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Bioinspired, Polymeric Photonic Crystals for High Cycling pH-Sensing Performance Xiang Fei, Tao Lu, Jun Ma, Wanlin Wang, Shenmin Zhu, and Di Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08724 • Publication Date (Web): 19 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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ACS Applied Materials & Interfaces

Bioinspired, Polymeric Photonic Crystals for High Cycling pH-Sensing Performance Xiang Fei1, Tao Lu1, Jun Ma2, Wanlin Wang3, Shenmin Zhu1*, Di Zhang1 1

State Key Laboratory of Metal Matrix Composites, School of Materials Science and

Engineering, Shanghai Jiao Tong University, Shanghai 200240, China 2

School of Engineering, University of South Australia, Mawson Lakes, SA5095 Australia

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College of Electronic Science and Technology and Guangdong Provincial Key Laboratory of

Optoelectronic Micro/Nano Optomechatronics Engineering, Shenzhen University, Shenzhen 518060, China KEYWORDS: photonic crystals, biotemplate, sensor, hydrogel, pH response

ACS Paragon Plus Environment

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ABSTRACT

Artificial photonic crystals (PCs) have been extensively studied to improve the sensing performance of poly (acrylic acid) (PAAc), as it can transform the PAAc volume change into optical signal which is easier to read. Nevertheless, these PCs are limited by the monostructure. We herein developed new photonic crystals (PCs) by coating acrylic acid and acrylamide (AAm) via in situ copolymerization onto Papilio paris wings having hierarchical, lamellar structure. Our PCs exhibited high performance of color tunability to environmental pH, as detected by reflectance spectra and visual observation. The introduction of AAm into the system created covalent bonding which robustly bridged the polymer with the wings, leading to an accurate yet broad variation of reflection wavelength to gauge environmental pH. The reflection wavelength can be tailored by the refractive index of the lamellar interspacing due to the swelling/deswelling of the polymer. The mechanism is not only supported by experimenta but proved by finitedifference time-domain simulation. Moreover, It is worth noting that the covalent bonding has provided the PCs-based pH sensor with high cycling performance, implying great potential in practical applications. The simple fabrication process is applicable to the development of a wide variety of stimuli-responsive PCs taking advantage of other polymers.


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Introduction Smart polymers have been extensively studied due to a great many potential applications in many fields, but it is a well-known problem to gauge the volumetric change of the polymers in swelling or deswelling. The current solution is to combine smart the polymers with photonic crystals (PCs); the PCs typically consist of periodic arrays of contrasting refractive index materials with unique light manipulation properties, and they can transform the polymer volume change into optical signals which is easier to read. In 1996, Asher1 reported a thermal-responsive PC by embedding a colloidal array of polystyrene spheres within poly (N-isopropylacrylamide) hydrogel; the diffraction wavelength was thermally tunable since the array lattice constant varies with temperature that can be triggered by polymer. The milestone work is followed by researchers using various polymers to produce stimuli-responsive PCs. Given a wide variety of smart polymers, many PCs have been developed which are responsive to pH,2,

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irons,4,

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magnetic6 & electric fields,7 mechanical force,8-10 humidity11 and vapors.12 Recent reviews have introduced responsive PCs ranging from one- to three-dimensional structure;13-15 these responsive PCs were synthesized by self-assembly, spin-coating and other methods. However, there lacks a simple, low-cost approach to the fabrication of high-performance responsive PCs with more precise structure. As a natural PC, readily accessible butterfly wings have brilliant colors due to their unique hierarchical structure. The structure and optical characteristic of these materials have been studied. Potyrailo16 reported optical responses from Morpho butterfly wings to different vapors such as water, methanol and ethanol. The structural colors of certain butterflies can cover both the visible and UV ranges, implying potential as communication and mating signals;17 this is because the butterfly wing has narrow width of the ridges, continuous multilayer and hierarchical


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structure exhibiting not only interference but diffraction and scattering. Such precise 3D structures, however, are currently beyond all of our fabrication capacity.18 From our perspectives, the combination of this biological PC structure with smart polymers would demonstrate novel or highly improved properties and thus lead to a wide range of new applications. Recently Xu et al.19 have reported a novel thermal-responsive PC by coating poly (N-isopropylacrylamide)-co-acrylic acid on the surface of Morph butterfly wing. The hydrogen bonding from acrylic acid promoted the formation of uniform-thickness coating on the template; the resulting PC exhibited a temperature response with a total 30 nm reflection wavelength variation. Zang et al.20 embedded the sunset moth scales into the interpenetrating polymer network (IPN) of chitosan and PVA; the IPN swelled and shrunk in different pH environments, leading to the microstructural changes of the scales which thus exhibited a desired pH-responsive behavior; nevertheless, the work is limited by low cycling stability because hydrogen bonding between IPN and moth scales is vulnerable to pH, hindering its application. Herein, we firstly report a novel pH-responsive PC of lamellar structures by mean of tailoring a coating of acrylic acid (AAc)-acrylamide (AAm) copolymer on the color spot of Papilio paris with glutaraldehyde (GA) crosslinking. Poly (acrylic acid) is widely studied as a pH-responsive hydrogel in many studies, and the introduction of AAm for copolymerization provides the macromolecular chain with amino groups for crosslinking with GA. Owing to the strong chemical bonding between natural template and hydrogel, these biological PCs show a distinct color response when tuned by external pH; such sensing process can be performed in many cycles. Our work provides an effective approach for the synthesis of pH-responsive polymeric PCs with stability and broad range wavelength variation in visible light region, which can be easily detected by naked eyes. Through partnering with other stimuli-responsive polymers, our


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PCs can be suitable candidates for bioassays, chemical sensor and other optical-based devices. Additionally, our results offer valuable data support for the artificial analogies of functional 3D PCs.


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Experimental Section Materials. Papilio paris butterflies were purchased from the Shanghai Natural Wild-Insect Kingdom Co. Ltd. Acrylic acid (AAc), acrylamide (AAm), potassium persulfate (KPS), glutaraldehyde (GA), citric acid monohydrate (C6H8O7·H2O) and hydrochloric acid (HCl) were supplied by the Sinopharm Chemical Reagent Co.,Ltd. Na2HPO4·12H2O, Na2CO3, NaHCO3, NaCl and NaOH were obtained from Shanghai Linfeng Chemical Reagent Co. Ltd. N, Nmethylenebis acrylamide (BIS) was purchased from Aladdin Chemistry Co. Ltd. AAc, AAm and KPS were all refined before polymerization. Preparation of poly (acrylic acid-co-acrylamide)-PCs. Original butterfly wings to be used as the templates were cleaned by a 10 wt% NaOH solution for 5 min at 60 °C, during which chitin transformed into chitosan through deacetylation in order to expose more amino groups on the template surface. The alkali-treated templates were carefully washed with deionized water to remove unreacted alkali solution. We made a precursor solution consisting of KPS (initiator, 0.04 g), GA (100 µL, 25 wt%), BIS (0.04 g), deionized water (5 mL), and mixture of AAc and AAm (0.5 g in total, the ratio of AAc to AAm is in Table S1). Then the treated templates were immersed into the precursor solution for 10 hr in a refrigerator at 4 °C to obtain an adsorption equilibrium. Later the templates were carefully taken out and wiped by filtration papers, and then they were sealed in a bottle. Oxygen in the bottle was gradually removed by using vacuum and N2 atmosphere. Through polymerization at 60 °C for 12 hr, we obtained polymer-PCs. Instruments and measurements. The morphology was analyzed by scanning electron microscopy using a JEOL JSM-6360LV at an accelerating voltage of 20 kV. The sample cross section was investigated by transmission electron microscopy using a FEI Tecnai G2 spirit


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Biotwin. Fourier transform infrared measurements were conducted by a Thermo Fisher Nicolet 6700 spectrophotometer. Thermal gravimetric analysis was performed by Perkin Elmer Pyris 1 from 25 to 900 °C at 20 °C·min−1. Keyence VHX-1000 was used to take digital optical photographs of these samples. A series of buffer solutions with pH 2.2 – 10.5 were prepared by using NaCl, Na2CO3, NaHCO3, Na2HPO4 ·12H2O and C6H8O7·H2O. A constant ionic strength of 0.2 M is used in the test. A prepared PC was soaked into a buffer solution for 10 min to reach swelling equilibrium, followed by measurements for pH-induced optical property using ARM Microscopic Angle-Resolved Spectroscopy made by the Shanghai FuXiang Optics Corporation, with reflected and incident light perpendicular to the surface of the samples. After the measurement the sample was thoroughly washed with deionized water until neutral, and it was then immersed into the next buffer solution for optical measurements. In our work, we collected at least 5 spectrum data for each sample and reported the mean value. Simulation. Reflectance simulation was conducted by finite-difference time-domain method by assuming smooth lamellae in scale and homogenous material properties (refractive index, layer thickness of polymer and template). The simulation model (Figure S4a) consisting of 10 layers with the periodic length 210 nm. In a periodic length, the chitin layer thickness is 70 nm and the coated polymer is 7.5 nm. All parameters is derived from the simplified cross-section TEM image of polymer-PC. Moreover, the influence of the template layer thickness and the number of them have been simulated (Figure S4b, c). Both layer number and thickness have slight influence on the spectral width. However, the peak intensity is enhanced with increase in the layer number, implying an increment of the signal to noise ratio (SNR) but the thickness of each layer poses no effect on the noise level. The refractive indices of polymer, water and template


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are fixed as 1.44, 1.33 and 1.56, respectively.

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During the simulation, the deformation of

template was ignored. We could not get the swelling parameters directly from TEM mesuremernt, because TEM needs samples be dried and embedded into resin thus it can’t keep the samples with swelling status. Thus, the swelling parameters in calculation are based on the experiments below: neat PAAc and P(AAc-co-AAm) hydrogels with the same protocol of materials (monomer, 0.5 g; KPS, 0.04 g; BIS, 0.04 g; deionized water, 5 mL) and processing method were prepared; we then investigated their swelling behavior. The polymer was soaked into a buffer solution for 10 min, followed by measurements for its volume and then immersed into next buffer solution. The processing was strictly consistent with what in optical measurements for polymer-PCs. The swelling ratio is defined as followed:

Swelling Ratio =


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Result and Discussion The detailed synthesis of pH-responsive PCs was described in the Experimental Section. In brief, a Papilio paris wing was alkali-treated and immersed into a precursor solution for ten hours to ensure the scale micro-structure to be filled with the solution. A layer of hydrogel was formed through radical polymerization, which produced a pH responsive PC (Scheme 1). The Papilio paris butterfly is one of typical natural biological species whose photonic crystal structure locates in its color spot (Figure 1a).22 Scales of butterfly wing are ~190 µm in length and 79 µm in width, being orderly arranged like roofing tiles (Figure 1b). Figure 1c reveals the morphology of a single scale. Ridges are well-distributed; the distance between adjacent two ridges was measured as 10 ±1 µm. TEM provided insight into the cross section structure of a single scale (Figure 1c inset). The cross-section of the scale with lamellar structure is similar to the Bragg Stack which comprises periodic stacked layers with alternating high and low refractive indices.23, 24 In Figure 1c inset, each lamella is about 75 nm in thickness and 140 nm in interlayer gap. As a major component of the scales, chitin has a refractive index (RI) of 1.56,25 whist the air having an RI of 1.0 actually fills throughout the scale structure; this accounts for the shining color of the butterfly wings. When the butterfly wing was immersed into the precursor solution, liquid covered the surface of scale as well as occupied space of gaps in these lamellae. Once the monomer reached the temperature to polymerize, the stimuli-responsive hydrogel would generate and attached themselves to the surface of lamellae. A sample P(AAc-co-AAm)-PC was prepared with a ratio of monomers AAc to AAm at 4: 1. To investigate the role of AAm, a contrastive sample PAAc-PC containing AAc only was also prepared. SEM was used to detect the surface morphology of scales coated with polymer. In


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Figure 2, the surface morphological characteristics of these samples (Figure 2b, c) are very close to the original butterfly wing (Figure 2a). No polymer agglomeration can be obviously seen in both PAAc-PC and P(AAc-co-AAm)-PC, which means the polymer uniformly coating on the surface. The uniform coating can be explained from the perspectives of bonding between the polymers and the template. For PAAc-PC, hydrogen bonding originated from AAc plays a major role.26 With regards to P(AAc-co-AAm) -PC, both AAm and the template provide sufficient amino groups for crosslinking with glutaraldehyde, which should build up a strong bonding between the polymer and the template. TEM was also employed to investigate the inner structure of the samples; since the samples were embedded within epoxy for microtomy, it is impossible to distinguish between polymer coating and epoxy under TEM. However, the lamellae structure was found to reserve well in both PAAc-PC and P(AAc-co-AAm)-PC (Figure 2 inset). Therefore, the template photonic structure was well inherited in our polymer samples. TGA was employed to further investigate the composition of PCs in air. When it rose stepwise to 900°C, the template residue which mostly is inorganic substance was recorded at 11.0 wt% whilst PAAc and PAAm was completely exhausted as both are polymeric (Figure S1a,b). A residue content of ~6.0 wt% was found for PAAc-PC and P(AAc-co-AAm)-PC (Figure 3). The obvious difference in residue supports strongly that templates have been successfully coated with the polymer. The decomposition of the original butterfly wing has two main thermal decomposed peaks respectively located at around 300 and 500 °C (Figure S1c). However, an obvious thermal decomposition peak at 400 °C found for neat PAAm (Figure S1b) can not be seen in P(AAc-coAAm)-PC, which means the complete copolymerization. Fourier transform infrared spectroscopy (FT-IR) was employed to confirm interface reaction between the template and the polymers (Figure 4a). All of the samples exhibit absorptions of


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amide groups at ~1640 cm-1 relating to C=O stretching, amide I, 1550 cm-1 corresponding to in plane bending in non-acetylated 2-amino glucose primary amine, amide II, and 1380 cm-1 pointing to N−H stretching vibration, amide III. The amide groups can be derived from either chitin or chitosan or AAm. The characteristic absorption at 1720 cm-1 for all samples except the template is attributed to C=O derived from carboxylic acid. All of those reveal that template is successfully coated with the designed polymers. Different to the template (black curve) and PAAc-PC (blue curve), the P(AAc-co-AAm)-PC graph demonstrates a new absorption peak at 1626 cm-1, which is most likely due to –C=N stretching vibration27 derived from Schiff Base between the polymer layer and the template. Based on the foregoing discussion, we herein propose a formation mechanism for coating the polymer on the template. In Figure 4b, some part of chitin can transfer into chitosan through treating the butterfly wing with alkali, which produces –NH2 on the surface of lamellae. On the other hand, the copolymerization of AAm with AAc also adds more –NH2 groups. Then GA is used as a cross-linker to bridge the polymers with the template, as proved by FT-IR. The sensing performance of all samples was evaluated through pH buffer solution ranging 2.2– 10.5. As indicated in the Experimental, all of the samples were immersed into the solution for 10 min prior to the detection of their reflectance spectrums. Figure 5, Figure S2 shows the sensing performance for each PC. In specific, the optical images demonstrate that the PC color changes obviously with pH, corresponding to the reflection spectra. For instance, PAAc-PC is green in color at pH 2.2, relating to the highest reflection peak at 550 nm; λmax increases with pH; when pH is 10.5, λmax moves to 600 nm and the sample color turns brown, exhibiting an obvious red shift in the visible light region (Figure 5a). Nevertheless, P(AAc-co-AAm)-PC shows a different pH-sensing behavior with a distinct transition at pH 6.2 (λmax = 531 nm); when pH is 2.2, λmax


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reaches 555 nm; it shows a blue shift when pH increases to 6.2. On the contrary, while pH is beyond the transition, there is a sharp red shift until λmax is 610 nm at pH 10.5. Relevant optical image shows the sample color transforming from green to brown, with the total variation of reflection wavelength up to 80 nm (Figure 5b). PAAc hydrogel is widely studied as a smart polymer sensitive to pH. The carboxyl groups of PAAc are deprotonated when pH is over its pKa. These groups repel each other, swelling the hydrogel. On the other hand, PAAm hydrogel can be protonated to generate –NH3+ groups when pH is below its pKa. We thus propose a mechanism to explain the optical phenomena of these PCs. As shown in Figure 1c inset, the template is quite similar to the one-dimensional Bragg stacks, so we calculated photonic band gap for these PCs by the Bragg law below28 mλ = 2 (ntemplatedtemplate + ninterdinter) sin θ

(1)

Where m is the diffraction order, λ is the wavelength of Bragg diffraction, and ntemplate and dtemplate are respectively the refractive index (RI) and thickness of the template lamellar. ninter is the average RI for the filler between two lamellae, dinter is its thickness, and θ is the Bragg glancing angle. In this study Bragg glancing angles are fixed to be vertical, and the detection of light is under normal incidence to the film plane for both convenience and reproducibility. dtemplate and dinter would not change because the chitin is rigid having a high Young’s modulus of 1.3 GPa. We also investigated the spectral properties of template, and could confirm that the butterfly wing structure did not change with similar variation of pH (Figure S3). Both of these point out that the template parameters ntemplate, dtemplate and dinter are constant. Thus the wavelength of Bragg diffraction is determined by ninter only in this system. When polymeric PCs


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were immersed into buffer solution, the filler between lamellae consists of polymer layers and water. Thus the average ninter can be calculated as followed:

ninter = nhydrogel

!"#

+ nwater

$%# !"#

(2)

The refractive indices of PAAc, PAAm and water are 1.44, 1.45 and 1.33, respectively. According to the equation (2), the value of ninter would rise with increase in hydrogel thickness. In other words, the swelling/deswelling behavior of hydrogel can change the average ninter of the filler, thus posing an effect on the optical properties of polymer-PCs. For PAAc-PC, the PAAc hydrogel shrank at pH 2.2; as pH value rises, the hydrogel swelled leading to an increase in thickness; correponsingly ninter increased, resulting a red shift. However, while AAm is added to copolymerize with AAc, the macromolecular chain has a certain amount of amino groups. At pH 2.2, the amino groups of P(AAc-co-AAm)-PC transform to –NH3+ which repel each other causing the hydrogel coating to swell slightly. In this condition, when pH increases from 2.2, the carboxylic groups start deprotonating and then the electronic pairs of –COO- and –NH3+26 attract each other, which shrinks the hydrogel coating leading to a blue shift. With further increase in pH, more –COO- groups are transformed but fewer –NH3+ groups can be reserved. At a certain pH value, the quantity of such electronic pairs reaches to maximum and exhibits a distinct transition in optical spectrum. When pH is beyond the transition point, a large quantity of –COO- groups would result in a rapid rise of λmax. The whole process is shown in Scheme 2, which is easier to understand. Therefore, on the basis of discussion above, the transition pH value should depend on the ratio of AAc to AAm. To verify this hypothesis, we prepared another P(AAc-co-AAm)-PC* by using a ratio of AAc to AAm at 1:4 which is far different to the former. As expected, P(AAc-co-AAm)-


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PC* exhibited a similar pH-responsive behavior. In addition, a different pH value of transition was measured as 7.4 (Figure S8). P(AAc-co-AAm)-PC* containing more AAm needs a higher pH to reach a maximum quantity of electronic pairs where the hydrogel shrinks maximum. At a pH value of 10.5, λmax of P(AAc-co-AAm)-PC* was gauged at 595 nm and the total variation of reflection wavelength was around 65 nm, both which are lower than those of P(AAc-co-AAm)PC. Hence, an appropriate ratio of PAAc to PAAm is paramount to create a color change, and such desired change is advantageous for a pH sensor towards practical applications. Here we contribute the unique pH sensing performance to the change in ninter which is the average refractive index of the interspacing between multilayers. It is obvious that ninter closely relates to the hydrogel coating thickness according to equation (2). To support this mechanism, we used the finite-difference time-domain (FDTD) method to simulate the optical property changes of PAAc-PCs and P(AAc-co-AAm)-PCs as the pH rises. In the FDTD simulation, the biotemplate parameters were borrowed from the TEM micrograph analysis, with the RI of each layer fixed. Only the hydrogel thickness was set as a variable due to swelling. To deduce the thickness variation of the hydrogel coating on lamellae, we prepared neat PAAc and P(AAc-coAAm) hydrogels using the same protocol of materials and processing method; we then investigated their swelling behavior (Figure S5). Based on these experimental data, the pH sensing behaviors of PAAc-PCs and P(AAc-co-AAm)-PC were calculated and shown in Figure 6a, b (red curves), Figure S6. The calculated pH response basically agrees with the experimental results (black curves in Figure 6a, b), where only a slight difference is seen. It can be explained from the fact that each lamella is not perfectly flat and thus the change of the coated hydrogel thickness is not exactly proportional to the swelling evolution in the multi-lamellar space. Another reason is that the amino groups of chitosan in the scale should promote the interactions.


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However, the interactions are not considered in the simulation developed using the neat polymer gel. The PCs were investigated further for their durability; their optical evolution in a number of pH environment was recorded for several cycles. Figure 7 shows the reversible change of λmax as pH varies; PAAc-PC performs well in the first few cycles but becomes unstable as the total variation of λmax gradually reduces in later (Figure 7a). With further investigation by FT-IR, the characteristic absorption peak at ~1720 cm-1 of aldehyde group derived from carboxylic acid became weaker and slightly shifted to 1726 cm-1 (Figure S7) for PAAc-PC, indicating that the PAAc layer partly delaminated from lamellar with hydrogen bond destroyed. However, P(AAcco-AAm)-PC exhibits satisfactory reversible performance comparing with the former (Figure 7b). As proved in FT-IR, the interface between the P(AAc-co-AAm) hydrogel coating and the template is reinforced by chemical bonding, while only hydrogen bonding supports the interface for PAAc-PC, which can be negatively influenced by pH. Therefore, it is essential to employ chemical bonding to build up interface. This conclusion is further verified by another sample P(AAc-co-AAm)-PC* demonstrating high durability with the pH variation (Figure S8).


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Conclusion In summary, polymeric pH-responsive photonic crystals (PCs) having lamellar structures were developed by engineering a polymer P(AAc-co-AAm) with Papilio paris butterfly wings. The butterfly wings are coined by nature evolution, and they are more accessible than artificial PCs. Our crystals P(AAc-co-AAm)-PC exhibited an obvious, designed color evolution (spanning ~80 nm in wavelength) at broad pH. The pH responsive behavior was induced by the change of average refractive index of the interspacing between multilayers, due to the change in polymeric hydrogel thickness which was proved by both experimental analysis and simulation. The introduction of acrylamide into the PCs formed a uniform thickness coating of hydrogel on the template by chemical bonding, which enhanced the cycling performance of P(AAc-co-AAm)PC. The strategy developed herein can be extended to other polymers for the development of a wide variety of stimuli-responsive PCs, which have a great potential for label-free chemicals, biological detections, pH analysis, and photonic switches.


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ASSOCIATED CONTENT Supporting Information. TGA of pure PAAc and PAAm. DTA of P(AAc-co-AAm)-PC Dependence of diffraction resonance on pH for the bio-template. Experimental and simulated reflectance spectra of PAAc-PC, P(AAc-co-AAm)-PC. Swelling behavior of neat PAAc hydrogel and P(AAc-co-AAm) hydrogel as pH rising. FTIR spectra of PAAc-PC before and after cycling test. Spectral properties and cycling perfortmance of P(AAc-co-AAm)-PC* (the mass ratios of AAc to AAm is 1: 4). This material is available free of charge via the Internet at http://pubs.acs.org AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors gratefully acknowledge the financial support from Ministry of Science and Technology of China (2016YFA0202900), the National Science Foundation of China (NO. 51072117,), Shanghai Science and Technology Committee (13JC1403300), Science and Technology Planning Project of Guangdong Province (2016A010103018).


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Scheme 1. Synthesis process of PCs based on Papilio paris wings. Original butterfly wings were cleaned by a 10 wt% NaOH solution. Then the alkali-treated templates were immersed into the precursor solution for 10 hr at 4 °C to obtain an adsorption equilibrium. Later the templates were carefully taken out and polymerized at 60 °C for 12 hr, then we obtained polymer-PCs.


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Scheme 2. Characteristic optical response and hydrogel swelling at different pH values: a) hydrogel swells slightly since amino groups protonate at a pH value 2.2, and digital image shows green color (λmax = 555nm); b) when pH value is 6.2, positive and negative ions attract each other so hydrogel shrinks drastically leading to reduction in the layer thickness; c) at pH 10.5 a large quantity of carboxylic acid deprotonate and the negative ions fill the whole hydrogel network and such electronic repulsion makes hydrogel swelling extremely.

Figure 1. Papilio paris wing: (a) digital micrographs of a butterfly wing, (b) SEM micrograph of scales array on the surface of skeleton and (c) single scale, inset: TEM micrograph of the scale cross-section with lamellar structure.


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Figure 2. SEM image of (a) template, (b) PAAc-PC and (c) P(AAc-co-AAm)-PC. The inset images are TEM of cross section, respectively.

Figure 3. TGA spectra of butterfly wing template and polymer-PCs.


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Figure 4. (a) FTIR spectra of template, PAAc-PC, P(AAc-co-AAm)-PC respectively, and (b) mechanism of hydrogel coated on the lamellae of template


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Figure 5. Spectral properties of polymer-PCs: (a) PAAc-PC and (b) P(AAc-co-AAm)-PC. The error bar corresponds to the 5 spectrum data on the same sample. Relevant digital optical photographs were taken at a certain pH.

Figure 6. pH responsive behaviors of a) PAAc-PCs and b) P(AAc-co-AAm)-PC, by experimentation (black curve) and the FDTD mothed (red curve). The error bar corresponds to the 5 spectrum data on the same sample.


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Figure 7. The reflectance wavelength reversibility after multiple pH cycles of (a) PAAc-PC and (b) P(AAc-co-AAm)-PC.


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