Near-Infrared Selective and Angle-Independent Backscattering from Magnetite Nanoparticle-Decorated Diatom Frustules

Abstract

Periodic heterogeneous structures exhibit color with brilliance through constructive interference of electromagnetic waves in accordance with Bragg’s law. However, the wavelength of diffracted light is strictly angle-dependent, and such periodic structures generate only iridescent color. Here we report that periodically porous, microellipsoidal shells of diatom Pinnularia sp. wavelength-selectively backscatter light at arbitrary incidence. The biosilica frustules could be approximated as many polygonal faces of two-dimensional photonic crystal slabs. Furthermore, surface decoration of the frustule with magnetite nanoparticles produced photonic band gaps in the near-infrared. Magnetite nanoparticle-decorated frustules behave as angle-independent near-infrared reflectors and near-infrared contrast agents in optical coherence tomography.