Cent. Eur. J. Chem. • 8(6) • 2010 • 1323-1330
DOI: 10.2478/s11532-010-0113-0
Central European Journal of Chemistry
pH impact on the sol-gel preparation of calcium
hydroxyapatite, Ca10(PO4)6(OH)2, using a novel
complexing agent, DCTA
Research Article
Irma Bogdanoviciene , Kaia Tõnsuaadu , Valdek Mikli ,
Inga Grigoraviciute-Puroniene1, Aldona Beganskiene1, Aivaras Kareiva1*
1
2
2,3
1
Department of General and Inorganic Chemistry,
Vilnius University, LT-03225 Vilnius, Lithuania
2
Laboratory of Inorganic Materials,
Tallinn University of Technology, 19086 Tallinn, Estonia
3
Centre for Materials Research,
Tallinn University of Technology, 19086 Tallinn, Estonia
Received 18 June 2010; Accepted 7 September 2010
Abstract: Aqueous sol-gel chemistry routes – based on ammonium hydrogen phosphate as the phosphorus precursor, calcium acetate
monohydrate as the source of calcium ions, and 1,2-diaminocyclohexanetetraacetic acid monohydrate (DCTA) as the complexing
agent – have been used to prepare calcium hydroxyapatite (HA). The sol-gel process was performed in aqueous solution at different
pH values followed by calcination of the dry precursor gels for 5 h at 1000°C. Phase transformations, composition, and structural
changes in the polycrystalline samples were studied by thermoanalytical methods (TG/DTA), infrared spectroscopy (IR), X-ray powder
diffraction analysis (XRD), and scanning electron microscopy (SEM). It was shown that pH adjustment has significant impact on the
apatite formation process and on the morphology and phase purity of the ceramic samples.
Keywords: Calcium hydroxyapatite • Aqueous sol-gel process • DCTA • Structural features
© Versita Sp. z o.o.
1. Introduction
Calcium hydroxyapatite, Ca10(PO4)6(OH)2, commonly
referred to as HA, is one of the calcium phosphate
based bioceramic materials that makes up the majority
of the inorganic components of human bones and teeth.
Synthetic HA is among the most important implantable
materials due to its biocompatibility, bioactivity and
osteoconductivity due to its analogy to the mineral
components of natural bones. For use in medical practice,
HA ceramics have conventionally been strengthened
and toughened in the form of granules, dense or porous
ceramic composites, coatings, whiskers, nanorods and
different pieces with complex shapes. Although these
materials can closely replicate the structure of human
bone, improvement of the materials’ properties is still
very much desirable [1]. The specific chemical, structural
and morphological properties of HA bioceramics are
highly sensitive to changes in chemical composition and
processing conditions [2-5].
The solid-state synthesis of HA from oxide or inorganic
salt powders usually requires extensive mechanical
mixing and lengthy heat treatments at high temperatures.
These processing conditions do not allow facile control
over grain size distribution in the resulting powders
[6-11]. Several wet-chemistry and/or soft chemistry
techniques such as polymerized complex routes
[12-14], hydrothermal syntheses [15-17], precipitation
methods [18-23] or spray-, gel-pyrolysis methods
[24,25] have been used to produce HA phases. Most of
these methods suffer from complex procedures and/or
mismatch in the solution behaviour of the constituents.
As a consequence, heterogeneity may be present in the
obtained ceramic, e.g. formation of significant amounts
of impurities. It has been well demonstrated that the
sol-gel process offers considerably better mixing of the
starting materials and excellent chemical homogeneity
and stoichiometry of the product [26-34]. Several solgel approaches, starting from nonaqueous solutions
of various calcium and phosphorus precursors, have
* E-mail: [email protected]
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pH impact on the sol-gel preparation of calcium hydroxyapatite,
Ca10(PO4)6(OH)2, using a novel complexing agent, DCTA
been used for the preparation of HA powders [35-46].
The major limitation for their application was found to
be the low reactivity and very low solubility in organic
solvents of calcium alkoxides, resulting in deviations in
the stoichiometry of the final materials.
However, a very efficient HA mechanochemical
synthesis method from biogenic raw materials has
recently been suggested [47]. The aqueous route of
sol-gel preparation also offers an effective and relatively
simple way to produce HA [2]. Recently, aqueous sol-gel
chemistry routes have been developed to prepare calcium
hydroxyapatite samples with different morphological
properties in which ethylenediaminetetraacetic acid
(EDTA) or tartaric acid (TA) were used as complexing
agents [48,49]. The monophasic Ca10(PO4)6(OH)2 was
obtained by calcination of precursor gels for 5 h at 1000oC.
These results have initiated the present work in which
the impact of the nature of the gelation agent used in the
sol-gel process on the HA formation process is studied.
The
compound
1,2-diaminocyclohexanetetracetic
acid monohydrate (DCTA) was selected – for the first
time, to our knowledge – as the complexing agent
for the preparation of calcium hydroxyapatite by an
aqueous sol-gel synthesis route. The aim of the work
was to elucidate the impact of the gel pH on the apatite
formation process and to study the obtained products’
phase purity and morphological features.
2. Experimental Procedure
In the sol-gel process, calcium acetate monohydrate,
Ca(CH3COO)2·H2O,
and
ammonium–hydrogen
phosphate, (NH4)2HPO4, were selected as Ca and P
sources, respectively, in Ca/P mole ratio 1.67. Firstly,
calcium acetate monohydrate (5.285 g; 0.03 mol) was
dissolved in distilled water under continuous stirring
at 65°C. In order to obtain water-soluble calcium
complexes and thereby avoid undesirable crystallization
of calcium phosphates, DCTA (11.2 g; 0.03 mol) was
dissolved in distilled water, and afterwards added to the
initial solution. The resulting mixture was stirred for 1 h
at the same temperature. Then, an aqueous solution of
(NH4)2HPO4 (containing 2.376 g; 0.018 mol of solute)
was added to the above mentioned solution. The pH of
the solution was ~5.7. To determine the impact of pH on
the formation of the final product, a solution of 10% NH3
(aq) was added to the reaction solution. Seven different
sol samples were synthesized at pH ~5.7, ~7.0, ~8.0,
~9.0, ~10.0, ~11.0, ~12.0. Finally, after slow evaporation
under continuous stirring at 65°C, the Ca–P–O sols
turned into transparent gels. The oven dried (100°C)
gel powders were ground in an agate mortar, heated at
10°C min-1 to 1000°C, and calcined at this temperature
for 5 h in air. The flow chart of the sol-gel synthesis of
calcium hydroxyapatite is presented in Fig. 1.
Figure 1. A schematic diagram of the steps involved in the sol-gel processes used for the preparation of Ca10(PO4)6(OH)2 ceramics
.
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I. Bogdanoviciene et al.
Figure 2.
TG curves and DTA profiles for the Ca-P-O precursor gels at different pH values in the sol-gel processing.
The dry gels were characterized by thermal analysis
(TG/DTA), and the calcination products by infrared
spectroscopy (IR), X-ray powder diffraction (XRD)
analysis and scanning electron microscopy (SEM).
The thermal analysis was performed using a Setaram
TG-DSC12 apparatus in an air flow of 50 mL min-1 at a
heating rate of 10°C min-1. The IR spectra of KBr pellets
containing the calcination products were recorded on a
Perkin-Elmer FTIR Spectrum BX II spectrometer. The
XRD studies were performed on a D8 (Bruker AXS)
diffractometer operating with Cu Kα1 radiation (step
size: 0.04; time per step: 5 s). A scanning electron
microscope JEOL JSM 8404 was used in order to study
the morphology and microstructure of the ceramic
samples.
3. Results and Discussion
3.1. Thermal analysis of precursor gels
It is well known that the temperature during the synthesis
strongly influences the reaction progress and the
properties of the resulting HA material. The TG and DTA
curves for five selected dry gels are shown in Fig. 2.
The behaviour of all recorded TG curves is similar. The
curves show four main weight losses in the temperature
ranges of 20-203°C (3.8-4.0%), 203-430°C (53.0-
54.0%), 430-630°C (19.5-20.0%) and 630-750°C (5.56.0%). The overall weight loss determined at 1000°C
was approximately the same in all analyzed dried gel
samples, ranging from 80.4 to 83.2%.
The DTA curves recorded for DCTA precursor
samples are also almost identical (see Fig. 2). The first
decomposition step is indicated by broad endothermic
peaks at 179-234oC on the DTA curves that indicate the
evaporation of absorbed and/or structurally-incorporated
water [2]. Exothermic peaks at ~281, ~329, ~384 and
~497oC are characteristic for the pyrolysis processes of
organic constituents. A broad, weak endothermic effect
at ~745oC corresponding to the mass loss at about 5.6%
that may be associated with the decarbonisation of the
desired Ca10(PO4)6(OH)2 phase [50]. In conclusion,
the synthesized gel samples from the sol solutions
with different pH values show almost identical thermal
behaviour, independent of pH. The temperatures of
calcination for intermediate steps to study the apatite
formation mechanism were chosen on the basis of
thermal analysis results.
3.2. Powder X–ray diffraction analysis
XRD patterns of the ceramic samples after calcination of
Ca-P-O gels obtained at pH 10 at different temperatures
are shown in Fig. 3. The calcination product of the
precursor gel at 400ºC remains amorphous. The
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pH impact on the sol-gel preparation of calcium hydroxyapatite,
Ca10(PO4)6(OH)2, using a novel complexing agent, DCTA
Relative intensity
1000°C - 5h
1000°C - 30min
600°C - 30min
400°C - 30min
20
25
30
35
40
45
2θ (°)
50
55
60
65
70
Figure 3. X-ray
(322)
(313)
(222)
(132)
(230)
(213)
(321)
(410)
(402, 303)
(004)
(113)
(203)
(212)
(130)
(221)
(131)
(301)
(211)
(112)
(300)
(202)
(102)
(210)
(201)
(002)
(202)
(111)
diffraction patterns of the ceramic products
obtained after calcination of Ca-P-O precursor gel derived
at pH~10 at different temperatures and duration.
pH ~12.0
Relative intensity
pH ~11.0
*
*
**
*
*
*
**
*
*
*
*
*
20
25
30
*
*
*
* *
*
* *
*
* *
35
pH ~10.0
pH ~9.0
*
*
*
*
pH ~8.0
*
*
*
pH ~7.0
*
*
40
2θ (°)
*
45
*
50
pH ~5.7
55
60
Figure 4. X-ray diffraction patterns of the HA samples synthesized
at 1000oC using different pH environment in the sol-gel
processing. Ca3(PO4)2 phase of the peak is marked with
(*).
Transmittance
pH ~12.0
pH ~11.0
pH ~10.0
pH ~9.0
crystalline phase appears after annealing Ca-P-O gel at
600ºC when the organic constituents are decomposed.
The most intensive diffraction line located at
2θ = 29.52o could be attributed to the Ca3(PO4)2 phase,
however, the less intensive peaks correspond to
calcium octaphosphate Ca8H2(PO4)6. Octaphosphate
is often found as a first precursor compound when HA
is precipitated in a solution. With increasing calcination
temperature up to 1000ºC and after annealing for
30 min, the XRD diffraction pattern contains lines of
different phosphates. However, the most intensive
peaks in the range of 2θ ≈ 31-33o are attributable to
calcium hydroxyapatite. Finally, with further annealing
up to 5 h at the same temperature the HA forms as a
main phase in the ceramic product. Therefore, the
formation of HA in solid state from Ca–P–O gels is a
time-consuming reaction as an annealing time up to
5 hours at a temperature not less than 1000°C is needed
for getting hydroxyapatite.
XRD analysis results show that the calcination
products at 1000oC of Ca–P–O gels obtained at different
pH are different. Their diffraction patterns are displayed
in Fig. 4. There is an obvious dependence of material
composition on pH. The sample synthesized at the
lowest pH, namely ~5.7, has three intensive peaks
belonging to the desirable Ca10(PO4)6(OH)2 phase.
In addition, there are many peaks corresponding to
β-Ca3(PO4)2 phase (PDF [9-0169]). The number and
intensity of the calcium phosphate peaks decrease with
increasing pH from 7 to 9. Finally, the polycrystalline
single-phase Ca10(PO4)6(OH)2 was obtained at pH
values 11.0 and 12.0. The three most intensive lines
are located between 2θ≈31–33° ((2 1 1) – 100%,
(3 0 0) – 62.8%, and (1 1 2) – 51.3%) in the singlephase samples. These results are in good agreement
with the reference data for Ca10(PO4)6(OH)2 (PDF [72–
1243]). All single reflexes (number of accepted peaks
– 42) from our XRD measurements were indexed. The
lattice parameters of the synthesized Ca10(PO4)6(OH)2
samples were obtained from the diffraction pattern by
fitting the peaks of identified reflections. The hexagonal
lattice parameters and cell volume determined for the
sample obtained from the pH 12.0 gel were found to be
a = 9.432(2) Å, c = 6.881(1) Å, and V = 530.14(3) Å3,
which correspond to a stoichiometric hydroxyapatite.
pH ~8.0
pH ~7.0
pH ~5.7
3500
3000
2500
2000
1500
1000
500
Wavenumber (cm-1)
Figure 5. FTIR spectra of HA samples synthesized at 1000oC using
different pH environment in the sol-gel processing.
3.3. Infrared spectra
IR spectroscopy is highly sensitive to the impurities and
substitutions in the structure of apatite [51]. The IR spectra
of HA samples obtained at 1000oC in the sol-gel process
using different pH environments are shown in Fig. 5. In
the spectra of calcined samples, a complex band from
the asymmetric stretching vibration of the phosphate
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I. Bogdanoviciene et al.
Figure 6.
SEM micrographs of HA samples synthesized at 1000oC from the sol prepared at pH ~5.7. Magnification (a) and (b) 1000X,
(c) 2000X, (d) 5000X.
group at 1000-1100 cm-1 dominates. In the spectra of
samples obtained from the gels with pH 5.7-10 there are
observed, in the region of symmetric stretching vibration
of phosphate group at 940-970 cm-1, two peaks at 969
and 942 cm-1 that are characteristic of β-Ca3(PO4)2 [52].
For the samples obtained from the gels with pH 11-12, a
peak at 963 cm-1, characteristic of HA, is observed. The
peak intensity at 630 cm-1 assigned to the OH librational
vibration increases with the increase in the pH value in
gel, as does the band from the stretch vibration of the
OH group at 3573 cm-1 in the hydroxyapatite spectra.
At the same time the intensity of the band at 3644 cm-1
assigned to Ca(OH)2 decreases. The peaks assigned
for carbonate substitution in apatite structure at 14001550 cm-1 are weakly expressed, which is characteristic
of apatites calcined above 1000°C. Therefore the IR
spectra prove the results of XRD analysis regarding
the formation of two or more phases due to calcination
of the gels with lower pH, as well as the positive
impact of higher pH in gel solutions on HA formation.
The positive impact of higher pH of the gel formation
environment on stoichiometric HA formation is similar
in the wet precipitation synthesis. The formation of non
stoichiometric apatite or other calcium phosphates at
pH < 9 is explained by the lack of the OH- ions needed
in HA structure. In solid-state hydrothermal synthesis,
the presence of water vapour is needed to increase the
amount of HA phase in sol-gel calcination products.
3.4. Scanning electron microscopy studies
Calcined Ca–P–O gel precursor powders were
investigated by scanning electron microscopy, from
which the grain size and typical morphologies of HA
could be observed. The morphological features of the
heated samples of DCTA gels are given in the SEM
pictures (Figs. 6-8). The product of calcination derived
at pH ~5.7 (Fig. 6) is made up of different size and
shape particles. According to phase analysis results,
few types of crystallites could be observed. The particles
are composed of agglomerates of elongated crystallites
1.5-4.0 μm necked to each other (Fig. 6a). However,
HA nanocrystallite particles with regular rod-shaped
structure and centrifugal habit with width of 70 nm and
length of 300 nm (Fig. 6c,d) [3,4] are also seen. Cubic
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pH impact on the sol-gel preparation of calcium hydroxyapatite,
Ca10(PO4)6(OH)2, using a novel complexing agent, DCTA
Figure 7. SEM micrographs of HA samples synthesized at 1000oC
Figure 8. SEM micrographs of HA samples synthesized at 1000oC
crystallites of 300-800 nm in size (Fig. 6b) could be
assigned to the Ca3(PO4)2 phase. The SEM micrographs
on Fig. 6 show that the morphology, particle size and
distribution are difficult to control by this process at low
pH value. With increasing pH of sol-gel processing up
to ~8.0 (see Fig. 7a), a more agglomerated material of
round-shaped crystallites was formed. Two types of
particles are observed: agglomerates of fine (0.5-1 μm)
and of coarser particles (1.5-8 μm), which construct a
porous structure. With increase in pH up to ~10.0 the
fine particle agglomerates disappear (Fig. 7b) and the
formation of conglomerates of weld crystallites (0.53.0 μm) of HA is observed. However, the formation of
a negligible amount of Ca3(PO4)2 as a side phase with
smaller cubic crystallites is also evident. The SEM
micrographs of monophasic HA powders obtained from
gel prepared at pH ~11.0 contain smaller (0.5-2.0 μm)
merged crystallites (Fig. 8). Moreover, it is obvious
that the particles are more rounded. When pH in the
gel was ~12.0, the HA particles were even finer (0.31.0 μm), remained rounded and merged into a porous
conglomerate.
In previous studies [48,49], it was found that HA
powders derived from the EDTA sol-gel route are
composed of agglomerates of broad size distribution. The
mean size of the aggregated grains was approximately
0.5–8 µm. The HA solids were composed of grains with
irregular shape and surface texture. When tartaric acid
was used in the sol-gel processing the formation of
nicely shaped ultrafine elongated particles (microrods
and/or microsticks) with length of ~2-10 µm and width of
1-4 µm have been obtained.
Therefore, it may be concluded that the nature of the
complexing agent and pH have an essential influence
in sol-gel process on the particle shape and size of HA
bioceramics.
from the sol prepared at (a) pH ~8.0 and (b) pH ~10.0.
Magnification 1000X.
from the sol prepared at (a) pH ~11.0 and (b) pH ~12.0.
Magnification 1000X.
4. Conclusions
DCTA was successfully used as complexing agent in the
preparation of calcium hydroxyapatite by an aqueous
sol-gel technique for the first time to our knowledge. An
aqueous sol-gel chemistry route based on ammonium–
hydrogen phosphate and calcium acetate monohydrate
as the sources of phosphorus and calcium ions,
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respectively, has been developed to prepare calcium
hydroxyapatite (HA). It was shown that proper selection
of solution pH in the sol-gel processing allows control
of the phase purity and grain size of the resulting
Ca10(PO4)6(OH)2 powders. The content of hydroxyapatite
in calcination products increases with increasing pH of
the sol solution. At lower pH values (6-9), a significant
amount of an impurity phase of Ca3(PO4)2 forms. The
amount of side phases monotonically decreases with
increasing pH from 5.7 up to 12.0. The morphological
properties could be also controlled by changing pH
in the sol-gel processing. HA particles with a size
0.3-1.0 μm merged into a porous conglomerate were
obtained after calcination of a gel with pH 12 for 5 h at
1000°C.
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