“You can functionalise the wire by coating it with a chemical that will attract a particular anti-body,” Professor Peter Ashburn told Electronics Weekly. “Anti-bodies are usually charged, so they alter conduction in the nanowire, which you can detect.”
A simple version might involve a 1µm wide silicon track on the surface of a chip, said Ashburn, but this might not be sensitive enough.
“To get high sensitivity, you need a large surface to volume ratio, so there are advantages in a 50nm wire,” he explained.
Beyond thinning the wire, to further increase sensitivity the University proposes to expose not just the top, but the sides of the wire as well to anti-bodies – the bio equivalent of a finfet – or even expose the whole wire by suspending it in space.
“We are going to look at a polysilicon finfet structure made on oxide or nitride using techniques that are used to make displays,” said Ashburn. “This is probably the most straight forward method to get anti-bodies on three sides.”
By exploiting anisotropic etching ‘spacer’ techniques used in chip-making, the team aims to construct 50nm wide wires using 1µm lithography. The fully-exposed wire approach may be examined by forming the sensor across a 2µm wide gap.
This is intrinsically more sensitive but, said Ashburn, free-standing nanowires may be too fragile to withstand normal liquid processing, and the jury is still out on the safety of free nanowires that escape into the environment.
Beyond the sensing structure, which the team aims to have prototyped in between 12 and 18 months, the three-year plan includes integrating electronics and micro-fluidics to produce a self-contained device that allows routine blood tests to be completed in a doctor’s surgery.
Ashburn feels that chemical coating technology is sufficiently developed to allow a single chip to have wires that detect different anti-bodies. “I can envisage three, four of five nanowires 100µm apart to identify different biomarkers.”
The chip will probably be disposable. “We could make it re-usable, but the doctor will somehow have to sterilise it,” he explained.
Towards the cost of the programme, Ashburn’s nanotechnology group and the University’s schools of medicine have been awarded £1,330,346 by the Government’s Nano Grand Challenge in Healthcare.