SiMPore Inc., a company commercializing nanotechnology invented at the University of Rochester, has developed an ultra-thin microscope slide that significantly improves high-resolution imaging of nanoscale materials such as proteins, viruses, and carbon nanotubes. This is the first commercial application of a unique nanomembrane initially reported in Nature in 2007.
These new slides, more commonly called windows for electron microscopy, are made of a proprietary silicon membrane so thin that it is invisible edge-on. The extreme thinness of the windows—less than 50 atoms thick—reduces background interference and improves contrast in images generated with transmission electron microscopes (TEM), making individual biological molecules, like proteins or viruses, easier to analyze.
Unlike conventional TEM windows, SiMPore's also have a pure silicon composition meaning that they can be subjected to intense plasma cleaning to remove contaminants, which further improves image quality.
TEM windows are effectively used as slides to support samples that will be imaged and analyzed with an electron microscope. Imaging is done by focusing a beam of electrons onto a sample, whereby some electrons transmit through the sample and others are scattered out of the beam. The electrons that emerge from the sample carry structural information about the sample that can be magnified by the lens system of the microscope and used to produce a detailed image of atomic scale features. While the best light microscopes have magnifications of up to 2,000 times, some electron microscopes can magnify objects at millions of times their actual size.
Christopher Striemer, now vice president of membrane development at SiMPore, discovered the membrane technology that underlies the new TEM windows while working with nanocrystalline silicon films for computer chip memory applications. By transforming those films into membranes only 15 nanometers thick, he could more precisely image the intricate crystalline structures of his samples using an electron microscope.
"These new TEM windows are extremely thin for atomic scale imaging and feature characteristics unique in this market," says Striemer. SiMPore currently manufactures both porous and nonporous varieties. The porous windows have an inherent crystalline structure, while the nonporous windows are amorphous, or non-crystalline, and each type of window offers distinct advantages over conventional carbon and silicon nitride windows.
The porous windows contain millions-to-billions of nanopores, ranging from 10 to 50 nanometers in diameter, and it is these pores that permit stable suspension of similarly sized materials, such as protein molecules and carbon nanotubes. "The fact that nanomaterials can be suspended across pores of similar scale and imaged without intervening background will lead to a better physical understanding of nanoscale structures," says Striemer.
Compared to widely used carbon windows, SiMPore's nonporous windows are more consistently thin, circumventing unpredictable variations in a window's thickness that would otherwise introduce additional background noise into images.
Researchers at SiMPore also point to the pure silicon composition of their membranes as offering a number of previously unavailable advantages.
"The ability to vigorously clean these new TEM windows with standard plasma cleaning tools will help researchers examine nanostructures at higher resolution without problematic contaminants," says Striemer. Plasma cleaning is simply not possible with carbon windows, which have a carbon film that overlays a copper lattice, as the organic structure of the window itself vaporizes.
Elemental analysis of samples containing nitrogen and carbon also becomes much simpler when using a pure silicon window since the composition of any background signal will be minimal and immediately distinguishable from the sample.
Additionally, silicon grids are electronically stable under high-current electron beams, which arise when trying to obtain high-magnification, high-resolution images of nanostructures using electron microscopy. Silicon impedes an excessive build-up of charge on both the window and the sample, a problem that has consistently plagued research undertaken with traditional windows. Charge build-up often results in degradation of both the window and the sample.
When the membrane technology was discovered in 2004, Striemer was doing doctoral research in the lab of Philippe Fauchet, professor of electrical and computer engineering. Following talks with James L. McGrath, associate professor of biomedical engineering, and his graduate student, Tom Gaborski, the group realized that since the membrane's holes were only nanometers in size, it might have an array of applications, particularly for nanofiltration. The group of researchers founded SiMPore in 2007 to work toward commercializing these applications.
In the future, the company will be exploring how its nanomembrane technology can augment bio-molecule and nanoparticle separation, improve filtering in hemodialysis, aid in the efficient culturing of stem cells, speed ion exchange in fuel cells, and even be used in micro-fluidic applications on next-generation computer chips.
The possibilities are far-reaching, and electron microscopy is only the tip of the iceberg, but nevertheless an avenue that researchers at SiMPore feel will greatly benefit those doing the latest work in nanotechnology.
"We believe these TEM grids will help those at the forefront of nanotechnology continue to push into new frontiers," says Striemer.
About SiMPore Inc.
SiMPore Inc. (www.simpore.com) is a Rochester, New York-based nanotechnology company which designs and produces membranes and membrane-enabled products based on its patent-pending platform technology—an ultra-thin nanoporous silicon membrane. SiMPore is developing products that include filters for separating and concentrating biological molecules and nanoparticles, cell culture substrates for growing cells, and electron microscopy grids for preparing and imaging samples at the nanoscale. In 2008, it won the Rochester Business Plan Competition and the Western New York Golden Horseshoe Business Plan Competition. In the spring of 2008, SiMPore also closed a $1.25 million investment round financed primarily by local Rochester high net worth individuals.