as the shell were synthesized successfully using a simple urea homogeneous precipitation method.. The up-conversion emission intensity was enhanced significantly comparing to that withou
Trang 1O R I G I N A L P A P E R
particles with enhanced optical properties
Hong Ha Thi Vu•Timur Sh Atabaev•
Nang Dinh Nguyen•Yoon-Hwae Hwang •
Hyung-Kook Kim
Received: 6 January 2014 / Accepted: 29 April 2014
Springer Science+Business Media New York 2014
Abstract Bifunctional magneto-optical nanocomposites
with Fe3O4nanoparticles as a core and erbium and lithium
codoped gadolinium (Gd2O3:Er3?, Li?) as the shell were
synthesized successfully using a simple urea homogeneous
precipitation method The fabricated Fe3O4@Gd2O3:Er3?,
Li?particles were characterized by X-ray diffraction, field
emission scanning electron microscopy, transmission
electron microscopy, photoluminescence spectroscopy and
quantum design vibrating sample magnetometry The
up-conversion emission intensity was enhanced significantly
comparing to that without Li? ions These bifunctional
composites are expected to be potentially applied for drug
delivery, cell separation and bioimaging
Keywords Luuminescence Bifunctional
nanocomposites Core–shell structure
1 Introduction
Over the past decade, bifunctional composite materials with both fluorescent and magnetic properties have been applied widely in biological and chemical fields, such as biomedical multitasking, magnetic biological separation, targeted-drug delivery and cell imaging [1 7] Iron oxide is considered as an ideal candidate for bio-applications due to their special magnetic properties, lack of toxicity and good biocompatibility [8 10] Recently, a range of bifunctional magnetic properties with luminescence nanophosphors with Fe3O4 nanoparticles as the core and the phosphor materials as the shell have been studied [10–16] Yu.et al [17] synthesized bifunctional magnetic-optical Fe3O4 @-SiO2/Y2O3:Yb3?, Er3? particles using a layer-by-layer method Wang et al [18] prepared Fe/Fe3O4@Y2O3:Eu nanocomposites and Yang et al [19] synthesized multi-functional Fe@C@Y2O3:Eu3? nanoparticles using a solvothermal method The synthesis process in these methods, however, was relatively complex and expensive Down-conversion materials display some intrinsic lim-itations (e.g the quantum dots are toxic and organic dyes exhibit broad absorption and emission bands and low photostability), whereas, up-conversion (UPC) materials have superior photostability and low toxicity Among the various of UPC materials, gadolinium oxide (Gd2O3 )-doped Er3?ions have attracted increasing attention Gd2O3
is a good host matrix for the UPC luminescence process because of its interesting physical properties, such as chemical durability, high melting point (*2,320C), thermal stability and low phonon energy (phonon cutoff
*600 cm-1) Also the Gd2O3 doped with Er3? has long been reported to be a strong green phosphor [20] On the other hand, Gd2O3 based phosphors show a weaker sta-bility against atmospheric H2O and CO2, which are known
H H T Vu T Sh Atabaev Y.-H Hwang (&)
H.-K Kim ( &)
Department of Nanomaterials Engineering and BK 21 Nano
Fusion Technology Division, Pusan National University,
Miryang 627-706, Republic of Korea
e-mail: yhwang@pusan.ac.kr
H.-K Kim
e-mail: hkkim@pusan.ac.kr
N D Nguyen
Department of Semiconducting Nanomaterials and Devices,
Faculty of Engineering Physics and Nanotechnology, University
of Engineering and Technology, Vietnam National University
Hanoi, 144 Xuan Thuy Street, Cau Giay, Hanoi, Vietnam
DOI 10.1007/s10971-014-3382-9
Trang 2as luminescence killers [21] Therefore, it is important to
enhance the luminescent properties of Gd2O3-based
phosphors
It is well know that the UPC emission intensity of
lumi-nescent materials is dependent on their intra 4f transition
probabilities, which are significantly affected by their crystal
field symmetry [22] Therefore, incorporation of additional
metal ions to the phosphors can be considered an effective
way to improve the luminescence Among them, the Li?ion
has a smaller cationic radius than other ions Therefore, it can
be doped easily into the host lattice, either substitutionally or
interstitially, and break the symmetry of the crystal field
around the rare-earth element, leading to an increase in
photoluminescence intensity [22–24] For example, Chen
et al reported the enhancement of UPC radiation in
Y2O3:Yb3?/Er3? nanocrystals by codoping with Li? ions
[25] Qian Cheng et al reported a 47 fold increase in visible
green UPC emission in Yb3? and Er3? codoped NaGdF4
nanoparticles by introducing Li?ions [26]
This study reports the synthesis and properties of
bifunctional optical-magnetic Fe3O4@Gd2O3:Er3?:Li?
particles with Fe3O4nanoparticles as the core, and Gd2O3
codoped Er3? and Li? as the shell using a facile urea
homogeneous precipitation method The relationship
between fluorescent and magnetic properties is discussed
The results revealed a significant increase in the UPC
emission in shell Gd2O3:Er3? layer by introducing Li?
ions The combined fluorescent and magnetic properties of
the nanoparticles highlight their potential applications in
bioseparation, drug delivery and luminescence labeling
2 Experimental
2.1 Reagents
Ferrous chloride tetrahydrate (FeCl24H2O), ferric chloride
(FeCl3, 97 %), polyethylene glycol (PEG, MR = 4,600),
sodium acetate (CH3COONa, 99.0 %), ammonium
hydroxide (28–30 % NH3 basis), gadolinium (III) oxide
(Gd2O3, 99.99 %), erbium oxide (Er2O3, 99.99 %), nitric
acid (HNO3, 70 %), urea ((NH2)2CO, 99–100.5 %), and
ethanol were purchased from Sigma-Aldrich and used as
received Purified deionized (DI) water was used in all
experiments
2.2 Synthesis of Fe3O4nanoparticles
Fe3O4nanoparticles were prepared using the reported
co-precipitation method [26] Firstly, FeCl3 (1.625 g) and
FeCl24H2O (0.955 g) at 2:1 molar ratio were dissolved in
50 ml of DI water at 70C PEG can displace H2O, [OH-]
adhering to them through coordination bond or hydrogen bond, reducing the surface free energy, preventing agglomeration from small particles to big particles, and dispersing particles stably in base solution Therefore, PEG (1 g) was then added as a surfactant and the resulting mixture was stirred of 30 min until complete dissolution The iron solution source was added slowly to a NH4OH solution with vigorously stirring for 45 min The black
Fe3O4precipitates were separated and rinsed several times with DI water The precipitates were dried in a vacuum oven at 50C for 24 h, and Fe3O4 nanoparticles were finally obtained
2.3 Synthesis of bifunctional Fe3O4@Gd2O3:Er3?:Li? nanocomposites
Gd2O3codoped with Er3?and Li?phosphor was coated on the magnetic Fe3O4using an urea homogeneous precipita-tion method [27, 28] In a typical procedure, appropriate amount of Gd2O3, Er2O3and LiOH4H2O were dissolved in nitric acid to form 0.001 mol of sample with composition of
Gd2O3:1 % Er3?,10 % Li? At the same time 0.001 mol of another sample with composition of Gd2O3:1 % Er3?was prepared and used as a reference The solutions were dried at 70C for 1 day to remove the excess of nitric acid After cooling to room temperature, samples were mixed with DI water (40 ml) and urea (0.5 g), and stirred vigorously for
10 min to form a clear solutions Subsequently, Fe3O4
nanoparticles (70 mg for each sample) were then added to the above solutions The mixtures were sonicated for 30 min and heated to 90C for 5 h under vigorous stirring The resulting suspensions were centrifuged at 7,000 rpm for
45 min The precipitates were washed 3 times with ethanol and DI water and dried overnight at 75C under vacuum Finally, the precipitates were calcined at 700C in air for 1 h
to produce final structures
2.4 Characterization
The crystal phase of the prepared samples was character-ized by X-ray power diffraction (XRD, Bruker D8 Dis-cover) using Cu Ka (k = 0.15405 nm) radiation within the range of 20–60 2h The morphology and composition of the samples were examined by transmission electron microscopy (TEM, JEOL JEM-2100F) The UPC emission spectra of the samples were recorded using a Hitachi F-7000 spectrophotometer using a commercially available power adjustable NIR (975 nm) diode laser The magne-tization measurements were performed using a quantum design vibrating sample magnetometer (QD-VSM option
on a physical properties measurement machine PPMS 6000) All measurements were carried out at room
Trang 33 Results and discussion
3.1 Crystal structure and morphology
The synthetic procedure is schematically illustrated at
Scheme1 The Fe3O4nanoparticles were firstly prepared by
using a co-precipitation method as the core, and then Gd2O3
codoped with Er3?and Li?phosphors shell layer was coated
on surface of the Fe3O4nanoparticles The structure of the
synthesized samples was examined by XRD Figure1shows
the XRD patterns of Fe3O4, Fe3O4@Gd2O3:1 % Er3? and
Fe3O4@Gd2O3:1 % Er3?, 10 % Li?nanocomposites after
calcination at 700C The XRD peaks for Fe3O4 were
indexed to the face-centered (Fd3m space group) cubic
structure of Fe3O4 (JCPDS 19-0629) [29] In the case of
Fe3O4@Gd2O3:1 % Er3? and Fe3O4@Gd2O3:1 % Er3?,
10 % Li? nanocomposites exhibited the characteristic
Gd2O3diffraction peaks (JCPDS no 88-2165) and additional
peaks that coincide with the strongest {311} reflection peak
of the cubic face-centered Fe3O4(marked with black star)
No additional peaks from the doped components were
detected due to the low concentration of codopant ions
Furthermore, all the diffraction peaks of the samples were
intense and sharp, indicating that the final product with high
crystallinity was obtained using this method, which is very important for the luminescence properties of the composites The well-known Debye–Scherrer’s equation was used to calculate the mean crystallite sizes of the samples The cal-culated mean crystallite sizes of the composites without and with Li?codoping were approximately 46.77 and 51.47 nm, respectively, indicating an increase in crystallite size upon
Li?codoping
The morphology and structure of the obtained products were examined by TEM study Figure2 revealed a core– shell structure of the Fe3O4@Gd2O3:1 % Er3?, 10 % Li? particles The Fe3O4cores were black spheres with a mean size of approximately 30 nm, and the Gd2O3:1 % Er3?,
10 % Li?shell showed a gray color with a mean thickness
of 15 nm The similar core–shell structure was also observed in the case of Fe3O4@Gd2O3:1 % Er3? com-posites (not shown)
3.2 Optical properties
Figure3presents the room temperature up-conversion UPC luminescence spectra of Fe3O4@Gd2O3:1 % Er3? and Fe
3-O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites under a
975 nm NIR laser with a 125 mW excitation power Emis-sion bands at 482–494 nm (blue), 512–581 nm (green) and
Scheme 1 Illustration
of the formation of
Fe3O4@Gd2O3:1 % Er3?,
10 % Li?NPs
Fig 1 The XRD patterns of Fe3O4, Fe3O4@Gd2O3:1 % Er 3? and
Fe3O4@Gd2O3:1 % Er 3? , 10 % Li ? composites calcinated at 700 C
Fig 2 TEM images of Fe3O4@Gd2O3:1 % Er 3? :10 % Li ? compos-ites calcinated at 700 C
Trang 4650–691 nm (far-red) were assigned to the 4F7/2?4I15/2,
2H11/2; 4S3/2?4I15/2, and 4F9/2?4I15/2 transitions,
respectively The emission intensity of sample codoped with
Li? was enhanced strongly (3.3 times) compared to that
without Li? ions The enhanced UPC emission intensity
may be because doping with a small radius of Li?ion can
diffuse easily to sites in/near Er3?or to the interstitial sites
in the Gd2O3 host lattice Thus, the banned electric-dipole
transitions of Er3? can be allowed for Li?-tailored
phos-phors since the inversion symmetry can be destroyed easily
by the interstitial Li?ions in the Gd2O3host lattice [26] On
the other hand, Li? codoping improves the crystallinity
significantly and produces a larger crystallite size [22] Thus,
the distortion of local symmetry around Er3?and enhanced
crystallinity of the composites favors the enhanced
lumi-nescence emission
Figure4 shows the room temperature up-conversion
UPC luminescence emission spectra of Fe3O4@Gd2O3:1 %
Er3?, 10 % Li? composites in a powder under a 975 nm
NIR laser at excitation powers ranging from 20 to
160 mW The overall emission intensity was increased
with increasing of the laser output power It is well-known
that the intensity of UPC photoluminescence, IUPC, for an
unsaturated mechanism is proportional to the power of n of
the excitation intensity, IEx:
IUPC/ In
Ex
where n = 2, 3,… is the number of pump photons absorbed
per upconverted photon emitted A plot of logIUPC versus
logIExyielded a straight line with a slope n The slopes were
equal to 1.57, 1.84, and 1.56 for blue (4F7/2?4I15/2), green
respectively, which are close to 2 This suggests that two-photons were needed to populate the higher lying levels
of Er3?
3.3 Magnetic properties
The magnetic properties of the pure Fe3O4and Fe3O4
@-Gd2O3:1 % Er3?:10 % Li? composite particles were examined using a QD-VSM at room temperature as it shown in Fig.5 Both samples displayed super-paramag-netic properties The saturation magnetization (Ms) value
of pure Fe3O4was equal to 44 emu/g After coating with the phosphor shell layer, the saturation magnetic value decreased to 4.89 emu/g This value was much lower than for pure Fe3O4 due to paramagnetic nature of the shell coating The inset in Fig.5b shows the magnetic separation
of an aqueous dispersion of the Fe3O4@Gd2O3:1 % Er3?,
10 % Li? nanocomposites The sample can be quickly directed and accumulated to the side of the glass vial near the magnet This suggests that the nanocomposite particles exhibit both magnetic and optical properties Therefore, sinthesized Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nano-composites have promising potential in many areas, including targeting, separation, visual tracking, etc
4 Conclusions
Bifunctional nanocomposites with Fe3O4 particles as the core and Er3?, Li?codoped Gd2O3as the shell were syn-thesized successfully using a facile and inexpensive urea
Fig 3 Up-conversion luminescence spectra of Fe3O4@Gd2O3:1 %
Er3? and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? composites excited
with a 975 nm NIR laser at 125 mW pump power
Fig 4 Dependence of upconversion emission intensity of Fe3O4
@-Gd2O3:1 % Er 3? , 10 % Li ? sample on the excitation power
Trang 5luminescence was enhanced significantly due to Li?
co-doping Strong green emission due to the 2H11/2?4I15/2
transition in Er3? was observed upon 975 nm NIR laser
excitation of the sample The magnetic properties of the
Fe3O4@Gd2O3:1 % Er3?, 10 % Li?composites were also
evaluated These bifunctional nanocomposites are expected
to have promising applications in fluorescence labeling,
drug delivery, cell separation and diagnostic analysis
Acknowledgments This work was supported by the National
Research Foundation of Korea (Grant No 2012R1A1B3001357).
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Fig 5 Magnetic hysteresis loops of Fe3O4 (a) and Fe3O4@Gd
2-O3:1 % Er 3? :10 % Li ? (b) nanoparticles measured at 300 K The
insets show the photographs of magnetic Fe3O4@Gd2O3:1 %
Er 3? :10 % Li ? nanocomposites dispersed in aqueous solution (left) without and (right) with external magnetic field