Reduced photon quenching in Ce-doped NaYF
4
:Yb/Ho
upconversion nanoparticles with core/shell structure
Shuai Ye (叶 帅), Jun Song (宋 军)*, Dong Wang (王 东), Yuliang Tian (田宇亮),
Junle Qu (屈军乐)**, and Hanben Niu (牛憨笨)
Key Laboratory of Optoelectronic Devices and Systems of Guangdong Province,
College of Optoelectronic Engineering, Shenzhen University, Shenzhen 518060, China
*Corresponding author: song jun@szu.edu.cn; **corresponding author: jlqu@szu.edu.cn
Received October 12, 2015; accepted December 11, 2015; posted online February 1, 2016
The use of red light or near-infrared radiation as a luminescent probe for in vivo bio imaging is crucial in order to
restrict the strong absorption of short-wavelength light below 600 nm in tissue. It is demonstrated that the emission
color of Yb/Ho codoped NaYF
4
nanoparticles can be tuned from green to red by incorporating Ce
3þ
ions. However,
compared with that of the NaYF
4
:Yb∕Ho nanoparticles, the photoluminescence intensity of the Ce
3þ
-tridoped
NaYF
4
:Yb∕Ho nanoparticles is drastically reduced. In this work, Ce
3þ
-incorporated core/shell NaYF
4
:
Yb
3þ
50%@NaYF
4
:Ho
3þ
0.5% nanoparticles are prepared. A strong red emission and a high-intensity ratio between
the red emission and green emission are obtained in these upconversion nanoparticles. The emission intensity in-
creases by a factor higher than 120 when compared with that of the NaYF
4
:Yb∕Ho∕Ce nanoparticles. This result
indicates that the Ce
3þ
incorporation into the NaYF
4
:Yb∕Ho nanoparticles promotes a strong quenching
effect and reduces the emission intensity; however, the quenching effect can be significantly reduced by
incorporating the Ce
3þ
ions into the core/shell NaYF
4
:Yb
3þ
50%@NaYF
4
:Ho
3þ
0.5% nanoparticles. A theoretical
model is proposed to explain the presence of the quenching effect in the NaYF
4
:Yb∕Ho∕Ce nanoparticles,
demonstrating that the quenching is mainly related to the interaction between the Yb
3þ
and Ce
3þ
ions.
OCIS codes: 160.2540, 160.4760.
doi: 10.3788/COL201614.021601.
Due to their interesting properties, including absence of
autofluorescence, low photobleaching, strong penetration
abilities, low toxicity, etc., rare-earth-doped upconversion
nanoparticles (UCNPs) have attracted increasing
interest
[1–5]
. These special features provide UCNPs with a
great potential for applications in several fields, such as
solar cells, solid-state lasers, boilables, and imaging
[6–10]
.
In particu lar, among the various applications, in vivo imag-
ing based on UCNPs is expec ted to be a promising photo-
luminescence imaging technique, as it provides high
sensitivity and spatial resolution, leading to predictive
models for potential clinical applications
[11,12]
. For in vivo
imaging, long-wavelength light (above 600 nm) is necessary
as the luminescent probe, as the tissue strongly absorbs
light with a short wavelength (below 600 nm)
[13]
. However,
three of the most important types of UCNPs, i.e., Yb/Er-,
Yb/Tm-, and Yb/Ho-codoped NaYF
4
nanoparticles, radi-
ate green, blue, and green light, respectively
[14–18]
.
Presently, most of the research on red-emission UCNPs
mainly focuses on the Yb/Er-codoped NaYF
4
nanopar-
ticles, as they show a strong green emission (∼550 nm)
along with a weak dark-red emission (∼660 nm). The
colorcanbetunedfromgreentoredusingdifferentmethods:
controlling the particle’s size or the Yb concentration
[19–22]
,
changing the surface ligands
[23,24]
, partially replacing the
Y site of the lattice by Mn
2þ
or Zr
4þ
ions
[25–30]
, etc.
Similar to the Yb/Er-codoped NaYF
4
nanoparticles, the
Yb/Ho-codoped NaYF
4
nanoparticles also show a strong
green emission and a weak red emission, and it is possible
to tune the emission color from green to red using different
methods. Unfortunately, the tuning seems very hard to
achieve. Until now, only one method has been reported
to succe ssfully tune the emission color from green to red
in Yb/Ho-codoped NaYF
4
nanoparticles by incorporating
Ce
3þ
ions and inducing a cross-relaxation process between
the Ho
3þ
and Ce
3þ
ions
[31–33]
. However, a significant
quenching effect due to the Ce incorporation was simulta-
neously produced, and the upconversion (UC) photolumi-
nescence intensity of the NaYF
4
:Yb
3þ
,Ho
3þ
1%, Ce
3þ
15% nanoparticles was several orders of magnitude lower
than that of the NaYF
4
:Yb
3þ
,Ho
3þ
1% nanoparticles.
In our previous research, core/shell structured
NaYF
4
:Yb
3þ
at NaYF
4
:Ho
3þ
1% nanoparticles were
proven to enhance the red emission by increasing the
Yb
3þ
concentration in the core
[34]
. Here, the Ce
3þ
ions
were incorporated into the core/shell structured NaYF
4
:
Yb
3þ
@NaYF
4
:Ho
3þ
nanoparticles to further enhance
the red emission; in addition, a method to eliminate the
quenching effect is proposed.
Two types of NaYF
4
:Yb
3þ
50%@NaYF
4
:Ho
3þ
0.5%
nanoparticles with incorporated Ce
3þ
ions were prepared
to investigate the influence of the Ce
3þ
ions. In addition,
NaYF
4
:Yb
3þ
20%, Ho
3þ
0.5% and NaYF
4
:Yb
3þ
20%,
Ho
3þ
0.5%, Ce
3þ
20% nanoparticles were also prepared
as the contrastive samples. All samples were prepared
by the procedure descripted in our previous work
[34]
.
Figure
1 shows the typical transmission electron micros-
copy morphologies of such nanoparticles. The mono-
dispersed NaYF
4
:Yb
3þ
20%, Ho
3þ
0.5%, Ce
3þ
20%
nanospheres exhibited an average size of ∼24 nm, similar
COL 14(2), 021601(2016) CHINESE OPTICS LETTERS February 10, 2016
1671-7694/2016/021601(5) 021601-1 © 2016 Chinese Optics Letters