Download Synthesis and Characterisation of Polymer

Synthesis and Characterisation of PolymerCoated Quantum Dots with Integrated Dye Molecules
Tobias Niebling, Sebastian Friede and Wolfram Heimbrodt
Diluted Magnetic Semiconductor Group of the Philipps-University of Marburg
Zulqurnain Ali, Feng Zhang, and Wolfgang J. Parak
Biophotonics Group of the Philipps-University of Marburg
Department of Physics and Material Sciences Center, Philipps-University of Marburg, Renthof 5, D-35032 Marburg, Germany
10-1
10-2
Hydrophobic nanoparticles dissolved in toulene are mixed with an appropiate amount
of polymer. The number of added polymer scales linearly with the area of the particle
surface.
In the plain amphiphilic polymer 75% of the anhydride rings have reacted with
hydrophilic backbone. To incorporate dye molecules an ATTO-dye into the polymer
shell, derivate with an amino-group was used. By the reaction of the amino-group with
the remaining anhydride rings the ATTO-dye is linked to the amphiphilic polymer. In
standard configuration 2% of the anhydride rings reacted with the ATTO-molecules, so
that in total 75% of the anhydride rings serve to link side chains and 2% of the anhydride
rings serve to link ATTO-dye.
ZnS
ZnS
+
40
60
80 100 120 140 160
time [ns]
dnQD
nQD
nt
=+
t
t feed
dt
QD
tt
t QD
A mixture of plain polymer-coated quantum dots and empty micelles does not influence
the decay behaviour of the quantum dot luminescence. The dye emission on the other
hand shows a decelerated non-exponential decay which indicates a reabsorption process
of the quantum dot emission.
dn ATTO
n ATTO
t
t
t
a
b
c
n
+ n ATTO exp (+ n ATTO exp(- )
)
)
=+ wreab QD , nATTO (t) = n ATTO exp (tATTO
t QD
tt
t ATTO
dt
The situation changes dramatically when the dye molecules are incorporated into the
polymer shell of the quantum dots. A direct transfer of the excitation from the quantum dot
to the dye molecule produces an additional relaxation channel resulting in an accelerated
decay of the quantum dot luminescence.
dnQD
nQD
nt
nQD
nt
n
= - wtrans QD +
= - eff +
t
t
tQD
tfeed
dt
QD
feed
n QD (t) = n exp (a
QD
650
Purification
All polymer coated nanoparticles are purified by gel electrophoresis. All samples run
on 2% agarose gel for one hour at 100 Volts.
The following configurations were used in this
study:
1.6
600
500
600
550
500
400 nm
0.4
dye
absorption
0.3
plain
polymer-coated
quantum dots
(d ≈ 3.3 nm)
0.2
quantum dots
absorption
0.1
0.0
1.5
2.0
2.5
3.0
3.5
4.0
io
n
polymer-coated
quantum dots
with embedded
dye molecules
mixture of
quantum dots
and dye
molecules
dye molecules
in empty micelles
1.8
2.0
2.2
2.4
t
eff
QD
)+n
b
QD
t
exp (- )
tt
550
500
450
quantum
dot
emission
dye
absorption
10-1
3.3 nm
3.4 nm
3.2 nm
0
10
20 40 60 80 100 120 140 160 180
3.3 nm
J ~ 4.4 · 1011 nm5
J ~ 9.6 · 1011 nm5
3.4 nm
1.8
5
J ~ 7.5 · 10 nm
J ~ 3.4· 1012 nm5
4.4 nm
4.4 nm
10-4
3.2 nm
2.0
2.2
2.4
2.6
2.8
photon energy [eV]
time [ns]
The quenching of the quantum dot luminescence with increasing dot size is clearly
observable. The effective quantum dot lifetime τeffQD changes from 19 ns, 12.2 ns, 5.57 ns to
4.5 ns. The transfer probability rises with increasing spectral overlap (mean dot diameter)
and the respective transfer probabilities are wtrans = 0 ns-1, 0.03 ns-1, 0.13 ns-1,and 0.25 ns-1.
N
∞
R0 6
wtrans =
t i = 1( ri )
1
4
J(l) = FD(l) eA(l) l dl
0
wavelength [nm]
The resulting effective Förster R0 radii are
0 nm, 3.25 nm, 4.1 nm and 4.8 nm for the
dot diameters of 3.2 nm, 3.3 nm, 3.4 nm,
and 4.4 nm, respectively, assuming a
mean number of the surrounding
acceptors N = 8, 9, 10, 15, the decay
time of the donor in absence of an
acceptor τ = 19 ns, the transfer rates wtrans
extracted from the fits and the mean
distances R between the nanoparicles
and dye molecules. (The polymer shell
adds on the order of 3 nm to 4 nm to the
particle radius.)
6
-5
2
-4
R0 ≈ 8.79 · 10 · k · n · QD · J(l)
in Å
750 700
650
600
550
500
14000
12000
10000
8000
6000
4000
2000
0
0
12
1·10
12
2·10
spectral overlap [nm5]
12
3·10
12
4·10
3.3 nm
3.4 nm
1.6
1.8
2.0
4.4 nm
2.2
2.4
2.6
photon energy [eV]
6
These radii scale with the spectral overlap between the emission of the quantum dots and
the absorption of dye molecules. This enhanced energy transfer for the increasing
spectral overlap is consistent with the trend of a decreasing quantum dot emission
intensity compared to the dye emission with increasing quantum dot size.
photon energy [eV]
qu
a
em nt
is um
si d
on ot
Polymer-coated
quantum dots
with embedded
dye molecules (EPA544)
700
650
dy
e
Quantum dots (EP577)
(diameter ~4.4 nm)
800
absorption [arb. units]
Quantum dots (EP544)
(diameter ~3.4 nm)
photoluminescence intensity
[arb. units]
750 700
is
s
Quantum dots (EP520)
(diameter ~3.3 nm)
w empty polymer micelles with dye molecules (PA)
w polymer-coated quantum dots without dye
incorporated in the polymer (EP)
w polymer-coated quantum dots with dye incorporated
in the polymer (EPA)
w a mixture of empty micelles and plain polymer-coated
quantum dots (PA + EP)
wavelength [nm]
em
Quantum dots (EP490)
(diameter ~3.2 nm)
600
100
10-3
Synthesis of empty micelles
For the preparation of empty polymer micelles as control samples the same procedure
as described above was carried out, but by using plain chloroform instead of a solution
of quantum dots in chloroform. As no quantum dots are present in solution the resultant
particles can be ascribed to empty micelles of the polymer.
t
A reduction of the quantum dot diameter leads to a blue-shift of the emission band of the
nanoparticles due to the quantum confinement effect and modifies the spectral overlap
between the quantum dot emission and the dye absorption.
wavelength [nm]
10-2
CdSe
exp ( -
a
ATTO
R 60 [nm6 ]
+
+
20
dye in
mixture
intensity [arb. units]
amphiphilic
polymer
hydrophobic
side chains
dye in polymer shell
of quantum dots
t
t ATTO )
The decay of the quantum dot
emission coated with polymer is a
combination of the intrinsic decay
of the quantum dot (tATTO » 19 ns) or
a feeding for energetically higher
states and a thermal activation of
lower dark states (tt » 70 ns).
t
t
a
b
n QD (t)= nQD exp () + n QD exp (- )
n ATTO (t) = n
photoluminescence intensity
[arb. units]
hydrophilic
backbone
dye in
empty
micelles
0
dn ATTO
n ATTO
=t ATTO
dt
quantum dots
in mixture
10-4
photoluminescence intensity
[arb. units]
Inorganic colloidal nanoparticles capped with hydrophobic surfactant molecules are
transferred to an aqueous solution. This is done by wrapping an amphiphilic polymer
around the nanoparticles, the hydrophobic side-chains (dodecylamine (C12 carbon
chain)) of the polymer intercalate the hydrophobic surfactant layer on top of the
nanoparticle surface, while the hydrophilic backbone (Poly(isobutylene-alt-maleic
anhydride)) of the polymer points towards the solution and thus warrants solubility in
aqueous solutions. The mixture is chosen so that 75% of the available anhydride rings
react with the amino-groups of backbone. Afterwards, the polymer still has 25% of
anhydride rings that can react with cross-linker or ATTO-dye.
The photoluminescence decay of
dye molecules in empty micelles
shows a nearly single exponential
decay with a decay time
tATTO » 4.5 ns.
quantum dots (d ≈ 3.3 nm)
with polymer shell
quantum dots with dye
molecules in polymer shell
10-3
Synthesis
CdSe
100
laser
Semiconductor nanoparticles have emerged as basis for promising sensors in
bioanalytics and markers for biolabeling. Common strategies to achieve probe
applications use the interplay between a central quantum dot and multiple
fluorophores.
To analyse a specific substance, a particular analyte could bind to its receptor which
induces a change in the donor acceptor distance or a change in the spectral overlap
between donor emission and acceptor absorption and thus modifies the energy
transfer between quantum dot and dye. A profound knowledge of the complex transfer
mechanisms is important for future applications.
photoluminescence intensity
[arb. units]
Energy Dynamics in Polymer-Coated Quantum Dots
Motivation
2.6
photon energy [eV]
The spectral overlap between the emission of the quantum dot and the absorption of the
dye molecule can be modified by choosing different sizes of the quantum dot. The
stronger quantum confinement leads to a blue-shift of the photoluminescence emission
with decreasing dot diameter.
Conclusions
It is possible to give a first description for the temporal behaviour of the
photoluminescence of polymer-coated quantum dots with integrated dye molecules
which is consistent with the observed redistribution trend of the emission intensities. This
can be done within a rate equation model that accounts for the interplay between the
excitation dynamics of the quantum dot and the dye molecules. For future applications, it
is still necessary to gain further insight into the underlying transfer processes.