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Dendrimer-like hybrid particles with tunable hierarchical pores† Xin Du,*a Xiaoyu Li,b Hongwei Huang,c Junhui Hed and Xueji Zhanga Dendrimer-like silica particles with a center-radial dendritic framework and a synergistic hierarchical porosity have attracted much attention due to their unique open three-dimensional superstructures with high accessibility to the internal surface areas; however, the delicate regulation of the hierarchical porosity has been difficult to achieve up to now. Herein, a series of dendrimer-like amino-functionalized silica particles with tunable hierarchical pores (HPSNs-NH2) were successfully fabricated by carefully regulating and optimizing the various experimental parameters in the ethyl ether emulsion systems via a one-pot sol–gel reaction. Interestingly, the simple adjustment of the stirring rate or reaction temperature was found to be an easy and effective route to achieve the controllable regulation towards center-radial large pore sizes from ca. 37–267 (148 ± 45) nm to ca. 8–119 (36 ± 21) nm for HPSNs-NH2 with particle sizes of 300–700 nm and from ca. 9–157 (52 ± 28) nm to ca. 8–105 (30 ± 16) nm for HPSNs-NH2 with particle sizes of 100–320 nm. To the best of our knowledge, this is the first successful regulation towards centerradial large pore sizes in such large ranges. The formation of HPSNs-NH2 may be attributed to the complex cross-coupling of two processes: the dynamic diffusion of ethyl ether molecules and the selfassembly of partially hydrolyzed TEOS species and CTAB molecules at the dynamic ethyl ether–water
Received 28th January 2015, Accepted 23rd February 2015
interface of uniform small quasi-emulsion droplets. Thus, these results regarding the elaborate regulation
DOI: 10.1039/c5nr00640f
of center-radial large pores and particle sizes not only help us better understand the complicated selfassembly at the dynamic oil–water interface, but also provide a unique and ideal platform as carriers or
www.rsc.org/nanoscale
supports for adsorption, separation, catalysis, biomedicine, and sensor.
1.
Introduction
In the past decade, great efforts have been devoted to the design and controllable synthesis of porous silica nanomaterials with various well-defined morphologies and structures
a Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, P. R. China. E-mail: [email protected] b Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China d Functional Nanomaterials Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, China † Electronic supplementary information (ESI) available: The schematic illustration of the experimental procedures, SEM images of the prepared HPSNs under varied APTES amounts at 20 °C, SEM images of as-prepared HPSNs under varied co-condensation silanes, size distribution histograms of the surface pores of HPSNs-NH2 (St1–St6 and St1′–St5′), digital images of CTAB-stabilized ethyl ether emulsion systems after vigorous stirring and after static placement, SEM and TEM images of the Sa3 product synthesized in the emulsion containing 20 mL of ethyl ether and 10 mL of ethanol, 15 mL of ethyl ether and 5 mL of ethanol and 10 mL of ethyl ether and 5 mL of ethanol (before gradient centrifugation). See DOI: 10.1039/c5nr00640f
This journal is © The Royal Society of Chemistry 2015
because of their many attractive structural and intrinsic features such as tunable particle size and pore size, tunable particle shape and pore structures, large specific surface area, high pore volume, low density, high mechanical and thermal stability, low toxicity, good biocompatibility, and easy functionalization.1–10 In fact, the size, shape, structure, composition, and surface functionalization of porous silica nanomaterials directly affect their physical and chemical properties and the resultant performance in their wide practical and potential applications, including molecule adsorption and separation, heterogeneous catalysis, nanoreactors, nanocasting, functional coatings, drug/gene/protein storage or delivery, cell imaging, and disease diagnosis.1–10 Among these, the delicate structural and morphological control of porous silica nanomaterials has become increasingly crucial for adjusting their properties and launching new functions; moreover it also affects and even determines their performance. Hierarchical structures discovered in natural species, such as lotus surface, shark skin, spider silk, butterfly wing, endow them with a variety of remarkable unique properties, and have inspired the rapid development of new artificial materials with multiscale structures and multiple functions.11–16 Recently, newly created dendrimer-like (dendritic) silica particles with a
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center-radial dendritic framework and hierarchical porosity, which are constructed by introducing additional center-radial large pores with a conical pore shape (i.e., gradually increasing the pore size from the particle center to the particle surface) into uniform mesopores, have attracted much attention.17–23 In comparison with conventional mesoporous silica particles, this type of particle has the integrated synergistic hierarchical porosity to simultaneously combine the advantages of the varied pore sizes, offering highly improved mass transport through the radial-oriented large pore channels, coupled with uniform small mesopores.17–23 Thus, this synergistic hierarchical pore structure favors the guest molecule, especially large molecules, to diffuse and access the active sites, thus it is an ideal scaffold or carrier for biological, medical, and catalytic applications.11–23 Several groups have synthesized this type of similar silica particles with hierarchical pores.17–23 For example, Polshettiwar et al. achieved the synthesis of silica nanoparticles with a fibrous morphology through a microemulsion system composed of cyclohexane as an oil phase and pentanol as a co-solvent, using cetylpyridinium bromide (CPB) or cetyltrimethylammonium bromide (CTAB) as a structure-directing agent under microwave-assisted hydrothermal conditions.19 Zhang’s group reported the facile large-scale synthesis of monodispersed MSNs with stellate-like or raspberry-like channel morphologies by employing cetyltrimethylammonium tosylate as a structure-directing agent and small organic amines as the basic catalysts via a templating sol–gel technique.21 Du et al. fabricated dendritic silica nanoparticles with hierarchical pores through a CTAB-stabilized ethyl ether emulsion system via a one-pot sol–gel reaction.22,23 However, there still exist a few disadvantages, such as a very large particle size, small center-radial pore size, very thick fiber, difficult large-scale synthesis, and, in particular, it is very difficult to control the sizes of the center-radial pores.17–23 It is worth noting that the elaborate regulation of center-radial pore sizes should be very significant for adjusting the porosity, permeability, and accessibility.17–23 Thus, the reliable synthesis of dendrimer-like silica particles with a tunable hierarchical porosity and particle size is highly desirable. Herein, based on our previously developed ethyl ether emulsion system,22,23 we successfully fabricate a series of dendrimerlike amino-functionalized silica nanoparticles with adjustable hierarchical pores (HPSNs-NH2) and particle sizes by systematically regulating the various experimental parameters. Moreover, it was found that the simple regulation of the stirring rate and reaction temperature is an easy and effective route to obtain a controllable regulation towards center-radial large pore sizes. In addition, a tentative mechanism is proposed for the formation of HPSNs-NH2 based on a systematic study and analysis.
2. Experimental section (materials and methods) 2.1.
Reagents and materials
Tetraethyl orthosilicate (TEOS, ≥98%), 3-aminopropyltriethoxysilane (APTES, ≥98%), 3-mercaptopropyl-
Nanoscale
trimethoxysilane (MPTMS), 3-glycidoxypropyltrimethoxysilane (GPTMS), cetyltrimethylammonium bromide (CTAB, ≥99%), aqueous ammonia (NH4OH, 30%), hydrochloric acid (HCl, 37%), ethyl ether, ethanol, chloroauric acid (HAuCl4·4H2O), sodium borohydride (NaBH4), and other chemicals not mentioned here were purchased from Sigma Aldrich. All the materials were of analytical grade and used as received without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ cm was used in all the experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. One pot synthesis of dendrimer-like aminofunctionalized silica nanoparticles with hierarchical pores (HPSNs-NH2) HPSNs-NH2 were prepared in an ethyl ether emulsion system based on the modified synthesis procedure in our previous study.23 Various experimental parameters (composition ratio, silane type, stirring rate, and reaction temperature) were regulated to investigate their effect toward the morphologies and structures of the final products (Table 1). In a typical procedure for sample Sa3, 0.5 g of CTAB was dissolved in an emulsion system composed of 70 mL of H2O, 0.8 mL of aqueous ammonia (NH4OH, 30%), 20 mL of ethyl ether and 10 mL of ethanol. After the mixture was vigorously stirred with a magnetic stirring rate of 1000 rpm (size of stirring bar: 35 mm (length) × 5 mm (diameter)) for 0.5 h at 20 °C (the temperature was controlled by a Thermoline Scientific BL-30 water bath), a mixture of 2.5 mL of TEOS and 0.1 mL of APTES was quickly poured into the abovementioned mixture. The resulting mixture was vigorously stirred with a magnetic stirring rate of 1000 rpm at 20 °C for 4 h. After that, 1 mL of HCl (37%) was added in order to stop the base-catalyzed reaction. A white precipitate was obtained by centrifugation at 4200 rpm (Sigma 3–18 centrifuge from Sigma Laborzentrifugen GmbH) for 12 min, which was washed with ethanol and pure water three times, and dispersed into ethanol. All the experiments were performed in triplicate. Although a slight difference appeared, the experimental results were basically reproducible. 2.3.
Particle purification and surfactant removal
The as-prepared product (undried) dispersed into ethanol was treated by ultrasonic dispersion at an ultrasonic power of 330 W for 30 min (soniclean from Soniclean Pty. Ltd.). The asprepared product was then purified by gradient centrifugation and the varying centrifugation rates (1500, 2000, 2500, 3000 and 3200 rpm) were investigated. To obtain products with large particle sizes (such as St1–St6), the particle suspensions were first treated by centrifuging at 2000 rpm for 3 min and then centrifuging at 2500 rpm for 3 min, and large particles and sheets could then be effectively removed. Furthermore, the abovementioned suspension containing nanoparticles was centrifuged at 8000 rpm for 3 min and the purified product was kept in ethanol or dried in air at 60 °C for analysis. To obtain products with small particle sizes (such as St1′–St5′), the particle suspensions were first treated by centrifuging at
This journal is © The Royal Society of Chemistry 2015
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Table 1 Synthesis parameters and properties of the prepared HPSNs-NH2 after purification by gradient centrifugation, followed by solvent extraction
Sample
Effect factors
Sa1 Sa2 Sa3b Sa4 Sa5 Sa6 SMPMS SGPMS Ss1 Ss2 Ss3 Ss4 St1 St2 St3 St4 St5 St6 St1′ St2′ St3′ St4′ St5′
Effect of APTES
MPTMS (0.1 mL) GPTMS (0.1 mL) Effect of stirring rate
Effect of reaction temperature
Ethanol (mL)
Diethyl ether (mL)
APTES (mL)
Temperature (°C)
Magnetic stirring rate (rpm)
Particle sizea (nm)
Fig.
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 5 5 5 5
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 15 15 15 15 15
0 0.05 0.1 0.2 0.3 0.4 0 0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
20 20 20 20 20 20 20 20 20 20 20 20 10 15 20 25 30 35 10 15 20 25 30
1000 1000 1000 1000 1000 1000 1000 1000 400 600 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
100–300 200–1000 300–600 500–1000 500–1000 irregular 150–250 irregular 500–1100 500–1000 500–900 300–600 300–600 300–600 300–600 300–600 300–700 300–700 100–260 100–260 100–280 100–300 100–320
S2a S2b S2c S2d S2e S2f S3a S3b 1a 1b 1c 1d 2a 2b 2c 2d 2e 2f 4a 4b 4c 4d 4e
a
Particle sizes are estimated by averaging 50 particles from the SEM images. b Sa3, Ss4 and St3 are the same sample, but are given different names and are placed at various positions in the table to allow a clear comparison.
3000 rpm for 3 min and then centrifuging at 3200 rpm for 3 min, and the large particles and sheets could then be effectively removed. The abovementioned suspension containing nanoparticles was also centrifuged at 10 000 rpm for 3 min and the precipitate was washed with ethanol three times. A subsequent CTAB template extraction was performed by treating the purified product in ethanolic HCl (15 mL of HCl (37%) in 120 mL of ethanol) by stirring at 70 °C for 24 h. The final purified and extracted products were obtained by washing with ethanol three times, and then these products were kept in water or ethanol for further use or dried in air at 60 °C for analysis. The products were also calcined in air at 550 °C for 5 h to obtain the calcined products (HPSNs-cal) for comparison. 2.4. Loading of Au nanoparticles in the HPSNs-NH2 (HPSNs-NH2@Au) 0.040 g of the purified and extracted HPSNs-NH2 (St3′) was dispersed in 40 mL of H2O. The color of the mixture was yellow after the addition of 0.36 mL HAuCl4·3H2O (20 mg mL−1) solution. After the mixture was magnetically stirred in a closed conical flask for 2 h at room temperature, a specific volume of fresh aqueous NaBH4 (10 mM) was added into the abovementioned mixture until the color turned pale red. Finally, the Au nanoparticles loaded precipitate (HPSNs-NH2@Au) was centrifuged, washed with water, and re-dispersed in 5 mL of water or dried in air at 60 °C. It is worth noting that the solution is colorless after centrifugation, suggesting that nearly all the
This journal is © The Royal Society of Chemistry 2015
HAuCl4 was reduced to Au nanoparticles, which were then immobilized in HPSNs-NH2. 2.5.
Characterization methods
Scanning electron microscopy (SEM) observations were carried out on a Philips XL30 field emission scanning electron microscope operated at 10 kV. The specimens were coated with a layer of platinum with a size of 5 nm by ion sputtering before the SEM observations. For the transmission electron microscopy (TEM) observations, the powder products were dispersed in ethanol by sonication for 10 min, and added on carbon-coated copper grids. After drying at 60 °C overnight, the TEM observations were carried out on a Philips CM200 transmission electron microscope at an acceleration voltage of 100 kV. Nitrogen adsorption–desorption measurements were carried out on a TriStar II surface area and porosity analyzer from Micromeritics at −196 °C using the volumetric method. Prior to the measurement, the products were degassed at 120 °C for at least 6 h. Brunauer–Emmett–Teller (BET) specific surface areas were calculated using adsorption data at a relative pressure range of P/P0 range of 0.05–0.25. The pore size distributions were estimated from the adsorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. The pore volumes were determined from the amounts of N2 adsorbed at the single point of P/P0 = 0.98. Fourier transform infrared (FTIR) spectra of the samples were taken on a Thermo Scientific NICOLET 6700 FTIR spectrometer at room temperature. 13C solid state cross-polarization magic-angle spinning
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nuclear magnetic resonance (13C-CP-MAS NMR) measurements were performed on a Bruker MSL-300 spectrometer operating at a frequency of 75.482 MHz for 13 C. The spectrometer was equipped with a 4 mm double air bearing, magicangle spinning probe suitable for MAS experiments. The proton 90° pulse time was 5.5 μs, the acquisition time was 45 ms, the cross-polarization time was 2 ms, and the relaxation delay was 3 s. The spectrum width was 50 kHz, and 4000 data points were collected over 2000 scans. All the spectra were referenced to the resonance of adamantane (δ) 38.23 ppm, and the samples were equilibrated to room temperature (22 °C) prior to all the measurements.
3. Results and discussion A series of HPSNs-NH2 were successfully fabricated using TEOS as silane precursor, APTES as co-condensation silane, NH3·H2O as basic catalyst, CTAB as mesopore template and emulsion stabilizer, ethyl ether as oil phase, and ethanol as co-solvent by a one-pot sol–gel synthesis procedure, as shown in Fig. S1.† Various experimental parameters, such as the size of the oil nanodroplets and the rate of hydrolysis and condensation of TEOS, can affect the stability of the CTAB-contained ethyl ether-in-water emulsion systems, thus affecting and determining the morphologies and structures of the final products. In our previous study,22,23 it was discovered that an increase in the volume ratio of ethanol to ethyl ether from 0 to
Nanoscale
∞ resulted in the structural evolution of the final products from hollow mesoporous nanospheres to varied particles with hierarchical pores (center-radial pores and uniform small mesopores) to conventional mesoporous nanospheres, and that the addition of APTES could endow the final particles with a better defined spherical morphology and larger mesopores.22,23 More specifically (Table S1†),23 too much ethanol (>15 mL) and ethyl ether (>30 mL) gave rise to the formation of irregular HPSNs-NH2 with a broad size distribution, and many large hollow microspheres. With a decrease in the volumes of ethanol (from 10 to 5 mL) and ethyl ether (from 20 to 10 mL), the particle size decreased gradually, and the center-radial pore structure could be retained very well. On further decreasing the volumes of ethanol (
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Cite this: DOI: 10.1039/c5nr00640f
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Dendrimer-like hybrid particles with tunable hierarchical pores† Xin Du,*a Xiaoyu Li,b Hongwei Huang,c Junhui Hed and Xueji Zhanga Dendrimer-like silica particles with a center-radial dendritic framework and a synergistic hierarchical porosity have attracted much attention due to their unique open three-dimensional superstructures with high accessibility to the internal surface areas; however, the delicate regulation of the hierarchical porosity has been difficult to achieve up to now. Herein, a series of dendrimer-like amino-functionalized silica particles with tunable hierarchical pores (HPSNs-NH2) were successfully fabricated by carefully regulating and optimizing the various experimental parameters in the ethyl ether emulsion systems via a one-pot sol–gel reaction. Interestingly, the simple adjustment of the stirring rate or reaction temperature was found to be an easy and effective route to achieve the controllable regulation towards center-radial large pore sizes from ca. 37–267 (148 ± 45) nm to ca. 8–119 (36 ± 21) nm for HPSNs-NH2 with particle sizes of 300–700 nm and from ca. 9–157 (52 ± 28) nm to ca. 8–105 (30 ± 16) nm for HPSNs-NH2 with particle sizes of 100–320 nm. To the best of our knowledge, this is the first successful regulation towards centerradial large pore sizes in such large ranges. The formation of HPSNs-NH2 may be attributed to the complex cross-coupling of two processes: the dynamic diffusion of ethyl ether molecules and the selfassembly of partially hydrolyzed TEOS species and CTAB molecules at the dynamic ethyl ether–water
Received 28th January 2015, Accepted 23rd February 2015
interface of uniform small quasi-emulsion droplets. Thus, these results regarding the elaborate regulation
DOI: 10.1039/c5nr00640f
of center-radial large pores and particle sizes not only help us better understand the complicated selfassembly at the dynamic oil–water interface, but also provide a unique and ideal platform as carriers or
www.rsc.org/nanoscale
supports for adsorption, separation, catalysis, biomedicine, and sensor.
1.
Introduction
In the past decade, great efforts have been devoted to the design and controllable synthesis of porous silica nanomaterials with various well-defined morphologies and structures
a Research Center for Bioengineering and Sensing Technology, Department of Chemistry & Biological Engineering, University of Science & Technology Beijing, Beijing 100083, P. R. China. E-mail: [email protected] b Key Laboratory of Green Process and Engineering, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, China c School of Materials Science and Technology, China University of Geosciences, Beijing 100083, China d Functional Nanomaterials Laboratory, Technical Institute of Physics and Chemistry, Chinese Academy of Science, Beijing 100190, China † Electronic supplementary information (ESI) available: The schematic illustration of the experimental procedures, SEM images of the prepared HPSNs under varied APTES amounts at 20 °C, SEM images of as-prepared HPSNs under varied co-condensation silanes, size distribution histograms of the surface pores of HPSNs-NH2 (St1–St6 and St1′–St5′), digital images of CTAB-stabilized ethyl ether emulsion systems after vigorous stirring and after static placement, SEM and TEM images of the Sa3 product synthesized in the emulsion containing 20 mL of ethyl ether and 10 mL of ethanol, 15 mL of ethyl ether and 5 mL of ethanol and 10 mL of ethyl ether and 5 mL of ethanol (before gradient centrifugation). See DOI: 10.1039/c5nr00640f
This journal is © The Royal Society of Chemistry 2015
because of their many attractive structural and intrinsic features such as tunable particle size and pore size, tunable particle shape and pore structures, large specific surface area, high pore volume, low density, high mechanical and thermal stability, low toxicity, good biocompatibility, and easy functionalization.1–10 In fact, the size, shape, structure, composition, and surface functionalization of porous silica nanomaterials directly affect their physical and chemical properties and the resultant performance in their wide practical and potential applications, including molecule adsorption and separation, heterogeneous catalysis, nanoreactors, nanocasting, functional coatings, drug/gene/protein storage or delivery, cell imaging, and disease diagnosis.1–10 Among these, the delicate structural and morphological control of porous silica nanomaterials has become increasingly crucial for adjusting their properties and launching new functions; moreover it also affects and even determines their performance. Hierarchical structures discovered in natural species, such as lotus surface, shark skin, spider silk, butterfly wing, endow them with a variety of remarkable unique properties, and have inspired the rapid development of new artificial materials with multiscale structures and multiple functions.11–16 Recently, newly created dendrimer-like (dendritic) silica particles with a
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center-radial dendritic framework and hierarchical porosity, which are constructed by introducing additional center-radial large pores with a conical pore shape (i.e., gradually increasing the pore size from the particle center to the particle surface) into uniform mesopores, have attracted much attention.17–23 In comparison with conventional mesoporous silica particles, this type of particle has the integrated synergistic hierarchical porosity to simultaneously combine the advantages of the varied pore sizes, offering highly improved mass transport through the radial-oriented large pore channels, coupled with uniform small mesopores.17–23 Thus, this synergistic hierarchical pore structure favors the guest molecule, especially large molecules, to diffuse and access the active sites, thus it is an ideal scaffold or carrier for biological, medical, and catalytic applications.11–23 Several groups have synthesized this type of similar silica particles with hierarchical pores.17–23 For example, Polshettiwar et al. achieved the synthesis of silica nanoparticles with a fibrous morphology through a microemulsion system composed of cyclohexane as an oil phase and pentanol as a co-solvent, using cetylpyridinium bromide (CPB) or cetyltrimethylammonium bromide (CTAB) as a structure-directing agent under microwave-assisted hydrothermal conditions.19 Zhang’s group reported the facile large-scale synthesis of monodispersed MSNs with stellate-like or raspberry-like channel morphologies by employing cetyltrimethylammonium tosylate as a structure-directing agent and small organic amines as the basic catalysts via a templating sol–gel technique.21 Du et al. fabricated dendritic silica nanoparticles with hierarchical pores through a CTAB-stabilized ethyl ether emulsion system via a one-pot sol–gel reaction.22,23 However, there still exist a few disadvantages, such as a very large particle size, small center-radial pore size, very thick fiber, difficult large-scale synthesis, and, in particular, it is very difficult to control the sizes of the center-radial pores.17–23 It is worth noting that the elaborate regulation of center-radial pore sizes should be very significant for adjusting the porosity, permeability, and accessibility.17–23 Thus, the reliable synthesis of dendrimer-like silica particles with a tunable hierarchical porosity and particle size is highly desirable. Herein, based on our previously developed ethyl ether emulsion system,22,23 we successfully fabricate a series of dendrimerlike amino-functionalized silica nanoparticles with adjustable hierarchical pores (HPSNs-NH2) and particle sizes by systematically regulating the various experimental parameters. Moreover, it was found that the simple regulation of the stirring rate and reaction temperature is an easy and effective route to obtain a controllable regulation towards center-radial large pore sizes. In addition, a tentative mechanism is proposed for the formation of HPSNs-NH2 based on a systematic study and analysis.
2. Experimental section (materials and methods) 2.1.
Reagents and materials
Tetraethyl orthosilicate (TEOS, ≥98%), 3-aminopropyltriethoxysilane (APTES, ≥98%), 3-mercaptopropyl-
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trimethoxysilane (MPTMS), 3-glycidoxypropyltrimethoxysilane (GPTMS), cetyltrimethylammonium bromide (CTAB, ≥99%), aqueous ammonia (NH4OH, 30%), hydrochloric acid (HCl, 37%), ethyl ether, ethanol, chloroauric acid (HAuCl4·4H2O), sodium borohydride (NaBH4), and other chemicals not mentioned here were purchased from Sigma Aldrich. All the materials were of analytical grade and used as received without further purification. Ultrapure water with a resistivity higher than 18.2 MΩ cm was used in all the experiments, and was obtained from a three-stage Millipore Mill-Q Plus 185 purification system (Academic). 2.2. One pot synthesis of dendrimer-like aminofunctionalized silica nanoparticles with hierarchical pores (HPSNs-NH2) HPSNs-NH2 were prepared in an ethyl ether emulsion system based on the modified synthesis procedure in our previous study.23 Various experimental parameters (composition ratio, silane type, stirring rate, and reaction temperature) were regulated to investigate their effect toward the morphologies and structures of the final products (Table 1). In a typical procedure for sample Sa3, 0.5 g of CTAB was dissolved in an emulsion system composed of 70 mL of H2O, 0.8 mL of aqueous ammonia (NH4OH, 30%), 20 mL of ethyl ether and 10 mL of ethanol. After the mixture was vigorously stirred with a magnetic stirring rate of 1000 rpm (size of stirring bar: 35 mm (length) × 5 mm (diameter)) for 0.5 h at 20 °C (the temperature was controlled by a Thermoline Scientific BL-30 water bath), a mixture of 2.5 mL of TEOS and 0.1 mL of APTES was quickly poured into the abovementioned mixture. The resulting mixture was vigorously stirred with a magnetic stirring rate of 1000 rpm at 20 °C for 4 h. After that, 1 mL of HCl (37%) was added in order to stop the base-catalyzed reaction. A white precipitate was obtained by centrifugation at 4200 rpm (Sigma 3–18 centrifuge from Sigma Laborzentrifugen GmbH) for 12 min, which was washed with ethanol and pure water three times, and dispersed into ethanol. All the experiments were performed in triplicate. Although a slight difference appeared, the experimental results were basically reproducible. 2.3.
Particle purification and surfactant removal
The as-prepared product (undried) dispersed into ethanol was treated by ultrasonic dispersion at an ultrasonic power of 330 W for 30 min (soniclean from Soniclean Pty. Ltd.). The asprepared product was then purified by gradient centrifugation and the varying centrifugation rates (1500, 2000, 2500, 3000 and 3200 rpm) were investigated. To obtain products with large particle sizes (such as St1–St6), the particle suspensions were first treated by centrifuging at 2000 rpm for 3 min and then centrifuging at 2500 rpm for 3 min, and large particles and sheets could then be effectively removed. Furthermore, the abovementioned suspension containing nanoparticles was centrifuged at 8000 rpm for 3 min and the purified product was kept in ethanol or dried in air at 60 °C for analysis. To obtain products with small particle sizes (such as St1′–St5′), the particle suspensions were first treated by centrifuging at
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Table 1 Synthesis parameters and properties of the prepared HPSNs-NH2 after purification by gradient centrifugation, followed by solvent extraction
Sample
Effect factors
Sa1 Sa2 Sa3b Sa4 Sa5 Sa6 SMPMS SGPMS Ss1 Ss2 Ss3 Ss4 St1 St2 St3 St4 St5 St6 St1′ St2′ St3′ St4′ St5′
Effect of APTES
MPTMS (0.1 mL) GPTMS (0.1 mL) Effect of stirring rate
Effect of reaction temperature
Ethanol (mL)
Diethyl ether (mL)
APTES (mL)
Temperature (°C)
Magnetic stirring rate (rpm)
Particle sizea (nm)
Fig.
10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 10 5 5 5 5 5
20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 20 15 15 15 15 15
0 0.05 0.1 0.2 0.3 0.4 0 0 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1
20 20 20 20 20 20 20 20 20 20 20 20 10 15 20 25 30 35 10 15 20 25 30
1000 1000 1000 1000 1000 1000 1000 1000 400 600 800 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000 1000
100–300 200–1000 300–600 500–1000 500–1000 irregular 150–250 irregular 500–1100 500–1000 500–900 300–600 300–600 300–600 300–600 300–600 300–700 300–700 100–260 100–260 100–280 100–300 100–320
S2a S2b S2c S2d S2e S2f S3a S3b 1a 1b 1c 1d 2a 2b 2c 2d 2e 2f 4a 4b 4c 4d 4e
a
Particle sizes are estimated by averaging 50 particles from the SEM images. b Sa3, Ss4 and St3 are the same sample, but are given different names and are placed at various positions in the table to allow a clear comparison.
3000 rpm for 3 min and then centrifuging at 3200 rpm for 3 min, and the large particles and sheets could then be effectively removed. The abovementioned suspension containing nanoparticles was also centrifuged at 10 000 rpm for 3 min and the precipitate was washed with ethanol three times. A subsequent CTAB template extraction was performed by treating the purified product in ethanolic HCl (15 mL of HCl (37%) in 120 mL of ethanol) by stirring at 70 °C for 24 h. The final purified and extracted products were obtained by washing with ethanol three times, and then these products were kept in water or ethanol for further use or dried in air at 60 °C for analysis. The products were also calcined in air at 550 °C for 5 h to obtain the calcined products (HPSNs-cal) for comparison. 2.4. Loading of Au nanoparticles in the HPSNs-NH2 (HPSNs-NH2@Au) 0.040 g of the purified and extracted HPSNs-NH2 (St3′) was dispersed in 40 mL of H2O. The color of the mixture was yellow after the addition of 0.36 mL HAuCl4·3H2O (20 mg mL−1) solution. After the mixture was magnetically stirred in a closed conical flask for 2 h at room temperature, a specific volume of fresh aqueous NaBH4 (10 mM) was added into the abovementioned mixture until the color turned pale red. Finally, the Au nanoparticles loaded precipitate (HPSNs-NH2@Au) was centrifuged, washed with water, and re-dispersed in 5 mL of water or dried in air at 60 °C. It is worth noting that the solution is colorless after centrifugation, suggesting that nearly all the
This journal is © The Royal Society of Chemistry 2015
HAuCl4 was reduced to Au nanoparticles, which were then immobilized in HPSNs-NH2. 2.5.
Characterization methods
Scanning electron microscopy (SEM) observations were carried out on a Philips XL30 field emission scanning electron microscope operated at 10 kV. The specimens were coated with a layer of platinum with a size of 5 nm by ion sputtering before the SEM observations. For the transmission electron microscopy (TEM) observations, the powder products were dispersed in ethanol by sonication for 10 min, and added on carbon-coated copper grids. After drying at 60 °C overnight, the TEM observations were carried out on a Philips CM200 transmission electron microscope at an acceleration voltage of 100 kV. Nitrogen adsorption–desorption measurements were carried out on a TriStar II surface area and porosity analyzer from Micromeritics at −196 °C using the volumetric method. Prior to the measurement, the products were degassed at 120 °C for at least 6 h. Brunauer–Emmett–Teller (BET) specific surface areas were calculated using adsorption data at a relative pressure range of P/P0 range of 0.05–0.25. The pore size distributions were estimated from the adsorption branch of the isotherm using the Barrett–Joyner–Halenda (BJH) method. The pore volumes were determined from the amounts of N2 adsorbed at the single point of P/P0 = 0.98. Fourier transform infrared (FTIR) spectra of the samples were taken on a Thermo Scientific NICOLET 6700 FTIR spectrometer at room temperature. 13C solid state cross-polarization magic-angle spinning
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nuclear magnetic resonance (13C-CP-MAS NMR) measurements were performed on a Bruker MSL-300 spectrometer operating at a frequency of 75.482 MHz for 13 C. The spectrometer was equipped with a 4 mm double air bearing, magicangle spinning probe suitable for MAS experiments. The proton 90° pulse time was 5.5 μs, the acquisition time was 45 ms, the cross-polarization time was 2 ms, and the relaxation delay was 3 s. The spectrum width was 50 kHz, and 4000 data points were collected over 2000 scans. All the spectra were referenced to the resonance of adamantane (δ) 38.23 ppm, and the samples were equilibrated to room temperature (22 °C) prior to all the measurements.
3. Results and discussion A series of HPSNs-NH2 were successfully fabricated using TEOS as silane precursor, APTES as co-condensation silane, NH3·H2O as basic catalyst, CTAB as mesopore template and emulsion stabilizer, ethyl ether as oil phase, and ethanol as co-solvent by a one-pot sol–gel synthesis procedure, as shown in Fig. S1.† Various experimental parameters, such as the size of the oil nanodroplets and the rate of hydrolysis and condensation of TEOS, can affect the stability of the CTAB-contained ethyl ether-in-water emulsion systems, thus affecting and determining the morphologies and structures of the final products. In our previous study,22,23 it was discovered that an increase in the volume ratio of ethanol to ethyl ether from 0 to
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∞ resulted in the structural evolution of the final products from hollow mesoporous nanospheres to varied particles with hierarchical pores (center-radial pores and uniform small mesopores) to conventional mesoporous nanospheres, and that the addition of APTES could endow the final particles with a better defined spherical morphology and larger mesopores.22,23 More specifically (Table S1†),23 too much ethanol (>15 mL) and ethyl ether (>30 mL) gave rise to the formation of irregular HPSNs-NH2 with a broad size distribution, and many large hollow microspheres. With a decrease in the volumes of ethanol (from 10 to 5 mL) and ethyl ether (from 20 to 10 mL), the particle size decreased gradually, and the center-radial pore structure could be retained very well. On further decreasing the volumes of ethanol (
