Diatoms : from dynamite to photonic crystal

In terms of materials science, diatoms open up new avenues for research and development. The highly porous architectures of their frustules have made it possible to use diatomites (fossil diatoms) as: adsorbers of toxic molecules and heavy metals, carriers of reactive molecules and catalysts, filters and membranes, vectors of active ingredients in agriculture and cosmetics. Because of their abundance and the structural evolution of their frustules in response to pollution, diatoms are also used as indicators of water quality. Since diatoms can be produced by aquaculture and present a wide diversity of nano-, micro-structures and morphologies, they are an easily accessible and sufficiently abundant source to obtain the quantities required for materials science studies.

In particular, over the last ten years, since the explosion of nanotechnologies, a great deal of research has been carried out into the multi-scale porous structures of diatoms. The potential applications of these new diatom-based materials or devices were extensively presented and discussed in the sixth lesson. Chemical and/or biochemical modification of diatom frustules leads to materials with a range of original properties. Porous silica frustules accept high levels of active ingredients, are biocompatible and easily functionalized with organic or biological components, enabling the development of hybrid therapeutic vectors for controlled drug delivery. Theranostic vectors that combine targeting, diagnostics and therapy have already been developed. Recent studies have also demonstrated that these microalgae can be a sustainable source of biofuels. The nanopore structures of frustules are similar to those of photonic crystals, in which photons propagate like electrons in a semiconductor crystal, generating zones of permitted and forbidden energy. Thanks to this property, frustules can be used as micro-lenses to optimize optical responses and concentrate light in energy conversion and storage devices such as photovoltaic cells, dye-sensitized solar cells, photoelectrochemical cells, photo-rechargeable batteries... On the other hand, the topotactic transformation of diatoms into various materials: semiconductors, conductors, piezoelectrics (_TiO2, _BaTiO3, Si, diatom-graphene nanocomposites, etc.) opens up a host of possibilities for the development of new materials.) opens up a wide range of possibilities for the development of materials with properties that can be applied in the energy and environmental fields. In particular, the photoluminescence or conduction properties of these materials are sensitive to the adsorption of gases, molecules or biomolecules. This sensitivity, amplified by the multi-scale porosity of frustules, makes it possible to develop high-performance gas sensors and biosensors. The transformation of silica frustules into the element silicon enables the development of Li-ion batteries with negative silicon electrodes, whose performance is promising.

Diatoms can also form stable colloidal suspensions, enabling the lithography of microelectronic circuit components and sensor arrays. These components can be chemically modified in a second step by ALD(Atomic Layer Deposition) , further diversifying the functionalities available.

Undoubtedly, a better understanding of diatom structure and genomics, their mechanical, optical and photonic properties, and their bio-mineralization process will lead to the nano-fabrication and engineering of new materials and devices. Diatoms will continue to play an important role in biology and materials science, as their beauty continues to inspire all who study them.