Materials Chemistry and Physics 115 (2009) 21–27
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Materials Chemistry and Physics
journal homepage: www.elsevier.com/locate/matchemphys
Effect of fluorides (KF and NaF) on the growth of dicalcium phosphate dihydrate
(DCPD) crystal
C. Sekar a,∗ , P. Kanchana a , R. Nithyaselvi a , E.K. Girija b
a
b
Department of Physics, Periyar University, Salem 636011, Tamilnadu, India
School of Physics, Madurai Kamaraj University, Madurai 625021, India
a r t i c l e
i n f o
Article history:
Received 12 February 2008
Received in revised form 21 May 2008
Accepted 5 November 2008
PACS:
87.85.−J
82.70.Gg
81.10.−h
Keywords:
Biocrystals
Dicalcium phosphate dihydrate
Additives
Fluorides
Crystal growth
a b s t r a c t
Dicalcium phosphate dihydrate (CaHPO4 ·2H2 O, DCPD also known as brushite) is the major component
of hard tissues like bone, teeth and a medicine for calcium supply. The effect of sodium fluoride and
potassium fluoride on the crystallization of DCPD in sodium meta silicate gel has been studied at room
temperature under the physiological pH (7.4). Addition of fluorides reduces the crystal size and the number of crystals grown when compared to pure system. Among the two fluorides, the KF suppresses the
crystal formation more drastically than that of NaF. In both the cases, the ‘Ca’ content in the DCPD crystals
was found to be less when compared to its pure form. The crystal morphology, elemental composition
and properties of the grown crystals were analyzed using SEM-EDAX, powder X-ray diffraction (XRD),
Fourier transform infrared spectroscopy (FTIR), and thermal analysis (TG-DSC).
© 2008 Elsevier B.V. All rights reserved.
1. Introduction
Materials play a key role in several biomedical applications,
and it is imperative that both the materials and biological aspects
are clearly understood for attaining successful biological outcome. Calcium phosphates (CaP) are the most ubiquitous family
of bioceramics well known for their use in biological applications
[1].
The chemical composition of calcium phosphates is roughly
equivalent to that of the inorganic matrix of human bone and is
found to be the most suitable as implant materials. Major phase
found in bone is hydroxyapatite (HAP, Ca10 (PO4 )6 (OH)2 ) and the
other commonly known phases are octacalcium phosphate (OCP),
tricalcium phosphate (TCP), dicalcium phosphate dihydrate (DCPD,
CaHPO4 ·2H2 O) and dicalcium phosphate DCP [2].
The crystallization of DCPD has attained considerable attention
due to the vital role played by it in the formation of metabolic, nonmetabolic stones, bones and teeth [3]. Brushite is used as an abrasive
constituent of toothpaste, a feed additive, constituent of fertilizers,
etc. [4]. The crystallization of DCPD was obtained mainly by solution
and gel method [5,6]. Anee et al. [7] have reported the influence of
∗ Corresponding author.
E-mail address: [email protected] (C. Sekar).
0254-0584/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.matchemphys.2008.11.020
cobalt and malic acid on the crystallization of DCPD by gel method.
They reported the presence of cobalt and malic acid modifies the
crystal morphology. Kalkura et al. [8] reported the crystallization
of DCPD in the presence of iron in agarose gel. Sivakumar et al. [9]
reported the effect of magnesium on the formation of DCPD. They
observed the presence of magnesium inhibited the crystallization
of DCPD crystals. LeGeros and LeGeros [10] grew single crystals of
brushite in silica gel, and reported its morphological changes due
to the addition of impurities. According to these authors, the presence of Sr2+ and P2 O7 4− causes marked effect on the crystal habit.
Abbona et al. [11] have studied the crystal habit and growth conditions of brushite crystals. In the present work, we have studied
the influence of fluoride on the crystallization of DCPD. Fluoride
in small amounts is essential to many living organisms, including
humans. But excessive exposure to fluoride in drinking-water, or in
combination with exposure to fluoride from other sources (foods,
beverages, etc.,), can give rise to a number of adverse effects. These
range from mild dental fluorosis to crippling skeletal fluorosis as
the level and period of exposure increases. Fluoride exists only in
combination with other elements as a fluoride compounds. It is
the most electronegative of all the elements, which means that it
has a strong tendency to acquire a negative change, and in solution forms F− ions. Fluoride ions have the same charge and nearly
the same radius as hydroxide ions and may replace each other in
mineral structures [12].
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C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
The motivation for the present work is to study the influence
of sodium fluoride (NaF) and potassium fluoride (KF) on crystal
growth of DCPD under in vitro conditions and the influence of these
fluorides on their growth behaviors.
2. Experimental procedure
The single test tube diffusion method was employed for growing DCPD crystals in
gel medium [2]. The analar grade chemicals were used for the present investigations.
Gel was prepared using silica gel (SMS) solution of specific gravity 1.03 g cm−3 and
was adjusted to a pH 7.2–7.6 by treating it with 10% glacial acetic acid and 0.6 M
of Na2 HPO4 solution. The solution was allowed to get at room temperature. After
gelation, 10 ml of 1 M CaCl2 ·2H2 O (pH 7.82) was poured carefully over the top of
the silica gel without disturbing the latter. For growing DCPD crystals in presence of
fluorides, the supernatant solution was prepared by mixing 1 M of calcium chloride
with 10 wt.% of KF and NaF each.
Growth features of DCPD crystals were studied using scanning electron microscope (SEM) and the elemental analyses were done using the OXFORD INCA energy
dispersive X-ray fluorescence spectrometer. Powder X-ray diffraction (XRD) pattern
was recorded on Bruker advanced diffractometer within the 2 range of 10–80◦ .
Thermal analysis of samples was performed using simultaneous TGA-DSC instrument with thermal solution versions 1.2J controller software. Data analysis was
carried out using TA Instrument Universal Analyser version 2.3C software.
Fig. 1. DCPD crystals in test tube: (a) pure, (b) KF added and (c) NaF added.
C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
23
Table 1
Comparison of pure and fluoride doped DCPD growth observations.
Growth of DCPD crystals
Time
Pure system
KF addition
NaF addition
Addition of supernatant solution
2h
Immediate white precipitate
About 5 mm white precipitate at
interface
Six circular rings (d = 3 mm)
Immediate white precipitate
About 5 mm white precipitate at
interface
Three rings (d = 4–5 mm)
11 rings and some DCPD crystals
between and below the rings
(d = 3 mm)
12 rings and size of the crystals
slightly improved
Four rings(d = 4–5 mm) no crystal is
formed
Immediate white precipitate
About 5 mm white precipitate at
interface
Two irregular circular ring
(d = 3 mm)
Three irregular circular ring and
observed DCPD crystals
3 days
7 days
13 days
20 days
20 days
No change in the rings and the size
of the crystals slightly improved
The distance between the 12
Liesegang rings were equal
Five rings and four to six DCPD
crystals grew between the
Liesegang rings
Seven rings and size of the crystals
increased
Distance between the total seven
Liesegang rings increased towards
bottom
No more improvement of rings and
many DCPD crystals grew beneath
the Liesegang rings
Size of the crystals slightly
improved
The distance between the total four
Liesegang rings are equal
d, thickness of the Liesegang ring.
3. Results and discussion
3.1. Crystal growth
The DCPD crystals have been successfully grown from pure and
fluoride added supersaturated solutions. In pure system, a dense
white precipitate has formed at the interface between the gel and
supernatant solution and it continued to grow in solution and inside
the gel medium to a thickness of about 5 mm. Rings of precipitate
(Liesegang rings) were observed just below the continuous precipitate inside the gel medium in both pure and fluoride added
experiments. The shape and the number of white discs were found
to be different in pure and fluoride added growth runs. Fig. 1 shows
the picture of DCPD crystals grown with and without fluorides.
When compared to pure system, NaF addition resulted in a fewer
white discs and they were found in the gel but close to the interface. In case of KF, there were large number of bulk white discs
inside the gel and its thickness increased when moving from the
top to the bottom of the test tube.
The DCPD crystals were observed in between the Liesegang rings
inside the gel. There was more number of crystals in pure system
when compared to the fluoride added growth experiments (Fig. 1).
Further, the number of crystals and total yield were less in fluoride
added experiments. Among the fluorides, KF suppresses the crys-
Fig. 2. DCPD crystal: (a) pure, (b) KF doped and (c) NaF doped.
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C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
Fig. 3. (1a and b) SEM picture of pure DCPD crystals; (2a–c) SEM picture of KF doped DCPD crystals; (3a and b) SEM picture of NaF doped DCPD crystals.
C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
tal formation more drastically than the NaF. KF is a well-known ‘F’
source in many chemical reactions. In this case, KF can be expected
to dissociate itself into K and F ions and the supernatant solution then becomes enriched with fluoride ions. This may be due
to the inhibition of crystal growth in the presence of KF. The physical observation made during the growth experiments are listed in
Table 1. In general, both pure and fluoride doped DCPD crystals have
platelet morphology and transparent in nature (Fig. 2).
3.2. SEM and EDAX studies
Fig. 3.1a shows the SEM picture of pure DCPD crystals. The crystals grew in platelet morphology with the growth layers parallel
to (0 0 1) surface. These platelets are known to grow by twodimensional nucleation mechanism and subsequent spreading of
the layers and these layers are parallel to the longer edges of the
crystals. In general crystal surfaces were clean and free from any
major defects (Fig. 3.1b).
Fig. 3.2a–c shows the surface feature of the DCPD crystals grown
in the presence of KF. Impurities and concentration gradients are
25
known to be the two main environmental habit modifiers. However, the KF addition does not seem to influence the morphology
of the DCPD crystals. It is well known that, the early stages of
growth takes place by mainly two-dimensional nucleation since
the supersaturation is very high. When the crystals reach a certain critical size, the surface is strained by internal stresses or by
pressure from other crystals forming imperfection on the surface.
Internal or external stresses help the dislocations present in the
crystal to concentrate into a small area. From imperfect area thus
formed, growth layers perfectly starts and spreads two dimensionally. This fact is applicable to the growth of thin layers and these thin
growth layers bunch together to form thick layers as represented in
Fig. 3.2a–c.
The morphology of NaF grown DCPD also had platelet shape
(Fig. 3.3a and b). In some cases spherulite crystals were observed on
the crystal surface as agglomerates. This could be possibly because
of the formation of the secondary phase dicalcium phosphate (DCP,
CaHPO4 ) whose characteristic morphology is spherulite.
The chemical composition of the grown crystal was estimated
by EDAX analyser equipped with SEM. Fig. 4a shows that the pure
Fig. 4. EDAX spectra of DCPD: (a) pure, (b) KF doped and (c) NaF doped.
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C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
Table 2
EDAX data of pure and fluoride doped DCPD crystals.
Element
Pure DCPD (atm.%)
KF addition (atm.%)
NaF addition (atm.%)
Ca
P
Na
F
K
53.30
43.87
2.44
–
–
50.64
46.64
–
–
1.30
43.51
40.91
2.31
12.87
–
1.21
1.08
1.06
Ca/P ratio
DCPD crystal is primarily composed of calcium and phosphorous.
The atomic calcium to phosphorus (Ca/P) ratio of the pure brushite
crystal is estimated as 1.21, which is higher than the expected value
of 1.00 according to chemical formula [1]. In addition to Ca and
P, a small amount of Na is detected in the as grown samples. As
gel media contains sodium in the form of silica gel, Na doping is
unavoidable during the growth. The elemental analysis of the crystals grown with KF addition indicate the Ca/P to be 1.085 and a small
amount of potassium (∼1.3%) (Fig. 4b).
Fig. 4c shows a representative EDAX spectrum of the crystals grown with NaF addition. It shows that the Ca/P ratio to be
1.06, which is in good agreement with the expected value of 1.00.
Interestingly, about 12.87 atm.% of fluoride and a small amount of
sodium (∼2.3 atm.%) were detected in these samples. Similar peaks
corresponding to ‘F’ was seen in the EDAX spectra of pure and KF
grown samples. This can be attributed to the ‘F’ impurities present
in the starting materials used for crystal growth. Contrary to the KF,
the NaF addition seems to promote the more F-doping in the DCPD
system. Several measurements were made at different points on
the crystals and the average atomic percentages of the individual
elements are shown in Table 2. It can be noticed that the addition of
fluorides (both KF and NaF) leads to the ‘Ca’ reduction when compared to pure system. The excess ‘Ca’ may react with ‘F’ leading to
the formation of CaF2 [13].
3.3. TG-DSC analysis
The TGA results of pure DCPD samples revealed a two-stage mass
loss in the temperature range between 152–199 and 422–470 ◦ C
as shown in Fig. 5. In the first stage, a sharp 19% mass loss was
observed, which could be attributed to the loss of adsorbed water.
The second stage shows a 5% mass loss due to the loss of lattice
water. There was no further change in weight on heating up to
800 ◦ C. This indicates the medium thermal stability of the sample.
Fig. 5. TG-DSC curve of pure DCPD crystals.
Fig. 6. TG-DSC curve of KF doped DCPD crystals.
The mass loss corresponds well with the DSC data, which reveals
the exothermic peak at 131, 182 and 447 ◦ C respectively.
The DCPD crystals grown in the presence of KF showed three
– stage mass loss in the temperature range between 128–180,
180–210 and 400–480 ◦ C respectively (Fig. 6). There was no further
change in weight on heating up to 800 ◦ C. The mass loss corresponds
well with the DSC data, which reveals the strong exothermic peaks
at 124, 152, 195, 276, 358, 397, 446, 582, and 654 ◦ C respectively.
Fig. 7 shows the TGA-DSC curves of the crystals grown in the
presence of NaF. At low temperatures, the TG result was similar
to that of KF grown crystals. The mass loss continued with the
increase in temperature and a significant drop in mass was observed
at 741 ◦ C. The TG results correspond well with the DSC data through
the appearance of sharp exothermic peaks at 126, 162, 191, 311, 444,
511, 635, 703 and 741 ◦ C respectively.
3.4. FTIR studies
The FTIR spectra of the DCPD crystals were recorded at
room temperature in the wave number range between 400 and
4000 cm−1 . Fig. 8 shows the spectra of pure and fluoride grown
DCPD crystals. They have a broad envelope between 2000 and
3900 cm−1 could be assigned to OH stretching of HPO4 2− and H2 O
Fig. 7. TG-DSC curve of NaF doped DCPD crystals.
C. Sekar et al. / Materials Chemistry and Physics 115 (2009) 21–27
27
Fig. 8. IR spectra of DCPD: (a) pure, (b) KF doped and (c) NaF doped.
Fig. 9. Powder XRD patterns of DCPD: (a) pure, (b) KF doped and (c) NaF doped.
Table 3
Vibrational band assignments of pure and fluoride doped DCPD.
−1
Vibrational frequency (cm
)
parameters and unit cell volume of the pure and doped samples are
given in Table 4. Fluoride ions have the same charge and nearly the
same radius as hydroxide ions and may replace each other in crystal
structures.
Assignment
526, 527, 576, 577, 666–668, 791–794,
872–874
986–990, 1060, 1063, 1067, 1129,
1134–1136, 1349, 1350, 1383, 1382
1651, 1650
2000–3900
O–P–O bending
HPO4 2− vibration
4. Conclusion
OH2 bending
O–H stretch of HPO4 2− and H2 O
Table 4
Lattice cell parameters of pure and fluoride doped samples.
Samples
Pure DCPD
KF addition
NaF addition
Volume (Å3 )
Crystal parameters (Å)
a
b
c
5.824
5.805
5.833
15.205
15.226
15.130
6.241
6.226
6.241
495.629
493.506
493.947
[11]. The H–O–H bending mode of lattice water molecules appears
at 1650 and 1651 cm−1 . The bands, which correspond to HPO4 2−
vibration, occurred at 986, 990, 1060, 1063, 1067, 1129, 1134, and
1136 respectively. The peaks appeared at 526, 527, 576, 577, 666,
668, 791, 794, 872 and 874 cm−1 were assigned to O–P–O bending
mode. The peaks at 1350 and 1382 cm−1 in the NaF grown crystals
and 1349 and 1383 cm−1 in the KF doped DCPD was not visible in
the undoped one and hence it might be attributed to the incorporation of fluorides into the crystal. The vibrational assignments of
DCPD crystals are given in Table 3 and the spectrum agrees well
with the reported result [1,5,7,8].
3.5. XRD analysis
Beevers [14] reported the structure of DCPD crystal to be monoclinic with space group I2/a and the lattice parameters a = 5.812 Å,
b = 15.180 Å, c = 6.239 Å, ˛ = = 90◦ , and ˇ = 116◦ 25 . Powder XRD
patterns of the pure and fluoride doped DCPD crystals are shown
in Fig. 9. The result is in good agreement with the standard data
(JCPDS NO. 11-293) and theoretically simulated pattern using lattice parameters by Beevers [14]. For doped crystals, the intensity of
some of the planes was less than that for the pure DCPD. The lattice
Large transparent crystals of pure DCPD have been grown by
gel method. Fluoride addition to the growth experiments reduces
the crystal size and total number of crystals grown. Among the
two investigated fluorides, KF suppresses the growth of DCPD more
significantly than the NaF.
The EDAX analyses show that the DCPD is primarily composed of
calcium and phosphorous in the ratio of 1.21 for pure samples and
that of fluoride doped crystals is found to be 1.06 (NaF) and 1.08 (KF)
respectively. The TGA results of the fluoride grown DCPD crystals
underwent several stages of mass loss suggesting the samples are
doped with impurities such as F, Na, and K. The FTIR spectra of
the undoped crystals confirmed the compound formation and the
fluoride added crystals revealed additional peaks corresponding to
fluorides. Powder XRD studies confirmed the good crystallinity of
the pure and fluoride grown crystals.
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