Materials Research Bulletin 47 (2012) 24–28

Effects of Eu3+ ions on the morphology and luminescence properties ofhydroxyapatite nanoparticles synthesized by one-step hydrothermal method

Suping Huang *, Jun Zhu, Kechao Zhou

State Key Lab of Powder Metallurgy, Centre South University, Changsha 410083, PR China

A R T I C L E I N F O

Article history:

Received 29 March 2011

Received in revised form 16 September 2011

Accepted 17 October 2011

Available online 21 October 2011

Keywords:

A. Nanostructures

B. Chemical synthesis

C. TEM

D. Luminescence

A B S T R A C T

In the present investigation, we have demonstrated the synthesis of sphere shaped HA phosphors by a

simple one-step hydrothermal method, and the structural, morphological, textural and optical properties

were characterized by XRD, TEM, XPS, ICP and PL spectroscopy. Results showed that the crystal size and

the ratio of length to radius of Eu3+-doped HA particles decreased with the increasing of Eu3+doped

concentration, and its PL emission intensities increased with the increase of Eu3+doped concentration.

When the Eu3+ amount was 7.5 at.%, the HA particle showed sphere shape, and the actual doping

concentration and the PL emission intensities reach the maximum. This indicated that sphere shaped HA

phosphors can be obtained by adjusting the Eu3+ doping concentration without adding any other reagent.

And it also indicated that the PL emission intensities were mainly dependent on the Eu3+ doped

concentration, and the effect of morphology was very little.

� 2011 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

During the past decade, a lot of efforts have been made todevelop novel techniques for gene therapy. In order to transfervarious exogenous genetic materials into target tissues, numerousgene vectors were used and characterized in the last decade. Manyviral vectors [1–4], some non-viral cationic liposomes [3,5,6], andother non-viral transfection agents [7,8] have been applied todeliver specific genes into the target tissues for treatment trials insome animal models. Of which nano-hydroxyapatite (nHA,Ca10(PO4)6(OH)2), as a new non-viral gene vector, has attractedconsiderable attention and has been used for practice of genetherapies due to its good biocompatibility with no cytotoxicity andnon-inflammatory properties [9,10]. Experiments showed thatnHA appeared with no cytotoxic effects when applied to Hela cellsline [11,12]. And it can successfully delivered therapeutic gene intothe inner ear of guinea pigs via an osmotic mini-pump for slowdelivery passing through a hole to scala tympani, which showed agreat protective effect on reducing hearing loss from excitotoxicy[13–15]. It was proved that nHA not only can transfer exogenousgene in vitro, but also it can transfer gene in vivo. However, thecomparatively low efficiency of transfection is one of the biggestconcerns for popularizing the nHA in clinic trials.

* Corresponding author. Tel.: +86 731 8883 6264; fax: +86 731 8883 0464.

E-mail address: huangsp1@mail.csu.edu.cn (S. Huang).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.10.013

It is well accepted that the gene transfect efficiency of genevector was strictly linked to their nanoscale dimensions [16,17],and the sphere shape of the materials is usually regarded to beeasier to entering target cells than any other morphology of vectorsin many cases. Therefore, the design and development ofluminescence functionalized HA with nano-sized and sphereshape should be able to reach this goal. So far, various techniques,such as chemical co-precipitation [18,19], sol–gel process [20,21],hydrothermal synthesis [22,23], emulsion and microemulsion[24–28], and microwave irradiation [29,30] have been employedfor the synthesis of Eu3+ doped hydroxyapatite. In the above-mentioned studies, researchers paid more attention on the effectsof Eu3+ doped concentration on the luminescence properties, littlework has been reported on the morphology of nHA on itsluminescence properties. On the other hand, the crystal morphol-ogy of HA was always rod-like shape, and sphere morphology of HAwas synthesized by adding some surfactants. In this paper, wepropose the synthesis of Eu3+ doped hydroxyapatite phosphorswith sphere morphology and narrow size distribution throughone-step hydrothermal treatment process without adding anyother reagents. And the influences of Eu3+ ions on the morphologyand luminescence properties were fully investigated by means ofXRD, TEM, XPS, and photoluminescence (PL) spectra.

2. Experimental

All reagents were analytical grade and used without furtherpurification. The Eu3+-doped HA nanoparticles were synthesized

S. Huang et al. / Materials Research Bulletin 47 (2012) 24–28 25

by hydrothermal method, and samples with Eu/Ca molar ratios of0 at.%, 1 at.%, 3 at.%, 5 at.%, 7.5 at.% and 10 at.% were prepared indifferent pH value and different reaction time. Eu(NO3)3 wereobtained by dissolving stoichiometric Eu2O3 in dilute HNO3 withvigorous stirring, the superfluous HNO3 in the solution was drivenoff until the Eu(NO3)3 were obtained. In a typical synthesis process,50 ml of a mixed solution of Ca(NO3)2�4H2O (0.2 M) and anappropriate amount of Eu(NO3)3 were introduced into 50 ml of(NH4)2HPO4 (0.2 M) solution. With the atomic ratio (Eu + Ca)/Pfixed at 1.667. And the pH of mixture was adjusted at 10 usingHNO3 and NH4NO3. After stirring for 30 min, the mixture wastransferred into a 200 ml Teflon-lined autoclave, and then theautoclave was sealed and maintained at 160 8C for certain time.After the reaction was complete, the precipitate was collected,washed with distilled water and ethanol several times to removethe ions possibly remaining in the final products, and finally driedat 100 8C for 12 h.

The X-ray diffraction (XRD) patterns of the synthesized sampleswere obtained by a Brucker D8-advance X-ray powder diffractom-eter (XRD) with Cu Ka radiation (l = 0.15418 nm). The transmis-sion electron microscopy (TEM) images were obtained using a JEM3010 transmission electron microscope. The X-ray photo-electronspectroscopy (XPS) was performed in a VG Scientific ESCALABMark II spectrometer equipped with two ultrahigh vacuum (UHV)chambers using Al Ka radiation (1486.6 eV). The XPS bindingenergies were calibrated with respect to the C 1s peak from acarbon tape at 284.6 eV. Photoluminescence (PL) and photolumi-nescence excitation (PLE) spectra were measured by a Hitachifluorescence spectrophotometer F-4500 equipped with a xenonlamp. The actual Eu3+ doping concentration of samples wereexamined by inductively coupled plasma (ICP) analysis.

3. Results and discussion

3.1. Structure and formation of Eu3+-doped HA

Fig. 1 shows the XRD patterns of as prepared Eu3+-doped HAsamples with different Eu3+-doped concentration. The diffractionpeaks of each sample are in agreement with the hexagonal HA inP63m space group (JCPDS No. 09-0432). No other phases relatedwith doped component can be detected, which indicated that Eu3+

ions had successfully doped into HA crystal. From Fig. 1 it can beseen that there were no peaks appeared in the range between 608

Fig. 1. X-ray diffraction patterns of Eu3+-doped HA phosphor with different

Eu3+concentration (a) 0 at.%, (b) 1 at.%, (c) 2 at.%, (d) 3 at.%, (e) 4 at.%, (f) 5 at.%, (g)

7.5 at.% and (h) 10 at.%.

and 808/2theta, which indicated that the sample is calcium-deficient HA. The calculated lattice constants and the actualdoping concentration of the as-prepared samples are summarizedin Table 1. As shown, the calculated lattice constants of a or b andthe lattice volume decreased with the increasing of Eu3+ dopingamount, when the Eu3+ doping amount was 7.5 at.%, they reachedthe minimal value, and then increased with further increasedconcentration. While the calculated lattice constants of c had nochanged in some extend. The ionic radius of Eu3+ is smaller thanthat of Ca2+, which may be responsible for the decrease of the cellparameters. Moreover, Table 1 also shows that the actual dopingconcentration increased with the increasing of Eu3+dopedconcentration. When the Eu3+doped concentration were7.5 at.%, the actual doping concentration reached the maximalvalue, and then decreased with further increased concentration.The trendline of lattice constants was just inversing to that of theactual doping concentration, which indicated that the dopingconcentration had greatly influenced on the crystal growthbehaviors of Eu3+doped HA.

XPS technique has been tested as a powerful tool forqualitatively determining the surface composition of one material.In the XPS spectrum (Fig. 2) of Eu3+ doped HA, the binding energy ofEu(3d, 1135.20 eV), Eu(4d, 123.65 eV), P(2p, 133.55 eV), Ca(2p,347.50 eV), O(1 s, 531.25) can be obviously found. By combinationof previous XRD results, it can be deduced that these signals can beassigned to Eu3+-doped HA.

3.2. Morphology of Eu3+-doped HA

The TEM images in Fig. 3 shows the morphology and size ofEu3+-doped HA with different Eu3+concentration. It can be seenthat all the samples are non-aggregated with narrow distribution.The pure HA particles were regular rod-like whiskers in diametersof 20–30 nm and lengths of 100–200 nm. When the Eu3+ dopingconcentration was added, the morphology of HA changed fromrod-like shape to sphere shape, and the ratio of length to radius ofsamples decreased with the increasing of Eu3+ doping amount.When the Eu3+ doping amount was 7.5 at.%, they reached theminimal value, and then increased with further increasedconcentration. The width of particles changed little, which wereconsistent with the XRD results.

Morphology changes of HA particles with the increase ofEu3+concentration can be explained that the presence of Eu3+

inhibited the crystal growth of HA possibly through substitutionthe active growth sites for crystal growth. It was well knownthat HA particles synthesized by hydrothermal method withoutadding any surfactants were usually rod-like shape. When Eu3+

doped into the HA crystal, the Eu3+would substitute the Ca2+ inboth hydroxyapatite Ca (I) and Ca (II) site, which may block theactive growth sites of the seed crystals. This resulted in thegrowth rate and the ratio of length to radius decreasing, namelythe morphology of HA changed from rod-like shape to sphereshape.

3.3. Luminescence properties of Eu3+-doped HA

Fig. 4 shows the excitation spectra of pure and Eu3+-doped HAnanoparticles monitored at 613 nm. It was found that there was nopeaks can be seen in the excitation spectrum of pure HA phosphors.In curves b–f shown in Fig. 4, it was found that there were fiveintense broad bands with a maximum at 361, 381, 394 nm, 414 nmand 465 nm, respectively. These peaks correspond to the directexcitation of the Eu3+ ground state into higher levels of the 4fmanifold, which can be ascribed to 7F0! 5D4 (361 nm), 7F0! 5G2

(381 nm), 7F0! 5L6 (394 nm), 7F0! 5D3 (414 nm), and 7F0! 5D2

(465 nm), respectively [27].

Fig. 2. XPS spectrum (a) and the high-resolution XPS spectra of Eu 3d (b), Eu 4d (c), P 2p (d), Ca 2p (e) and O 1p (f) of Eu3+-doped HA sample.

S. Huang et al. / Materials Research Bulletin 47 (2012) 24–2826

Fig. 5 shows the room temperature photoluminescence (PL)spectra of pure and Eu3+-doped HA nanoparticles doped withdifferent Eu3+ concentrations under 395 nm excitation. From Fig. 5it can be seen that there appeared the blue luminescence peaks in

Table 1The characteristics of Eu3+-doped HA nanoparticles with different Eu3+concentration.

Eu doping amount (at.%) 0 1

a or b (nm) 0.945339 0.945056

c (nm) 0.690291 0.689815

Vol (nm3� 103) 534.24 533.55

Actual doping concentration (at.%) 0 0.8871

Ratio of length to radius 10 7

pH = 10, time = 3 h.

the emission spectra at 450 nm and 475 nm in all the samples,which indicated that all the samples were calcium-deficient HAand the presence of Eu2+ ions [31]. Besides, The PL spectra of Eu3+-doped HA showed three weak emission peaks in the 566–582,

3 5 7.5 10

0.9447 0.944187 0.94246 0.943725

0.690227 0.689517 0.690495 0.689688

533.47 532.34 531.15 531.95

2.2686 3.6764 5.0691 3.4254

5.3 3.5 2.9 2.5

Fig. 3. The TEM graphs of Eu3+ doping HA with different Eu3+concentration (a) 0 at.%; (b) 1 at.%; (c) 3 at.%; (d) 5 at.%; (e) 7.5 at.%; and (f) 10 at.%.

S. Huang et al. / Materials Research Bulletin 47 (2012) 24–28 27

640–655, 680–700 nm range and two strong emission peaks in the582–604 and 604–625 nm range, which corresponded to the5D0! 7F0, 5D0! 7F3, 5D0! 7F4, 5D0! 7F1 and 5D0! 7F2 transi-tions of Eu3+ ions.

By combination of Figs. 4 and 5, it can be seen that the changetrend lines of the excitation spectra and the room temperature

photoluminescence (PL) spectra were same, namely the intensityof peaks increased with the increasing of Eu3+ doping amount,when the Eu3+ doping amount was 7.5 at.%, they reached themaximal value, and then decreased with further increasedconcentration. On the other hand, when the reference objectand the preparation conditions were same, the quantum efficiency

Fig. 4. Excitation spectra of pure HA and Eu3+-doped HA nanoparticles with

different Eu3+ concentrations monitoring the emission of Eu3+ at 613 nm.

Fig. 5. Room temperature photoluminescence (PL) spectra of Eu3+-doped HA

nanoparticles with different Eu3+ concentrations (ex = 398 nm).

S. Huang et al. / Materials Research Bulletin 47 (2012) 24–2828

was in proportion to the intensity of emission peaks. Therefore, thequantum efficiency of Eu3+-doped HA nanoparticles increased withthe increasing of Eu3+ doping amount, when the Eu3+ dopingamount was 7.5 at.%, they reached the maximal value, and thendecreased with further increased concentration.

4. Conclusions

The Eu3+-doped HA nanoparticles were synthesized by hydro-thermal method. It was found that the optimum Eu3+content in HAnanoparticles was 7.5 at.%. TEM images analysis showed that thehigher Eu3+ doping concentration, the smaller was the crystal size.And the morphology of HA changed from rod-like shape to sphereshape with the increasing of Eu3+ doping concentration. All of theEu3+-doped HA nanoparticles exhibited five sharp emission peaks

at 361, 381, 394 nm, 414 nm and 465 nm under 613 nm excitation.The PL emission intensities of Eu3+ increase with the increase oftheir concentration, reaching a maximum at the concentration of7.5 at.%, and then decrease with further increased concentration.Compared with Eu3+ doping concentration, effect of morphologyon the luminescence properties of Eu3+-doped HA was littler.

Acknowledgments

This project is financially supported by the foundation ofNatural Science Foundation of China (No: 51102285), and thefoundation of ‘‘The Frontier Research Project’’ of Centre SouthUniversity, China (201022100002, 2010QZZD0006).

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