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One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release
CLAY-03561; No of Pages 5 Applied Clay Science xxx (2015) xxx–xxx
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Applied Clay Science journal homepage: www.elsevier.com/locate/clay
One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release Shixiang Zuo a,b, Wenjie Liu a, Chao Yao a,c,⁎, Xiazhang Li a,c, Shiping Luo a, Fengqin Wu a,d, Yong Kong a, Xiaoheng Liu b,⁎⁎ a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China R&D Center of Xuyi Attapulgite Applied Technology, Changzhou University, Xuyi 211700, China d Changzhou Aotena New Materials S&T Co. Ltd., Changzhou 213164, China b c
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 18 August 2015 Accepted 20 August 2015 Available online xxxx Keywords: Halloysite Sodalite Nepheline Hollow mesoporous nanosphere Drug release
a b s t r a c t Hollow mesoporous sodalite nanospheres (HMSN) were facilely fabricated by a one-pot template-free hydrothermal route using natural halloysite as a silicon–aluminum source. The obtained products were characterized by X-ray diffraction, Fourier transform infrared spectrum, transmission electron microscope, field emission scanning electron microscope and N2 adsorption–desorption isotherm. Well-defined HMSN with a mesopore structure were successfully obtained when the hydrothermal temperature was 180 °C, the reaction time was 9 h and the NaOH concentration was 3 mol/L. The products began to gradually transform from HMSN to micronepheline hydrate when the time was extended to 12 h. The drug release results indicate that the modified HMSN have outstanding drug release performance. The capacity for aspirin loading can reach as high as 20.8% and its cumulative release can reach 65% when release time is ~50 h. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hollow mesoporous nanospheres (HMN) with penetrating porous matrix wall or shell geometries, which are considered to be one of the best effective carriers, have captured increasing interests due to their conspicuous features of low density, high specific surface, large void space fraction, and high chemical stability (Feng et al., 2008; Zhang et al., 2012). All these characteristics make them more suitable candidates for many potential applications in catalysis, separation/adsorption, and drug delivery etc. Some HMN, such as carbon, SiO2, SnO2 and FexOy, have been extensively studied by many researchers (Ding et al., 2011; Jin et al., 2012; Dai et al., 2014; Xin et al., 2014). Template-assisted synthesis has been one of the most common approaches to fabricate HMN due to its unique advantages against the shell thickness of a hollow sphere and its morphology, which can be effectively controlled using different templates and raw materials. For instance, SiO2-HMN are generally synthesized with various templates including inorganic colloids, surfactant/ vesicle templates, emulsion templates and polymeric micelle templates (Liu et al., 2010; Sasidharan et al., 2013). However, the removal of the templates is expensive and time consuming. Therefore, it is urgent to ⁎ Correspondence to: C. Yao, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (C. Yao).
find straightforward and economic routes to obtain HMN. Templatefree hydrothermal synthesis has been considered to be an effective methodology. It can be described as follows, firstly, the insoluble matters can redissolve under the circumstance of high temperature and high pressure without templates, and then the dissolved matters renucleate and grow at certain supersaturation. As a traditional zeolite, sodalite has been intensively investigated due to a wide range of applications including catalyst support, gas sorption, and hydrogen separation (Shanbhag et al., 2009; Zheng et al., 2009; Wang et al., 2014). Currently, micro-nano sized sodalite crystals with a solid structure can be synthesized by the hydrothermal process (Wei et al., 2008; Jiang et al., 2012). A few hollow sodalite nanospheres have been reported previously. Ji et al. (2011) fabricated hollow sodalite micro-spheres with a hole on the shell in a first-closed then-open system from the synthesis gels aged under ultrahigh nitrogen pressures. Firstly, the synthesis gels were transferred into a sealed autoclave with adjustable pressure, and then the pressure in the autoclave was slowly released by opening this valve. Although some templates are not used in a previous report, the above synthetic methods have some drawbacks in terms of complicated technology, high cost and long periodicity. As a naturally engineered aluminosilicate clay with a nanotube structure and a 1:1 Al:Si ratio (Yah et al., 2012a,b), halloysite has been universally used in functional materials (Zhou, 2011; Zhou and Keeling, 2013). Herein, a one-pot template-free strategy was adopted to obtain hollow mesoporous sodalite nanospheres (HMSN) using
http://dx.doi.org/10.1016/j.clay.2015.08.029 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
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S. Zuo et al. / Applied Clay Science xxx (2015) xxx–xxx
halloysite as a Si–Al source because of its advantages compared with previous work: (a) the raw material-halloysite is abundant and economical, and the tube-like and special crystal structure of which are efficiently utilized; (b) as a unique molecular sieve, HMSN have more excellent performance in the above applications compared to the other HMN. To date, there has been no report on the hydrothermal fabrication of HMSN. Due to their environmental friendliness and biocompatible nature, HMSN are expected to play a significant role in the drug loading and release. 2. Experimental 100 mL of HCl solution (1.0 mol/L) was poured into 200 mL of dispersion containing 9.6 g of halloysite. After stirring for 3 h at 80 °C, the obtained dispersion was filtered and washed using deionized water until neutral, and the acid–halloysite was dried at 80 °C for further use. 4.0 g of acid–halloysite was dispersed into 100 mL of NaOH solution (3.0 mol/L), and then transferred into a Teflon-sealed autoclave. The autoclaves were maintained respectively at different temperatures for different times. The products were separated by filtration, followed by washing with deionized water, and finally dried at 80 °C. Fourier transform infrared (FT-IR) spectrum was performed by a Nicolet Avatar 370 (Thermo Corporation, USA) from 4000 to 400 cm−1. X-ray diffraction (XRD) patterns were recorded on a D/Max 2500 PC X-ray diffractometer (Rigaku Corporation, Japan) with CuKα radiation of the X-ray wavelength of 0.15418 nm over a 2θ range (5–80°). The N2 adsorption–desorption isotherms and pore distribution (Brunauer–Emmett– Teller, BET method) were determined by a Micromeritics Corporation ASAP2010C surface area and porosimetry system. The morphologies of the as-obtained samples were observed by a JEOL Corporation (Japan)
JEM-2100 transmission electron microscope (TEM) and a Carl Zeiss Corporation (Germany) SUPRA-55 field emission scanning electron microscope (FE-SEM). First of all, the sodalite was modified with N-(2-aminoethyl)-3aminopropyltrimethoxysilane (KH792) under similar technological conditions (Zuo et al., 2013). Subsequently, 0.5 g of sodalite was impregnated in 20 mL of ethanolic solution containing 0.24 g of aspirin (ASP) at room temperature for 48 h. Afterwards, the above dispersion was filtrated and washed with ethanol several times. Finally, 0.1 g of ASP-sodalite was added into 50 mL of phosphate buffered solution (PBS; pH = 7.5) with constant rotation at 37 °C, and the effect of release time on the cumulative release of ASP was studied in detail. For comparison, halloysite was also investigated under the same technological conditions. The loading content of the drug and the cumulative release were determined according to the methods as reported by Liang (2012). 3. Results and discussion 3.1. XRD and FT-IR analysis To ascertain the structures of the products, the XRD patterns are shown in Fig. 1. As can be generally confirmed, the sodalite crystals have been mainly harvested at 180 °C for 3–9 h. The XRD reflections of halloysite appear at 2θ = 12.1°, 19.9° and 24.8° in accordance with reflection planes (001), (020), (110) and (002), respectively (JCPDS no. 74-1022) (Liu et al., 2013). However, the above reflections disappear after a hydrothermal reaction, and the new reflections can be observed clearly. The tiny new reflection at 2θ = 10.8° can be clearly found when the hydrothermal time reaches 12 h, suggesting that a very small
Fig. 1. (a) XRD patterns of products at 180 °C for different times; (b) XRD patterns of products at different temperatures for 9 h, and modified sodalite; (c) FT-IR spectra of products and modified sodalite.
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
S. Zuo et al. / Applied Clay Science xxx (2015) xxx–xxx
portion of nepheline hydrate begins to generate. Subsequently, the majority of sodalite crystals have been transformed into nepheline hydrate (JCPDS no. 72-1523) when the reaction time exceeds 24 h. In general, the hydrothermal products display a gradually changing process: halloysite → sodalite → nepheline. However, the crystalline phase of sodalite was not completely formed below the hydrothermal temperature of 180 °C (Fig. 1(b)). The (100), (211), (222) and (330) crystal planes are the major reflections of the obtained sodalite (JCPDS no. 81-0105), which are identical with those of the previous study (Naskar et al., 2011). The FT-IR spectrum of pristine halloysite has three important peaks that include inner AlO–H stretching at 3622 cm− 1 and 3694 cm− 1 and Al–Al–OH bending at 913 cm−1 (Wu et al., 2014) (Fig. 1(c)). The two peaks at 536 cm−1 and 470 cm−1 are attributed to the bending vibration of T–O–T (T = Al, Si), respectively (Luo et al., 2014). Another band at 763 cm−1 is identified as the wider absorption of T–O–T. However, the above peaks of halloysite disappear after hydrothermal treatment for 3–9 h, which can be seen from the FT-IR of the obtained product that the broadband at ~ 990 cm− 1 is assigned to the asymmetric stretching of T–O–T. The absorptions at 461 cm− 1 and 434 cm−1 are due to the bending vibration of O–T–O, and are consistent with those of the previous studies (Fan et al., 2008; Naskar et al., 2011), proving the formation of sodalite. When the hydrothermal time is prolonged to 24 h, the evident peak at 678 cm− 1 is assigned to the
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T–T stretching vibration of nepheline. After modification of the sodalite, the intensities of tiny N–H asymmetric blending vibrations at 1492 cm−1 and the C–N peak at 1257 cm−1 increase slightly, indicating that the –NH2 groups existing on the surface have altered the surface property of sodalite. 3.2. EM analysis TEM and FE-SEM photographs of halloysite and the hydrothermal products are presented in Fig. 2. Pristine halloysite is a cylindricalshaped tube with multilayer walls. Generally, the halloysite samples contain agglomerates of nanotubes with some irregularities in diameter, wall thickness, and morphology (Yah et al., 2012a,b) (Fig. 2a). Most importantly, the nanospheres are achieved successfully by a onepot hydrothermal pathway, and the hollow structure of these nanospheres can be distinctly discovered (Fig. 2b and c). These nanospheres with serious agglomeration have relatively uniform particle sizes (Fig. 2e), and their inner pore diameters are slightly different. The above results demonstrate the formation of hollow sodalite nanospheres (HSN). However, columnar micro-nepheline, as well as a few sodalite particles on their surface, can be found distinctly (Fig. 2d and f), which manifests that HSN are gradually converted to micro-nepheline in the entire reaction.
Fig. 2. Representative TEM and FE-SEM images of halloysite (a), sodalite (b, c, e), and nepheline (d, f).
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
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3.3. BET analysis The N2 adsorption–desorption isotherms of the three materials exhibit typical type-IV hysteresis, which is indicative of the presence of mesopores. The N2 volume adsorbed by the sodalite is slightly larger than that of halloysite in Fig. 3(a), corresponding to the complicated mesopores of sodalite as reflected in Fig. 3(b). The mesopore size of sodalite decreases while the pore volume of which increases in comparison with halloysite. A new mesopore of about 3 nm can be observed apparently, which is beneficial for the drug release. The N2 volume adsorbed and the pore distribution of modified sodalite remain hardly changed, showing that the pore structure of sodalite is not influenced by surface treatment. However, the N2 volume adsorbed and pore volume of the obtained nepheline mixture decrease significantly, which might be ascribed to the fusion of the mesopores from sodalite. The BET surface areas are measured to be 42.9 m2/g, 41.4 m2/g and 10.6 m2/g for HMSN, halloysite and nepheline mixture, respectively. All these results are in good agreement with those of TEM, reflecting the formation of HMSN. 3.4. Drug release The release profiles of ASP from the samples are described in Fig. 4. Obviously, HMSN exhibit the higher loading content of ASP as compared with halloysite due to the specific pore structure (Fig. 3(b)), suggesting that ASP is easily adsorbed into the channel of HMSN. The cumulative release of ASP from the halloysite–ASP displays an apparent increasing of release in PBS (98% within 10 h), illustrating a transient sustained drug release. However, the original mesopore almost disappears after the formation of nepheline hydrate due to the fusion of sodalite.
Fig. 4. The loading content of ASP and the cumulative release from halloysite, HMSN, HMSN-NH2 and nepheline.
Therefore, the drug loading and release performance of nepheline are not satisfactory. Above all, HMSN-NH2 are more favored for drug loading (20.8%), which is associated with the interaction on the internal or external surface between –NH2 and –COOH (Liang, 2012). HMSNNH2–ASP demonstrate a sustained and long-term release profile (65% within 50 h), which is contributed to their mesoporous structure and surface property. Actually, the interaction between HMSN-NH2 and ASP is regarded as a chemical adsorption, leading to the hard desorption, which is obviously different from HMSN. Therefore, HMSN-NH2 have preferable performance for drug release. 4. Conclusion Hollow sodalite nanospheres with a specific inner mesopore size of 3 nm have been successfully synthesized by a one-pot template-free strategy. The hydrothermal products gradually change from halloysite to HMSN, and then to micro-nepheline hydrate. The resulting HMSNNH2 show excellent drug release performance. The loading amount of aspirin is 20.8%, and its cumulative release can reach 65% when release time is about 50 h. This work is of significance in the guidance of preparing other analogous nanospheres, which will promise high activity and a cost-effective candidate in the application of drug release. Acknowledgments This work was financially supported by Technology Innovation Team of Colleges and Universities Funded Project of Jiangsu Province (2011-24), Production, Practical Experience and Research of Prospective Joint Research Projects of Jiangsu Province (BY 2014037-23), Natural Science Funds for Young Scientists of Jiangsu Province (BK 20130247), International Cooperation of Huaian City (HAC 2014014), and Technology Support Program of Changzhou City (CE 20135008, CE 20130033). References
Fig. 3. N2 adsorption–desorption isotherms (a), and pore distribution (b) of halloysite, sodalite, modified sodalite and nepheline.
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Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
Contents lists available at ScienceDirect
Applied Clay Science journal homepage: www.elsevier.com/locate/clay
One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release Shixiang Zuo a,b, Wenjie Liu a, Chao Yao a,c,⁎, Xiazhang Li a,c, Shiping Luo a, Fengqin Wu a,d, Yong Kong a, Xiaoheng Liu b,⁎⁎ a
School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China School of Chemical Engineering, Nanjing University of Science and Technology, Nanjing 210094, China R&D Center of Xuyi Attapulgite Applied Technology, Changzhou University, Xuyi 211700, China d Changzhou Aotena New Materials S&T Co. Ltd., Changzhou 213164, China b c
a r t i c l e
i n f o
Article history: Received 29 January 2015 Received in revised form 18 August 2015 Accepted 20 August 2015 Available online xxxx Keywords: Halloysite Sodalite Nepheline Hollow mesoporous nanosphere Drug release
a b s t r a c t Hollow mesoporous sodalite nanospheres (HMSN) were facilely fabricated by a one-pot template-free hydrothermal route using natural halloysite as a silicon–aluminum source. The obtained products were characterized by X-ray diffraction, Fourier transform infrared spectrum, transmission electron microscope, field emission scanning electron microscope and N2 adsorption–desorption isotherm. Well-defined HMSN with a mesopore structure were successfully obtained when the hydrothermal temperature was 180 °C, the reaction time was 9 h and the NaOH concentration was 3 mol/L. The products began to gradually transform from HMSN to micronepheline hydrate when the time was extended to 12 h. The drug release results indicate that the modified HMSN have outstanding drug release performance. The capacity for aspirin loading can reach as high as 20.8% and its cumulative release can reach 65% when release time is ~50 h. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Hollow mesoporous nanospheres (HMN) with penetrating porous matrix wall or shell geometries, which are considered to be one of the best effective carriers, have captured increasing interests due to their conspicuous features of low density, high specific surface, large void space fraction, and high chemical stability (Feng et al., 2008; Zhang et al., 2012). All these characteristics make them more suitable candidates for many potential applications in catalysis, separation/adsorption, and drug delivery etc. Some HMN, such as carbon, SiO2, SnO2 and FexOy, have been extensively studied by many researchers (Ding et al., 2011; Jin et al., 2012; Dai et al., 2014; Xin et al., 2014). Template-assisted synthesis has been one of the most common approaches to fabricate HMN due to its unique advantages against the shell thickness of a hollow sphere and its morphology, which can be effectively controlled using different templates and raw materials. For instance, SiO2-HMN are generally synthesized with various templates including inorganic colloids, surfactant/ vesicle templates, emulsion templates and polymeric micelle templates (Liu et al., 2010; Sasidharan et al., 2013). However, the removal of the templates is expensive and time consuming. Therefore, it is urgent to ⁎ Correspondence to: C. Yao, School of Petrochemical Engineering, Changzhou University, Changzhou 213164, China. ⁎⁎ Corresponding author. E-mail address: [email protected] (C. Yao).
find straightforward and economic routes to obtain HMN. Templatefree hydrothermal synthesis has been considered to be an effective methodology. It can be described as follows, firstly, the insoluble matters can redissolve under the circumstance of high temperature and high pressure without templates, and then the dissolved matters renucleate and grow at certain supersaturation. As a traditional zeolite, sodalite has been intensively investigated due to a wide range of applications including catalyst support, gas sorption, and hydrogen separation (Shanbhag et al., 2009; Zheng et al., 2009; Wang et al., 2014). Currently, micro-nano sized sodalite crystals with a solid structure can be synthesized by the hydrothermal process (Wei et al., 2008; Jiang et al., 2012). A few hollow sodalite nanospheres have been reported previously. Ji et al. (2011) fabricated hollow sodalite micro-spheres with a hole on the shell in a first-closed then-open system from the synthesis gels aged under ultrahigh nitrogen pressures. Firstly, the synthesis gels were transferred into a sealed autoclave with adjustable pressure, and then the pressure in the autoclave was slowly released by opening this valve. Although some templates are not used in a previous report, the above synthetic methods have some drawbacks in terms of complicated technology, high cost and long periodicity. As a naturally engineered aluminosilicate clay with a nanotube structure and a 1:1 Al:Si ratio (Yah et al., 2012a,b), halloysite has been universally used in functional materials (Zhou, 2011; Zhou and Keeling, 2013). Herein, a one-pot template-free strategy was adopted to obtain hollow mesoporous sodalite nanospheres (HMSN) using
http://dx.doi.org/10.1016/j.clay.2015.08.029 0169-1317/© 2015 Elsevier B.V. All rights reserved.
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
2
S. Zuo et al. / Applied Clay Science xxx (2015) xxx–xxx
halloysite as a Si–Al source because of its advantages compared with previous work: (a) the raw material-halloysite is abundant and economical, and the tube-like and special crystal structure of which are efficiently utilized; (b) as a unique molecular sieve, HMSN have more excellent performance in the above applications compared to the other HMN. To date, there has been no report on the hydrothermal fabrication of HMSN. Due to their environmental friendliness and biocompatible nature, HMSN are expected to play a significant role in the drug loading and release. 2. Experimental 100 mL of HCl solution (1.0 mol/L) was poured into 200 mL of dispersion containing 9.6 g of halloysite. After stirring for 3 h at 80 °C, the obtained dispersion was filtered and washed using deionized water until neutral, and the acid–halloysite was dried at 80 °C for further use. 4.0 g of acid–halloysite was dispersed into 100 mL of NaOH solution (3.0 mol/L), and then transferred into a Teflon-sealed autoclave. The autoclaves were maintained respectively at different temperatures for different times. The products were separated by filtration, followed by washing with deionized water, and finally dried at 80 °C. Fourier transform infrared (FT-IR) spectrum was performed by a Nicolet Avatar 370 (Thermo Corporation, USA) from 4000 to 400 cm−1. X-ray diffraction (XRD) patterns were recorded on a D/Max 2500 PC X-ray diffractometer (Rigaku Corporation, Japan) with CuKα radiation of the X-ray wavelength of 0.15418 nm over a 2θ range (5–80°). The N2 adsorption–desorption isotherms and pore distribution (Brunauer–Emmett– Teller, BET method) were determined by a Micromeritics Corporation ASAP2010C surface area and porosimetry system. The morphologies of the as-obtained samples were observed by a JEOL Corporation (Japan)
JEM-2100 transmission electron microscope (TEM) and a Carl Zeiss Corporation (Germany) SUPRA-55 field emission scanning electron microscope (FE-SEM). First of all, the sodalite was modified with N-(2-aminoethyl)-3aminopropyltrimethoxysilane (KH792) under similar technological conditions (Zuo et al., 2013). Subsequently, 0.5 g of sodalite was impregnated in 20 mL of ethanolic solution containing 0.24 g of aspirin (ASP) at room temperature for 48 h. Afterwards, the above dispersion was filtrated and washed with ethanol several times. Finally, 0.1 g of ASP-sodalite was added into 50 mL of phosphate buffered solution (PBS; pH = 7.5) with constant rotation at 37 °C, and the effect of release time on the cumulative release of ASP was studied in detail. For comparison, halloysite was also investigated under the same technological conditions. The loading content of the drug and the cumulative release were determined according to the methods as reported by Liang (2012). 3. Results and discussion 3.1. XRD and FT-IR analysis To ascertain the structures of the products, the XRD patterns are shown in Fig. 1. As can be generally confirmed, the sodalite crystals have been mainly harvested at 180 °C for 3–9 h. The XRD reflections of halloysite appear at 2θ = 12.1°, 19.9° and 24.8° in accordance with reflection planes (001), (020), (110) and (002), respectively (JCPDS no. 74-1022) (Liu et al., 2013). However, the above reflections disappear after a hydrothermal reaction, and the new reflections can be observed clearly. The tiny new reflection at 2θ = 10.8° can be clearly found when the hydrothermal time reaches 12 h, suggesting that a very small
Fig. 1. (a) XRD patterns of products at 180 °C for different times; (b) XRD patterns of products at different temperatures for 9 h, and modified sodalite; (c) FT-IR spectra of products and modified sodalite.
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
S. Zuo et al. / Applied Clay Science xxx (2015) xxx–xxx
portion of nepheline hydrate begins to generate. Subsequently, the majority of sodalite crystals have been transformed into nepheline hydrate (JCPDS no. 72-1523) when the reaction time exceeds 24 h. In general, the hydrothermal products display a gradually changing process: halloysite → sodalite → nepheline. However, the crystalline phase of sodalite was not completely formed below the hydrothermal temperature of 180 °C (Fig. 1(b)). The (100), (211), (222) and (330) crystal planes are the major reflections of the obtained sodalite (JCPDS no. 81-0105), which are identical with those of the previous study (Naskar et al., 2011). The FT-IR spectrum of pristine halloysite has three important peaks that include inner AlO–H stretching at 3622 cm− 1 and 3694 cm− 1 and Al–Al–OH bending at 913 cm−1 (Wu et al., 2014) (Fig. 1(c)). The two peaks at 536 cm−1 and 470 cm−1 are attributed to the bending vibration of T–O–T (T = Al, Si), respectively (Luo et al., 2014). Another band at 763 cm−1 is identified as the wider absorption of T–O–T. However, the above peaks of halloysite disappear after hydrothermal treatment for 3–9 h, which can be seen from the FT-IR of the obtained product that the broadband at ~ 990 cm− 1 is assigned to the asymmetric stretching of T–O–T. The absorptions at 461 cm− 1 and 434 cm−1 are due to the bending vibration of O–T–O, and are consistent with those of the previous studies (Fan et al., 2008; Naskar et al., 2011), proving the formation of sodalite. When the hydrothermal time is prolonged to 24 h, the evident peak at 678 cm− 1 is assigned to the
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T–T stretching vibration of nepheline. After modification of the sodalite, the intensities of tiny N–H asymmetric blending vibrations at 1492 cm−1 and the C–N peak at 1257 cm−1 increase slightly, indicating that the –NH2 groups existing on the surface have altered the surface property of sodalite. 3.2. EM analysis TEM and FE-SEM photographs of halloysite and the hydrothermal products are presented in Fig. 2. Pristine halloysite is a cylindricalshaped tube with multilayer walls. Generally, the halloysite samples contain agglomerates of nanotubes with some irregularities in diameter, wall thickness, and morphology (Yah et al., 2012a,b) (Fig. 2a). Most importantly, the nanospheres are achieved successfully by a onepot hydrothermal pathway, and the hollow structure of these nanospheres can be distinctly discovered (Fig. 2b and c). These nanospheres with serious agglomeration have relatively uniform particle sizes (Fig. 2e), and their inner pore diameters are slightly different. The above results demonstrate the formation of hollow sodalite nanospheres (HSN). However, columnar micro-nepheline, as well as a few sodalite particles on their surface, can be found distinctly (Fig. 2d and f), which manifests that HSN are gradually converted to micro-nepheline in the entire reaction.
Fig. 2. Representative TEM and FE-SEM images of halloysite (a), sodalite (b, c, e), and nepheline (d, f).
Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029
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S. Zuo et al. / Applied Clay Science xxx (2015) xxx–xxx
3.3. BET analysis The N2 adsorption–desorption isotherms of the three materials exhibit typical type-IV hysteresis, which is indicative of the presence of mesopores. The N2 volume adsorbed by the sodalite is slightly larger than that of halloysite in Fig. 3(a), corresponding to the complicated mesopores of sodalite as reflected in Fig. 3(b). The mesopore size of sodalite decreases while the pore volume of which increases in comparison with halloysite. A new mesopore of about 3 nm can be observed apparently, which is beneficial for the drug release. The N2 volume adsorbed and the pore distribution of modified sodalite remain hardly changed, showing that the pore structure of sodalite is not influenced by surface treatment. However, the N2 volume adsorbed and pore volume of the obtained nepheline mixture decrease significantly, which might be ascribed to the fusion of the mesopores from sodalite. The BET surface areas are measured to be 42.9 m2/g, 41.4 m2/g and 10.6 m2/g for HMSN, halloysite and nepheline mixture, respectively. All these results are in good agreement with those of TEM, reflecting the formation of HMSN. 3.4. Drug release The release profiles of ASP from the samples are described in Fig. 4. Obviously, HMSN exhibit the higher loading content of ASP as compared with halloysite due to the specific pore structure (Fig. 3(b)), suggesting that ASP is easily adsorbed into the channel of HMSN. The cumulative release of ASP from the halloysite–ASP displays an apparent increasing of release in PBS (98% within 10 h), illustrating a transient sustained drug release. However, the original mesopore almost disappears after the formation of nepheline hydrate due to the fusion of sodalite.
Fig. 4. The loading content of ASP and the cumulative release from halloysite, HMSN, HMSN-NH2 and nepheline.
Therefore, the drug loading and release performance of nepheline are not satisfactory. Above all, HMSN-NH2 are more favored for drug loading (20.8%), which is associated with the interaction on the internal or external surface between –NH2 and –COOH (Liang, 2012). HMSNNH2–ASP demonstrate a sustained and long-term release profile (65% within 50 h), which is contributed to their mesoporous structure and surface property. Actually, the interaction between HMSN-NH2 and ASP is regarded as a chemical adsorption, leading to the hard desorption, which is obviously different from HMSN. Therefore, HMSN-NH2 have preferable performance for drug release. 4. Conclusion Hollow sodalite nanospheres with a specific inner mesopore size of 3 nm have been successfully synthesized by a one-pot template-free strategy. The hydrothermal products gradually change from halloysite to HMSN, and then to micro-nepheline hydrate. The resulting HMSNNH2 show excellent drug release performance. The loading amount of aspirin is 20.8%, and its cumulative release can reach 65% when release time is about 50 h. This work is of significance in the guidance of preparing other analogous nanospheres, which will promise high activity and a cost-effective candidate in the application of drug release. Acknowledgments This work was financially supported by Technology Innovation Team of Colleges and Universities Funded Project of Jiangsu Province (2011-24), Production, Practical Experience and Research of Prospective Joint Research Projects of Jiangsu Province (BY 2014037-23), Natural Science Funds for Young Scientists of Jiangsu Province (BK 20130247), International Cooperation of Huaian City (HAC 2014014), and Technology Support Program of Changzhou City (CE 20135008, CE 20130033). References
Fig. 3. N2 adsorption–desorption isotherms (a), and pore distribution (b) of halloysite, sodalite, modified sodalite and nepheline.
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Please cite this article as: Zuo, S., et al., One-pot template-free fabrication of hollow mesoporous sodalite nanospheres for drug release, Appl. Clay Sci. (2015), http://dx.doi.org/10.1016/j.clay.2015.08.029