Photonic Crystals
Microfluidic Molding of Photonic Microparticles with Engraved Elastomeric Membranes Jae Young Sim, Jae-Hoon Choi, Jong-Min Lim, Soojeong Cho, Shin-Hyun Kim,* and Seung-Man Yang* This work is dedicated to Professor Yang, who passed away unexpectedly on September 26th 2013
A microfluidic approach to prepare photonic microparticles by repeated molding of photocurable colloidal suspension is reported. An elastomeric membrane with negative relieves which vertically separates two microfluidic channels is integrated; bottom channel is used for suspension flow, whereas water-filled top channel is used for pneumatic actuation of the membrane. Upon pressurization of the top channel, membrane is deformed to confine the suspension into its negative relieves, which is then polymerized by UV irradiation, making microparticles with mold shape. The microparticles are released from the mold by relieving the pneumatic pressure and flows through the bottom channel. This one cycle of molding, polymerization, and release can be repeatedly performed in microfluidic device of which pneumatic valves are actuated in a programmed manner. The microparticles exhibit structural colors when the suspension contains high concentration of silica nanoparticles; the nanoparticles form regular arrays and the microparticles reflect specific wavelength of light as a photonic crystals. The silica nanoparticles can be selectively removed to make pronounced structural colors. In addition, the microparticles can be further functionalized by embedding magnetic particles in the matrix of the microparticles, enabling the remote control of rotational motion of microparticles.
1. Introduction The periodic modulation of dielectric constant at half-wavelength of light exhibits opalescent colors through constructive interference of scattered light. Many creatures such as J. Y. Sim, Dr. J.-H. Choi, Dr. J.-M. Lim, S. Cho, Prof. S.-H. Kim, Prof. S.-M. Yang Department of Chemical and Biomolecular Engineering KAIST Daejeon 305–701, Korea E-mail: [email protected]; [email protected] J. Y. Sim, Dr. J.-H. Choi, Dr. J.-M. Lim, S. Cho, Prof. S.-M. Yang National Creative Research Initiative Center for Integrated Optofluidic Systems KAIST Daejeon 305–701, Korea DOI: 10.1002/smll.201400005 small 2014, DOI: 10.1002/smll.201400005
Morpho butterfly and peacock possess such periodic nanostructures and show beautiful structural colors. To mimic and use such structural colors, various approaches have been exploited.[1–4] Among them, colloidal self-assembly have provided economic and simple method to create three dimensional (3D) periodicity. However, conventional colloidal structures confined in a thin film limit the ease of processing and application area. To overcome such limitations, granular structure with internal periodic arrays of colloids have been prepared and intensively studied for their utilities in display color pigments[5] and colloidal barcoding systems.[6–8] The photonic granules are prepared by two distinct ways: Emulsion-templating and microfluidic flow lithography technique. In emulsion-templating method, monodisperse colloidal particles are confined in spherical emulsion drops and either of slow concentration of particles or interparticular repulsion leads to formation of spherical colloidal crystals.[9–12]
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Although this method provides high throughput, shape and optical property is limited to be isotropic and external fieldresponse is difficult to achieve. Microfluidic flow lithography have enabled the production of 2D microdisks whose shape is designed by photomask.[13,14] In addition, introduction of lock and release steps provides pseudo-3D microparticles.[15] By employing magnetic nanoparticles which form chainlike configuration under external magnetic field, photonic microdisks with a magnetic-response have been prepared by micromirror array-assisted flow lithography.[16,17] However, periodicity only occurs at 1D, limiting diversity of optical functionality and reflectivity. Therefore, there still remains a need for a new method to create photonic microparticles with 3D nanostructures for diverse functionalities in optical properties, shape and external field-response. In this paper, we report the microfluidic molding-featured microparticles with 3D internal periodic nanostructures using elastomeric membrane with negative relieves. The elastomeric membrane, separating flow channel and control channel, confines a photocurable suspension of silica particles into the negative relieves as it acts as monolithic microvalve.[18] Upon pressurization of the monolithic microvalve, the membrane collapses on the bottom of flow channel, thereby making ultra-thin film of suspension in all area except the negative relieves.[19] Therefore, UV irradiation on the valve polymerizes the resin only in the relieves, while ultra-thin film remains in a liquid state due to oxygen-inhibition. The polymerized microparticles are released from the mold by returning the valve off and flow through the bottom channel. In this method, microparticle shape is determined by that of the mold; this enables the featuring of microparticles with not only flat surface but even curved shape or potentially complex pattern depending on the shape of mold. In addition, silica particles in the resin form 3D arrays, which provide structural colors through constructive interference. The color can be more pronounced by selectively removing silica particles. Moreover, incorporation of magnetic particles leads to magnetic functionality, providing remote controllability over rotational motion of photonic microparticles.
2. Results and Discussion 2.1. Pneumatic Valve Actuation for Confinement of Photocurable Suspension Double-layered microfluidic device made of poly(dimethylsiloxane) (PDMS) is prepared by conventional photolithography and multilayer soft-lithography technique. The device is composed of two straight control channels on the top layer and T-shaped flow channel on the bottom layer as schematically illustrated in Figure 1a. One of control channel is positioned to be perpendicular to main flow channel, which is used for actuation of engraved elastomeric membrane for molding. The other control channel is positioned to be perpendicular to side flow channel which regulates the flow direction. To clearly show these three channels, we fill them with aqueous solution of food coloring dyes as shown in Figure 1b; red for two control channels and blue for flow
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channel, respectively. The pneumatic actuation of two control channels are shown in fluorescence microscope images of Figure 1c,d, where photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA) containing a red fluorescence dye, rhodamine 6G, is employed in the flow channel. To feed fresh resin into molding region (tagged with “A”), the molding valve is open and flow valve is closed at the same time as shown in Figure 1c, thereby making the resin flow only through main flow channel. For molding, the molding valve is closed and flow valve is open as shown in Figure 1d, thereby allowing the resin flows through side channel and preventing accumulation of pressure even for continuous feeding of photocurable suspension. The simultaneous operation of two pneumatic valves are precisely controlled by a JAVA-based programmable graphical user interface platform, which alternatively pressurizes the valve as needed with specific time intervals.[20,21] When the molding valve is open, the resin flows through overall volume of the channel as shown in Figure 1e, where negative relieves with circulardisk shape in the elastomeric membrane are also all occupied by resin as shown in the cross-sectional confocal microscope image of the inset. Upon actuation of molding valve, photocurable resin is isolated in all relieves shown in Figure 1f; the inset clearly shows that the resin is confined in the circular relieves and ultra-thin film of the resin might remain in the interstices between the relieves.
2.2. Repeated Fabrication of Polymeric Microparticles One cycle of microparticle fabrication is shown in Figure 2a–c. The photocurable resin which is continuously injected at constant flow rate of 100 μL/h can be isolated by actuation of molding valve at the pneumatic pressure of 15 psi as shown in Figure 2a, where 8 × 4 arrays of negative relief with circular disk shape are employed. Subsequent UV irradiation leads to photopolymerization of the isolated photocurable resin as shown in Figure 2b. During the polymerization, microdisks with slightly smaller than feature size are generated, while thin film of the resin on the surface of the PDMS mold remains unpolymerized as shown in the inset; this is attributed to oxygen-induced inhibition of polymerization.[22] This inhibition is beneficial for release of polymerized microparticles and complete isolation of the microparticles without formation of continuous film from ultra-thin residual layer of the resin in the interstices. This is confirmed as we relieve the applied pneumatic pressure and recover the undeformed membrane (Figure 2c). However, the oxygen-inhibition should be carefully controlled to provide a high definition of microparticles; smallest feature size of microparticles is determined by the thickness of the inhibition layer. In this work, the thickness is typically 5 μm as shown in Figure 2b and therefore, minimum radius of curvature is approximately 2.5 μm. This cycle can be repeated by automatic operation of control valves to improve production amount as shown in Movie S1 of the Supporting Information. Even for repeated molding process, all the microparticles are readily released with the flow stream. The resultant monodisperse circular microdisks are collected outside the channel as shown in
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Microfluidic Molding of Photonic Microparticles with Engraved Elastomeric Membranes
Figure 1. a) Schematic illustration of double-layered microfluidic devices which have one flow channel and two valve channels. b) Image of the microfluidic device. c,d) Fluorescence microscope images showing valve actuation and control of flow direction; c) photocurable resin flows through main channel for closed flow valve and open molding valve and d) the resin flows through side channel for open flow valve and closed molding valve. Molding region is tagged with ‘A’ and scale bars are 500 µm. e,f) Fluorescence microscope images showing molding region; e) resin flows for open molding valve and f) confinement of the resin upon molding valve actuation. Insets are confocal microscope images showing the cross-sections along the denoted lines. Scale bars are 100 µm.
Figure 2d. In this case, one cycle takes 6 s and therefore production rate is 320 particles per minute. The production rate can be increased by using the membrane with a larger number of relieves and decreasing the time required for one cycle; polymerization can be done in 0.5 s and resultant particles can be released in 1 s. Shape of microparticles is determined by that of mold. Although oxygen inhibition makes the particles slightly blunt, overall shape of particles is maintained if the smallest feature size of mold is larger than thickness of inhibition layer; otherwise, the smallest feature disappears during the polymerization. Therefore, we can prepare microparticles with distinct shapes. For example, circular, square, triangular, and stellate microdisks are prepared by using four distinct molds with same shape as shown in Figure 3a–d, respectively; the minimum size of microparticles containing such small 2014, DOI: 10.1002/smll.201400005
sharp corners is much larger than the thickness of oxygeninhibition, while the minimum size of circular disk can be much reduced because isotropic oxygen-inhibition enables the particles to maintain their circular shape. In addition, overall shape is not limited to disk, but more complicate morphologies can be achieved by employing molds with the corresponding negative profile. As simplest examples, we prepared microparticles whose top or side surface is not flat but curved as shown in Figure 4; the molds are prepared by thermal annealing of photolithographicallyfeatured structures made of positive photoresist and subsequent soft-lithography (see the Supporting Information for details). When we use mold with hemispherical crater, the resultant microparticles have hemispherical shape as shown in Figure 4b; resin confined in the mold is shown in the inset of Figure 4a. In the same fashion, triangular microparticles
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where d is spacing between (111) planes, neff is effective refractive index, and np and nm are the refractive indices of particles and medium, respectively. For D = 193 nm, φ = 0.35, np = 1.45 and nm = 1.4689, above equation provides λ = 591 nm, which slightly larger than peak wavelength; we attribute this small deviation (≈1.5%) to imperfect arrangement of particles to fcc lattice. The weak color is because of very small contrast of refractive index between silica (nsilica = 1.45) and ETPTA (nETPTA = 1.4689). Therefore, the color can be more pronounced by selective wet-etching of silica nanoparticles with a HF solution; the selective removal of silica nanoparticles leaves behind regular air cavities in the polymerized ETPTA matrix, which makes large refractive index contrast between scatters (nair = 1) and matrix and widens the stop band-width. During the etching, effective refractive index decreases, which leads to blue-shift of structural color. Figure 2. a–c) Still shot images and corresponding schemes showing a) isolation of resin, b) photopolymerization, and c) release of resulting microparticles. Inset in (b) shows formation Increase of reflectivity, widening of bandof microparticle within a mold where thin layer of resin on the surface of the mold remains width, and blue-shift of reflection color are unpolymerized. d) Optical microscope image of monodisperse circular microdisks which are all confirmed with a green curve and corcollected outside the channel. Scale bars are 100 µm. responding inset in Figure 5a; for np = 1, the Equation 1 provides λ = 535 nm, with inclined side wall and stellate microparticles with which is slightly larger than the peak wavelength of 520 nm. convex surface can be prepared as shown in Figure 4c and d, The cavity arrays are observed in a broken microparticle respectively. as shown in Figure 5b, where outer surface of microparticle do not have any pores. As we expected, structural color of 2.3. Fabrication of Photonic Microparticles with Structural Colors Monodisperse silica nanoparticles dispersed in ETPTA form nonclose-packed face-centered cubic (fcc) structures, which produces structural colors.[23] Therefore, microparticles with 3D periodic internal nanostructures and structural colors can be prepared by employing a photocurable suspension of silica nanoparticles in microfluidic molding process, instead of particlefree resin. For example, microparticle made from silica-ETPTA suspension with silica diameter, D, of 193 nm and volume fraction, φ, of 0.35 exhibit weak reddish colors and reflectance peak at 582 nm as shown in red curve of Figure 5a and corresponding inset. This peak position can be estimated with Bragg’s diffraction for stacked (111) planes of nonclose-packed fcc lattice: ⎛ π ⎞ λ = 2 dneff = ⎜ ⎝ 3 2φ ⎟⎠
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() 8 3
D( n 2pφ + n 2m (1 − φ ))
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Figure 3. a–d) Scanning electron microscope (SEM) images of microdisks with a) circular, b) square, c) triangular, and d) stellate shapes. Scale bars are all 20 µm. The insets show the corresponding fluorescence microscope images of molding region under actuation of molding valve. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Microfluidic Molding of Photonic Microparticles with Engraved Elastomeric Membranes
Figure 4. a) Fluorescence microscope image of molding region under valve actuation where array of hemispherical craters is used for molding. Inset shows cross-sectional confocal microscope image along with denoted line. b–d) SEM images of b) hemispherical microparticle, c) triangular microparticle with inclined side wall d) stellate microparticle with convex surface. Scale bars in (a) and (b–d) are 100 µm and 200 µm, respectively.
porous microparticles can be adjusted by employing different size of silica particles at the same volume fraction. For example, blue- and green- colored photonic microparticles are prepared from silica nanoparticles with D = 166 nm and 200 nm, respectively as shown in Figure 4c. Shape of photonic microparticles can be controlled by shape of mold in the same manner. We prepare square, circular, and triangular photonic microparticles with two distinct structural colors as shown in Figure 5d. It is noteworthy that the photocurable resin is not limited to ETPTA. Any photocurable resin which has highly polar acrylate group and similar refractive index to silica particles can induce a repulsive interparticle potential through the disjoining pressure of the solvation layer and weak electrostatic interactions; low refractive index contrast imposes diminishing van der Waals attraction. Under the repulsive interaction, silica particles form nonclose-packed fcc structures at high volume fraction. For illustrative purpose, we select additional three resins which are
Figure 5. a) The reflection spectra and optical microscope images of composite and porous photonic microparticles, where silica-ETPTA suspension with nanoparticle diameter of 193 nm and volume fraction of 0.35 is used. b) SEM image of a broken porous microparticle whose outer surface is smooth and interior contains air cavity arrays. Inset shows intact porous microparticles. Scale bar is 1 µm. c) The reflection spectra and optical microscope images of two distinct porous photonic microparticles which are made from silica nanoparticles with diameter of 166 nm and 200 nm at the same volume fraction of 0.35, respectively. d) Optical microscope images showing a collection of photonic microparticles. Scale bar is 100 µm. small 2014, DOI: 10.1002/smll.201400005
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poly(ethylene glycol) diacrylate (PEGDA: nPEGDA = 1.466), pentaerythritol triacrylate (PETA: nPETA = 1.483) and trimethylolpropane triacrylate (TMPTA: nTMPTA = 1.474) and prepare photonic crystal films using silica suspension. In all three cases, nonclose-packed fcc structures are prepared and pronounced structural colors are observed after removing silica nanoparticles from the polymerized matrix as shown in Figure S1 of the Supporting Information. Therefore, these combinations of silica and photocurable resin can be used for preparation of photonic microparticles.
2.4. Magnetic Manipulation of Photonic Microparticles Photonic microparticles can be rendered to be magnetoresponsive by incorporating magnetic particles. To accomplish this, barium ferrite (BaFe12O19) particles with diameter of 1 μm are additionally dispersed in silica-ETPTA suspension at low weight fraction of 0.5. Such a low fraction of the barium ferrite particles insignificantly influences the structural colors. The barium ferrite particles are weakly ferromagnetic, which can be aligned within the suspension by external magnetic field before polymerization as schematically illustrated in Figure 6a. Subsequent polymerization permanently fixes the orientation of all magnetic particles, which makes net magnetic moment in the molding-featured microparticles.[24] Therefore, microparticles can align along the direction of external magnetic field afterward and their rotational motion can be remotely controlled by the rotating magnetic field as shown in Figure 6b. The photonic microparticles can exhibit different appearance depending on alignment relative to direction of view. When the flat top or bottom surfaces are positioned to be perpendicular to the view, structural color from stacked (111) planes dominates; the pronounced reddish color appears in the first image of Figure 6c because of this. When the angle between surface normal and direction of view increases, the reflectivity decreases and the color changes slightly as shown in the second image. When the side surface of the microparticles faces to the view, the different color appears due to contribution of other crystal planes as shown in the last image. Thus, the magnetic manipulation of rotational motion could switch the reflection colors or reflectivity.
variety of shape and top surface profile can be prepared by corresponding negative relieves and the repeated production of the uniform microparticles can be achieved by automated actuation of molding and flow valves. In addition, structural color can be introduced into microparticles by forming internal nanostructures with silica particles. Especially, the colors become more pronounced by selective removal of silica particles. The photonic microparticles are further functionalized to be magneto-responsive, which enables the remote control of rotational motion of microparticles. This microfluidic molding technique to make photonic microparticles with desired shape and functionality will provide new opportunities to color displays operated at reflection mode and colloidal barcoding systems for bioassays.
4. Experimental Section Fabrication of Microfluidic Chip: PDMS microfluidic device with engraved elastomeric membrane is prepared using conventional photolithography using a negative photoresist (SU-8 10, Microchem) and multilayer soft-lithography. Three photomasks are used to prepare photoresist patterns; two of them are used for making flow channel and control channel, respectively and one mask is used to make the elastomeric membrane with negative relief structures. To make a flow channel whose top surface is engraved elastomeric membrane, two-step photolithography is performed. A negative photoresist (SU-8 10) is spin-coated on the silicon wafer at 1000 rpm to make a uniform film of 30 µm in thickness. After soft baking at 65 °C for 3 min and at 95 °C for 7 min, the photoresist film is exposed to UV light (wavelength 365 nm) through
3. Conclusions We develop a microfluidic approach to make photonic microparticles. With engraved elastomeric membrane integrated in double-layered microfluidic device, photocurable suspension are repeatedly molded, polymerized, and released to fabricate microparticles. A
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Figure 6. a) Schematic illustration for fabrication of magneto-responsive photonic microparticles. b,c) Optical microscope images showing the rotational motion of the photonic microparticles, where rotating external magnetic field is used. Images are taken at b) transmission mode and c) reflection mode. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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photomask for flow channel. The film is then baked again at 65 °C for 1 min and 95 °C for 3 min. Without development, the second SU-8 layer is spin-coated on the first layer at 2000 rpm to create an additional film with thickness of 20 µm. Then, soft baking is done at 65 °C for 2 min and at 95 °C for 5 min and UV is irradiated through the second photomask for making negative relieves. After another baking step, SU-8 film is subjected to development using propylene glycol monomethyl acetate (PGMEA, Aldrich) and rinsed with isopropyl alcohol (Merck). A mixture of PDMS base and curing agent in a 10:1 ratio is spin-coated on the prepared SU-8 pattern at 1000 rpm to make 20-µm-thick film of PDMS on the top of the pattern; the thickness is carefully selected to make the film rigid enough to prevent bending without pressurizing the valve and flexible enough to completely close the main channel with a moderate pressure. The PDMS and SU-8 pattern is cured for 2 h in a convection oven. Another SU-8 pattern for the control channel is fabricated by single-step photolithography and PDMS control channel is molded from the pattern. Subsequently, PDMS control channel is positioned on the top of PDMS-coated SU-8 mold for flow channel and bonded, where two channels are treated by oxygen plasma for 10 s before assembly and carefully aligned under observation with optical microscope. Finally, whole PDMS is peeled off from SU-8 mold and bonded with PDMS-coated glass slide after oxygen plasma treatment. Preparation of Photocurable Suspension: Monodisperse silica particles with different sizes are synthesized using Stöber method.[25] Silica particles are dispersed in ethanol and mixed with ETPTA (Aldrich) containing 5 wt% photoinitiator (Darocur 1173, Ciba chemical). Ethanol was selectively evaporated from the mixture in convection oven at 70 °C and we obtain photocurable suspension of silica particles. For all other resins of PEGDA, PETA, and TMPTA, we prepare suspension in an exactly same manner. To provide magnetic functionality, barium ferrite (BaFe12O19) particles with diameter of 1 µm are additionally dispersed into the ethanolic silica suspension and the suspension is subjected to same procedures to silica-ETPTA suspension. For observation with fluorescence and confocal microscopes, rhodamine 6G (Aldrich) is mixed with ETPTA solution containing photoinitiator. Synthesis and Characterization of Photonic Crystals: To polymerize photocurable resin confined in molds, we irradiate UV light from a mercury lamp mounted in an inverted optical microscope (Nikon, TE2000-U). To remove silica particles from polymerized matrix, microparticles are immersed in 5% hydrofluoric acid (Aldrich) for 12 h. Reflectance spectra of photonic microparticles are characterized by optical spectrometer (Ocean Optics, USB4000) mounted in optical microscope (Nikon, L150)
Supporting Information
Acknowledgements This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Education, Science, and Technology for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems”, and Mid-career Researcher Program (2014R1A2A2A01005813) through NRF grant funded by the MEST. The authors appreciate Jung Yoon Seo for providing barium ferrite particles.
[1] S. H. Kim, S. Y. Lee, S. M. Yang, G. R. Yi, NPG Asia Mater. 2011, 3, 25. [2] J. P. Ge, Y. D. Yin, Angew. Chem. In. Ed. 2011, 50, 1492. [3] J. F. Galisteo-Lopez, M. Ibisate, R. Sapienza, L. S. Froufe-Perez, A. Blanco, C. Lopez, Adv. Mater. 2011, 23, 30. [4] J. H. Moon, S. M. Yang, Chem. Rev. 2010, 110, 547. [5] S. H. Kim, J. M. Lim, W. C. Jeong, D. G. Choi, S. M. Yang, Adv. Mater. 2008, 20, 3211. [6] B. C. Tang, X. W. Zhao, W. D. Zhang, Q. R. Wang, L. F. Kong, Z. Z. Gu, Langmuir 2011, 27, 11722. [7] S. H. Kim, J. W. Shim, S. M. Yang, Angew. Chem. In. Ed. 2011, 50, 1171. [8] Y. J. Zhao, X. W. Zhao, C. Sun, J. Li, R. Zhu, Z. Z. Gu, Anal. Chem. 2008, 80, 1598. [9] O. D. Velev, A. M. Lenhoff, E. W. Kaler, Science 2000, 287, 2240. [10] S. H. Kim, S. Y. Lee, G. R. Yi, D. J. Pine, S. M. Yang, J. Am. Chem. Soc. 2006, 128, 10897. [11] S. H. Kim, Y. S. Cho, S. J. Jeon, T. H. Eun, G. R. Yi, S. M. Yang, Adv. Mater. 2008, 20, 3268. [12] S. H. Kim, J. G. Park, T. M. Choi, V. N. Manoharan, D. A. Weitz, Nat. Commun. 2014, 5, 3068. [13] D. Dendukuri, D. C. Pregibon, J. Collins, T. A. Hatton, P. S. Doyle, Nat. Mater. 2006, 5, 365. [14] S. E. Chung, W. Park, H. Park, K. Yu, N. Park, S. Kwon, Appl. Phys. Lett. 2007, 4, 041106. [15] K. W. Bong, D. C. Pregibon, P. S. Doyle, Lab Chip 2009, 9, 863. [16] H. Kim, J. Ge, J. Kim, S. E. Choi, H. Lee, H. Lee, W. Park, Y. Yin, S. Kwon, Nat. Photonics 2009, 3, 534. [17] H. Lee, J. Kim, H. Kim, J. Kim, S. Kwon, Nat. Mater. 2010, 9, 745. [18] T. Thorsen, S. J. Maerkl, S. R. Quake, Science 2002, 298, 580. [19] S. A. Vanapalli, D. Wijnperle, A. van den Berg, F. Mugele, M. H. G. Duits, Lab Chip 2009, 9, 1461. [20] J. M. Lim, J. P. Urbanski, T. Thorsen, S. M. Yang, Appl. Phys. Lett. 2011, 98, 044101. [21] J. P. Urbanski, W. Thies, C. Rhodes, S. Amarasinghe, T. Thorsen, Lab Chip 2006, 6, 96. [22] D. Dendukuri, P. Panda, R. Haghgooie, J. M. Kim, T. A. Hatton, P. S. Doyle, Macromolecules 2008, 41, 8547. [23] S. H. Kim, S. J. Jeon, G. R. Yi, C. J. Heo, J. H. Choi, S. M. Yang, Adv. Mater. 2008, 20, 1649. [24] S. H. Kim, J. Y. Sim, J. M. Lim, S. M. Yang, Angew. Chem. In. Ed. 2010, 49, 3786. [25] W. Stober, A. Fink, E. Bohn, J. Colloid. Interf. Sci. 1968, 26, 62.
Supporting Information is available from the Wiley Online Library or from the author.
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Microfluidic Molding of Photonic Microparticles with Engraved Elastomeric Membranes Jae Young Sim, Jae-Hoon Choi, Jong-Min Lim, Soojeong Cho, Shin-Hyun Kim,* and Seung-Man Yang* This work is dedicated to Professor Yang, who passed away unexpectedly on September 26th 2013
A microfluidic approach to prepare photonic microparticles by repeated molding of photocurable colloidal suspension is reported. An elastomeric membrane with negative relieves which vertically separates two microfluidic channels is integrated; bottom channel is used for suspension flow, whereas water-filled top channel is used for pneumatic actuation of the membrane. Upon pressurization of the top channel, membrane is deformed to confine the suspension into its negative relieves, which is then polymerized by UV irradiation, making microparticles with mold shape. The microparticles are released from the mold by relieving the pneumatic pressure and flows through the bottom channel. This one cycle of molding, polymerization, and release can be repeatedly performed in microfluidic device of which pneumatic valves are actuated in a programmed manner. The microparticles exhibit structural colors when the suspension contains high concentration of silica nanoparticles; the nanoparticles form regular arrays and the microparticles reflect specific wavelength of light as a photonic crystals. The silica nanoparticles can be selectively removed to make pronounced structural colors. In addition, the microparticles can be further functionalized by embedding magnetic particles in the matrix of the microparticles, enabling the remote control of rotational motion of microparticles.
1. Introduction The periodic modulation of dielectric constant at half-wavelength of light exhibits opalescent colors through constructive interference of scattered light. Many creatures such as J. Y. Sim, Dr. J.-H. Choi, Dr. J.-M. Lim, S. Cho, Prof. S.-H. Kim, Prof. S.-M. Yang Department of Chemical and Biomolecular Engineering KAIST Daejeon 305–701, Korea E-mail: [email protected]; [email protected] J. Y. Sim, Dr. J.-H. Choi, Dr. J.-M. Lim, S. Cho, Prof. S.-M. Yang National Creative Research Initiative Center for Integrated Optofluidic Systems KAIST Daejeon 305–701, Korea DOI: 10.1002/smll.201400005 small 2014, DOI: 10.1002/smll.201400005
Morpho butterfly and peacock possess such periodic nanostructures and show beautiful structural colors. To mimic and use such structural colors, various approaches have been exploited.[1–4] Among them, colloidal self-assembly have provided economic and simple method to create three dimensional (3D) periodicity. However, conventional colloidal structures confined in a thin film limit the ease of processing and application area. To overcome such limitations, granular structure with internal periodic arrays of colloids have been prepared and intensively studied for their utilities in display color pigments[5] and colloidal barcoding systems.[6–8] The photonic granules are prepared by two distinct ways: Emulsion-templating and microfluidic flow lithography technique. In emulsion-templating method, monodisperse colloidal particles are confined in spherical emulsion drops and either of slow concentration of particles or interparticular repulsion leads to formation of spherical colloidal crystals.[9–12]
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Although this method provides high throughput, shape and optical property is limited to be isotropic and external fieldresponse is difficult to achieve. Microfluidic flow lithography have enabled the production of 2D microdisks whose shape is designed by photomask.[13,14] In addition, introduction of lock and release steps provides pseudo-3D microparticles.[15] By employing magnetic nanoparticles which form chainlike configuration under external magnetic field, photonic microdisks with a magnetic-response have been prepared by micromirror array-assisted flow lithography.[16,17] However, periodicity only occurs at 1D, limiting diversity of optical functionality and reflectivity. Therefore, there still remains a need for a new method to create photonic microparticles with 3D nanostructures for diverse functionalities in optical properties, shape and external field-response. In this paper, we report the microfluidic molding-featured microparticles with 3D internal periodic nanostructures using elastomeric membrane with negative relieves. The elastomeric membrane, separating flow channel and control channel, confines a photocurable suspension of silica particles into the negative relieves as it acts as monolithic microvalve.[18] Upon pressurization of the monolithic microvalve, the membrane collapses on the bottom of flow channel, thereby making ultra-thin film of suspension in all area except the negative relieves.[19] Therefore, UV irradiation on the valve polymerizes the resin only in the relieves, while ultra-thin film remains in a liquid state due to oxygen-inhibition. The polymerized microparticles are released from the mold by returning the valve off and flow through the bottom channel. In this method, microparticle shape is determined by that of the mold; this enables the featuring of microparticles with not only flat surface but even curved shape or potentially complex pattern depending on the shape of mold. In addition, silica particles in the resin form 3D arrays, which provide structural colors through constructive interference. The color can be more pronounced by selectively removing silica particles. Moreover, incorporation of magnetic particles leads to magnetic functionality, providing remote controllability over rotational motion of photonic microparticles.
2. Results and Discussion 2.1. Pneumatic Valve Actuation for Confinement of Photocurable Suspension Double-layered microfluidic device made of poly(dimethylsiloxane) (PDMS) is prepared by conventional photolithography and multilayer soft-lithography technique. The device is composed of two straight control channels on the top layer and T-shaped flow channel on the bottom layer as schematically illustrated in Figure 1a. One of control channel is positioned to be perpendicular to main flow channel, which is used for actuation of engraved elastomeric membrane for molding. The other control channel is positioned to be perpendicular to side flow channel which regulates the flow direction. To clearly show these three channels, we fill them with aqueous solution of food coloring dyes as shown in Figure 1b; red for two control channels and blue for flow
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channel, respectively. The pneumatic actuation of two control channels are shown in fluorescence microscope images of Figure 1c,d, where photocurable resin of ethoxylated trimethylolpropane triacrylate (ETPTA) containing a red fluorescence dye, rhodamine 6G, is employed in the flow channel. To feed fresh resin into molding region (tagged with “A”), the molding valve is open and flow valve is closed at the same time as shown in Figure 1c, thereby making the resin flow only through main flow channel. For molding, the molding valve is closed and flow valve is open as shown in Figure 1d, thereby allowing the resin flows through side channel and preventing accumulation of pressure even for continuous feeding of photocurable suspension. The simultaneous operation of two pneumatic valves are precisely controlled by a JAVA-based programmable graphical user interface platform, which alternatively pressurizes the valve as needed with specific time intervals.[20,21] When the molding valve is open, the resin flows through overall volume of the channel as shown in Figure 1e, where negative relieves with circulardisk shape in the elastomeric membrane are also all occupied by resin as shown in the cross-sectional confocal microscope image of the inset. Upon actuation of molding valve, photocurable resin is isolated in all relieves shown in Figure 1f; the inset clearly shows that the resin is confined in the circular relieves and ultra-thin film of the resin might remain in the interstices between the relieves.
2.2. Repeated Fabrication of Polymeric Microparticles One cycle of microparticle fabrication is shown in Figure 2a–c. The photocurable resin which is continuously injected at constant flow rate of 100 μL/h can be isolated by actuation of molding valve at the pneumatic pressure of 15 psi as shown in Figure 2a, where 8 × 4 arrays of negative relief with circular disk shape are employed. Subsequent UV irradiation leads to photopolymerization of the isolated photocurable resin as shown in Figure 2b. During the polymerization, microdisks with slightly smaller than feature size are generated, while thin film of the resin on the surface of the PDMS mold remains unpolymerized as shown in the inset; this is attributed to oxygen-induced inhibition of polymerization.[22] This inhibition is beneficial for release of polymerized microparticles and complete isolation of the microparticles without formation of continuous film from ultra-thin residual layer of the resin in the interstices. This is confirmed as we relieve the applied pneumatic pressure and recover the undeformed membrane (Figure 2c). However, the oxygen-inhibition should be carefully controlled to provide a high definition of microparticles; smallest feature size of microparticles is determined by the thickness of the inhibition layer. In this work, the thickness is typically 5 μm as shown in Figure 2b and therefore, minimum radius of curvature is approximately 2.5 μm. This cycle can be repeated by automatic operation of control valves to improve production amount as shown in Movie S1 of the Supporting Information. Even for repeated molding process, all the microparticles are readily released with the flow stream. The resultant monodisperse circular microdisks are collected outside the channel as shown in
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Figure 1. a) Schematic illustration of double-layered microfluidic devices which have one flow channel and two valve channels. b) Image of the microfluidic device. c,d) Fluorescence microscope images showing valve actuation and control of flow direction; c) photocurable resin flows through main channel for closed flow valve and open molding valve and d) the resin flows through side channel for open flow valve and closed molding valve. Molding region is tagged with ‘A’ and scale bars are 500 µm. e,f) Fluorescence microscope images showing molding region; e) resin flows for open molding valve and f) confinement of the resin upon molding valve actuation. Insets are confocal microscope images showing the cross-sections along the denoted lines. Scale bars are 100 µm.
Figure 2d. In this case, one cycle takes 6 s and therefore production rate is 320 particles per minute. The production rate can be increased by using the membrane with a larger number of relieves and decreasing the time required for one cycle; polymerization can be done in 0.5 s and resultant particles can be released in 1 s. Shape of microparticles is determined by that of mold. Although oxygen inhibition makes the particles slightly blunt, overall shape of particles is maintained if the smallest feature size of mold is larger than thickness of inhibition layer; otherwise, the smallest feature disappears during the polymerization. Therefore, we can prepare microparticles with distinct shapes. For example, circular, square, triangular, and stellate microdisks are prepared by using four distinct molds with same shape as shown in Figure 3a–d, respectively; the minimum size of microparticles containing such small 2014, DOI: 10.1002/smll.201400005
sharp corners is much larger than the thickness of oxygeninhibition, while the minimum size of circular disk can be much reduced because isotropic oxygen-inhibition enables the particles to maintain their circular shape. In addition, overall shape is not limited to disk, but more complicate morphologies can be achieved by employing molds with the corresponding negative profile. As simplest examples, we prepared microparticles whose top or side surface is not flat but curved as shown in Figure 4; the molds are prepared by thermal annealing of photolithographicallyfeatured structures made of positive photoresist and subsequent soft-lithography (see the Supporting Information for details). When we use mold with hemispherical crater, the resultant microparticles have hemispherical shape as shown in Figure 4b; resin confined in the mold is shown in the inset of Figure 4a. In the same fashion, triangular microparticles
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where d is spacing between (111) planes, neff is effective refractive index, and np and nm are the refractive indices of particles and medium, respectively. For D = 193 nm, φ = 0.35, np = 1.45 and nm = 1.4689, above equation provides λ = 591 nm, which slightly larger than peak wavelength; we attribute this small deviation (≈1.5%) to imperfect arrangement of particles to fcc lattice. The weak color is because of very small contrast of refractive index between silica (nsilica = 1.45) and ETPTA (nETPTA = 1.4689). Therefore, the color can be more pronounced by selective wet-etching of silica nanoparticles with a HF solution; the selective removal of silica nanoparticles leaves behind regular air cavities in the polymerized ETPTA matrix, which makes large refractive index contrast between scatters (nair = 1) and matrix and widens the stop band-width. During the etching, effective refractive index decreases, which leads to blue-shift of structural color. Figure 2. a–c) Still shot images and corresponding schemes showing a) isolation of resin, b) photopolymerization, and c) release of resulting microparticles. Inset in (b) shows formation Increase of reflectivity, widening of bandof microparticle within a mold where thin layer of resin on the surface of the mold remains width, and blue-shift of reflection color are unpolymerized. d) Optical microscope image of monodisperse circular microdisks which are all confirmed with a green curve and corcollected outside the channel. Scale bars are 100 µm. responding inset in Figure 5a; for np = 1, the Equation 1 provides λ = 535 nm, with inclined side wall and stellate microparticles with which is slightly larger than the peak wavelength of 520 nm. convex surface can be prepared as shown in Figure 4c and d, The cavity arrays are observed in a broken microparticle respectively. as shown in Figure 5b, where outer surface of microparticle do not have any pores. As we expected, structural color of 2.3. Fabrication of Photonic Microparticles with Structural Colors Monodisperse silica nanoparticles dispersed in ETPTA form nonclose-packed face-centered cubic (fcc) structures, which produces structural colors.[23] Therefore, microparticles with 3D periodic internal nanostructures and structural colors can be prepared by employing a photocurable suspension of silica nanoparticles in microfluidic molding process, instead of particlefree resin. For example, microparticle made from silica-ETPTA suspension with silica diameter, D, of 193 nm and volume fraction, φ, of 0.35 exhibit weak reddish colors and reflectance peak at 582 nm as shown in red curve of Figure 5a and corresponding inset. This peak position can be estimated with Bragg’s diffraction for stacked (111) planes of nonclose-packed fcc lattice: ⎛ π ⎞ λ = 2 dneff = ⎜ ⎝ 3 2φ ⎟⎠
1
3
() 8 3
D( n 2pφ + n 2m (1 − φ ))
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1
2
(1)
Figure 3. a–d) Scanning electron microscope (SEM) images of microdisks with a) circular, b) square, c) triangular, and d) stellate shapes. Scale bars are all 20 µm. The insets show the corresponding fluorescence microscope images of molding region under actuation of molding valve. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Figure 4. a) Fluorescence microscope image of molding region under valve actuation where array of hemispherical craters is used for molding. Inset shows cross-sectional confocal microscope image along with denoted line. b–d) SEM images of b) hemispherical microparticle, c) triangular microparticle with inclined side wall d) stellate microparticle with convex surface. Scale bars in (a) and (b–d) are 100 µm and 200 µm, respectively.
porous microparticles can be adjusted by employing different size of silica particles at the same volume fraction. For example, blue- and green- colored photonic microparticles are prepared from silica nanoparticles with D = 166 nm and 200 nm, respectively as shown in Figure 4c. Shape of photonic microparticles can be controlled by shape of mold in the same manner. We prepare square, circular, and triangular photonic microparticles with two distinct structural colors as shown in Figure 5d. It is noteworthy that the photocurable resin is not limited to ETPTA. Any photocurable resin which has highly polar acrylate group and similar refractive index to silica particles can induce a repulsive interparticle potential through the disjoining pressure of the solvation layer and weak electrostatic interactions; low refractive index contrast imposes diminishing van der Waals attraction. Under the repulsive interaction, silica particles form nonclose-packed fcc structures at high volume fraction. For illustrative purpose, we select additional three resins which are
Figure 5. a) The reflection spectra and optical microscope images of composite and porous photonic microparticles, where silica-ETPTA suspension with nanoparticle diameter of 193 nm and volume fraction of 0.35 is used. b) SEM image of a broken porous microparticle whose outer surface is smooth and interior contains air cavity arrays. Inset shows intact porous microparticles. Scale bar is 1 µm. c) The reflection spectra and optical microscope images of two distinct porous photonic microparticles which are made from silica nanoparticles with diameter of 166 nm and 200 nm at the same volume fraction of 0.35, respectively. d) Optical microscope images showing a collection of photonic microparticles. Scale bar is 100 µm. small 2014, DOI: 10.1002/smll.201400005
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poly(ethylene glycol) diacrylate (PEGDA: nPEGDA = 1.466), pentaerythritol triacrylate (PETA: nPETA = 1.483) and trimethylolpropane triacrylate (TMPTA: nTMPTA = 1.474) and prepare photonic crystal films using silica suspension. In all three cases, nonclose-packed fcc structures are prepared and pronounced structural colors are observed after removing silica nanoparticles from the polymerized matrix as shown in Figure S1 of the Supporting Information. Therefore, these combinations of silica and photocurable resin can be used for preparation of photonic microparticles.
2.4. Magnetic Manipulation of Photonic Microparticles Photonic microparticles can be rendered to be magnetoresponsive by incorporating magnetic particles. To accomplish this, barium ferrite (BaFe12O19) particles with diameter of 1 μm are additionally dispersed in silica-ETPTA suspension at low weight fraction of 0.5. Such a low fraction of the barium ferrite particles insignificantly influences the structural colors. The barium ferrite particles are weakly ferromagnetic, which can be aligned within the suspension by external magnetic field before polymerization as schematically illustrated in Figure 6a. Subsequent polymerization permanently fixes the orientation of all magnetic particles, which makes net magnetic moment in the molding-featured microparticles.[24] Therefore, microparticles can align along the direction of external magnetic field afterward and their rotational motion can be remotely controlled by the rotating magnetic field as shown in Figure 6b. The photonic microparticles can exhibit different appearance depending on alignment relative to direction of view. When the flat top or bottom surfaces are positioned to be perpendicular to the view, structural color from stacked (111) planes dominates; the pronounced reddish color appears in the first image of Figure 6c because of this. When the angle between surface normal and direction of view increases, the reflectivity decreases and the color changes slightly as shown in the second image. When the side surface of the microparticles faces to the view, the different color appears due to contribution of other crystal planes as shown in the last image. Thus, the magnetic manipulation of rotational motion could switch the reflection colors or reflectivity.
variety of shape and top surface profile can be prepared by corresponding negative relieves and the repeated production of the uniform microparticles can be achieved by automated actuation of molding and flow valves. In addition, structural color can be introduced into microparticles by forming internal nanostructures with silica particles. Especially, the colors become more pronounced by selective removal of silica particles. The photonic microparticles are further functionalized to be magneto-responsive, which enables the remote control of rotational motion of microparticles. This microfluidic molding technique to make photonic microparticles with desired shape and functionality will provide new opportunities to color displays operated at reflection mode and colloidal barcoding systems for bioassays.
4. Experimental Section Fabrication of Microfluidic Chip: PDMS microfluidic device with engraved elastomeric membrane is prepared using conventional photolithography using a negative photoresist (SU-8 10, Microchem) and multilayer soft-lithography. Three photomasks are used to prepare photoresist patterns; two of them are used for making flow channel and control channel, respectively and one mask is used to make the elastomeric membrane with negative relief structures. To make a flow channel whose top surface is engraved elastomeric membrane, two-step photolithography is performed. A negative photoresist (SU-8 10) is spin-coated on the silicon wafer at 1000 rpm to make a uniform film of 30 µm in thickness. After soft baking at 65 °C for 3 min and at 95 °C for 7 min, the photoresist film is exposed to UV light (wavelength 365 nm) through
3. Conclusions We develop a microfluidic approach to make photonic microparticles. With engraved elastomeric membrane integrated in double-layered microfluidic device, photocurable suspension are repeatedly molded, polymerized, and released to fabricate microparticles. A
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Figure 6. a) Schematic illustration for fabrication of magneto-responsive photonic microparticles. b,c) Optical microscope images showing the rotational motion of the photonic microparticles, where rotating external magnetic field is used. Images are taken at b) transmission mode and c) reflection mode. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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photomask for flow channel. The film is then baked again at 65 °C for 1 min and 95 °C for 3 min. Without development, the second SU-8 layer is spin-coated on the first layer at 2000 rpm to create an additional film with thickness of 20 µm. Then, soft baking is done at 65 °C for 2 min and at 95 °C for 5 min and UV is irradiated through the second photomask for making negative relieves. After another baking step, SU-8 film is subjected to development using propylene glycol monomethyl acetate (PGMEA, Aldrich) and rinsed with isopropyl alcohol (Merck). A mixture of PDMS base and curing agent in a 10:1 ratio is spin-coated on the prepared SU-8 pattern at 1000 rpm to make 20-µm-thick film of PDMS on the top of the pattern; the thickness is carefully selected to make the film rigid enough to prevent bending without pressurizing the valve and flexible enough to completely close the main channel with a moderate pressure. The PDMS and SU-8 pattern is cured for 2 h in a convection oven. Another SU-8 pattern for the control channel is fabricated by single-step photolithography and PDMS control channel is molded from the pattern. Subsequently, PDMS control channel is positioned on the top of PDMS-coated SU-8 mold for flow channel and bonded, where two channels are treated by oxygen plasma for 10 s before assembly and carefully aligned under observation with optical microscope. Finally, whole PDMS is peeled off from SU-8 mold and bonded with PDMS-coated glass slide after oxygen plasma treatment. Preparation of Photocurable Suspension: Monodisperse silica particles with different sizes are synthesized using Stöber method.[25] Silica particles are dispersed in ethanol and mixed with ETPTA (Aldrich) containing 5 wt% photoinitiator (Darocur 1173, Ciba chemical). Ethanol was selectively evaporated from the mixture in convection oven at 70 °C and we obtain photocurable suspension of silica particles. For all other resins of PEGDA, PETA, and TMPTA, we prepare suspension in an exactly same manner. To provide magnetic functionality, barium ferrite (BaFe12O19) particles with diameter of 1 µm are additionally dispersed into the ethanolic silica suspension and the suspension is subjected to same procedures to silica-ETPTA suspension. For observation with fluorescence and confocal microscopes, rhodamine 6G (Aldrich) is mixed with ETPTA solution containing photoinitiator. Synthesis and Characterization of Photonic Crystals: To polymerize photocurable resin confined in molds, we irradiate UV light from a mercury lamp mounted in an inverted optical microscope (Nikon, TE2000-U). To remove silica particles from polymerized matrix, microparticles are immersed in 5% hydrofluoric acid (Aldrich) for 12 h. Reflectance spectra of photonic microparticles are characterized by optical spectrometer (Ocean Optics, USB4000) mounted in optical microscope (Nikon, L150)
Supporting Information
Acknowledgements This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Education, Science, and Technology for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems”, and Mid-career Researcher Program (2014R1A2A2A01005813) through NRF grant funded by the MEST. The authors appreciate Jung Yoon Seo for providing barium ferrite particles.
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Supporting Information is available from the Wiley Online Library or from the author.
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© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Received: January 2, 2014 Revised: May 17, 2014 Published online:
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