Biomedical Engineering Reference
In-Depth Information
Fig. 4.13
2) and
(pyridyl)
1
-PDI molecule (PP). Spectra are related to various confocal spots of the spin-coated
sample. It was possible to detect PL spectra (1 s binning time) of single QD (
a
), single PP (
c
),
and single nanoassemblies with PP and QD spectra simultaneously (
b
). Typical luminescence
intensities, spectral positions, and decay times are shown as a function of observation time of
60 s and longer.
Emission spectra of single TOPO-capped CdSe/ZnS QD (
d
CdSe
=
3.0 nm,
n
ZnS
=
0.6 kW cm
−
2
. A confocal scan of a spin-coated sample with
λ
=
465 nm;
P
=
exc
solution ratio
x
=
1 is shown on
top
. Adapted from [
162
]
τ
DA
≈
16 ns (based on measurements for 18 nanoassemblies). The shortening of
the decay time is clearly related to the observed decrease of the QD PL intensity
upon nanoassembly formation. While in a given sample the fluorescence intensity
of PP is nearly (see discussion of FRET) indistinguishable between isolated PP
and assembled PP, the QD PL intensity of single QD-PP assemblies is quenched
on average by 50% as compared to isolated QD in the same sample. This has
to be compared to the PL reduction to 0.88 observed in ensemble experiments at
x
1(seeFig.
4.11
). This indicates that at least the majority of the simultaneously
detected QD-PP spectra are in fact due to assembly formation. Taken together, these
facts prove that QD PL quenching is also observed on a single nanoassemblies level
and reflects directly the existence of an additional non-radiative channel for QD
exciton relaxation in these nanoassemblies.
Finally, the spectral distribution of the fluorescence of many single PP molecules
with respect to the one of many single “QD-PP” nanoassemblies is related to a
reduced conformational mobility of PP molecules on a QD surface [
94
]. It is known
[
130
,
134
] that individual PP molecules have different fluorescence spectra due to
=
