Page 174 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Reactive Oxygen Species Generation on Nanoparticulate Material 159

The lifetime of singlet oxygen in water is on average 3 µs due to deac-
tivation by collision with H O according to Eq. 9.
2
k d
3 2
1
O s d 1 H O h O s g d 1 H O ∗ (9)
2
2
2
g
2
The first-order decay constant for the deactivation of singlet oxygen in
5 1 1
water is k 3.2
10 M s where the characteristic lifetime is given
d
by 1/k d .
Singlet oxygen can be generated in nature by sensitization (type II
reaction) of dye compounds such as rose bengal or C 60 molecules, and
also by irradiation of naturally occurring humic acids in lakes and rivers
[1, 2]. Singlet oxygen reacts with molecules of biological significance such
as nucleic acids, lipids, and amino acids [3] with toxic consequences. For
1
example, rhodopsin reacts with O 2 s g d at pH 8.0 with a second-order
9 1 1
rate constant of 1.1
10 M s .
Superoxide, the one-electron reduction product of dioxygen (Table 5.1),
.
and its protonated conjugate acid, HO 2 , have the following equilibrium
relationship:
K a 2. 1
HO m O 2 1 H (10)
2
where pK 4.8. Superoxide is readily formed by electron transfer from
a
sensitized dyes (type I reactions) or by sensitized oxidation of secondary
2.
alcohols. In addition, O 2 is formed in aqueous suspensions of semicon-
ductor photocatalysts (e.g., ZnO or TiO 2 ) where oxygen is reduced by the
photo-excited conduction-band electrons on the surface of the metal oxide
or metal sulfide semiconductors. However, the characteristic lifetime of
superoxide in aqueous solution is short due to competition from its self-
reaction (i.e., dismutation) into oxygen and hydrogen peroxide.
? 2. H 1
HO 1 O 2 h H O 1 O 2 (11)
2
2
2
? ?
HO 1 HO h H O 1 O 2 (12)
2
2
2
2
2H 1
2.
2.
O 2 1 O 2 h H O 1 O 2 (13)
2
2
However, there is a pronounced difference in the rates of Eqs. 11 and
1 1
7
12. For example, k 11 9.7
10 M s ; this can be compared to k 12
5
1 1
1 1
8.3
10 M s and k 13 < 2 M s . As a consequence of the relative
slowness of Eq. 13, most living cells employ a protein “superoxide dis-
mutase” (SOD) to catalyze the reaction under biological pH conditions
(e.g., pH 7.8).
Hydroxyl radical, the three-electron reduction product of dioxygen, is
the most highly reactive oxygen species in terms of redox potential and