Biomedical Engineering Reference
In-Depth Information
Energy transfer QD
→
porphyrin
ZnS
Energy transfer porphyrin
→
oxygen
CdSe
Singlet oxygen
N
H
1
Δ
g
Scheme 4.2
Schematic presentation of a complex nanoassembly with a twofold energy transfer
processes from the QD to H
2
P(QD
→
H
2
P) and to oxygen (H
2
P
→
O
2
) followed by generation of
singlet oxygen
hand, in terms of electron tunneling across the ZnS shell even under the condition
of quantum confinement, and, on the other hand, by the influence of ligand dynamic.
FRET is—depending on the environment—often only a small part of the PL
quenching. The competition between FRET and non-FRET quenching processes
drastically depends on the solvent and ligand properties. “FRET quenching” in
“QD-Dye” nanoassemblies is effectively suppressed already in slightly polar
solvents which is often overlooked in literature data.
In addition based on the quantitative analysis of FRET efficiency in “QD-
porphyrin” nanoassemblies as well as on the direct comparative measurements of
near-IR emission of singlet oxygen
1
O
2
it has been proven that namely FRET
→
process QD
porphyrin is a reason of singlet oxygen generation by these nanocom-
posites in solutions at ambient temperature [
114
](Scheme
4.2
).
4.6.2
Non-FRET Quenching in QD-Dye Nanoassemblies
Photoinduced FRET and/or charge transfer (CT) has been reported in literature
for many QD-Dye assemblies. An unambiguous identification of such processes,
however, can only be identified by optical means if the luminescence of both
constituents is followed quantitatively. The observation of PL quenching and PL
lifetime shortening is not sufficient to identify FRET or CT. If the dynamics
observed for both constituents are not in agreement with each other, but PL
quenching is observed, non-FRET quenching has to be postulated [
62
-
65
,
74
,
75
].
How can such a process be modeled? In case that dangling bonds at the surface of
a QD are not saturated by appropriate ligands new electron-hole-related states are
generated in the band gap. Similar trap states might be formed due to surface or
