Biomedical Engineering Reference
In-Depth Information
forbidden (vibration-less) transition at high energy. We assume that the same
processes hold for the situation of H
2
P in toluene solution. Assembly formation
obviously “restores” the intrinsic H
2
P symmetry (a planar H
2
P ring) which has a
symmetry-forbidden optical transition. Probably the planarity is enforced when the
H
2
P molecules coordinate with the QD surface and interact with the closely packed
TOPO shell in the vicinity. The observed shifts depend obviously on the type of QD
and decrease with increasing molar ratio
x
[
64
]. For large molar ratios, the shifts
vanish from which we conclude that the spectroscopic properties are on average
determined by non-assembled H
2
P molecules. The critical molar ratio at which this
happens is in the order of
x
c
=
1-10 and increases with increasing QD size.
Finally, it may be concluded that spectral shifts proof assembly formation
immediately after the first titration step at
x
<
x
c
, conducted in our case for molar
ratiosaslowas
x
1-10 depending on QD
size [
64
]), the porphyrins become less and less complexed. It follows from studies
on single QD-PDI nanoassemblies [
74
,
94
] that one dye molecule causes a PL
quenching per QD that is at most two times stronger than in solution at
x
∼
0.1. At molar ratios
x
>
x
c
(with
x
c
=
1.
This implies that only a smaller number of dyes (about one in four [
74
]) becomes
attached to the QD surface at comparable molar ratios in solution. As it was shown
in Fig.
4.5
, for “QD-H
2
P” nanoassemblies, fluorescence lifetime measurements
support a complexation efficiency that is of the same order of magnitude: for
individual (m-Pyr)
n
-H
2
P molecules in absence of QDs in toluene, the fluorescence
decay is measured to be 8.4-9.3 ns [
62
,
111
-
113
]. At the first titration step at
x
=
0.25, the mono-exponential lifetime is increased to 11 ns [
123
] due to FRET
from the long-lived CdSe/ZnS QDs and possible diminishing of H
2
P fluorescence
quenching by molecular oxygen because of QD screening action. This fact implies
that, at these low molar ratios, most of the H
2
P molecules are attached to a QD,
whereas the complexation efficiency reduces considerably when the molar ratio is
increased. All these experiments show that already one attached dye molecule can
quench the PL intensity of a QD considerably. Further information on the number of
H
2
P per QD will be presented below from an analysis of the PL quenching following
a detailed Stern-Volmer description.
As was outlined above, the attachment of functionalized porphyrin molecules
to a QD surface manifests itself in noticeable QD PL quenching as well as in
related complex interface dynamics caused by non-radiative relaxation channels for
the exciton. Here, we demonstrate that QD PL quenching (as a manifestation of
the nanoassemblies formation) is also visible in experiments with single nanoob-
jects. Figure
4.9
shows the comparison of blinking statistics for two samples in
spin-coated toluene solution at 295 K: CdSe/ZnS QDs and “QD-(m-Pyr)
4
-H
2
P”
nanoassemblies both having the same initial QD concentration and being excited
within the QD first excitonic absorption band. Nanoassemblies were prepared at a
molar ratio
x
∼
=
[
C
Porphyrin
]/[
C
QD
]
=
10, at which the bulk QD PL quenching is about
40% [
62
].
It is seen from Fig.
4.9
a, b that for both cases blinking statistics show a power
law distribution for “on-” and “off-”times. Dark QD states are usually explained
by charged nanocrystals [
67
], and the heterogeneity (power law behavior [
124
]) is
