Hollow Silica Capsules with Well-Defined Asymmetric Windows in the

May 2, 2012 - coupling a large polystyrene sphere to an ensemble of small, polystyrene latex spheres. The hierarchical template in conjunction with a ...
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Hollow Silica Capsules with Well-Defined Asymmetric Windows in the Shell Bo Zhao and Maryanne M. Collinson* Department of Chemistry, Virginia Commonwealth University, Richmond, Virginia 23284-2006, United States ABSTRACT: A straightforward and effective approach to fabricate porous silica capsules with well-defined asymmetric windows in the shell using raspberry-like templates has been developed. This process begins with the formation of a hierarchical template by chemically coupling a large polystyrene sphere to an ensemble of small, polystyrene latex spheres. The hierarchical template in conjunction with a hard templating method and spin-coating leads to silica capsules with well-defined, asymmetric pores (windows) in the outer shell. Proof-ofprinciple of this approach has been demonstrated using a 1500/110 nm hierarchical template. The silica capsules thus produced were characterized with scanning electron microscopy and STEM. The diameter of the capsules was ∼1400 nm, and the outer opening of the windows was ∼100 nm in size, consistent with the diameters of the core and satellite spheres considering the shrinkage due to the calcination. The inner opening was ∼30 nm, which gives rise to an asymmetry factor, defined as the diameter of the outer window to the diameter of the inner window, of ∼3. In another example, surface-bound capsules with an asymmetry factor of ∼1 were made. Collectively, these windows can provide efficient pathways to connect the inside of the capsule to the outside and have potential for asymmetric diffusion and rectification.

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INTRODUCTION A key challenge in material design and fabrication is the need to control pore architecture, whether it be at the micro-, meso-, or macropore level or a combination of levels as in the case of hierarchical structures.1,2 Not only is size important, but also the distribution in size, shape, order of the pores (both longrange and short-range), interconnectivity, and number density. Such physical characteristics play an important role in the performance of these materials, particularly in applications that require efficient and/or controllable transport into and out of the porous structure. Inorganic porous capsules are one such structure.3−7 These materials have a morphology that consists of an interior void and a porous inorganic shell and have served as microreactors, drug storage containers, and adsorbents for applications ranging from chemical sensing, drug delivery, to separation and filtering.7−9 The pore channel(s) in the capsule shell plays an important role in this architecture, as this is the location where loading and releasing of the desired molecule(s) take place. During the past decade, there have been many reports on the preparation and characterization of hollow capsules with a porous shell using various approaches that include layer-bylayer (LBL) assembly, direct chemical deposition or adsorption, soft templating, and nanocasting.7−11 Many of those porous shells contain nonuniform, randomly distributed pores innate to the material itself while others have more directed porosity created via the use of a secondary template such as a surfactant/ block copolymer.12−17 These secondary pores are typically nanometers in size, which are not necessarily efficient for applications that require fast loading and releasing characteristics. To address this challenge, more recent work has focused on the introduction of a large, single hole into the polymer capsule structure18−25 and/or a collection of holes (windows) © 2012 American Chemical Society

of nonuniform size scattered around the surface of the capsule.26−31 These nanometer- to micrometer-sized holes are valuable because they can provide an efficient pathway to load and release a desired reagent(s). However, explicit control over the size of the windows, their number density, and how they are connected to the interior of the capsule are needed. In this work, hollow capsules with well-defined pore architecture and asymmetric windows that transverse the shell have been fabricated and characterized. The introduction of an asymmetric window compared to a more traditional cylindershaped window has several advantages. A large molecule can be trapped within the interior of the capsule, and providing its diameter is larger than the inner window, it will remain encapsulated while smaller ions or reagents can readily diffuse into/out of the capsule. A higher flux of such species into the capsule would be expected to take place because diffusion would be hindered only at the inner window rather than the entire length of a traditional cylindrical shaped pore, as noted before for conical nanopore membranes.32 For large analytes or macromolecular species, this feature would be particularly important. Asymmetric pores can also cause asymmetric diffusion to take place, particularly when the size of a given molecule/ion/particle approaches the diameter of the windows.33−35 Under the appropriate conditions, a molecule/ion/ particle can be transported more readily in one direction compared to the other, particularly at high concentrations. Asymmetric diffusion/transport of ions, for example, has been shown to play an important role in ion rectification observed in conically shaped nanopores.36−38 For example, a particle in bulk solution will pass through the window and into the capsule Received: January 13, 2012 Published: May 2, 2012 7492

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interior if the pore opening is larger than the particle diameter. A much larger particle would not, thus leading to pore clogging and the restriction of diffusion in one direction.33 At high concentrations pore sealing is also possible, leading to a constant amount of material encapsulated in the capsule until such point as the capsule is removed and placed in a different environment.33 The method described herein utilizes a sol−gel based, hard templating route and a raspberry-like bimodal, hierarchical template formed by chemically coupling a large polystyrene (PS) core sphere to an ensemble of small satellite spheres39 to fabricate hollow capsules with well-defined windows. The hard templating method together with a well-defined bimodal template ensures explicit control over pore size, with the size of the capsule controlled by the diameter of the core sphere and the size of the windows determined in part by the diameter of the satellite spheres. The interconnectivity between the inner void of the capsule with the external environment is guaranteed by the bonding between core and satellite spheres. Proof-ofprinciple is demonstrated in this work with the use of a 1500/ 110 nm hierarchical template. Capsules with an asymmetry factor defined as the ratio of the outer window diameter to inner window diameter of ∼3 have been fabricated and characterized. In addition, we show how the asymmetry factor can be changed by adding an organoalkoxysilane to the silica sol.

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Scheme 1. (A) Overview of the Fabrication Process; (B) Images of the Possible Encapsulation Style; (C) CrossSectional View of a Single Asymmetric Pore

EXPERIMENTAL SECTION

Materials. Tetraethoxysilane (TEOS, 99%), dimethyldiethoxysilane (DMDEOS, 97%), N-(3-(dimethylamino)propyl)-N′-ethylcarbodiimide hydrochloride (EDC, protein seq. grade), and N-hydroxysuccinimide (NHS, 98%) were purchased from Aldrich used as received. Aqueous suspensions of polystyrene (PS) microspheres functionalized with either carboxyl (diameter = 1.5 μm) or amine groups (diameter = 110 nm) were obtained from Invitrogen (formally Interfacial Dynamics Corp. (IDC), 4.1% w/v for PS-COOH, 2.1% w/v for PS-NH2). The PS-NH2 latex spheres contained no COOH groups. Capsule Fabrication and Characterization. The hierarchical template (1500/110 nm) was made by the carbodiimide coupling of COOH groups located on the core polystyrene sphere and NH2 groups on the satellite polystyrene spheres as previously described.39 The hierarchical templates, separated and dispersed in acetic acid, were washed several times with water after which the supernatant was removed and 0.5 mL of sol was added. The silica sol was prepared 2 days prior by hydrolyzing tetraethoxysilane (TEOS, 1.3 mL) in the presence of water (0.3 mL), ethanol (2 mL), and HCl (0.1 M, 0.3 mL). An alternative sol prepared in a similar fashion contained both dimethyldiethoxysilane (DMDEOS, 0.3 mL) and TEOS (1 mL). After soaking in the sol for 5 min, the templates were separated from the solution by centrifugation, redispersed in 25 μL of concentrated acetic acid by quick vortexing, and then immediately spin-coated onto a polystyrene coated glass slide at 8000 rpm. The spun slides were dried at 40−45% humidity at room temperature for 1−2 days to strengthen the silica framework. Upon immersion in chloroform, the underlying polystyrene layer was removed, thus releasing the silica-coated templates into solution; the polystyrene template also dissolved at the same time. A simplified overview of the fabrication process is shown in Scheme 1A. The silica capsules thus produced were collected and washed via filtration and calcined at 450 °C for 5 h (ramp of 5 °C/ min) to ensure all the polystyrene was removed and the silica structure strengthened. Scanning electron microscopy (SEM) and scanning transmission electron microscopy (STEM) were used to characterize the materials. As necessary, the materials were coated with a thin layer of gold prior to imaging.

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RESULTS AND DISCUSSION The fabrication process begins with the formation of a hierarchical template, defined as a template with morphology on different length scales.39 When coupled with a hard templating method, multimodal porous materials with welldefined, interconnected pores can be made. The size of the inner void and the windows (pores) will be dictated by the diameters of the core sphere and satellite spheres, respectively.39 In prior work, we described how these hierarchical templates can be used to make multimodal porous silica as well as bimodal porous gold films.39 The present study takes this initial work to the next level to show how such templates can be used to craft hollow silica capsules with well-defined, asymmetric pores, something that is not easy to do. Proof-ofprinciple is demonstrated in this work with the use of the 1500/ 110 nm hierarchical template. To form a porous capsule with a closely packed ensemble of well-defined windows, the silica sol must uniformly coat the inner sphere but, most importantly, not encase the entire template, as shown in Scheme 1B. If the silica sol polymerizes around the entire structure such as that shown in Scheme 1B(b,c), the result would be a macroscopically rough silica capsule with a bimodal inner pore structure but without clear openings (windows) to the external environment. It is because of this complexity that traditional sol−gel based methods for forming hollow silica capsules cannot be used. To circumvent the encapsulation problem, the sol-coated hierarchical latex spheres were first dispersed in acetic acid and then spin-coated on an appropriate substrate. The sheer act of spin-coating 7493

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causes excess sol to be flung off the template surface creating a thin shell. Most importantly, during spin-coating, the residual sol wicks down the sides on the satellite spheres,40−4240 thus exposing their tops while still producing the silica shell around the core. This spin-coating method works well for making silica nanowells on planar substrates that are open at the top and bottom from silica sols that have been also doped with latex spheres.40−4240 As the sol wicks down the spheres assembled in a close-packed array on the substrate, their tops are exposed, allowing for easy removal. The sacrificial polymer layer on the glass slide enables the capsules to be transferred back into solution, if so desired. Figure 1A shows the distribution of sol-coated templates on the PS−glass slide. As can be seen, there are dimers, trimers,

Figure 2. (A) SEM image of the porous capsules on glass after calcination. (B) SEM image of the porous capsules after they are released, calcined, and then redispersed on a glass slide.

coated on the polystyrene-coated glass slide, it makes contact with the surface resulting in the trapping of sol at this location. The small volume of trapped sol leads to the formation of the “button” upon gelation. As long as the capsules remain on the slide, the button cannot be easily seen (Figure 2A). When the capsules are removed from the surface, redispersed in solution, and then cast on another slide, the buttons become visible (Figure 2B) on the capsules with the bottom side facing up. The “buttons” can also be easily seen in the STEM images shown in Figure 3. The average size of the “buttons” is ∼600− 700 nm. Figure 1. (A) SEM image of silica-sol coated 1500 nm/110 nm hierarchical templates dispersed on the PS-coated glass slide. (B) SEM image of a single sol-coated hierarchical template.

and polymers, which inevitably form by using the spin-coating method. In Figure 1B, an SEM image of a single, sol-coated hierarchical template is shown. In this case, the silica sol is trapped in the space between the satellite spheres without covering the top of the satellite spheres. This thin silica coating will later transform into the silica shell of the capsule, and the shell windows will form when the satellite spheres are removed. The shell thickness formed is ∼50−60 nm as estimated from Figure 1B. Figure 2A shows SEM images of the porous capsules formed by directly calcining the glass slide/silica coated templates while Figure 2B shows the porous capsules released into solution (chloroform) followed by calcination. In both images, the diameters of the capsules are ∼1400 nm and the outer openings of the windows are ∼100 nm size, consistent with the diameters of the core and satellite spheres. The shrinkage associated with calcination causes these sizes to be slightly smaller than the raspberry templates used for fabrication (1500/110 nm). The acetic acid used to redisperse the sol-coated templates is necessary to help spread them evenly on the polystyrene-coated glass surface. For some applications such as in filtration and purification, the presence of aggregate structures would not be a limitation. In Figure 2B, what appears to be a “button” (a small circular raised area with several holes in its center) can be seen on many of the capsules. When the silica sol-coated template is spin-

Figure 3. STEM images of porous capsules at low magnification (A) and high magnification (B). The arrows indicate the location of the button.

To help understand the architecture of the capsule, a 3D model of a hierarchical sphere and its corresponding porous, hollow capsule were created (Figure 4A). The SEM image of a hierarchical capsule shown in Figure 4C is very similar to the model (Figure 4A): a capsule with a packed ensemble of asymmetric pores (windows) through the shell. The number density of pores in the shell is high because the satellite spheres 7494

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Figure 5. SEM images of capsules formed from a sol containing both TEOS and DMDEOS (A, B). Inner pores can be seen in the broken capsules shown in (C).

Figure 4. (A) Computer-generated 3D image of a hierarchical template and its corresponding capsule. SEM images of (B) a hierarchical template (1500/110 nm), (C) a porous capsule, (D) a broken capsule clearly showing the inner pores in the silica shell, and (E) STEM images of the broken shell. The inset is a magnified image of a collection of outer pores on the shell. Inset scale bar is 100 nm.

the sol was changed to form capsules with a different window size and asymmetry factor. In Figure 5, SEM images of hollow capsules prepared from the 1500/110 nm raspberry-like latex sphere from a sol that contains an organoalkoxysilane, specifically dimethyldiethoxysilane (DMDEOS) are shown. A SEM image of a collection of surface-bound capsules is shown in Figure 5A, while an image of a single capsule can be seen in Figure 5B. The outer window diameter is much smaller than that shown in Figure 4C: ∼10−40 nm in contrast to ∼100 nm. The inner window diameter as measured from broken capsules shown in Figure 5C is ∼15−45 nm, giving rise to an aspect ratio of ∼1. The addition of DMDEOS changes the degree at which the sol wets the satellite sphere, thus altering the ridge of silica that builds around it. In this case, the organically modified silica sol covers nearly the entire satellite spheres, leaving only a small opening at the top creating the outer window. It also better wets the underlying substrate, which can later transform into a thin layer of silica, making the capsules harder to remove from the surface.

are closely packed on the surface of the core sphere. The outer window diameter is larger than the inner window diameter, thus giving rise to an asymmetric pore (or window). A clear depiction of the inner window can be seen in Figure 4D, which shows the inside of a broken capsule obtained by sonication. The diameters of the inner holes estimated from this SEM image are ∼30 nm. A magnified image of the outer windows can be seen in the center inset, which shows the outer diameter to be ∼100−110 nm. Both these images indicate that the pores are asymmetric and transverse through the shell. The asymmetry factor, defined as the diameter of the outer window to the diameter of the inner window, is ∼3. The thickness of the shell, estimated from this image, is 75 nm given that the edges of the outer openings are a little thicker than the real shell. Similar results are also observed in the STEM images shown in Figure 4E. The pores (windows) are clearly open from outside to inside as evident from the bright centers, which are also ∼30 nm in diameter. In the button area, only the center has several openings. These openings form at the place(s) where the satellite spheres touch the polymer-coated substrate. Because of the relatively limited amount of material obtained using a spin-coating method (in contrast to a bulk solution method) to form the capsules and the limited information that can be obtained from a macroporous sample (in contrast to a microporous material), N2 adsorption/desorption isotherms were not collected. The STEM/SEM images clearly define the size of the macropores in the shell. Because the shell is mostly macroporous, the surface area is expected to be small as we have shown in prior work on multimodal silica powders.39 Both the size of the satellite spheres and the solution/ processing conditions can influence the diameter of the windows (inner and outer) and the asymmetry factor. The outside window diameter will depend on the size of the satellite sphere and the extent at which the sol wicks down the sphere.42 Sol viscosity and wettability will be important factors in the latter. The diameter of the pore leading to the interior of the capsule depends on the extent of contact the satellite sphere makes with the inner core sphere.42 In this work, the diameter of the hierarchical template was fixed and the composition of

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CONCLUSIONS In summary, this is the first report where the “windows” in the shell of silica porous capsules are well-defined and near uniformly distributed over the entire shell of the capsule. Both the inner pore and the outer pores have a relatively narrow pore size distribution due to the monodispersity of the PS particles used to make the hierarchical templates (coefficient of variation between 2% and 5%). The large size of the capsule (e.g., 1400 nm) ensures a large loading capacity and the accommodation of large particles/biological entities and provides ease of handling during separation and washing steps. Two important attributes of this structure are (1) the numerous direct channels with short diffusional passageways and (2) asymmetric windows. Collectively, these direct channels provide efficient pathways to connect the inside of the capsule to the outside. The asymmetric windows offer the potential for asymmetric diffusion and rectification, particularly when the diffusing particles are similar in size to the inner hole.33 Such particles could be drug-loaded nanocapsules, hydrophilic/hydrophobic nanoparticles, catalysts, polymers or microorganisms such viruses, DNA, and small cells. By confining transport to a given direction, the option for separation/concentration of a 7495

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given species in solution can be realized. It is envisioned that these capsules can be potentially useful for a number of applications including catalyst protection, purification, sequestration, pollution treatment, and filtration.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Fax: +1 804-828-8599; Tel: +1 804-828-7509. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the National Science Foundation for support of this work (CHE-0847613) and the VCU Nanomaterials Core Characterization facility. B.Z. thanks Altria for a research fellowship.

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