Biosynthesis of Silicon–Germanium Oxide Nanocomposites by the Marine Diatom Nitzschia frustulum

The biomineralization capacity of the photosynthetic marine diatom Nitzschia frustulum was harnessed to fabricate Si–Ge oxide nanocomposite materials. Germanium was incorporated into the diatom cell by a two-stage cultivation process. In stage 1, the N. frustulum cell suspension was grown up to cell density of 3 × 10⁶ cells/mL in 0.35 mM silicic acid within a bubble-column photo- bioreactor. In stage 2, when all of the soluble silicon was consumed,0.10 mM Ge(OH)₄ or a mixture of 0.020 mM Ge(OH)₄ and 0.25 mM Si(OH)₄ were added to Si-starved cells. The cells assimi- lated soluble germanium by a surge uptake mechanism. The cell mass was thermally annealed in air at 800°C for 6 h to oxidize carbonaceous materials. The thermally annealed cell biomass was characterized by TEM-EDS, FT-IR, and XRD. These measurements confirmed the formation nanostructured Ge–Si oxides composed of CaSiO₃ and Ca₃ GeO₅.