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Biomaterials Research (2011) 15(2) : 52-59

52

Biomaterials

Research

C The Korean Society for Biomaterials

Synthesis, Characterization and Electrical Properties of Hydroxyapatite Nanoparticles from Utilization of Biowaste Eggshells

Nadia Abdel Aal1, M. Bououdina2,3, A. Hajry2, A. A. Chaudhry3,4, J. A. Darr4, A. A. Al-Ghamdi5,E. H. El-Mossalamy6, Attieh A. Al-Ghamdi7, Yong Kiel Sung8,9*, and Farid El-Tantawy2,10

1Department of Chemistry, Faculty of Science, Suez Canal University, Ismailia, Egypt2Department of Physics, Faculty of Science, King Khalid University, Abha 9004, Kingdom of Arabia Saudi3Department of Physics, Faculty of Science, Universityof Bahrain, Kingdom of Bahrain4Department of Chemistry, University College London, 20 Gordon Street, London, WC1H 0AJ, UK5Department of Physics, Faculty of Science, King Abdulaziz University, Jeddah, Jeddah 21569, Kingdom of Saudi Arabia6Department of Chemistry, Faculty of Science, King Abdulaziz University, Jeddah, Jeddah 21569, Kingdom of Saudi Arabia7King Abdulaziz City for Science and Technology, Electronic, Communication & Photonics Research Program Riyadh, Kingdom of Saudi Arabia8ReSEAT Program, Korea Institute of Science and Technology Information, 66 Hoegiro, Dongdaemun-gu, Seoul 130-741, Korea9Department of Chemistry, College of Science, Dongguk University, 3Ga-26, Phil-dong, Chung-gu, Seoul 100-715, Korea10Department of Physics, Faculty of Science, Suez Canal University, Ismailia, Egypt(Received March 7, 2011/Acccepted May 4, 2011)

Hydroxyapatite(HAP) nanoparticles were successfully prepared from the biowaste chicken eggshells and phosphoricacid solution by chemical precipitation methods. The structures of HAP were characterized in terms of the X-raydiffraction (XRD), scanning electron microscope (SEM), and Fourier transform-infrared (FT-IR) spectroscopy. Theeffect of temperature on the crystallinity and lattice parameters of prepared HAP was monitored by X-ray powderdiffraction. Energy dispersive X-ray analysis revealed that the HAP had a Ca/P molar ratio of about 1.65 and 1.69for as prepared and sintered HAP at 1200oC (1 hour), respectively. The temperature dependence on the electricalconductivity of sintered HAP nanoparticle was investigated. Furthermore, dielectric properties such as dielectric con-stant and dielectric loss as a function of frequency for the sintered HAP were also studied.

Key words: hydroxyapatite, nanoparticle, biowaste, eggshell, precipitation, microstructure, electrical conductivity,biomaterials

Introduction

unctionalization of nanomaterials with chemical or biologi-

cal molecules exhibits novel properties for various potential

applications. Synthetic hydroxyapatite, Ca10(PO4)6(OH)2 (HAP),

is analogous to natural apatite, the major inorganic component

of bone and teeth.1-3) HAP has extensively been studied as

filler for bulk bone regeneration and for improving oesteointe-

gration of biomedical implants due to its bioactive and osteo-

conductive properties.4,5) However, an important issue for in

vivo application is its biocompatibility. When bone is lost due

to injury, the defects are sometimes filled with natural bone.

However, as natural bone is often of variable quality and if it is

not from the same person as the recipient, it may lead to pos-

sible disease transmission. Synthetic HAP with well defined

properties offers a good, safer alternative bone graft material

which can help the body to repair bone defects.6,7) Recently,

HAP has found interesting applications in other areas such as a

support for adsorption of bacteria or viruses, for ammonia

catalysis, as a catalyst support material, support bone ingrowth

and osseointegration when used in orthopaedic, dental and

maxillofacial applications.8-10) Thereof, the synthesis of HAP has

been a major subject for chemists and material scientists for

many years since it is one of the most important and versatile

biomaterials.11-14) With the ever-growing need to develop clean,

non-toxic and environmentally friendly techniques, HAP pow-

ders have been extracted using bio- products like corals,1,15)

cuttlefish shells,2,16) natural gypsum,3,17) natural calcite,4,18)

bovine bone,5,19) etc. Chemical analysis has shown that these

products considered as biowaste are rich sources of calcium in

the form of carbonates and oxide. One of such biowastes is

chicken eggshells. Everyday million tons of eggshells are gener-

ated as biowaste around the globe. Eggshell represents 11% of

the total weight of the egg and is largely composed of calcium

F

*Corresponding author: [email protected]

Synthesis, Characterization and Electrical Properties of Hydroxyapatite Nanoparticles from Utilization of Biowaste Eggshells 53

Vol. 15, No. 2

carbonate (minerals calcite) 97% and organic matter 3%.21,22)

This material is an unwanted byproduct after the production of

eggs and egg derivatives. Normally, manufacturers and factories

store the material or dispose of it as an unwanted polluting

industrial residues.23-26) The aim of the present work is to inves-

tigate the feasibility of synthesizing HAP nanoparticles by utiliza-

tion of biowaste eggshells and phosphoric acid using an

aqueous chemical precipitation method. The microstructure

prepared HAP were examined by means of X-ray powder dif-

fraction, scanning electron microscopy(SEM) and FT-IR spec-

troscopy. The effect of temperature on the crystallinity and

lattice parameters of HAP nanoparticles were displayed by X-

ray probe. The temperature dependence on the electrical con-

ductivity of HAP was investigated. Finally, dielectric constant

and dielectric loss of HAP against frequency were also studied

for their characterization.

Experimental

Preparation of HAP nanoparticlesHydroxyapatite(HAP) nanoparticles were prepared by chemi-

cal precipitation via aqueous slurries comprising of biowaste egg-

shells and phosphoric acid solution. In the first step, uncrushed

and washed raw eggshells were collected and mechanically

cleaned, followed by washing alternatively with 100 mL of dis-

tilled water and 100 mL of acetic acid for a total of three times

each followed by a final water wash. The treated eggshells were

then placed in an oven for a three stage thermal treatment.

These were 150oC for 5 hours (to remove the water and acid),

500oC for 3 hours (to remove any remaining organic residue)

and finally 1000oC for 2 hours to transform the eggshells to

calcium oxide (via loss of carbon dioxide).3,4) The CaO from

the heat-treated eggshells was transformed into HAP in a phos-

phate solution using the following procedure; 100 g of CaO

from cleaned and heat treated eggshells were dissolved under

stirring in 119 g phosphoric acid, then 7 g of urea was added

along with 15 g of ethylenediaminetetraacetic acid (EDTA). This

was then added to 1600 mL of distilled water to form a solu-

tion. The pH of the solution was then adjusted to 10 using

ammonia solution at 95oC. The reaction mixture was kept for

24 hours to allow nanorocrystalline growth. After this time, the

slurry was centrifuged (5000 rpm for 2 minutes) and the result-

ing sludge was thoroughly washed with distilled water until the

pH value decreased to 7 after re-centrifuging. The resulting

sludge was homogeneously suspended in distilled water and fil-

tered using Buchner funnel (20 cm) with application of mild

suction. After filtration, precipitate was dispersed in 100 ml eth-

anol (and ultrasonicated for 1 hour) and then dried in air for

24 hours. The powder was then further dried at 400oC, 500

to 1200oC (in 100oC intervals) using a muffle furnace and a

heating and cooling rate of 10oC min−1 with a 2 hour holding

time at each target temperature.

CharacterizationThe crystal phase and structure of the samples were deter-

mined by X-ray diffraction (XRD) using a Shimadzu diffractome-

ter (Shimadzu-Japan) with CuKα radiation (λ = 0.15418 nm).

The accelerating voltage of 35 KV and emission current of

30 mA was used. Variable temperature Powder XRD (PXRD)

data was collected using Shimadzu X-ray powder diffractometer

(model XRD-6000) equipped with vacuum heating unit (HA-

1001) from room temperature to 1200oC in air at 100oC inter-

vals. The heating unit consists of special sample holder with alu-

mina (Al2O3) sample holder, temperature controller (± 5K) and

Pt-Rh thermocouple to monitor temperature of the sample dur-

ing measurements. The X-ray diffraction data were analysed

using profile refinement method and Fullproof software.26) The

fraction of crystalline phase (Xc) in the HAP powders can be

evaluated by the following equation:9)

Xc = 1 − (V112/300/I300) (1)

where I300 is the intensity of (300) diffraction peak and V112/

300 is the intensity of the hollow between (112) and (300) dif-

fraction peaks of hydroxyapatite. Peak broadening of X-ray

reflection can be used to estimate the crystallite size in a

direction perpendicular to the crystallographic phase based on

the following Debye Scherrer equation:25)

(2)

where D is the size in Å measured using (002), k the shape

factor equal to 0.9, λ the wavelength of X-ray equal to

1.5418 Å, θ the diffraction angle equal to 12.92o for the reflec-

tion (002) and β1/2 is defined as the diffraction peak width at

half height expressed in radians. Microstructure and chemical

composition were determined using JEOL 6300 Scanning Elec-

tron Microscope (SEM) equipped with JEOL Electron Dispersive

Spectrometer (EDS) at a 5 KV accelerating potential. Prior to

observation, the surface of the samples was coated with a thin,

electric conductive copper film. Fourier transformed infrared

(FTIR) spectroscopy using FT-IR 1750 (Perkin-Elmer Instruments),

was performed in order to follow the chemical evolution from

the gel to HAP and to aid structural information gathered from

X-ray diffraction. To obtain the observable absorption spectra,

the dilution and homogenization of the dried, fine samples with

KBr (spectroscopic grade), was carried out using manual grind-

ing and mixing in an agate mortar. Discs (ca. 20 mm dia and

1 mm thick) were prepared in a uniaxial press under 5 tonnes

pressure. The spectrum was measured and recorded in the

wavenumber range of 400-4000 cm−1. Electrical conductivity

was measured by the four-point probe method. Rectangular

shape samples of dimensions 4 × 3 × 3 cm were cut from the

sintered pellets and then coated with indium electrodes. The

measurements (in the temperature range 273 to 700K) were

Dkλ

β1 2⁄

θcos--------------------------=

54 Nadia Abdel Aal et al.

Biomaterials Research 2011

performed in an in-house made cell which incorporate a fully

automatic computer–controlled system for temperature varia-

tion and data acquisition and processing. The dielectric proper-

ties such as dielectric constant and dielectric loss were carried

out using RLC bridges (Grounding RLC- Germany) with varying

frequency in the range 1 kHz to 10 MHz. The densification fac-

tor (DF) is calculated using the following relation:1,2)

(3)

where ρg is the green density of HAP and ρt is the density on

heating HAP for a time t. The green and sintered densities of

compacts were obtained by Archimedes method from the

measurement of dimensions and sample mass. The specific sur-

face area measurements were performed using BET method.

The flexural strength (σ) was determined as per ASTM D638.

The measurements were done using a universal testing machine

(Model TNE 5000) at a crosshead speed of 10 mm min−1. Rect-

angular specimens of 100 × 10 × 3 mm3 were used for deter-

mining flexural strength. The flexural strength (K) was calculated

using the following relation:5,8)

(4)

where PN is the load at failure, a is the span length, b and

d are the breadth and thickness of the specimen, respectively.

Results and Discussion

Figure 1(a-d) shows the XRD patterns of the HAP nanopow-

ders calcined at different temperatures from 400oC, 700oC,

1000oC and 1200oC, respectively. The HAP nanopowders

were characterized by broad peaks with no secondary phases

indicating that the synthesized HAP was pure and in a good

match to JCPDS pattern 09-432) with a crystallite size of about

22 nm as calculated using the Debye Scherrer equation. It is

seen that the crystallinity of HAP nanopowders increased with

heat treatment temperature. Broad diffraction peaks were

observed for the HAP powders heat-treated at 700oC. With

increase in heat-treatment temperature, these diffraction peaks

became sharper as expected, indicating the increase in crystal-

linity of HAP powders. Crystal structure refinements were car-

ried out using X-ray diffraction data for qualitative and

quantitative phase analyses. The Reitveld refinements confirm

the formation of only phase pure HAP. Typical structure refine-

DFρg

1– ρt1–

–

ρg1–

------------------------=

K3PNa

bd2---------------=

Figure 1. X-ray diffraction patterns of: (a) as-prepared HAP(Green HAP), (b) annealed HAP at 700oC, (c) annealed HAP at1000oC and (d) annealed HAP at 1200oC.

Figure 2. Rietveld profile refinements of: (a) green HAP and (b)HAP heat-treated at 1200oC. Continuous line: calculated pro-file, empty circles o: experimental recorded data, bottom line—: difference pattern of Iobs – Ical, bottom line | : correspondsto positions of diffraction peaks.


Vol. 15, No. 2

ment patterns of green i.e. as prepared and sintered HAP at

1200oC are shown in Figures 2(a) and 2(b), respectively. The

green HAP sample was heat-treated at different temperatures

to assess phase stability [see Figures 2(a) and 2(b)]. It is noted

also that the HAP powders are stable at high temperatures

and there was no secondary phase formation which is rather

surprising compared to the literature.10-15) The Reitveld refine-

ments also confirm the stability of the HAP phase without any

decomposition or formation of secondary phases (see Table 1

and Figure 2b).

Crystallite sizes were calculated from X-ray data and as

expected, increasing heat-treatment temperature resulted in an

increase in crystallite size as illustrated in Figure 3. a- and c-

axis lattice parameters of the HAP crystal structure at various

Figure 3. The crystallite size of HAP with sintering temperature.

Table 1. Crystal structure refinements results using Fullproof program for selected HAP samples

Lattice parameters(Å)

Atoms Coordinates Occupancy

x y z

Green HAP a = 9.413(1) Ca1(4f) 1/3 1/3 0.001 1.008

c = 6.882(1) Ca2(6h) 0.246(1) −0.008(1) 1/4 0.994

O1(6h) 0.328(1) 0.466(2) 1/4 1.038

O2(6h) 0.595(2) 0.471(2) 1/4 0.994

O3(12i) 0.334(1) 0.253(1) 0.081(1) 0.920

O4(4e) 0.0 0.0 1/4 0.531

P1(6h) 0.409(2) 0.369(1) 1/4 0.982

HAP 1200oC a = 9.424(1) Ca1(4f) 1/3 1/3 0.001 1.008

c = 6.887(1) Ca2(6h) 0.245(1) −0.005(1) 1/4 1.018

O1(6h) 0.326(1) 0.475(2) 1/4 1.052

O2(6h) 0.584(2) 0.469(2) 1/4 0.990

O3(12i) 0.332(1) 0.250(1) 0.073(1) 1.052

O4(4e) 0.0 0.0 1/4 0.516

P1(6h) 0.399(1) 0.363(1) 1/4 1.036

Figure 4. The variation of lattice parameters a-axis and c-axisof green HAP with temperature as obtained from in situ pow-der X-ray diffraction measurements.



temperatures are illustrated in Figure 4. For the in situ XRD, it

can be clearly seen that the a-axis decreases while c-axis

increases with increasing temperature. Figures 5(a) and 5(c)

show the images of green and heat-treated HAP powder,

respectively. Agglomerates of very fine particles were noticed

[Figure 5(a)] for the green sample while after heat-treatment at

1200oC, the particles become sub-100 micron in size [Figure

5(c)]. The chemical composition analysis was carried out using

EDS and the results are shown in Table 2. It was observed that

the ratio Ca/P for both the green and heat-treated samples is

ca. 1.69, which can be compared to a value of 1.67 which

would be expected for fully stiochiometric HAP.

Typical EDS spectra for green and heat-treated HAP are

shown in Figures 5(b) and 5(d), respectively. The chemical anal-

yses do not show any significant effect of heat treatment tem-

perature on the stoichiometry. No additional peaks that could

be attributed to impurities can be observed, which confirms

the formation of highly pure HAP.

Crystallinity, densification factor, surface area and flexural

strength of green and heat treated of HAP at different temper-

ature are reported in Table 3. Based on Table 3 results, higher

of densification factor tend to harden the sintered HAP bodied

due to the interlocking of crystalline phase and plate-like

microstructure of HAP. It is also noted that the crystallinity and

surface area of the HAP sample increase with the increase of

heat treatment temperature. The flexural strength mainly

depends on the density and grain size of HAP sample. Though

the grain size of sintered sample increases with heat treatment

Figure 5. SEM image of (a) as-prepared (Green) HAP, (b) its EDS spectrum, (c) SEM image of HAP sintered at T = 1200oC, and (d)its EDS spectrum.

Table 2. Chemical analysis using EDS technique for HAP

Elements Ca P O Ca/P

Green HAP 23.6(2) 14.2(1) 62.2(2) 1.65 (0.03)

HAP 1200oC 23.8(2) 13.8(1) 62.3(2) 1.72 (0.03)

Table 3. Fraction of crystalline phase (Xc), densification factor (DF), flexural strength and surface area for selected HAP samples

Sample code Crystallinity (%) Densification factor (%) Flexural strength(M Pa) Surface area (m2 g-1)

Green HAP 52 41 36 82

HAP, 700oC, 2hrs 59 49 47 84

HAP, 1000oC, 2hrs 70 67 76 87

HAP, 1200oC, 2hrs 77 88 98 90


Vol. 15, No. 2

temperature, the relative density becomes obviously higher

with low porosity, which results in the higher flexural strength.

In addition, the increase in the flexural strength with sintering

temperature can be elucidated by higher neck bonding

among HAP nanoparticles due to the formation of liquid

phase at high temperature.

The structure of the powder was analyzed using FT-IR spec-

troscopy after dried at 100oC, as shown in Figure 6. The FT-IR

spectra exhibit distinct strong bands for green and heat-treated

HAP at 1200oC for 2 hours. In the FT-IR analysis, mainly the

peaks for PO43− and OH− groups in the hydroxyapatite can be

identified in Figure 6. These FTIR spectra reveal peaks similar

to those expected for HAP.12) The peak at 3571 cm−1 (strong)

and 633 cm−1 correspond to the stretching mode (υs) and the

liberational mode (υL) of hydroxyl, respectively. The peaks at

1430 and 1410 cm−1 correspond to stretching mode (υ3) of car-

bonate in HA. It can be seen that the band decreases in inten-

sity in Figure 6 at 1200oC after heat-treatment for 2 hours. This

is due to a reduction in carbonate (as carbon dioxide) at heat-

treatment elevated temperatures. The band at 1043 cm−1 corre-

sponds to asymmetric P-O stretching (υ3) in phosphate. The

peak at 962 cm−1, corresponds to symmetric P-O stretching (υ1)

of phosphate. The weak peak at 880 cm−1 corresponds to

bending mode (υ2) of carbonate which also diminishes at high

temperatures analogous to the peaks at 1430 and 1410 cm−1.

Peaks at 602 and 564 cm−1 correspond to the bending mode

(υ4) of the O-P-O linkage. The temperature dependence of the

electrical conductivity (σ) of heat-treated HAP up to 700oC is

shown in Figure 7. It is seen that the conductivity increases with

increasing temperature. At relatively low temperatures the elec-

trical conductivity slightly increases. This can be explained as

frozen equilibrium between defects and charge carriers. With

increasing temperature, the conductivity rapidly increased.

There are two possible reasons for increasing conductivity with

temperature. Firstly, the hydroxyl ions migration through vacan-

cies in the center of Ca ion triangle planes along the c-axis of

the hexagonal structure of HAP. In addition, the transport of

OH− vacancies along the c-axis enhanced the conduction

mechanism in the conductivity of HAP. Secondly, the increase

of charge carrying proton density due to OH− and O2− leads to

the increase of conductivity as confirmed by FT-IR analysis

before. The conduction mechanism in HAP is due to either to

a proton hopping along the c-axis between electro active O2−

anions as follows:

OH− + OH

−↔ O2− + HOH (5)

And/or to an OH− ions interacting with the double bonded

oxygen of a PO43− groups such as

(6)

OH - PO4

[ ] 3-+ O 2- HPO

4[ ] 2-

+

O 2- PO4

[ ] 3- HOH+ +

Figure 7. Electrical conductivity versus temperature for sin-tered HAP nanoparticles.

Figure 6. The comparative study of FT-IR spectra of green HAPand HAP heat-treated at 1200oC for 2hrs.



In both cases, protons must be delocalized between O2−

anions in channels or PO43− groups. Dehydration appears to be

always initiated by thermally activated excess protons delocal-

ized on the conduction. In this case, excess protons move

through the lattice and become trapped onto OH− lattice sites.

H2O molecules are formed which may eventually be repelled

from the lattice, if they happen to be near the surface (or they

can be re-dissolved in apatite structure).

On the basis of σ(T) obtained, the activation energy (Ea) of

the charge conduction process can be determined according to

the following equation:20,21)

σ = σ0 e−Ea/KT (7)

where σ stands for electric conductivity, σ0 for pre-exponen-

tial factor, T for temperature, and K for the Boltzmann's con-

stant. The estimated value of Ea was ca. 1.77eV and this was in

good agreement with published data.14,15,16)

The hopping energy (Eh) was calculated using the following

relation:19,23)

(8)

The calculated value of Eh was about 2.91eV. Notably, the

value of Ea was much less than the value of Eh. This strongly

suggested that the conduction mechanism of HAP was gov-

erned by a proton hopping mechanism.21,24) The dielectric con-

stant and dielectric loss versus frequency for sintered HAP at

1250oC is presented in Figure 8. Notably, as we start increasing

the frequency, both the dielectric constant and dielectric loss

decrease. Dielectric absorption was not recorded beyond

106 Hz. There can be three possible reasons which evoked the

observed decrease of both dielectric constant and loss with

increasing frequency. Firstly, the relaxation time and ionic spe-

cies of dielectric polarization are greatly reduced by increasing

frequency, resulting in decreasing both dielectric constant and

dielectric loss. Secondly, the decrease of dielectric constant

with frequency is due to the coulomb interactions of the

charge carriers and the disorder within the structure, suggesting

a Maxwell Wagner mechanism.26) Thirdly, interfacial polariza-

tion is expected as dispersion occurs in the bulk properties

from the charging of interfaces within the matrix.

Conclusion

Hydroxyapatite nanoparticles can successfully be produced

by chemical precipitation technique from biowaste eggshells

and phosphoric acid solution as starting materials. Furthermore,

the use of biowaste eggshells via this method should also be

applicable for the synthesis of other bioactive calcium phos-

phate and related biomedical materials. It has good prospect

for commercialization as the CaO source is an unwanted and

sustainable industrial byproducts. The hydroxyapatite grain

gradually increased in size when the sample was heated from

100 to 1200oC, and the hydroxyapatite hexagonal phase was

not transformed to the other calcium phosphates phases up to

1200oC. The temperature dependence of the electrical conduc-

tivity showed an increase with increasing temperature, except

at low temperatures where a slight increase was observed. This

was explained as frozen equilibrium between concentrations of

defects and charge carriers. With increasing temperature, the

conductivity rapidly increased and a likely conduction mecha-

nism of a proton was proposed. The dielectric constant and

dielectric loss decrease as a function of frequency with increas-

ing frequency up to 106 Hz and then level off. This is due to

the relaxation time and ionic species of dielectric polarization

are greatly reduced by increasing the frequency.

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Figure 8. Dielectric constant and dielectric loss versus fre-quency for sintered HAP nanoparticles.


Vol. 15, No. 2

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