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amylose composite particles with core–shell structure
Polymer 53 (2012) 3297e3303
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Polymer journal homepage: www.elsevier.com/locate/polymer
Sol-gel synthesis of silica/amylose composite particles with coreeshell structure Yinhui Li a, Jiwen Hu a, *, Guojun Liu a, b, **, Jinheng Shi a, Wei Li a, Dingshu Xiao a a b
Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2012 Received in revised form 4 May 2012 Accepted 7 May 2012 Available online 18 May 2012
Synthesized and characterized were silica/amylose composite particles with interesting morphologies which we named as silica/amylase composite coreeshell particles. Amylose is soluble in hot water. Ethanol addition to a volume fraction fEtOH of 75% caused amylose to aggregate into globules with diameters w50 nm and smaller. After ammonia and tetraethoxysilane (TEOS) addition, TEOS underwent sol-gel reactions in the presence of amylose. The reactions eventually yielded coreeshell nanospheres with their core enriched by amylose and shell consisting mostly of fused SiO2-wrapped amylose or SiO2@amylose nanoparticles. Under a given set of experimental conditions, the equilibrium nanospheres had a well-defined size. The shell thickness increased as mS/mA increased, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added and mA denoted the amylose amount. After pyrolysis of the nanospheres prepared at relatively high mS/mA values, interesting hollow silica particles with nano-sized porous walls were obtained. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: SiO2 particles Morphological control Amylase
1. Introduction Silica/polymer composite particles have been attracted much attention for a long time due to their various applications in material science field that relate extremely with morphology and structures [1e4]. So far, there are many reports on preparation of silica/polymer composite particles with different polymer and morphology, some successful examples included are raspberry-like silica/poly(methyl methacrylate) composites [5], coreeshell silica/ poly[3-(triisopropyloxysilyl)propylmethacrylate]-block-poly[2(perfluorooctyl)ethyl methacrylate] superamphiphobic coatings [6], silica/polyamide hybrid composite films [7], silica/poly(styrene) coreeshell nanocomposites particles [8], silica/poly (N,N0 -methylenebisacrylamide) coreeshell composites materials [9], silica/ polysaccharide porous materials [10], sunflower-like silica/polypyrrole nanocomposites [11], hybrid silica/vesicles nanospheres or hybrid silica/micelles particles with coreeshell structure [12e14] etc. Among these silica/polymer composite particles, those from silica/polysaccharide (SiO2/polysaccharide) are of especially unique and attractive due to their nontoxic and biocompatibility properties
* Corresponding author. Tel.: þ86 20 85232136; fax: þ86 20 85232307. ** Corresponding author. Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China. Tel.: þ86 20 85232136; fax: þ86 20 85232307. E-mail addresses: [email protected] (J. Hu), [email protected] (G. Liu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.05.015
and may be potential for many applications. For example, silica/ starch particles have been used in the adsorption and purification of novel metal ions [15,16]. Porous silica/chitosan hybrid microspheres could be used as potential catalyst [17]. It’s well accepted that, for various applications, the morphology and structure of SiO2/polysaccharide composites are of critical importance. Accordingly, the design and controlled fabrication of SiO2/polysaccharide colloidal composites with tailored morphologies and structures have been made considerable efforts [18e22]. For example, Kabulov and coworker [19] studied that hybrid silica/ chitosan sorbents were proved that their structure determined their properties, the results were reflected on chromatographic behavior of the sorbents. Mann and coworkers [20] prepared silicalite-starch foams with pores up to 100 mm. First, they synthesized starch sponges with high internal macroporosites by freezing and thawing of starch gels. Second, the starch sponges were infiltrated with colloidal suspensions of silicalite nanoparticles. Final, silicalite-starch foams with pores were obtained. The pore size depended on the starch concentration and the silicalite loading. The research showed that the macroporous silicalitestarch hybrid sponges could have certain advantages over the corresponding calcined replicas according to increased structural organization and enhanced mechanical properties. Sailor and coworkers [21] prepared a pH responsive, chitosan-based hydrogel film. In detail, chitosan/SiO2 films were obtained by combining thermal oxidation of an electrochemically etched Si wafer with reaction of chitosan with glycidoxypropyltrimethoxysilane
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(GPTMS). Insulin trapped in the porous SiO2 layer underneath the hydrogel film will be release by the pH-dependent volume phase transition. The chitosan-based hydrogel in the top layer effectively blocks insulin release at pH 7.4, while insulin penetrates the swollen hydrogel layer at pH 6.0, leading to a steady release into solution. Liu and coworkers [22] reported the preparation of chitosanesilica nanocomposite membranes by cross-linking chitosan in situ on silica nanoparticles with sulfonic acid group on their surfaces. Cross-linking on chitosan increased its permselectivity in pervaporation dehydration of an ethanolewater mixture due to the reduce on its degree of swelling in water and an alcohol/water mixture. In addition, the addition of silica nanoparticles to chitosan supplied extra free volumes in polymer matrices with water permeation and would result in high permeation rates for complex membranes. According to Shchipunov and coworkers [23], the polysaccharide hydroxyl groups and the silanol groups could undergo hydrogen bonding and this accelerated and catalyzed silanol condensation. Aside from the work by Shchipunov et al., there have been other reports on the preparation of SiO2/polysaccharides composite particles including SiO2/starch [15,16,20], SiO2/chitosan [24e27], SiO2/alginate [28], SiO2/cyclodextrin [10,29] composite particles. Amylose, the linear soluble portion of starch consisting of (1/4)-linked a-D-glucopyranosyl units, was a polysaccharide and used in this study because it is abundant and inexpensive [30,31]. Also, amylose was known to be compatible with sol-gel chemistry. Reported in this paper is an interesting method to prepare silica/ amylose composite particles. The composite particles were prepared by sol-gelling tetraethoxysilane (TEOS) using ammonia as the catalyst in the presence of amylose nanoaggregates. The nanoaggregates were obtained by adding ethanol, a poor solvent for amylose, to an aqueous solution of amylose. TEOS first sol-gelled around the amylose nanoaggregates to yield SiO2-wrapped amylose or SiO2@amylose nanoparticles. The condensation of the surface silanol groups on different particles led to the clustering of these primary particles into coreeshell microparticles with their core enriched by amylose and shell consisting mostly of fused SiO2@amylose nanoparticles. To the best of our knowledge, composite particle morphologies as observed here were never reported before. Furthermore, most of the past studies did not examine the particle formation process by going beyond reporting the morphologies of the final particles. The composite coreeshell particles might have many potential applications in areas of absorption or separation for metal ions due to the rich hydroxyl groups on the surface of coreeshell SiO2@amylose nanospheres. Moreover, after pyrolysis and thermal degradation of amylose, hollow porous silica nanospheres with unique morphologies were produced, the hollow porous silica nanospheres could apply in catalysis, drug delivery system, chromatography, and others.
The eluant used was an aqueous 0.10-M NaCl solution at a flow rate of 0.6 mL/min. The system was calibrated using poly (ethylene glycol) standards with low polydispersity indices. 2.3. SiO2/amylase composite particle synthesis Amylose, 0.025 g, was stirred at 100 C for 30 min for dissolution in 5.0 mL of distilled water. After the mixture was cooled to 75 1 C, 15.0 mL of ethanol was dropped over 10 min into it. This turned the amylose solution cloudy. After the amylose aggregate solution was cooled over 0.5 h to room temperature, 14-M ammonia was added. Fifteen min later, TEOS was dropped over 10 min into the stirring mixture. This mixture was left at room temperature stirring for 18 h and still for another 12 h before it was centrifuged at 10,080 g for 15 min to settle the particles as a white precipitate. Particles were prepared at different mS/mA values, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added, and mA denoted the amylose amount used. Unless mentioned otherwise, the volume ratio used between ammonia and TEOS was 4/1. To prepare particles with mS/mA ¼ 2.5/1.0, 1.00 mL of ammonia, 0.25 mL or 2.3 102 mg of TEOS, and 25 mg of amylose in 5.0 mL of water were used. The mS/mA value was 2.5/1.0 in this case because one TEOS molecule yields one SiO2 unit corresponding to 26 wt% of TEOS. To prepare particles at mS/mA ¼ 1.0/ 1.0, 1.0/2.5, and 5.0/1.0, the TEOS amounts used were 0.10 mL, 0.040 mL, and 0.020 mL, respectively, while maintaining the amylose amount constant at 25 mg. 2.4. Composite particle formation process The composite particle formation process at mS/mA ¼ 2.5/1.0 and 1.0/2.5 was followed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In these cases, TEOS was added in one shot rather than dropwise. For TEM studies, samples taken at pre-designated times after TEOS additions were immediately aero-sprayed onto carboncovered TEM grids. Nitrogen was then blown on the grids to remove the residual solvent. For DLS studies, amylose particles containing ammonia were first pushed through a 0.45-mm filter into a light scattering cell. The desired amount of fresh TEOS in water/ethanol was then injected, in one shot, through a 0.1-mm filter into the light scattering cell. Measurements were then made at different times. 2.5. Transmission electron microscope (TEM)
Amylose [(C6H10O5)n, Tianjin Ruijinte Chemical Reagent Co. Ltd.], TEOS (Guangzhou Chemical Reagent Factory), ammonia (25e28% or w14 M in water, Tianjin Ruijinte Chemical Reagent Co. Ltd.), ethanol (>99.7%, Tianjin Fuyu Fine Chemical Co., Ltd.) were of analytical grade and were used as received. Water was doubled distilled.
To prepare TEM specimens of amylose aggregates, an amylose aggregate solution cooled to room temperature before ammonia addition was aero-sprayed or atomized, using a home-built device, onto Formvar-coated copper grids. To stain this sample, a drop of a 2 wt% uranyl acetate solution in water was equilibrated at room temperature with a sprayed and dried sample for 3 min before the liquid was wicked off using filter paper. TEM observations were made using a JEM-100CX II microscope operated at 80 kV. To prepare specimens of the SiO2/amylose composite particles, 10 mg of the particles were ultrasonicated for 15 min in 1 mL of water. After dispensing 1 drop on a Formvar-coated copper grid, the specimen was allowed to dry at room temperature for 24 h before TEM observation.
2.2. Size-exclusion chromatography (SEC)
2.6. Dynamic light scattering (DLS)
SEC analysis of amylose was performed at 40 C temperature on Waters 515 instrument equipped with Ultrahydrogel 250 columns.
Light scattering measurements were performed in a 1.2 1.2 4.5 cm3 quartz cell at 25 C on a Zetasizer Nano S90
2. Experimental section 2.1. Materials
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3. Results and discussion
An amylose solution at fEOH ¼ 75% was also aero-sprayed or atomized onto a Formvar-covered copper grid using a home-built device. Using this method, ethanol has evaporated from the atomized fine droplets during their flight from the spraying nozzle to a catching TEM grid and water has evaporated within w3 s after the spray landed on the TEM grid. Due to the fast solvent evaporation, the slow polymer chain motion, and the low polymer concentration and thus large distance between the different aggregates before solvent evaporation, the dried amylose aggregates have more or less retained their shapes in solution. A dried sample was then equilibrated with a drop of a uranyl acetate solution drop to negatively stain the droplets. Since amylose was insoluble in room-temperature water, the aggregates have survived this staining process as well. Fig. 1b shows a TEM image of an amylose aggregate specimen that was prepared using the method described above. Since the sample was negatively stained, the lighter domains corresponded to locations of the amylose aggregates. They must have mostly consisted of spheres and ellipsoids. The average particle diameter was 32 8 nm. The TEM diameter was substantially smaller than the DLS diameter because the former was the number-average diameter and the latter was the z-average diameter. Furthermore, sizes of the dry particles were obtained by TEM and those of the solvated particles were obtained from DLS. Regardless of the discrepancy, what was clear was that amylose did aggregate after ethanol addition. The reasonable TEM and DLS diameters also suggested that the aggregates survived the TEM specimen preparation and staining protocols and were observed by TEM. While not detected by DLS and TEM, unimolecular amylose chains must have co-existed with the aggregates. They were not detected by DLS probably because of their low scattering power. They were not seen by TEM because of the limited resolution of the instrument used.
3.1. Amylose and amylose aggregates
3.2. Composite particle formation
The amylose used in this study was analyzed by SEC. Based on polyethylene glycol standards, the number-average molecular weight Mn and polydispersity index Mw/Mn were 1.22 104 g/mol and 2.27, respectively, for the sample. The amylose dissolved in hot water but was insoluble in ethanol or room-temperature water. After ethanol addition at 75 C to a volume fraction fEtOH of 75%, amylose solutions turned cloudy, suggesting amylose aggregation. After sample cooling to room temperature, a DLS analysis was performed to yield an average hydrodynamic diameter dh of 78 nm and a polydispersity index of 0.025 for the aggregates. The size distribution of these aggregates is shown in Fig. 1a.
Composite particles were formed by adding TEOS into ammonia-containing amylose solutions in water/ethanol at fEtOH ¼ 75%. The sol-gel formation process was followed by analyzing samples that were taken at different times after TEOS addition in one shot to mS/mA ¼ 2.5/1.0 and were immediately aerosprayed. Fig. 2 shows TEM images of samples taken at different times. These samples were not stained because Si in SiO2 or silanol groups acted as an intrinsic staining agent. Comparing the TEM images for samples taken at different times allowed us to draw the following conclusions: First, “messy” background was seen only in Fig. 2a and b for samples taken 1 and 4 min after TEOS addition. The messy
Particle Size Analyzer. The light source used was a 633-nm HeeNe laser. The size and polydispersity values as well as the size distributions were automatically generated by the instrument after each measurement. 2.7. Atomic force microscopy (AFM) Specimens were prepared by aero-spraying solution samples onto silicon wafers. All samples were analyzed by tapping-mode AFM using a Veeco multimode instrument equipped with a Nanoscope IIIa controller. 2.8. Diffuse reflectance FT-IR spectroscopy (DRIFTS) DRIFTS were carried out on an Equinox 55 spectrometer using a Spectra-Tech diffuse reflectance accessory and a high temperature chamber with KBr windows. 2.9. Thermogravimmetric analyses (TGA) Thermogravimmetric analyses were done on a Netzsch TG-209/ Bruker Vector-22 instrument. SiO2/amylase particles, 3e5 mg, were placed in ceramic pot and heated from room temperature to 800 C at a rate of 10 C/min under nitrogen atmosphere. 2.10. Nitrogen sorption measurement The porosity of silica was performed on a 3H-2000PS1 instrument. Silica nanospheres, 0.39 g, were measured by static volumetric method. Absorbent was Nitrogen, degassing temperature was 90 C, degassing time was 120 min, and degassing mode was molecular replacement one.
Fig. 1. DLS hydrodynamic diameter distribution (a) and TEM image (b) for amylose aggregates.
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Fig. 2. TEM (aee) and AFM (f and g) images of SiO2/amylose particles aero-sprayed 1 min (a), 4 min (b), 6 min (c), 8 min (d), and 18 h (eeg) after TEOS addition at mS/mA ¼ 2.5/1.0 into an ammonia-containing amylose aggregate solution.
background seemed to consist of fused gray particles with or without a lighter core. The diameter of these particles was w20 nm. Second, particles with diameters w50 nm or shapes similar to these larger amylose particles of Fig. 1b were observed in Fig. 2a or b only and some of these were marked by orange arrows in these images. These particles in Fig. 2a seemed wrapped by SiO2 nanoparticles or were SiO2@amylose nanoparticles. Third, the SiO2@amylose nanoparticles were fused with other particles in Fig. 2a already. By 4 min after TEOS addition or in Fig. 2b, the SiO2@amylose particles had fused into nanospheres with a well-defined coreeshell structure. Fourth, the shape of these coreeshell nanospheres changed little with time after w4 min. Many of these particles seemed to be spherical (circular in the 2-d images) or ellipsoidal and possessed a singular core. Some particles assumed the shape of a double-yolk egg. There were also particles possessing irregularly-shaped cores. Fifth, the shell got darker and thicker with reaction time. Sixth, the particles aged at long reaction times were round and not hollow. This was clearly confirmed by the AFM topography image shown in Fig. 2f. The phase image in Fig. 2g suggested that the nanospheres surfaces were not perfectly smooth. Rather, the nanospheres shell seemed to consist of fused nanoparticles. The gray nanoparticles without a light core in the background in Fig. 2a and b were probably the SiO2 nanoclusters formed from the TEOS sol-gel reactions. The particles with a light core were formed probably because of deposition and condensation of SiO2 nanoclusters bearing surface SieOH groups on amylose molecules or small aggregates. These were SiO2-wrapped amylose nanoparticles or SiO2@amylose nanoparticles. The silanol groups of the SiO2 nanoclusters also condensed with the surface hydroxyl groups of the larger amylose aggregates seen in Fig. 1b, resulting in the larger SiO2@amylose nanoparticles marked by orange arrows in Fig. 2a. While amylose molecules or small aggregates were not detected by TEM and DLS, they must have existed because of the equilibrium between these species and the larger aggregates. Since the specific surface area was larger and the fraction of accessible hydroxyl
groups was higher for amylose molecules or smaller particles, they would more likely condense in the early stage of the sol-gel reaction with the formed silanol-bearing SiO2 nanoclusters than the larger particles. This condensation converted these species into thermodynamically different species and would have decreased the concentration of these species below their equilibrium concentrations and triggered the dissociation of the larger amylose aggregates. This probably explained why a higher fraction of smaller SiO2@amylose nanoparticles were seen in Fig. 2a and b than that of smaller amylose aggregates in Fig. 1b. These primary SiO2@amylose nanoparticles, regardless of their size, should bear SieOH and SieO groups on their surfaces. Comparing Fig. 2a and b suggested that they condensed further into larger aggregates. While the exact aggregation mechanism was not known, their fusion yielded eventually the coreeshell nanospheres as first seen in Fig. 2b. The particles looked lighter in the core because the core was amylose rich and darker in the shell because the shell was SiO2 rich. Comparing Fig. 2bee suggested that the coreeshell nanospheres did not undergo further fusion after w4 min or Fig. 2b probably because of the repulsion arising from their surface SieO groups. This repulsion force should increase with particle size due to the increase in the number of surface SieO groups. The existence of a particular average diameter for the particles under a given set of experimental conditions should not be surprising because the Stober process produced SiO2 particles with welldefined size and size distributions. The particle preparation conditions used here were similar to those reported by Stober and others except the use of amylose as a “dynamic” template, which was able to aggregate and dissociate reversibly. The wall of the nanospheres thickened with time initially probably because of the incorporation of SiO2 nanoclusters or SiO2@amylose nanoparticles that were continuously forming due to the TEOS sol-gel reactions. The further compacting and thickening of the shell at the later stage of particle synthesis, e.g. from
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Fig. 3. Plots of nanospheres dh distribution and TEM images 0.5 h (a, d), 8 h (b, e), and 18 h (c, f) after TEOS addition at mS/mA ¼ 2.5/1.0 into an ammonia-containing amylose aggregate solution.
Fig. 2d to e, occurred probably because of Oswald ripening, which involved the dissociation of certain particles and the growth of other particles. That the wall contained SiO2@amylose nanoparticles was fairly evident in Fig. 2bed because many white dots were seen in the walls of the particles in these TEM images. Due to the rapid formation and fusion of the initial SiO2@amylose particles, DLS could not be used to follow these processes. Rather, DLS was used to follow the later stage of the coreeshell SiO2/amylose particle formation process. The DLS specimens were prepared by pushing a TEOS solution through a filter into a filtered ammonia-containing amylose solution in a light scattering cell. Fig. 3 plots of SiO2/amylose particle dh distribution 0.5, 8, and 18 h after TEOS addition. The average particle diameter increased from 209 to 221 nm in the later stage of the reaction between 8 and 18 h probably due to shell thickening as observed by TEM.
3.3. Effect of varying mS/mA SiO2/amylose composite particles were prepared using different mass feed ratios mS/mA, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added and mA denoted the amylose amount used. The other conditions included the use of an aqueous amylose solution at 0.5 wt%, a volume ratio of 4/1 for ammonia and TEOS, and a reaction time of 18 h plus 12 h of sample aging. The prepared particles were purified by centrifugation and rinsing. After drying and redispersing in water by ultrasonication, they were sprayed on TEM grids for observation. Fig. 4 compares the TEM images of the composite particles prepared under these conditions at different mS/mA. A comparison of Fig. 4aec and Fig. 2e suggested that coreeshell nanospheres were always formed at mS/mA 1.0/2.5. Evidently, the
Fig. 4. TEM images of coreeshell nanospheres prepared at mS/mA ¼ 5.0/1.0 (a), 1.0/1.0 (b), 1.0/2.5 (c), and 1.0/5.0 (d).
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Fig. 5. FT-IR spectra of composite particles prepared at mS/mA ¼ 5.0/1.0 (a1), 2.5/1.0 (a2, b3)1.0/1.0 (a3, b2), 1.0/2.5 (a4), 1.0/5.0 (a5) and amylase (b1).
shell thickness increased with mS/mA. At mS/mA 2.5/1.0, the shells were thick enough and appeared rather smooth. At lower mS/mA values, the thickness of the shells was comparable to the size of their constituent SiO2@amylose particles and such a shell made of a particulate monolayer or sub-monolayer appeared rough. At mS/ mA ¼ 1.0/5.0, the coreeshell nanospheres structure was not obvious but might still have existed. Two of these seemingly coreeshell structures were marked by orange circles in Fig. 4d. The lack of a well-defined coreeshell structure in this case might be due to the lack of sufficient SiO2@amylose nanoparticles to cover the amylose core nanospheres. Our statistical analysis yielded the average TEM diameters of 326 40, 244 42, 161 23, and 109 19 nm for the coreeshell particles prepared at mS/mA ¼ 5.0/1.0, 2.5/1.0, 1.0/1.0, and 1.0/2.5, respectively. The corresponding average shell thicknesses were
115 11 nm, 69 5 nm, 44 8 nm, and 28 4 nm, respectively. Using these values, the core diameters were calculated to be 96 62 nm, 106 52 nm, 73 39 nm, and 53 27 nm. 3.4. Properties of the composite particles After drying, the SiO2/amylose nanospheres were used directly for diffuse reflectance FT-IR analysis. Fig. 5a compares the FT-IR spectra of amylose and the SiO2/amylose composite particles prepared at different mS/mA values. As mS/mA increased, the transmittance at 1100, 806, and 471 cm1 increased. Since these peaks corresponded to the asymmetric stretching, the symmetric stretching, and the distorted stretching of the SieOeSi bonds, this suggested that the SiO2 amount in the composite particles increased with mS/mA. This should not be surprising given the fast
Fig. 6. TGA curves of the SiO2/amylose and hollow SiO2 nanospheres (a), TEM image of pyrolyzed SiO2/amylose nanospheres prepared at mS/mA ¼ 2.5/1.0 (b), inset is image of part of an individual particle at high magnification. The mS/mA values used to prepare the SiO2/amylose nanospheres were 0/5.0 (1), 1.0/5.0 (2), 1.0/2.5 (3), 1.0/1.0 (4), 2.5/1.0 (5), 5.0/1.0 (6), and hollow SiO2 nanospheres (7), respectively, DLS hydrodynamic diameter distribution for starch solution 5 mg/mL (c), and the nitrogen sorption isotherms of hollow silica nanospheres (d).
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kinetics of the TEOS sol-gel reaction and the likely quantitative conversion of TEOS into SiO2. In addition, some amylose peaks as shown in Fig. 5b were changed. For example, the characterization peaks between 525 cm1 and 928 cm1 for amylose ring vibrations disappeared or were weakened. The shape of the broad band at 1450e1400 cm1 for amylose CH2-bending and hydroxyl group in-plane bending was changed [32]. These results suggested bond formation between amylose and SiO2. The dried SiO2/amylose nanospheres were pyrolyzed in an oxygen atmosphere. Compared in Fig. 6a are the thermogravimetric analysis curves with weight residuals normalized to those at 150 C. The weight at this temperature was taken as the intrinsic weight of the samples because absorbed moisture would have evaporated by then and amylose decomposition would not have set in. The tested amylose sample was essentially fully decomposed by 600 C. Between 150 and 600 C, silica would barely lose any weight. Evidently, the weight loss by 600 C increased as mS/mA increased, in agreement with expectation, for the SiO2/amylose nanospheres samples. Hollow SiO2 nanospheres didn’t remained amylase after degradation of it at high temperature because TGA curve displayed 7% of loss weight attributing to absorbing moisture in Fig. 6a7. The pyrolyzed samples were ultrasonicated in water and then a drop of this dispersion was placed on a TEM grid for drying and observation by TEM. Fig. 6b shows a TEM image for a pyrolyzed sample prepared using mS/mA ¼ 2.5/1.0. Many small pores with an average diameter of about 5 nm were seen in the walls of the particles. The porous structure was especially clear in the image of part of an individual particle at high magnification (inset in Fig. 6b). These pores were most likely derived from the burning of the original amylose nanospheres forming larger amylase aggregates. DLS data of single amylase nanospheres was displayed in Fig. 6c. This image also confirmed that the sol-gelling of TEOS in the presence of amylose and the subsequent pyrolysis of the amylose offered a viable route towards silica particles with interesting morphologies. The porosity was confirmed by the nitrogen sorption measurement in Fig. 6d. 4. Conclusion Ethanol addition to an amylose solution in hot water to an ethanol volume fraction fEtOH of 75% caused amylose to aggregate into globules. The larger globular aggregates with an average diameter w50 nm were detected by not only DLS but also TEM. After ammonia and TEOS addition, TEOS underwent sol-gel reactions. Analyzing samples taken at different reaction times using TEM indicated that the hydrolyzed TEOS first condensed with amylose molecules and aggregates to yield SiO2-wrapped amylose or SiO2@amylose nanoparticles. These particles then clustered into coreeshell nanospheres with an amylose-rich core. The shell consisted of fused smaller SiO2@amylose nanoparticles and the shell thickness increased with reaction time due to the gradual incorporation of SiO2@amylose nanoparticles into the shell. Under given experimental conditions, the equilibrium nanospheres had a welldefined size. The shell thickness increased as the TEOS to amylose feed mass ratio mS/mA increased. At high mS/mA values, the shell was thick and smooth. The shell was thin and had roughness
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comparable to the size of its constituent SiO2@amylose nanoparticles at low mS/mA values. After pyrolysis of silica/amylose particles prepared at relatively high mS/mA values, interesting hollow silica particles with porous walls were obtained. The solgelling of TEOS in the presence of amylose and the subsequent pyrolysis of the amylose offered a viable route towards silica particles with interesting morphologies and many potential applications. Acknowledgments We thank the National Natural Science Foundation of China (No 20474068, 51173204) for financial support. We also thank support from the Outstanding Overseas Chinese Scholars Funds of the Chinese Academy of Sciences as well as support from the Leading Talents Program of Guangdong Province. References [1] Dubey A, Choi M, Ryoo R. Green Chemistry 2006;8(2):144e6. [2] Rao GV, Lopez GP. Advanced Materials 2000;12(22):1692e5. [3] Kirsch BL, Chen X, Richman EK, Gupta V, Tolbert SH. Advanced Functional Materials 2005;15(8):1319e27. [4] Comes M, Marcos MD, Martínez-Máñez R, Sancenón F, Villaescusa LA, Graefe A, et al. Journal of Materials Chemistry 2008;18(47):5815e23. [5] Chen M, Wu LM, Zhou SX, You B. Macromolecules 2004;37(25):9613e9. [6] Xiong D, Liu G, Hong L, Duncan EJS. Chemistry of Materials 2011;23(19): 4357e66. [7] Chen Y, Iroh JO. Chemistry of Materials 1999;11(5):1218e22. [8] Zhang SW, Zhou SX, Weng YM, Wu LM. Langmuir 2005;21(6):2124e8. [9] Liu G, Yang X, Wang Y. Polymer 2007;48(15):4385e92. [10] Polarz S, Smarsly B, Bronstein L, Antonietti M. Angewandte ChemieInternational Edition 2001;113(23):4549e53. [11] Yang X, Dai T, Lu Y. Polymer 2006;47(1):441e7. [12] Du JZ, Armes SP. Langmuir 2008;24(23):13710e6. [13] Du J, Armes SP. Langmuir 2009;25(16):9564e70. [14] Yuan JJ, Mykhaylyk OO, Ryan AJ, Armes SP. Journal of the American Chemical Society 2007;129(6):1717e23. [15] Singh V, Singh SK, Pandey S, Sanghi R. Advanced Materials Letters 2010;1(1): 40e7. [16] Singh V, Singh SK, Pandey S, Kumar P. Journal of Non-Crystalline Solids 2011; 357(1):194e201. [17] Molvinger K, Quignard F, Brunel D, Boissiere M, Devoisselle JM. Chemistry of Materials 2004;16(17):3367e72. [18] Oliveira F, Monteiro SR, Barros-Timmons A, Lopes-da-Silva JA. Carbohydrate Polymers 2010;82(4):1219e27. [19] Rashidova SS, Shakarova DS, Ruzimuradov ON, Satubaldieva DT, Zalyalieva SV, Shpigun OA, et al. Journal of Chromatography B 2004;800(1e2):49e53. [20] Zhang BJ, Davis SA, Mann S. Chemistry of Materials 2002;14(3):1369e75. [21] Wu J, Sailor MJ. Advanced Functional Materials 2009;19(5):733e41. [22] Liu YL, Hsu CY, Su YH, Lai JY. Biomacromolecules 2005;6(1):368e73. [23] Shchipunov YA, Karpenko TY. Langmuir 2004;20(10):3882e7. [24] Yeh J-T, Chen C-L, Huang K-S. Materials Letters 2007;61(6):1292e5. [25] Yang YM, Wang JW, Tan RX. Enzyme and Microbial Technology 2004;34(2): 126e31. [26] Zou Y, Xiang C, Sun L-X, Xu F. Biosensors and Bioelectronics 2008;23(7): 1010e6. [27] Fei B, Lu H, Xin JH. Polymer 2006;47(4):947e50. [28] Coradin T, Livage J. Journal of Sol-Gel Science and Technology 2003;26(1e3): 1165e8. [29] Guo M, Jiang M, Pispas S, Yu W, Zhou C. Macromolecules 2008;41(24): 9744e9. [30] Hizukuri S, Takeda Y, Yasuda M, Suzuki A. Carbohydrate Research 1981;94(2): 205e13. [31] Zhang Y, Hu L, Han J, Jiang Z, Zhou Y. Microporous and Mesoporous Materials 2010;130(1e3):327e32. [32] Casu B, Reggiani M. Journal of Polymer Science Part C:Polymer Symposium 1964;7:171e85.
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Polymer journal homepage: www.elsevier.com/locate/polymer
Sol-gel synthesis of silica/amylose composite particles with coreeshell structure Yinhui Li a, Jiwen Hu a, *, Guojun Liu a, b, **, Jinheng Shi a, Wei Li a, Dingshu Xiao a a b
Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China Department of Chemistry, Queen’s University, 90 Bader Lane, Kingston, Ontario, Canada K7L 3N6
a r t i c l e i n f o
a b s t r a c t
Article history: Received 4 March 2012 Received in revised form 4 May 2012 Accepted 7 May 2012 Available online 18 May 2012
Synthesized and characterized were silica/amylose composite particles with interesting morphologies which we named as silica/amylase composite coreeshell particles. Amylose is soluble in hot water. Ethanol addition to a volume fraction fEtOH of 75% caused amylose to aggregate into globules with diameters w50 nm and smaller. After ammonia and tetraethoxysilane (TEOS) addition, TEOS underwent sol-gel reactions in the presence of amylose. The reactions eventually yielded coreeshell nanospheres with their core enriched by amylose and shell consisting mostly of fused SiO2-wrapped amylose or SiO2@amylose nanoparticles. Under a given set of experimental conditions, the equilibrium nanospheres had a well-defined size. The shell thickness increased as mS/mA increased, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added and mA denoted the amylose amount. After pyrolysis of the nanospheres prepared at relatively high mS/mA values, interesting hollow silica particles with nano-sized porous walls were obtained. Ó 2012 Elsevier Ltd. All rights reserved.
Keywords: SiO2 particles Morphological control Amylase
1. Introduction Silica/polymer composite particles have been attracted much attention for a long time due to their various applications in material science field that relate extremely with morphology and structures [1e4]. So far, there are many reports on preparation of silica/polymer composite particles with different polymer and morphology, some successful examples included are raspberry-like silica/poly(methyl methacrylate) composites [5], coreeshell silica/ poly[3-(triisopropyloxysilyl)propylmethacrylate]-block-poly[2(perfluorooctyl)ethyl methacrylate] superamphiphobic coatings [6], silica/polyamide hybrid composite films [7], silica/poly(styrene) coreeshell nanocomposites particles [8], silica/poly (N,N0 -methylenebisacrylamide) coreeshell composites materials [9], silica/ polysaccharide porous materials [10], sunflower-like silica/polypyrrole nanocomposites [11], hybrid silica/vesicles nanospheres or hybrid silica/micelles particles with coreeshell structure [12e14] etc. Among these silica/polymer composite particles, those from silica/polysaccharide (SiO2/polysaccharide) are of especially unique and attractive due to their nontoxic and biocompatibility properties
* Corresponding author. Tel.: þ86 20 85232136; fax: þ86 20 85232307. ** Corresponding author. Guangzhou Institute of Chemistry, Chinese Academy of Sciences, Guangzhou 510650, PR China. Tel.: þ86 20 85232136; fax: þ86 20 85232307. E-mail addresses: [email protected] (J. Hu), [email protected] (G. Liu). 0032-3861/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymer.2012.05.015
and may be potential for many applications. For example, silica/ starch particles have been used in the adsorption and purification of novel metal ions [15,16]. Porous silica/chitosan hybrid microspheres could be used as potential catalyst [17]. It’s well accepted that, for various applications, the morphology and structure of SiO2/polysaccharide composites are of critical importance. Accordingly, the design and controlled fabrication of SiO2/polysaccharide colloidal composites with tailored morphologies and structures have been made considerable efforts [18e22]. For example, Kabulov and coworker [19] studied that hybrid silica/ chitosan sorbents were proved that their structure determined their properties, the results were reflected on chromatographic behavior of the sorbents. Mann and coworkers [20] prepared silicalite-starch foams with pores up to 100 mm. First, they synthesized starch sponges with high internal macroporosites by freezing and thawing of starch gels. Second, the starch sponges were infiltrated with colloidal suspensions of silicalite nanoparticles. Final, silicalite-starch foams with pores were obtained. The pore size depended on the starch concentration and the silicalite loading. The research showed that the macroporous silicalitestarch hybrid sponges could have certain advantages over the corresponding calcined replicas according to increased structural organization and enhanced mechanical properties. Sailor and coworkers [21] prepared a pH responsive, chitosan-based hydrogel film. In detail, chitosan/SiO2 films were obtained by combining thermal oxidation of an electrochemically etched Si wafer with reaction of chitosan with glycidoxypropyltrimethoxysilane
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(GPTMS). Insulin trapped in the porous SiO2 layer underneath the hydrogel film will be release by the pH-dependent volume phase transition. The chitosan-based hydrogel in the top layer effectively blocks insulin release at pH 7.4, while insulin penetrates the swollen hydrogel layer at pH 6.0, leading to a steady release into solution. Liu and coworkers [22] reported the preparation of chitosanesilica nanocomposite membranes by cross-linking chitosan in situ on silica nanoparticles with sulfonic acid group on their surfaces. Cross-linking on chitosan increased its permselectivity in pervaporation dehydration of an ethanolewater mixture due to the reduce on its degree of swelling in water and an alcohol/water mixture. In addition, the addition of silica nanoparticles to chitosan supplied extra free volumes in polymer matrices with water permeation and would result in high permeation rates for complex membranes. According to Shchipunov and coworkers [23], the polysaccharide hydroxyl groups and the silanol groups could undergo hydrogen bonding and this accelerated and catalyzed silanol condensation. Aside from the work by Shchipunov et al., there have been other reports on the preparation of SiO2/polysaccharides composite particles including SiO2/starch [15,16,20], SiO2/chitosan [24e27], SiO2/alginate [28], SiO2/cyclodextrin [10,29] composite particles. Amylose, the linear soluble portion of starch consisting of (1/4)-linked a-D-glucopyranosyl units, was a polysaccharide and used in this study because it is abundant and inexpensive [30,31]. Also, amylose was known to be compatible with sol-gel chemistry. Reported in this paper is an interesting method to prepare silica/ amylose composite particles. The composite particles were prepared by sol-gelling tetraethoxysilane (TEOS) using ammonia as the catalyst in the presence of amylose nanoaggregates. The nanoaggregates were obtained by adding ethanol, a poor solvent for amylose, to an aqueous solution of amylose. TEOS first sol-gelled around the amylose nanoaggregates to yield SiO2-wrapped amylose or SiO2@amylose nanoparticles. The condensation of the surface silanol groups on different particles led to the clustering of these primary particles into coreeshell microparticles with their core enriched by amylose and shell consisting mostly of fused SiO2@amylose nanoparticles. To the best of our knowledge, composite particle morphologies as observed here were never reported before. Furthermore, most of the past studies did not examine the particle formation process by going beyond reporting the morphologies of the final particles. The composite coreeshell particles might have many potential applications in areas of absorption or separation for metal ions due to the rich hydroxyl groups on the surface of coreeshell SiO2@amylose nanospheres. Moreover, after pyrolysis and thermal degradation of amylose, hollow porous silica nanospheres with unique morphologies were produced, the hollow porous silica nanospheres could apply in catalysis, drug delivery system, chromatography, and others.
The eluant used was an aqueous 0.10-M NaCl solution at a flow rate of 0.6 mL/min. The system was calibrated using poly (ethylene glycol) standards with low polydispersity indices. 2.3. SiO2/amylase composite particle synthesis Amylose, 0.025 g, was stirred at 100 C for 30 min for dissolution in 5.0 mL of distilled water. After the mixture was cooled to 75 1 C, 15.0 mL of ethanol was dropped over 10 min into it. This turned the amylose solution cloudy. After the amylose aggregate solution was cooled over 0.5 h to room temperature, 14-M ammonia was added. Fifteen min later, TEOS was dropped over 10 min into the stirring mixture. This mixture was left at room temperature stirring for 18 h and still for another 12 h before it was centrifuged at 10,080 g for 15 min to settle the particles as a white precipitate. Particles were prepared at different mS/mA values, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added, and mA denoted the amylose amount used. Unless mentioned otherwise, the volume ratio used between ammonia and TEOS was 4/1. To prepare particles with mS/mA ¼ 2.5/1.0, 1.00 mL of ammonia, 0.25 mL or 2.3 102 mg of TEOS, and 25 mg of amylose in 5.0 mL of water were used. The mS/mA value was 2.5/1.0 in this case because one TEOS molecule yields one SiO2 unit corresponding to 26 wt% of TEOS. To prepare particles at mS/mA ¼ 1.0/ 1.0, 1.0/2.5, and 5.0/1.0, the TEOS amounts used were 0.10 mL, 0.040 mL, and 0.020 mL, respectively, while maintaining the amylose amount constant at 25 mg. 2.4. Composite particle formation process The composite particle formation process at mS/mA ¼ 2.5/1.0 and 1.0/2.5 was followed by dynamic light scattering (DLS) and transmission electron microscopy (TEM). In these cases, TEOS was added in one shot rather than dropwise. For TEM studies, samples taken at pre-designated times after TEOS additions were immediately aero-sprayed onto carboncovered TEM grids. Nitrogen was then blown on the grids to remove the residual solvent. For DLS studies, amylose particles containing ammonia were first pushed through a 0.45-mm filter into a light scattering cell. The desired amount of fresh TEOS in water/ethanol was then injected, in one shot, through a 0.1-mm filter into the light scattering cell. Measurements were then made at different times. 2.5. Transmission electron microscope (TEM)
Amylose [(C6H10O5)n, Tianjin Ruijinte Chemical Reagent Co. Ltd.], TEOS (Guangzhou Chemical Reagent Factory), ammonia (25e28% or w14 M in water, Tianjin Ruijinte Chemical Reagent Co. Ltd.), ethanol (>99.7%, Tianjin Fuyu Fine Chemical Co., Ltd.) were of analytical grade and were used as received. Water was doubled distilled.
To prepare TEM specimens of amylose aggregates, an amylose aggregate solution cooled to room temperature before ammonia addition was aero-sprayed or atomized, using a home-built device, onto Formvar-coated copper grids. To stain this sample, a drop of a 2 wt% uranyl acetate solution in water was equilibrated at room temperature with a sprayed and dried sample for 3 min before the liquid was wicked off using filter paper. TEM observations were made using a JEM-100CX II microscope operated at 80 kV. To prepare specimens of the SiO2/amylose composite particles, 10 mg of the particles were ultrasonicated for 15 min in 1 mL of water. After dispensing 1 drop on a Formvar-coated copper grid, the specimen was allowed to dry at room temperature for 24 h before TEM observation.
2.2. Size-exclusion chromatography (SEC)
2.6. Dynamic light scattering (DLS)
SEC analysis of amylose was performed at 40 C temperature on Waters 515 instrument equipped with Ultrahydrogel 250 columns.
Light scattering measurements were performed in a 1.2 1.2 4.5 cm3 quartz cell at 25 C on a Zetasizer Nano S90
2. Experimental section 2.1. Materials
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3. Results and discussion
An amylose solution at fEOH ¼ 75% was also aero-sprayed or atomized onto a Formvar-covered copper grid using a home-built device. Using this method, ethanol has evaporated from the atomized fine droplets during their flight from the spraying nozzle to a catching TEM grid and water has evaporated within w3 s after the spray landed on the TEM grid. Due to the fast solvent evaporation, the slow polymer chain motion, and the low polymer concentration and thus large distance between the different aggregates before solvent evaporation, the dried amylose aggregates have more or less retained their shapes in solution. A dried sample was then equilibrated with a drop of a uranyl acetate solution drop to negatively stain the droplets. Since amylose was insoluble in room-temperature water, the aggregates have survived this staining process as well. Fig. 1b shows a TEM image of an amylose aggregate specimen that was prepared using the method described above. Since the sample was negatively stained, the lighter domains corresponded to locations of the amylose aggregates. They must have mostly consisted of spheres and ellipsoids. The average particle diameter was 32 8 nm. The TEM diameter was substantially smaller than the DLS diameter because the former was the number-average diameter and the latter was the z-average diameter. Furthermore, sizes of the dry particles were obtained by TEM and those of the solvated particles were obtained from DLS. Regardless of the discrepancy, what was clear was that amylose did aggregate after ethanol addition. The reasonable TEM and DLS diameters also suggested that the aggregates survived the TEM specimen preparation and staining protocols and were observed by TEM. While not detected by DLS and TEM, unimolecular amylose chains must have co-existed with the aggregates. They were not detected by DLS probably because of their low scattering power. They were not seen by TEM because of the limited resolution of the instrument used.
3.1. Amylose and amylose aggregates
3.2. Composite particle formation
The amylose used in this study was analyzed by SEC. Based on polyethylene glycol standards, the number-average molecular weight Mn and polydispersity index Mw/Mn were 1.22 104 g/mol and 2.27, respectively, for the sample. The amylose dissolved in hot water but was insoluble in ethanol or room-temperature water. After ethanol addition at 75 C to a volume fraction fEtOH of 75%, amylose solutions turned cloudy, suggesting amylose aggregation. After sample cooling to room temperature, a DLS analysis was performed to yield an average hydrodynamic diameter dh of 78 nm and a polydispersity index of 0.025 for the aggregates. The size distribution of these aggregates is shown in Fig. 1a.
Composite particles were formed by adding TEOS into ammonia-containing amylose solutions in water/ethanol at fEtOH ¼ 75%. The sol-gel formation process was followed by analyzing samples that were taken at different times after TEOS addition in one shot to mS/mA ¼ 2.5/1.0 and were immediately aerosprayed. Fig. 2 shows TEM images of samples taken at different times. These samples were not stained because Si in SiO2 or silanol groups acted as an intrinsic staining agent. Comparing the TEM images for samples taken at different times allowed us to draw the following conclusions: First, “messy” background was seen only in Fig. 2a and b for samples taken 1 and 4 min after TEOS addition. The messy
Particle Size Analyzer. The light source used was a 633-nm HeeNe laser. The size and polydispersity values as well as the size distributions were automatically generated by the instrument after each measurement. 2.7. Atomic force microscopy (AFM) Specimens were prepared by aero-spraying solution samples onto silicon wafers. All samples were analyzed by tapping-mode AFM using a Veeco multimode instrument equipped with a Nanoscope IIIa controller. 2.8. Diffuse reflectance FT-IR spectroscopy (DRIFTS) DRIFTS were carried out on an Equinox 55 spectrometer using a Spectra-Tech diffuse reflectance accessory and a high temperature chamber with KBr windows. 2.9. Thermogravimmetric analyses (TGA) Thermogravimmetric analyses were done on a Netzsch TG-209/ Bruker Vector-22 instrument. SiO2/amylase particles, 3e5 mg, were placed in ceramic pot and heated from room temperature to 800 C at a rate of 10 C/min under nitrogen atmosphere. 2.10. Nitrogen sorption measurement The porosity of silica was performed on a 3H-2000PS1 instrument. Silica nanospheres, 0.39 g, were measured by static volumetric method. Absorbent was Nitrogen, degassing temperature was 90 C, degassing time was 120 min, and degassing mode was molecular replacement one.
Fig. 1. DLS hydrodynamic diameter distribution (a) and TEM image (b) for amylose aggregates.
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Fig. 2. TEM (aee) and AFM (f and g) images of SiO2/amylose particles aero-sprayed 1 min (a), 4 min (b), 6 min (c), 8 min (d), and 18 h (eeg) after TEOS addition at mS/mA ¼ 2.5/1.0 into an ammonia-containing amylose aggregate solution.
background seemed to consist of fused gray particles with or without a lighter core. The diameter of these particles was w20 nm. Second, particles with diameters w50 nm or shapes similar to these larger amylose particles of Fig. 1b were observed in Fig. 2a or b only and some of these were marked by orange arrows in these images. These particles in Fig. 2a seemed wrapped by SiO2 nanoparticles or were SiO2@amylose nanoparticles. Third, the SiO2@amylose nanoparticles were fused with other particles in Fig. 2a already. By 4 min after TEOS addition or in Fig. 2b, the SiO2@amylose particles had fused into nanospheres with a well-defined coreeshell structure. Fourth, the shape of these coreeshell nanospheres changed little with time after w4 min. Many of these particles seemed to be spherical (circular in the 2-d images) or ellipsoidal and possessed a singular core. Some particles assumed the shape of a double-yolk egg. There were also particles possessing irregularly-shaped cores. Fifth, the shell got darker and thicker with reaction time. Sixth, the particles aged at long reaction times were round and not hollow. This was clearly confirmed by the AFM topography image shown in Fig. 2f. The phase image in Fig. 2g suggested that the nanospheres surfaces were not perfectly smooth. Rather, the nanospheres shell seemed to consist of fused nanoparticles. The gray nanoparticles without a light core in the background in Fig. 2a and b were probably the SiO2 nanoclusters formed from the TEOS sol-gel reactions. The particles with a light core were formed probably because of deposition and condensation of SiO2 nanoclusters bearing surface SieOH groups on amylose molecules or small aggregates. These were SiO2-wrapped amylose nanoparticles or SiO2@amylose nanoparticles. The silanol groups of the SiO2 nanoclusters also condensed with the surface hydroxyl groups of the larger amylose aggregates seen in Fig. 1b, resulting in the larger SiO2@amylose nanoparticles marked by orange arrows in Fig. 2a. While amylose molecules or small aggregates were not detected by TEM and DLS, they must have existed because of the equilibrium between these species and the larger aggregates. Since the specific surface area was larger and the fraction of accessible hydroxyl
groups was higher for amylose molecules or smaller particles, they would more likely condense in the early stage of the sol-gel reaction with the formed silanol-bearing SiO2 nanoclusters than the larger particles. This condensation converted these species into thermodynamically different species and would have decreased the concentration of these species below their equilibrium concentrations and triggered the dissociation of the larger amylose aggregates. This probably explained why a higher fraction of smaller SiO2@amylose nanoparticles were seen in Fig. 2a and b than that of smaller amylose aggregates in Fig. 1b. These primary SiO2@amylose nanoparticles, regardless of their size, should bear SieOH and SieO groups on their surfaces. Comparing Fig. 2a and b suggested that they condensed further into larger aggregates. While the exact aggregation mechanism was not known, their fusion yielded eventually the coreeshell nanospheres as first seen in Fig. 2b. The particles looked lighter in the core because the core was amylose rich and darker in the shell because the shell was SiO2 rich. Comparing Fig. 2bee suggested that the coreeshell nanospheres did not undergo further fusion after w4 min or Fig. 2b probably because of the repulsion arising from their surface SieO groups. This repulsion force should increase with particle size due to the increase in the number of surface SieO groups. The existence of a particular average diameter for the particles under a given set of experimental conditions should not be surprising because the Stober process produced SiO2 particles with welldefined size and size distributions. The particle preparation conditions used here were similar to those reported by Stober and others except the use of amylose as a “dynamic” template, which was able to aggregate and dissociate reversibly. The wall of the nanospheres thickened with time initially probably because of the incorporation of SiO2 nanoclusters or SiO2@amylose nanoparticles that were continuously forming due to the TEOS sol-gel reactions. The further compacting and thickening of the shell at the later stage of particle synthesis, e.g. from
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Fig. 3. Plots of nanospheres dh distribution and TEM images 0.5 h (a, d), 8 h (b, e), and 18 h (c, f) after TEOS addition at mS/mA ¼ 2.5/1.0 into an ammonia-containing amylose aggregate solution.
Fig. 2d to e, occurred probably because of Oswald ripening, which involved the dissociation of certain particles and the growth of other particles. That the wall contained SiO2@amylose nanoparticles was fairly evident in Fig. 2bed because many white dots were seen in the walls of the particles in these TEM images. Due to the rapid formation and fusion of the initial SiO2@amylose particles, DLS could not be used to follow these processes. Rather, DLS was used to follow the later stage of the coreeshell SiO2/amylose particle formation process. The DLS specimens were prepared by pushing a TEOS solution through a filter into a filtered ammonia-containing amylose solution in a light scattering cell. Fig. 3 plots of SiO2/amylose particle dh distribution 0.5, 8, and 18 h after TEOS addition. The average particle diameter increased from 209 to 221 nm in the later stage of the reaction between 8 and 18 h probably due to shell thickening as observed by TEM.
3.3. Effect of varying mS/mA SiO2/amylose composite particles were prepared using different mass feed ratios mS/mA, where mS denoted the amount of SiO2 obtainable from the amount of TEOS precursor added and mA denoted the amylose amount used. The other conditions included the use of an aqueous amylose solution at 0.5 wt%, a volume ratio of 4/1 for ammonia and TEOS, and a reaction time of 18 h plus 12 h of sample aging. The prepared particles were purified by centrifugation and rinsing. After drying and redispersing in water by ultrasonication, they were sprayed on TEM grids for observation. Fig. 4 compares the TEM images of the composite particles prepared under these conditions at different mS/mA. A comparison of Fig. 4aec and Fig. 2e suggested that coreeshell nanospheres were always formed at mS/mA 1.0/2.5. Evidently, the
Fig. 4. TEM images of coreeshell nanospheres prepared at mS/mA ¼ 5.0/1.0 (a), 1.0/1.0 (b), 1.0/2.5 (c), and 1.0/5.0 (d).
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Fig. 5. FT-IR spectra of composite particles prepared at mS/mA ¼ 5.0/1.0 (a1), 2.5/1.0 (a2, b3)1.0/1.0 (a3, b2), 1.0/2.5 (a4), 1.0/5.0 (a5) and amylase (b1).
shell thickness increased with mS/mA. At mS/mA 2.5/1.0, the shells were thick enough and appeared rather smooth. At lower mS/mA values, the thickness of the shells was comparable to the size of their constituent SiO2@amylose particles and such a shell made of a particulate monolayer or sub-monolayer appeared rough. At mS/ mA ¼ 1.0/5.0, the coreeshell nanospheres structure was not obvious but might still have existed. Two of these seemingly coreeshell structures were marked by orange circles in Fig. 4d. The lack of a well-defined coreeshell structure in this case might be due to the lack of sufficient SiO2@amylose nanoparticles to cover the amylose core nanospheres. Our statistical analysis yielded the average TEM diameters of 326 40, 244 42, 161 23, and 109 19 nm for the coreeshell particles prepared at mS/mA ¼ 5.0/1.0, 2.5/1.0, 1.0/1.0, and 1.0/2.5, respectively. The corresponding average shell thicknesses were
115 11 nm, 69 5 nm, 44 8 nm, and 28 4 nm, respectively. Using these values, the core diameters were calculated to be 96 62 nm, 106 52 nm, 73 39 nm, and 53 27 nm. 3.4. Properties of the composite particles After drying, the SiO2/amylose nanospheres were used directly for diffuse reflectance FT-IR analysis. Fig. 5a compares the FT-IR spectra of amylose and the SiO2/amylose composite particles prepared at different mS/mA values. As mS/mA increased, the transmittance at 1100, 806, and 471 cm1 increased. Since these peaks corresponded to the asymmetric stretching, the symmetric stretching, and the distorted stretching of the SieOeSi bonds, this suggested that the SiO2 amount in the composite particles increased with mS/mA. This should not be surprising given the fast
Fig. 6. TGA curves of the SiO2/amylose and hollow SiO2 nanospheres (a), TEM image of pyrolyzed SiO2/amylose nanospheres prepared at mS/mA ¼ 2.5/1.0 (b), inset is image of part of an individual particle at high magnification. The mS/mA values used to prepare the SiO2/amylose nanospheres were 0/5.0 (1), 1.0/5.0 (2), 1.0/2.5 (3), 1.0/1.0 (4), 2.5/1.0 (5), 5.0/1.0 (6), and hollow SiO2 nanospheres (7), respectively, DLS hydrodynamic diameter distribution for starch solution 5 mg/mL (c), and the nitrogen sorption isotherms of hollow silica nanospheres (d).
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kinetics of the TEOS sol-gel reaction and the likely quantitative conversion of TEOS into SiO2. In addition, some amylose peaks as shown in Fig. 5b were changed. For example, the characterization peaks between 525 cm1 and 928 cm1 for amylose ring vibrations disappeared or were weakened. The shape of the broad band at 1450e1400 cm1 for amylose CH2-bending and hydroxyl group in-plane bending was changed [32]. These results suggested bond formation between amylose and SiO2. The dried SiO2/amylose nanospheres were pyrolyzed in an oxygen atmosphere. Compared in Fig. 6a are the thermogravimetric analysis curves with weight residuals normalized to those at 150 C. The weight at this temperature was taken as the intrinsic weight of the samples because absorbed moisture would have evaporated by then and amylose decomposition would not have set in. The tested amylose sample was essentially fully decomposed by 600 C. Between 150 and 600 C, silica would barely lose any weight. Evidently, the weight loss by 600 C increased as mS/mA increased, in agreement with expectation, for the SiO2/amylose nanospheres samples. Hollow SiO2 nanospheres didn’t remained amylase after degradation of it at high temperature because TGA curve displayed 7% of loss weight attributing to absorbing moisture in Fig. 6a7. The pyrolyzed samples were ultrasonicated in water and then a drop of this dispersion was placed on a TEM grid for drying and observation by TEM. Fig. 6b shows a TEM image for a pyrolyzed sample prepared using mS/mA ¼ 2.5/1.0. Many small pores with an average diameter of about 5 nm were seen in the walls of the particles. The porous structure was especially clear in the image of part of an individual particle at high magnification (inset in Fig. 6b). These pores were most likely derived from the burning of the original amylose nanospheres forming larger amylase aggregates. DLS data of single amylase nanospheres was displayed in Fig. 6c. This image also confirmed that the sol-gelling of TEOS in the presence of amylose and the subsequent pyrolysis of the amylose offered a viable route towards silica particles with interesting morphologies. The porosity was confirmed by the nitrogen sorption measurement in Fig. 6d. 4. Conclusion Ethanol addition to an amylose solution in hot water to an ethanol volume fraction fEtOH of 75% caused amylose to aggregate into globules. The larger globular aggregates with an average diameter w50 nm were detected by not only DLS but also TEM. After ammonia and TEOS addition, TEOS underwent sol-gel reactions. Analyzing samples taken at different reaction times using TEM indicated that the hydrolyzed TEOS first condensed with amylose molecules and aggregates to yield SiO2-wrapped amylose or SiO2@amylose nanoparticles. These particles then clustered into coreeshell nanospheres with an amylose-rich core. The shell consisted of fused smaller SiO2@amylose nanoparticles and the shell thickness increased with reaction time due to the gradual incorporation of SiO2@amylose nanoparticles into the shell. Under given experimental conditions, the equilibrium nanospheres had a welldefined size. The shell thickness increased as the TEOS to amylose feed mass ratio mS/mA increased. At high mS/mA values, the shell was thick and smooth. The shell was thin and had roughness
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comparable to the size of its constituent SiO2@amylose nanoparticles at low mS/mA values. After pyrolysis of silica/amylose particles prepared at relatively high mS/mA values, interesting hollow silica particles with porous walls were obtained. The solgelling of TEOS in the presence of amylose and the subsequent pyrolysis of the amylose offered a viable route towards silica particles with interesting morphologies and many potential applications. Acknowledgments We thank the National Natural Science Foundation of China (No 20474068, 51173204) for financial support. We also thank support from the Outstanding Overseas Chinese Scholars Funds of the Chinese Academy of Sciences as well as support from the Leading Talents Program of Guangdong Province. References [1] Dubey A, Choi M, Ryoo R. Green Chemistry 2006;8(2):144e6. [2] Rao GV, Lopez GP. Advanced Materials 2000;12(22):1692e5. [3] Kirsch BL, Chen X, Richman EK, Gupta V, Tolbert SH. Advanced Functional Materials 2005;15(8):1319e27. [4] Comes M, Marcos MD, Martínez-Máñez R, Sancenón F, Villaescusa LA, Graefe A, et al. Journal of Materials Chemistry 2008;18(47):5815e23. [5] Chen M, Wu LM, Zhou SX, You B. Macromolecules 2004;37(25):9613e9. [6] Xiong D, Liu G, Hong L, Duncan EJS. Chemistry of Materials 2011;23(19): 4357e66. [7] Chen Y, Iroh JO. Chemistry of Materials 1999;11(5):1218e22. [8] Zhang SW, Zhou SX, Weng YM, Wu LM. Langmuir 2005;21(6):2124e8. [9] Liu G, Yang X, Wang Y. Polymer 2007;48(15):4385e92. [10] Polarz S, Smarsly B, Bronstein L, Antonietti M. Angewandte ChemieInternational Edition 2001;113(23):4549e53. [11] Yang X, Dai T, Lu Y. Polymer 2006;47(1):441e7. [12] Du JZ, Armes SP. Langmuir 2008;24(23):13710e6. [13] Du J, Armes SP. Langmuir 2009;25(16):9564e70. [14] Yuan JJ, Mykhaylyk OO, Ryan AJ, Armes SP. Journal of the American Chemical Society 2007;129(6):1717e23. [15] Singh V, Singh SK, Pandey S, Sanghi R. Advanced Materials Letters 2010;1(1): 40e7. [16] Singh V, Singh SK, Pandey S, Kumar P. Journal of Non-Crystalline Solids 2011; 357(1):194e201. [17] Molvinger K, Quignard F, Brunel D, Boissiere M, Devoisselle JM. Chemistry of Materials 2004;16(17):3367e72. [18] Oliveira F, Monteiro SR, Barros-Timmons A, Lopes-da-Silva JA. Carbohydrate Polymers 2010;82(4):1219e27. [19] Rashidova SS, Shakarova DS, Ruzimuradov ON, Satubaldieva DT, Zalyalieva SV, Shpigun OA, et al. Journal of Chromatography B 2004;800(1e2):49e53. [20] Zhang BJ, Davis SA, Mann S. Chemistry of Materials 2002;14(3):1369e75. [21] Wu J, Sailor MJ. Advanced Functional Materials 2009;19(5):733e41. [22] Liu YL, Hsu CY, Su YH, Lai JY. Biomacromolecules 2005;6(1):368e73. [23] Shchipunov YA, Karpenko TY. Langmuir 2004;20(10):3882e7. [24] Yeh J-T, Chen C-L, Huang K-S. Materials Letters 2007;61(6):1292e5. [25] Yang YM, Wang JW, Tan RX. Enzyme and Microbial Technology 2004;34(2): 126e31. [26] Zou Y, Xiang C, Sun L-X, Xu F. Biosensors and Bioelectronics 2008;23(7): 1010e6. [27] Fei B, Lu H, Xin JH. Polymer 2006;47(4):947e50. [28] Coradin T, Livage J. Journal of Sol-Gel Science and Technology 2003;26(1e3): 1165e8. [29] Guo M, Jiang M, Pispas S, Yu W, Zhou C. Macromolecules 2008;41(24): 9744e9. [30] Hizukuri S, Takeda Y, Yasuda M, Suzuki A. Carbohydrate Research 1981;94(2): 205e13. [31] Zhang Y, Hu L, Han J, Jiang Z, Zhou Y. Microporous and Mesoporous Materials 2010;130(1e3):327e32. [32] Casu B, Reggiani M. Journal of Polymer Science Part C:Polymer Symposium 1964;7:171e85.