The luminescent properties of Eu3+-doped calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAp) nanopowders
prepared by the chemical precipitation and sol-gel method were studied. It was shown the possibility of
improving the properties of HAp as a fluorescent carrier of biological molecules and drugs by control of the
defect formation.
When you are citing the document, use the following link http://essuir.sumdu.edu.ua/handle/123456789/3542

Dotsenko, V.P.

Efryushina, N.P.

Zubar, E.V.

SocioEconomic Challenges Journal (Sumy State University)

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  
NANOMATERIALS: APPLICATIONS AND PROPERTIES 
Vol. 2 No 4, 04NABM09(4pp) (2013) 
 
 
2304-1862/2012/1(4)04NABM09(6) 04NABM09-1  2013 Sumy State University 
Creating Properties of Eu3 +-doped Calcium Hydroxyapatite as Biocompatible  
Fluorescent Probes  
 
E.V. Zubar, N.P. Efryushina, V.P. Dotsenko 
 
A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 86, Lustdorfskaya Doroga, 
65080 Odessa, Ukraine 
 
(Received 20 May 2013; published online 31 August 2013) 
 
The luminescent properties of Eu3+-doped calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAp) nanopowders 
prepared by the chemical precipitation and sol-gel method were studied. It was shown the possibility of 
improving the properties of HAp as a fluorescent carrier of biological molecules and drugs by control of the 
defect formation. 
 
Keywords: Calcium hydroxyapatite, Synthesis, Nanopowders, Europium, Luminescence. 
 
 PACS numbers: 78.47.jd, 81.07.Wx 
 
 
1. INTRODUCTION 
 
Fluorescence analysis is one of the most important 
techniques in biology and biomedicine due to its non-
invasive character and high sensitivity. In recent years, 
lanthanide-doped nanocrystals have been developed as 
a new class of luminescent optical labels that have be-
come promising alternatives to organic fluorophores 
and quantum dots for applications in biological assays 
and medical diagnostics [1]. In this context, calcium 
hydroxyapatite based nanophosphors might be good 
candidates, since this material is highly biocompatible, 
biodegradable, stable in biological buffer solutions [2]. 
It should be noted that Eu3 + is less sensitive than some 
other lanthanide ions to the decrease in luminescence 
intensity in the presence of water molecules or high 
vibration energy groups such as C-H or C-O [3]. So 
Eu3 +-doped calcium hydroxyapatite can be employed as 
a biological probe. HAp belongs to the hexagonal sys-
tem with the P63/m space group. In this lattice, two 
unequivalent calcium sites are present: Ca(I) with C3 
symmetry is surrounded by nine oxygen atoms, and 
Ca(II) with CS symmetry is coordinated to six oxygen 
atoms and one OH ˉ group [4]. It is accepted that the 
disordering of OH ˉ ions in c-axis columns causes a 
structural complexity of HAp. Due to the different 
structure of coordination polyhedra of Ca(I) and Ca(II) 
Eu3 + ions at these positions exhibit different spectral 
properties. Despite numerous publications devoted to 
study of the character of isomorphic substitution of 
alkaline earth atoms in this structure by lanthanide 
ions, there is no single opinion on the matter. Also in-
formation about the ability to manage the process of 
distributing Eu3+ ions on the positions of Ca in the HAp 
by the formation of the charge compensation defects is 
strongly limited. The aim of this work is to study the 
influence of the preparation route and formation of dif-
ferent types of defects on the distribution and lumines-
cent properties of Eu3 + ions in HAp. 
 
2. EXPERIMENTAL 
 
Samples of nominal formula  
Ca10(1 – x)Eu10x(PO4)6(OH)2 (x  0.01) used in the present 
work were prepared by two different methods. The 
samples were checked by X-ray diffraction (XRD) using 
Cu Kα radiation. No impurity phases were detected in 
the XRD patterns. Morphological investigations were 
carried out by scanning electron microscopy (SEM) on 
JEOL JSM 6390LV electron microscope. The emission 
and excitation spectra in UV-visible region were rec-
orded at room temperature using a Fluorolog Fl-3 
(Horiba Jobin Yvon) spectrofluorometer equipped with 
a xenon lamp. The Fourier-transformed infrared ab-
sorption spectra were recorded using a Bruker Vertex 
70 spectrometer. 
 
3. RESULTS AND DISCUSSION 
 
In this work, Eu3 +-doped Ca10(PO4)6(OH)2 with dif-
ferent particle sizes have been synthesized by chemical 
precipitation and sol-gel method. To prepare calcium 
phosphate nanoparticles by a precipitation process, an 
aqueous solution of (NH4)2HPO4 was slowly added to an 
aqueous solution of calcium nitrate tetrahydrate 
Ca(NO3)2·4H2O upon continuous stirring. The pH of the 
mixture was adjusted to 10 by adding NH4OH and the 
reaction temperature was maintained at 60 °C. The 
resulting precipitate was aged for 0.5 h under stirring, 
and then filtered, washed with distilled water and 
dried at 105 °C. The preparation of HAp powders by 
the sol-gel method involved the use of aqueous solution 
of calcium nitrate tetrahydrate Ca(NO3)2·4H2O and 
alcohol solution of trimethyl phosphate (CH3O)3PO as 
the precursors. Some amount of citric acid (as a chelat-
ing agent) was added into aqueous solution of 
Ca(NO3)2·4H2O. The solutions were mixed at room 
temperature and kept at pH 7.5 for 48 h. The obtained 
gel was dried at 190 °C for 2 h, and then fired at 600 °C 
for 1 h. In the both cases europium nitrate was added to 
the calcium nitrate solutions to obtain molar ratio of 
Eu / (Ca + Eu)  0.01. The luminescent properties of the 
samples were analyzed in the as prepared-state and 
after annealing at 800 °C for 2-3 h in air. The average 
sizes of crystallites (d) of the as-prepared polycrystal-
line materials were calculated from XRD-patterns by 
the Scherrer equation [5]. The reflections from the 
(300) and (002) planes were used. The obtained values 
 E.V. ZUBAR, N.P. EFRYUSHINA, V.P. DOTSENKO PROC. NAP 2, 04NABM09 (2013) 
 
 
04NABM09-2 
varied from 34 to 70 nm, in agreement with the results 
reported previously for HAp nanopowders prepared 
either by chemical precipitation method [6, 7] or by the 
sol-gel process [8]. A typical SEM image of HAp pre-
pared by the sol-gel method is presented in Fig. 1. It is 
seen that the material is composed of crystallites with 
an irregular form, and which exhibit a high degree of 
aggregation. 
 
 
 
Fig. 1 – SEM photograph of Ca10(1 – x)Eu10x(PO4)6(OH)2 
(x  0.01) synthesized by the sol-gel method 
 
Fig. 2 shows FTIR absorption spectra of HAp pre-
pared by precipitation from aqueous solution at 60 °C, 
precipitation followed by calcination at 1000 °C and the 
sol-gel method. Regardless of the synthesis method the 
spectra contain the stretching and bending vibration 
bands of OH ˉ groups at 3572 and 631 cm – 1, asymmeric 
(ν3) and symmetric (ν1) stretching vibration bands of 
the isolated PO43 – groups at 1090, 1039 and 960 cm – 1. 
The bending vibrations (ν4, ν2) of the PO43 – groups 
cause the bands at 601, 568 cm – 1 and 472 and  
435 cm – 1, respectively. In addition to these features, the 
FTIR spectrum of HAp prepared by the sol-gel method 
contains bands at 1457, 1416 and 873 cm – 1, which must 
be attributed to the stretching and bending vibrations of 
the CO32 – anions occupying both the OH ˉ and PO43 – 
positions [7-9]. The presence of these bands indicates 
that the sol-gel method causes the formation of car-
bonate substituted calcium hydroxyapatite of mixed AB-
type. Since these bands are absent in the FTIR absorp-
tion spectrum of HAp prepared by the chemical precipi-
tation method, one can expect that the incorporation of 
the CO32 – groups into the crystal lattice is a consequence 
of the use of citric acid as a reagent. 
Fig. 3 presents the luminescence spectra of Eu3 + 
ions in Ca10(PO4)6(OH)2 prepared by precipitation from 
aqueous solution at 60 °C, precipitation followed by 
calcination at 1000 °C and the sol-gel method at 600 °C. 
A comparison of the luminescence spectra shown in 
Fig. 3 indicates that the distribution of Eu3 + ions at Ca 
positions in the structure of Ca10(PO4)6(OH)2 depends 
strongly on the synthesis conditions. It is seen that the 
luminescence spectrum Eu3 + ions in HAp, obtained by 
precipitation from aqueous solution at 60 °C, consists of 
three main groups of bands in the 580-600, 608-630, 
685-715 nm regions. It is clear that this spectrum is 
due to 5D0 → 7FJ (j  1, 2, 4) transitions in Eu3 + ions. It 
is well known that the spectrum of luminescence of 
Eu3 + in Ca(II) positions in the apatite structure is 
characterized by an extremely intense band at 574 nm 
associated with 5D0 → 7F0 transition [10]. According to 
Chen and Liu [10], high energy and a relatively high 
intensity of 5D0 → 7F0 transition of Eu3 + ions in inor-
ganic crystals are due to the low point symmetry of the 
lattice sites occupied by Eu3 +. It is assumed that in the 
case of Ca10(PO4)6(OH)2 and BaFCl this effect is caused 
by compensation of the excess positive charge of Eu3 + 
ions in Са2 + / Ва2 + positions due to substitution О2 –
 → ОН − or О2 – → F – [10]. 
In the luminescence spectrum of Eu3 + ions in the 
HAp precipitated from aqueous solution, the band cor-
responding to 5D0 → 7F0 transition is absent (Fig. 3(1)). 
This means that the Eu3 + ions predominantly occupy 
Ca(I) positions. Fig. 3 shows that the high temperature 
calcining of the sample not only leads to a significant 
increase in the luminescence intensity, but is accompa-
nied by changes in the spectral composition of the 
emission. In particular, the fine structure appears in 
the spectrum. Moreover, very intense bands at 574 and 
627 nm are present. 
 
 
 
Fig 2 – FTIR absorption spectra of HAp prepared by precipita-
tion from aqueous solution at 60 °C (1), precipitation followed by 
calcination at 1000 °C (2) and the sol-gel method (3) 
 
From a comparison of the Eu3 + luminescence spec-
tra (Fig. 3) for the sample HAp precipitated from an 
aqueous solution and the product of its high-
temperature firing one can conclude that in the second 
case the ions Eu3 + are in both Ca(I) and Ca(II) posi-
tions. Actually, apart from a number of the 5D0 → 7F2 
emission bands of Eu3 + in Ca(I) position, a band with a 
maximum at 574 nm due to the transition 5D0 → 7F0 of 
Eu3 + in the Ca(II) positions is present in the spectrum 
of the product of high temperature firing. Note also 
that the bands corresponding to the 5D0 → 7F0 and 
5D0 → 7F2 transitions are of comparable intensity. 
The fact of increasing the integrated intensity of 
luminescence of Eu3 + is attributed to the increase in 
the solubility of europium oxide in the HAp and an 
 CREATING PROPERTIES OF EU3 +-DOPED CALCIUM HYDROXYAPATITE… PROC. NAP 2, 04NABM09 (2013) 
 
 
04NABM09-3 
 
 
Fig. 3 – The emission spectra of Ca10(1 – x)Eu10x(PO4)6(OH)2  
(x  0.01) prepared by precipitation from aqueous solution at 
60 °C (1), precipitation followed by calcination at 1000 °C (2) 
and the sol-gel method (3). The spectra were recorded at 293 K 
upon excitation at 396 nm 
 
increase of its crystallinity due to the high temperature 
firing. It is believed that in the precipitated from aque-
ous solution Eu3 +-doped HAp only restricted amount of 
the europium ions introduced into the crystal lattice. 
As mentioned above, the Eu3 + ions in the sample HAp 
precipitated from aqueous solution at 60 °C, preferably 
occupy Ca(I) positions. High-temperature annealing is 
accompanied by an increase in the solubility of Eu2O3 
and the changing character of the distribution of Eu3 + 
ions in the lattice. Karbowiak and Hubert [11] have 
studied the luminescent properties Eu3 + ions in fluo-
rapatite calcium Ca10(PO4)6F2 and found out that in the 
apatite obtained by solid state reaction, the ions Eu3+ 
predominantly occupy Ca(II) positions, whereas in apa-
tite prepared by precipitation from aqueous solution - 
position Ca(I). In the authors opinion, subsequent an-
nealing at 900 ºC of the sample precipitated from aque-
ous solution, resulted in the Eu3 + ion migration from 
the position Ca(I) to position Ca(II) [11]. According to 
Ref. [11], synthesis condition influence strongly on site 
preference of Eu3 + in HAp. Our data for calcium hy-
droxyapatite are in agreement with the results pre-
sented by the authors [11] for the Ca10(PO4)6F2, but our 
interpretation of the experimental results is different. 
The changing character of the distribution of Eu3 + ions 
at the positions of Ca due to high temperature firing we 
relate not so much to the migration of ions Eu3 +, but 
mainly with the introduction of an additional amount 
of lanthanide ions to the crystal lattice of HAp, i.e. with 
the increase in concentration of lanthanide ions. 
Fig. 3(3) shows that the luminescence spectrum of 
Eu3 + ions in Ca10(PO4)6(OH)2 prepared by the sol-gel 
method is generally similar to that of the product of 
high temperature calcination of the sample precipitated 
from aqueous solution. However, it can be noted slight-
ly lower relative intensity of the bands at 612 and 
616 nm which are most characteristic bands of Eu3 + 
ions in the Ca(I) position. The probable reason for this 
is the carbonate substitution in the structure of 
Ca10(PO4)6(OH)2 prepared by sol-gel method. As noted 
above, the sol-gel method results to the formation of 
carbonate substituted calcium hydroxyapatite of mixed 
AB-type, i.e. СО32 – substitute ОН − group (A-type) as 
well as РО43 – group (B-type). Substitution of B-type 
(СО32 – → РО43 –) leads to appearance of the excess posi-
tive charge which can be compensated by calcium va-
cancies. Also, calcium vacancies formation is the most 
likely mechanism for compensation of excess positive 
charge of Eu3 + ions in the Ca(I) position. However, the 
ability of the HAp structure to generate the calcium 
vacancies is limited and therefore the number of ions 
Eu3 + holding Ca(I) position in B-type carbonate substi-
tuted HAp is less than in pure HAp. On the other hand, 
the СО32 – → ОН − substitution can be considered as an 
independent mechanism to compensate the excess posi-
tive charge of Eu3 + in Ca(II) positions. Analyzing the 
spectrum in Fig. 3(3), we conclude that the compensa-
tion of the excess positive charge due to СО32 – groups 
in ОН − positions is not locally. Otherwise, the for-
mation of (EuСа(II)·СО3 ОН)х associates would lead to the 
appearance of additional bands in the spectra. Thus, 
the interaction of defects in carbonate substituted cal-
cium hydroxyapatite of mixed AB-type obtained by the 
sol-gel method promote occupancy of Ca(II) positions by 
Eu3 + ions. 
The excitation spectra for the emissions of Eu3 + in 
Ca(I) and Ca(II) positions are markedly different. The 
spectrum for Eu3 + ions in Ca(I) positions with N  9 
(ex  612 nm) is typical for Eu3 + in inorganic com-
pounds (Fig. 4a). It is dominated by an intense band 
with a maximum at 243 nm, which is caused by charge 
transfer from the 2p orbitals of О2 – to empty Eu3 + 4f6 
orbitals. As can be seen from Fig. 4, the excitation spec-
trum for the Eu3 + ions in Ca(II) position with N  7 
(ex  574 nm) has a fundamentally different character. 
It is characterized by a relatively intense charge trans-
fer band (CTB) 2р О → 4f6 Eu3 +, the maximum of 
which (max  312 nm) is substantially shifted to longer 
wavelengths compared with other alkaline-earth oxide 
compounds. Also, there is an intense band at 459 nm, 
corresponding to 7F0 → 5D2 transition in Eu3 + ions 
(Fig. 4b). One can expect that the long-wavelength po-
sition of the CTB (max  312 nm) of Eu3 + ions in Ca(II) 
position is due to a high degree of covalency of the Eu-
ligand bond caused by formation of the neutral 
(EuСа(II)·ООН)х associates. 
 
 
 
Fig. 4 – Exitation spectra for the Eu3 + emissions in product of 
high temperature calcination of the sample HAp precipitated 
from aqueous solution: a – em  612 nm, b – em  574 nm 
 E.V. ZUBAR, N.P. EFRYUSHINA, V.P. DOTSENKO PROC. NAP 2, 04NABM09 (2013) 
 
 
04NABM09-4 
Due to the availability of effective excitation sources 
in the 300-500 nm region HAp in which Eu3 + ions are 
preferably in Ca(II) positions should be used as a fluo-
rescent carrier of biological molecules and drugs. As fol-
lows from the spectra shown in Fig. 3 stabilizing factor 
Eu3 + ions mainly in the Ca(II) position is the introduc-
tion of СО32 – groups in the structure of the HAp. 
 
4. CONCLUSIONS 
 
Eu3 +-doped Ca10(PO4)6(OH)2 with different particle 
sizes have been synthesized by the chemical precipita-
tion and sol-gel method. The site preference of Eu3 + 
ions for the two calcium sites Ca(I), Ca(II) in the apa-
tite structure was studied by means of luminescent 
spectroscopy. The effect of synthesis conditions and 
Eu3 +-concentration on the distribution of lanthanide 
ions on Ca-positions in the structure of Ca10(PO4)6(OH)2 
was studied. It was found that at relatively low concen-
trations (<< 1 at. %) Ln3 + ions occupy mainly the Ca(I) 
positions. With increasing concentration of lanthanide 
ions their fraction in the Ca(II) position increases sig-
nificantly. 
It was demonstrated the prospects for improving 
the composition and properties of the Eu3 +-doped HAp 
as a fluorescent carrier of biomolecules and drugs by 
control the defects formation. In particular, it is shown 
that the carbonate substitution of mixed AB-type influ-
ences on the distribution of Eu3 + ions at calcium posi-
tions and promotes to the stabilization of Eu3 + ions in 
Ca (II) positions. 
 
 
REFERENCES 
 
1. F. Wang, X. Liu, Chem. Soc. Rev. 38, 976 (2009). 
2. J.Y. Chane-Ching, A. Lebugle, I. Rousselot, A. Pourpoint, 
F. Pelle, J. Mater. Chem. 17, 2904 (2007). 
3. A. Doat, F. Pelle, N. Gardant, A. Lebugle, J. Solid State 
Chem. 177, 1179 (2004). 
4. J.M. Hughes, M. Cameron, A. Mariano, Am. Mineral. 76, 
1165 (1991). 
5. H.P. Klug, L.E. Alexander, X-ray diffraction procedures for 
polycrystalline and amorphous materials (New York: Wil-
ley-Interscience: 1974). 
6. S.P. Mondejar, A. Kovtun, M. Epple, J. Mater. Chem. 17, 
4153 (2007). 
7. L.M. Rodriguez-Lorenzo, M. Vallet-Regi, Chem. Mater. 12, 
2460 (2000). 
8. C.L. Chu, P.H. Lin, S. Dong, D.Y. Guo, J. Mater. Sci. Lett. 
21, 1793 (2002). 
9. S. Angelov, R. Stoyanova, R. Dafinova, K. Kabasanov,  
J. Phys. Chem. Solids 47, 409 (1986). 
10. X.Y. Chen, G.K. Liu, J. Solid State Chem. 178, 419 (2005). 
11. M. Karbowiak, S. Hubert, J. Alloy. Compd. 302, 87 (2000). 
 


Creating Properties of Eu3 +-doped Calcium Hydroxyapatite as Biocompatible Fluorescent Probes

Electronic Sumy State University Institutional Repository

PROCEEDINGS OF THE INTERNATIONAL CONFERENCE  NANOMATERIALS: APPLICATIONS AND PROPERTIES Vol. 2 No 4, 04NABM09(4pp) (2013)   2304-1862/2012/1(4)04NABM09(6) 04NABM09-1  2013 Sumy State University Creating Properties of Eu3 +-doped Calcium Hydroxyapatite as Biocompatible  Fluorescent Probes   E.V. Zubar, N.P. Efryushina, V.P. Dotsenko  A.V. Bogatsky Physico-Chemical Institute, National Academy of Sciences of Ukraine, 86, Lustdorfskaya Doroga, 65080 Odessa, Ukraine  (Received 20 May 2013; published online 31 August 2013)  The luminescent properties of Eu3+-doped calcium hydroxyapatite (Ca10(PO4)6(OH)2, HAp) nanopowders prepared by the chemical precipitation and sol-gel method were studied. It was shown the possibility of improving the properties of HAp as a fluorescent carrier of biological molecules and drugs by control of the defect formation.  Keywords: Calcium hydroxyapatite, Synthesis, Nanopowders, Europium, Luminescence.   PACS numbers: 78.47.jd, 81.07.Wx   1. INTRODUCTION  Fluorescence analysis is one of the most important techniques in biology and biomedicine due to its non-invasive character and high sensitivity. In recent years, lanthanide-doped nanocrystals have been developed as a new class of luminescent optical labels that have be-come promising alternatives to organic fluorophores and quantum dots for applications in biological assays and medical diagnostics [1]. In this context, calcium hydroxyapatite based nanophosphors might be good candidates, since this material is highly biocompatible, biodegradable, stable in biological buffer solutions [2]. It should be noted that Eu3 + is less sensitive than some other lanthanide ions to the decrease in luminescence intensity in the presence of water molecules or high vibration energy groups such as C-H or C-O [3]. So Eu3 +-doped calcium hydroxyapatite can be employed as a biological probe. HAp belongs to the hexagonal sys-tem with the P63/m space group. In this lattice, two unequivalent calcium sites are present: Ca(I) with C3 symmetry is surrounded by nine oxygen atoms, and Ca(II) with CS symmetry is coordinated to six oxygen atoms and one OH ˉ group [4]. It is accepted that the disordering of OH ˉ ions in c-axis columns causes a structural complexity of HAp. Due to the different structure of coordination polyhedra of Ca(I) and Ca(II) Eu3 + ions at these positions exhibit different spectral properties. Despite numerous publications devoted to study of the character of isomorphic substitution of alkaline earth atoms in this structure by lanthanide ions, there is no single opinion on the matter. Also in-formation about the ability to manage the process of distributing Eu3+ ions on the positions of Ca in the HAp by the formation of the charge compensation defects is strongly limited. The aim of this work is to study the influence of the preparation route and formation of dif-ferent types of defects on the distribution and lumines-cent properties of Eu3 + ions in HAp.  2. EXPERIMENTAL  Samples of nominal formula  Ca10(1 – x)Eu10x(PO4)6(OH)2 (x  0.01) used in the present work were prepared by two different methods. The samples were checked by X-ray diffraction (XRD) using Cu Kα radiation. No impurity phases were detected in the XRD patterns. Morphological investigations were carried out by scanning electron microscopy (SEM) on JEOL JSM 6390LV electron microscope. The emission and excitation spectra in UV-visible region were rec-orded at room temperature using a Fluorolog Fl-3 (Horiba Jobin Yvon) spectrofluorometer equipped with a xenon lamp. The Fourier-transformed infrared ab-sorption spectra were recorded using a Bruker Vertex 70 spectrometer.  3. RESULTS AND DISCUSSION  In this work, Eu3 +-doped Ca10(PO4)6(OH)2 with dif-ferent particle sizes have been synthesized by chemical precipitation and sol-gel method. To prepare calcium phosphate nanoparticles by a precipitation process, an aqueous solution of (NH4)2HPO4 was slowly added to an aqueous solution of calcium nitrate tetrahydrate Ca(NO3)2·4H2O upon continuous stirring. The pH of the mixture was adjusted to 10 by adding NH4OH and the reaction temperature was maintained at 60 °C. The resulting precipitate was aged for 0.5 h under stirring, and then filtered, washed with distilled water and dried at 105 °C. The preparation of HAp powders by the sol-gel method involved the use of aqueous solution of calcium nitrate tetrahydrate Ca(NO3)2·4H2O and alcohol solution of trimethyl phosphate (CH3O)3PO as the precursors. Some amount of citric acid (as a chelat-ing agent) was added into aqueous solution of Ca(NO3)2·4H2O. The solutions were mixed at room temperature and kept at pH 7.5 for 48 h. The obtained gel was dried at 190 °C for 2 h, and then fired at 600 °C for 1 h. In the both cases europium nitrate was added to the calcium nitrate solutions to obtain molar ratio of Eu / (Ca + Eu)  0.01. The luminescent properties of the samples were analyzed in the as prepared-state and after annealing at 800 °C for 2-3 h in air. The average sizes of crystallites (d) of the as-prepared polycrystal-line materials were calculated from XRD-patterns by the Scherrer equation [5]. The reflections from the (300) and (002) planes were used. The obtained values  E.V. ZUBAR, N.P. EFRYUSHINA, V.P. DOTSENKO PROC. NAP 2, 04NABM09 (2013)   04NABM09-2 varied from 34 to 70 nm, in agreement with the results reported previously for HAp nanopowders prepared either by chemical precipitation method [6, 7] or by the sol-gel process [8]. A typical SEM image of HAp pre-pared by the sol-gel method is presented in Fig. 1. It is seen that the material is composed of crystallites with an irregular form, and which exhibit a high degree of aggregation.    Fig. 1 – SEM photograph of Ca10(1 – x)Eu10x(PO4)6(OH)2 (x  0.01) synthesized by the sol-gel method  Fig. 2 shows FTIR absorption spectra of HAp pre-pared by precipitation from aqueous solution at 60 °C, precipitation followed by calcination at 1000 °C and the sol-gel method. Regardless of the synthesis method the spectra contain the stretching and bending vibration bands of OH ˉ groups at 3572 and 631 cm – 1, asymmeric (ν3) and symmetric (ν1) stretching vibration bands of the isolated PO43 – groups at 1090, 1039 and 960 cm – 1. The bending vibrations (ν4, ν2) of the PO43 – groups cause the bands at 601, 568 cm – 1 and 472 and  435 cm – 1, respectively. In addition to these features, the FTIR spectrum of HAp prepared by the sol-gel method contains bands at 1457, 1416 and 873 cm – 1, which must be attributed to the stretching and bending vibrations of the CO32 – anions occupying both the OH ˉ and PO43 – positions [7-9]. The presence of these bands indicates that the sol-gel method causes the formation of car-bonate substituted calcium hydroxyapatite of mixed AB-type. Since these bands are absent in the FTIR absorp-tion spectrum of HAp prepared by the chemical precipi-tation method, one can expect that the incorporation of the CO32 – groups into the crystal lattice is a consequence of the use of citric acid as a reagent. Fig. 3 presents the luminescence spectra of Eu3 + ions in Ca10(PO4)6(OH)2 prepared by precipitation from aqueous solution at 60 °C, precipitation followed by calcination at 1000 °C and the sol-gel method at 600 °C. A comparison of the luminescence spectra shown in Fig. 3 indicates that the distribution of Eu3 + ions at Ca positions in the structure of Ca10(PO4)6(OH)2 depends strongly on the synthesis conditions. It is seen that the luminescence spectrum Eu3 + ions in HAp, obtained by precipitation from aqueous solution at 60 °C, consists of three main groups of bands in the 580-600, 608-630, 685-715 nm regions. It is clear that this spectrum is due to 5D0 → 7FJ (j  1, 2, 4) transitions in Eu3 + ions. It is well known that the spectrum of luminescence of Eu3 + in Ca(II) positions in the apatite structure is characterized by an extremely intense band at 574 nm associated with 5D0 → 7F0 transition [10]. According to Chen and Liu [10], high energy and a relatively high intensity of 5D0 → 7F0 transition of Eu3 + ions in inor-ganic crystals are due to the low point symmetry of the lattice sites occupied by Eu3 +. It is assumed that in the case of Ca10(PO4)6(OH)2 and BaFCl this effect is caused by compensation of the excess positive charge of Eu3 + ions in Са2 + / Ва2 + positions due to substitution О2 – → ОН − or О2 – → F – [10]. In the luminescence spectrum of Eu3 + ions in the HAp precipitated from aqueous solution, the band cor-responding to 5D0 → 7F0 transition is absent (Fig. 3(1)). This means that the Eu3 + ions predominantly occupy Ca(I) positions. Fig. 3 shows that the high temperature calcining of the sample not only leads to a significant increase in the luminescence intensity, but is accompa-nied by changes in the spectral composition of the emission. In particular, the fine structure appears in the spectrum. Moreover, very intense bands at 574 and 627 nm are present.    Fig 2 – FTIR absorption spectra of HAp prepared by precipita-tion from aqueous solution at 60 °C (1), precipitation followed by calcination at 1000 °C (2) and the sol-gel method (3)  From a comparison of the Eu3 + luminescence spec-tra (Fig. 3) for the sample HAp precipitated from an aqueous solution and the product of its high-temperature firing one can conclude that in the second case the ions Eu3 + are in both Ca(I) and Ca(II) posi-tions. Actually, apart from a number of the 5D0 → 7F2 emission bands of Eu3 + in Ca(I) position, a band with a maximum at 574 nm due to the transition 5D0 → 7F0 of Eu3 + in the Ca(II) positions is present in the spectrum of the product of high temperature firing. Note also that the bands corresponding to the 5D0 → 7F0 and 5D0 → 7F2 transitions are of comparable intensity. The fact of increasing the integrated intensity of luminescence of Eu3 + is attributed to the increase in the solubility of europium oxide in the HAp and an  CREATING PROPERTIES OF EU3 +-DOPED CALCIUM HYDROXYAPATITE… PROC. NAP 2, 04NABM09 (2013)   04NABM09-3   Fig. 3 – The emission spectra of Ca10(1 – x)Eu10x(PO4)6(OH)2  (x  0.01) prepared by precipitation from aqueous solution at 60 °C (1), precipitation followed by calcination at 1000 °C (2) and the sol-gel method (3). The spectra were recorded at 293 K upon excitation at 396 nm  increase of its crystallinity due to the high temperature firing. It is believed that in the precipitated from aque-ous solution Eu3 +-doped HAp only restricted amount of the europium ions introduced into the crystal lattice. As mentioned above, the Eu3 + ions in the sample HAp precipitated from aqueous solution at 60 °C, preferably occupy Ca(I) positions. High-temperature annealing is accompanied by an increase in the solubility of Eu2O3 and the changing character of the distribution of Eu3 + ions in the lattice. Karbowiak and Hubert [11] have studied the luminescent properties Eu3 + ions in fluo-rapatite calcium Ca10(PO4)6F2 and found out that in the apatite obtained by solid state reaction, the ions Eu3+ predominantly occupy Ca(II) positions, whereas in apa-tite prepared by precipitation from aqueous solution - position Ca(I). In the authors opinion, subsequent an-nealing at 900 ºC of the sample precipitated from aque-ous solution, resulted in the Eu3 + ion migration from the position Ca(I) to position Ca(II) [11]. According to Ref. [11], synthesis condition influence strongly on site preference of Eu3 + in HAp. Our data for calcium hy-droxyapatite are in agreement with the results pre-sented by the authors [11] for the Ca10(PO4)6F2, but our interpretation of the experimental results is different. The changing character of the distribution of Eu3 + ions at the positions of Ca due to high temperature firing we relate not so much to the migration of ions Eu3 +, but mainly with the introduction of an additional amount of lanthanide ions to the crystal lattice of HAp, i.e. with the increase in concentration of lanthanide ions. Fig. 3(3) shows that the luminescence spectrum of Eu3 + ions in Ca10(PO4)6(OH)2 prepared by the sol-gel method is generally similar to that of the product of high temperature calcination of the sample precipitated from aqueous solution. However, it can be noted slight-ly lower relative intensity of the bands at 612 and 616 nm which are most characteristic bands of Eu3 + ions in the Ca(I) position. The probable reason for this is the carbonate substitution in the structure of Ca10(PO4)6(OH)2 prepared by sol-gel method. As noted above, the sol-gel method results to the formation of carbonate substituted calcium hydroxyapatite of mixed AB-type, i.e. СО32 – substitute ОН − group (A-type) as well as РО43 – group (B-type). Substitution of B-type (СО32 – → РО43 –) leads to appearance of the excess posi-tive charge which can be compensated by calcium va-cancies. Also, calcium vacancies formation is the most likely mechanism for compensation of excess positive charge of Eu3 + ions in the Ca(I) position. However, the ability of the HAp structure to generate the calcium vacancies is limited and therefore the number of ions Eu3 + holding Ca(I) position in B-type carbonate substi-tuted HAp is less than in pure HAp. On the other hand, the СО32 – → ОН − substitution can be considered as an independent mechanism to compensate the excess posi-tive charge of Eu3 + in Ca(II) positions. Analyzing the spectrum in Fig. 3(3), we conclude that the compensa-tion of the excess positive charge due to СО32 – groups in ОН − positions is not locally. Otherwise, the for-mation of (EuСа(II)·СО3 ОН)х associates would lead to the appearance of additional bands in the spectra. Thus, the interaction of defects in carbonate substituted cal-cium hydroxyapatite of mixed AB-type obtained by the sol-gel method promote occupancy of Ca(II) positions by Eu3 + ions. The excitation spectra for the emissions of Eu3 + in Ca(I) and Ca(II) positions are markedly different. The spectrum for Eu3 + ions in Ca(I) positions with N  9 (ex  612 nm) is typical for Eu3 + in inorganic com-pounds (Fig. 4a). It is dominated by an intense band with a maximum at 243 nm, which is caused by charge transfer from the 2p orbitals of О2 – to empty Eu3 + 4f6 orbitals. As can be seen from Fig. 4, the excitation spec-trum for the Eu3 + ions in Ca(II) position with N  7 (ex  574 nm) has a fundamentally different character. It is characterized by a relatively intense charge trans-fer band (CTB) 2р О → 4f6 Eu3 +, the maximum of which (max  312 nm) is substantially shifted to longer wavelengths compared with other alkaline-earth oxide compounds. Also, there is an intense band at 459 nm, corresponding to 7F0 → 5D2 transition in Eu3 + ions (Fig. 4b). One can expect that the long-wavelength po-sition of the CTB (max  312 nm) of Eu3 + ions in Ca(II) position is due to a high degree of covalency of the Eu-ligand bond caused by formation of the neutral (EuСа(II)·ООН)х associates.    Fig. 4 – Exitation spectra for the Eu3 + emissions in product of high temperature calcination of the sample HAp precipitated from aqueous solution: a – em  612 nm, b – em  574 nm  E.V. ZUBAR, N.P. EFRYUSHINA, V.P. DOTSENKO PROC. NAP 2, 04NABM09 (2013)   04NABM09-4 Due to the availability of effective excitation sources in the 300-500 nm region HAp in which Eu3 + ions are preferably in Ca(II) positions should be used as a fluo-rescent carrier of biological molecules and drugs. As fol-lows from the spectra shown in Fig. 3 stabilizing factor Eu3 + ions mainly in the Ca(II) position is the introduc-tion of СО32 – groups in the structure of the HAp.  4. CONCLUSIONS  Eu3 +-doped Ca10(PO4)6(OH)2 with different particle sizes have been synthesized by the chemical precipita-tion and sol-gel method. The site preference of Eu3 + ions for the two calcium sites Ca(I), Ca(II) in the apa-tite structure was studied by means of luminescent spectroscopy. The effect of synthesis conditions and Eu3 +-concentration on the distribution of lanthanide ions on Ca-positions in the structure of Ca10(PO4)6(OH)2 was studied. It was found that at relatively low concen-trations (<< 1 at. %) Ln3 + ions occupy mainly the Ca(I) positions. With increasing concentration of lanthanide ions their fraction in the Ca(II) position increases sig-nificantly. It was demonstrated the prospects for improving the composition and properties of the Eu3 +-doped HAp as a fluorescent carrier of biomolecules and drugs by control the defects formation. In particular, it is shown that the carbonate substitution of mixed AB-type influ-ences on the distribution of Eu3 + ions at calcium posi-tions and promotes to the stabilization of Eu3 + ions in Ca (II) positions.   REFERENCES  1. F. Wang, X. Liu, Chem. Soc. Rev. 38, 976 (2009). 2. J.Y. Chane-Ching, A. Lebugle, I. Rousselot, A. Pourpoint, F. Pelle, J. Mater. Chem. 17, 2904 (2007). 3. A. Doat, F. Pelle, N. Gardant, A. Lebugle, J. Solid State Chem. 177, 1179 (2004). 4. J.M. Hughes, M. Cameron, A. Mariano, Am. Mineral. 76, 1165 (1991). 5. H.P. Klug, L.E. Alexander, X-ray diffraction procedures for polycrystalline and amorphous materials (New York: Wil-ley-Interscience: 1974). 6. S.P. Mondejar, A. Kovtun, M. Epple, J. Mater. Chem. 17, 4153 (2007). 7. L.M. Rodriguez-Lorenzo, M. Vallet-Regi, Chem. Mater. 12, 2460 (2000). 8. C.L. Chu, P.H. Lin, S. Dong, D.Y. Guo, J. Mater. Sci. Lett. 21, 1793 (2002). 9. S. Angelov, R. Stoyanova, R. Dafinova, K. Kabasanov,  J. Phys. Chem. Solids 47, 409 (1986). 10. X.Y. Chen, G.K. Liu, J. Solid State Chem. 178, 419 (2005). 11. M. Karbowiak, S. Hubert, J. Alloy. Compd. 302, 87 (2000).  

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Creating Properties of Eu3 +-doped Calcium Hydroxyapatite as Biocompatible Fluorescent Probes

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