Ink enables self-assembly sensor arrays

17 Jun 2002

Usually it's the application of ink which creates excitement rather than the ink itself. But scientists at Sandia National Laboratories have created an ink which allows for nanoscopic molecular recognition.

As it dries, the ink can be seen under a very powerful microscope to self-assemble into orderly layers of tiny nanoscopic pores, each leading to the next. Within those pores ligands (molecules which exhibit molecular recognition characteristics) can interrogate any gas, fluid, laser light, or electric or magnetic field which passes through. The nanostructure technology allows the direct writing, rather than the mechanical construction, of sensor arrays or photonic systems.

"If we can put a low refractive index coating on a substrate, then we can write these things," said Jeff Brinker, the project leader.Brinker is a senior scientist at Sandia National Laboratories and a professor at the University of New Mexico.

"We can make these things photo-sensitive so once they've been written we can further modify them optically in a localized way," he said. "We can write a waveguide structure with this approach."

The project has significant applications in lithography; the process avoids the needs for molds, masks, and resists in that field. The nanoscopic pores behave as miniature sensors or even valves, and prototypes have formed structures which would act as waveguides to direct laser light.

Work in the area has been undertaken for the past four years. "We're just harnessing the ability of the two-sided molecule to spontaneously organize into a variety of structures," Brinker noted.

The process is called evaporation-induced self-assembly and is based on the fact that two-sided detergent molecules with hydrophilic and hydrophobic portions spontaneously form spherical molecular assemblies. Other composites will co-organize with the two-sided molecules. "We end up with either porous materials or composites," Brinker said.

The writing process itself takes a matter of seconds. Other than the fact that the ink was written onto single crystal slab silicon wafers, the ink can be used on a commercial-off-the-shelf inkjet printer. "We had to reconfigure the inkjet a little bit to handle the silicon wafer rather than a piece of paper, but that's the only modification we used," Brinker remarked. "We just put this into the ink cartridge and it started writing."
Defined structures emerge within a pattern as the ink dries. "One thing that has been demonstrated by several groups now is the fact that we can put laser dye molecules in them," Brinker said of the pores which are between 1 and 5 nanometers in size. "They can put a higher concentration of these dyes in."

Details of the work were published in the May 4 issue of the journal Nature.