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Structural Color Tuning: Mixing Melanin-Like Particles with Different Diameters to Create Neutral Colors Ayaka Kawamura, Michinari Kohri, Shinya Yoshioka, Tatsuo Taniguchi, and Keiki Kishikawa Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00707 • Publication Date (Web): 02 Apr 2017 Downloaded from http://pubs.acs.org on April 4, 2017

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Structural

Color

Tuning:

Mixing

Melanin-Like

Particles with Different Diameters to Create Neutral Colors Ayaka Kawamura†, Michinari Kohri*,†, Shinya Yoshioka‡, Tatsuo Taniguchi† and Keiki Kishikawa† †

Division of Applied Chemistry and Biotechnology, Graduate School of Engineering,

Chiba University, 1-33 Yayoi-cho, Inage-ku, Chiba 263-8522, Japan ‡

Department of Physics, Faculty of Science and Technology, Tokyo University of

Science, 2641 Yamazaki, Noda, Chiba 278-8510, Japan

Corresponding Author *E-mail address: [email protected] (M. K.)

KEYWORDS: polydopamine, structural color, neutral color, colloidal crystals, amorphous structures

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ABSTRACT We present the ability to tune structural colors by mixing colloidal particles. To produce high-visibility structural colors, melanin-like core-shell particles, composed of a polystyrene (PSt) core and a polydopamine (PDA) shell, were used as the components. The results indicated that neutral structural colors could be successfully obtained by simply mixing two different sized melanin-like PSt@PDA core-shell particles. In addition, the arrangements of the particles, which were important factors when forming structural colors, were investigated by mathematical processing with a 2D Fourier transform technique and Voronoi diagrams. These findings will provide new insights for the development of structural color-based ink applications.

INTRODUCTION The development of structural color materials is an attractive topic in materials science due to its numerous potential applications and scientific significance.1-3 A number of studies have addressed the preparation of structural color materials from monodisperse colloidal particles. Recent investigations have demonstrated that there are two types of structural colors based on the assembly of spherical colloidal particles. Angle-dependent structural colors are obtained from close-packed hexagonal structures of particles (called colloidal crystal structures).4-8 In contrast, angle-independent colors are created from roughly packed arrays of particles (called amorphous structures).9-16 Most studies have been focused on the preparation of colloidal particle-based structural color materials from monodisperse colloidal particles, and their colors were changed by altering the size of the colloidal particles. However, it has been hard to create neutral colors and to strictly control them. While it would be of great interest to investigate the use of more than one particle to produce neutral colors, few studies were found in the literature. For example, Takeoka et al. reported the preparation of colloidal amorphous arrays by mixing two differently sized silica particles.17 While the obtained arrays had short-range order and exhibited angle-independent structural colors, more detailed studies will be necessary to obtain any desired structural colors with a high visibility. Inspired by nature, many functional materials have been developed.18-27 In nature, some of the beautiful structural colors are created by subcellular-sized structures of melanin, and the color of male peacock feathers is a typical example.28,29 Melanin is produced by


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several enzymatic reactions of DOPA (3,4-dihydroxyphenylalanine).30-34 In recent years, some researches focused on the use of polydopamine (PDA)35-49, which is prepared by the self-oxidative polymerization of dopamine (2-(3,4-dihydroxyphenyl) ethylamine), as a mimetic material of melanin to produce structural color materials.50-55 Among them, we reported the biomimetic design of structural color materials inspired by both building materials and the microstructure of peacock feathers.56 Bright structural colors were successfully observed from melanin-like core-shell particles, composed of a polystyrene (PSt) core and a PDA shell, compared to conventional materials such as silica particles or polymer particles. In this method, the coefficient of variation (CV) of the particles remained approximately 3% after PDA coating, indicating the formation of monodisperse PSt@PDA core-shell particles.56 Since PSt@PDA particles that mimic melanin act as both components of the structural color material and scattering absorbers, high-visibility structural colors were formed using a single component. Although structural colors could be controlled by altering the size of the melanin-like particles, altering the size required strict control of the synthetic conditions of the PSt@PDA particles. Herein, we report structural color tuning by mixing melanin-like particles with different diameters. The main objective of this study was to produce neutral structural colors by simply mixing two kinds of PSt@PDA core-shell particles. Structural color pellets composed of PSt@PDA core-shell particles were fabricated by evaporating solvents from the particle suspensions, and the structural colors and particle arrangements of the pellets were controlled by the feed ratios of the two particles. The effects of various parameters on the structural coloration were investigated. The arrangements of the particles in the pellet samples were also investigated by mathematical processing with a 2D Fourier transform technique and Voronoi diagrams. The findings from this study will provide a promising step toward the development of structural color-based ink applications.

EXPERIMENTAL SECTION Materials. Dopamine hydrochloride (DA) was obtained from Sigma-Aldrich. Tris(hydroxymethyl)aminomethane (Tris) and styrene (St) were obtained from Kanto Chemical. 2,2’-Azobis(2-amidinopropane) dihydrochloride (V50) was obtained from


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Wako Pure Chemical. Deionized water with a resistance of 18.2 MΩ.cm was obtained using a Millipore Simplicity UV system. St was dried over calcium hydride and distilled under reduced pressure. All other chemicals and solvents were of reagent grade and were used as received. Measurements. Scanning electron microscopy (SEM) micrographs of the samples were obtained using a scanning electron microscope (JSM-6510A; JEOL). Reflection spectroscopies were performed using a spectrophotometer (V-650; JASCO) equipped with a reflection spectroscopy unit (ARSV-732; JASCO) and a microscopic spectrophotometer (MSV-370; JASCO). Photographs of the samples were taken with a digital camera (OM-D; Olympus). Optical microscopic images were measured using a digital microscope (VHX-500F; KEYENCE). 2D Fourier transforms of the SEM images were carried out using ImageJ64. Voronoi diagrams were obtained using Wolfram Mathematica 11. Preparation of the PSt@PDA core-shell particles. Monodisperse PSt@PDA core-shell particles with different diameters were synthesized as described in our previous paper.56 Briefly, the PSt core particles were synthesized by a soap-free emulsion

polymerization

of

St

and

N-butyl-N-2-methacryloyloxyethyl-N,N-dimethylammonium bromide (C4DMAEMA). St (3.54 g, 34 mmol), C4DMAEMA (3.00–7.35 mg, 0.010–0.025 mmol), and V50 (0.136 g, 0.50 mmol) were added to a three-necked flask filled with 100 mL of deionized water and deoxygenated by purging with argon for 15 min. The polymerization was initiated by heating to 60 °C with stirring at 200 rpm in an argon atmosphere. After 10 h, the PSt core particles were separated and purified repeatedly by centrifugation (14,500 rpm for 30 min) and redispersion. DA (60 mg, 0.32 mmol), Tris (1.45 g, 12 mmol), and PSt core particles (0.12 g) dispersed in deionized water (120 mL) were stirred for 20 h at room temperature. The PSt@PDA core-shell particles were separated and purified repeatedly by centrifugation (14,500 rpm for 30 min) and redispersion. Fabrication of structural color pellets. Structural color pellets were fabricated by mixing and pouring 10 wt% PSt@PDA core-shell particle suspensions onto a silicone rubber plate and allowing the suspensions to dry at room temperature for 12 h.


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RESULTS AND DISCUSSION PSt core particles with different diameters (232, 251, and 287 nm) were prepared by soap-free emulsion polymerization of St and hydrophilic comonomers, which were added to control the core particle diameters.56 PSt@PDA core-shell particles were then prepared by DA polymerization in the presence of the core particles. The synthesized products were designated as P1, P2, and P3 particles. The details of the P1, P2, and P3 particles are summarized in Table 1. According to our recent paper, the PDA shell layer thicknesses were set to approximately 5 nm to produce bright structural colors.56 The particles obtained were highly monodisperse and created colloidal crystal structures after drying. As shown in Figure 1, pellet samples from the P1, P2, and P3 particles exhibited bright blue, green, and red structural colors, respectively. The maximum values of the reflection spectra (λmax) of the pellets from P1, P2, and P3, measured with a microscopic spectrophotometer, were 470, 521, and 603 nm, respectively, which corresponded to blue, green, and red colors (Table 1).

Table 1. Summary of the P1, P2, and P3 Particles. Sample

Size of PSt

name

core particles thickness [nm]a [nm]

a

PDA shell

a

Diameter of the

λmax [nm]b

PSt@PDA core-shell particles [nm]a

P1

232

4.5

241

470

P2

251

4.0

259

521

P3

287

5.5

298

603

Measured by SEM. b The λmax values of the pellets from P1, P2, and P3 were measured

with a microscopic spectrophotometer.

To investigate the effects of particle mixing on the structural coloration, pellet samples were prepared after water dispersions of P1+P2, P1+P3, and P2+P3 particles were allowed to dry (Figure 1a). The mixing ratios of PSt@PDA core-shell particles with different diameters were varied as 100/0, 80/20, 60/40, 50/50, 40/60, 20/80, and 0/100. The obtained pellets exhibited bright colors because the PDA layers effectively


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absorbed scattering light. Additionally, neutral colors were obtained relative to the naked eye. As shown in Figure 1b, the optical microscopy images of the pellets also showed their bright colors and uniformity.

Figure 1. (a) Digital camera images and (b) optical microscopic images of structural color pellets created by mixing P1+P2, P1+P3, and P2+P3 particles.

Figure 2a shows the reflection spectra of the pellets prepared from mixing of P1 and P2 particles with different feed ratios. The reflection spectra were redshifted by increasing the ratio of the P2 particles. While the reflection spectra from pellets created with P1 or P2 particles were narrower, the spectra from the P1+P2 mixed particles were broadened, suggesting the turbulence of the arrangements of the particles. Figure 2b is a plot of λmax as a function of the amount of larger (P2) particles, indicating the production of neutral colors by particle mixing. In the present experiments, we checked the differences in different parts of the samples. Five points were randomly selected, and reflection spectra were measured. The intensities of reflectance spectra and λmax were not changed,


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clearly indicating the uniformity of the samples. Similar trends appeared in the reflection spectra of pellets from P1+P3 and P2+P3 particles. These data clearly indicate that the structural colors could be easily controlled by altering the mixing ratio of the two particles. The averaged particle sizes of each pellet were measured using SEM images. In each sample, the size of 100 particles was randomly measured to calculate the size, size distribution and coefficient of variation (CV) of particles. The average diameters increased expectedly with the percentage of the larger particle (241, 244, 246, 251, 253, 256, and 259 nm), in good agreement with the calculated value that is determined from the seed ratio (241, 245, 248, 250, 252, 255, and 259 nm). Figure 2c shows the plots of λmax as a function of the average diameter of the samples. The reflection peaks were redshifted as the average size of the particles increased, in good agreement with the theoretical line (dotted line in Figure 2c) drawn according to the Bragg-Snell’s law as shown below.7 ଼

mλ = ට ݀ଶ (݊ଶ − sinଶ ߠ) ଷ

(1)

In equation (1), m is the order of diffraction (m = 1), λ is the wavelength of light, n is the refractive index of the PSt@PDA core-shell particles (The n of the particles was set to 1.59 because our recent study showed that the n of the PSt@PDA core-shell particles with thin-shell layers (ca. 5 nm) was nearly the same as the n of the core particles.56), d is the center-to-center distance between the nearest particle, and θ is the angle between the incident light and diffraction crystal planes (θ = 90). Mixing only two particles enabled controlling the average diameter of the particles and creating a variety of structural colors, indicating the usability of the present method. Regarding Figure 2a, there were great differences between the highest reflectance values of the pellets. As plotted in Figure 2d, the percentage of the highest reflectance of the pellets from only P1 or P2 particles was approximately 40 %. On the other hand, the highest reflectance percentages decreased sharply to approximately 10 % by mixing P1+P2 particles. Figure 2d also shows the CVs of particles. While CVs of the P1 and P2 particles were low, the CVs of the mixed particles were relatively high, suggesting the influence of the CVs of the particles on their reflectance.


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Figure 2. (a) Reflectance spectra of the pellets from the P1+P2 particles with different ratios. (b) λmax from the reflection spectra versus the amount of larger (P2) particles. In the present experiment, P2 particles were added to the P1 particles. (c) Plots of λmax as a function of the average diameter of the samples. In each sample, the size of 100 particles was randomly measured to calculate average diameter. Theoretical line (dotted line) drawn using Bragg-Snell’s law. (d) Maximum reflectance and coefficient of variation (CV) of the mixed particles as a function of the amount of P2 particles.

SEM measurements of the pellet surfaces were performed to investigate the effect of particle mixing on the arrangements in more detail. In this experiment, P1 and P3 particles were used as the components. While there was a great difference in their size, the obtained pellets exhibited neutral colors (Figure 1). Figure 3a shows the SEM images of the pellet surfaces prepared by mixing P1 and P3 particles. In Figure 3b, the P3 particles were colored pink. The P3 particles in the pellets could be arranged in a


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disordered state since the core-shell particles with PDA layers were well dispersed in water due to their high zeta potentials (approximately −60 mV).56 Particle arrangements are usually discussed using spatial information from SEM images.11,17,50,57 Thus, we obtained 2D Fourier transforms of the SEM images of the particles in the pellets to characterize their spatial information (Figure 3c). While sharp hexagonal peaks were observed for the close-packed structures, such as colloidal crystal structures, circular patterns were produced from the roughly packed amorphous structures. As expected, hexagonal peaks due to colloidal crystal structures were obtained from pellets created from P1 or P3 particles. On the other hand, circular patterns were observed from mixed particles pellet, indicating the formation of amorphous structures. In the present method, the size distributions (CVs) of the particles were relatively high (vide supra) since two particles with different diameters were mixed. When colloidal assemblies are prepared from particles with high CV values, it is reasonable to assume they will produce amorphous structures. We have once also reported that amorphous colloidal arrays were produced from assembling relatively monodisperse polydopamine particles (CV: 7 %).50,53

Figure 3. (a) SEM images of the pellet surfaces prepared by mixing P1+P3 particles. Scale bars: 1 µm. (b) P3 particles are marked in pink. (c) 2D Fourier transform spectra from the SEM images.


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A Voronoi diagram is a well-known concept in computational geometry, and its algorithms and applications have been studied.58-60 A Voronoi diagram of a set of points is a collection of regions that correspond to one of the sites, and all the points in one region are closer to the corresponding site than to any other site. We investigated the disorderly arrangements of the particles using a Voronoi diagram. The weights of the particle centers in the SEM images are shown in Figure 3a and were calculated using Mathematica (Figure 4a). The Voronoi diagrams were also prepared using Mathematica (Figure 4b). Each Voronoi cell is shown as a convex polygon. In these diagrams, hexagonal polygons due to close-packed structures of particles were painted in white. The five-sided polygon, seven-sided polygon, and other polygons were painted red, blue, and green colors, respectively. The Voronoi cells on the boundary of the analyzed square region (dark gray) were not included in this analysis. As shown in Figure 4b, many white polygons existed in the diagrams created from the monodisperse P1 or P3 particles, indicating the formation of close-packed colloidal crystal structures. In the diagrams from two mixed particles, the ratio of white polygons decreased, suggesting the formation of a disordered state (Figure 4c). While assembled structures in pellet surface were well discussed, it remains unclear the morphologies of samples in the Z-direction by mixing two-sized particles, which will also influence the structural coloration. Additional studies in progress include the investigation of the effect of Z-direction structures on coloration.


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Figure 4. (a) Calculation of the weights of the particles centers in the SEM images shown in Figure 3a. Scale bars: 1 µm. (b) Voronoi diagrams and (c) number of polygons. Red: five-sided polygon, white: hexagonal polygon, blue: seven-sided polygon, and green: others. The Voronoi cells on the boundary of the analyzed square region (dark gray) are not included in this analysis.

In Figure 5a, the maximum reflectance is plotted as a function of the percentage of white polygons. The figure suggests the existence of two regions. In the first region, both the reflectance (> 40 %) and percentage of white polygons (> 80 %) show high values, and the colloidal particles assembled to form closed-packed colloidal crystal structures. When light reflected on the submicron-sized particles, reflection spectra due to the particle sizes were produced. In this region, Bragg diffraction due to ordered structure also appeared, showing high reflection intensities. In the second region of the relatively lower area, the colloidal particles formed amorphous colloidal structures. While wavelength-selective light scattering due to the particles sizes was selectively enhanced, the diffraction was suppressed. As a result, the maximum reflectance showed a relatively low value. Saturated neutral colors, however, were observed by the human eye because the PDA layer effectively absorbed the multiscattered light.50,56 Figure 5b presents the relationship between the CV of the particles and the percentage of white polygons. The use of highly monodisperse particles with CVs below 3 % was advantageous for creating colloidal crystal structures, which is agreement with previous reports. In contrast, roughly packed amorphous structures were easily formed by using colloidal particles with > ca. 8 % variation coefficients. While the details of the relationship between the reflectance intensities and disorder of the arrangements remained unclear, spatial information of the particles was obtained by mathematical processing.


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Figure 5. Plots of (a) the maximum reflectance and (b) CVs of the particles as a function of the percentage of white polygons.

Figure 6 shows the International Commission on Illumination (CIE) 1931 chromaticity diagram, and the colors of each pellet are plotted. The colored area encompasses the color range perceivable by the naked eye. By controlling the mixing ratio of the two particles, we have successfully nearly obtained the full range of colors. In other words, gradient structural colors were produced by combining two sizes of particles. Furthermore, this method enables the preparation of high-visibility structural colors with a simple process. In the CIE diagram, the reflectance intensities become low as the plot moves toward the center. Thus, there remains the challenge to create near neutral structural colors with high reflective intensity. The simplicity of this method, however, facilitates the potential use for some practical applications, such as ink applications.


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Figure 6. The CIE chromaticity plot of the colors prepared by particle mixing.

CONCLUSION In summary, we developed a facile strategy to produce neutral structural colors. By mixing two different sizes of melanin-like PSt@PDA core-shell particles, bright structural colors were controlled with a full color range. The average size of the particles in the pellet samples is an important factor for controlling the structural coloration. Thanks to the high zeta potentials of the PDA-covered particles, melanin-like particles were well dispersed in the solvent. As a result, the two combined particles in the pellet were arranged in a disordered state. The arrangements of the particles strongly affected the reflectance intensities. The spatial information of particles in the pellets was investigated using 2D Fourier transforms of the SEM images. In addition, we demonstrated the usability of Voronoi diagrams to discuss the particle arrangements in solid conditions. This new type of bright structural color tuning method


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using melanin-like particles will be extremely important for the development of structural color ink applications.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] (M. K.) Notes The authors declare no competing financial interests.

ACKNOWLEDGEMENTS M. K. acknowledges the support of a JSPS KAKENHI (Grant Number 15H01593) in Scientific Research on Innovative Areas “Innovative Materials Engineering Based on Biological Diversity”, the Noguchi Institute, Konica Minolta Science and Technology Foundation, and a Chiba University Venture Business Laboratory project.

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