Thermoresponsive Hydrogel Photonic Crystals by Three-Dimensional Holographic Lithography**

Transcription

1 DOI: /adma Thermoresponsive Hydrogel Photonic Crystals by Three-Dimensional Holographic Lithography** By Ji-Hwan Kang, Jun Hyuk Moon, Seung-Kon Lee, Sung-Gyu Park, Se Gyu Jang, Shu Yang, and Seung-Man Yang* Hydrogels have been widely studied as biomaterials because of their hydrophilicity and biocompatibility. Many of them are responsive to chemical and physical stimuli, which is of potential significance in biomedical applications, including biosensors, [1] drug delivery, [2] and chromatography. [3] Recently, there has been growing interest in the fabrication of three-dimensional structured hydrogels. These structured hydrogels can mimic cellular environments and provide external platforms for studying various biomolecules. [4 7] Specifically, hydrogel photonic crystals (PCs), which have periodically modulated refractive indices in space, can possess tunable photonic bandgaps and have therefore been applied to optical sensors of various external stimuli including ph, [8,9] temperature, [9 12] humidity, [13] chemicals, [14] and photons. [11] Recently, various strategies have been studied to create 3D hydrogel PCs, including the use of colloidal crystal templates [8,9,11 13] and monodisperse hydrogel particles as building blocks for 3D structures. [10,12] Others have fabricated 2D patterns by conventional photolithography through using hydrogel photoresists. [15,16] However, the aforementioned methods require several time-consuming steps to produce the structures. Moreover, they often involve intrinsic defects during self-assembly and the loss or deformation of samples during the etching of templates, resulting in deteriorated optical sensitivities of PCs. In this Communication, we demonstrate a direct approach to fabricate 3D hydrogel PCs [*] Prof. S.-M. Yang, J.-H. Kang, S.-K. Lee, S.-G. Park, S. G. Jang National Creative Research Initiative Center for Integrated Optofluidic Systems Department of Chemical and Biomolecular Engineering Korea Advanced Institute of Science and Technology 335 Gwahangno, Yuseong-gu, Daejeon, (Korea) smyang@kaist.ac.kr Prof. J. H. Moon Department of Chemical & Biomolecular Engineering Sogang University 1 Shinsu-dong, Mapo-gu, Seoul, (Korea) Prof. S. Yang Department of Materials Science and Engineering University of Pennsylvania Philadelphia, PA (USA) [**] This work was supported by a grant from the Creative Research Initiative Program of the Ministry of Science and Technology for Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems. The authors also appreciate partial support from the Brain Korea 21 Program. J.H.K. thanks Jae-Hoon Choi for assistance with FTIR measurements. Supporting Information is available online from Wiley InterScience or from the authors. by using interference lithography. [17 21] To generate 3D interference patterns, we used a single refracting prism with a triangular pyramidal frustum shape. Compared to the use of multiple beams, the prism holographic lithography can abbreviate elaborate and complicated optical setups without sacrificing precision, [22,23] as well as avoid total internal reflection. [24] In addition, we showed thermo-responsive optical behavior by using hydroxyethyl methacrylate (HEMA)-based hydrogels. The structured hydrogels revealed viscoelastic deformation under temperature change, resulting in a shift in diffraction wavelength. In particular, we explored for the first time the viscoelasticity driven thermoresponsiveness in the hydrogel system by capturing directly the morphology evolution using chemical vapor deposition (CVD) of a thin film of silica on the hydrogel. In the experiments the beam, exposed normal to the truncated surface of a prism, was split into one central beam with wave vector k 0 and three refracted beams with k j ( j ¼ 1 3) from the sidewalls. Then, the four beams were recombined and focused on the bottom of prism and hydrogel photoresist. The wave vectors of the four beams inside the prism are k 0 ¼ 2pn/ l[ ], k 1 ¼ 2pn/l[ ], k 2 ¼ 2pn/l[ ], and k 3 ¼ 2pn/l[ ], where n is the refractive index of fused silica prism (n ¼ 1.48) and l is the wavelength of light. The polarization vectors of the four beams are E 0 ¼ [ ], E 1 ¼ [ ], E 2 ¼ [ ], and E 3 ¼ [ ]. The intensity ratio between the central beam and the three surrounding beams is 5.5:1:1:1. Figure 1a represents the iso-intensity surface of the interference pattern with these four beams. The pattern expresses the asymmetric face-centered cubic (FCC) lattice, where the atoms at lattice points are connected by bridges to their nearest neighbors. Here, the ratio of atom spacing on the (111) surface relative to the lattice spacing in the [111] direction is ca The interference pattern was transferred to the hydrogel photoresist, which was formulated by mixing poly(hemaco-mma) (molar ratio of ca. 7:3; 36 wt % to solvent), a cationic photoinitiator, triarylsulfonium hexafluorophosphate salts ( wt % to resin), and a crosslinker, tetramethoxymethyl glycoluril (TMMGU) (10 wt % to resin) in gammabutyrolactone (GBL). When the resist was exposed to the UV laser beams, the photoinitiator released photoacids, which activated methoxy groups in TMMGU to react with hydroxyl groups and crosslink poly(hema-co-mma). [25] The prepared random copolymer has a glass transition temperature (T g )of Adv. Mater. 2008, 20, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim 3061

2 Figure 1. Structure and optical reflectance of 3D hydrogel PCs via holographic lithography. a) Simulated 3D holographic interferential structure from the four-beam configuration constructed by the refractive prism. The front side of the image is the (111) plane. Typical experimental scanning electron microscopy images of b) the (111) plane of FCC lattices and c) 408 tilted cross-section of a fractured sample. The inset shows windows between lattices on the ( 110) plane. Scale bar: 500 nm. d) Reflectance spectrum of a nitrogen-dried hydrogel structure measured by Fourier transform IR at room temperature. around 55 8C, therefore, the exposed resist was post-exposure baked at 55 8C followed by development in methanol. Figure 1b and c show typical scanning electron microscopy (SEM) images of 3D hydrogel PCs fabricated by the prism holographic lithography. The spacing between atoms and the lattice spacing is around 680 nm and 770 nm, respectively. Comparing the spacing ratio to the simulated one (Fig. 1a), we estimated a shrinkage of 26% in the [111] direction during the development, which is due to the anchoring of the structure on the confined substrate. [26] The 3D hydrogel PCs exhibit a strong reflection at around 2.1 mm owing to the Bragg diffraction (Fig. 1d). From Bragg s law, the volume fraction of the hydrogel PCs was estimated around 0.69 with the lattice constant and the refractive index of poly(hema-co-mma) (n 1.50). Here, the volume fraction of hydrogel PCs can be controlled by the laser intensity (See Fig. S1 of Supporting Information). Specifically, the volume fractions were estimated as 0.31, 0.58, and 0.69 for the different laser intensities of 0.076, 0.099, and J cm 2. In the following experiment, the laser intensity was set at J cm 2. Also, we investigated optical reflectance only for light propagating in the [111] direction in the 3D structure. This is because the optical diffraction in this direction is strong and sensitive to the deformation of the structure. Also, it is easy to measure the reflectance because the (111) plane is facing. However, the structure should have optical responses in other directions because of the corresponding stop bands of the 3D periodic hydrogel structures. The crosslinked poly(hema-co-mma) has a high degree of hydration, which leads to increase in the physical volume. Moreover, HEMA-based hydrogels copolymerized with various functional groups, such as acrylic acid, glycidylmethacrylate, and acrylamide possess a conditional volume transition in response to various environmental stimuli. [27 31] For example, HEMA hydrogels with carboxylic acid groups shows phresponsive swelling, which enables the hydrogel PCs to be used as optical sensors for glucose [32] as well as ph sensors. [8,33] Here, we study the morphology evolution of poly(hema-co- MMA) and optical response during swelling and contracting. Specifically, we reveal temperature-sensitive responses of the hydrogel PCs, which are attributed to the viscoelastic deformation upon heating or cooling. First, we measured the reflectance spectra from the hydrogel PCs in the course of swelling by water. As noted from Figure 2a, the reflectance peak was red-shifted from 2.10 mm to 2.22 mm within 10 min after the hydrogel PC was exposed to humid air (100% relative humidity). Previously, the change of morphology during swelling has been estimated by the optical reflectance as well as direct observation. [32,33] In case of inverse opals of HEMA-based hydrogels, the lattice distance increased slightly along the normal direction to the substrate due to the pinning of the structure at the substrate while the volume of hydrogel increased at fixed lattice position. Here, we used direct method to explore the structural change in the course of swelling by capturing the structure with silica chemical vapor deposition (CVD). [34,35] Briefly, the hydrogel template was exposed to a precursor of silicon tetrachloride and a thin silica shell (thickness 40 nm) was formed on the hydrogel surface within a few minutes. Then, the solidified silica shell was mechanically strong enough to sustain the deformed morphology. In order to examine the morphological change, the magnified SEM images of the ( 110) plane were observed, as shown in Figure 2b d. Figure 2b shows a cross-sectional SEM image of dried hydrogel template, and the SEM images in Figure 2c and d show the inverted silica structures obtained from dried and swollen hydrogel templates, respectively. In Figure 2c and d, the enclosed shells of cylindrical shape were formed from the air cavities around unit atoms in the hydrogel template (Fig. 2b). It can be noted that the lattice spacing from the inverted silica structure (or swollen hydrogel) were elongated in the [111] direction by 10% relative to those from the dried hydrogel, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20,

3 Figure 2. a) Reflectance peak shift in response to the structural change with time after the dried sample was exposed in the humid air with 100% relative humidity. b) The cross-sectional SEM images of bare hydrogel PC at dry state. c,d) SEM images of inverted silica structures from hydrogel PCs after deposition of silica using dry and swollen hydrogel PCs, respectively. Hydrogels were dried in the presence of silica gel or swollen in moisture at room temperature. All scale bars are 500 nm. while the lateral dimensions remained unchanged. The expanded interlayer distance in the [111] directions resulted in the reflectance peak shift towards longer wavelengths. Given the refractive index of pure water (n ¼ 1.33) and poly(he- MA-co-MMA) (n ¼ 1.50), the average volume of the swollen hydrogel PCs could be estimated readily from Bragg s law. The result showed that the volume fraction of hydrogel was increased by about 9% from 69% of the dried sample after swelling. Meanwhile, the swollen hydrogel PCs exhibited thermosensitive optical behavior. The position of reflectance peak from swollen hydrogel PCs shifted toward shorter wavelength with increase in the ambient temperature; whereas no peak shift was observed in the dry hydrogel PCs (See Fig. S2 in Supporting Information). Specifically, when the temperature was increased from 25 8C to 508C, the peak wavelength was decreased from 2.2 mm to 1.8 mm, as shown in Figure 3a. The peak returned to the initial wavelength when the temperature decreased (Fig. 3b). However, at the temperature above 50 8C, the hydrogel PCs formed a transparent film and the peak disappeared. In this case, the peak was not recovered even when the sample was cooled (See Fig. S3a in Supporting Information). The shift of reflectance peak was reproduced after repeated heat cycles as shown in Figure 3c. Here, the maximum and minimum wavelengths were found to be reduced by 2 3% during a single heat cycle. Therefore, it can be deduced that the deformation was not perfectly restored during the cycle since the heating temperature (50 8C) was too high close to the temperature (55 8C) which would cause a permanent deformation of the structure. This temperature-responsive behavior could be explained by the fact that during swelling, hydrogels tend to deform in response to the temperature change owing to the lowered T g. Upon heating, the connecting bridges between the unit atoms are collapsed in order to lower the surface energy, and consequently the atoms are getting closer each other. The resulting decrease in the lattice spacing is shown in Figure 4. When the silica was deposited onto the swollen hydrogel PCs at high temperatures (ca. 40 8C), we only obtained the silica film from the top surface of the template (See Fig. S3b in Supporting Information). We believe that the decrease in the interlayer distance reduces the volume of air-cavity networks, therefore, blocking the penetration of silica precursors during the CVD process. The reversible transition during the heat cycle is due to the elasticity of crosslinked hydrogels. Here, we estimated the change in the lattice distance using Bragg law, considering the volume fraction is constant during the temperature-induced deformation. The lattice distance in the [111] direction could be repeatedly contracted and relaxed up to 160 nm in the cyclic heating or cooling between 25 and 50 8C. The decrease in the reflection peak intensity in Figure 3a during heating up was caused by thermo-responsive change in the void fraction of hydrogel PCs. As mentioned, the volume fraction of air-cavities of the lowest refractive index was reduced during heating up, which led to the decrease in the reflectance intensities. Indeed, when the hydrogel PC structure is collapsed eventually to film-like morphology at high temperatures, no appreciable reflectance is observed as shown in Figure S3a of Supporting Information. In summary, we demonstrated that the combination of prism holographic lithography and hydrogel photoresists offers a direct fabrication of 3D hydrogel PCs. In particular, we investigated the swelling of hydrogel PCs and thermoresponsive bandgap tuning. The morphology change in the course of swelling and the viscoelastic deformation were explored by the inversion of hydrogel PCs into silica structure. We found that the lattice distance was increased in the [111] direction during the swelling and the swollen PCs was deformed and recovered due to the nature of viscoelastic properties. This results in the shift of photonic stop-band in response to temperature change. We believe that the proposed method will provide simple and direct route for fabricating hydrogel PCs with various functional groups as well as integrating pre-patterned hydrogel PC with microfluidic chips for ideal optofluidic platforms. [36] Adv. Mater. 2008, 20, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

4 Figure 3. Optical spectra of the swollen hydrogel PCs with thermo-responsive photonic stop-band. Reflection spectra of hydrogel PC film exhibiting a) blue shift with increase in the ambient temperature and b) recovering red shift with decrease in the ambient temperature. c) The change of peak position during repeated heating and cooling cycle between 30 and 50 8C. In addition, tunable hydrogel PCs can have various bioapplications by changing functional groups in hydrogel photoresist that can trap and release biomaterials such as cells in response to external stimuli. Experimental Preparation of HEMA-Based Hydrogel Photoresist: Photocurable hydrogel solution was composed of random copolymer resin, crosslinker, photoinitiator (PI, or photoacid generator (PAG)) and solvent. First, the resin was synthesized by radical copolymerization of 2-hydroxyethyl methacrylate (HEMA, Aldrich) and methyl methacylate (MMA, Aldrich). The monomers with molar ratio of 7:3 (HEMA:MMA) were used. Poly(HEMA-co-MMA) photoresist was prepared by adding 10 wt % tetramethoxymethyl glycoluril (TMMGU, POWDERLINK 1174, SK CYTEC co.) as crosslinker and wt % triarylsulfonium hexafluorophosphate salts (Aldrich, or Cyracure UVI 6992 from DOW chemical co.) as photoinitiator. Prism Holographic Lithography: The hydrogel photoresist was filtrated by syringe filter (Whatman, 0.45 mm) and spin-coated (ShinuMST) on the SU-8 covered glass substrate. After softbaking, the thickness of the resultant film was about 13.5 mm. The single fused silica prism, which can form four-beam interference patterns, was put on the film. He-Cd Laser beam (Kimmon, 325 nm, 50 mw) controlled by an electronic shutter was focused on the surface of film via multi-faced prism. The interference pattern was transferred during exposure for s. After post-exposure baking for 5 10 min at 55 8C, the exposed film was developed in methanol to remove less crosslinked region. And then, the final hydrogel PC was obtained by washing with distilled water and drying under nitrogen gas. The thickness of the product was shrunken around 10 mm in the end of develop process. Chemical Vapor Deposition of Silica: We used simple alternative exposure to SiCl 4 vapor and the water following the procedure described in the literature [34, 35]: In the present case, since the swollen hydrogel PCs contained water inside, we only exposed the structure to SiCl 4 vapor. After two or three cycles of exposure, silica inverse opal was obtained by calcination at 500 8C for 3 h to remove the sacrificial hydrogel template. Characterization: Surface and cross-section images of the samples were taken using a field emission-scanning electron microscope (Hitachi S-4800). The reflectance spectra of the products were analyzed via a FT-IR & imaging microscope (IFS 66v/s & HYPERION TM 3000 by Bruker). A 15 IR objective lens was used and spot size was 200 mm. Temperature and humidity of the samples were controlled by using a hot plate convection oven (Thermolyne type 10600) and silica gel. Received: January 15, 2008 Revised: February 9, 2008 Published online: July 4, 2008 Figure 4. Simulated images of viscoelastic deformation during the temperature increasing or decreasing. The deformation changed the interlayer spacing in the [111] direction. The volume of hydrogel remained unchanged while the void volume of air changed. [1] J. H. Holtz, S. A. Asher, Nature 1997, 389, 829. [2] B. G. De Geest, C. Dejugnat, M. Prevot, G. B. Sukhorukov, J. Demeester, S. C. De Smedt, Adv. Funct. Mater. 2007, 17, 531. [3] A. Denizli, B. Salih, E. Piskin, J. Chromatogr. A 1996, 731, 57. [4] M. C. Cushing, K. S. Anseth, Science 2007, 316, [5] Y. Luo, M. S. Shoichet, Nat. Mater. 2004, 3, 249. [6] V. Jayawarna, M. Ali, T. A. Jowitt, A. E. Miller, A. Saiani, J. E. Gough, R. V. Ulijn, Adv. Mater. 2006, 18, 611. [7] J. H. Wosnick, M. S. Shoichet, Chem. Mater. 2008, 20, 55. [8] Y. J. Lee, P. V. Braun, Adv. Mater. 2003, 15, ß 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Adv. Mater. 2008, 20,

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