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Chitin-Nanofibers Extracted from Crab Shells in Broadband Visible Anti-Reflection Coatings with Controlling Layer-by-Layer Deposition and the Application for Durable Anti-Fog Surfaces Kengo Manabe, Chie Tanaka, Yukari Moriyama, Mizuki Tenjimbayashi, Chiaki Nakamura, Yuki Tokura, Takeshi Matsubayashi, KyuHong Kyung, and Seimei Shiratori ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b11786 • Publication Date (Web): 01 Nov 2016 Downloaded from http://pubs.acs.org on November 2, 2016

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ACS Applied Materials & Interfaces

Chitin-Nanofibers Extracted from Crab Shells in Broadband Visible Anti-Reflection Coatings with Controlling Layer-by-Layer Deposition and the Application for Durable Anti-Fog Surfaces

Kengo Manabe,† Chie Tanaka,† Yukari Moriyama,† Mizuki Tenjimbayashi,† Chiaki Nakamura,‡ Yuki Tokura,† Takeshi Matsubayashi,† Kyu-Hong Kyung,§ and Seimei Shiratori*,†‡

†

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan ‡

Department Applied Physics and Physico-Informatics, Fuculty of Science and Technology, Keio

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan §

SNT Co., Ltd., 7-1 Shinkawasaki, Saiwai-ku, Kawasaki, Kanagawa 212-0032, Japan

[email protected]*

ACS Paragon Plus Environment

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KEYWORDS biomass, layer-by-layer, anti-reflection, nanofiber, chitin, anti-fogging, self-assembly, durability

ABSTRACT

Reflection from various surfaces of many optical systems, such as photovoltaics and displays, is critical issue for their performances, and anti-reflection coatings play a pivotal role in a wide variety of optical technologies, which reduce light reflectance loss and hence maximize light transmission. With the current movement towards optically transparent polymeric media and coatings for anti-reflection technology, the need for economical and environmentally friendly materials and methods without dependence on shape or size has clearly been apparent. Herein, we demonstrate novel anti-reflection coatings composed of chitin nanofibers (CHINFs), extracted from crab shell as a biomass material, through an aqueous-based layer-by-layer self-assembly process to control the porosity. Increasing the number of air spaces inside the membrane led low refractive index, and precise controlling refractive index derived from the stacking of the CHINFs achieved the highest transmittance with investigating the surface structure and the refractive index depending on the solution pH. At a wavelength of 550 nm, the transmittance of the coatings was 96.4%, which was 4.8 % higher than that of a glass substrate, and their refractive index was 1.30.


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Further critical properties of the films were the durability and the anti-fogging performance derived from the mechanical stability and hydrophilicity of CHINFs, respectively. The present study may contribute to a development of systematically designed nanofibrous films which are suitable for optical applications operating at a broadband visible wavelength with durability and anti-fog surfaces.

INTRODUCTION

Sophisticated biointerfaces, such as the surfaces of lotus leaves,1-3 pitcher plants,4-6 jewel beetles,7,8 honeycomb,9,10 and natural opals11,12 have exceptional abilities to solve a wide variety of problems, and motivated by both pure scientific interest and industrial applications, many researchers have developed novel strategies for controlling micro/nanoarchitectures. Excellent examples of innovative design concepts to achieve optical advantages are anti-reflection (AR) of moth eyes,1315

which have a highly controlled refractive index, and glasswing butterflies Greta oto,16,17 which

have the small nanopillars covering the transparent regions of its wings. In particular, developing synthetic surfaces mimicking the AR of the moth eyes have been the subject of extensive biomimetics studies during the past decade.18-20 The performance of many optical systems, including large area, cost sensitive products such as displays, architectural glazing, photovoltaics, and solar thermal collectors, are highly dependent on the use of AR coatings which can effectively enhance the optical transparency with reducing


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light reflectance loss.21 AR coatings derived from single-layer should meet the following 2 requirements:22 𝑑 = 𝜆⁄4𝑛𝑓 𝑛𝑓 = (𝑛𝑎 𝑛𝑠 )1⁄2

(1) (2)

where 𝑑, 𝜆, 𝑛𝑓 , 𝑛𝑎 , and 𝑛𝑠 are the film thickness, wavelength of the incident light, the refractive index of the film, and the refractive index of air, and the refractive index of the substrate, respectively. In case that the substrate is glass, 𝑛𝑠 is 1.52, and the 𝑛𝑓 should be 1.23. Therefore, to fabricate low refractive index coatings, porosity should be introduced into a thin film, because such a natural material does not exist.23 To create much airspaces, porous films were created using silica nanoparticles or cellulose nanowires in the previous researches. Cohen et al. successfully achieved AR coatings via single and graded index designs by synthesized hollow silica nanoparticles with variable particle size and shell thickness.24 Kotov et al. reported strong AR properties having an origin in a novel highly porous architecture created by randomly oriented and overlapping the cellulose nanowires.25 Other study fabricated porous films through acid or base treatment after layer-by-layer (LbL) selfassembly.26,27 Also, the recent report from our group demonstrated wide-range control of the pore size of a porous polyelectrolyte structure achieved by metal ions.28 Despite considerable efforts, however, there remain many challenges to achieve the multi-functional AR films, such as durability and anti-fogging, that are composed of economical and environmentally friendly materials and methods without dependence on shape or size.29,30


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The idea to achieve such films was conceived from nanofibrous membrane derived from natural resources, chitin nanofibers (CHINFs). CHINFs have attracted much attention because of their high mechanical strength and biomass resource.31,32 The shells of crustaceans, represented by crabs, are composed of mineral salts, proteins, and chitin, which demonstrate high tensile strength and bendability.33 The mineral salts and the proteins can be easily removed by HCl and NaOH treatments, respectively.34 Chitin is a (1,4)-β-N-acetyl glycosaminoglycan-repeating structure and a semicrystalline biopolymer with nano-sized fibrillar morphology by strong hydrogen bonding derived from two hydroxyl groups and an acetamide group.35 Although CHINFs have no electrostatic charge on their surfaces, in the case of using only these procedures, the CHINFs surface can be transformed from chitin to chitosan by deacetylation, which results in a positive charge on the CHINFs, due to the presence of amine groups.36-38 As one of the typical examples, Isogai et al. designed unique AR coatings by combining advantages of anionic cellulose nanofibers and cationic CHINFs.39 Other example of utilizing CHINFs reported by our group was Nepenthes inspired slippery liquid-infused AR films with transmittance of 97% constructed from CHINFs and silica nanoparticles.40 However, to the best of our knowledge, few studies have achieved AR properties derived from porous structures consisting of CHINFs and water-soluble polymers instead of the fibers and the nanoparticles.41 Therefore, the primary aim of the present study was on developing a porous CHINFs/polymer film with a low refractive index for AR property. Here, to extract our CHINFs, the mineral salts and protein were removed by the abovementioned HCl and NaOH treatments at first. After the deacetylation treatment for the CHINFs, the solution-based LbL self-assemblies of CHINFs (positive charge) and poly(acrylic acid) (PAA, negative charge) achieved highly controlled optical properties and stability, derived from the nanoscale architecture and the electrostatic interactions, respectively. The coatings showed lower


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refractive index, which indicated the formation of a porous structure with film-thicknessdependent porosity.42-47 Additionally, incorporation of CHINFs opened the door for improvement of their abrasion resistance and anti-fogging properties derived from high mechanical strength and high hydrophilicity of chitin, respectively. The present study may demonstrate that crab shells have the potential to effectively develop functional thin films from natural biomacromolecules resources without disposing of them as waste materials.

EXPERIMENTAL SECTION

Materials. Crab shells (Kawai Hiryo, Iwata, Japan), PAA (Mw~5,000, 50 wt% aqueous solution, Sigma-Aldrich, St. Louis, MO, USA), and glass substrates (refractive index: 1.52, 76 × 26 mm, thickness: 1.0 mm, Matsunami Glass Ind., Ltd., Kishiwada, Japan) were used to produce AR films. All polyelectrolyte dipping solutions were made from ultra-pure water (Aquarius GS-500.CPW, Advantec Toyo Kaisha, Ltd., Japan), and the pH was adjusted using NaOH and HCl (Kanto Chemical Co., Inc., Tokyo, Japan). The glass substrates were washed in a KOH solution and water.6 NaClO2 and NaBH4 were purchased from the Kanto Chemical Co, Inc. (Tokyo, Japan). Poly(allylamine hydrochloride) (PAH; Mw=58,000, 10 mM, Sigma-Aldrich Co. LLC, MO, USA) and SiO2 nanoparticles (Nissan Chemical Industries, Ltd., Tokyo, Japan) were used to fabricate the films for comparison with the durability. For the refinement of CHINFs, the confirmed method in the previous studies was used.34,48,49 The extracted CHINFs were deacetylated in NaOH solution (33 wt%) containing 0.03 g NaBH4 with stirring for 4 h at 90°C.


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Preparation of the low refractive index layer. The low refractive index films were developed using the deacetylated CHINFs and 1 mg/mL PAA solution at different pH via LbL. The cationic CHINFs and anionic PAA were alternately deposited on the glass substrates by immersing into each solution with water rinsing for 3 min and drying in jet air from a distance of 5 mm at 0.05 MPa after the each deposition.

Characterization. To observe the morphology of CHINFs, transmission electron microscope (TEM; CM120, Royal Philips, Amsterdam, Netherlands) was used. Deacetylation ratio of the extracted CHINFs was masured by fourier-transform infrared spectroscopy (FT-IR; ALPHA-T, Bruker, Billerica, MA, USA), based on the previous study.50 Ellipsometry (Mary-102, FiveLab Co., Ltd., Saitama, Japan) determined the film thickness and the refractive index. Ultra-visible (UV-vis) spectrometer (UVmini-1240, Shimadzu Corporation, Kyoto, Japan) was used for transmittance measurements. Scanning electron microscope (SEM; S-4700, Hitachi, Ltd., Tokyo, Japan) and atomic force microscope (AFM; Nanoscope IIIa, Digital Instruments, Milano, Italy) images determined surface structures. Zeta-potential analyzer (ELS-8000, Otsuka Electronics Co., Ltd., Osaka, Japan) was used for zeta-potential measurements of the deacetylated CHINFs. X-ray photoelectron spectroscopy (XPS; JPS-9010TR, JEOL, Tokyo, Japan) with an MGKα laser was performed to investigate the chemical composition of the porous surface. For abrasion tests, the coatings were exposed to a piece of cotton moving across them at a distance of 30 mm and a speed of 50 mm/min with a 100 g/cm2 using an abrasion machine. After cooled in a refrigerator at


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temperatures below -20 degrees for 30 min, the half-coated glass substrate was returned to a roomtemperature environment.

RESULTS AND DISCUSSION Preparation of CHINFs solution. To develop CHINFs thin films, a CHINFs solution was prepared along with the procedure designed in the previous study (Figure 1a).34,48,49 After the deacetylation, the powder of CHINFs formed the aggregation of fibers as shown in the SEM image (Figure 1b). The TEM image and AFM image of the refined CHINFs derived from the diluted CHINFs solution showed that each CHINF was fully independent of the others (Figure 1c and d). It is known that chitosan is not protonated when the solution pH is over 6.5, and the zeta-potential of a CHINFs solution decreases to 0 as the pH increases (Figure 1e).51,52 The deacetylation ratio of the extracted CHINFs was 1.82%.


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Figure 1. (a) Outline of the experimental procedures to refine CHINFs and prepare the solution for LbL. (b) SEM image of CHINFs powder. (c) TEM and (d) AFM image of CHINFs refined from crab shells. (e) zeta potential of CHINFs with different pH values.


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Influence of changes in the pH. The surface structures of the LbL films changed drastically with changing solution pH for the CHINFs and PAA (Figure 2). Here, we examine them in different sections, region I and region II as follows. Region I: influence of CHINFs’ pH. When the solution pH for CHINFs was approximately pH 5-6, the surface of the fabricated LbL film was not homogeneous, and significant aggregation of the CHINFs was observed on the surface. Hereafter, the films in this study are expressed as (cationic polymer and its pH/anionic polymer and its pH)N (N: number of layers). In comparison with the surface structures of the (CHINFs 3.3/PAA 3.3)10, that of the (CHINFs 5.0-6.0/PAA 3.3)10 in the region I (Table 1) was more inhomogeneous, due to the aggregation of the CHINFs and the PAA (Figure 3a and b). When the CHINFs had low charge density in the solution at pH 5-6, the nanofibers were easily aggregated (Scheme 1a), resulting in the surface of the fabricated LbL films were not homogeneous, while the nanofibers were individually dispersed at high charge densities (Figure 1). The aggregation was a trigger to decrease the transmittance of the (CHINFs 5.0/PAA 3.3)10 (Figure 3c). Therefore, the films at this solution pH did not demonstrate any AR performances with the visual observation of the CHINFs aggregation.


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Figure 2. Surface structures of 10-bilayer films depending on the solution pH of CHINFs (vertical column, white boxes) and PAA (horizontal row, black boxes).


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Figure 3. The influence of changing the CHINFs' pH. The surface of (a) a (CHINFs 3.3/PAA 3.3)10, (b) a (CHINFs 5.0/PAA 3.3)10, and (c) transmittance results of both the films and a non-coated substrate.

Scheme 1. Possible conformation changes of (a) fibers and (b) weak polyelectrolyte chains at different charge densities depending on each solution pH.


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Table 1. Refractive index of each film depending on the solution pH. The vertical (white) and horizontal boxes (black) show the pH values of PAA and CHINFs, respectively. The shaded area indicates the films with the refractive index below 1.33.

The (CHINFs 3.3/PAA 3.3)10 film was homogeneous, because the CHINFs with a pH of 3.3 were well dispersed, due to the high charge density. In contrast, as the above mentioned CHINFs aggregation under the low charge density at pH 5.0, the (CHINFs 5.0/PAA 3.3)10 film was not homogeneous. These phenomena were investigated by diameters of the aggregates using dynamic light scattering (Table 2). As shown in the table, both the particle diameters depended on the zeta potential of CHINFs. The primary particle diameter slightly increased as the solution pH for the CHINFs increased. The primary diameters of the aggregates in the solutions with pH values


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of 3.3 and 6.0 were 145.5 nm and 201.3 nm, respectively. The secondary diameters were 1910.0 nm and 4114.7 nm. Considering the dissociation constant and zeta potential of the CHINFs and PAA solutions, the CHINFs and PAA were likely to form large aggregates around pH 6.0 while the secondary diameter of the aggregates in the solutions of pH 2.5, 4.0, and 5.0 were 2070.7 nm, 2079.3 nm, and 3952.3 nm, respectively. It was found that the occurrence of the inhomogeneous structures—i.e., aggregation between fibers—increased with increasing pH of the CHINFs solution (Scheme 1a). The refractive index of (CHINFs 3.3/PAA 3.3)10 film was 1.31, while that of (CHINFs 5.0-6.0/PAA 3.3)10 film was around 1.40 (Table 1). The SEM and refractive index results clearly demonstrated that there was a loss of porous structure, caused by the blocking of many of the pores by the aggregates. (CHINFs 5.0-6.0/PAA 3.3)10 does not have with antireflection properties. As a result, the refractive index of (CHINFs 5.0-6.0/PAA 3.3)10 was increased compared with that of (CHINFs 3.3/PAA 3.3)10 (Region I in Table 1).

Table 2. Average particle diameter at various pH values in the CHINFs solution. pH

2.5

3.3

4.0

5.0

6.0

Primary particle [nm]

156.8

145.5

158.4

171.5

201.3

Secondly particle [nm]

2070.9

1910.0

2079.3

3952.3

4114.7


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Region II: influence of PAA’s pH. In addition, as mentioned in a previous study, weak polyelectrolytes are likely to be affected by the solution pH,39 resulting in possible three configurations (loop, tail, and train) for their adsorption (Scheme 1). The loop and the tail configurations should appear at low cationic/anionic charge densities, while the only train configuration should occur at high cationic/anionic charge densities (Scheme 1b).53-55 In region II (Table 1), the multilayer films fabricated at pH 5.0-6.0 in PAA solution were almost flattened as they buried the porosity, and showed an increased refractive index of approximately 1.45. Comparing the shaded area with region II in the SEM image shown in Figure 2 and Table 1, it was found that more PAA was adsorbed on the surface of the previously deposited CHINFs layer, due to the large electrostatic forces, which results in the increase of the film refractive index, because the train configuration by PAA buried the porous CHINFs structures. The loop and the tail of PAA with dispersing CHINFs achieved the porosity of (CHINFs 3.3/ PAA 3.3)10.

Influence of both the pH. The pH regions in which the refractive index of the films was less than 1.33 are shown by the shaded area (Table 1). A careful examination of the table revealed that the diagonal pH values for the fabricated films corresponded to refractive indices of less than 1.33, and especially the (CHINFs 5.0/PAA 5.0)10 film demonstrated the lowest value 1.30 of refractive index. This showed that porous films were easily formed in these pH regions. We consider that the balance of the dissociation constant and the zeta potential of each polyelectrolyte caused the CHINFs to form uniform and porous structures with PAA, under the relatively strong electrostatic forces. Since the CHINFs and the PAA did not interact with each other in the shaded area shown in Table 1, no aggregation occurred. The LbL deposition of the (CHINFs/PAA) film under


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carefully controlling pH can achieve the formation of highly porous structures, and continuous (CHINFs/PAA) film deposition could be performed without burying the pore. Dependency of the number of bilayers and AR properties. Figure 4 shows the thickness and the refractive index of the (CHINFs 5.0/PAA 5.0) film as a function of the number of layers. As shown in these bars, the thickness of the (CHINFs 5.0/PAA 5.0) film increased linearly while the refractive index continued to decrease as the increase of bilayers and ceased to decline after 12 bilayers. The optical transmittance of each bilayer depended on the film thickness and the refractive index (Figure 5). The lower refractive index the film had, the higher transmittance value appeared at the peak, and the peaks were shifted as increasing the film thickness. All films demonstrated AR performances, resulting in the solid evidence that the porous structures derived from CHINFs and PAA with the carefully controlled pH were effective AR coatings.

Figure 4. The growth (gray bars) of the (CHINFs 5.0/PAA 5.0) film with the refractive index (black diamonds) as a function of the number of layers.


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Figure 5. UV–vis light transmittance spectra of the glass substrate and (CHINFs 5.0/PAA 5.0) films, for different numbers of layers (glass substrate, black line; 6-bilayer, red line; 8-bilayer, yellow line; 10-bilayer, green line; 12-bilayer, blue line; and 14-bilayer, purple line).

Evaluation of the durability. To achieve multi-functional properties, the surface of (CHINFs 5.0/PAA 5.0)10 film was investigated before and after an abrasion test (Figure 6). The SEM image indicated that the pores were obscured on the outermost surface of the film after the test. Even after the abrasion tests, the structure of the film was maintained, owing to the strength of the CHINFs. Figure 7 shows the relationship between the transmittance and the number of abrasion cycles. The squares are the transmittance of the glass substrate, and the circles are that of the fabricated films after the abrasion test. The transmittance of 96.4% was 4.8% higher than that of the bare substrate. After the abrasion test was performed for 3000 cycles, the transmittance of 95.0% was still 3.5% higher than that of


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the bare glass substrate. This clearly demonstrated that the fabricated film showed excellent resistance against abrasion. To confirm the abrasion resistance, the thickness and refractive index of the films for the number of abrasion cycles was measured (Figure 8). Although the refractive index increased after the abrasion tests, the refractive index remained approximately 1.35 after abrasion tests performed for 3000 cycles while the thickness remained the same after abrasion tests performed for 3000 cycles. For the comparison, (PAH/SiO2)10 was prepared via LbL. The SEM showed the frailty of the (PAH/SiO2)10 film after the abrasion tests for 10 cycles (see figures in supporting information), which should be caused the smaller contact area between the SiO2 nanoparticles than that in the films composed of CHINFs. The (CHINFs 5.0/PAA 5.0)10 film had abrasion resistance properties, due to the tangling of the CHINFs. This phenomenon suggested that their high mechanical stability is one of the advantages of CHINFs. Furthermore, the (CHINFs/SiO2)10 film and the (CHINFs/PAA+SiO2)10 film were also prepared for the comparison of durability. As is the case with the (PAH/SiO2)10 film, the former film was easily collapsed by the abrasion. However, the latter film maintained their optical property after the abrasion tests for 100 cycles, which indicates that the entanglements consisting of CHINFs and PAA should improve the abrasion resistance regardless of the existence of particles.


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Figure 6. SEM images taken (a) before and (b) after the abrasion tests for 3000 cycles.

Figure 7. Relationship between transmittance and abrasion cycles. The squares are the transmittance of the glass substrate, and the circles are that of the films after the abrasion tests.


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Figure 8. Thickness (gray bars) and refractive index (black diamonds) before and after abrasion tests performed for the number of abrasion cycles.

Anti-fogging performance. The (CHINFs/PAA) films also had anti-fogging properties, due to their hydrophilicity. After being cooled in a refrigerator at temperatures below −20°C, the uncoated glass was immediately covered with fog when it was moved into a room-temperature environment. The fog formed easily on the non-coated glass substrate, because of the aggregation of moisture droplets with each other. The reason for this was that small droplets from the moisture condensed easily on the uncoated part of the substrate, and strongly scatter the light. In contrast, the coated glass retained its high transparency (Figure 9). In the previous studies, the anti-fogging property is derived from the following 2 points; hydrophilic surfaces with water droplet contact angles of less than 40°, and strong hydrogen-bonding groups, such as hydroxyl and/or ether groups.29,56-58 Here, the contact angle of a 10-μL water droplet on the (CHINFs/PAA)10 was 15° (i.e., it was hydrophilic), and the


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CHINFs have hydrogen-bondings originating in hydroxyl and acetamido group. Therefore, the (CHINFs/PAA) film can meet both above mentioned criteria and showed anti-fogging properties; on the super hydrophilic part of the substrate with the strong hydrogen-bondings, the small droplets of moisture easily spread, and were therefore not visible.59,60

Figure 9. Photo image of the cooled glass slide with and without coating at room temperature after being cooled in a refrigerator at temperatures below −20°C.

CONCLUSIONS

In conclusion, highly porous structures composed of CHINFs like a flattened pile of matchsticks were developed using the layer-by-layer self-assembly method, and the precisely controlling pH of chitin nanofibers and PAA achieved the best condition to exploit the low refractive index, which


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results in the high-quality anti-reflection films. Furthermore, these films demonstrated the durability and antifogging properties. Although the investigation into the detailed mechanism of mechanical properties and stability, such as abrasion resistance, remain to be determined as future challenges, such further study of porous strategy derived from chitin nanofibers as biomass resources through environmentally friendly method could yield a prospective candidate for various practical applications in real world, especially as optical applications operating at a broadband visible wavelength with durability and anti-fog surfaces.

ASSOCIATED CONTENT Supporting Information. This material available free of charge via the Internet at http://pubs.acs.org/. XPS spectra of CHINFs; SEM images of the widespread LbL surfaces depending on the solution pH of CHINFs and PAA; Surface structures of each bilayer examined by AFM; Transmittance changes and SEM images before and after 100-cycle abrasion on (CHINFs 3.3/PAA 3.3)10 films; SEM images of (PAH/SiO2)10 after abrasion tests performed for 10 cycles; Transmittance changes and SEM images before and after 100-cycle abrasion on (CHINFs/PAA+SiO2)10 film; Transmittance changes before and after 100-cycle abrasion on (CHINFs/SiO2)10 film; List of anti-reflection/antifogging studies using LbL or other methods.

AUTHOR INFORMATION Corresponding Author


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*E-mail: [email protected]. Present Addresses †

Center for Material Design Science, School of Integrated Design Engineering, Keio University,

3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522 Japan ‡

Department Applied Physics and Physico-Informatics, Fuculty of Science and Technology, Keio

University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama, Kanagawa 223-8522, Japan §

SNT Co., Ltd., 7-1 Shinkawasaki, Saiwai-ku, Kawasaki, Kanagawa 212-0032, Japan

Author Contributions K.M. conceived, designed and carried out the experiments, and analyzed the data. K.M. and C.T. wrote the paper. K.M., C.T., and K-H.K. designed equipment. Y.M., M.T., C.N., Y.T. and KH.K. provided experimental support and support in data analysis. K.M., C.N., M.T., and T.M. built lists of the references in the supporting information. S.S. supervised the project, gave scientific advice and commented on the manuscript. Funding Sources We received funding from the Keio Leading-Edge Laboratory of Science and Technology (KLL) [KLL research grant No. 60 in 2015 and No. 34 in 2016, awarded to K.M.]. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT


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We would like to thank Dr. Kouji Fujimoto and Ms. Nanae Fukao who provided carefully considered feedback and valuable comments. We also owe a debt to Dr. Yoshio Hotta who provided technical and scientific help. Additionally, we thank Prof. Michael F. Rubner of MIT for useful discussions.

ABBREVIATIONS CHINF, chitin nanofiber; PAA, poly(acrylic acid); LbL, layer-by-layer; AR, anti-reflection.

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