Light Manipulation by Guanine Crystals in Organisms: Biogenic Scatterers, Mirrors, Multilayer Reflectors and Photonic Crystals

Abstract

Guanine crystals are widely used in nature to manipulate light. The first part of this feature article explores how organisms are able to construct an extraordinary array of optical "devices" including diffuse scatterers, broadband and narrowband reflectors, tunable photonic crystals, and imageforming mirrors by varying the size, morphology, and arrangement of guanine crystals. The second part presents an overview of some of the properties of crystalline guanine to explain why this material is ideally suited for such optical applications. The high reflectivity of many natural optical systems ultimately derives from the fact that guanine crystals have an extremely high refractive index-a product of its anisotropic crystal structure comprised of densely stacked H-bonded layers. In order to optimize their reflectivity, many organisms exert exquisite control over the crystal morphology, forming platelike single crystals in which the high refractive index face is preferentially expressed. Guanine-based optics are used in a wide range of biological functions such as in camouflage, display, and vision, and exhibit a degree of versatility, tunability, and complexity that is difficult to incorporate into artificial devices using conventional engineering approaches. These biological systems could inspire the next generation of advanced optical materials. Figure 2. Examples of guanine crystal-based optical systems with different functionality, and their corresponding architecture. a-c) The white widow spider (Latrodectus pallidus):The integument of the spider (a) is white due to coherent scattering from a thick dense layer of blocky guanine crystals ≈1 µm in size (b), located beneath the integument. Impinging light is scattered multiple times from neighboring randomly distributed crystals (c) before emerging. d-f) The Japanese Koi fish (Cyprinus carpio). The silvery reflection of the fish (d) arises from disordered multilayer reflectors located in the epithelial layer underneath the scales of the fish (e). The thin (≈20 nm) plate-like guanine crystals are not all parallel to each other, but there is local order within each crystal stack. f) The cytoplasm spacings are widely distributed. Constructive interference of different wavelengths reflected from individual stacks results in diffuse broadband reflection, appearing silvery to the human eye (d, inset). g-i) The neon tetra fish (Paracheirodon innesi): The brilliant blue color of the lateral stripe of the neon tetra (g) arises from ordered, well aligned multilayer reflectors (h,i) with narrow distribution of thicknesses of both the thin guanine crystals (≈22 nm) and the thicker cytoplasm spacings (≈155 nm). Constructive interference of selected wavelengths reflected from individual stacks results in narrow band reflection centered at 500 nm (g, inset). j-l) The panther chameleon (Furcifer pardalis): The observed skin color of the panther chameleon (j) originates from a combination of both pigments and guanine-based 3D photonic crystals. Light is reflected from small ≈130 n...

Synthesis of Bio‐Inspired Guanine Microplatelets: Morphological and Crystallographic Control

“…Despite the size variations, the thickness of these BGmPs is relatively constant, approximately 100 nm as measured from atomic force microscopy (AFM) imaging (Figure 1 b). This thickness value is close to the early measurements based on electron microscopy for a number of fish species, although recent measurements based on cryo‐scanning electron microscopy (SEM) revealed smaller thicknesses (≈20 nm) [13] . This may be due to the thickness variations within individual crystals or from species difference [39] .…”

Paradigm shift in eukaryotic biocrystallization

“…Interestingly, we noticed guanine microcrystals inside swarming acoelomate gastrotrichs (Platyzoa) in a sample from a peat bog (Movie S5). Using Raman microscopy, we also confirmed that guanine crystals serve as a refractile layer on fish scales 52 . In Fungi (Holomycota) we found crystalline guanine in Candida albicans, which has been tested for uptake of different purine compounds previously 53 .…”

Assembly of Guanine Crystals as a Low-Polarizing Broadband Multilayer Reflector in a Spider, Phoroncidia rubroargentea

“…29 In addition, the preferentially expressed face of these platy guanine crystals was identified in many organisms to be parallel to the hydrogen-bonding plane, i.e., the (100) plane. 2,29 This is quite different from the theoretically predicted preferred growth along the π−π stacking direction perpendicular to the (100) plane, which has also been demonstrated in in vitro experiments. 14,29−31 Due to the highly anisotropic crystal structure, the refractive indices of guanine crystals along different crystallographic orientations are dramatically different, with the lower refractive index of ∼1.46 along the molecular stacking direction and the higher refractive index of ∼1.83 in the (100) plane.…”