Page 213 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

198 Principles and Methods

in type I reactions. C can be reduced with up to five electrons in ben-
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zonitrile solvent with progressive reduction potentials of ( 0.36,
0.83, 1.42, 2.01, 2.60 V vs. SCE) [89, 90]. This affinity is due
to the extended -bonding that can spread the extra electrons across
the surface. In addition to its electron affinity, C 60 also has a stable
0
triplet state that is about 1.56 eV higher than C 60 . The higher energy
3
C 60 is more easily reduced because the reduction potential is raised
by this energy (1.56 V 0.42 V 1.14 V vs. SCE) [61]. As a conse-
3
quence, when an electron donor of lower reduction potential than C 60
is present, excitation to the triplet state plays an important role in
type I reactions (Eq. 103).
The electron transfer capabilities of -CD encapsulated C as opposed
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to free C 60 in propan-2-ol can be compared in terms of their bi-molecular
rate constants (Eq. 103). Interestingly enough, the rate constant is about
a factor of 2 slower in the encapsulating agent [72]. This is consistent with
the rate of oxygen quenching by -CD/C 60 [69]. Similar C 60 micellular sus-
pensions formed with the non-ionic surfactant Triton-X 100 form
monomeric or colloidal suspensions of C 60 depending on the preparation
method. The bi-molecular rate constant for reduction by a donor (Eq. 103)
is three orders of magnitude less than the free C 60 in toluene [83].
Independent measurement confirms that triton X encapsulation slows
reduction by one order of magnitude compared with -CD [70]. The
inability of the donor molecule to approach the surface of C 60 is likely due
to steric and charge repulsion effects [72]. PVP is another encapsulating
agent that has been used extensively to suspend C in aqueous solution
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at concentrations of up to 400 mg/L [68]. In the presence of adenosine 5 -
(trihydrogen diphosphate) (NADH), a C suspension has been shown to
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damage DNA; concurrently EPR and NBT detection confirms superox-
ide formation via type I reaction (Eq. 104) but at reduced rates from free
C 60 [9, 14, 91]. As noted previously for type II reactions, encapsulation
represents a tradeoff between triplet lifetime (Eq. 93) and quantum yield
of type I (Eq. 108) reactions.
As discussed earlier, the LUMO increases with the addition of
addends, and because C 60 is fully occupied in the HOMO, a reducing
electron must jump a larger and larger gap in order to complete the
reduction. This translates into increasingly more negative reduction
potentials that drop about 0.1 to 0.15 V for each additional addend
(Figure 5.31) [76, 78 90, 92]. Concurrently, the triplet energy
increases with the addition of addends (Figure 5.32) [86]. However,
this energy increase is not as dramatic as the decrease in reduction
potential, and upon the summation of these two effects a net decrease
in reduction potential for the triplet state of the increasingly func-
3
tionalized C 60 cage occurs. As C 60 is increasingly functionalized it
3
takes on electrons less readily than nonfunctionalized C 60 (Figure 5.33).