Utilizing the amidation reaction to address the "cooperative effect" of carboxylic acid/amine on the size, shape, and multicolor output of fluoride upconversion nanoparticles

Article abstract of DOI:10.1039/c1jm10985e

Here we elucidate the "cooperative effect" of carboxylic acid/amine on the size, shape, and multicolor output of fluoride upconversion nanoparticles (UCNPs) utilizing the amidation reaction. During the synthesis of NaYF₄ UCNPs using oleic acid (OA) and octadecylamine (OM) as ligands, the evolution of the reaction solution was monitored by FT-IR and proton nuclear magnetic resonance (¹H-NMR) spectroscopy. It is shown that N-octadecyloleamide (OOA) was formed and tightly bonded to the surface of the fluoride UCNPs. FT-IR and ¹H-NMR analyses point towards a two-step OOA formation mechanism where OA initially reacted with OM to form an acid-base complex (C₁₇H₃₃CO₂^{- +}NH ₃C₁₈H₃₇) in reaction mixture, which further transformed into OOA before the nucleation of fluoride UCNPs at elevated temperature. Importantly, we find that the interaction of OOA with the surfaces of the fluoride UCNPs was stronger than that of OM, and that OOA could strongly quench the green-emitting levels of excited Er³⁺ ions. A series of control experiments and analyses demonstrate that OOA was not only responsible for the control of the size and shape of NaYF₄: 20% Yb, 2% Er UCNPs, but also for the multicolor tuning of them. Those observations reveal that the "cooperative effect" between carboxylic acid and amine results from the amidation reaction and its product. This paper provides us with some new deep insights into the "cooperative effect" of carboxylic acid and amine during the synthesis of inorganic nanoparticles under thermolysis. Furthermore, other lanthanide-ion doped fluoride UCNPs with controlled size, shape and multicolor output can also be obtained in the same way.

Full text of DOI:10.1039/c1jm10985e

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PAPER
Utilizing the amidation reaction to address the ‘‘cooperative effect’’ of
carboxylic acid/amine on the size, shape, and multicolor output of fluoride
upconversion nanoparticles†
Wenbin Niu, Suli Wu* and Shufen Zhang
Received 7th March 2011, Accepted 6th May 2011
DOI: 10.1039/c1jm10985e
Here we elucidate the ‘‘cooperative effect’’ of carboxylic acid/amine on the size, shape, and multicolor
output of fluoride upconversion nanoparticles (UCNPs) utilizing the amidation reaction. During the
synthesis of NaYF UCNPs using oleic acid (OA) and octadecylamine (OM) as ligands, the evolution
4
1
of the reaction solution was monitored by FT-IR and proton nuclear magnetic resonance ( H-NMR)
spectroscopy. It is shown that N-octadecyloleamide (OOA) was formed and tightly bonded to the
1
surface of the fluoride UCNPs. FT-IR and H-NMR analyses point towards a two-step OOA
formation mechanism where OA initially reacted with OM to form an acid–base complex
ꢀ
+
(
C
17
H33CO
2
3 18
NH C H37) in reaction mixture, which further transformed into OOA before the
nucleation of fluoride UCNPs at elevated temperature. Importantly, we find that the interaction of
OOA with the surfaces of the fluoride UCNPs was stronger than that of OM, and that OOA could
3+
strongly quench the green-emitting levels of excited Er ions. A series of control experiments and
analyses demonstrate that OOA was not only responsible for the control of the size and shape of
NaYF : 20% Yb, 2% Er UCNPs, but also for the multicolor tuning of them. Those observations reveal
4
that the ‘‘cooperative effect’’ between carboxylic acid and amine results from the amidation reaction
and its product. This paper provides us with some new deep insights into the ‘‘cooperative effect’’ of
carboxylic acid and amine during the synthesis of inorganic nanoparticles under thermolysis.
Furthermore, other lanthanide-ion doped fluoride UCNPs with controlled size, shape and multicolor
output can also be obtained in the same way.
10,11
Introdution
size the and shape of the nanoparticles.
Carboxylic acids and
amines, as the most widely employed ligands, were usually
combined for the synthesis of colloidal nanoparticles with
various functions, which have been well-reported because
The synthesis of high-quality (with defined size, shape, physical
properties, etc.) inorganic nanomaterials has greatly advanced in
recent years. Up to now, the organometallic approach and its
alternatives in organic solvents at elevated temperature have
often been regarded as the relative successful synthetic methods
a combination of them could work well with inorganic nano-
2,3,12–16
particles and show the ‘‘cooperative effect’’.
However,
there is no generally accepted evidence and mechanism to
17
elucidate the observed ‘‘cooperative effect’’.
1–5
in the field. In these processes, ligands are necessarily used to
control the nucleation and growth as well as for the stabilization
Lanthanide-ion doped NaYF
4
upconversion nanoparticles
4,6–9
of the obtained nanoparticles.
Usually, the specific mixtures
(
UCNPs), as new alternative to conventional fluorescent nano-
18–23
of ligands were used, because they could facilitate the control of
particles, have attracted considerable interest.
Compared
with conventional quantum dots (QDs), they show unique
optical, physiological and chemical properties, such as high
photo-chemical stability, large anti-Stokes shifts, low toxicity,
improved signal-to-noise ratio, low photodamage and deep
State Key Laboratory of Fine Chemicals, Dalian University of Technology,
Dalian, 116012, China. E-mail: wusuli@dlut.edu.cn; Fax: +86 411
49896264; Tel: +86 411 49896265
†
Electronic supplementary information (ESI) available: TEM images of
penetration depth in biological specimens under infrared exci-
24–29
NaYF
with OA/OM 25/30, FT-IR spectrum of mixture of Y(CF
OA, RGR, upconversion mechanism, and the effect of OA/OOA,
OM/OOA and water on the upconversion spectra of NaYF : 20% Yb,
% Er, XRD patterns of NaLuF : 20% Yb, 2% Er and NaGdF : 20%
4
: 20% Yb, 2% Er and NaGdF
4
: 20% Yb, 2% Er UCNPs prepared
tation.
Fluorescent nanoparticles that can emit many colors
3
COO) and
3
30–32
could simplify the performance in multiple applications.
At
present, multicolor tuning of lanthanide-ion doped NaYF
UCNPs has been mainly achieved by compositional
4
4
2
4
4
Yb, 2% Er UCNPs prepared with various ratios of OA/OM, and the
synthesis and characterization of N-octadecyloleamide (OOA). See
DOI: 10.1039/c1jm10985e
20,33–37
37
changes.
However, the related work is still in its infancy.
Although we have demonstrated the multicolor tuning of
1
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lanthanide-ion doped NaYF UCNPs within a given composi-
description. NaLuF : 20% Yb, 2% Er/Ho UCNPs were prepared
4
4
8,39
3
ꢁ
tion in a previous report,
up to now, there are still no
at 300 C. The procedure for the synthesis and separation of Yb
investigations on the influence of the interaction between
carboxylic acids and amines on the multicolor output, size and
shape of lanthanide-ion doped fluoride UCNPs during
thermolysis.
and Er undoped NaYF4 nanoparticles with various ratios of
OA/OM was the same as that of NaYF
UCNPs.
4
: 20% Yb, 2% Er
Herein, we report on the interaction between oleic acid (OA)
and octadecylamine (OM) during the synthesis of UCNPs, and
Characterization
Upconversion fluorescence spectra were recorded on a Hitachi F-
500 fluorescence spectrophotometer under the excitation of
its effect on the size, shape and multicolor emissions of NaYF
4
:
4
2
0% Yb, 2% Er UCNPs with different ratios of OA/OM present
a 980 nm diode laser. TEM measurements were carried out on
a Tecnai G220 S-Twin microscope operated at 200 kV. Powder
X-ray diffraction patterns (XRD) were measured on a Rigaku
in the reaction solution, which we consider to be the model
system for colloidal nanomaterials in general. By means of
1
H-NMR and FT-IR spectra, we proved that the amidation
D/MAX-2400 with Cu-Ka radiation. The scanning step size was
ꢁ
reaction between OA and OM occurs before the nucleation of the
fluoride nanoparticles, and the binding of the amidation product
0
.02 2q. FT-IR spectra were measured on EQUINOX55 spec-
1
trometer. H-NMR spectra were recorded using a Varian
Mercury NMR Spectrometer (400 MHz). The samples were
4
to the particle surface, which finally influence the NaYF : 20%
Yb, 2% Er nanoparticles. This can reasonably explain the well-
reported ‘‘cooperative effect’’ between carboxylic acids and
amines in the synthesis of many colloidal nanoparticles with
various functions. By adjusting the ratio of OA/OM in the
reaction solution, the size, shape and multicolor output of
multiple lanthanide-ion doped fluoride UCNPs can also be
controlled.
3
prepared by adding the products into CDCl . The temperature-
controlled mantle was purchased from the Gongyi Yuhua
Instrument Co., Ltd.
Results and discussion
4
Characterization of NaYF : 20% Yb, 2% Er UCNPs
Experimental
Fig. 1 shows the X-ray diffraction (XRD) patterns of as-
prepared NaYF : 20% Yb, 2% Er UCNPs. The peak positions
4
Chemicals
and intensities of these particles agree well with calculated
All the chemicals were used directly without further purification.
Y O (99.99%), Gd O (99.99%), Lu O (99.99%), Yb O
pattern for cubic NaYF : Yb, Er crystal (JCPDS No.77-2042),
4
2
3
2
3
2
3
2
3
3
and variation of OA/OM ratio almost has no effect on phase.
(
(
99.99%), Er O3 (99.99%), Ho O (99.99%) and Na CO
2
The well-defined peaks indicate no other impurities were
detected.
2
3
2
99.99%) were purchased from Sinopharm chemicals reagents
Co., Ltd (Shanghai, China). OA (99%), OM (98%) and tri-
fluoroacetic acid (99%) were bought from Aladdin Chemistry
Co., Ltd. N-octadecyloleamide (OOA) was synthesized by
condensation reaction of oleic acid and octadecylamine (see ESI†
for more details).
TEM images of NaYF : 20% Yb, 2% Er UCNPs prepared
4
with various ratios of OA/OM are shown in Fig. 2. It is revealed
that those UCNPs were all nearly monodisperse and the mean
size and shape varied with the molar ratios of OA/OM in the
reaction solution (Fig. 2, 2a, 7.0 nm; 2b, 16.5 nm; 2c, 18.5 nm; 2d,
1
9.2 nm; 2e, 20.2 nm; 2f, 21.5 nm; Fig. S1 and Fig. S2 see ESI†).
When only OM was used, NaYF : 20% Yb, 2% Er UCNPs
exhibited nearly spherical morphology with an average size of
.0 nm in diameter (Fig. 2a). When the OA/OM ratio was 15/40,
Synthesis of lanthanide-ion doped NaYF
4
UCNPs
4
For a typical synthesis of NaYF
a mixture of Y (0.78 mmol), Yb
mmol) and Na CO (1.0 mmol) was dissolved in 10 mL of 50%
4
: 20% Yb, 2% Er UCNPs,
7
2
O
3
2
O
3
(0.20 mmol), Er (0.02
2 3
O
the size increased to about 16.5 nm and the particles showed
2
3
ꢁ
aqueous trifluoroacetic acid at 80 C. The residual water and acid
ꢁ
were slowly evaporated to dryness at 70 C in a three-necked
flask. Then, a mixture of OA (80 mmol) and OM (30 mmol) was
ꢁ
added. The reaction solution was heated to 120 C and main-
tained for 30 min under reduced pressure with magnetic stirring
to remove residual water and oxygen. Simultaneously, the flask
was purged periodically with dry nitrogen. The resulting opti-
ꢁ
cally transparent solution was heated to 285 C and maintained
for 1 h under a nitrogen atmosphere. After heating was stopped,
4
NaYF : 20% Yb, 2% Er UCNPs were precipitated by the addi-
tion of absolute ethanol and hexane. The products were isolated
by centrifugation. This procedure was repeated at least twice.
The resulting nanoparticles were dried under vacuum for 24 h.
NaYF : 20% Yb, 2% Er UCNPs with multicolor outputs were
4
obtained by changing the ratio of OA/OM while other conditions
were kept. The procedure for the synthesis of multicolored
Fig. 1 XRD patterns of NaYF : 20% Yb, 2% Er UCNPs prepared with
4
ꢁ
NaGdF
4
: 20% Yb, 2% Er UCNPs was similar to the above
various molar ratios of OA/OM, 285 C, 1h.
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J. Mater. Chem., 2011, 21, 10894–10902 | 10895

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Fig.
3
Upconversion spectra and corresponding photographs of
: 20% Yb, 2% Er UCNPs prepared with various molar ratios of
NaYF
4
3+
OA/OM (normalized to Er 650 nm emission; a, 0/55; b, 5/50; c, 15/40; d,
5/30; e, 27.5/27.5; f, 35/20; g, 40/15; h, 55/0). All of the UCNPs were
2
excited at 980 nm with a 338 mW diode laser.
observations indicated that different ratios of OA/OM in the
reaction solution resulted in various RGR and multicolor output
of NaYF
To investigate the effect OA/OM on the size, shape and
multicolor output of NaYF : 20% Yb, 2% Er UCNPs, we first
4
: 20% Yb, 2% Er UCNPs.
4
Fig. 2 TEM images of NaYF : 20% Yb, 2% Er UCNPs prepared with
4
various molar ratios of OA/OM: a) 0/55, b) 15/40, c) 25/30, d) 27.5/27.5,
ꢁ
monitored the interaction between OA and OM at different
1
stages by FT-IR and H-NMR spectroscopy, and a series of
e) 40/15, f) 55/0, 285 C, 1h.
control experiments were also carried out.
4
hexagonal morphology (Fig. 2b). The NaYF : 20% Yb, 2% Er
UCNPs became more regular in shape with sizes around 19 nm
as the ratio of OA/OM increased to 25/30 and 27.5/27.5 (Fig. 2c,
Interaction of OA and OM in the reaction solution
To test the details, we first investigated the reaction solution at
different stages by FT-IR spectroscopy (Fig. 4). The peaks at
2
d, and Fig. S2, see ESI†). While the size was not obviously
increased, the hexagonal edges of UCNPs disappeared and the
surfaces were smoothed in the presence of an excess of OA
(
Fig. 2e and 2f). These results were similar to what had been
40
observed in cobalt–iron-oxide nanoparticles.
Fig. 3 shows the upconversion spectra of NaYF : 20% Yb, 2%
4
Er UCNPs prepared with various molar ratios of OA/OM. We
can clearly observe that the intensity ratio of green to red emis-
sions (RGR) of these UCNPs varied with the increase of the
amount of OA in the reaction mixture (see ESI† Fig. S3). For the
4
NaYF : 20% Yb, 2% Er UCNPs prepared in OM, three emission
2
peaks around 520, 540, and 660 nm attributed to the H11/2
,
4
4
4
4
S
3/2 / I15/2, and F9/2 / I15/2 transitions respectively, were
19,20
observed, (Fig. 3a).
Interestingly, the introduction of a small
amount of OA led to a fast decrease of green emissions and
2
enhancement of red emission. The green emissions of H11/2,
4
4
S3/2 / I
transitions decreased with the increase of OA/OM
15/2
4
ratio until 15/40, in which the RGR of NaYF : 20% Yb, 2% Er
Fig. 4 FT-IR spectra of OA (a), OM (b), OOA (c), the equimolar
UCNPs was nearly zero (Fig. S3†) and a pure reddish color was
shown without filters (Fig. 3c). As the ratio of OA/OM further
increased, the green emissions again increased gradually. These
mixture of OA and OM (d), the reaction mixture when the reaction
ꢁ
ꢁ
ꢁ
solution was at 120 C for 30 min (e), at 250 C (f), and at 285 C for 1 h
(g) (OA/OM ¼ 27.5/27.5).
1
0896 | J. Mater. Chem., 2011, 21, 10894–10902
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ꢀ
1
2
921 and 2851 cm represent the intense asymmetric and
ESI† for details). Then, we further characterized the reaction
1
mixture at different stages by H-NMR spectroscopy (Fig. 5).
symmetric stretching vibrations of C–H bond of CH2 group.
When OA is mixed with OM in equimolar amounts at room
temperature, we could clearly observe that the characteristic
First, we took the sample from reaction mixture when the
ꢁ
temperature was raised to 250 C, because trifluoroacetates begin
3,44
ꢀ1
band of OA at 1710 cm disappeared, and that the strong peak
to decompose at this temperature.
1
Compared with the
ꢀ1
around 1560 cm ascribed to the asymmetric stretching vibra-
ꢀ
H-NMR signals of prepared OOA (Fig. 5c, iii and iv), the
reaction mixture also showed two strong characteristic peaks
around d 2.15 and 3.23 ppm (Fig. 5d), which were in agreement
with those of OOA. No strong signals of OA and OM were
exhibited. Due to the interaction between OOA and the nano-
41
tion of COO appeared (Fig. 4d), suggesting that the OA
molecules were deprotonated and transformed into carboxylate
ꢀ
anions (C17
H
33COO ) due to the promotion of octadecyl-
41
amine. That is to say, a mixture of carboxylate anions,
+
protonated octadecylamine (C H NH ) and acid–base
1
particles, the peak of the amide proton of OOA shifted from
39,45
8
37
3
ꢀ
+
complex (C H CO
1
NH C H ) was present in reaction
3 18 37
d 5.54 ppm to d 5.37 ppm.
These results suggested the
7
33
2
solution. The characteristic absorption bands of the reaction
mixture were similar to those of the equimolar mixture of OA
formation of a large amount of OOA in reaction solution.
ꢁ
Although trifluoroacetates begin to decompose at 250 C, no
ꢁ
and OM, after being maintained at 120 C for 30 min under
nanoparticles would be formed due to the unique delayed
46
reduced pressure (Fig. 4e). It is implied that a large amount of
ꢀ
nucleation pathway of NaYF
revealed that a large amount of OOA was formed before the
nucleation of NaYF : 20% Yb, 2% Er UCNPs.
4
: Yb, Er nanoparticles. It is
+
acid–base complex (C17
H33CO
2
3
NH C
18
H37) was formed in
ꢁ
the reaction mixture. When the temperature was raised to 250 C,
only a small amount of OA was detected, and two characteristic
4
With the rise of reaction temperature and prolongation of
time, the composition of the reaction solution was not varied
(Fig. 5e and 5f).
ꢀ1
bands for the amide groups appeared: 1634 cm for C]O
ꢀ1
stretching and 1536 cm for N–H bending vibrations (Fig. 4c
42,43
and 4f).
1
This indicates that the amidation reaction between
The above FT-IR and H-NMR measurements evidenced the
OA and OM forms N-octadecyloleamide (OOA). As the reaction
temperature and time further increased, the reaction solution was
not changed apparently (Fig. 4g).
reaction between OA and OM as the following process, during
the preparation of NaYF4 UCNPs at elevated temperature
(Scheme 1). This process was in agreement with the conventional
amidation mechanism via pyrolysis of amine and carboxylic
To demonstrate the formation of OOA in the reaction solu-
tion, we prepared OOA separately by the condensation reaction
47
acid, which further support our present results. Though the
1
of OA and OM and characterized it by FT-IR and H-NMR (see
formation of the acid–base complex has been proposed during
48
the preparation of nanoparticles in our previous report, in the
present work we demonstrated that formation of an acid–base
complex and its transformation into an amide. Usually, the
preparation of a range of amides by heating a mixture of
ꢁ
different amines and carboxylic acids can be achieved at 180 C
49
for 10–30 min. This temperature was lower than that for the
preparation of a series of inorganic nanoparticles by thermolysis.
In combination with our present results, it is implied that the
formation of the amide was a general phenomenon in a combi-
national usage of amine and carboxylic acid during the synthesis
of inorganic nanoparticles via thermolysis.
Analysis of surface ligands of nanoparticles
Subsequently, to investigate the effect of OOA on the size, shape
4
and multicolor tuning of NaYF : 20% Yb, 2% Er UCNPs, those
UCNPs prepared with different molar ratios of OA/OM were
separated from reaction solution and characterized by FT-IR
spectroscopy. FT-IR spectra (Fig. 6) clearly show the bonding
between ligand molecules and the surface atoms of nanoparticles.
ꢀ1
The strong characteristic band at 1561 cm attributed to the
asymmetric stretching vibration of carboxylate anions
Fig. 5 Schematic illustration of the condensation reaction between OA
1
and OM. The protonsare indicated on OA, OM and OOA (i–ix). H-NMR
spectra of OA (a), OM (b), OOA (c), reaction mixture taken from the
ꢁ
ꢁ
reaction system when the reaction solution was at 250 C (d), and at 285 C
for 1 h (e) with an equimolar ratio of OA/OM, and reaction solution after
Scheme 1 Schematic illustration of the evolution of OA and OM in
ꢁ
aging at 285 C for 1 h with molar ratio of 35/20 of OA/OM (f).
reaction solution.
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surface of nanoparticles (schematic illustration shown in Fig. 5).
1
Fig. 7 shows the H-NMR spectra of OOA and surface ligand
capped NaYF4 nanoparticles prepared with various ratios of
OA/OM. When OM was used alone in reaction solution, the
broad OM peaks were the only detectable signals in the spectrum
(
Fig. 7b). The characteristic protons of OM shifted from
d 2.64 ppm (Fig. 5b) to d 2.68 ppm, indicating the presence of an
39,53
interaction between OM and NaYF
4
nanoparticles.
1
Furthermore, it is well-known that the H-NMR peaks of surface
ligands in the field of nanoparticles are significantly broader than
those of the corresponding free ligands. This broadening is
a result of the inhomogenity of the chemical environment of the
53–55
ligands on the surface of nanoparticles.
The strongly
broadened peaks of all surface ligands further implied that they
Fig. 6 FT-IR spectra of OOA and surface ligands of NaYF
% Er UCNPs prepared with various ratios of OA/OM.
4
: 20% Yb,
were bound to the surface of the NaYF
present work (Fig. 7b–g).
4
nanoparticles in the
2
After addition of a given amount of OA to the reaction solu-
tion (OA/OM, 15/40), broad characteristic proton signals
appeared at d 2.10–2.20 and 3.19–3.28 ppm in the spectrum of the
ꢀ
50,51
ꢀ1
(
C
17
H
33COO ).
A broad band around 1640 cm ascribed to
the strong bonding interactions between OA and rare earth ions
46
4
obtained NaYF nanoparticles, which were in good agreement
was also shown (Fig. 6, 55/0), however, it overlapped with the
ꢀ1
with the prepared OOA (Fig. 7c, signals iii and iv). Compared
1
with the H-NMR spectrum of the prepared OOA, it is also
absorption band of the amide group at 1634 cm (Fig. 6, OOA).
Additionally, the characteristic bands for OM (around
ꢀ1
important to note that the peak of the amide proton of OOA
1
618 cm , Fig. 6, 0/55) were also buried in the broad ones of the
shifted from d 5.54 ppm to d 5.37 ppm due to the interaction with
carbonyl group. Although FT-IR spectroscopy has been widely
39,45
1
3,52
nanoparticles.
A strong characteristic band of OOA and
used for the characterization of surface ligands,
we found it
a very weak peak of OA, and no characteristic peak of OM were
exhibited. These results implied the following facts: firstly, most
of the OA has reacted with OM to form OOA when the OA/OM
ratio was 15/40. Secondly, OOA was bonded to the surface of the
nanoparticles. Thirdly, the content of OOA was much higher
difficult to identify the different ligands bound to the surface of
the nanoparticles that were prepared in the presence of amine
and carboxylic acid.
1
Herein, H-NMR spectroscopy was used to identify the
surface ligands. Due to the electron paramagnanetic resonance of
1
lanthanide ions, the H-NMR peaks of ligand capped NaYF
than that of OA amongst the surface ligands of the NaYF
nanoparticles. Forthly, the interaction of OOA with the NaYF
4
4
4
:
2
0% Yb, 2% Er nanoparticles could be strongly broadened,
nanoparticle surface was stronger than that of OM. Corre-
spondingly, a nearly pure reddish color output and hexagonal
which buried the characteristic peaks of different surface
39
ligands. So, the corresponding Yb and Er undoped NaYF4
nanoparticles were prepared with various molar ratios of
shape was achieved for NaYF : 20% Yb, 2% Er UCNPs
4
1
OA/OM and characterized by H-NMR spectroscopy in the
1
present report. H-NMR signals of OA, OM and OOA molecules
prepared with the molar ratio of OA/OM 15/40 (Fig. 2b and 3c).
In previous reports, there is an argument whether the amine is
present in the surfactant layer of the nanoparticles prepared in
the presence of carboxylic acid and amine. Although the direct
spectra evidence was lacking, some researchers proposed the
were used for probing the different ligands bonded onto the
41
presence of amine molecules on the surface of nanoparticles.
Our results can also clearly prove that amines were absent on the
surface of nanoparticles, because its binding was weaker than
that of the formed OOA.
In the case where OA/OM was 25/30 in the reaction solution,
similar results were also obtained for NaYF (Fig. 7d, e and f)
4
and NaYF : 20% Yb, 2% Er nanoparticles (Fig. 2c and 3d).
4
When the ratio of OA/OM further increased from 25/30 to 35/20,
it is observed that the peak intensity of OA increased (Fig. 7e).
From the integrated intensities of the a-protons of OOA (iv) and
OA (i), we calculate a ratio OA/OOA to be about 1 : 2 on the
particle surface. As OA further increased in the reaction solution
(
OA/OM, 40/15), the integrated intensities ratio of OA/OOA
increased to about 3 : 1. These results revealed that the relative
content of OA increased while the content of OOA decreased on
1
Fig. 7 H-NMR spectra of OOA (a), and isolated NaYF
the surface of NaYF , and the stronger coordination capability
4
4
nanoparticles
ꢁ
of OA. Correspondingly, we can observe that the green emissions
prepared with various ratios of OA/OM at 285 C for 1 h (b, 0/55; c,
15/40; d, 25/30; e, 35/20; f, 40/15; g, 55/0). The schematic illustration of
of NaYF : 20% Yb, 2% Er UCNPs increased and the shapes were
4
1
H-NMR signals is the same as that shown in Fig. 4.
smoothed gradually.
1
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Effect of OA/OM on the shape and size
disappeared and the surfaces were smoothed with further
increase of the ratio of OA/OM in the reaction solution. From
the above results and discussion, it is revealed that rare earth ions
reacted with OA or OM to form complexes at the beginning of
4
To investigate the pathways for the formation of NaYF : 20%
Yb, 2% Er UCNPs, a control experiment was carried out. The
procedure was the same as that used for the preparation of
reaction, which affects the size and shape of the NaYF
4
: 20% Yb,
NaYF
4
: 20% Yb, 2% Er nanoparticles. First, 2 mmol Y
was added into OA (110 mmol), which was heated to
20 C and maintained for 30 min under reduced pressure with
2
% Er UCNPs along with OOA. After the end of the reaction,
(
CF COO)
3
3
the complexes were exhausted to form NaYF
UCNPs stabilized by organic ligands.
4
: 20% Yb, 2% Er
ꢁ
1
magnetic stirring to remove residual water and oxygen. Then, the
sample was taken from solution and characterized by IR spec-
troscopy (see ESI† Fig. S4). We could clearly observe the char-
acteristic absorption band of the –COOH group of OA at
Effect of OA/OM on the multicolor output of NaYF : 20% Yb,
4
2
% Er UCNPs
ꢀ
1
1
711 cm . Compared with pure OA, there is an additional
ꢀ
For a given composition of NaYF : Yb, Er UCNPs, it is known
4
ꢀ1
absorption band at around 1596 cm ascribed to the –COO
that several factors mainly influence the probability of non-
3+
group. That indicates the formation of Y–oleate complexes at the
beginning of the reaction, although the absorption band was
radiative relaxation of Er ion from green-emitting levels to red-
emitting levels, and determine the RGR and color output of the
56,57
relative small due to the presence of a large amount of OA.
Similar results had also been obtained by Du and co-authors.
UCNPs, factors such as crystallographic phase, crystallinity,
24–28,62
57
surface defects, oxygen impurities, and ligands.
First, we
Moreover, they also reported that the addition of some alkyl-
amine into OA facilitated the formation of the reaction inter-
mediate partially substituted with alkylamine, due to the strong
affinity of the amine groups to the transition metals. These
results suggest that rare earth ions easily react with organic
ligands (OA, OM etc.) to form complexes at the beginning stage
of the reaction. That is to say, there are the rare earth ion–OA/
could easily exclude the effect of crystallographic phase because
all the products were in the cubic phase (Fig. 1). For the crys-
tallinity of nanoaprticles prepared in a thermolysis procedure,
the higher the reaction temperature, the higher the crystal-
10,63
linity.
Those products were prepared at the same reaction
temperature and time. Therefore, the crystallinity should not be
the most important factor influencing the multicolor output of
OM complexes in the reaction solution, in addition to proton-
+
NaYF : 20% Yb, 2% Er UCNPs.
4
ated carboxylate anions, octadecylamine (C18
H37NH
3
) and
Generally, the smaller the size of NaYF : Yb, Er UCNPs, the
4
62
more surface defects and the stronger the red emissions.
ꢀ
+
acid–base complex (C17
H
33CO
2
NH
In combination with H-NMR analyses, it is revealed that
a large number of nuclei formed and finally, small NaYF : 20%
3
C
18
H37).
1
Though the mean size of sample 2a (in Fig. 2a) was smaller than
samples 2b and 2c (in Fig. 2b and 2c), the green emissions of
sample 2a were dramatically higher than those of samples 2b and
2c. Additionally, the mean sizes of nanoparticles did not obvi-
ously change as the ratio of OA/OM increased from 25/30 to 55/
0, however, the green emissions increased gradually, implying
that the particle size and surface defects could be excluded as the
4
Yb, 2% Er upconversion nanoparticles were obtained due to the
relative weak chelating interaction between OM and the metal
ions, when only OM was used. As the ratio of OA/OM increased
from 15/40 to 25/30 in reaction solution, the size of the NaYF4:
2
0% Yb, 2% Er UCNPs and the relative content of formed OOA
42
increased. According to a recent report, the presence of amide
could lead to a decreased number of stable nuclei and an
increased size of the final products due to the existence of amide
functional group. Therefore, it is rational that the size of the
most important reasons for the multicolor tuning of NaYF : 20%
4
Yb, 2% Er UCNPs. Considering the whole growth process of
UCNPs under inert atmosphere and the compositional variation
57,64
of the reaction solution,
oxygen impurities should not be the
NaYF
4
: 20% Yb, 2% Er UCNPs increased due to the increased
major factor, and we propose that the ligands were mainly
responsible for the variation of RGR and multicolor output of
amount of OOA. A further increase of the relative content of OA
molecules in the reaction solution did not affect the size of
nanoparticles significantly (Fig. 2e and 2f), though the amount of
formed OOA decreased. This is because of the strong chelating
NaYF : 20% Yb, 2% Er UCNPs. Then, we investigated the
4
influences of ligands.
According to above results and discussion, the amidation reac-
tion between OA and OM occurred in the reaction solution.
Therefore, OOA and water—the products of the condensation
reaction—wereprobablythemostimportantfactorscontrollingthe
52,58,59
interaction between OA and the metal ions (Fig. S4†),
which prevented nucleation and decreased the number of stable
nuclei and finally, the size of particles did not vary greatly.
In the present work, we could observe that a large amount of
color emission of NaYF : 20% Yb, 2% Er UCNPs. Our result show
4
OOA was formed before nucleation of the NaYF
Er UCNPs (Fig. 4 and 5) and OOA could bind more strongly
than OM to the NaYF surface (Fig. 7). Although OM was
present in the reaction solution when the ratios of OA/OM were
5/40 and 25/30, the truncated octahedrons were formed due to
the selective binding of OOA to the surfaces of nano-
4
: 20% Yb, 2%
that the addition of water had no significant effect on the color
emission (see ESI† Fig. S5). Then, OOA may be mainly responsible
4
for the multicolor output of NaYF : 20% Yb, 2% Er UCNPs.
4
To test our supposition, we synthesized NaYF : 20% Yb, 2% Er
4
1
UCNPs in OOA instead of a mixture of OA and OM, based on the
3+
previousprocedure. Asweexpected, the green emissions ofthe Er
14,60,61
particles.
The hexagonal shape of NaYF : 20% Yb, 2% Er
4
ions were quenched completely and the red color output resulted
UCNPs are two-dimensional projections of the truncated octa-
(Fig. 8). It is indicated that OOA did quench the green-emitting
2
20
4
4
3+
hedral geometry. Because the OA could bind strongly and non-
11/2 3/2
levels ( H , S ) and populated the red ( F9/2) levels of the Er
52,61
selectively to the surface of nanoparticles,
the defined edges of
ion (see ESI† Fig. S6). This may be due to the vibrational energy of
ꢀ1
hexagonal NaYF 20% Yb, 2% Er UCNPs gradually
4
:
OOA (2920, 3300 cm , Fig. 4g) match the energy level differences
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Fig. 8 Upconversion spectrum of NaYF : 20% Yb, 2% Er nanoparticles
4
prepared in OOA.
2
4
4
4
4
of H11/2, S3/2 to F9/2, and I11/2 to I13/2 levels (ꢂ3000 and
ꢀ1
3+
300 cm , Fig. S6†) of the excited Er ion, which quenched the
4
green-emitting levels ( H11/2, S3/2) and populate the red-emitting
3
4
4
3+
level ( F9/2) of excited Er ions. Furthermore, two sets of exper-
iments were also conducted by controlling the amount of the pre-
Fig. 9 TEM images of NaGdF
4
: 20% Yb, 2% Er UCNPs prepared with
synthesized amide OOA used to prepare NaYF
4
: 20% Yb, 2% Er
various molar ratios of OA/OM: a) 0/55, b) 20/35, c) 35/20, d) 55/0,
ꢁ
2
85 C, 1 h.
UCNPs, as opposed to its in situ formation to stabilize the
nanoparticles. As what we had expected, it clearly shows that the
2
4
4
green emissions of H , S / I and the intensity ratio of
15/2
1
1/2
3/2
4
above method. Fig. 9 shows the TEM images of NaGdF : 20%
green to red emission of NaYF : 20% Yb, 2% Er UCNPs
4
Yb, 2% Er UCNPs prepared with different OA/OM ratios in the
reaction solution. We could observe that variation of the size and
decreased gradually as the ratio of OA or OM to OOA decreased
in reaction solution (see ESI† Fig. S7), which resulted in different
color outputs. These results indicated that OOA played a signifi-
shape was similar to that seen for NaYF : 20% Yb, 2% Er (Fig. 9,
4
9
a, 7.0 nm; 9b, 18.0 nm; 9c, 19.5 nm; 9d, 21.5 nm). Furthermore,
4
cant role in controlling multicolor output of NaYF : 20% Yb, 2%
the upconversion spectra show that the relative intensity of green
Er UCNPs in the present work.
1
In combination with H-NMR analysis of the surface ligands
Fig. 7), these experiments revealed that the ligands played
(
4
a major role in tuning the color output of NaYF : 20% Yb, 2%
Er UCNPs by this method, especially OOA—the product of
condensation reaction of OA and OM. OOA can strongly
4
quench the green-emitting levels ( H , S3/2) and populate the
4
1
1/2
4
3+
red-emitting level ( F9/2) of excited Er ions. As the ratio of OA/
OM increased from 5/50 to 27.5/27.5 in the reaction solution, the
relative content of OOA bound to the surface of the nanoparticle
was high, leading to the decrease of green emissions and the
increase of red emissions. By further increasing the ratio of OA/
OM in solution, the relative amount of OOA molecules of the
surface ligands decreased, which will result in increasing green
emissions. Then, the color output of NaYF
UCNPs was varied with the ratio of OA/OM.
A combination of carboxylic acid and amine exhibits the good
4
: 20% Yb, 2% Er
‘
‘cooperative effect’’, and has been well-reported in the synthesis
of inorganic nanoparticles with various functions via thermol-
ysis. Those observations revealed that the so-called ‘‘cooperative
effect’’ results from the amidation reaction of amine and
carboxylic acid and its products. These findings are more
complicated than what we expected.
Fig. 10 Upconversion spectra and corresponding photographs of
Synthesis of NaGdF : 20% Yb, 2% Er UCNPs
4
4
NaGdF : 20% Yb, 2% Er UCNPs prepared with various molar ratios of
3
+
To extend the generality of the above conclusions, we synthesized
OA/OM (normalized to Er 650 nm emission). All of UCNPs were
excited at 980 nm with a 400 mW diode laser.
multicolored NaGdF
4
: 20% Yb, 2% Er UCNPs based on the
1
0900 | J. Mater. Chem., 2011, 21, 10894–10902
This journal is ª The Royal Society of Chemistry 2011

View Online
to red emissions decreased initially and then increased by
increasing the ratio of OA/OM (Fig. 10), which was also similar
Conclusions
In conclusion, we have investigated the interaction of OA with
OM in the reaction mixture of fluoride UCNPs during ther-
molysis. The effect of OA/OM ratios on the size, shape and
multicolor tuning of lanthanide-ion doped fluoride UCNPs with
a fixed composition was also studied in detail. The results
demonstrated the formation of the acid–base complex
to the results of NaYF : 20% Yb, 2% Er UCNPs. These obser-
4
vations indicate that the size, shape and multicolor output of
NaGdF
way.
4
: 20% Yb, 2% Er can also be subtly tuned in the same
ꢀ
+
(
C
17
H33CO
2
3 18
NH C H37) in the reaction solution during the
4
Multicolor output of lanthanide-ion doped NaLuF UCNPs
preparation of the nanoparticles. Then, the acid–base complex
was further reacted to form OOA before the nucleation of the
fluoride UCNPs during thermolysis. It is revealed that the
‘‘cooperative effect’’ of carboxylic acid and amine originated
from their amidation reaction and its product—OOA—which
played a major role in controlling the size, shape and multicolor
Similarly, the multicolor output of lanthanide-ion doped
NaLuF UCNPs can also be tuned in the same way (Fig. 11). As
4
the ratio of OA/OM increases in reaction solution, the intensity
ratio of green to red emissions varied in NaLuF : 20% Yb, 2% Er
4
UCNPs. To further extend the generality of this method, we
synthesized Yb and Ho co-doped NaLuF
4
UCNPs in a mixture
output of NaYF : 20% Yb, 2% Er UCNPs, because of its
4
of OA and OM. We can observe the green emission and red
moderate binding capability to the particle surface and strong
3+
quenching of the green-emitting levels of Er ion. As the ratio of
5
5
5
5
emission, resulting from the S
2
/ I
Different color outputs and RGR in
: 20% Yb, 2% Ho nanoparticles can also be obtained by
8
and F
5
/ I
8
transitions
3+
33,36,65
of the Ho ion.
NaYF
OA/OM increased in the reaction solution, the relative content of
OOA bound to the particle surface was varied, which resulted in
4
varying the ratio of OA/OM. It is revealed that this method was
not only successfully extended to a series of fluoride hosts, but
also to other lanthanide ion doped fluoride UCNPs. These
results further supported our conclusions.
4
the multicolor output of the NaYF : 20% Yb, 2% Er UCNPs.
This approach can also be easily extended to the multicolor
tuning of a series of lanthanide-ion doped fluoride UCNPs. This
paper not only offers the deep understanding of the effect of OA/
OM on controlling the size, shape and multicolor tuning of
fluoride UCNPs, but also provides some new insights into the
well-reported ‘‘cooperative effect’’ of carboxylic acid and amine
during the synthesis of inorganic nanoparticles under thermol-
ysis. Moreover, these upconversion nanomaterials with multi-
colors are expected to find potential applications in bioimaging,
biosensing, light-emitting diodes, and other nanodevices.
Acknowledgements
This work was supported by the National Natural Science
Fundation of China (20836001, 20923006, 21076038), Innova-
tion Funds of Dalian University of Technology and the Program
for Changjiang Scholars and Innovative Research Team in the
University (IRT0711). We thank Tao Pang and Prof. Xixian Luo
(
Dalian Marine University, China) for help with collecting the
upconversion spectra data.
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