NANO LETTERS
Silica Hollow Spheres with Nano-Macroholes Like Diatomaceous Earth
2006 Vol. 6, No. 12 2925-2928
Masahiro Fujiwara,* Kumi Shiokawa, Ikuko Sakakura, and Yoshiko Nakahara Kansai Center, National Institute of AdVanced Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan Received September 29, 2006; Revised Manuscript Received November 13, 2006
ABSTRACT Artificial synthesis of hollow cell walls of diatoms is an ultimate target of nanomaterial science. The addition of some water-soluble polymers such as sodium polymethacrylate to a solution of water/oil/water emulsion system, which is an essential step of the simple synthetic procedure of silica hollow spheres (microcapsules), led to the formation of silica hollow spheres with nano-macroholes (>100 nm) in their shell walls, the morphologies of which are analogous to those of diatom earth.
Hollow cell walls of diatoms (“frustule”) are intricate speciesspecific siliceous microstructures and share certain characteristics such as macroholes (“stria”) often arranged regularly (as “areola”).1-3 Artificial fabrications of diatom-like silica and analogous materials are a challenging field of material science. For the preparation of artificial diatom-like materials, biological and biomimetic silicification processes have been applied.4-20 On the other hand, the productions of silica hollow particles by chemical methods are developed originally.21-23 We also reported a simple and effective preparation procedure of inorganic hollow spheres (microcapsules).24,25 For example, silica hollow particles are readily prepared using a water/oil/water (W/O/W) emulsion interface, where sodium silicate and a silica precipitant such as NH4HCO3 are added in the inner water phase (IWP) or the outer water phase (OWP), respectively.25 The hollow structure of these silica particles is formed at the same time as the particles are synthesized. Various compounds might be mixed in IWP with the sodium silicate. For example, the direct encapsulation of BSA (bovine serum albumin) and duplex DNA are achieved by their addition to IWP.26 Recently we also found that when some water-soluble polymers were added in IWP, silica hollow particles obtained had unique nano-macroholes in their shell walls. In this paper, we wish to show a simple chemical method that produces silica hollow particles with macroholes in their shell walls, the morphologies of which are analogous to those of diatomaceous earth. The key technology of this preparation was the addition of suitable water-soluble polymers to IWP of the W/O/W emulsion. * Corresponding author. Phone: +81-72-751-9253. Fax: +81-72-7519628. E-mail:
[email protected]. 10.1021/nl062298i CCC: $33.50 Published on Web 11/23/2006
© 2006 American Chemical Society
At first, we used a sodium polymethacrylate (average MW ∼6500) as an additive polymer to IWP. The pore properties of the shell walls of the silica particles thus prepared were estimated by nitrogen sorption isotherms, and the results are summarized in Table 1. A W/O emulsion of this IWP with an oil phase (OP) of n-hexane solution (including Tween 80 and Span 80) was mixed to OWP with NH4HCO3, leading the formation of a W/O/W emulsion system. The mixed solution became white turbid right after mixing. After a few minutes, silica hollow particles precipitated in the mixed solution. After aging for 2 h, the white precipitate was filtered, washed with deionized water and methanol, and dried at 100 °C. Field-emission scanning electron microscopy (FE-SEM) images of the silica hollow particles prepared in the absence of the polymethacrylate (sample 1)25 are shown in Supporting Information. Panels A and B of Figure 1 are FE-SEM images of a silica hollow particle synthesized with 1 g (about 3.3 wt % to 29.88 g of sodium silicate) of the polymethacrylate (sample 2). These silica hollow particles had the smooth and seamless silica shell as observed by FESEM images. According to nitrogen adsorption-desorption isotherms, these two particles had the mesopores in their shell walls. The peak pore diameters estimated from the BJH method were observed at 12.1 (sample 1) and 38.9 nm (sample 2), respectively. Figure 2 shows the nitrogen sorption isotherms (A) and the corresponding BJH pore size distributions (B) of various silica hollow particles (samples 1-5). When 2 g of the sodium polymethacrylate was added (sample 3), the mesopores observed in both samples 1 and 2 disappeared (Figure 2B). Alternatively, macroholes detectable by FE-SEM observation were found as shown in Figure 1D. The silica particles obtained with 3 g of the polymethacrylate
Table 1. Pore Properties of Silica Particles Prepared in the Presence of Sodium Polymethacrylate (Na PMA) Estimated by Nitrogen Sorption Isothermsa NaPMA sample (g) 1 2 3 4 5
0 1 2 3 4
NaPMA/sodium PV PA PPD silicate SSA (wt/wt %) (m2/g) (mL/g) (m2/g) (nm) 0 3.4 6.7 11.2 13.4
685 629 601 620 506
1.41 1.18 0.70 0.37 0.29
762 524 435 393 315
12.12 37.87 N/Ab N/Ab N/Ab
a Average MW ∼6500. SSA, BET specific surface area; PV, pore volume estimated by BJH method; PA, pore area estimated by BJH method; PPD, peak pore diameter estimated by BJH method. b BJH calculation method for pore diameter estimation is effective approximately from 2 to 60 nm. No clear peaks of pore diameters are found in this range of the distribution curves.
(sample 4) had larger macroholes in nanosize (macropores) observed in the FE-SEM images at a magnification of 60 000 (Figure 1F). In the FE-SEM images (panels G and H of Figure 1) of the silica hollow particles prepared with 4 g of the polymethacrylate (sample 5), the particle looked like a natural diatom. In the shell wall, several macroholes larger than 350 nm, similar to the “stria” of diatoms, are clearly seen even at a magnification of 6000 (Figure 1G). On the other hand, samples 3-5 had no larger mesopores that were found in samples 1 and 2 in the BJH pore size distribution plots (Figure 2B). A good correlation was found between the main pore (macrohole) size and the added amount of the sodium polymethacrylate. The pore sizes increased with the amounts of the polymer (12 nm with 0 g; 38 nm with 1 g; 120 nm with 2 g; 250 nm with 3 g; more than 350 nm with 4 g). Thus, the small mesopore (12 nm in size) in the silica shells of sample 1 (prepared without the polymer) was thought to be expanded by the addition of the polymethacrylate. For example, when 2 g of the polymethacrylate was added in IWP, the mesopore of the silica shells enlarged to the macropore (120 nm macrohole), which were not detectable by nitrogen sorption isotherms but observable by FESEM measurement. On the other hand, the addition of more than 4 g of the sodium polymethacrylate resulted in immediate gelation of the sodium silicate solution. Consequently, silica hollow particles were not obtained in this case. The analogous materials were obtained when sodium polymethacrylate with an average MW of ∼9500 was used. Panels A and B of Figure 3 are the FE-SEM images of silica hollow particles obtained in the presence of 4 g of this sodium polymethacrylate. The sizes of the macroholes in the shell wall exceeded 1 µm. The hollow structure of the particle was also confirmed based on the visibility of the shell wall on the opposite side. In Supporting Information, the FE-SEM images of silica particles obtained in the presence of from 1 to 3 g of this sodium polymethacrylate are shown. A good correlation between the main macrohole size and the added amount of the sodium polymethacrylate was observed, as is the case of the polymethacrylate with average MW of ∼6500. When 4 g of a sodium polyacrylate (average MW ∼8000) was mixed in IWP, smaller macroholes were formed in the shell wall as shown in panels C and D of Figure 3. The main 2926
Figure 1. FE-SEM images of silica hollow particles prepared by the W/O/W interfacial reaction method in the presence of sodium polymethacrylate (average MW ∼6500). Particles prepared in the presence of 1 g of sodium polymethacrylate (A, B, sample 2), 2 g of sodium polymethacrylate (C, D, sample 3), 3 g of sodium polymethacrylate (E, F, sample 4), and 4 g of sodium polymethacrylate (G, H, sample 5).
sizes of the macroholes were estimated to be about 100 nm. The arrangement of the macroholes in this particle was approximately similar to the samples obtained using the sodium polymethacrylates. It seems that analogous polymers such as sodium polymethacrylate and polyacrylate produced similar kinds of macroholes in the shell walls of silica hollow particles. The uses of other additive polymers such as sodium poly4-styrenesulfonate (average MW ∼1 000 000) and glycogen (from oyster) generated different kinds of characteristic silica hollow particles. In the case of sodium poly-4-styrenesulfonate (1 g addition), the silica hollow particles obtained looked like golf balls with dimples from 500 nm to 1 µm in Nano Lett., Vol. 6, No. 12, 2006
Figure 2. Nitrogen adsorption-desorption isotherms (A) and pore size distributions (B) of silica hollow particles prepared in the presence of sodium polymethacrylate (average MW ∼6500): 1, sample 1; 2, sample 2; 3, sample 3; 4, sample 4; 5, sample 5. The plots of pore size distributions (B) are calculated by the BJH method with the nitrogen adsorption-desorption isotherms (using adsorption branches).
diameter in their shell walls (Figure 3E, F). The addition over 1 g led to the gelation of the sodium silicate solution, resulting in no formation of hollow particles. According to the FE-SEM image of the cross section of the shell wall in Figure 3F, these macroholes in the shell wall were isolated inside the silica shell and were not opened to the outside. When 4 g of glycogen was used for silica hollow particle preparation, the macroholes from 200 to 500 nm in diameter were produced like as craters of the surface of the silica particle (Figure 3G). These macroholes looked not throughbores but divots in the shell wall (Figure 3H). On the other hand, as shown before, macroholes in the shell walls of the silica hollow particles prepared with the polymethacrylates penetrated to the inside of the hollow center. All silica particles prepared in this study were amorphous by X-ray diffraction measurement. Furthermore, silica hollow particles had no infrared absorptions of organic components and contained no combustible contents after thorough washing with water and methanol. (Representative IR spectrum and TGA profiles are shown in Supporting Information.) Therefore, the shell walls of the silica hollow particles were a common amorphous silica matrix free from the additive polymers. However, these polymers must participate in the formation of nano-macroholes in the shell walls, because no macroholes were created without them. In recent reports about the spherical particles with large pores,27-29 their macroholes are generated by the removal treatments of mixed components after the particles were complete. The elimination of a polystyrene nanoparticle,27 the evaporation of toluene solvent,28 or HF etching treatment29 is performed. Even in our case, the additive polymers might be eliminated from the silica shells during 2 h of aging and washing treatment. Then, we stopped the mixing of W/O/W emulsion in just 1 min and quickly filtered the white precipitate, when sample 5 was prepared. This sample has already had the macroholes in the shell wall (SEM images are in Supporting Information) and looked the same as sample 5 after 2 h of aging and thorough washing. Therefore, the formation of the Nano Lett., Vol. 6, No. 12, 2006
Figure 3. FE-SEM images of silica hollow particles prepared by W/O/W interfacial reaction method in the presence of various additive polymers: (A, B) sample prepared in the presence of 4 g of sodium polymethacrylate (average MW ∼9500); (C, D) Sample prepared in the presence of 4 g of sodium polyacrylate (average MW ∼8000); (E, F) sample prepared in the presence of 1 g of sodium poly-4-styrenesulfonate (average MW ∼1, 000 000); (G, H) sample prepared in the presence of 4 g of glycogen (from oyster).
macroholes found in the shell walls was accompanied by the particle production. In our proposed mechanism of silica hollow particle preparation,25 the W/O/W emulsion system created by the addition of W/O emulsion (IWP/OP) into OWP gradually collapses by the elimination of OP, intermingling IWP with OWP. The silica shell matrix is formed from sodium silicate and NH4HCO3 along the W/O/W interface during this combination process.25 On the other hand, in the case of direct encapsulation of BSA into silica hollow particles,26 only from 10 to 30% of BSA added in IWP was included in silica hollow particles, although BSA thus encapsulated was not released without the destruction 2927
Furthermore, the preparation period is only several hours which is short in comparison with biological and biomimetic syntheses of diatom-like materials.4-20 Acknowledgment. Authors thank Dr. Michiko Makino for her FE-SEM measurement. Supporting Information Available: Experimental procedures, additional SEM images, FT-IR spectrum, and thermogravimetric analysis profiles. This material is available free of charge via the Internet at http://pubs.acs.org. References
Figure 4. An expected diagram of the formation process of silica hollow particles with macroholes. W/O emulsion with water-soluble polymer in inner water phase (IWP) is added to the aqueous solution of NH4HCO3 as outer water phase (OWP), forming W/O/W emulsion. During the silica formation, polymers in IWP pass through the silica matrix to create the macroholes in the shell wall.
of the silica shell.26 These observations meant that approximately 70-90% of BSA in IWP escaped to OWP during the intermingling. It is thought that the additive polymers dissolved in IWP also spread to OWP when these IWP and OWP are mixing. From these results, we consider the formation mechanism of the macroholes in the silica shell as follows at the present stage (Figure 4). As no additive polymers were mixed in the silica particles, the polymers must permeate the shell walls of the particles that are forming along the W/O/W interface. The passage of the additive polymers through the W/O/W interface prevents the formation of the silica shell matrix in the domains where the polymers pass through. Finally those domains became macroholes in the shell walls of silica hollow particles (balloon in Figure 4). As the manners of the passage depend on the additive polymers, the macrohole structures vary with the polymers employed. In conclusion, we found that a chemical procedure using a W/O/W interfacial reaction fabricates the silica hollow particles with nano-macroholes that resembled natural diatom cell walls in morphology. The key technology of this process was the addition of water-soluble polymers such as sodium polymethacrylate to sodium silicate solution as inner water phase (IWP) of the W/O/W emulsion. Although the formation mechanism of macroholes in the silica shells is still unclear, it is likely that the additive polymers that permeated the silica matrix of the shell walls created the characteristic nano-macroholes. The diatom-like materials prepared by our method were produced directly, and do not require additional processes that are essential for other reported methods.27-29
2928
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NL062298I
Nano Lett., Vol. 6, No. 12, 2006
