Nanostrings enable electronic 'nose'

Physicists at Ludwig-Maximilians-Universitaet in Munich (LMU) have constructed a system of nanostrings made of non conducting material, in which each string can be electrically excited separately.

Thousands of these strings can be produced on a small chip and one of the devices that could be created with this system is a highly sensitive 'artificial nose' that detects various molecules individually.
When a molecule docks onto a string, the string becomes heavier and its oscillation slows down a tiny bit. "By measuring the period of oscillation, we could therefore detect chemical substances with molecular precision," explains LMU's Quirin Unterreithmeier. "Ideally, you would have several thousand strings sitting on a chip the size of a fingernail, each one for specifically recognising a single molecule – so you could build an extremely sensitive 'artificial nose', for example."
Nano electromechanical systems – or NEMS – systems involve strings with diameters of around 100nm. If these strings are coated with the right kind of chemicals, only one kind of molecule can dock onto each string.
Until recently, getting such systems to work has proven technically difficult; one problem being how to produce and measure the oscillations. While the nanostrings can be made to oscillate by magnetomechanical, piezoelectric or electrothermal means, this only works if the nanostrings are made of metal, or metal coated. This dampens the oscillations and prevents sensitive measurement.
The new approach avoids these difficulties. Unterreithmeier, along with Dr Eva Weig and Professor Jörg Kotthaus from LMU's Center for NanoScience, have constructed a NEMS in which silicon nitride nanostrings are excited individually by dielectric interaction using an oscillating inhomogeneous electric field.
The alternating electric field required for this stimulation was produced between two gold electrodes close to the string. Oscillations were measured by two other electrodes.
"We created this setup using etching techniques," said Dr Weig, "but this was easily done – even repeated ten thousand times on a chip. The only thing to do now is to make sure the strings can be addressed individually by a suitable circuit."

Graham Pitcher

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