Tiny Carbon Balls With Big Potential: The Uses of Fullerenes

Spheres, ovals, and tubes show up constantly in our daily lives, and their repeating geometries are easy to recognize. On the molecular scale, carbon can form these same shapes, creating hollow “cages” that scientists call fullerenes. Their cage-like structure allows scientists to use them in many different ways, from drug delivery to sports rackets. 

Fullerenes are the third allotropic form of carbon, where allotropes are different structural forms of a single element. They form closed, hollow structures, commonly resembling empty, cage-like spheres, ellipsoids, or cylindrical tubes. 

Every carbon atom has four valence electrons. Each carbon atom is bonded to three other carbon atoms by single covalent bonds. The last valence electron participates in a delocalized covalent pi (π) bond—a bond in which a pair of electrons is shared (delocalized) among multiple atoms at once. The delocalized electrons in a π bond are more exposed, which contributes to the versatile reactivity of fullerenes. The carbon rings of fullerenes are arranged in pentagonal or hexagonal rings. There are an infinite number of fullerenes that can exist, with two of the most common ones being C60 and C70. C60 and C70 are chemical formulas of fullerenes, and the numbers indicate the number of carbon atoms in each fullerene molecule. C60’s structure is similar to that of a soccer ball, and C70’s structure is similar to that of a rugby ball—a flattened sphere. The number of carbon atoms in a fullerene determines its structure. These structural characteristics give unique stability to fullerenes and enable them to be used in a variety of ways. 

C60 and C70, the first fullerenes to be discovered in history, were identified in 1985 by Robert F. Curl, Harold W. Kroto, and Richard E. Smalley. The scientists used a cluster beam apparatus, a machine that produces tiny clusters of atoms or molecules for study, to synthesize fullerenes. A powerful laser was shot at a piece of graphite, vaporizing it into a hot cloud of carbon atoms. A cooler, dense helium gas stream carried the carbon atoms down the apparatus, simultaneously cooling and stabilizing the carbon atoms into clusters. An ultraviolet laser pulse then ionized the clusters. Lastly, the clusters were detected using time-of-flight mass spectrometry, which measures their mass-to-charge ratio by observing how quickly the molecules travel a known distance. The most common fullerene formed was C60, with the second most common being C70. Fullerenes are extremely strong because their carbon atoms form stable covalent bonds and their spherical structure evenly distributes stress. 

The unique properties of fullerenes allow for many practical applications. Fullerene derivatives—fullerenes with attached functional groups—can serve as antiviral enzyme inhibitors. The large concentration of carbon atoms in fullerenes contributes to their highly hydrophobic characteristics. Fullerene derivatives are able to bind to the active site of HIV protein synthesizers called proteases. This prevents the binding of two amino acids, aspartate 25 and aspartate 125, which consequently inhibits viral replication and thus slows down the onset of AIDS. This opens new possibilities in fighting HIV progression. 

 Researchers discovered that regioisomeric bis-fulleropyrrolidines (a fullerene with two rings of an organic compound called pyrrolidine) bearing two ammonium groups were effective against HIV-1 and HIV-2 protease activity in 2005. In addition, researchers in 2021 discovered that fullerene derivatives can also inhibit HIV-1 reverse transcriptase. Reverse transcriptase is an enzyme that converts HIV’s RNA genome into double stranded DNA, which is a crucial step in hijacking the infected host cell’s genetic information. The inhibition of HIV-1 reverse transcriptase slows down HIV replication and infection progression. The ability of fullerenes to inhibit key HIV enzymes demonstrates their potential in HIV and AIDS treatments. 

The safe delivery of drugs or genes through the cell’s many membranes is crucial for many medical treatments. The transport of genes to the nucleus of the cell is limited by at least three membranes, making it especially difficult. To combat this, scientists are able to form fullerenes around drugs or genes needed for transport. The small, strong, cage-like structure of fullerenes protects the transported molecules. When modified by hydrophilic groups of atoms, fullerenes are more water-soluble, helping them travel better in bloodstreams or cellular fluid, while the hydrophobic fullerene core allows them to pass through the hydrophobic cell membranes. Modified fullerenes can also release drugs slowly, which can have many uses in the delivery of long-term treatment. Fullerenes show great promise as versatile drug carriers. 

On the other hand, the delocalized pi electron systems of fullerenes provide it with strong antioxidant abilities. Antioxidants are compounds that protect cells against free radicals, which are unstable molecules that damage DNA and cell parts. Oxidative stress occurs when there aren’t enough antioxidants to counter the damage of free radicals. Cancer radiotherapy produces free radicals that cause oxidative stress, leading to the death of cancer cells. However, many healthy cells are damaged in the process. Due to their antioxidative abilities, fullerenes can serve as radioprotectors—compounds that are applied before treatment to protect normal cells during radiotherapy. As a result, derivatives of fullerenes can protect red blood cells, heart muscle cells, colon cells, and protective lining cells from damage caused by radiotherapy. They can similarly protect organs such as the small intestine, lungs, and spleen. The antioxidative properties of fullerenes allow them to safeguard healthy cells during treatments like radiotherapy and expand their medical potential. 

The delocalized pi electron systems of fullerenes also allow them to absorb light efficiently, contributing to their enhanced photosensitizing abilities. In cancer photodynamic therapy (PDT), a photosensitizer absorbs light and accelerates chemical changes to kill cancer cells. PDT causes less damage to healthy cells than chemotherapy or radiotherapy. In experiments performed on mice, the pyrrolidinium-C60 derivative was able to kill three types of cancer cell lines. The effectiveness and less harmful side effects of PDT involving fullerene photosensitizers prompts a further exploration into the uses of fullerenes in cancer treatments. 

Furthermore, fullerenes’ stable, cage-like structure makes them useful in the encapsulation of harmful contrast agents during Magnetic Resonance Imaging (MRI). Contrast agents are compounds that are used to make body parts more visible during MRI scans. Gadolinium is a common contrast agent that is extremely toxic and can lead to fatal brain damage. Although gadolinium is combined with other substances to prevent leaching, conditions in the human body can break the complexes down and release gadolinium into the body, causing harmful effects. Modified fullerenes can stably enclose gadolinium ions and even withstand strong radiation, which effectively prevents gadolinium interaction with body tissue. Fullerenes can safely encapsulate harmful contrast agents, thus reducing risks in medical imaging.

Beyond their potential uses in the medical field, fullerenes can also be used in materials science and sports equipment. They are generally strong and lightweight, with carbon nanotubes (CNTs) having an added characteristic of flexibility. CNTs are often used as additives in sports equipment to provide strength while maintaining lightweight properties. For example, CNTs can be incorporated into tennis rackets with another allotrope of carbon, graphene, to increase shock absorption and reduce strain on athletes’ arms. CNTs can also be used to strengthen the midsoles and outsoles of running shoes, contributing to better cushioning and energy return for every step. In protective gear for sports like hockey and American football, CNTs and graphene are added to improve impact resistance without adding additional weight. These substances can also be used to strengthen bicycle frames to improve speed. By adding extra strength, stiffness, and flexibility without adding much weight, the incorporation of CNTs into sports equipment enhances both equipment longevity and performance. 

Despite having many uses and being widely researched, fullerenes are not used often. Fullerenes have been proven to be toxic in some cases: a 2007 study found that the exposure to fullerenes in the embryonic development of zebra fish led to malformations, induced cell death, and increased mortality rates among embryos. At high concentrations, fullerenes can potentially be carcinogenic, damage cells, cause inflammation, and induce long-term tissue injury. Additionally, in the materials industry, the incorporation of CNTs into sports equipment increases production costs, making equipment less affordable to consumers and limiting widespread adoption. While fullerenes and fullerene-derivatives offer unique advantages in medical treatments, bioengineering, and sports equipment, further testing is needed to determine their safety before they are widely used.