Biomedical Engineering Reference
In-Depth Information
the same time, Fig.
4.15
a shows that upon increasing the molar ratio
x
in solutions
of “QD-porphyrin” nanoassemblies the integrated fluorescence intensities of (m-
Pyr)
4
-H
2
P are noticeably larger than those being measured for individual porphyrin
molecules for the same excitation conditions (
465 nm, the donor QD strong
absorption). This finding reflects the presence of sensitization (enhancement) that
is typical for FRET. In addition, the fluorescence excitation spectrum of (m-
Pyr)
4
H
2
P in nanoassemblies (
λ
ex
=
λ
reg
=
719 nm) provides direct information about the
QD
porphyrin FRET process (Fig.
4.15
b). Nevertheless at a given molar ratio
x
,
there is no full coincidence between absorption and fluorescence excitation spectra
for “QD-porphyrin” nanoassemblies. It follows also from TRF experiments (see
Fig.
4.5
d) that only a small build-up component (1-2 ns) is observed [
62
,
123
]which
indicates an only weak role of a population mechanism of porphyrin molecules
via excited states of QD in nanoassemblies. Thus, the realization of FRET with
a quantum efficiency of 100% seems not to be the case for the above system.
Experimental results [
60
,
117
,
135
] and theoretical considerations [
115
,
136
]
suggest that nanocrystalline materials and organic-inorganic hybrids are best
described in the weak coupling regime. This allows the application of Foerster
theory [
137
], which typically is used in the limit of “localized oscillators” as a
result of the electric dipole approximation. The corresponding theoretical analysis
in the frame of Foerster model shows [
62
,
115
] that for the “QD-porphyrin”
nanoassemblies under study, the energy transfer “QD
→
porphyrin” should take
place within few nanoseconds time scale with the FRET efficiency
E
theor
FRET
→
8.
This conclusion is in contradiction to our findings but shows the following relation
between experimental and theoretical FRET rate constants:
k
DA
exper
∼
0
.
7-0
.
k
DA
theor
.
Thus, we have carried out a direct comparison of experimental quenching results
for QD emission and sensitization data for porphyrin fluorescence using a complete
set of titration points (Fig.
4.15
c). Based on the ideas presented in [
62
-
64
,
138
,
139
], the analysis of FRET efficiencies via sensitization effect has been carried at
every titration step upon variation of the
x
ratio using the following equation:
E
FRET
=[
ε
A
(
λ
ex
)
F
AD
−
F
A
]
/
[
ε
D
(
λ
ex
)
F
A
·
n
]
,
(4.5)
where
F
AD
corresponds to the A fluorescence intensity measured at
λ
em
[651 nm,
Q(0,0) band maximum of H
2
P] while exciting at the strong donor absorption
λ
ex
=
465 nm.
F
D
corresponds to H
2
P (acceptor) emission at the same
λ
em
and
λ
ex
in the absence of QD (donor).
n
is the number of A molecules per D,
ε
D
are
molar extinction coefficients of A and D, respectively, at the excitation wavelength
λ
ex
. It follows from Fig.
4.15
c that at every molar ratio
x
,
E
FRET
values calculated
from fluorescence enhancement (right scale) are significantly smaller than those
estimated from QD PL quenching efficiency [
E
QUENCH
=
ε
A
and
1
−
I
(x)/
I
0
]. It is seen also
that beginning from
x
>
6 there is a saturation tendency for experimental
E
FRET
values.
In fact, the experimental situation, when
E
FRET
E
QUENCH
, has been observed
not only for “QD-porphyrin” nanoassemblies with various CdSe/ZnS QDs [
62
-
64
]
but also for “QD-PDI” systems [
74
,
94
]. In order to verify the question whether the
