Biomedical Engineering Reference
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the PL quenching (and DTTP enhancement) main time component is shorter than
1 min, the time needed to register the spectra. On the other hand, the maximum
of PL quenching for
R
=
0:100 (only TEHOS) is about 0.5 which is very similar
as for
R
100:0, but with the difference that the related FRET is very weak. This
can be interpreted as following. At
R
=
0:100 it takes a long time to form “QD-
Dye” nanoassemblies, but FRET is very effective. At the other end of the relative
concentration range (
R
=
100:0) nanoassemblies are much more readily formed, but
non-FRET processes are more effective than FRET as has also been reported earlier
[
74
,
162
].
A qualitative model can be set up following recent arguments [
64
,
163
](dis-
cussed in Sect.
4.4.1
), namely that non-FRET quenching can be assigned to a
competition of dye-induced ligand detachment with subsequent assembly forma-
tion, which gives rise both to FRET and to formation of specific surface states
(related to remaining dangling bonds). These surface states give rise to non-FRET
quenching due to specific sites, probably localizing charges and thus quenching the
PL. Decreasing of polarity will destabilize charge localization [
106
]. On the other
hand, (TOPO) ligand detachment is less favorable in non-polar media, since the
polar ligands have a low solubility. For this reason there is not enough space to
attach large dye molecules to the QD surface and formation of assemblies might
take a very long time. That is why we observe FRET only after a long waiting
time but non-FRET quenching is relatively weak. For
R
=
100:1 we observe an
effective assembly formation within less than 1 min, since ligands are readily
detached from the surface. At the same time non-FRET effective surface states
are created. Obviously the optimum for effective FRET on a short time scale is
close to
R
=
70:30 most of the PL quenching results in FRET.
This observation points to the possibility that in mixed solvents not merely the
number of detached ligands has to be considered but also the specific structure of
the solvent shell enclosing the QD. Since TEHOS has long aliphatic chains structure
presented in Fig.
4.18
, these chains probably intercalate with the aliphatic TOPO
chains forming a stable TEHOS shell intercalated with the ligands. This shell is
only broken up at sufficiently high toluene concentrations
=
50:50. Even at
R
=
70%. Only then the
TEHOS shell is nearly absent and toluene may easily penetrate to the QD surface
and stabilizes the non-FRET quenching states.
These experiments in solvent mixtures demonstrate that the kinetics of assembly
formation is complex but nevertheless provide detailed information not only
on dynamic processes but also on the incorporation of QDs or nanoassemblies
into complex and heterogeneous self-organizing environments. Complementing
information has been reported via temperature-dependent experiments on similar
assemblies in organic glasses [
75
,
164
]. Though most of the properties are inherent
to the QDs themselves, dye molecules act as reporters or even initiate reorganization
processes especially of QD surface states or within the ligand shell. Such well-
defined experiments provide insights, why colloidal QD chemistry is often critically
dependent on small amounts of “impurities” during wet-chemical synthesis.
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