Water-soluble Yb3+, Tm3+ codoped NaYF4 nanoparticles: Synthesis, characteristics and bioimaging

Article abstract of DOI:10.1016/j.jallcom.2011.08.043

Water soluble NaYF₄ nanocrystals codoped with 20 mol% Yb ³⁺, 0.5 mol% Tm³⁺ were prepared by a facile solvothermal approach using polyvinylpyrrolidone (PVP) as a surfactant. The upconversion NaYF₄ nanocrystals were pure cubic phase with an average size of ～40 nm. They could be well redispersed in water to form a clearly transparent solution without obvious precipitation. With the excitation of a 980-nm diode laser, the nanocrystal solution presents bright violet and blue upconversion luminescence. These upconversion nanoparticles (UCNPs) were incubated with HeLa cells at 37 °C for 24 h, and bright blue upconversion luminescence were observed from the UCNPs endocytosed into the HeLa cells on a microscope equipped with a 980-nm fiber laser. These results indicated that the UCNPs had potential applications for biological imaging as luminescent probes.

Full text of DOI:10.1016/j.jallcom.2011.08.043

Journal of Alloys and Compounds 511 (2012) 70–73
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
3+
3+
Water-soluble Yb , Tm codoped NaYF nanoparticles: Synthesis,
4
characteristics and bioimaging
∗
∗
Huan Chen, Xuesong Zhai, Duo Li, Lili Wang, Dan Zhao , Weiping Qin
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science & Engineering, Jilin University, Changchun 130012, China
a r t i c l e i n f o
a b s t r a c t
Water soluble NaYF4 nanocrystals codoped with 20 mol% Yb3+, 0.5 mol% Tm3+ were prepared by a
facile solvothermal approach using polyvinylpyrrolidone (PVP) as a surfactant. The upconversion NaYF4
nanocrystals were pure cubic phase with an average size of ∼40 nm. They could be well redispersed
in water to form a clearly transparent solution without obvious precipitation. With the excitation of a
Article history:
Received 15 March 2011
Received in revised form 8 August 2011
Accepted 9 August 2011
Available online 7 September 2011
9
80-nm diode laser, the nanocrystal solution presents bright violet and blue upconversion luminescence.
◦
These upconversion nanoparticles (UCNPs) were incubated with HeLa cells at 37 C for 24 h, and bright
blue upconversion luminescence were observed from the UCNPs endocytosed into the HeLa cells on a
microscope equipped with a 980-nm fiber laser. These results indicated that the UCNPs had potential
applications for biological imaging as luminescent probes.
Keywords:
NaYF4
Upconversion
Water soluble
Nanoparticles
© 2011 Elsevier B.V. All rights reserved.
1
. Introduction
Upconversion (UC) is a nonlinear optical process that long-
However, the RE ions codoped NaYF4 materials should have the
following properties in order to meet the demands as lumines-
cent probes: (1) small size of sub-50 nm; (2) high dispersibility
in water; (3) good biocompatibility; (4) strong UC luminescence
in aqueous solutions [16,17]. It is worth noting that good water
solubility and strong UC luminescence are especially the key fac-
tors. Li’s group prepared oleic acid (OA)-capped UCNCs and used
Lemieux-von Rudloff reagent to oxide the C C double bond of OA
and then obtained the water soluble nanocrystals [18]. Zhang’s
group synthesized silica-coated polyvinylpyrrolidone (PVP)/NaYF4
nanocrystals which could dispersed in some of the most com-
monly used solvents, from weakly polar chloroform to strongly
polar water [19]. However, it is still a challenge to synthesize UCNPs
used as luminescent probes with good aqueous dispersibility and
strong UC luminescence intensity.
wavelength light can be converted to short-wavelength light by
absorbing two or more pump photons [1]. UC materials have been
widely used in many fields, including all-solid-state laser, full color
display, optical communication, bio-labeling and so on [2,3]. Very
recently, lanthanide (Ln)-doped UC nanoparticles (UCNPs) have
attracted great attention as luminescent probes for bioimaging
applications due to their remarkable advantages, such as large
anti-Stokes shifts, narrow emission bandwidth, long fluorescence
lifetime, and excellent photostability [4–7]. Compared with down-
conversion probes, such as fluorescent organic dyes and quantum
dots, the Ln-doped UC probes excited with near-infrared (NIR)
light suffer much less the background autofluorescence that is usu-
ally encountered by ultraviolet (UV) pumping [8,9]. It also reaps
the benefit of deep penetration into biological tissue by using a
pumping light in the biological optical window of 800–1200 nm
In this paper, we report on water soluble, cubic phase NaYF :
4
20 mol% Yb3+, 0.5 mol% Tm UC nanocrystals prepared by a facile
solvothermal method using PVP as the surfactant. These nanocrys-
tals have an average size of ∼40 nm with a narrow size distribution
and can be well dispersed in water to form a clearly transparent
solution. Under 980 nm excitation, the solution shows bright blue
and violet emission that can be clearly seen with naked eye in day
light. The HeLa cells could be labeled using these UCNPs as lumines-
cent probes. These results indicated that the UCNPs had potential
applications in biological field as luminescent probes.
3+
[
10,11]. Furthermore, compared with high-energy UV excitation,
low-energy NIR has the minimum photo-damage to the biological
specimens [12].
NaYF has been reported as the most efficient Ln-doped UC host
4
material [13]. Under the 980 nm excitation, visible or UV emis-
3+
3+
sions can be obtained via codoping Yb ions as sensitizers, Tm
ions, Er3+ ions, or other rare earth (RE) ions as activators [14,15].
2
.
Experimental
∗ Corresponding authors. Tel.: +86 0431 85168241 8325;
fax: +86 0431 85168241 8325.
The initial chemicals, including Y2O3, Yb2O3, Tm2O3 (all with purity > 99.99%,
E-mail addresses: dzhao@jlu.edu.cn (D. Zhao), wpqin@jlu.edu.cn (W. Qin).
Shanghai Shabo Chemical Technology Co., Ltd. China), hydrochloric acid (HCl,
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.08.043

H. Chen et al. / Journal of Alloys and Compounds 511 (2012) 70–73
71
3
7 wt%), ethylene glycol (EG, AR), polyvinylpyrrolidone K-30 (PVP, 58,000 g/mol)
and sodium fluoride (NaF, AR), were used without further purification.
2.1. Synthesis of sample
Y2O3, Yb2O3, and Tm2O3 were dissolved in dilute hydrochloric acid to form
RECl3 (RE = Y, Yb, and Tm) solutions. After excess HCl and water were completely
evaporated, the as-produced RECl3 powders were dispersed in EG. 1 mmol RECl3
(
79.5 mol% YCl3, 20 mol% YbCl3, and 0.5 mol% TmCl3) solutions were mixed together
3
+
and 0.5 g PVP K-30 was added under stirring (the exact ratio of PVP to RE is 1/116).
This solution was labeled as solution A. 0.21 g NaF (the molecular ratio of F to RE
−
3+
is 5/1) was dissolved in EG, then this NaF solution was added dropwise to the solu-
tion A. This mixture was kept stirring for at least 30 min, then it was transferred into
◦
a polytetrafluoroethylene autoclave and reacted at 150 C for 24 h. After cooling to
room temperature, the products was obtained by centrifugation and washed with
deionized water and ethanol. To remove the residual NaF completely, the products
should be washed with warm deionized water for several times. A part of precipi-
◦
tation was dried at 60 C for 2 h and the other was redispersed in deionized water
by ultrasonic treatment to get transparent solution.
2.2. Cell labeling with UCNPs
◦
The HeLa cells were cultured (at 37 C, 5% CO2) on glass chamber slides in PRMI
Roswell Park Memorial Institute) 1640 medium containing 10% fetal bovine serum
and 1% penicillin/streptomycin overnight in an incubator (Sanyo MCO-20AIC) for
4 h. The cells were gently washed three times with phosphate buffer solution (PBS).
Fig. 1. (a) XRD pattern of the as-prepared UCNPs and standard data of cubic-phase
NaYF4 (JCPDS No. 6-342).
(
2
Then the HeLa cells were incubated with 300 ␮g/mL UCNPs for another 24 h in CO2
incubator. After washing three times with PBS to remove excess UCNPs, the cells
were imaged by an Olympus IX71 microscope equipped with a 980-nm fiber laser.
2.3. Measurements
The XRD analysis was carried out with a powder diffractometer (Model Rigaku
RU-200b), using Ni-filtered Cu K␣ radiation (ꢀ = 1.5406 A˚ ) with 200 mA current and
5
0 kV voltage across the tube to generate powerful X-ray. The XRD measurement
◦
−1
◦
was performed at a scan rate of 18 min and step size of 0.02 . The morphology of
the nanocrystals was investigated by FE-SEM (JEOL, JSM-6330F) and TEM (H-8100
2
00KV). A 2 W 980 nm continuous wave (CW) diode laser was used as the excitation
source to study the UC luminescence of the sample. Their UC emission spectra were
recorded with a fluorescence spectrometer (Hitachi F-4500). The UC luminescent
pictures of the aqueous solution were taken by a Nikon digital camera (D300s). All
the measurements were performed at room temperature.
Fig. 3. (a) Molecular structure of PVP. (b) Scheme of the surface of PVP-modified
UCNPs. (c) FTIR spectrum of the PVP-modified UCNPs.
3
. Results and discussion
The XRD pattern of the NaYF : 20 mol% Yb3+, 0.5 mol% Tm3+
sample is shown in Fig. 1, where the diffraction peaks are in good
4
cubic phase Yb3+–Tm3+ codoped NaYF4 nanocrystals. In the SEM
image, we can see that the nanoparticles have small size contribu-
tion. In TEM image, it can be confirmed that the as-prepared UCNPs
have an average size of ∼40 nm. The small size of the nanoparticles
sufficiently satisfies the demands of bio-applications as lumines-
cent probes.
The molecular structure of PVP is given in Fig. 3(a). It is known
that both the nitrogen and oxygen atoms have lone pair electrons
which can combine with the empty orbital of metal ions. That
agreement with the data of cubic-phase NaYF nanocrystals (JCPDS
4
No. 6-342, ˛ = 5.448 A˚ ), and there no other peaks of impurities has
been detected. It indicates that the products is single-phase pure
cubic NaYF crystals and demonstrates the highly crystalline nature
4
of the nanocrystals. Although the as-prepared sample is cubic phase
which has been reported to be less efficient than hexagonal-phase
NaYF , the UCNPs we synthesized still has strong UC luminescence
4
intensity. Fig. 2(a) and (b) shows the SEM and TEM images of the
Fig. 2. (a) SEM and (b) TEM images of the UCNPs.

7
2
H. Chen et al. / Journal of Alloys and Compounds 511 (2012) 70–73
Fig. 4. (a) Upconversion spectrum of UCNPs under the excitation of 980 nm, insert: (left) photograph of the UCNPs dispersed in deionized water. (Right) Upconversion
3
+
3+
luminescence photograph of the same solution. (b) Energy level diagrams of Yb ions and Tm ions and the possible upconversion processes.
1
3
1
3
1
3
means the PVP molecules can well chelate with RE ions [20]. In
the reaction process, the PVP acts as a chelating agent and could
coordinate with RE ions. After the formation of UCNPs, it can act as
an amphiphilic surfactant and drive the UCNPs dispersing in ethy-
lene glycol or water. In our experiment, it can be supposed that the
remains of PVP on the surface of UCNPs can form hydrogen bond
with water molecules, as shown in Fig. 3(b). Presumably, this is why
the UCNPs could be well dispersed in water and kept transparent for
months. The photos of the UCNPs in water are shown in the insert of
Fig. 4(a). The capping ligands on the surfaces of UCNPs were exam-
ined by Fourier transform infrared (FTIR) spectrum, as shown in
Fig. 3(c). Some absorption bands in this spectrum confirm the pres-
ence of PVP molecules on the surfaces of UCNPs. The bands centered
D → F , G → H , and G → F , respectively. The UC mecha-
2
4
4
6
4
4
3
+
3+
nism of NIR-to-visible/UV in this Yb , Tm codoped NaYF system
was studied [21]. Fig. 4(b) describes schematically possible upcon-
4
3+
3+
verted processes in energy level diagrams of Yb and Tm ions.
Under the excitation of 980 nm NIR, Yb3+ ions, served as sensitiz-
3+
ers, absorb NIR photons and transfer the energy to Tm ions to
3
3
1
3+
populate the excited states ( H , F , and G ) of Tm ions. The
5
2
3
4
cross-relaxation process of 3F2, 3 → H (Tm3+): H → D (Tm
3
1
3+
)
6
4
2
1
may play the most important role in populating D . Thereafter,
2
1
2
2
3+
the state I can be populated by the process of F → F (Yb ):
6
5/2
7/2
1
1
3+
3+
3+
D → I (Tm ). In our experiments, the energy transfer from Yb
2
6
3+
to Tm is efficient, which resulted in effective population of Tm
ions in the high-energy state of G . From the spectrum in Fig. 4(a),
1
4
−1
at 2958, 2923, and 2852 cm are associated with the stretching
we can see that the strongest peak is centered at 479 nm, and there-
fore the UCNPs dispersed aqueous solution exhibits bright blue
luminescence under the excitation of 980 nm NIR, as shown in the
insert of Fig. 4(a). The excellent transparency also can be seen from
these photos. Through the solution the background words can be
well distinguished.
vibration of methyl (–CH ), the asymmetric (␯as), and symmet-
3
ric (␯s) stretching vibrations of methylene (–CH ), respectively.
2
−1
The band at 1290 cm originates from the stretching vibration of
carbon nitrogen bond (C–N) and the band at 1664 cm
−
1
orig-
inates from the vibration of carbon oxygen bond (C O). The
nitrogen and oxygen atoms can coordinate with the metal ions,
leading to PVP molecules adhered to the surfaces of UCNPs. It
For further investigation of their applicability as luminescent
◦
probes, the UCNPs were incubated with HeLa cells at 37 C for 24 h.
−
1
should be noticed that the C O groups (1664 cm ) on the sur-
faces of UCNPs can form hydrogen bonds with water molecules.
After washing with PBS to remove excess UCNPs, the living cells
were imaged on an Olympus IX71 microscope equipped with a 980-
nm NIR laser. Fig. 5(a) shows the bright-field images of the living
HeLa cells that endocytosed UCNPs. We can observe that the cell
morphology is good [22]. Then the sample was excited in situ by the
980-nm laser. We can clearly see the bright upconversion lumines-
cence in the image of Fig. 5(b). (For interpretation of the references
to color in this figure, the reader is referred to the web version of the
article.) The overlay images of the luminescence and the HeLa cells
are shown in Fig. 5(c). We can see that the blue luminescence well
overlaps the cells, which confirms that the UCNPs are successfully
used as luminescent probes to label the cancer cells with strong UC
luminescence. These results imply that the UCNPs are biocompat-
ible and can be used as potential luminescent probes for biological
imaging.
−1
The strong band around 3428 cm
can be attributed to the
O–H stretching vibrations. Above analysis indicates that PVP
molecules are well attached on the surface of the UCNPs, and
therefore leading to the well dispersibility of nanocrystals in the
solvents.
Both the as-prepared nanocrystals powders and their aqueous
solution can emit visible and UV luminescence under the NIR exci-
tation of 980 nm. We detect the UC spectrum using the 980 nm
continuous wave diode laser as pumping source. The power of the
focused laser is 140 mW. As shown in Fig. 4(a), there are character-
istic peaks of Tm3+ ions in the UC spectrum of the aqueous solution.
These peaks which center at 291, 350, 362, 450, 479, and 648 nm
are attributed to the transitions of 1I6 → H , I → F , D → H ,
3
1
3
1
3
6
6
4
2
6
Fig. 5. (a) Bright-field image and (b) luminescence image under the excitation of 980 nm and (c) the overlay image of HeLa cells labeled with the UCNPs.

H. Chen et al. / Journal of Alloys and Compounds 511 (2012) 70–73
73
4
. Conclusions
In conclusion, pure cubic-phase NaYF : 20 mol% Yb, 0.5 mol%
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