NANOTECHNOLOGY might be a multibillion-dollar industry in the making, but you’d never know it from a visit to Mark Hildebrand’s lab. His workers churn out elaborate microscopic structures for a salary that adds new meaning to the term “minimum wage”. “To get billions of them working costs about a dollar,” says Hildebrand.
The workers in his lab at the University of California in San Diego are diatoms – microscopic, single-celled algae that are well known for their beautiful and elaborate glass shells. Hildebrand isn’t interested in their aesthetic merits, though. He and other like-minded researchers think all sorts of applications could be found for diatoms in nanotechnology, from acting as tiny moulds to fashioning lenses for optical computers and growing gears for micro-robots.
To a nanotechnologist, diatoms seem almost heaven-sent. Not only do they turn out precision-made, three-dimensional structures quickly, cheaply and in enormous quantities, they do so at room temperature and pressure, and without a toxic chemical in sight. Nothing humans have invented can match them.
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Already some scientists are prospecting for diatom shells that naturally possess useful properties, such as gear-shaped components or a particular pore size. Others are working out how to convert the glassy shells into other, more useful materials while preserving their shape, or how to harness the microbes’ glass-moulding proteins to grow devices without the algae themselves. And biologists are trying to understand how diatoms sculpt their shells with such precision, in the hope that they will one day be able to persuade the tiny critters to build designer widgets to order.
Building minuscule 3D structures is one of nanotechnology’s biggest challenges. To build anything at present, materials scientists have to meticulously etch silicon wafers, painstakingly manufacture buckyballs and nanotubes to exact specifications, or laboriously push single atoms around using an atomic force microscope. Diatoms promise to put an end to all this hard labour. “The advantage with a diatom is that it grows complex structures in three dimensions directly,” says Richard Gordon, a diatomist at the University of Manitoba, Canada. “Most human technology can only do 3D by building layer after layer – and so far none can match the diatom for complexity.”
So far, more than 70,000 species of diatom have been documented, each with a uniquely shaped shell. The shells range in size from a millionth of a metre to nearly a thousand times as large, and can vary in structure from sparse skeletons of criss-crossing bars to barrels, pods, stars, triangles and elaborate discs that look like flying saucers. They also sport pores only a few nanometres across, plus ridges, spikes and bumps, often in dazzlingly elegant arrangements.
To some researchers these are ready-made for nanotechnology. Thomas Manning of Valdosta State University in Georgia, US, has described an innovative use for the pore-riddled shells of the diatoms Arachnoidiscus and Actinocyclus. He uses them as “ice cube trays” for growing nanocrystals.
His team started out trying to grow crystals of the manganese acetate cluster, also known as Mn12. Tiny crystals of Mn12 have many interesting magnetic properties so several research groups are exploring the idea of using them as computer memory storage devices. Each crystal can hold the same amount of information as a particle 100,000 times larger. In fact the crystals are small enough to start feeling the effects of quantum mechanics – a property that could make them useful in a quantum computer. But right now there is no good method for growing these crystals in large amounts.
Manning’s team hasn’t yet been able to make Mn12 crystals small enough because the diatoms they’re using turn out to have overly large pores. But he believes the technique has great potential for making crystals of other compounds, including the catalysts titanium oxide, vanadium oxide and zirconium oxide. The advantage of doing this would be that the smaller the crystals, the more surface area they have and the more effective they are. Nanoparticles of these compounds are already commercially available, but using diatoms would allow you to go even smaller and still make them in bulk, says Manning.
Another attraction of using diatom skeletons as reaction vessels is that they’re made of a material that chemists are accustomed to working with – glass. “They are like test tubes,” says Manning. “You can just flush them with acid or base to clean them and they won’t dissolve.”
While Manning sees chemical reaction vessels when he looks at diatoms, others see uses for the structures themselves. Mary Ann Tiffany at San Diego State University in California has found that diatoms create an array of intermediate structures as their shells mature, which in Coscinodiscus granii include hollow hexagons with pointed projections at their corners. “To me they look just like gears,” says Tiffany. “It’s not hard to picture breaking these off and putting a shaft through the middle.” Tiffany has also found discs from Pleurosira laevis and barrel hoops from Isthmia nervosa, which she argues could be harvested for micromachine components.
Of course, glass isn’t an ideal material for gears or other mechanical components. To solve that problem, ceramic chemist Ken Sandhage, now of the Georgia Institute of Technology in Atlanta, has invented a process to change diatom shells atom by atom from glass into other substances. Gordon has dubbed it the “Star Trek replicator”.
In a paper published in 2002 (Advanced Materials, vol 14, p 429), Sandhage reported that baking shells of the diatom Aulacoseira at 900 °C for four hours in the presence of magnesium gas transformed them from silica into the ceramic magnesium oxide while preserving their shape and size (see Figure). Sandhage says similar reactions can transform the shells into other useful compounds – titanium oxide, for example, which changes its electrical resistance when exposed to different gases. Because porous diatom shells have such a high surface area to interact with gases, such devices could make ultra-sensitive sensors. Sandhage has also transformed shells into biologically friendly calcium phosphate – the main constituent of bone – and says these could be used in drug-delivery devices or to shield implanted cells from the immune system. “What’s really attractive is that if you can find the shape you want, you can easily make millions and billions of them,” he says.
There are other potential uses for naturally occurring diatom shells, including as filters or additives in composite materials. But some researchers believe the real power of diatom nanotechnology will only be unleashed when they learn how to make designer shells. Nanotechnologists would reprogram the wee beasties to build their shells to human specifications. The result would be a virtually limitless array of new devices and materials.
But even the optimists admit this is a tall order that could take a decade or more to make a reality, because at present much about how diatoms sculpt glass with such precision remains a mystery.
What’s clear is that in their natural environment, diatom cells that are preparing to divide import dissolved silica in the form of silicic acid into a membrane-bound compartment known as the silica deposition vesicle or SDV. It then converts this silica into new shells for the daughter cells.
But what factors in the SDV control the final form of the shell? In 1999, Nils Kröger and his team at the University of Regensburg in Germany isolated a protein from the shell of the diatom Cylindrotheca fusiformis that turned out to be intimately involved in the process. When Kröger added the purified protein, which he dubbed silaffin 1A, to a solution of silicic acid in a test tube, it triggered the formation of silica spheres (Science, vol 286, p 1129). While this test-tube silica wasn’t nearly as well ordered as natural diatom shells, it suggested that silaffins are part of diatoms’ tool kit.
Kröger’s team has since discovered other silaffins, and they’re turning out to be highly unusual proteins. They are decorated with a strange array of chemical accessories, including positively charged polyamine chains, negatively charged sulphates and phosphates, and at least five different types of sugar molecule. Kröger thinks these attachments – rather than the protein itself – are what makes silaffins tick. Remove the phosphate, for example, and silaffin 1A no longer triggers silica aggregation in a test tube. “I think the protein is just a scaffold for all these modifications to be attached,” he says.
That complex array of decorations on silaffins may constitute a “code” for assembling 3D aggregates of the molecules, which then guide silica deposition. In October, Kröger announced some progress in deciphering this code. His team demonstrated that on its own, silaffin 1A creates silica spheres up to 700 nanometres in diameter. A second protein isolated from diatom shells, silaffin 2, cannot form any silica structure at all. But mix the two together in the right proportions and larger, shapeless blocks of silica form that are pockmarked with pores 100 to 1000 nanometres wide – sizes commonly seen in diatom shells (Proceedings of the National Academy of Sciences, vol 100, p 12075). “This was really astonishing to me,” says Kröger. “It may mean that fewer components than you might think are required to create a complex structure.”
Morley Stone and his group at the Air Force Research Laboratory in Dayton, Ohio, are already using isolated silaffins to make optical devices. They start by etching nanochannels into a polymer matrix, coat the inside of the channels with silaffins and then flood them with silicic acid. The result is regular rows of glass beads resembling rows of cabbages in a ploughed field. With these in place, the polymer’s power to bend light increases 50-fold, making it an attractive material for miniature lenses or diffraction gratings (Nature, vol 413, p 291).
So will nanotechnologists ever be able to crack the diatom code and use it to create devices to order? Eileen Cox, a diatomist at the Natural History Museum in London, is doubtful. She believes silaffins play a very limited role in the SDV, directing silica to precipitate in simple patterns or fibres. In time-lapse studies of the formation of new shells in various diatom species, she has found that as pores and other features form, cellular structures such as mitochondria compress the sides of the SDV and the glass seems to flow around those obstacles. She likens this to molten glass being poured into an ever-changing mould. Working out how the cell choreographs its contents to shape the glass will probably be very difficult.
Hildebrand is also sceptical. “There is no way to reconstitute everything diatoms do in a test tube,” he says. “To get the complete package, you need to work in the living organism.” He believes the key to ultimate control of silica formation will be a more complete understanding of diatom biology. And that work is already moving forward.
In 1997, Hildebrand’s lab identified transporter proteins that pull the silicic acid into the cell. By altering these conduits, he speculates, it may be possible to engineer the transporters to bring in related elements such as germanium, so that diatoms can incorporate new materials into their shells and bestow new electrical or optical properties on them. And later this year Hildebrand will publish the first genome sequence of a diatom, the 35 million DNA letters of Thalassiosira pseudonana. A second species, Phaeodactylum tricornutum, will also be sequenced this year.
Another promising avenue is genetic engineering. Imagine transferring a silaffin gene from a species with 70-nanometre pores into another species without those pores. If the spliced-in gene suddenly created pores of exactly that size, researchers would be in the business of designing shells with all sorts of intriguing properties.
So far, geneticists have successfully engineered three species of diatom. But the efficiency of gene transfer is very low and the technique seems of limited value. Ironically, it’s the glassy shell of the diatoms that seems to make them so resistant to taking up new genes. “It’s a good protective barrier,” says Hildebrand, “unfortunately.”
But even if genetic engineering fails to produce diatom shells to order, Gordon suggests that an evolutionary approach could succeed. He has proposed building a machine he calls a compustat that would select for ideal diatom shell shapes. Here’s how it would work. Researchers would start with a diatom whose shell had a shape close to the desired specifications. The cells would then be bombarded with chemicals to mutate their DNA and the resulting shell designs would be analysed by computers. Unaltered cells or those with unfavourable mutations would be zapped dead by lasers. Winners would be left to reproduce and breed. Through cycles of this selective breeding, Gordon thinks almost any shape could be achieved to design electrical, optical or micromachine components.
Hildebrand agrees that Gordon’s idea could work. However, he points out that many of the useful features of diatom shells are so small that light microscopes can’t distinguish them. And if you need electron microscopy or chemical tests for the selection process, the cells will need to be killed. That makes the selection process far more difficult.
And despite the enormous structural diversity that diatoms already have, some experts doubt they can keep pace with human imagination. Daniel Morse of the University of California, Santa Barbara, who works on silica-precipitating proteins from sponges, doubts we can create designer diatom shells at all. “I’m not sure there is a big envelope to change these shapes that will still be compatible with life or reproduction,” he says.
Even Ryan Drum, a retired diatomist who with Gordon has been championing the idea of using diatoms in nanotechnology, admits that what diatoms do in the wild and what they can be forced to do in an industrial vat may not be the same thing. “We don’t know if diatom workers will be predictable, reliable, or if they will have lazy days,” he says. “If you hire nanopersonnel to do a nanotask, you may always encounter problems with their nanopersonality.”
