Biomedical Engineering Reference
In-Depth Information
upon the replacement of (m-Pyr)
4
-H
2
P molecules by the Cu-containing derivative
(m-Pyr)
4
-CuP, and tetrahydroporphyrin (m-Pyr)
4
-THP (thus, changing the overlap
integral
J
(
ν
)=
0
d
ν
4
values by factor of 2.5) the quenching efficiency
of QD PL remains nearly the same [
90
,
101
]. So, the contribution of FRET
QD
f
D
(
ν
)
ε
A
(
ν
)
ν
porphyrin to the total QD PL quenching seems to be minor and hence is
negligible in most cases. For this reason we believe that the marked difference
between pyridines and dyes relates to thermodynamic and/or volume effects as will
be discussed later in detail.
The disagreement between PL quenching and FRET enhancement is also
observed via single nanoassembly detection. While the ensemble experiments allow
for a dynamic nanoassembly formation, this can be almost certainly ruled out
in single nanoobject experiments, since an effective diffusion of dyes or QD on
SiO
2
surfaces has not been observed experimentally [
65
,
74
,
75
,
94
]. Figure
4.13
b
(bottom) presents for a single nanoassemblies typical luminescence spectra and
PL decay times as a function of observation time. It is seen that in most cases
both types of spectra (QD and PP, respectively) show uncorrelated blinking and
bleaching events, which are a direct proof of the presence of in each case only one
emitter [
124
]. The observation of uncorrelated blinking is a strong indication that
the electronic interaction between QD and PP by either charge transfer or FRET is
quite weak.
Since the PL intensity of the QD is reduced on average by 50% [
74
,
94
]for
each identified single nanoassembly, the corresponding dye-induced quenching rate
should be (assuming
n
→
1) nearly equal to the (average) decay rate of the QD.
A close inspection of the time-resolved QD PL decay of single nanoassemblies
provides evidence for the shortening of the QD PL decay time. An average
decay time of the multi-exponential QD PL can be deduced applying a stretched
exponential fit (with
=
(
0
<
β
≤
1
)
) according to
exp
β
t
τ
I
(
t
)=
I
0
·
−
.
(4.6)
Correspondingly, an average decay time
τ
D
for a single QD can be calculated
according to
1
β
τ
D
=
τ
β
Γ
.
(4.7)
τ
D
values for single QDs and single “QD-PP” nanoassemblies as a
function of the observation time are shown as black circles in Fig.
4.13
a, b. The
spectral fingerprints of 90 QDs from 3 different samples have been used to compare
the decay times of free and assembled QDs [
74
]. Normalizing the histograms for
τ
Calculated
, and PL intensity
I
PL
to the number of emitters results as shown in Fig.
4.17
in probability distributions of
,
β
β
D
,and
I
PL
for isolated QDs and those for the
QDs in single “QD-PP” nanoassemblies, respectively. It is seen from Fig.
4.17
that the resulting decay time probability density of “QD-PP” nanoassemblies is as
τ
D
,
