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Compartment fabrication of magneto-responsive Janus microrod particles† Su Yeon Lee and Shu Yang*
Received 5th October 2014, Accepted 8th December 2014 DOI: 10.1039/c4cc07863b www.rsc.org/chemcomm
Monodispersed magneto-responsive microrod particles of variable magnetic/non-magnetic ratios and chemical compositions are created by compartment fabrication in a single poly(dimethylsiloxane) (PDMS) mold with periodic hole arrays. By labeling the two ends with green and red fluorescent dyes separately, we show that the particles can flip freely and reversibly in a confined geometry under the magnetic field, thereby displaying different patterned colors at the air–water interface.
Janus particles that impart considerably different chemical or physical properties within a single entity have attracted much attention for their unique assembly structures at the interface1–9 and potential applications, including drug delivery, biosensors, cell manipulation, microdisplay, and self-propulsion.10–15 Janus particles have been created by various approaches, including seeded emulsion polymerization, electrohydrodynamic jetting, vapor deposition, microcontact printing, and microfluidic technique.16–21 While most are synthesized from spherical particles, there have been increasing efforts to fabricate non-spherical particles with magnetic, electric, and optical characteristics for directed self-assembly or remote manipulation.22–25 For example, anisotropic amphiphilic polymeric particles with tunable multiphases and magneto-polymer composite particles have been fabricated by replication in nonwetting templates.26,27 Diblock and triblock rods with opposite wettability in the adjacent blocks are demonstrated. These particles become magnetoresponsive when Fe3O4 nanoparticles are introduced, which can align along the magnetic field, although they often appear as aggregates, possibly due to random dispersion of Fe3O4 nanoparticles in polymers and the less-defined interface between two adjacent blocks. Since the rod particles are molded in a laying-down fashion, the bottom of the particles obtained by this method is Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: (1) Experimental details, (2) rotational and flip motion of the magnetic Janus microparticles under an external magnetic field, (3) fabrication of magnetic Janus particles consisting of perpendicular magnetic moments in the adjacent blocks, and optical images of the assembled particles, (4) fabrication of the diblock and triblock Janus particles and SEM images. See DOI: 10.1039/c4cc07863b
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always flat. Monodispersed dumbbell-shaped particles have been prepared by two-step seeded emulsion polymerization.28,29 It is known that emulsion polymerization is complicated and it is difficult to control the size distribution for high throughput processes. Nonspherical magnetic microparticles with multiple functionalities are recently demonstrated using the stop-flow lithography (SFL) technique.30 Nevertheless, most Janus particles have limited functionalities and hydrodynamic mobility. Many magnetic particles lack precise control of the magnetic component vs. the nonmagnetic one in a single particle, thereby limiting the variety of possible structures and their practical applications. It will be highly desirable to mass-produce magneto responsive Janus particles of precisely controlled size and shape, anisotropy, chemistry, and magnetic composition in a single particle, leading to new active materials.31–33 Herein, we present a highly versatile method to fabricate monodispersed magneto-responsive microrod particles of variable magnetic/non-magnetic ratios, chemical composition and fluorescent labeling. The particles were created by compartment filling of silica colloids and magnetic iron oxide nanoparticles in a single poly(dimethylsiloxane) (PDMS) mold with periodic hole arrays, followed by infiltration of epoxy resin into the mold and UV curing to lock the rod structure. The particles were released from the substrate and dispersed into an aqueous solution for studying their assembly behaviors. This strategy allowed for organization of magnetic and non-magnetic materials into two or three different blocks of a single microparticle, where the block consisting of silica colloids could be fluorescently labeled separately. A clear boundary between the two adjacent blocks was achieved, leading to highly anisotropic magneto-responsiveness and tunable fluorescent colors within one particle. We showed that triblock Janus microparticles could flip freely and reversibly in a confined geometry under the magnetic field, thereby displaying two different fluorescent colors toward the air–water interface within a pattern. The square-array PDMS mold with cylindrical holes (10 mm diameter, 20 mm depth, and 15 mm pitch) was prepared by replica molding from 2D SU-8 micropillar arrays. For the production of magnetic Janus microparticles, silica particles (500 nm in diameter) were dispersed in ethanol to prepare one block, and the mixture of
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Fig. 1 Schematic illustration of the synthesis of magneto-responsive Janus microparticles. Silica colloids are partially filled into the PDMS mold with cylindrical holes. Silica particles and iron oxide nanoparticles were completely filled in the cylindrical hole of the PDMS mold. Epoxy resin was then back-filled into the PDMS mold by capillary force, and then a magnetic field is applied to the whole system. After UV curing of the epoxy resin, the PDMS mold is peeled off, leaving an array of micropillar structures. The micropillars are released from the substrate to obtain the anisotropic, magnetic Janus microparticles.
silica particles and iron oxide nanoparticles (a-Fe2O3, o50 nm in diameter) was dispersed in ethanol to prepare the other block. As shown in Fig. 1, 5 wt% silica particle suspension in ethanol was first dropped onto the PDMS mold, followed by blading the suspension across the PDMS mold using a glass slide to facilitate a uniform distribution of silica particles at the bottom of the mold. After complete evaporation of ethanol, an ethanol suspension consisting of silica colloidal particles and iron oxide nanoparticles was dropped onto PDMS mold in order to completely fill the PDMS mold, creating the magnetic part of the Janus microparticles. Epoxy resin was then backfilled into the PDMS mold by capillary force and photopolymerized under UV light while applying the magnetic field to the whole system to control the direction of the magnetic moment within the microparticle. After curing the epoxy resin, the PDMS mold was peeled off from the substrate, leaving a square array of hybrid micropillars on the substrate over a large area (ca. 3 cm2 ). Finally, the magnetic Janus micropillars were released from the substrate by scrapping and dispersed in water containing 1 wt% Pluronic F108, a polymeric surfactant, a PEO-b-PPO-b-PEO triblock copolymer (PEO, poly(ethylene oxide) and PPO, poly(propylene oxide)).34 Recently, we have reported the fabrication of anisotropic, magneto-responsive microparticles using this method,35 where the size and shape of magneto-responsive microparticles can be tailored using PDMS molds of different sizes and shapes. To generate magnetic anisotropy in a single particle, we prepared the microrod particles consisting of magnetic and non-magnetic components of variable lengths, the microscale analogues to those in a block copolymer. The magnetic block was composed of iron oxide nanoparticles, silica colloids and epoxy, and the non-magnetic one was composed of silica colloids and epoxy. The block length was controlled by the concentration of SiO2–Fe2O3 and SiO2 suspension filled within the cylindrical holes of the PDMS mold. To demonstrate the versatility of our method, we prepared a series of magnetic Janus microparticles with different length ratios of magnetic to nonmagnetic block (R) and surface functionality. As seen in optical
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Fig. 2 (a–d) Optical microscopy images of the assemblies of magnetic Janus microparticles with different length ratios of magnetic to nonmagnetic domains under the field. (a) 4 : 1, (b) 1.5 : 1, (c) 1 : 1, (d) 1 : 4. (e, f) Optical microscopy images of the magnetic Janus microparticles with two perpendicular magnetic moments (length ratio 1 : 1) without an external magnetic field. The inset of (e) shows the magnetic moment of each half domain in a single microparticle.
microscopy (OM) images (Fig. 2a–d), R within a single microparticle can be precisely tuned from 4 : 1 to 1 : 4. Because of the strong interaction between silica particles after their assembly in the confined PDMS channel, a clear boundary was observed between two blocks. When the magnetic block was the dominant one in the microparticle (e.g. R = 4 : 1), all the microparticles were aligned in the direction of the magnetic field (1 kG) parallel to the substrate (Fig. 2a). Indeed, such Janus microparticles could rotate and flip freely in an aqueous solution by rotating the magnetic field (see ESI,† Fig. S1). When the magnetic block length was reduced to R r 1.5 : 1, the Janus microparticles became less aligned (Fig. 2b and c). When R was further decreased to 1 : 4, the microparticles were randomly oriented much like those without magnetic particles (Fig. 2d) due to negligible magnetic moment generated within these particles. It has been shown that when embedding magnetic cubes with permanent magnetic moment in the Janus particles, the particles could self-assemble into clusters in the absence of an external field.12 Here, we created Janus magnetic microparticles with magnetic
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moments of adjacent blocks oriented perpendicular to each other in an individual microparticle with R = 1 : 1 (Fig. 2e and f). This was achieved by partial infiltration of silica–iron oxide particles, followed by UV curing of epoxy resin within the mold while applying a field parallel to the particle length. Then a second infiltration and UV curing was carried out in the same way as the first block, except that the magnetic field was applied perpendicular to the first one (see Fig. S2a, ESI†). Because two opposite magnetic moments were combined in a single microparticle, the magnetic microparticles in the aqueous suspension were assembled in a crossed manner or side-to-side fashion without application of an external magnetic field (Fig. 2e and f). As a reference, magnetic microparticles that had a perpendicular magnetic moment to the long axis of microparticles assembled side-to-side without an external magnetic field (Fig. S2b and c, ESI†). The use of silica particles in our system is particularly advantageous since a wide range of functionalities can be introduced on the silica surface via a wide range of silane chemistry. The combination of high magnetic responsiveness and selective surface functionalization can play an essential role in chemical detection for bioanalytical applications and microdisplays. As a proof-of-concept, we functionalized silica particles with fluorescein isothiocyanate (FITC, a green dye) or tetramethylrhodamine isothiocyanate (TRITC, a red dye) according to the literature.36 These fluorescent silica particles were then used to create the diblock microparticles of different magneticratios. In Fig. 3, the FITC- or TRITC-labeled silica particles were only used in the non-magnetic block. Upon the application of an external magnetic field parallel to the substrate, the magnetic Janus microparticles were aligned into chain structures in water (Fig. 3a, b, d, and e). As seen in the optical (Fig. 3a) and fluorescence (Fig. 3b) microscopy images, FITC-labeled particles showed emission of green fluorescence, which was clear in the non-magnetic block. Similarly, TRITC-labeled magnetic Janus microparticles were assembled into chain structures and the red fluorescent signals were noticeably present in the non-magnetic block according to the magnetic/ non-magnetic ratio (Fig. 3d and e). When the magnetic field was applied perpendicular to the substrate, the microparticles flipped and became normal to the substrate, showing the bright green and
Fig. 3 Optical and fluorescence microscopy images of the magnetic Janus microparticles (magnetic/nonmagnetic length ratio, 9 : 1) including FITClabeled silica particles in the nonmagnetic block under an external magnetic field (a, b) parallel and (c) perpendicular to the substrate, respectively. Fluorescence microscopy images of the magnetic Janus microparticles containing TRITC-labeled silica particles with (d) 4 : 1 (magnetic/nonmagnetic) length ratio and (e) 1.5 : 1 length ratio and subjected to a magnetic field (d, e) parallel and (f) perpendicular to the substrate.
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Fig. 4 Fluorescence microscopy images of the magnetic diblock (a–d) and triblock (e and f) Janus microparticles labeled with different fluorescent molecules of variable lengths. The length ratios of FITC to TRITC labeled blocks are (a) 1 : 4, (b) 4 : 1, (c) 1 : 1. (d) Fluorescence microscopy image of the diblock microparticles shown to be (a) dispersed in water on a planar substrate and subjected to a magnetic field parallel to the substrate. (e, f) Fluorescence microscopy images of the magnetic triblock microparticles with different ratios of two dye-labeled blocks (2 : 2 : 1).
red emission colors, respectively, where the fluorescence labeled block was facing upwards (Fig. 3c and f). Our method is highly versatile. We can introduce different functionalities in the microparticles without changing their magnetic nature and assembly under the field. This was achieved by compartmentalization of silica colloids labeled by two different fluorescent dyes in different blocks. The rest of the procedure was the same as shown in Fig. 1, where the magnetic moment was aligned in the same direction along the long axis of the rod particle. The length of the fluorescently labeled block was controlled by the concentration of FITC- or TRITC-labeled silica suspension. Three different types of Janus rod particles were produced by precisely tuning the block lengths as shown in Fig. 4a–c. For example, the magnetic Janus microparticles with an FITC block length of 4 mm and a TRITC block length of 16 mm were synthesized (Fig. 4a, R = 4 : 1). While each block displayed respective fluorescence colors, the entire microparticle was magnetically responsive. As seen in Fig. 4d, they could be aligned in chain structures under the magnetic field parallel to the substrate (Fig. 4d), the same as the observation in Fig. 2a. Using the same procedure, we created three-component, or triblock-like microparticles consisting of both green and red colors in a single magnetic particle by sequential deposition of dye-labeled silica particles into the PDMS mold, while the magnetic moment in both ends were aligned in the same direction along the long axis of the rod. As shown in Fig. 4e and f, three blocks were well arranged into a single microparticle without any overlap. This strategy could be potentially applied to design new drug carriers where multiple drugs can be incorporated into a single carrier. To demonstrate the magneto-responsiveness of the microparticles and their potential application in display, we synthesized triblock-like particles of different chemistry and magnetic responsiveness in the adjacent blocks, that is the FITC labeled magnetic responsive block, the particle-free non-magnetic block, and the TRITC labeled magnetic responsive block (see fabrication details in the ESI,† Fig. S3). As shown in Fig. 5a and b, three different blocks were clearly compartmentalized within each single microparticle, which could reversibly rotate and flip freely. The introduction of a particle-free block was critical here to separate two dye-labeled, magnetic blocks, preventing interference
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tailored size and functionality. B106 particles per cm2 was obtained in our system. However, by design of the template and scale up the wafer size containing the patterns, much larger number of particles can be obtained. These particles will be of great interest for potential applications, including biological sensors, drug carriers, single cell analysis, and field-responsive displays. The research is supported in part by the National Science Foundation (NSF)/EFRI awards, grant # EFRI-1038215 and #EFRI-1331583.
Notes and references
Fig. 5 (a, b) Fluorescence microscopy images of the magnetic Janus microparticles composed of three different blocks (FITC labeled/particle-free/TRITC labeled) dispersed in water on a planar substrate. The microparticles aligned in chain structures when the magnetic field was applied in parallel to the substrate. (c, d) Fluorescence microscopy images of the magneto-responsive display panel, where isolated triblock magnetic microparticles were aligned under an external magnetic field perpendicular to the substrate. The insets of (c) and (d) show a schematic illustration of the orientation of magnetic triblock microparticles under a magnetic field applied perpendicular to the substrate.
between two dye molecules. We then confined the aqueous dispersion of the triblock microparticles in a PDMS microwell in the shape of letter ‘‘N’’, which was replicated from a SU-8 master. When the external magnetic field was applied perpendicular to the microwell, the triblock magnetic microparticles flipped with the FITC-labeled block facing upwards, thus displaying the green letter ‘‘N’’ when illuminated by UV light (Fig. 5c). By reversing the magnetic field, the triblock magnetic microparticles re-oriented instantly, and displayed red emission in letter ‘‘N’’ under UV light (Fig. 5d). In summary, we have demonstrated a facile method to massproduce uniform magneto-responsive Janus microparticles of variable dimensions and chemical composition from a single PDMS mold. The well-controlled compartment filling of silica particles and iron oxide nanoparticles into the PDMS mold, followed by epoxy curing allowed us to create di- and tri-block microparticles in which the magnetic/ non-magnetic ratio could be precisely adjusted. The adjacent blocks had a clear boundary without any overlap, and therefore, they were highly responsive; they could be isolated and flipped freely and reversibly. When the magnetic field was applied in parallel to the substrate, the particles could be aligned to form the chain structures in the direction of the magnetic field. The degree of alignment, however, was found to be highly dependent on the length ratio of magnetic/ non-magnetic blocks. When the ratio was decreased to 1 : 4, the alignment was completely lost. Finally, we attempted to integrate magneto-responsiveness and fluorescent emission in a single particle. The use of silica particles in one of the blocks is especially advantageous since different functionalities, surface chemistry, and fluorescent dyes can be introduced into the silica block. We created triblocks with both ends consisting of silica particles and magneto responsiveness, which could flip in a confined geometry to display green and red colors under the magnetic field, respectively. Our strategy shown here is highly versatile and scalable to create field-responsive microparticles with
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1 Q. Chen, S. C. Bae and S. Granick, Nature, 2011, 469, 381. 2 Q. Chen, S. C. Bae and S. Granick, J. Am. Chem. Soc., 2012, 134, 11080. 3 Q. Chen, J. K. Whitmer, S. Jiang, S. C. Bae, E. Luijten and S. Granick, Science, 2011, 331, 199. 4 R. M. Erb, H. S. Son, B. Samanta, V. M. Rotello and B. B. Yellen, Nature, 2009, 457, 999. 5 L. Hong, A. Cacciuto, E. Luijten and S. Granick, Nano Lett., 2006, 6, 2510. 6 S. Jiang and S. Granick, Langmuir, 2009, 25, 8915. 7 B. Ren, A. Ruditskiy, J. H. Song and I. Kretzschmar, Langmuir, 2012, 28, 1149. 8 S. K. Smoukov, S. Gangwal, M. Marquez and O. D. Velev, Soft Matter, 2009, 5, 1285. 9 D. Zerrouki, J. Baudry, D. Pine, P. Chaikin and J. Bibette, Nature, 2008, 455, 380. 10 L. Baraban, D. Makarov, R. Streubel, I. Monch, D. Grimm, S. Sanchez and O. G. Schmidt, ACS Nano, 2012, 6, 3383. 11 I. Gorelikov, L. M. Field and E. Kumacheva, J. Am. Chem. Soc., 2004, 126, 15938. 12 X. H. Huang, I. H. El-Sayed, W. Qian and M. A. El-Sayed, J. Am. Chem. Soc., 2006, 128, 2115. 13 T. Nisisako and T. Torii, Adv. Mater., 2007, 19, 1489. 14 A. K. Salem, P. C. Searson and K. W. Leong, Nat. Mater., 2003, 2, 668. 15 S. Sanchez, A. A. Solovev, S. Schulze and O. G. Schmidt, Chem. Commun., 2011, 47, 698. 16 S. Bhaskar, J. Hitt, S. W. L. Chang and J. Lahann, Angew. Chem., Int. Ed., 2009, 48, 4589. 17 O. Cayre, V. N. Paunov and O. D. Velev, J. Mater. Chem., 2003, 13, 2445. 18 S. H. Kim, S. J. Jeon, W. C. Jeong, H. S. Park and S. M. Yang, Adv. Mater., 2008, 20, 4129. 19 Z. H. Nie, W. Li, M. Seo, S. Q. Xu and E. Kumacheva, J. Am. Chem. Soc., 2006, 128, 9408. 20 M. M. Rahman, F. Montagne, H. Fessi and A. Elaissari, Soft Matter, 2011, 7, 1483. 21 M. Yoshida, K. H. Roh, S. Mandal, S. Bhaskar, D. W. Lim, H. Nandivada, X. P. Deng and J. Lahann, Adv. Mater., 2009, 21, 4920. 22 J. W. Kim, R. J. Larsen and D. A. Weitz, J. Am. Chem. Soc., 2006, 128, 14374. 23 K. Liu, Z. H. Nie, N. N. Zhao, W. Li, M. Rubinstein and E. Kumacheva, Science, 2010, 329, 197. 24 J. C. Loudet, A. M. Alsayed, J. Zhang and A. G. Yodh, Phys. Rev. Lett., 2005, 94, 018301. 25 S. Park, S. W. Chung and C. A. Mirkin, J. Am. Chem. Soc., 2004, 126, 11772. 26 J. Nunes, K. P. Herlihy, L. Mair, R. Superfine and J. M. DeSimone, Nano Lett., 2010, 10, 1113. 27 J. Y. Wang, Y. P. Wang, S. S. Sheiko, D. E. Betts and J. M. DeSimone, J. Am. Chem. Soc., 2012, 134, 5801. 28 J. D. Forster, J. G. Park, M. Mittal, H. Noh, C. F. Schreck, C. S. O’Hern, H. Cao, E. M. Furst and E. R. Dufresne, ACS Nano, 2011, 5, 6695. 29 J. G. Park, J. D. Forster and E. R. Dufresne, J. Am. Chem. Soc., 2010, 132, 5960. 30 S. K. Suh, K. Yuet, D. K. Hwang, K. W. Bong, P. S. Doyle and T. A. Hatton, J. Am. Chem. Soc., 2012, 134, 7337. 31 K. Chaudhary, Q. Chen, J. J. Juarez, S. Granick and J. A. Lewis, J. Am. Chem. Soc., 2012, 134, 12901. 32 J. Kim, S. E. Chung, S. E. Choi, H. Lee, J. Kim and S. Kwon, Nat. Mater., 2011, 10, 747. 33 H. Lee, J. Kim, H. Kim, J. Kim and S. Kwon, Nat. Mater., 2010, 9, 745. 34 J. H. Moon, A. J. Kim, J. C. Crocker and S. Yang, Adv. Mater., 2007, 19, 2508. 35 S. Y. Lee and S. Yang, Angew. Chem., Int. Ed., 2013, 52, 8160. 36 A. Vanblaaderen and A. Vrij, Langmuir, 1992, 8, 2921.
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Compartment fabrication of magneto-responsive Janus microrod particles† Su Yeon Lee and Shu Yang*
Received 5th October 2014, Accepted 8th December 2014 DOI: 10.1039/c4cc07863b www.rsc.org/chemcomm
Monodispersed magneto-responsive microrod particles of variable magnetic/non-magnetic ratios and chemical compositions are created by compartment fabrication in a single poly(dimethylsiloxane) (PDMS) mold with periodic hole arrays. By labeling the two ends with green and red fluorescent dyes separately, we show that the particles can flip freely and reversibly in a confined geometry under the magnetic field, thereby displaying different patterned colors at the air–water interface.
Janus particles that impart considerably different chemical or physical properties within a single entity have attracted much attention for their unique assembly structures at the interface1–9 and potential applications, including drug delivery, biosensors, cell manipulation, microdisplay, and self-propulsion.10–15 Janus particles have been created by various approaches, including seeded emulsion polymerization, electrohydrodynamic jetting, vapor deposition, microcontact printing, and microfluidic technique.16–21 While most are synthesized from spherical particles, there have been increasing efforts to fabricate non-spherical particles with magnetic, electric, and optical characteristics for directed self-assembly or remote manipulation.22–25 For example, anisotropic amphiphilic polymeric particles with tunable multiphases and magneto-polymer composite particles have been fabricated by replication in nonwetting templates.26,27 Diblock and triblock rods with opposite wettability in the adjacent blocks are demonstrated. These particles become magnetoresponsive when Fe3O4 nanoparticles are introduced, which can align along the magnetic field, although they often appear as aggregates, possibly due to random dispersion of Fe3O4 nanoparticles in polymers and the less-defined interface between two adjacent blocks. Since the rod particles are molded in a laying-down fashion, the bottom of the particles obtained by this method is Department of Materials Science and Engineering, University of Pennsylvania, 3231 Walnut Street, Philadelphia, PA 19104, USA. E-mail: [email protected] † Electronic supplementary information (ESI) available: (1) Experimental details, (2) rotational and flip motion of the magnetic Janus microparticles under an external magnetic field, (3) fabrication of magnetic Janus particles consisting of perpendicular magnetic moments in the adjacent blocks, and optical images of the assembled particles, (4) fabrication of the diblock and triblock Janus particles and SEM images. See DOI: 10.1039/c4cc07863b
This journal is © The Royal Society of Chemistry 2015
always flat. Monodispersed dumbbell-shaped particles have been prepared by two-step seeded emulsion polymerization.28,29 It is known that emulsion polymerization is complicated and it is difficult to control the size distribution for high throughput processes. Nonspherical magnetic microparticles with multiple functionalities are recently demonstrated using the stop-flow lithography (SFL) technique.30 Nevertheless, most Janus particles have limited functionalities and hydrodynamic mobility. Many magnetic particles lack precise control of the magnetic component vs. the nonmagnetic one in a single particle, thereby limiting the variety of possible structures and their practical applications. It will be highly desirable to mass-produce magneto responsive Janus particles of precisely controlled size and shape, anisotropy, chemistry, and magnetic composition in a single particle, leading to new active materials.31–33 Herein, we present a highly versatile method to fabricate monodispersed magneto-responsive microrod particles of variable magnetic/non-magnetic ratios, chemical composition and fluorescent labeling. The particles were created by compartment filling of silica colloids and magnetic iron oxide nanoparticles in a single poly(dimethylsiloxane) (PDMS) mold with periodic hole arrays, followed by infiltration of epoxy resin into the mold and UV curing to lock the rod structure. The particles were released from the substrate and dispersed into an aqueous solution for studying their assembly behaviors. This strategy allowed for organization of magnetic and non-magnetic materials into two or three different blocks of a single microparticle, where the block consisting of silica colloids could be fluorescently labeled separately. A clear boundary between the two adjacent blocks was achieved, leading to highly anisotropic magneto-responsiveness and tunable fluorescent colors within one particle. We showed that triblock Janus microparticles could flip freely and reversibly in a confined geometry under the magnetic field, thereby displaying two different fluorescent colors toward the air–water interface within a pattern. The square-array PDMS mold with cylindrical holes (10 mm diameter, 20 mm depth, and 15 mm pitch) was prepared by replica molding from 2D SU-8 micropillar arrays. For the production of magnetic Janus microparticles, silica particles (500 nm in diameter) were dispersed in ethanol to prepare one block, and the mixture of
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Fig. 1 Schematic illustration of the synthesis of magneto-responsive Janus microparticles. Silica colloids are partially filled into the PDMS mold with cylindrical holes. Silica particles and iron oxide nanoparticles were completely filled in the cylindrical hole of the PDMS mold. Epoxy resin was then back-filled into the PDMS mold by capillary force, and then a magnetic field is applied to the whole system. After UV curing of the epoxy resin, the PDMS mold is peeled off, leaving an array of micropillar structures. The micropillars are released from the substrate to obtain the anisotropic, magnetic Janus microparticles.
silica particles and iron oxide nanoparticles (a-Fe2O3, o50 nm in diameter) was dispersed in ethanol to prepare the other block. As shown in Fig. 1, 5 wt% silica particle suspension in ethanol was first dropped onto the PDMS mold, followed by blading the suspension across the PDMS mold using a glass slide to facilitate a uniform distribution of silica particles at the bottom of the mold. After complete evaporation of ethanol, an ethanol suspension consisting of silica colloidal particles and iron oxide nanoparticles was dropped onto PDMS mold in order to completely fill the PDMS mold, creating the magnetic part of the Janus microparticles. Epoxy resin was then backfilled into the PDMS mold by capillary force and photopolymerized under UV light while applying the magnetic field to the whole system to control the direction of the magnetic moment within the microparticle. After curing the epoxy resin, the PDMS mold was peeled off from the substrate, leaving a square array of hybrid micropillars on the substrate over a large area (ca. 3 cm2 ). Finally, the magnetic Janus micropillars were released from the substrate by scrapping and dispersed in water containing 1 wt% Pluronic F108, a polymeric surfactant, a PEO-b-PPO-b-PEO triblock copolymer (PEO, poly(ethylene oxide) and PPO, poly(propylene oxide)).34 Recently, we have reported the fabrication of anisotropic, magneto-responsive microparticles using this method,35 where the size and shape of magneto-responsive microparticles can be tailored using PDMS molds of different sizes and shapes. To generate magnetic anisotropy in a single particle, we prepared the microrod particles consisting of magnetic and non-magnetic components of variable lengths, the microscale analogues to those in a block copolymer. The magnetic block was composed of iron oxide nanoparticles, silica colloids and epoxy, and the non-magnetic one was composed of silica colloids and epoxy. The block length was controlled by the concentration of SiO2–Fe2O3 and SiO2 suspension filled within the cylindrical holes of the PDMS mold. To demonstrate the versatility of our method, we prepared a series of magnetic Janus microparticles with different length ratios of magnetic to nonmagnetic block (R) and surface functionality. As seen in optical
1640 | Chem. Commun., 2015, 51, 1639--1642
Fig. 2 (a–d) Optical microscopy images of the assemblies of magnetic Janus microparticles with different length ratios of magnetic to nonmagnetic domains under the field. (a) 4 : 1, (b) 1.5 : 1, (c) 1 : 1, (d) 1 : 4. (e, f) Optical microscopy images of the magnetic Janus microparticles with two perpendicular magnetic moments (length ratio 1 : 1) without an external magnetic field. The inset of (e) shows the magnetic moment of each half domain in a single microparticle.
microscopy (OM) images (Fig. 2a–d), R within a single microparticle can be precisely tuned from 4 : 1 to 1 : 4. Because of the strong interaction between silica particles after their assembly in the confined PDMS channel, a clear boundary was observed between two blocks. When the magnetic block was the dominant one in the microparticle (e.g. R = 4 : 1), all the microparticles were aligned in the direction of the magnetic field (1 kG) parallel to the substrate (Fig. 2a). Indeed, such Janus microparticles could rotate and flip freely in an aqueous solution by rotating the magnetic field (see ESI,† Fig. S1). When the magnetic block length was reduced to R r 1.5 : 1, the Janus microparticles became less aligned (Fig. 2b and c). When R was further decreased to 1 : 4, the microparticles were randomly oriented much like those without magnetic particles (Fig. 2d) due to negligible magnetic moment generated within these particles. It has been shown that when embedding magnetic cubes with permanent magnetic moment in the Janus particles, the particles could self-assemble into clusters in the absence of an external field.12 Here, we created Janus magnetic microparticles with magnetic
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moments of adjacent blocks oriented perpendicular to each other in an individual microparticle with R = 1 : 1 (Fig. 2e and f). This was achieved by partial infiltration of silica–iron oxide particles, followed by UV curing of epoxy resin within the mold while applying a field parallel to the particle length. Then a second infiltration and UV curing was carried out in the same way as the first block, except that the magnetic field was applied perpendicular to the first one (see Fig. S2a, ESI†). Because two opposite magnetic moments were combined in a single microparticle, the magnetic microparticles in the aqueous suspension were assembled in a crossed manner or side-to-side fashion without application of an external magnetic field (Fig. 2e and f). As a reference, magnetic microparticles that had a perpendicular magnetic moment to the long axis of microparticles assembled side-to-side without an external magnetic field (Fig. S2b and c, ESI†). The use of silica particles in our system is particularly advantageous since a wide range of functionalities can be introduced on the silica surface via a wide range of silane chemistry. The combination of high magnetic responsiveness and selective surface functionalization can play an essential role in chemical detection for bioanalytical applications and microdisplays. As a proof-of-concept, we functionalized silica particles with fluorescein isothiocyanate (FITC, a green dye) or tetramethylrhodamine isothiocyanate (TRITC, a red dye) according to the literature.36 These fluorescent silica particles were then used to create the diblock microparticles of different magneticratios. In Fig. 3, the FITC- or TRITC-labeled silica particles were only used in the non-magnetic block. Upon the application of an external magnetic field parallel to the substrate, the magnetic Janus microparticles were aligned into chain structures in water (Fig. 3a, b, d, and e). As seen in the optical (Fig. 3a) and fluorescence (Fig. 3b) microscopy images, FITC-labeled particles showed emission of green fluorescence, which was clear in the non-magnetic block. Similarly, TRITC-labeled magnetic Janus microparticles were assembled into chain structures and the red fluorescent signals were noticeably present in the non-magnetic block according to the magnetic/ non-magnetic ratio (Fig. 3d and e). When the magnetic field was applied perpendicular to the substrate, the microparticles flipped and became normal to the substrate, showing the bright green and
Fig. 3 Optical and fluorescence microscopy images of the magnetic Janus microparticles (magnetic/nonmagnetic length ratio, 9 : 1) including FITClabeled silica particles in the nonmagnetic block under an external magnetic field (a, b) parallel and (c) perpendicular to the substrate, respectively. Fluorescence microscopy images of the magnetic Janus microparticles containing TRITC-labeled silica particles with (d) 4 : 1 (magnetic/nonmagnetic) length ratio and (e) 1.5 : 1 length ratio and subjected to a magnetic field (d, e) parallel and (f) perpendicular to the substrate.
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Fig. 4 Fluorescence microscopy images of the magnetic diblock (a–d) and triblock (e and f) Janus microparticles labeled with different fluorescent molecules of variable lengths. The length ratios of FITC to TRITC labeled blocks are (a) 1 : 4, (b) 4 : 1, (c) 1 : 1. (d) Fluorescence microscopy image of the diblock microparticles shown to be (a) dispersed in water on a planar substrate and subjected to a magnetic field parallel to the substrate. (e, f) Fluorescence microscopy images of the magnetic triblock microparticles with different ratios of two dye-labeled blocks (2 : 2 : 1).
red emission colors, respectively, where the fluorescence labeled block was facing upwards (Fig. 3c and f). Our method is highly versatile. We can introduce different functionalities in the microparticles without changing their magnetic nature and assembly under the field. This was achieved by compartmentalization of silica colloids labeled by two different fluorescent dyes in different blocks. The rest of the procedure was the same as shown in Fig. 1, where the magnetic moment was aligned in the same direction along the long axis of the rod particle. The length of the fluorescently labeled block was controlled by the concentration of FITC- or TRITC-labeled silica suspension. Three different types of Janus rod particles were produced by precisely tuning the block lengths as shown in Fig. 4a–c. For example, the magnetic Janus microparticles with an FITC block length of 4 mm and a TRITC block length of 16 mm were synthesized (Fig. 4a, R = 4 : 1). While each block displayed respective fluorescence colors, the entire microparticle was magnetically responsive. As seen in Fig. 4d, they could be aligned in chain structures under the magnetic field parallel to the substrate (Fig. 4d), the same as the observation in Fig. 2a. Using the same procedure, we created three-component, or triblock-like microparticles consisting of both green and red colors in a single magnetic particle by sequential deposition of dye-labeled silica particles into the PDMS mold, while the magnetic moment in both ends were aligned in the same direction along the long axis of the rod. As shown in Fig. 4e and f, three blocks were well arranged into a single microparticle without any overlap. This strategy could be potentially applied to design new drug carriers where multiple drugs can be incorporated into a single carrier. To demonstrate the magneto-responsiveness of the microparticles and their potential application in display, we synthesized triblock-like particles of different chemistry and magnetic responsiveness in the adjacent blocks, that is the FITC labeled magnetic responsive block, the particle-free non-magnetic block, and the TRITC labeled magnetic responsive block (see fabrication details in the ESI,† Fig. S3). As shown in Fig. 5a and b, three different blocks were clearly compartmentalized within each single microparticle, which could reversibly rotate and flip freely. The introduction of a particle-free block was critical here to separate two dye-labeled, magnetic blocks, preventing interference
Chem. Commun., 2015, 51, 1639--1642 | 1641
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tailored size and functionality. B106 particles per cm2 was obtained in our system. However, by design of the template and scale up the wafer size containing the patterns, much larger number of particles can be obtained. These particles will be of great interest for potential applications, including biological sensors, drug carriers, single cell analysis, and field-responsive displays. The research is supported in part by the National Science Foundation (NSF)/EFRI awards, grant # EFRI-1038215 and #EFRI-1331583.
Notes and references
Fig. 5 (a, b) Fluorescence microscopy images of the magnetic Janus microparticles composed of three different blocks (FITC labeled/particle-free/TRITC labeled) dispersed in water on a planar substrate. The microparticles aligned in chain structures when the magnetic field was applied in parallel to the substrate. (c, d) Fluorescence microscopy images of the magneto-responsive display panel, where isolated triblock magnetic microparticles were aligned under an external magnetic field perpendicular to the substrate. The insets of (c) and (d) show a schematic illustration of the orientation of magnetic triblock microparticles under a magnetic field applied perpendicular to the substrate.
between two dye molecules. We then confined the aqueous dispersion of the triblock microparticles in a PDMS microwell in the shape of letter ‘‘N’’, which was replicated from a SU-8 master. When the external magnetic field was applied perpendicular to the microwell, the triblock magnetic microparticles flipped with the FITC-labeled block facing upwards, thus displaying the green letter ‘‘N’’ when illuminated by UV light (Fig. 5c). By reversing the magnetic field, the triblock magnetic microparticles re-oriented instantly, and displayed red emission in letter ‘‘N’’ under UV light (Fig. 5d). In summary, we have demonstrated a facile method to massproduce uniform magneto-responsive Janus microparticles of variable dimensions and chemical composition from a single PDMS mold. The well-controlled compartment filling of silica particles and iron oxide nanoparticles into the PDMS mold, followed by epoxy curing allowed us to create di- and tri-block microparticles in which the magnetic/ non-magnetic ratio could be precisely adjusted. The adjacent blocks had a clear boundary without any overlap, and therefore, they were highly responsive; they could be isolated and flipped freely and reversibly. When the magnetic field was applied in parallel to the substrate, the particles could be aligned to form the chain structures in the direction of the magnetic field. The degree of alignment, however, was found to be highly dependent on the length ratio of magnetic/ non-magnetic blocks. When the ratio was decreased to 1 : 4, the alignment was completely lost. Finally, we attempted to integrate magneto-responsiveness and fluorescent emission in a single particle. The use of silica particles in one of the blocks is especially advantageous since different functionalities, surface chemistry, and fluorescent dyes can be introduced into the silica block. We created triblocks with both ends consisting of silica particles and magneto responsiveness, which could flip in a confined geometry to display green and red colors under the magnetic field, respectively. Our strategy shown here is highly versatile and scalable to create field-responsive microparticles with
1642 | Chem. Commun., 2015, 51, 1639--1642
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