Teeny-tiny wires capable of connecting with the kinds of molecules that make up the human body could be building blocks for what may be among the first nanotechnology applications: biological sensors for detecting glucose levels in diabetics, measuring hormone levels in menopausal women or identifying DNA at crime scenes.
Stanford chemists have taken an important step toward making such wires by synthesizing a material that conducts electricity faster and farther than earlier designs.
In the Feb. 23 issue of the journal Science, chemistry Associate Professor Christopher Chidsey, graduate students Stephen P. Dudek and Hadley Sikes, and several scientists from Brookhaven National Laboratory report the synthesis of organic molecules that conduct electricity about twice as far as the best such wires previously tested, and at least 3,000 times faster. The so-called "nanowires" made of oligophenylenevinylene, or OPV, are about 50,000 times shorter than a human hair is wide.
The Chidsey lab's earlier attempts to make nanowires produced a substance called oligophenyleneelthynylene, or OPE, which conducts electricity pretty well for about 3 nanometers (billionths of a meter). For practical applications, though, nanowires may need to conduct electricity farther. That's why Chidsey looked for a new material.
Chidsey and his students suspected that OPE's structure was not ideal for conducting electricity because it tends to twist, preventing easy movement of electrons. OPV, on the other hand, is nearly flat, with electrons in a cloud above and below the plane. Its planar structure may explain why OPV conducts electricity so much better than OPE.
To make the wires, Dudek strung single units of OPV into 1 to 5 unit chains that were about 1 to about 4 nanometers long. At one end of the wire is a sulfur atom that can stick to a gold plate. At the other end is an iron-laden molecule capable of giving and receiving electrons.
In a biological application, the end holding the iron would instead hold, say, an enzyme or piece of DNA capable of reacting with similar molecules in our bodies. The reaction would then cause a current to run through the wire to a computer chip. Dudek hopes to try detecting electrical changes in simple biological molecules as soon as this summer.
Although Chidsey's lab is not pursuing practical applications for the nanowires, Dudek envisions eventually attaching bits of DNA to the ends of the wires. Blood samples from a crime scene then could be exposed to the wires, where DNA in the blood would bind to corresponding pieces on the wires, sending an electrical signal to a computer chip that could determine whether the DNA is a good match for a particular suspect.
But such applications are far from reality yet because handling nanowires is not at all like handling ordinary electrical wires. This is chemistry: The wires are in solution and they are poured onto the gold plate where the sulfur end sticks, forming a single, invisible layer.
"It's like seaweed on a seafloor," Dudek says. "The wires are all aligned." By changing the electrical potential in the gold plate, Dudek can observe a current going through all the wires.
To measure the extraordinarily fast speeds at which OPV conducts electrons, Hadley Sikes, also a graduate student in Chidsey's lab, took the nanowires to Brookhaven. She found that electrons move across the smallest OPV wires in about 20 picoseconds. This is really fast equivalent to about 340 miles per hour.
Electricity moves through nanowires very differently from ordinary electrical wires. "If you add electrons to a typical metal wire, a domino effect moves them along the wire until they dump out the other end," says Chidsey. The electrons in metal wire move at a constant speed as they bump each other across the wire. Cut the length of a metal wire in half and it will take half as long for electrons to pass through it.
But organic nanowires don't conduct electricity that way. The rate of speed increases exponentially as the wires get shorter. For example, a 3-nanometer wire of OPV would conduct 950 times faster than a wire that's twice as long. That's because instead of bumping each other across the wire domino style, electrons "tunnel" through nanowires. When they tunnel, electrons bypass barriers they normally would not be able to climb without violating the law of conservation of energy. The chance they'll make it through to the other side drops exponentially with distance.
The OPV nanowire allows tunneling to occur relatively easily. In computer chips, tunneling is mostly a bad thing, Chidsey says: When electrons tunnel through a thin insulator around a circuit, they may cause it to short out. "I'm interested in seeing if we can understand and get control over tunneling through molecules," he says. And if he succeeds, tunneling may get a better reputation in electronics, as it may be harnessed for moving electrons between nanostructures.
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