Designing Responsive Photonic Crystal Patterns ... - ACS Publications

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Designing Responsive Photonic Crystal Patterns by Using Laser Engraving Youfeng Yue, and Takayuki Kurokawa ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b22498 • Publication Date (Web): 27 Feb 2019 Downloaded from http://pubs.acs.org on February 28, 2019

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ACS Applied Materials & Interfaces

Designing Responsive Photonic Crystal Patterns by Using Laser Engraving Youfeng Yuea,b*, Takayuki Kurokawab,c a

Graduate School of Life Science, Hokkaido University, Sapporo, Japan

b

Electronics and Photonic Research Institute, National Institute of Advanced Industrial Science

and Technology, Tsukuba, 305-8565, Japan. c

Faculty of Advanced Life Science, Hokkaido University, Sapporo, Japan.

d

Global Station for Soft Matter, Global Institution for Collaborative Research and Education,

Hokkaido University, Sapporo, Japan. Keywords photonic hydrogels, laser engraving, anisotropic diffusion, morphology change, ionic strength, shape morphing

ABSTRACT

Soft photonic crystals are periodic nanostructures that have attracted much attention for their applications in sensors, owing to their tunable structural colors in response to external stimuli. Patterned photonic crystals provide a novel strategy for constructing high-performance photonic materials with unique structures and functions. In this work, laser engraving is used for the first time to design patterns on a layered photonic hydrogel. This approach is based on the integration of laser power and chemical modifications to embed different polymer composites (polyelectrolyte

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and neutral polymers) along a prescribed laser-printed path. The polyelectrolyte and neutral composites show differential swelling or shrinking, causing a mechanical instability in the layered hydrogel. The resulted soft polymeric materials appear as synchronous tuning in the photonic band gaps in response to external stimuli. This approach is favorable for designing responsive photonic crystals with controllable optical properties and 3D shape transformation. Moreover, it is of great use in developing advanced photonic crystals for applications in sensors, soft actuators, and drug release. INTRODUCTION Nature is filled with structural colors in organisms (e.g., tropical fish, chameleons, and butterflies), which originate from the physical interaction of light with intrinsic periodic nanostructures.1-3 Inspired by these natural examples, many researchers have developed artificial photonic crystals with brilliant structural colors by using various inorganic, polymeric, and hybrid materials.4-8 Among these materials, soft photonic crystals (e.g., photonic hydrogels) have attracted much attention because they can swell or shrink in response to external stimuli such as chemicals,9-12 temperature,13 pH,14-15 Humidity,16 organic solvent,17 and mechanical stress, 18-20 thereby causing changes in photonic band gaps (PBGs) and structural colors. These features enable various applications of soft photonic crystals in sensors, actuators, and optical displays. However, improving the performance, e.g., the response speed, detection sensitivity, and selectivity, of soft photonic crystals for practical applications remains a challenge. Patterned soft photonic crystals provide a new strategy for constructing high-performance photonic crystal devices.21,22 Patterned photonic crystals reportedly have high response speed, sensitivity, and selectivity. For example, Gu et al. performed inkjet-printing on patterned alcohol-


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responsive photonic crystals that consisted of microdots assembled from colloidal nanoparticles.23 These patterned photonic crystals can detect alcohol vapor and change color in only 0.5 s.23 Song et al. developed a high-sensitivity colorimetric sensor with hydrophilic and hydrophobic patterns to improve the sensitivity of tetracycline detection.24 To create patterned photonic crystals, various fabrication techniques have been developed, such as photolithography with a mask,25 inkjet printing,23,26,27 spraying,28 patterned-substrateinduced self-assembly,29 and maskless printing.30 However, it is still necessary to develop fast, facile, and cost-effective methods for fabricating patterned photonic crystals with improved performances. Laser engraving a commonly used as an efficient method to mask patterns on a hard or soft material.31 However, the use of laser engraving to fabricate responsive patterned photonic crystals has not been demonstrated. Herein, we develop a method to fabricate colorimetric patterned hydrogels using laser engraving. The laser beam is like a pencil that prints arbitrary patterns on a planar hydrogel. We used a previously reported photonic hydrogel, which contains thousands of rigid poly(dodecyl glyceryl itaconate) (PDGI) bilayers in a soft polyacrylamide (PAAm) polymer matrix.4 The hydrogels have structural colors owing to the interference between reflections from two alternating interfaces. The outer surface of the hydrogel is hydrophobic because the PDGI bilayer membranes block the passage of water through them in the direction perpendicular to the layers. Here, the laser beam produces microscale dots in the PDGI bilayer, thereby causing the external aqueous solution to enter and exit the gel along the laser-printed path. Chemical modification is performed by hydrolysis to generate multiple composites in the polymer network along the laser-irradiated path. Subsequently, the different compositions embedded in the composite hydrogels undergo specific


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morphology or volume changes in response to external stimuli, which appear as synchronous changes in the PBGs and shape morphing. RESULTS AND DISCUSSION In a layered PDGI/PAAm hydrogel, thousands of hydrophobic PDGI bilayers are stacked alternately in PAAm polymer networks (Scheme 1). The PDGI bilayers (~4.7 nm) consist of two layers of homo-polymerized DGI molecules that have long hydrophobic alkyl chains.4 To characterize the surface properties of the photonic hydrogel, we measured the contact angle of water on the gel surface. The contact angle of a water droplet was approximately 120°, which indicates that the surface of the hydrogel is hydrophobic (Figure S1, in the Supporting Information). In contrast, for a PAAm hydrogel (without a PDGI layered structure), the surface contact angle is only 25°, indicating that PAAm is hydrophilic (Figure S1). Such hydrophobic surfaces on photonic crystals are self-cleaning, i.e., they allow quick dispersal of water and are widely found in natural organisms, e.g., on the wings of butterflies. The hydrophobic surface results in a unique water diffusion behavior. Compared to an isotropic material, e.g., a PAAm hydrogel, which exhibits the same diffusion in every direction, the layered hydrogel demonstrates anisotropic diffusion, where the diffusion coefficient (D) in the direction parallel to the layers is much larger than that in the direction perpendicular to the layers. The hydrophobic PDGI layer can be considered as a protective membrane that separates the interior of the hydrogel from the outside environment. However, it is also a limitation for some applications, e.g., in the rapid detection of external environmental stimuli. Thus, we design breaks in the PDGI bilayers by printing a pattern on the surface (Scheme 1). Laser engraving is selected because lasers are focused and can be controlled to produce small defects without destroying the


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photonic structure. The resulting defects act as high-transport channels for water or water-soluble molecules, making it possible for an aqueous solution or external molecules to pass through the bilayers via the laser defects in the direction perpendicular to the layers. During molecular diffusion, the hydrogel is further chemically modified by hydrolysis to change the neutral PAAm polymer network into a poly(acrylic acid) sodium salt (PAAcNa) polyelectrolyte. Thus, the patterned photonic hydrogel contains more than two embedded structural components that have different stimuli-responsive properties. As shown in Figure 1a, we applied a flower pattern to the photonic hydrogel using laser engraving. The laser-irradiated regions consist of small holes (dots) in the PDGI bilayer. The laserirradiated path appears greenish in color owing to the rapid evaporation of water surrounding the laser defects. The laser-defected regions were detected under a 3D laser scanning electron microscope, which revealed an average thickness decrease of ~150 μm. When the patterned photonic hydrogel was immersed in a 1M NaOH aq. solution, the solution first diffused into the gel surface from the laser-irradiated path and subsequently entered into the hydrophilic-layered structure. This controlled diffusion route produced a NaOH concentration gradient in the gel, where regions near the laser defects have higher NaOH concentration than those far from the defects. During the NaOH aq. diffusion, the temperature was increased to 50 °C for 2 min to trigger a reaction in which partial acrylamide groups (–CONH2) in the PAAm polymer were hydrolyzed into PAAcNa with anionic carboxyl groups (–COO-).32 Because the reaction degree is proportional to the NaOH concentration, PAAcNa–PAAm copolymers with different composition ratios were spatially embedded in the gel sheet. The region near the laser defects (e.g., point C in Figure 1b) has a higher PAAcNa/PAAm ratio than those far from the defects. To characterize this difference, we measured the PAAcNa composition using Fourier transform


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infrared (FT-IR) spectroscopy. The ratio of PAAcNa in PAAm gels can generally be estimated from the ratio of the absorbance (A) of the peaks at 1568 and 1670 cm−1, denoted as A1568 and A1670, respectively, in the FT-IR spectrum. The peak at 1568 cm−1 arises from the stretching vibration of carbonyl (C=O) in the anionic carboxyl groups (–COO−) of the PAAcNa polymers, and that at 1670 cm−1 results from the stretching vibration of carbonyl in the acrylamide groups (–CONH2) in PAAm. As shown in Figure 1c, there is no peak at 1568 cm-1 at position A (black line), indicating that no PAAcNa existed in the hydrogel before hydrolysis. However, the peak at 1568 cm−1 at position C is larger than that at position B, indicating that the regions near the laser defects have a higher PAAcNa content than those far from the defects. The PAAm polymer networks in the new patterned photonic hydrogels were impregnated with PAAcNa at different ratios, so that they show distinct stimuli-responsive capabilities. Next, we explored the color tuning of the patterned hydrogel under a single external stimulus (Figure 2a). When the hydrogel was immersed in a 1 M NaCl solution, the laser-irradiated path (regions with a high PAAcNa ratio) underwent rapid shrinkage and changed from red to blue owing to dehydration of PAAcNa in a solution with high ionic strength. Further, the regions inside the petals (low PAAcNa ratio) changed gradually from red to orange, owing to the retention of water by PAAm. Thus, a gradual color change from red to blue was observed in this sample. We also prepared another sample treated with NaOH for a short time (50 s) and explored their color tuning in NaCl solutions (Figure S2). It is found the color shift is small compared with the sample treated with NaOH for 2 min due to the relatively low PAAcNa ratio in the hydrogel. The color tuning can be estimated by the Bragg equation, λ=2ndsinθ, where n is the average refractive index of the hydrogel, d is the distance of the layer from the diffracting plane, and θ is the Bragg glancing angle. The Bragg equation implies that there are several approaches to regulate


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the structural color, such as changing the layer distance or the glancing angle. In this system, the refractive index change is small because the refractive index of the saline solution (1 M) is close to that of water (n = 1.33). Hence, the color tuning is primarily due to a change in the layer distance when the glancing angle is fixed. Then, we measured the thickness change of the patterned hydrogel upon immersion in water within and far from the defected region. Figure 2b shows the time dependence of the thickness variation of the hydrogel. In the first 100 s, the variations in thickness (T) in both the regions are linear with respect to time. Here, the layer distance (d, right-hand axis) was calculated using d = T/m, where m is the number of stacking layers, and T is the thickness of the gel. The layer number (m) was calculated as ~4800 in the red gel from Bragg’s equation (see experimental section). As the layer number in a hydrogel is constant after preparation, the variation in thickness corresponds to the change in the sum of the layer distance. The rate of change of d calculated from (the slope of the line in Figure 2b) is 0.8 nm/s and 0.12 nm/s per layer for the laser-defected regions and regions far from the laser defects, respectively. Thus, the laser-defected regions (with a high PAAcNa ratio) exhibit a six-fold difference in the swelling/shrinking speed compared to the background. This result indicates that the laser-patterned regions exhibit an improved response speed owing to the rapid infiltration of the aqueous solution from the laser-defected regions. The diffusion of NaCl ions into the gel follow the path shown in Figure 2c, where the ions pass through the defects and then proceed perpendicular to the hydrophilic PAAcNa–PAAm layers. The laser-defected regions appear blue owing to the strong contraction of PAAcNa in response to the increased ionic strength, resulting in a quick decrease in the layer distance. The regions far from the defects demonstrate less contraction owing to the low ratio of PAAcNa. The contraction


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causes a concavity to develop gradually on the upper surface of the hydrogels, which was measured by a 3D laser scanning electron microscope (Figure 2d). Figure 2e shows the reflection spectrum of the patterned photonic hydrogel reaching its equilibrium state in NaCl solutions with different ionic strengths. When the ionic strength was increased, the reflection spectrum of the hydrogel showed a blue shift. The maximum wavelength (λmax) shifted from 680 to ~500 nm when the ionic strength of the salt solution was increased from 0.001 to 0.05 M (Figure 2f). A further increase in the ionic strength did not change the peak wavelength. Moreover, spectral features at some wavelengths have shoulder peaks that are consistent with the different structural colors observed in the patterned gel in equilibrium states. In contrast, the unpatterned PDGI/PAAm hydrogel shows no obvious color tuning to changes in the ionic strength from 0.001 to 0.1 M (Figure S3). These results indicate that patterned photonic hydrogels are sensitive to and demonstrate naked eye distinguishable structural colors in low ionic strength solutions (10−3 to 10−1 M). The wavelength shift in the patterned hydrogel was accompanied by changes in the peak intensity and full width at half maximum (FWHM) (Figure 2g). In a high ionic strength solution, the hydrogel shows higher peak intensity and smaller FWHM values than in a low ionic strength solution, indicating that the layered structure in the hydrogels become ordered.33 The laser defects on the bilayers facilitate the rapid diffusion of molecules into and out of the gel. A dynamic pattern change was demonstrated by dropping NaCl solution (1 M) on the gel surface (Movie S1, Figure 3a). The defected regions of the gel initially appeared deep red owing to the water-swollen state of the PAAcNa polymer network. However, when NaCl solution was dropped on them, the color and volume of the hydrogels changed rapidly. In particular, the laser


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defected regions underwent strong shrinking with a distinct color change from red to green. During this process, NaCl ions were absorbed into the layered hydrogel from the defected regions. The opposite process, with molecule release, was also demonstrated by immersing another laserpatterned gel (swollen in 0.8 M NaCl solution) in water (Movie S2, Figure 3b). First, the gel demonstrated a weak green butterfly pattern because the polymer networks in the laser-defected regions were in a collapsed state. It is noteworthy that the patterned regions and background have a similar photonic structure but different ionic strength sensitivities. Therefore, upon immersion in water, a distinct color change from green to red was observed in the defected regions. During this process, the pre-absorbed salt ions in the gel were released from the laser-defected regions into the water. These unique molecules uploading and release processes may be applied in controlled drug delivery systems. The defected regions behave as drug release or uploading windows, and the other regions with hydrophobic bilayers act as storage reservoirs that support the storage or release of molecules from the laser-defected windows. Moreover, the amount of target molecules released or absorbed can be recognized by naked eyes from the color change of the materials. In addition to 2D surface pattern changes as demonstrated above, it is also possible to design 3D shape transformations by using this approach. For example, to design a bending shape, we cut the gel into a strip with a dimension of 3ൈ0.3ൈ0.12 cm3, and printed straight lines along its width (Figure 4a). Note that here we decrease the laser power so that the defects cannot fully destroy all the bilayers along the thickness (Figure 4b). After the same chemical treatment in NaOH solution, the side printed with lines has a higher composition of PAAcNa than the other side. The large swelling in the PAAcNa regions (compared to the PAAm side) produces more mechanical


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instability to the gel surface along the layers, which causes the gel to bend into a circle in water (Movie S3, Figure 4c). When the ionic strength of water increases, the shrinking in the PAAcNa regions causes the circle to deform again (Figure 4d, Figure S4). However, it achieved an equilibrium curved state in 0.5 M NaCl solution (without unfolded straight) even when the ionic strength was further increased (Figure S4). These results indicate that patterned gels can be used as soft actuators in water by increasing/decreasing the ionic strength (from 0 to 0.5M or vice versa). We also measured the photonic property of a typical hydrogel strip during the shape transformation (Figure S5). Other responsive 2D surface patterns or 3D shapes, e.g., saddle-shaped curvature can also be produced (Figure S6). The patterns in the layered hydrogel are angle dependent and stress sensitive. Figure 5a shows the optical images of the angle dependence of the patterns. The color becomes iridescent as the angle of view changes, indicating that photonic crystals have ordered arrays.34 The patterns on the gel can even disappear under mechanical stress. As shown in Figure 5b, the green butterfly pattern disappeared under compression and reversibly re-appeared in a relaxed state. The key point here is that the patterned regions are more responsive to the ionic strength than the background. Thus, initially the patterned regions show a green butterfly pattern while the background remains yellow. However, upon application of external stress, the patterned regions and background exhibit different mechanochromic capabilities; hence the patterns disappear under deformation owing to the nonuniform change in the photonic structures. Furthermore, the patterned photonic crystals, which contained a large amount of water, are not only flexible, iridescent, and stimuli-sensitive, but are also environment friendly. They may also be used in other fields, such as for body decoration, on the skin or nails (Figure 5c). CONCLUSIONS


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In this work, we developed a method to generate responsive patterned photonic crystals with unique structures and functions. A combination of laser engraving and chemical modification encodes multiple composites in a hydrogel sheet. The composite gels demonstrate local swelling or shrinking in different regions, thereby exhibiting color tuning in response to external stimuli. In particular, the laser-patterned regions allow rapid infiltration of aqueous solution and thus fast colorimetric detection/sensing. By controlling of the laser factors, such as power and routes, it is possible to create photonic crystals with programmable mechanical instability that can morph into a given target shape. This study opens new avenues for the use of responsive photonic crystals in potential applications such as biochemical sensing, tissue engineering, soft actuators, and drug release.

EXPERIMENTAL SECTION Photonic hydrogel synthesis. The anisotropic photonic hydrogels was synthesized according to the previous work.4 The DGI system was initially developed by Tsujii et al.35 For detail, the hydrogel was synthesized by free radical polymerization of the aqueous solution containing 0.10 M DGI, 0.027 mol% sodium dodecyl sulfate (relative to DGI), 2 M AAm, 2.5 mM N,N′-methylenebis acrylamide as cross-linkers of AAm and 2 mM Irgacure as an initiator. Before polymerization, the above precursor solution was injected into a reaction glass cell (0.5-mm spacing) by applying shear flow, which induced thousands of DGI bilayers aligned in the direction parallel to the glass surface. To stabilize the oriented bilayers, polymerization was performed by irradiating the ultraviolet light for 8 h at 50 °C under an argon gas atmosphere. After polymerization, the gel was immersed in water for 1 week to reach an equilibrium swelling state. The formation of the bilayer structure in


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water and hydrogel is shown in the Supporting Information. The layer distance of the gel, d, can determined from Bragg’s relation, 2ndsinθ = λ, where n and θ are the refractive index of water and the Bragg incident angle. The diffraction wavelength, λ of a red gel is about 580 nm. So, the layer distance, d ≈ 250 nm from Bragg’s relation From the thickness of the gel, T = 1.2 mm and the layer distance, we calculate the number of layers, m = T/d ≈ 4800 layers. Laser patterning on photonic gels. The fully swollen hydrogel was positioned on the stage of a laser (Universal Laser Systems). The patterns were designed on a controller (computer) connected to the laser machine. The controller converts the patterns into instructions so that patterns with a prescribed size can be engraved on the upper surface of the gels.


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Figures and captions

a)

Layered photonic hydrogel

b)

Laser beam

c) Patterned hydrogel

Hydrophobic

D

Laser defects: diffusion

Bragg diffraction shift

PDGI D

Laser

Hydrolysis

PAAm

Scheme 1. Scheme illustration of patterning a layered photonic hydrogel by using laser engraving. (a) Thousands of hydrophobic PDGI bilayers (red) alternating packed in the PAAm hydrogel (blue). The visible light scattering from nanostructure with periodic interfaces between PAAm and PDGI, results in structural color. (b) Laser patterning creates defects on the hydrophobic PDGI bilayers, which open a window for the external molecular diffusion. (c) The molecule diffusion and chemical modification (hydrolysis) enable the laser patented hydrogels contain different compositions along the laser irradiated path.


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a) Laser defects

A

500 μm

1 cm

500 μm

Height (μm) 260.0 177.1 88.6 0.0

Hydrolysis

b) B

C

c)

transmittance (%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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80 60

A

B

Laser intensity

A1568: C>B>A

C

40 2000

1600

1200

wavenumber (cm-1)

800

Figure 1. Characterizations of the patterned photonic hydrogels. (a) A flower pattern is printed on the layered photonic hydrogel by using laser engraving. The laser scanning microscope shows that the defects cause a depth difference of ~150 ߤm compared with the non-irradiation regions. (b) The diffusion of hot NaOH solution into the patterned hydrogel, cause a composition change in the polymer network. (c) The composition change is characterized by FT-IR spectra.


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a)

c)

Laser defects T2

T0 T1 T 2

T1

T0 = md0

Laser defect T0=T1=T2 (d0=d1=d2): homogeneous color

1.3

d)

270

1.2 240

1.1 1.0

210

0.9 0.8

T0᧸T1 T2 (d0

Designing Responsive Photonic Crystal Patterns ... - ACS Publications

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