27 Jul 2007
Changes in reflected light identify contaminants without needing to pre-treat the sample.
Stratophase, a spin-off from Southampton University in the UK, has developed a technique to simultaneously write a waveguide and a planar Bragg grating into an optical silicon chip. The technology has been used to fabricate the company's SpectroSens sensor, which can detect the presence of toxins using optical techniques rather than costly chemical tagging.
In the company's Direct Grating Writing process, two focused UV lasers are overlapped to give a small circular spot with an inherent linear interference pattern. Modulating the beam as it scans a photosensitive substrate allows a Bragg grating to be written, allowing complex variations to the period and contrast to be incorporated within a single grating if desired.
This technique allows gratings to be manufactured faster and more cheaply, since it avoids the need for a one-off mask to be made. Accuracy is also improved as grating structures are written simultaneously with the waveguide, removing any alignment error. The substrates are standard silicon wafers, with a germanium-doped silica glass top layer in which the waveguide circuit is fabricated.
The finished detector cell incorporates a sensing window above the grating that is chemically prepared with suitable antibodies, ready to bond with a chosen target agent. A fiber-optic cable travels under the window to illuminate it with visible or infrared light. When any target molecules bond to the prepared surface, small but measurable changes to the reflected light are caused, revealing the presence of the toxin.
Greg Emerson, developer of the chip, explained that this method avoids any expensive chemical tagging of the sample itself. Instead, a change to the physical properties at the surface of the chip is detected optically, speeding up the detection process.
"We are able to measure in real time the amount of mass bound to the top of the chip surface, and observe it without needing any fluorescing antibodies," he said. "All the process and information transfer is done optically, so it is completely immune to noise."
A particular advantage claimed for this method is a reduction in the number of false-positive detections. The ability to use several colors of light allows detailed information on the thickness of the layer binding to the surface to be obtained, helping to distinguishing the target species from other non-specific events.