NANOSCALE machines and circuits could one day be assembled by exploiting the way immune systems latch onto invading bacteria and viruses. The idea has already been successfully used to guide individual nanotubes into position on a metal surface.
Researchers desperately need a way to assemble nanoscale devices. Nanotubes made from various substances can be used in many ways when building these devices: as passive components such as structural supports or conducting wires, or as the basis for active elements such as transistors or light emitters. But connecting up even a single nanotube between two points in a circuit, say, is much easier said than done. That鈥檚 because a newly synthesised clump of nanotubes is much like a jumbled heap of lumber. The challenge is to pick one out and place it where you want it, says Rajesh Naik, biotechnology project leader at the US Air Force Research Laboratory in Dayton, Ohio.
Hiroshi Matsui, a chemist at Hunter College, part of the City University of New York, was struck by the fact that our immune systems are designed to perform an equally intricate task. When an antigen such as a fragment of protein from a virus, say, enters our body, the corresponding antibody immediately latches onto it. So he decided to see if he could use this mechanism to guide a nanotube to a precise location. His strategy was to apply antigens to the target area on a metal sheet, and stick the corresponding antibodies onto the nanotubes. When the two combine, the antibody鈥檚 natural tendency to latch onto its antigen should tack the nanotubes down in the right place.
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Matsui鈥檚 team began by coating a sheet of gold with a layer of alkylthiol, a type of substance to which antigens cannot bind. They then used the tip of an atomic force microscope to scrape away the coating along tracks where they wanted nanotubes to be placed (see Graphic). When they then bathed the etched sheet in a solution containing the antigen 鈥� for their experiment they chose the protein mouse immunoglobulin G (IgG) 鈥� it stuck to the exposed gold of the etched track but washed off the alkylthiol-coated regions.
The team then took nanotubes made from amino-acid building blocks and tagged them with antibodies to mouse IgG by incubating them in a solution containing the antibodies. When the antibody-tagged nanotubes were brought into contact with the gold sheet, the tubes nestled into the grooves etched and labelled with antigen (The Journal of the American Chemical Society, vol 126, p 8088).
The nanotubes did not attach to similar grooves labelled with a human IgG. 鈥淚f you want to make a more realistic device 鈥� to attach certain nanotubes to certain locations and other nanotubes to other places 鈥� we can pick hundreds of different antibodies and antigens,鈥� he says.
Matsui is hoping to build nanosensors to detect disease-causing micro-organisms. Such sensors would use conducting nanotubes that carry antibodies to particular pathogens 鈥� perhaps in a blood sample from a patient. Chemical changes when such antibodies bind to their antigens change conductivity which can be sensed. Individual detectors of this kind would be small enough to allow detectors for thousands of different pathogens to be packed onto a single chip, says Kimberly Hamad-Schifferli, an MIT engineer. 鈥淭hat would probably be the most compelling application of this.鈥�
