FTIR and EPR spectroscopic investigation of calcium-silicate glasses with iron and dysprosium

Accepted Manuscript FTIR and EPR spectroscopic investigation of calcium-silicate glasses with iron and dysprosium D. Eniu, C. Gruian, E. Vanea, L. Patcas, V. Simon PII: DOI: Reference:

S0022-2860(14)01223-X http://dx.doi.org/10.1016/j.molstruc.2014.12.020 MOLSTR 21170

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Journal of Molecular Structure

Received Date: Revised Date: Accepted Date:

29 September 2014 3 December 2014 3 December 2014

Please cite this article as: D. Eniu, C. Gruian, E. Vanea, L. Patcas, V. Simon, FTIR and EPR spectroscopic investigation of calcium-silicate glasses with iron and dysprosium, Journal of Molecular Structure (2014), doi: http://dx.doi.org/10.1016/j.molstruc.2014.12.020

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FTIR and EPR spectroscopic investigation of calcium-silicate glasses with iron and dysprosium D. Eniu1, C. Gruian2, E. Vanea2, L. Patcas2, V. Simon2* 1

Iuliu Haţieganu University of Medicine and Pharmacy, Department of Molecular Sciences, 400349 Cluj-Napoca, Romania 2 Babeş-Bolyai University, Faculty of Physics & Interdisciplinary Research Institute on Bio-Nano-Sciences, 400084 Cluj-Napoca, Romania

Abstract The sol-gel derived 50SiO2·30CaO·10Fe2O3·10Dy2O3 system was subjected to heat treatments at 500, 800 and 1200 oC in order to obtain crystalline phases of interest for biomedical applications. The structural changes were investigated by Fourier transform infrared spectroscopy (FTIR) and electron paramagnetic resonance (EPR). Both FTIR and EPR results support the development of wollastonite, hematite and magnetite crystalline phases desirable for samples bioactivity and heating possibility for hyperthermia treatment. Dysprosium addition was considered for subsequent radioactivation of the samples that could extend their application to thermoradiotherapy.

Keywords: calcium-silicate; iron; dysprosium; thermal treatment; FTIR; EPR.

______________________________________________________ *Corresponding author: E-mail: [email protected] Tel.: +40-264-405300; Fax: +40-264-59190

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Introduction Bioactive glasses containing radioactivable isotopes are considered potential candidates for radiotherapy treatment of bone cancer [1]. Systems containing radioactivable isotopes like yttrium or dysprosium have attracted much interest for biomedical applications, since they can be activated by neutron irradiation to radioactive isotopes with convenient properties for in situ radiotherapy [2–5]. Larger doses of radiation can thus be safely delivered to the target site and the exposure of the healthy tissue is considerably diminished [6]. On the other hand, glass ceramics containing magnetite (Fe3O4) were intensively studied considering their potential applications in hyperthermia treatment of cancer [7-9]. The ferromagnetic phase developed in vitroceramic biomaterials can produce a local heating through hysteresis losses or magnetic relaxation phenomena and can induce tumours necrosis [10]. Moreover, the simultaneous application of radiotherapy and hyperthermia was taken into account expecting a synergistic therapeutic effect [11]. The properties of biomaterials are strongly related to their structure and on these grounds the structure tailoring is an important issue for biomedical applications [12]. The development of wollastonite (CaSiO3) in glass-ceramic systems designed as biomaterials confers both bioactivity and biocompatibility [13, 14]. The present study aims to investigate the structural changes of the sol-gel derived calcium-silicate glasses containing dysprosium and iron oxides, upon thermal treatment at 500 800 and 1200 oC temperatures, by using Fourier Transform Infrared (FTIR) and Electron Paramagnetic Resonance (EPR) spectroscopic techniques.

Materials and methods The investigated calcium-silicate glass system 50SiO2·30CaO·10Fe2O3·10Dy2O3 (mol %) was prepared following the one step acid-catalysed sol-gel route. Tetraethyl orthosilicate 2

Si(C2H5O)4 (TEOS), calcium nitrate Ca(NO3)2·4H2O, iron nitrate Fe(NO3)3·9H2O, and dysprosium nitrate Dy(NO3)3·H2O were used as starting reagents. The sol was prepared by stirring TEOS with water and ethanol in 1:2:1 weight ratio, catalysed with HNO3 and heated to 80 ºC for about 30 min in a closed recipient to achieve the complete hydrolysis. After the gel formation, the samples were dried at 110 °C for 24 h and then heat-treated at 500 oC, 800 °C and at 1200 °C for 3 h. The aim of the thermal treatment was to facilitate the growth of magnetic iron oxides and of bioactivity enhancing wollastonite nanocrystalline phases. FTIR spectra were recorded in the range of 4000-400 cm-1 with a spectral resolution of 2 cm-1, in absorption mode, with a FT/IR-6200 Jasco Spectrometer by using the KBr pellet technique. The pellets were prepared by mixing 2 mg of powdered sample with 200 mg KBr. The EPR spectra were recorded on powdered samples with a Bruker Elexsys 580 EPR spectrometer, operating in X band. For EPR measurements, the samples were introduced in quartz tubes of 2 mm inner diameter to a height above the active height sampled by the resonator. The magnetic field was modulated at 100 KHz and the spectra were displayed as the first derivate of the absorption curve. All spectra were recorded at room temperature.

Results and discussion X-ray diffraction results regarding the structure of the investigated samples [15] proved that with increasing treatment temperature in 500 oC – 1200 oC range (Fig. 1) nanocrystalline magnetite, hematite and wollastonite phases are developed that recommend this system for potential biomedical applications. The crystallite sites estimated with Scherrer equation for all crystalline phases occurred after the applied heat treatments are below 40 nm. Additionally, more detailed information on structural changes were obtained by means of FTIR and EPR analyses. 3

The structural ordering upon heat treatment depends on the peculiarities of the elements occurring in the composition of the amorphous oxide compound. The properties regarding coordination number, field strength, Pauling electronegativity and single bond strength with oxygen of the cations entering in the investigated samples are summarized in Table 1. The cationic field strength is expressed by the ratio of the cation charge to the square of the ionic radius. The cations belonging to the conventional glass former oxides are characterized by high field strengths as compared to the cations entering as modifiers. In this approach, in the investigated system CaO, Dy2O3 and Fe2O3 are expected to play the role of modifier oxides in the silicate glass network built up from tetrahedral [SiO4] structural units. On the other hand, the higher the ionic field strength, the greater the stability of glass toward crystallization [19]. For nonmetallic glasses, it was also reported that the thermal stability against crystallization decreases with the electronegativity difference [20]. Considering the lowest field strength and electronegativity of calcium ions versus that of dysprosium and iron ions, it is expected to prevail the development of calcium silicate crystalline phases in the silicate vitreous network. The structural analysis accomplished in this study by FTIR spectroscopic technique is particularly focussed on the network structure. At wavenumbers between 4000 cm-1 and 2000 cm-1 only few differences are evidenced in FTIR spectra (Fig. 2a). The signal at 2358-2343 cm-1 is not related to samples; it comes from registration and can be used even as a reference. The broad band located between 3350-3600 cm-1 is assigned to O-H stretching vibration, and decreases with treatment temperature, but is still recorded also after 1200 oC. The very weak bands recorded from 500 and 800 oC treated samples between 2800 cm-1 and 3000 cm-1 correspond to the symmetric and asymmetric stretching vibrations of CH2 and CH3 groups belonging to alkoxide and solvent residues [21].

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The features recorded at lower wavenumbers evidence more differences due to increasing treatment temperature (Fig. 2b). The absorption band around 1640 cm-1 is commonly assigned to O-H bending vibration [22], but can occur also from vibrations of the SiO2 network and is often hidden by water O-H band [21]. In our spectra one remarkes that the band intensity does not decrease even after 1200 oC treatment, and is steady slightly shifted to lower wavenumbers with increasing treatment temperature (1647, 1637 and 1631 cm-1, for samples treated at 500, 800 and 1200 oC, respectively) that makes unlikely its assignment only to water molecules. The FTIR spectra (Fig. 2) reveal an absorption band at 1465 cm-1 due to C-H bending for methyl and methylene group and a characteristic very narrow band for nitrates at 1385 cm-1 [23, 24] only in the 500° C treated sample, and disappear after thermal treatment at higher temperatures. The spectra are dominated by a broad signal in the 750-1300 cm-1 spectral range, where the stretching vibrations of the Si-O-Si and Si-O-M (M = Ca, Fe) bridges occur [25, 26]. In 750-1300 cm-1 spectral range are recorded Si-O-Si bending vibrations around 800 cm-1, and at higher wavenumbers Si-O-Si asymmetric stretching vibration; Si-O-Si bending vibrations give absorption bands also in the range of 500 - 400 cm-1 [27, 28]. A comparison between the FTIR spectra of the samples treated at different temperatures and the infrared absorption bands recorded from γ-Ca2SiO4 (496, 520, 565, 819, 859, 953 cm-1) and β-CaSiO3 (452, 471, 508, 566, 642, 680, 904, 925, 964, 1019, 1056, 1087 cm-1) [29] crystalline phases sugessts, by the occurence of 520 cm-1 band, the growth of γ-Ca2SiO4 after 800 oC treatment. In the infrared spectrum of the 1200 oC treated samples only the band at 500 cm-1 could be further related to some extent to γ-Ca2SiO4 phase, but egually to β-CaSiO3, while the bands at 640, 682 and 904 cm-1 strongly prove the development of β-CaSiO3 phase. The spectra deconvolution in 750-1300 cm-1 range (Fig. 3) shows a good agreement between simulated and experimental spectra using four component bands. For the 500 oC heated 5

sample the large band is composed by the superposition of three components at 1210, 1004 and 877 cm-1. The dominant component occurs close to 1004 cm-1 and is characteristic for amorphous silica [30]. The components at 1210 cm-1 [31] and 877 cm-1 [26] are assigned to SiO-Si asymmetric stretching vibrations. The increase of treatment temperature leads to changes in the contributions of these components and to the forth component at 1097 cm-1 (Table 2). At the same time, one remarks that 1004 cm-1 component assigned to vitreous silica pronouncedly diminishes while that at 904 cm-1 pronouncedly increases. Moreover, after 800 oC treatment the large absorption band recorded at low wavenumbers, between 400-580 cm-1, contains at 445 and 520 cm-1 features (Fig. 2) that could include contributions from hematite (Fe2O3) [32]. After 1200 oC treatment well separated absorption bands occur at 1097, 1004, 904, 720, 500 and 436 cm-1 due to development of β-wollastonite (CaSiO3) and pseudowollastonite [33-35]. For the 1200 oC treated sample one observes around 595 cm-1 an absorption band apparently consisting of two components assigned to stretching vibrations of Fe-O bonds [36], typical for Fe-O vibration in magnetite (Fe3O4) [37]. The careful inspection of this band points out several contributions (Fig. 4) at different wavenumbers, denoting that the Fe-O bonds in nanocrystallized magnetite are affected by different vicinities. Actually, the data presented in Table 2 show the consistent diminution of the amorphous phase related to 1004 cm-1 component along with the increase of the components assigned to wollastonite. The ratio of amorphous components after 500 and 800 oC treatments is 66.8/20.7=3.2 and after 1200 oC treatment this ratio becomes 66.8/14.3=4.7. The components at 550 and 558 cm-1 are assigned to hematite [32], the prevalent component at 577 cm-1 to magnetite, [38], 595 cm-1 is typical for Fe-O vibration in magnetite (Fe3O4) [36]. An absorption band at 611 cm-1 was assigned to Fe-O bending vibrations of the 6

magnetite nanoparticles [39]. The component at 640 cm-1 occurs from β-CaSiO3 crystalline phase, as presented above. Dysprosium presence in the system should also be taken into account. Typical Dy-O vibrations occur at 550 or 555 cm-1 with a broad shoulder at 580 cm-1, and vibrations of Si-O-Dy bonds could occur at 1017 cm-1 [40]. The broad shoulder at 580 cm-1 could contribute beside magnetite to the the prevalent component at 577 cm-1. It was reported that large radius ions occuring in the vecinity of magnetically ordered iron oxide phases lead to a shift the Fe-O infrared absorption bands to higher wavenumbers [41]. For our system, beside the presence of calcium and dysprosium large radius ions, to this effect could also contribute the difference in the bond strengths of Dy-O and Fe-O, namely Dy-O strength is much higher than that of Fe-O (Table 1). The main lines from the EPR spectra of 500 and 800 oC treated samples are characterized by g = 4.24 and g = 2.02 resonance lines (Fig. 5). The first line is assigned to isolated Fe3+ ions disposed in sites of low symmetry characterized by high crystal fields, and that at g = 2.02 is assigned to Fe3+ ions disposed in sites of octahedral symmetry, with low crystal fields, or associated in clustes [42, 43]. A third EPR signal at g = 7.66 is also recorded from the 500 oC treated sample and is assigned to Fe3+ ions disposed in sites of axyal symmetry [44]. The EPR spectrum recorded from the 1200 oC treated sample consists of a single large resonance line at g = 2.54 given by Fe3+ ions disposed in randomly oriented iron oxides magnetite-maghemite nanoparticles [45]. Fe2+ ions species are also present along with Fe3+ ions but they are EPR silent at room temperature. The ferromagnetic magnetite (Fe3O4) and ferrimagnetic maghemite (γ-Fe2O3) are by far the most commonly employed magnetic materials for biomedical applications and both are of interest also for hyperthemia in cancer therapy [46].

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A very recently published paper [47] reports on iron oxide containing yttrium aluminosilicate microparticles tested intratumorally in an animal model of liver cancer for localized therapeutic hyperthermia. Based on yttrium activation, the hyperthermia can be combined with brachytherapy for a more efficient thermoradiotherapy.

Conclusions FTIR and EPR analyses on sol-gel derived 50SiO2·30CaO·10Fe2O3·10Dy2O3 samples subjected to heat treatments at 500, 800 and 1200 oC bring additional information to previous XRD results on structural changes induced by increasing treatment temperature. Nucleation and growth of hematite, magnetite and wollastonite phases diminishes the amorphous phase to one third by increasing the treatment temperature from 500 to 800 oC and almost to one fifth after 1200 oC treatment, according to infrared absorption bands deconvolution. Fe-O bond vibrations in magnetite recorded at several wavelengths around 595 cm-1 point out influences arising from further connections in different surroundings to which dysprosium ions could also contribute. The treatment temperature dependence of Fe3+ EPR spectra denotes that after 1200 oC treatment the iron ions are disposed in randomly oriented magnetite-maghemite nanoparticles.

Acknowledgments: This research was accomplished in framework of PN-II PCCA 78/2012 project granted by UEFISCDI—Romania.

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Figure and table captions

Fig. 1 X-ray patterns after thermal treatment at different temperatures. Fig. 2 FTIR spectra of 50SiO2·30CaO·10Fe2O3·10Dy2O3 system treated at different temperatures, shown in whole registration range (a), and expanded in 400-2000 cm-1 region (b). Fig. 3 Deconvolution of large absorption band recorded in 750-1300 cm-1 spectral range. Fig. 4 Infrared absorption band centered around 595 cm-1, for 1200 oC treated sample. Fig. 5 EPR spectra of 50SiO2·30CaO·10Fe2O3·10Dy2O3 system after treated at different temperatures.

Table 1

Coordination number, ionic radius, field strength, Pauling electronegativity and single bond strength for the cations from 50SiO2·30CaO·10Fe2O3·10Dy2O3 system.

Table 2

Treatment temperature dependence of different components, in 750-1300 cm-1 spectral range.

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m w

h - hematite m - magnetite w - wollastonite

h

Intensity (a.u.)

h m h m

h w

h m

mw w h

o

1200 C

o

800 C o

500 C

10

20

30

40

50

60

70

80

90

2θ (degree)

Fig. 1

14

1640

Absorbance (a.u.)

Absorbance (a.u.)

(b)

o

1200 C o

800 C o

500 C

1210 1097 1004 904 800 720 595 520 500 445 436

(a)

o

1200 C o

800 C o

500 C

4000

3000

2000

1000 -1 Wavenumber (cm )

0

2000

1500

1000

500

-1 Wavenumber (cm )

Fig. 2

15

904

1004

1097

1210

Absorbance (a.u.)

o

1200 C

o

800 C

o

500 C

1400

1200

1000

800

600

-1

Wavenumber (cm )

Fig. 3

16

550

576 580

558

595

615

600

640

Absorbance (a.u.)

660

640

620

560 -1 Wavenumber (cm )

540

Fig. 4

17

First derivative of absorption

g= 2.54

o 1200 C

o 800 C

g= 2.02

g= 4.24

g=7.66 o 500 C

0

1000 2000 3000 4000 5000 6000

Magnetic field (G)

Fig. 5

18

Table 1

Coordination number, ionic radius, field strength, Pauling electronegativity and single bond strength for the cations from 50SiO2·30CaO·10Fe2O3·10Dy2O3 system.

Cation

Coordination number

Shannon ionic radius (Å) [16, 17]

Cation field strength (Å−2)

Electronegativity (Pauling units) [18]

M–O strength (kJ/mol) [18]

Si4+

4 6

0.40 0.54

25 13.72

1.90

799.6 ± 13.4

Ca2+

6

1.14

1.54

1.00

402.1 ±16.7

8

1.26

1.26

6

1.05

2.72

1.22

607 ± 17

8

1.17

2.19

4

0.63

7.56

6

0.78

4.93

1.83

390.4 ± 17.2

8

0.92

3.54

Dy3+

Fe

3+

19

Table 2

Treatment temperature dependence of different components, in 750-1300 cm-1 spectral range.

Components wavenumber → Treatment temperature ↓ 500 oC

1012 cm-1

15.3

-

66.8

17.9 (877 cm-1)

800 oC

30.4

6.4

20.7

42.5

1200 oC

10.9

29.5

14.3

45.3

1097 cm-1

1004 cm-1

904 cm-1

Components contribution (%)

Highlights

-

Sol-gel derived SiO2-CaO-Fe2O3-Dy2O3 amorphous matrix partially crystallised CaSiO3, Fe2O3 and Fe3O4 phases are developed after heat treatment at 1200 oC The amorphous phase sinks to 1/3 after 800oC and to 1/5 after 1200oC treatments Iron ions are disposed in randomly oriented magnetite-maghemite nanoparticles The developed crystalline phases are desirable in glass-ceramic biomaterials

20