Hydroxyapatite-phosphonoformic acid hybrid compounds prepared by hydrothermal method

Applied Surface Science 290 (2014) 327–331

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Hydroxyapatite-phosphonoformic acid hybrid compounds prepared by hydrothermal method Thouraya Turki a , Masseoud Othmani b , Jean-Louis Bantignies c , Khaled Bouzouita a,∗ a b c

Institut Préparatoire aux Etudes d’Ingénieur de Monastir, Rue Ibn El Jazzar, 5019 Monastir, Tunisia Laboratoire de Physico-Chimie des Matériaux, Faculté des Sciences de Monastir, 5019 Monastir, Tunisia Laboratoire Charles Coulomb, UMR 5221 CNRS/UM2, Université de Montpellier 2, Place E. Bataillon, 34095 Montpellier, France

a r t i c l e

i n f o

Article history: Received 15 July 2013 Received in revised form 14 November 2013 Accepted 17 November 2013 Available online 23 November 2013 Keywords: Hydroxyapatite Phosphonoformic acid Functionnalization Hydrothermal method

a b s t r a c t Hydroxyapatites were prepared in the presence of different amounts of phosphonoformic acid (PFA) via the hydrothermal method. The obtained powders were characterized through chemical analysis, XRD, IR, 31 P MAS-NMR, TEM, and TG-TDA. The XRD showed that the PFA did not affect the apatite composition. Indeed, only a reduction of the crystallite size was noted. After grafting of PFA, the IR spectroscopy revealed the appearance of new bands belonging to HPO4 2− and carboxylate groups of the apatite and organic moiety, respectively. Moreover, the 31 P MAS-NMR spectra exhibited a peak with a low intensity assigned to the terminal phosphonate group of the organic moiety in addition to that of the apatite. Based on these results, a reaction mechanism involving the surface hydroxyl groups ( Ca OH) of the apatite and the carboxyl group of the acid was proposed. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Apatites are a large group of minerals whose main representative is hydroxyapatite (Hap), crystallizing mainly in the hexagonal structure (space group P63 /m). Thanks to its stability and flexibility, this structure can accommodate a great variety of substitutions–both cationic and anionic–leading to a wide variety of compounds which are used in various application fields. Hap is a major bioceramic used in orthopedic, maxillofacial, and dentistry surgery [1–4]. It is also utilized as a coating on metallic orthopedic and dental prostheses [5–7]. However, in comparison with its stoichiometric counterpart, the biological Hap differs significantly. Besides the fact that it is non stoichiometric and of low crystallinity, it contains different amounts of anions, such as F− , Cl− , CO3 2− , SO4 2− , and SiO4 4− , and cations such as Na+ , K+ , Sr2+ , Zn2+ , and Mg2+ . In addition to improving biocompatibility and bioactivity, these species affect the dissolution, thermal stability, morphological properties, as well as the surface reactivity of the synthetic HA [8–11]. These characteristics, particularly the surface ones, play an important role in the interactions between the implant and its biological environment. Therefore, to give a

∗ Corresponding author at: Institut Préparatoire aux Etudes d’Ingénieur de Monastir, Rue Ibn El Jazzar, 5019 Monastir, Tunisia. Tel.: +216 20 496 468; fax: +216 73 500 512. E-mail address: [email protected] (K. Bouzouita). 0169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.apsusc.2013.11.076

greater efficiency for Hap as a biomaterial, many researchers have attempted to modify its surface. Indeed, the surface modification is of great importance not only for the biomedical applications involving the mechanical, physical, and chemical properties but also for the tissue engineering. Apart from the biomedical applications, the modified hydroxyapatites have also been extensively investigated as a support for catalysts [12,13], adsorbent of rare earths and heavy metals for water purification [14], or in chromatography to purify and separate proteins and enzymes [15]. Two processes are usually implemented to modify the Hap surface. In the first one, there is adsorption of organic molecules, polymers and proteins on the Hap surface [16–19], mainly through the Van der Waals and electrostatic interactions, and the hydrogen bonding [20–22]. In the second process, the preceding species are covalently bonding to the hydroxyl groups available on the Hap surface. Two different functional groups, Ca OH and P OH, act as active sites [23,24], leading to organic–inorganic hybrids. These materials have the advantage of combining the properties of the mineral phase and those of the organic graft [25,26]. The modified surface materials have better mechanical properties, a good biocompatibility, and a bioactivity as well as a high osteoconductivity and osteoinductivity [27,28]. These materials are also used as drug delivery systems [29–31]. The aim of this work is to synthesize Hap via the hydrothermal method in the presence of different amounts of phosphonoformic acid (PFA). This acid is used in medicine for the prevention of

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calcification of vascular smooth muscle cells [32] and in antiviral drugs [33]. We firstly discussed the structural and morphological properties of the obtained compounds by means of X-ray diffraction and transmission electron microscopy. A mechanism for the surface modification is then discussed based on the results of the solid-state NMR and infrared spectroscopy. 2. Experimental

Table 1 Chemical analysis (±0.02) of Hap synthesized with and without PFA. Sample

wt.% Ca

wt.% P

Ca/P

wt.% C

Hap Hap-PFA(10) Hap-PFA(20)

37.41 36.92 36.89

17.45 17.85 18.06

1.65 1.60 1.58

– 0.13 0.22

3. Results and discussion

2.1. Powder synthesis 3.1. Chemical analysis

2.2. Powder characterization The calcium and phosphorus contents were obtained by ICPOES spectrometry (Horiba Jobin Yvon’s Activa Model), while that of carbon was determined according to the Anne method [34]. X-ray diffraction (XRD) analysis was performed by means of a Philips PW 1710 powder diffractometer equipped with a diffracted beam graphite monochromator and operating with the Cu-K␣ radi´˚ The samples were scanned in the 2 range ation ( = 1.5406 A). from 20 to 60◦ with a step size of 0.02◦ and a counting time of 1 s per step. The crystalline phases were identified by comparing the experimental XRD patterns to the standards compiled by the Joint Committee on Powder Diffraction and Standards (JCPDS cards). Mid-infrared measurements (400–4000 cm−1 ) (MFTIR) were carried out on a Brucker IFS 66 V spectrometer equipped with an N2-cooled MCT detector, a Globar source and a KBr beam splitter. The spectral resolution was 2 cm−1 and 64 scans were co-added for each spectrum. Measurements were performed using an attenuated total reflectance (ATR) device equipped with a diamond crystal allowing single reflection. Solid state 31 P MAS NMR spectra were recorded on a Brucker spectrometer Avance 300 at a resonance frequency of 121.5 MHz. The spinning rate of the sample at the magic angle was 8 kHz. The chemical shift was referenced to an external standard of an aqueous solution of 85% H3 PO4 . A thermal analysis was conducted in air from room temperature to 1000 ◦ C with a heating rate of 10◦ min−1 , using a SETARAM SETSYS 1750 equipment. The sample weights were in the 20–25 mg range. Transmission electron microscopy (TEM) investigations were carried out on a Zeiss EM10 microscope. The TEM specimen was prepared by dispersing a small amount of powder in ethanol. The obtained suspension is, then, submitted to ultrasonication, and a drop was transferred onto a holey carbon foil supported on a conventional copper microgrid.

The elemental analysis results of the as-prepared materials are listed in Table 1. The Ca/P molar ratio value of the pure Hap was close to the theoretical value, 1.67. However, for the grafted samples, the values were much lower. As the ICP method allows the determination of the total phosphorus content, the difference between the content of the grafted and nongrafted samples corresponds to the organic phosphorus. Therefore, the decrease of the Ca/P values with the increase of the used PFA content serves as an evidence for the surface modification. This evidence is also supported by the carbon analysis. In the absence of mineral carbonate in the apatite structure, as shown below, the determined carbon amount can only result from PFA fixed on the apatite’s particle surface (Table 1). As seen from this table, the carbon content increased with the increase of the PFA concentration in the starting solutions. 3.2. Thermal analysis The TG curves of the Hap and Hap-PFA(n) samples are displayed in Fig. 1. The three curves exhibit below 120 ◦ C, a first weight loss due to the removal of the water adsorbed on the solid. For the grafted samples, two weight losses were observed in the 100–670 ◦ C and 670–800 ◦ C ranges, respectively. They correspond to the combustion of the organic moiety and the decomposition of the phosphonate group of PFA, respectively [35]. The increase of the loss magnitude by increasing the PFA concentration in the starting solutions is in accordance with the carbon and phosphorus amounts determined by the chemical analysis (Table 1). The DTA curves of the samples are shown in Fig. 2. Each of them displayed an endothermic peak at 100 ◦ C, related to the departure of the adsorbed water. For the grafted samples, two exothermic effects, not observed for Hap, were occurred at 385 and 720 ◦ C, respectively. They correspond to the second and third weight losses observed on the TG profiles. 3.3. X-ray diffraction The XRD patterns of Hap synthesized with and without PFA are shown in Fig. 3. All the patterns exhibit only the reflections of 0

a -3

TG(mg)

The analytical grade raw materials Ca(NO3 )2 ·4H2 O and (NH4 )2 HPO4 employed for the synthesis of Hap were obtained from FLUKA while the phosphonoformic acid used for the preparation of the hybrids was purchased from SIGMA. Under nitrogen stream, a 0.75 M calcium nitrate solution (11 mL) was added to a 0.25 M diammonium hydrogen phosphate solution (20 mL), stirred vigorously. The pH of the mixture was adjusted to 9 by adding a concentrated ammonia solution. Then, the mixture was transferred to a Teflon vessel (50 mL) which was tightly-sealed and put in an oven whose temperature was raised to 120 ◦ C. The dwelling time span was of 15 h. After the furnace cooling to room temperature, the obtained products were filtered, washed with double distilled water, and dried overnight at 100 ◦ C. The hybrid compounds were prepared according to the same experimental protocol, except that an appropriate amount of PFA was added to the phosphate solution. In the following sections, the samples will be named as Hap-PFA(n), where PFA corresponds to the phosphonoformic acid and n is the value of the (PFA)/(Hap) molar ratio.

-6

b

-9

c -12

-15

0

200

400

T/ °C

600

800

Fig. 1. TG plots of: (a) Hap, (b) Hap-PFA(10), and (c) Hap-PFA(20).

1000

T. Turki et al. / Applied Surface Science 290 (2014) 327–331 8

c b

6

DTA ( v)

329

Hap-PFA(20)

4

Hap-PFA(10)

2

Hap

a

0 PFA 1700

-2 -4

0

200

400

T/ °C

600

800

1600

1500

1400

1300

Hap-PFA(20) Hap-PFA(10)

1000

L 3

Fig. 2. DTA plots of: (a) Hap, (b) Hap-PFA(10), and (c) Hap-PFA(20). as (COO

(C=O) 1600

-

) s (COO

1400

-

Hap

1

4 2

PFA

) 1200

1000

Wavenumber ( cm

800 -1

600

400

)

Fig. 4. IR spectra of PFA and Hap powders before and after modification with PFA. Inset: enlargement of 1700–1200 wave number domain.

Hap-PFA(20)

and parallel directions to the c-axis, respectively. The values of Dhkl were calculated using the Debye–Scherrer equation: D=

Hap-PFA(10)

K ˇ1/2 cos 

where,  is the wave length,  is the diffraction angle, K is a fixed constant equal to 0.9 for the apatite crystallites and ˇ1/2 is the line width at half maximum of a given reflection. The obtained values are reported in Table 2. As seen from this table, the crystallite size decreased with increasing acid amount. This confirms that in the presence of PFA, the crystallites were affected in the (a, b) plane and along the c direction, in agreement with previous works [37,38].

(002)

Hap

(310)

3.4. IR spectroscopy

20

30

40

50

60

Fig. 3. XRD patterns of Hap synthesized in the absence and presence of PFA.

an apatitic phase, indexed in the hexagonal system based on the hydroxyapatite (JCPDS card #01-074-0566). No secondary phases were detected in any of the patterns. However, with respect to those of pure Hap, the diffraction peaks of the grafted samples became broader with the increase of the PFA amount, indicating a decrease in the powder crystallinity. This is in agreement with several studies showing the inhibiting role of organic acids and phosphonates on the Hap crystallization [24,36]. Furthermore, the decrease in powder crystallinity can be corroborated by the evolution of the crystallite size as a function of the functionalization rate. Therefore, the line broadening of the (3 1 0) and (0 0 2) reflections was used to evaluate the crystallite size, Dhkl , in the perpendicular

Internal infrared vibrational modes are known to be very sensitive to chemical environment due to specific wave number dependence of the surface groups engaged in specific interactions with foreign species [39,40]. The middle IR spectra (1770–400 cm−1 ) of pure Hap and Hap-PFA(n) are shown in Fig. 4. The IR spectrum of PFA is also given here for comparison. The spectrum of Hap exhibited the absorption bands of PO3− 4 groups in an apatitic environment: s (961 cm−1 ), ıs (474 cm−1 ), as (1028–1086 cm−1 ) and ıas (564–600 cm−1 ). As for the OH libration band, indicative of hydroxyapatite, it is observed at 634 cm−1 . After the surface modification, new absorption bands appeared on the spectra. The 1200–1700 cm−1 range gives rise to clear PFA fingerprints without superimposition with Hap vibrational features (inset Fig. 4), indicating that all these bands belong to PFA. The bands at 1444 and 1547 cm−1 were assigned to the vibrational modes (as ) and (s ), respectively of the carboxylate group (COO− ). Compared to the PFA spectrum, these bands undergo a very significant shift, (s (COO− ) = −22 cm−1 ; (as (COO− ) = + 10 cm−1 ).

Table 2 Crystallite size values (Dhkl ) of the Hap–PFA(n) powders. Samples

ˇ1/2 (0 0 2)(◦)

D(0 0 2)

ˇ1/2 (3 1 0)(◦)

D(3 1 0)(nm)

D(0 0 2)/D(3 1 0)

Hap Hap-PFA(10) Hap-PFA(20)

0.164 0.343 0.530

49.7 23.8 15.4

0.421 0.956 1.850

20.1 8.8 4.6

2.47 2.70 3.34

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PO4

PO3 Hap-PFA(20)

Hap-PFA(10)

Hap 40

30

20

10

0

-10

-20

-30

-40

ppm Fig. 5. 31P NMR MAS spectra of Hap synthesized in the absence and presence of PFA.

This shift is due to specific interactions between Hap and the carboxylate group of PFA, suggesting that the functionalization has involved the latter group. One can note that this behavior is in agreement with calcium chelation [41]. As expected, increasing the molar ratio from 10 to 20 led to a significant enhancement of the intensity of the PFA IR features. When the molar ratio decreased, the downshift of COO− main IR contributions increased from 1557 to 1567(10) cm−1 , indicating a more efficient interaction in the latter case. Besides the bands belonging to PFA, another band is observed at 872 cm−1 , which is absent in the spectrum of the pure Hap. Its assignment was not straightforward. It is attributed to P–O–H 2− vibration in the HPO2− 4 group [23] as well as to CO3 (2 bending) of carbonated apatite. In the apatite’s IR spectra, the carbonate ions are characterized by absorption bands occurring in two regions −1 (out-of-plan bending vibration) and [42]: v2 CO2− 3 , 840–900 cm −1 (asymmetric stretching vibration). As in v2 CO2− 3 , 1350–1800 cm

all the spectra, no band corresponding to the mineral carbonate was observed in the latter region, therefore, the band at 872 cm−1 is due to the HPO2− 4 groups. The presence of these groups after grafting suggests that the acid was only linked to the surface Ca OH sites. Otherwise, the presence of this band indicates that the apatite was calcium-deficient.

Fig. 6. TEM images of: (a) Hap and (b) Hap-PFA(10); (scale-bars = 50 nm).

expected from the literature data [43]. After grafting, a broadening of this signal due to the small size of the powder crystallites was observed, in agreement with the XRD results. Furthermore, the spectra of the modified samples displayed a new signal with a low intensity, which was assigned to the phosphonate group of the organic moiety [44]. This confirms once more that the acid was bound to the HA surface Ca OH sites through its COO− group. 3.6. TEM analysis TEM micrographs of the samples are given in Fig. 6. As seen in Fig. 6a, the Hap crystallites were rod-shaped. After the grafting of the acid, the crystallite size was significantly reduced (Fig. 6b). Furthermore, the micrographs show that all the powders were agglomerated, and the agglomeration was more important for the functionalized samples.

3.5. Solid state NMR spectroscopy 4. Grafting mechanism The 31 P MAS NMR spectra of Hap and Hap-PFA(n) samples are presented in Fig. 5. As can be seen from this figure, the pure Hap exhibited only one peak positioned at around 2.8 ppm, as is

For the grafted Hap, XRD analysis showed that the apatite structure was not affected while the IR spectroscopy revealed the

Scheme 1. Mechanism of functionalization of phosphonoformic acid on hydroxyapatite surface.

T. Turki et al. / Applied Surface Science 290 (2014) 327–331

presence of new absorption bands: a band at 872 cm−1 and three bands in the 1200–1700 cm−1 region, attributed to the HPO2− 4 group of the apatite and the carboxylate group of PFA, respectively. Compared to pure PFA, the latter bands were shifted from their original positions, indicating that the carboxylate group was involved in the grafting of the acid onto the Hap surface. The results of the 31 P NMR MAS spectroscopy corroborate this assertion. Otherwise, the detection of the HPO2− 4 group after the grafting serves as an evidence of its non-participation in this process. Based on these findings, it is possible to propose the below mechanism for the functionalization of the phosphonoformic acid on the hydroxyapatite surface (Scheme 1). 5. Conclusion Hdroxyapatites were synthesized via the hydrothermal method in the presence of different amounts of phosphonoformic acid. XRD, IR, NMR, TG-DTA, TEM, and chemical and thermal analyses were used to investigate the structure, morphology, and composition of the obtained products. The overall results suggest that the grafting of the acid was made following a reaction between the carboxyl group and the hydroxyl groups of the metallic sites ( Ca OH) of the apatite surface. References [1] C. Rey, Calcium phosphates for medical applications, in: Z. Amjad (Ed.), Calcium Phosphates in Biological and Industrial Systems, Kluwer Academic Publishers, Boston, 1998, pp. 217–251. [2] S.K. Nandi, S. Roy, P. Mukherjee, B. Kundu, D.K. De, D. Basu, Orthopaedic applications of bone graft & graft substitutes: a review, Indian J. Med. Res. 132 (2010) 15–30. [3] P. Tschoppe, D.L. Zandima, P. Martus, A.M. Kielbassa, Enamel and dentine remineralization by nano-hydroxyapatite toothpastes, J. Dent. 39 (2011) 430–437. [4] V.S. Gshalaev, A.C. Demirchan, Hydroxyapatite, Synthetis, Properties, and Applications, Nova Science Publishers, Inc., USA, 2013. [5] M. Hashizume, H. Kobayashi, M. Ohashi, Preparation of free-standing films of natural polysaccharides using hot press technique and their surface functionalization with biomimetic apatite, Coll. Surf. B: Biointer. 88 (2011) 534–538. [6] J.D. Voigt, M. Mosier, Hydroxyapatite (HA) coating appears to be of benefit for implant durability of tibial components in primary total knee arthroplasty, Acta Orthop. 82 (4) (2011) 448–459. [7] G.E. Stana, A.C. Popaa, A.C. Galcaa, G. Aldicaa, J.M.F. Ferreira, Strong bonding between sputtered bioglass–ceramic films and Ti-substrate implants induced by atomic inter-diffusion post-deposition heat-treatments, Appl. Surf. Sci. 280 (2013) 530–538. [8] S. Nsar, A. Hassine, K. Bouzouita, Sintering and mechanical properties of magnesium and fluorine co-substituted hydroxyapatites, J. Biomater. Nanobiotechnol. 4 (2013) 1–11. [9] H.Y. Yang, S.F. Yang, X.P. Chi, J.R.G. Evans, I. Thompson, R.J. Cook, P. Robinson, Sintering behaviour of calcium phosphate filaments for use as hard tissue scaffolds, J. Eur. Ceram. Soc. 28 (2008) 159–167. [10] S. Cazalbou, C. Combes, D. Eichert, C. Rey, Adaptative physico-chemistry of biorelated calcium phosphates, J. Mater. Chem. 14 (2004) 2148–2153. [11] P. Ducheyne, Q. Qiu, Bioactive ceramics: the effect of surface reactivity on bone formation and bone cell function, Biomaterials 20 (1999) 2287–2303. [12] M. Zahouily, W. Bahlaouan, B. Bahlaouan, A. Rayadh, S. Sebtib, Catalysis by hydroxyapatite alone and modified by sodium nitrate: a simple and efficient procedure for the construction of carbon-nitrogen bonds in heterogeneous catalysis, Arkivoc (xiii) (2005) 150–161. [13] Z. Opre, D. Ferri, F. Krumeich, T. Mallat, A. Baiker, Aerobic oxidation of alcohols by organically modified ruthenium hydroxyapatite, J. Catal. 241 (2) (2006) 287–295. [14] S. Saoiabi, K. Achelhi, S. Masse, A. Saoiabi, A. Laghzizil, T. Coradin, Organoapatites for lead removal from aqueous solutions: a comparison between carboxylic acid and aminophosphonate surface modification, Coll. Surf. A: Physicochem. Eng. Asp. 419 (2013) 180–185. [15] B. Wlodarczyk, J. Pietrasik, M. Zaborski, Modification of hydroxyapatite with polymer brushes, in: Materials Science Forum, vol. 714, 2012, pp. 291–295. [16] H. Tanaka, Surface structure and properties of synthetic and modified calcium hydroxyapatite, in: A. Hubbard (Ed.), Encyclopedia of Colloid and Surface Science, Marcel Dekker, New York, 2002, pp. 5096–5107.

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