C60 fullerenes are oxidized by levels of ozone found in ambient air

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(This is a post by VTSuN member, Andrea Tiwari. You can reach Andrea at ajtiwari@vt.edu)

As we know, carbon is the basis of life on Earth – we are all “carbon-based life forms.” The carbon in our bodies, and in nearly everything around us, is closely bonded to other types of atoms – oxygen, hydrogen, nitrogen, and so on. In the right proportions, carbon bonded to other elements results in sugars, fats, proteins, and other building blocks of living, and some non-living, things.

There are, however, a few instances in which carbon is bonded to nothing else except carbon. For most of the 20^th century and prior, the pure carbon substances known to humanity were diamonds, graphite, and amorphous carbon. These materials have exactly the same composition, but they couldn’t be more different in their properties. While diamond is the hardest natural mineral known, graphite is soft enough to be used in pencil lead, and amorphous carbon (like soot) is soft. Diamond and graphite both have an ordered chemical structure, while amorphous carbon is very disordered in its structure. Graphite is unique among these forms of carbon in that it consists of two-dimensional sheets of carbon; neither diamond nor soot has this unusual characteristic.

Diamond and graphite

Our knowledge of carbon’s allotropes (different arrangements of atoms with the same chemical composition) took a big leap forward in 1985 with the discovery of fullerenes. Fullerenes are spherical molecules consisting only of carbon. The most common fullerene has the chemical formula C60 (since it is composed of 60 carbon atoms) and bears the name “buckminsterfullerene.” The scientists who discovered the C60 fullerene (several of whom were later awarded the Nobel Prize in Chemistry) named the molecule after the architect Buckminster Fuller, whose geodesic dome design bears a resemblance to the structure of the C60 molecule. Perhaps a more convenient way to think of a fullerene’s structure is to think of a soccer ball: every seam on the ball is a chemical bond between two atoms, and the atoms are located at the ‘corners’, where three faces meet. 

Soccer ball-like structure of fullerene

The family of fullerenes has since grown to include many different spheroidal molecules; carbon nanotubes, the tube-shaped cousins of C60, are also included in this family. These molecules can all be described as being nanoparticles, since they are smaller than 100 nm in at least two dimensions. These carbon-based nanoparticles are of great interest in the field of nanotechnology because they have very unique properties, attributable to their small size and unusual chemical structure. For instance, carbon nanotubes are semiconductive, can act as either thermal conductors or thermal insulators, and are stronger than stainless steel in several respects. These properties make nanotubes attractive candidates for use in a wide variety of electronic, structural, and medical applications.

Eight of the allotropes of carbon: a) Diamond b) Graphite c) Lonsdaleite d) C60 (Buckminsterfullerene) e) C540  f) C70 (see Fullerene) g) Amorphous carbon h) single-walled carbon nanotube

Fullerenes, specifically C60, have the ability to generate and absorb reactive oxygen species, which can be responsible for causing inflammation in living organisms. As such, fullerenes have been used in the fields of optics, computer memory, photovoltaics, and drug delivery to aid in treatment of a variety of diseases.

Left: Polymer fullerene photovoltaic cells (Hübler et al. 2011). Right: Specific eradication of cancer cells using functionalized fullerene and carbon nanotubes (CNTs) (Omidi 2011) 

While these innovations sound promising, we have insufficient knowledge as to the impact that carbon-based nanoparticles would have on the environment. As nanotechnology grows, unintentional release of engineered nanoparticles to environmental systems is inevitable. Anticipating this development, dozens of scientists have devoted themselves in recent years to studying the potential environmental impacts of engineered nanoparticles. Significant gains in our understanding of the environmental impacts of nanotechnology have resulted from these efforts, but the work is far from complete. One manner in which our knowledge is insufficient is understanding the role that the atmosphere will play in determining the behavior and effects of nanoparticles in the environment. If nanoparticles are released into the atmosphere, they can certainly be transported some distance before being deposited back to the Earth’s surface. But will the deposited nanoparticle be chemically unchanged during its transport, or will its time in the atmosphere have altered it?

Along with colleagues, I decided to focus on this question for my Ph.D. research. We set out to determine if C60-fullerenes would be affected by ozone in the atmosphere. While C60 is known to react with ozone at very high concentrations, nobody had explored what happens to C60 when it is exposed to the concentrations of ozone that we find outdoors in everyday situations. If C60 reacts with ozone, this reaction could possibly change the behavior and impact of C60 in the natural environment.

In a paper recently accepted for publication in the scientific journal Environmental Science & Technology (Oxidation of C60 Aerosols by Atmospherically Relevant Levels of O3), we show that C60 does indeed react with ambient levels of ozone. We positively identified three reaction products – C60O, C60O2, and C60O3 – the latter two of which had not previously been identified as products of this reaction occurring outside of a liquid system. We found that the presence of elevated humidity enhanced the reaction between C60 and ozone, and that exposure to ozone may enhance the ability of C60 molecules to exert oxidative stress on cells. It is also possible, although not yet proven, that C60 molecules bond together following reaction with ozone and make a new substance about which little is known. 

While much remains to be learned, we know now that the atmosphere can play an important role in determining how engineered nanoparticles could affect our environment. This makes sense…after all, the atmosphere is an important compartment of the environment, playing host to weather, providing us with oxygen to breathe (via plant expiration), and carrying our gas-phase and other airborne wastes away from us. Some of these gas-phase wastes act as greenhouse gases, and are contributing to anthropogenic climate change. Given the importance of the atmosphere in so many aspects of our lives, it makes sense that it would also be important with respect to the environmental impacts of nanoparticles.

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