Faculteit der Natuurwetenschap, Wiskunde en Informatica
Bachelor Scheikunde
The synthesis and properties of NaYF4 : Yb, Er
Upconverting Nanoparticles with a
Core and Core/Shell structure
Katja Goris (5969034)
June 2011
http://english.ciomp.cas.cn/rh/rp/200909/t20090914_37879.html
Supervisors:
Dhr. dr. H. Zhang
Mw. dr. Y. Wang
Dhr. prof. dr. W. J. Buma
HIMS photonic group
Summary
Scientists are searching for biomedical fluorescent labels already for many years. An example
of these labels are the quantum dots. Quantum dots can absorb short wavelength, high-energy
UV- light and emit at a longer wavelength low-energy visible light with a process called
downconversion. These quantum dots have a number of disadvantages like photobleaching
and leading to damage to the tissue, because of the need to use UV- light. A more recent
example of fluorescent labels are upconversion nanoparticles. These particles can absorb a
lower energy, longer wavelength of near-infrared light and with energy transfer of a
neighboring ion emit higher energy, shorter wavelength visible light with a process called
upconversion. These upconverting nanoparticles are much more resistant to photobleaching
and give less damage to the tissue because they are irradiated with near-infrared light, which
render them particularly useful for bio-imaging and bio-labeling applications. One of the holy
grails, scientists are currently pursuing is to improve the emission performance of these
upconversion nanoparticles. A promising route appears to be the use of core/shell
architectures.
In this report a comparison of the core and the core/shell structure is made by measuring the
intensity and the wavelengths distribution of the emission. Also the size and morphology of
the synthesized particles play an important role. They have been measured with TEM. The
particles synthesized in this report are the -NaYF4 : Yb3+, Er3+ nanoparticles. The size of the
synthesized particles is 10-20 nm for the core and 14-27 nm for the core/shell structure.
Comparing the emission spectra of the core and core/shell structure particles reveals that the
peak positions remain the same. However, there is a dramatic difference between the two
when the overall upconversion intensity and the relative green: red (G /R) emission ratio are
compared. The relative G/R ratio for the core 1:2.6 while for the core/shell structure 1:1.8.
In this report an evolving existent synthetic approach is used to synthesize the upconverting
nanoparticles (-NaYF4 : Yb3+, Er3+) [6]. During the synthesis various reaction parameters
such as temperature, concentration, reaction time and the solvent have been changed. The
reaction should be performed in a dry environment under nitrogen flow. The temperature
influences the size and shape of the particles, the pretreatment time has influence on the
certainty that all the water from the solvent is evaporated. The solvent makes sure that the
particles will be separated from each other to avoid clustering. The concentration of the
reactants determines the color of the emission from the nanoparticles.
2
Samenvatting
Wetenschappers zijn al jaren bezig naar onderzoek aan fluorescerende labels voor toepassing
in de biomedische wereld. Een voorbeeld van zo’n fluorescerende label is de quantum dots.
Deze deeltjes kunnen korte golflengte, hoge energie UV licht absorberen en emitteren
vervolgens de lange golflengte, lage energie zichtbaar licht met behulp van een zogenoemde
downconversion proces. De quantum dots hebben als nadeel dat ze last hebben van
photobleaching en beschadigen het weefsel in het lichaam, wanneer ze worden bestraald met
UV licht. Een recenter voorbeeld van fluorescerende labels zijn de upconverting nanodeeltjes.
Deze kunnen lange golflengte, lage energie NIR licht absorberen en door middel van energie
overdracht door een naastgelegen ion kortere golflengte, hoge energie zichtbaar licht
emitteren, met een zo genoemde upconverting proces. De upconverting nanodeeltjes zijn
resistent tegen photobleaching en brengen minder schade toe aan het weefsel in het lichaam,
door met NIR licht te worden bestraald. Deze eigenschappen maken de nanodeeltjes geschikt
voor biomedisch labelen en in beeld brengen van tissue. Een veelbelovende manier om de
luminescentie van de nanodeeltjes te verbeteren is de core/shell architectuur.
In dit verslag worden de core en de core/shell structuren met elkaar vergeleken door het
verschil in intensiteit van de emissie. Ook worden de grootte en de morfologie met elkaar
vergeleken door dit in beeld te brengen met TEM. De deeltjes gesynthetiseerd in dit verslag
zijn de -NaYF4 : Yb, Er nanodeeltjes. De core nanodeeltjes hebben een grootte tussen de 1020 nm en de core/shell nanodeeltjes tussen de 14-27 nm.
In het emissie spectra wordt de relatieve verhouding tussen de groene emissie piek en de rode
emissie piek laten zien (G/R ratio) De onderlinge relatieve verhouding tussen beide pieken
verschilt per structuur. Voor de core is de relatieve verhouding 1:2,6 en voor de core/shell
structuur is het 1:1,8.
Voor het synthetiseren van deze deeltje is een bestaande synthese gebruikt[6]. In dit verslag is
de synthese beschreven en vervolgens zijn er variabelen tijdens de synthese procedure
verandert. Zo zijn de temperatuur, de concentratie, de reactietijd en het oplosmiddel verandert
over verschillende procedures. De reactie moet plaatsvinden in een droge omgeving, onder
stikstofstroom. De temperatuur beïnvloed de grootte en vorm van het deeltje, de tijd zorgt
ervoor dat al het water uit de oplossing verdampt. De oplosmiddelen zorgen ervoor dat de
nanodeeltjes onderling niet gaan clusteren. De concentratie van de reactant bepaald de kleur
van de emissie van de nanodeeltjes.
3
Index
Summary
Samenvatting
P. 2
P. 3
1 Introduction
P. 5
-
P. 6
P. 8
P. 10
1.1 Quantumdots
1.2 Upconverting Nanocrystals
1.3 Downconversion and Upconversion process
o 1.3.1 Downconversion process
o 1.3.2 Upconversion process
-
1.4 Measurement Methods
P. 12
o 1.4.1 TEM
o 1.4.2 Emission spectroscopy
-
P. 10
P. 11
1.5 Common problems
1.6 The project
P. 12
P. 12
P. 13
P. 13
2 Experimental
P. 14
-
2.1 Procedure
o 2.1.1 Core Materials
o 2.1.2 Shell Materials
o 2.1.3 Core/Shell structure (washing process)
P. 14
P. 14
P. 15
P. 16
-
2.2 Measurement devices
P. 16
3 Results and Discussion
-
P. 17
3.1 Results and discussion by eye
3.2 Results and discussion of the emission spectra
3.3 Results and discussion of the TEM
4 Conclusion
5 Acknowledgement
6 Literature
P. 17
P. 21
P. 23
P. 25
P. 25
P. 26
4
1 Introduction
Since many years scientists search for biomedical fluorescent labels. The labels scientists are
looking for should have the appropriate properties for imaging, labeling and assays. These
assays measure processes like pharmacokinetics and pharmacodynamics, the presence of
biochemicals in organisms and in organic samples, enzyme activity, antigen capture, stem cell
activity and competitive protein binding. Imaging is about biological imaging, which
incorporates radiology, nuclear medicine, studies of radiological sciences, endoscopy, medical
thermography, medical photography and microscopy. The fluorescent labels contain
fluorophores, a component of a molecule which causes a molecule to be fluorescent. This part
will absorb energy at a specific wavelength and emit the energy of a different specific
wavelength.
There are two main ways to fluorescent emit light: the downconversion fluorescence and the
upconversion fluorescence. With a downconversion fluorescent the energy absorbed is higher
than the energy from the emitting light. With an upconversion fluorescent the energy
absorbed is lower than the energy from the light emitted. (figure 1)
Figure 1*: Downconversion fluorescence is emitted for example, as visible light when excited by ultraviolet light
or short wavelength visible light. It goes from short wavelength, higher frequency/energy to longer wavelength,
lower frequency/energy. The upconversion fluorescence is emitted for example, at visible light when excited by
near-infrared light. This goes from longer wavelength, lower frequency/energy to shorter wavelength, higher
frequency/energy.
For imaging for example the tissue in the human body fluorophores are attached to other of
the tissue molecules. As a result the tissue becomes fluorescent. For this process the size and
the chemical environment of the fluorophores are important. The fluorophores are tested in
vitro as well as in vivo. In vitro scientist uses tissue and cells in the lab. For the in vivo tests
scientists uses animals like mice, to see the results on living tissue or cells. Scientists have
found many different fluorophores. One which has been examined in detail are quantum dots.
5
1.1 Quantum dots
Quantum dots (QDs) have promising properties for biomedical fluorescence labeling,
imaging and assays. They give a bright emission and can emit different colors. However, the
QDs have some disadvantages. QDs are suffering from photobleaching, are very toxic and
they damaging the tissue in the body.
Photobleaching destroys eventually the fluorophore, due to the light necessary to stimulate the
fluorophore into fluorescence. This negative effect complicates the observation of the
fluorescence in time. When the QDs are antibody-linked, photobleaching is applied to quench
the autofluorescence and improve the signal-to-noise ratio. Autofluorescence is a natural
emission from biological molecules and is used to distinguish the light from the applied
fluorescence from the natural emission present.
In comparison with organic fluorophores, these QDs have unique optical and electronic
properties, such as size- and composition-tunable fluorescence emission from visible to
infrared wavelengths. They have large absorption coefficients across a wide spectral range
and very high levels of brightness and photostability.[2] The QDs with the ligand-specific
groups to attach to tumor cells are more efficient than those without these groups. There are
two ways of targeting tumor cells: active and passive targeting. Active targeting is much
faster than passive targeting. Passive targeting is based on the tumor permeation, uptake and
retention.[2] Active targeting occurs when there is a high affinity binding of QD antibodies to
tumor antigen. (figure 1.1.1)
Figure 1.1.1: Passive and active tumor targeting.[2]
6
The QDs are downconverting as they emit visible fluorescence when excited by ultraviolet
light or short wavelength visible light. In biomedical applications, long-term irradiation with
UV-light on living tissue or cells can cause DNA damage and cell death and significant
autofluorescence from biological tissues reduces the signal to background ratio. Quantum dots
have a short fluorescence lifetime, between 12 and 20 ns.[1] However, in practice the
fluorescent imaging operates under absorption limited conditions. Here the rate of the
absorption is the main limiting factor of fluorescence emission.[2] Because of the large molar
extinction coefficients, the QDs rate of absorption is fast and thereby also the emission rate
also. That is why the QDs will appear much brighter than other fluorescent probes.[2] Because
of the large Stokes shift and the broad excitation profile, it is possible to detected multiple
color QDs in a mouse with a single light source.[2] (figure 1.1.2)
Figure 1.1.2:[2] Quantum dots tested in vivo in a mouse. Here it shows that it is possible to detected multiple
color in a mouse with a single light source.
The quantum dots have a lot of advantages and have the right properties for imaging, labeling
and assays. But the disadvantage of the quantum dots, photobleaching, makes them vulnerable
in time. Chemically synthesized core-shell upconverting nanocrystals would possibly solve
the disadvantages associated with quantum dots.
7
1.2 Upconverting Nanocrystals
Upconverting nanocrystals are particles with a dimension less than 100 nm. They generate
higher-energy light from lower-energy irradiation. They emit detectable photons of higher
energy in near-infrared or visible range with irradiation of an near-infrared light source in a
upconverting process.[4] The upconverting nanoparticles are composed of transition metal,
lanthanide or actinide ions (figure 1.2.1), doped into a solid state host material.
Figure 1.2.1*: Periodic table with the Lanthanides and the Actinides, the most common used as part of the host
material are the Lanthanides Er, Yb, Ho and Tm in their most stable transition state (III). These atoms are the
emitter ions. The Lanthanide Yb is often used as an absorber ion and is also a part of the host material.
The upconverting nanoparticles were in the earlier 1960’s referred to upconverting phosphors
which have a size of 400 nm in diameter.[5] These upconverting phosphors were lanthanidecontaining ceramic particles that can absorb IR light and emit visible light. Due to
modifications in synthetic procedures the size of the particles has been reduced to less than
100 nm in diameter. The shape and structure determine their properties and functionalities and
generate a host of different nano composites.[5] Because of the size of these particles,
scientists started to call these particles upconverting nanoparticles.
The basic structure of an upconverting nanoparticle consists of lanthanide ions such as Er, Tm
and Yb embedded as a dopant in a matrix. The matrix consists of , for example NaYF4, these
crystals have the highest upconversion efficiency due to its low phonon energy. The crystals
are being used as matrix material for the upconverting nanoparticles. The dopants form the
emitter ion and the absorber ions (usually Ytterbium, Yb). Every dopant gives a different
emission. The ratio between these emitter ions determines the emission range. Erbium (Er),
Holmium (Ho) give a green emission and Thulium (Tm) gives a blue emission. The structure
of upconverting nanoparticles is a core/shell structure. (figure 1.2.2)
8
Figure 1.2.2:[3] The composition of the Core and Core/Shell structure. There can be seen that the dopant is
divided in the Core. A) Core nanocrystal, the ligands shielding the core from clustering to one another. B)
Core/Shell nanocrystal, the ligands shielding the core/ shell from clustering to one another.
The core and the core/shell structure are shielded by ligands to prevent mutual clustering. In
the core structure there is a relatively large number of surface dopant ions that are poorly
luminescent. In the core/shell structure all the dopant ions are confined in the interior core of
the crystal and participate in efficient luminescence.[3]
The upconverting nanoparticles should have a suitable size and suitable surface for
conjugation with biological molecules and exhibit high intensity emission as well.
To get high-quality nanocrystals, the growth dynamics of the nanocrystals needs to be
modulated by changing various reaction parameters such as concentration of the reactants,
time of reaction, temperature and ligand capping agents to obtain the necessary shape and
size, and also the surface properties needs to be engineered for attaching various ligands to
make them soluble in water and make them biocompatible. By use of near-infrared irradiation
they are less harmful to cells futhermore autofluorescence from biological tissues is
minimized. This increases the signal-to-noise ratio significantly. The upconverting particles
are chemically stable, resistant to photobleaching, nontoxic, and the luminescence is
independent of environmental effects. A large anti-Stokes shift separates discrete emission
peaks from the near-infrared excitation source. These particles are also non-polar and need to
be surface modified before their use in biological media. To understand the working process
of upconverting nanoparticles, some theoretical background is required.
9
1.3 Downconversion and Upconverting process
Luminescent materials absorb energy and emit the absorbed energy as radiation.[5] These
radiation can be categorized into two varieties based on the excitation and emission.
Downconversion is described in 1.3.1 and the upconversion processes is described in 1.3.2.
1.3.1 Downconversion process
The Downconversion process has a lower emission energy and a longer wavelength than the
excitation energy. This means that they obey the Stokes law. They have a Stokes shift, which
means there is a difference between the position of the band maxima of the emission and the
absorption spectra. When a system absorbs energy, the system is electronically excited. The
system can relax by losing a photon or losing heat. When the system emits the photon (losing
a photon) and the energy of that photon is lower than the absorbed energy, it is called the
Stokes shift and it obeys the Stokes law. When the emitted photon has more energy than the
absorbed photon, it is called an anti-Stokes shift and the process is called an upconverting
process. (figure 1.3.1)
A) Stokes Shift
B) Anti-Stokes Shift
Figure 1.3.1: Stokes shift and Anti-Stokes shift. A) is the Stokes shift and is the downconversion process. Here
the energy of the excitation is higher than the energy of emission. B) is the anti-Stokes shift and is an
upconversion process. Here the energy of excitation is lower than the energy of emission and thereby needs
another photon to emit the higher energy emission.
Most of the fluorophores like fluorophosphores and quantum dots are downconverting. The
rare process present under the fluorophores is the upconverting process.
10
1.3.2 Upconversion processes
Particles that undergo an upconversion process absorb low-energy light and emit high-energy
light by using another photon. The upconversion processes are divided into two classes:
excited state absorption (ESA) and energy transfer upconversion (ETU). With the ESA (figure
1.3.2a) process first a photon is absorbed when it has the right energy the excited metastable
energy level is populated. Then another photon is needed to excite it further to a higher lying
energy level and then the upconversion emission can take place.
ETU (figure 1.3.2b) is nearly the same process as ESA, only in the ETU process the photon is
coming through energy transfer between neighboring ions. First an ion is excited and will
relax to the ground state. During this relaxation the neighboring ion populates a higher level.
Then the ion is excited for the second time and energy transfer takes place, while the ion is
falling back to the ground state the neighboring ion populates an even higher state. Then it
will fall back to the ground state in the form of emission. Here the dopant concentration
determines the average distance between the neighboring dopant ions. Most of the time the
upconverting process is a two-photon excitation with two metastable excited states involved,
the first serving as an excitation reservoir and the second as an emitter. Theses upconversion
process have an anti-Stokes emission for upconversion processes 10-100 times kT (where k is
Boltzmann constant and T temperature) [3]
Figure 1.3.2: the two upconverting processes[3] A) The excited state absorption, here first a photon is absorbed
and the excited metastable level is populated, then another photon excite it to a higher lying level and then
emission takes place. B) The energy transfer upconversion, here an ion is excited twice and a neighboring ion is
populated excited metastable level to an higher level during the relaxation to the ground state. So upconversion
emission is taken place.
11
1.4 Measurement Methods
The properties of the upconverting nanoparticles can be measured by two main devices. It is
possible to measure the size and the surface with TEM, described in 1.4.1. The emission can
be determined using emission spectra, described in 1.4.2. There is also a possibility to
measure the fluorescence lifetime with for example a streak camera. Fluorescence lifetime is
the time where the electrons stay in the excited state and go to the ground state, while the
emission of the energy will take place. Fluorescence lifetime measurements are not discussed
in detail, because they are not used during the project.
1.4.1 TEM
For imaging the morphology and the structure of the upconversion nanoparticles, the
Transmission Electronic Microscopy (TEM) is used. TEM uses a technique like electron
microscopy, which uses electrons for imaging the morphology and structure of the particles.
The electrons are transmitted through a thin specimen and interacting with the specimen when
it passes through. The image is magnified and formed on an imaging device such as a
fluorescent screen, a layer of a photographic film, or detected by a sensor such as a CCD
camera.
TEM is a major analysis method that is used in a range of scientific fields such as physics and
biological sciences. TEM uses magnifications of one million or even more times and has a
high resolution. These properties make TEM useful for imaging atomic structures in the
material and has applications in cancer research, virology, material science and
nanotechnology.
TEM works with a typical accelerating potential of 100 till 400 kV. The specimen stage in the
TEM includes an airlock, which make it possible to insert the specimen holder in vacuum,
without too much increase of pressure in other parts of the TEM.
The specimen holders are adapted to hold a standard grid size (3.05 mm in diameter ring with
a thickness and mesh size from a few to 100 μm) upon which the sample is placed or a
standard size of self-supporting specimen. The sample is placed onto the inner meshed area
having diameter of approximately 2.5 mm. Usual grid materials are copper, molybdenum,
gold or platinum. The sample preparation is different for every specimen. The most important
claim is that the sample has to be electronically transparent because of the interaction between
the sample and the electronic beam.
1.4.2 Emission spectroscopy
Emission spectroscopy is a technique which examines the wavelengths of the photons emitted
by atoms or molecules. The photons are emitted during the transition from an excited state to
a lower state energy state of the molecule or atom. Each molecule or atom emits a
characteristic set of wavelengths associated with its electronic structure. The emission
spectrum thus forms a fingerprint of the molecule.
12
The light of the different wavelengths consists of electromagnetic radiation of these different
wavelengths. This light or energy is measured with a spectrometer. This is an instrument
which is used to separate the different wavelengths.
A type of emission spectroscopy is fluorescence spectroscopy. This is a type of
electromagnetic spectroscopy which analyzes fluorescence from a sample. It involves a beam
of light that excites certain molecules in the sample and the molecule will emit light as a
reaction. This technique is complimentary to the well known absorption spectroscopy.
Another known type of emission spectroscopy is phosphorescence.
1.5 Common problems
Now that it is known how to measure the properties of the upconverting nanocrystals
scientists search for a better way to synthesize them. The scientist has to make sure that all the
properties of the upconverting nanoparticles are present in the product. These properties are
the size of less than 100 nm in diameter, the biocompatibility, the emission of visible light and
the surface modification of the particle. The problem of these particles is the intensity of the
emission of the synthesized nanoparticles. The variation of size and surface of the
nanoparticles plays also an important role in some research. It is difficult to get all these
properties in one synthesis.
1.6 The project
Before using these particles in the biomedical industry, some properties have to become clear.
Why is it that the core/shell structure is used and not only the core? What is the effect on the
intensity of the emission when using the core/shell structure? How to synthesize these
nanocrystals? The answers to these questions have to become clear after this project. The aim
of the project is to find a synthesis for these nanocrystals and to compare the intensity of the
emission for the core structure and the core/shell structure. For finding the optimal synthetic
approach, an existing synthesis is used.[6]
13
2 Experimental
In the experimental part there is a general procedure described and the variable reaction
parameters chosen during the procedure in 2.1. In 2.2 the devices used for measuring the
properties size and surface and emission are described.
2.1 Procedure
During the project there have been some changes in the general procedure. The numbers,
italic between brackets, show the different variables in each procedure. These numbers will be
explained in table 1-3 for each procedure during this project.
2.1.1.Core Materials
Put (1) gram CF3COONa, (2) gram C(F3COO)3Y, (3) gram C(F3COO)3Yb and (4) gram
C(F3COO)3Er in a 3-neck flask with (5) ml 1-octadecene, (6) ml (solvent 1) and a stirring
bean. Heat the mixture up to (7)°C for about 30 minutes under nitrogen flow. After 30
minutes heat the mixture further to (8) °C and let it stir for 1 hour.
Table 2.1.1.: The variable explaining for the synthesis of the core materials
Procedure
1
2
3
4
5
6
Solvent
(gram) (gram) (gram) (gram) (ml)
(ml)
1
0.2725 0.3432 0.1045 0.0101 17
3
OleicA
Acid
0.1386 0.1712 0.0512 0.0051 8.5
1.5
OleicB
Acid
0.2725 0.3432 0.1045 0.0101 17
3
OleicC
Acid
0.272
0.3338 0.1024 0.0102 0
10
Oley
D
Amine
0.136
0.1669 0.0512 0.0051 10
10
OleicE
Acid
0.136
0.1669 0.0512 0.0051 10
10
OleicF
Acid
0.136
0.1669 0.0512 0.0051 10
10
OleicG
Acid
0.136
0.1669 0.0512 0.0051 10
10
OleicH
Acid
14
7
(°C)
110
8
(°C)
300
160
300
160
300
110
300
110
320
110
300
110
300
110
300
2.1.2. Shell Materials
Put (9) gram CF3COONa and (10) gram C(F3COO)3Y in a 3 neck flask with (11) ml 1octadecene, (12) ml (solvent 1) and a stirring bean. Heat the mixture up to (13) °C and let it
stir for about (14) minutes under nitrogen flow. After (14) minutes the mixture is added to the
core materials.
Table 2.1.2: The variable explaining for the synthesis of the Shell materials
Procedure
9
10
11
12
Solvent
13
(gram)
(gram)
(ml)
ml
1
(°C)
0.0681
0.0856
10
2
Oleic110
A
Acid
0.0340
0.0428
5
1
Oleic160
B
Acid
0.0681
0.0856
10
2
Oleic160
C
Acid
0.136
0.214
0
5
Oley
110
D
Amine
0.136
0.1669
5
5
Oleic110
E
Acid
0.136
0.1669
5
5
Oleic110
F
Acid
0.136
0.0512*
5
5
Oleic110
G
Acid
0.136
0.1669
5
5
Oleic110
H
Acid
*Here C(F3COO)3Yb is used instead of C(F3COO)3Y
15
14
(minutes)
30
30
30
30
30
>60
60
60
2.1.3. Core/Shell structure (washing process)
After 1 hour of stirring the core materials at (8) °C add the shell materials with a dropping rate
of 2.50 ml/min (during the adding of the shell materials the nitrogen flow should be turned
off) After adding, the nitrogen flow is turned back on and the mixture has to stir at (8) °C for
about 1 hour. Then let the mixture cool down to room temperature. Then pour the mixture in
(solvent 2). The particles precipitate in the solvent. Then aggregate the particles with (solvent
3) and use 2 minutes of centrifuging. Each time after centrifuging the particles have to be
dissolved in 2 ml (solvent 4) using an ultrasonic bath. If all the particles have been
aggregated, solve them in (solvent 4) and put the particles in an ultrasonic bath and wash them
twice with (solvent 5).
Table 2.1.3: Washing process for the Core/Shell structure
Procedure
8 (°C)
Solvent 2
Solvent 3
300
Ethyl alcohol
Ethyl
A
alcohol
300
Ethyl alcohol/
Ethyl
B
1
Acetone *
alcohol
300
Ethyl alcohol
Ethyl
C
alcohol
300
Ethyl alcohol
Ethyl
D
alcohol
320
Ethyl alcohol
Ethyl
E
alcohol
300
1:4
Acetone
F
(hexane:Acetone)
300
Ethyl
1:4
G
(hexane:Acetone) alcohol
H
300
Ethyl
1:4
(hexane:Acetone) alcohol
Solvent 4
Hexane
Hexane
Hexane
Hexane*2
Hexane
Hexane
Hexane
Hexane
Solvent 5
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
Ethyl
alcohol
*1One half of the mixture is participate in Ethyl alcohol and the other part in Acetone
*2Particles didn’t dissolve in Hexane
2.2. Devices for measuring
For the structure characterization of size and surface, the TEM was used. The TEM images
have been obtained with a Morgagni TM Transmission Electron Microscope (FEI Company).
The steady-state upconversion spectrum of upconversion samples was measured with a SPEX
Fluorolog 3 spectrometer using excitation from a CW semiconductor diode laser at 980 nm.
With a integration time of 0.1 seconds and a concentration of 100 mg dissolved in 6 ml
hexane.
16
3 Results and discussion
The results given in this section include all the particles synthesized. The emission spectra are
showed once, because they were all the same and only the intensity in the different procedures
were different. The TEM image is from procedure H, because the other particles did not
attach to the copper grid.
3.1. Results and discussion by eye
In table 3.1 the results of the synthesized particles are given by characterizations seen by eye.
Table 3.1: Results of the core/shell particles and notable events during synthesis
Procedure
Reaction
Color
Color
Color
Comments
parameter
emission
particles
particles
changed
(by eye)
(by eye)
(by eye)
from A
after
Before
after
washing
washing
washing
none
Green
Yellow
White
none
A
½ of the
Yellow
Yellow
Yellow
Probably too
B
concentration
much Er3+
of A
because of the
yellow emission
after washing
Temperature
Green
Yellow
Yellow
There is oxygen
C
from 110 °C
coming in the
to 160 °C
mixture by
adding the shell
materials into
the core
materials. Next
time a septum is
used to prevent
this.
Solvent from
None
Yellow
None
The particles
D
Oleic-Acid to
were not
Oley Amine
useable because
they didn’t
dissolve in the
hexane or
anything tried.
Amount of
Green
Brown
White*
There was too
E
reactants and
(bright)
much water left
the amount of
in the shell
solvent.
materials, after
adding it to the
core materials a
lot of gas arose.
17
Procedure
Reaction
parameter
changed
from A
Color
emission
(by eye)
after
washing
Green
F
Reaction time
to more than
60 minutes
with respect to
procedure E
G
C(F3COO)3Yb None
is used instead
of
C(F3COO)3Y
Color
particles
(by eye)
Before
washing
White
Color
particles
(by eye)
after
washing
White
White
None
Comments
Shell materials
were heated in
an oil bath for
more than one
hour, because
the oil bath was
heating very
slowly.
C(F3COO)3Yb
is used instead
of
C(F3COO)3Y,
when washing
for the second
time the
particles were
gone. This can
be that the time
of centrifuge
was too short.
After longer
centrifuge the
particles came
back.
This one was
perfect. Nothing
went wrong.
Reaction time Green
White
White
60 min, and
concentration
the same as E
*After a night standing they were white instead of the brown color soon after the washing
process
H
Table 3.1 shows the color of the emission by eye from different procedures after the washing
process. It also shows the color of the particles by eye, before and after the washing process
for different procedure A to H. There are also some comments given for each procedure
which were important for the change in the subsequent procedure.
Procedure A is the procedure with the results how they are supposed to be, that is green
emission and white particles after washing. But the particles before washing are yellow. By
changing the reaction parameters it will be examined if it is possible to get white particles
before the washing process.
18
First the concentration is changed in procedure B. The emission was yellow and the particles
before and after washing was yellow too. Because of the yellow emission we conclude that
this is probably because there is too much Er3+. The Er3+ gives an emission shifted to the red
compared with Yb3+ and Y3+.
In procedure C the concentration is the same as in A, but the temperature is raised from 110
°C to 160 °C. During this procedure it was noticed that the solution was brown, probably
because the solvent oleic acid was oxidized by oxygen. This happened because when the shell
materials were added into the core materials, the nitrogen flow was closed and there was
oxygen coming in the solution. The particles synthesized give a green emission but the color
of the particles was still yellow after washing. Because of the high temperature the oleic acid
oxidized much faster than before. So for the next time a septum is used to add the shell
materials into the core. The nitrogen flow can stay open and there is no oxygen coming in the
solution.
Because of the trouble with the oleic acid in procedure C, another solvent is tried in
procedure D, namely oley amine. This solvent makes it difficult to get results because the
synthesized particles did not dissolve in hexane. So it has to be concluded that with the
synthesis method used the solvent oley amine is not a good replacement for the oleic acid. So
in the next procedure oleic acid is used again.
In procedure E the amount of solvents, 1-octadecene and oleic acid is the same, but the
concentrations of the reactants were changed. The procedure gave brown particles, but a very
bright green emission. After washing the particles were still brown. The particles were left
during the night and it was found that the next morning the particles were white. During the
process there was too much water coming from the shell materials into the core materials,
which can cause oxidation of the solvent oleic acid.
So in procedure F the amount of solvents and reactants were the same as in E and the reaction
time of the shell materials was increased from 30 minutes to 60 minutes. This would be
enough time to evaporate all the water present in the solvent. The particles were white before
and after washing, and the emission was green. The washing process was also improved.
After some testing with a couple of solvents like hexane, acetone and ethyl alcohol, there was
found a good way to precipitate the nanoparticles in a mixture of 1:4 hexane:acetone.
Now that the right properties were found in the particles, another composition was tried.
Instead of C(F3COO)3Y, C(F3COO)3Yb was used. This is done in procedure G. This should
give a much brighter emission. Only this was not the case. The particles were quickly
dissolved in hexane and after one time of washing with ethyl alcohol all the particles were
gone. An explanation can be that the centrifuging time was too short. Indeed after longer
centrifuging the particles were back, but they did not give any good result on the emission.
Finally, to prove that the synthesis found after changing some reaction parameters is good
enough for making -NaYF4 : Yb3+, Er3+ nanocrystals there is procedure H. Procedure H has
all the positive changes in it. The temperature is kept the same as in procedure A, the reaction
time of the shell materials is from 30 minutes to 60 minutes, the solvent is oleic acid and the
concentration of the reactants and the solvents is the same as in procedure E. The
nanoparticles synthesized by procedure H were white before and after the washing process
and gave a green bright emission. The washing process is the one used by procedure F and the
particles only had to be washed once.
19
An example of the core (figure 3.1a) and the core/shell (figure 3.1b) structure synthesized by
procedure H can be seen in figure 3.1.
A) Emission by eye from core
structure, radiated with 980 nm
NIR light
B) Emission by eye from core/shell
structure, radiated with 980 nm
NIR light
Figure 3.1: The emission from the core and the core/shell structure. The core structure the color of the emission
looks Yellow and is very weak. The emission from the core/shell structure is the color green and it is very bright.
As can be seen in figure 3.1 the emission of the core/shell structure is much brighter and
greener light than the core. This result shows that the core/shell structure is more efficient for
the emission than the structure with only the core. To prove that these results are not only by
eye, the emission spectra have been taken.
20
3.2. Results and discussion of the emission spectra
From all the particles an emission spectrum is taken. The difference between these emission
spectra was the intensity of the peaks. Figure 3.2.1 gives the emission spectra of the core
materials of procedure H. The integration time was 0.1 seconds and the excitation wavelength
was 500 nm and a 2 nm slit was employed
Figure 3.2.1: The emission spectra of the core materials, with a band around the 550 nm (green) and a band
between the 650-675 nm (red)
Figure 3.2.1 shows a band at 550 nm (green light) and a band between the 650-675 nm (red
light). The relative ratio between those two peaks is 1:2.6. The red light is much greater than
the green light emission. This is also been seen in figure 3.1a. There the emission was almost
yellow instead of green.
There is also an emission spectrum of the core/shell structure of procedure H. These emission
spectra were also all the same and varied in intensity. The emission spectra of the particles
from procedure H are shown below in figure 3.2.2.
21
Procedure H gave the greatest intensity with a integration time of 0.1 seconds and a excitation
wavelength of 500 nm and a 2 nm slit was employed.
Figure 3.2.2: The emission spectra of the core/shell structure synthesized by procedure H, with a band around the
550 nm (green) and a band between the 650-675 nm (red)
Figure 3.2.2 shows the core/shell emission spectrum. There is a band around 550 nm and a
band between the 650-675 nm. The relative ratio between these bands is 1: 1.8. There is
relative more green emission than red emission as be seen in figure 3.1b. The red emission is
still larger than the green wavelength, which is something that is not seen by eye. But the
emission of the core/shell structure is much stronger than only the core structure.
22
3.3. Results and discussion of the TEM
To check the homogeneity and the size of the synthesized nanoparticles, TEM images have
been taken of the core (figure 3.3.1) and the core/shell (figure 3.3.2). These particles are
synthesized by procedure H. The other particles did not attach to the copper grid.
Figure 3.3.1: The TEM results of the synthesized core materials. The morphology and structure can be
determined by this image.
In figure 3.3.1 the size of the particles are between 10-20 nm. This is been calculated in the
following way:
the biggest particle is (0.5 cm x 200 nm) / (5.2 cm) = 19.23 nm and the smallest particle is
(0.3 cm x 200 nm) / (5.2 cm) = 11.54 nm. The shape of the particles is different among
themselves. Some of the particles are spherical, but some of them look more like nanoplates
or nanoellipses. This is hard to determine only on one image.
In figure 3.3.2 there is a TEM image of the core/shell structure particles. Here the size and the
shape are determined for the synthesized particles synthesized with procedure H.
23
Figure 3.3.2: The TEM results of the core/shell structure synthesized. The morphology and structure can be
determined by this image.
Figure 3.3.2 shows the size of the particles of the core/shell structure. The size of the particles
is between 14-27 nm. Because the biggest particle present is, (0.9 cm x 200 nm)/ (6.7 cm) =
26.87 nm and the smallest particle is, (0.5 cm x 200 nm) / (6.7 cm) = 14.93 nm. The shape of
these particles looks like nanoplates and nanospheres. Because these particles are larger than
the core structure particles, the synthesis of the core/shell structure can be considered
successful when synthesized with procedure H.
Now that procedure H is given the right properties of the particles, there are still some things
unknown. The shape and the surface of these synthesized particles are not perfect. There are
different shapes among the particles and the surface is not homogeneous enough for the
biocompatibility. In the future, a synthesis must more based on the shape and surface of the
particles and then combined with the findings of this project. Then the particles become more
and more perfect and they almost have all the properties to become great biomedical
fluorescent labels. And thereby hopefully save and help a lot of people from cancer in the
future.
24
4 Conclusion
The difference between the core and the core/shell structure is the brightness of the emission
of the green light. As seen in figure 3.2.1 and 3.2.2, both have an emission band between the
650-675 nm and a band around 550 nm. But the relative ratio of both bands are for the core
1:2.6 and for the core/shell 1:1.8. In combination with the results of lifetime studies[4] this
leads to the conclusion that the core/shell structure has a brighter emission in compare with
only the core structure. This can also be seen in figure 3.1. The procedures A to H show the
way to synthesize these core/shell particles. The best way to synthesize them is with
procedure H. The temperature is kept the same as in procedure A, the reaction time of the
shell materials is from 30 minutes to 60 minutes, the solvent is oleic acid and the
concentration of the reactants and the solvents is the same as in procedure E. Also the
washing process is improved with hexane:acetone (1:4) to precipitate the nanoparticles so
they become white. Procedure H gave white particles and a bright green emission. The
particles synthesized are between 14 and 27 nm for core/shell structure particles and for core
particles between 10-20 nm. The synthesized particles have a different morphology.
5 Acknowledgement
I want to thank my supervisor Yinghui Wang for helping me during the synthesis and the
information she gave me for making the synthesis possible. I also want to thank her for
guiding me in the lab and helping me with measuring the synthesized particles and for giving
me a glimpse of the Chinese culture. I want to thank Hong Zhang for giving me the
opportunity to be a part of his group and for giving me the project. I also wanted to thank him
for his support over the last three months. I wanted to thank Kai and the AMC group for
making it possible to measure with TEM. I also want to thank Marcel Plugge for helping me
with the program Igor for making the graphics. Eventually, I want to thank Dhr. prof. dr. W.
J. Buma for helping me with this report. I have learned a lot during this project, not only
scientifically, but most of it on personal development. This project has given me much more
self-confidence and I wanted to thank you all for given me a feeling of trust and being a part
of the group. Thank You All!
25
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Debapriya Banerjee, Yongsheng Liu, Xueyuan Chen and Xiaogang Liu, Analyst,
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[9] ‘The Active-Core/ Active-Shell Approach: A Strategy to Enhance the Upconversion
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Venkataramanan Mahalingam, Christopher G. Morgan and John A. Capobianco, Adv.
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* Sources images or figures
Figure 1:
http://www.google.nl/imgres?imgurl=http://www.chimachine.eu/images/infrarood.gif&imgrefurl=http://www.chi-machine.eu/infrarood-infraredfir/index.php&usg=__2MeJJj1fjY6mr0krS8UBixlaH2k=&h=322&w=671&sz=17&hl=nl&start=0&zo
om=1&tbnid=fNOG6OmqtGCm8M:&tbnh=90&tbnw=188&ei=dGb3TajqCYup8AO5ofW2Cw&prev
=/search%3Fq%3Delektromagnetisch%2Bspectrum%26hl%3Dnl%26biw%3D1259%26bih%3D627%
26gbv%3D2%26tbm%3Disch&itbs=1&iact=hc&vpx=130&vpy=252&dur=2168&hovh=155&hovw=
324&tx=212&ty=74&page=1&ndsp=18&ved=1t:429,r:9,s:0&biw=1259&bih=627
Figure 1.2.1:
http://www.google.nl/imgres?imgurl=http://imglib.lbl.gov/ImgLib/COLLECTIONS/BERKELEYLAB/images/XBD_9604-01788.lowres.jpeg&imgrefurl=http://www.lbl.gov/LBLPID/Nobelists/Seaborg/65th-anniv/29.html&usg=__BlroX3hcy6sbutT3f3Ag34_r1E=&h=458&w=640&sz=54&hl=nl&start=0&zoom=1&tbnid=xA_Bs8E
FBkPD-M:&tbnh=153&tbnw=221&ei=gmX3TaO2EIOEOp2mLwK&prev=/search%3Fq%3Dlanthanide%2Band%2Bactinide%26hl%3Dnl%26biw%3D1259%26bih
%3D627%26gbv%3D2%26tbm%3Disch&itbs=1&iact=rc&dur=125&page=1&ndsp=15&ved=1t:429,
r:2,s:0&tx=113&ty=104
26