m a te ri a l wo rld
Thinking thin for molecular filtration Ultrathin silicon membranes offer fast, effective sorting and separation.
S
CHRISTOPHER STRIEMER
they offer diffusive transport rates that based membranes probably cannot ometimes the limitations of current are an order of magnitude faster than match,” says Jongyoon Han of the Masstechnology aren’t obvious until polymer membranes, with regular, conachusetts Institute of Technology. “The something dramatically better comes trollable pore sizes. filtration speed, which comes naturally along. That’s what happened when The result, as Striemer and his cofrom the thickness and stability of the Christopher Striemer of the University material, is another imporof Rochester stumbled upon 500-nm thermal oxide tant and distinctive trait. a method to produce ultraPattern backside oxide mask Ideally, you would want to thin, nanoporous silicon Si wafer use pressure-driven filtration membranes that can be used to maximize the benefits. to filter and separate biomolThe data suggest that such ecules (Nature 2007, 445, operations certainly could 749–753). The new material Rapid thermal be possible, although it has outperforms conventional annealing not yet been fully demonpolymer membrane filters, strated.” raising the possibility of increasing the efficiency of nuStriemer and colleagues Etch pnc-Si membrane merous commercial and redid show that average pore Remove oxide search applications. size could be tuned to 7.3, masks Striemer wasn’t looking 13.9, and 21.3 nm by an~500 µm for a new filtration memnealing for 30 seconds at brane. He was in the final 715, 729, and 753 °C, remonths of his Ph.D. program Ultrathin pnc-Si starts with the original masked silicon wafer, proceeds spectively. “If we can show to a-Si deposition and thermal annealing at >700 °C, and finishes with a and had been seeking a new as fine control over pore final etching and removal of the oxide masks. way to suspend thin films for size as we think we can,” characterization by transmissays Striemer, “we can creworkers showed in a series of diffusion– sion electron microscopy. But when he ate cutoffs that just don’t exist today, filtration tests, is rapid, effective filtraannealed a thin layer of amorphous siliperhaps at every 10 kDa across a broad tion with minimal loss of filtrate. They con (a-Si) at >700 °C for 30 seconds, size range for proteins.” And because silihe discovered that voids formed sponta- also demonstrated efficient size-based con is easily functionalized, it can be diffusive separation of two proteins, neously among the nucleating silicon adapted for charge-based separations of bovine serum albumin and immunonanocrystals. The result was a layer of similarly sized macromolecules of differglobulin G, which differ in molecular porous nanocrystalline silicon (pnc-Si), ent charge. weight by a factor of ~2.3. Manufacturjust 15 nm thick and >200 µm across, Because the manufacturing process with abundant, well-defined pores of di- ers of conventional polymer membranes is thermodynamically driven, Striemer recommend a factor of 10 as the miniameter ~9–35 nm. says, there is a tradeoff in structural conmum for effective separation. “That wasn’t at all good for what I trol, especially compared with nanofabri“It’s an interesting new technology,” was trying to do,” Striemer says, but he cated filtration systems under developsays Charles Martin of the University of was intrigued. The pnc-Si membranes ment by Han and others. Nanofabricated Florida. Because the material is made were remarkably robust, and Striemer structures offer greater precision and tunwith standard microfabrication technolrecognized that the material had potenability, but the increased expense and efogy, he says, “the membranes might be tial as a molecular filter. fort involved may preclude their use for ideally suited for incorporation as sepaConventional nanoporous polymer bulk applications. However, pnc-Si memration elements in microfluidic devices.” branes can be arrayed for high-volume membranes suffer from fundamental Despite its thinness, a 40,000-µm2 limitations, including relatively low use. “It would take several weeks to make pnc-Si membrane can withstand a full at- a 4-inch filtration wafer with hundreds of transport rates, broad distributions of mosphere of differential pressure without billions of holes in it using an ion beam,” pore sizes, and thicknesses >10003 the size of the molecules they filter. The fracturing. “There are many different he says. “But with our process, they pnc-Si membranes are much thinner, in membrane systems, but this is a level of would all be formed in 30 seconds.” a the size range of macromolecules, and mechanical robustness that polymer—Thomas Hayden © 2007 AMERICAN CHEMICAL SOCIETY
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