Structure, Function, and Self-Assembly of Single Network Gyroid (I4132) Photonic Crystals in Butterfly Wing Scales

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Birgit Hausmann

Reference

V. Saranathan et. al. "Structure, Function, and Self-Assembly of Single Network Gyroid (I4<math>_1</math>32) Photonic Crystals in Butterfly Wing Scales" Proc. Nat. Acad. Sci., 107, 11676-11681 (2010)

Keywords

biological meta-materials, organismal color, biomimetics, biological cubic mesophases


Overview

Small angle X-ray scattering (SAXS) was used to identify the 3D photonic nanostructures (made of chitin and air) of five butterfly species from two families (Papilionidae, Lycaenidae) as single network gyroid (<math>I4_132</math>) photonic crystals. Photonic band-gap modeling was also performed. Butterflies first develop thermodynamically favored double gyroid precursors and transform these into a single gyroid network which is optically more efficient.

Results and Discussion

A periodic variation in the refractive index on the order of visible wavelengths gives butterflies its structural colors. Pinhole synchrotron small angle X-ray scattering (SAXS) has been used to investigate natural photonic materials with mesoscale (150–350 nm) scattering features (Fig. 2). The order of the lattice constant in these cases require small scattering angles and advanced X-ray optics. Five butterflies species were tested: Parides sesostris, Teinopalpus imperialis, (Papilionidae); Callophrys (formerly Mitoura) gryneus, Callophrys dumetorum, and Cyanophrys herodotus (Lycaenidae) (Fig. 1).

Fig. 1

Figure 2 shows SAXS patterns of these species which predict distinctly oriented crystallite domains that have also been indicated with SEM (Fig. 1 B and E). These results are consistent with the size of the X-ray beam (15 × 15 μm). The unit cell lattice constant of the butterfly scale photonic nanostructures was estimated as a > 300nm which is consistent with EM-estimated lattice parameters assuming a single gyroid space group, rather than a simple primitive or a single diamond symmetry. Higher-order reflections in T. imperialis, C. gryneus, and C. herodotus (Figs. 2 and 3) proves a high degree of order within the nanostructure.

Fig. 2
Fig. 3

Computer models of the single gyroid, simple primitive, and single diamond nanostructures were also performed, which produced simulated TEM and SEM projections of appropriate thicknesses and chitin volume fractions, along various lattice directions/cleavage planes, including (110), (111), and (211). SEM and TEM images of all five butterfly show characteristics of the single gyroid space group (Fig. 1 B and E, and Fig. S1 B, E, and H). Figure 4 compares the predicted reflectance from SAXS patterns with measured optical reflectance. The optical and X-ray scattering peaks agree reasonably well (< ∼ 15 nm) with a value of 1.16 for <math>n_{av}</math>, which corresponds to a chitin volume fraction of 0.25 (Fig. 4). Bandgap calculations indicate three closely spaced pseudogaps or partial photonic bandgaps along Γ-N (110), Γ-P (111), and Γ-H (200) directions for the butterfly photonic nanostructures. The Bragg attenuation lengths of the butterfly nanostructures were calculated to be between 3.9–4.4 a (Table S1). When the Bragg condition is met most of the light gets reflected while broader reflectance peaks seem to evolve from multiple scattering.

Fig. 4

Also, a model for the development of the 3D photonic nanostructure in the wing scale cells of lepidopteran pupae is presented. TEM images from developing pupae of C. gryneus (Fig. 5A) seem to indicate that "the cell plasma membrane and the SER membrane interact to form a pair of parallel lipid-bilayer membranes, separated by the cellular cytoplasm. The two parallel lipid-bilayer membranes of the developing scale cell form a pentacontinuous structure with a core-shell double gyroid morphology, similar to those seen in ABC triblock copolymer melts (Fig. 5 A and B)." The lipid-bilayer membranes of developing butterfly photonic scale cells seem also to define "the unique pentacontinuous volumes of a core-shell double gyroid (Ia3d) structure of the form ABCB′A′, in which A is the extracellular space, B is the plasma membrane, C is the cell’s cytoplasmic volume, B′ is the SER membrane, and A′ is the intra-SER space (Fig. 5A and Insets). As in a triblock copolymer system, the proposed ABCB′A′ core-shell double gyroid system in the butterfly scale has the lipid-bilayer membranes (B and B′) together with the intervening cytoplasmic space (C) forming the matrix phase (BCB′) of the system." While developing chitin is deposited and polymerized in the extracellular space (A) that is now within the peripheral outline of the scale cell and forms the core of one of the gyroid networks (Fig. 5C, red), which is enclosed by the plasma membrane (Fig. 5C, black). When the cell dies, the cellular cytoplasm and the membranes (BCB'A' blocks of the core-shell double gyroid) are replaced with air, leaving behind a single gyroid network of chitin (A) in air (Fig. 5C). The TEM section in Fig. 5A (which was taken from reference 11) indicates two different regions through the (110) plane. Initial chitin deposition is also visible in Fig 5A as dark lines in the spaces surrounding the double (right), while chitin rods are visible as dark spots in the center of the double rings (left, yellow square). Although SEM and TEM images are not sufficient to state on the chirality of the considered systems the given SEM images indicate single gyroid networks with both left- and right-handed chiralities (Fig. S8).

Fig. 5