Spectroscopic studies of noble metal incorporated lanthanide vanadates and YF3:Tb3+ nanoclusters encapsulated in hybrid hydrogels

Article abstract of DOI:10.1016/j.jallcom.2017.08.245

Extensive analysis of europium doped yttrium vanadate (YVO₄: Eu³⁺) and terbium doped yttrium fluoride (YF₃:Tb³⁺) has been evaluated by means of powder X-ray diffractions, scanning electron microscope and luminescence spectroscopy techniques. The results demonstrate noticeable difference in emission spectra for the sample of YVO₄: Eu³⁺ in the presence of Gd³⁺. As for the influence of the nanocrystal surface, the incorporation of silver content will also affect the intensity of the europium(III) luminescence. In addition, a hydrolytic sol-gel approach and a soft support matter (polyacrylic acid) were employed for the fabrication of YF₃:Tb³⁺ activated hydrogel. The emission spectra gave rise to the intensive green emissions centered at 545 nm due to ⁵D₄→⁷F₅ transition of Tb³⁺. Temperature elevation experiments were performed and could partially recover the green emissions.

Full text of DOI:10.1016/j.jallcom.2017.08.245

Accepted Manuscript
Spectroscopic studies of noble metal incorporated lanthanide vanadates and
3+
YF :Tb nanoclusters encapsulated in hybrid hydrogels
3
Yuhui Zheng, Chunxia Jiang, Jiansheng Huo, Jing Hu, Jinwei Gao, Xubing Lu,
Shuangyue Rao, Zhi Zeng, Qianming Wang
PII:
S0925-8388(17)32983-3
10.1016/j.jallcom.2017.08.245
JALCOM 43002
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 26 June 2017
Revised Date: 14 August 2017
Accepted Date: 27 August 2017
Please cite this article as: Y. Zheng, C. Jiang, J. Huo, J. Hu, J. Gao, X. Lu, S. Rao, Z. Zeng, Q.
3+
Wang, Spectroscopic studies of noble metal incorporated lanthanide vanadates and YF :Tb
3
nanoclusters encapsulated in hybrid hydrogels, Journal of Alloys and Compounds (2017), doi: 10.1016/
j.jallcom.2017.08.245.
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ACCEPTED MANUSCRIPT
Spectroscopic studies of noble metal incorporated lanthanide
vanadates and YF3:Tb3+ nanoclusters encapsulated in hybrid
hydrogels
Yuhui Zheng b, Chunxia Jiang b, Jiansheng Huo b, Jing Hu b, Jinwei Gaoc,
Xubing Lu c, Shuangyue Rao b, Zhi Zeng *b, Qianming Wang a, b
*
a Key Laboratory of Theoretical Chemistry of Environment, Ministry of Education, School of
Chemistry and Environment, South China Normal University, Guangzhou 510006, P. R.
China
b School of Chemistry and Environment, South China Normal University, Guangzhou 510006,
P. R. China
c Institute for Advanced Materials, Academy of Advanced Optoelectronics, South
China Normal University, Guangzhou 510006, P. R. China
* To whom the correspondence should be addressed. E-mail: qmwang@scnu.edu.cn;
zhizeng@scnu.edu.cn
Tel: 86-20-39310258; Fax: 86-20-39310187
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Abstract
Extensive analysis of europium doped yttrium vanadate (YVO4: Eu3+) and terbium doped
yttrium fluoride (YF3:Tb3+) has been evaluated by means of powder X-ray diffractions,
scanning electron microscope and luminescence spectroscopy techniques. The results
demonstrate noticeable difference in emission spectra for the sample of YVO4: Eu3+ in the
presence of Gd3+. As for the influence of the nanocrystal surface, the incorporation of silver
content will also affect the intensity of the europium(III) luminescence. In addition, a
hydrolytic sol-gel approach and a soft support matter (polyacrylic acid) were employed for
the fabrication of YF3:Tb3+ activated hydrogel. The emission spectra gave rise to the intensive
green emissions centered at 545 nm due to 5D4→7F5 transition of Tb3+. Temperature elevation
experiments were performed and could partially recover the green emissions.
Keywords: Nanophosphors; Luminescence; Hydrogel; Sol-gel
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Introduction
Fabrication of novel optical materials has received current interests due to their potentials
in organic light-emitting diodes, amplifier, biological diagnosis and environmental monitor
etc [1,2]. Lanthanide containing phosphors have been studied in view of their distinguished
spectroscopic features, such as characteristic line emissions and high color purity. As for the
inherent nature of lanthanides, its lifetimes lie in the range of microseconds to even
milliseconds. This feature will allow the use of time-gated measurements so that the
lanthanide emissions will rule out the influence of short-lived background signals [3].
However, these lanthanide based nanosized materials generally possess low emission
efficiency due to the limitations of excitation light. Therefore, how to improve the resultant
luminescence is the predominant issue towards the application of such materials. Recently,
the luminescence enhancement could be realized by the interplay of metallic surfaces and the
modification of photophyiscal properties adjacent to metallic nanostructures will shed new
insights into the physicochemical processes [3-5]. In this research, we reported the assembly
of YVO4: Eu3+/Gd3+ as a substrate to study the improved emission profiles with the
interactions of silver nanocrystals. YVO4: Eu3+/Gd3+ has been synthesized via a standard
hydrothermal method. The attached silver nanostructure has been formed by using trisodium
citrate as coupling reagent and reductant. Consequently, a substantial increase in emission
intensity located at red region was observed in YVO4: Eu3+/Gd3+ @ Ag composite material.
The second aim of this work is to investigate the effects of supported materials. Although
the phosphors were strongly emissive, the solid state powders that do not have controllable
shapes are difficult to collect and use in aqueous solution. In addition, it will also be
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inconvenient to recycle and wash by conventional solvents. These mentioned problems will
seriously prohibit their development of efficient optical devices. Therefore, it is highly
desirable to find improved ways to overcome the obstacles. It is well accepted that three
dimensional soft-scaffolds derived from organo- or hydrogels are particularly important in
tissue engineering, cosmetic, medical and drug delivery fields [6,7]. This self-organizing
network consists of a few molecular interactions like hydrogen bond, hydrophobic effects or
van der Waals forces. Numerous literatures focused on how to arrange the multiple
interactions reasonably in order to design new gel source materials [8,9]. Here in this work,
we tried to choose suitable soft matters to carry the luminescent species. Followed this idea,
the excellent emissive material (YF3:Tb3+) has been entrapped into the supramolecular
networks (sodium silicate-poly-acrylic acid hybrid hydrogels). To the best of our knowledge,
very limited papers of lanthanide luminescent gel have been reported [10,11]. Most of
lanthanide based soft materials were concentrated on organic complex structures [12,13]. It is
very rare to investigate the correlation between inorganic phosphor and the hosts. Additionally,
the resultant structure, photophysical properties and morphology of the new composite
material were fully discussed.
Experimental
Na2SiO3, acrylic acid, N,N’-methylene bisacrylamide and ammonium persulfate were
purchased from Aldrich company. AgNO3, sodium citrate, Polysuccinimide (PSI), NaOH,
NH4VO3, NH3·H2O (A.R.) and HNO3 (A.R.) were provided by the Shanghai Chemical
Reagent Company. Y(NO3)3 and Tb(NO3)3 were obtained by dissolving Y2O3 and Tb4O7 in
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dilute HNO3 solution under heating with agitation. The supersonic assisted microwave reactor
was commercially available and produced by Nanjing Xianou technology company, China
(XO-SM50). The X-ray powder diffraction was investigated on a Bruker D8 diffractometer
with Cu Kα radiation (λ= 0.1541 nm) in the range of 2θ =10–90o. Scanning electronic
microscope (SEM) was measured with JSM-6360LV. Energy dispersive X-ray spectrum was
measured with a Hitachi S4800 system. Luminescence spectra and lifetime decay curves were
measured on an Edinburgh FLS920 spectrometer. Dynamic light scattering was measured by
a BI-200SM machine (Brookhaven, USA).
Synthesis of YVO4: Eu3+/Gd3+ nano-particles
YVO4: Eu3+/Gd3+ powders were prepared by a hydrothermal method. 0.1g
Polysuccinimide (PSI) and 3mmol NaOH were dissolved in deionized water, then 0.1 mmol
NH4VO3 was added to above mixture with stirring, 0.1 mmol Ln(NO3)3 (Ln = Y, Eu, Gd; Y /
Eu / Gd = 0.95-x/0.05/x) solution was slowly added and stirred for 15 min. The precursor
solution was transferred into a 50 mL Teflon-lined autoclave and heated at 140
for 8 h.
After the autoclave was cooled to room temperature naturally, the YVO4: Eu3+/Gd3+ powders
were collected by centrifugation and washed with ethanol and deionized water several times,
finally dried at 80 for 24 h.
Synthesis of YVO4: Eu3+/Gd3+ @ Ag nano-particles
The above synthesized YVO4: Eu3+/Gd3+ sample was dispersed in deionized water for 30
min under supersonic irradiations. Then different volume (0, 0.5, 1.0, 1.5 and 2.0 mL) of
AgNO3 solution (0.5 M) was slowly added to above solution and stirred uniformly.
Meanwhile, sodium citrate solution was rapidly added to above solution. Subsequently the
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mixture was transferred to autoclave and heated at 100
for 3 h. The precipitates were
separated by centrifugation and washed with deionized water and ethanol for several times.
Synthesis of YF3:Tb3+
In a typical synthesis, 20 ml of 0.1 mol/L Ln(NO3)3 and 20 ml of 0.2 mol/L KF aqueous
solution were mixed. Subsequently, the mixture was transferred to a three neck flask which
adapted the reactor. Microwave power was set as 300 W and reaction temperature remained
o
to be 90 C. Supersonic power was 300 W with the reverse duty cycle 2 s. All the operation
time was fixed as 60 min. The as-prepared product was retrieved by centrifugation, washed
by deionized water and alcohol three times and then dried at 60 oC in vacuum.
Synthesis of YF3:Tb3+-sol-gel derived hydrogel
NaOH (1 mmol), Na2SiO3 (1 mmol) and acrylic acid (4 mmol) were added in 8 mL water
and the solution was stirred under an ice-water bath for 24 h. Then N,N-methylene
bisacrylamide (MBAM) (1.2 ꢀmol), ammonium persulfate (0.1 mmol) and YF3:Tb3+ (0.1 g)
were added and stirred for 1h. The reaction solution has been transferred to a sealed tube and
heated in water bath at 50oC for 3 h. The obtained gel was washed with ethanol for three
times and kept under room temperature for measurement uses. For the comparison purpose,
the D2O induced gel has been prepared in a similar procedure with the exception that water
was replaced by D2O.
MTT (methyl thiazolyl tetrazolium) assay
HeLa cells were cultured in DMEM media with 10% FBS and 1% antibiotics penicillin and
maintained at 37 °C with 5% CO2. The cell lines within replicate 96-well plates were
pre-cultured at standard culture condition for one day. After removal of the medium, cells
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were incubated with the hydrogel (0, 5, 10, 20 and 50 ꢀM) for another 24 h. Then 20 ꢀL MTT
solution was added to each well. After incubated with 4 h, the MTT solution was removed,
followed by adding DMSO (100ꢀL each well) to dissolve the formed crystals. The absorbance
was measured at 490 nm using a Polarstar microplate reader.
Results and discussion
The XRD patterns of as-derived YVO4: Eu3+/Gd3+ are given in Fig. 1. The intensive
peaks at 24.95o, 33.59o and 49.8o in the nanocrystals are attributed to the formation of a
tetragonal structure which agrees well with the standard card PDF # 17-0341 and no
impurities peaks have been found. Compared to the XRD pattern of YVO4: Eu3+/Gd3+, the
signals of YVO4: Eu3+/Gd3+ @ Ag contain the new diffraction peaks at 38.2°, 44.3°, 62.4°,
77.5° which are assigned to the (111), (200), (220), (311) lattice planes of standard card PDF
# 04-0783 of Ag. The above results confirm the presence of silver in the YVO4: Eu3+/Gd3+ @
Ag structure.
The crystal structure of synthesized YF3:Tb3+ is confirmed by X-ray powder diffraction
(XRD) (Fig. 2). The diffraction peaks indicate that the crystal belongs to the orthorhombic
phase, which was consistent with the standard JCPDS card (No. 74-0911). The signals of
sample prepared with hybrid hydrogels as hosts were very similar to the solid powders. Both
diffraction data have no other obvious impurity components. The result substantiated crystal
structure of YF3:Tb3+ has been formed and can be maintained within the hydrophilic matrix.
In SEM image (Fig. 3), YVO4: Eu3+/Gd3+ @ Ag nanostructures have very regular
spherical shape and a narrow size distribution was achieved. The average diameter of
nanoparticle was estimated to be between 70 and 80 nm. The particle size may find its
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potential functions in bio-medical studies. The dynamic light scattering is another effective
method for investigating the particle size (Fig. S1) and the corresponding results were
generally in agreement with the SEM observations. The sample was studied by energy
dispersive X-ray analysis to confirm the chemical compositions. The results revealed the
presence of Ag, Y, V, Eu, Gd and O (Fig. S2 and Table S1). No other impurity peaks can be
detected and it gave further evidence for the formation of YVO4: Eu3+/Gd3+ @ Ag phosphor.
The result of nucleation and growth for YF3:Tb3+ was given in Fig. 4. The solid phosphor
initially formed into many small building blocks with the diameter at 200-300 nm. We could
identify the existence of the particles when they were embedded into soft matter. Moreover,
extended fiber-like microstructures were also clearly seen.
The excitation spectra were monitored by fixing the emission wavelength at 615 nm (Fig.
S3). A dominant band with the maximum at 283 nm has been found and it was attributed to
the charge transfer band derived from oxygen moiety to vanadium atom. The very weak
signals at longer wavelength would be caused by the general f-f transitions inside europium
electronic configurations. The latter peaks were much smaller than the significant band of
vanadates, suggesting that the sensitization of Eu(III) ions was mainly relied on the energy
transfer from VO43- to Eu3+ ions. Upon the excitation at 283 nm, the emission spectra consist
of several narrow peaks and these signals were ascribed to the transitions from the excited 5D0
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level to the ground states of FJ (J = 1, 2, 3, 4), respectively (Fig. 5). Moreover, the electric
dipole transition at 615 nm (5D0→7F2) contributed to the majority of red emission. Based on
the incorporation of gadolinium ions, the emission intensity has been increased initially (from
x = 0 to x = 0.05, x refers to the concentration of gadolinium ions, molar ratio of Y / Eu / Gd
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= 0.95-x/0.05/x) and gradually been suppressed at higher concentrations (above x = 0.05).
With the aim of clarifying the influence of Ag addition on the photoluminescence spectra,
different volume of AgNO3 (0.5 M) was added to the reactive solution. We chose the emission
wavelength range from 500 to 750 nm under the excitation at 283 nm (Fig. 6). The
characteristic peaks at 591, 615, 653 and 703 nm were clearly observed, which were
5
corresponding to the radiation transitions of Eu3+ ions. The emission intensity of D0→7F2
transition has been significantly improved in the presence of silver shell (from 0 to 1.0 mL,
the volume refers to the amount of AgNO3 solution). Further addition of silver nitrate would
5
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lead to reduction in intensities of D0→7F1 and D0→7F2 transitions. Luminescence emission
decay files demonstrated valuable information concerning the excited state kinetics of the
fluorescent materials. The lifetime values in following Figure S4 were measured by
monitoring excitation and emission at 283 and 615 nm respectively. The corresponding
lifetime (τ) of the samples was provided in Table S2 and determined to be 935 µs, 660 µs and
276 µs for the three samples. It is evident that relatively shorter lifetime has been achieved for
YVO4: 0.05Eu3+/0.05Gd3+ incorporated by excessive amounts of silver contents and the
collected results were consistent with the photoluminescence studies. The results indicate that
the high concentration of silver with strong absorptions will show negative effects to the
energy transfer from vanadates to europium ions. In order to elucidate the influence of
microstructure, the shape evolution during the preparation process has also been demonstrated.
From the SEM image given in Figure S5, the first formed shape was a nanoparticle structure
with the size between 70 and 80 nm. The particles did not exist in an isolated state since more
silver nitrate was added (Figure S5b). Further incorporation of silver would induce serious
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agglomeration of particles and Figure S5c showed very high yields of the aggregation shape
structures. The particle aggregation might prefer to decay through non-radiative pathways and
the aggregation-caused quenching of light emission has been reported in several literatures
[14,15].
At room temperature, a broad band covering from 220 to 300 nm was observed in the
excitation file (Em = 545 nm, Fig. S6a). The excitation curve of the luminescent hydrogel
displayed the same feature as the powder and an intense band centered at 260 nm was
5
observed. The correspond transitions from the D4 excited state to the ground state were
5
presented in Figure 7. The characteristic D4→7FJ transitions (J=6, 5, 4, 3) of Tb3+ were well
5
resolved and the most intense peak would be D4→7F5 transition located in the green region.
The striking emission color can be noticed upon the irradiation of 254 nm UV lamp (Figure 8).
Both the excitation and emission lines showed that the intensity of hydrogel has been slightly
reduced after the encapsulation.
Temperature dependence steady-state fluorescence measurements were also explored for
the detection of its response behavior. From the gradual change graph, the luminescence
o
o
intensity has a significant improvement in the measuring temperature range (20 C to 80 C)
(Fig. 9). It has been proved that the intensities of lanthanide electronic transitions were very
sensitive to the external environment. Especially the so-called nonradiative deactivation
processes were closely related to high energy vibrations such as (OH, NH or even CH) [16].
In other words, the lanthanide binded water or solvent molecules will determine its
fluorescence efficiency. In a similar manner as described above, the changes in terbium
emission were investigated in the presence of heavy water (D2O). The replacement by D2O in
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the gelator solution does not affect the gel formation. Moreover, the lanthanide green
emission has been clearly switched on due to the temperature elevation as shown in Figure S7.
Therefore, it is estimated that the evaporation of H2O in soft networks in terms of the
enhancing temperature will dissociate the coordination interactions between hydroxyl moiety
and Tb3+, partial luminescence from terbium ions were recovered based on the release of
coordinating molecules.
In order to develop its potential in biomedical application, the biocompatibility of the
hydrated hydrogel in cellular culture engineering has been examined. The cytotoxicity of
YF3:Tb3+@SiO2-polyacrylic acid hybrid hydrogel in the presence of HeLa cells was evaluated
and methyl thiazolyl tetrazolium (MTT) assay was used (Fig. S8). Following incubation with
the hydrogel (100 ꢀM) for 24 hours, the cellular viabilities of HeLa cells were well above
90 %, indicating that the added luminescent species had very low cytotoxicity.
Conclusions
The main purpose of this study has been concentrated on the influence of dopants
composition (the replacement by gadolinium) and the covered silver content on the
photoluminescence features in YVO4: Eu3+. Addition of silver increased the charge transfer
bands and improved the europium emissions. The most suitable volume of AgNO3 during the
preparation process was determined to be 1.0 mL. Furthermore, YF3:Tb3+ nanoparticle has
been assembled by SiO2 shell through a sol-gel process and embedded in the polyelectrolyte
host (polyacrylic acid). Its terbium green emissions could be recovered during the temperature
dependence measurements. It is anticipated that this nanoparticles based on lanthanide and
noble metal or the nanophosphor activated luminescent hydrogel will provide promising
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future in the biomedical imaging fields.
Acknowledgements
Q. M. appreciates National Natural Science Foundation of China (No. 21371063), Science
and Technology Project in Guangzhou (2014J4100054), Natural Science Foundation of
Guangdong (2015A030313378) and Guangdong Science and Technology plan
(2013B010403025 and 2016A050502053). J. W. thanks supports from National Natural
Science Foundation of China (51571094), Guangdong province fund (2014B090915005) and
Innovation team project by Department of Education of Guangdong Province
(2016KCXTD009).
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Figure captions
Figure 1 XRD patterns for YVO4: 0.05Eu3+/0.05Gd3+ and YVO4: 0.05Eu3+/0.05Gd3+ @ Ag
samples.
Figure 2 XRD patterns for YF3:Tb3+ (a) and YF3:Tb3+@SiO2 -polyacrylic acid hybrid
hydrogel (b)
Figure 3 SEM image of YVO4: 0.05Eu3+/0.05Gd3+ @ Ag.
Figure 4 SEM images of YF3:Tb3+(a) and YF3:Tb3+@SiO2 -polyacrylic acid hybrid hydrogel
(b and c).
Figure 5 Emission spectra of YVO4: 0.05Eu3+/xGd3+ (x = 0, 0.01, 0.03, 0.05, 0.07, 0.1, x
refers to the concentration of gadolinium ions) samples (Ex = 283 nm).
Figure 6 Excitation and Emission spectra of YVO4: 0.05Eu3+/0.05Gd3+ @ Ag samples with
different content of Ag (0, 0.5, 1.0, 1.5 and 2.0 mL refer to the amount of AgNO3 solution).
Figure 7 Emission spectra of YF3:Tb3+ (a) and YF3:Tb3+@SiO2 -polyacrylic acid hybrid
hydrogel (b) (Ex = 260 nm).
Figure 8 Pictures of YF3:Tb3+@SiO2 -polyacrylic acid hybrid hydrogel under normal light
(left) and upon excitation at 254 nm (right).
Figure 9 Emission spectra of YF3:Tb3+@SiO2 -polyacrylic acid hybrid hydrogel at different
temperature (Ex = 260 nm).
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Figure 1
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Figure 2
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Figure 3
Figure 4
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Figure 5
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Figure 6
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Figure 7
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Left
Right
Figure 8
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Figure 9
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1. Silver content will decide the europium(III) luminescence.
2. Soft material was employed for the gel host.
3. Temperature increase will enhance green emissions.