Biomineral-Based Optical Metasurfaces and Cavities - Analysis and Sensing

New optical biomaterials such as biopigments and biominerals enable fascinating optical properties, such as bright and structural color and advanced camouflage. Arrays of isoxanthopterin nanoparticles in reflectors within shrimp eyes cause significant photonic backscattering, optimizing the sensitivity and spectral coverage of shrimp eyes.[1] Biominerals such as silicate in diatoms, strontium sulfate in radiolarians, goethite in limpets, magnetite in magnetotactic bacteria, and calcium phosphate in bones provide mechanical structure, sense magnetic and gravitational fields, and can store and mobilize iron. Biominerals are primarily non-toxic and environmentally friendly materials, generally with very high indices of refraction, especially for organic compounds. Biominerals are good candidates for optically sensing materials, because of their high index of refraction and non-toxic, scalable, and manufacturable properties. They are expected to be unregulated in the future due to lack of toxicological concern, unlike metal oxide nanoparticles, which are facing increasing amounts of regulation in manufacturing settings, due to concerns about potential carcinogenic nature from chronic exposure to workers in a manufacturing plant. Biominerals should be supply-chain resilient as well because they are straightforward and relatively simple to fabricate.<br/><br/>We have chemically synthesized the biomineral 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, forms spinel rods that have 10-30 nm diameters and lengths in the range of 100-300 nm as determined by Scanning Electron Microscopy (SEM), exhibits the correct elemental composition for goethite in Electron Dispersive Spectroscopy (EDS), and powder x-ray analysis matches reference data from x-ray crystallographic data. 2-D photonic crystals were formed using 508 nm PS beads and were very iridescent, producing different colors at different angles. These photonic crystals had a bandgap in the red and transmitted light therefore had a yellowish-green color. A film of goethite was dropcast onto this 2-D array and registered an interesting drop in signal intensity when exposed to ethanol vapor, the analysis of which will be reported in this presentation. 1-D photonic crystal arrays consisting of alternating 70 nm PVA and 100 nm goethite layers were fabricated, show the expected multilayer peaks in reflectivity (7 maxima between 800 nm and 2500 nm), and considerable backscattering above 1100 nm. For wavelengths less than 1100 nm, the silicon substrate attenuated the backscattered light. Exposing this multilayer to glucose liquid, ethanol vapor, and humidity all results in an attenuated signal and possibly a peak shift. This peak shift was modeled and the wavelength shift per refractive unit change at the surface estimated, for detecting molecules like ethanol, water, and glucose. Single Fabry-Perot cavity films of goethite were also fabricated, thickness and optical indices measured with profilometry, spectrophotometry, and ellipsometry, and sensing results from these F-P cavities will be reported. We will also explain how, in the future, to make the biomineral-based sensing more sensitive and selective, using an embedded specific receptor in the photonic crystal and in thin films. Potential optical systems and detection methods using biominerals will be discussed.<br/><br/>[1] Nat. Nanotech. 15 138 2020.