Journal of the Australian Ceramic Society (2022) 58:803–815
https://doi.org/10.1007/s41779-022-00730-5
RESEARCH
Theoretical and experimental characterization of Sn‑based
hydroxyapatites doped with Bi
A. Aksogan Korkmaz1 · Lana Omar Ahmed2 · Rebaz Obaid Kareem3 · Hanifi Kebiroglu4 · Tankut Ates5 ·
Niyazi Bulut4 · Omer Kaygili4 · Burhan Ates6
Received: 6 December 2021 / Revised: 19 January 2022 / Accepted: 24 February 2022 / Published online: 17 March 2022
© The Author(s) under exclusive licence to Australian Ceramic Society 2022
Abstract
This is the first report, including both theoretical and experimental results, on Bi and Sn co-doped hydroxyapatite
(HAp) structures. Sn content was kept at a constant amount of 0.22 at.%, and Bi content was changed from 0 to 0.44
at.% by using the steps of 0.11at.%. Theoretical results from density functional theory (DFT) calculations revealed an
increase in density from 3.154 g ­c m−3 to 3.179 g ­c m −3, as well as gradual decreases in the bandgap from 4.5993 eV
to 4.4288 eV and the linear absorption coefficient. The spectroscopic data obtained from both Raman and Fourier
transform infrared (FTIR) spectra confirmed the HAp structure for all the samples. The thermal behavior and morphology, as well as all X-ray diffraction (XRD) related parameters, were all considerably impacted by Bi-content.
In vitro assays showed that all the samples can be accepted as the biocompatible materials.
Keywords Hydroxyapatite · X-ray diffraction · Spectroscopic analysis · Bandgap
Introduction
Hydroxyapatite (HAp, ­C a 10(PO 4) 6(OH) 2) is a naturally
occurring mineral of calcium apatite [1]. HAp has a hexagonal crystal system. When it is pure, it is in the form
of white granules and can be found in colors such as
colorless, gray, yellow, yellowish-green depending on
the impurities it contains [2, 3]. HAp can be obtained
* Tankut Ates
tankut.ates@ozal.edu.tr
1
Department of Mining Technology, Malatya Turgut Özal
University, Malatya, Turkey
2
Department of Physics, Faculty of Science and Health, Koya
University, Koya KOY45, Kurdistan Region – F.R, Iraq
3
Physics Department, College of Science, University
of Halabja, 46018 Halabja, Iraq
4
Department of Physics, Faculty of Science, Firat University,
23119 Elazig, Turkey
5
Department of Engineering Basic Sciences, Faculty
of Engineering and Natural Sciences, Malatya Turgut Özal
University, Battalgazi, , Malatya, Turkey
6
Department of Chemistry, Faculty of Arts & Science, Inonu
University, 44280 Malatya, Turkey
naturally from bones such as bovine bone, fishbone, fish
shell, oyster shell, chicken eggshell, and coral [4].
HAp is widely used in biomedical applications (hard
tissue repair, substitution, augmentation and as a coating
for orthopedic implants) due to its chemical stability,
non-toxicity, high bioactivity and biocompatibility [5–8].
It is also used as an adsorbent in engineering barriers for
environmental repair. In addition, it has great potential
as a biomaterial for other applications such as catalysis,
chromatographic adsorption [9]. Further, it is also used
as an agent in bioimaging and as hyperthermia for cancer
[10].
HAp nanoparticles have attracted great interest for
biomedical applications due to their favorable stoichiometry and purity. HAp nanoparticles have not only an
ultrafine structure but also high surface reactivity similar to minerals found in bones. HAp nanoparticles show
enhanced densification and sintering properties due to
their high surface energy. Therefore, problems such as
micro-cracks can be avoided [11]. Due to its high wear
resistance, chemical stability, and chemical composition
similar to natural bone, HAp is more advantageous than
metal or polymer in bone-tissue applications [12]. However, in new bone formation, HAp has disadvantages such
as low bone-binding ability and susceptibility to bacterial
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Journal of the Australian Ceramic Society (2022) 58:803–815
Table 1 The moles of the
as-used chemicals
Sample
Ca(NO3)2·4H2O
(mmol)
Bi(NO3)3·5H2O
(mmol)
SnCl4·5H2O
(mmol)
(NH4)2HPO4
(mmol)
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
49.890
49.835
49.780
49.725
49.670
0.055
0.110
0.165
0.220
0.110
0.110
0.110
0.110
0.110
30.000
30.000
30.000
30.000
30.000
activity [13–16]. To improve its low bone-binding ability, its composites using some polymers (e.g., chitosan
and polyetheretherketone) or special glasses have been
used [17–19]. Additionally, some antimicrobial agents
a
b
Energy gap (Eg) = 4.5993 eV
20
­(Fe 3O4, ­Ag +, ­C u 2+, and ­Ti 4+, etc.) can be substituted in
HAp to give it better antibacterial properties in comparison with its un-doped form [16].
10
Energy (eV)
Energy (eV)
10
0
0
-10
-20
Energy gap (Eg) = 4.5376 eV
20
-10
G A
H
K γ
M L
H
0
10
20
30
40
50
60
70
-20
80
G A
K γ
H
M L
0
H
c 20
d
Energy gap (Eg) = 4.4946 eV
30
40
50
60
70
80
10
Energy (eV)
Energy (eV)
20
Energy gap (Eg) = 4.4562 eV
20
10
0
-10
-20
10
Density of states (electrons/eV)
Density of states (electrons/eV)
0
-10
G A
H
K
γ
M L
H
0
10
20
30
40
50
60
70
80
-20
G A
H
K γ
M L
0
H
Density of states (electrons/eV)
e
10
20
30
40
50
60
70
80
Density of states (electrons/eV)
Energy gap (Eg) = 4.4288 eV
20
Energy (eV)
10
0
-10
-20
G A
H
K γ
M L
H
0
10
20
30
40
50
60
70
80
Density of states (electrons/eV)
Fig. 1 The bandgap and density of states of the a) 0.22Sn-HAp, b) 0.11Bi-0.22Sn-HAp, c) 0.22Bi-0.22Sn-HAp, d) 0.33Bi-0.22Sn-HAp and e)
0.44Bi-0.22Sn-HAp structures
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Journal of the Australian Ceramic Society (2022) 58:803–815
In biological apatites, some cations (­ Na +, ­K +, ­M g 2+,
­Z n 2+, ­S r 2+, ­F e 2+ etc.) and anions ­( F −, ­C l −, ­C O 32− etc.)
can substitute at the HAp lattice. This reduces the Ca/P
ratio, possibly improving solubility and affecting the
crystal structure [6, 20]. Polarizability, electronegativity, valence and ionic radii are important factors in
cationic substitutions. The physical and chemical properties of HAp are affected by its surface morphology,
size and crystal structure [21, 22]. Even though some
cationic substitutions are minimal, significant changes
can be observed in the structural properties of HAp such
as lattice parameters, crystal structure, morphology, and
important properties such as thermal stability, magnetic
and mechanical properties [1].
HAp synthesis is an alternative way to provide better
control of material properties [23]. Various techniques
have been used to synthesize HAp, such as sol–gel,
hydrothermal, solid-state reaction, microwave irradiation, spray pyrolysis, hydrolysis, chemical precipitation,
emulsion or sonochemical process [5, 21, 24].
Although there are very few studies of Bi and Sndoped hydroxyapatite in the literature, there are many
studies in which different metals are doped. Bi and Sn
have no known biological role and are non-toxic. Especially, Bi, which is a heavy metal, has been widely used
in the most applications in the chemistry, physics and
materials science due to its non-toxic nature and different chemical and physical properties in comparison
with other elements [25]. Bi and its complexes have
been used in the biological applications as well as Sn
and its oxide form [26–30]. Ramesh et al. [31] investigated the sinterability of HAp with different amounts of
­Bi 2O3 doped (from 0.05% to 1% by wt.) and compared it
with the un-doped HAp. They investigated phase stability, relative density, Vickers hardness, fracture toughness and Young’s modulus. They determined that the
­B i 2O 3 additive had a negligible effect on HAp particle
morphology and crystal size. The best densification,
Young's modulus, hardness and fracture toughness values ​​were obtained in HAp doped with 0.5% by weight
­Bi2O3 when sintered at 1000ºC [31]. Moussa et al. [32]
investigated the effect of 0.5% Sn and Bi addition as
alloying on the microstructure and in vitro degradation
behavior of as-cast Mg-4wt% Zn alloy without and with
Ca-P coating. It was determined that the alloy containing Sn showed the lowest degradation rate, while the
alloy containing Bi showed the highest degradation rate
[32]. Ahmed et al. [33] synthesized Bi and Sr doped
nano-sized hydroxyapatites by precipitation microwaveassisted method for applications, particularly in bone tissue engineering. They determined that the B
­ i3+ and S
­ r2+
dual substitutions were successfully incorporated into the
HAp lattice. Bi–Sr-doped HAp has been demonstrated to
805
exhibit a high reactivity to E. coli. Radha and Sreekanth
[34] used Sn as an alloying element to evaluate its effect
on the mechanical and corrosion properties of Mg/HAp
composites. They determined that the addition of Sn also
plays an important role in increasing both the mechanical
and the corrosion resistance of Mg/HAp.
There are some reports on Bi- and Sn-doped HAps
as mentioned above, but to the best of our knowledge,
there is no study in the literature on the effects of Bi/
Sn co-dopants on the HAp structure. We present a more
detailed experimental and theoretical investigation report
on these samples the first time.
Materials and Methods
Synthesis and characterization
Di-ammonium
hydrogen
phosphate
((NH4)2HPO4, ≥ 98.0%, Sigma-Aldrich, France), calcium
nitrate tetrahydrate (Ca(NO 3) 2·4H 2O, 99%, Carlo-Erba,
France), tin (IV) chloride pentahydrate (­ SnCl 4 ·5H 2 O,
98.0%, Sigma-Aldrich, France) and bismuth (III) nitrate
pentahydrate (Bi(NO3)3·5H2O, ≥ 98.0%, Sigma-Aldrich,
France) were used as the starting chemicals of the wet
chemical synthesis of the Sn-based HAps doped with
Bi at different amounts (e.g., 0, 0.11, 0.22, 0.33 and
0.44 at. %). For each sample, the appropriate amount of
the as-mentioned chemicals given in Table 1 was used.
100 mL solution of Ca(NO 3 ) 2 ·4H 2 O, ­S nCl 4 ·5H 2 O and
Bi(NO 3)3·5H2O was prepared in a flask and poured into
a beaker. 100 mL solution of ­( NH 4) 2HPO4 was prepared
in another flask, then poured drop by drop into the first
solution and a milky solution was obtained. The pH value
of this solution was adjusted to the value of 10 by adding
an ammonia solution ­( NH4OH, Sigma-Aldrich, France),
this solution was stirred at 65 ºC for 90 min, and it was
put in an oven at 120 ºC for 20 h for drying. The as-dried
sample was calcined in an electric furnace at 850 ºC for
120 min.
X-ray diffraction (XRD) data were recorded by a
Rigaku Rad B-DMAX II diffractometer (Japan) at 40 kV
and 30 mA using Cu-Kα radiation having the wavelength
of 0.15406 nm, scanning at 2θ = 20–60°. Fourier transform infrared (FTIR) data were collected in the wavenumber range from 400 ­c m −1 to 4000 ­c m −1 by a PerkinElmer Spectrum One spectrometer (U.S.A.) using the
KBr method. To record the Raman spectra, Renishaw’s
inVia confocal microscope (United Kingdom) with a
532 nm diode laser was used. A Leo Evo-40 XVP scanning electron microscope (SEM, Germany) was used to
investigate the morphology. Differential thermal analysis
(DTA, Japan) was performed by using a Shimadzu DTA
13
806
50 from room temperature to 1000 °C at a heating rate
of 10 °C ­min −1.
In vitro biocompatibility assay of Bi/Sn co‑doped
HAp samples
Bi/Sn co-doped HAp samples in powder form were
weighed separately 0.1 g each under sterile conditions
and put into sterile tubes with a volume of 2 mL. Then,
1 mL of DMEM was added on each material and was
kept in the incubator for 72 h. L929 (Mouse Fibroblast)
cells were grown in a flask at 37 °C in 5% C
­ O2 incubator
by using high glucose DMEM medium, which is a mixture of 10% FBS and 1% Penicillin–Streptomycin. It was
inoculated into each well in the 96 well plate as 10,000
L929 cells. It was expected to stick and spread in the
­CO2 incubator until the next day. After 24 h, the medium
in the well plate was removed and discarded. Instead,
the media kept with the material for 72 h were carefully
filtered from the materials and added to the wells. Cells
were incubated with material-suspended media for 24 h
in the incubator.
MTT dye was dissolved in PBS (pH 7.4 in phosphate
buffer) to 5 mg/mL. After 24 h, the soaked medium in
the well plate was withdrawn and discarded. Instead, 90
µL of DMEM and 10 µL of 5 mg/mL prepared MTT
(3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium
bromide) mixture was placed in each well and incubated
in a ­CO 2 incubator for 4 h.
After the incubation, the medium with MTT in the
well plate was taken and discarded. 100 μL of DMSO
was added to each well. Well plates were read at 540 nm
wavelengths immediately after loading DMSO.
Results and Discussion
Theoretical results
Bandgap and density of states calculations
Our prior research includes theoretical calculations of
band structure and density of states [35]. Here in this
subsection, a brief overview will be given.
The formula can be used to calculate the density of
states (DOS) as a function of energy
∑ (
)
DOS(E) =
g E − 𝜀i
(1)
where g is a Gaussian with a fixed FWHM, E is the
total energy, and ε i is the energy of the ­ith molecular
orbital.
13
Journal of the Australian Ceramic Society (2022) 58:803–815
The interaction between atoms causes the atomic levels to break into numerous closely related levels, which
is what the band structure and density of states measurements are based on. These are the molecular energy
levels that have been examined in the system. The number of splitting in a solid molecule is determined by the
interatomic distances between two or more atoms. When
the number of nuclei in an atom grows, the number of
splitting grows as well, potentially resulting in a lot more
energy splitting due to overlapping spin-orbits and other
factors. The band structure and density of states of the
system may vary as a result of these splitting.
A density functional theory (DFT) tool was used in
conjunction with the CASTEP Software [36]. Figure 1a1e depicts the estimated density of state and band structure for all synthesized samples of tin (Sn)-based HAp
with doped bismuth (Bi) at various atomic percentages
such as 0.22Sn-HAp, 0.11Bi-0.22Sn-HAp, 0.22Bi0.22Sn-HAp, 0.33Bi-0.22Sn-HAp, 0.44Bi-0.22Sn-HAp.
The band gaps in all figures were measured at an interval of (G-H). Figure 1a shows the band structure calculations and DOS results for an Sn-based HAp molecule.
Tin (Sn) has the electronic configuration [Kr] ­5s24d105p2.
The closed-shell atomic levels of the Krypton configuration result in highly strong bound bands of the remaining
electrons of the Sn atom. The splitting of the 4d level is
bigger than that of the 5 s level due to the relatively small
and near constraint of the 5 s orbital. The energy bandgap of this structure was determined to be 4.5993 eV.
The d orbital contribution to the bands corresponds to
the region in the picture with a large density of states,
which is more dominating at positive energy and even in
a negative energy zone. The low contribution of s orbitals
to the DOS structure can also be expected. Figure 1a-e
shows the HAp molecule with Sn and Bi-doped to it. The
energy gap of the molecule was reduced to 4.4288 eV
from 4.5993 eV by adding atom Bi at various amounts.
The electron structure of bismuth is [Xe] 4­ f145d106s26p3.
Because of its higher energy, when paired with the
Bismuth and other atoms in the HAp molecule, this configuration minimizes the device's energy gap. As energy
levels rise, more splitting happens in general. By introducing the Bi atom into the doping HAp molecule, the
result of a greater energy contribution to the energy gap
can be seen very clearly in Fig. 1b-e.
The energy bandgap values drop as Bi is introduced
to the Sn-based HAp structure, as shown in the figures.
In comparison with the studies reported in the literature,
the bandgap value calculated for each element doped structure is lower than those of the reported ones for the pure
HAp as 4.92, 5.05 and 5.23 eV [37, 38]. For 0.40at%Ce
and 0.47at.%Ce-doped HAp structures, the bandgap values
were reported as 4.6078 eV and 4.5905 eV, respectively
Journal of the Australian Ceramic Society (2022) 58:803–815
Table 2 Comparison between
theoretical and experimental
values of the lattice parameters
and unit cell volume
807
Sample
Theoretical
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
Experimental
a (nm)
c (nm)
V (nm)
a (nm)
c (nm)
V (nm)3
0.9354
0.9348
0.9347
0.9331
0.9324
0.6797
0.6797
0.6794
0.6801
0.6803
0.5150
0.5144
0.5140
0.5128
0.5122
0.9410
0.9421
0.9358
0.9398
0.9363
0.6867
0.6876
0.6838
0.6867
0.6836
0.5266
0.5285
0.5186
0.5252
0.5190
[39, 40]. Additionally, this value was calculated as
5.1915 eV for 0.35at.%Sr-doped HAp [41]. In this work, we
calculated the bandgap value for 0.22at.%Sn-doped HAp
structure as 4.5993 eV. Although the amount of this dopant
is lower than those of both Ce and Sr-doped ones, the bandgap value for the Sn-doped structure is lower than those
structures. This result can be associated with the higher
electrical conductivity of Sn (0.0917 × ­106 Ω−1 ­cm−1) in
comparison with both Sr (0.0762 × ­106 Ω−1 ­cm−1) and Ce
(0.0115 × ­106 Ω−1 ­cm−1) [42].
The lattice parameters and volume of the unit cell values for all the as-modeled structures were also estimated.
These theoretical results are listed in Table 2 together
with the experimental ones to make a better comparison
among them.
3
0.33Bi-0.22Sn-HAp and 0.44Bi-0.22Sn-HAp, respectively. The densities of Sn (7.265 g ­c m −3 ) and Bi
(9.78 g ­c m −3) are higher than that of Ca (1.55 g ­c m −3)
[43]. Even though the amount of Sn was kept at a constant value, the addition of Bi caused an increase in the
density of the HAp system. Briefly, the higher amount
of Bi means the higher density.
The linear absorption coefficient (LAC) as a function of photon energy for each structure is illustrated in
Fig. 2. For all structures, with rising photon energy, a
downward tendency persists. Additionally, the LAC value
increases gradually with increasing amount of Bi. The
Indexed peaks: HAp (JCPDS 09-0432)
: β -TCP (JCPDS 09-0169)
0.44Bi-0.22Sn-HAp
Density and linear absorption coefficient calculations
1200
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
1000
710
800
0.33Bi-0.22Sn-HAp
Intensity (a.u.)
Using the as-modeled structures, both density and linear
absorption (or attenuation) coefficients were computed
and reported in this section. The density values were estimated as 3.154, 3.161, 3.168, 3.173, and 3.179 g ­cm−3 for
0.22Sn-HAp, 0.11Bi-0.22Sn-HAp, 0.22Bi-0.22Sn-HAp,
0.22Bi-0.22Sn-HAp
700
0.11Bi-0.22Sn-HAp
600
(211)
630
(322)
(313)
(213)
(321)
(410)
(402)
(004)
(222)
(312)
(203)
(302)
(113)
(212)
(310)
(210)
(200)
(111)
200
0.22Sn-HAp
(301)
7
(202)
6
(0 2 10)
5
(002)
610
(112)
(300)
620
400
(102)
µ (cm-1)
690
0
5
10
15
20
25
30
Energy (keV)
Fig. 2 The linear absorption coefficient dependence on the energy
and composition of the as-modeled systems
20
30
40
2θ (°)
50
60
Fig. 3 XRD patterns of the Sn-based HAp samples doped with Bi at
various amounts
13
808
Sample
XC%
DS (nm)
DWH (nm)
ε × ­10–4
σ (MPa)
u (kJ m
­ −3)
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
71.693
71.698
71.811
72.380
68.499
28.87
26.66
27.75
25.93
26.76
26.26
26.01
23.15
23.07
22.18
-2.085
-1.606
-8.318
-6.168
-9.509
-226.496
-317.347
126.002
-288.186
212.803
420.595
760.892
33.663
911.616
117.965
as-observed results are in very good agreement with the
literature and can be evaluated that due to their weakening property for electromagnetic radiation, all of the
samples are ideal candidates for radiation shielding and
medical applications. [35, 41, 44].
Experimental results
XRD analysis
Figure 3 shows the XRD results of Sn/Bi co-doped HAp
samples. For the 0.44Bi-0.22Sn-HAp, a single-phase
distribution of the HAp (JCPDS pdf no: 09–0432) was
observed. For the rest samples, the major phase is the
HAp, and the minor phase is the beta-tricalcium phosphate (β-TCP, JCPDS pdf no: 09–0169). The amounts of
the β-TCP phase were computed as 1.04%, 2.44%, 1.26%,
0.95% and 0% for 0.22Sn-HAp, 0.11Bi-0.22Sn-HAp,
0.22Bi-0.22Sn-HAp, 0.33Bi-0.22Sn-HAp and 0.44Bi0.22Sn-HAp, respectively. It was seen that the amount
of the as-mentioned phase was affected significantly by
Bi-content.
The calculation of the lattice parameters (a and c) and
unit cell volume (V) was carried out using the following
Eq. (2) and Eq. (3) [45]
(
)
1
4 h2 + hk + k2
l2
=
+
(2)
3
d2
a2
c2
V = 0.866a2 c
(3)
where d corresponds to the distance between adjacent planes. Both theoretical and experimental calculation results for the parameters of a, c and V are given in
Table 2. While the theoretical results imply that there is a
gradual decrease in the lattice parameter of the a and unit
cell volume, the experimental ones point out that there
are non-gradual changes in the lattice parameters and
unit cell volume. The addition of Bi to the HAp structure
affects significantly all of these three parameters.
Using the intensity of the pit (V112/300) between (112)
and (300) reflections and the intensity (I 300 ) of (300)
reflection, the crystallinity percent (XC%) value for each
sample was estimated by using Eq. (4) [46].
13
XC % =
(
1−
V112∕300
I300
× 100
)
(4)
The as-computed values of the crystallinity percent
for all the samples are listed in Table 3. The introduction of Bi into the HAp structure causes variations in the
crystallinity and the XC% values for all the samples are
in the range of 68.5%-72.4%.
For the estimation of the crystallite size for all the
samples, the following Scherrer (D S) and WilliamsonHall (DWH) equations were used, respectively [41]
0.8
0.0100
0.0075
1.0
1.2
1.4
1.6
1.8
1.4
1.6
1.8
0.44Bi-0.22Sn-HAp
0.0050
0.0025
0.0000
0.0075
0.33Bi-0.22Sn-HAp
0.0050
0.0025
0.0000
cos
Table 3 The calculation results
for the parameters of the
crystallinity percent, crystallite
size, lattice strain, stress and
anisotropic energy density
Journal of the Australian Ceramic Society (2022) 58:803–815
0.0075
0.22Bi-0.22Sn-HAp
0.0050
0.0025
0.0000
0.0075
0.11Bi-0.22Sn-HAp
0.0050
0.0025
0.0000
0.0075
0.22Sn-HAp
0.0050
0.0025
0.0000
0.8
1.0
1.2
4sin
Fig. 4 The βcosθ vs. 4sinθ plots of the Sn-based HAps doped with Bi
Journal of the Australian Ceramic Society (2022) 58:803–815
DS =
0.9𝜆
𝛽cos𝜃
𝛽cos𝜃 =
0.9𝜆
+ 4𝜀sin𝜃
DWH
809
0.9𝜆 4𝜎sin𝜃
+
DWH
Y
(5)
𝛽cos𝜃 =
(6)
where σ and Y are the lattice stress and Young’s
modulus, which can be calculated by using Eq. (8),
respectively.
where β, λ, ε and θ correspond the full width at half
maximum, wavelength of the incident X-rays, lattice
strain and angle of Bragg diffraction, respectively. Using
Eq. (6), the value of the DWH for each sample was found
from the intercept of the βcosθ vs. 4sinθ shown in Fig. 4.
The ε parameter was also calculated using the slope of
each plot in the same figure. As can be seen from the
results given in Table 3, the crystallite size is affected by
the amount of Bi. Non-gradual changes for the D S value
and a gradual decrease for the DWH value were observed.
All the as-estimated values of the ε are negative. This is
associated with the compressive strain [47].
Using the relation of the ε = σ/Y, a new relationship
can be derived as
(7)
[
h2 +
Y=
(
7.49 × 10−12 h2 +
)2
2
(h+2k)
3
(h+2k)2
3
+ 10.9 × 10−12
+
( )2 ]2
al
c
( )4
al
c
(
− 7.1 × 10−12 h2 +
(h+2k)2
3
)( )2
al
c
(8)
where the h, k and l are Miller indices. From the slope
of the βcosθ vs. 4sinθ Y −1 plots illustrated in Fig. 5, the
σ values of the samples were estimated. The positive and
negative values are respectively due to the tensile and
compressive stresses [48].
To compute the anisotropic energy density (u) value,
the following expression may be used [41]
𝛽cos𝜃 =
( )1∕2
2u
0.9𝜆
+ 4sin𝜃
DWH
Y
(9)
The u values were calculated from the βcosθ vs.
­25/2sinθ Y −1/2 plot shown in Fig. 6. All the as-estimated
0.0100
0.0075
0.0100
0.44Bi-0.22Sn-HAp
0.0075
0.0050
0.0050
0.0025
0.0025
0.0000
0.0075
0.0000
0.33Bi-0.22Sn-HAp
0.0075
0.0050
0.0025
0.0000
0.22Bi-0.22Sn-HAp
β cosθ
β cosθ
0.0000
0.0050
0.0025
0.0075
0.0050
0.0000
0.11Bi-0.22Sn-HAp
0.0075
0.0050
0.0050
0.0025
0.0025
0.11Bi-0.22Sn-HAp
0.0000
0.0000
0.0075
0.22Bi-0.22Sn-HAp
0.0025
0.0000
0.0075
0.33Bi-0.22Sn-HAp
0.0050
0.0025
0.0075
0.44Bi-0.22Sn-HAp
0.22Sn-HAp
0.0075
0.22Sn-HAp
0.0050
0.0050
0.0025
0.0025
0.0000
4.0x10-125.0x10-126.0x10-127.0x10-128.0x10-129.0x10-121.0x10-111.1x10-11
0.0000
3.0x10-6
4sin Y
Fig. 5 The βcosθ vs. 4sinθ Y
with Bi
−1
-1
-1
(Pa )
graphs of the Sn-based HAps doped
3.5x10-6
4.0x10-6
5/2
2 sin Y
4.5x10-6
-1/2
(Pa
5.0x10-6
5.5x10-6
-1/2
)
Fig. 6 The βcosθ vs. 2­ 5/2sinθ Y −1/2 plots of the Sn-based HAps doped
with Bi
13
810
Journal of the Australian Ceramic Society (2022) 58:803–815
Fig. 7 FTIR results of the asprepared HAps
0.44Bi-0.22Sn-HAp
Transmittance (a.u.)
0.33Bi-0.22Sn-HAp
3560
3570
3580
3590
0.22Bi-0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Sn-HAp
4000
3600
3200
2800
2400
2000
1600
1200
800
400
Wavenumber (cm -1)
FTIR and Raman spectroscopy results
Figure 7 shows FTIR plots of the as-produced HAp samples. The bands belonging to both phosphate and hydroxyl
groups were observed. The bands at the wavenumber positions of 466 ­cm−1, 561 ­cm−1, 594 ­cm−1, 957 ­cm−1, and
1018 ­cm−1 are related to the vibration modes of the phosphate group [50–52]. The bands detected at 636 ­cm−1 and
3569 ­cm−1 are related to the vibration mode of the hydroxyl
group [53, 54]. All the as-observed bands on the FTIR
spectra confirm the formation of the HAp structure for
each sample [55]. The intensities of the band of 3569 ­cm−1
related to the hydroxyl group for all the Bi-doped samples
are lower than that of the Bi-free sample. Also, the intensity of the band centered at 1018 ­cm−1, belonging to the
phosphate group, are significantly affected by Bi-content.
These changes in the band intensities support the influence
of the dopant(s) into the HAp structure.
Figure 8 illustrates the Raman spectra of the samples recorded over the range of 200—4000 ­c m −1 . For
all the samples, five intense bands at the positions
of 435 ­c m −1 , 585 ­c m −1 , 962 ­c m −1 having the highest
intensity, 1039 ­c m −1, and 3574 ­c m −1 were observed on
the Raman spectra. The bands observed at 435 ­c m −1 ,
585 ­cm−1, 962 ­cm −1 and 1039 ­c m−1 are associated with
the symmetric bending (υ2), antisymmetric bending (υ4),
13
symmetric stretching (υ1) and antisymmetric stretching
(υ3) modes of the phosphate group, respectively [56, 57].
The band detected at 3574 ­cm−1 is related to the stretching mode of the hydroxyl group [56]. With the adding
of Bi to the HAp structure, significant changes in the
intensities of the bands were detected. Especially for the
intensities of the bands of 962 ­cm−1 and 3574 ­cm−1, quite
obvious changes were seen.
Morphological investigations
SEM images and EDX results of the as-produced co-doped
HAps are shown in Fig. 9. All the samples are consisted of
0.44Bi-0.22Sn-HAp
Intensity (a.u.)
u values given in Table 3 are higher than the value of
12 kJ ­m−3 reported for the un-doped HAp [49].
0.33Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Sn-HAp
0
400
800
1200
1600
2000
2400
2800
Raman shift (cm -1)
Fig. 8 Raman spectra of the as-synthesized HAps
3200
3600
4000
Journal of the Australian Ceramic Society (2022) 58:803–815
811
Fig. 9 SEM images and EDX analysis report for each sample
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812
Journal of the Australian Ceramic Society (2022) 58:803–815
be caused by the deposition of this element on the HAp
surface. Additionally, the Ca/P molar ratios of the samples
were found to be 0.94, 0.99, 0.97, 1.05, and 0.94 for 0.22SnHAp, 0.11Bi-0.22Sn-HAp, 0.22Bi-0.22Sn-HAp, 0.33Bi0.22Sn-HAp and 0.44Bi-0.22Sn-HAp, respectively.
60
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
Heat flow (mW)
50
40
Differential thermal analysis
30
20
10
0
200
400
600
800
1000
Temperature (°C)
Fig. 10 DTA thermograms of the as-prepared HAp samples
stacked nanoparticles having sphere-like shapes. The EDX
results support the introduction of both Sn and Bi into the
HAp structure. With the addition of Bi, the as-detected Bi
content increases as expected, but the introduction of Bi
into the apatitic structure is limited. For this reason, it can
be said that the higher amounts of the dopant of Bi may
In vitro biocompatibility result of Bi/Sn co‑doped HAp
samples
60
Mouse fibroblast cells (L-929) of the genus Mus musculus
were used in the biocompatibility test of the formulations
carried out by the indirect method. Cell viability results
were given Fig. 11 and Table 4. The biocompatibility of
the HAp samples co-doped with Bi and Sn is above 80%.
The reduction of cell viability by more than 30% is considered as a cytotoxic effect for biomaterials according to
ISO-10993–5. Therefore, we can say the all the HAp samples have acceptable biocompatibility. The image of L-929
cells exposed with samples was also given Fig. 12. These
images were confirmed our cell viability result in term of
cell morphology.
40
Conclusions
120
Cell Viability (%)
DTA curves of the as-prepared HAp samples co-doped with
Sn and Bi are illustrated in Fig. 10. For all the samples, an
exothermic peak, which is due to the removal of the physically
adsorbed water molecule, which is weakly bound to the sample, is observed in the temperature range of 200–350 °C [58].
These peaks are detected as 214.50 °C (weak), 284.98 °C
(sharp), 214.53 °C (weak), 244.54 °C (sharp), and 227.04 °C
(sharp) for 0.22Sn-HAp, 0.11Bi-0.22Sn-HAp, 0.22Bi-0.22SnHAp, 0.33Bi-0.22Sn-HAp and 0.44Bi-0.22Sn-HAp, respectively. It can be concluded that Bi content affects significantly
the thermal behavior of the sample.
100
80
20
C
O
N
0.
11 0.2 TR
O
B 2
0. i-0 Sn- L
2 2 .2
H
B 2 S Ap
0. i-0 n33 .2
H
B 2 S Ap
0. i-0 n44 .2
H
B 2 S Ap
i-0 n
.2 - H
2 S Ap
nH
A
p
0
Fig. 11 Effect of the Bi/Sn co-doped HAp samples on the viability of
L929 mouse fibroblast cells
13
For the first time, HAp samples doped with Bi and Sn
were prepared and characterized theoretically and experimentally. Theoretically, adding Bi to a Sn-based HAp
Table 4 Effect of the Sn-HAp samples on the viability of L929
mouse fibroblast cells
Sample
Cell Viability %
CONTROL
0.22Sn-HAp
0.11Bi-0.22Sn-HAp
0.22Bi-0.22Sn-HAp
0.33Bi-0.22Sn-HAp
0.44Bi-0.22Sn-HAp
100.00 ± 6.35
89.63 ± 3.35
96.98 ± 8.58
82.32 ± 6.24
90.07 ± 8.12
97.28 ± 6.77
Journal of the Australian Ceramic Society (2022) 58:803–815
813
Fig. 12 Optical microscopic
images of L-929 cells treated
with the Bi/Sn co-doped HAp
samples
structure results in a progressive increase in density and
continuous decreases in the bandgap and linear absorption coefficient. The lattice parameters were affected by
the dopant of Bi. Significant variations in the crystallite size and crystallinity percent were detected. It was
observed that the addition of Bi to Sn-based HAp caused
the formation of the lattice strain and stress. Both Raman
and FTIR spectra verified the formation of the HAp
structure for each sample. Nano-sized morphology was
observed for all the samples. The thermal behavior of the
Sn-based HAp was affected significantly by Bi content.
All the as-prepared samples in this study can be accepted
as the biocompatible material and can be used in biological and medical applications.
Declarations
Conflicts of interest The authors declare that they have no conflict
of interest.
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