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Optics Express

Optics Express

  • Editor: Andrew M. Weiner
  • Vol. 21, Iss. 5 — Mar. 11, 2013
  • pp: 6561–6571
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Time-resolved fluorescence line-narrowing of Eu3+ in biocompatible eutectic glass-ceramics

D. Sola, R. Balda, M. Al-Saleh, J. I. Peña, and J. Fernández  »View Author Affiliations


Optics Express, Vol. 21, Issue 5, pp. 6561-6571 (2013)
http://dx.doi.org/10.1364/OE.21.006561


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Abstract

The spectroscopic properties of Eu3+ in biocompatible glass and glass-ceramic eutectic rods of composition 0.8CaSiO3-0.2Ca3(PO4)2 doped with 0.5 wt% of Eu2O3 are investigated to explore their potential applications as optical probes. The samples were obtained by the laser floating zone technique. Depending on the growth rate, they exhibit three (two crystalline and one amorphous) or two (one crystalline and one amorphous) phases. The crystalline phases correspond to Ca2SiO4 and apatite-like structures. At high growth rates the system presents an amorphous arrangement which gives a glass phase. The results of time-resolved fluorescence line narrowing spectroscopy obtained under excitation within the inhomogeneous broadened 7F05D0 absorption band allow to isolate the emission from Eu3+ ions in the crystalline and amorphous environments and to accurately correlate the spectroscopic properties with the microstructure of these eutectics.

© 2013 OSA

1. Introduction

Directionally solidified eutectics are composite materials grown from the melt by means of a controlled system. They have shown more relevant properties than their crystalline counterparts with the same composition due to the unique microstructures that can be created during the solidification process. The characteristics of this complex and homogeneous microstructure which can be controlled by the solidification conditions have provided them with excellent mechanical properties, microstructural stability and corrosion resistance up to temperatures very close to their melting point [7

7. J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). [CrossRef]

]. In addition to their excellent performance as structural materials, the unique features of the eutectic microstructure allow extending the use of these materials to functional applications by the addition of rare earths (RE) to the eutectic system. For instance, the contrast in the refractive index between the constituent phases allows to produce efficient light guiding in some eutectic ceramics [8

8. R. I. Merino, J. A. Pardo, J. I. Peña, G. F. de la Fuente, A. Larrea, and V. M. Orera, “Luminescence properties of ZrO2-CaO eutectic crystals with ordered lamellar microstructure activated with Er3+ ions,” Phys. Rev. B 56(17), 10907–10915 (1997). [CrossRef]

, 9

9. V. M. Orera, J. I. Peña, R. I. Merino, J. A. Lazaro, J. A. Valles, and M. A. Rebolledo, “Prospects of new planar optical waveguides based on eutectic microcomposites of insulating crystals: The ZrO2(c)-CaZrO3 erbium doped system,” Appl. Phys. Lett. 71(19), 2746–2748 (1997). [CrossRef]

].

The eutectic wollastonite (W)-tricalcium phosphate (TCP) binary system has been widely studied throughout the last decades since it belongs to the family of bioceramic materials. In particular, tricalcium phosphate, Ca3(PO4)2, is osteoconductive and bioactive (resorbable) whereas wollastonite, CaSiO3, is bioactive with osteostimulative properties [4

4. P. N. de Aza, F. Guitian, and S. de Aza, “Phase diagram of wollastonite-tricalcium phosphate,” J. Am. Ceram. Soc. 78(6), 1653–1656 (1995). [CrossRef]

, 5

5. P. N. De Aza, F. Guitián, and S. De Aza, “Bioeutectic: a new ceramic material for human bone replacement,” Biomaterials 18(19), 1285–1291 (1997). [CrossRef] [PubMed]

, 10

10. R. G. Carrodeguas and S. De Aza, “α-Tricalcium phosphate: Synthesis, properties and biomedical applications,” Acta Biomater. 7(10), 3536–3546 (2011). [CrossRef] [PubMed]

12

12. M. Magallanes-Perdomo, P. Pena, P. N. De Aza, R. G. Carrodeguas, M. A. Rodríguez, X. Turrillas, S. De Aza, and A. H. De Aza, “Devitrification studies of wollastonite-tricalcium phosphate eutectic glass,” Acta Biomater. 5(8), 3057–3066 (2009). [CrossRef] [PubMed]

]. Worth mentioning is the fact that this material presents a high reactivity in simulated body fluid (SBF) so that when soaked into SBF an alteration of the material, based on the solution of W into SBF and a pseudomorphic transformation of TCP into HA is produced, giving rise to a thin layer of porous material. In addition, this eutectic glass, when doped with rare earths, has shown excellent optical properties [13

13. J. A. Pardo, J. I. Peña, R. I. Merino, R. Cases, A. Larrea, and V. M. Orera, “Spectroscopic properties of Er3+ and Nd3+ doped glasses with 0.8CaSiO3-0.2Ca3(PO4)2 eutectic composition,” J. Non-Cryst. Solids 298(1), 23–31 (2002). [CrossRef]

15

15. R. Balda, R. I. Merino, J. I. Peña, V. M. Orera, and J. Fernández, “Laser spectroscopy of Nd3+ ions in glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition,” Opt. Mater. 31(9), 1319–1322 (2009). [CrossRef]

].

Recent results have shown that the transformation of this eutectic glass-ceramics doped with Nd3+ ions when soaked into SBF could be analyzed in vitro by using rare-earth ions as luminescence probes. Site-selective laser spectroscopy in the 4I9/24F3/2/4F5/2 transitions of Nd3+ ions were used to investigate the crystal field changes felt by Nd3+ ions as a consequence of the sample crystallization stage. The differences among the spectral features of the site-selective excitation and emission spectra of Nd3+ ions allowed discerning between crystalline and amorphous environments for the rare-earth ions as well as to attempt correlating the spectroscopic properties with the microstructure of these eutectic materials [16

16. D. Sola, R. Balda, J. I. Peña, and J. Fernández, “Site-selective laser spectroscopy of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 biocompatible eutectic glass-ceramics,” Opt. Express 20(10), 10701–10711 (2012). [CrossRef] [PubMed]

]. However, due to the complexity of the Nd3+ ion levels and to the spectral overlapping of the emissions coming from the rare earth ion in different amorphous and crystalline environments, it was impossible to isolate the emission corresponding to individual crystalline phases. With the aim of clearly identifying the rare-earth emission from the amorphous and crystalline phases in these eutectics, Eu3+ ions which are highly sensitive to the local environment, have been used as probe ions in this work. Since 5D0 state is non-degenerate under any symmetry, the structure of the 5D07FJ emission is only determined by the splitting of the terminal levels caused by the local crystal field. Moreover, as the 7F0 level is also non-degenerate, site-selective excitation within the 7F05D0 absorption band can be performed by using fluorescence line narrowing (FLN) spectroscopy in order to distinguish among different local environments around the Eu3+ ions [17

17. R. Balda, J. Fernández, J. L. Adam, and M. A. Arriandiaga, “Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu3+-doped fluorophosphates glass,” Phys. Rev. B 54(17), 12076–12086 (1996). [CrossRef]

19

19. C. Cascales, R. Balda, V. Jubera, J. P. Chaminade, and J. Fernández, “Optical spectroscopic study of Eu3+ crystal field sites in Na3La9O3(BO3)8 crystal,” Opt. Express 16(4), 2653–2662 (2008). [CrossRef] [PubMed]

].

The work investigates the microstructural and photoluminescence properties of the binary system CaSiO3-Ca3(PO4)2 in the eutectic composition doped with Eu3+ ions as a function of the crystallization stage of the host matrix. Although photoluminescence properties have been studied in calcium silicate [20

20. H. Nagabhushana, B. M. Nagabhushana, M. Madesh Kumar, K. V. R. Chikkahanumantharayappa, K. V. R. Murthy, C. Shivakumara, and R. P. S. Chakradhar, “Synthesis, characterization and photoluminescence properties of CaSiO3: Eu3+ red phosphor,” Spectrochimica Acta Part A 78(1), 64–69 (2011). [CrossRef]

23

23. Y. Fan, S. Huang, J. Jiang, G. Li, P. Yang, H. Lian, Z. Cheng, and J. Lin, “Luminescent, mesoporous, and bioactive europium-doped calcium silicate (MCS: Eu3+) as a drug carrier,” J. Colloid Interface Sci. 357(2), 280–285 (2011). [CrossRef] [PubMed]

], dicalcium silicate [24

24. Q. Yu, Y. Liu, S. Wu, X. Lü, X. Huang, and X. Li, “Luminescence properties of Ca2SiO4:Eu3+ red phosphor for trichromatic white light emitting diodes,” J. Rare Earths 26(6), 783–786 (2008). [CrossRef]

], apatite-like structure [25

25. A. Doat, M. Fanjul, F. Pellé, E. Hollande, and A. Lebugle, “Europium-doped bioapatite: a new photostable biological probe, internalizable by human cells,” Biomaterials 24(19), 3365–3371 (2003). [CrossRef] [PubMed]

28

28. B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin. 39(2), 97–103 (1987). [CrossRef]

], and tricalcium phosphate phosphors [29

29. K. Madhukumar, H. K. Varma, M. Komath, T. S. Elias, V. Padmanabhan, and C. M. K. Nair, “Photoluminescence and thermoluminescence properties of tricalcium phosphate phosphors doped with dysprosium and europium,” Bull. Mater. Sci. 30(5), 527–534 (2007). [CrossRef]

, 30

30. W. Xue, S. Zhai, and H. Zheng, “Synthesis and photoluminescence properties of Eu3+-doped γ-Ca3(PO4)2,” Mater. Chem. Phys. 133(1), 324–327 (2012). [CrossRef]

], most of these works deal with low spectral resolved luminescence of Eu3+ obtained under non-selective UV excitation to explore its intense red emission for applications in phosphor materials [20

20. H. Nagabhushana, B. M. Nagabhushana, M. Madesh Kumar, K. V. R. Chikkahanumantharayappa, K. V. R. Murthy, C. Shivakumara, and R. P. S. Chakradhar, “Synthesis, characterization and photoluminescence properties of CaSiO3: Eu3+ red phosphor,” Spectrochimica Acta Part A 78(1), 64–69 (2011). [CrossRef]

, 22

22. S. J. Dhoble, N. S. Dhoble, and R. B. Pode, “Preparation and characterization of Eu3+ activated CaSiO3, (CaA)SiO3 [A = Ba or Sr] phosphors,” Bull. Mater. Sci. 26(4), 377–382 (2003). [CrossRef]

, 24

24. Q. Yu, Y. Liu, S. Wu, X. Lü, X. Huang, and X. Li, “Luminescence properties of Ca2SiO4:Eu3+ red phosphor for trichromatic white light emitting diodes,” J. Rare Earths 26(6), 783–786 (2008). [CrossRef]

, 29

29. K. Madhukumar, H. K. Varma, M. Komath, T. S. Elias, V. Padmanabhan, and C. M. K. Nair, “Photoluminescence and thermoluminescence properties of tricalcium phosphate phosphors doped with dysprosium and europium,” Bull. Mater. Sci. 30(5), 527–534 (2007). [CrossRef]

, 30

30. W. Xue, S. Zhai, and H. Zheng, “Synthesis and photoluminescence properties of Eu3+-doped γ-Ca3(PO4)2,” Mater. Chem. Phys. 133(1), 324–327 (2012). [CrossRef]

] and drug release tracking [21

21. X. Kang, S. Huang, P. Yang, P. Ma, D. Yang, and J. Lin, “Preparation of luminescent and mesoporous Eu3+/Tb3+ doped calcium silicate microspheres as drug carriers via a template route,” Dalton Trans. 40(9), 1873–1879 (2011). [CrossRef] [PubMed]

, 23

23. Y. Fan, S. Huang, J. Jiang, G. Li, P. Yang, H. Lian, Z. Cheng, and J. Lin, “Luminescent, mesoporous, and bioactive europium-doped calcium silicate (MCS: Eu3+) as a drug carrier,” J. Colloid Interface Sci. 357(2), 280–285 (2011). [CrossRef] [PubMed]

, 25

25. A. Doat, M. Fanjul, F. Pellé, E. Hollande, and A. Lebugle, “Europium-doped bioapatite: a new photostable biological probe, internalizable by human cells,” Biomaterials 24(19), 3365–3371 (2003). [CrossRef] [PubMed]

]. Site-selective emission spectra of Eu3+-doped fluoroapatite and oxyapatite structures are reported in Refs. 27

27. M. Karbowiak and S. Hubert, “Site-selective emission spectra of Eu3+:Ca5(PO4)3F,” J. Alloy. Comp. 302(1-2), 87–93 (2000). [CrossRef]

and 28

28. B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin. 39(2), 97–103 (1987). [CrossRef]

where the presence of different crystal field sites for Eu3+ ions depending on the method of preparation are identified. In this work, and for the first time to our knowledge, time-resolved fluorescence line narrowing (TRFLN) spectroscopy of Eu3+ in the eutectic glassy as well as glass-ceramic stages are reported. For this purpose, the samples were fabricated by the laser floating zone technique (LFZ) which permits to obtain shaped fiber glasses and glass-ceramic materials up to 4 mm in diameter. Furthermore, it allows controlling the solidification rate by means of the growth rate, providing high axial and radial thermal gradients in the liquid-solid interface [7

7. J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). [CrossRef]

, 31

31. D. Sola, F. J. Ester, P. B. Oliete, and J. I. Peña, “Study of the stability of the molten zone and the stresses induced during the growth of Al2O3–Y3Al5O12 eutectic composite by the laser floating zone technique,” J. Eur. Ceram. Soc. 31(7), 1211–1218 (2011). [CrossRef]

33

33. F. J. Ester and J. I. Peña, “Analysis of the molten zone in the growth of the Al2O3-ZrO2 (Y2O3) eutectic by the laser floating zone technique,” Bol. Soc. Esp. Ceram. 46, 240–246 (2007). [CrossRef]

], of paramount importance in the final microstructure obtained, in such a way that glasses and glass-ceramic materials can be produced. The spectroscopic properties of Eu3+ in biocompatible glass and glass-ceramic eutectic rods of composition 0.8CaSiO3-0.2Ca3(PO4)2 doped with 0.5 wt% of Eu2O3 are investigated to explore their potential applications as optical probes and identify the different micro-structural phases present in the glass ceramic. The samples were obtained by the laser floating zone technique at different growth rates between 50 and 1000 mm/h. The microstructural analysis shows the existence of three (two crystalline and one amorphous) or two phases (one crystalline and one amorphous) depending on the growth rate. The crystalline phases correspond to Ca2SiO4 and apatite-like structures. At high growth rates the system presents an amorphous arrangement which gives a glass phase. The Ca2SiO4 crystalline structure observed in the glass-ceramic samples grown at low rate shows high reactivity when soaked into simulated body fluid giving rise to a porous layer.

The results of fluorescence line narrowing experiments demonstrate the existence of three different local environments around the Eu3+ ions in these eutectic samples. The line structure in the TRFLN spectra together with the remarkably different lifetime values of the 5D0 state as a function of excitation wavelength allows to isolate the emission from Eu3+ ions in the crystalline and amorphous environments and to accurately correlate the spectroscopic properties with the microstructure of these eutectics.

2. Experimental

2.1. Samples fabrication

The precursor rods were obtained from the powder mixture of wollastonite (CS) and tricalcium phosphate (TPC) in the eutectic 80% CaSiO3, 20% Ca3(PO4)2 mol% composition. Furthermore, 0.5 wt% of Eu2O3 was added to the eutectic composite to obtain the doped samples. The resulting powders were isostatically pressed at 200 MPa for 2 minutes to obtain ceramic rods which were sintered at 1200°C for 10 hours.

Glass and glass-ceramic eutectic rods were obtained by the laser floating zone technique (LFZ), which has been described elsewhere in detail [7

7. J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). [CrossRef]

, 16

16. D. Sola, R. Balda, J. I. Peña, and J. Fernández, “Site-selective laser spectroscopy of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 biocompatible eutectic glass-ceramics,” Opt. Express 20(10), 10701–10711 (2012). [CrossRef] [PubMed]

]. This technique permits to control the solidification rate, providing high axial and radial thermal gradients in the liquid-solid interface, of a great importance in the microstructure domain, and opens up the possibility of fabricating eutectic glasses and glass-ceramics [7

7. J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). [CrossRef]

, 31

31. D. Sola, F. J. Ester, P. B. Oliete, and J. I. Peña, “Study of the stability of the molten zone and the stresses induced during the growth of Al2O3–Y3Al5O12 eutectic composite by the laser floating zone technique,” J. Eur. Ceram. Soc. 31(7), 1211–1218 (2011). [CrossRef]

33

33. F. J. Ester and J. I. Peña, “Analysis of the molten zone in the growth of the Al2O3-ZrO2 (Y2O3) eutectic by the laser floating zone technique,” Bol. Soc. Esp. Ceram. 46, 240–246 (2007). [CrossRef]

]. The growth rates were varied in order to modify the crystalline character of the samples. Keeping the rod diameter constant at about 2.5 mm, the samples were grown in air at rates of 50 and 500 mm/h to yield glass-ceramic samples, and at 1000 mm/h to produce the glass sample. After growth they were annealed at 650°C for 5 h to relieve inner stresses.

The reactivity of the glass-ceramic samples was checked by an in vitro test carried out by immersing, for a three-month period, a glass-ceramic sample grown at 50 mm/h with diameter and length of 2.5 and 5 mm respectively, inside a polyethylene bottle with 100 ml of simulated body fluid (SBF) which was prepared according to the standard process [34

34. A. Oyane, H. M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, and T. Nakamura, “Preparation and assessment of revised simulated body fluids,” J. Biomed. Mater. Res. A 65A(2), 188–195 (2003). [CrossRef] [PubMed]

]. The sample was kept at human body temperature of 37 °C by means of a Memmer Beschickung-loading-model 100-800 stove.

2.2. Characterization techniques

The microstructure, composition, and crystalline features of the phases present in the samples were determined by means of scanning electron microscopy (SEM), Energy-dispersive X-ray spectroscopy analysis (EDX), and Electron backscatter diffraction (EBSD), with a field emission JEOL JSM-7000F microscope.

Resonant time-resolved FLN spectra were performed by exciting the samples with a pulsed frequency doubled Nd:YAG pumped tunable dye laser of 9 ns pulsed width and 0.08 cm−1 linewidth and detected by an EGG&PAR Optical Multichannel Analyzer. The measurements were carried out by keeping the samples temperature at 10 K in a closed cycle helium cryostat. For lifetime measurements, the fluorescence was analyzed with a 0.25 m Jobin-Ybon monochromator and the signal detected by a Hamamatsu R636 photomultiplier. Data were processed by a Tektronix oscilloscope.

3. Results and discussion

3.1 Compositional and microstructural characterization

In first instance, the chemical composition of the glass-ceramic samples was analyzed to verify that composition was around the eutectic point, 50.7 CaO + 31.0 SiO2 + 18.3 P2O5 in wt%, and to check the amount of RE the samples were doped with. Table 1

Table 1. Compositional analysis of the W-TCP eutectic glass-ceramics in wt% together with the growth rate, V, the theoretical eutectic composition and the nominal doping composition of Eu2O3.

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shows the processing features of the glass-ceramic samples and the amount of oxides present in each one. As can be seen, the composition of the samples as well as the percentage of doping material were close to the nominal values. Traces of Mg, Fe, and Al impurities were also found.

In order to analyse the influence of the rare earth nature on the microstructure of the glass-ceramic samples, a microstructural analysis was carried out by means of scanning electron microscopy (SEM). Regarding the morphology of the samples, the substitution of Nd2O3 by Eu2O3 did not produce any effect either on the arrangement or on the number of phases. As an example, Fig. 1
Fig. 1 Longitudinal-section of the glass-ceramic samples doped with 0.5 wt% of Eu2O3 grown at 50 mm/h (a) and 500 mm/h (b). The insets in the micrographs show the details of the microstructure in a cross-section view.
shows the longitudinal section of the glass-ceramic samples doped with 0.5 wt% of Eu2O3 grown at 50 mm/h (a) and 500 mm/h (b). The insets in the micrographs show the details of the microstructure in a cross-section view. The micrographs show how, as when doped with Nd2O3, the microstructure was made up of fibers with an irregular shape, aligned to the growing direction embedded in a matrix. In the same way, the number of phases present in the samples depended on the growth rate, so that at a low growth rate of 50 mm/h, the sample had three phases, i.e., the fibers (clear phase) and the matrix (dark and black phases), whereas at a high growth rate of 500 mm/h, there were only two phases in the sample, the fibers (clear phase) and matrix (dark phase).

With regards to the crystalline nature of the phases, EBSD analysis performed in the samples confirmed that the crystalline structures were the same as observed for the Nd2O3 case. Fibers (clear phase) were crystalline with an apatite-like structure in any case, whereas the dark phase turned from crystalline, with Ca2SiO4 structure, into amorphous as the growth rate increased. The black phase, which only appeared at low rate, was amorphous. Figure 2
Fig. 2 Cross-section micrograph of a glass-ceramic sample doped with 0.5 wt% of Eu2O3 grown at 50 mm/h. The insets show the electron backscatter diffraction patterns corresponding to an oxyapatite structure found in the clear phase, (1), and to the dicalcium silicate found in the dark phase, (2).
shows the cross-section micrograph of a sample grown at 50 mm/h doped with 0.5 wt% of Eu2O3. The insets show the electron backscatter diffraction patterns corresponding to the oxyapatite structure found in the clear phase, (1), and to the dicalcium silicate found in the dark phase, (2).

On the other hand, the Ca2SiO4 structure observed in the glass-ceramic samples grown at low growth rate has shown an excellent in vitro bioactivity, since when soaked into SBF, a bonelike hydroxyapatite (CHA) layer is formed on the surface [35

35. Z. Gou, J. Chang, and W. Zhai, “Preparation and characterization of novel bioactive dicalcium silicate ceramics,” J. Eur. Ceram. Soc. 25(9), 1507–1514 (2005). [CrossRef]

]. To ascertain the reactivity of the glass-ceramic sample grown at low rate, an in vitro preliminary test was carried out by soaking the sample grown at 50 mm/h into SBF for 3 months. Figure 3
Fig. 3 Longitudinal -section micrograph of a glass-ceramic sample doped with 0.5 wt% of Eu2O3 grown at 50 mm/h after an immersion period of three months in simulated body fluid (SBF).
shows the micrograph in a longitudinal-section view of the sample after the immersion period. It can be observed how the SBF has dissolved the two phases of the glass-ceramic matrix, the amorphous phase as well as the one with Ca2SiO4 structure, giving rise to a porous layer with a thickness of around 80 μm in which the fibers still remain in the sample. An EBSD analysis carried out in the sample confirmed that the apatite-like structure of the fibers was unaltered after the immersion period.

Therefore, we can conclude that the features of the LFZ growing technique permit to obtain glass-ceramic samples departing from a non-equilibrium state, resulting in microstructures not reported so far in this system that cannot be obtain by the conventional thermal treatments in furnace in which the substitution of the rare earth does not affect the microstructure of the glass-ceramic samples either in the morphology or in the crystalline structure.

3.2. Time-resolved fluorescence line-narrowing spectroscopy

As we have mentioned before, the optical properties of Eu3+ ions are highly sensitive to the local environment. Since the first excited state 5D0 is non-degenerate, the structure observed in the fluorescence spectra is only determined by the terminal ground state splitting. Therefore, the 5D07F0 transition clearly indicates there are different sites present in the host. Moreover, the 5D07F2 transition is hypersensitive to the chemical surroundings and is symmetry dependent. Hence it is possible to distinguish between crystalline and glassy environments by means of time resolved fluorescence line narrowing spectroscopy.

Time-resolved line-narrowed fluorescence spectra of the 5D07F0-2 transitions of Eu3+-doped samples were obtained at 10 K by using different resonant excitation wavelengths into the 7F05D0 transition with a time delay of 10 μs. Figure 4(a)
Fig. 4 5D0→F0, 1, 2 emissions of Eu3+ in the sample grown at (a) 50 mm/h and (b) 500 mm/h obtained at different excitation wavelengths along the 7F05D0 absorption band. Data correspond to 10 K.
shows the spectra corresponding to the 5D07F0,1,2 transitions for the sample grown at 50 mm/h obtained at different pumping wavelengths along the 7F05D0 absorption band. Depending on the excitation wavelength the emission spectra present different characteristics, mainly regarding the relative intensity of the transitions and the splitting. Under excitation between 572 and 576, the spectra show similar features to those observed in Eu3+-doped apatites [27

27. M. Karbowiak and S. Hubert, “Site-selective emission spectra of Eu3+:Ca5(PO4)3F,” J. Alloy. Comp. 302(1-2), 87–93 (2000). [CrossRef]

, 28

28. B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin. 39(2), 97–103 (1987). [CrossRef]

]. In these spectra, the 5D07F0 transition shows a strong intensity and the energy of the 5D0 level is higher than in other Eu3+-doped crystalline samples. Moreover, there is a significant splitting of the Stark components in which levels 7F1 and 7F2 overlap which indicates the presence of a highly asymmetric crystal field which produces mixing of the 7F1 and 7F2 states [27

27. M. Karbowiak and S. Hubert, “Site-selective emission spectra of Eu3+:Ca5(PO4)3F,” J. Alloy. Comp. 302(1-2), 87–93 (2000). [CrossRef]

, 28

28. B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin. 39(2), 97–103 (1987). [CrossRef]

]. In the apatite lattice Eu3+ can substitute the Ca2+ ions and therefore, as there are two inequivalent Ca sites, one ninefold coordinated with C3 symmetry and the other sevenfold coordinated with Cs symmetry, the splitting of these transitions indicates that Eu3+ ions in the apatite-like crystalline phase occupy the low symmetry sites. The presence of slightly different sites can be explained by a slightly different arrangement of charge compensating ions and defects.

On the other hand, while tuning the excitation pulse at other wavelengths (578 nm) the spectra consist of a superposition of emissions corresponding to Eu3+ ions in different phases. The spectrum obtained by exciting at 579.1 nm is only observed in this sample and according to the EBSD analysis, corresponds to Eu3+ ions in dicalcium silicate crystalline phase. As in the case of the spectra corresponding to the apatite-like structure, three Stark levels for the 5D07F1 transition and five levels in the hypersensitive 5D07F2 transition are observed, meaning that Eu3+ ions are located in crystal sites with C2v or lower symmetry.

As in the case of Nd3+ ions the dicalcium silicate phase appears only in the sample grown at low rate, in this case at 50 mm/h. This can be clearly observed in Fig. 5
Fig. 5 5D0→F0, 1, 2 emissions of Eu3+ for the glass-ceramic samples grown at 50 and 500 mm/h and for the glass sample obtained under excitation at 579.1 nm Data correspond to 10 K.
which shows the emission spectra obtained under excitation at 579.1 nm for the GC samples grown at 50 mm/h and 500 mm/h and for the glass sample. As can be seen the emission spectrum of the sample grown at 500 mm/h is quite similar to the one obtained in the glass sample, which indicates that in this sample at this wavelength we are exciting Eu3+ ions in the amorphous phase. However, in the case of the glass-ceramic sample grown at 50 mm/h, the spectrum shows sharp lines according to Eu3+ in the dicalcium silicate crystalline phase. This is clear from the excitation spectra corresponding to the 7F05D0 transition for the three samples (see Fig. 6
Fig. 6 Low temperature excitation spectra of the 7F05D0 transition for the glass-ceramic samples grown at 50 and 500 mm/h and for the glass sample obtained by collecting the luminescence at the 5D07F2 emission.
). As can be seen, the excitation spectrum of the sample grown at 50 mm/h shows a band centered around 574 nm which corresponds to the apatite-like crystalline phase, together with a narrow peak around 579.1 nm, the excitation wavelength at which Eu3+ ions in the dicalcium silicate crystalline phase are excited. In the case of the sample grown at 500 mm/h, the spectrum shows the band around 574 nm and a shoulder at the long wavelength side of the spectrum that corresponds to the Eu3+ ions in the amorphous phase.

The presence of Eu3+ in the crystalline and amorphous phases is also confirmed by lifetime measurements of the 5D0 level. As could be expected, if there are different sites for the Eu3+ ion, the lifetime of state 5D0 should depend on the excitation wavelength. Thus, the lifetime of the 5D0 state at different excitation wavelengths was measured and the luminescence at the highest intensity Stark component of the 5D07F2 transition collected.

The temporal decays obtained under excitation at which the crystalline phases are selectively resolved can be described by a single exponential function. In the sample grown at 50 mm/h, the lifetime obtained under excitation at 579.1 nm which corresponds to Eu3+ in dicalcium silicate crystalline phase is around 3 ms whereas the lifetime corresponding to the apatite-like crystalline phase displays a variation from 0.448 to 0.661 as the excitation wavelength increases from 571.6 to 576 nm. At other wavelengths when Eu3+ ions are simultaneously excited in different phases, the decays are not single exponentials and the lifetimes change from 0.423 to 2.5 ms depending on the excitation and emission wavelengths. As an example Fig. 7
Fig. 7 Experimental decays of the 5D0 level obtained under excitation at (a) 574 and (b) 579.1 nm and collecting the luminescence at the 5D07F2 emission for the sample grown at 50 mm/h.
shows the experimental decays obtained by exciting at 574 and 579.1 nm and collecting the luminescence at the highest Stark component of the 5D07F2 transition for the sample grown at 50 mm/h. In the case of the sample grown at 500 mm/h the decays obtained under excitation between 571 and 577 nm are single exponentials with lifetimes ranging between 0.47 and 0.77 ms. At longer wavelengths when Eu3+ ions in the apatite and amorphous phases are simultaneously excited, the decays are not single exponentials and lifetimes are longer. In the case of the glass sample the decays are single exponentials at all excitation wavelengths and the lifetime changes from 1.45 to 1.71 ms depending on the excitation wavelength. Therefore, as expected, lifetime measurements allow to identify the presence of Eu3+ ions in the different phases.

4. Conclusions

Eutectic glass and glass-ceramics samples of composition 0.8CaSiO3-0.2Ca3(PO4)2 doped with 0.5 wt% of Eu2O3 were fabricated by the laser floating zone technique. This growing technique permits to obtain glass-ceramic samples starting from a non-equilibrium state and ending in not yet reported microstructures in this system that cannot be obtained by conventional thermal treatments in furnace. The microstructural analysis showed that the nature of the rare earth the samples were doped with, did not produce any effect either on the arrangement or on the number of phases, so that the growth rate is the key factor involved in the crystallization process and hence in the size and number of phases present in the glass-ceramic samples as well as in their crystalline features. In particular, the glassy character of the glass-ceramic samples increases with the growth rate in such a way that at high growth rates a glass sample can be obtained. For the glass-ceramic samples, the existence of three (two crystalline and one amorphous) or two phases (one crystalline and one amorphous), depending on the growth rate, has been shown. The crystalline phases correspond to Ca2SiO4 and apatite-like structures. The Ca2SiO4 structure observed in the glass-ceramic samples grown at low growth rate has shown high reactivity when soaked into simulated body fluid, so that the SBF dissolved this crystalline phase giving rise to a porous layer in which the apatite-like structure phase remains unaltered in the sample after the immersion period.

Acknowledgments

This work was supported by the Spanish Government MEC under Projects No. MAT2009-14282-C02-02, FIS2011-27968, Consolider SAUUL CSD2007-00013, and Basque Country Government IT-331-07 and SAIOTEK S-PE12UN016. Daniel Sola thanks the JAE-DOC program and the Science and Technology Inter-Ministry commission of Spain and FEDER funds of the EC under project MAT2009-13979-C03-03 for the financial support of his contract.

References and links

1.

L. L. Hench, R. J. Splinter, T. K. Greenle, and W. C. Allen, “Bonding mechanisms at the interface of ceramic prosthetic materials,” J. Biomed. Mater. Res. 2, 117–141 (1971).

2.

L. L. Hench, “Bioceramics: From Concept to Clinic,” J. Am. Ceram. Soc. 74(7), 1487–1510 (1991). [CrossRef]

3.

K. De Groot and R. Le Geros, Significance of Porosity and Physical Chemistry of Calcium Phosphate Ceramics, P. Ducheyne, ed. (Ann. N.Y. Acad. Sci.,1988).

4.

P. N. de Aza, F. Guitian, and S. de Aza, “Phase diagram of wollastonite-tricalcium phosphate,” J. Am. Ceram. Soc. 78(6), 1653–1656 (1995). [CrossRef]

5.

P. N. De Aza, F. Guitián, and S. De Aza, “Bioeutectic: a new ceramic material for human bone replacement,” Biomaterials 18(19), 1285–1291 (1997). [CrossRef] [PubMed]

6.

M. Magallanes-Perdomo, A. H. De Aza, I. Sobrados, J. Sanz, and P. Pena, “Structure and properties of bioactive eutectic glasses based on the Ca3(PO4)2-CaSiO3-CaMg(SiO3)2 system,” Acta Biomater. 8(2), 820–829 (2012). [CrossRef] [PubMed]

7.

J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci. 51(6), 711–809 (2006). [CrossRef]

8.

R. I. Merino, J. A. Pardo, J. I. Peña, G. F. de la Fuente, A. Larrea, and V. M. Orera, “Luminescence properties of ZrO2-CaO eutectic crystals with ordered lamellar microstructure activated with Er3+ ions,” Phys. Rev. B 56(17), 10907–10915 (1997). [CrossRef]

9.

V. M. Orera, J. I. Peña, R. I. Merino, J. A. Lazaro, J. A. Valles, and M. A. Rebolledo, “Prospects of new planar optical waveguides based on eutectic microcomposites of insulating crystals: The ZrO2(c)-CaZrO3 erbium doped system,” Appl. Phys. Lett. 71(19), 2746–2748 (1997). [CrossRef]

10.

R. G. Carrodeguas and S. De Aza, “α-Tricalcium phosphate: Synthesis, properties and biomedical applications,” Acta Biomater. 7(10), 3536–3546 (2011). [CrossRef] [PubMed]

11.

P. N. de Aza, F. Guitian, and S. de Aza, “A new bioactive material which transforms in situ into hydroxyapatite,” Acta Mater. 46(7), 2541–2549 (1998). [CrossRef]

12.

M. Magallanes-Perdomo, P. Pena, P. N. De Aza, R. G. Carrodeguas, M. A. Rodríguez, X. Turrillas, S. De Aza, and A. H. De Aza, “Devitrification studies of wollastonite-tricalcium phosphate eutectic glass,” Acta Biomater. 5(8), 3057–3066 (2009). [CrossRef] [PubMed]

13.

J. A. Pardo, J. I. Peña, R. I. Merino, R. Cases, A. Larrea, and V. M. Orera, “Spectroscopic properties of Er3+ and Nd3+ doped glasses with 0.8CaSiO3-0.2Ca3(PO4)2 eutectic composition,” J. Non-Cryst. Solids 298(1), 23–31 (2002). [CrossRef]

14.

R. Balda, J. Fernández, I. Iparraguirre, J. Azkargorta, S. García-Revilla, J. I. Peña, R. I. Merino, and V. M. Orera, “Broadband laser tunability of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glass,” Opt. Express 17(6), 4382–4387 (2009). [CrossRef] [PubMed]

15.

R. Balda, R. I. Merino, J. I. Peña, V. M. Orera, and J. Fernández, “Laser spectroscopy of Nd3+ ions in glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition,” Opt. Mater. 31(9), 1319–1322 (2009). [CrossRef]

16.

D. Sola, R. Balda, J. I. Peña, and J. Fernández, “Site-selective laser spectroscopy of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 biocompatible eutectic glass-ceramics,” Opt. Express 20(10), 10701–10711 (2012). [CrossRef] [PubMed]

17.

R. Balda, J. Fernández, J. L. Adam, and M. A. Arriandiaga, “Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu3+-doped fluorophosphates glass,” Phys. Rev. B 54(17), 12076–12086 (1996). [CrossRef]

18.

C. Cascales, J. Fernández, and R. Balda, “Investigation of site-selective symmetries of Eu3+ ions in KPb2Cl5 by using optical spectroscopy,” Opt. Express 13(6), 2141–2152 (2005). [CrossRef] [PubMed]

19.

C. Cascales, R. Balda, V. Jubera, J. P. Chaminade, and J. Fernández, “Optical spectroscopic study of Eu3+ crystal field sites in Na3La9O3(BO3)8 crystal,” Opt. Express 16(4), 2653–2662 (2008). [CrossRef] [PubMed]

20.

H. Nagabhushana, B. M. Nagabhushana, M. Madesh Kumar, K. V. R. Chikkahanumantharayappa, K. V. R. Murthy, C. Shivakumara, and R. P. S. Chakradhar, “Synthesis, characterization and photoluminescence properties of CaSiO3: Eu3+ red phosphor,” Spectrochimica Acta Part A 78(1), 64–69 (2011). [CrossRef]

21.

X. Kang, S. Huang, P. Yang, P. Ma, D. Yang, and J. Lin, “Preparation of luminescent and mesoporous Eu3+/Tb3+ doped calcium silicate microspheres as drug carriers via a template route,” Dalton Trans. 40(9), 1873–1879 (2011). [CrossRef] [PubMed]

22.

S. J. Dhoble, N. S. Dhoble, and R. B. Pode, “Preparation and characterization of Eu3+ activated CaSiO3, (CaA)SiO3 [A = Ba or Sr] phosphors,” Bull. Mater. Sci. 26(4), 377–382 (2003). [CrossRef]

23.

Y. Fan, S. Huang, J. Jiang, G. Li, P. Yang, H. Lian, Z. Cheng, and J. Lin, “Luminescent, mesoporous, and bioactive europium-doped calcium silicate (MCS: Eu3+) as a drug carrier,” J. Colloid Interface Sci. 357(2), 280–285 (2011). [CrossRef] [PubMed]

24.

Q. Yu, Y. Liu, S. Wu, X. Lü, X. Huang, and X. Li, “Luminescence properties of Ca2SiO4:Eu3+ red phosphor for trichromatic white light emitting diodes,” J. Rare Earths 26(6), 783–786 (2008). [CrossRef]

25.

A. Doat, M. Fanjul, F. Pellé, E. Hollande, and A. Lebugle, “Europium-doped bioapatite: a new photostable biological probe, internalizable by human cells,” Biomaterials 24(19), 3365–3371 (2003). [CrossRef] [PubMed]

26.

X. H. Chuaia, H. J. Zhang, F. Sh. Li, Sh. Z. Lu, J. Lin, Sh. B. Wang, and K. Chi-Chou, “Synthesis and luminescence properties of oxyapatite NaY9Si6O26 doped with Eu3+, Tb3+, Dy3+ and Pb2+,” J. Alloy. Comp. 334, 211–218 (2002).

27.

M. Karbowiak and S. Hubert, “Site-selective emission spectra of Eu3+:Ca5(PO4)3F,” J. Alloy. Comp. 302(1-2), 87–93 (2000). [CrossRef]

28.

B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin. 39(2), 97–103 (1987). [CrossRef]

29.

K. Madhukumar, H. K. Varma, M. Komath, T. S. Elias, V. Padmanabhan, and C. M. K. Nair, “Photoluminescence and thermoluminescence properties of tricalcium phosphate phosphors doped with dysprosium and europium,” Bull. Mater. Sci. 30(5), 527–534 (2007). [CrossRef]

30.

W. Xue, S. Zhai, and H. Zheng, “Synthesis and photoluminescence properties of Eu3+-doped γ-Ca3(PO4)2,” Mater. Chem. Phys. 133(1), 324–327 (2012). [CrossRef]

31.

D. Sola, F. J. Ester, P. B. Oliete, and J. I. Peña, “Study of the stability of the molten zone and the stresses induced during the growth of Al2O3–Y3Al5O12 eutectic composite by the laser floating zone technique,” J. Eur. Ceram. Soc. 31(7), 1211–1218 (2011). [CrossRef]

32.

F. J. Ester, D. Sola, and J. I. Peña, “Thermal stresses in the Al2O3-ZrO2 (Y2O3) eutectic composite during the growth by the laser floating zone technique,” Bol. Soc. Esp. Ceram. 47, 352–357 (2008). [CrossRef]

33.

F. J. Ester and J. I. Peña, “Analysis of the molten zone in the growth of the Al2O3-ZrO2 (Y2O3) eutectic by the laser floating zone technique,” Bol. Soc. Esp. Ceram. 46, 240–246 (2007). [CrossRef]

34.

A. Oyane, H. M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, and T. Nakamura, “Preparation and assessment of revised simulated body fluids,” J. Biomed. Mater. Res. A 65A(2), 188–195 (2003). [CrossRef] [PubMed]

35.

Z. Gou, J. Chang, and W. Zhai, “Preparation and characterization of novel bioactive dicalcium silicate ceramics,” J. Eur. Ceram. Soc. 25(9), 1507–1514 (2005). [CrossRef]

OCIS Codes
(160.5690) Materials : Rare-earth-doped materials
(300.6320) Spectroscopy : Spectroscopy, high-resolution
(160.1435) Materials : Biomaterials

ToC Category:
Materials

History
Original Manuscript: December 10, 2012
Revised Manuscript: February 26, 2013
Manuscript Accepted: March 1, 2013
Published: March 8, 2013

Citation
D. Sola, R. Balda, M. Al-Saleh, J. I. Peña, and J. Fernández, "Time-resolved fluorescence line-narrowing of Eu3+ in biocompatible eutectic glass-ceramics," Opt. Express 21, 6561-6571 (2013)
http://www.opticsinfobase.org/oe/abstract.cfm?URI=oe-21-5-6561


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References

  1. L. L. Hench, R. J. Splinter, T. K. Greenle, and W. C. Allen, “Bonding mechanisms at the interface of ceramic prosthetic materials,” J. Biomed. Mater. Res.2, 117–141 (1971).
  2. L. L. Hench, “Bioceramics: From Concept to Clinic,” J. Am. Ceram. Soc.74(7), 1487–1510 (1991). [CrossRef]
  3. K. De Groot and R. Le Geros, Significance of Porosity and Physical Chemistry of Calcium Phosphate Ceramics, P. Ducheyne, ed. (Ann. N.Y. Acad. Sci.,1988).
  4. P. N. de Aza, F. Guitian, and S. de Aza, “Phase diagram of wollastonite-tricalcium phosphate,” J. Am. Ceram. Soc.78(6), 1653–1656 (1995). [CrossRef]
  5. P. N. De Aza, F. Guitián, and S. De Aza, “Bioeutectic: a new ceramic material for human bone replacement,” Biomaterials18(19), 1285–1291 (1997). [CrossRef] [PubMed]
  6. M. Magallanes-Perdomo, A. H. De Aza, I. Sobrados, J. Sanz, and P. Pena, “Structure and properties of bioactive eutectic glasses based on the Ca3(PO4)2-CaSiO3-CaMg(SiO3)2 system,” Acta Biomater.8(2), 820–829 (2012). [CrossRef] [PubMed]
  7. J. Llorca and V. M. Orera, “Directionally solidified eutectic ceramic oxides,” Prog. Mater. Sci.51(6), 711–809 (2006). [CrossRef]
  8. R. I. Merino, J. A. Pardo, J. I. Peña, G. F. de la Fuente, A. Larrea, and V. M. Orera, “Luminescence properties of ZrO2-CaO eutectic crystals with ordered lamellar microstructure activated with Er3+ ions,” Phys. Rev. B56(17), 10907–10915 (1997). [CrossRef]
  9. V. M. Orera, J. I. Peña, R. I. Merino, J. A. Lazaro, J. A. Valles, and M. A. Rebolledo, “Prospects of new planar optical waveguides based on eutectic microcomposites of insulating crystals: The ZrO2(c)-CaZrO3 erbium doped system,” Appl. Phys. Lett.71(19), 2746–2748 (1997). [CrossRef]
  10. R. G. Carrodeguas and S. De Aza, “α-Tricalcium phosphate: Synthesis, properties and biomedical applications,” Acta Biomater.7(10), 3536–3546 (2011). [CrossRef] [PubMed]
  11. P. N. de Aza, F. Guitian, and S. de Aza, “A new bioactive material which transforms in situ into hydroxyapatite,” Acta Mater.46(7), 2541–2549 (1998). [CrossRef]
  12. M. Magallanes-Perdomo, P. Pena, P. N. De Aza, R. G. Carrodeguas, M. A. Rodríguez, X. Turrillas, S. De Aza, and A. H. De Aza, “Devitrification studies of wollastonite-tricalcium phosphate eutectic glass,” Acta Biomater.5(8), 3057–3066 (2009). [CrossRef] [PubMed]
  13. J. A. Pardo, J. I. Peña, R. I. Merino, R. Cases, A. Larrea, and V. M. Orera, “Spectroscopic properties of Er3+ and Nd3+ doped glasses with 0.8CaSiO3-0.2Ca3(PO4)2 eutectic composition,” J. Non-Cryst. Solids298(1), 23–31 (2002). [CrossRef]
  14. R. Balda, J. Fernández, I. Iparraguirre, J. Azkargorta, S. García-Revilla, J. I. Peña, R. I. Merino, and V. M. Orera, “Broadband laser tunability of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 eutectic glass,” Opt. Express17(6), 4382–4387 (2009). [CrossRef] [PubMed]
  15. R. Balda, R. I. Merino, J. I. Peña, V. M. Orera, and J. Fernández, “Laser spectroscopy of Nd3+ ions in glasses with the 0.8CaSiO3–0.2Ca3(PO4)2 eutectic composition,” Opt. Mater.31(9), 1319–1322 (2009). [CrossRef]
  16. D. Sola, R. Balda, J. I. Peña, and J. Fernández, “Site-selective laser spectroscopy of Nd3+ ions in 0.8CaSiO3-0.2Ca3(PO4)2 biocompatible eutectic glass-ceramics,” Opt. Express20(10), 10701–10711 (2012). [CrossRef] [PubMed]
  17. R. Balda, J. Fernández, J. L. Adam, and M. A. Arriandiaga, “Time-resolved fluorescence-line narrowing and energy-transfer studies in a Eu3+-doped fluorophosphates glass,” Phys. Rev. B54(17), 12076–12086 (1996). [CrossRef]
  18. C. Cascales, J. Fernández, and R. Balda, “Investigation of site-selective symmetries of Eu3+ ions in KPb2Cl5 by using optical spectroscopy,” Opt. Express13(6), 2141–2152 (2005). [CrossRef] [PubMed]
  19. C. Cascales, R. Balda, V. Jubera, J. P. Chaminade, and J. Fernández, “Optical spectroscopic study of Eu3+ crystal field sites in Na3La9O3(BO3)8 crystal,” Opt. Express16(4), 2653–2662 (2008). [CrossRef] [PubMed]
  20. H. Nagabhushana, B. M. Nagabhushana, M. Madesh Kumar, K. V. R. Chikkahanumantharayappa, K. V. R. Murthy, C. Shivakumara, and R. P. S. Chakradhar, “Synthesis, characterization and photoluminescence properties of CaSiO3: Eu3+ red phosphor,” Spectrochimica Acta Part A78(1), 64–69 (2011). [CrossRef]
  21. X. Kang, S. Huang, P. Yang, P. Ma, D. Yang, and J. Lin, “Preparation of luminescent and mesoporous Eu3+/Tb3+ doped calcium silicate microspheres as drug carriers via a template route,” Dalton Trans.40(9), 1873–1879 (2011). [CrossRef] [PubMed]
  22. S. J. Dhoble, N. S. Dhoble, and R. B. Pode, “Preparation and characterization of Eu3+ activated CaSiO3, (CaA)SiO3 [A = Ba or Sr] phosphors,” Bull. Mater. Sci.26(4), 377–382 (2003). [CrossRef]
  23. Y. Fan, S. Huang, J. Jiang, G. Li, P. Yang, H. Lian, Z. Cheng, and J. Lin, “Luminescent, mesoporous, and bioactive europium-doped calcium silicate (MCS: Eu3+) as a drug carrier,” J. Colloid Interface Sci.357(2), 280–285 (2011). [CrossRef] [PubMed]
  24. Q. Yu, Y. Liu, S. Wu, X. Lü, X. Huang, and X. Li, “Luminescence properties of Ca2SiO4:Eu3+ red phosphor for trichromatic white light emitting diodes,” J. Rare Earths26(6), 783–786 (2008). [CrossRef]
  25. A. Doat, M. Fanjul, F. Pellé, E. Hollande, and A. Lebugle, “Europium-doped bioapatite: a new photostable biological probe, internalizable by human cells,” Biomaterials24(19), 3365–3371 (2003). [CrossRef] [PubMed]
  26. X. H. Chuaia, H. J. Zhang, F. Sh. Li, Sh. Z. Lu, J. Lin, Sh. B. Wang, and K. Chi-Chou, “Synthesis and luminescence properties of oxyapatite NaY9Si6O26 doped with Eu3+, Tb3+, Dy3+ and Pb2+,” J. Alloy. Comp.334, 211–218 (2002).
  27. M. Karbowiak and S. Hubert, “Site-selective emission spectra of Eu3+:Ca5(PO4)3F,” J. Alloy. Comp.302(1-2), 87–93 (2000). [CrossRef]
  28. B. Piriou, D. Fahmi, J. Dexpert-Ghys, A. Taitai, and J. L. Lacout, “Unusual fluorescent properties of Eu3+ in oxyapatites,” J. Lumin.39(2), 97–103 (1987). [CrossRef]
  29. K. Madhukumar, H. K. Varma, M. Komath, T. S. Elias, V. Padmanabhan, and C. M. K. Nair, “Photoluminescence and thermoluminescence properties of tricalcium phosphate phosphors doped with dysprosium and europium,” Bull. Mater. Sci.30(5), 527–534 (2007). [CrossRef]
  30. W. Xue, S. Zhai, and H. Zheng, “Synthesis and photoluminescence properties of Eu3+-doped γ-Ca3(PO4)2,” Mater. Chem. Phys.133(1), 324–327 (2012). [CrossRef]
  31. D. Sola, F. J. Ester, P. B. Oliete, and J. I. Peña, “Study of the stability of the molten zone and the stresses induced during the growth of Al2O3–Y3Al5O12 eutectic composite by the laser floating zone technique,” J. Eur. Ceram. Soc.31(7), 1211–1218 (2011). [CrossRef]
  32. F. J. Ester, D. Sola, and J. I. Peña, “Thermal stresses in the Al2O3-ZrO2 (Y2O3) eutectic composite during the growth by the laser floating zone technique,” Bol. Soc. Esp. Ceram.47, 352–357 (2008). [CrossRef]
  33. F. J. Ester and J. I. Peña, “Analysis of the molten zone in the growth of the Al2O3-ZrO2 (Y2O3) eutectic by the laser floating zone technique,” Bol. Soc. Esp. Ceram.46, 240–246 (2007). [CrossRef]
  34. A. Oyane, H. M. Kim, T. Furuya, T. Kokubo, T. Miyazaki, and T. Nakamura, “Preparation and assessment of revised simulated body fluids,” J. Biomed. Mater. Res. A65A(2), 188–195 (2003). [CrossRef] [PubMed]
  35. Z. Gou, J. Chang, and W. Zhai, “Preparation and characterization of novel bioactive dicalcium silicate ceramics,” J. Eur. Ceram. Soc.25(9), 1507–1514 (2005). [CrossRef]

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