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RE V
I E W
S
IN
AD V A
NC
E
Upconversion of Rare EarthNanomaterialsLing-Dong Sun, Hao Dong,
Pei-Zhi Zhang,and Chun-Hua YanBeijing National Laboratory for
Molecular Sciences, State Key Laboratory of Rare EarthMaterials
Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth
Materialsand Bioinorganic Chemistry, College of Chemistry and
Molecular Engineering, PekingUniversity, Beijing 100871, China;
email: [email protected], [email protected]
Annu. Rev. Phys. Chem. 2015. 66:61942
The Annual Review of Physical Chemistry is online
atphyschem.annualreviews.org
This articles doi:10.1146/annurev-physchem-040214-121344
Copyright c 2015 by Annual Reviews.All rights reserved
Keywords
anti-Stokes emission, 4f-4f transitions, nanomaterial, energy
transfer,optical application
Abstract
Rare earth nanomaterials, which feature long-lived intermediate
energy lev-els and intracongurational 4f-4f transitions, are
promising supporters forphoton upconversion. Owing to their unique
optical properties, rare earthupconversion nanomaterials have found
applications in bioimaging, thera-nostics, photovoltaic devices,
and photochemical reactions. Here, we reviewrecent advances in the
photon upconversion processes of these nanomate-rials. We start by
considering energy transfer models involved in the studyof
upconversion emissions, as well as well-established synthesis
strategies tocontrol the size and shapeof rare earth
upconversionnanomaterials. Progressin engineering energy transfer
pathways, which play a dominant role in de-termining upconversion
emission outputs, is then discussed. Lastly, repre-sentative
optical applications of these materials are considered. The aim
ofthis review is to provide inspiration for researchers to explore
novel upcon-version nanomaterials and extended optical
applications.
619
Review in Advance first posted online on January 30, 2015.
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RE: rare earth
UC: upconversion
1. INTRODUCTION
Rare earth (RE) elements, including the lanthanide family (from
lanthanum to lutetium), as wellas scandium and yttrium, are
important in functional materials. Because of their similar
electroncongurations ([Xe]4f n15d016s2), trivalentRE ions have
similar physical and chemical properties(13). Owing to the large
quantum number (n = 4, l = 3), the energy levels from the 4f
electronconguration are abundant, thus allowing for many
intracongurational transitions (Figure 1).However, the 4f-4f
intracongurational transition is parity forbidden for free RE ions.
As the REions are embedded in an inorganic lattice, the
parity-forbidden rule may be partially broken dueto the mix of
certain odd-parity congurations. Hence, the originally forbidden
transitions arepartially allowed.
Because of their abundant energy levels and intracongurational
transitions, RE ions are con-sidered promising luminescent centers
(47). Moreover, the intrinsic spectroscopic character ofRE ions
causes them to be less affected by their microsurroundings because
of shielding from the5s25p6 subshells (5). Moreover, excellent
photostability, a large anti-Stokes shift, long lumines-cence
lifetime, and sharp-band emission result from the unique 4f energy
levels. Three typicalenergy transfer modes have been established in
RE ionactivated materials in understanding theemission behavior,
namely downshifting, quantum cutting, and upconversion (UC).
The term photon UC refers to nonlinear optical processes in
which the continuous absorp-tion of two or more low-energy photons
leads to the emission of high-energy ones (anti-Stokesemission). In
1959, Bloembergen (8) began UC investigations with a device termed
an infraredquantum counter. Since then, numerous efforts have
contributed to enriching the family of UCmaterials. In contrast to
simultaneous two-photon absorption and second harmonic
generation,the UC process is realized via long-lived intermediate
energy levels. Furthermore, a continuous-wave laser with relative
low-power density (1103 W/cm) can trigger efcient UC emissions
(9).
5F5
5S24G7/2
2P1/2
4G5/2 5D
3P0
0
0
1
1 2
3
6P7/2
6I7/2
5D4
5D3
4F9/2
4F9/24S3/2
22H11/211/22H11/2
2H9
3H4
3F3
1G4
5
10
15
20
25
30
35
40
Ener
gy (1
03 c
m1
)
YbPr Nd Sm Eu Gd Tb Dy Ho Er Tm
1D2
1D2
1I6
2 2F5/2
Figure 1Energy-level diagrams of rare earth ions. Typical
upconversion emissive excited states are highlighted by redbold
lines.
620 Sun et al.
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UCN: upconversionnanomaterial
ETU: energy transferupconversion
In recent decades, RE-based UC materials have attracted
considerable attention owing totheir unique optical properties.
RE-based bulk materials have been successfully employed as dis-play
devices and compact solid state lasers (4, 1012). With the
development of nanoscienceand nanotechnology, RE-based UC materials
can now be designed on the nanoscale for morepromising prospects.
Unlike semiconductor quantum dots, whose luminescence exhibits high
re-liability with particle size, the emission bands of RE-based
upconversion nanomaterials (UCNs)are less affected. Moreover, UCNs
also exhibit high resistance to photobleaching and photoblink-ing,
micro-/millisecond lifetimes, and large anti-Stokes shifts (1318).
Owing to their specicnear-infrared (NIR) excitation, UCNs have
excellent penetration depth in biosystems, and thereis no
autouorescence from backgrounds. Because of these advantages,
RE-based UCNs havefound use in a wide range of applications, from
bioimaging (1921) and theranostics (2224) tophotovoltaic devices
(25) and photochemical reactions (26).
Despite these features, several aspects of UCNs are of great
concern to researchers. As isknown, the intrinsic energy transfers
of RE ions play a dominant role in determining UC emis-sion
efciency and color outputs. Energy transfer pathways must be
manipulated to generate thedesired UC emissions for applications.
Additionally, another challenge is the need to explore
newapplication elds to efciently utilize UC emissions.
Based on these considerations, we review recent investigations
in the eld of UCNs. In Sec-tion 2, we introduce the basic energy
transfer mechanisms in RE-related UC emissions. Next,Section 3
presents synthesis strategies to obtain high-quality UCNs.
Subsequently, we focusin Section 4 on typical approaches used for
energy transfer modulations, through which UCemissions as well as
excitations are selected. Section 5 discusses issues involved in
typical opticalapplications.
2. ENERGY TRANSFER MECHANISM
Figure 1 depicts the energy levels of RE ions and pathways to
realize UC emissions. In general,ve energy transfermechanisms are
involved in the photonUCprocess (Figure 2), namely excitedstate
absorption (ESA), energy transfer upconversion (ETU), photon
avalanche (PA), cooperativeenergy transfer (CET) upconversion, and
energy migration-mediated upconversion (EMU).
In an ESA process (Figure 2a), RE ions with multiple energy
levels can undergo the successiveabsorption of two or more
low-energy photons, resulting in the transition from the ground
toexcited state, and further to a higher excited state. High-energy
photons could be released withinsuch transitions. Although the ESA
process is simple and straightforward, the requirement isrigorous.
The absorption cross section of the excited ions should be adequate
to absorb the secondpump photon. However, its capability of
absorbing the second photon is generally rather low.
The ETU process (Figure 2b) includes two types of luminescent
centers, a sensitizer and anactivator. The absorption cross section
of the sensitizer is usually larger than that of the activator.Upon
excitation with the pump photons, the excited sensitizer transfers
energy to adjacent acti-vators resonantly. UC emission is generated
from the activator when electrons drop back to theground state. In
the ETU process, energy-level matching and a close spatial distance
are required.RE ions with abundant energy levels provide a great
advantage for ETU processes.
In 1979, Chivian and colleagues (27) discovered the PA process
(Figure 2c) using Pr3+
ionbased infrared quantum counters. The energy gap between the
intermediate state and theground state is in a mismatch with the
energy of the pump photon. Once electrons are excitedto the
intermediate state, an ESA process is likely to occur to populate
the higher excited state.Subsequently, resonant cross relaxation
takes place between the superexcited ion and adjacentground state
ion, yielding two ions in the intermediate state. Repeating the
cross-relaxation
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5I8
5I7
5I6
5I5
5I4
5F5
0
0
3P21
2
Ho3+Er3+ Yb3+Yb3+Yb3+ Tm3+2F7/2
2F5/2
4F3/25/27/2
2H9/2
4I15/2
4I13/2
4I11/2
4I9/2
4F9/2
4S3/2
2H11/2 5F4,5S2
5F3
3H6
3F4
3H5
3H4
3F3
1G4
5
10
15
20
25
30
35
40
Ener
gy (1
03 c
m1
) 1D2
1I6
f
ESA
a
ETU
b
PA
c
CET
d
EMU
e
Figure 2(ae) Basic energy transfer mechanisms of rare
earthrelated upconversion (UC) emissions: (a) excited state
absorption (ESA),(b) energy transfer upconversion (ETU), (c) photon
avalanche (PA), (d ) cooperative energy transfer (CET)
upconversion, and (e) energymigration-mediated upconversion (EMU).
The gray dashed line in panel e represents the core/shell
interface. ( f ) Energy-leveldiagrams and proposed UC energy
transfer pathways in the Yb3+-Er3+, Yb3+-Ho3+, and Yb3+-Tm3+
pairs.
process, exponential population of the intermediate state is
sure to occur, along with excitationabove the threshold. In this
case, PA-induced UC emissions are readily produced as long as
theconsumption of superexcited ions is less than that of ground
state ions.
Similar to ETU, two types of luminescent centers are required in
the CET process(Figure 2d ): a cooperative sensitizer and activator
(28). Themain difference between the two pro-cesses is the absence
of adequate long-lived intermediate energy levels in the activators
in CET. InCET,UCemission results from simultaneous energy transfer
from two sensitizers to one activator.Hence, its UC emission
efciency is approximately three orders of magnitude lower than that
ofETU (4). Sometimes cooperative UC emissions can be observed from
cooperative dimers (29, 30).
In 2011, Liu and coworkers (31) proposed the EMU mechanism,
based on energy transferwithin core/shell nanostructures. An EMU
process (Figure 2e) incorporates four types ofluminescent centers
with dened concentrations into separated layers: a sensitizer,
accumulator,migrator, and activator. Upon excitation with
low-energy photons, an ETU process occurs,
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populating the higher excited state of the accumulator. Then the
energy is donated to an adjacentmigrator in the same region,
followed by further energy transfer through the core/shell
interfaceto a migrator in the neighboring region. Finally, the
energy is trapped by an activator, giving outUC emissions as
electrons drop back to the ground state. Meanwhile, UC emissions
from theaccumulator ions can also occur.
As mentioned above, the UC efciency varies with different
mechanisms. Because of highemission efciency, ETU-motivatedUCNs
overwhelmingly dominate correlative studies. Amongthe RE ions, the
Yb3+ ion is the best choice for the sensitizer. The absorption
cross section of theYb3+ ion is 9.11 1021 cm2 (976 nm; 2F7/2
2F5/2), larger than most of the RE ions. Moreimportantly, the
energy-level diagram of the Yb3+ ion is quite simple, with only one
excited stateof 2F5/2, which matches well with those of many RE
ions. As for the activators, there are manychoices. Owing to their
ladder-like arrangement of energy levels and excellent level
matching withthe Yb3+ ion, the Er3+, Tm3+, and Ho3+ ions are ideal
activators for the ETU process (9, 32).The doping concentration of
the Yb3+ ion is usually kept at 20% or higher, whereas that of
theactivators is lower than 2%.
Figure 2f shows the energy-level diagrams and proposed energy
transfer pathways in theYb3+-Er3+, Yb3+-Ho3+, and Yb3+-Tm3+ pairs.
The number of photons involved in UC processesis obtained from
log-log diagrams of the UC emission intensity versus excitation
power density,so-called I-P curves (33, 34). We have
comprehensively investigated the UC properties inYb3+-Er3+-codoped
NaYF4 UCNs (35). Upon 980-nm excitation, Yb3+ ions absorb the
pumpphoton and undergo the 2F7/2 2F5/2 transition. Subsequently,
the excited Yb3+ ions donateas-absorbed energy to adjacent Er3+
ions resonantly, promoting Er3+ ions to generate the 4I15/2 4I11/2,
4I11/2 4F7/2, 4I13/2 4F9/2, and 4F9/2 2H9/2 upward transitions.
After electrons havepopulated these excited states, nonradiative
relaxations to the 2H11/2, 4S3/2, and 4F9/2 states occur,further
yielding the 2H11/2 4I15/2 (525 nm; green), 4S3/2 4I15/2 (545 nm;
green), 4F9/2 4I15/2 (655 nm; red), and 2H9/2 4I15/2 (415 nm;
violet) emissions. From the monitored I-Pcurves, the green and red
emissions are assigned to two-photon UC processes, and the
violetemission belongs to the three-photon transitions, consistent
with the proposed mechanisms.Similarly, two-photon green and red
emissions can also be generated in Yb3+-Ho3+-codopedUCNs via the
5F4, 5S2 5I8 (545 nm; green) and 5F5 5I8 (650 nm; red) transitions.
In addition,weak blue emission centered at 485 nm could also be
observed via the 5F3 5I8 transition (36).
In contrast to the two former cases, Yb3+-Tm3+-codoped UCNs may
involve more thantwo-photon UC processes (37). This may be
attributed to the discretely arranged energy levels ofTm3+ ions,
which reduce the possibility of nonradiative relaxations. As shown
in Figure 2f, whenelectrons of Yb3+ ions relax to the ground state
(2F5/2 2F7/2), the energy migrates to nearbyTm3+ ions to conduct
the 3H6 3H5, 3F4 3F2, 3H4 1G4, 1G4 1D2, and 1D2 3P2upward
transitions. After population on these excited states and several
nonradiative relaxations(3F2 3F3, 3F2 3H4, 3P2 1I6), the following
UC emissions are generated: two-photon3F3 3H6 (695 nm; red) and 3H4
3H6 (800 nm; NIR), three-photon 1G4 3F4 (645 nm;red) and 1G4 3H6
(475 nm; blue), four-photon 1D2 3F4 (450 nm; blue) and 1D2 3H6(365
nm; UV), and ve-photon 1I6 3F4 (345 nm; UV) and 1I6 3H6 (290 nm;
UV).
3. SYNTHESIS STRATEGY
The ideal host matrix should possess high chemical stability and
low phonon energy. Amongvarious RE compounds, RE uorides, such as
REF3, REOF, and MREFn (M = Li, Na, K,or Ba; n = 4 or 5), are
commonly considered as ideal host materials (9, 32). Additionally,
REuorides, with low phonon energies, are especially desirable for
UC studies and applications.
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The synthesis strategies of RE uorides have been well
established. Thermal decomposition andhydro(solvo)thermal synthesis
methods are efcient in controlling the uniformity of
hydrophobicUCNs. These routes allow products to be precisely tuned
in terms of size, shape, and compositionwithin the nanoscale.
3.1. Thermal Decomposition
The thermal decomposition method is an oxygen-free, organic
phase, synthetic process in whichthe precursors are dissolved and
decomposed in high-boiling-point organic solvents with
theassistance of surfactants. Ions are then generated to combine
into new nuclei at elevated tem-peratures (38, 39). By adding
different kinds of precursors, one can obtain various RE
uorides.Generally, the precursors are RE-based triuoroacetates and
oleates. Organic solvents usuallyconsist of surfactants and
coordination solvents, such as octadecene, oleic acid, and
oleylamine. Itis widely accepted that coordination solvents can cap
the surface of UCNs to control growth anddisperse them in organic
solvents. The thermal decomposition method can also be divided
intosingle-source and multisource precursor thermal decompositions
according to the uorine sourceprovided by the precursors.
3.1.1. Single-source precursor thermal decomposition. In 2005,
Yan and coworkers (38)reported the synthesis of LaF3 triangular
nanoplates using the thermal decomposition method(Figure 3a). They
used RE(CF3COO)3 as the precursor, which provided both RE and
uorineions upon decomposition. This strategy has been developed
into a universally applicable methodfor the synthesis of
high-quality and monodispersed UCNs, including NaREF4 (39, 40), as
well asREOF (41), LiREF4 (42), KREF4 (42), and BaREF5 (43) (Figure
3bf ). Single-source precursorthermal decomposition is also
suitable for the preparation of UCNs with a core/shell
structure(35). In addition, UCNs with a core/shell structure have
been prepared to great success usinga modied hot-injection
technique, in which a stock solution containing a reactive
precursor oftriuoroacetates is injected into the hot solvent at a
constant rate (44).
3.1.2. Multisource precursor thermal decomposition. RE and
uorine precursors are re-spectively provided by two or more kinds
of precursors in the multisource precursor thermaldecomposition
method. Generally, RE triuoroacetates, oleates, acetates, and
chlorides are em-ployed for RE ions, whereas HF, NH4F, NH4HF2, NaF,
and CF3COOH are used for uorineions. Chen and coworkers (45)
prepared -NaREF4 UCNs with this approach and chose NaFand RE
oleates as precursors. They controlled the morphology of UCNs by
simply regulating theratios of NaF-to-RE oleates or the ratio of
solvents. Li & Zhang (46) developed a facile and user-friendly
method for the synthesis of -NaYF4:Yb,Er/Tm UCNs in oleic acid and
octadecene.The reaction occurred in an anhydrous and oxygen-free
environment. In this work, a methanolsolution containingNaOH
andNH4F was added to a homogeneous solution of RECl3, oleic
acid,and octadecene. The most signicant part of this strategy is to
consume a stoichiometric amountof uoride reagents entirely at room
temperature, so as to decrease the HF gas and uorinatespecies at
high temperatures.
3.2. Hydro(solvo)thermal Method
The hydro(solvo)thermalmethod is another way to
yieldUCNswithwell-controlledmorphology.The strategy involves mixing
RE precursors with uoride precursors in an aqueous solution,sealing
and heating the solution in an autoclave (often lined with Teon).
RE nitrates, chlorides,and oxides are usually chosen as
REprecursors.HF,NH4F, andNH4HF2 are frequently employed
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a b c
d e f
g h i
[0001][0001][0001]
0.30 nm0.30 nm0.30 nm
(1120)(1120)(1120)
(111)(111)(111)
0.32 nm0.32 nm0.32 nm0.27 nm (002)
0.27 nm (002)
0.27 nm (002) 10 nm10 nm10 nm
10 nm10 nm10 nm2 nm2 nm2 nm
5 nm5 nm5 nm
(111)(111)(111)
0.32 nm0.32 nm0.32 nm
0.46 nm0.46 nm0.46 nm
(101)(101)(101)
11203030
50 nm 100 nm 200 nm
100 nm100 nm50 nm
100 nm 200 nm 1,000 nm
Figure 3Transmission electron microscopy images of (a) LaF3, (b)
-NaYF4, (c) -NaYF4, (d ) LaOF, (e) BaGdF5,and ( f ) LiErF4,
synthesized with the thermal decomposition method, and ( g) -NaYF4
and (h) -NaYF4,prepared with the hydro(solvo)thermal method. (i )
Scanning electron microscopy image of -NaYF4prepared with the
hydro(solvo)thermal method. Panel a reprinted with permission from
Reference 38.Copyright 2005 American Chemical Society. Panels b and
c reprinted with permission from Reference 40.Copyright 2006
American Chemical Society. Panel d reprinted with permission from
Reference 41.Copyright 2008 American Chemical Society. Panel e
reprinted from Reference 43 with permission of TheRoyal Society of
Chemistry. Panel f reprinted from Reference 42 with permission of
The Royal Society ofChemistry. Panels g and h reprinted with
permission from Reference 47. Copyright 2007 AmericanChemical
Society. Panel i reprinted with permission from Reference 48.
Copyright 2007 Wiley-VCHVerlag GmbH & Co. KGaA.
as uoride precursors for REF3 UCNs, whereas NaF and KF are used
for MREF4 (M = Na, K)UCNs. In this method, many experimental
parameters, such as the reactant concentration, dosageof RE ions,
temperature, reaction time, and pH value, can inuence the growth of
UCNs. NearlymonodispersedNaYF4 single-crystal nanoparticles (Figure
3g), hexagonal nanorods (Figure 3h),and ower-patterned nanodisks
(Figure 3i) have been synthesized with the
hydro(solvo)thermalmethod (47, 48).
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4. UPCONVERSION ENERGY TRANSFER MODULATION
Due to their critical effect on optical properties and further
applications, engineering UC energytransfer pathways has always
been a great concern. With regard to optical properties,
energytransfer modulation can be divided into three categories:
multicolor tuning, the enhancement ofUC emissions, and novel UC
excitations triggered by energy transfer.
4.1. Multicolor Tuning of Upconversion Emissions
In anETU-motivated system, efcient activators aremainly theEr3+,
Tm3+, andHo3+ ions.Thesefew choices determine the limited emission
color outputs. For example, green and red emissionscanbe
simultaneously obtained inYb3+-Er3+-codopedUCNs, generating
anoverall yellowoutput.Coincidently, green and red emissions with
similar wavelengths are also yielded in Yb3+-Ho3+-codoped UCNs,
which, to a large extent, limit the application of UCNs in
multicolor encodingand multiplexed analyte detection purposes.
Hence, investigations into multicolor tuning areessential. Numerous
methods have been developed, which can be classied as follows:
controllingthe RE doping concentration, screening the hostmatrix,
introducing extraneous energy levels, andincorporating energy
acceptors to undergo the luminescence resonant energy transfer
(LRET)process.
4.1.1. Controlling the rare earth doping concentration. The RE
doping concentration de-termines the number of luminescent centers
as well as their spatial distance in the inorganic hostmatrix.
Novel multiphoton cross relaxations and energy backtransfer from
activators to sensitizersusually occur when their doping
concentration changes.
In 2004, Capobianco and coworkers (49) demonstrated that the red
to green emission ratio en-hancedmonotonicallywith an
increasingdoping concentrationofYb3+ ions inY2O3:Yb,ErUCNs.They
attributed this phenomenon to the multiphoton cross relaxation of
4F7/2 (Er3+) + 4I11/2(Er3+) 4F9/2 (Er3+) + 4F9/2 (Er3+), where
4F9/2 is responsible for red emissions. Inspired by theoptical
advantage of -NaYF4:Yb,Er UCNs, our group systematically studied
their composition-dependent UC properties. By precisely tuning the
content of Yb3+ (1030%) and Er3+ (0.55%)ions, we discovered that
the red to green emission ratio increased with an elevated content
of bothYb3+ and Er3+ ions (35). Apart from the cross-relaxation
process, the energy backtransfer alsoenhances red emissions. Wang
& Liu (50) observed this phenomenon in -NaYF4:Yb,Er UCNswith an
elevated Yb3+ ion content (Figure 4a). They reasoned that the
energy backtransfer process4S3/2 (Er3+) + 2F7/2 (Yb3+) 4I13/2
(Er3+) + 2F5/2 (Yb3+) should contribute to the phenomenon.
As for Yb3+-Tm3+ codopedUCNs, theUC emission property also
exhibits a certain regularity.Our group studied UC emission proles
with various doping concentrations of Tm3+ ions (0.25%) in
-NaYF4:Yb,Tm UCNs. Spectral results showed that decreasing the
content of Tm3+
ions from 5% to 0.2% tended to enhance the four-photon UC
emissions more than it did thethree-photon UC emissions, resulting
in the transition of color output from blue to purple (37).Prasad
and coworkers (51) found that the ratio of ve-photon emission to
three-photon emissionincreased with the content of Yb3+ ions and
reached a maximum at 90%. Han and coworkers(52) observed that an
elevated doping concentration of Yb3+ ions enhanced ve- and
four-photonemissions compared with three- and two-photon
emissions.
4.1.2. Screening the host matrix. The host matrix provides the
doping sites for RE luminescentcenters. Besides the fundamental
adoption functionality, different host matrices lead to
variousspatial distances, as well as different local coordination
structures of luminescent centers.Moreover, the phonon energy,
which signicantly affects UC energy transfer pathways, differs
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420 490 560 630 700 420 490 560 630 700
Wavelength (nm)
Inte
nsit
y (a
.u.)
2 H9/
2
4 I15
/2
2 H11
/2
4 I 1
5/2
4 S3/
2
4 I15
/2
4 F9/
2
4 I15
/2
1 G4
3 F
41G
4
3 H6
1 D2
3 F
4
-NaYF4:Yb,Er -NaYF4:Yb,Er
Wavelength (nm) Wavelength (nm)
Inte
nsit
y (a
.u.)
Inte
nsit
y (a
.u.)
700600
Na/RE
RE/NaRE
Na
F
Na/RE4
F
Na/RE4
500 700600500
a
b
25% Yb40% Yb60% Yb
1.5% Er
0.2% Er0.5% Er0.8% Er1.2% Er
NaYF4:Yb,Er
Icode
Iref
Icode
Iref
Icode Iref
IcodeIref
500 600
+
+
700
Wavelength (nm)
500 600 700
Wavelength (nm)In
tens
ity
(a.u
.)In
tens
ity
(a.u
.)
cRhodamine B
S-0378
S-0378
Figure 4Multicolor upconversion (UC) emission modulation of
Yb3+-Er3+-codoped upconversion nanomaterials (UCNs) by (a)
controlling thedoping concentration of Er3+ ions and introducing
extraneous energy levels of Tm3+ ions, (b) screening host matrices
of -NaREF4and -NaREF4, and (c) incorporating organic dyes to
generate the luminescence resonant energy transfer processes
between theUCNs and the dyes. The digital photographs in panel a
show the UC emission outputs of the samples listed in the upper
spectra. Panela reprinted with permission from Reference 50.
Copyright 2008 American Chemical Society. Panel b reprinted with
permission fromReference 40. Copyright 2006 American Chemical
Society. Panel c reprinted with permission from Reference 68.
Copyright 2011Wiley-VCH Verlag GmbH & Co. KGaA.
in diverse host matrices. The branch ratio of UC emissions can
be effectively regulated afteralternation in host matrices.
In 2005, Soukka and coworkers (53) studied the effect of the
host matrix on the UC emissionsof Yb3+-Er3+-codoped systems. The
results showed that the red to green emission ratio of Er3+
ions in oxides and oxychlorides was larger than that in
oxysuldes, uorides, and uoride double
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salts. Huang and coworkers (54, 55) investigated Sc3+ ionbased
UCNs, NaScF4 and KSc2F7.In the two kinds of UCNs, apparent red
emission dominated the visible spectra, yielding a redcolor output.
Recently, Capobianco and coworkers (56) studied the UC emission
properties ofGdVO4:Yb,ErUCNs. Spectral results demonstrated that
green emission was the dominant visibleemission.Moreover, the
emission at 525 nm (2H11/2 4I15/2) wasmore intense than that at 545
nm(4S3/2 4I15/2), which was different from most cases. Recently,
Liu and coworkers (57) describeda new type of UCN by adopting an
orthorhombic crystallographic structure in which RE ionswere
distributed in arrays of tetrad clusters. The unique arrangement
enabled the preservationof excitation energy within the sublattice
domain and effectively enhanced the violet emission(415 nm; 2H9/2
4I15/2), which was conrmed as a four-photon UC process.
It is worth noting that different phase structures of the same
compound can also result indistinctive UC emissions. Our group
investigated the phase-structure-dependent UC emissionproperties of
NaYF4:Yb,Er UCNs (35). As shown in Figure 4b, there are two phase
structures ofNaYF4, namely cubic () and hexagonal (). From their
corresponding UC emission spectra, wecan see that the intensity of
the green emission is in the same proportion of that of the red
emissionof Er3+ in -NaYF4:Yb,Er UCNs, thus generating yellow
emission output. However, the greenemission dominates the visible
regime in -NaYF4:Yb,Er UCNs, yielding green emissionoutput.
Along with Er3+ ions, UC emissions of Tm3+ ions are also
dependent on the host matrix.Typically, the four- and ve-photon
emissions are small in proportion. Capobianco and coworkers(58)
presented a type of LiYF4:25%Yb,0.5%TmUCN, for which spectral
results exhibited intenseUV emissions with moderate Yb3+ ion
content.
4.1.3. Introducing extraneous energy levels. Abundant energy
levels provide RE ions withvarious energy transfer pathways, and
multicolor emissions can be generated when different REactivators
are appropriately codoped.Multiphoton cross relaxations occurring
within the codopedactivators can also tailor the original
emissions. After such energy transfers, novel emission bandsor
altered branch ratios appear.
Wang & Liu (50) were able to ne-tune visible UC emissions in
-NaYF4:Yb,Er,Tm UCNs(Figure 4a). Before doping with Er3+ ions, they
obtained blue emission output upon 980-nmexcitation. By subtly
increasing the concentration of Er3+ ions from 0.2% to 1.5%, they
graduallytuned the emission output from blue to white. Wang and
coworkers (59) prepared a series ofNaYbF4-based UCNs doped with
Er3+-Tm3+, Tm3+-Ho3+, and Er3+-Ho3+ pairs, respectively,with the
result of varying emission outputs and spectral ngerprints.
Except for multicolor emissions, Zhang and coworkers (60)
successfully tailored UC emissionin Yb3+-Ce3+-Ho3+-tridoped UCNs.
With an increasing doping concentration of Ce3+ ions(015%), the
color emission output obviously changed from green to red. The
spectral transfor-mation was ascribed to multiphoton cross
relaxations: 5I6 (Ho3+) + 5F5/2 (Ce3+) 5I7 (Ho3+)+ 2F7/2 (Ce3+) and
5S2/5F4 (Ho3+) + 5F5/2 (Ce3+) 5I5 (Ho3+) + 2F7/2 (Ce3+). Unusual
5G5 5I7 and 5F2/5K8 5I8 transitions from Ho3+ ions and 5d 4f
transitions from Ce3+ ionshave also been observed (61). Qin and
coworkers (62) demonstrated UC emission modulation inNaYF4:Er,Tm
UCNs. With the introduction of Tm3+ ions, the red to green emission
ratio ofEr3+ ions increased as a result of multiphoton cross
relaxation: 3F4 (Tm3+) + 4I11/2 (Er3+) 3H6(Tm3+) + 4F9/2 (Er3+).
Recently, Chan and coworkers (63) employed a combinatorial methodto
investigate the inuence of codoping with other RE ions on the UC
emissions of Er3+ ions.They found that almost spectrally pure green
emission can be generated from Er3+-Pr3+ andEr3+-Sm3+ coactivated
UCNs, whereas pure red emission can be yielded from Er3+-Ho3+-
and
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Er3+-Tm3+-codoped counterparts. Selective quenching of red
emission was proposed to accountfor the pure green emission,
whereas the pure red emission was induced by cross
relaxations:4I11/2 (Er3+) + 3F4 (Tm3+) 4I13/2 (Er3+) + 3H5 (Tm3+),
3F4 (Tm3+) + 4I11/2 (Er3+) 3H6(Tm3+) + 4F9/2(Er3+), 4I11/2 (Er3+) +
5I7 (Ho3+) 4I13/2 (Er3+) + 5I6 (Ho3+), and 5I7 (Ho3+) +4I11/2
(Er3+) 5I8 (Ho3+) + 4F9/2 (Er3+), respectively.
Recently, several transition metal ions have also been
incorporated as intermediate states forphoton upconverting.Mn2+
ions, which possess the unique 4T1 energy state, are employed to
tailorthe UC emissions of RE ions. First, Li and coworkers (64)
investigated the role of Mn2+ ions onthe UC emission of Er3+ ions
in KMnF3:Yb,Er UCNs. Spectral results showed that spectrallypure
red UC emissions were generated in this system. The authors
reasoned that the energytransfer Yb3+ (2F5/2) Er3+ (2H11/2, 4S3/2)
Mn2+ (4T1) Er3+ (4I15/2) enhanced red emissionwhile decreasing the
intensity of green emission. Later, Liu and coworkers (65) conrmed
thered emission output in KMnF3:Yb,Er UCNs. Moreover, they also
observed spectrally pure redemission in KMnF3:Yb,Ho UCNs and
spectrally pure NIR emission in KMnF3:Yb,Tm UCNs,which also
mediated energy transfer via the 4T1 state of Mn2+ ions.
4.1.4. Luminescence resonance energy transfer. LRET refers to
resonant energy migrationfrom UCNs to decorated energy acceptors,
such as quantum dots, organic dyes, and noble metals.The key
prerequisite for LRET is the spectral overlap between the emission
of UCNs and theabsorption of energy acceptors. With various
decorations of energy acceptors, UCN emissionscan be subtly tuned
via LRET.
Li and coworkers (66) preparedNaYF4:Yb,ErUCNsdecoratedwith gold
nanomaterials, whoseabsorption band heavily overlappedwith the
green emission of Er3+ ions.With an elevated amountof gold
nanomaterials, the green emission ofUCNsdecreased gradually. Zhang
and coworkers (67)encapsulated uorescein isothiocyanate (FITC),
tetramethylrhodamine isothiocyanate (TRITC),and quantum dots
(QD605) into silica shells coated outside of the NaYF4:Yb,Er/Tm
UCNs.The occurrence of LRET processes was conrmed by the reduction
of the typical emission ofNaYF4:Yb,Er/Tm UCNs and the emergence of
novel emission bands of FITC, TRITC, andQD605. Gorris and coworkers
(68) employed RhB and the dye S-0378 to modulate the green andred
emissions of NaYF4:Yb,Er UCNs, respectively (Figure 4c).
Furthermore, they also adopteduorescein and the dyeNIR-797 to
encode the blue andNIR emissions
ofNaYFF4:Yb,TmUCNs,respectively.
4.2. Enhancement of Upconversion Emissions
Determined by the parity-forbidden 4f-4f intracongurational
transitions, the absorption crosssection of RE ions is quite small.
Hence, the UC emission efciency of RE ions is far fromsatisfactory.
Many approaches exist, including forming core/shell structures,
tailoring the localcrystal eld, constructing building blocks with
noblemetals, and introducingNIR antenna ligands.
4.2.1. Constructing core/shell structures. UCNs prepared by wet
chemical methods are facedwith surface defects, as well as the
vibration of capping ligands, which are deleterious to UCemissions.
Boyer & van Veggel (69) observed a sharp decrease in the
quantum yield of UCNs withdecreasing particle size. The increasing
surface defects in smaller particles should account for
thephenomenon. Capobianco and coworkers (70) investigated the
effect of the hydroxyl group on theUC emissions of Er3+ ions. After
the removal of the original oleate ligands, the resulting
UCNsexhibited an enhanced red to green emission ratio in the UC
spectra. An efcient way to eliminatethe surface effect is the
formation of core/shell structures.
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Liu and coworkers (71) investigated the surface quenching effect
by comparing the branch ratioof blue to NIR emission intensity of
Tm3+ ions and the overall emission intensity before and aftershell
growth. van Veggel and coworkers (72) demonstrated novel epitaxial
layer-by-layer growthof NaYF4:Yb,Er UCNs with Ostwald ripening
dynamics. UC spectral results conrmed that theemission intensity
steadily enhanced with an increasing thickness of the shell layer.
Moreover, theUC emission intensity enhanced linearly with the shell
thickness (73). Recently, our group demon-strated the novel
generation of a heterogeneous core/shell structure,-NaYF4@CaF2 UCNs
(74).The successful growth of the CaF2 layer should be ascribed to
the same space group (Fm3m) and asimilar lattice constant, that is,
a = 5.448 A for -NaYF4 and a = 5.451 A for CaF2.We preciselytuned
the thickness of the CaF2 shell layer by controlling the molar
ratio of [Ca]/[RE]. Spectralresults showed that, when the molar
ratio of [Ca]/[RE] was 4, the overall integrated emissionintensity
was enhanced approximately 300 times (Figure 5a). Moreover, RE ion
leakage was sup-pressed after the epitaxial growth of the CaF2
layer. Very recently, Bednarkiewicz and coworkers
350 450 550 650 750
Wavelength (nm)
Inte
nsit
y (a
.u.)
a[Ca]/[RE] = 4[Ca]/[RE] = 2[Ca]/[RE] = 0
[Ca]/[RE] = 1
450 550 650 750
Wavelength (nm)
Inte
nsit
y (a
.u.)
c
450 500 550 600 650 700
Wavelength (nm)
Inte
nsit
y (a
.u.)
dUCNs
IR-806/UCNsIR-780/UCNsIR-806
500 550 600 650 700
Li+ 0
Li+ 5
Bulk
Wavelength (nm)
Inte
nsit
y (a
.u.)
b
Figure 5The enhancement of upconversion emissions of
Yb3+-Er3+-codoped upconversion nanomaterials (UCNs) by (a)
constructingcore/shell structures, (b) tailoring the local crystal
eld, (c) constructing building blocks with noble metals, and (d )
introducing anear-infrared antenna ligand. Panel a reprinted with
permission from Reference 74. Copyright 2012 Wiley-VCH Verlag GmbH
& Co.KGaA. Panel b reprinted with permission from Reference 80.
Copyright 2008 American Chemical Society. Panel c reprinted
fromReference 86 with permission of The Royal Society of Chemistry.
Panel d reprinted from Reference 93 by permission from
MacmillanPublishers Ltd, copyright 2012.
630 Sun et al.
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(75) conrmed a considerable enhancement in CET-mechanized
-NaYF4:Yb,Tb@CaF2 UCNs.The maximum enhancement factor was reported
to be 40.
In contrast to the optically inert shell layers mentioned above,
optically active shells notonly could enhance the UC emission
intensity, but also could generate novel UC emis-sions of
activators embedded in the shell layer. In Capobianco and coworkers
(76) workwith NaGdF4:Yb,Er@NaGdF4:Yb core/shell UCNs, the UC
emission intensity of the activecore/active shell UCNs increased
three times in green emission and 10 times in red emissioncompared
with that of NaGdF4:Yb,Er@NaGdF4 UCNs. Very recently, Liu and
coworkers (77)employed an epitaxial end-on growth technology to
synthesize multicolor microrods comprisingdifferent RE activators.
In this way, six kinds of multicolor-banded microrods were obtained
formulticolor barcoding. Chen and coworkers (78) described the
intriguing generation of UCNsby introducing energy transfer through
the core/shell interface. NaGdF4:Yb,Tm@NaGdF4:Eucore/shell UCNs
were prepared, and the UC emission property was investigated. Upon
980-nmexcitation, emissions from both Tm3+ and Eu3+ ions were
observed. Liu and coworkers (31, 79)further promoted the
application of energy transfer though core/shell interfaces. They
obtainedUC emissions from a series of activators without long-lived
energy levels, including Eu3+, Tb3+,Dy3+, and Sm3+ ions.
4.2.2. Tailoring the local crystal field. Because of destruction
in the local symmetry, tailoringthe local crystal eld is expected
to facilitate intracongurational transitions. An efcient way to
doso is to compensatewith optically inert non-RE ions.Hence,
several alkalimetal ions and transitionmetal ions are frequently
introduced to tailor the local crystal eld of luminescent
centers.
Owing to their smaller cationic radii, Li+ ions are supposed to
be randomly located at thelattice site or interstices among the
lattices. Zhang and coworkers (80) introduced Li+ ions toY2O3:Yb,
Er UCNs. They found that the UC emission intensity of Y2O3:Li,Yb,Er
UCNs showedan enhancement of two orders of magnitude compared to
Y2O3:Yb,Er UCNs (Figure 5b). Theyconcluded that the prolonged
lifetime of the 4I11/2 (Er3+) and 2F5/2 (Yb3+) states and the
extrapopulation of the 4I13/2 (Er3+) state favored the enhancement
of UC emissions. Following this,Wang & Nann (81) observed a
more than 30-fold enhancement of emissions after 80% Li+
doping in NaYF4:Yb,Er UCNs. Cai and coworkers (82) investigated
the Li+ iondependent UCproperties of -NaGdF4:Li,Yb,Er UCNs. They
observed 47 and 23 times the enhancement forthe green and red
emissions compared with the -NaGdF4:Yb,Er UCNs, respectively.
Besides Li+ ions, several transition metal ions, such as Bi3+
and Fe3+, have also been employedto tailor the local crystal eld
(83, 84). Except for doping with non-RE ions, the local crystaleld
could be tailored by other means as well. Recently, Hao and
coworkers (85) demonstrated aninteresting method to enhance UC
emissions by applying relatively low voltages to BaTiO3:Yb,Erlms.
They attributed this enhancement phenomenon to the promotion in
radiative probabilitiesinduced by the lower symmetry of Er3+ sites,
which took place after the introduction of the externalelectric
eld.
4.2.3. Constructing building blocks with noble metal
nanostructures. Nobelmetal nanopar-ticles, which have localized
surface plasmon resonance (LSPR) properties, have been
extensivelyinvestigated in terms of their interactions withUC
emissions. Themost frequently used plasmonicnoble metals are gold
and silver nanostructures. Two possible mechanisms have been
proposedto account for the enhancement of UC emissions caused by
the LSPR property of gold and silvernanostructures. On one hand,
the enhancement effect may be achieved as a result of the
amplica-tion of the local incident electromagnetic eld, which
arises from the coupling of excitation bandswith the surface
plasmon resonance. On the other hand, the increase in the
recombination rate
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by the surface plasmoncoupled emission could enhance the
emission efciency, which effectivelypromotes radiative and
nonradiative decay rates.
We reported the rst enhancement of the UC emission intensity
assisted with LSPR(Figure 5c) (86). In our design, silver nanowires
were employed to provide the LSPRs, andNaYF4:Yb,Er UCNs were chosen
as the UC emission generators. Spectral results showed thatthe
enhancement factor was 2.3 and 3.7 for the green and red emissions,
respectively. Duan andcoworkers (87) decorated gold nanoparticles
onto the surface of NaYF4:Yb,Tm UCNs. Theyfound a more than 150%
increase in the emission intensity of blue emission, whereas an
increaseof only 50% was observed for the red emission. Zhang and
coworkers (88) investigated the inter-action between the
NaYF4:Yb,Er@SiO2 UCNs and the gold shells by adjusting the LSPR
peaksof the composites. Spectral results demonstrated that UC
emissions were enhanced only in thecomposite whose LSPR peak was at
900 nm, nearest to the excitation wavelength of 980 nm. Suchresults
indicate that the excitation ux was increased via the local eld
enhancement effect.
The spatial distance between the UCNs and noble metals is
crucial for the enhancementeffect. Xu and coworkers studied the
separation distance-dependent UC emission prole
inNaYF4:Yb,Er@SiO2@Ag core/shell composites (89). The inuence of
silver nanoparticles on UCemission behavior was studied by
controlling the thickness of the SiO2 layer. The maximum UCemission
enhancement factor was observed at a separation distance of 10 nm.
Kagan and cowork-ers (90) constructed a metal oxideUCN trilayered
structure in which the thickness of the oxidespacer was tuned from
2 to 15 nm. The optimal thickness of the oxide spacer was 5 nm for
gold and10 nm for silver noble metals. Additionally, ne arrays of
noble metals have been demonstratedas efcient LSPR donors. May and
coworkers (91) observed over three times the enhancementof UC
emissions from the patterned gold surface. Compared with the smooth
gold surface, theyfound an approximately twofold magnication of the
excitation eld intensity. Recently, Nagpaland coworkers (92)
reported at least six times the enhancement of UC emissions of Er3+
ions on agold pyramid patterned surface, while fourteen times
quenching the UC emission on the at goldsurface.
4.2.4. Introducing NIR antenna ligands. Because of their large
absorption cross sections, an-tenna ligands have been frequently
used to enhance the downshifting emission intensity of Eu3+
and Tb3+ ions. Theoretically, appropriate NIR antenna ligands
are also capable of enhancingthe intensity of UC emissions of Er3+,
Tm3+, and Ho3+ ions. However, only one report has con-rmed the
feasibility of the sensitization effect fromNIR antenna ligands
(Figure 5d ). Hummelenand coworkers (93) prepared IR-806 dyes by
grafting carboxyl-containing chains to commercialIR-780 cyanine dye
molecules. The modied IR-806 molecules were capped on the surface
ofNaYF4:Yb,Er UCNs by a ligand exchange strategy. Upon excitation
with NIR photons (800 nm),the IR-806 dyes were excited and donated
energy to the Yb3+ ions embedded in the inorganicmatrix. Finally,
the energy was extracted by the Er3+ ions via the ETU process.
Spectral resultsexhibited a 3,300-fold enhancement compared with
the oleate-capped UCNs. Besides the signi-cant sensitization, the
excitation band was shifted to 800 nm, which corresponded to the
excitationof IR-806 molecules.
4.3. Energy TransferTriggered Novel Upconversion Excitation
Conventional ETU-mechanized UCNs, which are sensitized by Yb3+
ions, exhibit a narrow exci-tation band centered at 980 nm.
However, the small selection of excitations limits the exibility
offurther applications of UCNs. For example, the specic 980-nm
excitation heavily overlaps withthe absorption of water molecules,
which are the main ingredients of organisms.
632 Sun et al.
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400 500 600 700
Wavelength (nm)
Inte
nsit
y (a
.u.)
b
Nd3+Yb3+Yb3+Er3+4I9/2
4I11/2
4I13/2
4F3/2
4F5/2
2F5/2
2F7/2
4F7/2
4I11/2
4I15/2
2H11/2, 4S3/2
a ex = 980 nmex = 808 nm
Figure 6(a) Proposed energy transfer mechanism of Nd3+
ionsensitized upconversion (UC) emissions. Differentregions are
highlighted with different background colors. (b) UC emission
spectra of Er3+ ions upon 980-nmand 808-nm irradiation with the
same excitation power. Figure reprinted with permission from
Reference94. Copyright 2013 American Chemical Society.
To solve the pending question, we developed Nd3+ ionsensitized
UCNs (Figure 6) (94).After absorption of the 730-nm, 808-nm, and
865-nm NIR light, Nd3+ ions were excited, andthen the energy was
transferred to adjacent Yb3+ ions through a downshifting process.
Finally,activators trapped the energy and gave out UC emissions. To
avoid mutual quenching, Nd3+ andEr3+ ions were incorporated into
separated layers. We observed a similar excitation efciency of808
nm and 980 nm. Moreover, the heating effect induced by the
irradiation of 980-nm light waslargely minimized by the
illumination of 808-nm light. Along with incorporation into
separatedlayers (94, 95), tridoping with Nd3+, Yb3+, and Er3+ ions
in the same layer could also generatedistinguishable UC emissions
(96, 97). We note that the content of Nd3+ ions should be kept ata
low level to prevent considerable multiphoton cross relaxations
between Er3+ and Nd3+ ions.
5. APPLICATIONS
REUCNs have been adopted in a wide range of applications, from
bioimaging to photoresponsivedevices, owing to the unique NIR
excitation, large anti-Stokes shifts, sharp-band emissions,
andexcellent photostability. RE UCNs have miniscule biotoxicity,
remarkable penetration depth, anda high signal-to-noise ratio.
Therefore, they have been studied as popular luminescent
materialsfor bioimaging applications (21).
Various in vitro (98101) and in vivo (102106) models have been
employed to validate thebioimaging potentials of RE UCNs. For
example, Li and coworkers (103) synthesized
sub-10-nm-NaLuF4:Gd,Yb,Tm UCNs for sensitive in vivo imaging.
Excellent detection limits of 50 and1,000 UCN-labeled cells were
achieved for subcutaneous and intravenous injection,
respectively.Our group reported the in vivo imaging and toxicity
assessments of NaYF4:Yb,Tm UCNs withCaenorhabditis elegans
(106).Theworms exhibitedNIRUC luminescence in the gut upon
excitationof 980 nm. Furthermore, theUCNs can be excreted out as
thewormswere fedwith onlyEscherichiacoli again, and no signicant
difference in the ingesting of UCNs was observed between
thehermaphrodite and the male.
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PDT: photodynamictherapy
Due to low photon energy and efcient anti-Stokes emissions, RE
UCNs have also been em-ployed as energy transducers to upconvert
NIR light and trigger photochemical reactions. A seriesof
photoresponsive applications, such asNIR-triggered photodynamic
therapy (PDT), sensing anddetection, photoswitch, photorelease, and
photoisomerization, highlights the signicance of REUCNs.
In the PDT system, photosensitizers are activated by emission
fromUCNs and then combinedwith adjacent oxygen atoms, producing
reactive oxygen species, resulting in oxidative damage tocancer
cells. Liu and coworkers (107) presented the rst UCN-based in vivo
PDT. NaYF4:Yb,ErUCNs were decorated with chlorin e6 molecules to
trigger the PDT process. Upon 980-nmexcitation, visible emissions
from the UCNs were transferred to the photosensitizers. We
con-structed triple-functional NaGdF4:Yb,Er@CaF2 core/shell UCNs
loaded with hematoporphyrinand silicon phthalocyanine dihydroxide
molecules, respectively (108). The composites exhibitedexcellent
PDT efciency in HeLa cells upon 980-nm irradiation.
In sensing and detection studies, UCNs were decorated with
specic recognition site-containing chromophoric complexes. The UC
emissions could be turned on or off by the com-plexes via
anLRETprocess. For example, Li and coworkers (109) demonstrated in
vivo bioimagingof methylmercury (MeHg+) with cyanine-modied
NaYF4:Yb,Er,Tm UCNs. The nanocompos-ites showed high specicity and
sensitivity with a detection limit of 0.18 ppb. Recently, Zhang
andcoworkers (110) developed a novel ligase-assisted
signal-ampliable DNA detection scheme withhigh sensitivity and
specicity based on UCNs via an LRET process.
The rst NIR-triggered photoswitch application was reported by
Branda and coworkers (111).Photochromicmolecules, diarylthene
derivatives, which are sensitive toUV and visible light with
areversible ring-closing and -opening form,were
stimulatedwithUCemission fromNaYF4:Yb,Tmand NaYF4:Yb,Er
nanoparticles, respectively. They further demonstrated the
reversible remotecontrol of diarylethene derivatives with one type
of UCN (112). The strategy lies in embeddingEr3+ ions and Tm3+ ions
into separated layers of nanoparticles to yield UV and visible
emissionupon high and low excitation power, respectively.
Spectral match between the absorption spectra of photosensitive
molecules and emission spec-tra of UCNs is the key factor for
photorelease studies. Branda and coworkers (113) demonstratedthe
release of carboxylic acid molecules from 3,5-dialkoxybenzoin
compounds with NIR light.NaYF4:Yb,Tm UCNs were employed as photon
transducers to provide UV emissions upon NIRirradiation. An
apparent decrease in the absorption of reactants and a concomitant
increase inthe absorption of products suggested the occurrence of
photorelease. Xing and coworkers (114)demonstrated the photorelease
application in vivo. D-luciferin molecules were covalently bondedto
the NaYF4:Yb,Tm UCNs. Upon UV emission generated by the UCNs, these
molecules werereleased and gave out bioluminescence. Both in vitro
and in vivo imaging results demonstratedthe advantage of UCNs.
In the NIR-triggered photoisomerization system, azobenzene
molecules are typical photosen-sitizers. They are able to conduct
the trans-cis photoisomerization process upon exposure to UVand
visible light fromYb3+-Tm3+-codopedUCNs.Yu and coworkers (115)
observed the fast bend-ing of azotolane containing cross-linked
liquid crystal polymer lms covered with NaYF4:Yb,TmUCNs upon 980-nm
excitation. This occurred due to the photoisomerization of
azotolane unitsand a change in the alignment of the mesogens. Li
and coworkers (116) fabricated an NIR-light-responsive
self-organized helical superstructure by dopingNaGdF4:Yb,Tm@NaGdF4
UCNs andchiral azobenzenes into a liquid crystal host. By tuning
the excitation power density, they wereable to achieve reversible
NIR-light-driven red, green, and blue reections in a single thin
lm(Figure 7). Aside from the above-mentioned applications, UCNs
have also been employed astemperature sensors (117) and energy
transducers for solar cells (25, 118, 119).
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Inte
nsit
y (a
.u.)
Wavelength (nm)
a
250 300 350 400 450 500 550 600
Wavelength (nm)250 300 350 400 450 500 550 600
b
c0 min
5 min3 min1 min
10 min20 min
Abs
orba
nce
(a.u
.)
Wavelength (nm)250 300 350 400 450 500 550 600
d0 s
5 min1 min30 s
8 min10 min
20 min
Wavelength (nm)250 300 350 400 450 500 550 600
f0 s 60 s 120 s
Refle
ctio
n (%
)
0
10
20
30
40
50
400 450 500 550 600 650 700
Wavelength (nm)
e
0
10
20
30
40
50
400 450 500 550 600 650 700
Wavelength (nm)
240 s 60 s 0 s
Figure 7Upconversion emission spectra of upconversion
nanomaterials upon irradiation with a 980-nm laser at(a) 2 W mm2
and (b) 0.15 W mm2. (c) Trans-cis and (d ) cis-trans
photoisomerization process ofazobenzenes in cyclohexane upon
irradiation with a 980-nm laser at 2 W mm2 and 0.15 W
mm2,respectively. Corresponding reection spectra of a cholesteric
liquid crystal at room temperature uponirradiation with a 980-nm
laser at (e) 2 W mm2 and ( f) 0.15 W mm2 for different times.
Figure reprintedwith permission from Reference 116. Copyright 2014
American Chemical Society.
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6. CONCLUSIONS AND FUTURE OUTLOOK
In this review, we summarize recent developments of RE UCNs in
terms of energy transferpathways, synthesis strategies, energy
transfer modulations, and applications. RE-based uorides,which are
excellent host matrices for UC processes, have been synthesized
with high quality byvarious methods. By rationally dening the RE
luminescent centers, host matrices, and energy-level-matched energy
extractors, investigators can easily obtain multicolor UC
emissions. Addi-tionally, efcient approaches to enhance UC
emissions include constructing core/shell structures,lowering the
local symmetry, and introducing noble metals and NIR antenna
ligands. Further-more, we show the emerging progress of novel
excitations triggered by energy transfer fromNd3+
ions. In this way, the excitation bands of UC emissions are
greatly enriched. To date, various bio-logical models, such as
cells, bacteria, C. elegans, mice, and rabbits, have been used to
test andverify the bioimaging value of RE UCNs. The exciting
results conrm that UCNs are superiorto conventionally used quantum
dots and organic dyes.
Despite these accomplishments, there are still some remaining
questions. For example, thefabrication of bright RE UCNs with small
size (sub10 nm) is required for better bioimagingpurposes. However,
the surface effects determined by the large surface-to-volume ratio
are dele-terious for UC emission efciency. Presently, attention is
mainly given to Er3+, Tm3+, and Ho3+
ions. Other RE activators, such as Tb3+ and Eu3+ ions, have
distinctive spectral ngerprints andlonger luminescence lifetimes.
Hence, skillfully mastering the UC energy transfer routes in
otherRE ions is also meaningful and challenging. In addition, only
one type of NIR antenna ligand hasbeen reported; nonetheless, due
to the signicant enhancement effect, further investigations
intoantenna-sensitized UC emissions should be urgently explored.
Valuable applications of UCNsshould also be developed to make full
use of UC emissions.
SUMMARY POINTS
1. RE ions are excellent supporters for photon UC emissions
owing to abundant energylevels. With diverse energy transfer
pathways, numerous pairs of RE ions are capable ofgenerating UC
emissions.
2. RE UCNs exhibit large anti-Stokes shifts, sharp-band
emissions, excellent photostabil-ity, and long luminescence
lifetimes. Owing to these advantages, RE UCNs have beenadopted as
more promising luminescent materials compared with quantum dots
andorganic dyes.
3. Synthetic strategies for the creation of high-quality RE UCNs
are well developed tosome extent. The size and morphology of UCNs
could be purposefully controlled byvarious methods.
4. Multicolor RE UCNs are of signicance to multicolor imaging
and multiplexed detec-tion applications. Controlling the RE doping
concentration, screening the host matrix,introducing extraneous
energy levels, and incorporating energy acceptors are excellentways
to tune the branch ratio of UC emissions.
5. The enhancement of UC emissions is a general concern for
researchers. Constructingcore/shell nanostructures, tailoring the
local crystal eld, incorporating noble metals,and introducing NIR
antenna ligands are methods to enhance UC emissions.
636 Sun et al.
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6. The introduction of Nd3+ ions to conventional ETU-motivated
UCNs greatly enrichedUC excitations within the NIR range.
Simultaneously, the heating effect induced bylong-time irradiation
of 980-nm light is largely minimized by 808-nm irradiation,
whichhas an excitation efciency that is similar to that of 980-nm
light.
7. With their unique and fascinating optical properties, RE UCNs
are used in a wide rangeof applications, from bioimaging to
photoresponsive applications. UC emissions fromRE UCNs are expected
to trigger more intriguing applications in the future.
DISCLOSURE STATEMENT
The authors are not aware of any afliations, memberships,
funding, or nancial holdings thatmight be perceived as affecting
the objectivity of this review.
ACKNOWLEDGMENTS
This work was supported by the NSFC (21371011, 21331001) and
MOST of China(2014CB643800).
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