Quantitative Optical Properties of Sustainable Bio-Pigment Xanthommatin, Host Polymers and Biomaterials for Sensing Metasurfaces and Cavities—Nanophotonic Refraction, Absorption and Scattering

New optical biomaterials such as bio-pigments and biominerals enable very interesting properties, such as bright structural color and iridescence, dependency of perceived color on viewing angle. For example, isoxanthopterin nanoparticles in reflectors within shrimp eyes cause significant photonic backscattering that optimizes the sensitivity and spectral coverage of shrimp eyes.[1] The bio-pigment Xanthommatin has been found to create color in cephalopod chromatophore organs, has demonstrated natural and artificial photon scattering when encased in natural and artificial granules present in the chromatophores, has demonstrated good properties as a sustainable, natural ingredient for cosmetics [2], and can, because it is a small molecule, be combined with a variety of non-toxic polymer hosts. However, the latter add nanostructures complicating optical analysis, and so far, a quantitative determination of the indices of refraction of Xanthommatin (Xa) across a broad wavelength range has eluded researchers. Xanthommatin (Xa) is also robust for a pigment and can withstand unusual amounts of UV radiation [2].<br/><br/>We have fabricated Xa-bearing polymer films of PVA and found the PVA films to be too porous and rough for high-quality optical analysis. Instead, we use a polymer film platform of ethyl cellulose (EC), a sustainable material which, when fabricated using a specific protocol determined in our laboratory, can be made into a reasonably good optical material with indices of refraction determined by broadband (380 nm – 3300 nm) ellipsometry including depolarization analysis. As an alternative good optical quality polymer film platform, we also try SU-8 resist, which is less sustainable than EC. We also analyze the optical indices of a biomineral (goethite), which is both sustainable and robust against environmental conditions, in an EC matrix. We synthesized goethite (a-FeOOH), using a mixture of iron nitrate and tetramethylammonium hydroxide with an initial pH around 13. The reactant has a pure yellowish color and forms spinel rods having 10-30 nm diameters and lengths in the range of 100-300 nm as determined by Scanning Electron Microscopy (SEM). We have created nanophotonic devices – Fabry-Perot cavities and 1-D and 2-D metasurfaces with photonic crystal properties – using Xa and goethite, and have carried out sensing experiments in environments rich with glucose liquid, ethanol vapor, and humidity. The 1-D metasurface experienced overall signal attenuation and perhaps a small peak shift. This peak shift was modeled and the wavelength shift per refractive unit change at the surface estimated, for detecting such molecules. We comment further on their optical performance, detection bandwidth, and other capabilities for sensing molecules such as sensitivity and selectivity in future nanophotonic devices.<br/><br/>Prof. Richard M. Osgood, Jr., who passed away unexpectedly this past fall, was a leader in optical materials and optical devices and, while he did not work extensively in the biomaterials field, was a proponent of researching sustainable materials like biomaterials and some polymers. The perspective of Osgood’s past efforts, related to this research, will be briefly mentioned.<br/><br/>[1] Nat. Nanotech. 15 138 2020. [2] L. F. Deravi, N. C. Cox, C. A. Martin, JID Innovations 2 100081 2022.