Langmuir 2008, 24, 5663-5666
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Novel Fabrication of Janus Particles from the Surfaces of Electrospun Polymer Fibers Chi-Chih Ho, Wen-Shiang Chen, Tzung-Ying Shie, Jiun-Nan Lin, and Changshu Kuo* Department of Materials Science and Engineering, Frontier Material and Micro/Nano Science and Technology Center, National Cheng Kung UniVersity, Tainan 701-01, Taiwan ReceiVed February 13, 2008. ReVised Manuscript ReceiVed April 13, 2008 A novel synthetic approach for the efficient fabrication of Janus silica particles was demonstrated by embedment of zero-dimensional colloids on one-dimensional polymer fiber surfaces, followed by the surface modification on the exposed silica hemispheres. Electrospinning of poly(methyl methacrylate) and poly(4-vinyl pyridine) blends produced polymer fibers with high specific surface area and desired surface hydrophilicities. Fiber compositions determined the colloid adsorption density and uniformity. The colloid embedding resulted from the polymer softening was manipulated by the isothermal heat treatment. Subsequent silianization completed the amino functionalities on hemispherical surfaces of embedded silica colloids. Janus particles with uniform asymmetric chemical features were further labeled with gold nanoparticles before their recovery from fiber substrates. Fabrication of Janus particles, including colloid adsorption, temperature-driven embedding, and hemispherical surface modification, were investigated and are discussed.
Introduction Low-dimensional materials with asymmetric chemical/physical properties or geometric structures have attracted much interest because of their unique performance that cannot be achieved by homogeneous or symmetric materials. A zero-dimensional example is a particle with two asymmetric hemispheres and is often referred to as the Janus particle (JP)1 carrying not only two different functionalities but also the spatial distribution of these functions. In recent years, significant attention has been paid to Janus-type materials because of their potential applications in dual-functional devices,2 structure materials for 3D assembly,3–6 electronic paper and display applications,7 surfactants for emulsion systems,8 anisotropic imaging probes for both diagnostic and therapeutic purposes,9 bimetallic nanomotors,10 nanoprobes,11,12 and anisotropic plasmon materials.13 Crucial issues in the fabrication of Janus materials, including high productivities and uniformities of asymmetric features, were mostly determined by the synthesis pathways. Synthesis approaches of zero-dimensional Janus materials with micro- or nanostructure can be generally categorized into a direct dualsupplied method and an indirect template-assisted method. The direct dual-supplied method involves the formation of droplets consisting of two immiscible materials in a liquid or molten form. Biphasic particles with diameters on the order of tens of micrometers were then coejected via a spinning disk2 or a * Corresponding author. E-mail:
[email protected]. (1) de Gennes, P.-G. ReV. Mod. Phys. 1992, 64, 645. (2) Perro, A.; Reculusa, S.; Ravaine, S.; Bourgeat-Lami, E.; Duguet, E. J. Mater. Chem. 2005, 15, 3745. (3) Glotzer, S. C. Science 2004, 306, 419. (4) Nie, Z.; Li, W.; Seo, M.; Xu, S.; Kumacheva, E. J. Am. Chem. Soc. 2006, 128, 9408. (5) Hong, L.; Cacciuto, A.; Luijten, E.; Granick, S. Nano Lett. 2006, 6, 2510. (6) Suzuki, D.; Tsuji, S.; Kawaguchi, H. J. Am. Chem. Soc. 2007, 129, 8088. (7) Nisisako, T.; Torii, T.; Takahashi, T.; Takizawa, Y. AdV. Mater. 2006, 18, 1152. (8) Binks, B. P.; Fletcher, P. D. I. Langmuir 2001, 17, 4708. (9) Yoshida, M.; Roh, K.-H.; Lahann, J. Biomaterials 2007, 28, 2446. (10) Wang, Y.; Hernandez, R. M., Jr.; Bingham, J. M.; Kline, T. R.; Sen, A.; Mallouk, T. E. Langmuir 2006, 22, 10451. (11) Takei, H.; Shimizu, N. Langmuir 1997, 13, 1865. (12) Koo, H. Y.; Yi, D. K.; Yoo, S. J.; Kim, D. Y. AdV. Mater. 2004, 16, 274. (13) Wang, H.; Halas, N. J. Nano Lett. 2006, 6, 2945.
microfluidic system.14 The continuous ejection process in the direct dual-supplied method demonstrates the efficient production of Janus particles with moderate uniformities in terms of particle sizes and hemispheric features. The indirect template-assisted method, however, addresses the chemical or physical modifications of the hemispheric surfaces of existing monodisperse particles. Particle embedding on substrate surfaces is required to conceal one hemispheric surface and to modify the other exposed hemisphere with chemical functionalities or geometric shapes. Particle adsorption and embedding are usually conducted on 2D flat surfaces.15–17 However, this can potentially constrain production rates because of the limited working areas. Spherical substrates, therefore, offer an opportunity to expand the surface-to-volume ratio.18,19 For example, the effective fabrication of Janus silica particles has been demonstrated by using suspended wax microparticles as embedding vehicles.20 In this work, polymer fibers prepared by electrospinning were utilized as 1D substrates for indirect template-assisted fabrication. Electrospinning involves strong electrostatic attractions induced by a high voltage applied between a polymer medium and a counter electrode.21 Electrospinning occurs when highly charged polymer jets repel each other, causing micro- or nanoscale jet splitting, along with solvent evaporation and fiber solidification. Electrospun polymer fibers provide a large amount of surface area for colloid embedment or encapsulation,22 resulting in the superior production of Janus particles. In comparison with spherical substrates suspended in liquid media, polymer scaffolds with stable spacing among fibers furthermore allow gas-phase modification reactions, such as chemical vapor deposition (CVD), (14) Roh, K.-H.; Martin, D. C.; Lahann, J. Nat. Mater. 2005, 4, 759. (15) Cayre, O.; Paunov, V. N.; Velev, O. D. Chem. Commun. 2003, 18, 2296. (16) Paunov, V. N.; Cayre, O. J. AdV. Mater. 2004, 16, 788. (17) Cui, J.-Q.; Kretzschmar, I. Langmuir 2006, 22, 8281. (18) Takahara, Y. K.; Ikeda, S.; Ishino, S.; Tachi, K.; Ikeue, K.; Sakata, T.; Hasegawa, T.; Mori, H.; Matsumura, M.; Ohtani, B. J. Am. Chem. Soc. 2005, 127, 6271. (19) Kim, S.-H.; Heo, C.-J.; Lee, S. Y.; Yi, G.-R.; Yang, S.-M. Chem. Mater. 2007, 19, 4751. (20) Hong, L.; Jiang, S.; Granick, S. Langmuir 2006, 22, 9495. (21) Reneker, D. H.; Chun, I. Nanotechnology 1996, 7, 216. (22) Lim, J.-M.; Moon, J. H.; Yi, G.-R.; Heo, C.-J.; Yang, S.-M. Langmuir 2006, 22, 3445.
10.1021/la800282j CCC: $40.75 2008 American Chemical Society Published on Web 05/06/2008
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Table 1. Formulations of Electrospinning Solutions and the Average Diameters of Electrospun Fibers sample number PMMA/P4VP (by weight) DMF/MEK (by volume) concentration of polymers (wt %)a diameter ofelectrospun fibers (µm) JPF1-4-0 JPF2-3-1 JPF3-2-2 JPF4-1-3 JPF5-0-4 a
100/0 75/25 50/50 25/75 0/100
50/50 50/50 50/50 40/60 40/60
20.6 24.2 26.5 29.8 31.5
2.8 2.7 2.8 3.1 0.8
Polymers concentration is calculated on the basis of the weight fractions of polymers in the DMF/MEK solutions.
to be conducted. Time-consuming purification, such as filtration or centrifugation, can be replaced by simple liquid washing processes. Meanwhile, fiber substrates consisted of a hydrophobic poly(methyl methacrylate) (PMMA) and a hydrophilic poly(4vinyl pyridine) (P4VP) that achieved the desired fiber surface properties and reinforced the solvent resistance of polymer fibers. PMMA/P4VP fibers with two glass-transition temperatures expand the window of polymer softening. It could be further utilized to establish uniform and temperature-dependent colloid submerging.
Experimental Section Materials. Poly(methyl methacrylate) (PMMA, Mw ) 120 000, Aldrich) and poly(4-vinylpyridine) (P4VP, Mw ) 60 000, Aldrich) were employed in the fabrication of electrospun polymer fibers. Solvents, including N, N-dimethylformamide (DMF, from Fluka) and methyl ethyl ketone (MEK, from J.T. Baker), were used as received. Silica colloids with a uniform diameter of 500 nm were obtained from Alfa Aesar. An amino end-capped silane, 3-aminopropyltrimethoxysilane (3-APTMS, Fluka), was utilized for surface modification in a chemical vapor deposition (CVD) process. AgNO3, HAuCl4, and NaBH4 (Aldrich) were also used as received. Electrospun Polymer Fibers. Polymer fibers that served as the embedding substrates were electrospun from PMMA/P4VP polymer solutions. Electrospinning formulas, including the PMMA/P4VP compositions and DMF/MEK solvent ratios, are summarized in Table 1. It was known that diameters of electrospun fibers were determined by numerous electrospinning parameters. In this work, the regulation of fiber diameters was achieved by altering cosolvent ratios, solid concentrations, and/or the ejection flow rates in the range of 15-35 µL/min, as precisely controlled by a syringe pump. The electrospinning of pure P4VP (sample JPF5-0-4) produced a fiber diameter of 0.8 µm as a result of the lower solution viscosity in comparison with that of other samples containing PMMA. Except for JPF5-0-4, the diameters of electrospun polymer fibers were carefully adjusted to about 3 µm, which is 6 times larger than the 500 nm silica colloids used in this work. The ejection nozzle with an inner diameter of 0.9 mm was kept 15 cm away from a grounded collector covered with aluminum foil. Applied high voltage in the electrospinning process was fixed at 10 kV for all samples. Electrospun fibers from each 150 µL solution ejection, which is roughly equal to 30-45 mg of polymer solid content, were collected and stored in a desiccator for future use. Janus Particles. Silica colloids (2 g) were precleaned by 30 min of sonication in a NaOH/EtOH aqueous solution (150 mg of sodium hydroxide dissolved in 30 mL of H2O and 30 mL of ethanol). Centrifuge separation and DI water washing were repeated eight times to ensure the removal of base residue. The cleaned silica colloids were then dispersed in 200 mL aqueous solutions (i.e., 1 wt % of solid content) with pH values of 3, 4, 5, 6, 7, and 8, which were carefully adjusted with acetic acid and ammonium hydroxide. The colloid adsorption process was carried out at room temperature. Ten milligrams of polymer fiber mats was weighed and dipped into 20 mL of silica-suspended aqueous solutions as mentioned above. Sonication for a few seconds was applied to eliminate possible air trapped within the fiber mats. After 10 min of dipping, extra colloids with no direct contact with the fiber surfaces were removed by moderate rinsing with DI water several times. Sample fibers were dried under vacuum at room temperature to minimize moisture content
within the PMMA/P4VP substrates. Colloid adsorption densities and uniformities were calculated on the basis of SEM images of these samples. (See Supporting Information for details.) Colloid embedding on fiber surfaces was conducted by 4 h of isothermal treatment at temperatures of 105, 120, 135, and 150 °C, separately. Subsequent surface modification was performed using the silane-based CVD method. A silane atmosphere from 1 mL of 3-aminopropyltrimethoxysilane (APTMS) was generated at 100 °C under vacuum and then introduced into a 500 mL vacuum chamber where polymer fiber mats with embedded colloids were maintained at a constant temperature of 90 °C. The CVD process was extended for one hour at the same temperature to ensure silanization on surfaces of the silica colloids. Gold nanoparticles with a diameter of 10 nm were synthesized by the reduction of HAuCl4 in aqueous solution and utilized as nanosized labelers targeting the amino-functionalized silica colloids. Polymer fibers embedded with amino-modified silica colloids were immersed in a 20 mL aqueous solution containing gold nanoparticles for 10 min, followed by several water rinses. Embedded Janus particles were recovered by dissolving polymer fibers in a 20 mL cosolvent of acetone/ethanol (1:1 by volume), followed by centrifuge separation. Further colloid purification was conducted by washing with the same cosolvent four more times. Samples of Janus particles were suspended and kept in 20 mL aqueous solution for further investigation.
Results and Discussion Precleaned silica colloids from NaOH treatment and the DI water washing process provided silanol-enriched surfaces.23 In the absence of strong basic ions, silanol groups in acid environments served as proton donors (weak acid) to establish interactions with pyridine groups located on P4VP.24,25 However, the poor water resistance of P4VP, especially in acid environments, prohibited the use of pure P4VP fiber mats in the wet process. The incorporation of hydrophobic PMMA, therefore, became essential to the reinforcement of the polymer scaffolds in acid solution. Meanwhile, the morphologies of electrospun PMMA/P4VP fibers were carefully investigated by DFOM (darkfield optical microscopy) and SEM examinations . (See Supporting Information for details.) The results concluded P4VP domains with about 80 nm width were at least 6 times smaller than the silica diameter (500 nm). Evenly distributed silica colloids adsorbed on fiber surfaces, and consistent colloid embedding was achievable. Colloid adsorption on electrospun polymer fibers was investigated under the variations of PMMA/P4VP compositions, as well as the pH values of silica suspension solutions. Colloid numbers and adsorption uniformities were measured and summarized in Table 2. Figure 1 illustrates the overall profiles of the colloid adsorption on PMMA/P4VP fiber surfaces. Data marked in black represent the poor colloid adsorption caused either by (23) Yamanaka, J.; Yoshida, H.; Koga, T.; Ise, N.; Hashimoto, T. Langmuir 1999, 15, 4198. (24) Shenderovich, I. G.; Buntkowsky, G.; Schreiber, A.; Gedat, E.; Sharif, S.; Albrecht, J.; Golubev, N. S.; Findenegg, G. H.; Limbach, H. H. J. Phys. Chem. B 2003, 107, 11924. (25) Agarwal, G. K.; Titman, J. J.; Percy, M. J.; Armes, S. P. J. Phys. Chem. B 2003, 107, 12497.
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Table 2. Colloid Adsorption As Functions of Fiber Composition and Silica Suspension Solution pH solution pH sample number JPF1-4-0 JPF2-3-1 JPF3-2-2
3 a
2.78 (0.89)b 1.49 (2.30) 1.99 (3.45)
4
5
6
7
8
2.98 (2.75) 257.73 (1.17) 191.73 (0.51)
3.18 (2.290) 202.48 (0.39) 189.25 (0.19) 168.90 (0.35)
1.79 (2.34) 143.76 (0.35) 179.16 (0.26) 198.68 (0.10)
1.79 (3.32) 138.63 (0.17) 113.15 (0.33) 118.61 (0.29)
0.20 (4.90) 50.45 (0.56) 39.70 (0.58) 114.81 (0.20)
JPF4-1-3 JPF5-0-4
( )c ( )c fiber dissolved
a Average of colloid adsorption densities (number/100 µm2). b Unitless uniformities of corresponding colloid adsorptions. c Fiber dissolved in the solutions.
Figure 2. DSC thermogram of sample JPF3-2-2 illustrating two glasstransition temperatures at 121 and 149 °C. SEM images shown on the top demonstrate the controllable colloid embedding at 105, 120, 135, and 150 °C.
Figure 1. Colloid adsorption densities (right axis) as functions of PMMA/ P4VP composition (lower axis) and pH values of silica suspension solutions (left axis). (The insert SEM image shows an example of JPF32-2 fibers with uniform colloid adsorption at pH 6).
the hydrophobicity of the PMMA-enriched fiber surface or by the dissolution of P4VP in an acidic environment. The reduction of fiber diameters after low pH (
