Advances in nanotechnology such as tiny porphyrin tubes to make a broad range of nanodevices, and tiny bioelectronic circuits to make nanotech machines or sensors, could lead to new devices.
The tiny porphyrin tubes developed at Sandia may be used in a variety of new types of nanodevice, including ones that may be able to harness sunlight in order to split water molecules and produce hydrogen. The idea of using sunlight to split water at the nanoscale grew out of the research by Sandia researcher John Shelnutt into the development of hollow porphyrin nanotubes, which are micrometers long and 50-70nm in diameter, with about 20nm-thick walls.
Porphyrin nanotubes are made of oppositely charged porphyrin molecules that can self-assemble in water at room temperature. By contrast, carbon nanotubes are formed at high temperatures, and they have covalent bonds between carbon atoms. Porphyrin nanotubes lack the mechanical strength of carbon nanotubes, but they do have a wider range of optical and electronic properties. Porphyrin molecules are closely related to chlorophyll, the active part of photosynthetic proteins. Currently, carbon nanotubes frequently are modified by attaching porphyrins to increase their uses.
The porphyrin nanotubes have intense resonance light scattering and photocatalytic activities (see Fig). When exposed to light, some porphyrin nanotubes can grow metal structures to create a functional nanodevice. If the nanotubes are put into a solution with gold or platinum ions and exposed to sunlight, for example, their photocatalytic activity causes the reduction of the ions to the metal. Sandia researchers have been able to deposit platinum outside the nanotube and to grow a nanowire of gold inside the tube. This is the key part of a new nanodevice that may be able to split water into oxygen and hydrogen.
But water-splitting is only one of the possible applications of the nanodevices based on porphyrin nanostructures: Shelnutt said the tubes could be used as conductors, semiconductors and photoconductors, as well as electronic and photonic devices and chemical sensors.
Scientists at the University of Wisconsin are conducting research that could lead to new nanotech machines or a new class of more sensitive biological attack sensors. They have used single cells of bacteria to make tiny bioelectronic circuits that potentially could be used in such devices. They directed the bacteria down a narrow channel to a pair of electrodes, where the bacteria were trapped by mild electric currents, thus making them “bio junctions” that can be captured, interrogated and released any time. Using living bacteria could become the basis for new ways to assemble all kinds of nanodevices, the researchers said.
“One of the great challenges of nanotechnology remains the assembly of nanoscale objects into more complex systems,” said Robert Hamers, professor of chemistry at the University of Wisconsin. “We think that bacteria and other small biological systems can be used as templates for fabricating even more complex systems.”
Hamers said it may be possible to capitalize on the complex topography of the surface of a bacteria cell and on the cell’s interactions with antibodies so that scientists could construct much more complex nanoscale structures using the natural ability of cells to dock or adhere to different kinds of molecules. That would alleviate the painstaking manipulation of individual nanosized components such as microscopic wires and tubes, Hamers said.
“We spend a lot of time making tiny nanowires, and then we try to direct them in place, but it’s very hard,” he said. “But bacteria and other biological systems can be thought of as nature’s nanowires that can be grown easily and manipulated.”
