1
Microwave synthesis and characteristics of functionalized CdTe quantum dots for drug
delivery in the central nervous system
Science Department, Innovations High School, Chicago, Illinois, 60602
Laboratory for Product and Process Design
Departments of Bioengineering and Medicine
University of Illinois at Chicago, Chicago, Illinois 60607
David E. Ashkenaz, NSF-RET Fellow, 2012
Abstract
A rapid procedure for the production of tunable CdTe quantum dots (QDs) has been
adapted that employs microwave irradiation. CdTe QDs can be synthesized to a desired
size and wavelength, and can be used as a specific color label to investigate biological
processes, especially drug targeting and delivery. The synthesized CdTe-QDs will be
attached to gold coated magnetite nanoparticles (NPs) via thiol linkages. The purpose
of producing these QDs is for in-vivo detection of the localization of the gold coated
magnetite nanoparticles under the influence of a static external magnetic field.
Introduction
Quantum dots (QD) are specially sized
nanoparticles
or
semiconductor
nanocrystals, that have been used in an
extensive array of applications including
biological labeling, optical devices and
electronically active structures. QD
sensors have been developed for small
molecules and biomolecules that were
stable and exhibited high fluorescent
(quantum) yields that were very useful
for visualizing these specific structures
[1,2,3,4]. The QDs synthesized using
Group II-VI semiconductor elements
such as CdSe, CdTe and ZnSe [5,6],
and CdTe have been used as
luminescent probes [7] in biological
environments and were very efficient as
a label due to a narrow band gap (1.47
eV) with a large exciton Bohr radius of
up to 7.3 nm, providing appropriate
limits for quantum confinement [8].1
There are various ways to synthesize
Cd-X (X=Te, Se) QDs [9, 10, 11], yet
usage of QDs in biological applications
require that the particles be compatible
in a biological environment, which would
be an aqueous environment, where
water solubility is of prime importance.
The general synthesis route for QDs is
an organometallic synthesis procedure,
where hydrophobic reagents are used
and the particle is capped with a variety
of hydrophobic compounds [12, 13].
Earlier aqueous methods used long
reaction times, resulting in low
luminescence
arising
from
the
production of large numbers of surface
defects [14]. The methods used for
aqueous synthesis are thermal or
microwave (dielectric) heating, the latter
being the preferred method since both
particle defects and synthesis time are
reduced, and more consistent heating is
accomplished [15, 16, 17]. Toxic
solvents are also eliminated in the
microwave synthesis procedure, making
it a more earth-friendly protocol [18].
The microwave synthesis to be used
and adapted in this investigation uses a
tellurium (Te) source more stable in
2
aqueous conditions; sodium tellurite
(Na2TeO3). This precursor is air-stable,
so ambient conditions can be used in
the synthesis. Complicated, error
inducing,
manipulations
at
high
temperatures for extended time periods
(many hours up to 10) are absent, and
the QD synthesis can, therefore, take
place rapidly, in a matter of (up to) 30
minutes, depending on the desired QD
size [8].
An additional goal in this research is to
direct the microwave synthesis of CdTe
QDs and then conjugate them to gold
magnetic nanoparticles (QD-Au-Fe3O4NP) to form a QD-NP complex as shown
in the diagram seen in Fig. 1.
The conjugation procedure, briefly,
consists of 20ml of dry ethanol (200
proof) is mixed with equal volumes (2
ml) of the gold coated magnetite
nanoparticles and QDs in a closed glass
container. The pH is set to 8. This
mixture is sonicated for 5 minutes,
which is then followed by gentle mixing
for 48 hours using a rotamix. During this
entire procedure, light is avoided as
much as possible to prevent any
possible photo bleaching of the QDs.
[19]
and
is
biocompatible
and
magnetically active [20, 21]. The QDNP
complex
luminescence
and
movement will be examined and tracked
through location under UV irradiation
and
visualized
using
fluorescent
microscopy.
Materials and Methods
Materials
Quantum dot synthesis
s
QD
Figure 1
Au-Mag
NP
The top diagram indicates the
overall structure of the QD combined to the
gold coated NP. NP diameter is approximately
20nm and QD is 2-4nm. Lower diagram shows
the thiol bond formation and attachment of
various molecules to the linkages.
NaBH4, Na2TeO3, a 60 ML teflon vessel
and HOC(COONa)(CH2COONa)2 · 2H2O
were all purchased from Fisher
Scientific. HO2CCH(NH2)CH2SH and
HSCH 2CH2 COOH (MPA) were
obtained from Sigma-Aldrich. A 900W,
2450 MHz Samsung microwave oven
was obtained from the Bioengineering
Department at the University of Illinois
Chicago, and a standard curve was run
for temperature equivalents of power
settings A FS20H Ultrasonicator from
Beckman and the J7 Ultrascentrifuge
was from Beckman. The Water mixer
was from Thermo-line and the Shae-RBath was obtained from Lab-line. The
Syringe pump was from New Era Pump
Systems , Inc. The HRTEM was a JOEL
3010.
3
Gold coated
synthesis
magnetite
nanoparticle
NH4Fe(SO4)2·12 H2O and
(NH4)2Fe(SO4)2 · 6H2O were purchased
from Acros, along with C6H12O6 . HAuCl4
was purchased from Alfa Aesar, and
(C14H22O(C2H4O)n), with and H2SO4
were from Fisher Scientific, and NaOH
was from Sigma.
Methods
Quantum dots
To synthesize the QDs, 0.4 ML of 0.04
mol/L is diluted to 42 ML of (nanopure©)
water and then 100 mg-trisodium citrate
dehydrate, 4 ml of 0.01 M Sodium
tellurite, 119 mg of 3-methylproprionic
acid and 50 mg of sodium borohydride
under magnetic stirring, The molar ratio
of Cd/MPA/Te was 1:7:0.25, and 10 ML
of this solution was placed into a 60 ML
Teflon vessel. A variety of QDs were
synthesized
using
different
time/temperature lengths per degrees
centigrade, respectively (10-40 min and
89C to 130C) using a microwave device
set at 400W. The QD samples were
allowed to cool to 50C prior to
examination.
Gold coated magnetite nanoparticles
Gold magnetite nanoparticles were
synthesized using a stock solution of
ferric and ferrous ammonium sulfate
made up of 6.17g of ferric and 2.5g of
ferrous ammonium sulfate added to
0.4M sulfuric acid. 25 ML of the stock
solution was added to 250 mL-1 M
NaOH and was stirred and heated at
80C. 1.5 ml of TX100 was added and
then the stock solution was added,
dropwise (1 drop/sec) and the stirred
and heated (80C) for 0.5 hr more. The
particles were allowed to settle, and the
suspension was decanted, centrifuged
(4,000 rpm for 3 minutes) and washed in
water. All particles were combined and
0.5g of glucose was added to the
solution which was sonciated for 15 min,
and 2.7 ML of chloroauric acid was
added and the solution was heated
under slow stirring for 1 hour. The
particles were removed and covered.
Results
The results for experiments completed
to date will be shown in picture and
diagram form, The size of nanoparticles
grown
(synthesized)
is
generally
controlled through timed heating and is
governed by the Ostwald ripening
process, which is the dissolving of
smaller particles and the re-deposition
of these particles resulting in larger
particles as seen in Fig. 2. The injection
process for the synthesis of magnetite is
revealed in Fig. 3, showing the dropwise addition of the ferric and ferrous
reagents using a timed syringe pump.
After centrifugation, glucose is added as
a reductant and choroauric acid is
added
to
coat
the
magnetite
nanoparticles. The characteristic red
color of gold coated nanoparticles is
seen in the photograph in Fig. 4B. Fig. 5
shows a key process whereby Goldmagnetite-NPs are produced. The entire
synthetic process takes approximately 4
hours, and characterization of the
formation of gold-magnetite naoparticles
will be the next step in the research.
4
Figure 2
Ostwald ripening process,
where small particles dissolve then recombine
or re-deposit to form larger particles.
Figure 5
This figure reveals the 15
minute sonication of nanoparticles which is a
key process in the layering of gold upon the
magnetite nanoparticles.
100
T
e
m
p
A
B
Figure 4 After addition of the iron solution,
the formation of magnetite nanoparticles can be
seen in Fig. 4A, where a brown, foamy, solution
is seen due to TX100. B shows post-gold
coating, where the Au-magnetite-NPs have
been prepared after centrifugation washing
(4,000 rpm, 3 minutes) and 15 minute
sonication.
40
20
0
C
0
)
The
1
drop/sec
addition of ISS (iron stock solution)
to 1M NaOH and TX100 stirred at
80C. The TX100 is used to particle
dispersant. This process leads to
magnetite NP formation.
60
(
Figure 3
80
1 2 3 4 5 6 7 8 9 10
Power level
Figure 6
A comparison of temperature vs
microwave power levels was done to determine
the temperature relationship. 10 ml of H2O was
used as the sample and microwaved at various
power settings at 4 min/level.
The process of synthesizing QDs was
based on the preliminary determination
of the microwave temperature-power
profile, confirmation of the correct pH
necessary for synthesis and a time
course study of the production of
luminescent QDs/minute. As seen in
Fig. 6, there was an initial increase and
then a plateau of approximately 850 C
reached, regardless of the power
strength. The next procedure was to
verify the proper pH level for the
production of the highest luminescent
QDs. This was accomplished by
5
synthesizing QDs at pH 5 and pH 8 and
comparing
the
luminescence
(fluorescence)
with
a
(Zeiss)
fluorescence microscope and analyzing
the fluorescent output (particle density)
with Image J© software. Fig. 7 indicates
that the QDs produced at pH 8 have
higher maximum densities compared to
pH 5, most likely due to the
unsuccessful capping by MPA, leaving a
surface with multiple defects, thereby
producing a less 1,339,000 counts at pH
8 (6 min). The proper pH creates and
maintains the environment at and
around the surface of the QD and,
therefore, has a direct relationship to the
fluorescence produced by the structure.
The QDs were synthesized minute-wise,
to examine the time course of
luminescence. Fig. 9 shows these
results. As can be seen in the slides, a 4
minute time period produced a bright,
luminescent effect. The red picture
shows the same luminescence through
a red filter. The counts indicate a
substantial fluorescent quality. The
temperature reached in the micro-wave
apparatus and the wavelength (color)
observed in the QDs indicate a very
small particle size of the order of 2-3nm.
The theoretical size of CdTe QDs can
be obtained from the following equation
[13]:
X-ray diffraction pattern for 2nm CdTe
QDs is shown exhibiting 1.9 angstrom
lattice distance in the cubic structure
seen in the HRTEM.
2min
4min
6min
A
pH
5
B
pH8
CdTed = (0.98127 x 10-6)λ3 – (1.7147 x
10-3) λ2 + (1.0064) λ – 194.84
eq. 1
Where d=CdTe particle size and λ=peak
absorption wavelength. Using 500, 510,
520 or 530 nm for the wavelength, the
theoretical CdTe particle size estimate,
shown in Fig. 10, is 2.32, 2.65 and 2.82
nm, respectively. This has been
confirmed by EDS completed on 2 nm
CdTe QDs, and 300 kv HRTEM of 4nm
and 2nm samples as seen in Fig. 11. An
Figure 7
These pictures represent QD
formation at pH 8 (B) and pH 5 (A). Max density
analysis indicates much higher densities in B
compared to A. See Fig. 8.
6
1600000
1400000
1200000
1000000
800000
600000
400000
200000
0
535
Theoretical particle size
530
Ph 8
525
2
4
6
λ
520
Figure 8
Max density counts of
produced at 2, 4 and 6 minutes @ 850C.
QDs
515
510
505
500
495
0
1
2
3
4
QD particle size (nm)
Figure 10
Theoretical
QD
particle
size
calculated from eq. 1: CdTed = (0.98127 x 10-6)λ3 –
(1.7147 x 10-3) λ2 + (1.0064) λ – 194.84.
Figure 9
Slides (100x) of QDs produced
at 1, 2, 3 & 4 minutes. The actual fluorescence
(at 4 minutes) is seen below the slides
7
B-a
A
B-b
B-c
Figure 12
Figure 11
A. EDS of 2 nm CdTe QDs.
Note Cd and Te peaks identifying the QD. B.
300kv TEM of 4-5nm (a) and 2 nm (b) CdTe
QDs. Diffraction pattern indicates a 1.9
angstrom distance.
Quantum
dot
fluorescence
showing suppression of pressure induced
fluorescence. Similar exposure time (4 sec), 3
minute syntheses under low (left) and high
(right) pressures. High pressure was induced by
tightly capping the reaction vessel.
8
angstrom lattice crystal structure. As a
result of experimental data, it was
necessary to modify the synthesis
procedure. The temperature achieved in
the synthesis was approximately 85C in
2-3 minutes, and remained at that level
for syntheses continuing up to 6
minutes. This was due to the rapid
dielectric heating of the volume of
aqueous sample (5-20 ML), and data
was observed that prevented usage of
synthesis for longer than 2-3 minutes.
According to Guy-Lussac’s Law, the
pressure of a gas (vapor) is directly
proportional to temperature (P1/T1 =
P2/T2), and enormous pressure was
built up in the enclosed reaction vessel,
leading to very high temperatures being
Figure 13
Photographs of Qds conjugated to rapidly produced (95C in 2 minutes).
Fe3O42Au
NPs
and
conjugation
induced This resulted in fast evaporation and
fluorescence quenching observed in observed concentration of particles leading to
samples 2 and 4 minutes.
unusable conglomerations of QDs. Also,
a 2-3 minute synthesis time produced a
quenching of the fluorescence observed
Conclusion
(Fig. 12), possibly due to dilution of
sample, seen when using 20 ml vs 10
Luminescent quantum dots (QDs) have
ml sample size. Fig. 13 shows the
been successfully synthesized using a
successful initial conjugation of QDs to
modified microwave irradiation method
Fe3O4@Au NPs which were fluorescent
that produced fluorescent, stable QDs
structures. The fluorescence in the case
usable as functional imaging entities.
of QDs conjugated to Au containing
Since the QDs vary in size from 2 to 5
nanoparticles will be diminished by the
nm, an array of fluorescent colors can
effect of plasmon resonance and also by
be produced from green at the lower
the linker (QD to nanoparticle) distance;
size scale (e.g., 2nm) to red at the larger
the closer the distance, the lower the
scale (4-5 nm). These varying colors
QD fluorescence will appear. Note the
arise from the quantum confinement
dulling of the fluorescence scene at 4
effect with QDs and the ease of
minutes. These studies will be continued
tunability of size (and wavelength). The
leading to successful usage of QD
stability of QDs are on the order of
labeled nanostructures as tracking-drug
weeks vs days for comparably
delivery devices.
luminescent dyes, and can be used in
the tracking and delivery of biologically
important molecules or synthetic drug
carriers. Size tunability was observed
through HRTEM analysis, and showed
consistent QD structure with 1.9
9
Acknowledgements
I’d like to acknowledge that excellence
assistance from the LPPD Laboratory at
the University of Illinois Chicago, and
specifically, Ms. Indu Venugopal in the
biolengineering department and
Professor Andreas A. Linninger, Director
of the LPPD Laboratory and Director of
the CSTR RET Program. I’d also like to
thank the National Science Foundation
for the support provided with grants NSF
RET EEC 0743068 and EEC 1132694.
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