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Diatoms have a very interesting model system with high potential in the nano-structured production, there are almost 100 000 different species with unique frustule morphologies, and it is therefore important to note the various genetic variations, because it is these genes that govern the formation of frustules with 3D precision. With multifunctional properties, 105 different diatom species are formed in tens of nanometers, with hierarchical architectures. Formed frustule architectures (Figure 2 shows a range of frustules) have optical, mechanical and transport properties, and can also be used in making advanced devices for light harvesting, photonics, molecular separation, sensing, and drug delivery.
Figure 2: Diversity in shapes and structures of diatoms (Zyga, 2007)
But the question is, whether there are any other principles such as chemical / thermodynamic / kinetic influences which govern the diatoms, and how they are grouped into macro-structures such as forming clusters and colonies. It is especially interesting to focus on things such as; diatom frustules' potential for advanced materials with respect to their intricate structure, synthesis of novel silica-based material by biomimetics, and also principles responsible for colonizing; and how these can be exploited to serve as alternatives in nanofabrication techniques, sensing devices, etc. (Dusan Losic J. G., 2009) In this project we will particularly study the diatoms natural and induced ability to orient itself and the possibility of their attachment to the substrate. Also, relating the fine structure of diatoms to the different genes and RNA expressions present so that it can be used to construct high scale integrated micro- and nano- structures in 3D is also a part.
Diatomaceous lessons in nanotechnology and advanced materials
Use of silicon and silica is intense since a past few decades, and the diversity existing for innovation with this element raises scope for research. In 1988, Richard Gordon invented the word "Diatom Nanotechnology", referring to the ways diatoms create 3D nanostructures by controlled deposition of silica in their skeletons. Minute structures made by the diatoms which are beyond the capabilities of materials scientist are attracting the attention of nanotechnologist to learn from them a large number of concepts. (Bradbury, 2004) Current research is focused on fabrication and self-assembly methods for shaping elemental silicon in nanometer to micromenter length ranges, for widespread use in electrical, optical and structural materials. Various fields and application domains for diatoms in biotechnology and material sciences are creating interest for the nanotechnologist (Figure 3).
Figure 3: Various fields and applications for diatoms research (Pascal J Lopez, 2005)
Joanna Aizenberg of Lucent Technologies' Bell Laboratories in Murray Hill, N.J., says, "We can think of diatoms as living silicon chips". Semiconductor manufactures build micro- and nanoscale features for use in electronics and optics, a costly and time-consuming undertaking. Comparatively, diatoms build structures out of silicon with more proficiently. Although diatoms are doubtful to put the semiconductor industry out of business in the near future, their capability to create new materials with complex structures on a miniature scale may possibly serve as the foundation of a powerful technology in the near future. Because by regular means, making of silicon chips or other electronics require a lot of harsh chemicals and also produce much waste, whereas diatoms produce the same kind of results without using any chemical and at ambient temperatures. (Goho, 2004)
DIATOM BIOMINERILIZATION - FORMATION OF BIOLOGICAL NANOSTRUCTURES
Diatoms generate their cell walls by silica biomineralization, composed of silica and organic molecules the cell wall has a complex structure. Inorganic components constitute to about 97% of the cell wall compounds, particullarly silica along with trace quantities of aluminium and iron. And the main organic components constituting the diatom biosilica are silaffins, proteins posttranslationally modified by long-chain polyamines and oligo-N-methyl-propyleneamines. To achieve the biomineralization particular interactions , choosy organic moieties and biocompatible minerals are to be evolved.
The frustule of the diatom gets its flexiblity and toughness from the materials it is composed of; silaffins and polyamine proteins co-precipitated with slicic acid available in the aqueous enviroment form the organic material of the diatom frustule. This is stronger when compared to the comerically available bulk silica, one of the most brittle materials known. Along with , the nanoporous structure also enhancess the toughness in addition to the materials forming the frustule. (Frank Noll, 2002) (Mann, 2002)
Silica becomes increasingly used in chemical, pharmaceutical and nano-technological processes, which causes an increased demand for well-defined silicas and silica-based materials. Production of highly ordered silica under economic satisfaction, with cheap starting materials and ambient conditions is the current research target from the (nano-to-microscale) manufacturing perspective. This is already realized from the formation of diatom biosilica, where highly hierarchical ordered (Figure 4) meso- and macro- featured silica structures are produced. (Vrieling, et al., 2005)(Andre P. Garcia, 2010)
Figure 4: Hierarchical structure in a diatom along the hexagonal grid (Andre P. Garcia, 2010)
Rigid cell walls (frustules) out of amorphous silica are formed by diatoms, which is massively parallel and precise 3-D hierarchical assembly.The diatom silica structure formation is dynamic and nonequilibrium process, where optimization is necessary not only in the fomation stages but also in the final phase. Produced by natural biomineralization using organic templates with controlled growth of the inorganic phase; with well-defined structures, arranged from the nanometer to macroscopic length scale, we have diatoms, shells, bones and teeth as unique examples. Ability to fabricate inorganic materials into complex hierarchical patterns by bottom-up self-assembly process is possessed by diatoms. Inspite of our limited understanding of the hierarchical complexity found in nature, strategies for mimicking nature have partially succeeded in synthesizing human designed bio-inorganic composite materials. (Emilie Pouget, 2007) (Kenneth H. Sandhage S. M., 2007)
A major aspiration for this research is to utilize the diatom expertise in biogenic silica construction to build up strategies for bio-inspired nanofabrication of silicon based equipment. In this project, we try to report a synthesis methodology for utilising the highly ordered slica structures formed by the diatoms, and arranging them into a definite pattern, which could be useful for many mechanism such as slow dispensing of medicines or other nanotechnological applications. It can be, if possible, further be generalized as a rational preparation scheme with well-defined multiscale architectures for applications into biotechnology as well as nanotechnology.
CONVERSION OF DIATOMS INTO 3D STRUCTURES WITH NEW CHEMISTRIES - BIOMIMETICS
Diatoms have developed elegant solutions for precise 3D manufacturing, enabling the low-cost mass production of micro/nano-devices with complex structures under physiologically compatible and environmentally gentle conditions. Replacing the existing nanofabrication techniques, such as 2D planar lithography, they are beneficial because of minimal energy usage and minimal waste production. Understanding about the bioprocesses available for the synthesis of nanomaterials may perhaps give the potential to design and develop new nano-sized devices.
Biomimetics is the extraction of good designs from the nature, where diatom biomimetics stands high in the materials community. A number of strategies for creating materials with outstanding functional properties are evolved from nature. There have also been many experimental works where the diatom nanostructures are built or copied to create complex architectures in 3-dimensional, also which do not fail due to friction, wear, adhesion or lubricant loss such as man-made micro-electromechanical systems (MEMS).
Beyond Micromachining: the potential of diatoms
As the frustule of the diatoms with hierarchial 3D porous structures is well defined and could be best appropriate for many applications some such as heterogeneous catalysis or seperation technologies and more. These kind of features have inspired the design and production of new nanomaterials by the maintenance of diatom nanostructure and modification of the material chemistry. Thus, there are number of processes (as in Figure 5) which could be used to produce composite materials having higly defined hierarchical structures from a large variety of easily and commercially available diatom species.
Figure 5: Summary of process, for producing 3D nanostructured materials (Dusan Losic J. G., 2009)
A strategy known as BaSIC (bio-clastic and shape preserving inorganic conversion) has been devised by Sandhage et al. where gas/silica displacement reactions, conformal coating, or a combination of all are used for converting the diatom frustules' chemical composition without losing the bio-assembled 3S morphologies. By the approach, nano-engineering efficiencies of living cells such as diatoms are harnessed which can serve as alternatives in a number of commercial applications. The adaptive growth is supported by hierarchical structuring in place of complex fabrication techniques, with the facility of constant remodeling and counteractive advantages.
For example, if diatom Coscinodiscus wailesii is grown in the presence of nickel sulphate, then the pore size in the frustule becomes larger and this could be used in sensing applications (Figure 6), like as an optical sensor where the change in light could be detected and information about environmental conditions be obtained. (Townley, 2007)
Figure 6: (a,b) frustule pattern of diatom Coscinodiscus wailesii; (c,d) pore size becomes larger when grown in the presence of nickel sulphate (Townley, 2007)
Another interesting example to look at may perhaps be, a commercially available diatomaceous earth structure being mimicked, it could form porous carbon catalyst support or current carries in fuel cells. For this first the cell structure is to be templated and then empty space be filled with sucrose and polymerising with sulphuric acid and carbonization in reducing atmosphere and finnaly removing the silica by dissolving in sodium hydroxide solution gives the required mimic useful for fuel cells. (S. M. Holmes, 2006) We can say numerous opportunities for developing new strategies which could result in multifunctional materials by hierarchical assembly are made available by biomimetic materials research.
Biomimetic materials research, lessons from the biological world: about hierarchical structuring, growth and functional adaptation, on damage repair and self-healing provide enormous opportunities for rapidly growing and enormously promising fields. A major opportunity for bio-inspired material synthesis and adaptation into specific functions is because of the hierarchical structuring given by a material, instead of choosing a new material for providing the desired function. Pore size control ability and morphology control along with chemical composition changeability gives the potential to modify the structural, photonic, absorptive, diffusive and mechanical properties of the diatom frustule to outfit various applications.
Processes for the assembly of biologically fabricated microstructures into defined patterns are required for future device applications in optoelectronics and bio-nanotechnology, to take advantage of the nanoscale features and properties. (Fratzl, 2007) Nanotechnology industry always aims to design and manufacture devices in the nanometer proportions but the current limitations for the nanotechnological devices are economy and atomic level accuracy. Where the nanotechnologists are attracted to the ways by which diatoms make minute silica structures - which is impossible to achieve using current materials and technology. The nanotechnological applications could be linked to atomic level filtrations, biosensors and immune-isolations or micro fabrications. (Gordon, 1999)
For instance growing cultures of single-celled diatoms by directed assembly may provide an alternative for achieving a scale range that may not have been possible to be achieved by the latest micromachining process, this shows the potential of diatoms beyond micromachining. Unanticipated discovery from the observation of nature will gradually be replaced by systematic approaches in the near future, by application of engineering principles, for further development of bio-inspired ideas. The synthesis of advanced materials having complex shapes with hierarchial architectures is possible by biomimetic mineralization along with size, shape and morphology control under controlled settings.
DIATOMS AS A NEW STANDARD FOR 3D NANOSTRUCTURE ASSEMBLY
"Biogenesis of the diatom cell wall is well thought-out to be a prototype for the controlled production of nanostructured silica". (Sumper, 2006) In nature elegant examples exist having intricate 3D microstructures with nanoscale features. Of these striking biological examples of self-formed rigid structures are diatoms, single celled micro algae with cell walls composed of amorphous silica. Diatom frustules have reproducible features in the range of micro-scale to nanoscale, further they have nano sized assembly of silica particles assembled which could be directly utilized in a wide range of micro/nano technological applications (e.g., masks for lithographic patterning, catalyst supports, and gel filtration, so on).
Diatom nanotechnology has gained much attention because of its wide range of species, highly regular multi-scale structures, fast reproduction capabilities and also genetic manipulability. Diatoms produce wide range of three-dimensional structures, with high rate of growth, which may be of great use in the manufacturing of components for the nanotechnology, acting as an alternative to current lithographic techniques.
In other words, diatoms could be used as bio-factories to generate great number of 3D structures with identical shapes with reproducible micro-to-nanoscale features. (Kenneth H. Sandhage, 2002) A wide range of applications is anticipated by the capability to genetically engineer diatoms to synthesize exclusive frustules.
In the face of the technological and economic promises, for such devices with wide variety of applications, present micro/nano fabrication methods are largely based on layer-by-layer deposition techniques or micromachining by photolithography/chemical etching which are two dimensional (2D) in temperament. Such is not suitable for low cost mass production of 3D micro devices with intricate structures having micro/nano scale features. (Parkinson, 2005) New fabrication techniques with competent skills for yielding large volume of complex 3D structures in either silicon or non-silicon based compositions are required to develop for wider range of device applications. For that reason diatoms surpass modern engineering capabilities, forming intricately patterned silica shells by bio-mineralizing a mixture of proteins, amorphous silica and carbohydrates. (Andrew Bozarth, 2009) Advantage being direct fabrication of 3D structures is allowed, in place of the layer-by-layer deposition technique of the present day, where lithography is used to build microelectronics.
Generally possible approaches for using diatoms in nanotechnology are, either using naturally produced silica structures, or bio-mimicking the structure as we have seen in the biomimetics section earlier. Novel micro-scale devices with meso-scale to nano-scale features for a variety of applications are in progress to development with appreciable worldwide efforts. Applications may be in a wide range such as:
Telecommunications (e.g., optical sensors, actuators, and lenses),
Medicines (e.g., drug delivery capsules, in-vitro sensors, membranes for chemical purification),
Transportation (e.g., catalytic components, sensors, valves for aircraft and automobiles as catalysts),
Manufacturing (e.g., self-assembled devices, on-line sensors, micro-robots).
At the same time, economic feasibility for industrial production is also mainly interested in cheap culturing so that it is more practicable; this is possible with diatoms, since it just requires carbon dioxide, water, inorganic salts, and light to grow the cultures. Therefore for interested candidates, it would just need readily available fresh, sea or brackish water to make the media for cultures and then enrich it by merely adding a few organic compounds.
HIGH POTENTIAL FOR TECHNOLOGICAL APPLICATIONS
Need for precise porous structures or materials with individual surface properties is the current requirement of the industry. Biologists and diatomists together are studying the single-celled micro-algae since and long time and engineers are now discovering ways and means to exploit the advantages of these organisms. Many nanoscale applications can be suggested keeping in view the unique nano-pore structures, micro-channels and mainly silica microcrystal structure. For example, diatom frustules can be embedded into a metal-film membrane, for pinpoint drug delivery by magnetizing the diatoms or silica nano-powders could be produced. (Kit Mun Wee, 2005)
Diatoms have attracted the interest of nanotechnologist for the development of the discipline and practical applications. The self-formation of these species in silica on the micrometer scale with nanometer sized features in a wide variety of shapes and patterns creates an interest in the diatom development. These shapes can be recreated, under different topological combinations, and glued to substrates to create nano-substrates and nano-patterned nanomaterials or in other applications of nanotechnology. (Pappas, 2005) Angularity, rigidity, and inertness not available in other microbes give the diatoms inimitability in using the complex 3D arrays of silica pores for, sorting of particles, nano-fluidics because the molecule size that can be altered over the paths are in tens of nanometers in size and many more.
The frustules (rigid walls) of the diatoms consist of intircate 3D structures constituting of micro-scale pores and channels, this combination of silica chemistry on a high area is suitable for applications such as microscale total analysis system. The diatom frustule is chemically modified for antibodies attachment, could be used in immunoprecipitation; exceptionally cheap and renewable material, produced using only lignt and minimal nutrients. (Helen E. Towley, 2008)Therefore a detailed understanding of the diatom frustules from the nano scale level to the entire shell level may provide insights for, advanced combinations of nanostructured ceramic materials and light weight architectures for technological applications, because of their high degree of symmetry and complexity. We also have an alternative where, only symmetrical diatoms could be selected by "compustat selection" experiments (i.e. forced evolution) which has been suggested be Robert Gordon. Here only diatoms having the same kind of structure, size and other features specified by the user are matched and selcted while the others are killed to thrown them out of the analysis. (Hamm, 2005) (Gordon, 2003)
Making complex nanoscale three-dimensional structures at low cost and in large numbers is the key to the development of nanotechnology. Wide variety of structures in the silicified cell walls offers a great scope, where diatom silica can be converted into other materials by maintaining the nano-scale morphology, which upon desired specific in vivo manipulations determines use in nanotechnology. By introducing modified genes into the diatoms i.e. straightforward gene modification is possible but currently replacing the native diatom gene with modified copies is a problem. Therefore, by molecular genetics techniques diatom silicified structures can be modified according to the requisite and used for specific distinctive applications. (Hildebrand, 2005) (PhysOrg.com)
FABRICATION OF MICRO- AND NANOSCALE PATTERNS BY REPLICA MOLDING FROM DIATOM BIOSILICA USEFUL FOR SENSING
The highly ordered 3D porous silica structures hold a considerable promise for the biological or biomimetic fabrication of nanostructured devices and materials. Diatoms are relatively new sources of inspiration for design and fabrication of nano-structured materials. Inorganic structures are synthesized with ordered micro-to-nanoscale features. The frustules of the diatoms is decorated with nano-sized features such as pores, ridges, spikes and spines, composed in amorphous silica in a single-cell. Ability to convert diatom to semiconducting ceramics like TiO2, GeO2, ZrO2, or SnO2 by BaSIC, sol-gel chemistry, hydrothermal conversion or bioengineering gives scope for gas-sensing applications.
First the diatom replica is molded onto a soft and elastic polymer using poly(dimethylsiloxane) PDMS as a master, this is then used a mold to fabricate the actual structures of the diatoms in either polymers or ceramics or any other material. This demonstrates the use of diatoms for the rapid and simple fabrication of polymer nanostructures. The several hierarchical structures in the frustules with different feature sizes and patterns are copied. A wide range of potential applications of such structures include optics, photonics, catalysis, bio-sensing, drug delivery, filtration, bio-encapsulations and immunizations. Benefit is diatoms are cheap resources and remarkable number of masters with a range of structures and patterns across the micro- and nanoscale. (Dusan Losic, 2007)
We also have, another alternative, where three-dimensional nanostructured silica micro-assemblies formed by the diatoms can be converded into nanocrystalline silica replicas. A magnesiothermic reduction process at tempreature roughly 650°C are used for converting, then the structures are converted in to co-continuous, nanocrystalline mixture of slicon and magnesia by reaction with magnesium gas. These formed silicon replicas are photolumiscent and exhibited raphid changes in the impedance when exposed in gaseous nitric oxide (a possible application in microscale gas sensing).
Therefore, systheses of microporous crystalline silicon micro-assemblies with various three-dimensional shapes can be formed biologically. By combining the frustules morphology with nanotechnology, a nanoporous structure can be formed to be used as a sensor (e.g. NOx sensor) as shown in Figure 3 below; structures will be useful for obtaining the maximum sensitivity of the sensor. Such formed sensors have high sensitivity and rapid response rate. These silicon frustule replicas show great number of benefits of such open, porous, 3D strutures with high surface area.
Figure 3: Nanocrystalline silicone senor made by reduction of diatom silica structure using magnesium oxide for sensing NOx gas (Werong Yang)
Unique and explicit behavior of the diatoms is the formation of nanoporous biosilica inside the body, by collection Si from the water to form several micrometers to several hundreds of micrometers of biosilica, contacting nanometer to several hundred nanometer sized nanopores. These nanoporous silica known as frustules can be used in filters, carriers, support for chromatography, and building materials. Either synthetic silica templetes for sensors, elctronic, optical or biomedical applications can be made easily. (Zhihao Bao, 2007) (Werong Yang)