Observed through a microscope, dried ink appears as a jumble of particles. Now an ink that, as it dries, self-assembles into orderly layers of very tiny caves -- actually, nanoscopic pores -- each leading to the next, has been created. Within these caves, active molecules interrogate any gas or fluid, laser light, or electric or magnetic field passing through. The new ink can be printed easily and cheaply from ordinary inkjet printers or even written by lithographic pens. The dried result -- nanostructures that perform work -- could be considered intelligent ink.
"Our achievement should be of practical importance for those of some technical ability wishing to directly write -- rather than mechanically construct -- sensor arrays and fluidic or photonic systems," says project leader Jeff Brinker, a senior scientist at the Department of Energy's Sandia National Laboratories and a professor at the University of New Mexico.
A report of the work, published in the May 4 issue of the journal Nature, describes how researchers were able to use these self-assembling inks to write patterns of varying lengths, on a variety of surfaces, that possess external form and internal function.
The process, which avoids the need for molds, masks, and resists common to most lithographic processes, produces ink that in seconds becomes a functioning, self-assembled, nanoscopic material. Its caves -- nanoscopic pores -- behave as little sensors or even valves, as though one had created machines so small that, next to any of them, a grain of pollen would be roughly the size of a skyscraper. The research group's prototypes have already monitored the pH of fluids transported by capillary action, and formed structures that could act as wave-guides to direct laser light.
By linking computer-aided design (CAD) with an inkjet printer, it will be possible to create in seconds a functional nanostructure that was a drawing on a computer screen only moments before.
"We should be able to fabricate a substance that organizes itself to build a fluidic channel network instead of having to painstakingly design and cut one," says Brinker. "With positive ligands in the mix to act upon incoming chemicals, we would have the equivalent of an analytical machine that built itself instead of needing construction."
He also envisions making a valve simply by creating molecular pores that change shape due to external input. "We could write structures that position pore pathways in a second, sensors included." "People have used ink jet printers to print ceramic material into a substrate," says Brinker. "Here, inside each ink dot, the ink self-organizes into further function: pore networks, surface(s) decorated by organic functional ligands and mesoscopic pore channels. It's a self-creating functional factory." In effect, says Brinker, "we can combine thousands of different types of ink for different functionality. The process would work just the way color printers currently mix hundreds of different colored inks to get a blended result. With color-pattern software, we could make functioning materials with a variety of characteristics: say, strong, hard, and hydrophobic, with a low dielectric constant."
Ligands sprinkled into the ink could be used to sense light or heat, measure magnetic and electric fields, or filter gasses and liquids. The work is an extension of previous achievements by the research group, also reported in Nature. These involved using simpler forms of the same technique to produce self-organized materials on the nanoscale. They were used to form sensitive coatings, layers that produced the structure and strength of seashells, and nanospheres structured to selectively adsorb environmental molecules or dispense chemicals.
The process, called evaporation-induced self-assembly, is based on the scientifically well-known tendency of two-sided detergent molecules composed of water-loving (hydrophilic) and water-hating (hydrophobic) portions to spontaneously form spherical molecular assemblies. By including organic and inorganic materials, detergent self-assembly can be harnessed to create organic and inorganic nanostructures. Continued mild heating polymerizes these nanostructures and bonds their interfaces. The Sandia/UNM process is promising because it does away with the tedious, sequential deposition of individual organic and inorganic layers, a much longer process when it is even possible.
Other members of the team are Hongyou Fan, Aaron Stump, Victor Perez-Luna, and Gabriel Lopez from UNM's Center for Micro-Engineered Materials and Department of Chemical and Nuclear Engineering. Participating Sandia researchers included Yunfeng Lu (former UNM student and Sandia post-doc, now at Applied Materials Inc. in Santa Clara, Calif.), Scott Reed, Tom Baer, and Randy Schunk.
The work continues to interest major research societies. Hongyou Fan won a Materials Research Society Graduate Student Award silver medal for this work, and for his work on self-assembly Yunfeng Lu won the American Chemical Society's 31st annual Victor K. LaMer Graduate Award in Colloid and Surface Chemistry. Both are students of Brinker's. Previous winners of the prestigious LaMer award, from 1990 to 1999 respectively were students from Princeton, Purdue, Massachusetts Institute of Technology, Stanford, Harvard, University of California Berkeley, Cornell, University of Pennsylvania, Carnegie Mellon, and Harvard.
Funding for this project was provided by Sandia's Laboratory Directed Research and Development office, DOE, and the Department of Defense's Defense Advanced Research Projects Agency.
Sandia is a multiprogram DOE laboratory, operated by a subsidiary of Lockheed Martin Corp. With main facilities in Albuquerque, N.M., and Livermore, Calif., Sandia has major research and development responsibilities in national security, energy, and environmental technologies.
Journal
Nature