Rapid Synthesis of Sub-10 nm Hexagonal NaYF4-Based Upconverting Nanoparticles using Therminol 66

Article abstract of DOI:10.1002/open.201700186

We report a simple one-pot method for the rapid preparation of sub-10 nm pure hexagonal (β-phase) NaYF₄-based upconverting nanoparticles (UCNPs). Using Therminol 66 as a co-solvent, monodisperse UCNPs could be obtained in unusually short reaction times. By varying the reaction time and reaction temperature, it was possible to control precisely the particle size and crystalline phase of the UCNPs. The upconversion (UC) luminescence properties of the nanocrystals were tuned by varying the concentrations of the dopants (Nd³⁺ and Yb³⁺ sensitizer ions and Er³⁺ activator ions). The size and phase-purity of the as-synthesized core and core–shell nanocrystals were assessed by using complementary transmission electron microscopy, dynamic light scattering, X-ray diffraction, and small-angle X-ray scattering studies. In-depth photophysical evaluation of the UCNPs was pursued by using steady-state and time-resolved luminescence spectroscopy. An enhancement in the UC intensity was observed if the nanocrystals, doped with optimized concentrations of lanthanide sensitizer/activator ions, were further coated with an inert/active shell. This was attributed to the suppression of surface-related luminescence quenching effects.

Full text of DOI:10.1002/open.201700186

DOI: 10.1002/open.201700186
Rapid Synthesis of Sub-10 nm Hexagonal NaYF4-Based
Upconverting Nanoparticles using Therminolꢀ 66
Julia Hesse+,[a] Dennis T. Klier+,[b] Massimo Sgarzi,[a] Anne Nsubuga,[a] Christoph Bauer,[c]
Jçrg Grenzer,[d] RenØ Hübner,[d] Marcus Wislicenus,[e] Tanmaya Joshi,*[a] Michael U. Kumke,*[b]
and Holger Stephan*[a]
We report a simple one-pot method for the rapid preparation
of sub-10 nm pure hexagonal (b-phase) NaYF4-based upcon-
verting nanoparticles (UCNPs). Using Therminolꢀ 66 as a co-sol-
vent, monodisperse UCNPs could be obtained in unusually
short reaction times. By varying the reaction time and reaction
temperature, it was possible to control precisely the particle
size and crystalline phase of the UCNPs. The upconversion (UC)
luminescence properties of the nanocrystals were tuned by
varying the concentrations of the dopants (Nd3+ and Yb3+ sen-
sitizer ions and Er3+ activator ions). The size and phase-purity
of the as-synthesized core and core–shell nanocrystals were as-
sessed by using complementary transmission electron micros-
copy, dynamic light scattering, X-ray diffraction, and small-
angle X-ray scattering studies. In-depth photophysical evalua-
tion of the UCNPs was pursued by using steady-state and
time-resolved luminescence spectroscopy. An enhancement in
the UC intensity was observed if the nanocrystals, doped with
optimized concentrations of lanthanide sensitizer/activator
ions, were further coated with an inert/active shell. This was at-
tributed to the suppression of surface-related luminescence
quenching effects.
1. Introduction
Upconverting nanophosphors have the ability to absorb multi-
ple near-infrared (NIR) photons and convert them into shorter-
wavelength visible light.[1,2] They show narrow emission and
sharp excitation bandwidths, large anti-Stokes’ shifts, high
chemical, thermal, and photostability, excellent resistance to
photobleaching, and low long-term toxicity.[2,3] Due to their
unique photophysical properties, upconverting nanoparticles
(UCNPs) have potential use in diverse biomedical fields such as
bioimaging, photodynamic therapy, and bioanalytics.[4–13] To
date, the most frequently studied UCNPs consist of hexagonal
closed packed (hcp) b-NaYF4 crystal host lattice co-doped with
a sensitizer–activator combination of Ln3+ ions (primarily Yb3+
combined with Er3+, Tm3+, or Ho3+ as activator ions).[14–24] De-
spite the fact that progress has been made in the synthesis of
lanthanide-doped UCNPs, scalable size-controlled production
of phase-pure b-NaYF4 nanocrystals still remains a great chal-
lenge. In particular, reproducible synthetic routes that can pro-
vide access to sub-10 nm nanocrystals with strictly controlled
morphology, high crystal phase purity, and exceptional upcon-
version (UC) luminescence are highly desirable. Reported
methods for the synthesis of hcp b-NaYF4 particles with diame-
ters below 10 nm mostly rely on the use of additives such as
Gd3+ and Eu3+ ion in high concentrations. This results in ma-
nipulated host lattices, which has a detrimental effect on the
luminescent properties of the final materials.[25–29] Other popu-
lar strategies to obtain sub-10 nm UCNPs involve the use of an
additional high-boiling solvent such as oleylamine (OM)[30] or
trioctylphosphine oxide (TOPO)[31–34] in the reaction formulation
under harsh synthesis conditions (high temperatures and long
reaction times), which limits the choice of compatible solvent
systems that can be used.
[a] J. Hesse,+ Dr. M. Sgarzi, A. Nsubuga, Dr. T. Joshi, Dr. H. Stephan
Institute of Radiopharmaceutical Cancer Research
Helmholtz-Zentrum Dresden-Rossendorf
Bautzner Landstraße 400, 01328 Dresden (Germany)
E-mail: t.joshi@hzdr.de
h.stephan@hzdr.de
[b] Dr. D. T. Klier,+ Prof. Dr. M. U. Kumke
Institute of Chemistry (Physical Chemistry), University of Potsdam
Karl-Liebknecht-Straße 24–25, 14476 Potsdam (Germany)
E-mail: kumke@uni-potsdam.de
[c] C. Bauer
Physical Chemistry, Technische Universität Dresden
Bergstraße 66b, 01062 Dresden (Germany)
[d] Dr. J. Grenzer, Dr. R. Hübner
Institute of Ion Beam Physics and Materials Research
Helmholtz-Zentrum Dresden-Rossendorf
Bautzner Landstraße 400, 01328 Dresden (Germany)
[e] M. Wislicenus
Center Nanoelectronic Technologies
Fraunhofer Institute for Photonic Microsystems
Kçnigsbrücker Straße 178, 01099 Dresden (Germany)
[+] These authors contributed equally to this work
Supporting Information and the ORCID identification number(s) for the
author(s) of this article can be found under https://doi.org/10.1002/
open.201700186.
ꢁ 2017 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
This is an open access article under the terms of the Creative Commons
Attribution-NonCommercial License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited and is not used for commercial purposes.
As part of our quest to develop highly luminescent sub-
10 nm UCNPs suitable for imaging applications, herein we
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report the utilization of Therminolꢀ 66 (T66) as a reaction co-
solvent for the synthesis of sub-10 nm phase-pure b-NaYF4 par-
ticles. T66 is a commercially available, high-boiling organic
fluid that consists of a mixture of terphenyls, hydrogenated
terphenyls, and hydrogenated polyphenyls (Figure S1 in the
Supporting Information), and it is frequently employed as a
heat-transfer liquid.[35–37] Importantly, it is commonly used in in-
dustrial processes and has been previously used to prepare
quantum dots of various sizes with high reproducibility.[38–40]
Yb3+-sensitized UCNPs are limited in deep-tissue imaging
applications, as the absorption peak of Yb3+ ions is centered
at l=980 nm, which overlaps with the absorption band of
water molecules. Moreover, overexposure of biological species
with l=980 nm irradiation can cause serious overheating
issues, resulting in cell death and tissue damage. Thus, one of
our goals was to prepare UCNPs excitable in the first optical
transparency window (l=650–950 nm), which is characterized
by minimal water absorption and, consequently, offers maximal
transparency of blood and organic tissues[41,42] without com-
promising the UC efficiency.
In this context, previous studies[43] demonstrated that UC
under l=800 nm excitation could be achieved through Nd3+
co-doping. With the above in mind, we also envisioned using
T66 to synthesize sub-10 nm b-NaYF4 UCNPs incorporating
Nd3+ ions in both the core and shell, obviating the need for
any additional phase-transformation steps or additives. The
photophysical properties of the individual UCNPs were investi-
gated in detail to illustrate the suitability and versatility of T66
for the controlled synthesis of highly luminescent UCNPs.
Thus, owing to the superior optical properties of pure hexago-
nal-phase UCNPs, a binary solvent mixture of T66/OA (3:2, v/v)
was used to obtain sub-10 nm hexagonal UCNPs. To deliver
highly luminescent sub-10 nm UCNPs, the Yb3+, Er3+, and
Nd3+ contents in the core were systematically optimized. Addi-
tionally, a few monolayer thick Yb3+/Nd3+ (10/25%) active
shell was epitaxially grown around the active core by slow in-
jection (feeding rate: 2 mLhÀ1) of a precursor solution consist-
ing of lanthanide oleates. Construction of an active shell
around the luminescent core might serve to enhance the UC
luminescence further by minimizing surface-related quenching
effects and separating the Nd3+ ions from the activators in the
core, as already reported for other core–shell UCNPs.[50–56]
It was possible to control the crystalline phase and size of
the nanoparticles (NPs) by varying the time and temperature
of the reaction (see below). The temperature and time regimes
explored as part of this work allowed us to prepare UCNPs
with sizes between 5 and 30 nm. To note, size and phase for-
mation were unaffected by the concentration of the dopants
(Nd3+/Yb3+/Er3+) used during the synthesis. However, as men-
tioned before, our interest was in delivering sub-10 nm pure-
phase NPs. For this reason, the discussion in this manuscript
will be primarily confined to sub-10 nm particles, and UCNPs
of other sizes, if discussed, will only be for comparison pur-
poses. The preparation conditions of the UCNPs discussed in
this work are summarized in Table 1. The as-synthesized UCNPs
were easily dispersible in different nonpolar solvents, such as
n-hexane, cyclohexane, and chloroform.
Transmission electron microscopy (TEM) was used to obtain
the size distribution of the UCNPs. The TEM images show mon-
odisperse particles with uniform spherical morphology
(Figure 1 and Table 2). Statistical analyses revealed narrow par-
ticle-size distributions for both the core [sample A: (9Æ1) nm]
and core–shell particles [sample C: (11Æ1) nm]. Consequently,
growing a shell around the core particles resulted in a size in-
crease by about 2 nm. Larger particles were obtained with ex-
tended reaction times, and they remained monodisperse with
a spherical shape [sample B: (20Æ2) nm, Figure S2]. Dynamic
light scattering (DLS) measurements in n-hexane further corro-
borated the narrow size distributions of the particles [sam-
ple A: (9Æ3) nm, sample B: (23Æ6) nm, sample C: (12Æ3) nm;
Table 2 and Figure S3].
2. Results and Discussion
2.1. Morphology and Crystalline Structure
Oleic acid (OA)-stabilized, di- and tri-doped NaYF4 UCNPs, excit-
able at l=980 and 795 nm, were prepared by a one-pot co-
precipitation method by using a suitable combination of
Yb3+/Er3+/Nd3+ chlorides, ammonium fluoride (NH4F), and
sodium oleate. Reaction in neat T66 resulted in the formation
of ultrasmall (ꢀ5 nm) cubic-phase particles for all of the time
and temperature conditions explored in this study. In compari-
son, coordinating solvents such as OM and OA are known to
favor the growth of pure hexagonal-phase UCNPs.[15,17,44–49]
Table 1. Reaction conditions used for the preparation of UCNP samples discussed in this work.
Sample[a]
Composition
Solvent
Time [min]
Temperature [8C]
A
B
NaYF4: Nd3+/Yb3+/Er3+ (1/20/2%): core
NaYF4: Nd3+/Yb3+/Er3+ (1/20/2%): core
NaYF4: Nd3+/Yb3+ (25/10%): shell
NaYF4: Nd3+/Yb3+/Er3+ (1/20/2%): core
NaYF4: Nd3+/Yb3+ (25/10%): shell
NaYF4: Nd3+/Yb3+/Er3+ (1/20/2%): core
NaYF4: Yb3+/Er3+ (20/2%): core
T66/OA (3:2, v/v)
T66/OA (3:2, v/v)
10
60
5[a]
10
5[b]
10
10
319
320
305
319
305
303
319
T66/OA (3:2, v/v)
C
D
E[c]
T66 (neat)
T66/OA (3:2, v/v)
[a] Samples A–C and E were pure hexagonal (b-phase), whereas sample D crystallized in the cubic a-phase (refer to XRD analysis for more details). [b] Shell
precursor solution was added over 30 min at 3058C. [c] Crystalline phase and size distribution were obtained from the powder XRD and DLS data, respec-
tively (Figure S3).
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Figure 1. Bright-field TEM images of a) sample A [NaYF4:Nd3+/Yb3+/Er3+
(1/20/2%), core] and b) sample C [NaYF4:Nd3+/Yb3+/Er3+ (1/20/2%) @
NaYF4:Nd3+/Yb3+ (25/10%), core–shell]. The relative ratio of dopants in the
NPs is given as mol%, as obtained by inductively coupled plasma mass
spectrometry analysis.
Figure 2. X-ray powder diffractograms of synthesized phase-pure UCNPs:
sub-10 nm cubic (sample D, crystallite size: ca. 3.5 nm) and hexagonal (sam-
ple A, crystallite size: ca. 6 nm) UCNPs doped with Nd3+/Yb3+/Er3+ along
with the non-doped hcp (b) and cubic (a) reference bulk materials. The dif-
fractogram for the over-10 nm hexagonal UCNPs (sample B, crystallite size:
ca. 13 nm) is provided for comparison. The mentioned crystallite sizes were
obtained from the recorded diffractograms by using the Williamson–Hall
equation [Eq. (1)].
Table 2. Comparison of various size-distribution metrologies used for
representative samples of sub-10 nm (sample A) and over-10 nm (sam-
ple B) hexagonal phase UCNPs.[a]
Sample
Size distribution [nm]
TEM XRD
DLS (n-hexane)
SAXS (n-hexane)
A
B
9Æ1
6Æ1
9Æ3
8.8Æ0.5
22Æ2
20Æ2 13Æ1
23Æ6
[a] Sample A: [NaYF4:Nd3+/Yb3+/Er3+ (1/20/2%), core]; sample B:
[NaYF4:Nd3+/Yb3+/Er3+ (1/20/2%)@NaYF4:Nd3+/Yb3+ (25/10%), core–
shell].
methods employing different solvent combinations generally
need longer reaction times or additional ions (e.g. Li+, Ca2+
,
Mn2+, Gd3+) to trigger the phase transformation and to pro-
duce small-sized hexagonal crystals.[14,25,27–33,47,57,60–62]
Here, one should note that although the calculation method
based on Equation (1) has its limitations, for particles with an
average size of the order of 10 nm and below, the calculated
uncertainty was estimated to be about 1 nm and slightly in-
creases with decreasing particle size. Consequently, a compari-
son with dynamic light scattering (DLS detects the hydrody-
namic radius of a particle in solution), with TEM, or with small-
angle X-ray scattering (SAXS) analysis leads to a particle size
that always has to be larger than the corresponding crystallite
size determined by XRD (Table 2). A possible reason for small
particles is the strong influence of the surface and for larger
particles is that during particle growth single-crystallite UCNPs
are not always formed.
Phase and corresponding lattice-parameter determination
were performed by using X-ray powder diffraction (XRD) analy-
sis. In addition, by using the Williamson–Hall equation [Eq. (1)
in the Experimental Section, X-ray diffraction studies], we also
determined the average coherent crystallite sizes from the re-
corded diffractograms (see below). Here, the recorded XRD
patterns (Figure 2) indicate that (Yb3+/Er3+/Nd3+)-doped NaYF4
nanoparticles (NPs) crystallized either in the cubic face-cen-
tered cubic (fcc) a-phase (space group: Fm3m) or in the hexag-
onal b-phase (space group: P63/m).[17,57] The 3.5 nm sized
NaYF4 particles (prepared in pure T66) crystallized in the fcc a-
phase [ICDD(2016) card No. 04–013–7404],[58] whereas 6 and
13 nm sized particles (prepared in OA/T66 mixtures) grew in
the hcp b-phase [ICDD(2016) card No. 00–16–0334].[59] On the
other hand, small single hexagonal-phased (hcp) particles
could be readily produced by performing the reaction in a
binary solvent mixture of T66/OA (3:2, v/v) and by raising the
reaction temperature to 3178C or above. The lattice constants
of the 3.5 nm sized fcc UCNPs were slightly higher than the
reference data from the ICDD cards with a=5.54 Š [ICDD
(2016) card No. 04–013–7404: a=5.47 Š]. Similar behavior was
found for the hexagonal particles with a=6.01 Š and c=
3.53 Š [ICDD (2016) card No. 00–16–0334: a=5.96 Š, c=
3.53 Š].[15,59]
We also used the SAXS method to validate the particle-size
distributions. SAXS is a promising technique for measuring par-
ticle sizes below 10 nm, a point at which other methods may
reach their detection limit. It is sensitive to scattering density
variations and was used here to analyze the shape and the
size of the synthesized UCNPs. With SAXS, however, it is not
possible to measure the contributions of individual atoms to
the scattering signal but rather the changes in scattering con-
trast.[63] The measurements were performed by using a solution
of NPs in n-hexane (Figure 3). Therefore, the scattering contrast
was formed by changes between the NPs and the solvent. Ad-
ditionally, we assumed that there was negligible spatial correla-
tion between the individual particles in solution and that the
resulting scattering signal was the averaged sum of the scat-
tering signal of one single mean particle. A major advantage of
this technique exists in the possibility to observe NP shapes
It should be emphasized that upon using T66 as a co-sol-
vent, a reaction time as short as 10 min (at 3198C) was suffi-
cient to produce sub-10 nm pure hexagonal (b-phase) UCNPs.
In comparison, most of the reported thermal decomposition
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Figure 3. Integrated SAXS signals of a) sub-10 nm sample A [NaYF4:Nd3+
Yb3+/Er3+ (1/20/2%), core] and b) over-10 nm sample B [NaYF4:Nd3+/Yb3+
/
/
Er3+ (1/20/2%)@NaYF4:Nd3+/Yb3+ (25/10%), core–shell]. The measurements
were performed on NP solution in hexane. The model was simulated and
fitted to the measurements by assuming a filled nanoparticle in solution, al-
lowing certain size variation.
Scheme 1. Schematic representation of the energy levels involved in the UC
mechanism of a Yb3+ and Er3+ dopant ion system following excitation at
l=976 nm. Furthermore, the UC mechanism of a Nd3+, Yb3+, and Er3+
dopant ion system following excitation at l=795 nm is shown. The solid
lines pointing upwards represent energy absorption, the dashed lines repre-
sent energy-transfer processes, the wavy lines represent nonradiative relaxa-
tion processes, the blue dotted lines represent cross-relaxation (CR) process-
es between excited Er3+ species, and the solid lines pointing downwards
represent visible and NIR emissions. The excitation energy transfer from
Yb3+ ions to Er3+ ions occurs by long-range energy migrations from the
Yb3+ absorption site through the Yb sublattice.[67]
and sizes in situ, without any sample preparation-induced
effects.
[64]
The program “X+” was used to analyze the radially inte-
grated scattering pattern by assuming a model that represents
the UCNPs as filled spheres of known density. The sphere
radius was used as a single fitting parameter. In our case, the
values obtained from this readily available method matched
very well with the TEM and DLS results and even extended our
knowledge of NP shape.
2.2. Upconversion Luminescence Properties
2.2.1. Analysis of Di-doped (Yb3+/Er3+) b-NaYF4 UCNPs
The upconversion luminescence spectrum of b-phase UCNPcore
(b-NaYF4:Yb3+/Er3+ 20/2%) measured in cyclohexane shows
4
three intense bands centered at l=525 (2H11/2! I15/2 transition,
4
4
G1), 545 (4S3/2! I15/2 transition, G2), and 660 nm (4F9/2! I15/2
transition, R), upon excitation at l=976 nm, consistent with
excitation energy-transfer (ET) processes between the Yb3+
(sensitizer) and Er3+ (activator) ions (Scheme 1).[35–37,65,66]
Figure 4. Comparison of the UC emission intensities of UCNPcore b-
NaYF4:Yb3+/Er3+ (20/2%) UCNPs with different sizes, measured in cyclohex-
ane, upon excitation at l=976 nm. The Er3+ emission bands G1 and G2
(left) as well as R (right) are shown.
An increase in the UC emission intensity was observed upon
increasing the size of the particles (Figure 4). Furthermore, the
relative GG1+G2/R peak ratio of UCNPcore also decreased upon in-
creasing the NP diameter. These effects can be attributed to a
decrease in the surface-area-to-volume ratio and subsequent
reduction in the contributions from surface-related lumines-
cence quenching processes, resulting in a higher UC emission
intensity (Figure 5). As a consequence, the excited-state life-
time of the Er3+ ions increased, which caused the probability
for cross-relaxation processes between two excited Er3+ ions
to increase as well. This resulted in further population of the
4F9/2 energy level of the Er3+ ions, which thereby enhanced the
4
4
luminescence intensity of the F9/2! I15/2 transition (R). This
effect was independent of the particle shape, as the surface-
area-to-volume ratio (calculated by assuming UCNPs as
spheres or as hexagonal prisms) showed similar trends.[68]
The UC luminescence properties of UCNPcore b-NaYF4:Yb3+
/Er3+ (20/2%) covered with an inert shell of pure NaYF4
Figure 5. The GG1+G2/R peak ratio of the UC emission bands of UCNPcore b-
NaYF4:Yb3+/Er3+ (20/2%) as a function of the surface area to volume (SA/V)
ratio assuming a particle shape of hexagonal prism (left) and sphere (right).
The red circles mark UCNPcore covered with a nonactive shell of pure NaYF4
(UCNPcore@NaYF4).
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(UCNPcore@NaYF4) were also evaluated. Typically, the vibrational
coupling interaction between the solvent (or ligand) and the
lanthanide ions results in a decrease in the UC emission. This
ence of two different populations of erbium ions: the former is
located inside the NP (t2, bulk phase) and the latter (t1) lies on
the surface of the UCNPs. The R band showed a longer aver-
age decay time (tm =67 ms) than the pooled green emission
band (GG1+G2, tm =55 ms) [see Eq. (4) in the Experimental Sec-
tion, Luminescence Spectroscopy]. Furthermore, an increase in
the kinetics of UC luminescence was observed, and the rise ki-
netics were fivefold longer for R than for GG1+G2. The difference
in rise times resulted from the different population kinetics
and rate constants of the involved energy levels.
2
coupling is much more effective for the F5/2 level of the Yb3+
4
4
ions or the I11/2 and I13/2 levels of the Er3+ ions, which are cru-
cial for the population mechanism of the state responsible for
2
4
the UC emission (4F9/2, H11/2, or S3/2). In contrast, the probabili-
4
2
4
ty of vibrational coupling involving F9/2, H11/2, or S3/2 levels of
Er3+ is much lower because of the higher number of phonons
needed.[69] Shielding the sensitizer ions in the core is thus nec-
essary for reducing this vibrational coupling. Moreover, the
growth of a shell around the core of the UCNPs also serves to
passivate trap states due to lattice defects on the surface.
The core–shell UCNPs displayed higher UC emission than
the UCNPcore materials (Figure 6). Passivation of the UCNP sur-
face with a 1 nm thick shell led to a roughly twofold increase
in the UC luminescence intensity (this increase was also corro-
borated by an increase in the luminescence decay times, see
below), providing evidence for surface-related quenching ef-
fects. Moreover, the R transition increased its intensity as a
result of the presence of the shell, which thus caused the
GG1+G2/R peak ratio of UCNPcore@NaYF4 to be much lower than
that of UCNPcore of the same size (Figure 5, points circled in
red).
For UCNPcore@NaYF4, an increase in the UC luminescence
decay time was registered, which indicated a lower contribu-
tion from nonradiative deactivation processes. For
UCNPcore@NaYF4, no Er3+-based surface species were expected
to contribute to the overall luminescence. As a result, monoex-
ponential UC decay kinetics were observed (see Figure S5). Fur-
thermore, the presence of the inert shell had a distinct effect
of increasing the rise kinetics (Table 3). This was due to the fact
that the NaYF4 inert shell prevented nonradiative deactivation
of Yb3+ and Er3+ excited states caused by ligand and/or cyclo-
hexane CH oscillators.[71] Under these conditions, the excitation
energy was dissipated by means of long-distance migrations
through the Yb sublattice and was finally transferred to Er3+
which resulted in longer rise times than those observed for
,
[72]
UCNPcore
.
2.2.2. Analysis of Tri-doped (Yb3+/Er3+/Nd3+) b-NaYF4 UCNPs
The UC properties of tri-doped UCNPs containing Nd3+ ions as
a primary sensitizer, Yb3+ as bridging ions (secondary sensitiz-
er), and Er3+ as activator ions were also investigated. Here, the
Nd3+ ions first absorb photons in the NIR range (lex =795 nm,
4
4I9/2! F5/2 transition), and this is followed by relaxation to the
4F3/2 level. Next, an ET process sensitizes the bridging Yb3+
4
ions [4F3/2 (Nd3+)+2F7/2 (Yb3+)! I9/2 (Nd3+)+2F5/2 (Yb3+)],
Figure 6. Influence of the shell thickness on the UC emission intensity of
UCNPcore@NaYF4 (in cyclohexane) excited at l=976 nm. Er3+ transitions G1
and G2 (left) as well as R (right) are shown. The luminescence of core-only
b-NaYF4:Yb3+/Er3+ (20/2%) UCNPs is shown for comparison (black solid
trace).
4
which in turn transfer the excitation energy to the I11/2 state of
Er3+, followed by the UC processes described in Scheme 1.
Moreover, direct excitation of the Yb3+ ions at l=976 nm is
also possible in these systems, obviating the participation of
Nd3+ ions in the ET chain.
Under l=976 nm excitation, upon increasing the Nd3+
doping level from 1 to 5% in UCNPcore,Nd, a significant decrease
in the UC emission intensity was observed (Figure 7, left). One
explanation for the observed change in UC emission is that as
We also studied the luminescence decay kinetics of the G1,
G2, and R transitions for the UCNPs. Both the UCNPcore and
UCNPcore@NaYF4 particles exhibited luminescence decay times
on the microsecond timescale (Table 3). For data analysis of
UCNPcore, a biexponential decay law was used [see Eqs. (2) and
(3) in the Experimental Section, Luminescence Spectroscopy].
The complex decay kinetics[70] can be attributed to the pres-
Table 3. Upconversion decay kinetics of the pooled green emission
GG1+G2 and red emission R of UCNPcore and UCNPcore@NaYF4
(lex =976 nm).
Sample
Emission
Rise [ms]
tm [ms]
UCNPcore
GG1+G2
R
GG1+G2
R
1.7
10.7
7.1
54.9
66.7
75.0
90.4
Figure 7. Upconversion emission spectra of UCNPcore,Nd (in cyclohexane)
doped with 1 and 5% Nd3+ upon excitation at lex =976 nm (left) and
795 nm (right). Size of UCNPs: (9Æ1) nm.
UCNPcore@NaYF4
20.5
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the concentration of Nd3+ ions was increased, the probability
of parasitic energy back-transfer processes between the Er3+
and Nd3+ ions increases, which results in lower UC emis-
sions.[73] In contrast, under excitation at l=795 nm, an en-
hancement in the UC luminescence was observed as Nd3+
doping was increased (Figure 7, right). These results can be as-
cribed to the different roles of the Nd3+ ions in the two UC
processes. At lex =976 nm, the Nd3+ ions only act as quench-
ers for the excited Er3+ ions, whereas if the UCNPs are excited
at l=795 nm, the Nd3+ ions act as primary sensitizers and
absorb light directly. Moreover, the absorption cross section of
Nd3+ is 10-fold higher at lex =795 nm than that of Er3+ at l=
976 nm,[74] which is also beneficial to the efficiency of the UC
process.
Table 4. Upconversion decay kinetics of the pooled green emission
GG1+G2 and red emission R of UCNPcore,Nd and UCNPcore,Nd@NaYF4:
Nd3+/Yb3+ (25/10%) particles.
Sample
Emission Rise[ms] tm [ms] Rise[ms] tm [ms]
lex =795 nm lex =976 nm
UCNPcore,Nd
GG1+G2
R
UCNPcore,Nd@NaYF4:Nd3+/Yb3+ GG1+G2
(25/10%)
4.3
13.2
20.5
39.7
50.7
70.2 10.7
73.8
113.2 20.5
1.7
54.9
66.7
75.0
90.4
7.1
R
underline the minor contribution of nonradiative deactivation
processes in core–shell UCNPs. Moreover, at lex =795 nm, the
Nd3+ ions in the shell, present in a relatively high concentra-
tion, acted as harvesting units and further improved the
brightness of UCNP as a result of increased absorption per NP.
Insight into the nature of the upconversion mechanism was
obtained by analyzing the variation in integrated intensities of
the green (G1 and G2) and red (R) emission bands as a func-
tion of applied excitation power density (f). The results
showed that in UCNPcore,Nd, the G1 and G2 transitions were
based on a two-photon process at lex =976 nm (Figure S5).
The R transition, on the other hand, could be described as a
two-photon process as well as a three-photon process. The
UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%) particles behaved simi-
lar to the UCNPcore,Nd particles. There was no significant in-
crease in the number of photons (n), and the addition of Nd3+
(25 mol%) and Yb3+ (10 mol%) in the shell had no further in-
fluence on the UC population processes. Moreover, no
variations in n for the G1 and G2 transitions upon excitation
at l=795 nm (Figure S6) in either the core or core–shell
UCNPs were registered. However, a decreased probability of
three-photon processes was observed for R transitions in
UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%), which can be attribut-
ed to a significant change in the favored population mecha-
nism for R transition due to changes in population density of
the excited Er3+ levels.
Quantitative evaluation of the relative UC emission intensi-
ties upon excitation at l=795 and 976 nm was also performed
by using UCNPcore,Nd (1% Nd3+ doping). As can be observed
from Figure 8 (left), the UCNPcore,Nd emission intensity at l=
Figure 8. Upconversion emission spectra of UCNPcore,Nd (left, sample A) and
UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%) (right, sample C) upon excitation at
l=976 and 795 nm [power density (f)=85 mWcmÀ2 for both excitation
wavelengths; cUCNP =7 mgmLÀ1 in cyclohexane].
795 nm was only 6% of the corresponding UC emission inten-
sity at l=976 nm excitation. Lowering of the UC intensity can
be rationalized on the basis of an additional energy transfer
step involving the Nd3+ and Yb3+ ions that occurs under l=
795 nm excitation, which is anticipated to decrease the effi-
ciency of the entire ET chain. Covering the UCNPcore,Nd with an
active NaYF4:Nd3+/Yb3+ (25/10%) shell, however, significantly
enhanced the UC emission intensity at both excitation wave-
lengths (Figure 8, right). UC emission intensity at lex =795 nm
was calculated to be 47% of its UC emission upon excitation
at l=976 nm, which reaffirmed the ability of the active shell
to reduce Er3+–Nd3+ energy back transfer as a consequence of
the spatial separation between these two ions.[73]
The comparison between excitation wavelength did not
show large differences in the number of photons [e.g. n=1.8
4
4
(lex =795 nm) and n=1.9 (lex =976 nm) for the S3/2! I15/2
transition]. This is in contrast to recent findings by Skripka
et al.,[75] who described larger differences for n (e.g. for the
4
4S3/2! I15/2 transition, a reported decrease from n=1.98 at
lex =976 nm to n=1.24 at lex =806 nm). It should be under-
lined that the excitation conditions used in these two works
are very different. Whereas Skripka et al. reported on results for
excitation power density (f) >10 WcmÀ2 under continuous-
wave excitation, we used smaller f (<200 mWcmÀ2) under
pulsed excitation. Thus, in our case, direct excitation of Er3+ by
ground state absorption (GSA) or excited state absorption
(ESA) processes, which would lead to a decrease in n, was min-
imal, and the observed n values for both excitation wave-
lengths do not differ significantly. Additionally, the UCNPs in-
vestigated in the present study are smaller in size and endow-
ed with a thinner shell, resulting in different contributions
from surface-related processes due to a different surface-area-
to-volume ratio.
The luminescence decay kinetics of the G1, G2, and R transi-
tions of the UCNPcore,Nd and UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/
10%) particles were studied under excitation at l=795 and
976 nm (Table 4). Data analysis revealed that core–shell
UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%) exhibited longer decay
times than UCNPcore,Nd. In addition, the rise times for all UC
transitions were longer, which suggested an efficient long-
range energy migration took place between the sensitizer ions
in the shell and the activator ions in the core,[72] as already de-
scribed for UCNPcore and UCNPcore@NaYF4. Overall, these results
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At both excitation wavelengths, the intensity ratio G1/G2
was constant for the investigated range of excitation power
densities (13–85 mWcmÀ2) for the core and core–shell NPs
(Figure 9). The G/R ratio, on the other hand, showed a signifi-
cant decrease at higher f values upon excitation at l=
976 nm. This indicated a higher probability of cross-relaxation
processes involving Er3+ ions, which resulted in a higher popu-
action times and by varying the reaction temperature. Using
this protocol, sub-10 nm Nd3+/Yb3+/Er3+ tri-doped core and
related core–shell b-NaYF4 UCNPs were also synthesized. These
UCNPs showed efficient upconversion (UC) luminescence in-
tensity under irradiation at an intrinsically biocompatible wave-
length of 795 nm. The advantage of having different options
(e.g. having two sensitizer ions present in the UCNPs) to sensi-
tize the UC luminescence was demonstrated. In general, we
presented a highly time-saving method for the synthesis of
sub-10 nm UCNPs with improved luminescence properties that
hold great potential for various biomedical applications. The
ability to extend the synthetic methodology to the design of
core–shell UCNPs with a shell thickness of at least 1 nm high-
lights the versatility of the reported protocol. Our future re-
search will focus on understanding the exact role of Thermi-
nolꢀ 66 in the growth mechanism of the UCNPs and unraveling
the mechanisms underlying the involved phase-transition pro-
cesses. To expand greatly the applicability of this method in
the synthesis of highly luminescent ultrasmall UCNPs, we are
also exploring its use for readily delivering nanoparticles with
other lanthanide and host matrix compositions.
4
lation of the F9/2 level responsible for the red emission. This
effect was independent of the architecture of the particles
(core or core–shell UCNPs).
Figure 9. Plot of the UC emission intensity ratio G1/G2 (left) and G/R (right)
of UCNPcore,Nd and UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%) as a function of
the excitation power density (f) measured for lex =976 and 795 nm.
Experimental Section
Chemicals
In contrast, the power density dependency of the G/R ratio
was no longer observed if the UCNPs were excited at l=
795 nm (Figure 9, right). This behavior can be explained on the
basis of the influence of the Nd3+ ions on the saturation effect
in the ET chain. Nd3+ ions with their large absorption cross sec-
tion (1.2”10À19 cm2 at l=808 nm[74] vs. 1.2”10À20 cm2 for
Yb3+ ions at l=980 nm) possibly lower the saturation intensity
(which is inversely proportional to the cross section)[76] by
about one order of magnitude, and this leads to a constant
G/R ratio under the conditions of our study.
Yttrium(III) chloride hexahydrate (99.99%), ytterbium(III) chloride
hexahydrate (99.99%), neodymium(III) chloride hexahydrate
(99.99%), erbium(III) chloride hexahydrate (99.99%), ammonium
fluoride (ACS reagent ꢁ98.0%), sodium hydroxide (reagent grade
ꢁ98.0%), and oleic acid (technical grade 90%) were purchased
from Sigma–Aldrich and were used as received. Therminolꢀ 66
(T66) was supplied by JULABO GmbH (Seelbach, Germany, product
number: 8940131). All other organic solvents used were of the
highest grade available.
Despite the more efficient light-absorption ability, the G/R
transition band ratio of UCNPcore,Nd@NaYF4:Nd3+/Yb3+ (25/10%)
was lower than that calculated for UCNPcore,Nd because of the
higher rate of Er3+-based cross-relaxation processes, which are
favored in the presence of Nd3+ ions. Notably, variation in the
intensity ratio of the UC emissions bands was found to be
highly dependent on the lanthanide ions used and the compo-
sition of the host lattice. For instance, the excitation power
density did not have any effect on the G/R ratio of Er3+-doped
a-phase NaYF4 UCNPs (data not shown). Nevertheless, the exci-
tation power density could have a huge impact on the ratio of
emission bands and should always be taken into careful con-
sideration upon understanding the UC luminescence proper-
ties of UCNPs.
Synthesis
Sodium oleate: Oleic acid (OA, 50 mmol) was added dropwise to a
solution of sodium hydroxide (50 mmol) in absolute ethanol
(700 mL). The mixture was stirred overnight at RT. Sodium oleate
was obtained as a white solid after evaporation of the solvent
under reduced pressure at 408C.
Core UCNPs (UCNPcore,Nd): In a typical procedure, YCl3 (0.77 mmol),
NdCl3 (0.01 mmol), YbCl3 (0.2 mmol), and ErCl3 (0.02 mmol) were
dissolved in a mixture of T66/OA (3:2, v/v, 20 mL), and the mixture
was degassed for 45 min at 1208C under vacuum to obtain a
yellow homogenous solution. Following this, the solution was
cooled to 908C. Afterwards, sodium oleate (2.5 mmol) and ammo-
nium fluoride (4 mmol) were added at once under an argon atmos-
phere. A second degassing step at 908C was then performed to
generate anhydrous, oxygen-free conditions without premature
decomposition of NH4F. Subsequently, the solution was heated
(heat rate 108CminÀ1) up to a final temperature of 280–3258C
(Æ18C). The mixture was maintained at this temperature for 10–
60 min under an argon atmosphere and was then cooled rapidly
by a strong stream of air to give particles with a defined size
(Table 1). After cooling, the UCNPs were precipitated from the solu-
tion by the addition of absolute ethanol (25 mL) and were isolated
by centrifugation (3939”g, 5 min). The supernatant was discarded,
3. Conclusions
We demonstrated a facile method for the preparation of high-
quality sub-10 nm b-phase NaYF4 nanocrystals by introducing
Therminolꢀ 66 as a new reaction co-solvent. The size and mor-
phology of upconverting nanoparticles (UCNPs) could be pre-
cisely tuned to the 1–10 mmol reaction scale by using short re-
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and the white pellet was washed several times by dispersing it in a
minimal amount of n-hexane. The nanoparticles were then precipi-
tated again with the addition of ethanol and were centrifuged.
This washing procedure was performed to ensure elimination of
the reaction surfactants as well as any NaF impurities that were
formed. Finally, the purified oleate-coated UCNPs were dispersed
in n-hexane or chloroform (10 mL). The UCNPs could be stored at
room temperature and were colloidally stable for several months.
The molar compositions of lanthanide ions in the final UCNP sam-
ples were determined by inductively coupled plasma mass spec-
trometry (ICP-MS) and showed good agreement with the stoichio-
metric ratios used for the synthesis.
XRD analysis, hydrophobic nanoparticle samples (dissolved in n-
hexane) were precipitated in an excess amount of ethanol, and a
thick film of a concentrated solution was slowly dried on a silicon
wafer.
The crystallite size (L) was calculated from the X-ray diffractograms
by using the Williamson–Hall equation [Eq. (1)]:
2p
L
4p
l
1
Dq q
k ꢂ
e0 ꢂ q; q
ꢂ sin q
in which L is the coherent size of the crystallite, k is a shape factor
depending on particle morphology (here, 0.9 was used), l is the X-
ray wavelength, e’ is an equivalent to the microstrain in the crystal-
lite itself or corresponds to lattice constant variations of an infinite
number of crystallites, q is the Bragg angle of the X-ray diffraction
peak, and Dq is the full width at half maximum of the diffraction
peaks as a function of q. The reciprocal of the intercept Dq0 (L=
2kp/Dq0) yielded the average crystallite size, L. The values given in
Table 2 are the average crystallite sizes based on all isolated peaks
calculated for samples A and B.[77]
Core–shell UCNPs (UCNPcore,Nd@NaYF4:Nd3+/Yb3+ 25/10%): A shell
precursor solution was prepared as follows: T66 (8 mL), OA (4 mL),
YCl3 (0.65 mmol), NdCl3 (0.25 mmol), and YbCl3 (0.1 mmol) were
mixed in a 50 mL flask, and the mixture was heated at 1208C for
60 min under vacuum. Afterwards, the solution was cooled to
908C. Next, NH4F (4 mmol) and sodium oleate (2.5 mmol) were
added under an argon atmosphere. The mixture was stirred for an-
other 30 min under an argon atmosphere to dissolve the solids. Fi-
nally, the shell solution was stored at RT until further use.
To apply a shell on the synthesized core nanoparticles, the slow in-
jection method was used. First, a solution of core UCNPs (60 mg)
in T66/OA (3:2, v/v, 20 mL) was heated at 758C for 30 min. After-
wards, an argon atmosphere was applied, and the temperature
was elevated to the injection temperature of 3058C. Thereafter, the
shell precursor (1 mL) solution was added dropwise while control-
ling the injection velocity (2 mLhÀ1) by using a syringe pump. After
the addition of the precursor solution, the temperature was main-
tained at 3058C for 5 min before it was cooled to 758C. The purifi-
cation steps were the same as those described for the core UCNPs.
The molar compositions of the lanthanide ions in the final UCNP
samples were determined by ICP-MS and showed good agreement
with the stoichiometric ratios used for the synthesis.
SAXS Studies
SAXS measurements were performed on UCNP solutions in n-
hexane by using a modified Empyrean diffractometer equipped
with a two-dimensional side-by-side optics and an extended fine-
focus Cu tube with a point spot size of 150 mm. It was ensured
that the primary beam width remained almost constant over the
entire dynamic range and that no secondary maxima occurred. We
used a PIXcel3D 2”2 detector to register the scattering signal. The
setup allowed us to work without any beam stop; the detector dis-
tance was varied between 155 and 500 mm depending on the size
of the nanoparticles. Typical measurement times for one SAXS
measurement were about 5000 s.
Luminescence Spectroscopy
TEM
Steady-state as well as time-resolved upconversion luminescence
spectra were obtained by applying a wavelength tunable pulsed
Nd:YAG/OPO laser system (laser: Quanta Ray, Spectra-Physics,
Mountain View, CA, USA; OPO: GWU-Lasertechnik Vertriebsges.
GmbH, Erftstadt, Germany) operating at 10 Hz as the excitation
light source (at 26 mJ, 120 mWcmÀ2) and were recorded by using
an intensified CCD camera (iStar DH720-18V-73, Andor Technology,
Belfast, Great Britain) coupled to a spectrograph (Shamrock SR
303i, Andor Technology, Belfast, Great Britain) equipped with a
600 lines per mm grating blazed at 500 nm. The luminescence
measurements were collected in the so-called boxcar technique by
applying an initial gate delay of Dt=500 ns relative to the excita-
tion laser pulse and gate widths of dt=30 ms for signal accumula-
tion. For determination of the luminescence decay kinetics, the ini-
tial gate delay was stepwise increased and was used for the con-
struction of the intensity–time traces. Typically, 300 spectra were
recorded and used in determining the luminescence decay kinet-
ics. For the laser-power-dependent luminescence measurements, a
set of absorptive gray filters and a power meter FieldMax 2-TOP
with connected PowerMax PM10V1 (Coherent, Portland, OR, USA)
were used. The luminescence decay kinetics were evaluated by
using the following equation [Eq. (2)]:
Bright-field TEM analysis was performed by using an image Cs-cor-
rected FEI Titan 80–300 electron microscope operating at an accel-
erating voltage of 300 kV. The size distributions of the UCNPs were
determined from the acquired TEM images by using ImageJ
image-processing software, by analyzing 300 particles in each case
(version 1.50b, NIH, USA).
DLS Measurements
The hydrodynamic diameters of the prepared nanocrystals were
determined by DLS by using a Malvern Zetasizer Nano ZS Instru-
ment with a He–Ne laser (l=632 nm) (Malvern Instruments GmbH,
Germany). Measurements were performed at a constant tempera-
ture of 258C with a detection angle of 1738 and were processed
by means of the associated Zetasizer software version 7.12.
XRD Studies
The crystal structures and phase purities of the prepared nanoma-
terials were studied by powder XRD measurements by using an
Empyrean diffractometer from PANalytical equipped with a Gçbel
mirror using CuKa radiation (l=0.154 nm). The XRD patterns were
recorded by a PIXcel3D 2”2 detector in the 2q range of 1.3 to
130.18 by using a typical scanning step of 0.0138 per 592 s. For
ꢀ
ꢁ
ꢀ
ꢁ
Àt
Àt
I t
A
B1 ꢂ exp
B2 ꢂ exp
2
t1
t2
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[19] P. Du, A. M. Deng, L. H. Luo, J. S. Yu, New J. Chem. 2017, 41, 13855–
13861.
[20] P. Du, P. Zhang, S. H. Kang, J. S. Yu, Sens. Actuators B 2017, 252, 584–
591.
in which t1 and t2 represent the luminescence decay times. From
the coefficients B1 and B2, the relative fractions a1 and a2 were cal-
culated according to the following equation [Eq. (3)]:
[21] M. Y. Hossan, A. Hor, Q. Luu, S. J. Smith, P. S. May, M. T. Berry, J. Phys.
Chem. C 2017, 121, 16592–16606.
[22] M. Kaiser, C. Wurth, M. Kraft, I. Hyppanen, T. Soukka, U. Resch-Genger,
Nanoscale 2017, 9, 10051–10058.
[23] U. Kostiv, V. Lobaz, J. Kucka, P. Svec, O. Sedlacek, M. Hruby, O. Janousko-
va, P. Francova, V. Kolarova, L. Sefc, D. Horak, Nanoscale 2017, 9,
16680–16688.
[24] A. Pilch, C. Wurth, M. Kaiser, D. Wawrzynczyk, M. Kurnatowska, S. Ara-
basz, K. Prorok, M. Samoc, W. Strek, U. Resch-Genger, A. Bednarkiewicz,
Small 2017, 13, 1701635.
[25] S. Dühnen, T. Rinkel, M. Haase, Chem. Mater. 2015, 27, 4033–4039.
[26] N. J. J. Johnson, W. Oakden, G. J. Stanisz, R. S. Prosser, F. C. J. M. van Veg-
gel, Chem. Mater. 2011, 23, 4877–4877.
[27] X. M. Li, D. K. Shen, J. P. Yang, C. Yao, R. C. Che, F. Zhang, D. Y. Zhao,
Chem. Mater. 2013, 25, 106–112.
[28] B. Voss, M. Haase, ACS Nano 2013, 7, 11242–11254.
[29] B. Voss, J. Nordmann, A. Uhl, R. Komban, M. Haase, Nanoscale 2013, 5,
806–812.
[30] A. D. Ostrowski, E. M. Chan, D. J. Gargas, E. M. Katz, G. Han, P. J. Schuck,
D. J. Milliron, B. E. Cohen, ACS Nano 2012, 6, 2686–2692.
[31] M. Banski, M. Afzaal, A. Podhorodecki, J. Misiewicz, A. L. Abdelhady, P.
O’Brien, J. Nanopart. Res. 2012, 14, 1228.
B1 ꢂ t1
a1
3
B1 ꢂ t1 B2 ꢂ t2
The average decay time (tm) was calculated by the following equa-
tion [Eq. (4)]:
P
1 Bit2i
z
i
z
i
P
4
tm
1 Biti
Acknowledgements
We thank Andrea Scholz and Janine Partsch for excellent techni-
cal assistance. This work was supported by the Helmholtz Initia-
tive and Networking Fund [Functional Nanomaterials for Multi-
modality Cancer Imaging (NanoTracking), project ID: VH-VI-421],
a Helmholtz-Zentrum Dresden-Rossendorf fellowship to M.S., and
an Alexander von Humboldt Foundation research fellowship to
T.J.
[32] M. Banski, A. Podhorodecki, J. Misiewicz, M. Afzaal, A. L. Abdelhady, P.
O’Brien, J. Mater. Chem. C 2013, 1, 801–807.
[33] J. Shan, X. Qin, N. Yao, Y. Ju, Nanotechnology 2007, 18, 445607.
[34] G. F. Wang, W. P. Qin, J. S. Zhang, L. L. Wang, G. D. Wei, P. F. Zhu, R. J.
Kim, J. Alloys Compd. 2009, 475, 452–455.
Conflict of Interest
[35] Y. S. Chen, W. He, Y. C. Jiao, H. H. Wang, X. L. Hao, J. X. Lu, S. E. Yang, J.
Lumin. 2012, 132, 2247–2250.
The authors declare no conflict of interest.
[36] H. Dong, S. R. Du, X. Y. Zheng, G. M. Lyu, L. D. Sun, L. D. Li, P. Z. Zhang,
C. Zhang, C. H. Yan, Chem. Rev. 2015, 115, 10725–10815.
[37] D. T. Klier, M. U. Kumke, J. Mater. Chem. C 2015, 3, 11228–11238.
[38] S. Asokan, K. M. Krueger, A. Alkhawaldeh, A. R. Carreon, Z. Z. Mu, V. L.
Colvin, N. V. Mantzaris, M. S. Wong, Nanotechnology 2005, 16, 2000–
2011.
[39] N. Gresty, O. Masala, C. Newman, S. Whitelegg, N. Pickett (Nanoco Tech-
nologies), Patent US20140011317A1, 2014.
[40] W. Y. L. Ko, H. G. Bagaria, S. Asokan, K.-J. Lin, M. S. Wong, J. Mater. Chem.
2010, 20, 2474–2478.
[41] J. Shen, G. Y. Chen, A. M. Vu, W. Fan, O. S. Bilsel, C. C. Chang, G. Han,
Adv. Opt. Mater. 2013, 1, 644–650.
[42] Y. T. Zhong, G. Tian, Z. J. Gu, Y. J. Yang, L. Gu, Y. L. Zhao, Y. Ma, J. N. Yao,
Adv. Mater. 2014, 26, 2831–2837.
[43] Y. Zhang, Z. Z. Yu, J. Q. Li, Y. X. Ao, J. W. Xue, Z. P. Zeng, X. L. Yang, T. T. Y.
Tan, ACS Nano 2017, 11, 2846–2857.
[44] Q. Liu, Y. Sun, T. S. Yang, W. Feng, C. G. Li, F. Y. Li, J. Am. Chem. Soc.
2011, 133, 17122–17125.
Keywords: core–shell materials · lanthanides · nanostructures ·
photoluminescence · upconversion
[1] F. Auzel, Chem. Rev. 2004, 104, 139–173.
[2] G. Chen, H. Qiu, P. N. Prasad, X. Chen, Chem. Rev. 2014, 114, 5161–5214.
[3] A. Gnach, T. Lipinski, A. Bednarkiewicz, J. Rybka, J. A. Capobianco, Chem.
Soc. Rev. 2015, 44, 1561–1584.
[4] L. Prodi, E. Rampazzo, F. Rastrelli, A. Speghini, N. Zaccheroni, Chem. Soc.
Rev. 2015, 44, 4922–4952.
[5] A. Escudero, A. I. Becerro, C. Carrillo-Carrión, N. O. NfflÇez, M. V. Zyuzin,
M. Laguna, D. Gonzµlez-Mancebo, M. OcaÇa, W. J. Parak, Nanophotonics
2017, 6, 881–921.
[6] H. H. Gorris, U. Resch-Genger, Anal. Bioanal. Chem. 2017, 409, 5875–
5890.
[7] A. Gulzar, J. T. Xu, P. P. Yang, F. He, L. G. Xu, Nanoscale 2017, 9, 12248–
12282.
[8] X. Liu, C.-H. Yan, J. A. Capobianco, Chem. Soc. Rev. 2015, 44, 1299–1301.
[9] D. Mendez-Gonzalez, E. Lopez-Cabarcos, J. Rubio-Retama, M. Laurenti,
Adv. Colloid Interface Sci. 2017, 249, 66–87.
[45] R. Naccache, Q. Yu, J. A. Capobianco, Adv. Opt. Mater. 2015, 3, 482–509.
[46] C. Yan, H. Zhao, D. F. Perepichka, F. Rosei, Small 2016, 12, 3888–3907.
[47] X. S. Zhai, S. S. Liu, Y. L. Zhang, G. S. Qin, W. P. Qin, J. Mater. Chem. C
2014, 2, 2037–2044.
[10] U. Resch-Genger, H. H. Gorris, Anal. Bioanal. Chem. 2017, 409, 5855–
5874.
[48] P. Huang, W. Zheng, S. Zhou, D. Tu, Z. Chen, H. Zhu, R. Li, E. Ma, M.
Huang, X. Chen, Angew. Chem. Int. Ed. 2014, 53, 1252–1257; Angew.
Chem. 2014, 126, 1276–1281.
[49] H.-X. Mai, Y.-W. Zhang, L.-D. Sun, C.-H. Yan, J. Phys. Chem. C 2007, 111,
13730–13739.
[11] D. M. Wang, B. Liu, Z. W. Quan, C. X. Li, Z. Y. Hou, B. G. Xing, J. Lin, J.
Mater. Chem. B 2017, 5, 2209–2230.
[12] S. Wilhelm, ACS Nano 2017, 11, 10644–10653.
[13] X. J. Zhu, Q. Q. Su, W. Feng, F. Y. Li, Chem. Soc. Rev. 2017, 46, 1025–
1039.
[50] F. Vetrone, R. Naccache, V. Mahalingam, C. G. Morgan, J. A. Capobianco,
Adv. Funct. Mater. 2009, 19, 2924–2929.
[51] L. Lei, B. Xie, Y. Li, J. Zhang, S. Xu, J. Lumin. 2017, 190, 462–467.
[52] M. Quintanilla, F. Ren, D. Ma, F. Vetrone, ACS Photonics 2014, 1, 662–
669.
[14] J. A. Damasco, G. Y. Chen, W. Shao, H. Agren, H. Y. Huang, W. T. Song,
J. F. Lovell, P. N. Prasad, ACS Appl. Mater. Interfaces 2014, 6, 13884–
13893.
[15] M. Haase, H. Schafer, Angew. Chem. Int. Ed. 2011, 50, 5808–5829;
Angew. Chem. 2011, 123, 5928–5950.
[53] X. Huang, J. Lin, J. Mater. Chem. C 2015, 3, 7652–7657.
[54] S. Hao, G. Chen, C. Yang, W. Shao, W. Wei, Y. Liu, P. N. Prasad, Nanoscale
2017, 9, 10633–10638.
[55] X. Du, X. Wang, L. Meng, Y. Bu, X. Yan, Nanoscale Res. Lett. 2017, 12,
163.
[16] C. S. Mao, X. H. Yang, L. J. Zhao, Chem. Eng. J. 2013, 229, 429–435.
[17] F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu, H. Y. Chen, C. Zhang,
M. H. Hong, X. G. Liu, Nature 2010, 463, 1061–1065.
[18] D. Q. Chen, M. Xu, M. F. Ma, P. Huang, Dalton Trans. 2017, 46, 15373–
15385.
ChemistryOpen 2018, 7, 159 –168
www.chemistryopen.org
167
ꢁ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

[56] S. Han, R. Deng, X. Xie, X. Liu, Angew. Chem. Int. Ed. 2014, 53, 11702– [69] J. G. E. Bünzli, Basics of Lanthanide Photophysics, Vol. 7, Springer, Berlin,
11715; Angew. Chem. 2014, 126, 11892–11906.
[57] R. R. Qiao, C. H. Liu, M. H. Liu, H. Hu, C. Y. Liu, Y. Hou, K. C. Wu, Y. N. Lin,
J. M. Liang, M. Y. Gao, ACS Nano 2015, 9, 2120–2129.
[58] L. L. Wang, X. J. Xue, H. Chen, D. Zhao, W. P. Qin, Chem. Phys. Lett. 2010,
485, 183–186.
[59] Q. Zhang, B. Yan, Chem. Commun. 2011, 47, 5867–5869.
[60] D. D. Ma, L. J. Meng, Y. Z. Chen, M. Hu, Y. K. Chen, C. Huang, J. Shang,
R. F. Wang, Y. M. Guo, J. Yang, ACS Appl. Mater. Interfaces 2015, 7,
16257–16265.
2010.
[70] The average decay time of UCNPcore is composed of t1 =46 ms (a1 =
81%) and t2 =91 ms (a2 =19%) for G1+G2 as well as t1 =61 ms (a1 =
87%) and t2 =102 ms (a2 =13%) for R.
[71] A. Beeby, I. M. Clarkson, R. S. Dickins, S. Faulkner, D. Parker, L. Royle,
A. S. de Sousa, J. A. G. Williams, M. Woods, J. Chem. Soc. Perkin Trans. 2
1999, 493–503.
[72] F. Wang, R. R. Deng, J. Wang, Q. X. Wang, Y. Han, H. M. Zhu, X. Y. Chen,
X. G. Liu, Nat. Mater. 2011, 10, 968–973.
[61] J. X. Wang, T. Ming, Z. Jin, J. F. Wang, L. D. Sun, C. H. Yan, Nat. Commun.
2014, 5, 5669.
[73] X. J. Xie, N. Y. Gao, R. R. Deng, Q. Sun, Q. H. Xu, X. G. Liu, J. Am. Chem.
Soc. 2013, 135, 12608–12611.
[62] X. Xu, Z. Wang, P. P. Lei, Y. N. Yu, S. Yao, S. Y. Song, X. L. Liu, Y. Su, L. L.
Dong, J. Feng, H. J. Zhang, ACS Appl. Mater. Interfaces 2015, 7, 20813–
20819.
[63] L. A. Feigin, D. I. Svergun, Structure Analysis by Small-Angle X-ray and
Neutron Scattering, Springer, New York, 1987.
[64] T. Ben-Nun, A. Ginsburg, P. Szekely, U. Raviv, J. Appl. Crystallogr. 2010,
43, 1522–1531.
[65] W. Yu, W. Xu, H. W. Song, S. Zhang, Dalton Trans. 2014, 43, 6139–6147.
[66] W. Zheng, P. Huang, D. T. Tu, E. Ma, H. M. Zhu, X. Y. Chen, Chem. Soc.
Rev. 2015, 44, 1379–1415.
[74] Y. F. Wang, G. Y. Liu, L. D. Sun, J. W. Xiao, J. C. Zhou, C. H. Yan, ACS Nano
2013, 7, 7200–7206.
[75] A. Skripka, R. Marin, A. Benayas, P. Canton, E. Hemmer, F. Vetrone, Phys.
Chem. Chem. Phys. 2017, 19, 11825–11834.
[76] R. H. Page, K. I. Schaffers, P. A. Waide, J. B. Tassano, S. A. Payne, W. F.
Krupke, W. K. Bischel, J. Opt. Soc. Am. B 1998, 15, 996–1008.
[77] B. D. Cullity, S. R. Stock, Elements of X-ray Diffraction, 3rd ed., Prentice-
Hall, New York, 2001.
[67] C. Würth, M. Kaiser, S. Wilhelm, B. Grauel, T. Hirsch, U. Resch-Genger,
Nanoscale 2017, 9, 4283–4294.
[68] D. M. Liu, X. X. Xu, Y. Du, X. Qin, Y. H. Zhang, C. S. Ma, S. H. Wen, W. Ren,
E. M. Goldys, J. A. Piper, S. X. Dou, X. G. Liu, D. Y. Jin, Nat. Commun.
2016, 7, 10254.
Received: November 20, 2017
Version of record online January 25, 2018
ChemistryOpen 2018, 7, 159 –168
www.chemistryopen.org
168
ꢁ 2018 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim