2011 - Imitation of Variable Structural Color in Paracheirodon Innesi Using Colloidal Crystal Films

Imitation of variable structural color in paracheirodon innesi using colloidal crystal films

Hailin Cong,1,2*

Bing Yu,1,2

and Xiu Song Zhao2,3

1 College of Chemical and Environmental Engineering, Qingdao University, Qingdao 266071, China

2 Laboratory for New Fiber Materials and Modern Textile, Growing Base for State Key Laboratory, Qingdao University, China

3 Department of Chemical and Biomolecular Engineering, National University of Singapore, 4 Engineering Drive, 117576, Singapore

*[email protected]

Abstract: Spacing variation of adjoining reflecting thin films in iridophore is responsible for the variable interference color in the paracheirodon innesi. On the basis of this phenomenon, colloidal crystal thin films with different structures are fabricated from monodisperse poly(styrene-methyl methacrylate-acrylic acid) (PSMA) colloids. The relationship between the colors and structures of the films is investigated and discussed according to the principle of light interference. A two-layer colloidal film having uniform color is researched and it displays diverse colors before and after swelling by styrene (St), which can be used to mimic the variable structural color of the paracheirodon innesi.

©2011 Optical Society of America

OCIS codes: (310.6860) Thin films, optical properties; (330.1690) Color.

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1. Introduction

The beautiful colors of morpho butterfly wings and sea mouse setae are not produced from pigments but from the order array of small dots or fine lines [1–5]. The diffraction and scattering of light by the microstructures on the animal’s body tell us nature got wise to photonic arrays long ago. The colors generated by the microstructures are called structural colors, which are important not only in biology but also for the understanding of photonic crystals [6–10].

In the biological world, many animals including paracheirodon innesi, hercules beetle, paradise whiptail, blue damselfish, and tortoise beetle have variable structural colors in response to environmental stimuli due to interference of light on their ordered body microstructures, which is essential to the camouflage, predation, communication, and reproductive behaviors of the species [11–14]. For instance, the blue damselfish normally displays a structural color of blue, produced by interference of light on periodically stacked microstructures of reflecting plates in skin cells. Under stressful conditions, the fish changes its blue coloration rapidly to ultraviolet, triggered by the simultaneous spacing variation of adjoining reflecting plates, to evade its pursuers [15]. While the hercules beetle alters color by varying the amount of water in the cuticle and thereby the thickness variation of the thin skin films is responsible for the interference color [16].

In the photonic world, the ordered three-dimensional (3D) packing of monodisperse colloids to form colloidal or photonic crystals with tunable structural color has been extensively investigated recently [17–21], and it is believed that these materials could play an important role in the field of optical devices and communications [22–26]. By use of the changes of structural color generated from Bragg diffraction, a lot of functional and intelligent materials such as tunable band-gap materials, environment (pH, temperature, ionic, and stress etc.) sensitive materials, photonic papers, and color changeable coatings have been developed [27–33]. However, compared with the Bragg diffraction commonly taking place at these materials, the phenomenon of light interference is greatly neglected.

Like the interference of light appearing at the aforementioned fishes and beetles, this phenomenon could also be occurring at colloidal or photonic crystals and causing them to show variable structural colors. In this article we report that diverse colors are observed in colloidal crystal thin films upon perpendicular irradiation of white light. An explanation for this unique optical phenomenon according to the principle of light interference has been proposed, and the application of this phenomenon in bionic and photonic materials has been discussed preliminarily.

2. Materials and methods

PSMA copolymer latex was synthesized by emulsion copolymerization of styrene(St), methyl methacrylate(MMA), and acrylic acid(AA) according to a method described elsewhere [34]. The particle diameter of a typical sample was ~380 nm and almost monodisperse.

Stairlike colloidal thin films were fabricated by depositing the colloids on an inclined silicon substrate as described previously [34]. The two-layer colloidal film to mimic the variable structural colors of paracheirodon innesi was achieved by dipping method. First, a

suspension of the PSMA colloidal particles was diluted to 10 mg·mL1

using deionized water. Then, a silicon wafer (60 mm × 12 mm × 1 mm) was immersed vertically into the dispersion

and lifted up at a constant speed of 0.2 μm·s1

for 20 h, which was accurately controlled by a precision step motor (Micros VT-80, Germany). After the wafer was lifted out of the dispersion and dried in the air for 12 h, a large area of colloidal thin film (> 1 cm

2) with equal

thickness was obtained. All the experiments were operated at regular room condition. A reference sample of very thick 37-layer colloidal crystal film (thickness: ~11 μm) was made

using the same dipping condition with a colloid concentration of 300 mg·mL1

. A scanning electron microscope (SEM, Amray-1910FE, USA) was used to observe the structures and morphologies of the thin films. To reduce charging effects, samples were sputter-coated with


a thin layer of gold (5 nm) prior to the SEM exam. An Atomic force microscope (AFM,

Nanoscope III A, USA) attached with an optical microscope with CCD camera (Sony XC-999P, Japan) was used to study the structures and colors of the film under perpendicular irradiation of white light. The structural colors of the paracheirodon innesi and colloidal films were characterized using a UV-Vis reflection spectrometer with a fixed measuring angle of 90° (Ocean Optics USB-2000, USA).

3. Results and discussion

3.1 Variable structural color in paracheirodon innesi

The lateral stripe of the paracheirodon innesi looks cyan normally, while the color changes to yellow when the fish is excited or under stress [35–37]. Lythgoe and Oshima et al. have revealed that periodically stacked microstructures of guanine crystals inside oval iridophores form the lateral stripe [35–39]. As illustrated in Fig. 1, the oval iridophore contains periodically stacked thin platelets of guanine crystals, which leads to structural color on the basis of multilayer thin film interference [35–40]. Lythgoe et al. further confirmed the correspondence between the spacing of the platelets and the wavelength of the reflected light [35–37]. And they found that the inflow of water may cause swelling of the cell and consequently increase the spacing among the platelets, which leads to variable structural color due to the swell-caused spacing variation mechanism.

Fig. 1. Photos and illustration of the variable structural color in paracheirodon innesi.

Figure 2 shows the UV-Vis reflection spectra of the variable structural color in the lateral stripe of paracheirodon innesi. Before the swelling of the oval iridophores, the lateral stripe of paracheirodon innesi shows a structural color of cyan with a characteristic peak at 506 nm. After the swelling of the oval iridophores, the characteristic peak moves to 593 nm, and thus the lateral stripe of paracheirodon innesi shows a structural color of yellow. The variable structural color is attributed to swelling and deswelling of the iridophore cells.


450 475 500 525 550 575 600 625

Deswelling

Re

fle

cta

nce

(a

.u.)

Wavelength (nm)

506 nm 593 nmSwelling

Fig. 2. UV-Vis reflection spectra of the variable structural color in the lateral stripe of paracheirodon innesi.

3.2 Structure and color of colloidal thin films

Figure 3(a) is a micrograph of the formed colloidal thin film from ~380 nm colloids by inclined deposition method, which can be divided into B, C, D, E, and F stripe regions. We can see that the different regions show different colors under perpendicular irradiation of a white light. To understand the diverse colors of different regions in the film, detailed AFM

images of the BF regions of Fig. 3(a) are shown in Fig. 3(b)-(f), respectively. Form Fig. 3(b)-(d), we can see that the B, C and D regions are one-, two- and three-layer films of colloids with hexagonal array, respectively. The square arrays [Fig. 3(e) and 3(f)] were found in the E and F regions, which are transition areas between B, C and C, D regions, respectively.

Fig. 3. Optical and AFM images of the formed colloidal thin film by inclined deposition method: (a) image observed by the optical microscope on the AFM; (b), (c), (d), (e) and (f) images scanned at AFM tapping mode of the B, C, D, E and F regions of (a), respectively.


From Fig. 3, we can see that the film shown in Fig. 3(a) has a stairlike structure, and one

of the main differences of the BF regions should lie in the thickness. Figure 4 is an illustration of the formed colloidal thin film. Figure 4(a), 4(b) and 4(c) display plane, section

and stereo profiles, respectively. The BF regions in Fig. 4 are corresponding to those of Fig. 3(a). From Fig. 4(a), we can see that dislocation of colloids might be the reason for the formation of the E and F transition areas. According to Fig. 4(b), the thickness of the B, E, C, F and D regions can be calculated to be ~380 nm, ~650 nm, ~692 nm, ~916 nm and ~999 nm, respectively. And from Fig. 4(c), we can see that the square or hexagonal layer forms the (100) or (111) crystal plane of the face centered cubic packing (fcc), which is the most common packing mode in colloidal crystals [41–45].

Fig. 4. Schematic illustration of the stairlike colloidal thin film (d represents the diameter of colloids).

3.3 Thin film interference occurred in the colloidal films

Like the multilayer thin film interference of light taking place at the aforementioned paracheirodon innesi, the thin film interference should also occur in the colloidal thin films and is the cause of various colors appearing at different thicknesses of the film. The lights reflected from top and bottom surface of the film will undergo thin film interference when the thickness (t) and refractive index (n) of the film satisfy Eq. (1) as the refractive index of the film is less than that of the substrate [46]. In Eq. (1), λ is the wavelength of the reflected light and m is an arbitrary integer coefficient.

2 / ( 0.5)

0,1,2,3,...

nt m

m

(1)

The refractive index (n) of the colloidal thin film can be calculated according to Eq. (2) [47], where φ represents the void ratio of the film and np and nair represent the refractive indices of the PSMA and air, respectively. For the fcc packing of monodisperse spheres, the void ratio is ~26%, which is calculated from a colloidal crystal with many layers of sphere, and the influence of the voids at the surface and bottom layers is neglected. But for the


colloidal thin films with only a few layers of sphere, as shown in Fig. 4(b), the influence of voids at the surface and bottom layers cannot be neglected. According to geometry calculations, the void ratios in this case for the B, E, C, F and D regions are ~40%, 39%, 34%, 35%, and 31%, respectively. The np is ~1.59 due to ~90% PSt in the colloids composition (nPSt = 1.59) [34], and the nair equals 1.0. So the refractive indices of the B, E, C, F and D regions can be calculated to be ~1.38, 1.39, 1.42, 1.41, and 1.43, respectively.

2 2 2(1 ) p airn n n= - + (2)

For the B region of the film (t = 380 nm, n = 1.38), according to Eq. (1), the wavelength of the reflected light, which will be eliminated from the white light by the thin film interference, can be calculated to be 699 nm (m = 1) and 420 nm (m = 2). As a result of the complementary color theory [46], the color of this region will show green, which accords well with the result of the experiment [Fig. 3(a)]. If other values of m such as m = 0, 3, etc. are used, the calculated λ will be over the region of visible light that does not influence the film’s color. Bragg diffraction needs a set of lattice planes and cannot appear in one-layer colloidal films, so the green color shown in the B region is direct evidence of thin film interference occurring in the one-layer colloidal films.

For the C region of the film (t = 692 nm, n = 1.42), the wavelength of the reflected light, eliminated from the white light by the interference, can be calculated to be 561 nm (m = 3) and 436 nm (m = 4). As a result the color of this region will show orange, which accords well with the result of the experiment [Fig. 3(a)]. If other values of m such as m = 0, 1, 2, etc. are used, the calculated λ will be over the region of visible light that does not influence the film’s color.

For the E region of the film (t = 650 nm, n = 1.39), the wavelength of the reflected light, eliminated from the white light by the interference, can be calculated to be 722 nm (m = 2), 516 nm (m = 3), and 401 nm (m = 4). As a result the color of this region will show yellowish green, which accords well with the result of the experiment [Fig. 3(a)]. And for a film with more than two layers of colloid, like the D or F region, the color becomes complicated because Bragg diffraction begins to appear in the multilayer colloidal crystal film and disturbs the structural colors of thin film interference.

3.4. Imitation of the variable structural color of paracheirodon innesi

A two-layer thin film of the ~380 nm colloids fabricated by dipping method is shown in Fig. 5. The SEM images of the thin film are show in Fig. 5(b)-(d). Figure 5(c) and 5(d) are the details of the marked rectangular regions in Fig. 5(b) and 5(c) respectively, from which we can see that the film is composed by the hexagonal double layers. As shown in Fig. 5(a), the two-layer hexagonal film displays homogeneous orange color under the perpendicular irradiation of white light, which accords well with the color of the C region in Fig. 3(a).


Fig. 5. Optical and SEM images of the formed hexagonal two-layer colloidal thin film by dipping method: (a) optical image; (b) SEM image; (c) detail of the marked rectangular area of (b); (d) detail of the marked rectangular area of (c).

We fix the two-layer colloidal film on a specimen stage of the optical microscope, and scratch a “stripe” shape mark on the film using tweezers. We can see that the color of the film changes from orange [Fig. 6(a)] to green [Fig. 6(b)] as 10 μL of styrene (St) is trickled on the film (for 10 min at ~25°C), then the color will return to orange [Fig. 6(c)] about 20 min later when the styrene is all evaporated. Figure 6(d)-(f) shows SEM images of the marked film, Fig. 6(e) and 6(f) are the details of the marked rectangular regions in Fig. 6(d) and 6(e), respectively. From the SEM images shown in Fig. 6(d)-(f), we can confirm that the light yellow color of the “stripe” mark in Fig. 6(a)-(c) is attributable to the reflection of the light from the silicon substrate, and the orange (or green) color of Fig. 6(a) and 6(c) (or 6(b)) is attributed to the reflection of light from the two-layer colloidal film. The arrows shown in Fig. 6(b) indicate the widths of the mark. Compared to Fig. 6(a), they become narrow after the film is swollen by St. When the St in the film is all evaporated, the widths of the mark as well as the color of the film [Fig. 6(c)] are recovered.


Fig. 6. Optical and SEM images of two-layer colloidal films before and after swelling by St: (a) optical image before swelling; (b) optical image after swelling; (c) optical image after evaporation of St; (d) SEM image of (c); (e) detail of the marked region of (d); (f) detail of the marked region of (e).

Figure 7 is a schematic explanation for a two-layer colloidal thin film before and after being swollen by St. If the thickness before swelling is t0, it becomes t1 after swelling and turns back to t0 as the St is all evaporated. Correspondingly the color of the film is changed from orange to green and recovered to orange [Fig. 6(a)-(c)]. In our experiment, we apply insufficient amount of styrene to fill voids of colloidal crystal film, and wait 10 min to let the PSMA colloids absorb the styrene in voids by swelling. So after that, the remaining styrene in the voids of colloidal crystal should be very little, and the influence of it to the structural color can be ignored. The influence of absorbed St to the refractive index of PSMA colloids can also be ignored, because the refractive index of St is ~1.55, which is very close to that of PSMA colloids (~1.59).

Fig. 7. Schematic illustration of the variation in thickness of two-layer film before and after swelling by St (t0, the thickness before swelling; t1, the thickness after swelling).

Figure 8 shows the UV-Vis reflection spectra of the variable structural color in the two-layer colloidal thin film. Before the swelling by styrene, the film shows a structural color of orange with a characteristic peak at 603 nm. The band gap of 380 nm PSMA colloidal crystal is calculated at 453 nm according to the Bragg law [33], which is very close to the measured value (459 nm) shown in Fig. 8 from the very thick multi-layer colloidal crystal reference sample instead of the two-layer thin film. The band gap peak of 380 nm PSMA colloidal crystal is very different from the peak at 603 nm, indicates that the latter does not come from Bragg diffraction but from thin film interference. After swelling by styrene, the characteristic peak at 603 nm moves to 528 nm, and thus the film shows a structural color of green. The tunable interferential colors displayed by the two-layer colloidal film should be attributed to the swelling and deswelling the colloids, which mimics the swell-caused variable structural colors in the paracheirodon innesi very well.


450 475 500 525 550 575 600 625

Re

fle

cta

nce

(a

.u.)

Wavelength (nm)

528 nm 603 nmDeswelling

Swelling

Band gap of

colloidal crystal

Fig. 8. UV-Vis reflection spectra of the variable structural color in the two-layer colloidal thin film. (The dot line is the band gap of 380 nm PSMA colloidal crystal measured from a very thick 37-layer reference sample)

4. Conclusion

Oval iridophore in the lateral stripe of the paracheirodon innesi contains periodically stacked thin films of guanine crystals, which leads to variable structural color on the basis of multilayer thin film interference. This unique optical phenomenon is also observed in colloidal thin films composed of monodisperse PSMA colloids, and the structural colors caused by thin film interference of the films with different layer structures are different under perpendicular irradiation of white light. On the basis of the principle of thin film interference, a two-layer hexagonal colloidal thin film is achieved to mimic the variable structural colors of the paracheirodon innesi. After the two-layer film of ~380 nm colloids swollen by St, the thickness of the film increases, and the film’s color changes from orange to green. The color will return to orange again as the St in the film is all evaporated. The colorful display of the two-layer colloidal thin films caused by thin film interference may provide an easy way to make photonic and bionic materials.

Acknowledgements

This work is financially supported by the Natural Science Foundation of China (Grant 21004035, 21005042), the Natural Science Foundation of Shandong Province (Grant ZR2010BQ020), and the Promotive Research Fund for Excellent Young and Middle-aged Scientists of Shandong Province (Grant BS2010CL014). The authors are grateful to Professor W.X. Cao at Peking University for his help in finishing this paper. The Taishan Scholarships Program is acknowledged.


2011 - Imitation of Variable Structural Color in Paracheirodon Innesi Using Colloidal Crystal Films

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