Page 205 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

190 Principles and Methods

sup 5 P T D (108)
T
1
The overall quantum yield for the production of ROS ( O 2 and O 2 ) in
such a system can then be given as the sum of the quantum yields:
ROS 5 1 sup (109)

Kinetically, the triplet state is a key intermediary in photosensitizing
processes. The quantum yield for ROS (Eq. 109) cannot be maximized
unless triplet decay pathways (k ), oxygen quenching that does not lead
TD
to singlet oxygen (k k O 2 ), self quenching (k S 0 ), and triplet-triplet
SQ
TQ
T
annihilation (k T 1 ) are minimized as potential pathways for removal of
AN
the triplet-state sensitizer. Based on these kinetic limitations, the ideal
photosensitizer has three properties: the absorbance of low energy light
to create the singlet excited state efficiently; preferred conversion of
the singlet state to the triplet state due to intersystem crossing (Eq. 83);
and a low occurrence of non-ROS forming triplet removal pathways.
The nanomaterial class known as fullerenes holds great promise due to
properties that correspond to each of these desirable traits.

ROS production by fullerenes
Carbon-based nanomaterials such as fullerenes have been known to be
photoactive as photosensitizers from the first studies of their physical
properties [61]. Fullerenes, and in particular C , have been studied
60
intensively for applications in fields such as photodynamic therapy [62],
photovoltaics [63], and materials [64].
An advantage of using fullerenes, and in particular C 60 , as photosen-
sitizers in an engineered system is that they are highly stable. For
example, the carbon cage making up C 60 appears to be nearly impervi-
ous to degradation by oxidation or susceptible to enzymatic attack.
However, fullerenes may be modified in aqueous environments such as
in the presence of UV light
in the formation of epoxide derivatives of C 60
[65] or on the surface of a metal oxides such as TiO [66].
2
When fullerenes are illuminated under the appropriate wavelength,
0
the electrons are excited from the ground state ( C ) to the singlet
60
1
state (Eq. 80). The singlet state ( C ) can decay in three main manners:
60
fluorescence (Eq. 81), internal conversion (Eq. 82), and intersystem
crossing (ISC) (Eq. 83). The first two result in the ground state while
3
to the triplet state ( C ).
the latter leads to the relaxation of singlet C 60 60
Interaction of the singlet state with oxygen can also result in the triplet
state (Eqs. 86 and 87). Eq. 86 results in the production of singlet oxygen
3
via type II photosensitization. The triplet state, C , has a signifi-
60
1
cantly longer lifetime than C 60 in solution, allowing it to participate
in type II formation of singlet oxygen to a greater extent than does the