Biomedical Engineering Reference
In-Depth Information
were obtained within a time of 60 s after every addition of the aliquot. To prove this
operational procedure, long-term stability of the applied chemical systems including
the solvent quality has been checked within the time scale from 60 s to hours as well
as upon variation of the solvent (toluene, TEHOS or
n
-octane) [
64
]. The detailed
analysis of a whole series of these experiments leads to the following main results
and conclusions.
4.4.1
Dependence of PL Quenching of QDs and QD-Dye
Nanoassemblies on Long Observation Times
4.4.1.1
Development of Intrinsic PL Quenching with Time in Solvents
of Various Qualities
After sample preparation via diluting the originally concentrated stock solution
of QDs a decrease of PL intensity is observed with time. This decrease turns
out to depend on the kind of solvent, the respective purity as well as on the
absolute concentration of QDs and surface ligands in solution. Figure
4.23
shows
for uncapped CdSe QDs the role of different toluene purities and the influence of
the addition of (m-Pyr)
4
-H
2
P molecules at molar ratio
x
1.2.
It is clearly observed from Fig.
4.23
a that the amount of water governs the
stability of the PL over time. The poorly dried toluene results in an almost
complete PL quenching within 30 min. We also find that PL quenching of the
QDs is reduced upon an increased content of TOPO in the solution (molar ratio
y
=
800). However, the most stable PL is observed following
an extensive drying of the solvent, which reduces PL quenching considerably.
The related PL decrease is in all cases nearly exponential at early times. The
corresponding decay time constants are in the range 240-3,000 s for the given
examples.
The question regarding the long-term stability of the formed “QD-H
2
P”
nanoassemblies in solution was investigated by adding H
2
P in one step at
x
=
[
C
TOPO
]/[
C
QD
]
=
1.2
to the respective samples of uncapped CdSe QDs (see Fig.
4.23
b). Here, for all
solvent qualities, similar results were found: addition of the H
2
P aliquot to the
master solution of QDs results in an immediate decrease of the PL intensity faster
than our time resolution of about 60 s. In each of the samples, the initial fast PL
decrease is followed by a decrease of the PL intensities on similar time scales, but
it is no longer close to an exponential decay. As compared to the time behavior
of the corresponding samples before the titration step (see Fig.
4.23
a) the decay
times seem to be broadly distributed (similar to a power law) reflecting the presence
of different quenching processes, namely those already inherent in the initial QD
sample and those imposed by addition of the H
2
P resulting in assembly formation.
In this respect, the slower QD PL quenching in time for “QD-H
2
P” nanoassemblies
=
