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silica composite nanofibers membrane functionalized with mercapto groups by electrospinning
Materials Letters 64 (2010) 1295–1298
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Preparation of poly (vinyl alcohol)/silica composite nanofibers membrane functionalized with mercapto groups by electrospinning Shengju Wu, Fengting Li ⁎, Ran Xu, Shihui Wei, Hongtao Wang College of Environmental Science & Engineering, State Key Laboratory of Pollution Control and Resource Reuse Study, Tongji University, 1239, Siping Road, Shanghai, 200092, China
a r t i c l e
i n f o
Article history: Received 29 December 2009 Accepted 4 March 2010 Available online 7 March 2010 Keywords: Electrospinning Nanofibers membrane Nanomaterials Fiber technology Mesoporous Adsorption
a b s t r a c t Membranes of poly vinyl alcohol (PVA)/silica functionalized with mercapto groups are synthesized by electrospinning. Scanning electron microscopy (SEM) studies showed that the fiber diameters are in the range of 200–300 nm. The thickness of nanofiber decreases with an increase in calcination temperature. The results of Fourier transform infrared (FTIR) indicated that PVA/silica nanofibers are functionalized by mercapto groups via the hydrolysis poly-condensation method. N2 adsorption–desorption showed that organic molecules can be removed completely when the PVA/silica composite fibers are calcinated at 800 °C. The fibers calcinated at 800 °C were pure inorganic silica species with a mesoporous structure. These mercapto groups functionalized PVA/silica nanofibers have a great potential application in the field of adsorption of heavy metal ions. © 2010 Published by Elsevier B.V.
1. Introduction Adsorption technology is one of the most promising methods to extract heavy metal pollutants [1–3]. In recent years, silica-based mesoporous adsorbents have generated considerable interest due to their uniquely large specific surface area, regular pore structure and highly controllable surface properties [2,3]. Porous materials have been extensively studied because of their potential and demonstrated applications in many fields, such as adsorbents, catalyst support, electrode materials and separation media, etc. [4]. The preparation of sub-micrometer or even nanometer-sized fibers using electrospinning has been a focus of research interest in recent years [5,6]. Both polymers [7] and inorganic materials [8] can be electrospun to fibers. For example, it has been demonstrated that porous carbon fibers and metal oxide fibers can be readily synthesized via electrospinning [9]. Electrospun fibers with nanoscale pores have also been developed [6]. Nanoporous polyacrylonitrile fibers with ultrahigh surface area were fabricated by combining phase separation and electrospinning, these fibers had diameters of hundreds of nanometers with pore diameters from 8.1 nm to 23.7 nm [10]. Electrospun fibers with extremely long length and high specific surface area [11] have found extensive applications in many biomedical and industrial fields [12]. For example, electrospun fibrous membranes have shown great potential in adsorption of heavy metal ions from aqueous solutions [13]. Efficiency in removing heavy metal ions increases remarkably after the adsorbents have been modified by the functional groups of –NH2, –SH, –HSO3, respectively [1–3]. The
⁎ Corresponding author. Tel.: + 86 21 65983121; fax: + 86 21 65985059. E-mail address: [email protected] (F. Li). 0167-577X/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matlet.2010.03.012
reason is that the sulfur atom of the –SH group and heavy metal ions can form chelates [3]. Fiber mats of PVA/silica composite with different silica content were successfully prepared by electrospinning technique [14]. The fiber mats of organic–inorganic composite materials, which give some special properties of the organic–inorganic composite materials distinguished from the state of film and gels [14]. However, there are a few papers to report the preparation of membranes by electrospinning of poly (vinyl alcohol)/silica functionalized with mercapto groups. Here we report the synthesis of new nanoporous PVA/silica fibers functionalized with mercapto groups. The structure of the nanofibers was fully characterized by XRD, nitrogen adsorption–desorption analysis, FTIR and scanning electron microscopy (SEM). 2. Experiment The materials used for the synthesis of nanoporous PVA/silica fibers include 3-mercaptopropyltrimethoxysilane (MPTMS), tetraethyl orthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB), absolute ethyl alcohol and polyvinyl alcohol (PVA 1750). First, 2.19 g CTAB was dissolved in 7.37 g ethanol and vigorously stirred for 0.5 h at 60 °C. Second, 8.64 g distilled water and 1.54 g MPTMS were added into the mixture and further stirred for 0.5 h at 60 °C. Then, 6.64 g TEOS was slowly added into the solution mixture. Finally, 0.8 ml HCl (2 mol l− 1) was dropped slowly into the mixture and a silica gel was obtained after reacting for 2 h at 30 °C. 10.0 g of 10 wt.% PVA solution was dropped slowly into the silica gel, then the reaction proceeded in a water bath at 60 °C for another 4 h. Thus, a viscous gel of PVA/silica composites was obtained. The above gel of PVA/silica composites was put in a syringe. The positive terminal of a high voltage power supply was connected to the metallic syringe tip (a needle with diameter of 0.6 mm) while the
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S. Wu et al. / Materials Letters 64 (2010) 1295–1298
Fig. 1. SEM images of various fiber samples: PVA/silica composite fibers calcinated at (a) 200 °C; (b) 500 °C; (c) and (d) 800 °C.
negative terminal was connected to a conductive drum covered with aluminum foil as a collector of fibers. A voltage of 20 kV and a speed of 0.5 ml h− 1 were applied to the solution and a dense web of fibers was collected on the aluminum foil. The fibers thus formed were dried initially for 12 h at 60 °C under vacuum. Then the electrospun PVA/silica
fibers were refluxed in ethanol/HCl (molar ratio is 70:1) for 12 h at 70 °C to remove the template and dried for 6 h at 60 °C under vacuum. Finally, these nanofibers were calcinated in a muffle at 200–800 °C for 2 h. SEM images were recorded with a field emission XL-30 SEM. FTIR spectra of KBr powder-pressed pellets were recorded on a BRUKER
Fig. 2. FTIR spectra pattern of various PVA/silica nanofibers: (a) PVA/silica composite fibers; fiber samples calcinated at (b) 200 °C; (c) 500 °C; and (d) 800 °C.
S. Wu et al. / Materials Letters 64 (2010) 1295–1298
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Fig. 3. Nitrogen adsorption and desorption isotherms for (a) fibers extracted by ethanol/HCl (molar ratio of ethanol:HCl is 70:1); Nitrogen adsorption and desorption isotherms for (b) fibers calcinated at 800 °C; pore size distribution curves based on the Brunauer–Emmet–Teller (BET) method for (c) fibers calcinated at 800 °C; and XRD patterns for (d) fibers calcinated at 800 °C.
VECTOR 22 spectrometer. XRD patterns were obtained with a Siemens D5005 diffractometer. N2 adsorption–desorption isotherm was measured with a Quantachrome NOVA1000 system.
silica by means of FTIR and N2 adsorption–desorption, and still retained nanofiber morphology with diameters of 100–200 nm. 3.2. Fourier transform-infrared spectroscopy (FTIR)
3. Results and discussion 3.1. Scanning electron microscopy (SEM) SEM images of PVA/silica nanofibers spun at a voltage of 20 kV and calcinated at various temperatures are depicted in Fig. 1. After calcination, the diameter of the nanofibers decreases and their surface roughness increases compared to PVA/silica composite nanofibers before extraction by ethanol/HCl (Fig. 1). When the calcination temperature rises above 500 °C, the nanofibers begin to break. The higher the calcination temperature, the rougher the surface and the smaller the diameters of the nanofibers [15]. When the calcination temperature reaches 800 °C, the nanofibers break significantly. Furthermore, the samples in Fig. 1(c) and (d), proved to be pure
The FTIR spectra collected from PVA/silica composite fibers and those fibers calcinated at 200 °C are illustrated in Fig. 2(a) and (b). These two samples have similar FTIR peaks, indicating that there is no significant structural change in PVA/silica composite fibers upon calcination at 200 °C. The spectra of Fig. 2(a) and (b) show characteristic bands for mercapto group vibrational peak around 2559 cm− 1 [3], which proves that the silica skeleton has been successfully modified with an SH group by hydrolysis poly-condensation. When the sample calcinated at temperatures above 500 °C (Fig. 2(c)), all the peaks corresponding to organic molecules disappeared; only the peaks of νSi–O–H (3427, 1684 and 957 cm− 1) and νSi–O–Si (1084, 808 and 457 cm− 1) were observed [14]. The peaks corresponding to νSi–O–H, disappeared upon calcination at 800 °C, and the peak assigned to νSi–O–Si was enhanced (Fig. 2(d)). These results illustrate that the organic molecules can be removed
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S. Wu et al. / Materials Letters 64 (2010) 1295–1298
Table 1 The physical parameter of fibers.a Sample
1 2
Surface area (m2 g−1) 140.1 437.2
Average pore diameter (nm) N2 adsorption–desorption
SEM
6.34 7.43
4.1 6.6
Pore volume (cm3 g− 1) 0.243 0.843
a Sample 1 is fibers extracted by ethanol/HCl(molar ratio of ethanol:HCl is = 70:1), sample 2 is fibers calcinated at 800 °C.
from PVA/silica composite fibers when the calcination temperature is above 500 °C, and the fibers obtained above this temperature were pure inorganic silica species with mesoporous structure. The membranes of poly (vinyl alcohol)/silica nanofibers functionalized with mercapto groups have a great potential application in water treatment such as adsorption of heavy metal ions, dyes and other pollutants from aqueous solutions. Acknowledgements
completely from PVA/silica composite fibers when the calcination temperature is above 500 °C, and the fibers turn into pure inorganic silica species [15]. 3.3. N2 adsorption–desorption analysis We observed typical type IV isotherms with a N2 hysteresis loop in both samples, which is characteristic of a mesoporous structure. The XRD pattern of the silica nanofibers (Fig. 3(d)) indicated that the samples had hexagonal structures as pure silica. Only one strong peak was observed at 2θ = 1.60°, which corresponds to (100) reflection of pure silica. Comparing the N2 adsorption–desorption isotherm of fibers extracted by ethanol/HCl(molar ratio of ethanol:HCl is 70:1) and fibers calcinated at 800 °C, we see that the adsorption point of extracted fibers is lower at a relative pressure (P/P0) of 0.15. It indicates that the extracted fibers had a relatively wide pore size distribution and small pore diameters compared to calcinated fibers, which was confirmed by the pore analyses. The pore size distribution of these fibers was calculated based on BJH theory, as shown in Table 1. The data of Table 1 show that the surface area, pore diameter and volume of calcinated fibers increase significantly after the removal of organic groups in the mesoporous structure at high calcination temperatures (N500 °C). 4. Conclusions In the study, we demonstrated for the first time the preparation of PVA/silica nanofibers with pores functionalized with mercapto groups. The results of FTIR indicated that the mercapto groups were successfully modified to PVA/silica nanofibers by hydrolysis polycondensation. The organic molecules can be removed completely
The work was supported by State Key Special Funds for Water Program (Project Number is 2008ZX07421-002), China-American Cooperation for 10 + 10 program (2009DFA90740), and the Ministry of Science and Technology. The research was also sponsored by Science & Technology Commission, Shanghai. References [1] Mureseanu M, Reiss A, Stefanescu I, David E, Parvulescu V, Renard G, et al. Chem 2008;73:1499–504. [2] Yang H, Xu R, Xue XM, Li FT, Li GT. J Hazard Mater 2008;152:690–8. [3] Xue XM, Li FT. Microporous Mesoporous Mater 2008;116:116–22. [4] Bazuła PA, Lu AH, Nitz JJ, Schüth F. Microporous Mesoporous Mater 2008;108: 266–75. [5] Li D, Xia Y. Nano Lett 2004;4:933–8. [6] Peng M, Li DS, Shen L, Zheng Q, Chen Y. Langmuir 2006;22:9368–74. [7] Huang ZM, Zhang YZ, Kotakic M. Compos Sci Technol 2003;63:2223–5. [8] Rambuad F, Vallé K, Thibaud S, Julián-López B, Sanchez C. Adv Funct Mater 2009;19:1–10. [9] Macias M, Chacko A, Ferraris JP, Balkus KJ. Microporous Mesoporous Mater 2005;86:1–7. [10] Zhang LF, Hsieh YL. Nanotechnology 2006;17:4416–23. [11] Greiner A, Wendorff JH. Angew Chem Int Ed 2007;46:5670–3. [12] Liu HQ, Kameoka J, Czaplewski DA, Craighead HG. Nano Lett 2004;4:671–5. [13] Keyur D, Kit K, Li JJ, Zivanovic S. Biomacromolecules 2008;9:1000–6. [14] Shao CL, Kim HY, Gong J, Ding B, Lee DR, Park SJ. Mater Lett 2003;57:1579–84. [15] Shao CL, Kim HY, Gong J, Lee D. Nanotechnology 2002;13:635–7.
Contents lists available at ScienceDirect
Materials Letters j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / m a t l e t
Preparation of poly (vinyl alcohol)/silica composite nanofibers membrane functionalized with mercapto groups by electrospinning Shengju Wu, Fengting Li ⁎, Ran Xu, Shihui Wei, Hongtao Wang College of Environmental Science & Engineering, State Key Laboratory of Pollution Control and Resource Reuse Study, Tongji University, 1239, Siping Road, Shanghai, 200092, China
a r t i c l e
i n f o
Article history: Received 29 December 2009 Accepted 4 March 2010 Available online 7 March 2010 Keywords: Electrospinning Nanofibers membrane Nanomaterials Fiber technology Mesoporous Adsorption
a b s t r a c t Membranes of poly vinyl alcohol (PVA)/silica functionalized with mercapto groups are synthesized by electrospinning. Scanning electron microscopy (SEM) studies showed that the fiber diameters are in the range of 200–300 nm. The thickness of nanofiber decreases with an increase in calcination temperature. The results of Fourier transform infrared (FTIR) indicated that PVA/silica nanofibers are functionalized by mercapto groups via the hydrolysis poly-condensation method. N2 adsorption–desorption showed that organic molecules can be removed completely when the PVA/silica composite fibers are calcinated at 800 °C. The fibers calcinated at 800 °C were pure inorganic silica species with a mesoporous structure. These mercapto groups functionalized PVA/silica nanofibers have a great potential application in the field of adsorption of heavy metal ions. © 2010 Published by Elsevier B.V.
1. Introduction Adsorption technology is one of the most promising methods to extract heavy metal pollutants [1–3]. In recent years, silica-based mesoporous adsorbents have generated considerable interest due to their uniquely large specific surface area, regular pore structure and highly controllable surface properties [2,3]. Porous materials have been extensively studied because of their potential and demonstrated applications in many fields, such as adsorbents, catalyst support, electrode materials and separation media, etc. [4]. The preparation of sub-micrometer or even nanometer-sized fibers using electrospinning has been a focus of research interest in recent years [5,6]. Both polymers [7] and inorganic materials [8] can be electrospun to fibers. For example, it has been demonstrated that porous carbon fibers and metal oxide fibers can be readily synthesized via electrospinning [9]. Electrospun fibers with nanoscale pores have also been developed [6]. Nanoporous polyacrylonitrile fibers with ultrahigh surface area were fabricated by combining phase separation and electrospinning, these fibers had diameters of hundreds of nanometers with pore diameters from 8.1 nm to 23.7 nm [10]. Electrospun fibers with extremely long length and high specific surface area [11] have found extensive applications in many biomedical and industrial fields [12]. For example, electrospun fibrous membranes have shown great potential in adsorption of heavy metal ions from aqueous solutions [13]. Efficiency in removing heavy metal ions increases remarkably after the adsorbents have been modified by the functional groups of –NH2, –SH, –HSO3, respectively [1–3]. The
⁎ Corresponding author. Tel.: + 86 21 65983121; fax: + 86 21 65985059. E-mail address: [email protected] (F. Li). 0167-577X/$ – see front matter © 2010 Published by Elsevier B.V. doi:10.1016/j.matlet.2010.03.012
reason is that the sulfur atom of the –SH group and heavy metal ions can form chelates [3]. Fiber mats of PVA/silica composite with different silica content were successfully prepared by electrospinning technique [14]. The fiber mats of organic–inorganic composite materials, which give some special properties of the organic–inorganic composite materials distinguished from the state of film and gels [14]. However, there are a few papers to report the preparation of membranes by electrospinning of poly (vinyl alcohol)/silica functionalized with mercapto groups. Here we report the synthesis of new nanoporous PVA/silica fibers functionalized with mercapto groups. The structure of the nanofibers was fully characterized by XRD, nitrogen adsorption–desorption analysis, FTIR and scanning electron microscopy (SEM). 2. Experiment The materials used for the synthesis of nanoporous PVA/silica fibers include 3-mercaptopropyltrimethoxysilane (MPTMS), tetraethyl orthosilicate (TEOS), cetyltrimethyl ammonium bromide (CTAB), absolute ethyl alcohol and polyvinyl alcohol (PVA 1750). First, 2.19 g CTAB was dissolved in 7.37 g ethanol and vigorously stirred for 0.5 h at 60 °C. Second, 8.64 g distilled water and 1.54 g MPTMS were added into the mixture and further stirred for 0.5 h at 60 °C. Then, 6.64 g TEOS was slowly added into the solution mixture. Finally, 0.8 ml HCl (2 mol l− 1) was dropped slowly into the mixture and a silica gel was obtained after reacting for 2 h at 30 °C. 10.0 g of 10 wt.% PVA solution was dropped slowly into the silica gel, then the reaction proceeded in a water bath at 60 °C for another 4 h. Thus, a viscous gel of PVA/silica composites was obtained. The above gel of PVA/silica composites was put in a syringe. The positive terminal of a high voltage power supply was connected to the metallic syringe tip (a needle with diameter of 0.6 mm) while the
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Fig. 1. SEM images of various fiber samples: PVA/silica composite fibers calcinated at (a) 200 °C; (b) 500 °C; (c) and (d) 800 °C.
negative terminal was connected to a conductive drum covered with aluminum foil as a collector of fibers. A voltage of 20 kV and a speed of 0.5 ml h− 1 were applied to the solution and a dense web of fibers was collected on the aluminum foil. The fibers thus formed were dried initially for 12 h at 60 °C under vacuum. Then the electrospun PVA/silica
fibers were refluxed in ethanol/HCl (molar ratio is 70:1) for 12 h at 70 °C to remove the template and dried for 6 h at 60 °C under vacuum. Finally, these nanofibers were calcinated in a muffle at 200–800 °C for 2 h. SEM images were recorded with a field emission XL-30 SEM. FTIR spectra of KBr powder-pressed pellets were recorded on a BRUKER
Fig. 2. FTIR spectra pattern of various PVA/silica nanofibers: (a) PVA/silica composite fibers; fiber samples calcinated at (b) 200 °C; (c) 500 °C; and (d) 800 °C.
S. Wu et al. / Materials Letters 64 (2010) 1295–1298
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Fig. 3. Nitrogen adsorption and desorption isotherms for (a) fibers extracted by ethanol/HCl (molar ratio of ethanol:HCl is 70:1); Nitrogen adsorption and desorption isotherms for (b) fibers calcinated at 800 °C; pore size distribution curves based on the Brunauer–Emmet–Teller (BET) method for (c) fibers calcinated at 800 °C; and XRD patterns for (d) fibers calcinated at 800 °C.
VECTOR 22 spectrometer. XRD patterns were obtained with a Siemens D5005 diffractometer. N2 adsorption–desorption isotherm was measured with a Quantachrome NOVA1000 system.
silica by means of FTIR and N2 adsorption–desorption, and still retained nanofiber morphology with diameters of 100–200 nm. 3.2. Fourier transform-infrared spectroscopy (FTIR)
3. Results and discussion 3.1. Scanning electron microscopy (SEM) SEM images of PVA/silica nanofibers spun at a voltage of 20 kV and calcinated at various temperatures are depicted in Fig. 1. After calcination, the diameter of the nanofibers decreases and their surface roughness increases compared to PVA/silica composite nanofibers before extraction by ethanol/HCl (Fig. 1). When the calcination temperature rises above 500 °C, the nanofibers begin to break. The higher the calcination temperature, the rougher the surface and the smaller the diameters of the nanofibers [15]. When the calcination temperature reaches 800 °C, the nanofibers break significantly. Furthermore, the samples in Fig. 1(c) and (d), proved to be pure
The FTIR spectra collected from PVA/silica composite fibers and those fibers calcinated at 200 °C are illustrated in Fig. 2(a) and (b). These two samples have similar FTIR peaks, indicating that there is no significant structural change in PVA/silica composite fibers upon calcination at 200 °C. The spectra of Fig. 2(a) and (b) show characteristic bands for mercapto group vibrational peak around 2559 cm− 1 [3], which proves that the silica skeleton has been successfully modified with an SH group by hydrolysis poly-condensation. When the sample calcinated at temperatures above 500 °C (Fig. 2(c)), all the peaks corresponding to organic molecules disappeared; only the peaks of νSi–O–H (3427, 1684 and 957 cm− 1) and νSi–O–Si (1084, 808 and 457 cm− 1) were observed [14]. The peaks corresponding to νSi–O–H, disappeared upon calcination at 800 °C, and the peak assigned to νSi–O–Si was enhanced (Fig. 2(d)). These results illustrate that the organic molecules can be removed
1298
S. Wu et al. / Materials Letters 64 (2010) 1295–1298
Table 1 The physical parameter of fibers.a Sample
1 2
Surface area (m2 g−1) 140.1 437.2
Average pore diameter (nm) N2 adsorption–desorption
SEM
6.34 7.43
4.1 6.6
Pore volume (cm3 g− 1) 0.243 0.843
a Sample 1 is fibers extracted by ethanol/HCl(molar ratio of ethanol:HCl is = 70:1), sample 2 is fibers calcinated at 800 °C.
from PVA/silica composite fibers when the calcination temperature is above 500 °C, and the fibers obtained above this temperature were pure inorganic silica species with mesoporous structure. The membranes of poly (vinyl alcohol)/silica nanofibers functionalized with mercapto groups have a great potential application in water treatment such as adsorption of heavy metal ions, dyes and other pollutants from aqueous solutions. Acknowledgements
completely from PVA/silica composite fibers when the calcination temperature is above 500 °C, and the fibers turn into pure inorganic silica species [15]. 3.3. N2 adsorption–desorption analysis We observed typical type IV isotherms with a N2 hysteresis loop in both samples, which is characteristic of a mesoporous structure. The XRD pattern of the silica nanofibers (Fig. 3(d)) indicated that the samples had hexagonal structures as pure silica. Only one strong peak was observed at 2θ = 1.60°, which corresponds to (100) reflection of pure silica. Comparing the N2 adsorption–desorption isotherm of fibers extracted by ethanol/HCl(molar ratio of ethanol:HCl is 70:1) and fibers calcinated at 800 °C, we see that the adsorption point of extracted fibers is lower at a relative pressure (P/P0) of 0.15. It indicates that the extracted fibers had a relatively wide pore size distribution and small pore diameters compared to calcinated fibers, which was confirmed by the pore analyses. The pore size distribution of these fibers was calculated based on BJH theory, as shown in Table 1. The data of Table 1 show that the surface area, pore diameter and volume of calcinated fibers increase significantly after the removal of organic groups in the mesoporous structure at high calcination temperatures (N500 °C). 4. Conclusions In the study, we demonstrated for the first time the preparation of PVA/silica nanofibers with pores functionalized with mercapto groups. The results of FTIR indicated that the mercapto groups were successfully modified to PVA/silica nanofibers by hydrolysis polycondensation. The organic molecules can be removed completely
The work was supported by State Key Special Funds for Water Program (Project Number is 2008ZX07421-002), China-American Cooperation for 10 + 10 program (2009DFA90740), and the Ministry of Science and Technology. The research was also sponsored by Science & Technology Commission, Shanghai. References [1] Mureseanu M, Reiss A, Stefanescu I, David E, Parvulescu V, Renard G, et al. Chem 2008;73:1499–504. [2] Yang H, Xu R, Xue XM, Li FT, Li GT. J Hazard Mater 2008;152:690–8. [3] Xue XM, Li FT. Microporous Mesoporous Mater 2008;116:116–22. [4] Bazuła PA, Lu AH, Nitz JJ, Schüth F. Microporous Mesoporous Mater 2008;108: 266–75. [5] Li D, Xia Y. Nano Lett 2004;4:933–8. [6] Peng M, Li DS, Shen L, Zheng Q, Chen Y. Langmuir 2006;22:9368–74. [7] Huang ZM, Zhang YZ, Kotakic M. Compos Sci Technol 2003;63:2223–5. [8] Rambuad F, Vallé K, Thibaud S, Julián-López B, Sanchez C. Adv Funct Mater 2009;19:1–10. [9] Macias M, Chacko A, Ferraris JP, Balkus KJ. Microporous Mesoporous Mater 2005;86:1–7. [10] Zhang LF, Hsieh YL. Nanotechnology 2006;17:4416–23. [11] Greiner A, Wendorff JH. Angew Chem Int Ed 2007;46:5670–3. [12] Liu HQ, Kameoka J, Czaplewski DA, Craighead HG. Nano Lett 2004;4:671–5. [13] Keyur D, Kit K, Li JJ, Zivanovic S. Biomacromolecules 2008;9:1000–6. [14] Shao CL, Kim HY, Gong J, Ding B, Lee DR, Park SJ. Mater Lett 2003;57:1579–84. [15] Shao CL, Kim HY, Gong J, Lee D. Nanotechnology 2002;13:635–7.