Biomedical Engineering Reference
In-Depth Information
The TRF experiments indicate a change of the (3-exponentially fitted) emis-
sion decay times of both QD and (m-Pyr)
4
-H
2
P porphyrin upon nanoassemblies
formation. The main conclusion is that in cases of both CdSe/ZnS QDs and
CdSe QDs [
60
,
62
], the attachment of dye molecules leads to a PL mean decay
time shortening accompanied by a reduction of the PL efficiency. For “QD-H
2
P”
nanoassemblies, the initial two components of 19-22 and 7 ns are shortened, while
the fast component at
700 ps remains nearly constant [
62
]. These shortened
components reflect that there is an (static or dynamic) inhomogeneous distribution
of QDs, some of them showing non-radiative shortening of the intrinsic PL decay,
which is related to assembly formation. It turns also out from Fig.
4.5
b, d that,
especially at
x
∼
=
1, the fluorescence decay time of porphyrin in the presence of
QDs is longer (
0.2 ns).
With increasing molar ratio
x
, the characteristic decay time approaches the one of
the porphyrin. Further, for (m-Pyr)
4
-H
2
P molecules being attached to the surface
of CdSe QDs, a weak rising component (with amplitudes below 5%) is seen, the
characteristic time of which varies within the range of 1-2 ns. The longer decay
for porphyrin molecules being attached to CdSe QD surface at small
x
ratio may be
explained by two reasons: (1) the influence of QD on the radiationless rate constants
of the porphyrin molecule in the nanoassemblies [
62
] and (2) the reduction of the
porphyrin excited singlet state quenching by molecular oxygen due to the relative
screening effect caused by QD with a high hydrodynamic volume like it has been
observed upon interaction of porphyrin triads and pentads with molecular oxygen
in non-degassed liquid solutions at 295 K [
112
]. In its turn, the detection of a
weak rising component for (m-Pyr)
4
-H
2
P molecules in nanoassemblies reflects the
existence of energy transfer QD
τ
=
11.7
±
1.1 ns) than that for pure porphyrin (
τ
0
=
9.1
±
porphyrin. Taken together, all these facts indicate
that the formation of “QD-H
2
P” nanoassemblies manifests itself in PL quenching
of the CdSe counterpart, and this quenching is a dynamic process caused by the
increased non-radiative relaxation channels in QD excited states.
The detailed analysis of the main reasons (mechanisms) leading to the observed
QD PL quenching in various “QD-Dye” nanoassemblies will be presented below.
In this section, with respect to porphyrin moieties being used for the QD surface
attachment (see Fig.
4.2
), it is reasonable to outline the results of systematic
titration experiments [
62
,
65
] (which will be of interest for the following analysis
of the basic principles for self-assembled nanoassemblies). Like it has been done
previously with self-assembled multipophyrin arrays [
107
,
108
,
112
,
113
], the
strategy was: (1) to vary the number of pyridyl-rings from 1 to 4 including the
two variants (
m
-Pyr)
2
-H
2
Pand(
m
ˆPyr)
2
-H
2
P and (2) to replace the type of nitrogen
(N) position within the pyridyl ring from the
meta
-(
m
), to
ortho
-(
o
), and
para
-(
p
)
N position in the case of the fourfold
meso
-pyridyl-substituted H
2
P molecules. The
data presented in Fig.
4.6
a show that for CdSe/ZnS QDs under the same titration
conditions, the observed QD PL quenching depends strongly on the number and
type of pyridyl substituents. The most obvious observation is that (
o
-Pyr)
4
-H
2
P does
almost not quench the PL, while the quenching is strongest for (
p
-Pyr)
4
-H
2
P closely
followed by (
m
-Pyr)
4
-H
2
P. In addition, within the (
m
-Pyr)
n
-H
2
P manifold there is
a systematic increase of quenching efficiency depending on the number of pyridyl
→
