Biomedical Engineering Reference
In-Depth Information
Fig. 4.24
Long-time
development of the PL of
CdSe/ZnS QDs
(
d
CdSe
=
2)
and the FRET efficiency in
presence of (m-Pyr)
4
-H
2
P
molecules for “QD-H
2
P”
nanoassemblies at different
molar ratios. The values
I
(
x
)/
I
0
on the
left axis
were
normalized to the PL of the
individual sample prior
titration. The related FRET
efficiency (
right axis
)is
plotted at the bottom.
Adapted from [
64
]
3.0 nm,
n
ZnS
=
4.4.1.2
One-Step PL Quenching of CdSe/ZnS QDs in “QD-H
2
P”
Nanoassemblies at Various Molar Ratios
x
This example concerns the behavior of the PL intensity as a function of the waiting
time after one titration step followed by the formation of “QD-H
2
P” nanoassemblies
at a given ratio
x
. In these experiments, the long-term stability of the PL of core-
shell CdSe/ZnS QDs was investigated at various H
2
P concentrations (Fig.
4.24
).
Here, samples at different molar ratios
x
were prepared in one single titration step
which results in a PL quenching similar to the discussion above for the uncapped
QDs. The data for this quasi-static titration are presented as a ratio of
I
(
x
), the
intensity after titration, to
I
0,
the intensity just before the first titration event. It is
clearly seen that PL quenching increases with observation time. This effect is most
pronounced for high molar ratios (e.g., additional quenching of 15% over 3.5 h
at
x
=
11), whereas solutions at molar ratios
x
=
0-6 remain stable in PL intensity
within the range of 5-8% over 3.5 h.
The absolute efficiency of quenching increases with
x
, although again some
quenching is present in the absence of H
2
P. The comparison of the PL quenching
with the FRET build-up during time shows that the “QD-H
2
P” nanoassembly
formation has multiple (at least three) macroscopic time scales. All these effects
should be taken into account upon quantitative analysis of titration results with
respect to the efficiency of FRET and non-FRET quenching processes in “QD-Dye”
nanoassemblies.
4.4.1.3
Dye Attachment Dynamics and PL Quenching for CdSe/ZnS QDs
of Various Sizes
As was outlined in Sect.
4.3.2
for “QD-H
2
P” nanoassemblies based on CdSe/ZnS-
capped QDs of various sizes and (m-Pyr)
4
-H
2
P molecules, the Stern-Volmer
