Biomedical Engineering Reference
In-Depth Information
The above results clearly show that
K
c
values are increased with the number
of pyridyl rings, thus reflecting a dynamic equilibrium between the QD and
(Pyr)
n
-H
2
P. C o m p a r i n g
K
c
values with those obtained for various multiporphyrin
complexes [
107
,
108
,
112
,
113
] shows that with respect to twofold coordination they
are in a comparable range but being much larger as compared to the corresponding
onefold coordination. Like for porphyrin self-assembled triads, the importance of
the designing adapted coordination schemes has thus been also demonstrated for
formation of the CdSe/ZnS QD-porphyrin nanoassemblies. It is seen also that for
“QD-H
2
P” nanoassemblies the increase of
K
c
values with the number n of pyridyl
rings is less dramatic with respect to
K
c
differences of two to three orders of
magnitude found between two- or one-fold coordination in the case of porphyrin
triads and pentads. It means that in the case of “QD-H
2
P” nanoassemblies the
higher
K
c
values for two-point interaction reflect rather the competition with one-
point interaction of TOPO ligands, thus leading to ligand exchange. In the latter
case, allosteric effects seem to be not important, while they play a dominant role
for porphyrin triad and pentad formation. An analysis of this dynamic exchange
equilibrium will be discussed further on with respect to the quenching efficiency as
a function of the molar ratio
x
.
4.2.2.2
Nanoassemblies from QDs and Perylene Diimide Molecules
The principle of molecular self-assembly was transcribed also to “QD-PDI”
nanoassemblies based on amine-capped CdSe/ZnS QDs (QD-AM) and pyridyl
functionalized perylene bisimides (dye structures and corresponding abbreviations
are presented in Fig.
4.3
), implementing a detailed investigation by single molecule
spectroscopy (due to the high quantum yield of PDI and its large photostability).
The observation of QD PL quenching caused by FRET and non-FRET processes
was analyzed, even on a single assembly level, and the detailed analysis of this
quenching has been presented recently [
74
].
Figure
4.10
shows spectral properties and results of titration experiments for
a given CdSe/ZnS QD-AM and PP dye. Optical spectra of the five PDI dyes
used in this study are almost identical. It is clearly seen from Fig.
4.10
that upon
nanoassembly formation, QD-AM PL intensities change while spectral shapes
remain the same. This finding is very similar to the one discussed for “QD-
porphyrin” nanoassemblies. Titration of solution by various PP dye results, as shown
in Fig.
4.10
b, in instantaneous PL quenching of QD-AM.
Figure
4.11
collects experimental results on PL quenching for QD-AM upon
titration by all PDI dyes as a function of molar ratio
x
.
The strongest quenching is observed in case of DPP. Quenching is quantitatively
similar for TPP and DTPP and somewhat stronger than for PP and DAP. The ratio
I
x
/
I
0
at
x
1 becomes 0.88, 0.82, and 0.66 for PP, DTPP, and DPP, respectively (see
dashed lines in Fig.
4.11
). The surprising result is that DAP (without any pyridyl
group) quenches the PL almost to the same extent as, e.g., PP. In this case, we apt to
believe that the attachment might be accomplished for dyes without pyridyl groups
=
