Photon upconversion in Yb3+-Tb3+ and Yb3+-Eu3+ activated core/shell nanoparticles with dual-band excitation

Article abstract of DOI:10.1039/c6tc00413j

Exploring novel lanthanide-activated upconversion nanoparticles with distinctive spectral fingerprints and emission lifetimes has long been a great concern for extended optical applications. Herein, we report the study of photon upconversion emissions in Yb³⁺-Tb³⁺ and Yb³⁺-Eu³⁺ activated nanoparticles with near-infrared excitation. In these nanoparticles, a high content of Yb³⁺ is required for the simultaneous excitation of two Yb³⁺ ions, yielding a Yb³⁺ dimer with a higher excited energy to upconvert photons onto Tb³⁺ and Eu³⁺. The optimum doping concentration of Yb³⁺ ions for Yb³⁺-Tb³⁺ and Yb³⁺-Eu³⁺ pairs was determined to be 80% and 60%, respectively, which are much higher than that of Yb³⁺-Er³⁺/Tm³⁺ pairs. Notably, the upconversion emission lifetime of the as-prepared nanoparticles was prolonged to 2.3 ms (Tb³⁺) and 4.0 ms (Eu³⁺), respectively. Through the epitaxial growth of a Nd³⁺ doped shell layer, the upconversion emissions of Tb³⁺ and Eu³⁺ were intensified 25-fold. At the same time, an extra excitation band in the shorter near-infrared region from Nd³⁺ at 808 nm was achieved. Moreover, the emissions of Tm³⁺ were employed to compensate for those of Tb³⁺ and Eu³⁺ for multicolor emissions. These results highlight the upconversion emissions of Tb³⁺ and Eu³⁺ for potential multicolor imaging and multiplexed detection applications.

Full text of DOI:10.1039/c6tc00413j

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Jo uP rl ne aa sl eo fd oM na ot et r ai ad l jsu sCt hme am r gi si nt rs y C
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DOI: 10.1039/C6TC00413J
Journal Name
ARTICLE
3+
3+
3+
3+
Photon Upconversion in Yb –Tb and Yb –Eu Activated
Core/Shell Nanoparticles with Dual-Band Excitation
Received 00th January 20xx,
Accepted 00th January 20xx
a
a
a
a
b
b
Hao Dong, Ling-Dong Sun,* Ye-Fu Wang, Jia-Wen Xiao, Datao Tu, Xueyuan Chen and Chun-
Hua Yan*a
DOI: 10.1039/x0xx00000x
Exploring novel lanthanide activated upconversion nanoparticles with distinctive spectral fingerprints and emission
lifetimes has always been a great concern for extended optical applications. Here, we report the study on photon
www.rsc.org/
3+
3+
3+
3+
upconversion emissions in Yb –Tb and Yb –Eu activated nanoparticles with near-infrared excitation. In these
nanoparticles, high content of Yb3+ is required for the simultaneous excitation of two Yb3+ ions, yielding a Yb3+ dimer with
higher excited energy to upconvert photons onto Tb3+ and Eu . The optimum doping concentration of Yb ions for Yb3+–
3+
3+
Tb3+ and Yb –Eu pairs was determined to be 80% and 60%, respectively, which are much higher than that of Yb3+–
3+
3+
3+
3+
3+
Er /Tm pairs. Notably, the upconversion emission lifetime of as-prepared nanoparticles is prolonged to 2.3 ms (Tb ) and
.0 ms (Eu ), respectively. By epitaxial growth of Nd doped shell layer, upconversion emissions of Tb3+ and Eu3+ has
3+
3+
4
intensified by 25 times. In the meanwhile, an extra excitation band at shorter near-infrared region from Nd3+ at 808 nm
was achieved. Moreover, emissions of Tm3+ were employed to compensate those of Tb3+ and Eu3+ for multicolor emissions.
These results highlight the upconversion emissions of Tb3+ and Eu3+ for potential multicolor imaging and multiplexed
detection applications.
Tb3+ and Eu , typical luminescent centers for green and red
emissions respectively, exhibit distinctive spectral profiles and
much longer luminescent lifetimes compared with those of
3+
Introduction
Lanthanide activated upconversion nanoparticles (UCNPs)
have been extensively studied as optical probes and light
3+
3+
3+
3+
3+
Er /Tm /Ho . And Tb and Eu activated complexes and
down-shifting nanoparticles are usually employed as probes
1
–3
4–7
transducers for bioimaging,
theranostic,
sensing and
23–25
for bioassays and time-gated detections.
However,
8
,9
photovoltaic devices10,11 and photoactivation
due to their excellence in converting near-
detection,
reactions,
considering the unmatched energy diagram, it is difficult to
12,13
3
+
3+
sequentially pump NIR photons onto Tb and Eu as
infrared (NIR) light into visible and ultraviolet (UV) ones.1
4–17
3+
3+
3+
3+
upconverted in Yb –Er /Tm /Ho pairs. Recently, a series of
3+
Overwhelmingly substantial attention has been paid to Yb –
3+
3+
3+
3+
Tm , Gd , and Tb /Eu co-activated UCNPs were developed
3
+
3+
3+
Er /Tm /Ho activated UCNPs for their high emission
3+
3+
to produce typical emissions of Tb and Eu under 980 nm
1
8,19
efficiency.
Yet these emissions are fixed in wavelength, and
26
3+
3+
excitation. In these UCNPs, Tb and Eu were activated with
sequential energy transfer from Tm and Gd3+ in separated
emission lifetimes are limited to microseconds, thus many
extended applications such as multicolor imaging and
multiplexed detection are greatly hindered by limited choices
of sensitizer-activator.
novel UCNPs with distinctive spectral fingerprints and emission
lifetimes. However, matched energy level between Yb3+ and
3+
layers. The upconverting process is relatively complicated, in
3+
the meanwhile, spectral fingerprints from both Tm and
20–22
Therefore, it is desirable to explore
3+
3+
Tb /Eu were always simultaneously displayed in spectra.
In this work, we describe the photon upconversion in a
3+
3+
3+
3+
category of Yb –Tb and Yb –Eu activated nanoparticles,
activators, as
a prerequisite for efficient upconversion
which are sensitized by straightforward energy transfer from
emission, makes the exploration of novel UCNPs a daunting
challenge.
3
+
Yb
dimers (Fig. 1a) under NIR excitation. In these
3+
nanoparticles, high content of Yb is required to facilitate the
3+
simultaneous excitation of two adjacent Yb ions, resulting in
an Yb3+ dimer with higher virtual excited state to match that of
a
Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare
Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare
Earth Materials and Bioinorganic Chemistry, College of Chemistry and Molecular
Engineering, Peking University, Beijing 100871, China. Fax: +86-10-62754179; Tel:
3+
3+
3+
3+
Tb and Eu . Investigations on emissions from Tb /Eu with
3+
Yb
dimer sensitization have covered bulk materials,
+
86-10-62754179; E-mail: sun@pku.edu.cn, yan@pku.edu.cn
Key Laboratory of Optoelectronic Materials Chemistry and Physics, Fujian
especially glasses and polymers,27–29 and also nanoscale
b
30–37
materials.
Nonetheless, due to the low efficiency and
Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, China.
Electronic Supplementary Information (ESI) available: detailed experimental
3
8–41
prominent quenching from surface defects and ligands,
photon upconverison in Yb –Tb and Yb –Eu activated
3+
3+
3+
3+
procedures and characterizations. See DOI: 10.1039/x0xx00000x
This journal is © The Royal Society of Chemistry 20xx
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microscopy (HRTEM) measurements, energy dispersive
View Article Online
DOI: 10.1039/C6TC00413J
spectroscopy mapping (EDX-Mapping), linear EDX scanning
measurements were carried out with a JEOL JEM-2100F TEM
operated at 200 kV. NIR absorption spectra were recorded on
a SHIMADZU UV-3100 spectrophotometer. Powder X-ray
diffraction (XRD) patterns were recorded on a Rigaku D/MAX-
2
000 diffractometer (Japan) with a slit of 1/2° at a scanning
rate of 2° min 1, using Cu Kα radiation (λ = 1.5406 Å). ICP-AES

characterization was performed with a Leeman Profile SPEC.
Upconversion emission spectra were measured on a Hitachi F-
4
500 fluorescent spectrometer with the PMT voltage of 700 V.
Lasers used as excitation sources for 980 nm and 808 nm are
high-power multimode pump lasers (Hi-tech Optoelectronic Co.
Ltd., with a maximum power 5.0 W for both lasers). The
upconversion emission lifetime was measured on a customized
UV to mid-infrared steady-state and phosphorescence lifetime
spectrometer (FSP920-C, Edinburgh Instrument) equipped with
a tunable mid-band OPO pulse laser as excitation source (410 –
2
400 nm, 10 Hz, pulse width ≤ 5 ns, Vibrant 355II, OPOTEK).
3
+
3+
3+
3+
Fig. 1 (a) Simplified energy transfer diagram for Yb –Tb and Yb –Eu
3+
Digital photographs were taken with a NIKON D3000 camera.
activated nanoparticles with NIR excitation. Only partial energy levels of Nd
3
+
and A (A = Tb, Eu) are shown for clarity. (b, c) Typical TEM images of core and Synthesis
of
hexagonal
4 4
NaGdF :Yb,A@NaGdF :Nd,Yb
core/shell nanoparticles, respectively. (d) HRTEM image of the core/shell
nanoparticle. (e) Linear EDX scanning of a single core/shell nanoparticle and
nanoparticles (A = Tb, Eu)
corresponding elemental ratio analysis. (f) Absorption spectra of core (blue line) In a typical experiment, the hexagonal phased core/shell
and core/shell (red line) nanoparticles.
nanoparticles were derived from three steps, cubic phased
core, hexagonal phased core, and hexagonal phased core/shell
nanoparticles, respectively.
Synthesis of cubic phased core nanoparticles. A given amount
3 3 3
of RE(CF COO) (RE = Gd, Yb, A, 1 mmol) and CF COONa (1
mmol) were added to a mixture of OA, OM and ODE (40 mmol,
molar ratio 1:1:2) in a three-necked flask (100 mL) at room
UCNPs is very low-efficient. To address this issue, we have
systematically studied the photon upconversion processes in
3+
3+
3+
3+
Yb –Tb and Yb – Eu pairs, including the composition and
architecture correlated emissions. The doping concentration of
3
+
3+
3+
Yb and Tb /Eu , which is crucial for energy transfer
efficiency, has been precisely optimized. Moreover, core/shell
temperature. Then the slurry was heated to 110
water and oxygen with vigorous magnetic stirring under
vacuum. The solution was heated to 310 C and kept for 15
min under N atmosphere. After cooling to room temperature,
C to remove
3+
3+
architecture was introduced to separate Tb and Eu from
surface quenchers, facilitating the intensification of
upconversion emissions. Aiming at enriching the excitation
features, Nd is introduced to expand the NIR excitation band
to shorter wavelength at 808 nm.
of Tm were used to compensate those of Tb3+ and Eu for

2
3
+
the cubic phased nanoparticles were precipitated by adding
excess amount of ethanol into the solution, and then collected
by centrifugation. The final product was dispersed in 10 mL of
cyclohexane.
42–45
On this basis, emissions
3+
3+
releasing multicolor upconversion emissions. These results
3+
3+
suggest that Tb and Eu activated UCNPs should be
promising candidates for applications as multicolor bioimaging
and multiplexed applications.
Synthesis of hexagonal phased core nanoparticles. The 5 mL as-
prepared cubic phased NaGdF
redispersed in a 40 mmol OA, ODE mixture (molar ratio 1:1),
and an extra RE(CF COO) (RE = Gd, Yb, A, 0.5 mmol) and
CF COONa (0.5 mmol) were added. Then the slurry was heated
to 110 C to remove cyclohexane, water and oxygen with
vigorous magnetic stirring under vacuum, and formed a clear
solution. The solution was heated to 310 C and kept for 30
min under N atmosphere. Upon cooling to room temperature,
the hexagonal phased NaGdF :Yb,A nanoparticles were
4
:Yb,A colloidal solution was
3
3
3
Experimental Section
Materials


Rare earth oxides (RE = Gd, Yb, Tb, Eu, Nd, Y; China Rare Earth
Online Co., Ltd.), Oleic Acid (OA; >90 %, Sigma-Aldrich),
Oleylamine (OM; >80%, Acros), 1-octadecene (ODE; >90%,
Acros), trifluoroacetic acid (99%, Acros), trifluoroacetic acid
sodium salt (99%, Acros), ethanol (AR), and cyclohexane (AR)
were used as received without further purification. BALB/c
nude mice were bought from Vital River Co. Ltd., Beijing.
Instrumentation
2
4
collected by centrifugation after adding excess amount of
ethanol. Finally, the product was dispersed in 10 mL of
cyclohexane.
Synthesis of hexagonal phased core/shell nanoparticles. The 5
mL as-prepared hexagonal phased NaGdF
solutions were redispersed in a 40 mmol OA, ODE mixture
molar ratio 1:1), and an extra RE(CF COO) (RE = Gd, Nd, Yb, 1
4
:Yb,A colloidal
Low resolution transmission electron microscopy (TEM)
measurements were performed with a JEOL JEM-2100 TEM
operated at 200 kV. High resolution transmission electron
(
3
3
mmol) and CF
3
COONa (1 mmol) were added. Then the slurry
2
| J. Name., 2012, 00, 1-3
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was heated to 110
oxygen with vigorous magnetic stirring under vacuum, and
formed a clear solution. The solution was heated to 310 C and
kept for 30 min under N atmosphere. Upon cooling to room
temperature, the hexagonal phased
NaGdF :Yb,A@NaGdF :Nd,Yb nanoparticles were collected by
C to remove cyclohexane, water and
View Article Online
DOI: 10.1039/C6TC00413J

2
4
4
centrifugation after adding excess amount of ethanol. Finally,
the product was dispersed in 5 mL of cyclohexane.
Preparation of hydrophilic polyacrylic acid (PAA) modified
nanoparticles
A ligand exchange strategy was employed to transfer the
4
8
hydrophobic nanoparticles to aqueous phase. Typically, 1 mL
colloidal solutions containing as-prepared nanoparticles were
dispersed in
Dimethylformamide (DMF) (10 mL, volume ratio 1:1), then 50
mg NOBF was added. The slurry was stirred for 30 min at
a
mixture of cyclohexane and N,N-
4
room temperature to leave the OA ligands in the cyclohexane
phase while the naked nanoparticles transfer to the DMF
phase. After that, the cyclohexane phase was removed and the
nanoparticles were collected by centrifugation (18000 rpm, 15
min) after adding an excess amount of toluene. The naked
nanoparticles were redispersed in 10 mL DMF containing 10
mg PAA (25% saponification). The mixture was stirred
overnight to render PAA modification process. Finally, excess
amount of acetone was added to the solution to precipitate
PAA modified UCNPs. Hydrophilic UCNPs were collected by
centrifugation (18000 rpm, 15 min). The resulting PAA
modified UCNPs can be easily dispersed in aqueous phase.
Results and Discussion
As a proof-of-concept experiment, hexagonal NaGdF
Fig. 2 Upconversion emission spectra (a) and Logarithmic I
NaGdF :30%Yb,10%Tb@NaGdF :50%Nd,10%Yb nanoparticles under 980 nm
excitation. Inset image is the digital luminescence photograph. Marked with “*”,
P curves (b) of
4
was
4
4
46
chosen as the host matrix due to its low phonon energy.
3+
3+
3+
3+
3+
Activators Tb /Eu together with sensitizers Yb were co- emissions at 525 nm, and 655 nm come from the ppm impurity of Er in Yb
3+
precursors, respectively. (c, d) Upconversion emissions of Tb as a function of
the doping concentration of Tb3+ (c) and Yb (d), respectively.
doped in inner core region to ensure efficient sensitization,
3+
3+
3+
while sensitizers Nd and Yb were embedded in the
neighboring separated layer to absorb NIR light (Fig. 1a).
The UCNPs were synthesized with a modified thermolysis
method.47,48 Typical transmission electron microscopy (TEM)
images of as-prepared nanoparticles reveal the uniformity in
core/shell nanoparticles could be excited at these two
prominent NIR absorption bands.
Upconversion emission from NaGdF
4
:Yb,Tb nanoparticles
were firstly examined under 980 nm excitation. As depicted in
size and morphology. The mean particle size of NaGdF
Tb, Eu) core and NaGdF :Yb,A@NaGdF :Nd,Yb (A = Tb, Eu)
core/shell nanoparticles are 18.4 0.7 nm and 28.4 1.0 nm
4
:Yb,A (A
3
+
5
7
Fig. 2a, typical emissions from Tb , which come from D
J = 3 – 6) intra-configurational transitions, can be clearly
observed. This suggests that the excitation energy has
4 J
→ F
=
4
4
(


respectively, with a shell thickness of ~5 nm (Fig. 1b,c). High
resolution TEM (HRTEM) image shows a single crystal nature
3+
3+
transferred from Yb dimer to Tb . But for NaGdF
nanoparticles, as the control group without Yb ions, did not
exhibit any typical Tb emissions (Fig. S3) under 980 nm
excitation. This indicates that sufficient amount of Yb should
4
:Tb
3+
with a lattice spacing of 0.52 nm, corresponding to the (10̅
plane of hexagonal NaGdF (Fig. 1d). XRD patterns depicted in
10)
3+
4
3+
Fig. S1 further confirm the hexagonal structure of as-prepared
nanoparticles. Linear energy dispersive spectroscopy (EDX)
scanning (Fig. 1e) and EDX-mapping (Fig. S2) illustrate the
elemental distribution of as-prepared core/shell nanoparticles.
3+
be co-doped with Tb for efficient sensitization. Moreover,
3+
upconversion emission of Tb has intensified ~25 times after
depositing a shell layer as NaGdF :Yb,Tb@NaGdF :Nd,Yb for
the surface defects have been greatly minimized. In the
4
4
3+
It is evident that Tb ions are mainly distributed in core, while
5
7
meanwhile, emissions from D
3
→ F
J
(J = 4 – 6) transitions of
3+
Nd ions are located in shell, which is well correlated to the
expected core/shell structure. Moreover, apart from the NIR
3+
3+
Tb with an energy higher than that of Yb dimer also
5
emerged. The population of D
3
state should come from the
3+
absorption band at 980 nm from Yb , extra NIR absorptions of
cross-relaxation of Yb ( F5/2) + Tb3+ ( D
3+
2
5
) → Yb ( F7/2) + Tb
3+
2
3+
4
3+
Nd emerged with the epitaxial layer (Fig. 1f). This makes the
5
) due to the similar energy gap between F5/2 → 2F7/2 of
2
(
D
1
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Yb3+ (10400 cm ) and D
as a subsequent non-radiative relaxation from
Scheme S1).
–1
5
→ 5D
of Tb3+ (10113 cm ), as well
–1
4
1
View Article Online
DOI: 10.1039/C6TC00413J
5
5
1 3
D to D
(
The number of photons involved in the emission is crucial to
reveal the transition mechanism.4 From the logarithmic
9
excitation power dependent emission spectra (I

P curves, Fig.
2
b), it can be found that the slopes of the linear fits were
5
7
around 2 for all
4 J
D → F (J = 6, 5, 4, 3) emissions. This
suggests that two-photon upconverting process should be
involved in the emissions of Tb3+ under 980 nm excitation,
which is well correlated to the proposed energy transfer via
3+
Yb dimer as shown in Fig. 1a. It is noteworthy that the slopes
for these emissions should be similar because they are from
5
the same excited state of D
4
. Nonetheless, due to statistical
erros from the integrated intensity of the transitions,
5
7
especially the emission at 619 nm ( D
4
→ F
3
), the slope may
show a little deviation.
Subsequently, we studied the doping concentration of Tb3+
and Yb3+ dependent emissions, which significantly determines
the energy transfer efficiency (Fig. S4–S7). According to the
3
+
spectra in Fig. 2c, the optimum doping concentration of Tb is
3
+
19
1
0%. Compared with the Er (~2%) activated UCNPs, this was
3+
3+
largely elevated for Tb . The content of Yb ions, which
directly affect the formation of Yb dimers, should be a crucial
3+
5
7
3+
4 5
parameter. As shown in Fig. 2d, D → F emissions from Tb
3
+
increased ~130 times as Yb increased from 10% to 80%, and
further increment of Yb3+ saturated the upconversion
3+
emissions. Therefore, 80% Yb should be adequate for
sensitizing emissions of Tb via Yb3+ dimer, which is much
3
+
3+
Er pairs.19
3+
higher that in Yb

3+
Eu , which is widely used for commercial phosphors, has
also been studied. As depicted in Fig. 3a, typical Eu emissions
(J = 1, 2) transitions can be observed NaGdF
under 980 nm excitation. The population of D
from the non-radiative relaxation of D
activated by the energy transfer from Yb dimer (Scheme Upconversion emissions of Eu as a function of the doping concentration of Eu
state of Tb is directly activated by
energy transfer from Yb dimer, upconversion emission of
3+
Fig. 3 Upconversion emission spectra (a) and Logarithmic I
:30%Yb,10%Eu@NaGdF :50%Nd,10%Yb nanoparticles under 980 nm
excitation. Inset image is the digital luminescence photograph. Marked with “*”,
P curves (b) of
5
7
coming from D
0
→ F
J
4
4
5
3+
0
state of Eu is
, where D
3
+
3+
emissions at 525 nm, and 655 nm come from the ppm impurity of Er in Yb
5
5
5
1
→ D
0
1
is
3+
precursors. (b) Logarithmic I

P curves of Eu under 980 nm excitation. (c, d)
3
+
3+
3+
(c) and Yb3+ (d), respectively.
S2).50 Considering the D
5
3+
4
3+
3+
3+
3+
3+
Yb –Eu pairs should be less efficient than that of Yb –Tb
pairs. As shown in Fig. 3b, similar two-photon process
sensitized from Yb dimers is responsible for the emissions of
3+
3+
3+
Eu . Investigation on the Eu concentration dependent
emissions is shown in Fig.s 3c,d and Fig.s S8
that of Tb , the optimum doping concentration of Eu is 10%

S11. Similar to
3+
3+
3
+
(
Fig. 3c), and 60% of Yb content is desired to sensitize
3+
upconversion emissions of Eu (Fig. 3d).
Luminescent decay behaviors of Yb –Tb /Eu activated
3+
3+
3+
nanoparticles were subsequently studied. As shown in Fig. 4a,
3
+
green emission lifetime of Tb at 542 nm is 2.26 ms under 980
Fig.
4
Upconversion
decay
curves
of
and
3+
nm excitation, and that of Eu collected at 614 nm reaches NaGdF
.08 ms at 980 nm excitation (Fig. 4b). It is much longer than
that of Er doped UCNPs, which is in microseconds (Fig. S12).
Next, the decay behaviors of Yb –Tb activated nanoparticles
with varied doping concentration of Tb3+ and Yb are
employed for studying the composition dependent emission
lifetimes, which can reflect the energy transfer efficiency. As
shown in Fig. S13a, emission lifetime of Tb3+ prolonged by 0.06
4
:80%Yb,10%Tb@NaGdF
:30%Yb,10%Eu@NaGdF
4
:50%Nd,10%Yb
NaGdF
excitation.
4
4
:50%Nd,10%Yb nanoparticles under 980 nm
4
3+
3+
3+
ms as the doping concentration of Tb3+ increased from 5% to
3+
3+
1
0%. This suggests that the energy transfer efficiency of Yb
3+
dimer to Tb should enhance little with elevating content of
3
+
3+
Tb . In comparison, when the content of Yb increased from
4
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3+
3+
2
0% to 80%, the emission lifetime of Tb3+ prolonged noticeably differentiate the emissions for those of Tb and Eu . The
View Article Online
3+
3+
DOI: 10.1039/C6TC00413J
incorporation way of Tm was respectively studied by co-
should be the crucial factor to affect the energy transfer from doping and separation into shell. When Tb and Tm are co-
by 1.76 ms (Fig. S13b), indicating that the content of Yb
3+
3+
3+
3+
3+
3+
3+
Yb dimer to Tb by affecting the amounts of Yb dimers.
doped in core, typical emissions of Tb and Tm can be
Benefiting from Nd ions in the shell, as-prepared Tb3+ and simultaneously detected (Fig. S18,S19). In a similar way,
3+
Eu3+ activated nanoparticles are also expected to be excited multicolor emissions can also be noticed in Eu and Tm co-
3+
3+
4
2
with 808 nm NIR light. As shown in Fig. 5a and Fig. S14, doped nanoparticles (Fig. S20,S21). By fine-tuning of the
3+
3+
3+
3+
3+
typical emissions of Tb and Eu were dominant, and the content of Tm , upconversion emissions of Tb and Eu can
3
+
spectral profiles are almost the same to those under 980 nm be enhanced ~3 times. This indicated that Tm could act as a
3+
3+ 52
excitation. Moreover, these emissions are also derived from bridge to activate Tb and Eu .
two-photon absorption (Fig. 5b). It is postulated that Yb
dimer is activated from two nearby Nd3+ ions under 808 nm regions as NaGdF
3+
When Tb /Eu and Tm3+ were incorporated into separated
3+
3+
4
:Yb,A@NaGdF
4
:Yb,Tm@NaYF
4
:Nd,Yb (A = Tb,
3+
3+
3+
3+
3+
excitation. Upconversion emission lifetimes of Tb and Eu
Eu), emissions of Tb and Eu can be activated by Yb dimers
can also reach milliseconds of 2.69 ms and 3.96 ms under 808 in core as well as Gd and Tm3+ in shell. As shown in Fig. 6a
nm excitation, respectively (Fig. S15), respectively, which are and b, multicolor emissions from Tb /Eu and Tm can be
3+
3+
3+
3+
similar to those under 980 nm excitation.
released from these nanoparticles. Moreover, the content of
The doping concentration of Nd3+ and Yb3+ in shell should be Yb in core was found to be important for efficient multicolor
3+
an important parameter to affect the energy transfer under emissions, ~4 times enhancement can be obtained with 20%
4
2
3+
8
08 nm excitation. Spectral results (Fig. S16a) showed that Yb in core (Fig. S22

S25), which is relatively smaller than the
3
+
3+
3+
high content of Nd was favorable for the emission of Tb
optimum 80% and 60% Yb shown in Fig. 2d and 3d,
3+
due to sufficient near-infrared absorption. The emission respectively. This could be attributed to that Yb in the
intensity of Tb increased about four times as the content of adjacent shell layer assisted the formation of Yb dimers.
Nd increased from 10% to 50%, and green emission lifetime
of Tb increased from 2.55 ms to 3.17 ms (Fig. S16b), which Yb –Eu activated UCNPs should be promising candidates for
also suggests 50% Nd in shell are more suitable. The near- multicolor bioimaging.
infrared emission of Nd and Yb (Fig. S17a) also suggests for detection. As-prepared hydrophobic multicolor UCNPs
that 50% Nd facilitates the energy transfer from Nd to Yb , were functionalized with biocompatible polyacrylic acid (PAA)
3+
3+
3+
3+
3+
With the distinctive upconversion emissions, Yb –Tb and
3+
3+
3+
3+
53,54
Here, a nude mouse was employed
3+
3+
3+
3+
3+
55,56
3+
3+
which can be revealed from the prolonged emission lifetime of (Fig. S26).
For Tb /Tm co-activated nanoparticles, their
Yb (Fig. S17b). Yet when exciting Tb3+ directly, possible Tb
3+
3+
upconversion emission signals can be simultaneously detected
under 980 nm excitation (Fig. S27a). After adding a green filter,
Nd3+ back energy transfer may be significant with high Nd
3+
→
51
3+
concentration as observed in pervious report. High content the emission from Tm could be blocked, and only green
of Yb in the shell led to a prominent decreased emission (Fig. emission from Tb could be detected (Fig. S27b). For
S16c), for the over exposure of Yb to surface quenchers.
3+
3+
3+
3+
Moderate amount of Yb in shell layer is necessary to mediate
the energy from Nd (shell) to Yb3+ (core). And the optimized
composition for shell was determined as
NaGdF :50%Nd,10%Yb.
Intrigued by the success in NIR triggered emissions from
3+
4
3+
3+
3+
3+
Yb –Tb
and Yb –Eu
activated nanoparticles, their
potential in yielding multicolor emissions was subsequently
3+
investigated. Tm was introduced to compensate and
Fig. 6 (a, b) Typical multicolor upconversion emission spectra of NaGdF
4
:Yb,A
Fig.
NaGdF
5
Upconversion
emission
spectra
of
@
NaGdF :Yb,Tm@NaYF :Nd,Yb (A Tb, Eu) nanoparticles under 980 nm
4
4
=
4
:30%Yb,10%Tb@NaGdF :50%Nd,10%Yb (a) corresponding I–P curves (b)
4
excitation. Insets are corresponding TEM images, luminescent photographs, and
schematic design for photon upconversion energy transfer.
under 808 nm excitation. Inset is the luminescence photograph. Emissions of the
ppm impurity of Er3 in Yb precursors are marked with “*”.
+
3+
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 5
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Journal Name
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+
3+
10 B. M. van der Ende, L. Aarts and A. Meijerink,ViPewhyArsti.clCe hOenlmine.
Chem. Phys., 2009, 11, 11081–11095. DOI: 10.1039/C6TC00413J
Eu /Tm co-activated nanoparticles, upconversion emission
signal from these two centers can also be simultaneously
detected (Fig. S27c). And the imaging signals from Eu3+ could
be selected with a red filter (Fig. S27d). The above results
imply the feasibility of multicolor imaging from selected
lanthanide ions of such UCNPs. Moreover, multicolor in vivo
imaging could also be performed with 808 nm excitation (Fig.
S27e–h).
1
1
1
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In conclusion, we have described a novel series of Tb and
3+
3+
Eu activated nanoparticles sensitized via Yb dimers. The
3+
3+
3+
optimum doping concentration for Tb /Eu and Yb is greatly
3+
3+
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19 H. Dong, L. D. Sun, C. H and Yan, Chem. Soc. Rev., 2015, 44
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0 H. H. Gorris and O. S. Wolfbeis, Angew. Chem. Int. Ed., 2013,
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elevated compared with that of Er /Tm /Ho activated
UCNPs. And the upconversion emission lifetimes is prolonged
to several milliseconds, which could be beneficial to time-
gated detections. Moreover, with Nd introduced in shell,
these nanoparticles can also be triggered with 808 nm
excitation, and the upconverting process is the same to that
under 980 nm excitation. On this basis, emissions from Tm
are used to compensate with that of Tb3+ and Eu for
multicolor emissions and bioimaging. These achievements not
only offer new insights into modulation of lanthanide related 24 Y. S. Liu, D. T. Tu, H. M. Zhu and X. Y. Chen, Chem. Soc. Rev.,
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Acknowledgements
This work was supported by NSFC (Nos. 21425101, 21371011,
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Collaborative Research Team Program, and the CAS/SAFEA
International Partnership Program for Creative Research
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View Article Online
DOI: 10.1039/C6TC00413J
3
+
3+
3+
3+
Photon upconversion emission in a series of Yb –Tb and Yb –Eu activated
nanoparticles have been investigated in this work.