Microwave-assisted one-pot synthesis of water-soluble rare-earth doped fluoride luminescent nanoparticles with tunable colors

Article abstract of DOI:10.1016/j.jallcom.2012.02.095

Polyethyleneimine (PEI) functionalized multicolor luminescent LaF ₃ nanoparticles were synthesized via a novel microwave-assisted method, which can achieve fast and uniform heating under eco-friendly and energy efficient conditions. The as-prepared nanoparticles possess a pure hexagonal structure with an average size of about 12 nm. When doped with different ions (Tb³⁺ and Eu³⁺), the morphology and structure of the nanoparticles were not changed, whereas the optical properties varied with doped ions and their molar ratio, and as a result emission of four different colors (green, yellow, orange and red) were achieved by simply switching the types of doping ions (Eu³⁺ versus Tb³ +) and the molar ratio of the two doping ions.

Full text of DOI:10.1016/j.jallcom.2012.02.095

Journal of Alloys and Compounds 525 (2012) 154–158
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Microwave-assisted one-pot synthesis of water-soluble rare-earth doped
fluoride luminescent nanoparticles with tunable colors
Cong-Cong Mia, Zhen-huang Tiana, Bao-fu Hana, Chuan-bin Maob, Shu-kun Xua,∗
a Department of Chemistry, Northeastern University, Shenyang 110819, PR China
b Department of Chemistry & Biochemistry, Stephenson Life Sciences Research Center, University of Oklahoma, 101 Stephenson Parkway, Norman, OK 73019, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Polyethyleneimine (PEI) functionalized multicolor luminescent LaF3 nanoparticles were synthesized via
a novel microwave-assisted method, which can achieve fast and uniform heating under eco-friendly
and energy efficient conditions. The as-prepared nanoparticles possess a pure hexagonal structure with
an average size of about 12 nm. When doped with different ions (Tb3+ and Eu3+), the morphology and
structure of the nanoparticles were not changed, whereas the optical properties varied with doped ions
and their molar ratio, and as a result emission of four different colors (green, yellow, orange and red)
were achieved by simply switching the types of doping ions (Eu3+ versus Tb3 + ) and the molar ratio of
the two doping ions.
Received 15 November 2011
Received in revised form 9 February 2012
Accepted 13 February 2012
Available online 25 February 2012
Keywords:
Microwave-assisted synthesis
Multicolor luminescence
Nanomaterials
© 2012 Elsevier B.V. All rights reserved.
1. Introduction
[19]. Kuznik et al. investigated fluorescent properties of gadolin-
ium oxide powders doped with different rare earth ions (Er3+, Tb3+
,
Rare earth doped luminescent nanomaterials possess many
advantages, such as narrow emission peak, long fluorescence life-
time, high photochemical stability, and large stokes shift [1].
Moreover, it is easy to achieve multicolor emission under a sin-
gle excitation by changing doped rare earth ions or matrix [2]. In
recent years, synthesis and applications of rare-earth doped nano-
materials have attracted wide interest [3–5]. Rare earth doped
nanomaterials have broad applications in protein and DNA assays
[6], bio-labeling [7], optical imaging [8], magnetic resonance imag-
ing (MRI) [9] and clinical diagnostics. Over the last few decades,
various solution phase routes, including precipitation [10], thermal
decomposition [11], polyol-mediated process [12], micro-emulsion
[13], sol–gel [14], hydrothermal [15] and two phase method
[16] have been applied for the preparation of rare earth doped
nanoparticles. Most recently, Tang et al. reported the preparation
of GdVO4:Eu3+ nanocrystals with different morphologies through
hydrothermal method and investigated the effects of annealing
temperature and rare earth ions doping concentration on the lumi-
nescence intensity [17]. Chen et al. synthesized Pr3+ and Yb3+
ions co-doped down- and up-conversion transparent oxyfluoride,
during which the maximum quantum efficiency from Pr3+:3P0 to
Yb3+:2F5/2 was 153% [18]. Zheng et al. synthesized sphere-like
CaWO4:Mo/Eu,Tb microspheres, they found that proper doping
concentration of Mo extended excitation range of the microspheres
Dy3+). They found that singly doped Er3+ sample gave stronger
luminescence signals and others showed significantly larger decay
lifetimes [20].
Here, we describe a novel microwave-assisted solvothermal
approach for the fast and rapid preparation of small spherical single
phase LaF3 nanoparticles. As a kind of green synthetic method, in
recent years, microwave-assisted preparation has been developed
rapidly. In a microwave system, heating arises from either dipole
rotation or ion migration induced by the microwave field, which is
different from radiation, conduction and convection of traditional
heating method [21]. Consequently, compared with the conven-
tional methods, microwave irradiation shows several advantages,
including even and selective heating, shortened reaction time, less
side effects, smaller and narrower particle size, and higher yield
[22]. Due to the unique heating model, microwave-assisted method
has received more and more attention and been widely used in
the preparation and extraction of chemical, biological and medic-
inal materials. It has been employed to exploit new materials. As
a new heating source, microwave irradiation has been successfully
used in a number of chemical approaches to produce many types of
nanoparticles, such as metal [23], II–VI group semiconductors [24],
oxides [25], vanadium metal-organic frameworks [26], fluoride
[27], phosphates [28,29], and even for the grafting of poly(9,90-n-
dihexylfluorene) (PDHF) onto flat and nanopatterned crosslinked
photopolymer [30].
Rare earth fluoride has very low phonon energy, and can directly
stimulate the f–f transition of rare earth ions [31]. Therefore,
research on the synthesis and application of rare earth fluoride is of
∗
Corresponding author. Tel.: +86 24 83681343.
E-mail address: xushukun46@126.com (S.-k. Xu).
0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2012.02.095

C.-C. Mi et al. / Journal of Alloys and Compounds 525 (2012) 154–158
155
Scheme 1. Diagram illustrating the formation of PEI modified LaF3 nanoparticles.
great significance. Among them, the Eu3+ and Tb3+ ions doped inor-
ganic nanoparticles possess strong red and green luminescence,
respectively, and their applications in biological and medical field
have received much attention [32,33]. There are few reports on
the synthesis of rare earth doped fluoride materials, such as CeF3
and NaYF4 by microwave hydrothermal method [34,35] and syn-
thesis of LaF3 superfine powder from La(CH3COO)3 and NH4F by
microwave heating method [36]. In this work, one-pot synthesis
and modification of LaF3 nanoparticles via microwave solvother-
mal method was reported for the first time and PEI was used for
surface functionalization of the nanoparticles during preparation.
By this novel one-pot method, a four-color system was obtained
through controlling the doping amount of Eu3+ and Tb3+ ions in
the LaF3 matrix. X-ray diffraction (XRD), transmission electron
microscopy (TEM), photoluminescence (PL) and Fourier transform
infrared spectroscopy (FT-IR) were used to characterize the as-
synthesized nanoparticles.
The diagram for the formation of LaF3 nanoparticles was shown
in Scheme 1. During the experiment, a mixture of Ln(NO3)3·nH2O
and NaF were dispersed in ethylene glycol. With the elevating of
temperature, rare earth ions gradually reacted with F− ions to form
the tiny particles. After kept at the set temperature for a certain
time, the tiny nanoparticles grew up to single phase nanocrystals,
and could emit various luminescence colors such as green, yel-
low, orange and red when different rare earth ions were doped. In
addition, PEI was used to cover the surface of the nanoparticles to
improve their water-solubility, so finally the amino functionalized
hydrophilic nanoparticles were obtained.
Fig. 1. TEM (a), SAED (b) and XRD (c) patterns of the as-prepared LaF3:Ce,Tb
nanoparticles.
Preparation of LaF3:Eu nanoparticles was similar to that of LaF3:Ce,Tb. First,
La(NO3)3·6H2O (0.3900 g) was mixed with Eu(NO3)3 (2.25 mL, 0.2 mol·L−1) and NaF
(0.1890 g) in ethylene glycol. The solution was also stirred to be homogeneous and
treated at 150 ◦C for 30 min. The subsequent steps to obtain the products was carried
out just as procedure mentioned above for the preparation of LaF3:Ce,Tb nanopar-
ticles.
3. Results and discussion
3.1. Morphology and nanostructure of the nanoparticles
The morphology and electron diffraction patterns of the as-
prepared nanoparticles was characterized on a TECNAI G2 20
transmission electron microscope (TEM, FEI Ltd., USA), with an
accelerating voltage of 200 kV. Fig. 1a and b show the TEM
image and selected area electron diffraction (SAED) pattern of
as-synthesized LaF3:Ce,Tb nanoparticles. The nanoparticles are
uniform in shape with an average diameter of 12 nm. Size distribu-
tion of the nanoparticles showed that the average size was 15 nm,
2. Experimental
The LaF3 nanoparticles were synthesized with a MD6C-4H Microwave Digestion
System (Ying An Mei Cheng Company, Beijing China). Reagents used in the exper-
iment, such as ethylene glycol, sodium fluoride (NaF), lanthanum chloride (LaCl3),
cerium chloride heptahydrate (CeCl3·7H2O), terbium nitrate (Tb(NO3)3·6H2O),
europium oxide (Eu2O3), were purchased from Sinopharm Chemical Reagent Co.,
Ltd. (Shanghai, China). All of the reagents were of analytical grade and used as
received without further purification. PEI10000 (50%) was purchased from Sigma
(U.S.A.). Europium nitrate (Eu(NO3)3) was obtained by dissolving and evaporat-
ing europium oxide in nitric acid. Triple distilled water was used through out the
experiment.
Typically, for the synthesis of LaF3:Ce,Tb nanoparticles, La(NO3)3·6H2O
(0.4179 g), CeCl3·7H2O (0.0559 g), Tb(NO3)3·6H2O (0.1019 g) and NaF (0.1890 g)
were mixed in ethylene glycol (20 mL). The mixture was thoroughly stirred and
sonicated to form a homogeneous solution, then transferred to an autoclave, sealed
and kept at 120 ◦C for 30 min. After reaction, the autoclave was allowed to cool nat-
urally to room temperature and then opened. The final products were collected by
centrifugation at 8500 rpm for 5 min, washed with ethanol and water to remove any
remnants, and dried at 60 ◦C for 1 h. Preparation of the nanoparticles modified with
PEI was nearly same as the procedure mentioned above, except that 500 ␮L PEI was
used during the synthesis.
Fig. 2. Size distribution the as-prepared LaF3:Ce,Tb nanoparticles.

156
C.-C. Mi et al. / Journal of Alloys and Compounds 525 (2012) 154–158
Fig. 5. The excitation (a) and emission (b) spectra of as-prepared LaF3:Eu nanopar-
ticles.
Ce3+ and Tb3+ ions have been effectively doped into the crystal lat-
tices of the hexagonal LaF3 phase. According to the XRD data, we
could calculate the size of as-synthesized LaF3:Ce,Tb nanoparticles
through Debye–Scherrer equation (D = Kꢀ/ˇ cosꢁ, where D is the
diameter of the particle, K is normally equal to 0.89 and ꢀ is the
wavelength of X-ray radiation, ˇ is FWHM of diffraction peak and ꢁ
is the Bragg diffraction angle). Based on the calculation, the average
size of the prepared LaF3:Ce,Tb nanoparticles was 12 nm, which is
consistent with the result of TEM image shown in Fig. 1a.
As we all known, the doped ions will directly affect the
optical properties of materials so it is easy to achieve multi-
color emission under a single excitation by changing doped rare
earth ions. Here, by using Eu3+ ions to replace Ce3+ and Tb3+
ions, the LaF3:Eu nanoparticles were synthesized under the same
method. Morphology, structure and optical properties of the Eu3+
doped nanoparticles were also obtained from TEM, XRD and PL,
respectively. Fig. 3a is the TEM image of as-prepared LaF3:Eu
nanoparticles. It is clear that all of the nanoparticles are nearly
spherical, and the average size of the nanoparticles is about 13 nm.
The diffraction rings in Fig. 3b shows that the nanoparticles are
highly crystalline. Fig. 3c represents the XRD pattern of the as-
prepared LaF3:Eu nanoparticles and the standard pattern. It shows
that Eu3+ doped LaF3 nanoparticles were still phase-pure and pos-
sessed a hexagonal structure. Meanwhile, after calculation using
the Debye–Scherrer equation, the size of the as-prepared LaF3:Eu
nanoparticles was 13 nm. From the TEM images and XRD pat-
terns of the as-synthesizedCe3+/Tb3+ and Eu3+ doped nanoparticles,
Fig. 3. TEM (a), SAED (b) and XRD (c) patterns of the as-prepared LaF3:Eu nanopar-
ticles.
which was basically consistent with the TEM (Fig. 2). SAED pattern
in Fig. 1b shows a series of clear diffraction rings demonstrating
that the nanoparticles are highly crystalline.
The crystal phase of the as-prepared products was identified
by an X’Pert Pro diffractometer (Panalytical Co., Holland) using
◦
Cu K␣ radiation (ꢀ = 1.5406 A), at a scan rate of 8 min−1. Fig. 1c
shows the XRD pattern of the as-synthesized LaF3:Ce,Tb nanopar-
ticles and the standard pattern. It is clear that all the diffraction
peaks can be readily indexed to the hexagonal LaF3 (JCPDS card
01-076-0510). The results showed that the obtained nanoparticles
remained phase-pure LaF3 after Ce3+ and Tb3+ were doped at the
concentrations up to 10% and 15%, respectively, suggesting that the
˚
Fig. 4. The excitation (a) and emission (b) spectra of as-prepared LaF3:Ce,Tb
Fig. 6. Multicolor luminescent photograph of LaF3 nanoparticles doped with (a) 15
nanoparticles.
mol%Tb, (b) 25 mol% Tb, 5 mol% Eu, (c) 15 mol% Tb, 5 mol% Eu and (d) 30 mol% Eu.

C.-C. Mi et al. / Journal of Alloys and Compounds 525 (2012) 154–158
157
Fig. 7. TEM image (a) and IR spectrum (b) of LaF3:Ce,Tb nanoparticles one-pot synthesized and modified with PEI.
it can be concluded that there is neither obvious change in the
particle size, nor distinction in the crystal structure of the products
when different rare earth ions were doped.
emission due to the presence of two dopants with different molar
ratios.
3.3. One-pot modification of the nanoparticles
3.2. Optical properties of the nanoparticles
For the future biological application as a probe, the luminescent
nanoparticles usually need to be modified with amino or carboxyl
groups. Here, the synthesized nanoparticles were directly modi-
fied with PEI during the synthetic procedure. TEM image and IR
spectrum of the modified nanoparticles were presented in Fig. 7.
PEI is a long chain polymer, which can be adsorbed on the sur-
face of nanoparticles during preparation. It plays an important role
as a modifier. In addition, the steric hindrance from long chain of
PEI molecules can effectively inhibit the aggregation of nanoparti-
cles. After modification, the size of the nanoparticles was increased
slightly, reaching to 15–25 nm (Fig. 7a). Fig. 7b shows the IR spec-
trum of PEI modified nanoparticles. Peaks from 3517–3341 cm−1
and 1659–1380 cm−1 are attributed to the stretching and bending
vibration of NH2 groups, respectively, suggesting successful coating
of nanoparticles by PEI.
Photoluminescence excitation and emission spectra of the
nanoparticles were acquired on a LS-55 luminescence spectrom-
eter (Perkin-Elmer Co., USA). Digital photos were taken by a Nikon
camera from the samples dispersed in deionized water at a concen-
tration of 0.05 wt%. Fig. 4 shows the photoluminescence spectra of
the as-synthesized LaF3:Ce,Tb nanoparticles at room temperature.
In the excitation spectrum (Fig. 4a), the narrow peak at 252 nm
was contributed by 4f–5d transition of Ce3+ ions [37]. The emis-
sion spectrum located from 450 to 650 nm was mainly composed
of four peaks (Fig. 4b), which were separately corresponding to
the f-f transitions of Tb3+ from the excited 5D4 levels to the 7FJ
(J = 6, 5, 4, 3) levels [38,39]. After calculated, the quantum yield of
the synthesized nanoparticles was 60%, luminescence lifetime was
5.1 ms.
The excitation and emission spectra of as-synthesized LaF3:Eu
nanocrystals (monitored at 593 nm and 392 nm, respectively) are
shown in Fig. 5. The major excitation peak at 392 nm in Fig. 5a is cor-
responding to the 7F0–5L6 transition of Eu3+ ions. The weak peak
at 249 nm can be assigned to the charge transfer band between
Eu3+ and the surrounding oxygen anions, indicating that there is
a small amount of oxygen atoms existing within the crystal [40].
Fig. 5b is the emission spectrum of LaF3:Eu nanocrystals moni-
tored at 392 nm. The typical emission peaks at 593 and 615 nm were
attributed to 5D0–7F1 and 5D0–7F2 transition of Eu3+ ions, respec-
tively [32,41]. Through measurement, the luminescence lifetime of
the LaF3:Eu nanocrystals was 5.2 ms.
Recently, many studies have been performed for the fabrication
of multicolor nanoparticles, due to their important role in mul-
timodal bio-imaging [42]. Various colors can be achieved by the
adjustment of doped rare earth ions and their concentrations. Dur-
ing this experiment, four colors phosphors were obtained by doping
Ce3+/Tb3+ and Eu3+ ions into the crystal lattices of the hexagonal
LaF3 phase and controlling their relative amounts. The multicolor
luminescent photograph of Ce3+/Tb3+ and Eu3+ ions doped LaF3
nanoparticles with different concentrations taken at 252 nm exci-
tation is shown in Fig. 6. It is clearly shown that when doped with
Tb3+, the synthesized LaF3 nanoparticles showed bright green lumi-
nescence. The luminescence was changed to red when Eu3+ was
doped for the replacement of Tb3+ ions. Meanwhile, nanoparticles
co-doped with both Tb3+ and Eu3+ ions exhibit yellow and orange
4. Conclusions
In summary, we have demonstrated a novel method for the
preparation of Ce3+/Tb3+ and Eu3+ ions doped LaF3 nanoparticles.
The as-prepared nanoparticles were absolute hexagonal crystals,
with an average size of about 12 nm. A four-color system was
obtained through doping Ce3+/Tb3+ and Eu3+ ions into the crystal
lattices of hexagonal phase LaF3 and adjusting their doping con-
centrations. This system can not only be made water-soluble and
bearing amino groups for conjugation with antibodies, but also be
excited by one light to emit different visible colors, enabling its
future applications in biological labeling and imaging.
Acknowledgments
This work was funded by the National Science Foundation of
China (Grant No. 20875011), and the support from Northeastern
University on PhD students. CBM would like to thank the financial
support from the US National Science Foundation (DMR-0847758,
CBET-0854414, and CBET-0854465), National Institutes of Health
(R21EB009909-01A1, R03AR056848-01, and R01HL092526-01A2),
and Oklahoma Center for the Advancement of Science and Technol-
ogy (HR11-006#7823).

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