published
- Properties such as superconductivity and heat resistance are extensively studied to be used in nanomaterial development
What is buckminsterfullerene used for in commercial technologies?
As mentioned above, buckminsterfullerene or C60 was the first discovered fullerene molecule which gave name to whole new family. It is also the longest examined and the best described member and is predicted to have the most possible usage.
Basic properties of C60
- Smallest fullerene molecule
- Contains pentagonal/hexagonal rings
- High molecular symmetry
- Common in nature, present in soot
- Structure of truncated icosahedrons resembling an association football ball
- Exceptionally stable
- Resistant to radioactivity
- Resistant to chemical corrosion
- Accepts electrons and easily released them
Nowadays, buckminsterfullerenes are used as so called dry lubricants in coating applications. Nanosphere powders present composite coatings which are made from inorganic
fullerene-like material (IFLM). This material was developed to suppress friction and enhanced wear resistance in parts that are exposed to sliding or rolling contacts. After IFLM incorporation into a matrix, the particles enable independent control of friction and wear. IFLM strategy finds application for example in:
- Ball bearings
- Artificial joints
Main classes of IFLM-based coatings
- Metal matrix coatings
- Materials such as cobalt and nickel are used in their production by IFLM powder suspending in a plating bath solution
- Useful in high load applications that require hard coatings
- Based on aqueous coating techniques
- Polymer matrix coatings
- Softer matrix suitable for intermediate load-bearings applications
- Examples: Polyketone, Nylon, Polypropylene
- Matrix-less fullerene films
- Contain 85% of C60 and 15% of its derivative C70
- Thickness of 10nm
- Plated on metallic substrate coated by 1nm Ge sublayer before
- Amorphous (revealed by transmission electron microscopy)
- No variations in surface potential
The fullerene-coated surfaces have a high potential to be used in microbial fuel cell applications due to their ability to promote the formation of biofilm produced by studied organism if conducting basic surface is used.
What are fullerenes used for – a chemical point of view
Medical applications are usually highlighted because of the benefits which they can bring to financial sphere and their potential to improve the population life quality but on the other hand there are other useful applications of C60, that can improve living standard in a broad sense.
Because fullerenes and their derivates were discovered to have high electron affinity and they are able to transfer electrons, they can act as acceptors in solar cells system which is based on electron transport from light excited material (donor) to electrode. Whole process is mediated by the acceptor molecule. The example of commonly used acceptor is Phenyl-C61-butyryc acid methyl ester (PCMB) which is used with polythionphene (P3HT) as donor.
Hydrogen gas storage
Fullerenes are able to hydrogenate and dehydrogenate easily due to their one-of-a-kind molecule structure (consist of carbon atoms only). During hydrogenation, C=C double bonds which are present between carbon rings in C60 molecule are easily broken which gives rise to C-C single bonds and C-H bonds. The strength of C-H bonds is lower in comparison with C-C which enables relatively ease hydrogen release after exposure to heat. Up to 36 atoms of hydrogen can be hold by one C60 molecule.
Uses of C60 fullerenes in a pharmaceutical point of view
Since their discovery in 1985, fullerenes were extensively examined for possible biomedical applications. Investigations of their physical, biological and chemical properties have yielded important and promising information. The greatest problem in the medical field is that fullerenes are insoluble in aqueous medium and they posses high tendency to form aggregates. Scientists have tried to overcome these issues by using three main techniques as followed:
- Encapsulation in special carriers
- Suspension with the help of co-solvents
- Chemical functionalizatio
All known classes of chemicals have been used for the creation of derivatives that confirmed their high chemical activity and wide versatility of chemical reactions. They could be used for production of novel materials with possible use in medical field. To date, fullerenes and their derivatives are used in few medical applications including for example antiviral activity or drug and gene delivery. These and several more are discussed in more detail in the following text.
Based on the chemical properties like very high electron affinity and the large number of conjugated double bonds, fullerenes are known to act as excellent antioxidants. They are called „radical sponge” due to their ability to interact with a number of free radicals before being consumed. The major advantage of using them as an antioxidant is that they seem to be really safe and biocompatible that they can act within the cell.
2) Antiviral agents
Different modes of pharmaceutical actions have been described for compounds with antiviral activity which made them to be the center of medical interest. Fullerenes and their derivatives are believed to act as these compounds also termed as antiviral agents. One of their most exciting properties is the capability to suppress the human deficiency virus (HIV) replication which led to inhibition of acquired immunodeficiency syndrome (AIDS) manifestation. In more detail, it was observed that fullerene derivatives are able to inhibit HIV protease which in turn prevents HIV 1 replication and the subsequent development of the disorder.
Fullerenes posses a great potential for research and development of novel anti-HIV drugs.
The antiviral activity is strongly influenced by the relative position of side chains on C60. This was claimed by synthesis and subsequent characteristics of series fullerene derivatives which were found to really have antiviral activity, examples are listed below.
- Fulleropyrrolidines (contain 2 ammonium groups) were described as HIV-1 and HIV-2 antagonists
- Cationic, anionic and amino acids derivatives were seen to inhibit hepatitis C virus replication
- Amino acid derivatives of C60 were found to be able to inhibit human cytomegalovirus replication
- Water insoluble derivatives shown antiviral activity against enveloped viruses
3) Gene and drug delivery
The delivery of drugs means the transport of a chemical compound into the site, where it is supposed to have the right effect. For this purpose, it is crucial to use very safe but still effective molecules. Fullerenes seem to be nice carriers because they showed good biocompatibility, selectivity and they are still small enough for the right diffusion in organism, which is needed for their final localization
In the case of gene delivery, foreign DNA is introduced into the cells, which is supposed to bring the desired effect. For this purpose, DNA sequences are connected with amino acid derivatives of C60. In the proper site, these sequences are disconnected by loss or denaturation of these amino groups. Biochemical experiments have revealed better abilities compared to vectors which are commonly used for this application.
4) Photosensitizers in photodynamic therapy
This type of treatment could be another and maybe better option for cancer patients than classical anti-cancer therapy. In this case, C60 can be used as a non-toxic compounds which are able to target malignant cells.
Fullerenes become toxic after light exposure leading to cancer cell death.
New photosensitizers were tested using HeLa cells, their main features include:
- increased agility to be absorbed by cancer cells
- potential to still trigger cell death
- short-time stay in a body (preventive of unwanted cell death)
In 2006, highly water-soluble C60-N vinylpyrrolidine copolymer was described as a new photosensitizer. Radical polymerization enables covalent incorporation of C60 into poly (vinylpyrrolidin) chain which gave rise to most water-soluble fullerene reported so far.
In 2007, polyethyleneglycol (PEG)-conjugated fullerene containing Gd3+ ions was used for photodynamic therapy while combined with magnetic resonance imaging (MRI).
Recently, it was shown that monocationic fullerenes are highly effective in killing of cancer cells by illumination-induced apoptosis.
What are buckyballs used for in biomedicine?
Spherical fullerenes also termed buckyballs and their derivatives were found to have brought many benefits in different biomedical applications including:
- Suppression of important factors such as nitric oxide and TNF-alpha, responsible for neutrophilic lung inflammation which in turn attenuate this illnesses
- Protection of neurons against apoptosis in vitro and in vivo and prevention from brain tissue ischemia and iron-induced oxidative stress injuries
- Protection of visceral organs (heart, liver, kidney, etc.) from injuries caused by oxidative stress
- Cells protection against the ultraviolet A irradiation (they act as so called cytoprotectors and in this case they can bind to Reactive Oxygen Species (ROS) which subsequently leads to cell damage protection
- Inhibition of cellular apoptosis caused by oxidative stress
- Cellular imaging and bio-distribution detection
- Serum protein profiling
Fullerene C60 uses and applications based on its redox properties
The basal cellular respiration processed from mitochondrial oxidative phosphorylation is known to cause the endogenous production of reactive oxygen species (ROS) including hydroxyl radical or superoxid anion.
ROS are involved in cell signaling and form the necessary part of energetic metabolism in organisms. When the levels of ROS are too high or cellular antioxidant defense system does not work properly, oxidative stress occurs. This situation might lead to damage of cellular proteins, lipids and nucleic acid which are probably involved in pathogenesis of some illnesses as neurodegeneration, cancer or atherosklerosis.
The extrinsic antioxidants could help cells to reduce oxidative stress by removing ROS.
Due to its hollow spherical structure with 30 carbon-carbon (C=C) double bonds, C60 can react with at least 15 benzyl radicals or 34 methyl radicals while forming stable adducts. Moreover, it was discovered to be able to react with many superoxides without being consumed and finally, C60 was considered to act as most efficient “free radical sponge”. However, more research groups have been worked on development of water soluble or other derivatives.
What can fullerenes be used for in orthopaedic research
Nowadays, among the other medical specializations, orthopaedic seems to be the most promising for fullerenes and their derivatives usage. They are supposed to have importance in treatment of many skeletal disorders such cartilage degeneration, osteoporosis or intervertebral disc degeneration (IVDD)
· Cartilage degeneration treatment
The loss of functional chondrocytes leads to cartilage degeneration. The treatment is based on derivation of new chondrocytes from progenitor cells but improvement of this method is still needed because not enough functional cells could be produced.
Water-soluble C60 was explored to have positive effect on promotion of primary embryonic limb bud cells chondrogenesis. This effect was proportional to the concentration of the compound. The mechanism by which buckminsterfullerene is able to influenced chondrogenesis remains elusive. It might be due to its anti-oxidative properties or it might concentrate other substances known to promote the whole process of chondrocytes differentiation.
Another in vitro study revealed that water-soluble fullerene derivative is able to down-regulate production of matrix-degradating enzymes in chondrocytes from patients with osteoarthritis when cultured with specific factors such as interleukin-1 beta or H2O2.
Furthermore, C60 significantly elevated the biosynthesis of chondrogenesis supporting molecules such as proteoglycans or collagen type II. The other possible use of fullerene in cartilage degeneration therapy is its injection directly into tissue. Scientists predict that it might act as so called “molecular bearing” to coat, lubricate and even protect the function of joint cartilage.
· Osteoporosis therapy
Osteoporosis is a skeletal illness with following characteristics: reduced bone mineral density, Deterioration of micro architecture of the skeleton and increased risk of fracture
The C60 derivatives have high potential to be used in osteoporosis treatment because of their strong affinity to the calcium phosphate mineral hydroxyapatite of bone. It means that C60-based vectors could bring traditional bone promotion agents to destructive bone tissue.
· IVDD therapy
IVDD is believed to cause the low back pain, one of the most common problems associated with musculoskeletal disorders. It is very likely that mitochondrial-derived ROS contribute to manifestation of IVDD. It was described that free radical scavengers are important part in process of IVDD prevention.
Experiments with C60 derivative fullerol (because of its known ability to act as a free radical scavenger) were performed and revealed that it is able to effectively reverse the matrix degradation under the specific conditions.
· Radiculopathy treatment
Spinal nerve root inflammation which is associated with back pain could be caused by IVDD. Fullerol was observed to be able to suppress inflammatory responses of dorsal root ganglia via decreasing ROS levels.
Current and future potential uses of fullerenes
Although many possible uses of this novel family of carbon allotropes are still under extensive research, some of them were successfully applied in commercial sphere. To date, there is no product based on fullerenes only but these molecules were used as substances added into existing materials, because they are able to improve material properties or bring some new advantages to existing products.
Fullerenes were used as additives in cosmetics for developing the so called UV whitening cream. In this product the addition of C60 enhanced the absorption through skin and helped protect skin against the consequences of oxidative stress such as dark spots or wrinkles.
There is a high potential for fullerenes to be used in each field, but another research is required. The main challenge is to develop product which would be better, cheaper and even more environment friendly in comparison with products which are commonly available.
The challenges in fullerene research
Though some fullerenes are known to be useful in biomedicine or pharmaceutics, they cannot be commonly used because of the relatively high price of their production in large scales.
There are also toxicity issues that have to be solved. Comprehensive studies revealed that toxicity of C60 is not only time and dose-dependent, but even depend on various other factors. This fact is very limiting for their common use in pharmacology. The toxicity has to be tested for each novel application, which is costly and time consuming.
When all challenges will be overcome, these novel carbon allotropes will probably become one of the most used carbon allotropes in the world.
The possible future of fullerenes
- microscopic ball bearings
- new cancer treatments based on buckyballs filled with radioactive atoms which ensure the integrity of isotopes after injection by carbon barrier
- neuronal protectors used for treatment of Parkinson’s and Alzheimer’s diseases
- superpower full battery made by lithium and fluorine atoms which would create energy when combined inside buckyballs
- powerful rocket fuels
- conducting polymers for substitution of metal ones
- new class of catalytic converters
- the base for new types of plastics created by stringing buckyballs together followed by adding of different atoms or chemical groups to carbons which would enable to alter the molecule in million ways
- another uses in plastics and other organic compounds with carbon backbones
- many others because they can replace most of the molecules which are used at present