Micro-organisms : nanoparticle factories ?

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Plants and their extracts, as well as a wide variety of micro-organisms such as yeasts, fungi, microalgae and bacteria, are capable of generating nanoparticles of very different compositions (oxides, metals, sulfides, etc.). These microfactories can effectively trap metal salts and molecular compounds and then transform them into solids by soft chemistry to produce a variety of micro- and/or nanoparticles (gold, silver, calcite, CdS, magnetite, _Zn3(_PO4₎₂), often well calibrated. We have examined the main biosynthetic pathways and formation mechanisms of these nanomaterials. The key players in precursor-material transformation depend on the nature of the microorganism or biomass used. In the case of plants (leaves, roots, stems, flowers, etc.), these players are secondary metabolites (alkaloids, flavonoids, saponins, steroids, tannins, etc.), which act as complexing reducers. In the case of algae, fungi and yeasts, these players are respectively: polysaccharides carrying numerous hydroxyl groups that reduce and stabilize nanoparticles, extra- or intracellular enzymes (reductases) that initiate the biomineralization process, oxidoreductases linked to the cell membrane and quinones. In the case of bacteria, the cell reduces metal ions via specific enzymes such as NADH- or nitrate-dependent reductases to form nanoparticles. The choice of bacterial system makes it possible to select bacteria that can work at high or low temperatures. Using these properties, small companies have sprung up in the USA. They are producing larger quantities of nanomagnets and magnetic powders. These plants mainly use bacteria containing metal oxidoreductases that enable the production of a wide variety of metal oxides, such as ferrites of iron, cobalt, nickel, chromium, manganese and zinc, rare-earth oxides (Nd, Gd, Er, Ho, Tb) and uranium, and metals (gold, silver, palladium, selenium). This strategy has a number of advantages: the products are formed outside the cell, making them easier to harvest without damaging the cells, and therefore easier to recycle.

The materials and systems developed from these strategies are highly diverse. For example, the electrons produced by the metabolic activity of bacteria can be a source of electricity generation, enabling the development of fuel biopiles or microbe-based electrode materials. Arsenic sulfide nanotubes produced by Shewanella sp. display interesting electrical and optical properties. The formation of biofilms based on reducing bacteria enables the development of coatings for metal anticorrosion. Bacillus cereus-gold nanoparticle composites can be used to generate highly efficient electrically-sensing humidity biosensors, while hybrid micromotors can be constructed by functionalizing molecular sieve components and then coupling them with Escherichia coli bacteria. The movement of these living bacterial-zeolite nanocomposites can be easily visualized by optical microscopy with fluorescence detection.