Diatoms are single-celled microalgae producing silica-based cell walls. This biosilica is mechanically very stable, contains hierarchically arranged pores, and a relatively large surface area mainly due to mesoporosity. Mesopores are well-suited cavities for catalyst attachment and enable efficient reactant diffusion through the material. To mimic the extraordinarily high catalytic efficiencies of supramolecular enzyme complexes, we utilized layer-by-layer mineralization to simultaneously immobilize on diatom biosilica three enzymes that catalyze an artificial cascade reaction . The effect of relative placement of the enzymes in the silica layers was determined. To investigate the influence of biosilica structure on enzyme activity we used in vitro and in vivo methods for immobilizing enzymes in (i) biosilica from different diatom species, and (ii) in different regions of the biosilica of one diatom species. The experimental data reveal a substantial effect of biosilica morphology on enzyme activity.
Additionally, we investigated catalytic performance of gold nanoparticle(AuNP)-biosilica composites that were produced through covalent attachment in vitro . The AuNPs were homogeneously distributed on the biosilica surface and exhibited higher catalytic activities compared to AuNPs on synthetic silica materials. To avoid the harsh conditions for the AuNP synthesis, we established a biosynthetic route (“green synthesis”) to produce AuNPs using the diatom Stephanopyxis turris . Surface enhanced Raman spectroscopy (SERS) was used to determine the environment and localization of AuNPs in S. turris. The in vivo produced AuNPs were rather heterogeneous with respect to shape and size, and thus we aim to elucidate the mechanism underlying the nanoparticle biosynthesis to improve AuNP properties.