Biomedical Engineering Reference
In-Depth Information
dye molecules. The presence of various in part ligand-related trap states of various
nature has recently been suggested by numerical calculations [
46
,
56
-
58
].
Summarizing all these arguments, non-FRET quenching corresponds to the
removal of ligands at specific surface sites accompanied by the formation of intra-
band states and possibly the creation of new (PL quenching) trap states. Hence, for
tuning the PL properties, it does not need a mayor exchange of the ligand shell, but
already very few dye molecules can induce very effectively PL quenching. This
conclusion is supported by results [
71
] showing that the luminescence quantum
yields of QDs and the photobrightening are determined primarily by the surface
stoichiometry and by the resultant selective surface adsorption of passivants. At this
point we conclude from the long-time behavior of PL quenching and its dependence
on the amplitude of the electronic wave function at the outer interface, that both
processes play an essential role in PL quenching. If ligand depletion in combination
with intra-band state formation plays the leading effect for non-FRET quenching it
is not surprising that the solvent has an essential impact on PL quenching as will be
discussed in more detail in the following section.
4.4.2
Influence of Solvent Composition on FRET
and Non-FRET Contributions to PL Quenching
As we have discussed in detail, ligand dynamics and non-FRET processes depend
critically on the kind of the solvent. Most of the FRET investigations on QD-
Dye nanoassemblies have been performed in toluene. Addition of polar acetone
increases considerably the PL quenching [
62
]. This is understood according to
recent calculations, which show that ligand binding occurs via electrostatic forces
[
106
].
On the other hand, quenching is effectively suppressed in non-polar
n
-octane
[
64
]. Figure
4.26
shows the PL quenching of core-shell QDs CdSe/ZnS for the
two solvents, toluene and
n
-octane, as a function of the molar ratio
x
. It is clearly
seen that PL quenching is only observed in toluene, whereas the QD PL intensity
stays more or less constant in
n
-octane indicating that in the latter case the ligand
shell is stabilized against the H
2
P molecules. This is easily understood since the
solubility of TOPO is largely reduced in
n
-octane while that of H
2
P is increased as
compared to toluene. This will shift the equilibrium constants
K
M
and
K
L
towards
each other. In that case the ligand shell remains intact preventing H
2
P becoming
attached effectively.
To show the influence of solvent properties, we analyze the formation “QD-
Dye” nanoassemblies based on TOPO-capped CdSe/ZnS QDs and DTPP molecules
(dye structure is shown in Fig.
4.3
) in TEHOS (structure is presented in Fig.
4.18
)
since an efficient FRET QD
DyeisasshowninFig.
4.18
observed for single
nanoassemblies in this solvent [
140
]. Figure
4.27
shows QD PL quenching as a
function of waiting time following titration by DTPP at various molar ratios
x
.Itis
→
