A simple technique for stamping patterns invisible to the human eye onto a special class of nanomaterials provides a new, cost-effective way to produce novel devices for a wide range of applications including drug delivery, chemical and biological sensors, solar cells and battery electrodes. The technique, developed by Vanderbilt University engineers, works with porous nanomaterials that are riddled with tiny voids that give them unique optical, electrical, chemical and mechanical properties. Imagine a stiff, spongelike material filled with holes that are too small to see without a special microscope. There are nanoporous forms of gold, silicon, aluminum and titanium oxide, among others. Simple stamping The processing required to transform the materials into devices has been complex and expensive, which has been a major obstacle. Vanderbilt graduate student Jason Ryckman demonstrating the operation of a diffraction-based biosensor produced out of a nanoporous material by the new imprinting process. (Image: Anne Raynor, Vanderbilt University) Now Sharon M. Weiss, associate professor of electrical engineering, and her colleagues have developed a rapid, low-cost imprinting process that can stamp out a variety of nanodevices from these intriguing materials. "It's amazing how easy it is. We made our first imprint using a regular tabletop vise," she said. "And the resolution is surprisingly good." The traditional strategies used for making devices out of nanoporous materials are based on the process used to make computer chips. This must be done in a special cleanroom and involves painting the surface with a special material called a resist, exposing it to ultraviolet light or scanning the surface with an electron beam to create the desired pattern and then applying a series of chemical treatments to either engrave the surface or lay down new material. The more complicated the pattern, the longer it takes to make. About two years ago, Weiss got the idea of creating premastered stamps using the complex process and then using the stamps to create the devices. Weiss calls the new approach direct imprinting of porous substrates (DIPS). DIPS can create a device in less than a minute, regardless of its complexity. So far, her group reports that it has used master stamps more than 20 times without any signs of deterioration. Process can produce nanoscale patterns The smallest pattern that Weiss and her colleagues have made to date has features of only a few tens of nanometers, which is about the size of a single fatty acid molecule. They have also succeeded in imprinting the smallest pattern yet reported in nanoporous gold, one with 70-nm features. The first device the group made is a "diffraction based" biosensor that can be configured to identify a variety of organic molecules, including DNA, proteins and viruses. The device consists of a grating made from porous silicon treated so that a target molecule will stick to it. The sensor is exposed to a liquid that may contain the target molecule and then is rinsed off. If the target was present, then some of the molecules stick in the grating and alter the pattern of reflected light produced when the grating is illuminated with a laser. According to the researchers' analysis, when such a biosensor is made from nanoporous silicon, it is more sensitive than when made from ordinary silicon. The Weiss group collaborated with colleagues in the chemical and biomolecular engineering departments to use the new technique to make nanopatterned chemical sensors that are 10 times more sensitive than another type of commercial chemical sensor called Klarite, which is the basis of a multimillion-dollar market. The researchers have also demonstrated that they can use the stamps to make precisely shaped microparticles by a process called "overstamping," which essentially cuts through the nanoporous layer to free the particles from the substrate. One possible application for microparticles made this way from nanoporous silicon is a lithium-ion battery anode, which could significantly increase the battery's capacity without adding a lot of weight. Vanderbilt University has applied for a patent on the DIPS method. For more information, visit: www.vanderbilt.edu