NIR Luminescence Enhancement of YVO4:Nd Phosphor for Biological Application

Article abstract of DOI:10.1007/s10895-020-02649-1

This work reports two systematic studies related to yttrium vanadate (YVO₄) phosphors. The first evaluates how the annealing temperature and V⁵⁺/Y³⁺ molar ratio determine the emergence of a single YVO₄ tetragonal phase, whereas the second concerns the optimal Nd³⁺ concentration to improve the infrared emission properties for bio-labelling applications. The YVO₄:Nd phosphors were synthesized by adapting the non-hydrolytic sol–gel route. For the first study, samples containing different V⁵⁺/Y³⁺ molar ratios (1.02, 1.48, 1.71, or 3.13) were obtained. For the second study, YVO₄:Nd phosphors containing different Nd³⁺ concentrations (1.0, 3.0, 5.0, or 10.0% in mol) were prepared. X-ray diffractometry and RAMAN spectroscopy results revealed that, regardless of the heat-treatment temperature, the V⁵⁺/Y³⁺ molar ratio of 1.48 was the best composition to avoid undesired phases like Y₂O₃ and V₂O₅. Photoluminescence results indicated that the sample containing 3.0% in mol of Nd³⁺ and annealed at 1000?°C presented the best infrared emission properties. This sample displayed an intense broad band in the ultraviolet region, which was ascribed to the VO₄^3? charge transfer band, as well as several bands in the visible and infrared regions, which were attributed to the Nd³⁺ intraconfigurational f-f transitions. Regardless of the excitation wavelength (ultraviolet, visible, or near-infrared), the mean radiative lifetime was about 12.00 μs. The prepared phosphors presented absorption and emission bands in the biological window (BW) regions, which are located between 750 and 900?nm and between 1000 and 1300?nm, so they are candidates for applications in medical imaging and diagnoses.

Full text of DOI:10.1007/s10895-020-02649-1

Journal of Fluorescence
https://doi.org/10.1007/s10895-020-02649-1
ORIGINAL ARTICLE
NIR Luminescence Enhancement of YVO :Nd Phosphor
4
for Biological Application
1
1
1
2
Júlio Tadashi Tanaka & Susane Bonamin Moscardini & Willian Eurípedes do Nascimento Melo & Helena Brunckova
&
1
1
Eduardo José Nassar & Lucas Alonso Rocha
Received: 25 May 2020 /Accepted: 9 November 2020
#
Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract
This work reports two systematic studies related to yttrium vanadate (YVO ) phosphors. The first evaluates how the annealing
4
5
+
3+
temperature and V /Y molar ratio determine the emergence of a single YVO tetragonal phase, whereas the second concerns
the optimal Nd concentration to improve the infrared emission properties for bio-labelling applications. The YVO :Nd phos-
phors were synthesized by adapting the non-hydrolytic sol–gel route. For the first study, samples containing different V /Y
molar ratios (1.02, 1.48, 1.71, or 3.13) were obtained. For the second study, YVO :Nd phosphors containing different Nd
4
3
+
4
5
+
3+
3
+
4
concentrations (1.0, 3.0, 5.0, or 10.0% in mol) were prepared. X-ray diffractometry and RAMAN spectroscopy results revealed
5
+
3+
that, regardless of the heat-treatment temperature, the V /Y molar ratio of 1.48 was the best composition to avoid undesired
3
+
phases like Y O and V O . Photoluminescence results indicated that the sample containing 3.0% in mol of Nd and annealed at
2
3
2 5
1
000 °C presented the best infrared emission properties. This sample displayed an intense broad band in the ultraviolet region,
3−
which was ascribed to the VO charge transfer band, as well as several bands in the visible and infrared regions, which were
4
3
+
attributed to the Nd intraconfigurational f-f transitions. Regardless of the excitation wavelength (ultraviolet, visible, or near-
infrared), the mean radiative lifetime was about 12.00 µs. The prepared phosphors presented absorption and emission bands in the
biological window (BW) regions, which are located between 750 and 900 nm and between 1000 and 1300 nm, so they are
candidates for applications in medical imaging and diagnoses.
3
+
.
.
.
4
Keywords Infrared emission; photoluminescence; sol–gel Phosphors YVO Nd
Introduction
achieve emission in regions of interest for use in red (YVO :Eu)
4
[
7–12], blue (YVO :Tm) [13], or orange (YVO :Sm) [14, 15]
4 4
The YVO matrix, which was first introduced by Levine and
emitting devices, for example. Besides emitting in the visible re-
gion, some lanthanide ions display excitation and emission in the
infrared region, which is very attractive for medical imaging ap-
plications. Nd perfectly fits these requirements: Nd -doped ma-
trixes can be effectively excited in the near infrared region (at about
800 nm, which corresponds to the first biological window, desig-
nated I-BW), and they exhibit intense emission lines around
1060 nm (which matches the second biological window, designat-
ed II-BW), which paves the way for applications in living tissue
imaging and nanothermometry [16–25].
4
Palila in 1964 [1], has been extensively studied due to its proper-
−
1
ties, such as low-energy phonon (around 890 cm ) [2]. In addi-
tion, its optical, mechanical, and thermal characteristics are funda-
mental for light emitters [3, 4] and laser hosts [5, 6]. Moreover,
when this matrix is doped with lanthanide ions, it presents highly
efficient luminescence to excitation by an electron beam, leading
to applications in lamps, displays, scintillation detectors, TVs, and
cathode ray [7, 8]. Different lanthanide ions have been used to
3+
3+
Luminescent bio-imaging is widely employed in biomedical
sciences. Its applications range from morphological analysis of
the anatomical structure to sensitive measurements of intracellu-
lar molecular events, and it has several advantages like improved
selectivity and sensitivity as compared to conventional imaging
methods [26, 27]. Among the various processes that can be used
to obtain inorganic matrixes, the non-hydrolytic sol–gel route
*
Lucas Alonso Rocha
lucas.rocha@unifran.edu.br
1
2
Universidade de Franca, Av. Dr. Armando Salles Oliveira, 201, São
Paulo CEP 14404-600 Franca, Brazil
Institute of Materials Research, Slovak Academy of Sciences,
Watsonova 47, 040 01 Kosice, Slovakia

J Fluoresc
Table 1 Nominal composition of the prepared samples
stands out because it provides strict control of stoichiometry,
powder morphology, and phase purity; the cations are distributed
all over the structure; and the oxo bonds originate from oxygen
atoms of donors other than water [28–31]. Furthermore, this
route dismisses the need for solvents and may reduce or eliminate
the formation of residual metal-OH groups. The mechanisms of
the non-hydrolytic sol–gel route have been extensively discussed
in the literature [28, 32, 33]. Here, aiming at medical imaging
applications, we report the optimal conditions to obtain a single
Sample
VCl
3
YCl
3
NdCl
3
Y/V
Re*/V
(mol%) (mol%) (mol%) (molar ratio) (molar ratio)
YVO-a-1
YVO-b-1
YVO-c-1
YVO-d-1
YVO-b-3
YVO-b-5
50.0
59.0
62.5
75.0
59.0
59.0
49.0
40.0
36.5
24.0
38.0
36.0
31.0
1.0
1.0
1.0
1.0
3.0
5.0
10.0
0.980
0.678
0.584
0.320
0.644
0.610
0.525
1.00
0.695
0.600
0.333
0.695
0.695
0.695
3+
YVO tetragonal phase doped with different Nd concentrations
4
by adapting the non-hydrolytic sol–gel route.
YVO-b-10 59.0
*
Re = Rare-earth elements
Experimental
Preparation of the Precursor Chloride Solutions
The matrixes were synthesized from yttrium oxide (Y O ,
photoluminescence data were obtained at room temperature, un-
der continuous Xe lamp (450W) excitation in a Horiba Jobin
Yvon Fluorolog-3 spectrofluorimeter equipped with an excita-
tion and emission double monochromator and an R 928
Hammatsu photomultiplier. Emission was collected at 90° from
the excitation beam. The slits were placed at 2.0 and 0.5 nm for
excitation and emission, respectively; the bandpass was 0.2 nm,
and the integration time was 0.5 ms. An emission filter was
employed (transmittance 100% for λ > 830 nm). All the analyses
were carried out at room temperature.
2
3
Aldrich, 99.99%), vanadium oxide (V O , Aldrich, 99.6%),
2
5
and neodymium oxide (Nd O , Aldrich, 99.9%). Absolute
2
3
ethanol (99.5%) was used as oxygen donor. By using Y O ,
2
3
V O , and Nd O , the respective chlorides were obtained by
2
5
2 3
dissolution of these oxides in concentrated hydrochloric acid
at a temperature of approximately 70 °C for 3 h, under stirring.
Absolute ethanol was added until a pH of 5.5 was achieved.
After cooling, more ethanol was added, until a final volume of
2
5 mL was obtained. The final concentration of the YCl ,
3
−
1
VCl , and NdCl solutions was 0.1 mol.L .
Results and Discussion
3
3
3
+
Non-hydrolytic Sol–gel Synthesis of the YVO :Nd Phosphor
First, we evaluated how the annealing temperature and the Y
4
5
+
and V concentrations affected the structural properties and
The matrixes were synthesized by the non-hydrolytic sol–gel
method described by Acosta et al. [32] and modified by us
determined the emergence of a pure YVO tetragonal phase.
4
3
+
For this study, the Nd concentration was 1.0% in mol.
Figure 1 shows the X-ray diffractogram patterns of the
samples annealed at 600, 800, or 1000 °C. All of them pre-
[12]. In a two-neck round-bottom flask, the precursor solution
was prepared by homogenizing a mixture of ethanolic solu-
tions containing YCl , VCl , and EuCl and 0.7 mol of abso-
sented the characteristic peaks of the YVO tetragonal phase
3
3
3
4
lute ethanol at the desired proportion (Table 1). The solution
was left under reflux and argon atmosphere at 110 °C for 4 h.
The condenser was adapted to a thermostatic bath and main-
tained at -2 °C. After reflux, the mixture was cooled and aged
at room temperature for 24 h. The solvent was removed in a
rotary evaporator, under vacuum, and the material was ther-
mally treated at 600, 800, or 1000 °C for 4 h.
(JCPDS 16–250) with spatial group I41 / a m d (D4h) and
crystal lattice parameters a = b = 7.123 and c = 6.291. The
three main peaks at 2θ = 25.0, 33.5, and 49.7° corresponded
to the Miller indices (200), (112), and (312), respectively. The
3
+
sample containing the highest Y concentration (YVO-a-1)
displayed not only the peaks attributed to the YVO phase, but
4
also peaks related to the undesired Y O cubic phase (JCPDS
2
5
3
+
4
1-1105). As for the samples prepared with smaller Y
Characterization
amounts (YVO-c-1 and YVO-d-1), the extra peaks were as-
cribed to the V O orthorhombic phase (JCPDS 41-1426).
2
5
X-ray diffraction was performed at room temperature on a
Rigaku Geigerflex D/max-c diffractometer operating with
monochromated CuKα radiation (λ = 1.54Ǻ). The
diffractograms were recorded in the 2θ range from 10–80° at a
resolution of 0.05°. Raman spectroscopy was conducted on a
micro-Raman Jobin Yvon Horiba Spex spectrometer. The spec-
YVO-b-1 was the only sample that showed peaks that were
exclusively related to the YVO tetragonal phase, indicating
4
that the Y/V molar ratio of 0.678 was the optimal ratio to
obtain the pure YVO tetragonal phase.
4
For all the samples, we estimated the average crystallite
size from the X-ray peak width by using Scherrer’s formula
(Dsc = 0.9 λ / β cosθ, where λ is the wavelength of the Cu Kα
radiation (1.5406 Ǻ), θ is the diffraction angle of the most
−
1
tra were recorded from 100 to 1,200 cm , in two cycles. The
samples were excited with a He-Ne laser at 632.8 nm. The

J Fluoresc
a)
1
8
6
000o
00
00
o
C
b)
0
1000
800
600
C
0
C
C
C
C
o
0
YVO4 (JCPDS 16-250)
YVO4 (JCPDS 16-250)
Y O3 (JCPDS 41-1105)
2
*
*
*
o
*
1000
C
*
*
*
*
o
*
1000 C
o
o
800
C
800
C
o
600
C
o
600
C
JCPDS # 16-250
JCPDS # 41-1105
JCPDS # 16-250
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2
θ (degree)
2θ (degree)
c)
d)
0
0
1
8
6
000
C
1000
C
0
0
00
C
800
600
C
C
0
0
00
C
YVO4 (JCPDS 16-250)
YVO4 (JCPDS 16-250)
V O5 (JCPDS 41-1426)
V O5 (JCPDS 41-1426)
2
2
o
o
1
000
C
1000
800
C
o
o
C
8
00 C
o
600 C
o
6
00 C
JCPDS # 16-250
JCPDS # 41-1426
JCPDS # 16-250
JCPDS # 41-1426
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2
θ
(degree)
2θ (degree)
Fig. 1 X-ray diffractogram patterns of the prepared samples: a IVO-a-1, b YVO-b-1, c YVO-c-1, and d YVO-d-1. The markers represent the crystalline
phases of Y (star) and V (circle)
O
2 5
2 5
O
3
+
intense peak, and β is the full width at half maximum of the
Bragg peaks) [10, 34]. Table 2 lists the corresponding values.
In the case of sample YVO-a-1, the mean crystallite size was
crystal size of 50 nm for YVO :Eu phosphors annealed
4
a 1000 °C. Another work showed that crystallite size
depended on the doping ion (Eu or Bi ) concentration
[2]. In our previous works, we reported that annealing
3
+
3+
2
3 nm regardless of the annealing temperature. However, this
sample presented the smallest crystallite size when compared
temperature, precursors, and catalysts influenced the
3
+
3+
to the samples containing higher Y concentrations. This fact
structural and luminescent properties of YVO :Eu pre-
4
5
+
can be ascribed to the difference in the ionic radii of V
pared by the conventional sol–gel route. The crystallite
sizes were estimated to range from 6 to 55 nm, with par-
ticle sizes ranging from 25 up to 50 nm depending on the
parameters that were used in the synthesis [10, 12].
Once the X-ray diffraction results of sample YVO-b-1
3
+
3+
(
0.335 Ǻ) and Y (0.892 Ǻ) and to the limiting Y concen-
tration, which indicated that the low Y amount inhibited
crystallite growth along the process [35, 36].
3
+
As for the other samples (YVO-b-1, YVO-c-1, and
YVO-d-1), the crystallite size values increased for the
demonstrated that the pure YVO tetragonal phase was
4
3
+
3+
samples annealed above 800 °C. Nevertheless, the Y
formed, we investigated the effects of the Nd concentration
concentration did not affect the crystallite size values of
these samples significantly. Other works have reported
similar results. For instance, Matos et al. [37] reported
and annealing temperature, aiming to obtain better lumines-
cence properties such as higher infrared emission intensity,
longer lifetime, and wide excitation spectrum range, without
Table 2 Average crystallite size of the samples
YVO-a-1
00 °C
23
YVO-b-1
600 °C
23
YVO-c-1
600 °C
23
YVO-d-1
600 °C
27
6
800 °C
20
1000 °C
23
800 °C
46
1000 °C
46
800 °C
46
1000 °C
46
800 °C
46
1000 °C
46
Dsc (nm)

J Fluoresc
structural changes. Figure 2 depicts the X-ray diffractograms
patterns of sample YVO-b-1 annealed at 600, 800, or 1000 °C
and containing 1.0, 3.0, 5.0, or 10.0% in mol of Nd . As
observed in Fig. 1, all the samples presented the characteristic
bonds within the anionic groups [39]. Table 1, in supplemen-
tary information, lists the Raman vibrational mode values for
the samples YVO-b-1, YVO-b-3, YVO-b-5, and YVO-b-10,
calcined at different temperatures.
3
+
peaks of the YVO tetragonal phase (JCPDS 16–250).
Concerning the samples YVO-b-3, YVO-b-5, and YVO-b-
For the samples calcined at 600 °C, the bands at 280, 402,
and 690 cm referred to the formation of the yttrium oxide
and oxychloride phases. These bands disappeared, giving rise
4
−
1
1
0 calcined at 600 °C, we detected peaks ascribed to the
V O orthorhombic phase (JCPDS 41-1426). However, this
to the simple YVO phase, as proven by X-ray diffraction
2
5
4
undesired phase did not emerge in the samples calcined at
higher temperatures. In addition, the samples containing 3.0,
[40]. Thus, YVO possibly originated above 600 °C via
4
solid-state reaction between yttrium oxychloride and V O .
2
5
3
+
− 1
5
.0, and 10.0% in mol of Nd had the main peaks displaced
The band around 994 cm is associated with V O vibrations
2 5
probably because of the difference between the ionic radii of
Nd (0.995 Å) and Y (0.892 Å), which distorted the unit
[41] and was observed for samples containing 5 and 10% in
mol of Nd and calcined at 600 °C. This fact indicated that
3
+
3+
3+
cell of the YVO tetragonal phase. [36, 38]. We did not verify
YVO formation involved V O formation, which was also
4
4
2 5
these displacements for YVO-b-1 because it contained a low
Nd concentration.
observed in the X-ray diffractograms.
3
+
Depending on the difference between the ionic radii of the
replacing and the replaced ions, doping ions may cause crystal
lattice expansion or contraction, which is reflected in the
Raman spectroscopy measurements helped to confirm
3
+
changes in the YVO structure as a function of Nd concen-
4
3
+
tration. All the samples presented intense bands, which im-
plied strong interactions between the ions that arose mainly
from the stretching and bending of the shorter metal–oxygen
Raman band displacements [40, 42, 43]. In this work, Y
3
+
replacement with Nd increased the crystal lattice, as proven
by the broadening and shifting of the Raman bands [44]. The
a)
0
b)
0
1
8
6
000
C
1000
C
0
0
00
00
C
C
800
600
C
C
0
0
YVO4 (JCPDS 16-250)
YVO4 (JCPDS 16-250)
V O (JCPDS 41-1426)
2
5
o
1
000 C
o
o
800 C
1
000 C
o
800
C
o
6
00 C
o
600
C
JCPDS # 16-250
JCPDS # 41-1426
JCPDS # 16-250
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2
θ
(degree)
2θ (degree)
c)
0
d)
0
1
8
6
000
C
1000 C
0
0
00
00
C
C
800
C
600 C
0
0
YVO4 (JCPDS 16-250)
V O (JCPDS 41-1426)
YVO (JCPDS 16-250)
4
2
5
V O (JCPDS 41-1426)
2
5
o
1
000 C
o
1
000 C
o
800
C
o
800
C
o
600
C
o
600
C
JCPDS # 16-250
JCPDS # 41-1426
JCPDS # 16-250
JCPDS # 41-1426
10
20
30
40
50
60
70
80
10
20
30
40
50
60
70
80
2
θ
(degree)
2θ (degree)
2 5
Fig. 2 X-ray diffractograms patterns of the prepared samples: a IVO-b-1, b YVO-b-3, c YVO-b-5, and d YVO-b-10. The crystalline phase of V O is
represented by the circle symbols

J Fluoresc
literature has attributed these facts to defects caused by the
larger ionic radius of the doping Nd as a function of con-
centration [45–47].
wavelength due to lattice distortion. Increasing concentration
3
+
3+
of Nd distorted the D symmetry caused by the larger ionic
2
d
3
+
3+
radius of Nd as compared to Y [50].
The full width at half maximum (FWHM) band of the sam-
ples YVO-b-1 and YVO-b-3, containing 1 and 3% in mol of
Nd , respectively, decreased as a function of the heat-
treatment temperature, which was ascribed to the increase in
The maximums values were 325 nm for the samples con-
3
+
taining 1 and 3% mol of Nd and 317 nm for the samples
3
+
3+
containing 5 and 10% mol of Nd . This shift indicated a
3
+
distorted site symmetry for Nd , as previously reported
3
+
YVO matrix crystallinity. For these samples, we did not verify
[10–12]. Furthermore, the Nd concentration impacted the
4
any influence of the doping ion concentration due to its small
CTB intensity. Therefore, the decreasing ratios between
CTB and the hypersensitive bands at 520 nm ( I → K
9/2 13/
3+
4
2
amount. As for the samples containing 5 and 10% mol of Nd ,
−
1
4
4
2
4
the bands at 808, 822, and 882 cm , corresponding to VO
; G ) and 590 nm ( I → G ; G ) [51] also suggested
9/2 9/2 7/2 5/2
4
2
3
+
deformation, asymmetric mode, and symmetric stretching, re-
spectively, were approximately 30% broader than the samples
distortions in the Nd symmetry site. The calculated CTB/
520 nm ratios were 49.7, 20.3, 8.0, and 7.1, whereas the cal-
culated CTB/590 nm ratios were 14.0, 4.1, 3.3, and 1.8 for the
samples containing 1, 3, 5, and 10% mol of Nd
respectively.
Figure 4 contains the emission spectra of the Nd -doped
samples excited at different wavelengths (ultraviolet, visible,
and infrared regions). Regardless of the excitation wave-
length, all the samples presented infrared emission bands at
3+
containing 1 and 3% mol of Nd , which attested to the distortion
3
+
in the YVO matrix reticulum [43].
,
4
We accomplished photoluminescence measurements
only for the samples calcined at 1000 °C because they
presented more interesting properties, as observed by the
X-ray diffraction and RAMAN spectroscopy results.
3+
3
+
Figure 3 illustrates the excitation spectra of the Nd ion
4
4
doped into the YVO matrix, at different concentrations.
1064 and 1340 nm, which were ascribed to the F → I
3/2
and F → I
concentration shifted the F → I transition slightly
11/2
4
11/2
3+
4
4
3+
Table 3 summarizes the maximum values of the observed
bands and their transitions.
transitions of Nd , respectively. The Nd
3
/2
13/2
4
4
3
/2
The excitation spectra presented a broad band around
[10–12]. For samples YVO-b-1 and YVO-b-3, the maximum
intensity emerged at 1067 nm, whereas for the samples YVO-
b-5 and YVO-b-10, the maximum intensity appeared at 1065
5
+
2−
3
20 nm, which was assigned to the V – O charge transfer
3
−
band (CTB) of VO4 . According to the molecular orbital
1
3+
theory, this band is associated with transitions from the A
and 1064 nm, respectively. Lanthanide ions (Ln ) can replace
2
1
1
1
1
1
3+
(
T ) ground state to the A ( D) and B ( D) excited states
Y
in the YVO structure, to occupy D symmetry sites.
4 2d
1
1
1
3−
of the VO4 [48]. Energy transfer occurred more easily be-
tween the VO group and Nd due to the different ionic
radii of V (0.355 Å) and Nd (0.995 Å) [11]. The CTB can
be used to measure the covalent character of the Ln -ligand
bond [12, 49]. The higher the energy of the CTB, the higher
However, the band displacements could also be ascribed to
3−
3+
3+
the Nd symmetry distortion to D and C due to its larger
4
2
2v
5
+
3+
ionic radius [50].
Sample YVO-b-3 presented the most intense emission
3
+
3
+
bands, which suggested that 3.0% mol of Nd was the opti-
mal concentration, as confirmed in Fig. 4.
3
+
the Ln -ligand interaction. The CTB shifts to higher
Fig. 3 Excitation spectra (λem
:
5
5
5
4
4
3+
I9/2
2
G7/2
2
2.0x10
1.5x10
1.0x10
5.0x10
1
064 nm) of the Nd ion doped
into the YVO matrix calcined at
000 °C, at different
H9/2
4
G5/2
4
4
1
CTB
F3/2
concentrations. a YVO-b-1,
b YVO-b-3, c YVO-b-5 and
d YVO-b-10
4
4
S3/2
d)
c)
F7/2
2
4
4
K1
3/2
4
4
4
D3/2
2
4
2
G9/2
b)
a)
K
15/2
D5/2
G7/2
G11/2
D1/2
4
G9/2
F5/2
2
2
D5/22(D,P)3/2
4
F9/2
P1/2
0
.0
300
400
500
600
700
800
900
Wavelength (nm)

J Fluoresc
Table 3 Transitions of the bands
observed in the excitation spectra
illustrated in Fig. 3 and their
1
Wavelength (nm)
Energy (cm−
)
Transitions
3
3
4
4
5
5
6
7
8
8
20
31,250
28,248
23,200
21,321
Charge transfer band
wavelengths and energy values
4
4
4
4
4
4
4
4
4
4
4
4
54
I
I
I
I
I
I
I
I
I
9/2 → D3/2; D5/2; D1/2
2
2
31
9/2 → D5/2; P1/2
2
4
2
2
69
9/2 → K15/2; G11/2; G9/2; (D,P)3/2
2
4
4
20 and 528
19,342 and 18,939
16,949
9/2 → K13/2; G9/2; G7/2
2
4
90
9/2 → G7/2; G5/2
4
79
14,727
9/2 → F9/2
4
4
49
13,351
9/2 → S3/2; F7/2
2
4
04 e 831
75
12,437 and 12,033
11,428
9/2 → H9/2; F5/2
4
9/2 → F3/2
3
+
Figure 5 shows the intensity values of the Nd emission
radiative lifetime was much higher as compared to other ex-
citation wavelengths.
4
4
4
4
bands related to the F → I
and F → I
transi-
3
/2
11/2
3/2
13/2
3
+
tions at 1064 and 1344 nm, respectively. Increasing Nd
As mentioned above, the transparency regions of the bio-
logical window (BW) can be divided into three parts: I-BW,
from 750 to 950 nm; II-BW, from 1000 to 1350 nm; and III-
BW, from 1500 to 1800 nm. On the basis of the images only,
I-BW may suffer interference from the autofluorescence of
tissues, but, as excitation source, it is better suited for biomed-
ical applications than ultraviolet (UV) light because it pene-
trates deeper into tissues and causes less photodamage to
concentration above 3.0% mol decreased both infrared emis-
sion bands. We also observed this behavior when we analyzed
the radiative lifetime (τrad) values regardless of the wave-
length excitations, as shown in Table 4. However, sample
YVO-b-3 sample presented a very high emission band at
1
8
064 nm (which matches the II-BW) when it was excited at
08 nm (which corresponds to the I-BW). Besides that, the
5
a)
b) 8.0x10
4
4
YVO-b-1
YVO-b-3
YVO-b-5
YVO-b-10
4
4
YVO-b-1
YVO-b-3
YVO-b-5
YVO-b-10
F3/2
-
I11/2
F3/2
-
I11/2
6
6
6
5
2.0x10
1.5x10
1.0x10
5.0x10
YVO-b-3
YVO-b-3
5
5
5
6
.0x10
4
.0x10
YVO-b-10
YVO-b-10
2.0x10
4
4
4
4
F3/2
-
I13/2
F
- I
3/2 13/2
YVO-b-1
YVO-b-5
YVO-b-1
YVO-b-5
0
.0
1
0.0
1000
000
1100
1200
1300
1400
1500
1600
1100
1200
1300
1400
1500
1600
Wavelength (nm)
Wavelength (nm)
c) 1
.0x10
.0x10
.0x10
.0x10
.0x10
4
4
YVO-b-1
5
4
4
4
4
F - I
3/2 11/2
YVO-b-3
YVO-b-5
YVO-b-10
YVO-b-3
8
6
4
2
YVO-b-10
4
4
F3/2
-
I13/2
YVO-b-5
YVO-b-1
0.0
1000
1100
1200
1300
1400
1500
1600
Wavelength (nm)
3+
Fig. 4 Emission spectra of Nd doped into the YVO
4
matrix calcined at 1000 °C, at different concentrations, excited at (a) 320 nm, (b) 590 nm, and (c)
8
08 nm

J Fluoresc
8
7
6
5
4
3
2
a)
10
a)
λexc: 320 nm
λexc: 590 nm
λexc: 808 nm
804 nm
1
1
1
1
1
1
0
0
0
0
0
0
8
72 nm
YVO-b-1
YVO-b-3
YVO-b-5
YVO-b-10
Samples
750
800
850
900
I-BW wavelength (nm)
8
7
6
5
4
3
2
b)
10
λexc: 320 nm
λexc: 590 nm
b)
1
1
1
1
1
1
0
0
0
0
0
0
λ
exc: 808 nm
1
067 nm
1088 nm
1344 nm
1340
YVO-b-1
YVO-b-3
YVO-b-5
YVO-b-10
Samples
3+
4
4
Fig. 5 Nd emission intensity values (a) F3/2 → I11/2 (1064 nm) and
4
4
1000
1050
1100 1320
1330
1350
(
b)
3/2
F → I13/2 (1340 nm), obtained under different excitation
wavelengths
II-BW wavelength (nm)
Fig. 6 Excitation (a) and emission (b) spectra of sample YVO-b-3 em-
biological systems. On the other hand, II-BW and III-BW are
suitable for NIR images in large tissue depths [16–25]. Thus,
the ideal luminescent biolabeling should be excited in the I-
BW and emit light in the II-BW or III-BW regions [19].
Figure 6 shows the luminescence spectra of the YVO-b-3
sample emphasizing the I-BW and II-BW regions.
phasizing the I-BW and II-BW regions
1067, 1088, and 1344 nm with FWHM of 6, 9, and 6 nm,
respectively, confirming the potential application of this phos-
phor in medical imaging and diagnoses [16–25].
The five bands fit the biological windows, with maximum
at 804 and 872 nm with FWHM of 12 and 9 nm, respectively,
for I-BW. As for II-BW, the maximum bands values were
Conclusion
The non-hydrolytic sol–gel methodology has been used to
prepare diverse oxide-based matrixes under mild conditions,
Table 4 Radiative lifetime (λem: 1064 nm) under different excitation
wavelengths
3
+
doped with dopants at different levels. In this work, the Y /
5
+
V
molar ratio and the annealing temperature proved to be
Samples
τrad (µs)
τrad (µs)
τrad (µs)
important parameters to obtain the YVO tetragonal phase,
4
(λexc = 320 nm)
(λexc = 590 nm)
(λexc = 808 nm)
directly affecting purity and crystallite size.
3
+
We also investigated how the Nd concentration affected
the luminescent properties of the prepared phosphor. In addi-
tion to excitation bands in the visible region, all the samples
presented luminescence bands around 750–950 and 1000–
YVO-b-1 12.85 ± 1.29
YVO-b-3 12.26 ± 1.23
YVO-b-5 11.96 ± 1.20
YVO-b-10 10.54 ± 1.05
10.64 ± 1.06
12.69 ± 1.27
10.31 ± 1.03
9.72 ± 0.97
3.49 ± 0.35
11.18 ± 1.12
6.84 ± 0.68
1.10 ± 0.11
1350 nm, which fit the first and second biological windows,

J Fluoresc
3
+
5+
5
6
7
8
9
.
O’Connor JR (1966) Unusual crystal-field energy levels and effi-
cient laser properties of YVO4:Nd. Appl Phys Lett. https://doi.org/
respectively. According to the results, the Y /V molar ratio
3
+
of 0.678, doping with 3.0% mol of Nd , and annealing at
000 °C was the optimal composition to obtain pure Y /
1
0.1063/1.1754631
3
+
1
V
.
Shur JW, Kochurikhin VV, Borisova AE et al (2004)
Photoluminescence properties of Nd:YVO4 single crystals by
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tetragonal phase with high luminescence intensity and
longer radiative lifetime in the NIR range.
.
.
.
Supplementary Information The online version contains supplementary
material available at https://doi.org/10.1007/s10895-020-02649-1.
Acknowledgements The Laboratoire des Matériaux Inorganiques/
Clermont-Ferrand/France (E. PETIT) is also acknowledged for the use
of the Raman spectrometer.
1
467. https://doi.org/10.1364/AO.5.001467
Huignard A, Gacoin T, Boilot JP (2000) Synthesis and lumines-
cence properties of colloidal YVO4:Eu phosphors. Chem Mater.
https://doi.org/10.1021/cm990722t
Author Contributions All authors contributed to the study conception
and design. Study design was performed by Júlio T. Tanaka and Lucas
10. Saltarelli M, Luz PP, Matos MG et al (2012) Influence of catalyses
on the preparation of YVO4:Eu3 + phosphors by the sol–gel meth-
odology. J Fluoresc 22:899–906. https://doi.org/10.1007/s10895-
011-1028-7
(
A) Rocha. Data collection and statistical analysis were performed by
Júlio T. Tanaka, Susane (B) Moscardini, Willian E. Do N. Melo and
Lucas (A) Rocha. Data interpretation and manuscript preparation were
performed by Júlio T. Tanaka, Susane (B) Moscardini, Eduardo J. Nassar
and Lucas A. Rocha. Literature search and funds collection were per-
formed by Willian E. Do N. Melo, Helena Brunckova, Eduardo J.
Nassar and Lucas A. Rocha. The first draft of the manuscript was written
by Eduardo J. Nassar and Lucas A. Rocha and all authors commented on
previous versions of the manuscript. All authors read and approved the
final manuscript.
11. Matos MG, Rocha LA, Nassar EJ, Verelst M (2016) Influence of
3+
3+
Bi ions on the excitation wavelength of the YVO :Eu matrix.
4
Opt Mater (Amst) 62. https://doi.org/10.1016/j.optmat.2016.09.035
12. Saltarelli M, Matos MG, de Faria EH et al (2015) Preparation of
YVO4:Eu3 + at low temperature by the hydrolytic sol–gel method-
ology. J Sol-Gel Sci Technol 73:283–292. https://doi.org/10.1007/
s10971-014-3525-z
13. Ningthoujam RS, Singh LR, Sudarsan V, Dorendrajit Singh S
(2009) Energy transfer process and optimum emission studies in
luminescence of core-shell nanoparticles: YVO 4:Eu-YVO 4 and
surface state analysis. J Alloys Compd. https://doi.org/10.1016/j.
jallcom.2009.05.044
Funding This study was financed in part by the Coordenação de
Aperfeiçoamento de Pessoal de Nível Superior - Brasil (CAPES) -
Finance Code 001; Conselho Nacional de Desenvolvimento Científico
e Tecnológico (CNPq, grants: 302702/2018-0 L.A.R. and 302668/2017-9
E.J.N.); São Paulo Research Foundation (FAPESP, grant: 2011/09823-4
E.J.N.); Grant Agency of the Slovak Academy of Sciences through pro-
ject VEGA No. 2/0037/20.
1
4. Xiao X, Yan B (2008) In-situ chemical co-precipitation composition of
hybrid precursors and luminescence of Y1-xGdxNbO4: RE3+ (RE =
Tb, Eu) micron crystalline phosphors. J Alloys Compd 456:447–451.
https://doi.org/10.1016/j.jallcom.2007.02.098
5. Zhang H, Fu X, Niu S, Xin Q (2008) Synthesis and luminescent
properties of nanosized YVO4:Ln (Ln = Sm, Dy). J Alloys Compd.
https://doi.org/10.1016/j.jallcom.2007.02.134
6. Skripka A, Benayas A, Marin R et al (2017) Double rare-earth
nanothermometer in aqueous media: Opening the third optical
transparency window to temperature sensing. Nanoscale. https://
doi.org/10.1039/c6nr08472a
1
Data Availability The data that support the findings of this study are
available from the corresponding author, upon reasonable request.
1
Compliance with Ethical Standards
Conflict of Interest The authors declare no conflict of interests.
Code Availability Not applicable.
1
1
1
7. Kolesnikov IE, Tolstikova D V., Kurochkin A V., et al (2015)
Concentration effect on structural and luminescent properties of
3
+
YVO
4
:Nd nanophosphors. Mater Res Bull. https://doi.org/10.
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016/j.materresbull.2015.06.023
8. Suo H, Zhao X, Zhang Z, Guo C (2017) 808 nm light-triggered
thermometer-heater upconverting platform based on Nd3+-sensi-
tized yolk-shell GdOF@SiO2. ACS Appl Mater Interfaces.
https://doi.org/10.1021/acsami.7b12753
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