Biomedical Engineering Reference
In-Depth Information
surface ions and the ones of the ligand anchoring group. Similar conditions also hold
for the formation of nanoassemblies with participation of H
2
P and PDI molecules
via pyridyl anchoring groups.
A versatile scheme for the preparation of “QD-Dye” nanoassemblies in non-
polar liquids or low polar environments at ambient conditions for bulk experiments
is based on quantitative titration steps: into low concentrated (
C
QD
∼
10
−
7
M)
QD solutions dye molecules are added sequentially in steps of 10
L from a
10
−
5
highly concentrated (
C
Dye
∼
M) stock solution up to the wanted molar ratios
x
[
C
Dye
]/[
C
QD
](where
C
Dye
and
C
QD
denote the concentration of the species),
thus varying the number of dye molecules per QD. Details of the experimental tech-
nique, steady-state, and time-resolved measurements for bulk solutions and single
nanoassemblies as well as theoretical approaches are presented in corresponding
papers [
62
,
65
,
74
,
90
,
94
,
114
,
115
].
=
4.2.2
Spectral-Kinetic Manifestations of Nanoassembly
Formation
4.2.2.1
Semiconductor QDs and Porphyrin Molecules
It should be mentioned that the formation of “QD-Dye” nanoassemblies with
various subunits shows some common spectral-kinetic features which may vary
depending on the physicochemical properties of interacting QDs and attached dye
molecules. In the case of nanoassemblies based on CdSe/ZnS QDs and functional
porphyrin molecules, typical spectral-kinetic data of titration experiments are
collected in Figs.
4.4
and
4.5
. It is seen from Fig.
4.4
that upon molar ratio
x
increase, besides QD absorption bands (
556 nm) a linear increase of the
porphyrin absorption bands takes place. Noteworthy, in all cases QD absorption
remains constant, while the PL (at
λ
max
=
585 nm) is considerably quenched upon
titration by (Pyr)
4
H
2
P molecules. No such effects are observed when using various
porphyrins without pyridyl rings.
Time-resolved PL measurements show that emission of CdSe or CdSe/ZnS QDs
in the absence of dyes is characterized by a non-exponential decay [
116
-
120
]. The
decay time distribution shows in the given example a strong and broad contribution
centered at 19-22 ns which is related to the “intrinsic” PL of CdSe QDs. The center
and width of this intrinsic decay times depend noticeably on the excitation and
detection wavelength and/or the corresponding spectral bandwidths.
In this respect, time-resolved fluorometry (TRF) was applied also to compare the
excited-state depopulation kinetics of semiconductor QDs in absence and presence
of attached (
m
-Pyr)
4
H
2
P molecules. Figure
4.5
summarizes the TRF results ob-
tained at the near-band-edge emission peak wavelength of the QD (
λ
max
=
λ
em
=
580 nm)
and the Q
x
(0, 0)-band emission wavelength of porphyrin (
λ
em
=
653 nm) at molar
ratios
x
=
1, 2, and 4.
