ISSN 0012-5008, Doklady Chemistry, 2006, Vol. 411, Part 1, pp. 219–222. © Pleiades Publishing, Inc., 2006.Original Russian Text © A.S. Fomin, S.M. Barinov, V.M. Ievlev, I.V. Fadeeva, V.S. Komlev, E.K. Belonogov, T.L. Turaeva, 2006, published in Doklady Akademii Nauk, 2006,Vol. 411, No. 3, pp. 348–351.

219

Calcium hydroxyapatite (HA) Ca

10

(PO

4

)(OH)

2

is abasic component of the mineral part of bone tissue,enamel, and dentine. Synthetic HA is biocompatiblewith the human body, is able to combine with bone tis-sue, and is highly resistant to dissolution by extracellu-lar fluids. As a result, synthetic HA is widely used inmedicine as a material for bone implants, drug carriers,a component of bone cements, and a component thatensures biocompatibility of polymer-based compositematerials [1–3]. Much recent effort has been devoted todeveloping the methods of synthesis of nanosized HA(NHA). As distinct from microcrystalline HA, nano-sized HA powders have a high specific surface andtherefore exhibit enhanced activity toward chemicaland biological interactions in the human body. NHAbetter corresponds to the inorganic part of bone tissue,which consists of HA crystals up to 20 nm in diameterand about 50 nm in length [4]. NHA is used as an initialhighly active component of bone cement: the interac-tion of NHA with orthophosphoric acid leads to thecrystallization of brushite and in situ hardening of thecement paste [5]. NHA is introduced as a reinforcingphase into polymer-based materials, thus ensuring theirbiocompatibility and enhanced mechanical properties[6–8]. NHA is expected to be promising as drug carri-ers, especially for intravenous administration and celltherapy [9]. The nanosized HA has an enhanced solu-bility, which can be used for fabrication of resorbablescaffolds in cell technologies for repair of damagedbone tissues.

Different methods of synthesis of NHA are known[10–12], and precipitation in aqueous solutions ofbiopolymers seems to be among the most promisingmethods [7, 13]. In this work, we synthesized NHA in

aqueous gelatin solutions of variable concentration andstudied the synthesis products.

EXPERIMENTAL

The synthesis of NHA was carried out in a gelatinsolution at 10

°

C by the following reaction:

(1)

To 200 mL of a 1 M

Ca(NO

3

)

2

solution, 200 mL ofammonia and 100 mL of gelatin solution with a concen-tration from 0 to 5.5 g/L were added. Then, 200 mL of

(NH

4

)

2

HPO

4

was added dropwise over 10 min. Theresulting mixture was stirred for 2 h and allowed to agefor a day. The pH of the medium was maintained above10.5. After that, the precipitate was separated on aBüchner funnel and dried at

120°ë

. The powder wasscreened through a 60-

µ

m sieve, disaggregated by trit-uration in ethanol, and calcined at

900°ë

for 1 h.The powders thus obtained were studied by X-ray

powder diffraction (Shimadzu XRD-6000), the BETmethod of measuring specific surface areas (TristarMicromeretics), transmission electron microscopy(TEM, EMB-100BR and PREM-200), electron diffrac-tion (EG-100M), and FT IR spectroscopy (Avatar, sam-ples were powders in KBr). For TEM studies, suspen-sions of NHA powders in distilled water were preparedby ultrasonication (UZDN-1). The suspensions wereapplied to a thin (30 nm) amorphous carbon film anddried in air.

RESULTS AND DISCUSSION

According to X-ray powder diffraction, the majorphase in the synthesized products is HA and theremaining part is a phase amorphous to X-ray diffrac-tion. Figure 1 shows a plot of the full width at half max-imum (FWHM) and the intensity of the (002) reflectionof the X-ray diffraction pattern of HA versus the gelatinconcentration in a solution in the course of synthesis.

10Ca NO3( )2 6 NH4( )2HPO4 8NH4OH+ +

= Ca10 PO4( )6 OH( )2 20NH4NO3 6H2+ .+

Nanosized Hydroxyapatite Synthesized by Precipitation in a Gelatin Solution

A. S. Fomin, S. M. Barinov,

Corresponding Member of the RAS

V. M. Ievlev, I. V. Fadeeva, V. S. Komlev, E. K. Belonogov, and T. L. Turaeva

Received May 3, 2006

DOI:

10.1134/S0012500806110073

Institute of Physicochemical Problems of Ceramic Materials, Russian Academy of Sciences, Ozernaya ul. 48, Moscow, 119361 RussiaVoronezh State Technical University, Moskovskii pr. 14, Voronezh, 394026 Russia

CHEMICAL TECHNOLOGY

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FOMIN et al.

The peak is broadened as the gelatin concentrationincreases to 2.8 g/L, which points to a low crystallinityand/or a small size of granules. As mentioned in [7], anincrease in the gelatin concentration in a solutiondecreases the driving force of HA crystallization, i.e., isfavorable for amorphization of the precipitate. Thegranule size, calculated by the Scherrer formula

(2)

(

K

= 0.9 is a constant,

λ

is the wavelength and

β

is theFWHM [14]), is 32 nm at a gelatin concentration of2.8 g/L. This value is in agreement with the TEM dataand the estimate of the specific surface of the powders(Fig. 2). In particular, the powder synthesized at this

LKλ

β θcos---------------=

gelatin concentration has a maximum specific surfacearea of

47.4

±

0.3

m

2

/g. The range of specific surfaceareas corresponds to a granule size of approximately20–30 nm (spherical model). The decrease in specificsurface at high gelatin concentrations is presumablycaused by the formation of a large number of crystalli-zation sites, which facilitates agglomeration of gran-ules. The qualitative estimates of the particle size areconsistent with direct electron microscopic data.Figure 3 shows TEM images of HA particles and theelectron diffraction pattern. The table presents the inter-planar spacings (

d

hkl

) calculated from electron diffrac-tion patterns and the JCPDS card no. 84-1998 data onthe HA structure. These data demonstrate that the crys-talline phase in the powders is HA. The average size is

0.23

0

3300

3350

2950

3000

3050

3100

3150

3200

3250

54321

Inte

nsity

, arb

. uni

ts

0.26

0.27

0.25

0.24

0.22

FWH

M, d

eg

Gelatin concentration, g/L

0 5 64321

20

30

40

50

Gelatin concentration, g/L

Specific surface, m

2

/g

70 nm 4000 3000 10002000

Wavenumber, cm

–1

Intensity, arb. units

357035402077 2002

163414581413

1089963 874

637605

567

476

2

1

Fig. 1.

FWHM and intensity of the (002) line of HA as afunction of the gelatin concentration in a solution.

Fig. 2.

BET specific surface area of powders as a functionof the gelatin concentration in a solution.

Fig. 3.

TEM image of HA nanoparticles and (inset) the elec-tron diffraction pattern of the powder synthesized in a gela-tin solution with a concentration of 5.5 g/L.

Fig. 4.

IR spectra of the samples synthesized (

1

) withoutgelatin and (

2

) in the gelatin solution with a concentrationof 2.8 g/L.

DOKLADY CHEMISTRY

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NANOSIZED HYDROXYAPATITE 221

about 32, 10, 18, and 24 nm for the powders synthe-sized at gelatin concentrations of 1.5, 2.8, 4.5, and5.5 g/L, respectively. The sizes of particles obtainedthrough synthesis in an aqueous solution without gela-tin are close to 100 nm.

Figure 4 shows the IR spectra of powders synthe-sized without gelatin (spectrum

1

) and in a solutionwith a gelatin concentration of 2.8 g/L (spectrum

2

).Both spectra show the

éç

–

absorption band at3570 cm

–1

and the

ν

1

,

ν

4

, and

ν

3

bands of phosphategroups at 960–963, 520–660, and 1030–1090 cm

–1

,respectively. The spectrum of the sample synthesized ina gelatin solution shows absorption bands at 1410–1460 cm

–1

and a band at 874 cm

–1

, which can beassigned to, respectively, the

ν

3

and

ν

2

modes of car-bonate groups. The gelatin chemically bound to HAdecomposes at

514°ë

[7], which can lead to saturationof HA with carbonate groups to produce carbonate-substituted HA.

In the course of precipitation of HA in a gelatinsolution, the concentration of the latter is a decisive fac-tor in nucleation of HA crystals [7]. When HA is syn-thesized in an aqueous solution without gelatin, a

homogeneous reaction of

Ca

2+

with

ê

ions takesplace. In a gelatin solution, the calcium ions in the solu-tion first react with the carboxyl groups of gelatin, andnucleation of crystals of a new phase therein is inducedby gelatin active sites by the heterogeneous reactionmechanism. Then, phosphate ions are attached to thenascent calcium complexes and critical nuclei of thenew phase are formed, which grow to give HAnanocrystals. It is likely that the crystallization site den-sity, which depends on the gelatin concentration, is ofcrucial importance for the size of particles of the pre-cipitated phase. At a high density of crystallizationsites, growing particles interact with one another,agglomerate, and become larger.

Our findings show that nanosized HA powders witha particle size of up to 10 nm can be obtained in aque-ous gelatin solutions at a definite concentration of thelatter.

ACKNOWLEDGMENTS

This work was supported by the Russian Foundationfor Basic Research (project no. 06–03–32192) and theCouncil for Grants of the President of the Russian Fed-eration for Support of Young Russian Scientists.

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O43–

Interplanar spacings (

d

hkl

) calculated from electron diffrac-tion patterns and the JCPDS card no. 84-1998 data on the HAstructure

JCPDS data HA powders un-der consideration

d

hkl

, nm

hkl I

, arb. units

d

hkl

, nm

0.8155 100 171

0.52561 101 47

0.47083 110 24

0.40775 200 63

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0.35070 201 26

0.34372 002 360 0.340

±

0.004

0.31674 102 91

0.30823 120 174 0.306

±

0.004

0.28125 121 999 0.281

±

0.004

0.27761 112 532

0.27183 300 614 0.267

±

0.004

0.26280 202 209 0.258

±

0.004

0.25278 301 42

0.23541 220 2

0.22947 122 57

0.22617 310 215 0.224

±

0.003

0.22271 221 16

0.22060 103 3

0.21485 311 63

0.21321 302 11 0.211

±

0.003

0.20604 113 45

0.20387 400 10 0.204

±

0.003

0.19976 203 35

0.19546 401 8

0.19422 222 283

0.18894 312 131 0.191

±

0.002

0.18708 320 41

0.18389 123 311

0.18052 321 161 0.181

±

0.002

0.17795 410402 125

0.17535 303 125

0.17186 004 137 0.170

±

0.002

0.16816 104 10

0.16432 231223 56 0.163

±

0.002

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FOMIN et al.

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