Biomedical Engineering Reference
In-Depth Information
“QD-H
2
P” and “QD-PDI” nanoassemblies in toluene [
63
,
74
]. This non-FRET
quenching mechanism has not been considered in most of the related publications
and is connected with the extension of the wave function of the exciton to the outside
of the QD core.
In such a study, the experimental background was the comparative analysis of
the QD PL quenching by one type of porphyrin (m-Pyr)
4
-H
2
P molecule for QDs of
different sizes as well as having different ZnS capping layers. For the analysis of
the PL quenching as a function of the number of H
2
P molecules per QD for various
QD sizes, the well-known Stern-Volmer formalism [
139
] was modified. In our more
generalized approach, the PL quenching can be described by
∞
I
0
I
=
1
+
K
(
x
)
·
dx
,
(4.10)
0
where
I
(
x
)and
I
0
represent the QD PL emission intensity in the presence and
absence of quenchers, respectively. The corresponding results are presented in
Fig.
4.20
a.
In this approach, the Stern-Volmer function
K
depends explicitly on the molar
ratio
x
and is expressed as the first derivative of the experimental data plotted in
a Stern-Volmer representation. Further, the Stern-Volmer “constant”
K
(
x
)
(
x
)
can be
written as
K
(
x
)=
k
q
(
x
)
·
τ
0
,
(4.11)
where
k
q
corresponds to the total quenching rate induced by the number of attached
quencher molecules and
τ
0
is the intrinsic PL lifetime of the QD in absence of
quencher molecules (known from measurements [
63
] or extrapolated from literature
[
117
]). It is seen from Fig.
4.20
athat
I
0
/
I
(
x
) does not show a linear correlation with
x
over the total H
2
P concentration range. The double logarithmic plot presented for
K
(
x
) values shows the clear dependence of PL quenching on QD size (Fig.
4.20
b). It
can be seen that
K
(
x
) indeed initially constant but becomes smaller around a critical
molar ratio (which we name
x
c
[
63
,
64
]). Additionally, this critical molar ratio
x
c
increases systematically with the diameter of the QD. A non-constant
K
(
x
) indicates
eviation from a normal Stern-Volmer relationship in case of a bimolecular reaction.
The overall interpretation of the above findings presented recently in [
64
]is
that QD PL quenching upon titration by porphyrin molecules occurs in two steps:
(1) Immediately after titration, “QD-H
2
P” nanoassemblies are effectively formed,
which results in both PL quenching of the QD PL and low-effective FRET to the
adsorbed dye molecules [
63
]. (2) More dye molecules become attached during a
following-up waiting time. However, increased PL quenching and FRET do not
follow the same dependence on
x
. Clearly, the titration (assembly formation) causes
predominantly other quenching mechanisms than those related to FRET. As it is
seen from Fig.
4.20
c, the maximal FRET efficiency does not exceed
12-15% over
the total titration range, and becomes sequentially lower with increasing QD size.
This makes us to believe that the dynamics initiated by a single titration step are not
∼
