Transition-Metal Salt-Containing Silica Nanocapsules Elaborated via

May 17, 2013 - ... and Department of Physics, Hangzhou Normal University, Xuelin Street ... Zhejiang Medical College, Binwen road 481, Hangzhou, 31005...
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Transition-Metal Salt-Containing Silica Nanocapsules Elaborated via Salt-Induced Interfacial Deposition in Inverse Miniemulsions as Precursor to Functional Hollow Silica Particles Zhihai Cao,*,† Liu Yang,† Quanlin Ye,† Qinmin Cui,‡ Dongming Qi,§ and Ulrich Ziener∥ †

College of Materials, Chemistry and Chemical Engineering and Department of Physics, Hangzhou Normal University, Xuelin Street 16, Hangzhou, 310036, China ‡ Department of Pharmacy, Zhejiang Medical College, Binwen road 481, Hangzhou, 310053, China § Key Laboratory of Advanced Textile Materials and Manufacturing Technology, Ministry of Education, Zhejiang Sci-Tech University, Hangzhou, 310018, China ∥ Institute of Organic Chemistry III − Macromolecular Chemistry and Organic Materials, University of Ulm, Albert-Einstein-Allee 11, Ulm, 89081, Germany S Supporting Information *

ABSTRACT: Aqueous core−silica shell nanocapsules were successfully prepared using liquid droplets containing transition-metal salt as templates in inverse miniemulsions. The formation of the silica shell was attributed to the interfacial deposition of silica species induced by the presence of the transition-metal salt. In addition to the control of the particle morphology, the incorporated transition-metal salts could be used to derivatize the particles and confer additional functionalities to the hollow silica particles. To demonstrate the derivatization, the magnetic hollow silica particles were prepared by converting iron salts to magnetic iron oxides by heat treatment. The particle morphology, size, and size distribution were characterized by transmission electron microscopy and scanning electron microscopy. The results show that the particle properties strongly depend on the type and the amount of salts, the amount of tetraethoxysilane (TEOS), the pH of the droplets, and the ratios of 2-hydroxyethyl methacrylate to aqueous HCl solution. The specific surface area and pore properties were characterized by N2 sorption measurements. The pore properties and specific surface area could be tuned by varying the amount of salt. Levels of elements and of iron oxides in the magnetic hollow particles were measured by energydispersive X-ray spectroscopy. Iron was distributed homogenously with silicon and oxygen in the sample. The magnetization measured by a magnetic property measurement system confirmed the successful conversion of the iron salts to magnetic iron oxides.

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INTRODUCTION Nanocapsules or hollow nanoparticles, structures that are able to encapsulate versatile functional cargos,1−3 have received considerable attention in the past decades because of their promising applications in medicine, biology, coatings, catalysis, cosmetics, and so forth. Silica capsules exhibiting high mechanical, thermal, and chemical stabilities, good biocompatibility, low toxicity, as well as tunable shell permeability may be the most investigated inorganic capsules in recent years.4,5 A series of synthetic methods based on the templating technique for elaborating silica nanocapsules or hollow silica particles have been reported. These methods include emulsion droplet,6−9 hard templating,10−13 micelle templating,14,15 liposome templating,16 and self-templating.17,18 The hard templating technique allows a high degree of control of the particle size, size distribution, and shell thickness. Recently, Nandiyanto and co-workers reported a surfactant-free technique to prepare © XXXX American Chemical Society

monodisperse, mesoporous-free silica capsules with controllable size and shell thickness.12,13 Compared with the hard templating technique, the emulsion droplet technique shows higher flexibility, allowing removal of the template via evaporation or solvent displacement, and enabling direct encapsulation of functional cargos by dissolution in the templates. Liang et al. prepared Janus hollow spheres with various external and internal surfaces by using three silicon alkoxysilanes and emulsion droplets as templates.6 Duran et al. prepared oil-filled silica nanocapsules in microemulsions that strongly absorb lipophilic drugs. These nanocapsules have potential applications for drug detoxification therapy.7 However, the preparation of microemulsions is often tedious and time-consuming. It also requires a large amount of surfactants. Received: December 23, 2012

A

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specific amount of HClaq or water. The hydrophilic organic compound was introduced to control the size and size distribution of the nanodroplets. The low-HLB surfactant P(E/B)-PEO was dissolved in 12.5 g of cyclohexane, and the resulting cyclohexane solution of P(E/ B)-PEO was used as the continuous phase. After the solution was preemulsified under strong magnetic stirring for 15 min, the resulting crude emulsion was sonicated in an ice bath to prepare the inverse miniemulsions. Sonication was done by applying a pulsed sequence (work 12 s, break 6 s) for 15 min, using a Scientz JY92-II DN sonifier at 67% maximum power (600 W). Pulsed-mode sonication was applied in all experiments to avoid overheating during the process. The formulations for the prepared silica nanocapsules are listed in Table 1.

Zoldesi et al synthesized silica capsules by using silicone oil droplets as templates.9 The sizes of the silica capsules or hollow particles prepared through most of the reported emulsion templating techniques lie in the micrometer range.19−22 Miniemulsions offer the advantage of directly encapsulating liquid hydrophilic or hydrophobic compounds to prepare nanocapsules through droplet nucleation mechanism and restricted molecular net diffusion.23−28 In addition, the size of droplets in miniemulsion systems is several tens to several hundreds of nanometers, a range between the dimensions of microemulsions and suspensions.29,30 The preparation of silica capsules in miniemulsion systems has been reported. Wu et al. synthesized silica nanocapsules in direct miniemulsions (oil-inwater) through a sol−gel process of tetraethoxysilane (TEOS) using a liquid mixture of octane and TEOS31 or silicone oil32 as template. Fickert et al. prepared thiol- or amine-functionalized silica nanocapsules via hydrolysis and condensation reaction of silicon alkoxides at the interface of droplets in direct miniemulsions.33 Cao et al. synthesized submicrometer silica capsules with a hydrophilic liquid core via an interfacial sol−gel process in inverse miniemulsions (water-in-oil).34 The use of cationic surfactants such as cetyltrimethylammonium bromide (CTAB)31,33,34 or surface-active silane like octadecyltrimethoxysilane32 is critical to inducing interfacial hydrolysis and condensation to access the capsule morphology. However, the introduction of CTAB to the system often causes colloidal instability of dispersions, especially in systems with a high solid content. In the present work, silica nanocapsules with a hydrophilic core loaded with transition-metal salt were successfully fabricated in inverse miniemulsions. The formation of the silica shell is attributed to the promoted interfacial deposition of the silica species because of the presence of transition-metal salts in the droplets. The introduction of salts and organic compounds in the droplets enabled a high degree of control of the size and size distribution of silica capsules, as well as the colloidal stability of the systems. The shell thickness and pore properties could be tuned by varying of synthetic parameters, such as the amount of TEOS and salts. Moreover, the incorporated transition-metal salts may be converted to functional compounds. For example, Fe(BF4)2 could be converted to magnetic iron oxides to confer additional functionalities (e.g., magnetization) to the hollow silica particles.

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Table 1. Formulations of the Prepared Silica Nanocapsulesab run

organic compound/ amount (g)

cosolvent/ amount (g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.0 HEMA/1.25 HEMA/0.75 HEMA/0.5 HEMA/0 AA/1.0 AAm/1.0 HEMA/1.0 HEMA/1.0

HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.5 HClaq/0.25 HClaq/0.75 HClaq/1.0 HClaq/1.5 HClaq/0.5 HClaq/0.5 water/0.5 HClaq/0.5

salt Co(BF4)2 NaBF4 CoCl2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Co(BF4)2 Fe(BF4)2

salt amount (mmol)

TEOS amount (g)c

1.00 1.00 0.50 0 0.25 0.50 1.00 1.00 1.00 0.25 0.25 0.25 1.00 1.00 1.00 1.00 1.00 1.00 1.00 1.00

0.67 0.67 0.67 0.67 0.67 0.67 0.34 1.01 1.34 0.34 1.01 1.34 0.67 0.67 0.67 0.67 0.67 0.67 0.67 1.01

a

P(E/B)-PEO (3 wt % with respect to the polar mixture) was used to stabilize all inverse miniemulsions. bCyclohexane (12.5 g) was used as the nonpolar solvent in all runs. cAn inverse miniemulsion (3 g) was used to prepare silica nanocapsules in each run. Preparation of Silica Nanocapsules. The preparation of silica nanocapsules is presented in Scheme 1. A measured amount of TEOS was added to the prepared inverse miniemulsions (see Table 1). The TEOS-containing inverse miniemulsion was placed in a closed glass reactor, and then magnetically stirred at 300 rpm in a 40 °C oil bath for 24 h. The hydrolysis and condensation of TEOS and the precipitation of silica species were confined at the surface of the droplets because of the presence of transition-metal salts in the droplets; this led to the formation of a capsule morphology. Calcination of Silica Nanocapsules. The dispersions with silica nanocapsules were purified by five-cycle centrifugation and redispersion in cyclohexane. The dried and purified samples were heated to 550 °C at a heating rate of 2 °C·min−1, and then kept at 550 °C for 5 h in air to remove the organic components. The hollow silica nanoparticles were obtained. Preparation of Magnetic Hollow Silica Nanoparticles. The dried silica nanocapsules with iron salts were calcined to remove the organic components. Calcination was done using the same conditions as those for the other hollow silica nanoparticles. To obtain the magnetic hollow silica nanoparticles, the calcined samples were continuously processed in a tube furnace at 350 °C for 1 h under a mixed-gas atmosphere (5% hydrogen and 95% nitrogen), and the samples thereafter were kept at 150 °C for 2 h in an oven. γ-Fe2O3 was

EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (AR), aqueous hydrochloric acid (HClaq, 1 mol·L−1), 2-hydroxyethyl methacrylate (HEMA), acrylic acid (AA), and acrylamide (AAm) were purchased from Aladdin Chemistry Co. Ltd. and used as received. Cobalt(II) tetrafluoroborate hexahydrate (Co(BF4)2·6H2O, 99.0%, Aldrich), iron(II) tetrafluoroborate hexahydrate (Fe(BF4)2·6H2O, 99.0%, Aldrich), sodium tetrafluoroborate (NaBF4, 99.99%, Aladdin Chemistry Co. Ltd.), cobalt chloride hexahydrate (CoCl2·6H2O, 99.99%, Aladdin Chemistry Co. Ltd.), and cyclohexane (99.5%, Shanghai Lingfeng Chemical Reagent Co. Ltd.) were used as received. Poly(ethylene-co-butylene)b-poly(ethylene oxide) (P(E/B)-PEO) with number-average molecular weight of 6200 g·mol−1 (as determined by 1H NMR spectroscopy) was synthesized according to the literature.35 The lengths of the hydrophobic (E/B) and hydrophilic (EO) blocks are 4000 and 2200 g·mol−1, respectively. These lengths resulted in a P(E/B)-PEO hydrophilic−lipophilic balance (HLB) of 7. Demineralized water (Milli-Q grade; resistivity: 18 MΩ) was used. Preparation of Inverse Miniemulsions. A hydrophilic organic compound (HEMA in most cases) and metal salt were dissolved in a B

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Scheme 1. Schematic Representation of the Preparation of the Inverse Miniemulsions, Silica Nanocapsules, and Hollow Silica Particles

the type of iron oxides expected to form in the magnetic hollow silica nanoparticles.36 Characterization. Transmission Electron Microscopy (TEM). TEM measurements were performed on a JEOL JSM-1230EXT20 microscope operated at 80 kV. One droplet of the original dispersion diluted in 2 mL of cyclohexane was placed on a 400 mesh carboncoated copper grid and dried at room temperature. The numberaverage size and number-average shell thickness of the capsules were determined by counting 100 capsules in the TEM images. Field Emission Scanning Electron Microscopy (FESEM). FESEM measurements were performed on an Ultra 55 microscope (Carl Zeiss SMT Pte Ltd.) with an accelerating voltage of 2 kV. The powder of each dried samples was placed on a conductive film. Energy-dispersive X-ray (EDX) spectroscopy and element mapping measurements were done on an energy-dispersive spectroscopy analyzer (Oxford Instruments INCA PentaFET*3) operated at 15 kV. Nitrogen Sorption Measurements. Nitrogen sorption measurements were carried out at 77 K on a Quantachrome Autosorb-1-C automated gas sorption apparatus. The specific surface area was calculated through the Brunauer−Emmett−Teller (BET) equation. The pore size and pore size distribution were derived from the adsorption isotherm curves by using the Barrett−Joyner−Halenda (BJH) method. Measurement of Magnetic Properties. Magnetic properties were measured on a Quantum Design Magnetic Property Measurement System (MPMS-VSM) magnetometer. The field dependence of magnetization was measured at 300 K in a magnetic field sweep between 1 T and −1 T. Temperature-dependent magnetization was measured between 10 and 300 K using zero-field-cooling (ZFC) and field-cooling (FC) procedures. In the ZFC measurement, the sample was cooled from 300 to 10 K without applying an external field. After reaching 10 K, an external field of 100 Oe was applied, and the magnetic moments were recorded as the temperature was increased. For the FC measurements, the sample was cooled from 300 to 10 K under the 100 Oe field, and then the magnetic moments were recorded as the temperature was increased.

in triggering the interfacial deposition of silica in the following step. To demonstrate preparation of the magnetic hollow silica nanoparticles, the incorporated iron salts were converted to magnetic iron oxides. HClaq (1 mol·L−1), a reactant and catalyst, was preloaded in the droplets for the hydrolysis and condensation reaction of TEOS. A measured amount of hydrophilic organic compound, HEMA, was added to the dispersed phase to control the droplet size and size distribution. Since TEOS could dissolve in the continuous phase, it was directly mixed with the inverse miniemulsions. The dissolved TEOS molecules could diffuse to the surface of the nanodroplets and undergo hydrolysis and condensation reaction to form the silica shells. The formation of silica nanocapsules was confirmed by TEM. The capsule morphology could be clearly observed in the TEM images (Figure 1a,b). Comparing with

Figure 1. TEM images of the silica nanocapsules (a and b) and calcined hollow silica particles redispersed in water (e). SEM images of the silica hollow silica particles (c and d) (a and b, images at different magnifications; c, image after synthesis; d, image after calcination; see Table 1, run 1).

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RESULTS AND DISCUSSION Formation of Silica Nanocapsules via Interfacial Hydrolysis and Condensation Reaction Triggered by Transition-Metal Salts. In the present work, we focused on the preparation of silica nanocapsules in inverse miniemulsions using a soft template. Synthesis was via salt-induced interfacial deposition of silica species. As shown in Scheme 1, the inverse miniemulsion composed of a hydrophobic continuous phase and a hydrophilic dispersed phase loaded with transition-metal salt was first prepared. The hydrophilic transition-metal salts Co(BF4)2 or Fe(BF4)2 were added to the dispersed phase to improve the droplet stability by suppressing the molecular net diffusion. More importantly, these salts played a significant role

the as-synthesized sample (Figure 1c), the silica particles had a well-preserved morphology after calcination (Figure 1d). The number-average shell thickness of the silica capsules was 18.7 ± 4.1 nm, and the size of the majority of capsules was in the range of 120 to 220 nm. The hollow silica capsules exhibited a good dispersibility in water, as shown in Figure 1e. To confirm the role of cations and anions in the formation of the capsule morphology, we used NaBF4 and CoCl2 in place of Co(BF4)2. Because of their limited solubility in the polar mixture, NaBF4 and CoCl2 were added in amount of 1 and 0.5 C

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shells is not yet fully understood. A thorough investigation on the mechanism of shell formation and the precise interaction between the silica species and salt is underway. The N2 adsorption−desorption isotherm is shown in Figure 3. The isotherm shows a type-IV behavior. The specific surface

mmol, respectively. As shown in Figure 2a,b, the capsule morphology could not be obtained in both systems. This means

Figure 3. N2 adsorption−desorption isotherm of the calcined hollow silica particles (see Table 1, run 1).

area of the sample was around 170 m2·g−1, and the pore size distribution was broad. A relatively small peak centered at about 20 nm appeared in the curve of the pore size distribution, implying that there were some mesoscale pores in this sample (Figure S1a, see the Supporting Information). However, the hysteresis loop between the adsorption and desorption branches was mainly observed in the region of high pressure (p/p0 > 0.75); this indicates that the majority of the pores present in this sample were aggregated. The porous shell of silica capsules was also observed at high magnification by SEM (Figure S1b). Influence of the Amount of Salt on the Particle Morphology. The silica nanocapsules were assumed to form via transition-metal-salt-induced interfacial deposition in the sol−gel process. This assumption is supported by the dependence of the particle morphology on the amount of salt. As shown in Figure 4, the particle morphology changed from solid to porous, and finally to capsule morphology when the amount of salt was increased from 0 to 0.5 mmol. The sol− gel process of TEOS under acidic conditions produces mainly silica oligomers and polymers containing unreacted silanol groups. These silica oligomers and polymers displayed relatively high solubility in the water/ethanol mixture, and they could dissolve and diffuse freely into the dispersed phase to form solid silica particles. Therefore, only solid silica particles were obtained in the system without transition-metal salts (Figure 4a,b). With the introduction of transition-metal salts in the droplets, the deposition of silica species could be promoted once the silica oligomers or polymers bearing silanol groups encounter transition-metal ions (Co2+) and BF4− at the interface. This process depends on the amount of Co2+ and BF4−. The precipitation rate may be accelerated by increasing the amount of transition-metal salts, which can suppress the diffusion of silica species into the dispersed phase. On the other hand, the amount of transition-metal salts determines the maximum amount of precipitates of silica species at the interface. As shown in Figure 4c,d, porous silica particles were formed in the system with lower amount of salts (0.25 mmol). This formation indicates insufficient suppression of the diffusion of silica species into the dispersed phase. As mentioned earlier, both the precipitation rate of silica species

Figure 2. TEM images of the silica nanoparticles synthesized in the systems with (a) NaBF4 and (b) CoCl2 as salts (see Table 1, runs 2 and 3). EDX spectra (c) of the silica nanocapsules before and after washing with water (see Table1, run 1).

that both Co2+ and BF4− contributed to the formation of the silica shell. To determine the strength of the interaction between the silica species, Co2+, and BF4−, we washed the silica capsules with water through a centrifugation−redispersion cycle. The element composition of the washed silica capsules was determined by EDX spectroscopy. The peak for cobalt at about 6.9 keV in the spectrum of the original sample is absent in the spectrum of the washed sample (Figure 2c). This result clearly indicates that the interaction between the ions and silica species was relatively weak and was most probably contributed by secondary interactions. On the basis of the above-mentioned results, we propose that the formation of silica shell may be caused by the interaction between Co2+ (or Fe2+) and the incompletely condensed silica oligomers or polymers; this interaction was mediated by BF4−, which promotes the precipitation of silica species at the oil/water interface. This interaction is analogous to that between CTAB and silica species under highly acidic conditions.37 Under acidic conditions, a relatively large amount of silanols remain in the hydrolyzed and condensed silica species; in addition, some of the silanols can be protonated to form −Si(OH2)+, leading to the formation of positively charged silica species (I+). The interactions between the neutral (I0) and positive silica species and CTAB are established via weak hydrogen bonding mediated by halide ions in the species I+X−S+ and I0X−S+. Because of the presence of the secondary interaction between ions and silica species, the precipitation of the hydrolyzed and condensed silica species at the interface was promoted; this suppressed the diffusion of silica species into the interior of the droplets. The precipitation rate apparently depended on the concentration of Co2+ and BF4−; therefore, the capsule morphology formed only in the systems with a relatively large amount of Co(BF4)2. The formation mechanism of silica D

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smoother than that of the silica particles synthesized in the systems containing salts. This may be evidence for the difference in the precipitation rate of silica species caused by the difference in salt levels in the dispersed phase. With the addition of the salt, the droplet sizes decreased significantly relative to that of the system without Co(BF4)2. However, the size became independent of the salt content when more than 0.25 mmol of Co(BF4)2 was used.38 In the present case, the composition of the droplets was similar to that in ref 38. Therefore, we believe that the same rule of the dependence of particle size on the amount of salt still applies to the present study. In fact, the sizes of silica particles prepared in the systems with 0.025, 0.05, and 1 mmol of Co(BF4)2 are similar, according to the TEM and SEM results shown in Figures 1 and 4. Because of the similar size of the droplet templates, we believe that the amount of Co(BF4)2 did not have an obvious influence on the thickness of the silica capsules. Figure 5a shows type-IV isotherms for the silica particles with various amounts of salts. The specific surface area, pore size, and pore size distribution strongly depended on the salt content (Figure 5b,c). The silica particles synthesized in the system without salt displayed the lowest specific surface area (10.9 m2·g−1), which is consistent with the nonporous structure observed by TEM and SEM (Figure 4a,b). Upon addition of 0.25 mmol of Co(BF4)2, the specific surface area increased significantly to 397.9 m2·g−1, in agreement with the porous structures observed by microscopy (Figure 4c,d). The mean pore size of the sample with 0.25 mmol of Co(BF4)2 was around 5.8 nm (Figure 5c), which is indicative of the formation of a mesoporous structure in this sample. Further increase in the amount of salt led to a reduction of the specific surface area (Figure 5b), probably because of the decrease in pore density.

Figure 4. TEM (a, c, and e) and SEM (b, d, and f) images of silica particles and nanocapsules synthesized in inverse miniemulsions with various amounts of salt (a and b, 0 mmol of Co(BF4)2; c and d, 0.25 mmol; e and f, 0.5 mmol; the TEOS amount was 0.67 g; see Table 1, runs 4−6).

and the maximum amount of precipitates increased with the increase in the amount of the transition-metal salt. Consequently, the capsule morphology was successfully formed in the system with relatively higher salt content (Figure 4e,f). According to the SEM results in Figure 4, the surface of the silica particles synthesized in the system without salts was much

Figure 5. N2 adsorption−desorption isotherm (a), specific surface area (b), and pore size distribution (c) of the calcined silica hollow particles with various amounts of salt (the isotherm data of the samples with 0.25 and 0.50 mmol of Co(BF4)2 are shifted upward by 100 cm3·g−1; see Table 1, runs 1, 4−6). E

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of Co(BF4)2 was added to the dispersed phase. In addition, the shells of the silica capsules displayed good stability (Figures 1c and 6d,f). Although the shell thickness of the silica nanocapsules could be tuned by the variation of the amount of TEOS in the systems with 1 mmol of Co(BF4)2, the particle morphology changed from capsule to solid particle with the increase in the TEOS amount in systems with lower salt content. For instance, 0.25 mmol of Co(BF4)2 led to solid particles when more than 1.01 g of TEOS was used (Figure 7). This trend suggests that

The isotherms for the samples with 0.5 and 1.0 mmol of Co(BF4)2 suggest that aggregated pores dominated in these two samples. Tuning the Thickness and Stability of the Shell by Changing the Amount of TEOS. The thickness and stability of the silica shell could be tuned by varying the amount of TEOS. Although silica nanocapsules could be formed by using 0.34 g of TEOS, most of them were deformed, as shown in Figure 6a,b. This deformation could be ascribed to the

Figure 7. Dependence of particle morphology on the amounts of TEOS in the systems with 0.25 mmol of Co(BF4)2 (a, 0.34 g; b, 1.01 g; c, 1.34 g; see Table 1, runs 10−12). Figure 6. TEM (a, c, and e) and SEM (b, d, and f) images of silica nanocapsules with various amounts of TEOS (a and b, 0.34 g of TEOS; c and d, 1.01 g of TEOS; e and f, 1.34 g of TEOS; the amount of Co(BF4)2 was 1 mmol; see Table 1, runs 7−9).

the amount of salts used in this series of experiments was insufficient to confine the reaction and precipitation of silica species to the interface with the increase in the amount of TEOS. Once all cobalt ions were consumed, the produced silica species diffused into the dispersed phase. As a result, most of the particles with a solid morphology were produced in the systems with a high amount of TEOS and low amount of salt (Figure 7b,c). Tuning the Size and Size Distribution of Silica Nanocapsules. The size and size distribution of silica nanocapsules were determined by the droplet template. In the inverse miniemulsions, the size and size distribution of droplets depended on the composition of the dispersed phase. As shown in Figure 4, the introduction of salts could reduce the particle size and narrow the particle size distribution. This is consistent with our previously reported findings.38 In addition, we found that the size and size distribution strongly depended on the ratio of HEMA to HClaq. A very broad size distribution of silica capsules appeared in the system that used only HClaq as cosolvent. This broad distribution could be reasonably ascribed to the relatively high interfacial tension in this system. With the addition of HEMA, the size distribution was significantly narrowed, and the size of the silica capsules was accordingly reduced (Figure 8). The dominant products of all the systems with various ratios of HEMA to HClaq were silica nanocapsules (Figure 8). However, when the mass ratio of HEMA to HClaq was set at 5:1, the capsule morphology of some particles became less distinct. Some silica species probably diffused into the interior of the dispersed phase because of the increase in the interactions between cobalt ions and HEMA, which weaken the participation of cobalt ions in the interaction with the silica species.

Table 2. Shell Thickness and Stability of the Silica Nanocapsules with Various Amounts of TEOS run

TEOS amount (g)

5 1 6 7

0.34 0.67 1.01 1.34

shell thickness (nm)

shell stability

± ± ± ±

low high high high

8.4 18.7 23.1 37.1

1.8 4.1 4.9 8.1

formation of a thin shell (8.4 ± 1.8 nm, see Table 2) due to the low amount of TEOS used. The instability of the shell is further confirmed by the TEM image of the aqueous dispersion of the calcined nanocapsules (Figure S2). After the harsh treatment of calcination and sonication, the shells of most silica nanocapsules were broken. The shell thickness increased to 18.7 ± 4.1 nm when the amount of TEOS was increased to 0.67 g (Figure 1). Correspondingly, the shell stability of silica capsules was obviously improved. These changes indicate that the shell of silica nanocapsules remained intact after calcination and sonication (Figure 1d,e). The shell thickness further increased to 23.1 ± 4.9 and 37.1 ± 8.1 nm with the increase in the amount of TEOS to 1.01 and 1.34 g, respectively (see Figure 6c,e and Table 2). In the range of the TEOS amount used, the particles always possessed a capsule morphology when 1 mmol F

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Figure 8. TEM images of silica nanocapsules prepared via an interfacial sol−gel process with different mass ratios of HEMA to HClaq (a, 0:1.5; b, 0.5:1.0; c, 0.75:0.75; d, 1.25:0.25; the TEOS amount was 0.67 g; see Table 1, runs 13−16).

Figure 9. TEM image of silica nanocapsules prepared via an interfacial sol−gel process in the system using water as cosolvent (see Table 1, run 19).

probably produced by homogeneous nucleation could also be seen in the TEM image (Figure 9). Preparation of Magnetic Hollow Silica Particles. Among the various magnetic materials, maghemite (γ-Fe2O3) represents an important class of magnetic iron oxide materials because of its applications in diverse technologies, such as magnetic fluids, data storage, catalysts, and bionanotechnology. Fe(BF4)2 was used as the precursor salt to magnetize the hollow silica particles, control the particle morphology, and form the magnetic iron oxides. The capsule morphology of the particles could be clearly distinguished in the magnetized sample (Figure S5a). The systems with Fe(BF4)2 behaved similarly as those with Co(BF4)2. For instance, the relationship between the particle morphology and the amount of TEOS were similar in both systems (Figure S6). The Fe(BF4)2containing silica nanocapsules were further treated using the protocol described in the Experimental Section. The shell stability was well-preserved after the heat treatment (Figure S5b). The successful conversion of iron salts to magnetic iron oxides was confirmed by magnetic measurements. Figure 10a shows the field-dependent magnetization (M−H) curve with saturation magnetization (at 1 T), remanent magnetization, and coercivity of 1.8 emu·g−1, 0.05 emu·g−1, and 10 Oe, respectively. The curve suggests that the synthesized particles exhibited weak ferromagnetic and soft magnetic behaviors. Furthermore, the ZFC-FC curves (see the inset) do not show the superparamagnetic features as usually displayed by magnetic nanoparticles (Figure S7a). The absence of such properties is probably due to the confinement (or pinning) of magnetic nanoparticles in the silica nanoshell, which were used to overcome thermal instability. The type of iron oxide expected to occur in the magnetic nanoparticles was γ-Fe2O3.36 The atomic ratio of Fe to Si derived from the EDX spectrum was 1:24.6 (Figure 10b); thus, the content of γ-Fe2O3 in the magnetic hollow particles was about 5.14 wt %, which is close to the theoretical value of 5.70 wt % if we assume that all TEOS and Fe(BF4)2 were converted to SiO2 and γ-Fe2O3, respectively, after heat treatment. The saturation magnetization of the magnetic hollow silica particles was about 1.8 emu·g−1; thus, the saturation magnetization of pure γ-Fe2O3 in the magnetic hollow particles was about 35.0 emu·g−1. The reduced

AAm and AA were also employed in place of HEMA, but only limited success was achieved (Figure S3). In the case of AAm, the size distribution of silica particles was very broad and only solid silica particles were formed. Well-defined silica capsules could be synthesized in the case of AA, but a relatively large amount of free silica particles was also observed in the samples probably because of the relatively high solubility of AA in the continuous phase.39 Influence of pH Value on the Particle Properties. A miniemulsion is a kinetically stable system. If the nanodroplets are not solidified in time, the size and size distribution of droplets would change because of Ostwald ripening. The formation of a silica shell on the surface of droplets could be expected to improve the droplet stability. Therefore, the hydrolysis and condensation rate may play a decisive role in controlling the size and size distribution of the final products. In the system using HClaq as cosolvent, a silica shell was formed on the surface of droplet templates in 5 min (Figure S4). Therefore, it is reasonable to assume that the initial size distribution of droplet templates could be well-preserved. On the contrary, when water was used as cosolvent, the size distribution of silica capsules became very broad; moreover, the size of silica capsules was larger than that of capsules prepared in the system with HClaq as cosolvent (Figure 9). The pHdependent hydrolysis and condensation of silica precursors could be catalyzed by both acids and bases. In the system with water as cosolvent, the hydrolysis and condensation of TEOS was relatively slow; this led to the formation of larger silica capsules with a broader size distribution. This observation is consistent with our previous report on the preparation of silica capsules with a liquid template.34 In the previous study, silica capsules could not be obtained when water was used as cosolvent to replace the ammonia solution. In the present case, the droplet stability was expected to improve compared with that in ref 34 because of the presence of Co(BF4)2. In addition, the improvement could partly result from the weak acidity of the dispersed phase (pH = 3.9). Consistent with these assumptions, the silica capsules formed but their size distribution became broader than that of capsules synthesized under more acidic conditions. Some small solid particles G

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above results suggest that the γ-Fe2O3 nanoparticles were probably confined in the shell of the silica nanocapsules. The attachment of γ-Fe2O3 nanoparticles to the silica capsules is also confirmed by the fact that the magnetic silica capsules redispersed in water could be collected by a bulk magnet (Figure S8) to form a transparent dispersion. The good water dispersibility as well as the magnetic properties may open potential applications for the particles in bionanotechnology.

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CONCLUSION A facile protocol for the preparation of silica nanocapsules via interfacial deposition induced by transition-metal salts was successfully carried out using aqueous droplets as templates in inverse miniemulsion. The colloidal stability could be highly controlled in the entire synthesis (Figure S9). With the increase in the amount of salt in the droplets, the particle morphology changed from solid to porous, and finally to capsule morphology. In addition, the size of silica particles was reduced and the size distribution was narrowed with addition of salt. Silica particles with mesoporous structure were formed upon introduction of 0.25 mmol of Co(BF4)2. With the increase in amount of salt, aggregated pores dominated in the shell of the silica nanocapsules. The maximum specific surface area of the silica particles (398 m2·g−1) was obtained in the sample with 0.25 mmol of Co(BF4)2. With the increase in the amount of salt to 0.5 and 1.0 mmol, the specific surface area of the silica capsules decreased to about 226 and 170 m2·g−1, respectively. The shell thickness could be tuned within 8 to 37 nm by varying the amount of TEOS. The introduction of a suitable amount of hydrophilic compounds, such as HEMA, could assist the control of size and size distribution of silica particles. Very low pH of the droplets was required; adjusting the pH to highly acidic conditions was done using HClaq. A preliminary experiment to convert the loaded salts to magnetic materials was carried out by using Fe(BF4)2 as model salt. The magnetic hollow silica particles have potential applications in magnetically separable materials, controlled drug delivery carriers, or in bionanotechnology. Although we showed only two successful examples of salts (Co(BF4)2 and Fe(BF4)2), we believe that the salts that can successfully promote the precipitation of silica species under acidic conditions (except hydrofluoric acid) can be used to induce interfacial deposition to prepare silica nanocapsules.

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Figure 10. Magnetization curves (a), EDX spectrum (b), and element mapping (c) of magnetic hollow particles (see Table 1, run 20).

ASSOCIATED CONTENT

S Supporting Information *

Additional TEM images, tables, and graph. This material is available free of charge via the Internet at http://pubs.acs.org.

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magnetization with respect to the saturation magnetization of 80 emu·g−1 for bulk samples indicates a high fraction of surface canted spins and disordered spins due to broken bonds and nanoscale antiferromagnetic exchange interactions that were disrupted. In fact, the magnetization was not completely saturated even in the maximum field of 5 T and at 10 K (data not shown). The saturation magnetization of γ-Fe2O3 nanoparticles in the magnetic hollow particles was close to the saturation magnetization of γ-Fe2O3 nanoparticles (28.6 emu·g−1) synthesized via the protocol in the Supporting Information (Figure S7b). The element mapping result shown in Figure 10c clearly indicates that iron homogeneously distributed with silicon and oxygen in the sample. Therefore, the formation of separated and bulk γ-Fe2O3 could be excluded in the present case. Combined with the magnetic data, the

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial supports from National Natural Scientific Foundation of China (NNSFC) project (51003023 and 11104054), the Hangzhou Normal University high-level talents start-up fund (2011QDL04 and 2011QDL03), and open Foundation of Zhejiang Provincial Top Key Academic Discipline of Applied Chemistry and Eco-Dyeing & Finishing H

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Engineering (YR2012007) are gratefully acknowledged. We greatly thank Dr. Yifeng Shi and Dr. Xungao Liu for the helpful discussion on the N2 adsorption data, synthesis of magnetic hollow particles, and the formation mechanism of silica nanocapsules.

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