Biomedical Engineering Reference
In-Depth Information
Obviously, strong variations of
E
FRET
are observed for on one and the same
assembly, since
I
QD
depends on quenching processes other than FRET. Notably,
FRET values (obtained in TEHOS) are essentially higher with respect to those
(
E
FRET
≤
0.1) obtained for the same “QD-DPI” nanoassemblies both from ensem-
ble (toluene) and from single nanoobject detection (on an SiO
2
surface) [
74
,
94
]as
well as for “QD-porphyrin” nanoassemblies (in toluene) [
62
-
64
]. We have related
these previous findings of low FRET efficiencies to the presence of competing non-
FRET processes, which open a quenching pathway other than energy transfer by
merely attaching a dye molecule to the QD surface [
62
-
65
,
74
,
75
,
90
,
94
,
101
,
127
]. This will be discussed in more detail later on.
Finally, in the hypothetical case of a photoinduced charge (hole or electron)
transfer process in “QD-porphyrin” or “QD-PDI” nanoassemblies the fluorescence
of the dye should be also quenched. However, fluorescence parameters (efficiency
ϕ
F
and decay time
) for (m-Pyr)
4
-H
2
P and PDI molecules upon complexation
with QDs remain practically the same with respect to those measured for indi-
vidual constituents under the same conditions [
62
,
101
]. In addition, the titration
of identical QD solutions by (m-Pyr)
4
-H
2
P and (m-Pyr)
4
THP (hole acceptor)
as well as by H
2
P(mˆPyr)
2
(Ph)
2
and electron acceptors (mˆPyr)
2
H
2
P(5FPh)
2
or
(mˆPyr)
2
H
2
P(Anthraquinone)
2
gives the same result for PL quenching [
62
,
90
].
Thus, the independence of PL quenching efficiency on redox properties of porphyrin
and the absence of the porphyrin fluorescence quenching in “QD-porphyrin”
nanoassemblies rules out a dominant role of photoinduced charge transfer processes
with participation of molecular orbitals of the porphyrin macrocycle for PL
quenching for the systems under study.
τ
4.3.2
Non-FRET Photoluminescence Quenching
and Quantum Confinement
It is well documented that because of quantum confinement [
10
-
13
] the exciton in
a QD is highly sensitive to local charges or distortions of the charge distribution.
Therefore, the PL quantum efficiency is sensitive to the involved interfaces, such
as inorganic shell structures [
142
,
143
], surfactants [
83
,
144
-
146
]andsolvent
molecules [
147
] as long as the exciton wave function extends beyond the QD core.
In this respect, it would be tempting to probe the interactions locally by distorting
the interfacial properties with a local probe. Such controlled distortions will unravel
microscopic details of either non-radiative processes occurring at the QD interface
or the influence of chemical bonds at the QD surface. Organic dye molecules
attached to the surface might act as such probes. Several experiments upon the effect
of organic molecules exchanging the surfactant layer have been reported [
144
-
146
].
In this section we show that direct surface labeling of colloidal semiconductor QD
with only a few pyridyl-functionalized molecules reveals the nature of only recently
identified PL quenching mechanism which has been identified to be dominant for
