A little light detection

A

spin-out has developed a sensor that can detect the presence of anthrax or other biological hazards by monitoring the change in light refraction caused by the presence of harmful molecules.

SpectroSens sensor chip surfaces are coated with antibodies designed to attract specific antigens. When one sticks to the surface, a slight change in the colour of reflected light occurs, enabling the sensor to detect a target agent. An optical fibre takes the signal to a readout unit.

Greg Emerson, developer of the chip, said many other sensing technologies detect biohazards using expensive tagging techniques. By this method, technicians have to inject a fluorescent antibody, called a tag, into their test sample to see whether the antigen bound to the antibody.

'Our technology doesn't need that,' he said. 'We are able to measure in real time the amount of mass bound to the top of the surface, so we can observe that binding without any fluorescing antibodies.'

SpectroSens technology differs from other non-tagging sensing techniques that use reflectivity measurements, such as surface plasmon resonance (SPR), by its ability to work at multiple wavelengths simultaneously. Emerson said with multiple measurements, technicians are able to access more spectral information about a possible hazard in the environment.

'The analogy I like to use is that if you look at a train head-on, you see the front of it but you have no idea how long the train is,' said Emerson. 'Whereas if you step five steps to the side, you can at least get some perspective of its depth, size and colour. You can't say for absolute certainty its length but you have much more information to work on.'

This additional information can help ward against false detection, said Emerson. 'If you are monitoring biological hazards for homeland security, you don't want to give a false positive in an airport,' he said. 'Likewise, you don't want to give a false negative.'

SpectroSens chips are small (about 10mm x 20mm) monolithic devices with no moving parts and are made from silicon and glass. Each chip can contains one or more sensing areas, typically 1-2mm long and 10 micrometres wide.

The sensitivity of the chips is application- specific. However, the sensitivity to bulk liquid refractive index is 10-6, which gives SpectroSens chips a sensitivity equal to or better than SPR. The key functional element of the SpectroSens is the detection chips. These can be coated with a wide range of immunoassays or other chemically selective surface coatings.

While the sensitivity of the sensors are comparable to SPR, Emerson said the chips' self-contained structure makes it a much more robust technology than SPR. 'For example,' he said, 'an SPR couldn't be used on the back of a truck, whereas our unit can be used on the side of an engine block while it's running.'

Other than homeland security, Emerson said there are many other applications, such as drug detection, where the chips can be used in an array format to screen reactivity between potential drugs and their targets. He added that the sensors could also be used in food and beverage manufacturing, where contamination and liquid composition need to be controlled. 'There are a many factories where the pipes can go on for kilometres and these devices, because they're based on optical fibres, can easily spread that kind of distance and go back to a central hub,' he said. 'Obviously all that process and information transfer is done optically, so it is completely immune to noise.'

Currently all the information from the sensors is fed back to a transducer base, but Emerson said the long-term goal for Stratosphase could be to develop a handheld device.