Bio-Inspired Crystalline Core-Shell Guanine Spherulites.

Abstract

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 ( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mi>Δ</mi> <mi>n</mi> <mo>≈</mo> <mspace></mspace> <mn>30</mn> <mo>%</mo></mrow> <annotation>$\Delta n \approx \ 30\%$</annotation></semantics> </math> ) 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 (( <math xmlns="http://www.w3.org/1998/Math/MathML"> <semantics><mrow><mi>Δ</mi> <mi>n</mi> <mo>≈</mo> <mn>25</mn> <mo>%</mo></mrow> <annotation>$\Delta n \approx 25\%$</annotation></semantics> </math> ) 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.