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

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.