World Journal of Nano Science and Engineering, 2012, 2, 41-46
http://dx.doi.org/10.4236/wjnse.2012.22007 Published Online June 2012 (http://www.SciRP.org/journal/wjnse)
A Method to Improve the Up-Conversion Fluorescence of
Polymer Modified NaYF4:Yb,Er(Tm) Nanocomposites
Weina Cui1, Siyu Ni1, Shunan Shan2, Xingping Zhou1*
1College of Chemistry, Chemical Engineering and Biotechnology, Donghua University, Shanghai, China
2College of Life and Environment, Shanghai Normal University, Shanghai, China
Received October 3, 2011; revised November 11, 2011; accepted December 15, 2011
ABSTRACT
The modification of NaYF4:Yb,Er(Tm) nanoparticles synthesized in the presence of an ionic surfactant is critical to
their application in biological fields for better solubility and biocompatibility. In this work, NaYF4:Yb,Er(Tm) was
transformed from insoluble, inactive to hydrophilic, biocompatible via ligand exchange modification with polyacrylic
acid (PAA). Ligand exchange was carried out at room temperature when a colloidal solution of NaYF4:Yb,Er(Tm) in
tetrahydrofuran (THF) was treated with excess PAA. The PAA modified NaYF4:Yb,Er(Tm) nanoparticles got better
surface properties but with declined inner up-conversion fluorescence. Generally, coating an analogous layer of material
outside the core nanoparticles can improve the optical properties of the core. Accordingly, NaYF4:Yb,Er(Tm)/NaYF4
nanoparticles were synthesized before PAA modification to avoid the optical intensity decaying. The result of fluores-
cence test proved that the water soluble NaYF4:Yb,Er(Tm)/NaYF4/PAA nanocomposites had a sound up-conversion
property compared with that of NaYF4:Yb,Er(Tm)/PAA. Furthermore, the up-conversion fluorescence property of the
nanocomposite varied with the doping ratio of Er(Tm) to Yb and the possible mechanism for this change was also dis-
cussed.
Keywords: Doping Ratio; Fluorescence; Ligand Exchange; Polyacrylic Acid; Sodium Yttrium Fluoride;
Surface Modification
1. Introduction
Up-conversion fluorescence materials are attracting much
attention, owing to their unique optical properties and
potential applications. Among all the up-conversion fluo-
rescence materials, hexagonal-phase NaYF4:Yb,Er(Tm)
is one of the most efficient 980 nm near-infrared (NIR)
to visible (green and blue) up-conversion phosphors [1,2].
In bulk, the low phonon energy of the host strongly sup-
presses multiphonon relaxation process in the emission
centers, leading to up-conversion emission. Yb3+ work-
ing as a activator with Y3+ sensitizes Er3+(Tm3+) to emit
green (blue) and red lights. In the recent decade, the re-
search of up-conversion materials was mainly focused on
the biological and medical fields such as low-intensity IR
imaging and sensitive bioprobes [3,4].
Biological application of the up-conversion fluores-
cence materials presents new challenges of the develop-
ment of nano-sciecnce and nano-technology, especially
the surface modification study of nanoparticles. As well
known, ligand exchange is a versatile way to modify the
surfaces of nanoparticles for better solubility and bio-
compatibility. Dykstra et al. [5] prepared the CdSe/ZnS
nanoparticles by an organometallic route in the presence
of trioctylphosphine oxide (TOPO) and then used (PEG)-
phosphine oxide copolymer to remove the TOPO for
obtaining hydrophilic nanoparticles. Zhang et al. [6] chose
polyacrylic acid and poly-allylamine to replace the ori-
ginnal hydrophobic ligand on magnetic nanoparticles at
an elevated temperature in a glycol solvent and rendered
it high water solubility. Lin et al. [7] successfully synthe-
sized at 150˚C the OA-capped PbS QDs with emission in
the NIR and modified the PbS by ligand exhange with
polyelectrolytes for the application of deep-tissue in vivo
imaging.
Ligand exchange is also suitable for the modification of
NaYF4. In our work, PAA was chosen as the ligand polymer
because of its excellent properties including hydrophilicity,
abundant reactant groups (carboxyl) and non-toxicity. In
order to eliminate the quenching effect from polymer, the
nanoparticles of NaYF4:Yb,Er(Tm)/NaYF4 core-shell were
firstly synthesized before PAA modification [8]. The middle
NaYF4 layer protected the inner core and its crystal structure,
thus enhancing the fluorescence property. The final resultant
of NaYF4:Yb,Er(Tm)/NaYF4/PAA nanocomposites prom-
*Corresponding author.
C
opyright © 2012 SciRes. WJNSE

W. N. CUI ET AL.
42
ised to have better comprehensive properties, which made
it possible to be applied in biological fields.
To the best of our knowledge, fluorescence properties
of the NaYF4 with a size of several nanometers after
modification are barely discussed, although the research
about the up-converting fluorescence of NaYF4 core ma-
terial has been widely carried out. However, in our work,
fluorescence properties of the NaYF4:Yb,Er(Tm)/PAA
core-shell nanoparticles and the NaYF4:Yb,Er(Tm)/ NaYF4/
PAA core-shell-shell nanocomposites were mainly ex-
plored. It is more significant to study the fluorescence of
polymer modified NaYF4, as the existence of polymer
often causes the quenching effect which is one of the
most important influence factors for a bio-labeling mate-
rial. Besides that, the color changing rules of NaYF4:Yb,
Er(Tm)/NaYF4/PAA nanocomposites were also revealed
in this article. It is the first time to explore the relation-
ship in details between lanthanide doping ratio and fluo-
rescence property of the nanoparticles after modificatioin
with PAA.
2. Experimental
2.1. Synthesis of the Host Material: Hexagonal
NaYF4 Nanoparticles
The synthesis basically followed the routes below. Briefly,
the yttrium perchlorate (Y(ClO4)3) was prepared at the
beginning by dissolving the rare earth oxide in perchlo-
rate acid. Ten mL of 0.4 M Y(ClO4)3 was dispersed in 60
mL cyclohexane. Under vigorous stirring in a three-neck
flask, the mixture was heated to 40˚C and maintained for
20 min after the addition of some amount of the surface-
tant sodium oleate of 0.4 M. Then, 20 mL of 0.8 M sodium
fluoride was added into this reaction system and main-
tained at the same temperature for 60 min. The final mix-
ture was standing for 15 min to gain an obvious interface
between oil and water phases. The product was dispersed
homogeneously in 60 mL of cyclohexane (oil layer).
NaYF4 nanoparticles were precipitated by centrifugation
and ultrasonication treatments for the cyclohexane solu-
tion and washed by ethanol and hydroxide sodium for at
least 3 times.
2.2. Synthesis of NaYF4:Yb,Er(Tm)
Nanoparticles and NaYF4:Yb,Er(Tm)/
NaYF4 Core/Shell Nanoparticles
The synthesis of NaYF4:Yb,Er(Tm) is similar to that of
NaYF4 nanoparticles. The lanthanides were put into the
reaction system in the form of rare earth perchlorate in-
cluding Y(ClO4)3, Yb(ClO4)3, Er(ClO4)3 and Tm(ClO4)3.
Ten mL mixture of a designated molar ratio of Y to Ln
(0.4M Y(ClO4)3 and Ln(ClO4)3 (Ln = Yb, Er(Tm)) was
dispersed in 60 mL cyclohexane. The reaction condition
and other procedures are the same as those of NaYF4
mentioned above. NaYF4:Yb,Er(Tm) nanoparticles were
precipitated by centrifugating half of the cyclohexane (oil
layer) suspension.
Recently, several similar methods have been reported
in the literature for synthesizing NaYF4:Yb,Er(Tm)/
NaYF4 core/shell nanoparticles [9-11]. In this paper, we
also take this way to produce NaYF4:Yb,Er(Tm)/NaYF4
core/shell nanoparticles. In a typical process, five mL 0.4
M Y(ClO4)3 and 5 mL of sodium oleate solution (0.4 M)
were then put into another half of the cyclohexane sus-
pension and stirred for 20 min. Ten mL of 0.8 M sodium
fluoride was added and vigorously stirred for 60 min. All
the reactions were carried out at 40˚C. After centrifuga-
tion, the resulting nanoparticles of NaYF4:Yb,Er(Tm)
and NaYF4:Yb,Er(Tm)/NaYF4 core/shell composites were
dried in vacuum at 60˚C for at least 12 h.
2.3. PAA Surface Modification of NaYF4:Yb,Er
(Tm) and NaYF4:Yb,Er(Tm)/NaYF4
Core/Shell Nanoparticles via Ligand
Exchange
The NaYF4:Yb,Er(Tm)/PAA and NaYF4:Yb,Er(Tm)/
NaYF4/PAA were prepared by mixing a solution of ex-
cess PAA with that of NaYF4 nanoparticles in tetrahy-
drofuran (THF) and then stirred overnight. Herein, THF
is good solvent for hydrophilic PAA and hydrophobic
NaYF4 nanoparticles, and the other organic solvents don’t
reach the same effects as THF does. Taking the synthesis
of NaYF4:Yb,Er/PAA as an example, PAA (84.7 mg)
was mixed with NaYF4:Yb,Er(20.8 mg) in THF(15.0 mL)
in a 50 mL conical flask and this mixture was stirred at
30˚C overnight. The resultant was homogeneous and
ivory white in color. This conical flask was then placed
into a boiling water bath to evaporate the THF. After the
solvent evaporation, a white solid was obtained. This
white solid was then washed by sodium hydroxide and
alcohol for 3 times. NaYF4:Yb,Er/PAA was not soluble
in cyclohexane but can readily be transferred to methanol
or water to form robust colloidal solutions.
2.4. Characterization
TEM images of the NaYF4:Yb,Er(Tm) nanoparticles and
NaYF4:Yb,Er(Tm)/NaYF4 core/shell nanocom-posites
were collected on a JEOL JEM 3010 transmission elec-
tron microscope. Powder X-ray diffraction spectra were
acquired with a D8 advance X-ray diffractometer, with
Cu Kα radiation at 1.5406 Ǻ. Fourier transform infrared
spectroscopy (FTIR) was performed to further charac-
terize the composition and structure of the nanocom-
posites. Samples of the modified nanoparticles were
ground with KBr and then compressed into slices. The
spectrum was taken from 500 to 3500 cm–1.

W. N. CUI ET AL. 43
2.5. Fluorescence Property
Up-conversion fluorescence spectra were obtained on a
LS-55 luminescence spectrometer (Perkin-Elmer) with
an external 980 nm laser diode (1 W, continuous wave
with 1 m fiber, Beijing Viasho Technology Co.) as the
excitation source in place of the Xe-lamp in the spectro-
meter. For comparison, all the nanoparticles were milled
to powder and compressed into the groove.
3. Results and Discussion
3.1. Size Analysis of the Core Nanoparticles by
XRD and TEM
Figure 1 shows the X-ray diffraction (XRD) result of the
host material NaYF4. The well-defined peaks indicated
the high crystallinity of the nanoparticles. The peak posi-
tions and intensities of XRD patterns matched closely
with those of hexagonal β-NaYF4 in JCPDF (28 - 1192)
[9]. From the line breadth of the diffraction peaks, the
crystallite size of the samples was estimated to be appro-
ximately 6.9 nm for NaYF4 nanoparticles based on the
Debye-Scherrer formula.
Figures 2(a) and (b) show the transmission electron
microscopy (TEM) images of NaYF4:Yb,Er(Tm) core
and NaYF4:Yb,Er(Tm)/NaYF4 core/shell nanoparticles,
respectively. It was calculated that the average sizes of
them were 8.2 nm and 12.0 nm, respectively. The doping
ratio of Lanthanide Yb,Er (Tm) is so tiny compared with
yttrium that Figure 2(a) also can represent the TEM image
of NaYF4 host materials. So the size of NaYF4 (8.2 nm)
from TEM is basically consistent with the result (6.9 nm)
from Debye-Scherrer formula.
3.2. FTIR Spectra of NaYF4:Yb,Er(Tm)
Nanoparticles before and after PAA
Modification
To demonstrate the capping of PAA on the nanoparticles,
FTIR spectra of the NaYF4:Yb,Er(Tm)/PAA (Figure
3(c)) were compared with those of NaYF4:Yb,Er(Tm)
without surface modification (Figure 3(a)) and pure
PAA (Figure 3(b)). In Figure 3(a), the intense bands in
the vicinity of 2920 cm–1 and 2850 cm–1 are due to the
antisymmetric and symmetric vibration of the -CH2, and
the bands in 1545 cm–1 and 1457 cm–1 are contributed to
C = O antisymmetric and symmetric stretching vibrations
of carboxylate groups. The band position and shape are
the same as the FTIR spectra of sodium oleate [12], so it is
concluded that the nanoparticles are capped with sodium
oleate before modification. It is obvious that the nano-
particles after ligand exchange (Figure 3(c)) have no
absorption band of -CH2 and two new bands appear near
1600 cm–1 and 1400 cm–1 corresponding to the R-COO–
antisymmetric and symmetric stretching vibrations. The
Figure 1. XRD pattern of NaYF4 nanocrystals synthesized
with sodium oleate.
(a) (b)
Figure 2. TEM images of (a) NaYF4:Yb,Er(Tm) and (b)
NaYF4: Yb,Er(Tm)/NaYF4.
Transmittance
Figure 3. FTIR spectra of (a) NaYF4:Yb,Er(Tm); (b) Pure
PAA; and (c) NaYF4:Yb,Er(Tm)/PAA.
appearance of R-COO– stretching vibration can also be
found in the other nanocomposites, such as polyacrylic
acid modified aluminum oxide and ferroferric oxide [13,14].
This observation clearly indicates that the sodium oleate
on the surface of nanoparticles has been successfully
replaced by PAA.

W. N. CUI ET AL.
44
3.3. Fluorescence Properties of NaYF4:Yb,Er
(Tm), NaYF4:Yb,Er(Tm)/PAA, and NaYF4:
Yb,Er(Tm)/NaYF4/PAA
Figure 4 shows the up-conversion luminescence mecha-
nism of NaYF4:Yb,Er(Tm). Up-conversion is an effec-
tive avenue for the generation of visible emission with
near-infrared (NIR) excitation, which is based on sequen-
tial photon absorption and energy transfer steps [15].
In the case of NaYF4:Yb,Er, under the excitation of
980 nm light, which can be absorbed by Yb3+, the elec-
trons of Er3+ are first excited from the 4I15/2 to higher
level via excitation energy transfer from Yb3+ to Er3+ and
then to the 4F7/2 level by absorbing the energy of another
electron from Yb3+ (
2F5/2). The excited electrons of the
4F7/2 (Er3+) level then decay to the emitting 2H11/2 (the line
at 540 nm), and 4F9/2 levels (the line at 650 nm), mainly
through nonradioactive process. Meanwhile, as for NaYF4:
Yb,Tm, the emission process is a little different. An Yb3+
ion in the 2F7/2 ground state absorbs a 980 nm photon and
transits to excited state 2F5/2. When it drops back to the
ground state (2F5/2-2F7/2), energy is transferred to an adja-
cent ground state (3H6) Tm3+ ion. After the multistep ex-
citation energy transfer, the Tm3+ is promoted to the 1D2
and 1G4 emission levels. These excited electrons on the
emitting levels (1D2 and 1G4) then drop back to the
low-energy levels (3F4 and 3H6) via fluorescence emis-
sion [16,17]. But the 1D2 level of Tm3+ commonly cannot
be populated by the photon from Yb3+ via energy transfer
to the 1G4 due to the large energy mismatch between
them [18].
Figures 5 and 6 show the up-conversion fluorescence
spectra of NaYF4:Yb 10%, Er (Tm) 3% before and after
surface modification with a 0 - 1500 mw adjustable 980
nm laser. In Figure 5, the straight line represents the
NaYF4:Yb,Er core material, where the green emission
around 540 nm and the red emission around 650 nm are
attributed to the transitions of 2H11/2-4I15/2 and 4F9/2-4I15/2
for the Er3+ ions. It is obvious that the emission band of
dash line representing NaYF4:Yb,Er/PAA is very weak
due to the quenching effect from organic PAA. Neverthe-
less, the emission intensity of NaYF4:Yb,Er/NaYF4/PAA
showed as the dot dash line is dramatically increased by
the protection of middle NaYF4 layer. Especially the red
emission band at 650 nm is almost as high as that of the
NaYF4:Yb,Er nanoparticles before modification.
In the case of NaYF4:Yb,Tm (shown in Figure 6), the
red emission appears at the same position and the blue
emission is shifted to 470 nm arising from the 1G4 to 3H6
energy transition. Likewise, the emission intensity of
NaYF4:Yb,Tm/NaYF4/PAA is higher than that of NaYF4:
Yb,Tm/PAA. So it is concluded that the middle NaYF4
layer works as a protecting layer which can reduce the
influence on fluorescent intensity probably caused from the
organic modification.
Figure 4. Schematic energy level diagrams: upconversion
excitation and visible emission schemes for the Er3+,Tm3+
and Yb3+ ions.
4
4
4 4
Figure 5. Fluorescent spectra of nanoparticles doped with
Yb3+,Er3+ before and after PAA modification.
4
4
4 4
Figure 6. Fluorescent spectra of nanoparticles doped with
Yb3+,Tm3+ before and after PAA modification.

W. N. CUI ET AL. 45
3.4. Influence of Yb/Er(Tm) Doping Ratio on
the Up-Conversion Property
Figures 7 and 8 show the up-conversion fluorescence
spectra of NaYF4:Yb,Er/NaYF4/PAA and NaYF4:Yb,
Tm/NaYF4/PAA under 980 nm excitation, respectively.
It is clear that all samples show strong emissions in the
visible region. Upon addition of more Er3+ to Yb3+, there
is a marked increase in the emission ratio of red to green,
as can be seen by comparison among Figures 7(a)-(c).
This effect is probably related to the increased possibility
of the two-ion Er3+ cross-relaxation process (4I15/2, 4S3/2-
4I13/2, 4I9/2) that occurs at higher Er3+ concentrations. This
process depopulates the green-emitting 4S3/2 state, thereby
increasing the red to green ratio [19]. But in the case of
NaYF4:Yb,Tm/NaYF4/PAA, the result is just contrary
(Figures 8(a)-(c)). With the increase of Tm3+ to Yb3+
ratio, the red to blue emission is reduced. It shows that at
higher Tm3+ concentration, the Tm3+ cross-relaxation proc-
ess of 3F2, 3H4-3H5, 1G4 increases, which plays an impor-
tant role in populating 1G4 Level, thus enhancing the blue
emission [17]. It is very significant to find out the color
changing rules versus lanthanide doping ratio for fulfill-
ing the different practical requirements. For example, in
the fields of in vivo imaging and labeling, the variety of
color emission can provide more than one binding site to
image several bi-molecules at the same time. The fluo-
rescence property is the key for the material to be suc-
cessfully applied and then further arduous work should be
done in the future.
Figure 7. Fluorescent spectra of NaYF4:Yb,Er/NaYF4/PAA
of different Er3+ doping ratios: (a) 1.0% Er3+; (b) 3.0% Er3+;
(c) 10.0% Er3+.
Figure 8. Fluorescent spectra of NaYF4:Yb,Tm/NaYF4/PAA
of different Tm3+ doping ratio: (a) 1.0% Tm3+; (b) 3.0%
Tm3+; (c) 10.0% Tm3+.
4. Conclusion
Polymer modification of NaYF4:Yb,Er(Tm) can improve
the surface property of nanoparticles but decrease the
fluorescent intensity of the inner material because of
quenching effect. In this study, we described a method to
alleviate the quenching effect by coating another NaYF4
layer outside NaYF4:Yb,Er(Tm) inner core to resist the
influence from PAA (polyacrylic acid). Photolumines-
cence checking showed that the up-converting fluores-
cent intensity of NaYF4:Yb,Er(Tm)/NaYF4/PAA was
obviously higher than that of NaYF4:Yb,Er(Tm)/PAA,
thus proving the efficiency of this method. Another focus
of this paper here was mainly on the relationship between
the specific red/green (blue) emission value and Yb/Er
(Tm) doping ratio. In summary, when doping the Yb and
Er ions, the higher Er3+ concentrations increased the red
to green emission ratio. In contrary, the higher Tm3+ cen-
centrations just weaken the red emission but enhanced
the blue one. The controllable fluorescence efficiency and
hydrophilicity with desirable carboxylic functional groups
provided by PAA give the core/shell/shell nanocompo-
sites a great potential to be bio-probes.
5. Acknowledgements
The authors thank Shanghai Municipal Natural Science
Foundation (Grant 10ZR1400600), Shanghai Municipal
2010 Nanometer Special Project (Grant 1052nm06400),
and the Fundamental Research Funds for the Central
Universities for their financial supports.

W. N. CUI ET AL.
46
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