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A humidity sensitive two-dimensional tunable amorphous photonic structure in the bivalve ligament of Meretrix linnaeus
Journal of Structural Biology 192 (2015) 457–460
Contents lists available at ScienceDirect
Journal of Structural Biology journal homepage: www.elsevier.com/locate/yjsbi
Technical Note
A humidity sensitive two-dimensional tunable amorphous photonic structure in the bivalve ligament of Meretrix linnaeus Weigang Zhang a,⇑, Gangsheng Zhang b a b
College of Materials and Chemical Engineering, Chuzhou University, Chuzhou 239000, PR China College of Material Science and Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 10 October 2015 Accepted 10 October 2015 Available online 23 October 2015 Keywords: Bivalve ligament 2D tunable photonic structure Humidity sensitive Complex structural color
a b s t r a c t A humidity sensitive two-dimensional tunable amorphous photonic structure (2D TAPS) in the bivalve ligament of Meretrix linnaeus (LML) was reported in this paper. The structural color and microstructure of LML were investigated by reflection spectra and scanning electron microscopy, respectively. The results indicate that the LML has complex structural colors from blue to orange in the wet state from ventral to dorsal, which are derived from the aragonite fiber diameter increases continuously from ventral to dorsal of the ligament. The reflection peak wavelength of the wet LML can blue-shift from 522 nm to 480 nm with the air drying time increased from 0 to 60 min, while the reflectivity decreases gradually and only a weak reflection peak at last, relevant color changes from green to light blue. The structural color in the LML is produced by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins, in which the diameters of the aragonite fiber and the inter-fiber spacing are 104 ± 11 nm and 126 ± 16 nm, respectively. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, and the regulation achieved through dynamically tune the degree of order and lattice constant of the ligament in the different wet states. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction In recent years, there has been increased interest in tunable photonic structures due to their important applications in the fields of optical transducers and high sensitive detectors (Nishijima et al., 2007; Potyrailo et al., 2007; Xu and Guo, 2013; Tang et al., 2014). From our knowledge, humidity-, electrical-, magnetic-, and thermo-sensitive three-dimensional tunable gel photonic structures have been discovered (Weissman et al., 1996; Barry and Pierrewiltzius, 2006; Arsenault et al., 2007; Ge et al., 2007). But there are few previous studies about twodimensional tunable photonic structures (Leonaid et al., 2000; Kim and Gopalan, 2001). In addition, as we all know gel materials have obvious deficiencies in mechanical strength and hardness, so if we can find a two-dimensional tunable photonic band gap material which is mainly composed of inorganic materials, will greatly improve the mechanical properties and application prospects of such materials. The bivalve ligaments are mainly composed of aligned aragonite fibers and proteins (Kahler et al., 1976), have good mechanical
⇑ Corresponding author. E-mail address: [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jsb.2015.10.007 1047-8477/Ó 2015 Elsevier Inc. All rights reserved.
properties (Ono et al., 1990). Three kinds of two-dimensional photonic structures in the bivalve ligaments with blue or golden structural color had been reported by our research group (Zhang, 2007; Zhang et al., 2009; Zhang and Huang, 2010). But the twodimensional tunable photonic structure in the bivalve ligament with complex structural colors from blue to orange has not been reported. In this paper, the microstructure and reflection spectra of LML in the different wet states were systematically investigated. We first discovered blue to orange structural colors in LML, and confirmed these colors were caused by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, revealing that LML is a new kind of humidity sensitive 2D TAPS.
2. Experimental 2.1. Materials Samples of Meretrix linnaeus were obtained from Guangxi in southern China. After removing the soft body, the shells were washed with distilled water and dried at room temperature for
458
W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
5 days. Then we carefully removed the ligament. Next, using a single blade, we broke the ligament in different directions for SEM observation, and using a few ligaments for optical observation and reflection spectra measurement. 2.2. Characterization Optical photos of the ligament were taken using a GL-99 stereomicroscope, which was connected to a CCD camera, light source power was 70 W of HL2013 quartz lamp, the angle between incident light and observation surface was 60°. The reflection spectra of LML were measured by using an AvaSpec-2048 fiber optical spectrometer with analyzing software of Avasoft 7.1. A halogentungsten lamp and a reflection probe were used as light source and probe, respectively. The incident light and the test surface were perpendicular, the distance between the probe and the test surface was about 1 mm, and the probe surface area was about 1 mm2. A white board was used as a reference for the reflectivity. The microstructure of the ligament was observed by using an S3400 scanning electron microscope (SEM) operated at 30 kV accelerating voltage. 3. Results and discussion 3.1. Color observation We can see distinct color in the ligament under the natural drying condition, and the color has a significant stratification phenomenon, the ligament can show light blue, blue, blue green, and golden from ventral to dorsal, respectively (Fig. 1b). Also the colors of LML will gradually become darker or lighter with the incident light direction changing, and when the incident angle is changed into a certain extent, the colors will disappear, leaving only golden in the dorsal area, most area only to see its brown background color (Fig. 1a), revealing that the colors of LML belong to structural color. The color of LML in the wet state also has a significant stratification phenomenon, and it can present obvious red-shift from light blue, blue, blue green, and golden in the dry state to blue, green, yellow green, and orange in the wet state, respectively (Fig. 1b and c). In addition, Fig. 1b and c also clearly show that the ligament can present obvious longitudinal expansion in the wet state, and the expansion rate of ventral ligament is significant larger than dorsal ligament. The measured results show that the expansion rates of ventral and dorsal ligaments are 35% and 18%, respectively.
are inconsistent, the large layer can reach 68.5 lm, but the small layer is only 18.6 lm. The difference between the adjacent layers is significant, but the difference between the interphase layers is relatively small. Fig. 2a also shows that the position relationship between the aragonite fibers and the growth lines exists a visual angle about 62°, but not previous thought vertical (Kahler et al., 1976). The aragonite fibers orientation of adjacent layers is inconsistent, which exists a visual angle about 124°, but the aragonite fibers orientation of interphase layers is consistent. From Fig. 2d we can see that the cross-sectional shapes of the aragonite fibers are hexagon or rounded shape, the fiber’s diameter is highly uniform with an average value of 104 ± 11 nm, and the inter-fiber spacing (lattice constant) is 126 ± 16 nm. We can also see that the aragonite fibers are packed in short-range order (Fig. 2d), revealing that the LML resembles a two-dimensional amorphous photonic structure with short-range order. 3.3. Reflection spectra Fig. 3 shows the reflection spectra of LML, it can be seen that the reflection peak wavelength of wet LML (0 min) at 522 nm, which belongs to the green light range, so the performance of the corresponding color is green. This is consistent with above optical observation. In addition, the reflection peak wavelength can blueshift from 522 nm to 480 nm with the drying time increases from 0 to 60 min, while the reflectivity decreases gradually and only a weak reflection peak at last, so the corresponding color can only present light blue. 3.4. Mechanism discussion The SEM observation results show that the LML has two-dimensional amorphous photonic structural characteristics, so we can use the following formulas of photonic band gap of two-dimensional photonic crystal to calculate the reflection peak wavelength of LML. As the refractive indices of water (1.326) and protein (1.3) are close, so we still take 1.3 as the reflective index of the protein in the wet state, and we take 35% as the expansion rate of LML in the wet state. Using these parameters and the following formulas of photonic band gap, the reflection peak wavelength of wet LML is calculated at 514 nm, which belongs to the blue light range, and nearly equal to the measured reflection peak wavelength. The measured and calculated results indicate that the green structural color in the wet LML is produced by a twodimensional amorphous photonic structure consists of aligned aragonite fibers and proteins.
3.2. Structural characterization
Filling coefficient of aragonite fibers : f ¼
Fig. 2 shows the SEM images of LML, it can be clearly seen that the ligament is closely stacked by columnar aragonite fibers, and the fibers are wrapped in the protein, but the protein is not significant due the thickness is extremely thin. Fig. 2a shows that the ligament has obvious layered structure, the thickness of the layers
Dielectric constant : e ¼ fn1 þ ð1 f Þn22 1 e0 ¼ 2 fn1 þ ð1 f Þn2 2
pffiffiffi 2 3 L1 2 p2
ð1Þ
2
Fig. 1. Optical photos of LML in different states, the arrows indicate the direction of the incident light, (a) change incident angle, (b) dry state, (c) wet state.
ð2Þ
W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
459
Fig. 2. SEM images of LML, (a) longitudinal section, (b) a partial magnified image of (a), (c) cross section of aragonite fibers, (d) a partial magnified image of (c).
Fig. 3. Reflection spectra of LML dried in air for different times.
¼ Average refraction index : n
rffiffiffiffiffiffiffiffiffiffiffiffi e þ e0 2
Reflection peak wavelength : k ¼
ð3Þ
2pn m
Reflection peak wavelength in the wet state : k ¼
ð4Þ 2pð1 þ rÞn m ð5Þ
where L1 is aragonite fiber’s diameter, P is inter-fiber spacing, n1 is the refractive index of aragonite (1.67) (Brink and Berg, 2005), n2 is the refractive index of protein (1.3) (Tan et al., 2004), r is the expansion rate of LML in the wet state, m = 1. The color observation results show that LML can present obvious red-shift phenomenon from ventral to dorsal, which is mainly due to the aragonite fiber diameter increases continuously from ventral to dorsal of the ligament, but no significant change for the inter-fiber spacing (Bevelanger and Nakahara, 1969). According to the formulas of photonic band gap and the above variation of the aragonite fiber in the ligament, we can see that the average refractive index of the ligament will increase continuously with aragonite fiber diameter increases continuously from ventral to dorsal, leading to the reflection peak wavelength increases continuously from ventral to dorsal, so the structural color can present obvious red-shift phenomenon from ventral to dorsal of the ligament. Swelling test shows the water can obviously tune the microstructure of LML, we speculate the process of expansion as shown in Fig. 4. It can be seen two variations for the ligament from the dry state to the wet state. (1) Due to the swelling effect, the lattice constant of the ligament significantly increases from p to p(1 + r), leading to the reflection peak wavelength in the wet state is significantly higher than the reflection peak wavelength in the dry state. The change process of the ligament from wet state to dry state, the lattice constant becomes smaller, leading to the reflection peak wavelength gradually blue-shifted, and the observed color gradually blue-shifted too. In contrast, the ligament changed from dry state to wet state, the lattice constant becomes
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W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
Fig. 4. Structural characteristics of LML in different states, (a) dry state, (b) wet state.
larger, leading to the reflection peak wavelength gradually redshifted, and the observed color gradually red-shifted too. (2) Due to the swelling effect, the relatively disordered arrangement structure of the ligament in the dry state (Fig. 4a) changed into the ordered tetragonal lattice arrangement system of the ligament in the wet state (Fig. 4b). According to the Debord’s findings of geltype tunable photonic band gap materials: the higher the degree of order, the greater the intensity of reflection peak (Debord and Lyon, 2000). Which indicates that the reflection peak intensity of the ligament in the wet state is significantly higher than the reflection peak intensity of the ligament in the dry state. The change process of the ligament from wet state to dry state, the lattice arrangement gradually becomes disordered, leading to the reflection peak intensity gradually weakened, and the observed color gradually faded. In contrast, the ligament changed from dry state to wet state, the lattice arrangement gradually becomes ordered, leading to the reflection peak intensity gradually increased, and the observed color gradually becomes darker. In summary, the water has significant regulatory roles for the reflection peak intensity and photonic band gap range of LML, revealing that LML has obvious humidity sensitive twodimensional tunable amorphous photonic structural characteristics, belongs to a new kind of natural humidity sensitive two-dimensional tunable photonic band gap material. Compared to the gel material, due to the ligament is mainly composed of inorganic aragonite fibers, it has higher mechanical properties, so it has higher application values as a humidity detector in the fields of research and actual production. In addition, the bivalve ligament after a long biological evolution in the nature, with beautiful microstructure, can provide new ideas and templates for the field of biomimetic materials to prepare high-performance two-dimensional photonic band gap materials which are mainly composed of inorganic materials. 4. Conclusions In summary, the complex structural colors from blue to orange of wet LML are produced by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, and the regulation achieved through dynamically tune the degree of order and lattice constant of the ligament in the different wet states. Revealing that LML is a new kind of humidity sensitive 2D TAPS which is mainly composed of inorganic aragonite fibers, such kind of 2D TAPS can be used as a
humidity sensitive detector, and it may yield very useful template for artificial structures. Acknowledgement We appreciate the financial support of the National Natural Science Foundation of China (Grant No. 40772033). References Arsenault, A., Puzzo, D., Manners, I., Ozin, G., 2007. Photonic-crystal full-colour displays. Nat. photonics 1, 468–472. Barry, R.A., Pierrewiltzius, 2006. Humidity-sensing inverse opal hydrogels. Langmuir 22, 1369–1374. Bevelanger, G., Nakahara, H., 1969. An electron microscope study of the formation of the ligament of Mytilus edulis and Pinctada radiata. CMc. Tiss. Res. 4, 101–112. Brink, D.J., Berg, N.G., 2005. An investigation of green iridescence on the mollusk Patella granatina. J. Phys. D Appl. Phys. 38, 338–343. Debord, T.D., Lyon, L.A., 2000. Thermo responsive photonic crystals. J. Phys. Chem. B 104, 6327–6331. Ge, J.P., Hu, Y.X., Yin, Y.D., 2007. Highly tunable superparamagnetic colloidal photonic crystals. Angew. Chem. Int. Ed. 46, 7428–7431. Kahler, G., Sass, R.L., Fisher, F.M., 1976. The fine structure and crystallography of the hinge ligament of Spisula solidissima (Moilusca: Bivaivia: Mactridae). J. Comp. Physiol. 109, 209–220. Kim, S., Gopalan, V., 2001. Strain-tunable photonic band gap crystals. Appl. Phys. Lett. 78, 3015–3017. Leonaid, S.W., Mondia, T.P., Vandriel, H.M., Toader, O., John, S., 2000. Tunable twodimensional photonic crystals using liquid-crystal infiltration. Phys. Rev. B 61, 2389–2392. Nishijima, Y., Ueno, K., Juodkazis, S., Mizeikis, V., Misawa, H., Tanimura, T., Maeda, K., 2007. Inverse silica opal photonic crystals for optical sensing applications. Opt. Express 15, 12979. Ono, K., Kikuch, Y., Higashi, K., 1990. Elastic anisotropy of bivalve hinge ligament. J. Biomech. 23, 307–312. Potyrailo, R.A., Ghiradella, H., Vertiatchikh, A., Katharine, D., Janes, R., Eric, O., 2007. Morpho butterfly wing scales demonstrate highly selective vapour response. Nat. Photonics 1, 123–128. Tan, T.L., Wong, D., Lee, P., 2004. Iridescence of a shell of mollusk haliotis glabra. Opt. Express 12, 4847–4854. Tang, B.T., Zheng, X.X., Lin, T., Zhang, S.F., 2014. Hydrophobic structural color films with bright color and tunable stop-bands. Dyes Pigm. 104, 146–150. Weissman, J., Sunkava, H.B., Tse, A., Albert, S.T., Sanford, A.A., 1996. Thermally switchable periodicities and diffraction from mesoscopically ordered materials. Science 274, 959–960. Xu, J., Guo, Z.G., 2013. Biomimetic photonic materials with tunable structural colors. J. Colloid Interface Sci. 406, 1–17. Zhang, G.S., 2007. Photonic crystal type structure in bivalve ligament of Pinctada maxima. Chin. Sci. Bull. l52, 1136–1138. Zhang, G.S., Huang, Z.Q., 2010. Two-dimensional amorphous photonic structure in the ligament of bivalve Lutraria maximum. Opt. Express 18, 13361. Zhang, W.G., Yan, J., Wang, G., Li, H.X., Zhang, G.S., 2009. A natural humidity sensitive two-dimensional tunable photonic band gap material and its optic properties. J. Inorg. Mater. 24, 57–60 (in Chinese).
Contents lists available at ScienceDirect
Journal of Structural Biology journal homepage: www.elsevier.com/locate/yjsbi
Technical Note
A humidity sensitive two-dimensional tunable amorphous photonic structure in the bivalve ligament of Meretrix linnaeus Weigang Zhang a,⇑, Gangsheng Zhang b a b
College of Materials and Chemical Engineering, Chuzhou University, Chuzhou 239000, PR China College of Material Science and Engineering, Guangxi University, Nanning 530004, PR China
a r t i c l e
i n f o
Article history: Received 16 July 2015 Received in revised form 10 October 2015 Accepted 10 October 2015 Available online 23 October 2015 Keywords: Bivalve ligament 2D tunable photonic structure Humidity sensitive Complex structural color
a b s t r a c t A humidity sensitive two-dimensional tunable amorphous photonic structure (2D TAPS) in the bivalve ligament of Meretrix linnaeus (LML) was reported in this paper. The structural color and microstructure of LML were investigated by reflection spectra and scanning electron microscopy, respectively. The results indicate that the LML has complex structural colors from blue to orange in the wet state from ventral to dorsal, which are derived from the aragonite fiber diameter increases continuously from ventral to dorsal of the ligament. The reflection peak wavelength of the wet LML can blue-shift from 522 nm to 480 nm with the air drying time increased from 0 to 60 min, while the reflectivity decreases gradually and only a weak reflection peak at last, relevant color changes from green to light blue. The structural color in the LML is produced by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins, in which the diameters of the aragonite fiber and the inter-fiber spacing are 104 ± 11 nm and 126 ± 16 nm, respectively. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, and the regulation achieved through dynamically tune the degree of order and lattice constant of the ligament in the different wet states. Ó 2015 Elsevier Inc. All rights reserved.
1. Introduction In recent years, there has been increased interest in tunable photonic structures due to their important applications in the fields of optical transducers and high sensitive detectors (Nishijima et al., 2007; Potyrailo et al., 2007; Xu and Guo, 2013; Tang et al., 2014). From our knowledge, humidity-, electrical-, magnetic-, and thermo-sensitive three-dimensional tunable gel photonic structures have been discovered (Weissman et al., 1996; Barry and Pierrewiltzius, 2006; Arsenault et al., 2007; Ge et al., 2007). But there are few previous studies about twodimensional tunable photonic structures (Leonaid et al., 2000; Kim and Gopalan, 2001). In addition, as we all know gel materials have obvious deficiencies in mechanical strength and hardness, so if we can find a two-dimensional tunable photonic band gap material which is mainly composed of inorganic materials, will greatly improve the mechanical properties and application prospects of such materials. The bivalve ligaments are mainly composed of aligned aragonite fibers and proteins (Kahler et al., 1976), have good mechanical
⇑ Corresponding author. E-mail address: [email protected] (W. Zhang). http://dx.doi.org/10.1016/j.jsb.2015.10.007 1047-8477/Ó 2015 Elsevier Inc. All rights reserved.
properties (Ono et al., 1990). Three kinds of two-dimensional photonic structures in the bivalve ligaments with blue or golden structural color had been reported by our research group (Zhang, 2007; Zhang et al., 2009; Zhang and Huang, 2010). But the twodimensional tunable photonic structure in the bivalve ligament with complex structural colors from blue to orange has not been reported. In this paper, the microstructure and reflection spectra of LML in the different wet states were systematically investigated. We first discovered blue to orange structural colors in LML, and confirmed these colors were caused by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, revealing that LML is a new kind of humidity sensitive 2D TAPS.
2. Experimental 2.1. Materials Samples of Meretrix linnaeus were obtained from Guangxi in southern China. After removing the soft body, the shells were washed with distilled water and dried at room temperature for
458
W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
5 days. Then we carefully removed the ligament. Next, using a single blade, we broke the ligament in different directions for SEM observation, and using a few ligaments for optical observation and reflection spectra measurement. 2.2. Characterization Optical photos of the ligament were taken using a GL-99 stereomicroscope, which was connected to a CCD camera, light source power was 70 W of HL2013 quartz lamp, the angle between incident light and observation surface was 60°. The reflection spectra of LML were measured by using an AvaSpec-2048 fiber optical spectrometer with analyzing software of Avasoft 7.1. A halogentungsten lamp and a reflection probe were used as light source and probe, respectively. The incident light and the test surface were perpendicular, the distance between the probe and the test surface was about 1 mm, and the probe surface area was about 1 mm2. A white board was used as a reference for the reflectivity. The microstructure of the ligament was observed by using an S3400 scanning electron microscope (SEM) operated at 30 kV accelerating voltage. 3. Results and discussion 3.1. Color observation We can see distinct color in the ligament under the natural drying condition, and the color has a significant stratification phenomenon, the ligament can show light blue, blue, blue green, and golden from ventral to dorsal, respectively (Fig. 1b). Also the colors of LML will gradually become darker or lighter with the incident light direction changing, and when the incident angle is changed into a certain extent, the colors will disappear, leaving only golden in the dorsal area, most area only to see its brown background color (Fig. 1a), revealing that the colors of LML belong to structural color. The color of LML in the wet state also has a significant stratification phenomenon, and it can present obvious red-shift from light blue, blue, blue green, and golden in the dry state to blue, green, yellow green, and orange in the wet state, respectively (Fig. 1b and c). In addition, Fig. 1b and c also clearly show that the ligament can present obvious longitudinal expansion in the wet state, and the expansion rate of ventral ligament is significant larger than dorsal ligament. The measured results show that the expansion rates of ventral and dorsal ligaments are 35% and 18%, respectively.
are inconsistent, the large layer can reach 68.5 lm, but the small layer is only 18.6 lm. The difference between the adjacent layers is significant, but the difference between the interphase layers is relatively small. Fig. 2a also shows that the position relationship between the aragonite fibers and the growth lines exists a visual angle about 62°, but not previous thought vertical (Kahler et al., 1976). The aragonite fibers orientation of adjacent layers is inconsistent, which exists a visual angle about 124°, but the aragonite fibers orientation of interphase layers is consistent. From Fig. 2d we can see that the cross-sectional shapes of the aragonite fibers are hexagon or rounded shape, the fiber’s diameter is highly uniform with an average value of 104 ± 11 nm, and the inter-fiber spacing (lattice constant) is 126 ± 16 nm. We can also see that the aragonite fibers are packed in short-range order (Fig. 2d), revealing that the LML resembles a two-dimensional amorphous photonic structure with short-range order. 3.3. Reflection spectra Fig. 3 shows the reflection spectra of LML, it can be seen that the reflection peak wavelength of wet LML (0 min) at 522 nm, which belongs to the green light range, so the performance of the corresponding color is green. This is consistent with above optical observation. In addition, the reflection peak wavelength can blueshift from 522 nm to 480 nm with the drying time increases from 0 to 60 min, while the reflectivity decreases gradually and only a weak reflection peak at last, so the corresponding color can only present light blue. 3.4. Mechanism discussion The SEM observation results show that the LML has two-dimensional amorphous photonic structural characteristics, so we can use the following formulas of photonic band gap of two-dimensional photonic crystal to calculate the reflection peak wavelength of LML. As the refractive indices of water (1.326) and protein (1.3) are close, so we still take 1.3 as the reflective index of the protein in the wet state, and we take 35% as the expansion rate of LML in the wet state. Using these parameters and the following formulas of photonic band gap, the reflection peak wavelength of wet LML is calculated at 514 nm, which belongs to the blue light range, and nearly equal to the measured reflection peak wavelength. The measured and calculated results indicate that the green structural color in the wet LML is produced by a twodimensional amorphous photonic structure consists of aligned aragonite fibers and proteins.
3.2. Structural characterization
Filling coefficient of aragonite fibers : f ¼
Fig. 2 shows the SEM images of LML, it can be clearly seen that the ligament is closely stacked by columnar aragonite fibers, and the fibers are wrapped in the protein, but the protein is not significant due the thickness is extremely thin. Fig. 2a shows that the ligament has obvious layered structure, the thickness of the layers
Dielectric constant : e ¼ fn1 þ ð1 f Þn22 1 e0 ¼ 2 fn1 þ ð1 f Þn2 2
pffiffiffi 2 3 L1 2 p2
ð1Þ
2
Fig. 1. Optical photos of LML in different states, the arrows indicate the direction of the incident light, (a) change incident angle, (b) dry state, (c) wet state.
ð2Þ
W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
459
Fig. 2. SEM images of LML, (a) longitudinal section, (b) a partial magnified image of (a), (c) cross section of aragonite fibers, (d) a partial magnified image of (c).
Fig. 3. Reflection spectra of LML dried in air for different times.
¼ Average refraction index : n
rffiffiffiffiffiffiffiffiffiffiffiffi e þ e0 2
Reflection peak wavelength : k ¼
ð3Þ
2pn m
Reflection peak wavelength in the wet state : k ¼
ð4Þ 2pð1 þ rÞn m ð5Þ
where L1 is aragonite fiber’s diameter, P is inter-fiber spacing, n1 is the refractive index of aragonite (1.67) (Brink and Berg, 2005), n2 is the refractive index of protein (1.3) (Tan et al., 2004), r is the expansion rate of LML in the wet state, m = 1. The color observation results show that LML can present obvious red-shift phenomenon from ventral to dorsal, which is mainly due to the aragonite fiber diameter increases continuously from ventral to dorsal of the ligament, but no significant change for the inter-fiber spacing (Bevelanger and Nakahara, 1969). According to the formulas of photonic band gap and the above variation of the aragonite fiber in the ligament, we can see that the average refractive index of the ligament will increase continuously with aragonite fiber diameter increases continuously from ventral to dorsal, leading to the reflection peak wavelength increases continuously from ventral to dorsal, so the structural color can present obvious red-shift phenomenon from ventral to dorsal of the ligament. Swelling test shows the water can obviously tune the microstructure of LML, we speculate the process of expansion as shown in Fig. 4. It can be seen two variations for the ligament from the dry state to the wet state. (1) Due to the swelling effect, the lattice constant of the ligament significantly increases from p to p(1 + r), leading to the reflection peak wavelength in the wet state is significantly higher than the reflection peak wavelength in the dry state. The change process of the ligament from wet state to dry state, the lattice constant becomes smaller, leading to the reflection peak wavelength gradually blue-shifted, and the observed color gradually blue-shifted too. In contrast, the ligament changed from dry state to wet state, the lattice constant becomes
460
W. Zhang, G. Zhang / Journal of Structural Biology 192 (2015) 457–460
Fig. 4. Structural characteristics of LML in different states, (a) dry state, (b) wet state.
larger, leading to the reflection peak wavelength gradually redshifted, and the observed color gradually red-shifted too. (2) Due to the swelling effect, the relatively disordered arrangement structure of the ligament in the dry state (Fig. 4a) changed into the ordered tetragonal lattice arrangement system of the ligament in the wet state (Fig. 4b). According to the Debord’s findings of geltype tunable photonic band gap materials: the higher the degree of order, the greater the intensity of reflection peak (Debord and Lyon, 2000). Which indicates that the reflection peak intensity of the ligament in the wet state is significantly higher than the reflection peak intensity of the ligament in the dry state. The change process of the ligament from wet state to dry state, the lattice arrangement gradually becomes disordered, leading to the reflection peak intensity gradually weakened, and the observed color gradually faded. In contrast, the ligament changed from dry state to wet state, the lattice arrangement gradually becomes ordered, leading to the reflection peak intensity gradually increased, and the observed color gradually becomes darker. In summary, the water has significant regulatory roles for the reflection peak intensity and photonic band gap range of LML, revealing that LML has obvious humidity sensitive twodimensional tunable amorphous photonic structural characteristics, belongs to a new kind of natural humidity sensitive two-dimensional tunable photonic band gap material. Compared to the gel material, due to the ligament is mainly composed of inorganic aragonite fibers, it has higher mechanical properties, so it has higher application values as a humidity detector in the fields of research and actual production. In addition, the bivalve ligament after a long biological evolution in the nature, with beautiful microstructure, can provide new ideas and templates for the field of biomimetic materials to prepare high-performance two-dimensional photonic band gap materials which are mainly composed of inorganic materials. 4. Conclusions In summary, the complex structural colors from blue to orange of wet LML are produced by a two-dimensional amorphous photonic structure consists of aligned aragonite fibers and proteins. Water can reversibly tune the reflection peak wavelength and reflectivity of this photonic structure, and the regulation achieved through dynamically tune the degree of order and lattice constant of the ligament in the different wet states. Revealing that LML is a new kind of humidity sensitive 2D TAPS which is mainly composed of inorganic aragonite fibers, such kind of 2D TAPS can be used as a
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