- Email: [email protected]
Synthesis and characterization of SiO2–CaO–P2O5 hollow nanospheres for biomedical applications
Materials Letters 67 (2012) 273–276
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of SiO2–CaO–P2O5 hollow nanospheres for biomedical applications G.S. Pappas, P. Bilalis, G.C. Kordas ⁎ Institute of Materials Science, NCSR Demokritos, Agia Paraskevi Attikis, GR 153 10, Greece
a r t i c l e
i n f o
Article history: Received 6 June 2011 Accepted 24 September 2011 Available online 29 September 2011 Keywords: Sol–gel preparation Hollow nanospheres Bioceramics Biomaterials
a b s t r a c t A ternary system of SiO2–CaO–P2O5 hollow nanospheres has been successfully prepared by sol–gel method using polystyrene (PS) nanospheres as template. The inorganic shell was produced using tetraorthosilicate (TEOS) as the silica source and tri-calciumphospate as calcium and phosphorus sources, respectively. The positive surface charge of the template and the [template]/[TEOS] ratio were the key parameters for the creation of a stable primary inorganic network and the further growth of the shell. The removal of the polymeric core through a thermal treatment procedure created an inner void space with mean diameter 250 nm while the outer mean diameter was 330 nm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of inorganic nanospheres and microspheres with hollow structure is a continuously growing area with many possible applications in the fields of catalysis, separation and drug delivery [1,2]. The synthetic routes to these materials cover a wide range from template to template-free methods [3,4]. Nanosized hollow silica spheres with holes in the wall, denoted as silica nanobottles, have been successfully prepared by assembly of functional polymer nanospheres with tetraethoxysilane and removal of the core by thermal treatment [3]. In our previous work we successfully synthesized titania hollow nanospheres and studied the loading and release ability of anticorrosion inhibitors [5]. Uniform gold hollow nanospheres with tunable surface plasmon resonance controlled by interior-cavity sizes were fabricated by using cobalt nanoparticles as sacrificial templates [6]. In order to control pore size and shape, block copolymers and surfactants have been used as templates also [7,8]. In drug delivery area, the majority of the systems that are now investigated are polymeric materials that exhibit response to external stimuli. Despite that, inorganic nanospheres can also be applicable in the field of biotechnology [9]. Furthermore, the surface area of these materials can promote the formation and growth of hydroxyapatite when immersed in simulated body fluid [10,11]. In this paper we propose the synthesis of hollow nanospheres of the system SiO2–CaO–P2O5 (referred as SiCaP) with fine structure, very thin layer, large inner cavity and composition with relatively high concentration in Ca and P atoms. To our knowledge this system has not been synthesized before at the sub-micron range. The sub-micron diameter of the
⁎ Corresponding author. Tel.: + 30 2106503301; fax: +30 2106547690. E-mail address: [email protected] (G.C. Kordas). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.089
material is important when the challenge is the good packaging of the material at the site of application. 2. Experimental In order to synthesize the hollow SiCaP nanospheres the following reagents were obtained from commercial sources. Tetraethyl orthosilicate (TEOS 98%, Fluka), Tri-calcium phosphate (Fluka), 2,2-Azobis (2-methylpropionamidine) dihydrochloride (AMPA, Aldrich), ethanol and ammonium hydroxide NH4OH 33% (Panreac) were used as received . Milli-Q grade water was used in all stages of synthesis and cleaning. Styrene (Aldrich) was double distilled under reduced pressure prior to use, in order to remove the stabilizer and stored at −20 °C in a glass bottle filled with N2. The sol–gel method used to produce the core-shell structure of PS@SiCaP. Polymer templates with positive surface charge were synthesized as described elsewhere [12]. In a typical preparation, 0.6 g of the as prepared PS template was dispersed in 228 ml of ethanol under magnetic stirring in room temperature. 12 ml of Milli-water was added to the dispersion and after 10 min, 5 ml of TEOS was added to the flask. The mixture was left for 45 min under stirring in order to get TEOS hydrolyzed before condensation starts. A small amount of NH3 solution was added to start the condensation and after 40 min 1.25 g of tri-calcium phosphate was dissolved. The resultant mixture was stirred for 20 h, after which the nanospheres were collected by centrifugation, washed three times with ethanol and dried at 65 °C overnight. The removal of the polymeric core was achieved through a thermal treatment (calcination) of the solids at 525 °C for 5 h. The morphology of the samples was observed by Scanning Electron Microscopy (SEM) with a Philips Quanta Inspect microscope coupled with an energy dispersive X-ray analyzer. Infrared transmission spectra
274
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
were obtained on a Perkin Elmer Spectrum 100 FTIR Spectrometer equipped with a UATR accessory. Crystal phase characterization was conducted with X-ray powder diffraction analysis (XRD) on a SIEMENS D-500 diffractometer equipped with a CuKα lamp (λ = 1.5418 Å). Dynamic Light Scattering measurements were performed on a Zetasizer Nano ZS (Malvern Instruments). Pore size distribution and specific area were calculated from N2 adsorption–desorption isotherms. The measurements were performed on a Quntachrome Autosorb-1 porosimeter. 3. Results and discussion The SEM micrographs of the PS template and the sample SiCaP before calcination (SiCaP-BC) are shown in Fig. 1a and b, respectively. PS template was nearly monodispersed with a mean diameter 250 nm. In sample SiCaP-BC, the diameter increased with the shell formation to 330 nm and no significant polydispersity was observed. In Fig. 1c, the micrograph of the same sample after calcination at 525 °C (SiCaP-AC1) is shown. The diameter was not changed, indicating that the shrinkage of the network during thermal treatment was not significant. The shell was very thin with smooth and homogenous surface. A hole on the shell of some nanospheres was formed during the removal of the polystyrene core. Shell thickness was calculated from the difference between the template and core-shell structure diameters and was found to be 40 nm. Repeating the synthesis using slightly larger templates (~285 nm, Supplementary Fig. S1), the formed layer had smaller thickness (~23 nm) and didn't break after calcination at 400 °C for 5 h (SiCaP-AC2, Fig. 1d). The transparency of nanospheres in Fig. 1d is due to the very thin shell and the high voltage operating electron beam from the instrument. Also the difference of the density between the hollow cavity and the shell can be observed in this micrograph. The EDX analysis showed the composition of the sample after the removal of the core. Fig. 2b demonstrates the EDX graph for the elements analysis and Table 1 shows the weight percentage of the elements and oxides.
Elemental mapping was performed in the sample SiCaP-AC1, in order to confirm the homogenous distribution of the elements in the structure (Supplementary Fig. S2). The spectra comparison between the sample before (SiCaP-BC) and after calcination, at 525 °C (SiCaP-AC1) is shown in Fig. 2a. Before calcination, the peaks at 698, 755, 1452, 1493 and 3026 cm − 1 and the peak at 2918 cm − 1 correspond to the phenyl and the CH2 groups of the polystyrene core, respectively [12]. There are three characteristic peaks at 1058, 796 and 447 cm− 1, belonging to the three fundamental vibration bands for the silica structure. The peak at 965 cm− 1 derives from Si–OH stretch vibrations [13,14]. After calcination (SiCaP-AC1), only a small part of the 965 cm− 1 band remains, converted to a shoulder in that range, while the primary peak at 1058 cm− 1 increases slightly and shifts to 1039 cm− 1. The peak at 796 cm− 1, before calcination, shifts to 807 cm − 1, indicating the transformation of the Si–OH groups to Si–O–Si and Si–O–Ca bridges [15]. According to literature, the nucleation of HAp starts from the Si–OH groups while the Ca2+ ions that flow from the material, increase the concentration of the solution (body fluid) [16]. The sharp peaks at 602 and 562 cm − 1 from the crystalline structure of the PO−4 tetrahedral are present in both samples. The PO−4 tetrahedral has an additional peak at 1033 cm− 1 that overlaps with the anti-symmetric stretch of Si–O–Si and shifts the main peak at 1067 cm − 1 to lower wavenumbers. The presence of absorbed water (moisture) in sample before calcination, is supported by the appearance of the bending mode at 1640 cm− 1 and the stretching mode at 3400 cm − 1. Dynamic Light Scattering measurements were performed in order to estimate the value of ζ-potential of the polystyrene template and the samples after the shell formation and thermal treatment (SiCaP-AC1). The ζ-potential of PS template has positive value, 35 mV, showing the colloidal character of the material. The primary hydrolyzed species have negative charge and are attracted very fast on the positive surface of the PS nanospheres. The forming silica network is the key factor for a stable shell after the core removal. After the coating, ζ-potential has a
Fig. 1. SEM micrographs of a) polystyrene template, b) SiCaP before calcination, c) SiCaP-AC1 after calcination at 525 °C and d) SiCaP-AC2 after calcination at 400 °C.
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
275
Fig. 2. a) Infrared spectra of the sample before and after calcination at 525 °C and b) EDX analysis after calcination at 525 °C.
negative value of −53 mV indicating the creation of the shell and after the thermal treatment decreases to −37 mV indicating the presence of terminating −OH groups which were decreased with calcination. The XRD analysis of the sample SiCaP-AC1 showed the presence of a crystal phase corresponding to the calcium phosphate hexagonal apatite structure and some tri-calcium phosphate. The broad peak between 20°–24° 2θ comes from the amorphous structure of the silica network. The broad peaks at 25.8° and 31.8° correspond to (002) and (211) reflections and the weaken intensities at 46.8°, 49.7° and 53.5° correspond to (222),(213) and (004) reflections with poor crystallinity of the apatitelike structure, respectively (JCPDS # 9-432) (Fig. 3). Other reflections are missing due to poor crystallinity and broadening of the peaks. According to literature, the broad peak at 32° consists of the reflections from (211) at 31.87°, (112) at 32.18° and (300) at 32.87° [17]. Deconvolution of this peak was performed using Lorentzian profiles (Supplementary Fig. S3). There is a significant difference in the specific area, between the sample before and after calcination (SiCaP-AC1), calculated with B.E.T equation. The specific area before calcination is 9.9 m2/g and increased to 23.8 m 2/g after calcination. The isotherm of the sample after the calcination, showed a hysteresis loop on the desorption branch which is Type IV, characteristic for mesoporous materials. This result indicates the formation of a defined pore network and the contribution of the hollow structure (inner shell surface) to the increase of the specific area. The relative low specific area compared to other similar structures is due to the very thin shell thickness. Before calcination the pore volume and diameter was 0.056 cm3/g and 2.14 nm, respectively. After calcination only the pore volume increased to 0.194 cm3/g while the pore diameter did not change. Also the total pore volume calculated at 0.99 of the relative pressure increased from 0.061 cm3/g to 0.193 cm3/g for the sample before calcination. These results demonstrate that there is a significant increase in the number of the pores without changing the pore diameter.
Table 1 EDX analysis of the elements and oxides. Oxide
Wt.%
Elements
Wt.%
SiO2 CaO P2O5
59.83 24.49 15.67
O Si Ca P
41.37 30.97 19.81 7.85
Fig. 3. a) XRD pattern of the sample SiCaP-AC1 after calcination and b) Adsorption–desorption isotherms of the samples before and after calcination at 525 °C.
276
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
4. Conclusions
References
Hollow nanospheres of the ternary system SiO2–CaO–P2O5 have been successfully prepared through a sol–gel/template route. Fine structures were obtained after calcination of the material and the outer mean diameter was 330 nm with a shell thickness 40 nm, approximately. The nanospheres consist of an amorphous silica phase and an apatite-like crystal phase. A cavity, of about 250 nm, was created after the core removal and the specific surface area and total pore volume were increased significantly. The above properties make this material suitable for bone tissue regeneration applications due to the biocompatible chemical composition. Also, a smart drug delivery system could be produced for targeted cancer therapy, through a surface modification (e.g. SI-ATRP).
[1] Kim S-W, Kim M, Lee WY, Hyeon TJ. Am Chem Soc 2002;124:7642–3. [2] Sudeshna Chandra KC, Barick D, Bahadur. Oxide and hybrid nanostructures for therapeutic applications. Adv Drug Deliv Rev 2011, doi:10.1016/j.addr.2011.06.003. [3] Zhang G, Yu Y, Chen X, Han Y, Di Y, Yang B, et al. J Colloid Interface Sci 2003;263: 467–72. [4] Yang Hua Gui, Zeng Hua Chun. J Phys Chem B 2004;108:3492–5. [5] Kartsonakis IA, Danilidis IL, Pappas GS, Kordas GC. J Nanosci Nanotechnol 2010;10:1–9. [6] Liang Han-Pu, Wan Li-Jun, Bai Chun-Li, Jiang Li. J Phys Chem B 2005;109: 7795–800. [7] Lee Yong-Geun, Chul Oh, Yoo Sang-Ki, Koo Sang-Man, Oh Seong-Geun. Micropor Mesopor Mater 2005;86:134–44. [8] Khanal A, Inoue Y, Yada M, Nakashima K. J Am Chem Soc 2007;129:1534–5. [9] Slowing Igor I, Vivero-Escoto JL, Chia-Wen Wu, Lin Victor S-Y. Adv Drug Deliv Rev 2008;60:1278–88. [10] Zhao Shan, Li Yanbao, Li Dongxu. Micropor Mesopor Mater 2010;135:67–73. [11] Yun Hui-suk, Kim Sang-hyun, Lee Soyoung, Song In-hyuck. Mater Lett 2010;64: 1850–3. [12] Kartsonakis IA, Liatsi P, Danilidis I, Bouzarelou D, Kordas G. J Phys Chem Solids 2008;69:214–21. [13] Le Yuan, Min Pu, Chen Jian-feng. Mater Res Bull 2006;41:1714–9. [14] Civalleri B, Garrone E, Ugliengo P. Chem Phys Lett 1998;294:103–8. [15] Saravanapavan P, Hench Larry L. Solids 2003;318:1–13. [16] Tadashi Kokubo. Acta Mater 1998;46:2519–27. [17] Müller Lenka, Müller Frank A. Acta Biomater 2006;2:181–9.
Acknowledgments This work was funded by the European Research Council (ERC) under the project “NANOTHERAPY” with reference number 232959. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.09.089.
Contents lists available at SciVerse ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/matlet
Synthesis and characterization of SiO2–CaO–P2O5 hollow nanospheres for biomedical applications G.S. Pappas, P. Bilalis, G.C. Kordas ⁎ Institute of Materials Science, NCSR Demokritos, Agia Paraskevi Attikis, GR 153 10, Greece
a r t i c l e
i n f o
Article history: Received 6 June 2011 Accepted 24 September 2011 Available online 29 September 2011 Keywords: Sol–gel preparation Hollow nanospheres Bioceramics Biomaterials
a b s t r a c t A ternary system of SiO2–CaO–P2O5 hollow nanospheres has been successfully prepared by sol–gel method using polystyrene (PS) nanospheres as template. The inorganic shell was produced using tetraorthosilicate (TEOS) as the silica source and tri-calciumphospate as calcium and phosphorus sources, respectively. The positive surface charge of the template and the [template]/[TEOS] ratio were the key parameters for the creation of a stable primary inorganic network and the further growth of the shell. The removal of the polymeric core through a thermal treatment procedure created an inner void space with mean diameter 250 nm while the outer mean diameter was 330 nm. © 2011 Elsevier B.V. All rights reserved.
1. Introduction Synthesis of inorganic nanospheres and microspheres with hollow structure is a continuously growing area with many possible applications in the fields of catalysis, separation and drug delivery [1,2]. The synthetic routes to these materials cover a wide range from template to template-free methods [3,4]. Nanosized hollow silica spheres with holes in the wall, denoted as silica nanobottles, have been successfully prepared by assembly of functional polymer nanospheres with tetraethoxysilane and removal of the core by thermal treatment [3]. In our previous work we successfully synthesized titania hollow nanospheres and studied the loading and release ability of anticorrosion inhibitors [5]. Uniform gold hollow nanospheres with tunable surface plasmon resonance controlled by interior-cavity sizes were fabricated by using cobalt nanoparticles as sacrificial templates [6]. In order to control pore size and shape, block copolymers and surfactants have been used as templates also [7,8]. In drug delivery area, the majority of the systems that are now investigated are polymeric materials that exhibit response to external stimuli. Despite that, inorganic nanospheres can also be applicable in the field of biotechnology [9]. Furthermore, the surface area of these materials can promote the formation and growth of hydroxyapatite when immersed in simulated body fluid [10,11]. In this paper we propose the synthesis of hollow nanospheres of the system SiO2–CaO–P2O5 (referred as SiCaP) with fine structure, very thin layer, large inner cavity and composition with relatively high concentration in Ca and P atoms. To our knowledge this system has not been synthesized before at the sub-micron range. The sub-micron diameter of the
⁎ Corresponding author. Tel.: + 30 2106503301; fax: +30 2106547690. E-mail address: [email protected] (G.C. Kordas). 0167-577X/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2011.09.089
material is important when the challenge is the good packaging of the material at the site of application. 2. Experimental In order to synthesize the hollow SiCaP nanospheres the following reagents were obtained from commercial sources. Tetraethyl orthosilicate (TEOS 98%, Fluka), Tri-calcium phosphate (Fluka), 2,2-Azobis (2-methylpropionamidine) dihydrochloride (AMPA, Aldrich), ethanol and ammonium hydroxide NH4OH 33% (Panreac) were used as received . Milli-Q grade water was used in all stages of synthesis and cleaning. Styrene (Aldrich) was double distilled under reduced pressure prior to use, in order to remove the stabilizer and stored at −20 °C in a glass bottle filled with N2. The sol–gel method used to produce the core-shell structure of PS@SiCaP. Polymer templates with positive surface charge were synthesized as described elsewhere [12]. In a typical preparation, 0.6 g of the as prepared PS template was dispersed in 228 ml of ethanol under magnetic stirring in room temperature. 12 ml of Milli-water was added to the dispersion and after 10 min, 5 ml of TEOS was added to the flask. The mixture was left for 45 min under stirring in order to get TEOS hydrolyzed before condensation starts. A small amount of NH3 solution was added to start the condensation and after 40 min 1.25 g of tri-calcium phosphate was dissolved. The resultant mixture was stirred for 20 h, after which the nanospheres were collected by centrifugation, washed three times with ethanol and dried at 65 °C overnight. The removal of the polymeric core was achieved through a thermal treatment (calcination) of the solids at 525 °C for 5 h. The morphology of the samples was observed by Scanning Electron Microscopy (SEM) with a Philips Quanta Inspect microscope coupled with an energy dispersive X-ray analyzer. Infrared transmission spectra
274
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
were obtained on a Perkin Elmer Spectrum 100 FTIR Spectrometer equipped with a UATR accessory. Crystal phase characterization was conducted with X-ray powder diffraction analysis (XRD) on a SIEMENS D-500 diffractometer equipped with a CuKα lamp (λ = 1.5418 Å). Dynamic Light Scattering measurements were performed on a Zetasizer Nano ZS (Malvern Instruments). Pore size distribution and specific area were calculated from N2 adsorption–desorption isotherms. The measurements were performed on a Quntachrome Autosorb-1 porosimeter. 3. Results and discussion The SEM micrographs of the PS template and the sample SiCaP before calcination (SiCaP-BC) are shown in Fig. 1a and b, respectively. PS template was nearly monodispersed with a mean diameter 250 nm. In sample SiCaP-BC, the diameter increased with the shell formation to 330 nm and no significant polydispersity was observed. In Fig. 1c, the micrograph of the same sample after calcination at 525 °C (SiCaP-AC1) is shown. The diameter was not changed, indicating that the shrinkage of the network during thermal treatment was not significant. The shell was very thin with smooth and homogenous surface. A hole on the shell of some nanospheres was formed during the removal of the polystyrene core. Shell thickness was calculated from the difference between the template and core-shell structure diameters and was found to be 40 nm. Repeating the synthesis using slightly larger templates (~285 nm, Supplementary Fig. S1), the formed layer had smaller thickness (~23 nm) and didn't break after calcination at 400 °C for 5 h (SiCaP-AC2, Fig. 1d). The transparency of nanospheres in Fig. 1d is due to the very thin shell and the high voltage operating electron beam from the instrument. Also the difference of the density between the hollow cavity and the shell can be observed in this micrograph. The EDX analysis showed the composition of the sample after the removal of the core. Fig. 2b demonstrates the EDX graph for the elements analysis and Table 1 shows the weight percentage of the elements and oxides.
Elemental mapping was performed in the sample SiCaP-AC1, in order to confirm the homogenous distribution of the elements in the structure (Supplementary Fig. S2). The spectra comparison between the sample before (SiCaP-BC) and after calcination, at 525 °C (SiCaP-AC1) is shown in Fig. 2a. Before calcination, the peaks at 698, 755, 1452, 1493 and 3026 cm − 1 and the peak at 2918 cm − 1 correspond to the phenyl and the CH2 groups of the polystyrene core, respectively [12]. There are three characteristic peaks at 1058, 796 and 447 cm− 1, belonging to the three fundamental vibration bands for the silica structure. The peak at 965 cm− 1 derives from Si–OH stretch vibrations [13,14]. After calcination (SiCaP-AC1), only a small part of the 965 cm− 1 band remains, converted to a shoulder in that range, while the primary peak at 1058 cm− 1 increases slightly and shifts to 1039 cm− 1. The peak at 796 cm− 1, before calcination, shifts to 807 cm − 1, indicating the transformation of the Si–OH groups to Si–O–Si and Si–O–Ca bridges [15]. According to literature, the nucleation of HAp starts from the Si–OH groups while the Ca2+ ions that flow from the material, increase the concentration of the solution (body fluid) [16]. The sharp peaks at 602 and 562 cm − 1 from the crystalline structure of the PO−4 tetrahedral are present in both samples. The PO−4 tetrahedral has an additional peak at 1033 cm− 1 that overlaps with the anti-symmetric stretch of Si–O–Si and shifts the main peak at 1067 cm − 1 to lower wavenumbers. The presence of absorbed water (moisture) in sample before calcination, is supported by the appearance of the bending mode at 1640 cm− 1 and the stretching mode at 3400 cm − 1. Dynamic Light Scattering measurements were performed in order to estimate the value of ζ-potential of the polystyrene template and the samples after the shell formation and thermal treatment (SiCaP-AC1). The ζ-potential of PS template has positive value, 35 mV, showing the colloidal character of the material. The primary hydrolyzed species have negative charge and are attracted very fast on the positive surface of the PS nanospheres. The forming silica network is the key factor for a stable shell after the core removal. After the coating, ζ-potential has a
Fig. 1. SEM micrographs of a) polystyrene template, b) SiCaP before calcination, c) SiCaP-AC1 after calcination at 525 °C and d) SiCaP-AC2 after calcination at 400 °C.
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
275
Fig. 2. a) Infrared spectra of the sample before and after calcination at 525 °C and b) EDX analysis after calcination at 525 °C.
negative value of −53 mV indicating the creation of the shell and after the thermal treatment decreases to −37 mV indicating the presence of terminating −OH groups which were decreased with calcination. The XRD analysis of the sample SiCaP-AC1 showed the presence of a crystal phase corresponding to the calcium phosphate hexagonal apatite structure and some tri-calcium phosphate. The broad peak between 20°–24° 2θ comes from the amorphous structure of the silica network. The broad peaks at 25.8° and 31.8° correspond to (002) and (211) reflections and the weaken intensities at 46.8°, 49.7° and 53.5° correspond to (222),(213) and (004) reflections with poor crystallinity of the apatitelike structure, respectively (JCPDS # 9-432) (Fig. 3). Other reflections are missing due to poor crystallinity and broadening of the peaks. According to literature, the broad peak at 32° consists of the reflections from (211) at 31.87°, (112) at 32.18° and (300) at 32.87° [17]. Deconvolution of this peak was performed using Lorentzian profiles (Supplementary Fig. S3). There is a significant difference in the specific area, between the sample before and after calcination (SiCaP-AC1), calculated with B.E.T equation. The specific area before calcination is 9.9 m2/g and increased to 23.8 m 2/g after calcination. The isotherm of the sample after the calcination, showed a hysteresis loop on the desorption branch which is Type IV, characteristic for mesoporous materials. This result indicates the formation of a defined pore network and the contribution of the hollow structure (inner shell surface) to the increase of the specific area. The relative low specific area compared to other similar structures is due to the very thin shell thickness. Before calcination the pore volume and diameter was 0.056 cm3/g and 2.14 nm, respectively. After calcination only the pore volume increased to 0.194 cm3/g while the pore diameter did not change. Also the total pore volume calculated at 0.99 of the relative pressure increased from 0.061 cm3/g to 0.193 cm3/g for the sample before calcination. These results demonstrate that there is a significant increase in the number of the pores without changing the pore diameter.
Table 1 EDX analysis of the elements and oxides. Oxide
Wt.%
Elements
Wt.%
SiO2 CaO P2O5
59.83 24.49 15.67
O Si Ca P
41.37 30.97 19.81 7.85
Fig. 3. a) XRD pattern of the sample SiCaP-AC1 after calcination and b) Adsorption–desorption isotherms of the samples before and after calcination at 525 °C.
276
G.S. Pappas et al. / Materials Letters 67 (2012) 273–276
4. Conclusions
References
Hollow nanospheres of the ternary system SiO2–CaO–P2O5 have been successfully prepared through a sol–gel/template route. Fine structures were obtained after calcination of the material and the outer mean diameter was 330 nm with a shell thickness 40 nm, approximately. The nanospheres consist of an amorphous silica phase and an apatite-like crystal phase. A cavity, of about 250 nm, was created after the core removal and the specific surface area and total pore volume were increased significantly. The above properties make this material suitable for bone tissue regeneration applications due to the biocompatible chemical composition. Also, a smart drug delivery system could be produced for targeted cancer therapy, through a surface modification (e.g. SI-ATRP).
[1] Kim S-W, Kim M, Lee WY, Hyeon TJ. Am Chem Soc 2002;124:7642–3. [2] Sudeshna Chandra KC, Barick D, Bahadur. Oxide and hybrid nanostructures for therapeutic applications. Adv Drug Deliv Rev 2011, doi:10.1016/j.addr.2011.06.003. [3] Zhang G, Yu Y, Chen X, Han Y, Di Y, Yang B, et al. J Colloid Interface Sci 2003;263: 467–72. [4] Yang Hua Gui, Zeng Hua Chun. J Phys Chem B 2004;108:3492–5. [5] Kartsonakis IA, Danilidis IL, Pappas GS, Kordas GC. J Nanosci Nanotechnol 2010;10:1–9. [6] Liang Han-Pu, Wan Li-Jun, Bai Chun-Li, Jiang Li. J Phys Chem B 2005;109: 7795–800. [7] Lee Yong-Geun, Chul Oh, Yoo Sang-Ki, Koo Sang-Man, Oh Seong-Geun. Micropor Mesopor Mater 2005;86:134–44. [8] Khanal A, Inoue Y, Yada M, Nakashima K. J Am Chem Soc 2007;129:1534–5. [9] Slowing Igor I, Vivero-Escoto JL, Chia-Wen Wu, Lin Victor S-Y. Adv Drug Deliv Rev 2008;60:1278–88. [10] Zhao Shan, Li Yanbao, Li Dongxu. Micropor Mesopor Mater 2010;135:67–73. [11] Yun Hui-suk, Kim Sang-hyun, Lee Soyoung, Song In-hyuck. Mater Lett 2010;64: 1850–3. [12] Kartsonakis IA, Liatsi P, Danilidis I, Bouzarelou D, Kordas G. J Phys Chem Solids 2008;69:214–21. [13] Le Yuan, Min Pu, Chen Jian-feng. Mater Res Bull 2006;41:1714–9. [14] Civalleri B, Garrone E, Ugliengo P. Chem Phys Lett 1998;294:103–8. [15] Saravanapavan P, Hench Larry L. Solids 2003;318:1–13. [16] Tadashi Kokubo. Acta Mater 1998;46:2519–27. [17] Müller Lenka, Müller Frank A. Acta Biomater 2006;2:181–9.
Acknowledgments This work was funded by the European Research Council (ERC) under the project “NANOTHERAPY” with reference number 232959. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.matlet.2011.09.089.