Nano-structures which reflect or scatter certain colours are abundant in nature. For example, butterflies and birds use them to generate striking structural colours used in sexual display and camouflage. Reflectors are also used in animal eyes to produce images or to boost light-sensitivity. Many of these nanostructures are made from organic crystalline materials and over the last decade, the utility of crystalline materials in biological reflectors has been explored in a variety of systems.
In a previous study, it was discovered that prawns and crayfish use crystalline isoxanthopterin, a pteridine analogue of guanine, in their tapeta. A tapetum is a reflective structure which underlies the retina in many animal eyes and which functions to back-reflect photons not absorbed by the retina on the first pass, enhancing sensitivity. In the shrimp L. vannamei, this tapetum reflector is formed from a dense assembly of spherical 300 nm core-shell isoxanthopterin nanoparticles that are packed periodically over length scales of a few microns. These particles are constructed from lamellae of thin crystalline plates of isoxanthopterin assembled around an aqueous core much like the layers of an onion. Crystalline isoxanthopterin is highly birefringent, meaning light waves of different polarization experience different refractive indices as they propagate through the material. Curious about the implications of this structure and the birefringence of isoxanthopterin, we proceeded to study these nanoparticles in greater detail. Much to our surprise, we discovered that the arrangement of crystalline plates within each nanoparticle is such that light polarized tangential to the surface of the particle experiences a much higher refractive index than fields polarized normal to the nanoparticle surface.
Spherical structures of this kind, characterized by two refractive indices – one radial and one tangential, are commonly found in crystallized polymers and are known as spherulites. Spherulites possess one particularly interesting property- although birefringent, the particles possess the same rotational symmetries as an optically isotropic sphere. Intrigued by these particles, we thoroughly searched the literature and found a body of work that developed a framework to calculate light scattering from spherulites. After a few weeks spent writing, debugging and running the scattering programs, we had our first results – isoxanthopterin spherulitic shells scatter more efficiently in backward directions than isotropic shells of similar refractive indices.
Inspired by this finding, we proceeded to calculate the optical properties of close-packed assemblies of the nanoparticles using numerical electromagnetic simulations. From these simulations we learnt that the assemblies of spherulites reflect efficiently across a wider band of wavelengths in the blue region of the visible spectrum than assemblies of optically isotropic particles. Further calculations indicated that this enhanced range of efficient reflection is a consequence of the increased refractive index contrast in the spherulite assemblies. The high efficiency of these reflecting assemblies meant the tapetum could be thin, enabling better packing of sensory elements in the eye.
The organism appears to have evolved the ability to control the structural properties of the tapetum over several length scales, as illustrated in the Figure above. The alignment of the crystalline isoxanthopterin platelets, the diameter and core-shell ratio of the nanoparticles, and the periodicity of the assembly together determine optical properties of the assembly. This provides inspiration not only to search for new crystalline biomaterials with interesting optical properties but also to study means of organizing these materials at the nanoscale to engineer optical properties.
Our paper titled "A highly reflective biogenic photonic material from core–shell birefringent nanoparticles" has been published in Nature Nanotechnology (https://doi.org/10.1038/s41565-019-0609-5).
The optical properties of spherulite photonic crystals have been described in more detail in Yallapragada, V. J. & Oron, D., Opt. Lett. 44, 5860 (2019).
Preliminary results on isoxanthopterin structures in shrimp eyes are described in Palmer, B. A. et al., Proc. Natl. Acad. Sci. U. S. A. 115, 2299–2304 (2018).
Our scattering calculations are based on Roth, J. & Dignam, M. J., J. Opt. Soc. Am. 63, 308–311 (1973).