Rapid bacterial detection using novel optical waveguide sensor

Citation

(2004), "Rapid bacterial detection using novel optical waveguide sensor", Sensor Review, Vol. 24 No. 1. https://doi.org/10.1108/sr.2004.08724aaf.006

Rapid bacterial detection using novel optical waveguide sensor

Rapid bacterial detection using novel optical waveguide sensor

Keywords: Bacteria, Optics, Waveguide, Sensors

Bacterial infections represent one of the world's most serious health problems: they account for as many as 40 per cent of the 50 million annual deaths occurring worldwide and over 90 per cent of the 81 million infections from food that occur annually in the US arise from bacteria. Thus, there is a growing demand for effective bacterial detection and screening techniques. The most critical features of such techniques are speed of response and sensitivity, as it is believed that the infectious dose of some strains of Escherichia coli and Salmonella is as low as ten cells. The most widely used bacterial sensing techniques which offers adequate sensitivity involve either a traditional agar culture or an immunoassay but these can take between 18 and 24 h to complete, making them unsuitable for, say, on-line screening in a food production plant or the rapid diagnosis of an infection.

Optical sensing techniques using evanescent fields (i.e. the light that “leaks” outside a waveguide as it travels via total internal reflection) have been suggested as candidates for the rapid detection of biomolecules and micro-organisms and several products have appeared on the market. These include the Swedish BIAcore instrument and the American BioStar, which can rapidly and sensitively detect viruses and proteins, typically in around 5-20min. However, these are small molecules and organisms (~ 3-200 nm in diameter), and as bacteria are typically sized in the 0.5-5 µm range, they cannot be detected by an evanescent optical field which decays exponentially from the waveguide and only extends to about 100-200nm into the sample volume.

Now, researchers from the Optics and Fluid Dynamics Department at the Danish Risø National Laboratory have developed a technique that overcomes this limitation by designing a grating waveguide with a far greater optical penetration depth. This is achieved through the use of the so-called “reverse symmetry” (Figure 1).

Figure 1 Comparison of depth sensing capabilities of conventional and reverse symmetry techniques (RisøNational Laboratory)

In this arrangement, a polystyrene film waveguide, with a thickness of 160 nm and a refractive index of 1.57, is supported by a 1 µm- thick substrate layer of nanoporous silica, which is supported by a 1.6 mm glass plate. The refractive index of the silica layer is 1.22, lower than that of water (1.33), hence reverse symmetry is achieved. A sinusoidal surface- relief grating with a periodicity of 479 nm and a profile depth of 10 nm is imprinted into the polystyrene film to couple light from the source, a helium-neon laser with a wavelength of 632.8 nm, into the waveguide. To bind the bacteria onto the sensor surface, the waveguide is coated with a 4 nm layer of poly-L-lysine (PLL), a protein that imparts a positive charge to the surface, resulting in strong electrostatic binding to the cells. The entire sensor system is mounted on a high-precision, computer controlled goniometer, as shown in the schematic in Figure 2.

Figure 2 Sensor arrangement (Risø National Laboratory)

In the reverse symmetry configuration, the evanescent field penetrates further into the sample and by simply altering the thickness of the waveguide film it is possible to adjust the depth of the optical penetration. As the penetration depth has no theoretical upper limit, it is possible to detect large cells such as bacteria and perhaps even parasites and mammalian cells (~ 5-50 µm in diameter), as well as the smaller molecules and organisms detected by conventional evanescent field sensing methods.

The sensor has been tested with a bacterial solution, prepared by suspending an E. coli K12 colony from an agar plate in 10 ml phosphate- buffered saline, yielding a concentration of 3 x 10⁷ cells/ml. Following a 40 min period during which a buffer solution was applied, the bacterial solution was allowed to flow past the sensor's surface for 45 min, which led to a continuous change in the positions of the TE and TM peaks, caused by the accumulation of bacteria on the surface. The research group estimates that the detection limit of the sensor is approximately 60 cells/mm², which is almost three orders of magnitude better than that achieved by alternative optical detection methods.

This is an important technological development, as the ability to probe deeply into biological samples with an evanescent field, combined with the greatly reduced response times and the high inherent sensitivity, opens up a whole range of biochemical sensing possibilities. These include water quality testing, environmental monitoring, screening patients for diseases, detecting biological warfare agents and the quality control of foods during processing and production.

For further information, please contact: Henrik C. Pedersen, Senior Scientist. Tel: +45 4677 4550; E-mail: henrik.pedersen@risoe.dk