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Preparation of SiO2 nanowires from rice husks by hydrothermal method and the RNA purification performance
Chemical Physics Letters 662 (2016) 42–46
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Preparation of SiO2 nanowires from rice husks by hydrothermal method and the RNA purification performance Meiyan Huang a, Jianping Cao a, Xing Meng a, Yangsi Liu b, Wei Ke a, Jialiang Wang c, Ling Sun a,b,⇑ a
Material and Industrial Technology Research Institute Beijing, Boda Building North, No. 28 Life Science Park Road, Changping District, Beijing 102206, China Beijing Guyue New Materials Research Institute, Beijing University of Technology, 100 Ping Le Yuan, Chaoyang District, Beijing 100124, China c Shanghai Ribo-Technology Company Limited, Room 407, 4th Building of Juke Biotech Park, No. 333 Guiping Road, Xuhui District, Shanghai 200233, China b
a r t i c l e
i n f o
Article history: Received 29 June 2016 In final form 7 September 2016 Available online 9 September 2016 Keywords: SiO2 nanowires Hydrothermal method Composite filter paper RNA purification
a b s t r a c t In this study, SiO2 nanowires were prepared by using rice husks as silicon source via a hydrothermal method. The microstructure, thermal stability and morphology of SiO2 nanowires were characterized by X-ray diffraction, infrared spectroscopy, thermal gravimetric analysis and scanning electron microscope. SiO2 nanowires with a diameter of 30–100 nm were obtained and the formation mechanism of SiO2 nanowires during the hydrothermal reaction was proposed. The SiO2 nanowires were introduced into membrane spin columns to isolate RNA and the values of A260/280 and A260/230 were 2.0– 2.1 and 1.8–2.0, respectively, which shows the SiO2 nanowires were effective for RNA purification. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanomaterials with large length-todiameter aspect ratio, high storage density and good integration features, have been widely applied in nano-sized electronics, optics, optical integrated circuits, biological devices, and so on [1,2]. SiO2 nanowires are typical 1D inorganic nanomaterials which show many unique properties, such as high insulation, good fluorescence, and high active surface area, endowing themselves high potential in the catalysis, waveguide, sensing and photoluminescence applications [3–10]. Many techniques have been reported to obtain SiO2 nanowires, including template method [11], chemical vapor deposition [12–14], electrostatic spinning [15], metal catalysis [16–19], and thermal evaporation [20,21]. These methods are either easy to introduce more complex ingredients into the reaction system, or of high cost as equipment for high temperatures and pressures is normally needed. Hydrothermal method, however, attracts more attention for SiO2 nanowires preparation since it allows of mild reaction conditions, simple instruments and facile operation [22]. Easy-to-get low-cost raw materials are also necessary to produce SiO2 nanowires. Rice husks, occupying 20% weight of rice and containing 17–20% silica, are the largest number of
⇑ Corresponding author at: Material and Industrial Technology Research Institute Beijing, Boda Building North, No. 28 Life Science Park Road, Changping District, Beijing 102206, China. E-mail address: [email protected] (L. Sun). http://dx.doi.org/10.1016/j.cplett.2016.09.012 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
by-products in rice processing [21,23]. According to Food and Agriculture Organization of the United Nations, the global rice production is more than 740 million tons per year and China tops the production list, accounting for more than 30% of the total amount [24]. Therefore, effective recycling and reusing of rice husks are favorable for environmental protection and sustainable development from a global view. By far, there have been considerable studies on the preparation of silica nanoparticles by using rice husks [8,25], but rice husk-derived SiO2 1D nanowires are rarely reported. Herein, SiO2 nanowires were massively fabricated from rice husks by a hydrothermal method, and then, the nanowires were mixed with pulp to form composite filter papers. The growth mechanism and RNA purification ability of SiO2 nanowires were also studied. We believe the hydrothermal fabrication and RNA purification performance of SiO2 nanowires endue rice husks the great potential in bio-engineering and industrialization.
2. Experimental 2.1. Materials Rice husks are agricultural by-products, and they were cleaned and dried before experiments. Nine hydrated ferric nitrate (Fe (NO3)39H2O), citric acid (C6H8O7), hydrochloric acid (HCl) and ethane diamine (C2H8N2) are analytical grade and they were used without further purification. Deionized water was used throughout
M. Huang et al. / Chemical Physics Letters 662 (2016) 42–46
the experiments. The pulp was purchased from Shandong Feicheng Zhongheng paper company Co., Ltd. 2.2. Preparation of SiO2 nanowires
2.2.1. Purification and calcination of rice husks A certain amount of rice husks were immersed in diluted citric acid (1 wt%) solution and stirred intensively. They were transferred to an autoclave and maintain at 80 °C for 3 h to remove impurity elements, such as K, Na, and Mg. The rice husks were then washed several times until they became neutral and dried thoroughly. The purified rice husks were calcined at 700 °C for 2 h and ground to get rice husks ashes (RHA). 2.2.2. RHA/Fe2O3 intermediates 2 g RHA were mixed with 12.12 g Fe(NO3)39H2O by sufficiently grinding and the mixture were calcined at 400 °C for 4 h to form RHA/Fe2O3 composite powders. 2.2.3. Hydrothermal reaction 4 g RHA/Fe2O3 powders were put in an 200 ml autoclave with 104 ml ethylene diamine solution (C2H8N2:H2O = 8:5, vol:vol). The autoclave was heated up to 200 °C and the hydrothermal reaction was lasted for 4 days. 2.2.4. Post-treatment The resultant was centrifuged and washed by using deionized water several times, after that the wet powders were stirred in 12.5% HCl solution at 60 °C for 3 h to remove Fe2O3. The final precipitate was centrifuged, washed, and dried to get the milk white SiO2 nanowires. The preparation procedures from rice husks to SiO2 nanowires were illustrated in Fig. 1. 2.3. Preparation of SiO2 nanowires composite filter paper A certain amount of SiO2 nanowires (40–80 wt%) and pulp were mixed vigorously by a fiber standard dissociator for 5–10 min to get uniform slurry. The slurry was processed into filter papers by a paper-making machine (PL6-C, Xianyang Taisite Instrument Company). As-made SiO2 nanowires/pulp composite filter papers were tailored and fitted into spin columns, as shown in Fig. 2, for RNA purification. 2.4. Characterization The morphology of silica nanowires was obtained using a Hitachi S-3400N scanning electron microscope (SEM) with an
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accelerating voltage of 15 kV. X-ray diffraction (XRD) patterns were collected on a Rigaku DMAX-RB 12 KW powder diffractometer with Cu Ka radiation (k = 1.5406 Å) at 35 mA and 40 kV, and the scanning rate was 6°/min, 2h angle ranged from 5° to 80°. The synthesized materials were also characterized with Fourier transform infrared spectroscopy (FT-IR 850, Tianjin Gangdong SCI. & TECH. Development Co., Ltd.) and thermogravimetric analyzer (TGA, HCT-3, Beijing Henven Scientific Instrument Factory) with a working temperature ranging from 30 to 1200 °C. The BET characterization was finished by a high-performance automatic gas sorption analyzer with 6 channels (model: Autosorb-6iSA, USA). The RNA purification efficiency of the SiO2 nanowires was tested by a Thermo Scientific NanoDrop 2000/2000C spectrophotometer. 3. Results and discussion The crystal structures of RHA, RHA/Fe2O3 composites and SiO2 nanowires were revealed by their XRD patterns as shown in Fig. 3. Only one peak at 2h = 22.5° can be seen in both patterns of RHA and SiO2 nanowires, which is the characteristic absorption peak of amorphous SiO2. For RHA/Fe2O3 composites, strong characteristic absorption peaks appear at 2h = 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.2°, 62.6° and 63.9°, respectively, which correspond to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) crystal planes of a-Fe2O3. The absence of SiO2 phases in RHA/Fe2O3 composites may attribute to the major component of Fe2O3, which overlaid SiO2 signals. BET results were summarized in Table 1. Compared with RHA and RHA/Fe2O3 composites, SiO2 nanowires have the largest surface area and pore volume. This could be due to the special 1D structure of nanowires with high length-to-diameter aspect ratio and the stacking of SiO2 nanowires, which may generate a lot of pores. The over-loading of Fe2O3 in RHA/Fe2O3 composites decreased the surface area and pore volume, making them even lower than RHA. Fig. 4 shows FT-IR spectra of SiO2 nanowires under different hydrothermal reaction times (1d, 2d, 3d and 4d). The strong and wide absorption band at 1097 cm 1 is the SiAOASi antisymmetric stretching vibration peak. The bands at 800 cm 1, 586 cm 1 and 467 cm 1 are the symmetric stretching vibration peaks of SiAO, and the intensity of band at 960 cm 1 is associated with the bending vibration of SiAOH. These bands aforementioned are all the characteristic absorption peaks of SiO2. The wide band at 3432 cm 1 is the antisymmetric stretching vibration peak of AOH, which can be attributed to the adsorbed water and constitution water. The band near 1634 cm 1 is the HAOAH bending vibration peak. The FT-IR spectra of SiO2 nanowires for 3d and 4d are quite similar, which means SiO2 nanowires could be matured after hydrothermal reaction for 3d.
Fig. 1. Schematic diagram of the formation of SiO2 nanowires from rice husks.
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Fig. 2. Structure diagram of spin columns for RNA purification.
Fig. 3. XRD patterns of RHA, RHA/Fe2O3 composites and SiO2 nanowires.
Table 1 BET results of RHA, RHA/Fe2O3 composites and SiO2 nanowires. Samples
BET surface area/(cm2/g)
Pore volume/(cm3/g)
RHA RHA/Fe2O3 composites SiO2 nanowires
256.33 55.08 381.85
0.36 0.17 1.03
Fig. 4. IR spectra of SiO2 nanowires under different hydrothermal reaction times.
TGA were used to give further information on the phase structure of SiO2 nanowires under different hydrothermal reaction times as present in Fig. 5. The weight loss for all samples is rapid before 100 °C because of the evaporation of surface adsorbed water. After dehydration, TGA plots become less steep and the weight loss is stale and gentle. During this period, hydroxide groups (AOH) on SiO2 surfaces are removed gradually with the temperature rising. The total weight loss increases with hydrothermal reaction time extending since more nanowires will be generated and more water and hydroxide groups can be attached. The TGA plots of 3d and 4d are almost the same, meaning the similar weight loss behavior, and it is in good agreement with the FT-IR results. The morphological evolution of SiO2 nanowires grown for different reaction time is demonstrated in Fig. 6. Many tiny crystals were aggregated and constituted large clusters after reaction for 1d. Some short and fine wire-shaped SiO2 sprouted from the cluster matrix, which were the embryos of SiO2 nanowires (Fig. 6a). The clustered crystals were reduced and a lot of nanowires were formed after 2d (Fig. 6b). The SiO2 nanowires became more and more obvious as the extension of reaction time until 4d (Fig. 6c– d). From Fig. 6d, the diameter of SiO2 nanowires was estimated to be 30–100 nm after a 4d reaction. We believe that Fe2O3 plays a vital role for SiO2 nanowires formation in the C2H8N2-assisted hydrothermal reaction, which is analogous to the growth process of GeOx/C2H8N2 nanowires [26]. The growth mechanism of SiO2 nanowires is demonstrated in Scheme 1. First, C2H8N2 molecules concentrate on the surface of Fe2O3 for the coordination between C2H8N2 and Fe3+. The lone pair of electrons on the N atoms transfers to Fe3+ due to its weak oxidative ability in an alkaline solution. Meanwhile, due to the oxidation tendency of C2H8N2, the O atoms in AOH groups on
Fig. 5. TGA of SiO2 nanowires under different hydrothermal reaction time.
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Scheme 1. Schematic diagram of the growth mechanism of SiO2 nanowires.
Table 2 RNA purification results of SiO2 nanowires/pulp composite filter paper and Qiagen 74104. RNA purification indicators
A260/280
A260/230
40% SiO2 composite filter paper 60% SiO2 composite filter paper 80% SiO2 composite filter paper Qiagen 74104
2.04 2.06 2.11 2.06
1.79 1.93 1.86 1.97
the surface of SiO2 associate with the H atoms in ANH2 to form (NAH OASi) H-bonding. Then, adjacent SiAOH bonds begin to condense at 200 °C in aqueous solution, producing primary C2H8N2–SiO2 hybrid units on the Fe2O3 surface via (NAH OASi) H-bonding. The hybrid units will be detached while other C2H8N2 molecules continue combining with Fe3+. The attached C2H8N2 will hinder the in-plane growth of SiO2 nanowires, but favor the anisotropic growth along the vertical direction, resulting the 1D nanostructures, that is SiO2 nanowires. Five pieces of SiO2 nanowires composite filter papers were integrated in spin columns individually for RNA purification and the intensity ratios at the highest absorption peaks A260/A280 and A260/A230 were tested. The results were shown in Table 2. The values of A260/A280 and A260/A230 are the common indicators for measuring the purity of RNA and they are in the range of 2.0– 2.1 and 1.8–2.0 for the composite filter papers, respectively, when
the content of SiO2 nanowires is 40–80 wt%. These results are equivalent to commercial products, such as Qiagen RNA extraction kit 74104, which implies the SiO2 nanowires composite filter papers have good RNA purification ability and can be an alternative candidate in this biological application. Since the SiO2 nanowires were synthesized from economical rice husks, we believe this study endows such worthless nature products the great potential for recycling and reusing. 4. Conclusions As a cheap but high grade silicon source, the biomass rice husk were taken in consideration as the raw materials for high-tech application. Large-surface-area and high-pore-volume SiO2 nanowires with the diameter of 30–100 nm were successfully prepared via a hydrothermal method. Fe2O3 and C2H8N2 are key elements for the SiO2 nanowires formation during the hydrothermal process. The SiO2 nanowires composite filter papers show equivalent RNA purification performance to commercial products, which sheds light on the reclaiming, application and industrialization of rice husks. References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (1999) 435.
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[2] S.S. Wong, E. Joselevich, A.T. Woolley, C.L. Cheung, C.M. Lieber, Covalently functionalized nanotubes as nanometer-sized probes in chemistry and biology, Nature 394 (1998) 52. [3] F. Marlow, M.D. McGehee, D.Y. Zhao, B.F. Chmelka, G.D. Stucky, Doped mesoporous silica fibers: a new laser material, Adv. Mater. 11 (1999) 632. [4] Q.S. Huo, D.Y. Zhao, J.L. Feng, K. Weston, et al., Room temperature growth of mesoporous silica fibers: a new high-surface-area optical waveguide, Adv. Mater. 12 (1997) 974. [5] F. Adam, J. Andas, Amino benzoic acid modified silica – an improved catalyst for the mono-substituted product in the benzylation of toluene with benzyl chloride, J. Colloid Inter. Sci. 311 (2007) 135. [6] M.T. Tsay, F.W. Chang, Characterization of rice husk ash-supported nickel catalysts prepared by ion exchange, Appl. Catal. A: Gen. 203 (2000) 15. [7] F. Adam, P. Retnam, A. Iqbal, The complete conversion of cyclohexane into cyclohexanol and cyclohexanone into a simple-silica chromium heterogeneous catalyst, Appl. Catal. A: Gen. 357 (2009) 93. [8] F. Adam, A.E. Ahmed, L.M. Sia, Silver modified porous silica from rice husk and its catalytic activity potential, J. Porous Mater. 15 (2008) 434. [9] Y.W. Wang, C.H. Liang, G.W. Meng, X.S. Peng, L.D. Zhang, Synthesis and photoluminescence properties of amorphous SiOx nanowires, J. Mater. Chem. 12 (2002) 651. [10] J.Y. Lou, L.M. Tong, Z.Z. Ye, Modeling of silica nanowires for optical sensing, Opt. Express 13 (2005) 2135. [11] L.J. Chen, J.D. Liao, Y.J. Chuang, et al., Characterization of crystalline silica nanorods synthesized via a solvothermal route using polyvinylbutyral as a template, J. Nanopart. Res. 13 (2011) 783. [12] Z.W. Pan, Z.R. Dai, C. Ma, et al., Molten gallium as a catalyst for the large-scale growth of highly aligned silica nanowires, J. Am. Chem. Soc. 124 (2002) 1817. [13] Z.W. Wang, Z.Y. Xue, G. Wang, J.Y. Dai, et al., Catalyst-free approach for growth of graphene sheets on high-density silica nanowires by CVD, Mater. Lett. 162 (2016) 165. [14] D.Q. Zhang, X. Zhang, J.L. Wei, G.X. Gu, G. Xiang, Growth of tapered silica nanowires with a shallow U-shaped vapor chamber: growth mechanism and structural and optical properties, J. Appl. Phys. 117 (2015) 1.
[15] C.L. Shao, H.Y. Kim, J. Gong, B. Ding, S.J. Park, et al., Fiber mats of poly(vinyl alcohol)/silica composite via electrospinning, Mater. Lett. 57 (2003) 1579. [16] W.L. Liu, W.J. Chen, et al., Effect of Fe metal on the growth of silicon oxide nanowires, Int. J. Miner. Metall. Mater. 16 (2009) 317. [17] Z. Jiang, T. Xie, B.Y. Geng, G.Z. Wang, G.S. Wu, X.Y. Yuan, et al., Synthesis of core-shell nanowires of FeCoNi alloy core with silicon oxide layers, Inorg. Chem. Commun. 7 (2004) 812. [18] M. Paulose, O.K. Varghese, C.A. Grimes, Synthesis of gold-silica composite nanowires through solid-liquid-solid phase growth, J. Nanosci. Nanotechnol. 3 (2003) 341. [19] H. Chen, L. Zhao, X.T. Wang, X. He, F.C. Wang, et al., Hybrid one-dimensional nanostructure based on biomorphic porous SiO2 via in-situ catalytic pyrolysis of rice husk, Ceram. Int. 41 (2015) 6089. [20] Y.J. Chen, J.B. Li, Y.S. Han, Q.M. Wei, J.H. Dai, A novel morphology of SiOx nanowires with a modified diameter, Appl. Phys. A: Mater. 74 (2002) 433. [21] S. Pukird, P. Chamninok, S. Samran, P. Kasian, et al., Synthesis and characterization of SiO2 nanowires prepared from rice husk ash, J. Metal. Mater. Min. 19 (2009) 33. [22] L.W. Lin, Y.H. Tang, L.Z. Pei, L.B. Zhu, et al., Smooth silicon oxide nanowires under supercritically hydrothermal conditions, J. Non-Cryst. Solids 353 (2007) 159. [23] T.H. Liou, Evolution of chemistry and morphology during the carbonization and combustion of rice husk, Carbon 42 (2004) 785. [24] FAO rice market monitor, December 2015, Volume XVIII-Issue No. 4.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Preparation of SiO2 nanowires from rice husks by hydrothermal method and the RNA purification performance Meiyan Huang a, Jianping Cao a, Xing Meng a, Yangsi Liu b, Wei Ke a, Jialiang Wang c, Ling Sun a,b,⇑ a
Material and Industrial Technology Research Institute Beijing, Boda Building North, No. 28 Life Science Park Road, Changping District, Beijing 102206, China Beijing Guyue New Materials Research Institute, Beijing University of Technology, 100 Ping Le Yuan, Chaoyang District, Beijing 100124, China c Shanghai Ribo-Technology Company Limited, Room 407, 4th Building of Juke Biotech Park, No. 333 Guiping Road, Xuhui District, Shanghai 200233, China b
a r t i c l e
i n f o
Article history: Received 29 June 2016 In final form 7 September 2016 Available online 9 September 2016 Keywords: SiO2 nanowires Hydrothermal method Composite filter paper RNA purification
a b s t r a c t In this study, SiO2 nanowires were prepared by using rice husks as silicon source via a hydrothermal method. The microstructure, thermal stability and morphology of SiO2 nanowires were characterized by X-ray diffraction, infrared spectroscopy, thermal gravimetric analysis and scanning electron microscope. SiO2 nanowires with a diameter of 30–100 nm were obtained and the formation mechanism of SiO2 nanowires during the hydrothermal reaction was proposed. The SiO2 nanowires were introduced into membrane spin columns to isolate RNA and the values of A260/280 and A260/230 were 2.0– 2.1 and 1.8–2.0, respectively, which shows the SiO2 nanowires were effective for RNA purification. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction One-dimensional (1D) nanomaterials with large length-todiameter aspect ratio, high storage density and good integration features, have been widely applied in nano-sized electronics, optics, optical integrated circuits, biological devices, and so on [1,2]. SiO2 nanowires are typical 1D inorganic nanomaterials which show many unique properties, such as high insulation, good fluorescence, and high active surface area, endowing themselves high potential in the catalysis, waveguide, sensing and photoluminescence applications [3–10]. Many techniques have been reported to obtain SiO2 nanowires, including template method [11], chemical vapor deposition [12–14], electrostatic spinning [15], metal catalysis [16–19], and thermal evaporation [20,21]. These methods are either easy to introduce more complex ingredients into the reaction system, or of high cost as equipment for high temperatures and pressures is normally needed. Hydrothermal method, however, attracts more attention for SiO2 nanowires preparation since it allows of mild reaction conditions, simple instruments and facile operation [22]. Easy-to-get low-cost raw materials are also necessary to produce SiO2 nanowires. Rice husks, occupying 20% weight of rice and containing 17–20% silica, are the largest number of
⇑ Corresponding author at: Material and Industrial Technology Research Institute Beijing, Boda Building North, No. 28 Life Science Park Road, Changping District, Beijing 102206, China. E-mail address: [email protected] (L. Sun). http://dx.doi.org/10.1016/j.cplett.2016.09.012 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
by-products in rice processing [21,23]. According to Food and Agriculture Organization of the United Nations, the global rice production is more than 740 million tons per year and China tops the production list, accounting for more than 30% of the total amount [24]. Therefore, effective recycling and reusing of rice husks are favorable for environmental protection and sustainable development from a global view. By far, there have been considerable studies on the preparation of silica nanoparticles by using rice husks [8,25], but rice husk-derived SiO2 1D nanowires are rarely reported. Herein, SiO2 nanowires were massively fabricated from rice husks by a hydrothermal method, and then, the nanowires were mixed with pulp to form composite filter papers. The growth mechanism and RNA purification ability of SiO2 nanowires were also studied. We believe the hydrothermal fabrication and RNA purification performance of SiO2 nanowires endue rice husks the great potential in bio-engineering and industrialization.
2. Experimental 2.1. Materials Rice husks are agricultural by-products, and they were cleaned and dried before experiments. Nine hydrated ferric nitrate (Fe (NO3)39H2O), citric acid (C6H8O7), hydrochloric acid (HCl) and ethane diamine (C2H8N2) are analytical grade and they were used without further purification. Deionized water was used throughout
M. Huang et al. / Chemical Physics Letters 662 (2016) 42–46
the experiments. The pulp was purchased from Shandong Feicheng Zhongheng paper company Co., Ltd. 2.2. Preparation of SiO2 nanowires
2.2.1. Purification and calcination of rice husks A certain amount of rice husks were immersed in diluted citric acid (1 wt%) solution and stirred intensively. They were transferred to an autoclave and maintain at 80 °C for 3 h to remove impurity elements, such as K, Na, and Mg. The rice husks were then washed several times until they became neutral and dried thoroughly. The purified rice husks were calcined at 700 °C for 2 h and ground to get rice husks ashes (RHA). 2.2.2. RHA/Fe2O3 intermediates 2 g RHA were mixed with 12.12 g Fe(NO3)39H2O by sufficiently grinding and the mixture were calcined at 400 °C for 4 h to form RHA/Fe2O3 composite powders. 2.2.3. Hydrothermal reaction 4 g RHA/Fe2O3 powders were put in an 200 ml autoclave with 104 ml ethylene diamine solution (C2H8N2:H2O = 8:5, vol:vol). The autoclave was heated up to 200 °C and the hydrothermal reaction was lasted for 4 days. 2.2.4. Post-treatment The resultant was centrifuged and washed by using deionized water several times, after that the wet powders were stirred in 12.5% HCl solution at 60 °C for 3 h to remove Fe2O3. The final precipitate was centrifuged, washed, and dried to get the milk white SiO2 nanowires. The preparation procedures from rice husks to SiO2 nanowires were illustrated in Fig. 1. 2.3. Preparation of SiO2 nanowires composite filter paper A certain amount of SiO2 nanowires (40–80 wt%) and pulp were mixed vigorously by a fiber standard dissociator for 5–10 min to get uniform slurry. The slurry was processed into filter papers by a paper-making machine (PL6-C, Xianyang Taisite Instrument Company). As-made SiO2 nanowires/pulp composite filter papers were tailored and fitted into spin columns, as shown in Fig. 2, for RNA purification. 2.4. Characterization The morphology of silica nanowires was obtained using a Hitachi S-3400N scanning electron microscope (SEM) with an
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accelerating voltage of 15 kV. X-ray diffraction (XRD) patterns were collected on a Rigaku DMAX-RB 12 KW powder diffractometer with Cu Ka radiation (k = 1.5406 Å) at 35 mA and 40 kV, and the scanning rate was 6°/min, 2h angle ranged from 5° to 80°. The synthesized materials were also characterized with Fourier transform infrared spectroscopy (FT-IR 850, Tianjin Gangdong SCI. & TECH. Development Co., Ltd.) and thermogravimetric analyzer (TGA, HCT-3, Beijing Henven Scientific Instrument Factory) with a working temperature ranging from 30 to 1200 °C. The BET characterization was finished by a high-performance automatic gas sorption analyzer with 6 channels (model: Autosorb-6iSA, USA). The RNA purification efficiency of the SiO2 nanowires was tested by a Thermo Scientific NanoDrop 2000/2000C spectrophotometer. 3. Results and discussion The crystal structures of RHA, RHA/Fe2O3 composites and SiO2 nanowires were revealed by their XRD patterns as shown in Fig. 3. Only one peak at 2h = 22.5° can be seen in both patterns of RHA and SiO2 nanowires, which is the characteristic absorption peak of amorphous SiO2. For RHA/Fe2O3 composites, strong characteristic absorption peaks appear at 2h = 24.1°, 33.1°, 35.6°, 40.8°, 49.4°, 54.2°, 62.6° and 63.9°, respectively, which correspond to (0 1 2), (1 0 4), (1 1 0), (1 1 3), (0 2 4), (1 1 6), (2 1 4) and (3 0 0) crystal planes of a-Fe2O3. The absence of SiO2 phases in RHA/Fe2O3 composites may attribute to the major component of Fe2O3, which overlaid SiO2 signals. BET results were summarized in Table 1. Compared with RHA and RHA/Fe2O3 composites, SiO2 nanowires have the largest surface area and pore volume. This could be due to the special 1D structure of nanowires with high length-to-diameter aspect ratio and the stacking of SiO2 nanowires, which may generate a lot of pores. The over-loading of Fe2O3 in RHA/Fe2O3 composites decreased the surface area and pore volume, making them even lower than RHA. Fig. 4 shows FT-IR spectra of SiO2 nanowires under different hydrothermal reaction times (1d, 2d, 3d and 4d). The strong and wide absorption band at 1097 cm 1 is the SiAOASi antisymmetric stretching vibration peak. The bands at 800 cm 1, 586 cm 1 and 467 cm 1 are the symmetric stretching vibration peaks of SiAO, and the intensity of band at 960 cm 1 is associated with the bending vibration of SiAOH. These bands aforementioned are all the characteristic absorption peaks of SiO2. The wide band at 3432 cm 1 is the antisymmetric stretching vibration peak of AOH, which can be attributed to the adsorbed water and constitution water. The band near 1634 cm 1 is the HAOAH bending vibration peak. The FT-IR spectra of SiO2 nanowires for 3d and 4d are quite similar, which means SiO2 nanowires could be matured after hydrothermal reaction for 3d.
Fig. 1. Schematic diagram of the formation of SiO2 nanowires from rice husks.
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Fig. 2. Structure diagram of spin columns for RNA purification.
Fig. 3. XRD patterns of RHA, RHA/Fe2O3 composites and SiO2 nanowires.
Table 1 BET results of RHA, RHA/Fe2O3 composites and SiO2 nanowires. Samples
BET surface area/(cm2/g)
Pore volume/(cm3/g)
RHA RHA/Fe2O3 composites SiO2 nanowires
256.33 55.08 381.85
0.36 0.17 1.03
Fig. 4. IR spectra of SiO2 nanowires under different hydrothermal reaction times.
TGA were used to give further information on the phase structure of SiO2 nanowires under different hydrothermal reaction times as present in Fig. 5. The weight loss for all samples is rapid before 100 °C because of the evaporation of surface adsorbed water. After dehydration, TGA plots become less steep and the weight loss is stale and gentle. During this period, hydroxide groups (AOH) on SiO2 surfaces are removed gradually with the temperature rising. The total weight loss increases with hydrothermal reaction time extending since more nanowires will be generated and more water and hydroxide groups can be attached. The TGA plots of 3d and 4d are almost the same, meaning the similar weight loss behavior, and it is in good agreement with the FT-IR results. The morphological evolution of SiO2 nanowires grown for different reaction time is demonstrated in Fig. 6. Many tiny crystals were aggregated and constituted large clusters after reaction for 1d. Some short and fine wire-shaped SiO2 sprouted from the cluster matrix, which were the embryos of SiO2 nanowires (Fig. 6a). The clustered crystals were reduced and a lot of nanowires were formed after 2d (Fig. 6b). The SiO2 nanowires became more and more obvious as the extension of reaction time until 4d (Fig. 6c– d). From Fig. 6d, the diameter of SiO2 nanowires was estimated to be 30–100 nm after a 4d reaction. We believe that Fe2O3 plays a vital role for SiO2 nanowires formation in the C2H8N2-assisted hydrothermal reaction, which is analogous to the growth process of GeOx/C2H8N2 nanowires [26]. The growth mechanism of SiO2 nanowires is demonstrated in Scheme 1. First, C2H8N2 molecules concentrate on the surface of Fe2O3 for the coordination between C2H8N2 and Fe3+. The lone pair of electrons on the N atoms transfers to Fe3+ due to its weak oxidative ability in an alkaline solution. Meanwhile, due to the oxidation tendency of C2H8N2, the O atoms in AOH groups on
Fig. 5. TGA of SiO2 nanowires under different hydrothermal reaction time.
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Scheme 1. Schematic diagram of the growth mechanism of SiO2 nanowires.
Table 2 RNA purification results of SiO2 nanowires/pulp composite filter paper and Qiagen 74104. RNA purification indicators
A260/280
A260/230
40% SiO2 composite filter paper 60% SiO2 composite filter paper 80% SiO2 composite filter paper Qiagen 74104
2.04 2.06 2.11 2.06
1.79 1.93 1.86 1.97
the surface of SiO2 associate with the H atoms in ANH2 to form (NAH OASi) H-bonding. Then, adjacent SiAOH bonds begin to condense at 200 °C in aqueous solution, producing primary C2H8N2–SiO2 hybrid units on the Fe2O3 surface via (NAH OASi) H-bonding. The hybrid units will be detached while other C2H8N2 molecules continue combining with Fe3+. The attached C2H8N2 will hinder the in-plane growth of SiO2 nanowires, but favor the anisotropic growth along the vertical direction, resulting the 1D nanostructures, that is SiO2 nanowires. Five pieces of SiO2 nanowires composite filter papers were integrated in spin columns individually for RNA purification and the intensity ratios at the highest absorption peaks A260/A280 and A260/A230 were tested. The results were shown in Table 2. The values of A260/A280 and A260/A230 are the common indicators for measuring the purity of RNA and they are in the range of 2.0– 2.1 and 1.8–2.0 for the composite filter papers, respectively, when
the content of SiO2 nanowires is 40–80 wt%. These results are equivalent to commercial products, such as Qiagen RNA extraction kit 74104, which implies the SiO2 nanowires composite filter papers have good RNA purification ability and can be an alternative candidate in this biological application. Since the SiO2 nanowires were synthesized from economical rice husks, we believe this study endows such worthless nature products the great potential for recycling and reusing. 4. Conclusions As a cheap but high grade silicon source, the biomass rice husk were taken in consideration as the raw materials for high-tech application. Large-surface-area and high-pore-volume SiO2 nanowires with the diameter of 30–100 nm were successfully prepared via a hydrothermal method. Fe2O3 and C2H8N2 are key elements for the SiO2 nanowires formation during the hydrothermal process. The SiO2 nanowires composite filter papers show equivalent RNA purification performance to commercial products, which sheds light on the reclaiming, application and industrialization of rice husks. References [1] J.T. Hu, T.W. Odom, C.M. Lieber, Chemistry and physics in one dimension: synthesis and properties of nanowires and nanotubes, Acc. Chem. Res. 32 (1999) 435.
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