Biomedical Engineering Reference
In-Depth Information
From a comparison of calculated FRET efficiencies for “QD-DPP” and “QD-
DTPP” nanoassemblies (Fig.
4.12
b) as well as QD PL relative quenching for these
two dyes it has been concluded that the DPP molecule is oriented with the long
axis nearly perpendicular to the QD surface, while DTPP molecule prefers a more
tangential orientation [
74
]. Thus, it follows from the presented results that PDI dye
molecules may be sorted into two classes, namely DPP-type one-pyridyl, DTPP-
type three-pyridyl coordination via nitrogen lone pairs or electrostatic interaction
between partial negative charge densities at the N-atom of pyridyl with surface Zn
2
+
ions [
106
], like for “QD-porphyrin” nanoassemblies, and a third namely the DAP-
type intercalation into the AM capping shell. PP and TPP are mixed cases since they
are subject to a competition between pyridyl-type coordination and intercalation.
However, it became also evident from the experimental data [
74
,
94
] that the FRET
efficiency is quite low. Though relative FRET efficiencies can convincingly be
related to various QD-Dye assembly geometric structures and to different relative
orientations of the PBI dyes with respect to the surface, the surprisingly low FRET
efficiency in these nanoassemblies will be discussed separately.
With respect to “QD-PDI” nanoassemblies, we would like to show how the
spectral detection of single objects may give information about distinct nanoassem-
bly geometries and conformational mobility of PDI molecules attached to the QD
surface. Figure
4.13
shows spectra of single PP and TOPO-capped CdSe/ZnS QD
obtained via confocal microscopy for spin-coated samples [
162
]. The spectra can be
divided into three classes, namely QD PL (Fig.
4.13
a), PP fluorescence (Fig.
4.13
c),
and the sum of the two spectra (Fig.
4.13
). All types of spectra show typical single
quantum object fingerprints, that is luminescence intermittency (blinking) [
124
]and
spectral fluctuations (shown at the bottom). The experimental luminescence decay
times averaged over 500 ms are shown as a function of observation time on the
right side of each spectrum. In case of PP a nearly mono-exponential decay varying
between 4 and 6 ns is detected for different single molecules, while the decay
time for QDs is fluctuating between 50 and 5 ns (experimental lower limit in time
resolution). The observation of the sum of the two spectra shown in the middle has
been assigned to the formation of “QD-PP” nanoassemblies.
From Fig.
4.13
b it is seen that at
t
57 s PP fluorescence is bleached while
the QD remains emitting because of higher photostability in comparison with an
organic dye. In addition, the PL band of this quantum dot is blue shifted during
observation time which might be caused by photooxidation of the QD core and is
enhanced by the attached dye molecule [
75
,
94
,
129
]. While the fluorescence of
single PP molecules decays almost mono-exponentially, the PL of single QD-AM
(independent whether free or assembled) decays clearly multi-exponential as has
been reported for many similar cases [
130
-
133
]. It should be noted also that PL
average decay times
≈
of single QD (fitted by a stretched exponential function)
show a tendency to decrease with increasing observation time, and this behavior is
related to the spectral blue shift of the PL. The average decay time for single un-
complexed QD-AM was measured to be
τ
D
5 ns (based on measurements
for 20 QDs), while for QD in single nanoassemblies (QD-AM)-PP we obtain
τ
D
≈
20
.
