Inorganic Preforms of Biological Origin: Shape-Preserving Reactive Conversion of Biosilica Microshells (Diatoms)

The attractive three-dimensional (3-D) self-assembly characteristics of certain microorganisms may be coupled with the chemical versatility of synthetic processing to yield a revolutionary biologically enabling fabrication paradigm known as Bioclastic and Shape-preserving Inorganic Conversion (BaSIC). Nature provides impressive examples of the precise and scalable assembly of 3-D mineral (bioclastic) structures. A stunning variety of nanostructured 3-D silica assemblies are generated by diatoms (unicellular algae). Diatoms form rigid cell walls (frustules) comprised of inter-connected networks of amorphous silica. Each diatom species forms a 3-D frustule with a particular hierarchical structure; that is, the micro-scale frustule shape and patterned nanoscale frustule features (pores, channels, protuberances, etc.) are specific to a given diatom species. Because the frustule morphology is replicated with a high degree of fidelity upon diatom reproduction, sustained culturing of a particular diatom species can yield enormous numbers of frustules of similar shape (e.g., >1 trillion replicas in 40 reproduction cycles). Such genetically precise and massively parallel 3-D self-assembly under ambient conditions exceeds the capabilities of current synthetic protocols. However, the SiO2 chemistry of diatom frustules is not appropriate for a variety of devices. With BaSIC, diatom frustules (and other bioclastic structures) can be converted into a variety of new functional chemistries through shape-preserving displacement reactions, conformal coating approaches, or combinations thereof (although this chapter will focus on the use of displacement reactions). If continued research on the genetic manipulation of diatoms (see Chapter 3 in Volume 1) leads to tailorable frustule morphologies, then such genetic engineering may be coupled with the BaSIC process to enable the manufacturing of low-cost 3-D Genetically Engineered Micro/nano-devices (3-D GEMs).