Sensing the vibes

US researchers attempt to make highly-sensitive ‘vibrating plank’ sensors for micro-organisms which are ideal for use in environmental monitoring, in hospitals and for security purposes.

One of the odd things about nanotechnology is that many of the sub-micron sized devices, although apparently the highest of hi-tech, actually use some of the simplest engineering techniques.

At Purdue University in Indiana, for example, Rashid Bashir’s research group is trying make highly sensitive sensors for micro-organisms which are basically an array of vibrating planks. In this case the planks are a few microns long and 20nm thick, and could be used in environmental monitoring, in hospitals and for security purposes.

The sensors work by monitoring the frequency at which the planks — more correctly called nanocantilevers — vibrate. They are made from silicon, and coated with antibodies which bind to specific viruses, bacteria and other pathogens.

Each cantilever has a characteristic resonant vibration frequency, dictated by its mass and mechanical properties. When these micro-organisms stick to the surface of the nanocantilever, the mass increases, which also increases the resonant vibration frequency. This is the opposite of what happens with thicker cantilevers — an extremely unexpected result.

Bashir’s team, based at the university’s Birck Nanotechnology Centre, was also surprised to find that, when they were coating the silicon structures with the antibodies, the longer nanocantilevers attract a greater density of antibodies than shorter ones. This is significant, because the cantilevers seem to work best if the antibody coating is roughly the same thickness as the cantilever itself.

To attach the antibodies, the silicon nanocantilevers are immersed in a protein solution. Longer structures were expected to attract more antibodies, said Bashir, ‘but instead of simply attracting more antibodies because of their greater length, they also contained a greater density of antibodies’. The density was also greater at the free end of the cantilever than at the bound end.

The team found that the resonant vibration was fastest with the coating thickness equal to the cantilever thickness; also, the frequency increases as the cantilevers get longer. This, said Bashir, will be essential for nanoengineers designing structures that include cantilevers.

The anomalous increasing of the resonant vibration results from both the thickness of the cantilever itself and the relationship between the thicknesses of the cantilever, its antibody coating, and the amount of pathogens that stick to the coating.

When micron-thick cantilevers are used in detectors, the pathogens sticking to the surface act as vibration dampers and reduce the resonant frequency slightly.

But because the nanocantilevers are only a hundredth of the thickness, their mass is very small, so adding a coating has a much more profound effect.

‘The conclusion is that when the attached mass is as thick as the cantilever, then you not only affect the mass but also affect a key property called the net stiffness constant,’ explained Bashir. When this increases, the vibration frequency goes up.

Because the nanocantilevers are coated with specially-chosen antibodies, the vibration only changes when the antigens specific to that antibody are present.

This, said Bashir, makes them useful for environmental monitoring, in hospitals and for security purposes. A single 1cm-square chip could hold thousands of cantilevers, each responding to a different pathogen.