Organic Crystals and Optical Functions in Biology: Knowns and Unknowns

Organic crystals are widely used by animals to manipulate light for producing structural colors and for improving vision. To date only seven crystal types are known to be used, and among them β-guanine crystals are by far the most widespread. The fact that almost all these crystals have unusually high refractive indices (RIs) is consistent with their light manipulation function. Here, the physical, structural, and optical principles of how light interacts with the polarizable free-electron-rich environment of these quasiaromatic molecules are addressed. How the organization of these molecules into crystalline arrays introduces optical anisotropy and finally how organisms control crystal morphology and superstructural organization to optimize functions in light reflection and scattering are also discussed. Many open questions remain in this fascinating field, some of which arise out of this in-depth analysis of the interaction of light with crystal arrays. More types of organic crystals will probably be discovered, as well as other organisms that use these crystals to manipulate light. The insights gained from biological systems can also be harnessed for improving synthetic light-manipulating materials.

Bio-Inspired Crystalline Core-Shell Guanine Spherulites

Spherical particles with diameters within the wavelength of visible light, known as spherulites, manipulate light uniquely due to their spatial organization and their structural birefringence. Most of the known crystalline spherulites are branched, and composed of metals, alloys, and semi-crystalline polymers. Recently, a different spherulite architecture is discovered in the vision systems of decapod crustaceans - core-shell spherulites composed of highly birefringent (Δ n ≈ 30 % $\Delta n \approx \ 30\%$ ) organic single-crystal platelets, with exceptional optical properties. These metastructures, which efficiently scatter light even in dense aqueous environments, have no synthetic equivalence and serve as a natural proof-of-concept as well as synthetic inspiration for thin scattering media. Here, the synthesis of core-shell spherulites composed of guanine crystal platelets ((Δ n ≈ 25 % $\Delta n \approx 25\%$ ) is presented in a two-step emulsification process in which a water/oil/water emulsion and induced pH changes are used to promote interfacial crystallization. Carboxylic acids neutralize the dissolved guanine salts to form spherulites composed of single, radially stacked, β-guanine platelets, which are oriented tangentially to the spherulite surface. Using Mie theory calculations and forward scattering measurements from single spherulites, it is found that due to the single-crystal properties and orientation, the synthetic spherulites possess a high tangential refractive index, similarly to biogenic particles.

Surface-Guided Crystallization of Xanthine Derivatives for Optical Metamaterial Applications

Numerous bio-organisms employ template-assisted crystallization of molecular solids to yield crystal morphologies with unique optical properties that are difficult to reproduce synthetically. Here, a facile procedure is presented to deposit bio-inspired birefringent crystals of xanthine derivatives on a template of single-crystal quartz. Crystalline sheets that are several millimeters in length, several hundred micrometers in width, and 300-600 nm thick, are obtained. The crystal sheets are characterized with a well-defined orientation both in and out of the substrate plane, giving rise to high optical anisotropy in the plane parallel to the quartz surface, with a refractive index difference Δn ≈ 0.25 and a refractive index along the slow axis of n ≈ 1.7. It is further shown that patterning of the crystalline stripes with a tailored periodic grating leads to a thin organic polarization-dependent diffractive meta-surface, opening the door to the fabrication of various optical devices from a platform of small-molecule based organic dielectric crystals.

A highly reflective biogenic photonic material from core-shell birefringent nanoparticles

Spectacular natural optical phenomena are produced by highly reflective assemblies of organic crystals. Here we show how the tapetum reflector in a shrimp eye is constructed from arrays of spherical isoxanthopterin nanoparticles and relate the particle properties to their optical function. The nanoparticles are composed of single-crystal isoxanthopterin nanoplates arranged in concentric lamellae around a hollow core. The spherulitic birefringence of the nanoparticles, which originates from the radial alignment of the plates, results in a significant enhancement of the back-scattering. This enables the organism to maximize the reflectivity of the ultrathin tapetum, which functions to increase the eye's sensitivity and preserve visual acuity. The particle size, core/shell ratio and packing are also controlled to optimize the intensity and spectral properties of the tapetum back-scattering. This system offers inspiration for the design of photonic crystals constructed from spherically symmetric birefringent particles for use in ultrathin reflectors and as non-iridescent pigments.

The Organic Crystalline Materials of Vision: Structure-Function Considerations from the Nanometer to the Millimeter Scale

Vision mechanisms in animals, especially those living in water, are diverse. Many eyes have reflective elements that consist of multilayers of nanometer-sized crystalline plates, composed of organic molecules. The crystal multilayer assemblies owe their enhanced reflectivity to the high refractive indices of the crystals in preferred crystallographic directions. The high refractive indices are due to the molecular arrangements in their crystal structures. Herein, data regarding these difficult-to-characterize crystals are reviewed. This is followed by a discussion on the function of these crystalline assemblies, especially in visual systems whose anatomy has been well characterized under close to in vivo conditions. Three test cases are presented, and then the relations between the reflecting crystalline components and their functions, including the relations between molecular structure, crystal structure, and reflecting properties are discussed. Some of the underlying mechanisms are also discussed, and finally open questions in the field are identified.

Ocellar structure is driven by the mode of locomotion and activity time in Myrmecia ants

Insects have exquisitely adapted their compound eyes to suit the ambient light intensity in the different temporal niches they occupy. In addition to the compound eye, most flying insects have simple eyes known as ocelli, which assist in flight stabilisation, horizon detection and orientation. Among ants, typically the flying alates have ocelli while the pedestrian workers lack this structure. The Australian ant genus Myrmecia is one of the few ant genera in which both workers and alates have three ocellar lenses. Here, we studied the variation in the ocellar structure in four sympatric species of Myrmecia that are active at different times of the day. In addition, we took advantage of the walking and flying modes of locomotion in workers and males, respectively, to ask whether the type of movement influences the ocellar structure. We found that ants active in dim light had larger ocellar lenses and wider rhabdoms compared with those in bright-light conditions. In the ocellar rhabdoms of workers active in dim-light habitats, typically each retinula cell contributed microvilli in more than one direction, probably destroying polarisation sensitivity. The organisation of the ocellar retina in the day-active workers and the males suggests that in these animals some cells are sensitive to the pattern of polarised skylight. We found that the night-flying males had a tapetum that reflects light back to the rhabdom, increasing their optical sensitivity. We discuss the possible functions of ocelli to suit the different modes of locomotion and the discrete temporal niches that animals occupy.

Optically functional isoxanthopterin crystals in the mirrored eyes of decapod crustaceans

The eyes of some aquatic animals form images through reflective optics. Shrimp, lobsters, crayfish, and prawns possess reflecting superposition compound eyes, composed of thousands of square-faceted eye units (ommatidia). Mirrors in the upper part of the eye (the distal mirror) reflect light collected from many ommatidia onto the photosensitive elements of the retina, the rhabdoms. A second reflector, the tapetum, underlying the retina, back-scatters dispersed light onto the rhabdoms. Using microCT and cryo-SEM imaging accompanied by in situ micro-X-ray diffraction and micro-Raman spectroscopy, we investigated the hierarchical organization and materials properties of the reflective systems at high resolution and under close-to-physiological conditions. We show that the distal mirror consists of three or four layers of plate-like nanocrystals. The tapetum is a diffuse reflector composed of hollow nanoparticles constructed from concentric lamellae of crystals. Isoxanthopterin, a pteridine analog of guanine, forms both the reflectors in the distal mirror and in the tapetum. The crystal structure of isoxanthopterin was determined from crystal-structure prediction calculations and verified by comparison with experimental X-ray diffraction. The extended hydrogen-bonded layers of the molecules result in an extremely high calculated refractive index in the H-bonded plane, n = 1.96, which makes isoxanthopterin crystals an ideal reflecting material. The crystal structure of isoxanthopterin, together with a detailed knowledge of the reflector superstructures, provide a rationalization of the reflective optics of the crustacean eye.