NanoFermentation™: A bioprocess for manufacturing inorganic nanomaterials
by Robert J. Lauf
(Nanowerk Spotlight Guest Writer Series) The high cost and limited quantity of many "nanomaterials" create barriers to their commercialization because potential end users are reluctant to develop applications for materials when they are not confident that the materials will be available in useful quantities. Material suppliers are equally reluctant to invest in new manufacturing technologies without a clear end market, forming a vicious cycle that is hard to break.
NanoFermentation™, developed by Tommy Phelps and his team at Oak Ridge National Laboratory, is the first system to use industrial bioprocessing methods to manufacture nanometer-scale inorganic engineering materials rather than organic compounds. NanoFermentation™ harnesses the natural metabolic processes of metal-reducing bacteria to create tailored, single-crystal nanoparticles of important engineering materials, particularly ferrites.
Metal-reducing bacteria have been known for years, from a range of anaerobic environments, including lake sediments, the Antarctic, the ocean floor, hot springs, and deep subsurface geologic structures. Briefly, these bacteria have evolved metabolic processes suited for anaerobic conditions. Rather than relying on the oxidation of hydrogen or carbon in the presence of air, the bacteria reduce Fe(III) to Fe(II) or other similar reactions in the presence of an electron donor such as hydrogen or glucose. Because their electron transfer processes employ specialized proteins such as metal-reductases, it was long believed that the bacteria would always produce very pure magnetite, and in fact the presence of pure magnetite in a rock layer is considered a possible indicator that the rock is of biogenic origin.
Our discovery that the bacteria can make mixed metal oxides has created a breakthrough for large-scale synthesis of nanoscale powders.
Bacterial cells surrounded by magnetic iron oxide nanocrystals. (Source: Oak Ridge National Laboratory)
The photo shows three bacterial cells (rounded cylindrical bodies) surrounded by magnetic iron oxide nanocrystals they have produced in a laboratory fermentor. Two key aspects of the process are evident: First, the amount of product is very large compared to the biomass. Second, the product is formed outside the cell, so it can be harvested without damaging the cell culture. Indeed, a single cell culture can make a product of one composition in one run, and then make a different composition in the next run simply by receiving a different feedstock. The NanoFermentation™ process can use both low-temperature (e.g., Shewanella alga) and high-temperature (e.g., Thermoanerobacter ethanolicus) bacterial cultures.
Nanoparticles have been produced from many different simple and complex metal oxides, including
iron, cobalt, nickel, chromium, manganese, and zinc;
palladium;
rare earths (neodymium, gadolinium, erbium, holmium, and terbium); and
uranium.
We attribute the homogeneity and phase purity of the product to the action of the bacterial membrane, which effectively forms an electrode at a relatively fixed redox potential. The redox potential controls the crystal chemistry of the stable phase as it forms. Because of the vast number of individual bacterial cells and their high surface area, the electrode is effectively huge and is intimately dispersed throughout the volume of the fermentor. When the desired product has formed, the process automatically stops because the product phase is in equilibrium and will no longer accept electrons from the bacteria.
The most immediate applications of NanoFermentation™ are focused on nanoscale particles of doped ferrites. Because each particle is a single crystal containing a single ferromagnetic domain, the material exhibits giant paramagnetism. The powders can be used for magnetic media, ferrofluids, magnetorheological media, radar-absorbant materials and coatings, microfluidic heat transfer systems, and xerographic toner. We now have evidence that our latest improvements in NanoFermentation™: allow us to make rare-earth ferrites with tailored Curie points that will enable further advances in ferrofluids and "lab on a chip" technologies.
NanoFermentation™ can also create many other mixed transition metal oxides, including cobalt, chromium, manganese, zinc, uranium, and rare earth compounds. We expect that these materials will find use in catalysts, pigments, fluids, transport systems, and other product niches where precisely controlled fine oxide particulates are needed.
We expect NanoFermentation™ to follow a long-term development trajectory similar to that of sol-gel processing. As more creative people become aware of the process and its inherent flexibility, they will push the envelope outward. For example, a potential customer recently contacted us to propose a collaborative effort to integrate our nanomaterials into their proprietary water purification concept, with important implications for both military and municipal water systems.
The large number of potential bacterial species creates wide opportunities to develop other interesting ceramic nanomaterials. We are presently developing a conceptual framework for adapting NanoFermentation™ to synthesize non-oxide particulates, including semiconductors.
The most exciting prospect is that NanoFermentation™ will create a "virtuous cycle" in which the availability of these new materials in economic quantities will spur the development of new applications, helping to drive the field of nanotechnology to its fullest potential.
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