Twisting the light away

Reducing the size of spectroscopic biosensors by integrating waveguides into microchannel systems is generally a good idea, but it is limited because the beam from the light source typically intersects with the analyte at a relatively small window. However, Linan Jiang and Stanley Pau of the University of Arizona, Tucson, College of Optical Sciences, devised a biosensor with a twist, maximizing the light-analyte interaction area by using a long spiral waveguide comprising several loops.

By forming a waveguide alongside a fluidic channel in a spiral configuration, researchers at the University of Arizona, Tucson, maximized the interaction area between the source light and the analyte undergoing spectrographic analysis. Courtesy of Linan Jiang.

The researchers made their waveguide using a photosensitive polymer to form a 40-μm-thick × 50-μm-wide solid core. They used polydimethylsiloxane to create a microchannel structure and placed it directly on top of the waveguide, enclosing the top and side surfaces. When they forced liquid carrying an analyte into the channel, it surrounded the core, becoming a dynamic, fluidic cladding. Importantly, the evanescent wave of the light passing through the core overlaps the fluid inside the channel by about 100 nm — along the entire length of the waveguide.

Using this technique, they made two configurations: a 30-mm straight waveguide and a 110-mm-long waveguide that they twisted into a spiral coil with several loops to fit in a 4-cm² space.

They tested both systems using various concentrations of food coloring dispersed in water as the analyte. They coupled a 650-nm laser diode made by AixiZ Service and International LLC of Houston to one end of the waveguide, using an objective and a multimode fiber made by Thorlabs Inc. of Newton, N.J., and measured the output at the opposite end using a power meter from Newport Corp. of Irvine, Calif., to obtain absorbance data.

The investigators found that the absorbance for the spirally configured waveguide was more than double that for the straight layout. They ascribed the difference in sensitivity to the increased length of the spiral path.

Compared with a conventional spectrophotometer, the system devised by the researchers measured concentration levels of approximately 10 percent versus 0.1 percent. The waveguide sensor, therefore, would optimally be used for samples with high concentrations and without excessive dilution. It should be practical for applications such as medicine and food testing and for hazardous waste management.

Applied Physics Letters, March 12, 2007, Vol. 90, 111108.