Magnetic and optical properties of NaGdF4:Nd3+,
Yb3+,
Tm3+
nanocrystals
upconversion/downconversion
luminescence
with
from
visible to near-infrared second window
Xianwen Zhang1,†, Zhi Zhao2,†, Xin Zhang3, David B.
Cordesd4,5, Brandon Weeks3, Bensheng Qiu6, Kailasnath
Madanan7,8, Dhiraj Sardar7, and Jharna Chaudhuri1, *
1Department
of Mechanical Engineering, 3Department of
Chemical Engineering, and 4Department of Chemistry &
Biochemistry, Texas Tech University, Lubbock, Texas
79409, United States
2Hefei
National Laboratory for Physical Sciences at the
Microscale, and 6School of Information Science and
Technology, University of Science and Technology of
China, Hefei, Anhui
5EaStCHEM
230026, China
School of Chemistry, University of St.
Andrews, St. Andrews, KY16 9ST, U.K.
7
NaGdF4: Nd3+, Yb3+, Tm3+ nanocrystals that demonstrate dual-mode
Department of Physics and Astronomy, University of
photoluminescence when excited by near-infrared (NIR) light: 1):
Texas at San Antonio, San Antonio, Texas 78249, United
Downconversion emission in NIR second/first diagnostic window
States
allowing for improved light penetration; 2): Upconversion emission at
8International
School of Photonics, Cochin University of
Science and Technology, Kochi, 682022, India
†These
authors contributed equally.
visible light band for the convenient naked-eye or Si-CCD camera
detection.
Nano Research
DOI (automatically inserted by the publisher)
Research Article
Magnetic and optical properties of NaGdF4:Nd3+, Yb3+,
Tm3+ nanocrystals with upconversion/downconversion
luminescence from visible to near-infrared second
window
Xianwen Zhang1,†, Zhi Zhao2,†, Xin Zhang3, David B. Cordes4,5, Brandon Weeks3, Bensheng Qiu6,
Kailasnath Madanan7,8, Dhiraj Sardar7, and Jharna Chaudhuri1()
1
Department of Mechanical Engineering, 3 Department of Chemical Engineering, and 4 Department of Chemistry &
Biochemistry,
Texas Tech University, Lubbock, Texas 79409, United States
2 Hefei National Laboratory for Physical Sciences at the Microscale, and 6 School of Information Science and
Technology,University of Science and Technology of China, Hefei, Anhui 230026, China
5 EaStCHEM School of Chemistry, University of St. Andrews, St. Andrews, KY16 9ST, U.K.
7 Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States
8 International School of Photonics, Cochin University of Science and Technology, Kochi, 682022, India
† These authors contributed equally.
Received: day month year
ABSTRACT
Revised: day month year
We have designed and synthesized NaGdF4:Nd3+, Yb3+, Tm3+ magnetic
nanophosphors with combined dual-mode downconversion (DC) and
upconversion (UC) photoluminescence upon 800 nm excitation.
Hexagonal-phase NaGdF4: Nd3+, Yb3+, Tm3+ nanocrystals (NCs) with an average
size of 21 nm were synthesized using a solvothermal approach. Nd 3+, Yb3+, Tm3+
triple-doped NaGdF4 NCs exhibit a broad range of photoluminescence peaks
covering near infrared first/second window (860-900, 1000, and 1060 nm), and
visible spectra including blue (475 nm), green (520 and 542 nm) and yellow (587
nm) by the excitation of 800 nm. A mechanism of unique circulation of energy
over Gd3+ sublattices as bridge ions and finally trapped by the initial activator
ions (Nd3+) was proposed. Penetration depth studies indicate that NIR emission
is easily detected even at a large tissue thickness of 10 mm. These
paramagnetic nanophosphors demonstrate a large magnetization value of 1.88
emu/g at 20 kOe and longitudinal relaxivity value of 1.2537 mM-1S-1 as a
T1-weighted magnetic resonance imaging contrast agent. These NaGdF4: Nd3+,
Yb3+, Tm3+ NCs are promising for applications in biological and magnetic
resonance imaging.
Accepted: day month year
(automatically inserted by
the publisher)
© Tsinghua University Press
and Springer-Verlag Berlin
Heidelberg 2014
KEYWORDS
near-infrared second
window;
photoluminescence;
energy transfer;
nanocrystals
Nano Res.
1. Introduction
Fluorophores are one of the powerful non-invasive
imaging probes and are used for visualizing
morphological details of bio-species from living
cells, tissues and animals with subcellular
resolution [1, 2]. However, the spatial resolution is
limited by tissue penetration depth because of high
absorption
and
scattering
as
well
as
autofluorescence that occurs in biological tissues
[3]. To overcome these problems, “biological
transparency window” in the near-infrared range
at 750-850 nm, called the first near-infrared
window (NIR I), not only allows for improved
photon penetration through tissue but also
minimizes the effects of tissue autofluorescence
and light scattering [4, 5]. Many commercially
available probes lie within this region including
commonly used cyanine dyes such as indocyanine
green (ICG) and Cyanine 5.5 [6, 7]. Unfortunately,
they are limited due to a high photobleaching rate
when used in high intensity cell imaging studies.
Moreover, the organic dyes are vulnerable to
chemical and metabolic degradation impeding the
long-term cell tracking experiments [8]. The second
near-infrared window (NIR II at 1000-1400 nm) is
promising due to deep light penetration, minimal
autofluorescence and negligible light scattering,
which
could
significantly
improve
the
signal-to-noise ratio [9, 10]. Simulation and
modeling studies have predicted that fluorophores
at the NIR II window have higher tissue
penetration than those at the NIR I window [9, 11].
Currently, there is a scarcity of available
alternatives emitting in this beneficial region.
Quantum dots (QDs) such as Ag2S, PbSe, PbS,
CdHgTe, and single-walled carbon nanotube are
other candidates for imaging in NIR II [12-18].
However, the potential in vivo toxicity and
flickering emission of the NIR-emitting QDs limit
the biomedical application [19].
Rare-earth (RE) ions, typically trivalent, doped
nanocrystals (NCs) have attracted stimulating
interest due to narrow emission band widths, long
luminescence lifetime, biocompatibility, nontoxicity
as well as the potential applications in diverse
fields such as bio-imaging [20-28], solid-state
phosphors for display [29-31], etc. To date,
multifunctional NCs that exhibit two or more
different properties are highly desirable for many
important technological applications such as
multifunctional imaging, and simultaneous
diagnosis and therapy due to their versatile
functionality [20-22]. The Nd3+ ion with an
absorption around 800 nm is considered as a good
candidate to achieve high downconversion (DC)
quantum efficiency and significantly improve the
penetration depth for deep-tissue imaging due to
the NIR DC emission around 850-1100 nm [32, 33].
Besides, Nd3+ ions under the excitation of 800 nm
can also overcome the overheating issues in
comparison with the upconversion (UC) sensitizer
Yb3+ ions under the excitation of 980 nm in the UC
process [34]. Yb3+ ions can play a role of an
energy-transfer bridging ions between an energy
donor (Nd3+) ion and energy acceptor RE3+ ions
(Er3+, Ho3+ and Tm3+) with the emission at visible
region under excitation at 800 nm [35, 36].
Therefore, the Nd3+ sensitized DC/UC dual-mode
NCs combining the merits of both above
mentioned
Nd3+ doped
DC
system
and
3+
3+
3+
Yb -Nd -RE UC system at the same time are
attractive because of deep NIR light penetration,
low thermal effect, and the observations with the
naked eyes or widely equipped Si-CCD cameras of
the microscope under the single excitation around
800 nm. A recent study by Li et al. [33] has shown
that DC signal from Nd3+ sensitized dual-mode
nanomaterials can still be detected even from the
back side of the mouse under the excitation of 800
nm and the heat effect of 800-nm light is obviously
lower than that of 980 nm at the same power
density.
This
proof-of-concept
experiment
suggested that the 800 nm excited low thermal
effect UC/DC dual-mode nanoprobe not only can
be used for the NIR (800 nm)-to-Visible (540
nm) in-vitro bioimaging, but also show great
penetration depth at the “NIR biological window”
in
in-vivo imaging. Moreover,
when
these
fluorescence NCs are co-doped with gadolinium
ions (Gd3+), they are capable of being used as
magnetic resonance imaging (MRI) contrast agents
facilitating an excellent spatial resolution and
depth for in vivo imaging [27, 28].
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Here, we report on successful synthesis of
triple-doped NaGdF4: Nd3+, Yb3+, Tm3+ NCs, with
an average size of 21 nm, which demonstrated DC
(NIR I to NIR II)/ UC (visible light) dual-mode
photoluminescence and a large magnetization
value. The mechanism of unique energy migration,
over Gd3+ sublattices as bridge ions and ultimately
trapped by the initial activator ions (Nd3+), was
proposed. NaGdF4: Nd3+, Yb3+, Tm3+ NCs with
multi-modality functions of optical and magnetic
properties of Gd3+ show excellent potentials to
bridge gaps in resolution and depth of imaging as
multiplexed luminescent nano-biolabels and MRI
contrast agent.
2. Experimental
2.1 Synthesis of NaGdF4: Nd3+, Yb3+, Tm3+ NCs
Solutions containing a total amount of 1 mmol of
rare-earth nitride were used for the preparation of
hexagonal phase NCs of Gd(NO3)3•6H2O,
Nd(NO3)3•6H2O,
Yb(NO3)3•6H2O,
and
Tm(NO3)3•5H2O (Sigma Aldrich, 99.9%). In a
typical synthesis of hexagonal phase NaGdF4: Nd3+,
Yb3+, Tm3+ NCs, 5 mL deionized water, 30 mmol
NaOH (Sigma Aldrich, 99.9%), 10 mL ethanol and
15 mL oleic acid (Alfa Aesar, 99%), were mixed by
stirring at room temperature to get an even
solution. Subsequently, a total amount of 1 mmol of
rare-earth nitride hexahydrates and adequate
1mmol NaF aqueous solution (Sigma Aldrich,
99.9%) were added to the mixed solution to form
an emulsion. After stirring at room temperature for
twenty minutes, the mixed reactants were
transferred into a 60 mL autoclave, sealed and
heated at 190 °C for 12 hours. The system was then
allowed to cool to room temperature. The products
were deposited at the bottom of the vessel. The
precipitate was washed with ethanol several times.
The similar synthesis process was also applied to
NaYF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2
mol%) NCs. The NCs were dispersed in hexane for
TEM and UV-VIS-NIR absorption characterizations,
and were dried in a vacuum oven at 60 oC for 4 h
for X-ray diffraction (XRD), photoluminescence
spectra and decay curves measurements.
2.2 Preparation of aqueous dispersion of NaGdF4:
Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%)
NCs:
Oleate-capped NCs were dispersed in 10 mL of
hexane in the vial. 0.1 M HCl and 20 ml deionized
water was then added into it; its pH value was
decreased to 2. The mixture of powders and liquid
was sonicated for about 0.5 h while maintaining the
pH value of 2 by adding 0.1 M HCl every 10 min.
The carboxylate groups of the oleate ligand were
gradually protonated to yield oleic acid. After
completion of this process, the aqueous solution
was mixed with diethyl ether to remove the oleic
acid by extraction. The procedure was repeated
four times until the solution become totally
transparent. The ligand-free NCs were finally
dispersed in deionized water for MRI experiments.
2.3 Characterization and measurements
The powder XRD patterns of the synthesized
samples were recorded using a Rigaku Ultima III
diffractometer with Cu-Ka radiation operating in a
parallel-beam geometry. The nanoparticles were
characterized using a high-resolution transmission
electron microscopy (JEM-2100). NaGdF4: Nd3+,
Yb3+, Tm3+ NCs were separated by diluting them in
hexane and were dropped onto holey carbon film
supported on 400-mesh copper grids. Inductively
coupled plasma atomic emission spectrometry
(ICP-AES) (Optima 7300DV) was used to determine
Gd3+, Nd3+, Yb3+, and Tm3+ contents in NaGdF4: Nd3+
(3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs. The
photoluminescence spectra and decay curves of the
samples
were
measured
using
a
steady-state/lifetime spectrofluorometer (JOBIN
YVON, FLUOROLOG-3-TAU), in conjunction with
an 800 nm laser diode. For photoluminescence
measurement, fine powdered phosphor samples
were pressed into a square cell volume of
approximately 14×7×2 mm3 to ensure uniform
thickness and distribution of powders. Quantum
yield (QY) was measured in a calibrated integrating
sphere setup and power density used was 100 W/
cm2 [32]. The synthesized samples were also spin
coated on a quartz plate and used for measuring
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solid state UV-VIS-NIR absorption spectra with a
Lambda 1050 UV / VIS/ NIR (Perkin-Elmer, U.K.)
spectrometer. The magnetic measurements were
carried out with a superconducting quantum
interference device (SQUID) magnetometer at the
room temperature. T1-weighted MRI were acquired
on a 1.5 T MR scanner (Symphony; Siemens
Medical systems, Erlangen, Germany) using a
head-coil. Imaging parameters were as follows:
Sequence: spin echo; repetition time (TR): 150, 450,
750, 1050, 1350, 1650, 1950, 2250 ms; echo time (TE):
12 ms; number of excitations (NEX): 2; field of view:
230×31; and slice thickness: 3 mm.
2. Results and Discussion
electron microscopy (TEM) images of the resulting
NCs. As shown in Figure 2a, NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs are
uniform and nearly hexagonal in shape with an
average diameter of about 21 nm without
aggregation. The average size of 21 nm is suitable
for the bioimaging application [27]. In a further
investigation, the high resolution TEM (HRTEM)
image in Figure 2b demonstrates lattice fringes in
the individual nanoparticles and shows highly
crystalline nature of NCs. The lattice fringes
indicate the interplanar distance of 0.52 nm which
can be indexed to the d-spacing value of (101̅0).
The corresponding Fast Fourier Transform (FFT)
pattern of the NCs (inset in Figure 2b) taken along
the [101̅0] zone axis reveals that the FFT pattern is
a characteristic of the hexagonal NaGdF4 in
agreement with the lattice spacing of the (101̅0)
planes of the hexagonal-phase NaGdF4. ICP-AES
measurement shows the ratio of Gd: Nd: Yb: Tm is
95.05%: 2.83%: 1.74%: 0.16%.
Figure 1 XRD pattern of NaGdF4: Nd3+ (3 mol%), Yb3+ (2
mol%), Tm3+ (0.2 mol%).
Figure 2 (a) TEM image of NaGdF4: Nd3+ (3 mol%), Yb3+ (2
mol%), Tm3+ (0.2 mol%), and insert is the histogram of size
distribution from 320 particles; (b) HRTEM image of NaGdF4:
Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%), and inset is
the corresponding FFT.
To confirm the composition and the crystallinity
of the synthesized NaGdF4: Nd3+, Yb3+, Tm3+, X-ray
diffraction (XRD) pattern of the samples was
studied. Figure 1 shows the XRD pattern of
NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2
mol%). All of the diffraction peak positions and
intensities were in good agreement with the data
for the reference hexagonal phase (JCPDS card No.
27-0699) [37]. No impurity crystalline phase was
found in the diffraction pattern. The pattern
revealed that highly pure NaGdF4 NCs with good
crystallinity had been formed.
Figure 2 shows representative transmission
Figure 3 demonstrates the absorption spectrum
of NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+
(0.2 mol%) in the range of 500-1000 nm. There are
three main absorptions in the NIR range around
960 nm (Yb3+), 870 nm (Nd3+) and 800 nm due to
Nd3+ and Tm3+ ions corresponding to Nd3+: 4I9/2 (4F5/2, 2H9/2) and Tm3+: 3H6 - 3F4 transitions [38]. The
spectrum shows multiple absorption peaks
assigned to the f-f transitions of Nd3+ from the
ground 4I9/2 state to the excited states and has
intense absorption near 800 nm. Therefore, the
4f-electrons of Nd3+ as an initial activator ion can
be directly excited to the 2H9/2 state under laser
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Nano Res.
excitation at 800 nm.
be attributed to 1G4 - 3H6 and 1D2 - 3H5 radiative
transitions of Tm3+ according to the calculation of
energy level of Nd3+ and Tm3+ [41]. In this process
of DC and UC, Nd3+ ions act as a light-harvesting
antenna or activator to absorb 800 nm excitation
light and subsequently transfer energy to the
neighboring Yb3+ and Tm3+ resulting in a broad
range of visible to NIR II photoluminescence
emission.
Figure 3 Absorption spectrum of NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%).
As shown in Figure 4, NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs exhibit DC
and UC photoluminescence in a wide range of
emission spanning from visible to NIR II regions
upon
800
nm
excitation.
Three
DC
photoluminescence bands originating from Nd3+
and one broad band from Yb3+ are clearly
resolved (right): they are maxima at 860-900, 1060,
1330 and 980 nm, corresponding to the transitions
4F3/2 - 4I9/2, 4F3/2 - 4I13/2, 4F3/2 - 4I15/2 and 2F5/2 - 2F7/2,
respectively. It is proven that under the 800 nm
excitation the energies can be efficiently
transferred from Nd3+ to Yb3+ since the Nd3+: 4F3/2
- (4I9/2, 4I11/2) emissions show excellent
superposition on the absorption spectrum of Yb3+
at 1060 nm [39, 40].
The striking results
obtained here are that the excitation light at 800
nm and the NIR bands peaked at 860-900 nm lie
within the NIR I window, and the emissions
around 1000 and 1060 nm are spanning NIR II
window, both of which are ideal for bioimaging.
The UC spectrum (left) exhibits two bands
centered at 515-537 and 565-600 nm from Nd3+,
corresponding to 4G7/2 - 4I9/2 and 4G7/2 - 4I11/2
transitions. The other two bands centered on
475 and 542 nm in the UC emission spectrum can
Figure 4 UC and DC photoluminescence emission spectra of
NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%)
under 800 nm NIR excitation.
Figure 5 (a) DC and (b) UC emission spectra of NaGdF4: Nd3+
(3 mol%), NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%) and
NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%); (c)
DC and (d) UC emission spectra of NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%) and NaYF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%).
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Scheme 1 Proposed energy transfer mechanisms in NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs.
Figure 5a and b compares the DC and UC
emission spectra, respectively, for single doped
NaGdF4:Nd3+
(3
mol%),
double
doped
NaGdF4:Nd3+ (3 mol%), Yb3+ (2 mol%), and
triple-doped NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%),
Tm3+ (0.2 mol%) under800 nm excitation. It is
observed that the addition of Tm3+ in the
triple-doped sample produces approximately
two-fold magnitude enhancement of intensity at
the range of 950-1030 nm (NIR II), as compared to
that of the co-doped NaGdF4:Nd3+, Yb3+ sample
(Figure 5a). This is due to the absorption of Tm3+
ions corresponding Tm3+: 3H6 - 3F4 transitions
around 800 nm as shown in Figure 3. Thus, Tm3+ is
capable of absorbing extra light to enhance the
emission intensity. Likewise, the apparent
enhancement of intensity in the range of 500-530
and 570-600 nm in UC (Figure 5b) due to the
addition of Tm3+ is observed. Moreover, the blue
band centered at 475 nm, corresponding to the
transition of 1G4 - 3H6 of Tm3+, appears after doping
Tm3+ into NaGdF4:Nd3+ (3 mol%), Yb3+ (2 mol%). In
contrast, when the host matrix of NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) is replaced
with NaYF4 synthesized using the same method,
the integrated intensity of Yb3+ around 1000 nm,
especially the three Nd3+ bands, corresponding to
860-900, 1060, and 1330 nm, remarkably decreases
(Figure 5c). In Figure 5d, emission from the
reference sample NaYF4: Nd3+ (3 mol%), Yb3+ (2
mol%), Tm3+ (0.2 mol%) shows the enhanced UC
centered at 542 nm originating from 1D2 -3H5
transition of Tm3+ and simultaneous reduction of
the emission at the range of 510-540 nm arising
from the transition of 4G7/2 - 4I9/2 of Nd3+ as
compared to photoluminescence emission from
NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2
mol%). Apparently, Gd3+ ions in NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs play a
critical role in DC NIR and UC visible emissions
arising from Nd3+ and Tm3+, indicating that Gd3+
ions significantly contribute to the energy transfer
process.
By investigating the roles of doping ions Nd3+,
Yb3+ and Tm3+, the energy transfer process, in
which a circulation mechanism of energy is
proposed for the NaGdF4: Nd3+, Yb3+, Tm3+ NCs
under 800 nm excitation, is shown in Scheme 1. It is
assumed that there exists an energy transfer
circulation of Nd3+-Yb3+-Tm3+- Gd3+- Nd3+ under the
excitation of 800 nm. Scheme 1 shows that at first,
both Nd3+ and Tm3+ ions excited by a 800 nm laser
have corresponding
Nd3+: 4I9/2 - (2H9/2, 4F5/2) and
3+
3
3
Tm : H6 - F4 transitions. Nd3+ (2H9/2, 4F5/2) excited
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Nano Res.
states subsequently relax quickly to the next-lower
4F3/2 level by the multi-photon relaxation. The
radiative transition of Nd3+: 4F3/2 - 4I13/2 takes place
giving rise to narrow NIR II DC emission around
1060 nm and weak NIR I DC emission around 890
nm. Energy transfer UC of Nd3+ ions occurs via
dipole-dipole interaction of two excited photons at
the 4F3/2 state. In this process one Nd3+ photon losses
energy and falls down to the 4IJ (J = 11/2, 13/2)
states while the other one gains energy and is
populated to the high 4G7/2 or 4G9/2 state followed by
nonradiative relaxation to 4I9/2 and 4I11/2 states, and
as a result the UC of Nd3+ takes place [42, 43]. On
the other hand, an energy transfer (ET 1 in Scheme
1) from Nd3+ to nearby accumulator Yb3+ occurs
through a pair of transitions [38, 39]: Nd3+: 4F3/2 4I11/2 and 4I9/2; Yb3+: 2F7/2 - 2F5/2. The Yb3+ ions at the
2F5/2 state
radiatively relax to the 2F7/2 state
generating a broad band around 980 nm.
Subsequently, the energies at the 2F5/2 state of Yb3+
are transferred to the 4f-electron which is at the 3F4
excited state of Tm3+ resulting in the Tm3+: 3F4 - 1G4,
1G4 - 1D2 and 1I6 transition (ET 2 in Scheme 1) [38,
40]. From the 1G4 and 1D2 levels, the Tm3+ ions
transfer to 3H6 and 3H5 states leading to UC blue
light emission centered at 475 and 542 nm,
respectively. To enhance a radiative relaxation to
the 3H5 state resulting in the UC green light
efficient circulation process, Gd3+ ions are needed
to bridge the energy transfer (ET 3 and ET 4,
respectively in Scheme 1) from 1I6 state of the
accumulator Tm3+ to the 2P1/2 state of Nd3+ followed
by a nonradiative relaxation to aforementioned 4F3/2
and 4G7/2 states, contributing to Nd3+ DC/UC
emissions and next circulation. To validate the
third process of Tm-Gd (ET 3), Gd3+ was replaced
by inactive Y3+. The enhancement UC emission of
Tm3+ (1D2 - 3H5) and reduction of Nd3+ (4G7/2 - 4I9/2)
were simultaneously observed in Figure 5d.
Recently, Liu et al.[44, 45] reported that the
migratory Gd3+ ions can extract the excitation
energy from high-lying energy states of Yb3+/Tm3+
pair as photon accumulators followed by energy
hopping through the Gd3+ ions and trapping of the
migrating energy by the activator ions, Nd3+,
embedded in host lattices in the process of UC. To
further verify the process of energy transfer Gd-Nd
(ET 4 in Scheme 1), absorption spectra of undoped
NaGdF4, NaGdF4: Nd (3%), and emission spectrum
of NaGdF4: Nd (3%) were investigated as shown in
Figure 6. There are two absorptions around 274 nm
(Gd3+) and 350 nm (Nd3+), respectively. The overlap
between the emission spectrum (blue line) of Gd 3+
in NaGdF4: Nd excited at 274 nm and the
absorption spectrum (red line) of Nd 3+ in NaGdF4:
Nd at 346-360 nm, allows for Förster-Dexter energy
transfer [41] from Gd3+ to Nd3+ ions. Nd3+ ions play
roles not only as an energy-transfer accumulator
but also as the initial activator in this circulation
process.
In order to understand the UC mechanisms
involved in the emission of Nd3+ (4G7/2 - 4I9/2 and 4G7/2
- 4I11/2) and Tm3+ (1G4 - 3H6 and 1D2 - 3H5), the
emission intensity of these bands, Iemission, was
recorded as a function of laser pump power (P) and
plotted using Iemission ∝ Pn relation with n as the
number of incident photons in the emission of an
upconverted photon. The emission spectra were
taken up to 334 mW pump power focused in 3.14
mm2 area at the surface of NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%). The slope values,
1.77 (4G7/2 - 4I11/2), 1.79 (4G7/2 - 4I9/2), 1.72 (1D2 - 3H5) and
1.88 (1G4 - 3H6) shown in Figure 7, indicate the
involvement of two incident photons in the UC
emission.
Figure 6 Absorption spectra of NaGdF4 and NaGdF4: Nd (3%),
and emission spectrum of NaGdF4: Nd (3%). Inset: magnified
spectrum at the range of 330-380 nm.
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Figure 7 ln P - ln (Iemission) plots between excitation power and
emission intensity for UC bands of NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%).
Figure
8
Photoluminescence
emission
spectra
of
3+
NaGdF4-based NCs with different concentration of Tm
or
Yb3+. (a) DC and (b) UC emission spectra of NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2, 1, 2 mol%); (c) DC and (d)
UC emission spectra of NaGdF4: Nd3+ (3 mol%), Yb3+ (2, 5,
10, 20 mol%), Tm3+ (0.2 mol%).
In an attempt to probe the influence of Yb3+/Tm3+
pair in NaGdF4: Nd3+, Yb3+, Tm3+, we conducted a
series of control experiments by varying the doping
concentrations of Yb3+ and Tm3+ to compare the
photoluminescence emission intensity (Figure 8).
With the increase in Tm3+ concentration (0.2-2
mol%), a gradual decrease in DC and UC emission
intensity was observed which is mainly attributed
to the suppression of the DC/UC efficiency by
virtue of the elevated Tm3+ doping concentration.
High doping concentration of Tm3+ lead to
deleterious cross-relaxations between the adjacent
dopant ions resulting in the quenching of the
excitation energy and thereby weak emissions [46,
47]. In NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+
(0.2 mol%) with optimized dopant concentration of
Yb3+ and Tm3+, the NIR II emissions centered at 980
and 1060 nm originate from the 2F5/2 and 4F3/2 levels,
respectively. The decay curves of the Yb3+: 2F5/2-2F7/2
emission at 980 nm and Nd3+: 4F3/2-4I13/2 emission at
1060 nm were plotted in Figure 9. As shown in
Figure 9a, the average DC lifetime of 763 s for Yb3+
around 980 nm is longer than that of 40 s for Nd3+
at 1060 nm, indicating NaGdF4: Nd3+ (3 mol%), Yb3+
(2 mol%), Tm3+ (0.2 mol%) NCs may be used as an
ideal luminescence probe to eliminate the
interference from the background fluorescence
around 980 nm [48]. The DC emission absolute QY
of 1.06% was measured, which exhibits much
higher value than the UC QYs in the range of 0.005%
to 0.3% for NaYF4: 2% Er3+, 20% Yb3+ nanoparticles
[49].
Previous observations by Wen et al. [50] show
the
possibility
of
using
a
NaYbF4:Nd
@Na(Yb,Gd)F4:Er @NaGdF4 as a potential UC
bioprobe by penetration depth experiments of pig
skins. Herein, a proof of penetration depth
experiment through tissue was conducted by
placing the NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%),
Tm3+ (0.2 mol%) NCs under different thickness of
pig skins ranging from 0 to 10 mm. The emission
spectra in the NIR and visible light region for each
thickness are shown in Figure 10a and b by the
excitation at 800 nm, respectively. It can be seen
that the NIR light is more easily discernible even at
the largest tissue thickness of 10 mm than visible
light, which cannot be detected even at the
thickness of 3 mm. Therefore, the fact that both the
excitation and emission of NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs are in the
biological window of optical transparency,
combined with their long lifetime, high quantum
efficiency and deep penetration, makes these NCs
extremely promising as NIR bioimaging probes.
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Figure 9 Room temperature photoluminescence emission
decay of NaGdF4-based NCs. Decays of DC emission at 980
nm for Yb3+ (a) and 1060 nm for Nd3+ (b) in NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%).
Figure 10 Results of penetration depth experiments performed
with NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%)
NCs covered with pig skin tissues of varying thickness by
excitation at 800 nm: (a) DC emission spectra at NIR range.
Inset: magnified spectrum for the thickness of 10 mm; (b) UC
emission spectra at visible light range.
Apart from the excellent DC and UC emission,
NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2
mol%) NCs, as a function of applied field (-20 to
+20 kOe), also exhibit a linear correlation with a
magnetization value of 1.88 emu/g at 20 kOe,
suggesting that the NCs are paramagnetic at room
temperature (Figure 11a). The paramagnetism is
generated from the intrinsic magnetic moment of
Gd3+ ions having non-interacting and localized
nature. As a comparison, the magnetization value
of NaGd/YbF4 NCs is 0.79 -1.56 emu/g (at 20 kOe)
and that of undoped NaGdF4 NCs is 1.85 emu/g (at
20 kOe) as reported [51-53]. The multifunctional
NCs would help combine the advantages of
fluorescent probes and MRI contrast agents while
avoiding the disadvantages of the other.
Figure 11 (a) Room temperature magnetization of NaGdF4:
Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs; (b) Plot
of 1/T1 as a function of Gd3+ concentration for NaGdF4: Nd3+
(3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs; (c)
T1-weighted MRI of NaGdF4: Nd3+ (3 mol%), Yb3+ (2 mol%),
Tm3+ (0.2 mol%) NCs at various Gd3+ concentrations in water.
To verify whether the NaGdF4: Nd3+ (3 mol%),
Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs can be used for
MRI bioimaging, these nanoparticles were
transferred from the organic phase to the aqueous
phase. The longitudinal relaxation times (T1) and
T1-weighted MRI of hexagonal NaGdF4: Nd3+ (3
mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) NCs were
measured in aqueous solutions with different Gd3+
concentrations. From the plot of 1/T1 as a function
of Gd3+ concentration (Figure 11b), the longitudinal
relaxivity value of NaGdF4: Nd3+ (3 mol%), Yb3+ (2
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mol%), Tm3+ (0.2 mol%) NCs were determined to be
1.2537 mM-1S-1. In the proof-of-concept application
as a T1-weighted MRI contrast agent, representative
T1-weighted MRI of the NCs suspensions clearly
show the positive enhancing effect on T1-weighted
sequences as the Gd3+ concentration increases as
shown in Figure 11c thus indicating that NaGdF4:
Nd3+ (3 mol%), Yb3+ (2 mol%), Tm3+ (0.2 mol%) can
serve as an effective T1-weighted MRI contrast
agent.
Acknowledgements
We like to acknowledge the support of the
National
Science
Foundation
(NSF)
Grant
#MRI0922898 for the TEM work. We also thank the
support from the Anhui Provincial Natural Science
Foundation of China (1308085QA06) and the
National Science Foundation Partnership for
Research and Education in Materials (NSF-PREM)
grant (No. DMR-0934218). Thanks to Archis
3. Conclusions
Marathe for assistance with preparation of the
In summary, tri-doped NaGdF4: Nd3+, Yb3+, Tm3+
NCs synthesized demonstrated UC and DC
dual-mode photoluminescence when excited at 800
nm. The integrated intensity of DC and UC
emissions was remarkably influenced by the
concentration of Yb3+ and Tm3+ in NaGdF4: Nd3+,
Yb3+, Tm3+ NCs. The optimized doping
concentrations of Yb3+ and Tm3+ were 2% and 0.2%,
respectively
to
obtain
maximum
photoluminescence intensities in both visible and
NIR ranges. An energy transfer mechanism
explaining a process of energy circulation was
appropriately proposed by investigating the roles
of doping ions, Yb3+, Tm3+ and Gd3+. To generate an
efficient circulation process, Gd3+ ions are needed
to bridge the energy transfer from the accumulator
Tm3+ ions to the initial activator Nd3+ ions. Nd3+
plays roles not only as an energy-transfer
accumulator but also as the main activator. In
addition, decays of photoluminescence emissions
at 980 and 1060 nm in NaGdF4: Nd3+ (3 mol%), Yb3+
(2 mol%), Tm3+ (0.2 mol%) with optimized doping
concentration were studied. The NIR emission
offered deeper penetration depth of the pig skin
than visible light by a comparison of luminescence
signal strength. The NaGdF4: Nd3+ (3 mol%), Yb3+ (2
mol%), Tm3+ (0.2 mol%) NCs were found to be
paramagnetic with a magnetization value of 1.88
emu/g at 20 kOe and longitudinal relaxivity value
of 1.2537 mM-1S-1 as T1-weighted MRI contrast
agent. Due to its excellent solubility and stability in
water solution, the current work paved the way for
the potential application of bioimaging and
magnetic resonance imaging.
manuscript.
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