"Fullerenes are virtually insoluble
in water, yet most biological and environmental systems
are based around water," Fortner noted. "Researchers
thought fullerenes couldn't be transported by water
because they are so hydrophobic. We thought they would
simply stick to soil or other organic material. But
research shows this is actually not the case. When
fullerenes, such as C60, come in contact with water,
they form aggregates at the nanoscale. We call it
nano-C60."
In their study, Fortner and his colleagues
devised several novel applications of imaging techniques
to characterize the physical and chemical formation
of nano-C60 particles in water mixed with the organic
solvent THF. Using cryoTEM (transmission electron
microscopy), researchers froze samples of the solution
and examined slices of them to determine the effects
of various parameters on particle size.
The nano-C60 particles that formed
were 20 to 500 nanometers across and retained the
same properties as C60 molecules – a determination
researchers made using nuclear magnetic resonance
imaging. This finding is significant because pure
C60 is recoverable, therefore enhancing the promise
of sustainable fullerene production practices, Fortner
noted. Also, electron and powder diffraction techniques
revealed that nano-C60 has a particular crystalline
structure. These findings reinforced previous research
done elsewhere.
"Our work builds on previous
findings in an environmental engineering context,"
Fortner explained. "We wanted to know about nano-C60
formation under a variety of ambient natural conditions."
Researchers found that changing the
pH of the water with which C60 is mixed affected particle
size. A higher pH, such as 9, yielded smaller particles,
and a lower pH, such as 5, yielded larger particles.
Also, the rate at which C60 is mixed with water affected
particle size. A slower rate resulted in larger particles,
and a faster rate produced smaller ones.
"This research is the first to
show control over the particle formation processes
based on these parameters," Fortner said. Researchers
also examined the stability of these particles as
a function of ion concentration in the water. Because
nano-C60 particles rely on a negatively charged surface
to remain suspended in water, the presence of elevated
concentrations of ions, such as dissolved NaCl (table
salt), can render the surface neutral. If that happens,
nano-C60 particles sink to the bottom of the solution
container and form a solid glob, Fortner explained.
"In the laboratory, scientists
typically use de-ionized water, but that's not the
case in nature," Fortner said. "At some
level, salt is present, even in groundwater. We found
that even in water with the normal salt concentration
of groundwater, nano-C60 particles still remain suspended
for months. However, in simulated sea water, the particles
are neutralized and sink in a matter of hours."
Researchers don't yet know the full implications of
this finding, Fortner added.
"We've just developed a conceptual
model so far, and it doesn't take into account all
of the unknowns or heterogeneity found in the environment,"
Fortner said. "We studied the most controlled
situations and got preliminary data."
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TECHNICAL CONTACTS:
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