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In this review we highlight recent advances in the understanding of biosilica production, biomodification of diatom frustules and their subsequent applications in bio/chemical sensors, and as a model membrane for filtration and separation.

Understanding the morphogenesis of mineralized structures found in shells, bones, teeth, spicules and plant cell walls is difficult because of the complexities underlying biomineralization, and the requirement of accurate models for pattern formation. Here, we investigated the spatial and temporal development of siliceous structures found in a model diatom species, Phaeodactylum tricornutum, for which the entire genome has been sequenced and transformation is routine. Analyses of pattern formation revealed that the process of silicification starts from a 'pi-like' structure that controls the spatial organization of a sternum upon which regular instabilities are initiated and developed. Detailed analyses also demonstrate that morphogenesis of silica is nonuniform. We also tested the sensitivity of pattern formation to perturbation of proton pumps, and found that selective inhibitors of H(+)-V-ATPases affect silica biomineralization both quantitatively and qualitatively. Morphometric analyses of valves purified from isogenic populations of cells show that the morphometric noise of several traits is under exquisite regulation, explaining why the overall valve pattern is reproducibly maintained. Altogether our analyses demonstrate that silica morphogenesis is a robust but nonuniform process, and allow us to propose a model for the dynamic growth of materials within a spatially controlled geometry.

The control of silica growth by living organisms such as diatoms is known to involve the templating effect of several biomolecules working concomitantly. However, until now, biomimetic studies involving model molecules have mainly been performed with single templates. We show here that the addition of two biopolymers, gelatin and alginic acid, to silicate solutions allows the formation of complex structures resulting from the combined templating effect of both components at different scales. Gelatin is able to activate silica formation resulting in hybrid aggregates at the nanoscale. Alginic acid does not interfere with silica condensation but is able to control silica morphology through the assembly of these gelatin-silica aggregates at the microscale. For all materials, calcination up to 700 degrees C degrades the polymer component of the hybrid material and opens macroporosity in the silica network. In parallel, the high thermal stability of gelatin allows a good preservation of initial silica nanoparticle size upon heating whereas a coarsening process is observed in the sole presence of alginate. These results correlate well with previous models of biosilicification and suggest that the use of multiple templates is a suitable approach to elaborate more complex silica architectures.

Silicon biomineralization is a widespread mechanism found in several kingdoms that concerns both unicellular and multicellular organisms. As a result of genomic and molecular tools, diatoms have emerged as a good model for biomineralization studies and have provided most of the current knowledge on this process. However, the number of techniques available to study its dynamics at the cellular level is still rather limited. Here, new probes were developed specifically to label the pre-existing or the newly synthesized silica frustule of several diatoms species. It is shown that the LysoTracker Yellow HCK-123, which can be used to visualize silica frustules with common filter sets, presents an enhanced signal-to-noise ratio and allows details of the frustules to be imaged without of the use of ionophores. It is also demonstrated that methoxysilane derivatives can be coupled to fluorescein-5-isothiocyanate (FITC) to preferentially label the silica components of living cells. The coupling of labeling procedures might help to address the challenging question of the process of frustule exocytosis.

Pore channels of polycarbonate membranes were recently used as biomimetic models to study the effect of confinement on silicate condensation, leading to the formation of silica tubes exhibiting a core-shell structure. In this work, we preimmobilized poly-L-lysine on the membrane pores, leading to modification of the tube shell formation process and variation in core particle size. These data strengthen previous assumptions on the role of confinement on silica growth, i.e., interfacial interactions and perturbation of the diffusion coefficient. They also suggest that this approach is suitable to investigate in more detail the contribution of confinement effects on silica biomineralization.

Several strains of diatom exhibit a long-term viability in silica gels and demonstrate the ability to dissolve the silica in their surroundings.

Polymer membranes were used as biomimetic environments to study the effect of confinement on silica formation. Within membrane pores, silica tubes were formed, consisting of a dense silica shell incorporating nanoparticle aggregates. The shell structure does not depend on the membrane pore size, suggesting that its formation proceeds via interfacial interactions with the pore surface. In contrast, the size of primary nanoparticles within core aggregates is influenced by pore dimensions, indicating an effect of confinement on the diffusion-limited growth of silica. A parallel can be drawn with reported roles of confinement in biomineralization processes, providing a basis for future developments in biosilicification mimetic approaches and biofunctional nanomaterials design.