Biomedical Engineering Reference
In-Depth Information
a
b
Fig. 4.23
Influence of sample conditions on the time-dependent PL quenching for uncapped CdSe
QDs (
d
CdSe
=
3.5 nm) in toluene. (
a
) Influence of solvent composition. Tol (aq) and Tol (sec)
correspond to toluene spectroscopic grade as-supplied and 48 h dried over a molecular sieve,
respectively. The addition of tri-
n
-octyl phosphane oxide (TOPO) in large excess (molar ratio of
y
1.2. PL
intensities of CdSe are re-normalized with respect to the time of addition of H
2
P at about 30 min.
This time was set to zero. The time-dependent increase of the H
2
P fluorescence (FRET efficiency,
relative units,
right axis
)isalsoshown(
lower curves
) which proves the formation of “QD-H
2
P”
nanoassemblies. The absolute concentration
C
of uncapped CdSe QDs is
C
=
[
C
TOPO
]/[
C
QD
]
=
800). (
b
) Influence of the addition of (m-Pyr)
4
-H
2
P at molar ratio
x
=
10
−
7
M. Adapted
=
2
×
from [
64
]
in comparison with that for merely QDs may be explained by effective PL quenching
in nanoassemblies competing with that caused by solvent impurities.
The increase of the H
2
P fluorescence (Fig.
4.22
b) upon titration can be clearly
explained by FRET from the QD to the attached H
2
P molecule. It levels off as
soon as it cannot compete any more with other ingrowing quenching processes.
Hence, aside from the confirmation conducted in earlier publications [
62
,
63
,
101
,
127
], FRET is a measure for the formation kinetics of “QD-H
2
P” nanoassemblies.
It should be mentioned also that for CdSe QD
+
H
2
P solutions of the same molar
10
−
7
,
ratio
x
=
1.2 prepared at different initial concentrations of QDs (
C
QD
=
1
×
10
−
7
M) the PL of the QDs is continuously decreasing, with the
quenching most pronounced for the most dilute solutions.
10
−
7
2
×
and 4
×
