Article pubs.acs.org/cm
Surface Intaglio Nanostructures on Microspheres of Gold-Cored Block Copolymer Spheres Minsoo P. Kim,†,∥ Kang Hee Ku,†,∥ Hyeong Jun Kim,† Se Gyu Jang,‡ Gi-Ra Yi,*,§ and Bumjoon J. Kim*,† †
Department of Chemical and Biomolecular Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Republic of Korea ‡ Korea Electricity Power Research Institute (KEPRI), Daejeon 305-760, Republic of Korea § School of Chemical Engineering, Sungkyunkwan University, Suwon, 440-746 Republic of Korea S Supporting Information *
ABSTRACT: The confined self-assembly of block copolymers (BCPs) can be used to generate hierarchically structured composite microspheres. In this study, microspheres of goldcored BCP (Au-BCP) spheres were first produced from an evaporative toluene-in-water-emulsion in which polystyrene-bpoly(4-vinylpyridine) (PS-b-P4VP) BCPs incorporated with Au precursors (AuCl4−) were dissolved in toluene. Interestingly, the addition of cetyltrimethylammonium bromide (CTAB) into the microspheres resulted in the selective extraction of Au precursors within the Au-BCP spheres near the surface of the microsphere because of the strong electrostatic attraction between the CTAB and Au precursors. Therefore, regular-patterned porous nanostructures on the surface of Au-BCP microspheres were formed, of which the size could be varied by controlling the molecular weight of the PS-bP4VP polymers. In addition, the depth of the pores could be modulated independently by tuning the amounts of Au precursors that were incorporated into the Au-BCP spheres (λ). This method was then generalized using other additives (i.e., thiolterminated molecules) that had a favorable interaction with the Au precursors, producing both controlled inner and surface morphologies of the microspheres. Pores at surface could be used to successfully load various metal nanoparticles, potentially making them useful in optical, catalytic, and drug-delivery or therapeutic applications. KEYWORDS: porous microsphere, block copolymer microsphere, porous nanostructure, gold−block copolymer hybrids
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INTRODUCTION Ordered porous nanostructures are of great interest because of their large surface area and hollow features for loading valuable materials.1,2 Nanoporous particles with open pores at the surface have received significant attention because of their potential utility for separation, packing materials in chromatography, drug delivery, confined nanoreactors with catalytic activity, and small-molecule carriers.3−7 Hierarchically structured porous particles would be optimal systems for simultaneous or sequential performance of multiple functions, which includes biomedical probes and treatment, complicated energy conversion, and storage devices.8 Self-assembled structures of block copolymers (BCPs) are attractive and powerful templates for the creation of ordered nanostructures,9−14 which provide rich nanoscopic morphologies depending on their molecular weight and the volume fraction of each block.15 Recently, through the encapsulation of BCPs into an emulsion and evaporation, new types of colloidal spheres have been produced in which unusual morphologies were organized because of their confining geometry and the presence of surfactants at the interface.16−21 Furthermore, with a high-affinity block in BCP with metallic precursors, various © 2013 American Chemical Society
metallic precursors can be selectively incorporated into such BCP colloidal particles.9,22−24 The metallo-dielectric structures have been of considerable interest because of their potential uses in multifunctional biomedical probes, broadband scattering, or self-assembled metamaterials.25−29 Structured nanopores could be produced from BCP structures by removing one of the blocks. For instance, poly(methyl methacrylate), polybutadiene, and polylactide have been introduced as removable blocks, which could be removed by various etching processes including UV/ozone treatment.30−35 However, porous BCP structures have been also produced using a nondestructive approach of surface reconstruction of a thin film. For instance, thin films of poly(styrene-b-4-vinylpyridine) (PS-b-P4VP) or poly(styreneb-2-vinylpyridine) (PS-b-P2VP) can be applied to produce porous structures by exposing them to solvents that have different affinities for each block.36−39 Recently, another effective approach for the supramolecular assembly of BCPs Received: August 26, 2013 Revised: October 5, 2013 Published: October 7, 2013 4416
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to porous structures was reported using small molecules or metal ions that noncovalently bond to BCPs. These small molecules can be extracted selectively, leaving well-defined porous structures.40−43 Herein, we developed a facile route to fabricate hierarchically structured metal-BCP microspheres with ordered nanoporous surfaces in which the pore size, depth, and spacing were all precisely controlled. In detail, microspheres of gold-cored BCP (Au-BCP) spheres were prepared by emulsifying a mixed oil solution of PS-b-P4VP polymers with Au precursors into an aqueous surfactant solution and then slowly evaporating the oil. Interestingly, the addition of cetyltrimethylammonium bromide (CTAB) into the suspension of the Au-BCP microspheres resulted in regularly ordered nanoporous surfaces on the BCP microspheres. When the ratio (λ) of Au precursors to the PS-bP4VP spheres was about 0.3, surface morphological transitions were clearly observed from dotted patterns to ordered pore structures. Moreover, we demonstrated that the surface nanostructure can be controlled in terms of pore size and the spacing between the pores by changing the molecular weight (Mn) of the PS-b-P4VP chains, whereas the pore depth was controlled independently by varying the λ value. In addition, the inner structures of the Au-BCP microspheres can be controlled by choosing other additives (i.e., thiol-terminated molecules) that have a smaller size and stronger affinities for the Au precursors.
Figure 1. SEM images of raspberry-like microspheres that consist of Au-BCP spheres with two different Mn of PS-b-P4VP BCP: (a) PS190kb-P4VP45k and (b) PS25k-b-P4VP7k. The SEM images in panels c and d are the corresponding Au-BCP microspheres obtained after treatment with CTAB in aqueous solution. The morphological transition from the raspberry-like microsphere to the porous one with regularly ordered nanopores was observed upon treatment with the CTAB molecules. Scale bars are 100 nm.
Both the surface and inner structures of the Au-BCP microspheres were characterized by transmission electron microscopy (TEM), as shown in Figure 2. When PS-b-P4VP polymers were dissolved in toluene, spherical micelles were formed that consisted of a PS corona and P4VP core because toluene has a much higher affinity with PS than P4VP.44 Because of the strong electrostatic attraction between the Au precursors (AuCl4¯) and the protonated P4VP chains,33,45 the Au precursors were incorporated into the protonated P4VP core of the PS-b-P4VP spheres, which provided high contrast for the TEM measurement. The diameter of the P4VP core in the sphere of PS190k-b-P4VP45k (λ = 1.0) was determined to be 41.2 ± 9.9 nm, which is much larger than that of the P4VP core (λ = 0.0) without Au precursors (30.1 ± 7.5 nm). In the case of PS25k-b-P4VP7k, the core diameter for λ = 1.0 was 15.5 ± 2.1 nm. This difference in the core diameter resulted in a difference in the size of the porous structures, as shown in Figure 2. Thus, the sizes of the porous nanostructures in the Au-BCP microspheres can be tuned by using different Mn of PS-bP4VP BCPs. To examine better the porous features on the surface of AuBCP microspheres, cross-sectional TEM image was obtained, as shown in Figure 3. The Au-BCP microspheres were dropped onto an epoxy film and dried. A thin layer of Pt (3 nm) was deposited onto them for clear observation of their surface structures. The samples were then microtomed at room temperature to yield a 50 nm thick film. The P4VP cores appear as dark regions in the Au-BCP microspheres because of the selective staining of the P4VP chains by iodine vapor as well as the presence of the Au precursors. Of particular interest was the surface structure, which showed ordered nanopores and the P4VP domains surrounding the pores. In addition, the P4VP domains surrounding the pores contained no Au precursor, which was in contrast to other P4VP domains inside. The quarternized ammonium on CTAB attracts anionic Au precursors electrostatically.46−48 The attraction force between the anionic Au precursor and the CTAB was expected to be comparable to that between the Au precursor and protonated
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RESULTS AND DISCUSSION For preparing the microspheres of Au-BCP spheres, we first dissolved two different BCPs with either high Mn, PS190k-bP4VP45k (Mn = 235 kg/mol, PDI = 1.18, and f PS = 0.81), or low Mn, PS25k-b-P4VP7k (Mn = 32 kg/mol, PDI = 1.10, and f PS = 0.78), respectively, in toluene. After stirring 1 wt % PS-b-P4VP solutions at room temperature for 1 day, Au precursors (HAuCl4·3H2O) were added to the solutions, and the mixed solutions were stirred at room temperature for another day. The amount of Au precursor incorporated into the BCP sphere was controlled by tuning the mole ratio (λ) of Au precursor to the P4VP unit of PS-b-P4VP during the formation of the AuBCP spheres. The 1 wt % BCP solution with Au precursors (λ = 1.0) was emulsified into 0.1 wt % of a surfactant solution (Pluronic F108) via high-shear mixing at 25 000 rpm for 3 min. The toluene was then evaporated at a pressure of 10 mbar for 20 min. During the evaporation, the P4VP domains swelled near the microsphere surface and burst through the surface PS brush coating because the hydrophilic P4VP chains have much stronger affinity with water than the nonpolar PS chains. Thus, swollen P4VP formed mushroom-shaped domains on the surfaces of the microspheres, which were ordered on the tens of nanometer scale. As shown in Figure 1a,b, raspberry-like microspheres were successfully produced using PS190k-b-P4VP45k or PS25k-bP4VP7k, respectively. When they were treated with the CTAB solution, a dramatic change in the surface morphology was observed where structured nanopores were formed at the surfaces in both cases, as shown in Figure 1c,d. These pores were found to be uniform in size and ordered. Interestingly, the sizes of the pores on two different cases consisting of PS190k-bP4VP45k and PS25k-b-P4VP7k were clearly different, with the PS190k-b-P4VP45k microspheres having a much larger pore. Scheme 1 describes the overall procedure used to prepare the porous Au-BCP microspheres with nanostructured pores. 4417
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Scheme 1. Schematic Illustration of the Preparation of the Au-BCP Microspheresa
a
PS-b-P4VP micelles with Au precursors were encapsulated in a toluene-in-water emulsion. Through the slow evaporation of toluene, raspberry-like microspheres of Au-BCP spheres were produced. Finally, Au precursors near the surface were selectively extracted by CTAB treatment, producing Au-BCP microspheres with nanoporous surfaces.
pyridine group. However, the population (or concentration) of CTAB infiltrated together with water into P4VP-core overwhelmed that of the pyridine groups. The infiltration of CTAB, hence, resulted in the removal of Au precursors from P4VP cores that were exposed to the aqueous CTAB solution. To test the above hypothesis, we tried to extract the Au nanoparticles (NPs) after chemical reduction with NaBH4. In the UV−vis absorption spectra (Figure 3b), the maximum absorption peak of microspheres before chemical reduction was about 325 nm, which was attributed to the ligand-to-metal charge transfer.49,50 In contrast, the Au-BCP microspheres after chemical reduction exhibited a maximum intensity around 580 nm, indicating aggregate formation of the Au NPs (Figure 3c). Interestingly, the chemically reduced Au NP-BCP microspheres
Figure 2. TEM images showing Au-BCP microspheres of (a) PS190k-bP4VP45k and (b) PS25k-b-P4VP7k after extraction of Au precursors with CTAB. Scale bars are 50 nm.
Figure 3. (a) Cross-sectional TEM image of Au-BCP microsphere of PS190k-b-P4VP45k spheres with Au precursors after CTAB treatment (λ = 1.0). The P4VP domains appeared darker than the PS domains because of selective staining by iodine vapor. A thin layer of Pt (3 nm) was deposited onto the microspheres for the clear observation of their surface structures. The scale bar is 50 nm. (b, c) UV−vis spectra of the Au-BCP microspheres before and after chemical reduction with NaBH4, respectively. The insets show the UV−vis spectra of the BCP micelles that contain the Au precursors and the Au NPs, respectively. 4418
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had no change in the surface structure after CTAB treatment (Figure S1). Therefore, the strong affinity between the Au precursor and the CTAB molecule was a critical requirement for the formation of the nanoporous surface on the microspheres. For quantitative analysis of the surface nanoporous structures, a series of Au-BCP microspheres was prepared with different λ values, ranging from 0 to 1, to tune the amounts of Au precursors incorporated within the PS190k-bP4VP45k spheres. The TEM images in Figure 4a−e show the monolayers of the Au-PS190k-b-P4VP45k spheres with different λ values: (a) 0.0, (b) 0.2, (c) 0.3, (d) 0.5, and (e) 1.0. The diameter of the P4VP core increased gradually from 30.1 ± 7.5 to 41.2 ± 9.9 nm as the λ value increased from 0 to 1, indicating that the core volume expanded upon incorporation of the Au precursors. Another piece of evidence of the volume expansion by infiltration of Au precursors was provided by the change in the spacing between the P4VP cores (d), which was measured by grazing-incidence X-ray scattering (GIXS) (Figure S2). The toluene solutions of Au-BCP micelles with different λ values were emulsified, and microspheres were produced via an evaporation process in which all microspheres had dotted surface structures regardless of the λ values (Figure S3). However, after CTAB treatment, a dramatic difference in the surface morphology was observed as a function of the λ value (Figures 4f−j). Ordered porous structures were observed only when the λ value was greater than 0.3. In addition, for λ = 1.0, nanoporous features on the microspheres became more evident than at λ = 0.3 or 0.5. In contrast, for λ < 0.3, the microspheres maintained their protruding dot patterns even when additional CTAB was added and the mixture was stirred for a longer time (Figure 4g). Therefore, λ = 0.3 was determined to be the critical λ value (λc) for the formation of porous surface structures on Au-BCP microspheres. To understand better the surface porous structures and their formation mechanism, their surface structures were quantitatively evaluated as a function of the λ value. For λ = 0.3, 0.5, and 1.0, TEM images were taken as shown in Figure 5. The pore size (18.2 ± 2.4 nm) and the spacing between pores (52.4 ± 7.2 nm) were almost constant irrespective of the λ value. However, the pore depth increased significantly as λ increased. The pore depth was 14.0 ± 2.3, 16.1 ± 3.2, and 22.4 ± 2.0 nm for λ = 0.3, 0.5, and 1.0, respectively. Figure 5 shows the pore depths of surface nanostructures plotted as a function of λ. The surface structures on the microsphere adopted the dotted pattern observed previously for λ < λc, whereas the surface structure was transformed to a porous structure for λ ≥ λc. Therefore, we could modulate the pore depth on the microspheres by changing the amount of Au precursor incorporated in the P4VP core. Recently, in a thin film of BCP micelles, porous structures were demonstrated by micelle opening via a selective swelling of one of the blocks.39,51,52 The degree of the pore opening was dependent on the degree of the selective swelling by the solvent.49 Our nanoporous surfaces resembled the surface nanostructures formed through the process of partial opening. Scheme 2 shows a diagram of the proposed nanoporeformation mechanism on the microspheres. First, the Au-BCP micelles could form raspberry-like microspheres via an emulsion-encapsulation and evaporation process (Scheme 2a). It is noteworthy that the dot size was found to be constant regardless of the λ value. When CTAB was added to the AuBCP microspheres in water, the CTAB diffused into the P4VP
Figure 4. TEM images showing the monolayered structure of AuPS190k-b-P4VP45k spheres with different λ values: (a) 0.0, (b) 0.2, (c) 0.3, (d) 0.5, and (e) 1.0. The corresponding average diameters of the P4VP cores were measured as (a) 30.1 ± 7.5, (b) 31.1 ± 7.5, (c) 32.5 ± 8.3, (d) 35.4 ± 7.6, and (e) 41.2 ± 9.9 nm. (f−j) SEM images showing the porous microspheres of the Au-PS190k-b-P4VP45k spheres prepared under identical conditions after treatment with CTAB but with different λ values: (f) 0.0, (g) 0.2, (h) 0.3, (i) 0.5, and (j) 1.0. The final morphology of the microspheres clearly is dependent on the λ values, producing nanoporous microspheres only when λ was greater than 0.3. The scale bars are 100 nm.
dots, and the Au precursors in the P4VP dots were then extracted out because of the strong electrostatic attraction between the Au precursors and CTAB molecules (Scheme 2b).38 In aqueous solution, the P4VP dots remained swelled by water even after removal of the precursors. Then, when the water was dried, the P4VP dot at the outer surface of the microsphere shrinks, and a layer of P4VP surrounding the internal void was formed with an open hole on the outer surface of the glassy PS domain (Scheme 2c). The volume of extracted Au precursors was determined mainly by their initial 4419
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nanoreactors, and molecule carriers. We have tested three different types of metal precursors (i.e., HPt2Cl6·6H2O, AgNO3, and FeCl3). Within the pores of the Au-BCP microspheres, they were loaded, producing heterogeneous metal-decorated hybrid BCP microspheres (Figures 6 and S6). In addition, energy-dispersive X-ray spectroscopy (EDX) measurements confirmed the presence of these metals only within the pores at the surfaces of the particles. For example, cross-sectional TEM image (Figure 6c) and electron mapping images (Figure 6d−f) of Pt-loaded Au-BCP microspheres revealed the selective incorporation of Pt atoms within the pores at the surface of the microspheres.
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CONCLUSIONS We successfully developed a simple and efficient route for fabricating microspheres of Au-BCP spheres with nanoporous surfaces. Nanoporous surfaces were produced because of the selective interaction between CTAB and the Au precursors followed by surface reconstruction of the P4VP domains. We demonstrated that the pore size could be controlled by different Mn of PS-b-P4VP, whereas the pore depth could be controlled independently by varying the λ values. Furthermore, we were able to control the inner morphology by using other additives that had different sizes and affinities for the Au precursor. Finally, we successfully fabricated heterogeneous, metaldecorated hybrid Au-BCP microspheres by filling the P4VP domains at the surface with metal precursors of Pt, Ag, or Fe. Therefore, these materials hold great promise for use in optical, catalytic, and biological applications.
Figure 5. Plot of the pore depths of Au-BCP microspheres as a function of the λ value. TEM images are of Au-BCP microspheres with nanoporous surfaces for λ = 0.3, 0.5, and 1.0, respectively. The scale bars are 50 nm.
amount in the P4VP core. Therefore, the pore depth can be controlled by tuning the λ value, as described in Scheme 2. The structured porous surfaces can be also produced with other additives, including thiol compounds, as long as they have a strong affinity with the AuCl4− ions. The addition of thiol terminated poly(ethylene oxide) (PEO-SH) with a relatively large size (Mn= 2 kg/mol) resulted in regularly ordered nanopores on the surface of the BCP microspheres (Figure S5a,d). In this case, only the Au precursors in the BCP spheres near the surface of the microspheres were selectively removed. Interestingly, when small size mercaptobutanol was used instead, all of the Au precursors inside the microspheres were removed completely (Figure S5b,e). By controlling the amount of mercaptobutanol and the reaction time, the Au precursors were removed from only a few layers near the surface. (Figure S5c,f). This observation clearly demonstrates that the extraction process is highly dependent on the diffusion of the agents. The surface nanopores can be used to load various bio- and chemical-reagents, which makes them useful for drug delivery,
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EXPERIMENTAL SECTION
Preparation of Au-BCP Microspheres using PS-b-P4VP. To prepare Au-BCP microspheres, first Au precursors (HAuCl4·3H2O, purchased from Aldrich) were infiltrated into a 1 wt % PS-b-P4VP micelle solution in toluene at different molar ratios (λ = 0.0−1.0) to P4VP units. Two different BCPs of high Mn, PS190k-b-P4VP45k (Mn = 235 kg/mol, PDI = 1.18, and f PS = 0.81) and low Mn, PS25k-b-P4VP7k (Mn = 32 kg/mol, PDI = 1.10, and f PS = 0.78), were used. Then, 0.25 mL of a 1 wt % micelle solution containing the Au precursors was
Scheme 2. Schematic Diagram of the Formation Mechanism of Porous Microsphere with Different λ Valuesa
a
(a) Au-BCP micelles form raspberry-like microspheres via an emulsion-encapsulation and evaporation process. (b) When CTAB is added to the Au-BCP microspheres in water, the CTAB diffuses into the P4VP dots, and the Au precursors in the P4VP dots are extracted out because of the strong electrostatic attraction between the Au precursors and CTAB molecules. (c) When the water is dried, the swelled P4VP at the outer surface shrinks, and a layer of P4VP surrounding the internal void was formed with an open hole on the outer surface of the PS domain. 4420
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Figure 6. (a)Normal and (c) cross-sectional TEM images of Au-BCP microspheres after loading the Pt precursor within the surface pores. (b) EDX spectrum obtained in the region identified by the red arrow. Elemental mapping of (d) Pt only, (e) Au only, and (f) Au + Pt in the Pt-loaded AuBCP microspheres. The elemental mapping was performed using the cross-sectional sample (c) of Au-BCP microspheres. The scale bars are 50 nm. emulsified in 4.5 mL of distilled (DI) water that contained 0.1 wt % Pluronic F108 (PEO-b-PPO-b-PEO, Mn = 14 600 g/mol from SigmaAldrich) using a homogenizer at 25 000 rpm for 3 min. The aqueous emulsion was diluted with DI water, and the organic solvent was evaporated at 40 °C under a pressure of 10 mbar for 20 min. Au-BCP microspheres with dotted surface structures were obtained after washing with DI water to remove the remaining surfactants by repeated centrifugations performed at 13 000 rpm for 20 min. Preparation of Au-BCP Microspheres with Controlled Porous Nanostructures. To prepare the Au-BCP microspheres with porous surfaces, 10 mg of CTAB (Sigma-Aldrich) and 0.125 mg of Au-BCP microspheres were added to 1 mL of the aqueous solution. The mixture was stirred at 25 °C for 24 h. The mixture was then purified by washing with DI water and by repeated centrifugations at 13 000 rpm for 20 min. The products were redispersed in DI water and used for further characterization. As additives for the dissolution of the Au precursors in the P4VP core, mercaptobutanol (Sigma-Aldrich) and PEO-SH (thiol-terminated poly(ethylene oxide), Mn = 2 kg/mol, PDI = 1.05 from Polymer Sources Inc.) were also used. Characterization of Au-BCP Microspheres with Controlled Porous Nanostructures. Field-emission scanning electron microscopy (FE-SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL 2000FX) were used to observe the surface and internal structures of the Au-BCP microspheres. To visualize the surface structures of the microspheres using FE-SEM, the samples were prepared by drop-casting particle suspensions onto Si wafers, which were then sputtered with gold. To investigate the surface structures of the microspheres by TEM, the samples were prepared by dipping TEM grids coated with a 20−30 nm thick carbon film into the particle suspensions. The grids were then dried in air. The sizes and distributions of the PS-b-P4VP spheres were determined by TEM analysis. To investigate the porous nanostructures of the microspheres by cross-sectional TEM, the samples were prepared by drop-casting particle suspensions onto an epoxy film and allowing the solvent to dry. After the epoxy-supported films were coated with Pt, the films were cured in an oven at 60 °C for 12 h. The epoxy-supported films were then microtomed with a diamond knife at room temperature into 50 nm slices. The prepared samples were exposed to iodine vapor to selectively stain the P4VP domains of PS-b-P4VP. The pore size and depth on the microspheres were measured from cross-sectional TEM images. To characterize the optical properties of Au-BCP micro-
spheres, UV−vis absorption spectroscopy (Cary 50 Conc UV−vis spectrophotometer) was performed. GIXS measurements were performed on beamline 9A in the Pohang Accelerator Laboratory (South Korea). X-rays with a wavelength of 1.1010 Å were used. The incidence-angle X-ray beam was set in the range 0.13−0.18°, which was between the critical angles of the Au-BCP micelle films and the silicon substrate. A 0.5 wt % of the micelle solution in toluene was spin-coated onto silicon wafer that was treated by oxygen plasma.
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ASSOCIATED CONTENT
S Supporting Information *
Additional SEM and TEM images of Au-BCP microspheres, GIXS images of thin films of Au-PS190k-b-P4VP45k spheres, and TEM and EDX images of other metal-decorated Au-BCP microspheres are included (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (G.-R.Y.). *E-mail:
[email protected] (B.J.K.). Author Contributions ∥
These authors contributed equally to this work.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We dedicate this article to late Prof. Seung-Man Yang for his lifelong contribution to colloid and interface science. This research was supported by the Korea Research Foundation Grant funded by the Korean Government (2012R1A1A2A10041283, 2013R1A2A2A01016539, and 2010-0029409), an MKE grant (Sunjin-002), and the Global Frontier R&D Program on Center for Multiscale Energy System (2012M3A6A7055540). 4421
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