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Waveguide senses bacteria fast

24 Jul 2003

A grating-coupled device can detect the presence of E coli bacteria in a fraction of the time taken by standard test equipment.

Scientists in Denmark have built an optical waveguide sensor that detects bacteria using evanescent waves. The sensor could have a multitude of applications in food safety, medical diagnostics and detection of biological war agents (Optics Letters 28 1233).

Henrik Pedersen and colleagues at Risø National Laboratory used the device to monitor levels of Escherichia coli K12 cells in a solution.

The current standard test for ‘rapid’ bacteria detection, called enzyme-linked immunosorbent assay (ELISA), takes 18-24 hours to give a result and the Risø team believes that there is a large demand for much faster bacteria detectors.

Their detector makes use of the evanescent field - the light that “leaks” outside a waveguide as a beam of light travels via total internal reflection. Normally, its intensity decays exponentially away from the waveguide’s surface, and this restricts its penetration depth to no more than 200 nm.

This means that only a small fraction of bacterial cells, which are normally around 2 µm in size, can be probed, severely limiting the detector’s sensitivity. As a result, optical sensors are rarely used in bacteria detection.

The Risø team’s new waveguide design uses so-called reverse symmetry to improve sample penetration. In this configuration, a nanoporous silicon layer is sandwiched between a glass support and the polystyrene film waveguide. A grating with a periodicity of 479 nm and a profile depth of 10 nm is imprinted into the polystyrene film to couple light from a helium-neon laser into a detector lying perpendicular to the waveguide.

With this new geometry, the evanescent field penetrates further into the sample. In fact, the penetration depth has no theoretical upper limit so proteins (3-20 nm), viruses (50-200 nm), bacteria (0.5-5 µm) and even mammalian cells (up to 50 µm) could all be detected, say the team.

“Simply by choosing the correct thickness of the waveguide film, one can adjust the penetration depth,” they claim.

In Pedersen’s experiment, the bacterial solution containing Escherichia coli K12 was flowed past the sensor surface for 45 minutes. After that, the change in refractive index caused by the bacteria could be seen. The Risø team estimates the detection limit of the sensor at 60 cells per mm2, which is almost three orders of magnitude better than previous optical detection methods.

Michael Hatcher is technology editor of Opto & Laser Europe magazine.

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