ONE of the great nanotechnology challenges has been solved by chemists who have worked out how to place individual nanowires onto silicon chips reliably and accurately. The team behind the breakthrough has shown off the technique by building a device that could one day identify diseases from blood samples in a fraction of a second.
Christine Keating and colleagues at Pennsylvania State University in University Park developed their technique using nanowires made of rhodium. They first took a silicon wafer and carved an array of microwells into it, for the nanowires to sit in. A pair of electrodes added to each well enables a powerful electric field to form across its length.
The team then divided the rhodium nanowires into groups and coated each group with strands of DNA designed to bind to a specific disease marker.
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With the silicon chip immersed in ethanol, they released one group of DNA-coated nanowires into the fluid and switched on the electric fields in some of the microwells. The interplay between the dielectric properties of ethanol and the nanowires creates a force that pushes the nanowires towards the wells (see diagram). 鈥淥ur DNA-coated rhodium nanowires then snap into place due to the higher field strength there,鈥� says Keating.
The team rinses the chip, then releases another batch of nanowires coated with DNA strands that bind to a different disease marker. This time, they electrify a different set of microwells, and so on. In this way, the team places nanowires designed to detect certain diseases at specific sites on the chip. In tests, the team coated nanowires in DNA sequences that bind to nucleic acids from the genomes of hepatitis B, hepatitis C and HIV (Science, vol 323, p 352).
By labelling the nanowires with fluorescent groups, the team could see where they ended up. They found that 99 per cent of the nanowires settled precisely where they were meant to be, with electrostatic forces holding them firmly in place.
鈥�99 per cent of the wires settled exactly where they were meant to, held by electrostatic forces鈥�
鈥淗aving many copies of each type of nanowire in the array is important for avoiding false negatives and false positives, and enables screening for multiple disease targets at once,鈥� says Keating.
The device will work as a detector because a nanowire鈥檚 resistance changes when nucleic acids bind to it, and this can be picked up by circuitry that the group is planing to add.
With nanotechnologists always looking for new ways to build on the nanoscale, the Penn State team anticipates that the technique will find uses beyond biosensors, because it can add chemical, biological and optoelectronic components into traditional silicon electronics.
Geoff Thornton of the London Centre for Nanotechnology at University College London says: 鈥淲e don鈥檛 have a means of positioning a nanowire precisely where we want it right now.鈥� Indeed, nanowires must often be grown in situ, exactly where they are needed, which can present all sorts of thorny design challenges. But perhaps not for much longer.
