Materials Letters 106 (2013) 452–455

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Materials Letters

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Facile synthesis and in vitro bioactivity of monodispersed mesoporousbioactive glass sub-micron spheres

Qing Hu, Xiaofeng Chen n, Naru Zhao, Yuli LiSchool of Materials Science and Engineering, South China University of Technology, Guangzhou 510641, PR China

a r t i c l e i n f o

Article history:Received 6 March 2013Accepted 20 April 2013Available online 30 April 2013

Keywords:BiomaterialsSol–gel preparationBioactive glassMonodispersedMesoporous sub-micron spheres

7X/$ - see front matter & 2013 Elsevier B.V. Ax.doi.org/10.1016/j.matlet.2013.04.075

esponding author. Tel.: +86 20 2223 6283; faxail address: chenxf@scut.edu.cn (X. Chen).

a b s t r a c t

This paper reports a facile method for fabricating monodispersed mesoporous bioactive glass sub-micronspheres (MBGS) using dodecylamine (DDA) as a catalyst and template agent in sol–gel process. Themorphology, structure and in vitro bioactivity of MBGS were investigated by various methods. The resultsindicate that using DDA as the structure directing agent and hydrolysis catalyst is in favor of preparingthe MBGS with mesoporous surface structure, large specific surface area (362.073 m2 g−1), relativelyhomogeneous particle size (�560 nm) and good apatite-forming activity. The unique structure andproperties may turn MBGS into a good candidate as a drug delivery carrier or an injectable biomaterialfor bone tissue regeneration.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

Bioactive glasses (BGs) have been the promising biomaterialsfor bone tissue regeneration because of their good bioactive,resorbable, osteoproductive properties [1]. In the past few years,scientists have been focused on developing various BGs materials[2–4]. Many studies suggested that the bone-bonding ability ofBGs can be due to the formation of a hydroxycarbonate apatite(HCA) layer on the BGs surface when contacting with the simu-lated body fluid (SBF).

On the other hand, it is well known that the sol–gel process is aversatile technology for producing inorganic particles with differ-ent morphologies and sizes by controlling the hydrolysis andcondensation of organic precursors [5]. It was found that thebioactivity of sol–gel derived BG is better than that of the melt-derived BG due to its unique micro/nano-scale structure and largesurface area [6]. However, the problems for sol–gel BGs are stillunsolved, such as the severe agglomeration and irregular shape ofBGs particles. Previous studies have shown that regular sphericalBGs possess better physical-co-chemical and biological propertiescompared to the irregular BGs [7,8], exhibiting their potentialapplications in bone tissue regeneration and drug delivery sys-tems. In recent years, including our groups, several papers havereported the preparation and properties of spherical BGs particles[9–11].

However, most of the BG spheres are still suffering the multi-step preparing process, low specific surface area or aggregated

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morphology, which would weaken their bioactivity and physico-chemical properties when used as the drug carrier or biopolymerreinforcement [12]. Therefore, it is necessary to develop novel andfacile technology to fabricate mono-dispersed bioactive glassmicrospheres (MBGS) with homogeneous particle size and highspecific surface area.

Here, for the first time, we show a facile sol–gel method to prepareMBGSwith a relatively homogeneous particle size using dodecylamine(DDA) as a catalyst and template agent. DDA is a kind of organic weakbase with amphipathy, which is usually used as template to synthetizemesoporous materials [13–15]. In addition, the in vitro apatite-formingbioactivity of the prepared MBGS was also studied.

2. Experimental

Preparation of MBGS: MBGS were prepared by improved sol–gelmethod using DDA as a catalyst and template agent. The molarcomposition of MBGS was 80% SiO2, 16% CaO and 4% P2O5. Briefly,the precursor solutions of MBGS were prepared as follows: 4 gDDA was dissolved in deionized water (DW, 25 ml) and absoluteethyl alcohol (ETOH, 80 ml) under stirring. When DDA wasdissolved completely, tetraethyl orthosilicate (TEOS, 16 ml),triethylphosphate (TEP, 1.22 ml) and calcium nitrate tetrahydrate(CN, 3.39 g) were added to the above solutions in order for 30 mininterval with stirring magnetically. The resulted solution wasvigorously stirred together for another 3 h at room temperature,and the clear solution gradually turned into opaque due to theformation of a white precipitate. The white precipitate wascollected by filtration and rinsed with ETOH and DW, dried undervacuum at room temperature for 24 h. The MBGS was obtained

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after removing templates and organic components by sintering inair at 650 1C for 3 h (2 1C/min).

Characterizations: Specific surface area, pore volume wereevaluated using multipoint Brumauer–Emmett–Teller (BET) N2

absorption technique at 77.3 K [16]. The pore size and pore sizedistribution were calculated by the Barrett–Joyner–Halenda (BJH)method using desorption isotherm branch. The morphology,

Fig. 1. SEM micrographs at different magnification (a),

Fig. 2. TEM images of MBGS at diffe

microstructure and particle size distribution of samples weredetermined using field emission scanning electron microscope(FE-SEM, Nova NanoSEM430, FEI, USA, 15 kV), transmission elec-tron microscopy (TEM, JEM-2100HR, Japan, 100 kV) and ZetaszierNano-ZS (Marvin, England).

The in vitro bioactivity of the obtained MBGS was tested byimmersing samples in SBF [17] (Na+ 142.0, K+ 5.0, Mg2+ 1.5, Ca2+

(b), (c) and particle size distribution of MBGS (d).

rent magnification (a) and (b).

Fig. 3. N2 adsorption-desorption isotherm and pore size distribution (inset) ofMBGS.

Fig. 4. SEM micrographs (a) and (b), FT-IR spectra (c) and XRD patte

Q. Hu et al. / Materials Letters 106 (2013) 452–455454

2.5, Cl− 147.8, HCO3− 4.2, HPO42− 1.0, and SO4

2− 0.5 mmol L−1) at aconcentration of 1 mg/ml at 37 1C to monitor the formation of HCAon the surface of the sample with time. Once removed from theincubation, the solids were taken out, washed with acetone, driedin air and characterized using FE-SEM, Fourier transform infraredspectroscopy (FT-IR, Nexus, Nicolet Co., USA) and powder X-raydiffraction (XRD, X’pert PRO, Panalytical, the Netherlands) with CuKa (1.548 Å).

3. Results and discussion

In our study, MBGS were successfully synthesized by improvedsol–gel method using DDA as a hydrolysis catalyst. MBGS exhibitregularly spherical morphology and relatively narrow particle sizedistribution (510–590 nm) with a mean diameter of 560 nm, asshown in Fig. 1. Moreover, the obtained MBGS have favorablemono-dispersibility, which, perfectly solves the agglomeration pro-blem of the conventional sol–gel BGs. DDA is a kind of organic weakbase, in the ethanol/water solvent, the condensation reaction is

rns (d) of MBGS before and after soaking in SBF for 1d and 3d.

Q. Hu et al. / Materials Letters 106 (2013) 452–455 455

controlled by using base catalysts which prevents the particles fusingtogether and further favours the formation of spherical particles[18,19]. And the particle size distribution and average size can betuned by controlling the rate of hydrolysis and condensation.

As shown in Fig. 1c, the surface of MBGS was relatively rough. Itwas supposed that a bioactive glass sphere was composed by theaccumualtion of numerous nano-particles, which result in nanos-cale surface morphology and relatively homogeneous sized meso-pore distribution. Furthermore, the TEM image (Fig. 2) can provethis assumption. The formation of such mesoporous structure canbe explained through self-assembly between neutral organicsurfactant and neutral inorganic ions (SI) [20]. DDA was used asthe structure directing surfactant (S) and TEOS as the inorganicprecursor (I). The SI assembly relies on hydrogen bonding betweenS and I at the micelle interface, and will proceed during theformation of silica oligomer generated by the hydrolysis of TEOS.After calcination, DDA was removed, leaving behind the mesopor-ous structure. As a result, in this study, DDA is not just a basehydrolysis catalyst but also a mesoporous structure derivedtemplate.

The specific surface area and mesopore structure of MBGS wereobtained by N2 absorption–desorption isotherm (Fig. 3). Thesample exhibited type IV isotherm patterns with H3-type hyster-esis loop associated with slot-shape mesopore according to IUPACclassification [21].The specific surface area, the total volume andmean pore size are 362.073 m2 g−1, 0.221 cm3 g−1 and 2.352 nm,respectively. The specific surface area of our MBGS is much higherthan the reported BG microsphere in the literature.

The representative morphologies of the samples after soakingin SBF for 1d, 3d are shown in Fig. 4. After soaking in SBF for 1d,the surface of MBGS became coarser, covered by flaky apatiteprecipitates (Fig. 4a). With the increase of soaking time, the flakyprecipitates continued to grow. After 3d of soaking, flower-likelayer almost covered the whole surface (Fig. 4b). Additionally, thebioglass microspheres aggregated together and interconnectedwith each other by the deposit, which is confirmed to be hydro-xyapatite (HA) crystal according to the FT-IR and XRD results.

The FT-IR spectra of the samples before and after soaking in SBFfor 1d and 3d were summarized in Fig. 4c. Before soaking, thespectrum shows characteristic absorption bands corresponding toSi–O–Si bonding at 1060, 798 and 480 cm−1. After 1d of soaking inSBF, the weak P–O vibrations of phosphate groups near 562, 603,and 968 cm−1 indicating the initial presence of poor crystalline HAlayer. Furthermore, the absorption bands of phosphate groupsbecame more and more intense with the increase of soaking time,indicating the continuous growth of the HA layer. The formation ofthe crystalline HA layer after immersing in SBF was also demon-strated by XRD analysis (Fig. 4d). Before soaking, only one smallwide peak at around 241 was detected, indicating amorphousnature of MBGS. After 1d of soaking in SBF, new peaks wereobserved at 2θ¼26 1(002), 321 (211), 391 (310), 461 (222), 491 (213)and 531 (004) corresponding to the poor crytallinity for HA (JCPDS09–0432), suggesting the initial presence of nanocrystalline of HA

layer. Additionally, the characteristic peaks of HA became moreand more intense with longer immersing time, which also indi-cated the growth of HA layer. Based on the analysis of SEM, FT-IRand XRD, it is evident that MBGS possess good apatite-formingbioactivity.

4. Conclusions

MBGS with a mean particle size of 560 nm and high specificsurface area were successfully synthetized by using DDA as acatalyst and template agent based the sol–gel method. This studyeffectively solved the problems of the severe agglomeration,irregular shape and low specific surface area of the conventionalsol–gel BGs particles. The MBGS also possessed good in vitroapatite-forming bioactivity. These novel strucure and property ofMBGS may turn them into a good candidate as drug carrier,biopolymer reinforcements and injectable biomaterial for bonetissue regeneration.

Acknowledgments

This work was supported by the Key Project of the NationalNatural Science Foundation of China (Grant no. 50830101), NationalNatural Science Foundation of China (Grant no. 51072055, Grant no.51172073, Grant no. 51202069), the National 973 project of China(2011CB606204), Research Fund for the Doctoral Program of HigherEducation of China (20110172110002) and the Fundamental ResearchFunds for the Central University (2012ZP0001).

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